Current protection is a very important protection measure in power distribution systems, and zero-sequence current protection and residual current protection are the two main protection methods.
Zero-sequence current protection: In the power distribution system, when the three-phase currents are running in balance, their phasor sum is zero, that is, the sum of the three-phase currents is equal to zero. If the three-phase current is unbalanced, zero sequence current will occur.
Zero-sequence current protection uses this principle to determine whether there is a fault in the system by detecting whether the three-phase current is balanced. When a single-phase ground fault occurs, the current of the faulted phase will increase sharply, while the current of the other two phases will decrease. By detecting the changes in the current of each phase, the location and type of the fault can be determined.
Residual current protection: It is a protection method based on the principle of zero sequence current. It mainly realizes fault detection by detecting changes in neutral line current.
In a three-phase four-wire circuit , the neutral current is the sum of the three-phase currents. When the circuit is operating normally, the neutral current is zero. When a single-phase ground fault occurs in a circuit, the current of the faulted phase will flow back to the power supply through the neutral wire , causing the neutral current to be non-zero. By detecting changes in neutral line current, the location and type of fault can be determined.
So which one is better, ground fault protection, zero sequence current protection or residual current protection?
(1) Zero sequence current: In = Iu + Iv + Iw
1. From a formula point of view, the zero sequence current is the phasor sum of the three-phase currents, so the zero sequence current protection can only be used in three-phase circuits , and the single-phase circuit is bypassed!
2. When the line is running normally, there will be three-phase unbalanced current and harmonic current, which will be reflected in the neutral line. When a ground fault occurs, In will increase significantly, but compared with the line overcurrent, it is still Smaller.
3. In order to avoid protection malfunction during normal operation, the zero sequence current setting value Ig needs to be larger than these three-phase unbalanced currents and harmonic currents, so the setting value is often tens or hundreds of amperes.
4. If the setting value is too large, it can neither prevent ground arc fire (more than 300mA) nor personal electric shock (not more than 30mA). Therefore, when a ground fault occurs, the sensitivity is better than that of overcurrent protection. In fact, it can only be used for Protect distribution lines from insulation damage caused by heat.
5. Because of the above problems, zero-sequence current protection is often only used at the main incoming line switch on the low-voltage side .
(2) Residual current: Ie = Iu + Iv + Iw + In
1. Judging from the formula, the residual current also adds the neutral line current to the phasor, thus harmonizing the three-phase unbalanced current and harmonic current in the loop. The measured current value is usually only There is a leakage . This current is very small, in the milliamp level.
2. The residual current protection setting value Ia only needs to avoid the normal ground leakage current of the protected circuit, so it is also at the milliamp level, which will greatly improve the sensitivity of the protection.
3. When the setting value is 100mA and 300mA, it can prevent electrical fire; when the setting value is 30mA, it can be used to prevent personal electric shock.
4. RCD can be used in three-phase and single-phase circuits. It has wider application and stronger protection capabilities. It is often used in a wide range of primary, secondary and tertiary power distribution, and is also used most at the end of power distribution. .
From the above analysis, it can be seen that residual current protection has higher sensitivity and wider applicability than zero-sequence current protection, and can better protect the safe operation of the distribution system. Therefore, in actual power distribution systems, residual current protection is usually used as the main ground fault protection method.
At the same time, we can also see that although zero-sequence current protection has some limitations, it can also be used as an effective protection measure in specific application scenarios, such as the low-voltage side main line switch. Reasonable configuration of zero sequence current protection and residual current protection can improve the reliability and safety of the entire power distribution system.
In addition, no matter which current protection method is used, the correct setting value and configuration are also crucial. When installing and using these protective devices, reasonable settings should be made based on the actual situation to ensure that they can effectively function and prevent various faults and abnormal conditions that may occur in the power distribution system.