1.0 INTRODUCTION

1.1 The techniques / procedures being adopted for both preventive maintenance and condition based monitoring in respect of EHV transformers and reactors is briefly discussed in this section.

2.0 GENERAL MAINTENANCE

2.1 A power transformer in a sub-station is not only one of the costliest equipment but is also one of the most important links of the power system. If the power transformer is required to give a trouble free service it should receive proper attention for its maintenance. General maintenance, which is normally required to be done on transformers, is as under:

1. Regular inspection of the external surface of transformer for any dirt and dust and when required the same may be cleaned

2. Regular inspection of the external surface for any damages due to rust

3. Possible rust damages when noticed are to be removed and surface treatment restored in the original state by means of primer and finished paints for minimizing risk of corrosion and its subsequent spreading

4. Before carrying out any maintenance work ensure proper safety procedures as per utility practice and ensure the following:

a) The transformer and the associated equipment should be taken out of service, isolated and properly earthed

b) Obtain a permit to work / sanction for carrying out tests from the shift engineer

c) Obtain the keys for the transformer area

Following checks may be carried out

1. Check for signs of corrosion
   

2. Check all joints for any sign of leakage
   
3. Check for any sign of mechanical damage
   
4. Check oil levels
   
5. Check that surrounding areas are clean and tidy

All results must be entered in the proper format for comparison during future tests.

Silica Gel Breather

2.2 Check the colour of the silica gel breather so as to prevent any deterioration of silica gel breather. It is recommended to replace the same when half to two third of the silica gel has become saturated and become pink in colour. Failure to comply this will result in decreasing the drying efficiency of the breather. Silica gel breather could be reactivated while wilst in its charge container or it can be emptied into a shallow tray. It is required to be heated in a well ventilated oven and a temperature of 130-138 degrees till the entire mass achieve the original blue colour. Immediately after reactivation the new silica gel must be placed in a sealed container to avoid any absorbance of moisture while cooling.

Drycol Breather

2.3 Following checks may be carried out

• Check for operation of counter reading - This may be done on a regular basis so as to see the functioning of the counter.

• Check for defrost current indication and counter reading - This is being done to ensure the water is being ejected from the breather. Check the same by pressing the button. Also, check that 2 red neon lights are on and the ember neon light is off.

Conservator Oil Level - Visual Checks

2.4 Visual checks may be carried out on regular basis for conservator oil levels. If the level is normal no action is required. In the event of above or below normal level action has to be taken to add or remove some of the oil. The correct oil filling level is normally is to be specified on the information plate. At a temperature of 45 degree C the conservator should be half filled. If the level shows the value full oil must be drained off. If it is low oil must be added immediately.

Check for Marshalleing Cubicle and Chaos

2.5 Following checks may be carried out and all results may be recorded in the format of comparison during future checks.

Condition of paint work


Operation of door handles


Operation of doors and hinges


Condition of door seal


Door switches working


Lights working


Heater working


Thermostats working


Operation of heating and lighting switches


Mounting of equipment secure


Manual operation of switches satisfactory


Checking of tightness of cable terminations


Checking of operation of contractors (isolating the trip signal, if any)


HRC fuses and their rating


Operation of local alarm annunicator 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 and replacement, if found missing.

Note: Transformer / shunt reactor need not be taken out of service / isolated or earther while carrying out the above checks.

Valve Operation Checks

2.6 Following checks may be made either at the time of erection or after a major overhaul. All results must be recorded in the log for comparison during future tests.

1. Check each value for free operation
   

2. Check that each valve is padlocked where applicable
   
3. Check that each valve is adequately greased
   
4. Check that each valve returns to its "in service" operating position (open or closed)

Cooling System

2.7 Regular inspection may be carried out of the cooling surfaces and when required clean same from the dirt, insects, and leaves or any other air borne dirt. This is important as it affects the fan cooling. Cleaning is normally done by water flushing at high pressure. As regards cleaning of internal cooling surfaces, no major are considered necessary so long the oil is in good condition. In the event of setting of sludge formation of the oil the sludge may get deposited from the horizontal surfaces in radiators and coolers. The same may be flushed internally with clean oil in connection with oil exchange. In the event the sludge doesn't gets loosed the flushing may be done first with petrol and then with oil. However, this may be carried out in consultation with the supplier.

2.8 Regular inspection of the cooler banks may be made. The cooler can be cleaned by taking out the tube packets and thereby making them assessable for cleaning. For any increase in sound level of fan retighten all mounting supports.

Cooling System - Fans - Controls

2.9 Fan control are designed to operate both manually and automatically. The automatic function is related to the load and energisation or both. The following controls are required to be checked.

1. Manual Control - Fan operation should observed after turning the switch to ON position for a brief period. Oil pump should be checked by observing the flow through gauges. In case of any malfunctioning manufacturers may be consulted.

2. Temperature Control - Remove the temperature bulbs from its well on the side / top of the transformer. Set the master controller to the automatic position. The temperature of the bulb should be slowly raised by using a temperature control calibration equipment for observe for proper calibration / operation.

3. Load Control - Check the secondary current of the controlling CT for proper operation. Shot the secondary of CT (if the transformer is energized). Remove the secondary lead from the control circuit and inject the current to the control circuit. Vary the level of the current to observe the proper operation.

Cooling System - Fan - Visual Inspection

2.10 Following visual inspection checks may be carried out without taking a shut down of the transformer to check that the fans are operating at a designed speed, airways are not blocked and guards and blades are not damaged.

1. Visual check for contamination of motor and fan blades
   

2. Check for build up of moisture in the motor
   
3. Check bearing lubrication
   
4. Check for correct rotation
   
5. Check for unusual noises
   
6. Check for corroding parts

Cooling System - Pumps-Visual Checks

2.11 Following visual inspection checks may be carried out without taking a shut down of the transformer

1. The transformer and associated equipment need not be out of service or isolated while carrying out visual checks on the pumps.
   

2. Obtain a 'Permit to Work' from the Shift Engineer
   
3. Obtain keys to the transformer compound and marshalling kiosk
   
4. All results must be recorded in a log for comparison during future tests in service.
   
5. Following checks should be carried out

a) Check for correct rotation
b) Check for unusual noises/abnormal vibration - replacement of rotor and bearings
c) Check for corroded parts
d) Check for electrical problems

Winding Temperature Indicators - Test

2.12 Following tests may be carried out:

1. Cooler control, alarm and trip test
   

2. Temperature indication calibration of WTI bulb
   
3. Secondary induction test

Before carrying the tests it may be ensure that the transformer and the associated equipment is deenergised, isolated and earthed.

Cooler control, alarm and trip test

The setting of temperatures should be as per the approved scheme. The values given below are indicative values. However, these values are not to be taken for granted and are to be verified with manufacturers instruction manual.

- Access the local winding temperature indicator and set the temperature indicator pointer to the first stage of cooling value (65 degree C).

Check that the fans of those coolers set to first stage are operating.

- Set the temperature indicator pointer to second stage cooling value (80 degree C).

Check that the fans of those coolers set to second stage are working.

- Set the temperature indicator pointer to the alarm value (110 degree C).

Check with the control room that the alarm signal has been received.

- Set the temperature indicator pointer to the trip value (125 degree C).

Check with the control room that the trip signal has been received.

Temperature indication calibration of WTI bulb

Remove the WTI bulb from the transformer pocket and insert the bulb into the calibrated temperature controlled bath.

Raise the temperature of the bath in 5 degree steps and check the response of the WTI after 10 minutes. This may be continued up to a maximum temperature of 130 degree C. The tolerance permitted for temperature indication is ± 3 degree C.

Lower the temperature of the bath in 5 degree step and check the response of the temperature indicators after 10 minutes. At the same time check the transducer output. The tolerance indicated for temperature indication is ± 3 degree C.

Check the alarm and trip switch setting by rotating the pointer slowly to the set temperatures. These settings will be indicated using a multi-meter. Record the values at which the switches operated.

Once these checks are completed return the bulb to the pocked in the transformer cover. Do not forget to bring the maximum level pointer to match the temperature indicator.

Secondary induction test

Ensure the cooler supply isolator is switched to the OFF Position.

Replace the winding temperature indicator bulb in the calibrated temperature controlled bath and maintain a constant temperature of 50 degree C.

Inject the rated current into the appropriate terminals on the winding temperature indication test panel then check and record the resultant gradient is the same as the specified figure (26 degree ± 2 degree C).

Oil Temperature Indicator - Test

Remove the OTI bulb from the pocket on the transformer lid and insert them into the calibrated temperature controlled oil bath.

Increase the temperature of the oil bath in 20 degree C steps from O degree C up to a maximum temperature of 120 degree C. Check and record OTI readings against bath temperatures up the range (tolerance ± 3 degree C).

Access the oil temperature indicator and rotate the pointer slowly to the alarm value (95 degree C) and the trip value (110 degree C) and check their operation. Using a resistance meter, across the switches.

Gas and Oil Actuated Relay - Test

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 flat-controlled contact that gives an alarm signal.

Following tests may be carried out:

1. Gas and oil relay inclination (Only at the time of pre-comissioning)
   

2. Gas and oil relay alarm
   
3. Gas and oil relay trip
   
4. Gas and oil relay surge at pump energisation

Before conducting above tests ensure that transformer and associated equipment is deenergised, isolated and earthed.

Check the stability of the alarm and trip contacts of the buchholz relay during oil pump start by both manual and automatic control to ensure spurious alarms and trips do not result.

2.13 Bushsings

Regular cleaning of the bushing porcelene from dirt and dust should be carried out in the areas where the air contains impurities such as salt, cement, smoke or chemical substances, the frequency may be increased.

2.14 Connectors

To avoid prohibited temperature rise in the electrical connection of the transformer, all screw joints should be checked and retightened. Use of thermovision camera may be made for any hot-spots in the joints.

2.15 Maintenance of Insulating Oil

One of the most important factor responsible for the performance of the transformer is the quality of the oil. Normally insulating oil is subjected to dielectic and moisture contents at site for monitoring the condition of the oil.

Test for dielectric strength (BDV)

Using a BDV test kit, adjust the electrodes (12.5 mm dia) sot that a gap of 2.5 mm is between them. Carry out six tests on the oil, stirring the oil between each breakdown and allowing it to settle. Take the average result of the six figure and this should be used for acceptance criteria (i.e. 60 kV)

Tests for moisture content (ppm)

Using an automatic moisture content test set and a suitable syringe that has been flushed, inject a sample of the oil into the test set. Depending upon the make of the test set the moisture figure may be indicated by mg H2O. if this is the case the figure may be divided by weight of the sample injected in grams. This will give in parts per million (ppm). Typically the moisture content should be less than 15 ppm for transformers in service.

The recommended values of insulating oil for new / unused oil before filling in the equipment (as per IS: 335/1983) and after filling in the equipment (as per IS:1866/1983) are given below in Table 1 & 2.

Table 1

S. No.	Chracteristics / Property	IS 355/1983
1	Appearance	Clear & transparent, free from suspended matter or sediments
2	Colour
3	Density at 29.5° C, Max.	0.89 g/cm3
4	Kinematic Viscosity at 27° C, Max	27 cst
	Kinematic Viscosity at 40° C, Max	< 9 cst
5	Interfacial tension (IFT) 29.5° C, Min.	0.04 N/m
6	Flash point, Pensky Martin (Closed), Min.	140° C
7	Pour point, Max.	- 6° C
8	Acitity, Neutralisation value
a.	Total acidity, Max.	0.03 mg KOH / g
b.	Inorganic acidity / Alkalinity	NIL
9	Corrosive Sulphur	Non-corrosive
10	Di-electric strength (Breakdown Voltage), Min.
a.	New unfiltered oil	30 kV, rms
b.	After filteration	60 kV, rms
11	Dielectric dissipation factor (Tan d ) DDF at 90° C, Max.	0.002
12	Specific resistance (resistivity)
a.	At 90° C, Min.	35*1012 W -cm
b.	AT 27° C, Min.	1500*1012 W -cm
13	Oxidation Stability
a.	Neutralisation value after oxidation, Max.	0.40 mg KOH / gm
b.	Total sludge after oxidation, Max.	0.10% by weight
14	Ageing characteristics after accelerated ageing (Open Breaker method with copper ctalyst)
a.	Specific Resistance (resistivity)
i.	At 27° C, Min.	2.5*1012 W -cm
ii.	At 90° C, Min.	0.2*1012 W -cm
b.	DDF at 90° C, Max.	0.2
c.	Total Acidity, Max.	0.05
d.	Total sludge value, Max. % by weight	0.05
15	Prewsence of oxidation inhibitor	Max. 0.05% treated as absence of oxidative inhibitor
16	Water content
a.	New unfiltered oil	50 ppm
b.	After filtration	15 ppm
17	PCB content	<2 ppm
18	SK value	4 to 8%
19	Dissolved gas analysis (DGA)	Not applicable

Table 2

S.No.	Chracteristics / Property	IS 1866/1983
1	Appearance	Clear & transparent free from suspended matter or sediments
2	Interfacial tension (IFT) 29.5° C, Min.	0.018 N/M, Min.
3	Flash point, Pensky Martin (closed), Min.	125° C, Min
4	Total acidity, Max.	0.5 mg KOH/g
5	Di-electric strength (breakdown voltage) BDV Min.	Below 72.5 kV – 50 kV Min. 72.5 to including 145 kV – 40 kV, Min. 145 kV & above – 30 kV Min.
6	Dielectric dissipation factor (Tan d ) DDF at 90° C, Max.	Below 145 kV – 0.2 Max. 145 kV & above – 30 kV Min.
7	Specific resistance (resistivity) – At 90° C, Min.	0.1*1012 W -cm
8	Water content, Max.	Below 145 kV – 25 ppm Max. 145 kV & above – 35 ppm Max.
9	Dissolved gas analysis (DGA)	145 kV & above – as per IS 10593 latest rev.

Prior to energisation of transformer, the oil sample shall be tested for properties and acceptance norms as given in Table 3.

Table 3

3.0 MAINTENANCE TESTS RECOMMENDED FOR TRANSFORMERS / REACTORS

3.1 Measurement of Insulation Resistance of Transformer/Reactor

The measurement of insulation resistance is carried out to check the healthiness of the transformer insulation. This test is the simplest and is being widely used by the electrical utilities. This test indicates the condition of the insulation i.e. degree of dryness of paper insulation, presence of any foreign containments in oil and also any serious defects in the transformer. The measurement of insulation resistance is done by means of megger of 2.5 kV for transformer windings with voltage rating of 11 kV and above and 5 kV for EHV transformers.

All safety instructions have to be followed as per the utility practice before carrying out this test. It has also to ensured that high voltage and low voltage windings are isolated along with the concerned isolaters. In case transformer is having a tertiary windings, ensure the isolation are the same prior to commencement of the test. Also the jumpers and lighting arrestors connected to the transformer have to be disconnected prior to start of testing after issue of PTW/SFT.

Following precautions may be taken while conducting the above test.

1. Bushing porcelain may be cleaned by wiping with a piece of the dry cloth.
   

2. When using a megger, observe the usual accident preventive rules.
   
3. As the windings possess a substantial capacitance, the current carrying cords should only be touched after the electric charge have been removed from them.
   
4. Connecting wires from the bushing line lead and tank to megger shall be as short as possible without joints and shall not touch tank or each other.

Maintenance/testing procedure:

IR measurements shall be taken between the windings collectively (i.e. with all the windings being connected together) and the earthed tank (earth) and between each winding and the tank, the rest of the windings being earthed. Following measurements are relevant for Auto-transformer, three winding transformer and reactor.

For auto-transformer	For shunt reactor	For winding transformer
HV/LV+E	HV/E	HV/LV+TV+E
IV/HV+E		LV/HV+TV+E
LV/HV+LV+E		TV/HV+LV+E
HV/IV		HV+TV/LV+E
IV/LV		LV+TV/HV+E
HV/LV		HV+LV/TV+E

Note.: HV - High voltage, IV - Intermediate voltage, LV-Low voltage, TV - Tertiary voltage windings, E - Earth

Record date and time of measurement, sl.no., make of megger, oil temperature and IR values at intervals of 15 seconds, 1 minute and 10 minutes. The live terminal of the equipment shall be connected to the winding under test.

Evaluation of Test Results

Check the IR values with the values given in the test certificate by the manufacturer. These values may be used as bench marks for future monitoring of the IR values. The IR values vary with the type of insulation, temperature, duration of application of voltage and to some extent on apply voltage. The IR values in air will be nearly 15 to 20 times more than in the transformer oil at the same temperature. The following table can be used for IR conversion with temperature.

Minimum insulation values for one minute resistance measurements for transformers may be determined by using the following empirical formula:

R = CE / Ö kVA
Where
R - Insulation resistance in MW
C - 1.5 for oil filled transformers at 20° C assuming that the oil is dry, acid free and sludge free.
E - Voltage rating in V of one of the single face windings (phase to phase for delta connected and phase to netural for wye connected transformers)
KVA - Rated capacity of the winding under test.

IR test results below this minimum value would indicate probable insulation breakdown.

i) The following IR values may be considered as the minimum satisfactory value at 30° C at the time commissioning, unless otherwise recommended by the manufacturer.

Even if the insulation is dry, IR values could be low due to poor resistivity of the oil. The IR values increases with the duration of the application of the voltage. The increase in IR value is an indication of dryness of the insulation. The ratio of 60 second IR value to 15 second IR value is called absorption proportion. For oil transformers with Class A insulation with reasonably dried condition polarization index at 30° C will be more than 1.3. Polarisation index test is the ratio meteric test, insensitive to temperature variation and may used to predict insulation system performance even if charging currents (i.e. capacitive, absorption or leakage currents) have not be diminished to zero. Since leakage current increases at a faster rate with the presence of moisture then does absorption current, the megohm reading will not increase with time as fast with insulation in poor condition as with insulation in good condition. The polarisation index is the ratio 10 minute to 1 minute megaohm readings. The values given below are guidelines for evaluating transformer insulation:

3.2 Measurement of Tan Delta and Capacitance of Bushings of Transformers / Reactors

The above measurement gives an indication of the quality and soundness of the insulation in the bushings. For obtaining accurate results of tan delta and capacitance without removing the bushings from the transformers a suitable test set capable of taking measurement by ungrounded specimen test method shall be employed. This utilizes the test tap of the bushings and a tan delta/capacitance test set. Both tan delta and capacitance can be measured using the same set-up. Portable capacitance and tan delta bridge from any reputed manufacturer could be used for this test. Portable test set include measuring bridge such as SCHERING Bridge or transformer ratio arm bridge, power supply and standard capacitor in one enclosure.

Proper safety instructions as per utility practice and necessary isolation required is to be done prior to commencement of this test. Following precautions may be observed during this test:

1. Measurement may be made on low voltages preferably below 10 kV. It is preferred to have the bridge frequency different but close to operating power frequency, so that stray power frequency currents do not interfere with the operation of the instrument.
   

2. Measurement shall be made at similar conditions as that of the previous measurement. In the event of measurement being made a varying temperature correction factor have to be applied wherever applicable.
   
3. Porcelain of the bushing should be clean and dry. Remove any dirt or oil with clean dry cloth.
   
4. Test shall not be conducted when there is a condensation on the porcelain. Relative humidity in excess of 75% is preferred.
   
5. Connection to the overhead bus at the bushing need to be removed, only if the bus line affect the readings considerably.
   
6. Terminals of the bushings of each windings to be shorted together using bare braided copper jumper. Transformer windings not being tested shall be grounded.
   
7. Follow the safety precautions recommended by the instrument manufacturer.

Maintenance Procedure

1. It may be ensured that the test specimen has been isolated from other equipments.
   

2. Keep the test set at least 6 feet (180 cm.) away from the test specimen.
   
3. To avoid any damage to the test set, always set the capacitance multiplier dial to the SHORT, the capacitance measuring dials to their respective 'O' position.
   
4. Set UST - GST switch to UST position.
   
5. Set interference suppressor switches in off position.
   
6. Connect the ground terminal of the test set to a low impedance earth ground (to earth mat of the sub-station).
   
7. Connect the control unit to the high voltage unit using two 5 feet long shielded cables. Screw the pluges down fully on the receptacles.
   
8. Connect the external interlock cable to the 'interlock' terminal of the test set.
   
9. Connect the high voltage cable with Black boot/sheath to the high voltage terminal of the high voltage unit. Connect the pig-tail for the outer shield to the black binding post (ground) on the high voltage unit. Screw down the plug shell fully on the receptacle.
   
10. With the main breaker switched OFF plug the input power cord into the test set power receptacle and into a 3 wire grounded power receptacle having the appropriate voltage rating and current capacity.
    
11. Connect the crocodile clip of the HV cable to the top terminal of the bushing. Unscrew the test tap cover, insert a pin in the hole of the central test tap stud by pressing the surrounding contact plug in case of 245 kV OIP Bushing and remove the earthing strip from the flange by unscrewing the screw (holding earth strip to the flange body) in case of 420 kV OIP Bushing. Connect the LV cable to the test tap (strip/central stud) to the C & Tan & Kit through a screened cable and earth the flange body.

Evolution of Test Results

A large percentage of electrical equipment failure has been reported due to deteriorated condition of the insulation. A large number of these failures can be anticipated in advance by regular application of this test. Changes in the normal capacitance of insulation indicates abnormal conditions such as presence of moisture, layer short circuits or open circuit in the capacitance network.

The interpretation of the dilecting measurement are based on observing the difference:

1. Between measurements on the same unit after successive intervals of time.

2. Between measurements on similar part of a unit, tested under the same conditions around the same time e.g. several identical transformers or one winding of a three-phase transformer tested separately.

3. Between measurements made at different test voltages on one part of a unit; an increase in slope (tip up) of DF vs Voltage curve at a given voltage in an indication of ionization commencing at that voltage.

An increase of DF accompanied by a marked increase in capacitance usually indicates presence of excessive moisture in the insulation. An increase of DF alone may be caused to thermal deterioration or by contamination other than water. Surface of the insulator petticoats must be cleaned otherwise any leakage over terminal surfaces may add to the losses of the insulation itself and may if excessive, give a false indication of its condition.

Maximum value of tan delta of class insulation i.e. paper insulation, oil impregnated is 0.007. Rate of change of tan delta and capacitance is very important. Capacitance value can be within + 10%, - 5% in capacitance value.

The temperature correction factor to be applied for temperature other than 20° C is given in the following table which is based on IEEE 57 standard.

Ambient temperature in ° C	Temperature correction factor
10	0.80
15	0.90
20	1.00
25	1.12
30	1.25
35	1.40
40	1.55
45	1.75
50	1.95
55	2.08
60	2.42
65	2.70
70	3.00

3.3 Capacitance and Tan Delta Measurement of Winding Insulation of Transformer / Reactor

The above measurement is carried out to ascertain the general condition of the ground and inter-winding insulation of transformers and reactors. Portable capacitance and tan delta bridge from any reputed manufacturer may be used for carrying out this test. All safety instructions as per utility practice and isolation required may be followed before the commencement of this test. Following precautions need to be taken:

1. Never connect the test set to energized equipment.
   

2. The ground cable must be connected first and removed last.
   
3. Heart patients should not use this equipment.
   
4. The ground terminal of the input supply card (green lead) must be connected to the protective ground (earth) terminal of the line power source.
   
5. Keep the high voltage plugs free from moisture, dust during installation and operation.
   
6. Adequate clearance (Min 1 foot i.e. 30 cms) are maintained between energized conductor and ground to prevent any arc over.
   
7. It should be ensured that test specimen is de-energised and grounded before making any further connection and no person may come in contact with HV output terminal or any material energized by the output.

Testing Procedure

For the purpose of this test, the voltage rating of each winding under test must be considered and test voltage selected accordingly. If neutral bushings are involved, there voltage rating must also be considered in selecting the test voltage. Measurement should be made between in each inter winding combination (or set of 3 phase winding in a 3 phase transformer) with all other windings grounded to tank or ground all the other windings guarded. In the case of 2 winding transformer measurement should be made between each winding and ground with the remaining winding grounded. For 3 winding transformer measurement should be made between each winding and ground with 1 remaining winding guarded and second remaining winding grounded. Finally measurement should be made between all winding connected together and grounded tank.

1. Ensure that test specimen is isolated from other equipments.
   

2. Position the test set at least 6 feet (180 cm) away from the test specimen to be tested.
   
3. To prevent damage to the test set always set the capacitance multiplier dial to the SHORT position, the capacitance measuring dials to their 'O' position.
   
4. Set UST - DST switch to UST position.
   
5. Set interferance suppressor switches in off position.
   
6. Connect the ground terminal of the test set to a low impedance earth ground.
   
7. Connect control unit to the high voltage unit using two 5 feet long shielded cables. Screw the plugs down fully on the receptacles.
   
8. Connect the low voltage cable with red boot/sheath to the 'CxL red terminal' of the test set. Make sure the connector locks to the receptacle.
   
9. Connect the external interlock cable to the 'interlock terminal of the test set.
   
10. Connect the high voltage cable with Black boot/sheath to the high voltage terminal of the high voltage unit. Connect the pit - tail for the outer shield to the black binding post (ground) on the high voltage unit. Screw down the plug shell fully on the receptacle.
    
11. With the main breaker switched OFF, plug the input power card into the test set power receptacle and into a 3 wire grounded power receptacle having the appropriate voltage rating and current capacity.
    
12. Connect the Crocodile dlip of the HV cable to the HV terminal and LV cable to the LV terminal of the test specimen.
    
13. For ICTS: Tan delta and capacitance measurement of windings should be done in combination of HV+IV/LV+TANK+G; HV+IV+LV/TANK + G; LV/HV+IV+TANK+V in GST Test mode.
    
14. For Reactors: Tan delta and capacitance measurement of windings should be done in combination of HV/TANK+G in GST Test Mode.

Evolution of Test Results

The evolution of test results is similar to the one described in Clause 3.2

Measurement of Winding Resistance

The purpose of this test is to check for any abnormalities due to lose connection, broken strands and high contact resistance in tap changers as a pre-commission checks and compare the major values with the factory test values. The frequency of carrying out this test is yearly. The measurement of the winding resistance has to be carried out with the help of Kelvin Double bridge / transformer ohm meter. All safety instructions as per the utility practice and isolation required is to be carried out before the commencement of this test. Following precautions are needed to be taken:

• To reduce the high inductive effect, it is preferable to use a sufficiently high current to saturate the core. This will reduce the time required to get a stabilized results.
  

• It is necessary to measure accurately the temperatures of the windings.
  
• Care shall be taken that self inducting effects are minimized.
  
• Ensure that direct current circulating has settled down before the measurement is done.

Testing Procedure

The connection shall be as given in Figure 1.

Figure 1: Kelvin Double Bridge Method

• Resistance shall be measured between the line and the netural terminal and average of 3 sets of reading shall be taken as tested value for star connected winding with netural broughtout.
  

• The resistance of the HV side is measured between HV terminal and IV terminal then between IV terminal and the netural for star connected auto transformers.
  
• For transformers provided with delta connected windings such as tertiary windings of auto transformers, measurement shall be made between pairs of line terminals and resistance per windings shall be calculated per formula given below:

Resistance per winding = 1.5 x Measured value

• Winding temperature may be recorded during resistance measurement and resistance at 75 degree C may be calculated as per the following formula.

R₇₅ = R_t (235+75) / (235+t)

Where R_t = Resistance measured at windings temperature t

Evolution of Test Results

The resistance values obtained shall be compared with factory test value in case of pre-commissioning and the pre-comissioning value in case the test is being done the routine maintenance.

3.3 Operation Checks And Inspection / Maintenance Of Oltc

Operation Checks

i) Tab Changer Hand Operation

• Check the up and down and operation of tab changer for the full range before attempting electrical operation.
  

• Ensure the handle that handle interlock shall not allow electrical operation when the handle is inserted.
  
• Check for any discontinuity during tab changing operation by connecting an annlogue multimeter at cross HV and LV bushing and change the tab position from maximum to minimum.
  
• In the event of single phase tab changer check their tab position agree and are reach simultaneously at motor drive unit head

ii) Limit Switches

• Check the operation of limit switches at both ends of the range

iii) Maintaining Circuit

• Check the maintaining circuit for correct sequence by hand winding unit half way through a tab and then remove handle. Engergise the drive motor and check that the motor continues to drive the tab changer in the same direction.

iv) Drive Motor

• Check the direction of rotation and measure the start and running currents in both rise and lower mode operation with the tab changer in mid position. Record their values. Set the motor overload to the ten percent above running current

v) Raise and Lower Control

a) Step by step relay operation

• Check that the tab changer moves one at a time by pushing raise or lower push button

b) Out of step relay

• Move one tab changer in the 3 phase bank to the position which is out of step with the other two. Check the tab changer faulty alarm is activated. Similarly repeat for other two phases.

• Check the satisfactory operation of local/remote switch

vi) Tab Change Position Indicator

• Check for accurate indication of mechanical position indicator in all positions.

vii) Tab change in complete alarm

• Check the operation of tab changer in complete alarm including the flag relay by winding the unit by hand half way through a tab change and monitoring their correct operation and time to operate.

viii) Operation counter

• Check the tab changer operation counter for correct operation and record reading.

ix) Remote indication

• Check the remote indication and control facility provided to the outgoing terminals of the marshalling kiosk

x) Tab changer (surge protective relay)

• Check the tripping function of the relay. Open the cover and press button trip and check that all circuit breakers of the transformer operate correctly. Press push button 'reset', close the cover and tighten it.

Inspection / Maintenance of Tab Changer

Generally the temperature of OLTC compartments is a few degree Celsius less than the main tank. In case the temperature is found to be higher than this indicates a sign of internal problem and the OLTC compartment need to be opened. Prior to opening of OLTC compartment the same should be thoroughly inspected for external symptoms of potential problems. Also, inspect the integrity of paint, weld leakes, oil seal integrity, pressure release device and liquid level gage prior to opening of OLTC.

Following de-engerisation, close all the walls between oil conservator, transformer tank and tab changer head. Then lower the oil level in diverter switch oil compartment by draining the oil for internal inspection. Upon entering the OLTC compartment check for gaskit deterioration if any, compartment floor for any debris which may indicate abnormal wear.

Following items may be checked and manufacturer's engineer consulted for details of maintenance.

a. Function of control switches
b. OLTC stopping on position
c. Fastener tightness
d. Signs of moisture such as rusting, oxidation or free standing water
e. Mechanical clearances as specified by manufacturer's instruction booklet
f. Operation and condition of tap selector, changeover selector and arcing transfer switches
g. Drive mechanism operation
h. Counter operation
i. Position indicator operation and its co-ordination with mechanism and tap selector position
j. Limit switch operation
k. Mechanical block integrity
l. Proper operation of hand-crank and its interlock switch
m. Physical condition of tap selector
n. Freedom of movement of external shaft assembly
o. Extent of arc erosion on stationary and movable arching contacts
p. Inspect barrier board for tracking and cracking
q. After fitting with oil, manually crank throughout entire range
r. Oil BDV and moisture content (PPM) to be measured and recorded

Finally, the tap selector compartment should be flushed with clean transformer oil carbonization which may have been deposited should be removed. Min BDV should be 50 kV and moisture content should be less than 20 PPM.

3.4 Vibration Measurements (Shunt - Reactors)

The movement of the core coil assembly and ciling structure produced by the time varying magnetic forces results in the vibration of the shunt reactor bank and the ancillary equipment. For measuring the vibration the testing conditions shall be as under.

• Reactor under test shall be completely assembled in normal operating conditions.
  

• Reactor shall be energized at rated voltage and frequency
  
• Shunt reactor to be mounted on a level surface which will provide bearing for the base there by eliminating the generation of abnormal tank stresses.

Vibration shall be measured by transducers, optical detectors. The accuracy of the measuring equipment should be ± 10% at second harmonic of the exciting frequency. The peak to peak amplitude shall be determined by direct measurement or calculated from the acceleration or velocity measurement.

Test procedure

• Minimum number readings required shall be 48. Addition readings if required shall depend upon the size and the complexicity of core construction.

• All the four sides of the tanks shall be divided into 12 rectangle areas of approximately equal sizes. Area shall be marked and numbered for reference. Point of maximum excursion of each marked position shall be located for detecting the vibration pattern. Measurement for vibration shall be made at these points.

Evolution of results

The average amplitude of all local maximum points shall not exceed 60 µm (2.36 mils) peak to peak. The maximum amplitude within any rectangular area shall not exceed 200 µm (7.87 mils) peak to peak.

3.5 Maintenance Tests on Neutral Governing Reactors

Following tests should be conducted

• Winding resistance measurement
  

• IR measurement
  
• Core clamp insulation measurement

3.6 Test on Current Transformers Mounted in Tarrots

Following tests should be carried out

• Quality check
  

• Ratio test
  
• Excitation test
  
• Insulation resistance test

4.0 CONDITION MONITORING TECHNIQUES

4.1 As per CIGRE Working Group dissolved gas analysis using gas chromatograph and PD measurements are considered most effective techniques. Test on presence of moisture in oil, dielectric strength of oil and also loss angle measurement may be considered as medium degree of effectiveness. Hot-spots detection and turn ratio measurement have a low degree of effectiveness. Newer techniques such as FFA is being adopted for determining the deterioration in paper insulation.

Dissolved Gas Analysis (DGA)

This has been considered as one of the important tools by all the power utilities for detecting any incipient fault in the transformers and the reactors. Any abnormal or electrical stress in the transformers causes decomposition of the oil and / or paper insulation, thereby producing certain gases. These gases comeout and gets collected in the Buchholz relay when the quantity is more. However, these gases dissolved in the oil if the quantity is less. The composition and the quantity of gases generated is dependent on the severity of the fault. As such regular monitoring of these gases gives useful information about the healthiness of the transformers / reactors and prior information about the type of fault can be had by observing the trend of the various gas content. The gases which are of interest are hydrogen, methane, ethane, ethylene, accetelene, carbon mono-oxide, carbon di-oxide, nitrogen and oxygen. The equipment used for determining the content of these gases in oil is vacuum gas extraction apparatus and Gas Chromatograph. All the dissolved gases are first extracted from oil by string it under vacuum and total gas content in percentage is measured. These gases are then introduced in Gas Chromatograph for measurement of each component. Tables given below show the relationship of the evolved gas with temperature and type of fault.

Relationship with Temperature

Methane (CH4) > 1200 C

Ethane (C2H6) > 1200 C

Ethylene (C2H4) > 1500 C

Acetylene (C2H2) > 7000 C

Associated faults with different gases

Oil Overheating: C2H4, C2H6 , CH4 Traces of acetylene with smaller quantity of Hydrogen may be evolved

Overheated Cellulose: CO 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 involves an oil-impregnated structure. However principal gas shall be CO.

Partial discharge in oil: H2, CH4 Ionisation of high stressed area where gas / vapour filled voids are present or ‘wet spot’ produces Hydrogen and methane and small quantity of other hydrocarbons.

Arcing in Oil: C2H2, H2 Large amount of Hydrogen and acetylene are produced with minor quantities of methane and ethylene in case of arching between the leads, lead to coil and high stressed area.

Interpretation of DGA is not only a science but also an art. There is no precise interpretation methods available which can tell the exact location and type of fault. The various interepretation method available provide only guidelines to take an engineering graduate about the equipment. Besides DGA results, other considerations such as past history of the transformer, parameters, loading pattern are also taken into account. Some of the methods used for DGAR ratio analysis as per IEC 559, IEEE standard C 57.104-1991, Doernenberg Ratio method, Rogers ratio method, ANSI standard. It may be mentioned that DGA results may give misleading results unless certain precautions are taken during sampling procedures, type of sampling bottles, cleanliness of bottles, duration of storage, method of gas extraction, good testing equipment and skilled manpower. Annexures A, A1, A2, A3 give the oil sampling procedures required to be followed, information to be furnished along with the samples, and other additional data inputs required for DGA.

Continuous Monitoring

4.2 DGA is carried out at pre-set interval and any fault developed within that interval can't be ascertained till the transformer has actually failed. It is generally seen that some type of fault may take less than 1 year to progress from onset to failure whereas some others may remain in a stable stage for much longer period but have a potential of rapid increase. One of the latest technique widely used online gas monitor system has a membrane which allows preferably lighter molecules to pass through and be detected in gas reaction cell (HYDRAN). Recently some companies developed Fourer transform Infra Red (FTIR) detectors which will detect most of the gases which are of interest and also quantify their amount. However, these are quite expensive as compared to cost of DGA.

Furfurladehyde Analysis (FFA)

4.3 Overheating of transformers can lead to cellulose decomposition and generation of carbon-monoxide and carbon-dioxide. These gases are also produced during the decomposition of the oil. Therefore, the analysis of the gases and the measurement of carbon-monixide and carbon-dioxide will give a unambiguous indication0 of paper de-gradation. It is also well known that aging process of paper reduces several oil soluble by-products most notably furanoid compounds (FFA). Monitoring of furanic compounds by annula sampling of the oil and its analysis using High Performance Liquid Chromatography (HPLC) has been used for condition monitoring on a routine basis for some years.

4.4 FFAs are extracted from oil either by solvent extraction or solid phase extraction and measured by HPLC by uv detectors. The major FFA present in the oil is 2-Furfural and other are present in a very low or un-dected levels. 2-Furfural can be measured colormertically by using spectro-photometer. This method is quite accurate and is very rapid. FFA may be used as a complimentary techniques to DGA for condition monitoring.

Frequency Response Analysis

4.5 Condition monitoring techniques like DGA, FFA or PD measurement can to some extent give an idea about the condition of the transformer. However, FFA is generally employed to find any movement in the winding. This being a new technique and is still to be adopted by the power utilities. However, Central Transmission Utility (POWERGRID CORPORATION) is employing this technique.

Recovery Voltage Measurement

4.6 Recently recovery measurement are gaining momentum for monitoring for solid insulation in transformers. During this measurement humidity content in the transformers is determined. A pre-defined DC voltage in the range of 2 kV is applied to the winding under test keeping the other 2 winding shorted. The voltage is applied for a given time (tc) and discharged by short-circuiting for a given time again (td) and then by opening the shor-circuit and the voltage is allowed to build up. The peak voltage (Vr) attained is measured. The above procedure is repeated several times for different values of (tc) and (td) by keeping (tc) and (td) constant. A series of curve is obtained for various levels of humidity in winding.

Partial Discharge Measurement (PD)

4.7 This is very accurate method for determining the health of a transformer. However, as on today no equipment on commercial basis is available for measurement of PD. Presently, this method is not being used by any utility. However, as and when the testing equipment is available this technique would provide precise information and the location of the fault in the transformer.

5.0 DE-GASSING OF TRANSFORMER OIL

5.1 Transformers and reactors in operation are always subjected to thermal electrical stresses which results in de-gradation of both oil and paper insulation. The de-gradation produces moisture. Besides moisture may also enter the transformer from atmosphere due to improper breathing, exposure of oil and winding during maintenance etc. Moisture in association with oxygen present in air it breathes and damages the oil. All these results in the deterioration of electrical, chemical and physical properties of the oil. Similarly combustable gases produced/generated due to abnormal thermal electrical stresses in the transformers results in the breakdown of oil, cellulose insulating material. Most of the gases gets dissolved in oil. When the quantity of these gases dissolved in the oil exceeds a certain percentage then oil gets saturated. At this stage, it is recommended to degass the equipment since further gas generation will lead to operation of buckle relay and any air bubble inside the transformer will damage the solid insulation.

5.2 The filtration and degassing is recommended when the moisture present in oil or percentage gas content in the transformers exceed the violating norms. The capacity of the high vacuum oil filtration plant is dependent on the quantity of oil and the recommended quantity is given below:

Oil capacity	Recommended capacity of plant
Up TO 20 KL	2 KLPH
20 KL to 50 KL	4 KLPH
More than 50 KL	6 KLPH

5.3 Following precautions need to be taken during filtration and degassing.

• Don't use rubber hose since sulphur in rubber hose may react with oil
  

• In the event of transformer provided with air cell in the conservator, the conservator should be isolated.
  
• Don't allow the oil temperature to go above 60 degree C.
  
• Don't mix the OLTC oil with main tank oil while filtring the oil.
  
• Care to be taken to ensure no mixing of parafinic or naphthenic oil. In case the machine has been used for other type of oil it is recommended that the machine need to be flused with the 50-100 L of the same oil for which the machine is to be used.
  
• Read the maintenance manuals of transformers and filtration plant and all precautions stipulated by the manufacturer have to be followed.

Procedure:

Before starting the operation ensure the following

• Ensure that the vaccum plant of adequate capacity and in good working condition is chosen.
  

• Check the plant for proper earthing.
  
• Ensure that all switches in the control panel are in off conditin
  
• Check the quality and quantity of the oil in vacuum compartment
  
• Check the direction and rotation of all pumps
  
• Connect the cooling water supply
  
• Connect the inlet and outlet hoses to the respective inlet and outlet walls of the transformers and the plants

Before commencing the oil processing ensure plant has been evacuated for evacuation of the plant manufacturers instruction may be followed.

For filtration and degassing of the transformer, first the inlet to the machine is taken from the bottom of the transformer tank and the outage is connected to the top of the tank. This connection is changed at an interval of 12 hours for effective filtration.

Ensure that all hose connections are air tight and cool water is circulated through the condenser for effective removal of water.

The inlet and outlet valves of the transformers to the plant are open after evacuating the system for some time.