Difference between zero sequence current transformer and residual current transformer

What is a zero-sequence current transformer

Zero-sequence current transformer is a single-turn through-core current transformer, which is generally used for supporting power protection equipment such as small current grounding line selection device and microcomputer harmonic elimination device.

What is a residual current transformer

The residual current transformer is a signal detection element in the leakage protector. Its main function is to detect the residual current (ground fault current such as electric shock and leakage) through the main circuit of the transformer core, and convert the residual current of the primary circuit into two. The output voltage of the secondary circuit generally adopts a hollow ring-shaped transformer. When installing, pass the three-phase four-wire primary circuit through the zero-sequence current transformer to detect the vector sum of the current in the primary circuit.

When an electric shock accident, equipment leakage, ground fault, or three-phase to ground leakage current imbalance occurs in a three-phase circuit, the current vector sum of the residual current transformer is not zero, and the residual current detector collects the residual current signal.

3. What are the similarities between the zero-sequence current transformer and the residual current transformer?

Residual current transformer is also called leakage current transformer. Similar to the principle of zero-sequence current transformers, both are based on Kirchhoff's current law, but the requirements for use range and installation are different.

The zero-sequence current transformer is generally suitable for TN grounding systems, and the setting current of the residual current transformer protection operation can be from mA level to A level, and it has a very high operating sensitivity. ﹣CS system grounding system is applicable. During installation, the three-phase wires A, B, and C pass through a residual current transformer together with the N wire.

The low-voltage system is a directly grounded neutral system. When a single-phase ground fault occurs, there must be zero-sequence current flowing through the earth. Zero-sequence current transformers do not measure small currents, while residual current transformers measure small currents. So its action sensitivity is higher than the zero-sequence current transformer.

4. What are the functions of the zero-sequence current transformer and the residual current transformer?

Zero-sequence current transformer is used for zero-sequence protection. It is generally used for power protection equipment such as small current grounding line selection device, microcomputer harmonic elimination device, etc.

Residual current transformers are used for residual current protection (also known as leakage current protection). Residual current transformers are one of the most critical components in leakage protectors.

5. How does the zero-sequence current transformer and residual current transformer work?

If the zero-sequence current transformer does zero-sequence protection?

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 supplier 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 The impedance ZPE and the contact resistance supplier Zf, that is, ZS = Z1 + ZPEN + ZPE + Zf, the 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 it is set When appropriate, the zero-sequence current when a ground fault occurs can be detected 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 supplier's currents of the entire system's non-faulty system capacitors and capacitors, so it is easy to detect the ground-fault current. Therefore, a zero-sequence current protection device can be used to monitor the relative ground One earth fault. During installation, the three-phase wires A, B, and C pass through a zero-sequence C.T together, and a low-sequence C.T can also be installed on the neutral line N.

如何 How does the residual current transformer do residual current protection?

The residual current transformer detects the leakage current, and the neutral current does not belong to the leakage current. In other words, the residual current transformer needs to enclose the three live wires and one neutral wire (if any). When there is no leakage current, the vector sum of the currents of the live and neutral wires is equal to zero. The vector sum is the leakage current.

3. The connection is correct. As long as the leakage current does not reach the alarm value, the large neutral current will not alarm.

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