AC Output Bus Fault

Under short circuit conditions on the AC output bus, again the AC and DC systems respond to the fault. What this response is, however, is completely dependent upon the output frequency of the AFD and inverter. The fault as it appears on the DC bus can create extreme ripple with a waveform characteristic that is not easily understood, and thus only a transient level simulation can help in this matter. For an AFD that has a diode or thyristor front end and a thyristor output, the fault will even manifest on the input of the AFD. Thus again, a simulation with transient simulation of the rectifier and inverter bridges is essential to simulate such fault conditions.

Thyristor Blocking

One other condition that controls the fault current time is thyristor blocking (shutoff) under fault conditions. Most likely, a thyristor device will stop firing when it senses a short circuit condition. This time is up to manufacturers, and may be a software or hardware timing system. This is a detail left typically for detailed transient simulations of a well-modeled bridge.

Thyristor/Diode Fuse Protection

Under conditions of a DC bus fault, most likely current limiting fuses (typically in series with each thyristor or diode) will blow to mitigate the fault. To simulate this, again a fully detailed model of the bridge is needed; with fuses input into each component as it physically exists. Given that bridges can have both series and parallel combinations of thyristors and diodes, significant detail is needed in the specification and construction of the bridge model.

More Information

Short Circuit Reference (ANSI)
Faulting a Bus