How to measure the polarity of CT？

There are many methods to determine CT polarity, and we often use the following two testing methods: 1. DC method; 2. Instrument methods.

1. DC method

The wiring for DC mode is shown in the following figure. Connect the positive pole of the dry battery or battery to the primary winding P1 of the CT, connect the negative pole to P2, connect S1 on the secondary side of the CT to the positive pole of the pointer ammeter (or turn the pointer multimeter to the milliampere position), and connect S2 to the negative pole. After connecting the circuit, if the pointer of switch S is positive when closed and reverse biased when open, then P1 and S1 are the same named terminals, and the polarity of the current transformer decreases. If the pointer swings in the opposite direction to the above, the polarity will increase.

2. Instrument method

The instrument method mainly uses a transformer ratio polarity tester to measure CT polarity. Its internal constant voltage source has a wide measurement range and is easy to carry. It can not only directly display the polarity and transformation ratio of CT, but also measure the direct resistance and excitation characteristics of CT. It is simple and intuitive, and is currently widely used. The wiring diagram is shown in the following figure, and the specific wiring method is as follows: the red line (P1) on the primary side of the tester is connected to the polarity end of the CT primary side (P1), and the black line (P2) is connected to the polarity end of the CT primary side (P2); The red wire (S1) on the secondary side of the tester is connected to the polarity end of the CT secondary side (S1), and the black wire (S2) is connected to the polarity end of the CT secondary side (S2). Note: There are two parallel S1 and S2 on the tester, but their internal logic operations are different, used for polarity discrimination and ratio calculation.

Comparison of Two CT Polarity Measurement Methods

1. Characteristics of DC method: For polarity measurement of single-phase voltage or current transformers, the principle of DC method is simple, the wiring of measuring equipment is simple, the operation is simple, and it is suitable for single-phase detection and judgment. Transformer polarity. For many polarity measurements, the DC method requires frequent plug-in testing wiring and the cooperation of many people, which is both time-consuming and laborious. At the same time, there are potential safety hazards, so it is not suitable for polarity measurement of multiple transformers.

2. Characteristics of the instrument method: In the polarity detection of three-phase group voltage or current transformer connections, it has the advantages of less measurement time, high measurement accuracy, simple and intuitive judgment basis, convenient operation, and the ability to display polarity and displacement. At the same time, the ratio can greatly improve the efficiency of detection work. It is an advanced polarity detection method that has been widely used.

5. Precautions for CT polarity measurement

In phase separation testing, when testing a certain phase, please note that there is no change without testing.

For multi winding CT, each winding should be tested separately without omission.

3. Meet the special polarity requirements for protecting manufacturers.