School of Engineering and the Built Environment
Honours Project (Engineering)
MHH623549
Project Title: “The Use and Accuracy of the Arrhenius
Equation in Electrical Plant Lifespan
Predictions”
Programme/Option: Electrical Power Engineering (Hons)
Author: Lee Manson
Student Number: S1631921
Supervisor(s): Dr Geraint Bevan
Non-Disclosure Agreement:
Yes /
No.
Except where explicitly stated all work in this report, including the appendices, is my
own
Signed:
Date: 25/04/2019
ABSTRACT
The Author was seeking to investigate the use and accuracy of the Arrhenius equation in
estimating the remaining useful lifespan of electrical insulation. This is common industry
practice, which is shown in the real-world application completed within, and it is used
throughout international and national standards.
To verify a relationship and check the accuracy of it, a practical experiment was attempted
which accelerated the ageing of polyvinylchloride cable insulation to trend a practical lifespan
focused on the effects of thermal stress only, which is deemed to be the dominant stress in
electrical installations. Then a calculated Arrhenius estimation was completed for comparison
to the practical experiment results.
This proved to be inconclusive due to the main properties and changes seen in the ageing of
polyvinylchloride insulation, which are mechanical rather than electrical, and due to the time
and budget constraint’s to the investigation these mechanical/chemical changes were not tested
or trended which led to a less than accurate practical estimation.
The author concludes that whilst there is an observable and measurable change within the
materials which appears to follow a linear pattern related to the temperature and time used, and
in essence this describes the Arrhenius relationship, the experiment completed for this project
has been unable to comprehensively prove this relationship and therefore has not checked
the accuracy of using it in these estimations.
The author then proceeds to highlight future work that is required to be done to complete the
goal of this investigation.
Page | i
ACKNOWLEDGEMENTS
I would like to take the opportunity to acknowledge help and support given to me during the
completion of this report (and the degree) that marks a seven-year journey from college to
university, and with that in mind I would like to thank all the lecturers and staff that have helped
me get to this point, from Glasgow Kelvin College (Springburn) and GCU. A few people
deserve special mention such as George McGuire who gave me a second chance after my
biggest setback during my 1st HNC attempt, and from Springburn also -Fraser, Brian, Faris,
Parky, Nirmal etc. who all went the extra mile in assisting me, from GCU I have received lots
of help from Michelle McCourt in SEBE support particularly during my Industrial Placement
with EDF Energy and Dr Geraint Bevan who has made this year achievable even though it has
by far been the toughest yet, his calming influence helped talk me off a metaphorical ledge time
and again.
From EDF Energy I’d like to thank the Electrical Group Team who have all offered advice and
help at some stage over the last year. A special mention goes to Dougie McIntosh, my
Group/Branch Head who has been very understanding of my part time student requirements
and helped me with my project topic and Sean O’Neill who has been a great source of advice
whilst having to sit next to me and listen to all my gripes and moans.
To all my friends, I appreciate the understanding and support you have all given me these last
few years, in particular - Dave Mulholland and John Hodgson who was my boss all my time at
college when I struggled with deadlines and the demand of full-time study and 20/30 hours of
work a week.
Thanks to my Mum, Brothers and Sisters who have been patient with me having had to listen
to all of my moaning about the workload (see the pattern).
To my friend and study partner for the last five years, Michael Gilfeather, thanks for keeping
me going, especially during our 3rd year (which was a shock to the system coming to university
from college). It’s been tough at times for both of us but now we are working in this field and
starting to reap the benefits of all these choices we have made, I hope to continue this path and
we can both see the rewards for finally eating that bloody elephant. Cheers Mate!
To Keith and Chris who have been incredibly supportive during my studies, helping whenever
possible especially with the girls, I want to say a big thanks. You do for family as we say but
it’s always appreciated and hopefully reciprocated.
Finally, the biggest thanks and recognition goes to my loving partner Sheena and my daughters
Sophie, Hannah and Katie. Without you I would not have been able to accomplish any of this
and you have all sacrificed for my choices over these years, so I hope I can repay that support
and faith. I love you all so much and I promise no more academic study!
Page | ii
TABLE OF CONTENTS
ABSTRACT ............................................................................................................................................... i
ACKNOWLEDGEMENTS ...................................................................................................................... ii
FIGURES .................................................................................................................................................. 4
TABLES ................................................................................................................................................... 5
ABBREVIATIONS ................................................................................................................................... 6
1
INTRODUCTION ..................................................................................................................... 7
2
SCOPE ....................................................................................................................................... 9
3
THE ARRHENIUS EQUATION AND REAL-LIFE ENGINEERING ................................... 9
3.1
LITERATURE REVIEW ...................................................................................................... 9
3.1.1
The Origins of the Arrhenius Equation .............................................................................. 9
3.1.2
Electrical Insulation & the Arrhenius Equation ............................................................... 10
3.1.3
Nuclear Qualification & Remaining Life Prediction ....................................................... 11
3.1.4
Rotating Machines & Stresses ......................................................................................... 12
3.1.5
Literature Review Summation ......................................................................................... 12
3.2
REAL WORLD ARRHENIUS APPLICATION ................................................................ 13
3.2.1
Arrhenius Equation Input Parameters (HNB 11kv Motor Stator Windings) ................... 15
3.2.2
Operational Data and Determination of Main Motor Winding Temperature Profiles ..... 17
3.2.3
Calculation Method For Thermal Life Expired Based on the Arrhenius Equation ......... 19
Page | 1
3.2.4
Calculated Thermal Life .................................................................................................. 20
3.2.5
Temperature Profile Charts .............................................................................................. 21
3.2.6
Operation in Normal Conditions ...................................................................................... 23
3.2.7
Operation in Outage Conditions at Elevated Temperatures ............................................. 24
3.2.8
Discussions of 11kV Motor Evaluation ........................................................................... 25
3.2.9
Recommendations of 11kV Motor Evaluation (Purss & Manson, 2018) ........................ 28
3.3
PRACTICAL EXPERIMENT ............................................................................................. 29
3.3.1
Cable Specifications......................................................................................................... 29
3.3.2
Methodology .................................................................................................................... 30
3.3.3
AGEING & TESTING .................................................................................................... 31
3.3.4
RESULTS ........................................................................................................................ 34
4
DISCUSSION .......................................................................................................................... 41
5
CONCLUSIONS AND FURTHER WORK............................................................................ 42
6
REFERENCES ........................................................................................................................ 44
7
BIBLIOGRAPHY .................................................................................................................... 46
8
APPENDICES ......................................................................................................................... 47
8.1
Appendix A – HYB/TOR 11kV motor stator ageing test programme and applicability of
results to HNB/HPB 11kV motors. ...................................................................................... 47
8.2
Appendix B – Base Data from HYB/TOR 11kV Motor Lifetime R&D Project ................. 50
8.3
Appendix C – Data Analysis Results (valid August 2018) .................................................. 51
Page | 2
8.4
Appendix D - Case study STATOR 12 versus STATOR 12.2 ............................................ 52
8.5
Appendix E - % Thermal Life Expired Charts .................................................................... 54
8.6
Appendix F – 11kV Motor Stator Thermal Life Going Forward by Reactor Location ....... 73
8.7
Appendix G – Details from BS EN 60216-8:2013 Suggested Temperatures and durations
for accelerated ageing tests. ................................................................................................. 74
8.8
Appendix H – Excel Data capture from Calculated Arrhenius Service Levels ................... 75
Page | 3
FIGURES
Figure 1 - Cable Ageing Samples – 1m and 5 m coils ............................................................................ 32
Figure 2 - Electrical Test Circuit ............................................................................................................. 32
Figure 4 - Week 1 @ 120 degrees Celsius .............................................................................................. 35
Figure 3 - Week 2 @ 120 degrees Celsius .............................................................................................. 35
Figure 5 - Week 3 @ 120 degrees Celsius .............................................................................................. 35
Figure 6 - Week 5 @ 120 degrees Celsius .............................................................................................. 36
Figure 7 - Week 4 @ 120 degrees Celsius .............................................................................................. 36
Figure 8 - Week 6 @ 120 degrees Celsius .............................................................................................. 36
Figure 9 - Week 7 @ 120 degrees Celsius .............................................................................................. 37
Figure 10 - Thermal Capture of Sample from beginning to end (aged @ 120 degrees Celsius for 7
weeks). ..................................................................................................................................... 39
Figure 11 - Electrical Test Circuit and Thermal Capture ........................................................................ 39
Figure 12 - Arrhenius Equation Variation used to calculate service time @ rated temperatures ............ 41
Figure 13 - Service life @ Ea = 60 kj/mol .............................................................................................. 75
Figure 14 - Service life @ Ea = 130 kj/mol ............................................................................................ 75
Page | 4
TABLES
Table 1 - Simplified Example ................................................................................................................. 19
Table 2 - Stators >70% Thermal Life Expired ........................................................................................ 20
Table 3 - Winding Temperature vs. % Thermal Life Expired................................................................. 23
Table 4 - R3 Outage 2015 / R4 Outage 2017 .......................................................................................... 25
Table 5 - Testing Temperatures & Approximate Lifespan of Insulation ................................................ 31
Table 6 - Tactile Inspection Notes .......................................................................................................... 34
Table 7 - Weight loss over 8-week period/two temperatures .................................................................. 38
Table 8 - Suggested exposure temperatures and times ............................................................................ 74
Page | 5
ABBREVIATIONS
A
AGR
CO2
EDFE
EPR
eV
GCU
HNB
HPB
hrs
HSE
HV
HYB
IEEE
IR
K
kj/mol
kV
kW
m
MΩ
NDA
OIP
PLEX
PVC
RCD
rpm
SFE
TOR
XLPE
Page | 6
Amperes
Advanced Gas-cooled Reactor
Carbon Dioxide
EDF Energy
Ethylene Propylene Rubber
electron Volt
Glasgow Caledonian University
Hunterston B Power Station
Hinkley Point B Power Station
Hours
Health & Safety Executive
High Voltage
Heysham 2 Power Station
Institute of Electrical and Electronics Engineers
Insulation Resistance
Kelvins
Kilo joules per mol
kilo Volt
kilo Watt
Metres
Mega Ohms
Non-Disclosure Agreement
Oil Impregnated Paper Insulation
Plant Life Extension
Polyvinylchloride
Residual Current Device
Revs per minute
Super Fluid Extraction
Torness Power Station
Cross-Linked Polyethylene
1
INTRODUCTION
Electrical Plant such as transformers, cables and motors all have electrical insulation within
their construction. This insulation is used to protect the electrical circuit from leakage currents,
to protect anyone working on or located near the circuit and it provides an element of
mechanical protection. There are many different types of material used in the construction of
electrical insulation but they all have one thing in common – they restrict the flow of current in
an electrical circuit by having very high impedance. A perfect insulator would completely
inhibit the flow of current but, due to minor imperfections and small amounts of charge carrying
materials located within, a perfect insulator is impossible to manufacture therefore materials
with a very high resistivity are used.
From the UK government’s Health and Safety Executive the following information can be
found, the home page for the HSE website states “Electricity is a familiar and necessary part of
everyday life, but electricity can kill or severely injure people and cause damage to property.”
From this statement, which features prominently within the advice given, the priority given on
electrical safety is people first then property. As mentioned above electrical insulation is used
to provide protection in both cases. Due to the physical laws of electricity, specifically
Maxwell’s equations for magnetic force and Faradays equations for electric field theory, when
you are dealing with large amounts of power, which consists of high levels of voltage and
current, physically large electrical plant is required to handle the electrical and magnetic stress
and this investment in plant is expensive, not to mention the large amounts of investment seen
in the transportation of electrical power over large distances.
Once an electrical system has been designed, considering that it needs to be safe and as efficient
as is practicable, it is then put into service to generate, transport and deliver electrical power to
the consumer or ‘load’. At this point manufacturers will have defined a maximum useful
lifespan of the cable, generator or motor etc. before it is no longer capable of safely and
efficiently delivering the power required and this time limit will be based on rigorous testing
performed during its design. The lifespan given by the manufacturers after this testing will be
on the cautious side such as the minimum lifespan seen from all the samples tested, this is to
ensure safety when operating and to protect the manufacturers from any litigation based on
figures given to customers being too generous. The data given is fine in a laboratory setting
however in the real-world environment the plant usually does not see constant, continuous load.
Often the service life of the plant item is dynamic, which is to say that it experiences cyclical
or random levels of loads and ambient/ environmental conditions.
To address this uncertainty and to take full commercial advantage of the investment in the plant
there are a variety of maintenance plans adopted to ensure that the plant continues to be safe
and efficient for as long as possible. The maintenance plans used in industry are routinely
scheduled (planned maintenance), condition based (condition monitoring) or reliability centred
maintenance (predictive maintenance) that seek to address any potential issues seen by the plant
due to different types of stress in service and extend the lifespan whenever it is safe to do so.
As with most engineering solutions each of these types of maintenance plans have pros and
cons related to them which will not be discussed within the scope of this investigation.
Page | 7
Generally, this report will focus on predictive maintenance plans as this is the most common
trend adopted by engineers due to the relatively low amount of financial investment required to
adhere to it. Specifically, the report will investigate a predictive calculation that is used in a
variety of industries (and has been for decades) to provide an accurate lifespan for materials
that encounter thermal stress during service whether normal or abnormal and seeks to explain
the mechanical/chemical breakdown of these materials, this calculation is called the Arrhenius
Equation after the engineer who brought it to prominence.
The use of the Arrhenius equation in predictive maintenance has been chosen as the focus of
this report as thermal breakdown has long been accepted as one of the dominant factors in
ageing of electrical plant, whether that thermal stress comes from ambient (environmental)
temperatures or from within the conductors (Ohmic heating or ‘Copper Losses’). How this
report will investigate the Arrhenius Equation can be seen in Section 2.
Page | 8
2
SCOPE
This report will investigate the use and accuracy of the Arrhenius equation in electrical
engineering. To achieve this it will investigate current literature regarding the subject matter
and include the equations in use from inception until present day, there will be an example of a
real-world engineering problem from a nuclear power plant that is utilising the Arrhenius
equation, with details from the author’s employer EDF Energy (EDFE), and there will be a
practical experiment on low voltage cables to check the accuracy of the Arrhenius equation
which the report will critically analyse and present the results. Finally, there will be a conclusion
on the use and accuracy of the Arrhenius equation which will provide an idea of how successful
the investigation was and any future work that could be completed which would improve upon
the investigation.
3
3.1
THE ARRHENIUS EQUATION AND REAL-LIFE ENGINEERING
LITERATURE REVIEW
3.1.1
The Origins of the Arrhenius Equation
A search using IEEE Xplore database provides the extensive list of journals, standards and
reports that discuss the Arrhenius Equation in the context of engineering. These include the
modelling of brain tissue heating caused by direct cortical stimulation and the estimation of
maximum operating temperatures for copper wire bonds, this shows that the various uses of the
Arrhenius equation are not necessarily involving the electrical aspect. Svante August Arrhenius
whose name the equation bears was in fact a physicist who received the Nobel Prize for
chemistry in 1903.
The Arrhenius Equation has been used in the study of kinetic energy and chemical reactions
since the mid nineteenth century. Arrhenius, and other scientists of the time such as (van't Hoff,
M. J. H., 1884) and (Hood, 1885), realised that there was a noticeable dependence on
temperature for chemical reaction rates.
This is shown in (Logan, 1982) which states “…and in 1889 Arrhenius showed that temperature
and rate constant could be correlated by one simple equation, which still bears his name…”.
The most familiar form of that equation is:
𝐄
𝐤 = 𝐀. 𝐞−(𝐑.𝐓)
Where,
k = is the rate constant in units depending on the global order of reaction
A= the pre-exponential factor, a constant for each chemical reaction
E = the activation energy in kilo-joules per mol
R = the universal gas constant in kilo-joules per kelvin per mol
T = absolute temperature in kelvin
e = Euler’s constant
Page | 9
3.1.2
Electrical Insulation & the Arrhenius Equation
From this earlier work up to modern times there is rarely a journal article, paper or standard
surrounding thermal ageing or stress in electrical plant/machines that doesn’t directly or
indirectly reference the Arrhenius equation. This shows just how important the work was and
since the widespread inclusion of electricity into modern life, and the plant and machines that
come with it, there have been investigations into ageing mechanisms for insulation materials.
Operational experience shared amongst engineers during the early years of electrical power
distribution showed a possible correlation between thermal stress and lifespan, but it was not
really until 1930 when Montsinger established the 8-Degree Rule of Thumb, that industry
noticed the benefits that could be achieved using the Arrhenius equation (or work based upon
it).
Paper (Montsinger, 1930) stated “It is shown by the use of the thermal laws that without
increasing the maximum or hot spot temperature, transformers can be overloaded 1 per cent for
each degree centigrade by which the ambient is below 30°C (air) for self-cooled transformers,
25°C (water) for water-cooled transformers.” Also quoted is the more common “These tests
show that the rate of aging is roughly double for each 8°C increase in temperature.”
Once this theory was established this became the industry standard for evaluating the remaining
lifespan for electrical plant such as transformers, cables and motors to aid in the planning of
maintenance and potential replacements and whilst not the only method available to engineers
it is still widely in use for guidance in estimating plant lifespan.
This theory developed further when in 1948 Thomas W. Dakin proposed the paper “Electrical
Insulation Deterioration Treated as a Chemical Rate Phenomenon” (Dakin, 1948). In this paper
Dakin examined changes in electrical/physical properties of electrical insulation and proposed
a method of interpreting them during thermal ageing related stress. It also discussed the forms
of deterioration within insulation such as slow oxidation, brittle hardening of insulation due to
loss of plasticiser and excessive cross linking of polymer chains and a purely thermal or
internally depolymerisation of plastic insulation. Dakin’s method assumed that the observed
changes are resulting from internal reactions that obey theoretical laws and that is it possible to
know the reason for electrical failure, i.e. a short circuit that is usually in the form of mechanical
failure due to lower strength or flexibility.
Dakin states in (Dakin, 1948) that “it is important to note that the absolute temperature scale is
used, consequently the temperature intervals at higher temperatures have, percentagewise, less
effect on a reaction than the same temperature intervals at a lower temperature. This fact is
neglected in the commonly used rule of thumb, where the temperature interval for which a
reaction rate doubles is used”
As can be seen within more modern approaches, such as (Duran & Duarte, 2012) when
attempting to assess the lifespan of electrical plant or machines there are a few steps to ensure
the estimation is as accurate as practicable. Firstly, a list of the assets to be evaluated is collated
Page | 10
then secondly, all the potential stresses that plant may see in normal and fault conditions are
listed and finally the various methods for estimation are identified. Paper (Duran & Duarte,
2012) also states “In 1948 T. W. Dakin published some articles which raised a more precise
mathematical expression to calculate the deterioration of the insulation from the Arrhenius
equation.”
The list of estimation methods investigated in (Duran & Duarte, 2012) are listed below;




The Accelerated Ageing Factor (FAA) for thermally or non-thermally upgraded
dielectric materials (British Standards Institute, 2018; IEEE, 2012) (based on the
Arrhenius equation and Dakin’s proposition).
Ageing due to number of operation cycles (Kam & Ledwich, 2009) which applies
mainly to switching and protection equipment.
The Delta-T criteria (Lindquist, Bertling & Eriksson, 2005), which evaluates the
temperature increase in the contacts of electrical equipment by constant current flow.
Two-parameter Weibull distribution to model dielectric materials ageing (Stone &
Lawless, 1979)
This work from (Dakin, 1948) and (Duran & Duarte, 2012) highlight the common use of the
Arrhenius equation, and Dakin’s proposition based on it, in the estimation of the lifespan of
electrical insulation when it is experiencing thermal stress.
3.1.3
Nuclear Qualification & Remaining Life Prediction
As seen in (Robertson & Lamont, 2015) , The first commercial nuclear reactors were developed
in the 1950’s which was the generation-1 reactors, and since the late 70’s- early 80’s the
generation 2 reactors, advance gas-cooled reactors (AGRs) in the UK, have been connected to
the grid generating electricity. There are many ageing mechanisms as previously discussed, and
the same mechanisms apply to any nuclear power plant, but there is also the presence of ionising
radiation in certain areas of the plant, within the reactor and any fuel storage or preparation
area.
As stated in (Banford & Fouracre, 1999) “it is of major importance to know how these materials
will respond and how any resulting ageing phenomena can be monitored”. Insulation types that
are most commonly found in cables used in nuclear power stations are Polyvinyl chloride
(PVC), Cross-linked polyethylene (XLPE), ethylene propylene rubber (EPR) amongst others
which are based on polymers. This is of interest as this type of accelerated ageing due to dose
rate of ionising radiation influences electrical insulation, however due to the difficulty in
obtaining radioactive material for testing purposes the effects of radiation have had to be
excluded from this experiment, this will be mentioned in the further work section within the
main body of the report.
Page | 11
3.1.4
Rotating Machines & Stresses
Paper (Brancato, 1978) states that the role of insulation is to withstand stress, separating the
conductors from the outside environment and protecting the circuit, people and the system. This
stress that can be seen is multifactor, as mentioned in Section 1, and one final type of aging
mechanism or stress is vibration.
Rotating machines by their very nature encounter vibration, this is due to the specific nature of
motors, pumps or generators etc. which is to rotate at great speeds, for example 3000 revs per
minute (rpm) for any electrical generator in the UK. This rotating causes vibration and whilst
this investigation is focusing on thermal stress, the real-world application example will
investigate 11kV induction motors, the reason for which will be discussed further in Section
3.2, therefore it is necessary to note that these motors experience a lot of stress due to the
location and environment they are situated in.
Vibration in electrical equipment can sometimes be tolerated, particularly in rotating machines
however, large amounts of vibration can indicate that there are problems or deterioration of the
equipment, electrical insulation in most modern machines comprises layers of oil impregnated
paper or cast resin and when there are contaminants within the layers or poor manufacturing
practices then the final product is imperfect. These imperfections can become worse brought
about by constant vibration and the potential for resonance when the frequency of the vibration
resonates with the materials and contributes to overall damage.
3.1.5
Literature Review Summation
Paper (IEEE, 2014) section 4 discusses the various stressors seen by electrical plant, whether
environmental or operational and that a typical concern is the potential for synergistic effects
between temperature and radiation in some polymers, which is to say that the effects for thermal
and radiation caused degradation can combine to be greater than the sum of the individual
effects.
Within the further work section of this project report these ageing mechanisms will be
considered, but for the purposes of this investigation the main stresses experienced by the
motors, and the possible detrimental effects caused by them, will be listed here:


Mechanical (including vibration seen at full speed 3000rpm) – this can loosen
fasteners on linkages causing loss of electrical contact integrity and possible heatrelated degradation from the poor connections, it also causes wear in moving parts due
to misalignment. It can be mitigated for in the design stage and monitored for in a
condition-based maintenance regime to trend
Electrical (caused by the 11kV, 3 phase supply) – this is from the strength of the
electrical field flowing through the conductors and insulation causing mechanical stress
– again this can be mitigated in the design stage with conservative estimates, meaning
the conductors and insulators have adequate electrical field strength.
Page | 12



CO2 gas atmosphere (instead of air) – this hasn’t been fully investigated other than
to assume that an air atmosphere at varying pressures is more onerous due to oxidation
effects on the plant, therefore testing in air provides further conservatisms in the design
stage which mitigates these effects.
Doses of Ionising Radiation – in (Brancato, 1991), Brancato states “although radiation
does tend to deteriorate insulation, in mild environments where the total dose absorbed
during the lifetime of the equipment does not exceed 10,000 rads there appears to be
no perceptible effects on the insulation lifetime expectancy.” It’s worth noting that in
the context of Brancato’s report he was discussing plastics in general.
Thermal (both from ambient heating and from Ohmic heating) - (Brancato, 1991) also
states that “the electrical endurance qualities of insulation materials are affected by
temperature and time. Relatively moderate temperatures can cause failure if applied for
very long periods of time”. For example, if a cable had been poorly specified or the
load connected is increased without thought to overloading, this increased current will
cause more heat to be generated, without an insulation system sufficient enough to take
this heat away it will deteriorate the insulation quicker than expected leading to
spurious tripping of protection devices or even destructive faults such as a short circuit.
Of these stresses, from experience over 40 years of service, EDF Energy has determined that
the most onerous of these stresses are the Thermal stresses, High pressure differences and the
ionising radiation from the reactor core.
3.2
REAL WORLD ARRHENIUS APPLICATION
In the early 2000’s all of the Advanced Gas Cooled Reactors (AGRs) had their original designed
generating life extended due to the condition of the plant and ancillary systems. Paper (Banford
& Fouracre, 1999) states that “There is a general desire to continue operating the nuclear power
stations for as long as possible. To do so, however, the utilities must convince the relevant
regulatory authorities, as well as themselves, that continued operations are acceptably safe”.
This work was completed to ensure plant life extension (PLEX) was not only safe but
commercially viable.
Now, nearing decommissioning, the same utility companies are looking at similar work to
ensure that any major work that would be routinely completed can be deferred due to the plants
closure date if it is safe to do so. Two power stations that are due to be decommissioned in 2023
are Hunterston B (HNB) and Hinkley Point B (HPB) power stations and each station have two
AGRs onsite. The B moniker indicates that each site has had power stations on site before;
these were the 1st generation Magnox stations Hunterston A and Hinkley Point A which are
owned and operated by Magnox.
As part of the design of the AGRs there were large 11kV electrical induction motors installed
for safety use, to ensure that an NDA was not required the specific duty for these 11kV motors
has intentionally been omitted however these motors are important to safe and efficient
operation of the power station.
Page | 13
To qualify an accurate life expectancy when the plant was being designed, and to test the
thermal endurance of the motor insulation systems, test models of reduced sizes called
‘motorettes’ were designed using the same materials to scale. These motorettes could be
exposed to accelerated ageing mechanisms for investigation giving a realistic approximation of
the life sized motors ability and limits and this is discussed in (Brancato, 1978). These motors
were rigorously tested using test rigs in the early 80’s and part of these tests were for the motor
stator windings and the insulation used. Paper (EDF Energy, 1982) states that the motor stators
were tested at an accelerated temperature, voltage and various depressurisation rates, to
adequately qualify it for a temperature indices (TI) value of 155°C for 3000 hours, validating
its class F rating and giving it a failure time of 20,000 hours at continuous hot spot loading.
This equates to approximately 30 years for the motor stators as the average temperature rate is
approximately 80°C.
At HNB & HPB that 30-year period has since been surpassed due to PLEX and there has been
work done to assess the stator windings, using the Arrhenius modelling and all the motor
winding temperature data that has been accumulated, to provide a judgement that the motors
do not require to be rewound as the 30 year lifespan prediction was for a constant temperature
throughout the working life whereas in real-life the motors see a variety of temperatures which
are usually much lower than the class F rated temperature. Each rewinding would cost
approximately £600,000 and there are over 40 of these motors within the fleet so that would be
a substantial outlay for the business so close to decommissioning with a lifespan of only 5 to
10 years required.
The report calculated the expired thermal life as a proportion of the substantiated life, using an
industry recognised method and took account of any operation at elevated winding
temperatures. The thermal life expired profile for each winding is presented and the machines
with the highest expired thermal life are identified. This investigation also presents
recommendations for 11kV safety motors operation going-forward, including operation in low
pressure CO2 and air.
Report (Rollo, 2007) states that although the 11kV motors are not considered susceptible to
simultaneous common mode failure while operating under normal conditions; abnormal
conditions such as would be experienced during a severe depressurisation event would
simultaneously expose all 11kV motor winding insulation systems to an increased mechanical
stress as the CO2 tries to escape from the winding insulation layers. Under such circumstances
common mode failure could occur if the 11kV motors were to be operated beyond their present
level of substantiation and it was therefore concluded that the motors are vulnerable to a
common mode failure if operated beyond 30 years.
Report (Smyth, 2017) presents the results of a recent ageing test programme which extended
the substantiated life of the HYB/TOR power station’s 11kV stator windings and in doing so,
also extended the capability of the HNB 11kV stator windings as the test pieces and test
conditions are considered representative for the HNB motor windings.
Page | 14
As discussed previously when looking at conventional air-cooled motors the ageing of winding
insulation materials is affected by many factors such as temperature, electrical and mechanical
stresses, vibration, and contamination from moisture/dirt/chemicals. In modern insulation
systems, high operating temperatures can cause an oxidation reaction (in air-cooled machines)
resulting in the chemical bonds in the organic insulation compounds progressively breaking
under the thermally induced vibration. When bond breakage occurs, oxygen often attaches to
the broken bonds and the reduced bond strength leads to progressive delamination of composite
insulation or embrittlement. In the 11kV motors (which normally operate submerged in 40bar
Carbon Dioxide, CO2), the mechanisms which cause the insulation systems to age are complex.
For example gas pressure cycling is considered to influence insulation delamination and the
production of voids, however the windings operate in an oil vapour saturated environment,
which results in oil penetrating and filling voids in the insulation (Kennedy, Sterling & Hains,
2008).
Compared to conventional machines, it is recognised that 11kV safety motor operation in a
non-oxidising CO2 atmosphere is less onerous than an air environment and will lessen the
thermal aging affects (Schwarz, 1973). During operation in air, the HNB machines are known
to operate with elevated winding temperatures. Although most of these elevated temperatures
are below the thermal classification of the 11kV stator insulation system (Thermal Class 155
(F) in accordance with BS EN 60085) they have a considerable influence on thermal life as at
high temperatures thermal life is used up at a significantly higher rate, challenging the original
assumptions used to support the 30-year substantiated life.
As mentioned previously the concept of the “10°C rule of thumb” for insulation thermal ageing
calculations is a well-known industry approximation developed following early studies on the
relationship between time and temperature and their effect on insulation. The “10°C rule”
approximates that the thermal life of the insulation is reduced by one half for each 10°C increase
in temperature and that for every 10°C reduction in temperature the insulation thermal life is
approximately doubled. However, when material specific Activation Energy (E) values are
known, the Arrhenius equation will provide a more accurate prediction of thermal life.
The Arrhenius equation is adopted by international standards as the basis for evaluating the
thermal performance of electrical insulation materials in air and is used within EDFE to
calculate thermal ageing, where the material activation energy (E) is available. For the case
where E is unknown, the “10°C rule” remains a useful conservative approximation for
calculating thermal ageing. Several input parameters and the E for the HNB 11kV main motor
winding insulation are used to calculate the service life (t2) for a range of service temperatures
(T2). This, along with operational temperature data from the stations Ferranti Data Logging
System allows the proportion of thermal life used to be calculated (expressed throughout this
report as the ‘Percentage Thermal Life Expired’).
3.2.1

Arrhenius Equation Input Parameters (HNB 11kv Motor Stator Windings)
Service Temperature (T2): Operational data gathered from a Ferranti Data Logging
System provides a log of 11kV winding temperatures at a frequency of once per hour for
Page | 15
each operational 11kV motor. The assessment and processing of the operational data for
use in the thermal ageing calculations is discussed in detail within section 3.2.2

Activation Energy (E): All 11kV stator windings at HNB utilise a Novobond SX
insulation system. Report (Rollo, 2016) has determined the E in air for Novobond SX
as 1.20 eV. Theory and references indicate ageing in a non-oxidising environment is less
onerous than in an air environment. Therefore, using the Arrhenius Equation and the
above E value, derived from test conditions in air, to determine the thermal life expired
whilst operating in CO2, provides an additional degree of conservatism for the proportion
of the thermal life ageing attributed to CO2 operating conditions. This conservatism is
not realised in the proportion of thermal ageing when operating in air.

The Ageing Temperature (T1) and the Ageing Time (t1) : A test programme has been
undertaken recently (Smyth, 2017) which included accelerated ageing of 11kV
motorettes and end winding models for a prolonged period, to extend the winding
substantiated life to support a 11kV motor winding life extension safety case for TOR
and HYB Power Stations. The results detailed within (Smyth, 2017) have been used to
provide an equivalent base Ageing Temperature (T1) of 160°C and base Ageing Time
(t1) 1777.54hrs for the HNB Novobond SX insulation system, i.e. continuous operation
of the insulation system at 160°C would result in its thermal life being 1777.54hrs.
All winding thermal life calculations throughout the report used this base data to determine the
percentage thermal life expired for each specific 11kV motor operational temperature profile.
It should be noted that the accelerated ageing and tests were reported to be completed
successfully and there was nothing in the final test and inspection results to suggest that the test
pieces could not have undergone further ageing, giving a further extension to the thermal life
capability.
The calculations throughout this note use operational temperatures to determine a percentage
thermal life expired for the HNB 11kVs motors.
Assessing the expired (and hence remnant) thermal life in percentage terms is a move away
from the current expression of substantiated life in terms of years. This change in emphasis
means that the remaining life of the HNB 11kV main motor windings should not be thought of
in terms of operational years, but as a percentage figure which will diminish at a variable rate
determined by winding operational temperature profile.
As such, the author has concluded that careful management of HNB 11kV winding
temperatures can be used to prolong the remaining operational life.
Page | 16
3.2.2
Operational Data and Determination of Main Motor Winding Temperature
Profiles
Operational data gathered from the Ferranti Data Logging System and other historical Station
Records has been used to assess the percentage of thermal life expired for each of the main
11kV motors. The assessment of percentage thermal life expired for the pre-2001 (pre-Data
loggers) operational period is discussed in detail in Section 3.2.3. The records and data
provided by Hunterston are assumed to be accurate and correct.
The Data Logging System provides a log of 11kV motor stator winding temperatures at a
frequency of once per hour for each in service motor. This data has been assessed by the author,
see Section 11.5 - Appendix E, looking at the temperatures for HNB Reactors R3 and R4 from
June 2001 onwards.
R3 Ferranti data analysed – 28/06/2001 to 22/08/2018
R4 Ferranti data analysed – 28/06/2001 to 22/08/2018
The stator winding temperatures logged by Ferranti utilise thermocouples, located between the
stator winding conductors in the stator slot, which measure the temperature on the outer surface
of the winding insulation. In low pressure CO2 and air at atmospheric conditions, the stator
iron (core) losses have the greatest influence on stator winding temperatures. These losses
along with the reduced mass coolant flow in low pressure conditions results in the highest stator
winding temperatures occurring during operation in air. Under these conditions, at low current
levels and therefore low resistance heating of the conductor, the measured temperature at the
thermocouple position on the outer surface of the winding insulation closely represents the hot
spot temperature as the primary heat source stator iron loss is external to the winding insulation.
Operation with improved cooling at 40bar CO2 results in the motors running cooler even at the
higher levels of conductor current heating. In this case the hot spot temperature is expected to
occur at the centre of the coil cross-section due to the high winding currents rather than at the
thermocouple location on the outer surface of the winding insulation. To account for the
insulation thermal gradient a margin should be added to the measured temperatures when the
machine is operating in 40bar CO2 conditions.
Historically in winding life calculations a margin has been added to the peak measured
temperature, however due to the relatively high temperatures experienced by the main motor
windings at HNB when operating at low pressure CO2 or in air at atmospheric conditions, it is
deemed overly conservative to use peak temperatures in this assessment.
The Ferranti data has been processed by the author in excel, see Section 11.5 - Appendix E, to
determine a temperature profile for each individual motor winding, applying the following
rules:
Page | 17
•
Throughout the winding thermal life analysis detailed within this report, where the
Ferranti logged winding temperatures are below 100°C (representative of 40bar CO2
operation) an additional 10% has been added to the measured temperature values to
account for the hot spot temperature adjustment. Temperatures equal to and above
100°C are generally representative of operation in low pressure CO2 or air at
atmospheric conditions, where the measured temperature closely represents the hot
spot temperature and as such no margin has been added. This approach should ensure
reasonable conservatism within the subsequent motor thermal life calculations,
without being overly pessimistic.
•
Following the addition of 10% to Ferranti logged winding temperatures below 100°C,
all low temperatures have been increased to a minimum value of 70°C. This ensures
that for all percentage thermal life expired calculations detailed within this note a
minimum Service Temperature (T2) of 70°C has been utilised adding further
conservatism to the thermal life assessment.
•
The small number of stator winding temperatures recorded by Ferranti as 200°C are
‘full scale’ readings and are typically associated with conditions where the motor is
not running and therefore not considered real, as such they have been given a nominal
value of 70°C.
The post-process temperature profiles for each individual motor stator winding are presented
in the charts within Section 11.5 - Appendix E (Purss & Manson, 2018) and have been used for
all percentage thermal life expired calculations within the report.
Page | 18
3.2.3
Calculation Method For Thermal Life Expired Based on the Arrhenius Equation
To assess the percentage thermal life expired of each motor stator winding, the service life (t2)
(in hours) has been calculated using the Arrhenius equation and the parameters detailed in
Section 3.2.1 above. The service life (t2) and the percentage of thermal life expired is calculated
for each hour within the data period using the post-process temperature profiles, see Section
11.5 - Appendix E,(Purss & Manson, 2018) with the hourly temperature values rounded up to
the next whole number. These hourly percentage thermal life expiry figures can then be
summed to give a total percentage thermal life expired for the data period, and hence establish
the remnant thermal life for each motor stator.
Percentage thermal life expired for an hour = 100 / service life t2 applicable to the hour and
temperature in question = 100/(1777.5/e(E/k*(1/(160 +273.15)-1/(winding temperature +273.15)))) cited from
previous work within (Cowan & Potter, 2009)
Motor stator winding temperature
Calculated % thermal life expired per hour
120.5°C
0.0023367% @121°C
121.9°C
0.0025552% @122°C
122.1°C
0.0027930% @123°C
If these three temperatures were experienced for a total of 5000hrs, 1000hrs and 3000hrs
respectively,
Total Percentage thermal life expired = (0.0023367x5,000) + (0.0025553x1,000) +
(0.0027930x3,000) = 22.62%
Therefore, Total Percentage remnant thermal life = 77.38%
Table 1 - Simplified Example
Pre-2001 Operational History
During the production of Reference (Cowan & Potter, 2009), HNB quality plans and data
records were used to establish the operational history of the motors. The total number of hours
each motor was installed in a reactor location since the motors most recent stator rewind until
start of the Ferranti logging period was established with worst case estimates being used where
the motor reinstatement dates were not clear. The pre-2001 (pre-Ferranti) operational history
established for Reference (Cowan & Potter, 2009) has been confirmed by station as correct and
has been used in this report.
In this report it is assumed that the motors experienced similar conditions in terms of main
motor winding temperatures for the pre-2001 period (i.e. pre-Ferranti logged period) as during
the Ferranti logged period. This leads to the assumption that the same thermal life expiry rate
also applies to the pre-2001 period and as such a proportion of the calculated percentage thermal
life expired for the Ferranti period has been added to represent the pre-2001 period. For
example, if during the Ferranti logged period 50,000 service hours were calculated to use up
Page | 19
10% of thermal life, then 150,000 service hours during the pre-2001 period would use a further
30% of thermal life and the total percentage thermal life expired would therefore be 40%.
To make the percentage thermal life expired calculations more accurate, the station could
consider reviewing the historical operating practice with the aim of determining appropriate
winding temperature profiles for the pre-2001 period, however it is considered that the approach
used in this report will be sufficiently representative and, due to the conservatisms introduced
previously, a safe assumption that can be made.
3.2.4
Calculated Thermal Life
The table in Section 11.3 - Appendix C (Purss & Manson, 2018) gives the results of the data
analysis undertaken and details the calculated percentage thermal life expired for each motor
stator winding.
Observations:
 Eight stators are greater than 70% thermal life expired, see Table 2 below.
Present
Total % thermal life Service years since
Location
expired
last rewind
STATOR 12
Spare
137.18%
26.67
STATOR 20
4A2
95.93%
27.85
STATOR 3.2
3C2
92.34%
17.78
STATOR 02
3C1
87.30%
18.13
STATOR 08
Spare
84.93%
18.73
STATOR 18
4B2
77.04%
22.94
STATOR 12.2 3A2
74.52%
27.10
STATOR 05
3A1
72.08%
19.92
Table 2 - Stators >70% Thermal Life Expired
Stator ID
Note: Although STATOR 12.2 has over 27 years’ service (~235,000 service hours), it
is only ~74% thermal life expired. This is attributed to relatively low winding
temperatures during operation in CO2 from 2001 onwards.

From station records, the earliest date of motor stator installation following the last
stator rewind was 29/03/1982 for STATOR 20. Analysis of the data shows that
STATOR 20 is calculated to be 95.93% thermal life expired. Several other stators have
around 23 service years (~200,000 service hours) or greater namely STATOR 12.2,
STATOR 12 (spare), STATOR 11, and STATOR 18, and the percentage thermal life
expired calculated for these motor stator stators is 74.52%, 137.18%, 64.65% and
77.04% respectively.
Page | 20

Spare STATOR 12 is more than 100% thermal life expired and should not be
redeployed unless rewound.

The average rate of thermal life expiry per service year is assessed to be highest for the
following motor stators:
o
STATOR 21: 21.36% thermal life expired in 3.93 service years (average
thermal life expiry rate = 5.43% per service year)
o
STATOR 03: 21.04% thermal life expired in 4.00 service years (average
thermal life expiry rate = 5.26% per service year)
o
STATOR 07: 14.73% thermal life expired in 2.82 service years (average
thermal life expiry rate = 5.22% per service year)
o
STATOR 03.2: 92.34% thermal life expired in 17.78 service years (average
thermal life expiry rate = 5.19% per service year)
The average thermal life expiry rates detailed above result in a total life capability far less than
the original 30-year substantiated life stated for HNB machines in Reference (Rollo, 2007).
The main reason for this is that the motor winding thermal life was originally based on operating
at, or below, 80°C. Prolonged operation at the high temperatures experienced during low
pressure operation has considerable influence on the thermal life. As shown in Section 11.3 Appendix C, stators with similar service hours can have significantly different calculated
thermal life capability due to the varying stator winding temperature profiles. The case study
within Section 11.4 - Appendix D compares STATOR 12.2 and STATOR 12 windings and
clearly demonstrates the significant impact of operation at elevated temperatures.
3.2.5
Temperature Profile Charts
The charts in Section 11.5 - Appendix E show the post-process stator winding temperature
profile and percentage thermal life expired profile for each individual motor stator winding for
the Ferranti data period.

The charts are presented by motor machine ID rather than by reactor Location, so an
individual chart could contain operational data from both reactors and/or multiple
Locations.

The charts present the post-process stator winding temperatures used within the
percentage thermal life expiry calculations, rather than the actual Ferranti logged
winding temperatures. As such and as per Section 3.2.4 above, the charts present the
following temperature conservatisms:
Page | 21
o
An additional 10% has been added to Ferranti logged winding temperatures
below 100°C to allow for the hot spot temperature.
o
Following the addition of 10% to the Ferranti logged winding temperatures, all
low temperatures have been increased to a minimum of 70°C.
o
All temperature values have been rounded up to the next whole number i.e.
121.2°C is rounded up to 123°C. Where applicable this rounding occurs
following the addition of 10% to the Ferranti Logged winding temperatures.
o
Stator winding temperatures logged at 200°C ‘full scale’ (refer earlier
explanation) have been given a nominal value of 70°C.
Observations:

Typically, motor Ferranti logged winding temperatures range from 75°C to 100°C and
are associated with normal running in 40bar CO2 (the temperature profile charts within
Section 11.3 - Appendix E (Purss & Manson, 2018) show the post-process winding
temperatures where 10% has been added to logged temperatures below 100°C to
account for the hot spot temperature).

Periods of high temperature operation (Ferranti logged winding temperatures >100°C)
can generally be attributed to outages and motor operation at low pressure conditions.

The majority of peak winding Ferranti logged winding temperatures are around 140°C,
with a small number in the 140 - 150°C range. A maximum Ferranti logged winding
temperature of 173°C was recorded on STATOR 02 in August 2006.

The gradient of the percentage thermal life expired (red) line is equivalent to the
thermal life expiry rate. During periods of high temperature operation, the gradient of
the percentage thermal life expired line is steep, indicating that winding thermal life is
being used up at a significantly higher rate.

The starting point for the percentage thermal life expired profile is not always zero.
This is to account for any service period prior to 2001 for which data is not available.
As explained in Section 3.2.3 above, the assumption has been made that the same
thermal life expiry rate applies to the pre-2001 period as the post-2001 Ferranti data
period. As such a proportion of the calculated percentage thermal life expired for the
Ferranti data period has been added to the starting point of the percentage thermal life
expired profile to represent the pre-2001 period.
Page | 22

For motors which have been rewound post 2001, the chart data starts at the post-rewind
installation date and the percentage thermal life expired profile starts at zero.

Significant data gaps within the charts can be attributed to time periods where the motor
stator has not been installed in a reactor Location (i.e. is stored as a spare or is
undergoing maintenance in the motor stator workshop).

No stator winding temperature profile has been provided for STATOR 01 as this stator
is currently a spare and is due to be rewound.
3.2.6
Operation in Normal Conditions
Motor winding temperatures in 40bar CO2 are cooler and typically less variable, with Ferranti
logged winding temperatures ranging from approximately 75°C to 100°C. These lower
temperatures also have less impact on percentage thermal life expired as shown in Table 2
below, which illustrates the percentage of the entire substantiated thermal winding life used per
day, month and year if operating continuously at the specified winding temperatures.
Ferranti
Logged
Winding
Temperature*
63.6°C
Winding
Hot
Spot
Temperature
70°C
%
Thermal
Life Expired
per Day
0.00029
% Thermal Life
% Thermal Life
Expired
per
Expired per Year
Month
0.01
0.11
72.7°C
80°C
0.00093
0.03
0.34
81.8°C
90°C
0.00274
0.08
1.00
100°C
100°C
0.00767
0.23
2.80
110°C
110°C
0.02033
0.61
7.42
120°C
120°C
0.05126
1.54
18.71
130°C
130°C
0.12340
3.70
45.04
140°C
140°C
0.28470
8.54
103.92
150°C
150°C
0.63157
18.95
230.52
160°C
160°C
1.35050
40.52
492.93
Table 3 - Winding Temperature vs. % Thermal Life Expired
*Where Ferranti logged winding temperatures are below 100°C (representative of 40bar CO2
operation) an additional 10% has been added to the measured temperature values to account for
the hot spot temperature adjustment. Temperatures equal to and above 100°C are generally
representative of operation in low pressure CO2 or air at atmospheric conditions, where the
measured temperature closely represents the hot spot temperature and as such no margin has
been added.
Page | 23
From Table 3, if the HNB motors operated continuously at their original estimated operating
temperature of 80°C (hot spot temperature 88°C), as given in Reference [28], the thermal life
expiry rate per year is less than 1% and the motors have a winding thermal life capability of
more than 100 years based on results from the latest HYB/TOR ageing test programme.
Based on a revised representative Ferranti logged winding temperature of 100°C for normal
running in 40bar CO2 and the results from the latest test programme, the thermal life expiry rate
per year is 2.8% and the HNB motors have a winding thermal life capability of over 35 years,
if run continuously at this temperature.
3.2.7
Operation in Outage Conditions at Elevated Temperatures
To quantify ageing during outages when running at elevated temperatures, detailed assessments
of two recent outages are presented in Table 4. This will also help with forecasting ageing
during future outages. Table 4 show the motor stator winding percentage thermal life expired
during the 2015 Reactor 3 and the 2017 Reactor 4 Statutory Outage periods. The temperature
profile associated with each motor stator winding has been analysed for these outage periods
and used to calculate the percentage thermal life expired for each individual motor winding
during the outage period only. Only motor stators which remained in location throughout the
outage periods are considered. Motor stator exchange locations are marked.
Analysis shows that for normal ageing at a continuous Ferranti logged winding temperature of
100°C (judged representative of normal running in 40bar CO2), motor stator winding insulation
thermally ages by approximately 2.8% (see Table 2) of its total substantiated thermal life over
the period of a year. This contrasts with the figures provided for low pressure outage conditions
in Tables 3 and 4 which, despite the shorter period, are significantly higher.
Observations:

During the R3 2015 outage period of 63 days, STATOR 03 winding in location 3C2
had the highest percentage thermal life expiry figure of 5.91% (>12 times the rate of
normal ageing over the period).

motor STATOR05 had the lowest percentage thermal life expiry figure of 2.55%
during the Reactor 3 2015 outage (>5 times the rate of normal ageing over the period).

During the R4 2017 outage period of 55 days, STATOR20 winding in location 4A1
had the highest percentage thermal life expiry figure of 6.93% (>16 times the rate of
normal ageing).

The lowest percentage thermal life expiry figure was 0.95% for STATOR21 during the
R4 2017 outage (>2 times the rate of normal ageing).
Page | 24

The average percentage thermal life expired during the 2015 R3 outage and 2017 R4
outages were 3.93% and 4.39% respectively (>8 and >10 times respectively the rate of
normal ageing).
The high temperatures experienced by the motor stators during operation in air at atmospheric
conditions has resulted in a significant percentage of winding thermal life being used up in a
relatively short period of time. In addition, the variability in motor stator outage operation and
operating conditions, and hence their temperature profile, has resulted in variability in the
percentage thermal life expired figures for both the R3 2015 and the R4 2017 outage periods.
R3 Outage 2015 28/09/15 to 29/11/15
(63 days duration)
% thermal life
Location – Motor
expired during
Stator ID
outage period
3A1 - STATOR 05
2.55%
3A2 - STATOR 12.2 2.97%
3B1 - STATOR 05.2 4.93%
3B2 - STATOR 11
5.47%
3C1 - STATOR 02
Exchanged
3C2 - STATOR 03
5.91%
3D1 - STATOR 04
4.51%
3D2 - STATOR 07
Exchanged
Table 4 - R3 Outage 2015 / R4 Outage 2017
R4 outage 2017 08/09/17 to 31/10/17
(55 days duration)
% thermal life
Location - Motor
expired during
Stator ID
outage period
4A1 - STATOR 03
Exchanged
4A2 - STATOR 20
6.93%
4B1 - STATOR 17
Exchanged
4B2 - STATOR 18
1.19%
4C1 - STATOR 21
0.95%
4C2 - STATOR 15
6.63%
4D1 - STATOR 06
3.96%
4D2 - STATOR 19
Exchanged
Insulation systems can run at elevated temperatures without short term risk of failure; however,
insulation ageing is accelerated. Careful consideration should be given to the motor stators’
future outage operating regime, specifically with respect to the duration of operation in low
pressure CO2 or air at atmospheric conditions, as operation at higher temperatures greatly
impacts thermal life. When the reactor is at low pressure, operation of motor stators with a
calculated expired thermal life of greater than 90% (identified in Section 7.1) should be
minimised and avoided entirely where possible.
3.2.8
•
Discussions of 11kV Motor Evaluation
A test programme has been undertaken recently, Reference (Smyth, 2017), which
included accelerated ageing of 11kV motor stator motorettes and end winding models
for a prolonged period, to extend the winding substantiated life to support a motor stator
winding life extension safety case for TOR & HYB Power Stations. The results of this
test programme have been used (Section 8.1 - Appendix A gives a justification for use)
to provide an equivalent winding insulation qualified life of 1777.54hrs at 160°C for
the HNB Novobond SX insulation system (see Section 8.2 - Appendix B).
Page | 25
•
The HYB/TOR ageing test programme has increased the substantiated thermal life for
the HNB motor stator winding by approximately 75% for a given constant temperature
(See Appendix B). Without this extension there would have been more machines near
to, or exceeding, the 100% thermal life expired status than currently reported.
•
Assessing the expired (and hence remnant) thermal life in percentage terms is a move
away from the current expression of substantiated life in terms of years. This change
in emphasis means that the remaining life of the HNB 11kV main windings should not
be thought of in terms of operational years, but as a percentage figure which will
diminish at a variable rate determined by winding operational temperature profile. As
such, careful management of HNB 11kV motor stator winding temperatures can be
used to prolong the remaining operational life. It should be noted that calculating
substantiated life in terms of years remains valid for machines where the spread of
operational temperatures is small and therefore calculating thermal life based on a peak
temperature would give a representative result, as is the case for HYB/TOR.
•
Typical motor winding temperatures logged by the Ferranti ranged from 75°C to 100°C
and can be associated with normal running in 40bar CO2 (the temperature profile charts
within Section 8.5 - Appendix E show the post-process winding temperatures where
10% has been added to Ferranti logged temperatures below 100°C to account for the
hot spot temperature).
•
Periods of high temperature operation (Ferranti logged winding temperature >100°C)
can generally be attributed to outages and moto operation within a low-pressure
environment. Most peak winding temperatures are around 140°C, with a small number
in the 140 to 150°C range. A maximum Ferranti logged winding temperature of 173°C
was recorded on STATOR 02 in August 2006.
•
Stators with similar service years can have significantly different calculated thermal
life expiry figures. This is due to the varying temperature profiles and the considerable
influence prolonged operation at the high temperatures experienced during low
pressure operation has on the thermal life.
HNB 11kV motor stators with the highest percentage thermal life expiry figures:
o
STATOR 12 (spare), ~137% thermal life expired, ~26 service years
o
STATOR 20 (4A2), ~95% thermal life expired, ~27 service years
o
STATOR 03 (3C2), ~92% thermal life expired, ~17 service years
o
STATOR 02 (3C1), ~87% thermal life expired, ~18 service years
Page | 26
o
STATOR 08 (spare), ~84% thermal life expired, ~18 service years
o
STATOR 18 (4B2), ~77% thermal life expired, ~23 service years
•
The average thermal life expiry rates stated in this note, demonstrate that the total
thermal life capability is far less than the 30-year substantiated thermal life the HNB
machines are currently considered to have. The main reason for this is that the motor
winding life was originally based on operating at, or below, 80°C. Prolonged operation
at the high temperatures experienced during low pressure operation has considerable
influence on the thermal life.
•
If the HNB 11kV motors operated continuously at their original estimated operating
temperature of 80°C (hot spot temperature of 88°C), the motors would have a winding
thermal life capability of more than 100 years based on results from the latest
HYB/TOR ageing test programme.
•
Based on a revised representative Ferranti logged winding temperature of 100°C for
normal running in 40bar CO2, the HNB 11kV motors have a winding thermal life
capability of at least 35 years, if run continuously at this temperature.
•
The high temperatures experienced by the motor stators during outage operation in low
pressure conditions can result in a significant percentage of winding thermal life being
used up in a relatively short period of time:
o
Continual operation at a Ferranti logged winding temperature of 100°C would
result in 2.80% of thermal life being used up per year.
o
Continual operation at a Ferranti logged winding temperature of 130°C would
result in 45.04% of thermal life being used up per year.
•
Variability in motor stator outage operating conditions and hence their temperature
profile has resulted in variability in the percentage thermal life expired figures for both
the R3 2015 and the R4 2017 outage periods. The average percentage thermal life
expired during the 2015 R3 outage and 2017 R4 outages were 3.93% and 4.39%
respectively.
•
The variability and uncertainty over future outages make it difficult to predict thermal
life expiry rates going forward.
Page | 27
3.2.9
Recommendations of 11kV Motor Evaluation (Purss & Manson, 2018)
•
At date of issue, HNB Reactor 3 and Reactor 4 are off-line while work is completed to
assess the graphite core and develop the longer-term safety case. Depending on the
11kV motor operating regime during this extended shutdown, the calculated thermal
life expiry figures detailed within this investigation may now (at date of issue) be
significantly higher. It is recommended that HNB provide up to date temperature data
for both Reactor 3 and Reactor 4 such that the percentage thermal life expiry figures
can be updated, and more accurate calculations of thermal life can be made.
•
STATOR 20 currently in location 3C1 is calculated to be >95% thermal life expired.
HNB should provide up-to-date temperature data for this location as a priority. The
stator winding temperatures of this stator should be closely monitored and managed.
•
Spare STATOR 12 is more than 100% thermal life expired and should not be
redeployed unless rewound.
•
When the reactor is at low pressure, the number of 11kV motors operated should be
minimised and the operation of any with a calculated expired thermal life of greater
than 90% should be minimised and avoided entirely where possible.
•
Careful consideration should be given to future outage motor operating regime,
specifically with respect to the duration of operation in low pressure conditions.
•
The stator winding temperatures of motor stators predicted to reach >90% thermal life
expired before the end of station life should be closely monitored and managed, and
should not exceed 155°C. To maximise remaining life, operation of these stators with
the reactor at low pressure should be minimised and avoided entirely where possible.
•
Several motors stators are predicted to remain at <60% thermal life expired until the
proposed end dates and where possible, these stators should be targeted for any required
high temperature operation in air or low-pressure conditions.
•
The key points and recommendations identified within this investigation should be
disseminated to relevant station personal to ensure the appropriate management of
11kV motor stator winding life.
Page | 28
3.3
PRACTICAL EXPERIMENT
The purpose of this practical element of the project is to provide a real-life example of a reaction
rate in the insulation of electrical plant experienced linearly in time due to thermal stress.
This is required as the real-world plant discussed in section 3.2 deals with 11kv motors which
are important to safety and electricity generation, and due to this importance, there has never
been a failure of these stators due to insulation faults or ageing/degradation. Therefore, to
investigate how accurate lifespan estimation can be when using the Arrhenius equation data, a
practical failure is required for investigation.
To gather this data an experiment in accelerated ageing of electrical plant has been attempted,
taking into consideration any limitations/restrictions in place such as the timescale of the
project, the small budget and facilities available.
As mentioned previously in Section 3.1.5 and within (Carfagno & Gibson, 1980) there are many
factors of stress that cause ageing, Section 4 of (Carfagno & Gibson, 1980) discusses various
theories of ageing and well-established models relating ageing to stress such as the Arrhenius
model. Due to the limitations mentioned this practical experiment focused on thermal stress as
this is considered one of the higher contributing factors and it is also relatively easy to work
with. How this was completed is described in Section 3.3.2 – Methodology, but major steps are
listed below to put the following sections into context:





3.3.1
To address the limited resources available to the author and to ensure the project stays
within the timescale given a low voltage cable (see Section 3.3.1) was aged artificially
until it couldn’t perform its duty.
This was done using an oven located on site within a university lab.
Several samples were aged to account for random failures or imperfections in
manufacturing.
To ensure no preconceptions are inherent in the results no targets or goals are being set,
the (British Standards Institute, 2005b) was used and the outcomes recorded.
The data from this was then compared to a calculated Arrhenius model for the same
cable and the findings discussed.
Cable Specifications
The cable chosen for sampling is a Nexans Single Core, 1.5mm2, Solid conductor, nominal
rating of 10A with a PVC conductor insulation. This cable was chosen due to the relatively low
cost and accelerated ageing times required. To be more accurate to the electrical plant discussed
in section 3.2 a different piece of plant would be used (discussed in Section 5 - further work),
however this would require significantly longer for testing than the scope of this project allows.
Page | 29
It has a 6491X cable code (H07V-U) – which is held to the following Standard (British
Standards Institute, 2011) which relates to Class 1 – 1.5mm2 to 10mm2 with an Insulation type
T1 as in Standard (British Standards Institute, 2006) which gives specifications for PVC
insulating compounds that manufacturers are required to adhere to (Maximum temp rated for
70°C).
3.3.2
Methodology
Using (British Standards Institute, 2005b) as a guide, the cable described in section 3.3.1 above
was aged in an accelerated manner to simulate ageing caused by thermal stress.
This standard (British Standards Institute, 2005b) states “although originally developed for use
with electrical insulating materials and simple combinations of such materials, the procedures
are considered to be of more general applicability and are widely used in the assessment of
materials not intended for use as electrical insulation”. This shows the flexibility of the
Arrhenius equation and accelerated ageing tests.
Within (British Standards Institute, 2005b) there is a table of recommended temperatures and
duration for baking when performing these ageing tests further details can be seen in Section
8.7 – Appendix G, the following procedures are recommended also:
a. Prepare suitable specimens – for this practical experiment there will be five specimens
of one metre in length and one specimen of five metres in length, all specimens will
consist of three single cores taped together (this will enable insulation resistance testing
between cores to be performed easily). This choice was made to accommodate the oven
and space available to the author.
b. Subject groups of specimens to ageing at several fixed levels of elevated temperature
either continuously or cyclically – the first batch of specimens will be aged for 4 weeks
at 110°C at which point the temperature will be increased at 10°C intervals and for a
time relevant to temperature being used, see guide Table 5 and (British Standards
Institute, 2005b) for reference. Then the second batch (five metres in length) will be
aged at 120°C and then increase in 20°C intervals. This will continue until a point at
which the specimens would fail in service for whatever reason (mechanical/electrical).
c. Subject specimens to a diagnostic procedure in order to reveal the degree of ageing,
diagnostic procedures may be destructive or non-destructive – for this experiment the
diagnostic procedures chosen will be from (British Standards Institute, 2005a) with
advice from (IEEE, 2014) – Visual/Tactile Test, DC resistance of conductor, weight of
specimens to check polymer loss, insulation resistance (including Degree of
Polymerisation and Dielectric Response Measurement), functional tests at near load
current, thermography at load current. (Further details of each test will be in section
3.3.3)
Page | 30
d. Extend the heat exposure until the specified end point – this end point will be for a
period of at least 8 weeks to achieve the point where 60 years’ service would be
approximated in ageing through thermal stress based on the temperatures being used,
but until the cable cannot complete its service would be the defining end point.
e. Report the test results, showing the kind of ageing procedure (continuous or cyclical)
and diagnostic procedure – this will be in section 3.3.4
f.
Evaluate these results numerically and present graphically - this will be in section 3.3.4
g. Express the complete information in abbreviated numerical form. By means of
temperature index and halving interval - this will be in section 3.3.4
These procedures were followed, and the results shown in section 3.3.4 and a mathematical
model of the same cable was created using the Arrhenius equation and the activation energy for
PVC insulation which was compared with the practical experiment data. This should gave a
good example of how accurate the Arrhenius equation method is and validated the findings for
the real-world example in section 3.2.
Rated temperature
Gives approximate lifespan Gives approximate lifespan
of (years)
of (hours)
>70°C
20+
175200+
Using ‘10°K rule of thumb’.
71°C > 80°C
10
87600
81°C > 90°C
5
131040
91°C > 100°C
2.5
43800
101°C > 110°C
1.25
21900
111°C > 120°C
0.625
10950
121°C > 130°C
0.3125
5475
131°C > 140°C
0.15625
2737.5
141°C > 150°C
0.078125
1368.75
151°C > 160°C
0.0390625
684.375
161°C > 170°C
0.01953
342.1875
171°C > 180°C
0.009766
171.094
Table 5 - Testing Temperatures & Approximate Lifespan of Insulation
3.3.3
AGEING & TESTING
To perform the ageing experiment the author made use of an oven within the university
premises in one of the labs within the Charles Oakley building. The oven was available for
continuous baking which allowed for a more comprehensive ageing test and in turn made the
choice between cyclical or continuous.
Page | 31
As discussed in the previous section batch one would contain five samples of one metre length,
one of these specimens was wound around a one-inch pipe to create a coil. The purpose of this
coil was to check the effect of bending on the insulations thermal strength and to check if the
electrical degradation was different. Figure 1 below shows the specimens from batch one and
two.
Figure 1 - Cable Ageing Samples – 1m and 5 m coils
To begin with the specimens were measured and taped together, then each specimen was
weighed to allow any weight lost from the dielectric to be measured.
To test the cables using the thermal camera (thermography) and for load current functional tests
it was necessary to put the cables into an electrical circuit (see Figure 2). This circuit consisted
of an electrical supply protected from overloads with a thermal trip. This was then connected
to one end of a test specimen through a 13A fuse and that in turn was connected to a protected
junction that has a residual current device (RCD) built in to prevent short circuits. Finally, a
static resistive load was connected to the RCD junction through another 13A fuse. The load
was an electric iron rated at 2kW which when supplied from 240V gives a constant load of
8.33A, this was sufficiently close enough to the maximum cable rating of 10A to provide a
good test of its electrical capabilities during ageing.
Protected
Thermal CutOut Electrical
Protective
Residual Current
Device for Load
Aged
Cable for
Testing
13A Fused
Plugs
Fixed
Electrical
Load
Figure 2 - Electrical Test Circuit
Page | 32
The aim of the experiment is to test the condition of the cables before during and after the
ageing process, this will enable a trending of the data to provide an expected end of life for the
cables that can be compared with the Arrhenius modelling.
As mentioned previously, tests will be conducted to monitor the cables condition, these were
taken from (British Standards Institute, 2005a) with consideration from EDF Energy technical
guidance note 099 – (summarised below for cables that have a PVC, XLPE or OIP dielectric
and operate at less than 1kV). This guidance note recommends the following inspections for
walkdowns and routine maintenance of the cables and to monitor its condition.
a. Examine for mechanical damage, crushing or movement of any armour wires.
b. Where cables are installed above ground all cable supports and cleats should be
examined for corrosion, security and any fretting between cable and supports.
c. PVC, PE cable sheaths should be examined for any crazing, cracking, etc.
d. Cable terminations should be examined for evidence of corrosion or overheating at the
bolted connections where accessible.
e. Bonding leads and earth straps should be examined for corrosion, mechanical damage
or overheating.
f. The cable insulation resistance should be measured. Armoured cables should be
measured phase to earth. Unarmoured cables should be measured between phases. NB:
IR measurements can vary greatly with temperature, e.g. The resistivity of PVC drops
by approximately 50% with a 10°C increase of temperature. It is recommended that
repeat measurements are conducted at a similar time of day to minimise variations in
environmental conditions. Ambient conditions at the time of measurement should be
recorded to enable normalisation of the results if required.
Where electrical testing has proved inconclusive non-destructive mechanical or chemical
micro-sampling should be considered. From these sources the investigation will use the
following combination of tests for trending:






Visual/Tactile Test – the cables will be examined and any colour variation or damage
to the surface spotted will be recorded.
DC resistance of conductor – this will be tricky to get accurate due to the cable ends
being exposed once tested and the conductor baked without insulation on it causing
damage to the out layer of copper, possibility of a false reading.
Weight of specimens – weighed at beginning, during and end of the ageing process to
trend weight loss if present.
Insulation Resistance – measured between cores of the specimens using a Fluke IR
tester with a range up to 20MΩ at 500VDC test voltage, reading taken immediately,
called the spot test.
Polarisation Index – taking the IR value at 1 minute and 10mins, then putting the
10min/1min to give a ratio.
Dielectric Absorption Ratio – taking the IR value at 30 secs then at60secs, then putting
the 60secs/30secs to give a ratio.
Page | 33


3.3.4
Functional tests at near load current – using circuit shown in Figure 2 to allow for
testing of functional capability.
Thermography at load current – using the circuit shown in Figure 2 to enable thermal
images to be taken which will show any hotspots and potential degradation.
RESULTS
There are three processes in the ageing of PVC dielectric - oxidation, dehydrochlorination and
plasticiser loss. It was observed within (Jakubowicz, Yarahmadi & Gevert, 1999) that the two
main components of this chemical/mechanical change were the dehydrochlorination (discussed
in the next section) and plasticiser loss (discussed in Section 3.3.4.3).
3.3.4.1
Visual/Tactile Examination
Dehydrochlorination of the insulation causes discolouration (Ekelund, Edin & Gedde, 2007)
therefore it can be possible to use the colour change to track ageing in an effective, albeit crude
trending pattern. To attempt this the PVC insulation was photographed once per week during
baking. This should show a trend in colour from new to aged corresponding to the temperature
and time. During these intervals the specimens will be handled which will allow a tactile
inspection to check on the flexibility of the samples, is there any noticeable cracking or
variances in the surface etc.
Week / Sample @ 120
Flexibility
Cracking/Breaking
degrees Celsius
Week 1, Batch 1 – 1 metre
Very flexible, as new
None
Very flexible, cores initially
Week 2, Batch 1 – 1 metre
None
sticking together out of oven
Still flexible but a noticeable
difference since last week,
Slight treeing appearing
Week 3, Batch 1 – 1 metre
cores sticking together more
over bends on some samples
and requiring pulling apart
at times.
Shiny coating on surface
which cracks once bent, no
Week 4, Batch 1 – 1 metre
Much stiffer to bend
apparent damage to
insulation other than this
Unable to bend fully, very
If forced the insulation
Week 5, Batch 1 – 1 metre
stiff
breaks
Unable to bend fully, very
If forced the insulation
Week 6, Batch 1 – 1 metre
stiff – brittle in places
breaks
Can be bent but is very
brittle, exposed conductor at
No real force required,
ends are all coated in black
insulation very brittle and
Week 7, Batch 1 – 1 metre
soot(?), conductor doesn’t
cracks along bends every
break but insulation very
time
fragile
Table 6 - Tactile Inspection Notes
Page | 34
The following images show the PVC insulation from week one until week seven, displaying
the change in colour linked to dehydrochlorination and eventual mechanical failure of the
insulation as seen in week seven.
Figure 4 - Week 1 @ 120 degrees Celsius
Figure 3 - Week 2 @ 120 degrees Celsius
Figure 5 - Week 3 @ 120 degrees Celsius
Page | 35
Figure 7 - Week 4 @ 120 degrees Celsius
Figure 6 - Week 5 @ 120 degrees Celsius
Figure 8 - Week 6 @ 120 degrees Celsius
Page | 36
Figure 9 - Week 7 @ 120 degrees Celsius
From the images above, when Dehydrochlorination occurs there is a marked difference within
the cable’s appearance, including a darkening of the insulation colour to the point which gets
darkest after approximately week 6 and at this point the insulation becomes very brittle when
returned to ambient temperature from the oven. This could be a crude method to track ageing
within the insulation, but chemical sampling would be able to make a hydrochlorination level
against time curve possible giving a much more accurate trending tool.
3.3.4.2
Resistance of Conductor
During the experiment it was considered that a measure of the conductor’s resistance during
the ageing would be useful data to have, monitoring its electrical integrity before, during and
after the process. However, it became apparent that the exposed ends of conductor were being
aged also and becoming coated in something that gave false readings. To ensure accurate data
it would be necessary to strip small amounts of insulation from the ends of the specimens and
unfortunately this would also affect the weight – causing issues with recording plasticiser loss.
Due to the importance of monitoring plasticiser loss it was decided to forego recording
conductor resistance during the ageing process.
3.3.4.3
Weight of Specimens
Pure PVC is rigid and brittle, therefore to be a practical insulator the addition of various
plasticizers (phthalates and adipates) is required to make it malleable and workable. Weight
loss tracking shows the amount of plasticiser loss caused by the ageing process (accelerated or
otherwise) and this is linear with time when the rate is controlled by an evaporation process
Page | 37
(Jakubowicz et al., 1999). Due to its increased mass, the 5-metre specimen was used for weight
loss tracking. This ensured a noticeable change (in grams) over the ageing time – this is shown
in the following images. To check the linearity of this weight loss the 5m samples were aged
for 8 days at 140°C.
Day / Specimen
Weight (g)
Day 1 @ 140 degrees, 5m sample
324
Day 2 @ 140 degrees, 5m sample
318.5
Day 3 @ 140 degrees, 5m sample
316
Day 4 @ 140 degrees, 5m sample
313
Day 5 @ 140 degrees, 5m sample
308
Day 6 @ 140 degrees, 5m sample
308
Day 7 @ 140 degrees, 5m sample
306
Day 8 @ 140 degrees, 5m sample
306
Table 7 - Weight loss over 8-week period/two temperatures
Whilst this can be trended it is a crude method for evaluation of lifespan remaining. This is
because to be as accurate as possible it requires chemical examination such as super fluid
extraction SFE. This test was not conducted due to the restrictions on the project. Therefore, an
estimation of lifespan based on the weight loss can be an indication but not an accurate trending
tool. Table 7 shows that after 4 days @ 140°C weight loss plateaus around 308g to finally settle
at 306g. It would be safe to assume from this that there is very little plasticiser left within the
insulation to evaporate and hence the cable has reached its maximum lifespan, although in a
real-world application this cable would be replaced closer to the time when it had reached
lifespan and plasticiser loss equivalent to around day 5.
It’s worth noting that with a ventilated fan oven for ageing, the activation energy required per
molecule to begin the plasticiser losses and dehydrochlorination is less than in stagnant air,
such as behind a wall and representative of an electrical installation, hence the timescales for
service time will be much lower. This can be corrected for using a different activation energy
level.
3.3.4.4
IR Spot Values, Dielectric Absorption Ratio, & Polarisation Index
As discussed within Section 3.3.3. Insulation Resistance tests were going to be performed on
the specimens to provide the main trending tool for comparison with the Arrhenius model for
PVC cabling. However, it became apparent from lab tests and further research that the nature
of PVC ageing causes mechanical degradation much sooner than electrical property changes.
Due to the limitations of the project a Fluke tester with a maximum capability of 20MΩ
readings was being used to trend IR values, but this range is much too low to observe electrical
changes caused by ageing. For electrical changes to be observed a high voltage laboratory
would be required with methods of measuring IR levels far more than the tester used in this
experiment, see (Ekelund et al., 2007), and the ability to measure Partial Discharges or Leakage
Current with a test voltage being greater than 3kV to enable partial discharges to be detectable.
Page | 38
Due to the schedule there wasn’t enough time before the project deadline to complete these
tasks within the HV lab at GCU. Therefore, for this report there have been no IR values or
Ratios recorded.
3.3.4.5
Functional Tests and Thermography
As previously discussed, a test circuit (see Figure 2 for more details) was used to perform a
functional test at almost full rated load current and to allow for thermal images to be recorded,
using the FLIR infra-red camera.
Figures 6 and 7 below have confirmed that mechanical failure of the insulation has little effect
on the electrical ability of the cables. There is no increase in temperature for the aged cable
when handling the same electrical load even though at that point there was cracks forming in
the insulation and bare conductor in places. Due to the condition of the insulation it would be
unsuitable for use and would require changing so this would be considered at its end of useful
life even with the electrical attributes being acceptable.
Figure 11 - Electrical Test Circuit and Thermal Capture
Figure 10 - Thermal Capture of Sample from beginning to end (aged @ 120
degrees Celsius for 7 weeks).
Page | 39
3.3.4.6
Arrhenius Equation Model of PVC Cable Insulation
Taking into consideration the fact that PVC cables have been used in industry since the
1980’s approximately there has been around 40 years of operating experience of these
materials. There are varied suggestions of useful remaining lifespan within industry journals,
papers and message boards, the variations are due to the fact that most PVC insulated cables
have been oversized when being specified. This occurred due to electricians and engineers
using British standards such as the wiring regulations BS7671 (currently on the 18th edition)
which are more concerned with voltage drop and current carrying capability for safety
reasons.
This means that for a lighting circuit which is protected by a 6amp mini circuit breaker or
residual current device and which typically only see operating current of 3 amps, specifying a
minimum cable size of 1.5mm2 PVC cables which can cope with a maximum rating of
10amps (related to a constant temperature of 70°C) is considered highly over rated.
Due to this most PVC cables within electrical installations (which are rarely over ambient
temperature of 25/30°C – domestic or industrial) will have a lifespan well in excess of the
20,000 hours @ 89°C that they have been tested to. This means most cables will outlive the
circuit/installation that they are part of.
However, for completeness an attempt at specifying a service life for PVC insulated cables is
included below using the Arrhenius Equation and an activation energy taken from (Linde &
Gedde, 2014). For completeness both ventilated and stagnant air activation energies have
been used, but to represent the fan oven the closest value would be the ventilated air (60
kj/mol), the service temperature chosen is the rating for the cables of a maximum of 70°C.
Ventilated and Stagnant Air Conditions
T1 = ageing temperature (K) = 413.15 K (140°C)
T2 = service temperature (K) = 343.15 K (70°C)
E1 = activation energy (eV), ventilated conditions = 0.622 eV (60 kj/mol)
E2 = activation energy (eV), stagnant air conditions = 1.347 eV (130 kj/mol)
t1 = ageing time @ T1 = 168 hours
k = Boltzmann constant (eV/K) = 8.617 x 10-5 eV/K
Page | 40
Figure 12 - Arrhenius Equation Variation used to calculate service time @ rated temperatures
Using the equation in Figure 12, t2 (service lifespan) can be calculated for E1 & E2. (see
Section 8.8 - Appendix H for the excel screenshots of data).
(i)
t2 for ventilated air = gives a service time of 0.68 years for a constant temperature
of 70°C
(ii)
t2 for stagnant air = gives a service time of 43.19 years for a constant temperature
of 70°C
As can be seen from (ii) above, this would be an accurate estimation of a PVC insulated cable
within a normal electrical installation.
4
DISCUSSION
It was expected to be able to trend the electrical changes within the cables to provide an
extrapolated natural lifespan at ambient temperature, but due to the nature of PVC ageing shown
in paper (Ekelund et al., 2007) mechanical changes in the dielectric will be the limiting factors
long before any electrical deterioration.
Mechanical changes happen due to thermal induced ageing, such as compressor modulus,
elongation at break and density changes. These mechanical changes are monitored using visual
or tactile inspections, indenter modulus tools to measure surface hardness, tensile strength
equipment, infrared spectroscopy to monitor the changes in the polymer and how much
plasticiser has been lost, and other chemical observations. Of these only the visual and tactile
inspections were completed due to the time and budget constraints, which means that the results
are inconclusive and require more work to verify them or to provide more data which could be
used to create a trend pattern.
Page | 41
Due to the electrical bias and experience of the author these mechanical changes weren’t
considered prior to conducting this experiment which has highlighted a latent error in which
even using papers and standards as a guide to ensure no preconceived notions or ideas become
prevalent, the unconscious awareness of the author discounted the mechanical aspects of the
material and focused on the electrical service requirement for the cable to conduct the
experiment. This error could be mitigated for future work by working in a small team of
electrical and either mechanical or chemical engineers.
Therefore, the equipment and measurements required to accurately predict lifespan using the
practical experiment completed within this report have not been completed and these would
need to be finished before any conclusive/comprehensive statement regarding the accuracy of
the Arrhenius equation in predicting thermal stress ageing can be made.
5
CONCLUSIONS AND FURTHER WORK
The work completed so far in this project has been inconclusive in corroborating the Arrhenius
equations use in predictive maintenance. This is due to the choice of practical experimentation,
the samples chosen and the test methods available to the author at this time and within the
original time/financial constraints. There is an observable and measurable change within the
materials when they are subjected to thermal stresses and this seems to follow a linear pattern
related to the temperature and time used, which in essence is the description of the Arrhenius
equation, but the experiment completed for this project has been unable to comprehensively
prove this relationship.
For example if using the colour changes and tactile response results from the practical
experiment (@120°C) there was a big mechanical difference in the insulation, but as can be
seen in the thermography and IR results, there was no discernible change in the cables electrical
properties therefore as long as the cable wasn’t moved, by all accounts and purposes it would
fulfil its duty. Also looking at the practical experiment for plasticiser loss (@140°C) the weight
seemed to plateau after 4/5 days, this would suggest at that point the plasticiser has all but gone
from the insulation, but again from IR and thermography tests the cable would complete its
duty as long as the cable wasn’t moved. This proves an uncertainty and one that could only be
cleared up with chemical examination and mechanical tests of the insulation in question.
The author feels safe however in assuming that the discussions and recommendations made
within the real-world Arrhenius investigation are still valid. This is due to the fact that
Arrhenius has, and is still being used, within industry for ageing models and estimating lifespan
of materials for the last 50+ years, also the British Standards Institute has included the
estimation model using Arrhenius within its own cable, ageing and insulation standards (cited
throughout this report) all of which have been revised within the last 3 years to remain relevant
and valid.
Page | 42
To achieve the goals of this project, this further work would need to be completed;

A representative motorette sample (windings, insulation layers – epoxy resin, oil
impregnated paper etc.) used in the ageing experiment to check the accuracy of the
calculations within the real-world example.

Mechanical tests added to the electrical tests performed in the practical experiment to
provide a better trending of insulation deterioration.

Chemical test added to trend the structure of the insulation throughout to increase
estimations of lifespan.

Utilising a HV test lab to also measure a more sensitive IR values, leakage current and
partial discharge within the motorette sample for more accurate electrical data.

A wider variety of samples tested; with a longer timescale required to test the materials,
with test temperature as close to the service or rated temperature as practicable for
accuracy.

All of the trends collated and corrected to the one service life per service temp. This
would allow for the most accurate real world estimation.

Finally this accurate real-world estimation compared to a calculated one using
Arrhenius. This would give an accuracy margin for the Arrhenius calculation and
answer the question posed by this project with a high degree of confidence.
Page | 43
6
REFERENCES
BANFORD, H.M. & FOURACRE, R.A., 1999. Nuclear technology and ageing. IEEE
Electrical Insulation Magazine. 15(5), pp.19-27. Available from: 10.1109/57.793826.
BRANCATO, E.L., 1991. Life Expectancy of Motors. IEEE Electrical Insulation Magazine.
7(6), pp.14-22.
BRANCATO, E.L., 1978. Insulation Aging - A Historical and Critical Review
. IEEE Transactions on Electrical Insulation. (Vol EI-13, Vol 4), pp.308-317.
BRITISH STANDARDS INSTITUTE., 2018. BS IEC 60076-7:2018 - Power transformers.
Loading guide for mineral-oil-immersed power transformers.
BRITISH STANDARDS INSTITUTE., 2011. BS EN 50525-2-31:2011 - Electric cables. Low
voltage energy cables of rated voltages up to and including 450/750 V (U0/U). Cables for
general applications. Single core non-sheathed cables with thermoplastic PVC insulation.
BRITISH STANDARDS INSTITUTE., 2006. BS EN 50363-3:2005+A1:2011 - Insulating,
sheathing and covering materials for low voltage energy cables. PVC insulating compounds.
BRITISH STANDARDS INSTITUTE., 2005a. BS EN 50395:2005+A1:2011 - Electrical test
methods for low voltage energy cables.
BRITISH STANDARDS INSTITUTE BS EN 60216-2:2005: Electrical insulating materials.
Thermal endurance properties. Determination of thermal endurance properties of electrical
insulating materials. Choice of test criteria. , 2005b.British Standards Institute Available
from: https://bsol.bsigroup.com/en/Bsol-Item-Detail-Page/?pid=000000000030077957.
CARFAGNO, S.P. & GIBSON, R.J., 1980. Review of equipment aging theory and technology
Final report. United States: Available from:
http://inis.iaea.org/search/search.aspx?orig_q=RN:12582123.
COWAN, D.K. & POTTER, J., 2009. Estimated Insulation Life on 11kV Motor Stator
Windings
. EDF Energy.
DAKIN, T.W., 1948. Electrical Insulation Deterioration Treated as a Chemical Rate
Phenomenon
. AIEE Transactions. 67(1), pp.113-122. Available from: https://ieeexplore-ieeeorg.gcu.idm.oclc.org/document/5059649.
DURAN, I.C.&O.G. DUARTE. , 2012.A survey of methods of estimating lifetime and aging
of assets in substations In:Anonymous 9th IET International Conference on Advances in
Power System Control, Operation and Management (APSCOM 2012), Stevenage, UK: IET,
pp.107.
EDF ENERGY., 1982. Plant Substantiation Document
. NNC National Nuclear Corporation Limited.
EKELUND, M., EDIN, H. & GEDDE, U.W., 2007. Long-term performance of poly(vinyl
chloride) cables. Part 1: Mechanical and electrical performances. Polymer Degradation and
Page | 44
Stability; Polymer Degradation and Stability. 92(4), pp.617-629. Available from:
10.1016/j.polymdegradstab.2007.01.005.
HOOD, J.J., 1885. LIII. On retardation of chemical change. The London, Edinburgh, and
Dublin Philosophical Magazine and Journal of Science. 20(126), pp.444-456. Available
from: 10.1080/14786448508627784.
IEEE., 2014. IEEE Guide for Assessing, Monitoring, and Mitigating Aging Effects on
Electrical Equipment Used in Nuclear Power Generating Stations and Other Nuclear
Facilities (1205-2014). USA: IEEE.
IEEE., 2012. IEEE Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage
Regulators - Redline (C57.91-2011). Piscataway, USA: IEEE.
JAKUBOWICZ, I., YARAHMADI, N. & GEVERT, T., 1999. Effects of accelerated and
natural ageing on plasticized polyvinyl chloride (PVC). Polymer Degradation and Stability.
66(3), pp.415-421. Available from: 10.1016/S0141-3910(99)00094-4.
KAM, S.-. & LEDWICH, G. A database Alternative Transient Program simulated waveforms
of shunt reactor switching cases with vacuum breakers on motor circuits. , 2009.Australian
Journal of Electrical and Electronics Engineering.
KENNEDY, A., STERLING, R.&HAINS, A., 2008. Justification and Application of 11kV
Motor Stator Moisture Withstand Limits including Interpretation of Spraywater test results
. EDF Energy.
LINDE, E. & GEDDE, U.W., 2014. Plasticizer migration from PVC cable insulation – The
challenges of extrapolation methods. Polymer Degradation and Stability. 101(1), pp.24-31.
Available from: 10.1016/j.polymdegradstab.2014.01.021.
LINDQUIST, T.M., BERTLING, L. & ERIKSSON, R. Estimation of disconnector contact
condition for modelling the effect of maintenance and ageing. , 2005.IEEEAvailable from:
10.1109/PTC.2005.4524406.
LOGAN, S.R., 1982. The origin and status of the Arrhenius equation. Journal of Chemical
Education. 59(4), pp.279. Available from: 10.1021/ed059p279.
MONTSINGER, V.M., 1930. Loading Transformers By Temperature. AIEE Transactions.
49(2), pp.776-790. Available from: https://ieeexplore.ieee.org/document/5055572.
PURSS, J. & MANSON, L., 2018. Assessment of Thermal Life of the 11kV Motor Stator
Windings
. EDF Energy.
ROBERTSON, L.K.&L.A. LAMONT. , 2015.An overview of nuclear power In:Anonymous
2015 5th International Youth Conference on Energy (IYCE), Iyce, 2015.IEEE, pp.1-6.
ROLLO, J., 2016. Justification for the Thermal Ageing Calculations used to assess 11Kv
Motor Stators Substantiated Lifespan
. EDF Energy.
Page | 45
ROLLO, J., 2007. Review the requirement to introduce a 11kV motor rewind program at
HNB
. EDF Energy.
SCHWARZ, K.K., 1973. SUBMERGED GAS-CIRCULATOR MOTORS FOR
ADVANCED GAS-COOLED REACTORS. Proceedings of the Institution of Electrical
Engineers. 120(7), pp.777-785. Available from: 10.1049/piee.1973.0168.
SMYTH, T.P., 2017. HYB and TOR - 11kV Motor Insulation Tests: Phase 3
. EDF Energy.
STONE, G.C. & LAWLESS, J.F., 1979. The Application of Weibull Statistics to Insulation
Aging Tests. IEEE Transactions on Electrical Insulation. EI-14(5), pp.233-239. Available
from: 10.1109/TEI.1979.298226.
VAN'T HOFF, M. J. H., 1884. Etudes de dynamique chimique. Recueil Des Travaux
Chimiques Des Pays-Bas. 3(10), pp.333-336. Available from: 10.1002/recl.18840031003.
7
BIBLIOGRAPHY
E. L. Brancato, "Insulation Aging - A Historical and Critical Review", IEEE Transactions on
Electrical Insulation, (Vol EI-13, Vol 4), pp. 308-317, Aug 1, 1978.
EDF Energy Documents have been included as references and hard copies can be supplied at
request whenever protective markings allow. The data within this report has been altered to be
less specific to avoid the need for a non-disclosure agreement.
Page | 46
8
APPENDICES
DATA from EDF Energy regarding the real-world Arrhenius equation example. This has been
copied from the original report, (Purss & Manson, 2018), for information purposes only. As
before, a hard copy of the reference is available on demand.
8.1
Appendix A – HYB/TOR 11kV motor stator ageing test programme and
applicability of results to HNB/HPB 11kV motors.
This Appendix contains a summary review of the recent ageing test programme (Reference
A2), which extended the qualified life of the HYB/TOR 11kV motor windings. The assessment
considers the applicability of the test models and conditions such that the results may be used
in the calculation of HNB (and HPB) 11kV motor winding thermal life.
Test models v HNB 11kV motor windings
 The motorettes and end winding models were manufactured by the motor OEM, ATB
Laurence Scott, to an essentially similar insulation specification (N257F7) as the present
HNB motor windings. The HNB motors have been rewound (at least once) by Laurence
Scott to the same specification since the machines were originally supplied. The ground
wall insulation is glass backed mica paper referred to as Novobond SX. ‘SX’ refers to the
insulation tape resin Epoxy Novolac SX and it is thermal ageing of this resin which is
important because it plays a significant role in maintaining the mechanical integrity of the
conductor insulation to resist the effects of pressurised CO2 / pressure cycling and therefore
withstanding a rapid depressurisation fault.

The final coating resin on the test programme models is Epoxylite 235SG, which is a 2part epoxy chosen to replace the original Sterling 006 1073 because it is obsolete. Epoxylite
235SG was deemed to be equivalent to 006 1073. The HNB (and HPB) motor windings
are coated with Sterling 003 1010 isophthalate polyester varnish. The difference in final
coating material is much less important for thermal ageing and resistance to CO2 effects as
it does not play a significant role in the electric or mechanical strength capability of the
insulation system.
It is known that early in the life of the resin coating, cracks appear in the resin on the
endwindings due to the pressure cycling and this is the case with both types of final coating
material. During ageing the test models are subjected to a high-pressure atmosphere saturated
with oil to simulate the presence of bearing oil in the real motor compartment due to leaks from
the bearings. The resin cracks are a potential penetration route for water, but the high moisture
cycle and water spray tests gave no failures. This is in part due to the waterproofing action of
the oil which has been forced into the insulation layers under action of pressurised CO2 and this
is common to the test models and actual machines.
It is worthy of note that part of the Project mitigations, for the HYB/TOR ageing programme
not delivering an extended winding life, was a study to assess the feasibility of rewinding
HYB/TOR stators at the HPB rewind facility. ATB Laurence Scott concluded (Reference A1)
that isophthalate polyester varnish was a proven system with ATB LS referring to the rewinds
Page | 47
on HPB/HNB motors through both in service experience and simulated boiler tube leak water
spray testing for HPB/HNB (see mention below of the HPB/HNB water spray testing).
Ageing test conditions (Reference A2)
 Test models underwent accelerated ageing at elevated temperature in pressurised CO2,
normal moisture levels and were subjected to regular pressure cycles representative of
normal reactor pressurising and depressurising rates. Also included were several rapid
depressurisations.





Testing Environment - The accelerated ageing was completed in a pressurised CO2
environment. No ageing testing was conducted in an air environment; however, the test
results are considered applicable to the HNB winding thermal life calculations for periods
of operation in an air environment on the following basis. The thermal classification of
the HNB 11kV motor stator insulation system is Thermal Class 155 (F) in accordance with
BS EN 60085, meaning that the insulation should have an average life of 20,000 hours
when operating at 155°C (Reference A5). The test results from the accelerated ageing in
CO2 provide a qualified life base ageing temperature and duration for the winding thermal
life calculations detailed within this report (Section XX - Appendix B) and it has been used
to determine an equivalent thermal life capability of 2,587hours at 155°C. The thermal
life capability derived from the test results is much more conservative than the 20,000hours
and therefore bounds motor operation in an air environment. It should be noted that the
accelerated ageing and tests were completed successfully and there was nothing in the final
diagnostic test and inspection results to suggest that the test pieces could not have
undergone further ageing, giving a further extension to the thermal life capability.
Rapid Depressurisation Rate reproduced the pressure profile for a HNB major breach
curve as closely as reasonably practicable and was chosen as depressurisation at HNB
would occur more rapidly than at HYB/TOR (Figure 3 Reference A2). The test profile
was 4.8 bar/min to 20.69 barg, followed by 1.8 barg/min to 6.9 barg, then 0.5 bar/min until
depressurised.
Test voltages during ageing and water spray testing - The test models were subjected to
test voltages based on the motor rated voltage 11 kV so equally applicable to HNB.
Moisture concentration levels - For normal ageing in pressurised CO2, the moisture level
was based on the maximum of the normal concentration range specified in Reference A3
for HYB/TOR i.e. 194 wppm and was finally chosen to be 205 wppm on average over a
test dwell period. The maximum level specified in Reference A3 for HNB is 124 wppm
(HPB 112 wppm) so the test moisture level bounds the HNB operation. For a test cycle
with high moisture level cycle followed by rapid depressurisation, (to simulate water
ingress conditions following a boiler tube failure) the test simulated water saturated
conditions so it is equally applicable to HNB.
Voltage Endurance and Spray Testing (VEST)
o All test pieces endured 2off VESTs after they reached an equivalent of 42+ yrs. –
one to simulate 3.3 kV VSD/VFC operation and one for 11 kV.
o
Page | 48
VEST consists of enduring the appropriate voltage for 26.4 (24+10%) hrs dry plus
26.4hrs fully wetted (spray on) plus 26.4 hrs of “wet” (left wet following the spray
on period but spray is turned off).
o
The four oldest motorettes endured another 2off VESTs (based on 3.3 kV and 11
kV) at their final 54+ yr. ages. There is no reason to suggest that the next two oldest
(~50 years old) motorettes wouldn’t have been able to endure similar additional
VESTs but project time constraints prevented additional testing.
Water spray testing carried out several years ago for HPB/HNB for the Boiler Tube Leak safety
case development (see Reference A4 which quotes HINB/R/MSR/PLEX/002) established a
safety case claim of: Short term spray rate: 0.1kg/s per motor for up to 30 minutes, Longer term
spray rate: 0.03kg/s per motor for up to 6 hours. Cumulative durations achieved under wetted
conditions were 16 hrs, 18.5 hrs, 25 hrs for different test samples/configurations. In this case
HNB withstand against water ingress is bounded by the HYB/TOR test programme.
Conclusion - The recent test programme to extend the winding substantiated life to support a
motor winding life extension safety case for TOR and HYB Power Stations included
accelerated ageing of motorettes and end winding models. The test pieces and test conditions
are considered representative for the HNB (and HPB) 11kV motor windings and the test results
are appropriate for use in the HNB winding thermal life calculations in this report.
Appendix A References:
A1. ATB LS.2016.1624W04.01 (CDMS Records) Stator Impregnation system design study,
September 2016.
A2. 201424-TR-000037 (HYB CDMS Controlled Documents), Heysham 2 & Torness Power
Stations. Motor Insulation Tests: Review of Phase 3 Test Conditions, Issue 01, 19 February
2016.
A3. BEG/SPEC/ENG/BEOM/211 Control of AGR Coolant Composition.
A4. DAO/REP/JICC/018/AGR/08, Hunterston B and Hinkley Point B, Justification and
Application of 11kV Motor Moisture Withstand Limits including Interpretation of Spray
water Test Results, August 2008.
A5. Electrical Insulation for Rotating Machines: Design, Evaluation, Aging, Testing and
Repair, Greg C. Stone / Edward A. Boulter / Ian Culbert / Hussein Dhirani, 2004.
Page | 49
8.2
Appendix B – Base Data from HYB/TOR 11kV Motor Lifetime R&D Project
Test results are detailed within table 6-5 ‘Summary of Test Piece Ages’ of report Reference
[18] and are used to provide a qualified life base ageing temperature and duration for the
winding thermal life calculations detailed within this report. It should be noted that the
accelerated ageing and tests were completed successfully and there was nothing in the final
diagnostic test and inspection results to suggest that the test pieces could not have undergone
further ageing, giving a further extension to the thermal life capability.
Heysham 2 and Torness motor stators have a mix of Novobond S motors and SX motors. The
test pieces, subject of Reference [18], were made from SX, the S material being no longer
available. It was considered appropriate by the project to simulate the slightly more inferior S
material by basing the test durations on the S activation energy. For the purposes of calculating
the winding thermal life for HNB motor stators which only contain SX material, it is necessary
to derive a base ageing test duration which can then be used to calculate service life based on
the SX activation energy.
Service Temperature: 92.4°C (365.55 K) - Section 2.2 of Reference [18]
Average Claimed Service Age: 54.8years (480,048 hours)
Novobond S Activation Energy E = 1.13eV
Novobond SX Activation Energy E = 1.2eV
Solving the Arrhenius Equation for t1 assuming an ageing test temperature of 160°C (433.15
K) and E of 1.13eV, gives an ageing test duration of 1777.54 hours. Therefore, the winding
insulation has a qualified thermal life of 1777.54 hrs at 160°C. All winding thermal life
calculations throughout this note use this base test temperature and duration to determine the
equivalent insulation thermal life capability (in hours) for the HNB insulation material
Novobond SX.
For example, if a HNB motor stator ran continuously at 121°C, the insulation (Novobond SX)
thermal life capability is calculated using the Arrhenius Equation as follows:
Novobond SX Activation Energy (E) = 1.2eV
T1base = (273.15 + 160) K
T2 = (273.15 + 121) K
t1base (the ageing time at T1base) = 1,777.54hrs
k (the Boltzmann constant) = 8.617x10-5eVK-1
Solving the Arrhenius Equation for t2 results in a thermal service life capability = 42,795 hours.
It should be noted that the qualified life of 1777.54 hrs at 160°C is equivalent to ~3800 hrs life
at 150°C. This can be compared against the original Torness and Heysham 2 winding
substantiation test programme in the 1980s which maintained the test motorette coils at 150°C
for 2160 hrs (5 days in every 7 of the 3,000-hour programme) Reference [21]. Therefore, the
recent ageing test programme increased the substantiated thermal life capability of the HNB
stator winding by approximately 75% for a given constant temperature.
Page | 50
Gas circ stator ID
Present Berth
STATOR
Hunt
05
3A1
Date of rewind
28/08/1993
Installation date
Data start date
Data hours
(Ferranti data period)
Data years
(Ferranti data period)
Hours pre-2001
(pre-Ferranti)
29/08/1993
05/07/2002
STATOR
Hink 12
STATOR
Hink 05
STATOR
Hunt
11
3B2
STATOR
Hunt
02
3C1
16/09/1999
Complete 2015
17/09/1999
28/06/2001
30/09/2012
30/09/2012
29/10/2015
29/10/2015
112356
139008
51694
24694
11.97
12.83
15.87
5.90
2.82
0
125892
46451
16715
0
0
11.27
0.00
14.37
5.30
1.91
0.00
0.00
19.92
27.10
5.90
26.35
18.13
17.78
5.90
2.82
53.91
43.53
13.54
29.38
61.77
82.43
22.89
14.73
18.17
30.99
0
35.27
25.53
9.91
0
0
22/06/1989
3B1
prior to R3 stat
2012
20/01/1984
27/06/1996
23/06/1989
28/06/2001
30/09/2012
30/09/2012
20/01/1984
05/07/2003
28/06/1996
28/06/2001
130484
138681
51694
104893
14.90
15.83
5.90
43989
98723
Years pre-2001
(pre-Ferranti)
5.02
Total service years since rewind
% thermal life expired during
Ferranti data period
% thermal life expired pre-2001
(pre-Ferranti)
Total % thermal life expired
Gas circ stator ID
Present Berth
Date of rewind
Installation date
Data start date
Data hours
(Ferranti data period)
Data years
(Ferranti data period)
Hours pre-2001
(pre-Ferranti)
Years pre-2001
(pre-Ferranti)
Total service years since rewind
% thermal life expired during
Ferranti data period
% thermal life expired pre-2001
(pre-Ferranti)
Total % thermal life expired
72.08
STATOR
Hunt
03
4A1
prior to R4 stat
2011
02/08/2011
02/08/2011
74.52
STATOR
Hunt
20
4A2
13.54
STATOR
Hunt
17
4B1
64.65
STATOR
Hunt
18
4B2
28/03/1982
29/03/1982
06/07/2001
completed 2015
12/10/2017
12/10/2017
07/11/1987
08/11/1987
28/06/2001
35061
121388
7550
96432
4.00
13.86
0.86
0
122584
0.00
92.34
STATOR
Hunt
15
4C2
STATOR
Hunt
22.89
STATOR
Hunt
06
4D1
STATOR
Hunt
07
3D2
14.73
STATOR
Hunt
19
4D2
21/06/2000
21/06/2000
06/07/2001
completed 2016
22/10/2017
22/10/2017
34399
134372
133217
7309
11.01
3.93
15.34
15.21
0.83
0
104540
0
8355
9276
0
13.99
0.00
11.93
0.00
0.95
1.06
0.00
4.00
27.85
0.86
22.94
3.93
16.29
16.27
0.83
21.04
47.73
5.26
36.97
21.36
62.86
47.19
2.97
0
21.04
48.2
95.93
0
5.26
40.07
77.04
0
21.36
3.91
66.77
3.28
50.47
0
2.97
SPARE
SPARE
SPARE
SPARE
Date of rewind
01/08/1994
01/06/2003
11/09/1986
planned 2018
Installation date
Data start date
Data hours
(Ferranti data period)
Data years
(Ferranti data period)
Hours pre-2001
(pre-Ferranti)
01/08/1994
06/07/2001
01/06/2003
01/06/2003
11/09/1986
06/07/2001
0
0
113505
116150
132735
0
12.96
13.26
15.15
0.00
STATOR
Hunt 10
STATOR
Hunt 12
Hunt 01
STATOR
50579
0
100907
0
Years pre-2001
(pre-Ferranti)
5.77
0.00
11.52
0
Total service years since rewind
18.73
13.26
26.67
0.00
58.75
66.53
78
0
26.18
0
59.18
0
84.93
66.53
137.18
0.00
Page | 51
3C2
08/07/2000
08/07/2000
06/07/2001
STATOR
Hunt 08
Total % thermal life expired
87.30
STATOR
Hunt 21
4C1
prior to R4 stat
2014
20/09/2014
20/09/2014
Gas circ stator ID
Present Berth
% thermal life expired during
Ferranti data period
% thermal life expired pre-2001
(pre-Ferranti)
STATOR
Hink 03
04
3D1
prior to R3 stat
2012
3A2
8.3 Appendix C – Data Analysis Results (valid August 2018)
8.4
Appendix D - Case study STATOR 12 versus STATOR 12.2
11kV motor stators STATOR 12.2 and STATOR 12 have a similar number of service years but
a significant difference has been calculated in their percentage thermal life expired. Comparing
these two machines clearly demonstrates the effect that service temperature has on the thermal
life of the stator windings.
Stators STATOR 12.2 and STATOR 12 were both deployed following rewind in the late
1980’s. Prior to the Ferranti data period each machine had approx. 11.5 years of service.
Approximately 135,000 hours’ worth of logged Ferranti data has been analysed for each stator
winding, with both machines having approximately 27 years total service. STATOR 12 was
withdrawn from Reactor 4 in October 2017, whereas STATOR 12.2 is currently installed in
Reactor 3 location 3A2.
Table C1 – STATOR 12 vs. STATOR12
Stator ID
STATOR 12.2
STATOR 12
Present location
3A2
SPARE
Installation date
23/06/1989
11/09/1986
Data start date
28/06/2001
06/07/2001
Data years (Ferranti data period)
15.83
15.15
Years pre-2001 (pre-Ferranti)
11.27
11.52
Total service years (since rewind)
27.10
26.67
% thermal life expired during Ferranti data period
43.53
78.00
% thermal life expired pre-2001 (pre-Ferranti)
30.98
59.18
Total % thermal life expired
74.52
137.18
Page | 52
The temperature profiles for STATOR 12.2 and STATOR 12 are included in Section 11.5 Appendix E. Since 2001, both machines have continuously been deployed into a reactor
location, other than a period of approximately a year for each, where the machine was
withdrawn for maintenance.
At first glance the temperature profiles for each machine look similar, with STATOR 12 service
temperatures generally only slightly higher than those of STATOR 12.2. Further data analysis
supports this, as the average Ferranti logged temperature of the STATOR 12.2 winding is 88°C
compared with a modestly higher 93°C average temperature for the STATOR 12 winding.
However close examination of the two charts shows that the extended/additional periods at
elevated temperatures (>140°C) experienced by STATOR 12 significantly impacts the
percentage thermal life expiry rate (gradient of the red line). Again, further data analysis
supports this, with STATOR 12.2 winding temperatures >140°C for only 5 hours accounting
for 0.06% of expired thermal life, and STATOR 12 winding temperatures >140°C for 1,302
hours, accounting for 25% of expired thermal life.
In conclusion, although stators STATOR 12.2 and STATOR 12 have a similar number of
service years, the periods of elevated temperatures experienced by STATOR 12 have resulted
in its percentage thermal life expired being nearly double that of STATOR 12.2.
Page | 53
8.5
Appendix E - % Thermal Life Expired Charts
STATOR 05 in HNB location 3A1 as of August 2018
STATOR
Page | 54
STATOR 12.2 in HNB location 3A2 as of August 2018
STATOR
Page | 55
STATOR 05.2 in HNB location 3B1 as of August 2018
STATOR
Page | 56
STATOR 11 in HNB location 3B2 as of August 2018
STATOR
Page | 57
STATOR 02 in HNB location 3C1 as of August 2018
STATOR
Page | 58
STATOR 03.2 in HNB location 3C2 as of August 2018
STATOR
Page | 59
STATOR 04 in HNB location 3D1 as of August 2018
STATOR
Page | 60
STATOR 07 in HNB location 3D2 as of August 2018
STATOR
Page | 61
STATOR 03 in HNB location 4A1 as of August 2018
STATOR
Page | 62
STATOR 20 in HNB location 4A2 as of August 2018
STATOR
Page | 63
STATOR 17 in HNB location 4B1 as of August 2018
STATOR
Page | 64
STATOR 18 in HNB location 4B2 as of August 2018
STATOR
Page | 65
STATOR 21 in HNB location 4C1 as of August 2018
STATOR
Page | 66
STATOR 15 in HNB location 4C2 as of August 2018
STATOR
Page | 67
STATOR 06 in HNB location 4D1 as of August 2018
STATOR
Page | 68
STATOR 19 in HNB location 4D2 as of August 2018
STATOR
Page | 69
STATOR 08 Spare as of August 2018
STATOR
Page | 70
STATOR 10 Spare as of August 2018
STATOR
Page | 71
STATOR 12 Spare as of August 2018
STATOR
Page | 72
8.6
Appendix F – 11kV Motor Stator Thermal Life Going Forward by Reactor Location
R3 Berth
% life expired
at August
2018
3A1
STATOR
HUNT 05
3A2
72.08%
HINK 12
STATOR
3B1
74.52% STATOR
HINK 05 13.54%STATOR
HUNT 11
3B2
Planned
deployment
of
HUNT 10
STATOR
R3 2019 Stat
Outage
3C2
HINK 03
STATOR
22.64%
75.63%
83.98%
88.94%
11.90%
21.00%
25.44%
34.54%
78.43%
87.53%
99.20%
108.30% Planned
deployment
of
HINK 12
STATOR
2022
91.74%
23.80%
37.34%
90.33%
2023
94.54%
26.60%
40.14%
93.13%
2024
2025
97.34%
100.14%
29.40%
32.20%
42.94%
45.74%
95.93%
98.73%
Planned
deployment
of HUNT 18
R4 Berth
4A1
% life expired STATOR
HUNT 03
at August
2018
2019
R4 2020 Stat
Outage
2021
2022
2023
2024
2026
4A2
21.04%STATOR
HUNT 20
23.84%
32.94% Planned
deployment
of HUNT 08
35.74%
38.54%
41.34%
44.14%
46.94%
SPARE
SPARE
% life expired STATOR
HUNT 12
137.18%STATOR
HUNT 01
at August
2018
4B1
95.93%STATOR
HUNT 17
5.26%STATOR
HUNT 18
98.73%
94.03%
96.83%
99.63%
102.43%
105.23%
108.03%
4B2
8.06%
17.16% Planned
deployment
STATOR
of HUNT 11
19.96%
22.76%
25.56%
28.36%
31.16%
SPARE
SPARE
0.00%STATOR
HUNT 08 84.93%STATOR
HUNT 10
3D1
3D2
92.34% STATOR
HUNT 04 22.89% STATOR
HUNT 07 14.73%
66.53%
9.10%
2020
R3 2021 Stat
Outage
81.18% Planned
deployment
of HUNT 01
3C1
64.65%STATOR
HUNT 02
87.30%
96.40%
101.44%
31.99%
23.83%
104.24%
83.62%
34.79%
43.89%
26.63%
35.73%
111.10%
86.42%
46.69%
38.53%
113.90%
89.22%
49.49%
41.33%
116.70%
119.50%
92.02%
94.82%
52.29%
55.09%
44.13%
46.93%
4C1
4C2
77.04%STATOR
HUNT 21
21.36%STATOR
HUNT 15
4D1
4D2
66.77%STATOR
HUNT 06
50.47%STATOR
HUNT 19
2.97%
79.84%
73.75%
24.16%
33.26%
69.57%
78.67%
53.27%
62.37%
5.77%
14.87%
76.55%
79.35%
82.15%
84.95%
87.75%
36.06%
38.86%
41.66%
44.46%
47.26%
81.47%
84.27%
87.07%
89.87%
92.67%
65.17%
67.97%
70.77%
73.57%
76.37%
17.67%
20.47%
23.27%
26.07%
28.87%
66.53%
Predicted % life used per non-outage year
2.8% This figure is based a years continual operation with winding temperatures at 100°C
Predicted % life used per outage year
This figure is based on an outage of 55 days duration with winding temperatures continuously at 130°C and the remainder of the
9.1% year with winding temperatures continuously at 100°C.
Page | 73
8.7
Appendix G – Details from BS EN 60216-8:2013 Suggested Temperatures and durations for
accelerated ageing tests.
Table 8 - Suggested exposure temperatures and times
Page | 74
8.8
T1 =
T2 =
t1 =
k=
Ea =
Appendix H – Excel Data capture from Calculated Arrhenius Service Levels
413.15
343.15
168
8.62E-05
0.622
K
K
hours
eV/K
eV
Ventilated Air = Ea = 60 kj/mol
t2 =
23.1657 hours
t2 =
46.3314 hours
t2 =
92.66279 hours
t2 =
185.3256 hours
t2 =
370.6512 hours
t2 =
741.3023 hours
t2 =
1482.605 hours
t2 =
2965.209 hours
t2 =
5930.419 hours
t2 =
11860.84 hours
t2 =
23721.68 hours
t2 =
47443.35 hours
t2 =
94886.7 hours
t2 =
189773.4 hours
0.965237
1.930475
3.86095
7.721899
15.4438
30.8876
61.7752
123.5504
247.1008
494.2016
988.4031
1976.806
3953.613
7907.225
days
days
days
days
days
days
days
days
days
days
days
days
days
days
0.002644
0.005289
0.010578
0.021156
0.042312
0.084624
0.169247
0.338494
0.676988
1.353977
2.707954
5.415908
10.83182
21.66363
years
years
years
years
years
years
years
years
years
years
years
years
years
years
150 degrees Celsius
140 degrees Celsius
130 degrees Celsius
120 degrees Celsius
110 degrees Celsius
100 degrees Celsius
90 degrees Celsius
80 degrees Celsius
70 degrees Celsius
60 degrees Celsius
50 degrees Celsius
40 degrees Celsius
30 degrees Celsius
20 degrees Celsius
Figure 13 - Service life @ Ea = 60 kj/mol
T1 =
T2 =
t1 =
k=
Ea =
413.15
343.15
168
8.62E-05
1.347
K
K
hours
eV/K
eV
Stagnant Air = Ea = 130 kj/mol
t2 =
1475.452 hours
t2 =
2950.905 hours
t2 =
5901.81 hours
t2 =
11803.62 hours
t2 =
23607.24 hours
t2 =
47214.48 hours
t2 =
94428.96 hours
t2 =
188857.9 hours
t2 =
377715.8 hours
t2 =
755431.7 hours
t2 =
1510863 hours
t2 =
3021727 hours
t2 =
6043453 hours
t2 =
12086907 hours
Figure 14 - Service life @ Ea = 130 kj/mol
Page | 75
61.47719
122.9544
245.9087
491.8175
983.635
1967.27
3934.54
7869.08
15738.16
31476.32
62952.64
125905.3
251810.6
503621.1
days
days
days
days
days
days
days
days
days
days
days
days
days
days
0.168431
0.336861
0.673723
1.347445
2.69489
5.389781
10.77956
21.55912
43.11825
86.23649
172.473
344.946
689.8919
1379.784
years
years
years
years
years
years
years
years
years
years
years
years
years
years
150 degrees Celsius
140 degrees Celsius
130 degrees Celsius
120 degrees Celsius
110 degrees Celsius
100 degrees Celsius
90 degrees Celsius
80 degrees Celsius
70 degrees Celsius
60 degrees Celsius
50 degrees Celsius
40 degrees Celsius
30 degrees Celsius
20 degrees Celsius