Online Drying Systems with Moisture Monitoring, IEC 61850 Integration
and Online OLTC Filtration Systems for Unmanned Substations
Yogesh Sood Advisor (Application Technology), www.ptssglobal.com
INTRODUCTION
HISTORY
A number of Electrical Utilities in India are trying
to meet the growing demands of power by adding
more overhead lines and adding substations.
Substantial
cost
savings
result
in
deploying unmanned substations as compared
to the conventional manned substations. Once
the substation becomes unmanned, there will not
be any need for posting maintenance staff and it
can be operated remotely.
We all know the 3 main ageing accelerators of
transformer are DISSOLVED MOISTURE,
OXYGEN & HEAT. ‘If it doesn’t break, fix it’ and
‘distinguishing the broken from the un-broken’ is
the issue which has to be tackled smartly and
intelligently addressed, these expressions in the
old days were neglected because of limited
resources and inadequate knowledge. In the era
of today, one cannot avoid any situation when
the transformer and oil results are not in
international acceptable norms. This could be
very critical to the health of the grid all together. It
also introduces new risks.
The failure rate of significant assets like
Transformers is expected to grow to 6-7%. There
are, of course, different opinions, including more
pessimistic views, but most agree that the future
“is not what it used to be”. Things will definitely
get worse, much worse.
The average replacement cost of, say, a 100
MVA transformer can total about 4-5 Crores (and
can take approx. 12 months to build or replace).
Similar costs could incur heavy loss for different
rating of transformers annually in future;
therefore, saving the transformer health is
very important. The good news is that ‘Most
Transformer Failures (>50%) Are Preventable’
using the latest preventive maintenance
techniques. “Only real-state (real time) eliminates
all disadvantages”. Online Transformer Drying
System and OLTC Filter System are the best
preventive maintenance equipment’s.
The unmanned substation concept is the
successful working model of the Power Grid
Corporation India Limited (PGCIL) and slowly
being adopted by few state utilities who have
initiated pilot projects of various capacities to
study the viability of the Substations.
Substation Automation is the Key and many
Advances and Innovations has taken place in the
recent years which has helped the Power Sector
to integrate the Substation at all levels using
Intelligent Electronic Devices (IED’s), control and
automation capabilities within the substations as
well as from remote users using SCADA to
control power-system (switchyard) devices.
Online drying system plays a very important role
in the Substation automation and specially in the
Substations which are unmanned or expected to
be unmanned in the near future, where we not
only get the real time RS(Relative Saturation) &
Moisture PPM values with real temperature but
have an integrated system to remove moisture
and
conductive
contaminants
from
the
Transformer.
The
latest
IEC
61850
communication protocol is the pre-requisite in the
PGCIL specifications to supply Online Drying
Systems in any of their Substations. Hundreds of
ODS are in operation in PGCIL where the key
data from the HMI of the Online Dryout Systems
have been integrated with the SCADA from
various Contractors like Siemens, GE, ABB,
Schneider etc.
How does water affect solid insulation in
transformers?????
The Kraft paper used for transformer insulation is
similar to the brown paper used in grocery bags.
“Imagine setting your paper bag of soup cans
and a dozen eggs down in a water puddle, then
picking it up five minutes later. What do you think
will have happened to the paper’s tensile
strength?”
Operating experience with transformer insulation
over many years has shown that moisture in
microscopic amounts – not liters – is the cause of
more electrical breakdowns than any other
impurity. Recognition of the importance of
extremely small amount of moisture has grown
immeasurably with the increase in voltage stress,
load and reduced BILs (Basic Insulation Levels).
weight of cellulose paper. The %age of moisture
in the transformer can be accessed using ppm,
temperature equilibrium curves and more than
4% moisture is dangerous for transformer
operating at 90DegC.
Indeed, moisture ‘constitutes a hazard not only to
the dielectric performance of the oil itself but also
to insulations that are immersed in the oil’. Thus,
we should all agree that water is “enemy
number one” What is not known or generally
accepted is that water problem may begin before
what is laid down/revealed in any national or
international standard.
There are 5 processes practiced to achieve the
complete drying of wet Power Transformers
1. Vapour phase system (off line)
• Can be used in workshop environment
only.
• Lot of piping, gas cutting and bolting
works required.
• Required to dismantle the transformer,
transporting to workshop from site and
back to site for re erection.
• Down time and interruption.
• Manpower required.
2. Conventional off line dry out system
(stream line filter)
•Interruption required.
• Long time multiple circulations with high
temperature and vacuum will remove the
aromatic fractions from the transformer oil
(i.e.) Natural oxiditation inhibitors.
• Winding shrinkage.
• Reduction of winding compression
pressure after multi circulations.
• For many transformers jacking of
winding at site poses certain risks.
• Manpower required.
• IR & Tan Delta of insulation cannot be
achieved to the desired level.
3. Low frequency heating (offline dry out)
• Service interruption.
• Winding shrinkage.
• Cumbersome process
• Manpower required.
4. Oven heating dry out
• This option requires shipping and untanking.
• Drying core and coil in vacuum oven.
• Insulation may be in the end of its useful
life.
• Oven drying in oven would cause the
insulation to become more brittle.
• Increase the risk of insulation damage
during re-tanking.
5. Heating the winding by circulating
current (short circuiting secondary
winding)
• Applicable only for small transformers.
“The electrical user …. recognized the electrical
hazard which is presented by the presence of
water but rarely does he recognize the equally
disastrous effect which traces of moisture have
on the longtime usefulness of the insulated
equipment, even when the amount of moisture is
not sufficient to cause dielectric difficulties” as
stated by one of the experts.
Therefore, the moisture content of a
transformer’s solid insulation plays a major role in
determining a transformer’s length of life. Every
time the moisture content of the solid insulation
doubles, the expected life of the transformer is
cut by half. Throughout a transformer’s operating
life, moisture will accumulate in the solid
insulation. This moisture originates either from:
outside the transformer or from within, as the
liquid and solid insulation age and oxidize over
time. Cellulose insulation has the affinity to
absorb the moisture up to 8-9% moisture per dry
weight (%M/DW). International standards say it
should be less than 1% for ideal healthy
operating transformer. Who knows how much the
manufacturer has supplied? What is the present
%age?? Keep in mind that just an extra half
percent for a total of 1.0% M/DW, will cut your
transformer’s expected operating life to half.
“Taking out the moisture out of the transformer
insulation is as difficult as to dry the telephone
directory kept in the bucket full of water for more
than a week and taken out for drying all pages
without opening.”
Rule of thumb says that the weight of the
cellulose paper in transformer is 0.3 times the
KVA rating of the transformer in pounds and it
has to be maintained to less than 1%moisture/dry
SOLUTIONS FOR REMOVING TOO MUCH
WATER FROM THE CELLULOSE
• 1/3 of name plate KVA and impedance
voltage is required at site.
• Not suitable for EHV transformers
To avoid the above costly and cumbersome
processes and to protect your transformer
investment through the most use of a safer, more
effective and less costly equipment, automatic
online moisture removal systems without heat
and vacuum process should be practiced.
MOISTURE DETERMINATION
The present method of determining the dryness
of transformer insulation is to measure the
moisture content of an oil sample and use the
well-established
moisture
equilibrium
characteristics between oil and paper insulation.
However, the dynamics of the moisture
movement between the paper and the oil during
temperature cycling is significant and, unless
taken into consideration, can cause significant
errors in moisture assessment. In addition, the
actual determination of the moisture content of
the oil is full of uncertainties. Although Karl
Fischer method is the standard practiced for
measuring the water content of the oil, human
error and a number of uncertainties associated
with the oil sampling procedure reduce the
reliability of measurements and could lead to
incorrect conclusions.
Transformer insulation system comprises of
several different grades, and physical conditions
of oils vary considerably from one unit to another.
Even without these complications, moisture
migration between cellulose and oil is a complex,
temperature dependent process: time constants
for the exchange of moisture between cellulose
and oil are different in each direction; moisture in
the cellulose is not evenly distributed; and not all
the moisture in the cellulose is available for
transfer in the oil. In addition, dissolved moisture
in oil can form precipitate during rapid cool-down
periods and become free water, which may not
re-dissolve. Therefore, any method of moisture
determination based on a single measurement
without regard to those co-founding factors may
provide a false indication to the insulation's
integrity. The results of moisture saturation
measurements by capacitive probes eliminate
the frequent discussions that question the validity
of Karl Fischer titration (which suffers from
severe errors resulting into a poor accuracy) in
aged oils specifically.
HOW DOES THE
TRANSFORMER?
WATER
ENTER
THE
The main source of water contamination is
atmospheric moisture.
1. Residual after processing:
• Manufacturing
• Installation
• Maintenance
2. Leaks, through weak points of
Transformer
3. Transformer prevention system
• Ineffective
dryers
breathing
conservators
• Ruptured
Bladders/diaphragm
–
sealed conservators
4. Byproduct of cellulosic degradation.
5. Water Ingress Points: Cooler plugs, Cooler Gaskets and valves,
Lid Gasket, Electrode Shaft, Manhole
Gaskets, Bushing Gaskets, Gauges and
plugs, Valves.
Large amounts of rainwater can be sucked into a
transformer in a very short time (several hours),
when there is a rapid drop of pressure (after a
rapid drop of temperature that can be induced by
rain) combined with insufficient sealing.
Some valuable moisture interpretation tips to
access the transformer condition and to take
predictive maintenance actions for life
extension of large transformers: -
Interpretation of absolute saturation of solid
insulation: Percent M/DW of
Cellulose
Classification
<0.5%
New Transformer
>0.5-1.5%
Dry Insulation
>1.5-2.5%
Medium Wet Insulation
>2.5-4%
Wet Insulation
>4%
Very Wet Insulation
Interpretation based on top oil relative
saturation:Condition of
Cellulose
Insulation
RS of water in oil after
three days at
GOOD
<5
FAIR
>5<8
PROBABLY WET
>8<12
WET
>12
60-70DegC, %age.
PARTICLE CONTAMINATION
The particles in oil range from microscopic to
visible range. Large particles usually settle down.
Time constant of particle sedimentation depend
on oil viscosity. Hence an oil sample taken from a
transformer at high temperature may contain only
small suspended particles.
Suspended particles are usually those above
0.45 µm. The visible range starts at about 50 µm.
Manufacturing contaminants:
Cellulose fibers, iron, aluminum, copper and
other particles resulting from manufacturing
processes are naturally present in the
transformer’s oil. Non conductive mode
particles presumable would be present in a 5
to 50 micron range – easily removable with
0.5 micron filters (filter selection is very
important)
IMPORTANT
The fewer the particles, the weaker the effect of
water on the dielectric strength of the oil. Hence
removing particles could be a task of priority to
maintain dielectric safety margin of insulation
having an excessive level of moisture
contamination.
Efficient
processing
shall
incorporate drying and filtering procedures
simultaneously.
The most important parameter, which determines
effectiveness of the online process, is relative
rate of contaminant removed per one pass,
namely:
Ratio of input and output water,
Ratio of particles,
Ratio
of
oil
aging
characteristics
(neutralization number, interfacial tension,
PF, resistivity)
SAFETY ISSUES
Recommendations for some safety measures:
Ensure automatic tripping of the system with
automatic shutdown controls by integrating
electro valves, pressure sensor, flow sensor,
gauges, air bleed valves, moisture sensor,etc.
ADVANTAGES
Benefits of the Energized processing :-
Transformer remains in service
No switching scheduling or costs.
No transformer downtime.
No auxiliary power equipment needed.
Reduced transformer drying expense
Less costly compared to heat and vacuum.
Minimal energy consumption.
Unattended operation saves labor expense.
Improved Transformer Condition
IR/TanDelta values improvement after drying.
Increased oil dielectric strength.
Increased protection from hot spot damage.
Increased
safety
margin
for
higher
transformer loading.
Oil power factor and acidity improvement.
Transformer Life extended
Removing
moisture
retards
insulation
degradation.
Transformer oil life is enhanced
Since online processing through specialized
catridges is a vacuum less process and no
external heating is applied during circulation,
it will not destruct the oil, inherent oxhidation
inhibitors (aromatic fractions) will not be
removed. It retains the colour of the oil.
INSTALLATION AND IMPLEMENTATION
Installing the Online moisture removal
systems is very quick and easy. It can be
installed both on old wet Transformers as
well as new Transformers. Just take the unit
near the transformer and simply connect the
discharge hose to the top of the Transformer
and the suction hose to the bottom drain port
of the Transformer. Now, you are ready to
start the online dry out process on the
energized Transformer. Few of the Power
Utilities like PGCIL, NTPC, BBMB, PSEB,
HPSEB etc. have used this online processing
for drying their wet Transformers. Various
case studies are available.
MOISTURE MONITORING
Monitoring of the moisture ppm values along
with the online processing gives an Engineer
a confidence about the safe and healthy
condition of the Transformer. Moisture ppm
should be restricted to less than 8ppm and
RS less than 5% preferably at 600C oil
temperature.
Thus a perfect online processing equipment
should deliver the best of dehydration i.e
removal of dissolved moisture to less than
5ppm with simultaneous removal
of
contamination of 0.3/0.5/1.0microns from the
OLTC/Main tank coupled with continuous
monitoring of moisture.
SCHEMATIC FLOW of ODS
Connections of Online Drying Systems and
Online OLTC Filtration Systems
IEC 61850 DATA From ODS
Pre and Post Results of a Reactor
SN
Particulars
Before
After
01.
Moisture (PPM)
12ppm
6ppm
02.
RS
8%
5%
03.
%M/DWi
2.0%
1.6%
04.
Insulation
Tan
Delta/Capacitance
0.621/8587
0.402/8562
05.
Oil Tan Delta/PF
0.02296
0.0143
06.
IFT
(Interfacial
Tension)
28.8Dynes/cm
31.3Dynes/
07.
Sp.
Resistivity
@90DegC
0.819x1012
Ohm-cm
1.9x1012 Oh
cm
08.
Acidity (NN)
0.039407
0.024
09.
Weight
Cartridges
47.45Kgs
66.8Kgs
OLTC FILTER SYSTEM
of
Power Factor of Press Board(Before & After)
Using Above results.
3.
4.
CASE STUDIES
1.
More Case studies are available on request
and can be requested at info@ptssglobal.com
2.
References:1. SD Myer’s Guide for Transformer
maintenance
2. Ben Taylor’s article on Online
processing
of
High
Voltage
Transformers
3. Doble Laboratory Diagnostics