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Tips for Current Transformer Turns Ratio and Polarity Test

1.The purpose of the test:

It is important to check whether the transformer ratio and polarity are consistent with the nameplate value, because the wrong polarity judgment can mislead the wiring error, which in turn leads to the wrong indication of the instrument, and more seriously, the misoperation of the direction relay protection. Measuring the turns ratio can check the correctness of the primary and secondary relationship of the transformer, and provide a basis for the calculation of the correct action of relay protection and protection tuning.

2.The instrument selection

JYH C CT PT analyzer

3.Site test steps and requirements

The DC method measures the polarity of the current transformer, and the automatic ratio tester measures the ratio and polarity.

3.1 Concept of polarity of current transformers

The polarity means that in the case of the same magnetic flux of the core, the primary winding and the secondary winding will sense the electromotive force. Both ends that reach a high or low potential at the same time become the homopolarity.

Current transformers are generally expressed in terms of negative polarity. First, select one end of the primary winding as the starting end. When the primary winding current i1 flows instantaneously from the starting point, the end of the secondary winding current i2 is marked as the secondary winding. Polarity is usually denoted by "+" and "-".

3.2 Dc method measures the polarity of current transformers

At the moment of closing, if the pointer box is biased towards "+", and the pointer is biased towards "-" when opening, then P1 and S2 are Homonymous terminals, and the current transformer is of reduced polarity.

3.3 CT PT analyzer measures current transformer ratio and polarity

The primary windings are connected to testers L1 and L2, respectively

The measured secondary windings are connected to testers K1 and K2 respectively

Untested secondary windings are short-circuited to ground

4, Test precautions:

4.1 Precautions for determining the polarity of transformers by the DC method

The homopolar terminals of the dry cell and the meter should be connected to the same terminal of the winding.

The instrument used has a zero position in the center. If you use an ordinary DC instrument, if the displacement in the negative direction (without a scale) is small and not easy to observe, you can reverse the positive and negative ends of the instrument and repeat the measurement. The gauge pointer swings positively, but is recorded in a negative direction.

This operation is repeated several times to avoid misjudgment.

4.2 Precautions for measuring ratio and polarity using a transformer comprehensive characteristic tester

The test power supply should be the same as the operating power supply of the instrument used.

To prevent residual charges from affecting the measurement results, the transformer polarity must be fully discharged before testing.

In the CT ratio and polarity laboratory, the untested secondary windings of the current transformer should be short-circuited to prevent open circuits, prevent untested secondary windings from generating high voltages, and endanger the safety of person and test equipment.