Although thyristors have many advantages, their ability to withstand overvoltages and overcurrents is a major weakness of thyristors, and appropriate protection measures must therefore be taken in all thyristor devices.

Overcurrent protection of thyristor

Due to the small heat capacity of the thyristor, once the overcurrent occurs, the temperature will rise in sudden and may burn out the PN junction, causing a short circuit or open circuit inside the element. 

The causes of thyristor overcurrent are as follows:

Overload or short circuit at the load end;

A thyristor is broken down and shorted, resulting in overcurrent of other components;

The trigger circuit works improperly or is disturbed, which causes the thyristor to be triggered by mistake and causes overcurrent.

The overcurrent capacity of a thyristor, such as a 100A thyristor, is very poor. This means that when the thyristor of 100A has an overcurrent of 400A, it is allowed to last only 0.02 s, otherwise it will be damaged by overheating. 

It can be seen that the thyristor is allowed to withstand a certain amount of overcurrent in a short period of time, so the role of overcurrent protection. In the event of an overcurrent,  the overcurrent is cut off within the allowed time to prevent component damage.

How to prevent thyristor overcurrent burning? There are 3 ways:

1. Use high speed fuse

The conventional time delay fuse links, which is used to protect the thyristor, but due to its long fuse melting duration, will probably not have been melted after the thyristor has been burnt out, thus cannot provide protection efficiently.

Therefore, a high speed fuse specially designed to protect the thyristor must be used.High speed fuses are with silver melting fuse element,  at the same overcurrent  passes, high speed fuse before the thyristor will be damaged to protect thyristor timely. This is the main measure of thyristor overcurrent protection. 

There are three assembly methods for fast acting fuses.

1, To prevent secondary damage to supplied power

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• To protect from condenser short circuit or IGBT short circuit … Applicable to FUSE A

• To cope with accidents from condenser short circuit, IGBT short circuit, and diode short circuit … Applicable to FUSE 1 and 3.

• To cope with condenser short circuit, IGBT short circuit, diode short circuit, earth short circuit … Applicable to FUSE 1, 2, and 3, and also to FUSE 4, 5, 6, 7, 8, and 9.

2, To prevent diode chips from being damaged

• If you would like to prevent explosion or ignition of chips with fewer fuses:

• To prevent damage to a chip by adverse DC current … Applicable to FUSE A.

• To prevent damage to a chip by supplied power current … Applicable to FUSE 1 and 3.

• To prevent both of the above … Applicable to FUSE A, 1, and 3.

• If you would like to reuse sound chips as well as to prevent explosion or ignition of chips … Applicable to FUSE 4, 5, 6, 7, 8, and 9.

3, To prevent explosion and short-circuit mode of IGBT or thyristor chips

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• If you would like to protect with fewer fuses … Applicable to FUSE B.

• If you would like to reuse sound chips (only for thyristors) … Applicable to FUSE C, D, E, F, G, and H.

• For devices from a few kilowatts to tens of kilowatts, fuses are often applied to FUSE 1, 3, and A.

For Bussmann high speed fuse, pls visit: 

https://bussmannpower.com/product/list-fast_acting_fuse_Bussmann-cn.html

You can ask our engineers to select the proper fuse.

Also with Ferraz Shawmut fuse cross reference part numbers.

Caution: The current ration of the fuse shall be as close as possible to the effective value of the actual operating current and shall be selected on the basis of the current ration (mean value) of the protected element.

2. Overcurrent relay

Install a DC overcurrent relay at the output end (DC side). Or in the input end (ac side) through the current transformer connected to the sensitive over-current relay,  in the case of over-current fault action,  so that the input switch trip. 

For example: ABB, 2TLA010026R0000, relay

This kind of protection measure is effective for overload, but in the case of short circuit fault, due to the over-current relay action and automatic switch trip need a certain time, if the short circuit current is relatively large, this kind of protection method is not very effective.    

3. Overcurrent cut-off protection

The over-current signal is used to shift the trigger pulse of the thyristor so that the conduction Angle of the thyristor is reduced or the triggering is stopped.