Current transformers should undergo polarity testing before and after commissioning and major overhauls to prevent incorrect polarity during wiring, which could cause malfunctions in relay protection and metering circuits, and prevent accurate measurements. Therefore, a polarity test must be performed before commissioning.
Polarity Relationship Characterization: All outgoing terminals marked L1, K1, and C1 have the same polarity at the same instant.
There are many methods for measuring the polarity of current transformers. The following three methods are commonly used in practice: ① DC method; ② AC method; ③ Instrument method.
DC Method: Connect the positive terminal of a 1.5-3V dry cell battery to the primary coil L1 of the current transformer, and the negative terminal to L2. Connect the secondary side K1 of the current transformer to the positive terminal of a milliammeter, and the negative terminal to K2. After connecting the wires, close K and the milliammeter needle deflects positively; open it and the needle deflects negatively. This indicates that the terminal of the current transformer connected to the positive terminal of the battery and the terminal connected to the positive terminal of the milliammeter are of the same polarity, i.e., L1 and K1 are of the same polarity, meaning the current transformer is subtractive. If the needle deflects in the opposite direction, it is additive.
AC Method: Connect the primary and secondary coils L2 and the secondary side K2 of the current transformer with a wire. Apply a 1-5V AC voltage (using a small range) to the secondary side. Measure the values of U2 and U3 with a voltmeter below 10V. If U3 = U1 - U2, it is subtractive.
U3 = U1 + U2, it is additive. Note: During the test, keep the applied voltage as low as possible to avoid excessive current damaging the coil. For clear readings, choose a voltmeter with a smaller value. When the current ratio is below 5, the AC method is simpler and more accurate. Do not use this method for transformers with a current ratio exceeding 10, because the values of U2 and U3 are close, making it difficult to distinguish the magnitudes on the voltmeter. Therefore, this method is generally not recommended for polarity measurement.
Instrument Method: Most current transformer calibrators have polarity indicators. Before measuring the error of a current transformer, the instrument can check the polarity beforehand. If the indicator does not show any indication, it means the polarity of the current transformer under test is correct (reduced polarity).