Principle of zero-sequence current transformer

- Jan 08, 2020-

The specific application of zero-sequence current protection is to install a current transformer (CT) on each of the three-phase lines, or to pass the three-phase wires together through a zero-sequence CT, or to install a zero-sequence CT on the neutral line N. These CTs detect the three-phase current vector sum, that is, the zero-sequence current Io, IA + IB + IC = Io, when the three-phase load connected to the line is fully balanced (no ground fault, and regardless of the line, electrical equipment, etc.) Leakage current), Io = 0; when the three-phase load connected to the line is unbalanced, then Io = IN, and the zero-sequence current at this time is the unbalanced current IN; when a ground fault occurs on a certain phase, a single The phase-to-earth fault current Id, the zero-sequence current IO = IN + Id detected at this time, is the vector sum of the three-phase unbalanced current and the single-phase grounding current.

Zero-sequence current protection is generally suitable for use in TN grounding systems. Because when one-phase grounding occurs, the Id loop impedance of the TN-S system includes phase line impedance Z1, PE line impedance ZPE and contact impedance Zf, that is, Zs = Z1 + ZPE + Zf; for TN-C system, Id loop impedance Including phase line impedance Z1, PEN line impedance ZPEN and contact resistance Zf, that is, ZS = Z1 + ZPEN + Zf; for TN-CS system, the Id loop impedance includes phase line impedance Z1, PEN line impedance ZPEN, PE line impedance ZPE and contact The resistance Zf, that is, ZS = Z1 + ZPEN + ZPE + Zf, produces a single-phase ground fault current Id = 220 / ZS, which is significantly larger than the three-phase unbalanced current when there is no fault. As long as the setting is appropriate, the ground fault can be detected. The zero-sequence current at the time to cut off the fault circuit. For IT systems, it is generally used in industrial and mining enterprises that have high requirements for power supply reliability and do not need to immediately cut off the power supply circuit for single-phase grounding, but need to send insulation damage monitoring signals to maintain power supply for a period of time. Three-phase three-wire distribution line

When single-phase grounding, the zero-sequence current flowing on the fault line is the sum of the capacitor currents of the entire system and the non-fault system. Therefore, it is easy to detect the ground-fault current. Therefore, the zero-sequence current protection device can be used to monitor the first ground fault relatively. .

TT grounding system is often used in three-phase four-wire power distribution systems for industrial and agricultural and civil building lighting and power hybrid power supply. It is often found that the three-phase unbalanced current is large. When one-phase grounding occurs, the Id loop impedance includes the phase Line impedance Z1, PE line impedance ZPE, load-side ground resistance RA and power-side ground resistance RB, contact impedance Zf, that is, ZS = Z1 + ZPE + RA + RB + Zf, ground fault current Id = 220 / ZS, due to RA + RB >> Z1 + ZPE + Zf, and the values of RA + RB are generally large. It is obvious that the fault loop impedance of the TT system is large, and the single-connection fault current Id is much smaller than the unbalanced current, and it is difficult to detect the fault. Current, so it is not suitable for TT grounding system.

For zero-sequence CT installation with zero-sequence current protection, it must comply with relevant process standards. For IT grounding systems, when a single-phase ground fault occurs, the grounding current may not only flow back along the conductor surface of the failed cable, but may also flow back along the conductor surface of the non-faulty cable. The zero-sequence CT is grounded. In this way, it is possible to ensure that the capacitor currents of the faulted phase and the non-faulted phase pass through the ground point, which can prevent the protection device from malfunctioning when the fault is outside the zone, and can also ensure that the device operates reliably when the fault occurs.

For IT grounding systems, zero sequence CTs are generally installed on the neutral line N. Zero sequence CTs on the low-voltage side busbar must be installed on the busbar between the neutral line N and the working ground point (or repeated grounding). . For example, the zero-sequence CT is installed on the N-line busbar of the power distribution screen. Because the metal shell of the power distribution screen is generally directly connected to the ground electrode, when the busbar is shorted to ground, the fault current Id will be generated along the metal shell of the power distribution screen. → The grounding wire → the neutral point of the transformer flows without passing through the zero-sequence CT, and the required protection function cannot be achieved, which is easy to be neglected during construction on site.

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