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

Photocoupler

First, the current transformer (CT) must be selected with the correct accuracy class and ratio as specified in the design. It is crucial to ensure that the polarity markings are correctly applied. The primary coil's start terminal is marked as L1, and the end is L2. The secondary coil's start terminal is labeled K1, while the end is K2. If the secondary coil has an intermediate tap, the first end is still K1, and the taps are labeled K2, K3, etc. For CTs with multiple secondary coils, a serial number should be added before the "K" marking at each outgoing terminal, such as 1K1, 1K2, 2K1, 2K2, and so on.

Second, to prevent an open circuit on the secondary side of the CT, no fuse should be installed on the secondary side. All wiring connections must be properly and securely made to avoid any loose or faulty connections. This ensures safe and reliable operation of the system.

Third, the secondary winding of the CT must be reliably grounded, and only one grounding point is allowed. In differential protection circuits, the grounding is typically done through the protection panel. This helps maintain the integrity of the protection system and prevents potential faults from causing damage.

Fourth, if a CT’s secondary winding is not in use, it should be short-circuited and then grounded. This practice minimizes the risk of dangerous voltage buildup and protects both equipment and personnel from electrical hazards.

Regarding the voltage transformer (VT), attention must be paid to the polarity during secondary wiring. The main secondary winding is usually labeled with A as the start terminal and X as the end. The auxiliary secondary winding may have different labeling, such as AD and XD. The main protection circuits are generally connected to the basic secondary winding (A and X), while insulation monitoring systems are connected to the auxiliary winding (AD and XD). The neutral point of one or two windings is often marked with a "D."

To prevent a short circuit on the secondary side of the VT, a fuse is typically installed in the PT terminal box. However, for excitation transformers, fuses are usually omitted to avoid false trips. The secondary side of the VT, including the neutral point and phase B, is generally grounded via the terminal block inside the terminal box. The grounding point must be secure and reliable, and over-grounding should be avoided to prevent potential issues in the system.