What is the zero sequence transformer in the switchgear? what's the effect? -Solutions - Huaqiang Electronic Network

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Principle: Zero-sequence current protection operates based on Kirchhoff’s Current Law, which states that the sum of all currents entering a node in an electrical circuit must equal zero. Under normal conditions, the vector sum of the three-phase currents is zero, so no signal is generated in the secondary winding of the zero-sequence current transformer (ZCT), and the protective relay remains inactive. However, during a ground fault, the imbalance in phase currents creates a magnetic flux in the toroidal core of the ZCT. This flux induces a voltage in the secondary winding, triggering the actuator to activate and trip the circuit breaker, isolating the faulty section from the power supply. Function: The primary function of this system is to detect earth faults or leakage currents. When such a fault occurs—such as an electric shock or insulation failure—the protection mechanism quickly disconnects the power supply to prevent damage to equipment and protect human life. Use: Zero-sequence current transformers can be installed in different configurations. One common method is to pass all three phase conductors through the same toroidal core. Alternatively, a single ZCT can be placed around the neutral conductor (N) to monitor the vector sum of the three-phase currents. This setup allows for efficient detection of unbalanced currents that may indicate a fault. How It Works: In a balanced system, the currents flowing through the primary side of the ZCT (the main circuit lines) are equal in magnitude but opposite in direction. As a result, their magnetic fields cancel each other out, and no output is produced on the secondary side. However, if an unbalanced current exists—such as when a leakage occurs due to a fault or contact with a person—the magnetic field is no longer balanced. This imbalance causes a current to be induced in the secondary winding, which is then amplified and used to trigger the protective device. Importance: Zero-sequence protection is essential in ensuring electrical safety, especially in systems where grounding is critical. It helps identify and respond to dangerous conditions before they cause harm. Whether it's a small leakage or a major fault, the system acts swiftly to isolate the affected area and maintain the integrity of the overall electrical network.