1. Selection and characteristics of short-circuit protection components
In the short-circuit protection design of power distribution cabinet, fuses and circuit breakers are key protection components. Fuses use the fuse element to quickly melt under short-circuit current to cut off the circuit. It has an inverse time characteristic. The larger the short-circuit current, the shorter the melting time. For example, common high-breaking capacity fuses can withstand a huge current impact at the moment of short circuit and melt quickly to effectively protect the circuit. Circuit breakers achieve short-circuit protection through electromagnetic releases and thermal releases. The electromagnetic release acts instantly when the short-circuit current reaches the set value, causing the circuit breaker to trip. Its action speed is extremely fast and can cut off the fault current within a few milliseconds. The thermal release is mainly for overload conditions and acts after a certain period of time. Reasonable selection of parameters such as the rated current and breaking capacity of fuses and circuit breakers is the basis for ensuring the effectiveness of short-circuit protection.
2. Sensitivity and selectivity of short-circuit protection
The sensitivity of short-circuit protection is crucial. It requires that the protection device can act quickly and reliably when a short-circuit fault occurs. The sensitivity is measured by calculating the ratio of short-circuit current to the operating current of the protection device. Generally, the ratio is required to be greater than the specified sensitivity coefficient. At the same time, in order to ensure the selectivity of the power system, that is, when a short-circuit fault occurs, only the protection device closest to the fault point will be operated, while the protection devices in other non-fault areas will not be operated, it is necessary to selectively coordinate the protection devices at all levels in the power distribution cabinet. For example, time differential coordination or current differential coordination is adopted. In the time differential coordination, the operating time of the upper protection device is longer than that of the lower protection device by a certain time difference, so that when the lower protection device can quickly cut off the short-circuit fault, the upper protection device will not malfunction. Through precise calculation and setting, the balance between the sensitivity and selectivity of short-circuit protection is achieved.
3. Ways to quickly cut off the fault current
In order to quickly cut off the fault current, in addition to selecting suitable protection components and the correct coordination method, it is also necessary to optimize the internal electrical connection and wiring of the power distribution cabinet. Use low-impedance busbars and cable connections to reduce the transmission impedance of the fault current in the circuit, thereby reducing the voltage drop at the fault point, which is conducive to the protection device to quickly detect the fault and operate. For example, copper busbars are used, which have good conductivity and low resistivity. At the same time, in the design of the action mechanism of the protection device, technologies such as fast tripping and pre-energy storage are used to improve the action speed. For example, some advanced circuit breakers use electromagnetic repulsion mechanisms. At the moment when the short-circuit current is generated, the electromagnetic force is used to quickly push the contacts to separate, so as to achieve ultra-fast disconnection of the fault current, greatly shorten the fault duration, and reduce the harm of the short circuit to the power system.
4. Testing and verification of short-circuit protection design
After the short-circuit protection design of the power distribution cabinet is completed, strict testing and verification are required. Through the short-circuit test, the actual short-circuit fault situation is simulated to detect whether the protection device can quickly cut off the fault current according to the design requirements, and verify whether the selectivity and sensitivity of the protection meet the standards. For example, in a professional electrical laboratory, short-circuit test equipment is used to apply different degrees of short-circuit current to the power distribution cabinet to observe the fusing of the fuse, the tripping time and sequence of the circuit breaker, etc. According to the test results, the short-circuit protection design is adjusted and optimized to ensure that the power distribution cabinet can reliably respond to short-circuit faults in actual operation and ensure the safe and stable operation of the power system.
