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. 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 (this is an analysis well-suited for EMTP).
| Short Circuit Reference (ANSI) | |
| Faulting a Bus |
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