The low-impedance busbar current differential protection commonly used in China is not suitable for high-voltage busbars. When the busbar external fault is saturated, a large unbalanced current will appear in the busbar differential relay, which may cause the busbar protection to malfunction. High-impedance bus current differential protection solves this problem better, but in the internal fault of the busbar, the secondary side of the current transformer may have excessive voltage, which is unfavorable for the reliable operation of the relay, and requires the transmission of TA. The characteristics are identical and the ratio is the same, which is difficult for the substation to be expanded.

 

The medium impedance bus current differential protection effectively combines the high impedance characteristic and the ratio braking characteristic, which significantly reduces the load resistance of the bus differential circuit, and better ensures that the TA fault is not disturbed outside the zone. The internal fault is correct and fast. It uses the current instantaneous value as the measurement comparison. The measuring component and the differential component are mostly integrated circuits or rectifying relays. When the busbar internal fault occurs, the moving speed is extremely fast, about 1 to 3 ms. Before the 1/4 cycle, the TA will not be 100% saturated, and the primary side current can be well transmitted. There is no special requirement for the TA, and the TA ratio can be inconsistent. This is the main choice for all domestic power grids.

 

The main feature of the microcomputer differential protection that the manufacturer has introduced is the ability to make full use of the computer for digital calculation. It is convenient to realize the current instantaneous value differential principle with the ratio braking characteristic and the complex ratio differential principle. The microcomputer mother-slave has a unique detection method for TA saturation, and has strong anti-TA saturation ability. Several foreign manufacturers use waveform discrimination or compensation method to eliminate the influence of TA saturation, that is, the sampling point of TA linear transmission before 1/4 cycle Waveform processing or discrimination is performed with a certain algorithm to ensure the selectivity of protection. In the domestic practice, the synchronous identification method is often used to overcome the influence of TA saturation, and the saturation condition is identified by discriminating whether the differential action and the fault occur synchronously. It has the self-adaptive ability to identify the busbar operation mode, which theoretically omits the trouble of introducing the isolation blade auxiliary contact (for double busbar wiring). At the same time, the microcomputer differential protection has a self-test function, and the reliability is further improved. More importantly, the microcomputer mother-slave has a communication interface, which can be easily interconnected with the monitoring system, complete the remote transmission and remote control of information, and realize automation. Of course, the advantage of the microcomputer differential protection has the convenience of debugging and setting is self-evident. Therefore, the busbar protection is the same as the line protection, and the protection of the microcomputer type is the trend and direction. For substations that use a tiered distributed automation system, the decentralized microcomputer differential protection is more compatible with its structure.

 

It should be pointed out that the current microcomputer differential protection action speed is not as fast as the medium impedance busbar protection, and the operation time is longer when the fault occurs in the out-of-zone fault zone, and the stability and maturity of the operation need to be improved. Before solving these problems, the logic circuit and signal loop of the impedance bus differential protection in the busway are mechanized, which not only exerts the advantages of medium impedance protection, but also has the advantages of microcomputer protection self-test and communication interface. The ideal choice.