PRE Commissioning Procedures Tr Rev01 PGCIL

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

PRE-COMMISSIONING PROCEDURES FOR SUBSTATION

INTRODUCTION Energization of the system will be made step by step in zones as small as possible bound by the disconnectors installed in the system. Energisation is the first time connection of the Substation equipment to the high voltage system. It is to be done stepwise in the smallest possible zones (subsystems) as permitted by the design (position of disconnectors). During the energisation, only the very small excitation currents will flow. Only very little power will be required to supply the corona losses and the no load losses of the equipment. Overall procedure, safety rules, dispatch procedures, switching sequences, observations, passing criteria and documentation of results are treated in this document. Once all the zones have been energized in sequence, the energisation is concluded with an 8 hour charging to ascertain the capability of the system. The detailed instructions/test procedures for each zone to be energized will be given in separate section of this documentation. The contents of this document are as following: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 1.0

Subsystem limits Overall procedures Auxiliary system Protective system Dispatch procedures Switching sequences Energisation De-energisation Observations and duration Passing criteria Documentation SUBSYSTEM LIMITS Main Circuits: • • • • • • • • •

AC equipment: AC Switch yard, Control and Protection Station auxiliaries: Station auxiliary power Telecommunications (PLCC) Air Conditioning and Ventilation Telephone system (Exchange) Illumination Fire protection Others 1

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

2.0

OVERALL PROCEDURE Instructions for the work and supervision are taken care of by the test leader. However, all switching and operational activities will be executed by the regular operators incharge under the IE Rules. First it is to be ascertained that the subsystem to be energized is ready for operation and has been properly handed over (released) in writing. This will include all safety aspects, protections, control systems, auxiliaries and final inspection. Then the subsystem is prepared according to switching order, halted at the last step before closure of the ultimate Circuit Breaker. Before proceeding, the status is given a final check including confirmation with the dispatch centre i.e. CPCC/RLDC. Thereafter, energisation is executed by closure of the Circuit Breaker. Inspection as well as instrument readings are to be taken. Successive subsystems are then energized without special urgency but also without unnecessary delay. When the whole system has been energized, including the AC line, it will be kept in this state for 8 hours or more for observation with continuous inspection and monitoring. However, recommendation of the Manufacturers may be checked.

3.0

SAFETY PROCEDURES Energisation implies an abrupt and serious change of working conditions of the plant/ system. In order to avoid serious accidents, thorough information must be circulated to all the personnel and rigorous discipline enforced. This should include a statement that trespassing of safety barriers, even if accident does not occur, must be cause of immediate dismissal and removal from the plant area. All equipment tests and pre-commissioning tests must have been completed, terminated and documented. The subsystem must be formally declared ready for energization and handed over for operation in writing. The subsystem must be bound by warning signs and Safety barriers according to the IE Rules. Guards shall be posted in suitable positions. The subsystem must have been thoroughly cleaned and all non-regular objects (ladders, tools, vehicles, work-benches, debris etc) removed. It is necessary that the Commissioning in-charge (test leader) takes a final, thorough inspection of all areas 2

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

involved to ascertain that this has really been executed. No deviations whatsoever are to be tolerated. This is necessary in order to maintain the discipline and the respect for safety rules. 4.0

AUXILIARY SYSTEM All auxiliary systems needed for regular operation of the unit must be declared tested and formally in writing handed over for operation. A thorough inspection by the test leader for the confirmation of the readiness of the auxiliary system shall immediately precede the energization.

5.0

PROTECTIVE SYSTEM Before energization it must be ascertained that all protective systems for the unit to be energized are operative. This includes confirmation that the protections have been properly tested and that the tests have been documented. It also includes verification by inspection or otherwise, if necessary by repetition of trip test, that the protections are actually functionally enabled. This verification serves to prevent that energization takes place of a unit where a protection has been disabled for test or other reason.

6.0

DISPATCH PROCEDURES All operational activities (switching, power changes, changes in redundant reserves and alike) must be coordinated and communicated with the system dispatcher i.e. CPCC/RLDC.

7.0

SWITCHING PROCEDURES For each activity the instructions to the operators and the communications to the dispatchers will be made in writing or by confirmed telephone messages. The format established by POWERGRID for switching orders will be followed. When preparing the switching orders for the first energization, the only assumption permitted is that nothing is known about the status of the system. The implication of this is that each and every activity must be listed and described so that complete and unambiguous conditions become the result. Examples of such switching orders will be part of the following instructions for the different subsystems. Every attempt will be made to get them complete and self contained. However, local conditions may deviate from those assumed, they shall be mistrusted and considered 3

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

suspect at the best as guiding examples. They must be scrutinized in detail and double checked before application. 8.0

ENERGIZATION Execution of the energization is simply the last event in the switching sequence, switching of the close control button for the relevant Circuit Breaker.

9.0

DE-ENERGIZATION Instructions about de-energization are to be given only if this is part of the test. Otherwise de-energization will be considered part of regular operation.

10.0

OBSERVATIONS AND DURATION Visual and audible inspection (look and listen) of the relevant equipment and reading of permanent instrumentation will be made. The system should be charged at least for 8 hours. During this time continuous monitoring and inspection will be maintained in control room, auxiliary systems areas and switchyard.

12.0

PASSING CRITERIA Neither insulation breakdown nor protective system actions must occur. No irregular equipment behaviour noise, vibration, high temperature is permitted. Corona discharges may not be “unreasonable”. Local discharges that may be attributable to sharp points shall be carefully located and recorded. After termination of the energisation the equipment shall be closely inspected and the points rounded or covered. No unscheduled change of system or of equipment is permitted during the 8 hour energized condition.

13.0

DOCUMENTATION Switching and operational activities will be recorded in regular manner in the operators log. Likewise all readings of permanent instruments. Copies of this log, notes on special observations from inspections and other measurements will constitute the test records.

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POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

TESTING AND COMMISSIONING PROCEDURES FOR BAY EQUIPMENT INTRODUCTION All the equipment after erection/assembly at site, should be tested in order to check that it has not been damaged during transport, erection/assembly to such an extent that its future operation will be at risk. The significance of various tests with brief procedure has been elaborated in the subsequent sections of this document. Regarding the detailed testing methods / procedures for conducting various pre-commissioning tests refer to Doc. No. D-2-03-XX-01-01: Maintenance Procedures for Switchyard Equipments-Part 1: EHV Transformers & Reactors and Doc No. D2-03-XX-01-01 –Part 2: Other Switchyard Equipments. The instruction manuals of various equipments being used for the test shall also be consulted in case of any difficulty in operation of the testing instrument. The results of the test shall be documented on the test record formats as mentioned below, which are also part of this documentation: Sl. No. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

FORMAT NO. No. CF/CT/01/R-2 DATED 01/08/07 No. CF/CVT/02/R-2 DATED 01/08/07 No. CF/BAY/03/R-2 DATED 01/08/07 No. CF/SA/04/ R-2 DATED 01/08/07 No. CF/CB/05/ R-2 DATED 01/08/07 No. CF/ICT/06/ R-2 DATED 01/08/07 No. CF/SR/07/ R-2 DATED 01/08/07 No. CF/ISO/08/ R-2 DATED 01/08/07 No. CF/WT/09/ R-2 DATED 01/08/07 No. CF/C&P/10/R-2 DATED 01/08/07

EQUIPMENT CURRENT TRANSFORMER CVT BAY/FEEDER SURGE ARRESTOR CIRCUIT BREAKER ICT SHUNT REACTOR ISOLATOR/GROUND SWITCH WAVE TRAP Control & Protection

These formats have all the possible test recordings to be performed at site before energization. NOTHING SHOULD BE TAKEN FOR GRANTED AND NO CONNECTION SHOULD BE ASSUMED TO BE CORRECT UNTIL IT HAS BEEN TESTED AND PROVED. A Commissioning team should be constituted as per procedures laid down in Works & Procurement Policy and Procedures (Clause 2.15.4.6&7 of WPPP Vol. II), which shall oversee/ coordinate with erection agency/ manufacturer for the pre-commissioning test and charging of the equipment The pre-commissioning tests after erection shall be carried out in accordance with latest technical specifications issued by CC/Engg., Pre-Commissioning procedures / formats issued by CC/OS and Guidelines/ instruction from the Equipment Manufacturer. (Clause 2.15.4.8(iv) of WPPP vol. II) The test results shall be checked by Commissioning team. After all pre-commissioning checks and tests are found to be acceptable taking into account permissible deviation limits, the commissioning team, in consultation with Regional O&M shall give clearance for charging. (Clause 2.15.4.8 (viii) of WPPP vol. II). In case of deviations in commissioning test results beyond permissible limit as per guidelines/ standards in vogue, the commissioning team shall 5

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

submit its report along with recommendations to the concerned Site In-charge. In such an event, the test charging will be carried out only after obtaining approval of Head of O&M of the Region based on a proposal of execution site in consultation with Regional OS. (Clause 2.15.4.8 (ix) of WPPP vol. II). Please Note: It is preferable to have manufacturer representative during charging and get the charging clearance from equipment manufacturer after review of the test results by them. The test records had to be signed by the responsible personnel from the OEM, the supplier, the erection agency and the POWERGRID representative. The test formats/records are to be distributed as follows: 1.

Regional O&M office

2.

Corporate Operation Services

3.

Corporate Engineering

4.

Manufacturer/ Supplier of the equipment

5.

Erection Agency.

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POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

1.0

TRANSFORMER AND REACTOR

1.1

Following points to be checked After Receipt of transformer / reactor at Site:

1.1.1

N2 pressure and Dew point to be checked after receipt of transformer at Site. It should be within permissible band (as per graph provided by manufacturer & given below)

1.1.2

The data of impact recorder shall be analyzed jointly in association with the manufacturer. In case the impact recorder indicates some serious shocks during shipment, further course of action for internal inspection, if necessary shall be taken jointly.

1.1.3

Oil Samples shall be taken from oil drums/ tanker received at site and sent to our Lab for oil parameter testing. The copy of test certificate of routine testing at oil refinery should be available at site for comparison of test results.

1.1.4

Unpacking and Inspection of Accessories to be carried out taking all precautions so that the tools used for opening do not cause damage to the contents. Fragile instruments like oil level gauge, temperature indicators, etc. are to be inspected for breakage or other damages. Any damaged or missing components should be reported to equipment manufacturer, so that the same can be investigated or shortage made up as per the terms/ conditions of the contract.

1.1.5

Core Insulation Test shall be carried out to check insulation between Core (CC&CL) and Ground.

Figure -1 : Graph depicting variation of N2 gas pressure w.r.t temperature 7

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

1.1.6

1.1.7

After receiving the accessories at site same should be inspected and kept ready for immediate erection: •

If erection work can not start immediately due to some reasons, then accessories should be repacked into their own crates properly and packing list should be retained.

•

All packings should be kept above ground by suitable supports so as to allow free air flow underneath. The storage space area should be such that it is accessible for inspection, water does not collect on or around the area and handling/transport would be easy. Immediately after the receipt of main unit and also the accessories, same should be inspected and if found satisfactory, the unit should be erected completely and filled with dry transformer oil as per the instruction.

•

It is preferable to store the main unit on its own location/foundation. If the foundation is not likely to be ready for more than three (3) months, then it should be stored at suitable location preferably filled with oil. Fill the oil as per instructions up to tank oil level mark. Fill the gap above oil with dry air and fit silica gel breather on any opening on the cover, with suitable pipe. The transformer preferably be provided with oil conservator including oil-level indicator and breather.

•

If the transformer is to be stored up to three (3) months after arrival at site, it can be stored with N2 filled condition. N2 pressure and dew point to be monitored on daily basis so that chances of exposure of active part to atmosphere are avoided.

•

In case of storage of transformer in oil-filled condition, the oil filled in the unit should be tested for BDV and moisture contents once in every three months. The oil sample should be taken from bottom valve. If BDV is less and moisture contents are more than as given for service condition then oil should be filtered.

During erection, the active part of transformers should be exposed to atmosphere for minimum time and either dry air generator should be running all the time or dry air cylinders may be used to minimize ingress of moisture. The transformer should be sealed off after working hours. It is practical to apply a slight overpressure overnight with dry air or N2 inside – less than 300 mbar ( 30 kPa or 0.3 atmospheres). Next day the pressure is checked and suspected leaks may be detected with leak detection instruments , with soap water or with plastic bags tightened around valves (being inflated by leaking air) TABLE 1- Variation of Dew Point of N2 Gas Filled in Transformer Tank w.r.t Temperature For transformers with a gas pressure of 2.5- 3 PSI, the acceptable limits of dew point shall be as under: (Source: Courtesy BHEL, Bhopal) Temperature of Insulation in °F 0 5 10 15 20

Maximum Permissible dew point in °F -78 -74 -70 -66 -62

Temperature of Insulation in °C -17.77 -15.0 -12.22 -9.44 -6.66

Maximum Permissible dew point in °C -61.11 -58.88 -56.66 -54.44 -52.22 8

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 110 120 130 140

1.2

-58 -53 -48 -44 -40 -35 -31 -27 -22 -18 -14 -10 -6 -1 +3 +7 +16 +25 +33 +44

-3.33 -1.11 +1.66 +4.44 +7.44 +9.99 12.77 15.55 18.33 23.11 23.88 26.66 29.44 32.22 34.99 37.75 43.33 48.88 54.44 59.99

-49.99 -47.22 -44.44 -42.22 -39.39 -37.22 -34.99 -32.77 -29.99 -27.77 -25.55 -23.33 -21.11 -18.33 -16.11 -13.88 -8.88 -3.88 +0.55 +5.55

Insulating Oil When oil is dispatched to site separately it is usually in sealed steel drums. The oil to be used to filling and topping up must comply with IS 335 (latest revision). Before oil is accepted samples have to be taken and checked with respect to dielectric strength and dielectric losses. The latter is important, since dirty transportation vessels can significantly contaminate the oil. High dielectric losses cannot be removed by filter treatment, such lots have to be rejected. If the oil is supplied in railroad or trailer tanks, one or two samples are sufficient. If the oil is delivered in 200 litres drums, the following scheme for checking is recommended. Number of drums delivered 2 to 5 6 to 20 21 to 50 51 to 100 101 to 200 201 to 400

No. of drums to be checked 2 3 4 7 10 15

1.2.1 Samples from Oil Drum Check the seals on the drums. The drum should first be allowed to stand with bung (lid) vertically upwards for at least 24 hours. The area around the bung should be cleaned & clean glass or brass tube long enough to reach to within 10mm of the lower most part of the drum should be inserted, keeping the uppermost end of the tube sealed with the thumb while doing so. Remove the Thumb thereby allowing oil to enter the bottom of 9

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

the tube. Reseal the tube and withdraw an oil sample. The first two samples should be discarded. Thereafter, the sample should be released into a suitable receptacle. 1.2.2 Parameters of Transformer Oil The oil as received at site for filling and topping up in the transformer must comply with POWERGRID specification for acceptance criteria. The oil sample from the transformer tank, after filling in tank before commissioning should meet the following specifications as per IS: 1866 – 2000 (latest Revision) given in table-2 below: Table-2 Sr. No. 1. 2. 3. 4. 5. 6. 7 8. 9. 10. 11. a. b. 12.

Property Breakdown Voltage (BDV) (kV rms)-Min Moisture content –(ppm) Max Dielectric Dissipation Factor (Tan δ) at 90 ºC and 40-60Hz -Max Resisitivity at 90 ºC-Min ( x 1012 ohm-cm) Interfacial Tension (mN/m) -Min Viscosity at 27ºC-Max – cSt Density at 29.5ºC gm/cm3 - Max Flash point ºC – Min Pour point ºC – Max Neutralization Value mg KOH/gm of oil –Max Oxidation Stability of Uninhibited Oil Neutralization Value mg KOH/gm of oil –Max Sludge percent by mass – Max Oxidation Stability for Inhibited Oil

Highest Voltage of Equipment 170 kV 40 50 60 20 15 10 0.015 0.015 0.01 6 35 27 0.89 140 -6 0.03 0.4 0.1

6 35 27 0.89 140 -6 0.03

6 35 27 0.89 140 -6 0.03

0.4 0.4 0.1 0.1 Similar values as before filling

1.3 Oil Filling (For detailed Oil filling procedure, please refer to Appendix-2 of Doc. No. D-2-03-XX-01-01: Maintenance Procedures for Switchyard Equipments-Part 1: EHV Transformers & Reactors) Introduction: For transformer dispatched with gas filled from the work, the filling of oil inside the tank is done under vacuum. Transformer of high voltage ratings and their tanks are designed to withstand partial or full vacuum. Manufacturer’s instructions should be followed regarding the creating of full or partial vacuum during filling the oil in the tank. Final tightness test with vacuum (i.e. leakage test or Vacuum Drop Test): Before the oil filling is started, a final check is made of the tightness of the complete transformer with its cooling equipment by applying vacuum. When any vacuum is applied to a transformer, empty of oil, a leakage test must be carried out to ensure that there are no leaks on the tank which would result in wet air being drawn into the transformer. The procedure is as follows: •

Connect the vacuum gauge to a suitable valve of the tank. 10

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

• • • • • • •

Connect the vacuum pump to another opening. Evacuate the transformer/ reactor tank until the pressure is below 3 mbar ( 0.3 kPa or about 2 mm of Hg) Shut the vacuum valve and stop the pump. Wait for an hour and take a first vacuum reading – say P1 Take a second reading 30 minutes later- say P2 Note the quantity of oil, V tonnes according to the rating plate The leakage rate for any transformer can be calculated from the formula:

Leak rate (mbar litre /sec) = (Main tank oil volume in litres)* (drop in vacuum i.e. change in pressure in mbar)/ Time in seconds The leak rate observed at various points of time should not exceed the limits as given below: Time in hours After 1 hour End of 3rd –4th hour End of 7th –8th hour End of 11th –12th hour

Allowable leak rate in mbar litre/ sec 100 75 50 25

If the leak rate is high, it indicates excessive leakage, then all gasketted joints should be checked for any leakage. As soon as 25 millibar-litre /sec leak rate is achieved, re-start the vacuum pump, open the valve and continue pumping until the required vacuum (1.3 mbar or less) is reached and maintained for the specified period. This vacuum must be maintained for forty-eight hours before and during the subsequent oil filling operations by continuous running of the vacuum pumps. When filling a transformer with oil it is preferable that the oil be pumped into the bottom of the tank through a filter press or other reliable oil drying and cleaning device should be interposed between the pump and the tank. In case the transformer is provided with an On Load Tap Changer (OLTC), while evacuating the main transformer tank, the diverter switch compartment may also be evacuated simultaneously so that no undue pressure is allowed on the tap changer chamber. While releasing vacuum, the tap changer chamber vacuum should also be released simultaneously. For this one pressure equalizer pipe should be connected between main tank and tap changer. Manufacturer’s instruction manual should be referred to protect the air cell/diaphragm in the conservator during evacuation. The oil must pass from the oil tanker, or settling tanks, through an oil conditioning plant to the top oil filter valve and the oil must conform to the quality requirements as given in POWERGRID- Transformer Oil Specification. The oil flow at the entry valve must be controlled to maintain a positive pressure above atmospheric and to limit the flow rate if necessary to 5000 litres / hour, or a rise in oil level in the tank not exceeding one metre / hour (as measured on the oil level indicator)

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POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

Continue oil filling until the level reaches approximately 200 mm above the ambient oil level indicated on the magnetic oil level gauge in the expansion vessel. Then, release the vacuum, with dry air of dew point -40deg C or better. The diverter tank can now be topped up at atmospheric pressure. Reconnect oil outlet hose to valve on flange on tap changer diverter head. Reinstate breather and very slowly top up the diverter switch such that the correct level is reached in the diverter expansion vessel. In the event the expansion vessel is overfull drain oil from flange into a suitable container until the correct level is reached. When the vacuum filling of the transformer and diverter tank is complete, the cooling system/ Radiator bank can be filled (WITHOUT VACUUM) at atmospheric pressure, via an oil processing plant. Oil must be admitted, very slowly, through the bottom cooler filter valve, with the cooler vented at the top and the top cooler filter valve unblanked and open to atmosphere. As the oil level reaches the vents and open valve, close off and shut down the processing plant. Note: Care must be taken not to pressurise the coolers/ radiators. Upon completion, open the top cooler isolating valve in order to equalise the pressure in the cooler with the transformer tank. This will also allow contraction or expansion of the oil as the ambient temperature changes. Before oil filling into conservator inflate the air cell to 0.5 PSIG i.e. 0.035kg/cm2 max. slightly by applying pressure (N2/Compressed dry air) so that it can take shape. Release pressure. Fit breather pips but do not fit breather in position, instead put wire mesh guard over and flange of the pipe. This will ensure free air movement from the air cell to the atmosphere. Use flow meter / indicator on outlet of filter machine and regulate the flow using the valve to limit oil filling rate to 2000 litres per hour max. in case filter capacity is more. Push oil slowly into conservator through the transformer via valve No. 5 (valve 2,3 & 4 to remain open) till the oil comes out first through valve Nos.2 & 3 (close these valves) and then through valve No. 4. Allow some oil to come out through valve No.4. Oil should come out freely into the atmosphere. This will ensure that oil air inside the conservator is expelled out and the space surrounding the air cell is full of oil. (Close valve No. 4). During all these operations valve No.1 to remain closed. Drain oil from the conservator by gravity only through valve No. 1 or through drain valve of the transformer via valve No. 5. Do not use filter machine for draining oil from the conservator. Also do not remove buchholz relay and its associated pipe work, fitted between the conservator and the transformer tank while draining oil. Stop draining oil till indicator of magnetic oil level gauge reaches position-2 on the dial, which is corresponding to 30°C reading on the oil temperature indicator.

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POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

Figure-2 : General Arrangement For Oil Conservator

1.4

Drying of Transformer/ Reactor

The drying of a new Transformer/ Reactor required when the moisture get absorbed by the solid insulation used in Transformers/ Reactors due to various reasons. The process of drying out a transformer is one requiring care and good judgment. If the drying out process is carelessly or improperly performed, a great damage may result to the transformer insulation through over heating, etc. In no case shall a transformer be left unattended during any part of the dry out period. The transformer should be carefully watched through out the dry-out process and also observations shall be carefully recorded. When the transformer is dried out it is necessary to ensure that fire fighting equipment is available near the transformer. Various methods can be adopted for drying out a transformer depending upon the facilities available/provided at site.

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POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

1.4.1 DRYING OUT OF TRANSFORMER / REACTOR BY HOT OIL CIRCULATION USING HIGH VACUUM OIL FILTER MACHINE 1.4.1.1 PURPOSE

When the equipment is exposed to atmosphere during erection, atmospheric moisture is absorbed by the solid insulation. This moisture in the paper will reduce the dielectric strength and the ageing will be faster than the normal. The purpose of this procedure is to remove the moisture from the transformer / reactor when water is absorbed in the cellulose. When the equipment is opened and the winding insulation is exposed to atmosphere, it is a must that it should be dried out. Please Note that this procedure is applicable only for low wet equipments and for moderate or high wet equipments, procedure given in section 1.4.2 to be followed. 1.4.1.2 PRECAUTIONS:

1. Rubber hoses should not be used for oil since the sulphur in the rubber would react with oil. 2. If the transformer / reactor is fitted with air cell in the conservator, the conservator should be isolated while doing oil filtration / degassing. 3. At any point of time it is seen that the oil temperature should not be more than 60°C, otherwise the property of the oil gets deteriorated due to oxidation. 5. It is to be taken care that there is no mixing of parafinic or naphthenic oil. If the machine is already used for other type of oil, the machine needs to be flushed with 50 -100 litres of the same oil for which the machine is to be used. 6. The manual of the filter plant needs to be read thoroughly before starting the operation and all the additional precautions stipulated by the manufacturer of the plant needs to be taken into account. 7. Similarly the particular transformer / reactor manual also to be read carefully and all additional precautions mentioned need to be taken into consideration. 8. Coolers are to be isolated during drying process. 9. It is to be ensured that the tank will withstand the full vacuum. If not vacuum needs to be regulated according to the manufacturer's instruction. 1.4.1.3 PREPARATION

* For drying out operation of transformer / reactor the coolers are to be isolated so as to prevent heat loss during the process. * The transformer / reactor tank has to be completely logged with a tarpaulin or insulating cover to avoid heat loss. 14

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

* It is advisable to provide electric heaters at the bottom as well as at the sides of the transformer / reactor tank outside but within the insulation cover so that the process is speeded up. * A motorised megger (5KV) to be connected to the HV terminals to ground in case of reactor or HV + LV to ground in case of transformer for measurement of insulation resistance during the drying out. * The oil level in the equipment should be just above the winding and a vacuum gauge to be connected at the top of the transformer / reactor to read the applied vacuum. 1.4.1.4 PROCEDURE

Drying out a transformer using an oil filtration plant is the most practical method of drying at site compared to all other methods. In this method the drying out is achieved by circulating hot oil through a stream line filter. Connect the inlet of the filter machine to the bottom of the transformer / reactor tank. Connect the outlet to the top. Connect one vacuum pump to the top of the tank after lowering the oil level slightly to just above the winding. Run the oil filtration plant as well as the vacuum pump connected to the top of the transformer tank to reach a vacuum of about 5 torr. The heaters in the oil filtration plant are switched on so that the oil temperature at any point will not go more than 60°C. Now the main tank oil is circulated through the oil filtration plant and also getting heated up. The insulation resistance of the winding is measured at an interval and the same is noted down against the oil temperature in the transformer tank. The inlet oil temperature as well as the outlet oil temperature from the filtration plant are also noted down at the regular interval. It is seen that initially when the temperature starts shooting up, the IR value starts coming down. This indicates that the moisture drops are getting distributed in the winding and the oil in form of vapour. After several circulation of oil through the machine, the insulation resistance becomes steady. On further continuation, the IR value starts rising. This indicates that the moisture is being expelled from the winding and oil. The graph will be as shown in the figure-3 given below: 1.4.1.5 Stopping Criterion: The completion of the drying of the transformer / reactor is decided based on the following results. 1. Steady increase of IR value of the windings. 15

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

2. BDV of the oil is more than 70 kV. 3. Moisture content of the oil is less than 10 PPM. 4. PI value is more than 2.

1.4.2

If it is established that the Transformer winding is wet and N2 purging is required, it is recommended to use the procedure given in section 1.4.3. The brief procedure for dry out of wet winding of Transformer/ Reactor by Vacuum pulling, N2 filling and heating is described in 1.4.3. However, for the detailed procedure, please refer to the Doc. No. D-2-03-XX-01-01: Maintenance Procedures for Switchyard Equipments-Part 1: EHV Transformers & Reactors

Figure-3 : Graph of readings taken during Drying out process of a Power Transformer

1.4.3

PROCEDURE FOR DRY OUT OF WET WINDING OF TRANSFORMER/ REACTOR BY VACUUM PULLING, N2 FILLING AND HEATING

1.4.3.1 Isolation Required: All the openings of transformer main tank like openings for coolers/radiators, conservator, OLTC etc. is to be properly isolated and totally blanked. 1.4.3.2 Procedure: a. Fill the main transformer/ reactor tank with Nitrogen (at least with the dew point of –50 °C or better) until it comes to a positive pressure of 0.15 Kg/cm2. It is kept at least 48 hrs. At the end of 48 hrs, dew point of N2 at outlet is measured. If the dew point is not within acceptable limits as per Table-I, dry out method should be continued. b. Heating from outside: While N2 circulation is in progress, the heaters are to be installed around the transformer tank as shown in the Annexure. The heaters are to be kept ON until we achieve a temperature of about 75–80 deg C of the core & winding of transformer.

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POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

c. Vacuum Pulling: After ascertaining that there is no leakage, pull out vacuum and keep the transformer/reactor under near absolute vacuum (1-5 torr) for about 96 hours running the vacuum pump continuously. The duration of vacuum can vary between 48 to 96 hrs. depending upon the dew point being achieved. Keep Vacuum machine ON and collect condensate for measurement. Observe the rate of condensate collection on hourly basis. Depending on the value of rate of condensate, continuation of further vacuum shall be decided. d. Nitrogen Circulation: Then the vacuum is broken with dry nitrogen. The dew point of nitrogen at the inlet is to be measured and should be of the order of - 50 °C or better. When the nitrogen comes to the positive pressure of 0.15 kg/cm2, it is stopped and kept for 24 hours. Heating from outside is to be continued while N2 circulation is in progress. Then the nitrogen pressure is released and the outlet nitrogen dew point is measured. If the dew point is within acceptable limits as per Table-I then the dryness of transformer is achieved. If not again the transformer is taken for vacuum treatment and then nitrogen is admitted as mentioned above and tested. The cycle to be continued till desired dew point as per Table-I is achieved. e. Periodicity: Periodicity of vacuum cycle may vary between 48-96 hrs. Initially two N2 cycles may be kept for 24 hrs. After that it may be kept for 48 hrs depending upon dew point being achieved. After completion of drying process, oil filling and hot oil circulation to be carried out before commissioning. Please ensure standing time as per table given below before charging. Transformer HV Rated Voltage (in kV) Up to 145kV 145 kV and up to 420kV Above 420 kV

Application of Vacuum & holding for (before oil filling ) (in Hours) 12 HRS 24 HRS

STANDING TIME After Oil circulation and before energising (in Hours) 12 HRS 48 HRS

36 HRS

120 HRS

After the expiry of this time, air release operation to be carried out in Buchholz relays and other release points given by the manufacturers before charging. For subsequent activities, proceed to format no. CF/ICT/06/ R-2 DATED 01/08/07 for ICT & format No. CF/SR/07/ R-2 DATED 01/08/07 for Reactor.

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POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

1.5 RELATION BETWEEN DIFFERENT UNITS (CONVERSION OF UNITS):

Pressure: 1 bar = 105 Pa = 750 Torr = 14.5 psi = 1.02kp/cm2 1 Torr = 1.33 mbar = 0.133 kPa 1 kPa(kilo-Pascal) = 103 Pa = 10 mbar = 7.501 Torr(mm of mercury) 1 MPa = 106 Pa 1 atmosphere=0.1 MPa=1.02kp/cm2= 14.5 psi Force: 1 kp = 9.807 N Weight: 1 ton=1000 kg=2200 lbs Temperature: 5 * (° F - 32) 9 9 F = * (° C )+ 32 5

°C= °

Volume: 1m3= 1000 litres=260 US gallons=220 Imp gallons 1 litre =0.26 US Gallons 1 US Gallons = 3.78 litres 1 litre =0.22 Imp Gallons 1 Imp Gallons Benchmarks: 1-mm mercury (Torr) is about 1 millibar or 0.1 kPa 1 m3 of oil weights 0.9 tons –say 1 ton 1000 US gallons of oil weights 3.5 tons

18

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

1.6 CHECK LIST FOR ENERGISATION OF TRANSFORMER/ REACTOR 1.6.1 PRELIMINARY CHECKS

1. Release air at the high points, like oil communicating bushings, buchholz petcock, tank cover and the cooling devices including headers, radiators, pumps, expansion joints etc. of the transformer 2. Check the whole assembly for tightness and rectify where necessary. 3. Check the general appearance and retouch the paint work if need be. 4. Check that the valves are in the correct position : • Tank : valves closed and blanked • Cooling circuit : valves open • Conservator connection : valves open • By-pass: valves open or closed as the case may be. • On-load tap changer : valves open 5. Check that the silica gel in the breather is blue and that there is oil in the breather cup ( oil seal ) 6. Check the oil level in the main conservator and the conservator of on-load tap changer, bushing caps, flanges, turrets, expansion bellows as per manufacturer’s recommendation. Level should correspond to 35º C mark on oil level gauges for BHEL transformers 7. Check the bushings : • Oil level ( bushings fitted with sight-glasses) • Adjustment of spark-gaps /arcing horn –gaps, if provided • Conformity of connection to the lines ( no tensile stress on the terminal heads) • Bushing CT secondary terminals must be shorted and earthed, if not in use. • Neutral bushing effectively earthed 8. Check the on-load tap changer : • Conformity of the positions between the tap changer control cubicle and the tap changer head • Adjustment of the tap-changer control cubicle coupling • Electric and mechanical limit switches and protective relays • Step by step operation- local and remote electrical operation as well as manual operation and parallel operation, if any • Signalling of positions 9. Check the quality of the oil: • Draw off a sample from the bottom of the tank • Carry out DGA and oil parameters test ( i.e. BDV, Moisture content, resistivity & tan δ at 90°C and IFT ) before energisation

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POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

10 Extraneous materials like tools, earthing rods, pieces of clothes, waste etc. should be removed before energisation 1.6.2 CHECKING OF AUXILIARY AND PROTECTIVE CIRCUITS

1. Check temperature indicator readings and their calibrations 2. Check the setting and working of the mercury switches of winding and oil temperature indicators and presence of oil in the thermometer pockets • Follow the same procedure for the thermal replicas 3. Check the operation of the buchholz relay and the surge protective relay of the tap-changer : • Alarm and tripping • Protections and signals interlocked with these relays 4. Check the insulation of the auxiliary circuits in relation to the ground by 2 kV megger for 1 min. 5. Check the earthing of the tank and auxiliaries like cooler banks at two places. 6. Measure the supply voltages of the auxiliary circuits 7. Check the cooling system : • Check the direction of rotation of the pumps and fans • Check the working of the oil flow indicators • Check the setting of the thermal overload relays • Go through the starting up sequences, control and adjust, if necessary, the relay time delays 8. Check that there is efficient protection on the electric circuit supplying the accessories and tightness of all electrical connections 9. Check the heating and lighting in the cubicles 10. Check the differential protection, over-current protection, restricted earth fault protection, over-fluxing protection etc. are in service and settings are as per CC/Engg recommendations. After the inspection / tests are completed, the transformer may be energised from the incoming side on NO LOAD. The initial magnetising current at the time of switching will be very high, depending upon the particular moment in the cycle. The transformer should always be soaked for few hours under constant care i.e. keep it energised for twelve hours. Excessive vibrations of radiator parts etc. should be located and corrected. The transformer hum should be observed for any abnormality. After that it may be checked for gas collection. Should the gas prove to be inflammable, try to detect the cause which may probably be an internal fault? If the breaker trips on differential /REF, buchholz or any other protective device, the cause must be investigated thoroughly before re-energizing the transformer/ reactor. After successful charging, performance of transformer / rector should be checked under loading; OTI/WTI readings should be monitored for 24 hours and ensured that they are as per loading. DGA samples may be sent as per Standard practice (after 24 hrs of energisation, one week, 15 days, one month and three months after charging, thereafter as per normal frequency of 6 months). Loading data may be forwarded to CC/OS and manufacturer (if requested by them).

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POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

2.0

PRE-COMMISSIONING CHECKS /TESTS FOR TRANSFORMERS & REACTORS Once oil filling is completed, various pre-commissioning checks/ tests are performed to ensure the healthiness of the Transformer/ Reactor prior to its energisation. Various electrical tests to be performed and their significance are given below.

Sr. Name of Test No. Check point

/ Purpose of test/ check

1.

Core insulation tests

To check the insulation between Core (CC&CL) and Ground

2.

Operational Checks Operational Checks on cooler bank (pumps & Fans), Breathers on protection System (silica gel or drycol), MOG, temperature gauges (WTI/OTI), gas actuated relays (Buchholz, PRD, SPR etc.) and simulation test of protection system

3.

Insulation Resistance measurement

4

Capacitance and Tan Measurement of C1 & C2 Capacitance and Tanδ in UST mode. δ measurement of Changes in the normal capacitance of an insulator indicate bushings abnormal conditions such as the presence of moisture, layer short -circuits or open circuits in the capacitance network.

5.

Capacitance and Tan Dissipation factor/Loss factor and capacitance measurement of δ measurement of winding is carried out to ascertain the general condition of the windings ground and inter-winding insulation

6.

Turns ratio (Voltage To determine the turns ratio of transformers ratio) measurement

7.

Vector Group Polarity

8.

Winding resistance To check for any abnormalities due to loose connections, broken measurement strands and high contact resistance in tap changers

9.

Magnetic test

10.

Floating Neutral This test is conducted to ascertain possibility of short circuit in a point measurement winding.

11.

Exciting/Magnetising To locate defect in magnetic core structure, shifting of windings, current measurement failures in turn to turn insulation or problems in tap changers. These conditions change the effective reluctance of the magnetic circuit thus affecting the current required to establish flux in the core

Test reveals the condition of insulation (i.e. degree of dryness of (IR) paper insulation), presence of any foreign contaminants in oil and also any gross defect inside the transformer (Failure to remove the temporary transportation bracket on the live portion of tapchanger part)

& To determine the phase relationship and polarity of transformers

Balance This test is conducted only in three phase transformers to check the imbalance in the magnetic circuit

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POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

12.

Vibration To measure the vibrations of core /coil assembly in the tank of measurement of Oil- the reactor. Movement of the core-coil assembly and shielding immersed Reactor structure caused by the time–varying magnetic forces results in vibration of the tank and ancilliary equipment. These vibrations have detrimental effects such as excessive stress on the core-coil assembly

13.

Operational on OLTCs

14.

Tests/ Checks on To ascertain the healthiness of bushing current transformer at the Bushing Current time of erection Transformers (BCTs)

15.

Frequency Response To assess the mechanical integrity of the transformer. Analysis (FRA) Transformers while experiencing severity of short circuit current measurement looses its mechanical property by way of deformation of the winding or core.

16

On-Line moisture-in- To estimate the moisture (water) content in paper insulation of oil measurement transformers through on-line moisture measurement kit (DOMINO / VAISALA)

17

Dissolved Gas Oil sample for DGA to be drawn from transformer main tank Analysis (DGA) of before commissioning and after 24 hrs. of charging. Subsequently oil sample

18.

Thermovision Infra- A thermo vision Camera determines the temperature distribution red scanning (IR on the surface of the tank as well as in the vicinity of the Jumper thermography) connection to the bushing. The information obtained is useful in predicting the temperature profile within the inner surface of tank and is likely to provide approximate details of heating mechanism. Thermovision scanning of transformer to be done at least after 24 hrs. of loading and repeated after one week.

2.1

checks To ensure smooth & trouble free operation of OLTC during operation.

CORE INSULATION TEST This test is recommended before the unit is placed in service after receipt at site or following modifications to the transformer that could affect the integrity of its core insulation and at other times, when indicated by DGA (key gases being ethane and/or ethylene and possibly methane) or usually during a major inspection. For core-insulation to ground test, remove the cover of the terminal block, Disconnect the closing link that connects the two terminals CL-G. Apply 1.0 kV direct voltage between CL and CC + G (core grounding strap). The tank shall be grounded during the test. The insulation value after 1min. test-time should be minimum 10 MΩ for new transformer at the time of commissioning and minimum 1000kΩ for service aged transformers. Core insulation resistance is generally more than 100 MΩ for new assembled transformer when tested at factory. For Shunt Reactors, the measurement to be done by 3.5kV (DC) megger, Min value should be 1000kΩ.

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POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

2.2

• • • • •

• • • • 2.3

OPERATIONAL CHECKS ON PROTECTION SYSTEM Operational Checks on Breathers (Conventional Silcagel or Drycol as supplied with the transformers) Visual check of MOG of Main Conservator Marshalling Box & Kiosk Checks Valve Operational Checks Checks on Cooling System
Checks on cooling fans-rotation, speed & Control (Manual /temp /load) setting checks
Checks on Cooling pumps- rotation, vibration/noise, oil flow direction Checks on temperature Gauges (OTI/WTI-Calibration and Cooler Control , alarm & trip setting tests Checks on gas actuated (SPRs/ PRDs/ Buchholz) relays –Operational checks by simulation as well as shorting the respective contacts as applicable Checks on tightness of Terminal connectors - micro-ohm measurement of each connection Checks on Transformer protection (differential, SEF, Over-current & transformer stability tests etc.) INSULATION RESISTANCE (IR) MEASUREMENT

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. Before taking measurements the neutral should be disconnected from earth. Following table gives combinations of IR measurements for auto-transformer, three -winding transformer & Shunt Reactor. For Auto-transformer HV + IV to LV HV + IV to E LV to E

For 3 winding transformer HV + IV to LV HV + LV to IV HV + IV +LV to E

For

Shunt Reactor HV to E

Where HV-High voltage, IV-Intermediate voltage, LV-Low voltage/Tertiary voltage windings, E- Earth

Insulation resistance varies inversely with temperature and is generally corrected to a standard temperature (usually 20 °C) using table (Source: BHEL instruction Manual) as given below. Difference in temperatures ( °C) 10 20 30 40 50

Correction Factor (k) 1.65 2.6 4.2 6.6 10.5

Unless otherwise recommended by the manufacturer the following IR values as a thumb rule may be considered as the minimum satisfactory values at 30°C (one minute measurements) at the time of commissioning. 23

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

Rated Voltage class of Minimum desired IR value at 1 minute winding (Meg ohm) 11kV 300 MΩ 33kV 400 MΩ 66kV & above 500 MΩ The ratio of 60 second insulation resistance to 15 second insulation resistance value is called dielectric absorption coefficient or Index (DAI). For oil filled transformers with class A insulation, in reasonably dried condition the absorption coefficient at 30°C will be more than 1.3. Polarization Index Test is ratiometric test, insensitive to temperature variation and may be used to predict insulation system performance. The polarization index test is performed generally by taking mega ohm readings at the following intervals at a constant dc voltage: 1 min and then every minute up to 10 min. The polarization index is the ratio of the 10 min to the 1 min mega ohm readings. PI= R10 / R1 (dimensionless), Where PI is Polarisation Index and R is resistance The following are guidelines for evaluating transformer insulation using polarization index values: Polarization Index Less than 1 1.0-1.1 1.1-1.25 1.25-2.0 2.0 – 4.0 Above 4.0

Insulation Condition Dangerous Poor Questionable Fair Good Excellent

A PI of more than 1.25 and DAI of more than 1.3 are generally considered satisfactory for a transformer when the results of other low voltage tests are found in order. PI less than 1 calls for immediate corrective action. For bushings, an IR value of above 10,000 MΩ is considered satisfactory. 2.4

CAPACITANCE AND TAN δ MEASUREMENT OF BUSHINGS C& Tan δ measurement of bushings shall be done at 10kV with fully automatic test kit so as to have reliable test result. •

For 3-Ph auto-transformer, short together all 400kV, 220kV and Neutral (isolated from earth) Bushings. Also short all 33kV Bushings and earth the same. Measurement of C1 Capacitance and Tanδ δ: Connect the crocodile clip of the HV cable to the top terminal of the shorted HV/IV bushings. 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) of the bushing under test to the C & TAN δ KIT through a screened cable and earth the flange body. Repeat the test for all Bushings by changing only LV lead connection of the kit to test tap of the Bushing which is to be tested 24

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

• •

•

•

2.5

Measurement of C2 Capacitance and Tanδ δ : HV lead to be connected to the test tap of the bushing under test (if required additional crocodile type clip may be used) and LV of the kit to be connected to the ground. HV of the bushing is to be connected to the Guard terminal of the test kit. Test to be carried out in GSTg mode at 1.0kV. For measurement of 33kV Bushing Tan Delta, earth HV/IV Bushings (already shorted). Apply HV lead of the Test kit to shorted 33kV Bushings and connect LV lead of the test kit to Test tap of the Bushing under test. Measurements shall be made at similar conditions as that of a previous measurement. The oil-paper insulation combination of bushings exhibit fairly constant tan delta over a wide range of operating temperature. Hence, effort is to be made for testing at temperature near to previous test and Correction factor need not be applied. Do not test a bushing (new or spare) while it is in its wood shipping crate, or while it is lying on wood. Wood is not as good an insulator as porcelain and will cause the readings to be inaccurate. Keep the test results as a baseline record to compare with future tests. It is to be ensured that Test Tap points are earthed immediately after carrying out the measurements for that particular Bushing and earthing of test tap to be ensured by carrying out continuity test. CAPACITANCE AND TAN δ MEASUREMENT OF WINDINGS The combination for C & tanδ measurement of winding is same as that of measurement of IR value. The summery of probable combination is given below:

AutoTransformer (Two winding) HV HV + IV to LV HV HV + IV to E TV LV to E

Test Mode

Shunt TestFo 3 winding Reactor Mode Transformer

USTHV HV to E GSTg GSTg

GST

HV to LV1 HV to LV2 LV1 to LV2 HV to Ground LV1 to Ground LV2 to Ground

Test Mode UST UST UST GSTg GSTg GSTg

Table 2: Combination for C & tanδ measurement of winding for various transformers/ Shunt Reactor

•

•

Ensure that test specimen is isolated from other equipments. Removal of Jumpers from Bushings is Pre-Requisite for C & Tan δ Measurement of Windings. For ICTs (Auto-Transformers): Shorting of all three phase Bushings (400kV&220kV) and neutral to be done. In case of single phase, 400kV, 220kV and neutral Bushings to be shorted Capacitance and Tan δ measurement of windings should be done in following combinations: Test No. 1.

Winding Combination HV-IV/LV

Test mode UST

Cap Symbol CHL

Test lead Connection

Remarks

HV lead of test kit to HV/IV bushings of transformer LV lead of test kit to LV

25

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

2. 3. 4

5 6.

•

•

2.6

HV-IV/ LV+G HV-IV / LV with Guard HV-IV/LV

GST GSTg

CHL +C HG C HG

UST

CHL

bushing of transformer -do- do-

LV to Guarded

be

LV lead of test kit to HV/IV bushings of transformer HV lead of test kit to LV bushing of transformer -do-

LV/ HV-IV GST CHL +C LG +G LV/ HV-IV GSTg C LG -doHV to with Guard Guarded Table 3:.Winding combination for C & tan δ measurement for auto transformer

be

Measurement inter-check can be done by calculating C1= C2-C3 & C4= C5- C6 & DF1=C2DF2-C3DF3 / C2-C3= C4DF4-C5DF5 / C4-C5 Where C stands for capacitance and DF for dissipation factor or tan δ and attached suffix (1…6) denotes the sr. no. of test in above table. For Reactors: All 400kV and neutral Bushings to be shorted. HV of the test kit to be connected to shorted Bushings and LV of the test kit to be connected to Earth connection. Measure the Capacitance and tan Delta in GST mode. Neutral connection with earth/ NGR to be isolated before the test. TURNS RATIO (VOLTAGE RATIO) MEASUREMENT Ratio measurements must be made on all taps to confirm the proper alignment and operation of the tap changers. The test should preferably be performed by Transformer turns ratio (TTR) meter. They have a hand crank power supply, with the voltages commonly used being very low, such as 8-10 V and 50-60 Hz, so that the test may be performed on a transformer even when the oil is removed. Two windings on one phase of a transformer are connected to the instrument, and the internal bridge elements are varied to produce a null indication on the detector, with exciting current also being measured in most cases. Results of the transformation turns or voltage ratio are absolute, and may be compared with the specified values measured during factory testing. The turns ratio tolerance should be within 0.5 % of the nameplate specifications. For three phase Y connected winding this tolerance applies to phase to neutral voltage. If the phase-to-neutral voltage is not explicitly indicated in the nameplate, then the rated phase-to-neutral voltage should be calculated by dividing the phase-to-phase voltage by √3. Open turns in the excited winding will be indicated very low exciting current and no output voltage. Open turns in the output winding will be indicated by normal levels of exciting current, but no or very low levels of unstable output voltage. The turns ratio test also detects high-resistance connections in the lead circuitry or high contact resistance in tap changers by higher excitation current and a difficulty in balancing the bridge 26

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

2.7

VECTOR GROUP & POLARITY The test shall be conducted with three phase supply (415V, 3 phase supply) and voltmeters. By the measured voltage data it should ensure that the desired conditions of vector group and polarity are fulfilled

Example for D Y1

CONNECT 1U to 2U MEASURE 1W-2V, 1W-2W, 1U-2W, 1V-2V, 1V-2W VOLTAGE RELATION

1U

1W-2V= 1W-2W

2U

1W-2V< 1U-1W

1V 2W

2V

1V-2V