How to detect the polarity of a CT?
Thread Content
This post was last edited by Benbenxuan on 2010-6-15 at 13:36. There are three CTs; how can one determine their polarity before use? Secondly, in which range is the current change linear, and what happens when the current change is not linear beyond that point? What is the change in flux?
First, the reduced-polarity connection method is generally used for CTs in China at present. That is, if the instantaneous direction of the primary current is from CT P1, then the instantaneous direction of the secondary current is from S1. Generally, the one-time polarity P1 and P2 can be seen on the appearance of the on-site CT. Second, regarding the determination of CT polarity, there are generally two methods used on-site. 1) By applying a current in the primary circuit and measuring the currents on either side or all three sides of the differential protection to determine whether the CT polarities are compatible. 2. Use a battery to manually operate the circuit on the CT secondary side; connect a microammeter to the CT secondary side and observe the direction of the pointer’s deflection in order to determine the polarity of the CT. Third, it should be emphasized that determining the polarity of a CT alone is meaningless; for example, the polarities of the CTs on both sides of a differential protection system must be matched properly for accurate results. The polarity of CTs used for measurement and metering also needs to match the voltage polarity to yield correct results. Similarly, CTs used for protections related to loss of magnetism, power, impedance, etc., must also be aligned with the voltage polarity to function correctly.