MAINTENANCE OF EHV SUB-STATION

Prepared & Presented By Mr. Santosh

Bagade.
 The Important equipments in a Sub-
station

9. Sub-station battery
1. Power transformer
10. Battery charger
2. Circuit breaker
11. Control and relay
3. Current transformer
panels
4. Isolator
12. RTCC panel
5. Potential transformer
13. AC distribution board
6. Lightening arrester
14. DC distribution board
7. Wave trap
15. Switch yard
8. Coupling capacitor
16. Marshalling box

2
 Recommended Maintenance Schedule
For Power Transformer

No Item to be Inspection note Frequency Action required

inspected

a Ambient temp. To be noted. Daily To be recorded.

b Winding temp. Read and record temp. Hourly If found abnormal, shut down the
And oil temp. transformer and investigate

c Load and voltage To be noted Hourly To be recorded. On log -sheet.

d Oil level Check oil level in main Weekly If low, top up with dry oil. Check
and the OLTC transformer for oil leakage
conservator
e Oil level in Check Weekly If low, top up with dry oil. Check
Bushing transformer for oil leakage

f Relief Diaphragm Visual inspection Monthly Replace if cracked or broken

3
 Recommended Maintenance Schedule
For Power Transformer
Contd…

No Item to be Inspection note Frequency Action required

inspected
g Dehydrating Check for oil passage and Monthly If found pink reactivate or replace
Breather colour of the agent the agent (silica -jel)

h Bushing Examine for cracks and dirt. Quarterly Clean or replace

i Oil check Check for dielectric strength Half yearly Take suitable action to restore the
and water content quality of oil
j Cooler fan Lubricate bearings; check Half yearly Replace burnt or worn out
bearings, motor gearbox; examine contacts; contacts or other parts
and control controls and interlocks.
mechanism

k Oil in cooler Test for pressure Half yearly If pressure is abnormal take
corrective action
l Oil in Check for sludge Yearly Filter or replace with dry fresh
transformer oil
4
 Recommended Maintenance Schedule
For Power Transformer
Contd…

No Item to be Inspection note Frequency Action required

inspected

m Oil filled bushing. Test oil, Take Tan delta test Yearly Filter or replace with dry fresh oil
for condenser bushing
n Gasket Check visually Yearly Tighten bolts evenly

o Cable box Check for cracks, compound Yearly Replace if defective

leakage
p Relay and alarm Examine Relay and alarm Yearly Clean components, replace
circuit contacts contacts and fuses if necessary

q Earth resistance Measure Yearly Take suitable action if Earth

resistance found high
r O.L.T.C. Check OLTC, RTCC for Quarterly Clean and grease all moving
overhauling proper functioning contacts, check oil in diverter
arrangements
5
 Recommended Maintenance Schedule
For Power Transformer
Contd…

No Item to be Inspection note Frequency Action required

inspected

s Buchholz relay Check contacts and float Monthly Rectify or replace defective
contacts
t I. R. test of Measure by megger Yearly Take suitable action if found low.
winding (Filter / dry out the TF)

u Overall To be done under shelter Once in 5 Wash by hosing down with clean
inspection and in dry atmosphere, years dry oil. Filter and dry out the
including transformer
lifting of core

v Sludging Test oil for all values. Once in 10 Replace oil if tests are not
year satisfactory

6
 Min. Safe IR Values In Meg. Ohms At Different
Temp. Of Power Transformer By 5 kV Megger.

Rated voltage of the Temp. 30C Temp. 40 C Temp. 50 C Temp. 60 C

winding
66 kV and above 600 300 150 75

22 kV and 33 kV 500 250 125 65

6.6 kV and 11 kV 400 200 100 50

Below 6.6 kV 200 100 50 25

The insulation resistance measurement is a very easy method for diagnosing the health of the
transformer. If Insulation Resistance values are poor, carry out drying out process for the transformer.

7
 Chart Showing Various Tests To Be Carried
Out On Oil In Transformer.

N Characteristic Equipme Test Method Test Suggested Permissible

O nt (Ref to IS: or a t Initial l i m i t
Voltage Appendix) Field Periodicity satisfactory
/ La of tests for use
b
1 Electric Strength 145 kV IS-6792-1972 F/L After filling or 50 kV (Min)
(breakdown And Average of 6 refilling prior to
voltage kV) above break-downs on energizing, then 40 kV (Min)
72.5kV one on one axal after 3 months
and less filling with and one year. 30 kV (Min)
than 145 2.5mm gap
kV Below spacing
72.5 kV
2 Water content 145 kV IS-335-1983 L After filling or re 25 ppm
(ppm) and -filling prior to
above energizing, then 35 ppm
below after 3 months
145 kV and one year.
8
 Chart Showing Various Tests To Be Carried Out
On Oil In Transformer.

N Characteristic Equip. Test Method Test Suggested Permissible

Voltage (Ref to IS) at Initial l i m i t
O Field Periodicity satisfactory
/Lab of tests for use

3 Specific All IS-6103-1971 L After filling or re 0.1 X 1012

Resistance voltages -filling prior to 0.2 X 1012
( Resistivity ) ohm en-energizing,
-cm then after 3 0.3 X 1012
at 90 C months and 2 Ohm-cm (min)
years. at 90 C
4 D i el ec t r i c 145 kV & IS-6262-1971 L After filling or 0.2 (Max)
Dissipation above. refilling prior to 1.0 (Max)
Factor (Tan Delta) below energizing, then
at 90C. 145 kV after two years.

9
 Chart Showing Various Tests To Be Carried Out
On Oil In Transformer.

N Characteristic Equipme Test Method Test Suggested Permissible

nt (Ref to IS) at Initial l i m i t
O Voltage Field Periodicity satisfactory
/Lab of tests for use

5 N eu t r a l i z a t i o n All IS-1 4 4 8 (P : 2 ) F or L -do- 0 . 5 m g

value voltages 1976 KOH/g
(total acidity). (Max)

6 Sediment and or All Appendix A L -do- No sediment

precipita ble voltages or perceptible
sludge. sludge
should
be detectable.

10
 Chart Showing Various Tests To Be Carried Out
On Oil In Transformer.

N Characteristic Equip Test Method Test Suggested Permissible

Voltage (Ref to IS) at Initial l i m i t
O Field Periodicity satisfactory
/Lab of tests for use

7 Flash point A l l IS-1448 - 1970 L -do- Decrease in

voltages flash pt. 15C
Max of the
initial value
Min value 125
C
8 Interfacial A l l IS-:6104-1971 L -do- 0.018 N/m
Tension voltages (Min)
At 27C
9 Dissolved Gas 145 kV & IS-9434-1979 After filling or IS:
Analysis (DGA) above L refilling prior to 10593/1983
energizing, then
after, a month
and 1 year after

Action :- If permissible limit is violated, recondition the oil, or if economic, replace the oil.
11
DISSOLVED GAS ANALYSIS

The transformer in operation is subjected to various stresses like

thermal and electrical, resulting in liberation of gasses from the
hydro carbon mineral oil, which is used as an insulant and coolant.
The components of solid insulation also take part in the formation
of gases which are dissolved in oil. An assessment of these gases
both quantitatively and qualitatively would help in diagnosing the
internal faults in the transformer. Gases may be formed in the oil filled
transformer due to natural ageing but also to a greater extent, as a
result of internal fault.
12
 DISSOLVED GAS ANALYSIS

. The process of gas formation includes oxidation, insulation decomposition, oil

breakdown and electrolytic action. In case of fault, its type and severity may often
be inferred from the composition of gases and the rate at which they are formed. In
the case of incipient fault, the gases formed remain partially dissolved in the liquid
insulation, free gases will be found only in special cases. The dissolved gases
divided between gaseous and liquid phases by diffusion. Diffusion and achievement
of saturation both take time, during which serious damage to the equipment can
occur undetected. So the periodic analysis of oil samples for dissolved gases
forms method of detecting incipient faults. Study of DGA, therefore may help in
taking predictive and preventive maintenance of transformer.
13
 The Permissible Concentrations Of Dissolved
Gases In the Oil 0f A Healthy Transformer

GAS Less than 4 years in 4 To 10 Years More than 10 Years in

service service
Hydrogen 100/150 ppm 200/300 ppm 200/300 ppm

Methane 50/70 ppm 100/150 ppm 200/300 ppm

Acetylene 20/30 ppm 30/50 ppm 100/150 ppm

Ethylene 100/150 ppm 150/200 ppm 200/400 ppm

Ethane 30/50 ppm 100/150 ppm 800/1000 ppm

Carbon monoxide 200/300 ppm 400/500 ppm 600/700 ppm

Carbon dioxide 3000/3500 ppm 4000/5000 ppm 9000/12000 ppm

14
METHODS OF TRANSFORMER
RECONDITIONING

There are three different methods of

transformer reconditioning

1) Recycling process.

2) The flushing process.

3) The oil-spray method.

15
 METHODS OF TRANSFORMER
RECONDITIONING

First method:- Recycling process

Indirect drying by circulating oil and simultaneous oil conditioning

and heating up of the transformer

In this process moisture is transported by the oil, only to the

degassing tank and from there to the vacuum pump. The effect of
this procedure is relatively week and requires high temperature and
long processing time.
This process should be used in case of transformers which are not
vacuum tight.
16
 METHODS OF TRANSFORMER
RECONDITIONING

Second method:- Flushing process

This process is used for transformers with a vacuum proof tank

Where as recycling process extracts moisture from the
insulation via the oil, the flushing process on the other hand
ensures faster and more intense drying, as the live transformer
components are exposed directly to vacuum by draining the oil.
The ageing products are carried away during heating and
flushing phases.
This process requires an additional tank, capacity of which,
must be at least equal to the oil volume of the transformer, for
receiving the oil.
17
METHODS OF TRANSFORMER
RECONDITIONING

Third method:- The Oil-spray process

This process was used originally for final draying out of the
transformer.
The basic principal of this process is that the insulation is
heated up by hot oil, spread into the pre exhausted
transformer, so that water vapour can be pumped out at the
same time . The oil spraying dries the insulation faster.
For this process preparations are required such as fitting of
nozzles etc.

18
Important equipments and materials required for
TRANSFORMER RECONDITIONING

The important equipments and material required for

transformer reconditioning.
1) A three stage filter machine with additional vacuum
pump.
2) One oil tank of capacity at least equal to oil volume of
the transformer.
3) Glass wool mats and tarpaulin for clothing the
transformer.
4) fresh transformer oil.
5) Equalizing piping to equalize vacuum in main tank and
the OLTC / DIVERTER tank.
19
 Transformer conditioning time requirements

The time required for conditioning and regenerating a transformer depends on

a large number of out side factors. The following data may there be taken as
indicative only.

Weight of oil days required for days required for days required for
(tonnes) Recycling. Flushing. Oil-spray.

15 10 7 3

100 35 20 8

20
 Routine Maintenance Of the Circuit Breakers

 Based on the medium of arc quenching the C.B s are classified as

1) Air break C.B 2) BOCB 3) MOCB. 4) ABCB. 5) SF6 CB. 6) VCB.
The two types of principals of interruptions are used in circuit breakers.

a. HIGH RESISTANCE INTERRUPTION.:- In the ABCB the resistance of the arc is

increased by lengthening and cooling it down to such an extent that the
system voltage is no longer able to maintain the arc. And the arc gets
extinguished. This technique is employed in air blast circuit breakers and in
d.c. circuit Breakers.

b. LOW RESISTANCE OR ZERO POINT INTERRUPTION. : - In this process, the arc

gets extinguished at natural current zero of the alternating current wave and
is prevented from restriking again by rapid build up of dielectric strength of
the contact space. This process is employed in almost all the alternating
current circuit breakers.
21
 Types Of Operating Mechanism

1) Motor operated. Spring charging mechanism..

2)Hydraulic mechanism.
3)Pneumatic mechanism.
a) For all types of operating mechanisms: check nut-bolts, cotter pins, split pins
, operating levers, latches, cams, toggle levers. Lubricate and grease bearings,
sliding surfaces, rollers, spring where ever required. Check electric motor and
fuses, limit switches. Tripping and closing coils.
b) For hydraulic mechanism: check hydraulic fluid level. If necessary top up,
check hydraulic pressure, nitrogen pressure
c) For pneumatic mechanism: check air pressure leakage, drain off
condensation water once in a weak.
22
 Maintenance Of Current Transformer And
Potential Transformer

The CT and PT are stationary equipments so they do not need much

maintenance.
a) During planed outage observe oil level and check dielectric strength.
check oil leakage if any. If oil leakage is seen, take action to stop the oil
leakage and top up with dry oil if not hermetically sealed.
b) Clean the porcelain insulator and the CT/ PT body.
c) Check the primary and secondary connections for tightness.
d) Measure I. R. values
For 400 kV and 220 kV CTs and PTs measure capacitance and dielectric
dissipation factor (tan delta ) once in a year. The tan delta value at 20C
should not exceed 0.007 for 400 kV. If exceeds, reconditioning is required.
23
 Maintenance Of Wave Trap
1) Check jumper connections. If loose, tighten the nut-bolts.
2) Many a times the upper and lower lids of the WT are broken and bird’s
nests are found. Remove nests and replace the lids.
3) The main coil, lightening arrester and tuning device to be checked as per
manufacturer’s instruction manual.

Maintenance Of Coupling Capacitor

1) Check jumper connections. If loose, tighten the nut-bolts.

2) Clean porcelain body.
3) Measure IR value.
4) Check line matching unit, lightening arrester and earth switch, co- axial
cable in proper condition 24
 Maintenance of Isolator

During planed outages the isolator maintenance as below shall be carried

out.
 Lubricate all bearings. Tighten all nuts and bolts.
 Check jaws and moving contacts for healthiness. Check and maintain
proper alignment. Tighten loose nuts and bolts. Check all pipes and
guides of the operating mechanism. Apply petroleum jelly for jaws and
moving contacts. Check locking and inter- locking arrangements.
 If motor operated, check healthiness of the electrical system.
 Check semaphore indications.
 Check jumper connections for tightness and healthiness.
 If the isolator is with earth switch, check its alignment and check
mechanical and electrical interlocks function properly.

25
 Maintenance of Lightening Arrester

The lightening arrester requires very little maintenance.

 Clean porcelain insulator. Check tightness of the jumper connection.
 Check surge counter reading.
 Check arcing rings for proper position.
 Measure earth resistance. It should be less than 2 ohms. If it is more, the
earth electrodes and the earth pits should be renewed with common salt
(NaCl) char coal, black cotton soil and water. Betonate powder can also be
used.
 Measure IR values. If found low, replace the LA.

26
 Maintenance of Sub-station Battery

This is very very important equipment in the sub-station. There are

batteries of 30 volts, 48 volts, 110 volts and 220 volts for protection
and for carrier communication.
The routine maintenance for the battery is as under.

 Clean the battery terminals. Clean the cell-to-cell connecting strips.

Tighten the nut-bolts of the connectors. Apply petroleum jelly to the
nut-bolts and the connectors. Check sp. Gravity of the electrolyte by
hydrometer. Conf irm sp. Gravity is 1200 +- 5. top up electrolyte level
by distilled water to the specif ied mark. check voltage of each cell by
cell voltage tester, which should be 2.15 volts to 2.20 volts.

 Take capacity test of the battery. If full ampere- hour capacity is not
received, go for discharge and charge cycles till 100 % capacity is
achieved.
27
 Maintenance of Battery Charger

In the sub-station battery charger of 30 volts ,48 volts, 110 volts and
220 volts are provided, for the charging of the battery of respective
capacity.
The battery charger is almost a maintenance free equipment.

 Check whether the charger is clean and free from dust and spider nets.
 Check whether the charger works on both boost and float mode
operation satisfactorily and whether the boost mode can even work on
float mode if such facility is provided.
 Check whether amp meter, volt meter, Earth leakage current meter on
the charger panel are working satisfactorily.
 Check that the annunciation scheme is working.

28
 Maintenance of Control and Relay Panels
The Control and relay panels do not require much maintenance. See
that the panels are vermin proof. The panels are cleaned and kept
dust free. The indication lamps and the semaphore indications are in
working condition. See that the relays and meters are tested
annually as per the schedule.

Maintenance Of AC And DC Distribution Board

The AC and DC distribution boards also do not require daily maintenance.
Keep clean and vermin proof. Check fuses for proper size. Keep cable
connections in trim condition. Maintain switches in good operating
condition.

29
 Maintenance of Switchyard

 Keep the switch yard clean and free from grass and weed. Keep
cable trenches clean.
 Maintain all steel structures in rust free condition, by painting where
ever required.
 During outage, check nut-bolts of all earthing system for tightness.
 Provide watering to earth pits, once in a week in dry season.
 During outage, check nut-bolts of all clamps and connectors of the
conductor and equipment hard ware for tightness. Replace
damaged nut-bolts and clamps and connectors
 During outage, measure earth resistance of all the earth pits once in
a year in dry season and renew the earth pits having high earth
resistance. We should maintain the earth mat resistance below 1.0
ohm for major sub- station.
 Maintain yard lighting in good condition.

30
 Thermographic Scanning

Carry out thermo graphic scanning of all power connectors so that hot spots
will be detected early. This enables planning of preventive maintenance
either by outage or by hot line maintenance or even by bare hand technique.
This modern technique facilitates us to focus the maintenance activity on
the defective electrical joints only, avoiding un desired maintenance. This
reduces outage period, avoids wastage of time, wastage of manpower,
wastage of money on labor, and most importantly saves tremendous
revenue loss and consumer complaints because of elimination of
breakdowns and long outages.
Preventive maintenance is always better than the disaster management. The
proverb “ a stitch in time saves nine. “ is very true.
31
THERMAL IMAGES...... continued

32
33
34
Thank you !
35
Q & A Session...