DC short circuit calculations are performed for steady state conditions, using branch resistances, source resistances and short circuit output of converter equipment. The inductance and therefore the rise time of DC short circuit currents have been neglected.
Also, the rectifier/inverter output source model does not fully support switching on the input side. For example, if a DC generator source is switched off on the input side of an inverter, the output will not see the difference due to the user-entered FLA x Mult value. This requires you to manually change inverter/rectifier output currents when switching input sources in or out. One exception: If ALL input side sources are removed, then the inverter/rectifier gives zero output. But if any source remains, then the inverter/rectifier output is based on its own user-entered output entry. This means you are required to add an input side source for the inverter to have any effect in short circuit.
The Thyristor and Diode models for the AFD, Rectifier and Inverter are not modeled in detail to simulate actual behavior under short circuit conditions. To do so would require an extreme level of time simulation detail to properly model the interaction of the AC and DC systems.
Under short circuit conditions on the DC bus, a Rectifier actually pulls current from the AC system dependent upon the location of the fault on the DC system. If the fault is directly on the DC bus of the inverter, then a bolted three-phase fault appears on the AC side with little to no distortion. The fault current is then limited by the system Thevenin short circuit impedance and the equivalent resistance of the Thyristor or Diode bridge.
If the fault is located some significant distance from the rectifier after a DC cable, then both the AC and DC side fault currents will not be fully bolted, and we will have an intermediate condition where only a transient simulation can determine the fault level for us. This, and the fact the DC side current has excessive ripple lead to assigning such simulations to transient methods, which are presently beyond the scope of EasyPower to do (an analysis suited well for EMTP).
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.
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.
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 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.
| ANSI Short Circuit Reference | |
| Faulting a Bus |