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CURRENT TRANSFORMERS ROUTINE TEST PROCEDURE

CURRENT TRANSFORMERS ROUTINE TEST PROCEDURE
C57.13.5 Test procedures for current transformers.

Inter-turn overvoltage test
The test shall be performed in accordance with one of the following procedures. If there is no agreement between the manufacturer and user, the choice of the procedure shall be left to the manufacturer. If secondary protective devices are provided, these devices shall be installed for the inter-turn overvoltage test.

Procedure A
With the secondary windings open-circuited (or connected to a high impedance device which reads the peak
voltage), a substantially sinusoidal current shall be applied to the primary winding. The test current shall be increased until one of the following conditions is reached:

   a) The r.m.s. value of the applied current is equal to the maximum primary current corresponding to the continuous thermal current rating factor (RF), or
   b) The peak value of the secondary induced voltage is equal to:
      1) 280 V for metering rated current transformers, or
      2) 2.8 times the secondary terminal voltage rating for relaying rated current transformers

The test shall be maintained for 60 s.

Procedure B
With the primary winding open-circuited, a test voltage (at an elevated frequency not exceeding 400 Hz) shall be impressed to the terminals of each secondary winding. Both the test voltage and frequency shall be increased until one of the following conditions has been reached:

   a) The peak value of the applied secondary test voltage is equal to:
      1) 280 V for metering rated current transformers, or
      2) 2.8 times the secondary terminal voltage rating for relaying rated current transformers
Or,
   b) The r.m.s. value of the secondary current is equal to the continuous thermal current rating factor (RF)

The test duration shall be in accordance with 4.1.8.

At the frequency of 400 Hz, if the voltage achieved with the secondary current value corresponding to the continuous thermal current RF is lower than the value provided in a) above, then the voltage obtained shall be regarded as the test voltage.

The gas pressure of the gas-filled transformer may be at any setting equal to or less than the pressure given in Clause 5 for the test.

The transformer shall be considered as having met the requirements if no external or internal disruptive discharge or collapse of voltage is observed, and, if provided, the secondary protective devices remain inoperative during the test.

Accuracy test for the metering rated current transformers
  • The test shall be performed in accordance with 6.11 and 8.1 of IEEE Std C57.13-1993.
  • The calibration of the test system provided in 4.3 shall apply.
  • The current transformer shall be demagnetized in accordance with 8.2 of IEEE Std C57.13-1993 before the accuracy test.
  • The gas pressure of the gas-filled transformer may be at any setting for the test.
  • The transformer shall be considered as having met the requirements if the performance is within the limits of the accuracy class.

Resistance measurement of relaying rated secondary windings
The resistance of relaying rated windings including tap portions shall be measured in accordance with 8.5 of
IEEE Std C57.13-1993. The measured value shall be corrected to 75 °C.

The gas pressure of the gas-filled transformer may be at any setting for the test.

The resistance data obtained shall be compared with the design values to ensure proper construction.

Performance characteristics of relaying rated current transformers
The test comprises the following:

   a) With the primary winding open-circuited as shown in Figure 5, a substantially sinusoidal voltage equal to the secondary terminal voltage rating, corrected with the additional voltage due to the winding resistance voltage drop, shall be applied to the full secondary winding. The excitation current expressed as a percentage of the 20 times the rated secondary current shall be recorded.
   b) The short-circuit impedance of the lowest tapped portion of the secondary winding shall be measured in accordance with item a) of 8.3.1.2 of IEEE Std C57.13-1993.
   c) The turns-ratios of the secondary winding and the tapped portions to the primary winding shall be measured.

The gas pressure of the gas-filled transformer may be at any setting for the test.

The transformer shall be considered as having acceptable performance characteristics if:

   1) The percent excitation current is not higher than 10%,
   2) The short-circuit impedance is not higher than 1.05 times the resistance of the lowest tapped portion of the measured secondary winding,19 and
   3) The turns-ratios are in accordance with the marked ratios.

source: C57.13.5 IEEE Trial-Use Standard of Performance and Test Requirements for Instrument Transformers of a Nominal System Voltage of 115 kV and Above
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Comments

  1. Miracle ElectronicsOctober 4, 2016 at 5:45 PM

    Unlike a voltage transformer, the primary current of a current transformer is not dependent of the secondary load current but instead is controlled by an external load. The secondary current is usually rated at a standard 1 Ampere or 5 Amperes for larger primary current ratings.
    Power Transformers in India | Wire Harness Manufacturer in India

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