What makes protection firing so brilliant?

Protection firing, sometimes also called “firing through”, is a VFD protection
concept used in some medium voltage drives. The principle is well described in its name:
In case of a severe fault the VFD will turn on all the active switches (such as power
semiconductors in the inverter stage, crowbar thyristors etc). Thus, one or multiple short
circuits are created within shortest time. It may sound a bit scary but it is actually
an ingenious concept. It provides outstanding protection of the VFD itself. Moreover,
protection firing also makes good for the transformer and motor. Wonder why or how?

The concept of protection firing is relatively simple, extremely robust and proven in the field as it is being used for two decades.

First of all, protection firing allows fuseless VFD design. It means that no fuses are used for the protection of the VFD. That is certainly a big advantage as

Medium voltage fuses are subject of aging
Standard fuses are often too slow to protect semiconductor equipment
After a fault fuses need to be replaced (cost, time to replace)
Protection firing is much faster than any fuse

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Moreover, protection firing is extremely efficient in eliminating an arc fault. Thus, hardware damages are minimized and there is a huge contribution in terms of personal safety. Protection firing itself, eventually combined with additional sensors (such as Arc Guard) enables fast arc detection and elimination. The effect is similar as when using an external short circuiting device (e.g. UFES). However, in this case it is an inherent part of the VFD. Therefore, no extra hardware is required and no engineering effort needed.

Protection firing also minimizes dangerous dynamic forces acting on transformer windings in case of a fault. Thanks to that, so called “catapulting” or “telescoping” of the windings can be prevented. The telescope effect is known as a killer of many converter transformers hence its prevention is of crucial importance.

Last but not least, protection firing is helpful in minimizing machine transient torques and corresponding mechanical stress of the machine itself and all other shaft components including the driven load. This is something that mechanical engineers certainly appreciate.

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