1. High sensitivity, less affected by fault resistance. In the event of a high resistance ground fault, the zero-sequence current protection can still operate.
2. It will not malfunction when the system oscillates. The zero-sequence current direction protection is not afraid of system oscillation, because the oscillation system is still a symmetrical system, so there is no zero-word current.
3. Under the condition that the zero-word network of the power grid is basically stable, the protection range is relatively stable. Since the zero sequence impedance of the line is generally 3-3.5 times larger than the increasing sequence impedance, the zero sequence current changes significantly when the start and end of the line are short-circuited. The zero sequence current changes steeply with the location of the line protection grounding fault point, and its instantaneous The section protection range is relatively large, and can reach 70%-80% of the full line for general long and medium-long lines, and its performance is similar to distance protection.
4. Zero sequence voltage and zero sequence current will not appear during normal operation of the system and phase-to-phase short circuit. Therefore, the operating current of the delay period of the zero-word protection can be set smaller, which is beneficial to improve its sensitivity. In addition, the coordination between zero-sequence current protection is only determined by the impedance distribution of the zero-word network, and is not affected by load flow and generator startup and shutdown. As long as the zero-sequence network remains basically stable, a good protection effect can be obtained.
5. Simple structure and working principle. Zero-sequence current protection takes a single amount of current as the action amount, and only needs a relay to respond to any phase ground fault in the three phases, so it is easy to operate and maintain, and its correct action rate is higher than other complex protections. In the step-down transformer with Y/â–³ connection, the faults after the delta winding side will not reflect the zero-sequence current on the star winding side, so the action time limit of the zero-word current protection does not need to be coordinated with the subsequent line protection of this type of transformer. A shorter action time limit can be obtained.
1. When the current loop is disconnected, it may cause the protection to malfunction.
2. When asymmetrical operation of the power system occurs, zero-sequence current will also appear, which may cause the zero-sequence current protection to start.
3. For parallel lines close to geographical locations, when one line fails, zero-sequence current may be induced in the other line, causing the zero-word direction relay on the opposite direction to malfunction.
4. Since the zero-sequence directional relay current loop usually has no zero-sequence current and zero-sequence voltage, it is not easy to detect circuit breakage. When the zero-sequence voltage of the relay is taken from the open triangle side of the voltage transformer, it is not easy to check it with a more intuitive method. The correctness of the direction makes it easier to cause protection rejection and malfunction when the power grid fails due to problems with the AC circuit.
5. For short circuits or large changes in operating modes, the protection often fails to meet the requirements of system operation.
1200W Human Sensor PTC Fan Heater
1200W Human Sensor Ptc Room Heater,Ptc Fan Heater,Ptc Tower Heater,Ptc Electric Fan Heater
Foshan Shunde Josintech Electrical Appliance Technology Co.,Ltd , https://www.josintech.com