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Home / Technical Articles / Pre-commissioning checks and tests of high voltage current transformers (CTs)

Checks and tests applicable for CTs

Once erection is completed, various pre-commissioning checks and tests are performed to ensure the healthiness of the switchyard equipment prior to their energization. This technical article explains various major electrical tests applicable for current transformers (CTs).

Pre-commissioning checks and tests of high voltage current transformers (CTs)
Pre-commissioning checks and tests of high voltage current transformers (CTs) - photo credit: OMICRON

Let’s name these tests to be performed on CTs along with their brief description. Later, they will be explained in details.

Table of content:

No.Name of test / check pointPurpose of test / check
1.Polarity test for CTTo ascertain whether the polarity markings are correct or not as per drawing.
2.Magnetization characteristics of CTTo prove that the turns of CTs secondary windings are not short circuited and to check healthiness of CT cores. The magnetizing currents at KPV (Knee point voltage) should be less than the specified value. The ratio of secondary and primary voltage should also be measured.
3.Ratio test for CTThe ratio errors of the primary to the secondary currents should within specified ratio errors.
4.IR measurement of CT (Primary & Secondary windings)Changes in the normal insulation resistance (IR) value of CT indicate abnormal conditions such as presence of moisture, dirt, dust, crack in insulator of CT and degradation of insulation.
5.DGA test of CT oilThis test should be conducted after 30 days of commissioning. The purpose is to identify evolving faults in the CT and DGA values should be comparable with factory values (if available).


All measures and precautions should be undertaken to prevent occurrence of unsafe acts. All the personnel involved should be thoroughly apprised about the safe procedures to be adopted while performing various activities including carrying out tests in the switchyard.

Adequate fire-fighting system as per procedures and their healthiness is to be ensured before charging. Warning signs and safety barriers should be positioned.

1. Polarity Test For CT

A centre zero voltmeter is connected across CT secondary. A 1.5 Volt battery is touched to primary of CT. The deflection of pointer should be similar in case of each CT core. At any instant current entering the primary from P1 the current should leave secondary from the terminal marked S1.

A setup shown in the Figure 1 can show whether the polarity markings are correct or not.

CT polarity test setup
CT polarity test setup

When the key is pressed, current enters the primary through terminal P1, the voltmeter connected as shown, should read positive. A general arrangement of polarity test setup is indicated in Figure 2.

General arrangement of polarity test setup
Figure 2 – General arrangement of polarity test setup

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2. Magnetization Characteristics Of CTs

Important Precautions

  1. There should be no joints in testing leads / cables.
  2. It should be ensured that whole testing equipment along with testing procedures are available at testing site. Testing must be carried out in presence of testing personnel only.

Test Equipment

Voltage source of 5 kV, voltmeter of range 0 to 5 kV, ammeter, of range 0 to 500 Amps, testing leads/cables etc.

Test Procedure

Make connections as per diagram shown below (Figure 3). After making proper connections, applied voltage is increased from zero to rated Knee Point Voltage in steps of 25%, 50%, 75% and 100%. Measure the current drawn by the CT secondary core at respective applied voltages and record the test results.

Connection diagram for measuring magnetization characteristics of CTs
Figure 3 – Connection diagram for measuring magnetization characteristics of CTs

Knee Point Voltage is normally defined as the voltage at which 10% increase in the applied voltage causes 30 to 50% increase in secondary current.

The magnetization current at rated Knee Point Voltage should not be more than the specified/designed value. A curve can be drawn between applied voltage and magnetizing current.

Typically, the curve drawn should be like the one given below in Figure 4.

Typical magnetization curve of current transformer
Figure 4 – Typical magnetization curve of current transformer

From the curve it can be implied that up to rated KPV (Knee Point Voltage), the VI curve should be almost a straight line.

However, if this line is not linear, this indicates that the magnetizing characteristics are not desirable. If the slop of the curve starts increasing, it indicates that magnetizing induction becomes low and total primary current is utilized in exciting the core alone. Consequently, out put of CT secondary disappears.

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3. Ratio Test For Current Transformer

Ratio and polarity tests can be carried out using either primary current injection or secondary voltage injection. For multiple ratio CT’s, tests should be carried out on all ratios as marked on the manufacturer’s nameplate. For primary current injection, a current is applied to the primary side of the CT and corresponding secondary current measurements are made at each tapping of each core.

The ratio check has to be carried out as indicated in Figure 5 below.

Connection scheme for ratio test of current transformer
Figure 5 – Connection scheme for ratio test of current transformer

It is customary to conduct this in conjunction with the primary injection test. Current is passed through the primary circuit with the secondary winding circuit to its normal circuit load. The ratio of the primary to the secondary currents should approximate closely to that stamped under CT identification plate.

Alternatively, ratio test is to be conducted as per the following method as shown in Figure 6.

Alternative connection for Ct ratio test
Figure 6 – Alternative connection for Ct ratio test

Apply voltage from secondary of the CT and measure voltage in primary winding. Increase voltage in secondary up to rated KPV/ ISF and by recording Primary Voltage, compute ratio of V1/ V2. The ratio should match with the specified value.

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4. Insulation Resistance (IR) Measurement Of Current Transformer

Important Precautions

  1. There should be no joints in testing cables.
  2. Test leads should not touch any live part.
  3. Megger body should be earthed (if separate terminal is provided).
  4. Surface/terminals should be cleaned.
  5. IR measurement should be carried out preferably in dry and sunny weather.
  6. Never connect the test set to energized equipment.
  7. The ground terminal must be connected first and removed at last.
  8. High voltage plugs should be free from moisture during installation and operation.
  9. If oil traces are found on the surface of CT, the same should be cleaned by Methyl Alcohol only. Petrol or diesel should never be used.
  10. It should be ensured that whole testing equipment along with testing procedures are available at testing site. Testing must be carried out in presence of testing engineer only.
  11. After testing with high voltage, test terminals must be grounded before being touched by any personnel.
  12. Test leads should be properly screened/ shielded.

Connect the Megger as per Figure 7 given below. Connect the HV terminal to the Primary terminal of CT by using crocodile clip for firm grip

Typical Arrangement for insulation resistance (IR) measurement
Figure 7 – Typical Arrangement for insulation resistance (IR) measurement

Carry out the measurement as per standard procedure given by the kit supplier. A test voltage as specified is applied as per the above connections and successive readings are taken. Values of IR should be recorded after 15 seconds, 60 seconds and 600 seconds.

Ambient temperature and weather conditions are to be recorded.

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5. DGA Test of CT Oil

Current transformers are vital components in both the transmission and distribution of electrical power. The early detection of incipient faults in transformers is extremely cost effective by reducing unplanned outages. The most sensitive and reliable technique used for evaluating the health of oil filled electrical equipment is dissolved gas analysis (DGA).

Insulating oils under abnormal electrical or thermal stresses break down to liberate small quantities of gases.The qualitative composition of the breakdown gases is dependent upon the type of fault. By means of dissolved gas analysis (DGA), it is possible to distinguish faults such as partial discharge (corona), overheating (pyrolysis) and arcing in a great variety of oil-filled equipment.

Information from the analysis of gasses dissolved in insulating oils is valuable in a preventative maintenance program. A number of samples must be taken over a period of time for developing trends.

Oil samples to be collected in 300 ml bottles or similar and to be sent for testing. Test results should be comparable to factory values.

Data from DGA can provide:

  1. Advance warning of developing faults
  2. A means for conveniently scheduling repairs
  3. Monitor the rate of fault development

NOTE – A sudden large release of gas will not dissolve in the oil and this will cause the Buchholtz relay to activate.

Transformer Oil Sampling – Introduction

As always, safety first. We start our Transformer Oil Sampling series with a number of highlights on health, environmental and safety considerations when performing these tasks. Please these are only guidelines and do not replace adequate competency and skills.

Always follow the relevant company and legislative requirements for your particular work context.

Transformer Oil Sampling – Bottle Sample

There are some sampling techniques better suited for certain types of oil tests than others. The techniques described in this video should be sufficient to sample oil when common chemical and physical properties are analysed, such as Breakdown Voltage, Interfacial Tension, Acidity (or Neutralization Number), Colour and Visual Inspection.

Transformer Oil Sampling Training – Glass Syringe

A sampling technique proven to produce repeatable results when testing the Dissolved Gas content or Moisture (Water) Content in the oil is by sampling with a Glass Syringe.

In this video you’ll learn how to execute this technique.

Transformer Oil Sampling Training – The Buchholz Relay

In this video, the fourth and final of the Oil Sampling series, you’ll learn how to extract a gas and oil sample from a Buchholz gas accumulation relay through a sampling device.

Go back to Content Table ↑


  1. Pre-Commissioning Procedures & Formats for Switchyard Equipment by POWER GRID CORPORATION OF INDIA LTD
  2. A guide to transformer oil analysis by I.A.R. GRAY Transformer Chemistry Services

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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.


  1. Tibor
    Mar 27, 2023

    Please keep in mind that checking polarity using “flick test” will magnetise the core and could lead to nuisance operation of protection. Polarity check by sawtooth or ac source is a way to mitigate this. At the very least, user should demagnetise the core following this test

  2. Robert Todd
    Dec 02, 2021

    For the DGA Test for CT Oil it mentions that several samples should be taken to see a trend. Is there any specific time frames or periodic time when samples should be taken?

  3. Manuel Bolotinha
    Apr 30, 2021

    It would be interesting to know the standards used in these checks and tests

  4. Phunu Jyoti Dutta
    Sep 30, 2020

    It is very much informative to all Electrical Engineer.

  5. Abhijeet
    Sep 09, 2020

    Very nice and informative article….!

  6. Nwala Stanley
    Sep 08, 2020

    Pls I need the video for this program,
    I love it,
    Any other causes on this topic?
    I need it

  7. Umar
    Jun 27, 2020

    i would like to know that if Imag value is increases than the specific value at site test i.e. Imag= 0.089 and the specified value is <0.050 in a TPS class and CT analyser Omicron as per the standard is giving CT assessment Pass, what should be done? either it is acceptable or not? and what could be the reasons for increase in Imag?

  8. Cristian B
    Jun 20, 2020

    What about arc test?

  9. k s meena
    Oct 05, 2019

    Very useful Portal , my knowledge increase in electrical engineering

  10. Sonu Pankaj
    Oct 04, 2019

    Buchholtz relay are active when interturn fault occurs but some time due to excessive heating air bubbles form which give also a alarm for operator to check

  11. Andrews Antwi-Gyan
    Oct 03, 2019

    Great explanatexplanations

  12. John Prosper Abofra
    Oct 03, 2019


  13. Sushil kumar Singh Singh
    Oct 03, 2019

    I am requests sir electrical engineering

  14. Ramesh Upadhya Dr.
    Oct 02, 2019

    As I remember –
    Knee point has Imag increase of not more than 50% for 10% increase in exciting voltage . Your graph shows Imag. of 30% at knee point .
    Is there any amendment for knee point definition.
    Knee point is normally considered for class PS or equivalent class X(or PX) . Is it required in other than class PS ?

  15. Shyamkant Sitaram Pawar
    Oct 02, 2019

    “C &TanDelta Measurement of CT,PT is important test. It should be carried out before Commissioning. All the utilities introduced said test as precommissioning test.

  16. Kumaravelavan
    Oct 02, 2019


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