In a three-phase four-wire electrical system, the vector sum of the three phase currents is theoretically zero, meaning Ia + Ib + Ic = 0. When a current transformer is installed in such a system, no current is induced because the magnetic fields cancel each other out. However, if an electric shock or leakage occurs in the circuit, a leakage current will flow through the system. At this point, the vector sum of the three-phase currents is no longer zero: Ia + Ib + Ic = I_leakage.
This imbalance creates a magnetic flux in the core of the transformer, which induces a voltage in the secondary winding. This voltage is then sent to the amplifying circuit of the detection unit, where it is compared with a pre-set threshold value for the protection device. If the detected current exceeds this threshold, the sensitive relay is triggered, causing the circuit breaker or protective component to disconnect the power supply.
The type of transformer used in this setup is known as a zero-sequence current transformer. It detects the unbalanced current, also referred to as the zero-sequence current, which is a key indicator of faults such as ground leakage or insulation breakdown. This mechanism plays a vital role in ensuring electrical safety and preventing potential hazards in power systems. The design of the transformer allows it to respond only to the unbalanced current, making it highly effective for fault detection in three-phase systems.
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