DC fault protection

A fault in the DC power system is detected on the rectifier side when the following two conditions occur:

• The DC voltage falls below a specified threshold for a period of time (adjustable);
• No low AC voltage is detected on the rectifier side (characterizing a fault on the AC side).

This detection is done in the “Low AC voltage detection” function described earlier. When a fault is detected on the DC side, the rectifier α angle is forced to an adjustable value greater than 90^o so that the rectifier operates as an inverter to de-ionize the fault. This forced α is removed after an adjustable period of time. This operation will be repeated until the fault is cleared or the converter is blocked by the protection system after the operation has been repeated
a number of times.
This function is implemented for simulating faults on the DC side. The function does not discriminate between the faults on the AC side of the inverter and commutation failures on the inverter, both causing a drop in direct voltage. One way to disable the protection is to assign a very large value to “detection time” parameter. No action is taken on the inverter side since it is already in safe condition.