Why does the transformer trip?

A ground fault occurs on the low-voltage side, causing the zero-sequence protection on the high-voltage side to activate and trigger a trip; it is also possible that the low-voltage switch fails to operate

Could you help analyze the reasons in detail? Our project is under construction, and no actions have been taken regarding the low-voltage system; as a result, the transformer tripped and all power supply for the construction work was cut off. We suspect that the setting values are incorrect

Is the setting of the switch protection on the low-voltage side correct? The tests passed, with no failures in operation. Is the setting value of the zero-sequence protection on the high-voltage side too low? If the setting is incorrect, it is a setting issue. Is the timing coordination appropriate?

The protection on the low-voltage side is usually provided by a frame circuit breaker, which has two types of protection: one with a long delay and another with a short delay. These correspond to overcurrent and instantaneous fault protection respectively. However, its sensitivity is practically negligible; it will only trip when a short-circuit condition is reached, that is, when the T1 time is reached. As for why the high-voltage side trips after a short circuit, it’s because the setting parameters for the high-voltage side protection are more precise, and it offers a variety of protection functions such as overcurrent, instantaneous fault, overload, and low-voltage zero-sequence protection. The tripping in your case might be caused by low-voltage zero-sequence protection or instantaneous fault protection on the high-voltage side. Check whether the contacts of the low-voltage switch have stuck together; I’ve encountered such a problem before with ABB frame circuit breakers – a busbar breakage caused an instantaneous fault trip on the high-voltage side, and the contacts of the low-voltage circuit breaker could no longer open.

It’s likely that the protection setting value for the low-voltage incoming line switch is too low! Or there is a fault in the low-voltage protection circuit, preventing the protection from tripping!

Furthermore, if it is a outlet cable, then it is definitely because the protection trip setting for the outlet switch has not been adjusted properly.

Check the setting of the protection settings for the low-voltage outgoing main circuit breaker; it should ensure selectivity

This post was last edited by zyz8438 on 2010-8-22 08:18. Actually, there are still a few things that need to be clarified: The 95 cable is located on the load side of the switch on the low-voltage side of the transformer, right? Is this cable the output cable of the transformer, or a cable from one of the feed circuits on the low-voltage side bus of the transformer? If the load side of the transformer is equipped with a busbar and has several feed circuits, then the circuit breaker on the low-voltage side of the transformer will not be fitted with an instantaneous protection device. Because when a feeder cable short-circuits and explodes, the short-circuit current is almost identical to that of the busbar, making it impossible for the quick-break protection to provide selectivity. Additionally, the wiring configuration of this box-type substation is unclear. If it is not a Y/Y connection, then a single-phase ground fault on the low-voltage side will result in an overcurrent protection response rather than a zero-sequence protection action on the high-voltage side!

The main issue is that the setting values are not properly adjusted – the high voltage is too low while the low voltage is too high. It is necessary to first check whether the setting values are appropriate; if needed, verify the operation and setting values of the circuit breaker and the low-voltage switch. If there are problems with both of these, it could be due to the low-voltage switch failing to operate or a fault in the control circuit, among other possibilities