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This post was last edited by chenjinfeng on 2020-3-27 at 14:25. Faults in the medium and low voltage busbars caused by arc flash short circuits in switchgear occur from time to time, and there have also been incidents in which main transformers were damaged due to the impact of external short-circuit currents, resulting in significant economic losses. On the other hand, users are placing ever higher demands on the reliability of power supply. Therefore, 35 kV switchgear is equipped with specialized rapid busbar protection – arc flash protection. The principle of the arc light protection device is quite simple; its operation is based on two distinct factors that arise during a fault: arc light and current increase. A trip command is issued when both arc flash and current increase are detected simultaneously. In other words, when a system failure occurs, the arc sensor converts the arc signal into an electrical signal, which is then transmitted to the main unit via the I/O auxiliary unit. The main unit detects this current signal; once it reaches the activation threshold, it sends out a trip signal. In the event of an arc short-circuit inside a switchgear cabinet, the internal arcing time specified in Appendix AA of IEC298 is 100 ms. In other words, the total time during which the arc can burn within the switchgear cabinet – that is, the sum of the time it takes for the protection system to activate and the time required for the circuit breaker to disconnect the fault – must be less than 100 ms in order to ensure proper protection of the switchgear cabinet. Currently, switchgear sold on the market is basically manufactured in accordance with the IEC298 standard; in other words, it can withstand an arc burning time of 100 ms. Requirements for the dynamic stability time of transformers and the operating time of medium- and low-voltage bus protection: According to national standards, the permissible thermal stability time for transformers rated at 110 kV and above is 2 seconds, while the dynamic stability time is 0.25 seconds. However, in reality, during a short-circuit fault at the low-voltage side outlet, the tripping time of the overcurrent backup protection is often above 2 seconds, which is significantly higher than the required 0.25 seconds for the transformer’s dynamic stability – and this is also a major cause of transformer damage. Existing protection methods for medium- and low-voltage busbars and their problems: Transformer backup overcurrent protection: This is currently the most widely used protection method for medium- and low-voltage busbars in China. Considering the coordination with feeders and bus sectionalizers, the protection trip time is generally set between 1.0 and 1.4 seconds; in some cases it is even longer, reaching over 2.0 seconds. The time required for this action is far from sufficient to meet the requirements for quickly eliminating faults in medium and low voltage busbars. Feeder overcurrent protection to lock out transformer overcurrent protection: In recent years, the use of feeder overcurrent devices to lock out transformer overcurrent protection has become quite widespread. Compared with the traditional transformer backup overcurrent protection method, this approach offers a faster response time, with a typical response time of 300–400 ms. However, for applications requiring fault removal within 100 ms, it obviously fails to meet the requirements. High-impedance busbar protection using the circulating current principle: The typical operating time for this type of protection is 35–60 ms. Considering the circuit breaker’s opening time, this speed of operation is also slow for the requirement to eliminate faults within 100 ms. Moreover, this protection method involves complex wiring, requires high standards for the quality of TA, and is very difficult as well as uneconomical to install on 6–35 kV busbars with numerous outgoing connections. Furthermore, since its protection range is limited by the installation location of the TA, it cannot protect against faults at the cable joints in the cable rooms, where the likelihood of failures is higher. Therefore, it is not suitable for use in medium and low voltage busbar protection. From a practical perspective, the existing protection methods clearly cannot meet the requirements for quickly eliminating busbar faults or ensuring an adequate coverage area for protection. Therefore, it is necessary and urgent to install a rapid busbar protection system in medium and low voltage busbars as well as switchgear. Arc flash protection can quickly isolate faults in medium and low voltage busbars, ensuring the safe operation of power transmission and distribution networks.