Discussion on short-circuit current breaking capacity of low-voltage side circuit breakers of distribution transformers

Since the data of the system short-circuit capacity and short-circuit impedance need to be provided by the power supply department, the electrical design of most civil construction projects does not make accurate short-circuit current calculations at present.

Correct method: Calculate the short-circuit current of the low-voltage side for the transformers of different capacities of each project, including at least the peak value of the short-circuit current: ip

Steady State Short-Circuit Current RMS: Ik

When choosing a circuit breaker, at least the rated ultimate short-circuit breaking capacity of the circuit breaker should be guaranteed: Icu

Not less than: Ik

For each project, a cable with the smallest cross-sectional area and a short path should be selected for short-circuit current thermal stability verification, and a load terminal with the longest distribution path should be selected for voltage drop analysis. If the cable cross-sectional area does not meet the requirements, the cable cross-sectional area should be adjusted.

Under the condition that the system capacity is infinite, the circuit current parameters at the outlet of the low-voltage side of the transformer corresponding to different capacity and short-circuit impedance voltage parameters must be greater than the short-circuit capacity of the actual project whose short-circuit capacity of the system is not infinite. It can be used to check the low-voltage distribution. The breaking capacity of the electrical circuit breaker.

For another example, it can be seen from the selection table for short-circuit current of low-voltage copper core XLPE cables that for transformers of 1000kVA and below, the cross-sectional area of the distribution cable on the low-voltage bus shall not be less than 10mm2; for transformers greater than 1000kVA, the The cross-sectional area of the distribution cable shall not be less than 16mm2.

For large-scale or important public construction projects, short-circuit current calculation is recommended when short-circuit current has a great influence on equipment selection.