Short circuit current contribution of a VFD
Electric motors and generators contribute to the total fault level at the connection point. Their short circuit current is typically in the range 4…6-times the nominal current of the machine. How is the situation with a variable frequency drive? The answer is rather simple and yet sometimes confusing the community. Let us look what is the short circuit current contribution of a VFD.
Why do we need to know the short circuit level?
Calculation of short-circuit current is a standard discipline. The purpose is to determine the fault level (and optionally also the waveshape) at different locations. Such information is used for system dimensioning (dynamic and thermal effects of short circuit current), for rating of components (e.g. breaking capability of a circuit breaker, selection of power cables etc) or for parameterization of protection settings. In most cases we are interested in the maximum short circuit current level. However, minimum level is of interest as well, for instance for proper fault detection.
Sources of short-circuit current
In an industrial network or a utility grid there are typically multiple sources of short-circuit current. Obvious sources are the feeders. Also the load can be source of short-circuit current. An electric motor operated across the line is a short-circuit current contributor to the grid fault. With the growing installed base of variable frequency drives (VFD) the frequent question is how (much) is the short circuit current contribution of a VFD.
The fault behavior of the VFD as seen from the grid side depends on the type of VFD:
Diode front end (DFE) drives only allow power and current flow in one direction: from the grid to the motor. Opposite direction is not possible. Thus, DFE drives have no current contribution to a fault happening in the grid.
Active front end (AFE) drives in contrast allow bi-directional power flow and can contribute with their short circuit to the total fault level. However, the waveform is different than the one from machines. The first current peak can be relatively high but its duration is just a few milliseconds. The VFD continuously measures the grid voltage and current (to keep synchronized with the grid and to control the power factor) and thus immediately detects the fault and protects itself by blocking the pulses and opening the incoming circuit breaker (or eventually blowing the fuse if fuses are used). Consequently, the VFD contributes to the first peak of the short circuit current in the grid but then disappears very fast. When talking e.g. about 1 second fault level the current contribution of a AFE type of VFD is almost negligible.
The statement about VFD contribution is since few years also officially provided in IEC 60909 Short circuit currents in three phase a.c. systems:
“Reversible static converter-fed drives (for example, rolling mill drives) are considered for three-phase short circuits only, if the rotational masses of the motors and static equipment provide reverse transfer of energy for deceleration (a transient inverter operation) at the time of short circuit. Then they contribute only to the initial symmetrical short-circuit current Ik” and to the peak short-circuit current ip. They do not contribute to the symmetrical short-circuit breaking current Ib and the steady-state short circuit current Ik.”
A special case would be a bi-directional (AFE) type of VFD that provides the grid support functionality to the grid. Example of such application may be a frequency converter in a variable speed pumped storage plant. In such scheme, the VFD supplies the motor in pump mode and transfers power from the generator into the grid in turbine mode. In case of a far fault in the grid (which is seen at VFD grid connection point as a voltage dip), the VFD injects certain amount of reactive current into the grid to help to recover. Such grid supporting function takes as much time as needed (typically from some tens up to few hundred of milliseconds) and the VFD injects current into the grid. However, the injection normally stays below the nominal current level of the VFD. Power semiconductor converters do not have inherent overloadability and cannot provide high short circuit current in the way rotating electric machines do.
Summary
Long story short – the take away is simple:
- DFE drives do not contribute any current into a grid fault. It is inherently impossible since the diodes allow only one direction of current flow: from the grid into the dc link.
- AFE drives do make a contribution to the grid fault. In case of a physically near fault the converter typically provides a short peak current (transient) that is quickly detected by the VFD control and protection system. The current contribution of a VFD is much smaller compared to an electric motor of same power rating.
References
[1] Short-circuit currents in variable speed drive systems, https://mb-drive-services.com/short-circuit-currents-in-variable-speed-drive-systems/
[2] How to calculate the short circuit current in a correct way, https://mb-drive-services.com/how-to-calculate-the-short-circuit-current/
[3] IEC 60909 Short circuit currents in three phase a.c. systems
