Schematic diagram of zero-sequence current protection

The principle wiring of the three-section zero-order current protection is shown in Figure 16. The identical current transformer is installed on the three phase of the protected line, connecting their secondary windings in parallel with each other and then connected to the coil of the current relay.

When normal operation and alternating fault occur, there is no zero order current in the grid, so IR=0, relay does not act, only if grounding fault occurs, zero order current appears. If its value exceeds the fixed value, the relay will act.

In practice, due to the inconsistent excitation characteristics of the three current transformers, a large unbalanced current occurs. To prevent the protection device from misoperating correctly in this case, the protected action current is usually rectified against the maximum unbalanced current.

As with the current protection for alternating short circuits, the zero-order current protection also adopts phase protection, usually in three sections. The current "four unified" protection screen adopts the four-section type. Figure 16 shows the schematic wiring diagram for the three-stage zero-sequence current protection.

Transient zero sequence electric flow speed break (zero sequence section of, composed of KA1, KM and KS7), generally take ground short circuit at the end of the protection line, flow through the protection device 3 times 1.3 times the maximum zero sequence current 3Iom, the protection range is not less than 15% -25% of the total length of the line.

The fixed current of zero sequence section (composed of KA3, KT4 and KS8) generally takes 1.2 times of the rectification current of zero sequence section of the primary line, and the time limit of 0.5s, ensures reliable action in the single-phase ground at the end of this line.

The rectified current of zero sequence segment (composed of KA5, KT6 and KS9) may be 1.2 times of the zero order (or) segment or greater than the maximum unbalanced current of three-phase short circuit, whose sensitivity requires the next terminal failure.

Figure 16 Schematic diagram of zero-sequence current protection

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