Voltage transformer and CT polarity
In the power system, the role of the current transformer is to change the large current into the small current, isolate the secondary loop connected to the relay and measuring instrument from the high voltage system of the primary current, and convert the primary current to the secondary current value of 5A or 1A standards. The polarity of CT is closely related to current protection, especially in the agricultural power system, current protection plays a leading role, so it is necessary to master the relationship between polarity and protection.
Voltage transformer are used in AC circuits where the direction of current changes over time. The polarity of the CT means that the polarity of the primary side is the same as that of a certain end of the secondary side at a certain time, that is, it is positive or negative at the same time. This polarity is called the same polarity end or the same name end and is represented by symbols "*", "-" or ".". (also can be understood as the relationship between the direction of primary current and secondary current). According to regulations, the first end of the primary coil of the CT is labeled L1 and the tail end is labeled L2. The first end of the secondary coil is labeled K1 and the tail end is labeled K2. In the wiring L1 and K1 are called homopolar terminals, L2 and K2 are also homopolar terminals. The three labeling methods are shown in the figure below.
