Determining and verifying CT polarity is critical for accurate metering and correct protection relay operation. Incorrect polarity can cause meters to read negative power, protection relays to misoperate, and differential protection schemes to fail. Several methods can be used to determine and verify CT polarity.
The most basic method is visual inspection of the CT markings. Manufacturers carefully label one side "positive" (often P1 or H1) on the primary, matching a "positive" coil connection on the secondary (denoted S1 or X1)-. The polarity mark on the secondary side is typically a round dot near one of the two secondary wiring terminals-. If the markings are clear and the CT has not been modified, the polarity can be determined from these factory markings.
For definitive verification, a DC method can be used. Connect a DC source (such as a battery) to the primary winding through a switch, and connect a DC voltmeter or galvanometer to the secondary winding. When the switch is closed, observe the direction of the induced voltage. The convention is: If the meter deflects in the positive direction when the primary circuit is closed and in the negative direction when opened, the CT has subtractive polarity (P1 and S1 are the same polarity). If the deflection is reversed, the CT has additive polarity-.
An AC method using a phase-angle meter or oscilloscope can also be used. Under low-level injection or small normal load, measure the phase relationship between the primary current and the secondary current. For subtractive polarity, the secondary current should be in phase with the primary current (accounting for the CT ratio). A phase difference approaching 180 degrees indicates reversed polarity-.
In the field, the most practical verification method is to observe the power reading on the energy meter or the current reading on the protection relay after the system is energized. If electricity is consumed, the power must be displayed without a sign; if electricity is supplied, the power must be displayed with a negative sign. A negative reading during consumption indicates that one or more CTs are installed backwards or that the S1 and S2 leads are reversed.
If measurements appear incorrect, re-check CT orientation and secondary polarity wiring before assuming a device or meter failure-. For differential protection schemes, polarity verification is even more critical. The currents entering both sides of the protected equipment must be compared with respect to their direction, and any polarity error will cause the relay to see a false differential current-. Always verify polarity through commissioning tests before placing protection systems into service.