A
REPORT
ON
CONDITION MONITORING
OF LIGHTNING ARRESTERS
By:
Nasim Uddin
Chief Electrical Locomotive Engineer
North Central Railway
Allahabad
INDEX
Sr.
No.
1.
Contents
Page No.
Introduction
1
Principle of Operation of Lightening
Arrester
Energy Handling Capacity and Thermal
Stability of ZnO Arrester
Important Definitions
1
5.
Information required for Selection of a
Zinc Oxide LA
5
6.
Provision of LAs in Traction
Stations and Rolling Stock
5
7.
Failure Modes of LA
5
8.
Existing
methods
of
conditions
monitoring of LA
Monitoring of leakage current in LAs
6
10.
Proposed method for measurement of
leakage current
9
11.
Case study of Lightning Arrester failure
10
12.
Experience of other Utilities
10
13.
Conclusion
11
14.
References
12
15.
Annexure-1
13
2.
3.
4.
9.
Sub-
4
4
7
A Report on condition monitoring of Lightning Arresters
1. Introduction
High voltage power systems including traction power supply experience over
voltages, which are generated through occurrence faults, switching operations
and lightning discharges. The duration of the over voltages vary from a few
micro seconds few seconds depending on the type of surges. Similarly, the
magnitude of over voltages varies from 1.5 to 4 times of the normal operating
voltages. Under these severe overvoltage conditions, the insulation of the
power system/equipments undergoes stresses that could lead to catastrophic
failure. Hence it is imperative that the power system equipments are protected
from these over voltages at the time of occurrence. Using a device with
variable impedence with respect to voltages can provide the protection of
power equipments from overvoltages. This kind of overvoltage protection
device is connected in parallel to the system/equipments to be protected. An
effective surge Protection device must satisfy the following conditions:
•
Conduct only during over voltages.
•
Low power loss under normal operating conditions.
•
High energy handling capability.
•
High reliability and long life.
Usually, metal oxide (Zinc Oxide) type of Lightning Arrester (LA) is used for
protection of system/equipments from overvoltage.
2. Principle of Operation of Lightening Arrester
The primary function of a zinc oxide surge arrester is to protect the power
equipments from overvoltages and to absorb electrical energy resulting from
lightning or switching surges and from temporary over voltages. These gapless
metal oxide arrestors consist are of active part, which is a highly non-linear
ceramic resistor, made of essentially zinc oxide. Fine Zinc Oxide crystals are
surrounded by other metal oxides (additives). The microstructure at an
element of LA is shown in Fig. 1.
As shown in Figure 1 that Zinc Oxide elements are made by mixing Zinc Oxide
with small amount of additive materials such as Ba2 O3. The mixture is, then,
granulated, compacted and fired or baked. The equivalent electrical circuit of
an LA is a Parallel Combination of capacitance and variable resistance. The
current flowing through LA is the total leakage current (It) having capacitive
leakage current (Ic) and resistive leakage current (Ir) components. Normal
operating voltages cause ageing of Zinc Oxide elements/blocks whereas
Switching/Lightning overvoltages may cause overloading of all or part of the
Zinc Oxide blocks. Due to various electrical stresses, the granulated
layers/barriers break down causing the conduction. The increase in the
voltage stresses on healthy granulated layers results in the higher resistive
leakage current (Ir) amounting to higher total leakage current in LA.
The non-linear characteristics of these Zinc Oxide blocks is shown in Fig. 2.
In Fig.2, X-axis is in logarithmic scale. This special characteristic is the heart
of protection technology.
The lower linear part A is temperature dependent, and exhibits a negative
temperature coefficient. The arrester is designed in such a way that the
applied operating voltage gets located around point O. This results in a
continuous resistive current of few micro amps flowing through the resistor
elements. Under overvoltage condition, the voltage increases and shifts
operating point momentarily for overvoltage duration to point near B. This
results in a resistive current of few milli amperes flowing through the resistor
elements. As soon as the overvoltage disappears, the operating point will shift
back to O. In the event of transient switching or lightning overvoltages, the
operating point will shift to portion C.
For the transient of a few micro seconds, it will draw current in the rage of 5/
10 KAmps. In the event of very high lightning current of the order of 40 to 100
KAmps peak, the operating point will shift to portion D. However, on expiry of
transient of few milliseconds the operating point will come back to point O.
Thus, the operating point of these arresters is normally located at point O
called Maximum Continuous Operating Voltage (MCOV) & the point B of Fig. 2
indicates approximately the rated voltage of arrester. The arrester can stay at
point O, i.e., MCOV, all along its life but it can stay at point B (fault
condition), i.e.. Rated Voltage, for only 10 seconds (It is presumed that system
breakers will operate to isolate the fault within 2 seconds). The energy that
gets dissipated, i.e. (1²R) during continuous or overvoltage condition decides
the size (dia.) of ZnO resistor element.
Various types of Zinc Oxide elements typically used in LAs are shown in
figure-3
The active part of Zinc Oxide arresters is made up of a column of stacked
cylindrical resistor. The number of resistors in the stack depends upon the
continuous operating voltage of the arrester. The column is installed in a
hermetically sealed either in porcelain or in polymer housing. In the railway
applications particularly for a 39 KV class lightning arrestors, 12 nos. Zinc
Oxide blocks/elements of 3.25 KV rating each are used.
3. Energy Handling Capacity and Thermal Stability of ZnO Arrester
The application of ZnO ceramic elements for over voltage protection also calls
for energy handling capacity which is defined as Energy E= V x I x t.
The energy handling capacity of the commercial ZnO arresters is in the order
of 150 Jouls/cm³.
The Energy absorption and dissipation being dependent on this specific
energy handling capacity, the energy discharge of Zinc Oxide Block is
basically dependent on its volume.
The rated voltage of Zinc Oxide element is proportional to the height. The
energy level increases with the increase in area of the Zinc Oxide block.
Concept of thermal behaviour of ZnO arrester is an important application
consideration. Thermal capability of a design takes advantage of overvoltage
capability. The thermal capability of ZnO arrester depends on the assembly
structure of the arrester.
As long as the heat generated from the ZnO elements due to continuous
operating voltages and surges is less than the thermal power dissipation of
the housing, the elements will remain in an undamaged condition, capable of
performing their protective function.
4. Important Definitions
4.1.
4.2.
4.3.
4.4.
4.5.
4.6.
4.7.
Arrester discharge current
The current that flows through an arrester resulting from an impinging
surge.
Arrester discharge voltage
The voltage that appears across the terminals of an arrester during the
passage of discharge current,
Lightning overvoltage
The crest voltage appearing across an arrester or insulation caused by
a lightning surge.
Maximum continuous operating voltage (MCOV) rating
The maximum designated root-mean-square (rms) value of power
frequency voltage that may be applied continuously between the
terminals of the arrester.
Rated Voltage
The rated voltage is the maximum power frequency voltage that is
applied in the operating duty test for 10 seconds.
Standard lightning impulse
The wave shape of the standard impulse used is 1.2/50 µA.
Temporary overvoltage
An oscillatory overvoltage, associated with switching or faults (for
example, load rejection, single-phase faults) and/or nonlinearities
(ferroresonance effects, harmonics), of relatively long duration, which is
undamped or slightly damped.
5. Information required for Selection of a Zinc Oxide LA
For selection of an appropriate ZnO arrester, the following information need to be
collected:
(i)
nominal system voltage.
(ii)
maximum system voltage.
(iii) system overvoltages (type, magnitude and frequency of occurrence) from
past records.
(iv) ambient conditions (temperature, pollution level etc.).
(v)
grounding condition of the system.
(vi) insulation strength of the equipment/system to be protected.
(vii) the distance between the arrester and the equipment to be protected and
(viii) geographical location of installation etc.
6. Provision of LAs in Traction Sub-Stations and Rolling Stock
In the earlier stage of railway electrification, Silicon Carbide (SiC) Gap type
lightning arrestors were provided on 132 KV as well as 25 KV side of the Traction
Sub-Station in order to protect the traction equipments including the
transformers. However, the presence of series gaps in this class of arresters
reduced both the degree of protection and reliability. With the advent of Zinc
Oxide type gapless lightning arresters with superior voltage-current
characteristics, the gap type Las were extinct. However, there is a large number
of gap type LAs still in service in Traction Sub-Stations, which are to be replaced,
by Zinc Oxide type over a period of time.
Similarly, rod type gap (ET-1 & ET-2) arresters were provided on 25 KV on the
roof of the electric locomotives in order to protect the Power equipments of the
Locomotives, CLW has now cut in the provision the Zinc Oxide type Lightning
arresters in place of rod-type gap (ET-2).
7. Failures Modes of LA
Approximately 5000 nos. of Zinc Oxide type Lightning Arresters each on 132 KV
and 25 KV side of Traction Sub-Stations are in service. LAs provided on 132 KV
side are identical in design as on 25 KV side except the number of Zinc Oxide
blocks used for different voltage applications. Similarly, about, 500 nos. Zinc
Oxide LAs have been provided on electric locomotives, so far.
Incidentally, the exact rate of failure of LAs on Indian Railways is not accurately
available except the cases of bursting of LAs.
Further, the failure rate of Power equipments such as Transformers etc. are also
not fairly known on account of defective LAs or unprotected circuit in service.
However, as per the field survey of power utilities and cases reported by Central
Railway for bursting cases of dc Lightning Arrester on dc EMUs, a general pattern
of porcelain housed LA failures have been identified which is summarised in
Figure-4.
100
90
s 80
e
r
lu
i 70
a
f 60
l
a
t 50
o
t 40
f
o 30
e
g 20
a
% 10
0
90
Ingress of moisture
Premature ageing
of ZnO blocks
10
1
2
Causes of failures
Figure - 4
8. Existing methods of conditions monitoring of LA
As per the international norms, various techniques are available for the health
monitoring of Las in service. Some of the techniques are mentioned below:
a.
b.
c.
Insulation Resistance measurement.
Total leakage current measurement.
Third Harmonic resistive leakage current monitoring.
Presently, the health of an LA provided for Railway applications is monitored by
periodically measuring the Insulation Resistance (IR) by using 1000V meggar.
Usually, the allowable range of IR value for LA should not be less than 1000 Mega
Ohms. The measurement of IR value of an LA apparently gives an indication of
degradation due to ingress of moisture. In addition to this, some of the high
voltage Las are provided with an Ammeter connected in series to indicate the
total leakage current flowing through the LA while in service.
The measurement of Insulation Resistance of LA does not provide any significant
information about the health/degradation of metal oxide elements while in
service. The insulation resistance of an LA may remain high even though the LA
might be on the verge of failure due to various reasons including the ingress of
moisture. Hence, the value of IR of an LA cannot be taken as a criterion for
accurate monitoring the health of an LA. Therefore, practically no real time
system for monitoring the health of LAs exists on Indian Railways. Even, if the
measurement of total leakage current is monitored, then also a database is
required to be built up by the Railways in order to prescribe a threshold limit for
various make of Lightning Arrestors for taking the decision of replacing the LA.
9. Monitoring of leakage current in Las
9.1.
Significance of leakage current flowing through LA
In normal service, the Metal Oxide Lightning Arrestors are exposed to
different kinds of stresses such as the normal operating voltage,
temporary overvoltages, switching overvoltage, lightning overvoltages
and external pollution.. All these stresses, separately or together in
different combinations, may cause an increase in the resistive
component of the continuous leakage current through the arrester.
This increase may exceed the critical limit and cause arrester failure,
for instance if the arrester rated voltage has been selected too low, if
ZnO blocks have been cracked or punctured due to overvoltages or if
some special fault situations that cause high temporary overvoltages
have occurred in the network. In addition, general ageing may also be
the reason for increased leakage current and often accelerated due to
pollution on the arrester housing.
Thus, the measurement of leakage current flowing through LA under
normal situations gives the information about the real operating
condition of an LA, which may help to:
•
•
•
•
prevent arrester failures by replacing aged arresters before breakdown.
increase the safety for the utility/maintenance staff.
avoid disturbances in the electric power supply.
reduce the risk for damages to other equipment due to arrester
failures, for instance transformer bushings.
9.2.
Importance of monitoring of Resistive leakage current
As discussed in para-7, the measurement of total leakage current
flowing through an LA under normal conditions is also used as one of
the health monitoring techniques. However, the total leakage current
measurement does not indicate the severity of degradation of Zinc
Oxide elements as the resistive current (Ir) is only 15-25% of the total
leakage current. Hence, a sharp increase in resistive current due to
degradation/ageing of Zinc Oxide blocks does not affect the total
leakage current considerably. The higher resistive leakage current may
ultimately bring in the LA to thermal instability and may result in
complete failure/breakdown of the Arrester. Hence, the resistive
leakage current is the true indicator of health of an LA in service.
9.3.
Influence of temperature and operating voltage
The resistive leakage current depends on the arrester temperature (in
practice the ambient temperature) and the operating voltage. If the
temperature and voltage are not taken into account (uncorrected
values), the measured value of the resistive leakage current strongly
varies. Thus, uncorrected leakage current data should not be trusted to
give reliable information about the arrester condition.
However, by using arrester system data and measuring the ambient
temperatures and operating voltage at the same time as the condition
monitoring is performed, it is possible to recalculate the leakage
current data to a common reference. The resistive leakage current
values will then be approximately the same independent of the test
conditions. In other words – by taking account of the ambient
temperature and operating voltage, measurements performed under
different conditions can be directly compared and the measured values
will be a reliable indicator of the arrester conditions.
9.4.
Third harmonic resistive leakage current
A voltage-current characteristics of a typical metal oxide LA when a
sinusoidal voltage is applied to it, is shown in the figure-2, The nonlinear characteristics of Zinc Oxide blocks, then introduce a third
harmonic resistive current in the leakage current. This current
component is therefore generated by the arrester itself and will be an
indicator of changes in the non-linear characteristics of Zinc Oxide
blocks for a period of time due to ageing phenomenon.
The resistive current consists of fundamental, third harmonic, fifth
harmonic and seventh harmonic components. The harmonic contents
depend on the magnitude of the resistive current and on the degree of
non-linearity of the voltage current characteristics of zinc oxide blocks.
Further, the harmonic contents also are the function of temperature of
LA. The third harmonic is the largest harmonic contents of the resistive
current and most commonly used for monitoring purposes.
In addition to the above, the harmonic contents in operating voltage
also increase the harmonic contents in the leakage current. The system
harmonics will interfere with the harmonics generated by the arrestor
itself. This implies that with the presence of harmonics contents in the
system voltage, if any, cannot be ignored for the purpose of evaluation
of the resistive leakage current. In other words – if the harmonic
contents of the system voltage are ignored, then it will not be known if
an apparent increase in the third harmonic resistive leakage current (Ir)
is really due to ageing phenomena of the arrester or it is a ‘false’
increase due to varying harmonic contents with time.
Thus, it is necessary that a measurement of third harmonic resistive
current duly compensated for third harmonics present in the system be
made for monitoring the health of an LA in service. Further, it is not
only the measurement of third harmonic resistive current for one time,
but also a data base for this resistive leakage current are to be built up
for monitoring the periodical changes due to normal/abnormal ageing
phenomena. Sudden rise in third harmonic resistive current or very
high value of third harmonic resistive current indicates degradation of
Zinc Oxide blocks and calls for a corrective actions required to be taken
in advance in order to prevent a catastrophic failure of LA in service.
Thus, it is recommended that Railway should go for a periodical
measurement of third harmonic resistive leakage current through LA in
service.
10.
Proposed method for measurement of leakage current
As already explained in the previous Para that only third harmonic resistive
leakage current is important for evaluation of the performance of LA in
service. Depending on the value and trend in rise of third harmonic resistive
leakage current, a decision can be taken either for close monitoring or for
replacement of an LA. Various kinds of instruments are now available in the
market, which can be used for measurement of third harmonic resistive
leakage current flowing through an LA under normal working conditions. A
schematic diagram for the measurement of third harmonic leakage current by
using such instruments is shown in figure –5.
Figure –5
Presently, the availability of such indigenous measuring instruments is
limited. The imported instruments are quite expensive and are in the range of
Rs.10 to 15 lakhs. Therefore, there is a need to develop a cost effective
equipment indigenously so that each depot/shed can keep it for measurement
of third harmonic resistive current periodically say every year on all the
arrestors under its jurisdiction. The value of third harmonic resistive current
recorded during the measurement are to be analysed and a data base is to be
developed (LA make wise) so that a threshold/critical value of third harmonic
resistive leakage current can be decided for taking the corrective actions for
LAs in service
11.
Case study of Lightning Arrester failure:
11.1.
Failure Investigations
Zinc oxide Lightning Arrester commissioned on 08.10.2001 on 25 KV side
failed/burst on 07.07.04 at Traction Sub-Station in Bhandai of Agra Division
on North Central Railway. RDSO/Lucknow made an investigation into the
failure of this LA and the following observations were made:
• Rusting marks were noticed inside the arrester at diaphragm, stack plate and
fasteners.
• Flash over marks were noticed on the Zinc Oxide blocks.
• Rubber gasket on the top casting was found to be eccentric at its seating
between ceramic and the top cover plate.
For the above observations, the following conclusions were drawn to explain
the failure of the above Lightning Arresters:
¾ Flash over and short circuit of the metal blocks took place due to ingress
of moisture, which was confirmed by looking at the rusted parts of the
Lightning Arresters.
¾ The ingress of moisture was due to improper fitment of the gasket between
the porcelain housing and the top cover.
Hence, from the above observations, it was confirmed that the above Lightning
Arrester failed on account of ingress of moisture due to improper fitment of
rubber gasket between porcelain housing and the top cover. A few
photographs taken for the failed Lightning Arresters are reproduced and
placed at Annexure-1.
11.2.
Corrective actions suggested
The following corrective actions are suggested to avoid the ingress of moisture
into the Zinc Oxide type of Lightning Arrester with ceramic housing:
i)
Quality of the neoprene rubber gaskets and the dimensions including the
concentricity are to be ensured before the fitment on the Lightning
Arresters.
ii) Use of stainless steel fasteners in order to prevent the rusting/corrosion.
iii) Use of ‘O’ ring in place of gasket between porcelain housing and the top
cover for better sealing.
12.
Experience of other Utilities
Power Grid Corporation of India Limited (PGCIL) reported that about 200
Lightning Arresters provided on 400 KV and 200 KV side were monitored and
results obtained during sample checks are as below:
System
voltage
400 kV
220 kV
Range of THRC*
Make A
Abnormal
Normal range
range
10-100 µA
150-400
µA
20-40 µA
200-400
µA
Range of THRC*
Make B
Normal
Abnormal
range
range
30-150 µA
300-500
µA
20-40 µA
200-400
µA
Remarks
Five
surge
arresters were
showing upto
2100 µA
Three Surge
arresters were
showing upto
1000 µA
* THRC – Third Harmonic Resistive Current
The value of the third harmonic resistive leakage current was recorded generally
in the range of 10-15 micro amperes for new LAs whereas for 12 to 15 years old
LAs, the value was in the range of 200-300 micro ampere.
A similar kind of study is also to be made by RDSO/ Railways in order to arrive
at a threshold/ critical value of Third harmonic resistive leakage current in order
to take necessary corrective actions for those LAs, which are having large third
harmonic resistive leakage current.
13.
Conclusion
As explained earlier, LA is one of the most vital devices used to protect the power
equipments such as transformers etc. against over voltages including lightning
surges. Therefore, it is quite imperative that the health/condition of the LAs
provided on electrical rolling stock and Traction Sub-Stations is to be monitored
at a regular interval by measuring third harmonic resistive leakage current with
the help of measuring instruments. Further, a database for the third harmonic
resistive leakage current for all the LAs (make wise) is to be developed and
analysed. A threshold/critical value of third harmonic resistive leakage current is
then to be specified based on the database developed over a period of time.
As discussed earlier, the following major reasons have been identified for LA
failures:
1.
Ingress
of
moisture
through
sealing
system/gasket
leading
to
degradation/flash overs of Zinc Oxide blocks.
2. Accelerated ageing/degradation of Zinc Oxide blocks possibly due to
manufacturing defects.
In order to avoid the ingress of moisture into the porcelain housing of LAs
through the sealing system, the LAs with polymeric housing have been developed
and about 150 such LAs have already been put into the service on the electric
locomotives built by CLW a few years ago. No failures of these LAs with polymeric
housing have been reported so far. However, a close monitoring is required for
these kind of LAs in order to establish their efficacy against the ingress of
moisture etc.
In the light of the foregoing discussions, the following practices for maintenance
and condition monitoring of LAs in service are suggested:
Sr.
No.
1.
2.
3.
Proposed Practices
Periodicity
Cleaning of LA housing
Every year
Measurement of Insulation Resistance (IR) Every year
Value of LAs by using 1000 V meggar –
usually the IR value should not be less than
1000 M.Ohms.
Measurement of Third harmonic resistive Every year
leakage current through LA as proposed in
Para-6
4.
5.
14.
Preparation of database (LA make wise) by To
be
RDSO/ Railways for Third harmonic regularly
resistive leakage current for deciding the
threshold/critical valve for each type of LAs.
Decision is to be taken for either close
monitoring or replacement of LAs in service depending on the value/ steep rise/ trend in
the rise of Third harmonic resistive leakage
current.
updated
References
1) Distribution Arrester Research; M.V.Lat, J.Kortshinski; IEEE Transaction on
Power Apparatus and Systems, Vol. PAS-100-No.7, July 1981.
2) Maintenance of Surge Arrester by Portable Arrester Leakage Current Detector;
S.Shirakawa, F.Endo, H.Kitajima, S.Kobayashi,, K.Kurita; IEEE Transactions
on Power Delivery, Volume 3, No.3 July 1988.
3) Surge Arrester Monitorintg and Failure Investigations; Ravindra Kumar Tyagi,
S.Victor, Narendra Singh Sodha; PGCIL, Gurgaon.
4) Surge Arrester Application and Selection Guide; IEEMA-20-2000.
5) IEC 99-4. Metal Oxide Surge Arresters without gaps for ac systems.
6) RDSO’s Specification No. EIRS/SPEC/LA/0005, July 1999. Specification for
10 KA, 25 KV Gapless Surge Arrester.
7) Leaflets of M/s Crompton Greaves Limited.