(a)
External Cleaning
The insulators of the transformer bushings/ circuit
breaker / CT / CVT / isolator shall
be cleaned from salt and dirt/dust deposition together with the cleaning of the
other insulators in the substation.
The time interval for this cleaning shall be based on the polluting atmosphere. For installations
with higher atmospheric/saline pollution, cleaning frequency may be increased and and these may be suitably protected against pollution.
(b)
Rust Protection
Some parts of the operating mechanism are made of steel and are surface treated against rust. In spite of the good rust protection, minor corrosion will occur after some years, especially when the breaker / isolator is standing in strong corrosive surroundings. The rust stains shall be sand papered away and new rust protection shall be painted or sprayed on. As rust protection, grease C or Tectyl 506 is recommended.
(c)
Lubrication
For lubrication, the lubricants recommended by
manufacturers shall primarily be used. This is especially important in cold climates
with temperatures below - 25°C.
The bearings of the breaker and operating
mechanism of isolator
and breaker are to
be lubricated with grease G although these normally do not need lubrication
before the major overhauls. Plain
bearings in mechanism details such as arms, links and link gears are also to be lubricated with grease G. These bearings shall be regularly lubricated with a few drops of'
oil B. The teeth in the gear shall be lubricated with grease G. Dryness of driving
mechanism may lead to maloperation and failure.
(d)
Treatment of Contact Surfaces
The contacts of breaker / isolator / ground switch
shall be treated according to the following directions:
·
Silvered
contact surfaces: Silvered contact surfaces shall be cleaned, if
necessary, with a soft cloth and
solvent (trichloro ethane). Steel brushing or grinding is not allowed.
· Copper surface:
Copper surfaces shall be clean and oxide free. If necessary, they
shall
be cleaned with cloth and solvent (Trichloroethane) or steel brushing -
After steel brushing, the surface
shall always be cleaned of loose particles and dust.
·
Aluminium
surfaces: Aluminium contact surfaces shall be cleaned with steel brush or emery cloth. The surface is very
thoroughly cleaned of particles and dust with a dry I cloth. After this, a thin layer of Vaseline is applied.
This shall be done within 5 minutes after the cleaning. The joint
shall be assembled within 15
minutes.
(e)
Moving Contact
Surfaces
·
Silvered: Cleaned if necessary, with soft cloth and solvent
(trichloro ethane). No steel brushing.
·
Non-silver coated:
Cleaned as silvered surfaces,
can be steel brushed. After steel
brushing they shall
be thoroughly cleaned
of loose particles and dust.
·
Lubrication:
Lubricant - Grease K is applied in a very thin layer on the surfaces of the male contact
and the puffer cylinder. The superfluous grease is carefully
removed.
2.
TRANSFORMERS AND REACTORS
In order to provide long and trouble free service, it
is important that a careful and regular
supervision and maintenance of the transformer and its components is carried
out. The frequency and extent of
such a supervision and maintenance is dependent on the experience, climatic conditions, environment, service
conditions, loading pattern etc. All work
done on transformers should be recorded in history register for future
reference. Efforts have been made to
cover all important maintenance practices for transformers and reactors
in this chapter with details of interpretation of test results.
I.
General Supervision
(a)
Dirt and Dust
The external transformer surfaces shall be inspected
regularly; and when required cleaned
of dust, insects and other airborne dirt. Transformers/ reactors installed near polluting industry/cement plants, etc.,
need special care and more frequent cleaning of the bushings and other components. All Marshalling Boxes and OLTC
cubicle are to be kept properly
closed so that there will not be entry of dust
inside, which is difficult to clean.
(b)
Rust and Treatment
A regular inspection is to be carried out of the
external surface treatment of the transformer tank and radiators. Possible rust damages
are removed and the surface
treatment restored to original state by means of the primer and finish-paints of the transformer to minimize the risk of
corrosion and its subsequent spreading. These checks also include looking
for signs of oil leaks on gasket
areas and welded areas containing oil. The touch-up paint as and when required as per site
condition and re-painting is recommended once in five years. However
transformers in coastal
areas and more corrosive atmosphere may require more frequent painting.
(c)
Check for any Signs of
Mechanical Damage
Checks must be carried out for mechanical damage to the fabrications and associated
equipment. Particular attention should be given to vulnerable areas such as radiators. If damage is seen on the equipment, a decision must be taken as to its seriousness. It may be necessary to take
corrective actions such as the replacement of an item of equipment.
(d)
Check on all
Joints for Signs of Leakage
All joints, both welded and gasketed, must be checked
for signs of oil leakage. If there is
any doubt of a leak, the area must be cleaned of oil, using a suitable solvent (methyl alcohol) and sprayed with liquid
chalk. This will promote the flow of
the leak and give a good indication
as to the exact location of the leak, if in fact there is one. If a leak is suspected on a gasket, the joint
must be tightened until such time that it can be changed with a new gasket. If a leak is apparent at a welded
joint once again clean the area and
apply liquid chalk and allow to dry. This will highlight the point exactly if
in fact there is a leak. It must be properly repaired
with welding procedures when convenient. Prior
to leaving the leak, it must be highlighted with a marker, or something similar,
so it is not lost when permanent
repair takes place.
Other areas commonly associated with oil leaks are
drain plugs in radiators, valves in
the oil management and cooling
system and the gas and oil actuated
relay.
(e)
Check for Oil Level
It is good practice to check all oil levels associated
with the equipment. This will incorporate
the expansion vessel and all oil filled bushings. Also the oil in the oil seal should be maintained. Some bushings in
transformers will be below the conservator oil
level and some above. If there is leakage in bushing at the oil end, the
level will be low or high depending
upon the level of conservator. External leak on bushing will lead to indicate low oil level. This is to be
observed accordingly and if there is leak, action is to be initiated
immediately as bushing failure may lead
to failure of entire transformer.
OLTC oil conservators are always kept at lower level
compared to the main conservator tank so that OLTC oil will not
mix with main tank oil. An increase in level of oil in OLTC conservator tank indicates internal
leakage and action is to be taken accordingly.
After energizing of the transforn1er, a certain settling may appear in sealing joints. This applies especially to sealing
joints with plain gaskets that are not placed in grooves. These should therefore
be re-tightened. For correct torque for tightening the bolt, the manufacturer's recommendations are to be followed.
(f)
Check on the Surrounding Areas
Once all the checks are completed, a check should be
made to ensure that all materials or
tools, used for maintenance work, have been removed. All clothes and other debris must be disposed off. The
transformer compound should be left in a clean and tidy condition.
II.
Checks on Breathers
(a)
Checks on Silica Gel Breather
In open breathing transformer, the breather plays
active role in maintaining the transformer
dry by admitting dry air when transformer breathes. In transformers having air cell or diaphragm, the breather ensures
dry air inside the air cell or above the diaphragm.
The silica gel inside the breather should become pink from bottom to top over a period of time. Any de-colorization at
top or sides indicates leakage in container and need to be attended immediately. In order to prevent severe
deterioration of the silica gel, it
is recommended that it is replaced when half to two thirds of the silica gel
has become saturated and turned pink
in colour. Failure to do so will severely retard the drying efficiency of the breather. The silica gel
can be reactivated by heating it to 130°C-140°C in a ventilated oven until it has achieved the bright blue
colour. Check that the oil level is correct
in the oil cup at the breather
base and fill oil if the level is found low.
Note: Do not exceed the temperature stated above otherwise the colour
impregnation will be destroyed
and the silica gel will turn black.
Immediately after re-activation the loose silica gel
must be placed in a sealed container
to prevent absorption of moisture on cooling. The silica gel should be stored
in sealed condition until required for use.
Self indicating (blue) silica gel contains the dye
cobalt chloride which has been classified
carcinogenic by an European Commission directive and ia a banned substance because
of its potential health hazards.
In Europe the silica gel breathers are to be disposed
in 'Class l' disposal locations for hazardous waste products
or incinerated.
An alternative to the blue self-indicating silica gel
is SILICA GEL ORANGE with an organic
indicator. The colour changes from orange to light yellow as it absorbs moisture. The specifications of silica gel orange are as shown below:
|
Parameter |
Specification |
|
|
Adsorption capacity |
RH 50% (min) |
20 |
|
RH 80% (min) |
30 |
|
|
Appearance |
Orange |
|
|
Loss at heating
up % (max) |
4 |
|
|
Colour change |
RH 50% |
Light yellow |
|
RH 80% |
Colourless or Slight
yellow |
|
In view of above use of blue silica-gel may be phased
out.
(b)
Drycol Breather
Check (If Available)
Drycol breathers are provided in some transformers
where air cell is not provided. It
condenses the moisture inside the conservator and brings it out as water
droplets. Silica gel breather
will also be provided for these transformers. The following checks
need to be carried out for drycol breathers:
Operation of counter
reading: Check on a regular
basis that the counter is functioning.
Record the figures each time a check is made so that a progressive check is recorded.
Defrost current condition
indicates that water is still being ejected
from the breather
Press the test button and check that a defrost current is being indicated. Check that the two red neon lights are ON and the amber neon light is OFF.
Release the test button and check that the counter has
advanced one count and that freeze current is indicated.
III Checks
for Conservator
(a)
Visual Check for Conservator Oil Level
The transformer oil conservator is provided with an
oil-level indicator graduated from 0
to 1 or min to 6 or "low" to "full" with grading depending
on the manufacturer. Normally the
face of oil gauge or dial of Magnetic oil level Gauge (MOG) is marked at the 35°C
(or normal). These indications are relative
to temperature of the operating equipment. The oil
level indicated should be recorded along with top oil
temperature.
If corrected oil level is normal, no additional action
is required, whereas if it is above
or below the normal level, it may be necessary to remove or add some oil. The correct
oil-filling level is specified on an information plate that is placed on the transformer Rating plate panel. At an oil
temperature of + 45°C, the conservator should be half filled. If the level exceeds the
"full" oil must be drained off. If the value is "low" or "min", oil must be filled in.
(b)
Leakage Testfor
Air Cell
Normally leakage test for air cell fitted inside the
conservator is carried out before installing
the conservator in its position or at the time of major overhaul. During
service, the leakage in the cell or
in the sealing of the conservator can be detected by the oil level in the prismatic oil level indicator, if
provided, on the conservator. If there is no leakage, the prismatic oil level indicator will show "Full" oil
level. However, in case of leakage, the
oil level in the prismatic oil
level gauge shall be lower
than "Full" level.
For Releasing Air from Conservator Fitted with Air Cell
Pressurize the Air Cell up to the maximum pressure
as specified by the manufacturer and open the air vent valves
provided on the top of the conservator until oil starts coming out. Then close the valves. Release pressure from
the Air Cell and refit breather.
For Releasing Air from Conservators Fitted with Diaphragm
Type Air Sealing
Open the Air Release Valve provided on the top of the
diaphragm and start filling oil into the conservator, preferably from the valve provided
at the bottom of the conservator. Filling of oil from the oil filling valve at the bottom of the transformer tank is
avoided because it may result in entry of air into the transformer which may get trapped
in the winding and result in unnecessary accumulation of air in the Buchholz Relay at some later stage.
Continue filling oil into the conservator until it is
full and oil starts coming out of the
Air Release Valve. Close the Air
Release Valve after ensuring that all the air has come out from the oil portion below the diaphragm.
Slowly drain the oil from the conservator until the
oil level as indicated on the oil level gauge
corresponds to the transfom1er oil temperature.
Before making the leakage test of air cell for the,
transformer in service, oil should be
drained out to the lower level of conservator. Apply pressure as specified by
the manufacturers to inflate the air
cell. Adjust the pressure after 6 hrs, if required. Check temperature and maintain the air
cell at almost the same temperature for 24 hrs. If there is no loss
of pressure during 24 hrs, it means the air cell is not having leak.
(c)
Caution
Any heating process like welding, grinding etc. are not
allowed on the assembled conservator fitted with air cell diaphragm
as it is highly sensitive
to heat.
IV.
Check for Cubicle and Marshalling Kiosk and Valves
Marshalling Cubicle and Kiosk Check
The following need to be checked and ensured while inspecting and checking the
. Marshalling Boxes.
·
Condition of paint
·
Operation of door handles,
Hinges
·
Condition of door seal.
·
Door switches
·
Lights and heaters
·
Thermostats
·
Operation of heating
and lighting switches
·
Secure mounting of equipment
·
Checking of tightness
of cable terminations
·
Checking of operation of contactors
·
HRC fuses and their rating
·
Operation of local alarm annunciation by pushing push buttons provided
for lamp test, acknowledge, reset, system test, mute
etc. to cover all system function.
·
Source change over test
check by putting off power
sources alternatively.
·
Check for plugs for dummy holes, glass windows and replacement, if found missing/
broken.
V.
Checks for Auxiliaries
(a)
Cooling System
The cooling surfaces of radiators shall be inspected
regularly and when required cleaned
of dust, insects, leaves or other air borne dirt. The cleaning is suitably
carried out by means of water
flushing at high pressure. Precaution should be taken to cover the fan- motor so that water may not go inside.
Alternatively cleaning can be done with cleaning solution and cloth.
The fan-motors are provided with permanent - lubricated
bearings and double sealing rings.
The motor bearings are axially clamped with spring-washers. If the sound level of the fan increases, first tighten
all mounting supports and in case any abnormal
sound is noticed in fan
motor, then action should be taken for repair! replacement.
(b)
Cooling System-Fans-Controls
Fan controls are designed to operate both manually and
automatically with set temperature.
Manual, Control is to be turned 'ON' to operate cooling system for checking. Oil pumps need to be checked by observing
their flow gauges. Measurement of pump current reveals
any abnormality. Any significant imbalance
of current between
the terminals greater
than 15-20% is indicative of the problem
with the pump motor. Checking for correct rotation of fans and
pumps to be ensured as reverse rotation may not provide desired result.
(c)
Calibration of OTI
/ WTI
Temperature indicators in transformers are not only used for indication
purpose they are used as protective
device also. The accuracy of these devises is to be ensured for correct
operation of alarm and tripping
and also to prevent mal operation. The temperature
bulb is to be removed from its well on the side/ top of transformer. Using a temperature controlled calibration
instrument in oil bath the
temperature of the bulb should be
slowly raised in steps of 5°C and
observed for temperature reading. If the temperature
deviation is more than ± 5°C
compared to the standard thermometer reading,
the thermometers are to be replaced with healthy one.
(d)
Checking of Cooler Control,
Alarm and Trip Settings
Setting of temperature should be as per approved
scheme. Access the local winding/ oil temperature indicator and
rotate the temperature indicator pointer slowly to the first stage cooling value (say 65°C). Check that the fans of
those coolers set to first stage are
operating. Continue rotating the pointer to the second stage cooling value (say 8O°C). Check that the fans of those
coolers set to second stage are operating. Continue rotating the pointer to the
alarm value (say 110°C). Check
with the control room to ensure that the alarm signal has been
received. Continue rotating the pointer to the trip value (say 125°C). Check with the control room to ensure that
the trip signal has been received.
(e)
Gas Pressure
Relay
There are two types of gas pressure relays. The most
common type is mounted at the
transformer top body. Internal
arcing in liquid filled
electrical power equipment
generates excessive gas pressures that can severely
damage equipment and present extreme hazards to personnel. The gas pressure relay is intended to
minimize the extent of damage by
quickly operating and venting out the pressure. It will reset when the pressure becomes normal. A pointer is
provided to indicate the operation of this relay and the relay is connected for tripping the transformer on
operation. There will be oil spillage whenever the relay operates. Smaller transformers are provided
with explosion vent where the diaphragm will rupture due to heavy internal pressure and releases
the
pressure. The diaphragm
needs to be replaced when it operates.
There are some transformers
fitted with sensitive sudden pressure relay, which operates on rate of change of differential pressure and trips the equipment.
(f)
Buchholz Relays
The use of gas-operated relay as protection for
oil-immersed transformers is based on
the fact that faults as flashover, short-circuit and local overheating normally
result in gas-generation. The gas-bubbles gathering
in the gas-operated relay affect a float- controlled contact that gives an alarm signal.
For testing of the contact functions, buchholz relays
are provided with a test knob on the
cover. Unscrew the protective cap and press down the knob by hand. The spring loaded knob with a pin inside the relay
actuates first the alarm device and then the
tripping device. After testing, screw on the protective cap again.
Checking the operation of Buchholz relay in case of low
oil level is carried out by closing
step valve in both sides of the relay and draining of oil through oil drain
valve provided in Buchholz relay.
First alarm and then trip contact should operate to indicate healthiness.
To check the relay for oil surge, manufacturers
recommendations for particular relays to be followed.
(g)
Bushings
Bushings are most failure prone in any transformer/
reactor. Failure of bushings could
lead to the fire in transformer and total damage. For uniform voltage
distribution across capacitance
graded bushings, bushing porcelains shall be cleaned from dust and dirt during shutdown maintenance. In areas
where the air contains impurities as salt, cement
dust, smoke or chemical substances, shorter intervals are required.
VI.
Operational Checks
and Inspection / Maintenance of Tap Changer
EHV Transformers are provided with tap changer to have
voltage control. To enable operation
of taps during service, On- Load Tap Changers (OLTC) are provided in EHV transformers. OLTCs may be located in
either the high voltage winding or the low voltage
winding, depending on the requirements of the user, the cost effectiveness of
the application and tap changer
availability. OLTC being a current
interrupting device requires periodic inspection and
maintenance. The frequency of inspection is based on time in service,
range of use and number of operations.
(a)
Precautions
This testing shall be carried out during shutdown
period and all testing shall be done
under total de-energisation condition. Ensure the isolation of transformer for
high voltage and low voltage side
with physical inspection of open condition of the concerned isolators/ disconnectors. In case tertiary is also connected, ensure the isolation of the same prior to commencement of testing
(b)
Tap Changer
Hand Operation
Check hand operation of the tap changer up and down the full range before
electrical operation is attempted and that the handle interlock switch will not allow electrical operation
while the handle
is inserted. In addition where single phase tap changers are employed check their tap
positions agree and are reached simultaneously at motor drive unit head. Continuity check should be done for any
discontinuity during tap changing
operation by connecting an analogue multi meter across HV and IV bushing in case of auto transformers and relevant
winding in case of two winding transformers and change the tap positions from maximum to minimum.
(c)
Maintaining Circuit
Check the maintaining circuit for correct sequence by
hand winding unit half way through a
tap and then remove the handle. Energize the drive motor and ensure that the motor continues to drive the tap changer in the same direction.
(d)
Drive Motor
With the tap changer in mid position check the
direction of rotation and measure the
start and running currents in both the raise and lower mode of operation and
record their values.
Set the motor overload
to 10% above running current
(e)
Out of Step
Relay
Move one tap changer is in the three-phase bank to be one position out of step with other two. Check the tap
changer faulty alarm is activated. Repeat for other two phases.
Hold the raise and lower push buttons in following a
tap change to ensure it only moves one tap at a time hence checking
the step by step relay.
(f)
Tap Change Incomplete Alarm
Check the operation of the tap changer incomplete
alarm, including the flag relay, by
winding the unit by hand half way through a tap change and monitoring their
correct operation and time to operate.
(g)
Remote Indication
Check the remote indication and control facility
is proved to the outgoing
terminals of the marshalling kiosk.
(h)
Tap Changer
(Surge) Protective Relay
Check the tripping function of the relay. Open the cover and press button
"Trip". Check that all
circuit breakers of transformer operate properly. Press push Button
"Reset" close the cover and tighten it.
(i)
Inspection and Maintenance of OLTCs
Normally the temperature of the OLTC compartment may be
few degrees Celsius less than the
main tank. Any temperature approaching or above that of the main tank indicates an internal problem. Prior to
opening the OLTC compartment, it should be inspected
for external symptoms of potential problems. Such things as integrity of paint, weld leaks, oil seal integrity, pressure
relief device and liquid level gauge are all items which should be inspected prior to entering
the OLTC.
Following de-energisation, close all valves between oil
conservator, transformer tank and
tap-changer head, then lower the oil level in the diverter switch oil
compartment by draining of oil for
internal inspection. Upon opening the OLTC compartment, the door gasket should be inspected for signs of
deterioration. The compartment floor should be
inspected for debris that might indicate abnormal wear and sliding
surfaces should be inspected for signs of excessive wear.
Finally, the tap selector compartment should be flushed
with clean transformer oil and all
carbonization, which may have been deposited, should be removed. Min BDV should be 50 kV and moisture content
should be less than 20 PPM.
Dissolved Gas Analysis (DGA)
Transformer undergoes electrical, chemical and thermal stresses during its service life which may result in slow evolving incipient faults inside the transformer. The gases generated under abnormal electrical or thermal stresses are hydrogen (H2), methane(CH4), ethane(C2H6), ethylene (C2H4), acetylene (C2H2), carbon_monoxide (CO), carbon dioxide (CO2), nitrogen (N2) and oxygen (02) which get dissolved in oil. Collectively these gases are known as FAULT GASES, which are routinely detected and quantified at extremely low level, typically in parts per million (ppm) in dissolved Gas Analysis (DGA). Most commonly method used to determine the content of these gases in oil is using a vacuum Gas Extraction apparatus/ Head Space Sampler and gas chromatograph.
DGA is a powerful diagnostic technique for detection of
slow evolving faults inside the
transformer by analyzing the gases generated
during the fault which gets dissolved
in the oil. For Dissolved Gas Analysis to be both useful and reliable, it is essential that sample taken for DGA should
be representative of lot, no dissolved gas be
lost during transportation and laboratory analysis be precise and
accurate. Effective fault gas
interpretation should basically tell us first of all, whether there is any incipient
fault present in the transformer. If
there is any problem, what kind of fault it is. Whether the fault is serious
and the equipment needs to be taken out of service for further investigation.
DGA can identify deterioration of insulation oil and
hot spots, partial discharge, and
arcing. The health of oil is reflective of the health of the transformer
itself. DGA analysis helps the user
to identify the reason for gas formation and materials involved and indicate
urgency of corrective action to be taken.
The evolution of individual gas concentrations and
total dissolved combustible gas (TDCG) generation over time and the rate of change
(based on IEC 60599 and IEEE C 57-104 standards) are the key indicators of a developing problem. Some of the recognized interpretation techniques are discussed below:
Individual Fault Gases Acceptable Limits
When no
previous DGA history of Transformer is available, to ensure that a transformer is healthy or not, the DGA results are
compared with the gassing characteristics exhibited by the majority
of similar transformer or normal population. As the transformer ages and
gases are
generated, the normal levels for 90% of a typical transformer population can determined. From these values and based on
experience, acceptable limits or threshold levels have been determined as given in table (as per IEC 60599) below:-
|
Transformer Type |
Fault Gases (in
µ1/1) |
||||||
|
No OLTC |
H2 |
CH4 |
C2H6 |
C2H4 |
C2H2 |
CO |
CO2 |
|
60-150 |
40-110 |
50-90 |
60-280 |
3-50 |
540-900 |
5100- 13000 |
|
|
Communicating OLTC |
75-150 |
35-130 |
50-70 |
110-250 |
80-270 |
400-850 |
5300- 12000 |
The values listed in this table were obtained from specific networks.
Values on other networks
may not exactly indicate healthiness.
“Communicating OLTC” means that some oil and/or gas communication is possible between
the OLTC compartment and the main tank or between the respective conservators. These gases may contaminate
the oil in the main tank and affect the normal
values in these types of equipment. “NO OLTC” refers to transformers not
equipped with an OLTC, or equipped
with an OLTC but not communicating with or leaking to the main tank.
However it is improper to apply threshold level concept without
considering the rate of change of the
gas concentration in Dissolved Gas Analysis. When an abnormal situation
is indicated by above table, a testing
schedule is devised
with increased sampling
frequency.
Total Dissolved
Combustible Gas (TDCG)
Limits
|
TDGC limits,
PP |
Action |
|
<or = 720 |
Satisfactory operation, Unless individual gas acceptance values are exceeded |
|
721-1920 |
Normal ageing/slight decomposition, Trend to be established to see if any evolving incipient fault is present. |
|
1921-4630 |
Significant decomposition, Immediate action to establish trend to see if fault
is progressively becoming worse. |
|
>4630 |
Substantial decomposition, Gassing
rate and cause
of gassing should
be identified and appropriate corrective action such as removal from service
may be taken. |
TDCG includes
all hydrocarbons, CO and H2 and does not include
CO2 which is not a combustible gas.
Evaluation of Gases
The temperature at which the fault gas evolves is given in the table below:
Relationship with temperature
Methane CH4>120oC
Ethane (C2H6)>120oC
Ethylene (C2H4)>150oC
Acetylene
(C2H2)>700oC
Faults Associated with Different Gases
|
Oil Overheating |
C2H4, C2H6,CH4 |
|
Overheated Cellulose Traces of acetylene with smaller quantity of Hydrogen may be
evolved. Large quantity of Carbon-Di-Oxide (CO2) and Carbon Monoxide (CO) are evolved
from overheated cellulose. Hydrocarbon gases such as
Methane and Ethylene will be formed if the fault
involved oil impregnated structure. |
CO |
|
Partial discharge in Oil (Corona) Ionization of high stressed area where gas/vapour filled
voids are present or ‘wet
spot’produces Hydrogen and methane and small quantity
of other hydrocarbons like ethane and ethylene. Comparable amounts of carbon mono-oxide
and di- oxide may result due to discharges in cellulose. |
H2,CH4 |
|
Arcing in Oil Large amount of Hydrogen and acetylene are produced with minor quantities of methane and ethylene
in case of arcing between the
leads, lead to coil and high stressed area. Small amounts of carbon
mono-oxide and di-oxide may also be formed, if fault
involves cellulose. |
C2H2,H2 |
It is to be understood that there is no definite
interpretation method available, which can indicate
the exact location
and type of the! fault.
The different interpretation methods only provide
guidelines to make expert interpretation about the equipment. Apart from the DGA results
various other factors are taken into consideration such as past history of the transformer, grid condition, loading
patterns, voltage and frequency profile,
etc.
3.
CIRCUIT BREAKERS
Circuit breakers basically consists of two main parts, the interrupting chambers and the operating mechanism. The
interrupting chambers normally do not require routine preventive maintenance other than cleaning but operating
mechanism do require proper upkeep.
Circuit breaker interrupting chamber is an enclosed
unit mostly filled with oil or SF6
gas. Lower voltage Circuit breakers have vacuum interrupting chambers also.
There is stress on the contacts
during fault current interruption and damages may happen in arcing contacts or main contacts. The
breaker interrupting chamber is recommended to be opened only based on
condition monitoring tests
or as per advice
of the manufacturers.
(b)
Operating Mechanisms
Normally circuit breakers
have pneumatic, hydraulic
and spring operating
mechanisms. As operating force is required for closing and tripping of
circuit breakers, there can be
combination of these mechanisms in one circuit breaker. Since operating mechanisms have a number of moving parts, they need more maintenance such as
greasing, lubrication, cleaning, setting of limit switches, etc.
Compressors/ oil pumps/ spring charging
motors also require
maintenance. Other maintenance on particular operating mechanism such as air compressor
maintenance, nitrogen priming pressure checking
in hydraulic mechanism, checking of over travel, checking of gaps in operating plunger of close/ trip coils etc. are to
be carried out as the case may be and as specified by the manufacturers.
(c)
SF 6 Gas
Most of the higher voltage circuit breakers adopt SF6
in interrupting chamber. The density
of SF6 gas is about five times that of air and heat dissipation is also much
more than air. At atmospheric
pressure, dielectric strength of SF6 gas is about 2.4 times that of air and at about 3 kg/cm2 it is same as
that of oil. As SF6 is Green House gas, it needs to be handled carefully
and should not be let to the atmosphere.
(d)
Emptying and Re-filling
of Gas
The breaker is evacuated by means
of the gas treatment equipment that purifies and also compresses the gas for storage, so that it can be
reapplied. For economic and ecologic reasons,
SF6 contained in electrical equipments, should not be vented into atmosphere. Prior to
the gas removal, the quality of the
SF6 gas should be tested.
Operational contamination should be absorbed with
suitable filter unit provided in the gas
handling plant. Such filters/ sieves should already be installed into
the SF6 gas maintenance/handling
unit. When SF6 is suctioned from a gas compartment, the gas is passed automatically through
filters, which dry and purify the gas.
(e)
Evacuation of SF6 Gas Circuit Breakers
After maintenance/overhaul of the circuit
breaker, it should be evacuated
by vacuum PUI1l] before
filling in the SF6 gas so that SF6
gas does not mix with ambient air and
also humidity and dust particles are removed from the Breaker. With vacuum pump, a final vacuum must be reached less than 5 mbar.
4.
PREVENTIVE MAINTENANCE OF CURRENT TRANSFORMERS
(a)
Visual Inspection
Current transformers are normally filled with oil and
have oil impregnated paper insulation
for both primary and secondary winding. Careful inspection is to be made for any trace of oil leakages.
Oil leakages are more prone through cemented
joints or secondary terminal box due to improper
sealing of terminal studs. As CTs have less oil quantity small leakage may lead to exposure of paper insulation
and subsequent moisture absorption.
If bellows are provided in CTs, the position of bellow
indicates either leakage of oil or
expansion due to internal gas generation. Both the conditions are serious for
the life of the CTs and immediate action to be initiated for rectification.
Visual inspection is also to be carried
out on the healthiness of terminal connections, condition of porcelain,
development of cracks, chippings, cleanliness of insulator surface etc.
(b)
Maintenance of Gaskets
Marshalling boxes, CT terminal boxes are to be properly
sealed to prevent any dust, rain
water and insects. Door gaskets are to be changed periodically to give proper sealing.
All door bolts/ latches are to be properly
tightened and never left loose.
(c)
Secondary Terminals Connections
Stud type terminals are preferred in Marshalling box
cable terminals. This gives better grip even if more than one wire is connected
to one terminal. But pin type terminals are also provided
in some cases. Since tightness of wires may become loose due to vibration, climatic
condition, it is required to check tightness
of terminals periodically to avoid maloperation/
non-operation due to improper contacts. All terminals of unused CT secondary terminals are to be properly shorted to
avoid development of abnormal voltage
and subsequent failure of CTs. The tan δ test tap is to be properly earthed
to avoid damage to insulation.
Primary Terminals
Thermovision scanning indicate
proper connection of primary terminal.
If thermovision is not carried
out, physical checking of terminal connection is to be done with proper torque. All corona shields are
to be provided and any damaged corona shield
to be replaced with new one. As CT primary carries heavy current, any
loose joint may lead to arcing and welding of terminal connectors.
5.
CAPACITANCE VOLTAGE
TRANSFORMERS/ POTENTIAL TRANSFORMERS/ CAPACITOR
COUPLING
(a)
Visual Inspection
The bel10ws provided in most of the CVTs are not
visible from outside. CTs/ CVTs and
CC are also oil fil1ed equipments and oil leak is to be observed. If oil leak
is observed in anyone stack, the
entire CVT is to be replaced. CVTs are tuned units and replacement of anyone stack is not recommended to avoid phase angle errors.
(b)
Electro-Magnetic Unit
Electro-Magnetic Unit (EMU) of CVT houses the secondary transformer, Compensating reactor and ferro resonance suppression circuit.
The colour of oil indicated through the gauge:
glass gives some indication of the healthiness of the internal
components. Any abnormal
heating may also be observed
through Thermovision scanning.
(c)
Secondary Voltage
Deviation in secondary Voltage of CVT is clear
indication of failure of capacitor elements.
Necessary action to be taken to replace CVT if secondary voltage in anyone CVT is abnormal (may be +2V and -4V).
Continuing the equipment in service beyond this stage may lead to failure/
bursting of CVTs.
(d)
Other Maintenance
Maintenance of Marshalling box gaskets, tightening of secondary terminal
connections and tightening of primary terminal connections, etc., are
also to be ensured for healthy
operation. It is to be ensured that al1 extra holes at Marshalling boxes are properly plugged and kept
vermin proof. The anti-condensation heater and the thermostat are to be kept in working condition
to keep inside of the panel dry.
6.
DISCONNECTORS/ISOLATORS
Disconnectors have main current carrying arms and operating mechanism for connection and' disconnection. Being are off-line devices, they
are normal1y air break type. Normally
horizontal double break, Horizontal center break, Pantograph, Vertical break Disconnectors are in use for EHV isolations.
The alignment of Disconnectors is very important for
smooth operation. The limit switches,
the healthiness of auxiliary
contacts needs to be checked periodical1y. The
main contacts are to be inspected and made smooth if any pitting marks
seen. The corona shields are to be kept smooth
and shining and checked for tightness of fitting. Damaged
corona rings should be replaced. All moving parts are to
be lubricated for smooth operation.
The gear
mechanism and motor normal1y do not require any maintenance and manufacturer's recommendation should be referred for maintenance of
gears.
Earth Switches
The earth switch is a
safety device and smooth operation is to be ensured by proper
alignment. The earth blade contacts are to be cleaned properly for proper
contact and contact resistance to be
measured to ensure healthiness. The earth connection from blade to earth is to be carefully checked.
All the joints to be tightened and flexible copper braid connections are provided and healthiness is to be
ensured. All moving parts to be lubricated for smooth operation.
7.
LIGHTING ARRESTER/SURGE ARRESTERS
Surge arresters are to be maintained to give protection to other connected
switchyard equipments. Cleaning
of porcelain insulators is very much required for uniform
voltage distribution. Voltage grading rings are to be properly positioned and checked
for tightness and any damaged
rings to be replaced. Healthiness of surge monitors is to be checked and if found
defective the same may either be replaced with
healthy one or shorted to minimize
earth resistance. Healthiness of earth connections to be
checked as it plays a vital role on the operation of the surge arrester.
Normally it is not recommended that
if one stack fails it is replaced with healthy stack. It is always a good practice to change the entire arrester as
the stressed stacks will start failing along with the new stack.
8.
BATTERY AND BATTERY CHARGERS
Substations generally use Lead Acid batteries/for DC
batteries for DC supply. More and
more maintenance free batteries are now offered for substation applications which require less maintenance. As DC system is vital part of substation during emergency, upkeep of battery system is very important.
Cell containers are to be kept always clean to avoid
surface leakage. Any leakage is to be
attended immediately. Vaseline / white petroleum jelly is to be applied on
battery terminal and inter-cell
connectors, nuts and bolts to avoid sulphate deposit. The rubber seal at the base of the terminals
and on cell lid is to be fitted properly
and to be replaced if damaged. All connections are to be checked for tightness.
All vent plugs and level indicators to be maintained
for healthiness. Maintaining level of electrolyte in flooded cells is of very important
to avoid sulphation and permanent damage
of the cells. Distilled
water is to be added
to make up to the level.
If VRLA battery is used, the battery room temperature
is to be maintained using air conditioner
as the temperature plays vital role
on the performance of the battery.
(a)
Battery Chargers
Battery charger is to be maintained for keeping the battery always charged and also to supply normal DC load for
operation. If the charge / discharge ammeter does not show current on the charge side, then the float charger is not
giving output. Defect should be
located and corrected. In case of failure of float charger, the boost charger may be used as float charger as per design.
Charger panel is to be kept clean, free from dust and
all terminals to be checked periodically
for tightness. The battery maintenance and condition monitoring is to be carried out as per schedule to keep the DC system
in healthy condition.
(b)
BATTERY CAPACITY
TESTING
This procedure describes
the recommended practice
of capacity testing
by discharge in the battery.
All testing should follow the safety requirements.
(c)
INITIAL REQUIREMENTS
The following list gives the initial requirements for all battery
capacity tests except
otherwise noted.
(a) Equalize the battery if recommended by the manufacturer and then return
it to float for a minimum of 72 h, but less
than 30 days, prior to the start of the test.
(b) Check all battery connections and ensure that all connections are proper and clean.
(c) Record the specific gravity
and float voltage
of each cell just prior to the test.
(d) Record the electrolyte temperature of lO% or more of the cells to establish
31 average temperature.
(e) Record the battery terminal
float voltage
(f)
Take adequate precautions (such as isolating the battery to be tested from the
batteries and critical loads) to ensure that a
failure will not jeopardize other systems or equipment.