Precautions for transformers in secondary wiring - Solutions - Huaqiang Electronic Network

Photocoupler

First, the current transformer (CT) must be selected and installed according to the design specifications. The accuracy class and turns ratio of the secondary winding should match the project requirements precisely. The polarity marking method is as follows: the primary coil's first end is marked L1, and the second end is L2. The secondary coil's first end is K1, and the second end is K2. If there is an intermediate tap on the secondary coil, the first end is still K1, while the taps are labeled K2, K3, etc. For CTs with multiple secondary windings, each outgoing terminal should be labeled with a serial number before the "K" designation, such as 1K1, 1K2, 2K1, 2K2, and so on. Second, to prevent the secondary side of the CT from becoming open-circuited, no fuses should be installed on the secondary side. All wiring connections must be correct and securely fastened to avoid any potential issues. Third, the secondary winding of the CT must be properly grounded, and only one grounding point is allowed. In differential protection systems, the grounding is typically done through the protection panel. Fourth, if a CT's secondary winding is not in use temporarily, it should be short-circuited and then grounded to ensure safety and prevent voltage buildup.

Regarding the voltage transformer (VT), special attention should be paid to the polarity during secondary wiring. It is important to distinguish between the main secondary winding and the auxiliary secondary winding. Typically, the head end of the main secondary winding is labeled A, and the tail end is X. For the auxiliary secondary winding, the head end is usually labeled AD, and the tail end is XD. Main protection circuits are connected to the A-X main secondary winding, while insulation monitoring systems are connected to the AD-XD auxiliary winding. The neutral point of one or two phases is often marked with "D". To prevent short circuits on the secondary side of the VT, a fuse is generally installed in the PT terminal box. However, for excitation transformers, fuses are not used to avoid false trips. The secondary side of the VT (neutral point and phase B) is typically grounded via the terminal block inside the terminal box. The grounding point must be reliable and secure, and over-grounding should be avoided to maintain system integrity.