Protection for commutation failures

Variation of Gamma cf when Detecting a Commutation Failure

The protection against commutation failure is provided on the inverter only. The reason is the firing angle on the inverter side is large and, therefore, the inverter is more prone to commutation failures.
The detection of commutation failures is based on the following principle. In normal state, the currents (in pu) on the AC side and on the DC side are nearly equal. A commutation failure instantaneously creates a short circuit on the DC side and the DC current increases quickly. The inverter then has a null voltage and hence, the current drops on the AC side.

When a commutation failure is detected, the protection decreases the upper limit of alpha by gamma_cf as shown in equation 7. This keeps alpha away from the area where the risk of commutation failure is high. gamma_cf then takes on the form shown in Figure 11-12. gamma_cf increases quickly (adjustable first-order time constant) when a commutation failure is detected for quick protection, but decreases more slowly (adjustable first order time constant) to avoid successive commutation failures.

It is possible to simulate commutation failures by preventing the current from switching from one thyristor to its neighbor. To do so, a given thyristor (1 of 12 as per user’s choice) is prevented from receiving the firing pulse for a given period of time (adjustable).