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Home / Technical Articles / Testing and commissioning of HV power transformers, circuit breakers, CTs and VTs

Testing and commissioning procedures

As you already know, substations are the points for controlling the supply of power on different routes by means of various equipment such as power transformers, circuit breakers, CTs/VTs, compensating equipment, isolators, etc. The various circuits are joined together through these components to busbar systems at the substations.

Testing and commissioning of HV power transformers, circuit breakers, CTs and VTs
Testing and commissioning of HV power transformers, circuit breakers, CTs and VTs

This technical article provides guidance to substation personnel in carrying out testing and commissioning oF high voltage power transformers, circuit breakers, current and voltage instrument transformers.

One of the prime requirements of a good substation is that it MUST have regular maintenance. Testing and commissioning procedures are crucial and ensure safety of the operation and maintenance personnel.

Table of content:

  1. Power transformers
    1. Pre-commissioning tests on transformer
    2. Testing of auxiliary protective devices
    3. Testing OLTC (On Load Tap Changer)
    4. Tests for reference (Signature)
    5. Tests on main protective relays
  2. Circuit breakers
  3. Current transformers (CTs)
  4. Voltage transformers (CVTs / IVTs)

1. Power transformers

After installation of transformer on the plinth, accessories viz. radiators, headers, bushings, air blowers, oil pumps with connecting pipes, OLTC drive mechanism and the connecting pipes to diverter switch compartments, marshalling box, etc. have to be mounted on the transformer as per the manufacturers’ drawings.

Oil should be admitted into the transformer from its bottom valve and suction side to be connected to the top valve to evacuate nitrogen/dry air inside the transformer as the oil fills from the bottom.

It is preferable to fill the oil received from the works (in barrels) into an oil tanker of sufficient capacity and filter this oil separately. This helps in removing moisture absorbed in the oil during transport and storage. Fairly moisture free filtered oil now can be admitted from the oil tanker to the transformer.

The following check list provides various checks to be conducted for transformer assembly.

1Availability of erection drawings and literature at site.Yes/No
2Whether all components available at site in good condition.Yes/No
3Rinsed all the piping, radiators, conservator etc. with 60kV tested oil and blocked with dummy plates.Yes/No
4Filtered the oil for 60kV BDV and recorded in the erection register.Yes/No
5Measured the IR values of HV, MV condenser type bushings(with a 5kV Megger testing device) and observed more than 5000 MΩ after removing coverings, wrappers etc and cleaning.Yes/No
6Washed with hot oil (in case of other bushings of plain porcelain type).Yes/No
7Assembled the bushing after fixing corona shield and removing links and bends in pull through leads, on the turret at proper incline.Yes/No
8Complete assembly of HV, LV, Ter. & Neutral bushings done correctly.Yes/No
9Completed all piping work, conservator, explosion vent, equalizer pipes etc. as per the drawing and filled with oil , Bucholz relay checked.Yes/No
10Filled radiator after washing individually and ensuring removal of blanking plates and free movement of butterfly valves both top and bottom.Yes/No
11Measured IR values with temperature after filling of oil and compared with factory test value and recorded in the register.Yes/No
12For drying out of Transformer:
Applied proper lagging around the transformer. Fire fighting equipment kept at site.Yes/No
Filter machine cleaned and filled with transformer oil.Yes/No
Filter connected with outlet into the conservator and inlet from the bottom tank.Yes/No
Filter heaters switched on and the filter temperature maintained less than 60°C and filter vacuum maintained at 755 mm of mercury.Yes/No
Dehydration process for 7 days maintained oil temperature 60°C in the transformer (thermometer pockets filled with oil).Yes/No
13Filled the radiators with 60kV BDV tested oil. Opened the bottom butterfly valve provided between main tank and radiators after opening top air release valve to communicate with main tank.Yes/No
14All gases released from different release points in the order of ascending heights.Yes/No
15HV, LV, Tertiary Neutral earthing provided. Body earthing at two sides, earthing of DM and FCC provided.Yes/No
16Petroleum jelly applied in clamps and connectors in the transformer bushing studs and checked the tightness.Yes/No
17Blue silica gel crystals of 2.5 to 4mm size filled in the breather. Breather filled with oil after removing bottom cup, transit protection cover, cork packing etc.Yes/No
18Removed blanking plates on explosion vent pipe and fixed diaphragm.Yes/No
19In case of transformers provided with thermo-siphon filter and air-cell (pronol bag) breathing arrangement checked up as per manufacturers recommendations.Yes/No
20All air release points and other points thoroughly checked to ensure that there is no oil leakage.Yes/No
21Connecting test taps of all 132,220,400kV condenser bushings.Yes/No
22Oil level in main& OLTC conservators upto the mark.Yes/No
23Filling of oil in oil packets of OTI and WTI.Yes/No
24Welding of wheel stoppers to the rails.Yes/No

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1.1 Pre-commissioning Tests On Transformer

After installation and assembly at site, the following pre-commissioning tests on transformer should be conducted. These are basically low voltage tests.

  1. Ratio
  2. Polarity and vector group
  3. Measurement of magnetizing current and no load losses (3-phase 415V to be supplied on low voltage side of the transformer)
  4. Measurement of short circuit current, load losses and impedance at all taps (3-phase 415V supply to be given on the high voltage side and the other side shorted)
  5. Magnetic balance test
  6. Measurement of winding resistance at all taps
  7. Insulation Resistance and polarization index

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1.2 Testing of Auxiliary Protective devices

Checking of operation of alarms and trips of following auxiliary relays:

  1. High temperature settings for oil and winding temperatures
  2. Bucholz relay and OLTC oil surge relay
  3. Pressure relief device
  4. Magnetic oil gause (MOG) of conservator
Checking for running /stopping of cooler fans and/or oil circulating pumps in groups at the set temperatures/pressures. Conduct all checks and tests on turret mounted CTs as indicated under CTs.

You might find interesting reading an article about the most important alarms coming from a substation.

6 alarms coming from a substation transformer you MUST take very seriously

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1.3 Testing OLTC (On Load Tap Changer)

The following checks should be performed for OLTC:

  1. Checking for manual and electrical operation (both local & remote and lower & raise) of On Load Tap Changer and for continuity between the operations.
  2. Check for operation of limit switches at the extreme tap positions and mechanical interlock between manual and electrical operation.
  3. Check for Master/Follower and out of step relay operation of transformers if parallel operation is intended.

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1.4 Tests for reference (Signature)

The following tests should be done before commissioning and the results compared with the factory test results. These results should be kept as reference values for comparison with the future test results of these tests while conducting, as a part of O&M later.

  1. Measurement of Capacitance and tanδ of windings, Condenser Bushings
  2. Dissolved Gas analysis
  3. Furan Analysis (to measure degree of polymerization)
  4. Frequency Response Analysis
  5. Transient Recovery Voltage (TRV) Measurement
  6. Oil sample test for Dielectric strength, acidity, specific resistance, moisture content and tanδ

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1.5 Tests on main protective relays

The following protective relays connected to the transformer HV/LV/Tertiary sides shall be tested for proper operation duly adopting the settings communicated by the power systems:

  1. Over current / Earth fault relays (directional/non-directional).
  2. Operation of the instantaneous elements for the set current value.
  3. Differential relays with bias and restraint features including through current stability.
  4. Restricted Earth fault relays.
  5. Local Breaker Backup (LBB) relays.
  6. Over flux relay.
  7. Under and Over voltage relays.

Testing procedures for protective relays are not described in this article, but you can read about this subject in details here.

Commissioning tests of protection relays at site (before set to work)

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2. Circuit breakers

Physical checks for any damages to components of the circuit breaker should be made. Tightness of connections at all jumpers, flanges, joints in pipes, valves etc. should be checked. Check for oil/ gas/ air leakages at concerned valves/pipes and all oil seals, gaskets should be made. Checking and tightening of all foundation bolts should be done.

The following pre-commissioning tests/checks shall be conducted:

  1. Measurement of insulation resistance – Phase to earth, Between phases, across contacts with breaker open.
  2. Measurement of capacitance and tanδ of voltage grading capacitors. The grading capacitors are oil filled paper capacitor type.
  3. Purity and dew point measurement of SF6 gas at rated pressure of the breaker and at atmospheric pressure.
  4. In ABCBs Dew point of air to be measured. Air serves as insulation as well as arc quenching medium.
  5. Measurement of CB close, open, close-open, timings with CB Operation analyzer.
  6. Measurement of Dynamic contact Resistance and contact travel.
  7. Operation of Pole Discrepancy relay:
    To measure the difference in closing and opening times of different poles of CB where the poles are individually operated (220kV and above). Simultaneous closing of all the three poles in case of a three pole/ gang operated breaker (132kV and below).
  8. Check for proper working of carrier inter trip and auto-reclose features (for feeder breakers).
  9. Check for capacitor tripping device where provided.
  10. Measurement of resistance and current drawn by close coil and trip coils I and II.
  11. Checking of interconnecting cables to bay marshalling box.
  12. Checking of operating mechanism.
  13. Checking of operation counter and all mechanical indications.
  14. Checking of all Non-return valves/ Safety valves.
  15. Checking of local/remote breaker operations.

Check for correct operation of following operational lockouts at set pressures:

  1. SF6 gas pressure- alarm and lockout at different set pressures.
  2. Pneumatic operating system – Automatic start/stop of air compressor at set pressures.
  3. CB closing lockout.
  4. CB operational lockout.
  5. CB auto-reclose lockout.

HV Circuit Breaker Testing (VIDEO)

Circuit breakers allow the flow of energy to be controlled by safely switching currents on and off at all voltage levels of the energy grid. The interrupter and the mechanical drive are the main components that are subject to wear and aging.

In order to ensure the proper operation of a circuit breaker throughout its lifetime, diagnostic tests are performed, such as resistance, timing, minimum pick-up, travel, and power factor.

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3. Current transformers (CTs)

Physical observation of the CTs for proper installation with respect to the drawings and substation layout should be made. Tightness of bolts of support structures to the foundation and CT base mounting to the support structure shall be ensured.

Oil leakages in case of oil filled CTs and SF6 gas leakages/rated pressure of gas in case of SF6 gas filled CTs to be observed. Bellows provided for taking care of oil expansion shall be checked for proper functioning.

The following checks and electrical tests shall be conducted:

    1. Checking of tightness of all electrical connections including connections made in the marshalling box.
    2. Checking for tightness of CT secondary terminals and checking healthiness of secondary terminal bushings.
    3. Checking for short circuiting the secondary terminals of such CT secondary windings which are not required. Shorting should be done at secondary terminals in the CT secondary terminal box.
    4. Checking for earthing of common point of three phases of each secondary winding at one point only, preferably at the marshalling box.
    5. Measurement of IR values including Di-electric absorption ratio and Polarization index of primary to body and primary to secondary with a 5 kV Megger testing device.
    6. Measurement of IR values between secondary winding of each core to body and between different secondary windings of the CT with a 0.5 kV Megger testing device.
  1. Measurement of capacitance and tan∂ of the CT.
  2. Measurement of secondary winding resistance of each secondary winding.
  3. Testing for Magnetization characteristics (knee point voltage) of CT cores.
  4. Ratio test for approved CT ratios in all the cores including complete wiring in the secondary side to ensure correct ratio and continuity of the wiring.
  5. Check for polarity of CTs, to ensure correct directional sensitivity of metering and protection system.

Each CT should be individually tested to verify the polarity markings on primary and secondary windings. Test circuit is given below.

CT polarity test
Figure 1 – CT polarity test

Ammeter “A” is a robust moving coil permanent magnet center zero type instrument. Primary winding is given DC supply momentarily from a low voltage battery through a push button. When push button is pressed with the above markings on CT and ammeter, the ammeter should give a +ve flick, indicating correct polarity.

Multi meters and Digital ammeters can also be used. CT secondary connections to transformers, busbars, and feeders:

3.1 Busbar CT’s secondary connections

Busbar CTs secondary connections
Busbar CTs secondary connections

3.2 Transformer CT’s secondary connections

Transformer CT's secondary connections
Transformer CT’s secondary connections

3.3 Line CT’s secondary connections

Line CT's secondary connections
Line CT’s secondary connections

Where CT secondary connections are to be made in star, the star point should be made as follows.

Irrespective of the polarity on the primary side, star point on secondary side shall be on line side for feeders and on the equipment side for the bus bars, and transformers as shown in the figures above. In case of a feeder protection, if P1 is towards the bus, then S2s’ are to be shorted and if P2 is towards bus then S1s’ are to be shorted.

Testing protection current transformers with CT Analyzer (VIDEO)

Important parameters of a protection current transformer are its ratio, its excitation curve, the knee point, its winding resistance, and the burden.

CT Analyzer determines all relevant current transformer data during a test duration of approximately one minute: it measures the burden of the secondary circuit, the winding resistance and the excitation curve, calculates the accuracy limiting factor for rated and actual burden, determines its ratio, amplitude and phase angle accuracy, and verifies the polarity.

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4. Voltage transformers (CVTs / IVTs)

Visual observation for any cracks/breakages must be made. Mounting of capacitor stacks, electromagnetic units, and terminal box shall be checked for correctness as per the drawing.

Tightness of foundation bolts connecting the support structures to the foundation and the bolts connecting base of the VTs/CVTs to the supporting structures should be ensured.

The following checks shall be made:

  1. Check for tightness of all electrical connections on primary and secondary sides of CVTS/VTs including connections in the marshalling box.
  2. Check for oil leakages and oil level.
  3. Ratio test –voltage in all the secondary windings should be measured.
  4. Insulation resistance of primary to body and primary to secondary windings to be measured by a 5 kV Megger testing device.
  5. Insulation resistance of secondary to body and between secondary windings to be measure by a0.5 kV Megger testing device.
  6. Measurement of capacitance and tan ∂ of CVTs.
  7. Checking of HF point in case of CVTs. Caution given on the name plate by the manufacturer should be followed.
  8. Check for adopting the approved ratios and connections for different secondary windings.

Testing voltage transformers (VIDEO)

Voltage transformers (VTs) are used around the clock throughout the energy grid, and their life span can reach up to 40 years. During that time, the effects of aging, regular operation, network faults, or external influences may affect the integrity of a voltage transformer.

Utilities perform various tests during commissioning and maintenance to find any irregularities that have been caused by transportation, installation or daily operation.

Go back to Content Table ↑

Source: Technical reference guide by APTRANSCO

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


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  3. Ngor Rasmei
    Dec 29, 2020

    Could you share testing and commissioning of LV?

    Nov 04, 2019

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    Oct 07, 2019

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    • Rene
      Jun 29, 2020

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  6. Armando García
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  8. Very well written and great content
    Sep 13, 2019

    I would appreciate the complete article in a pdf format, with the videos in a downloadable link, if at all possible?

    I wish to share this with my technicians when we do training sessions.

  9. S.Soma Sekhar
    Sep 13, 2019

    Thanks for the information of testing and commissioning details .it will help for the all the students

  10. S.Soma Sekhar
    Sep 13, 2019

    Protection system is the very important in the entire electrical systems

  11. Mariyappa
    Sep 12, 2019

    I have Diploma in Electrical Background.I don’t have experience about the Testing & Commission.

    Sep 12, 2019

    Thanks so much for this helpful informations to the testing/commissioning Engrs. More grace to your elbow.

    • Debasish Halder
      Sep 13, 2019

      Sir plz send me your transformer testing PDF from this Gmail I’d

  13. Abrar
    Sep 12, 2019

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  14. Abrar
    Sep 12, 2019

    Explained in short summary but clear.

  15. Vidyadhar R. Kulkarni
    Sep 12, 2019

    Sir, It was nice information. I did my engineering in the year 1988 .I am fond of reading electrical and electronics engineering related articles.Thanks Sir.

  16. Subrat Ghosh
    Sep 12, 2019


    Thanks for sharing hands on details. This can only be done by a person , who has put his efforts in field job. Very practical advice. I am circulating to all my colleagues who are engaged in commissioning.

    Keep on sharing good stuff. Kudos

    Sep 11, 2019

    Very nice information and helpful for people working in HV substations.

  18. Satyender kumar
    Sep 11, 2019

    Sir please update all pre commissioning tests results table of each equipment like CTS pts,transformers , relays,cb

  19. Mukund Pawar
    Sep 11, 2019

    Portal very much good and informative

  20. Georgios Kafes
    Sep 11, 2019

    Thanks for the site. It is really helpful.

    • Edvard
      Sep 11, 2019

      You’re welcome Georgios, thank you.

    • Upendra kumar Gupta
      Sep 12, 2019

      B.A +ITI and 4 year ka experience hai mall ht and Lt panel

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