How to avoid the transformer inrush current during automatic transfer switch operation?

Dear sea friend, the situation you mentioned almost certainly does not exist, unless it is under abnormal operating conditions! Firstly, during normal operation, it is clearly inappropriate for multiple transformers to operate at no load or for multiple transformers to operate at half load; that unit does not permit such a mode of operation either. Generally speaking, it is understandable for a machine to be running without a load, but it is clearly inappropriate for multiple machines to operate without a load at the same time. Under normal circumstances, when adjusting the system protection settings, we certainly do not take this situation into account. If we really have to consider this situation, of course there are ways to solve it. After all, the inrush current during no-load operation of transformers follows certain patterns. Generally, the protection setting can be set at 3.5 to 4.5 times the rated current of the transformer; this takes into account the inrush currents of all no-load transformers on that bus, as well as the current generated by self-starting loads (when they are activated). Based on this, it is possible to estimate the maximum current value ； At the same time, by comparing it with the calculated value of the incoming line short-circuit current, if it is less than that value, then we need not consider the effect of the transformer’s inrush current ； When the current exceeds this value, since the excitation inrush current of the transformer lasts only a few cycles (less than 100 milliseconds), we can temporarily increase the delay time of the short-circuit quick-break protection to more than 100 milliseconds, thereby avoiding the impact of this excitation inrush current. Of course, this approach may have the downside of increasing the severity of the short-circuit fault. Once normal operation is restored, the delay can be canceled directly.

Thank you very much for your analysis. In such a scenario, the 10KV power supply feeds all 6 transformers in the plant; each pair of transformers serves as a backup to one another, with one operating under load while the other is idle. As a result, three transformers operate without any load. If there is a failure in this 10KV power supply, the backup 10KV power supply comes online to supply power to the entire plant. At that point, the issue of inrush current in the transformers needs to be taken into consideration – this current is likely to be greater than the quick-break current rating of the incoming switch. Assuming the rated current for each of the six transformers is 80.

I understand the OP's question. Under normal operating conditions, each of the two power supplies operates with half of the transformers. When a power supply circuit fails, the automatic transfer device comes into action; at this point, the transformer connected to the functioning power supply continues to operate under load, while the transformers connected to the failed power supply must be subjected to another power-on surge. Worried that the inrush current is very high, asking how to set it appropriately? 1. First, after the automatic transfer switch operates to close the circuit, the inrush current that occurs as the transformer connected to the faulty power source is re-energized is not an excitation inrush current. The generation of inrush current usually occurs when a no-load transformer is powered on, due to the presence of residual magnetism. The situation regarding the closing of the automatic backup switching device is different: the load connected to the transformer is not disconnected, and because the power outage lasts only a short time, the motors, due to their high inertia, generate a back EMF against the transformer. As a result, there is no residual magnetism in the transformer, and hence no inrush current. 2. After the automatic transfer switch operates, an inrush current does indeed occur when the transformer connected to the faulty power source is powered again. This inrush current is mainly caused by the decrease in back EMF as the speed of the motors on the load side drops, leading to an inrush current during reclosing, as well as the current required to restart those motors.