Fault current capability of DFIG generators 1

[rockman7892]

I've been looking at several wind projects that use Type 3 (DFIG) and Type 4 (Inverter) generator types. From what I've read both of these types of generators have limited fault current contribution capabilities due to the presence of the inverters.

I've been familiar with limited fault current contribution on inverters and thus makes sense for the Type 4 gens but curious about how fault current is limited on the DGIG type gens. Since the stator is still directly connected to the system doesn't that allow fault contribution similar to a typical sync gen?
re
Perhaps what I'm missing is the role that the rotor plays in fault contribution in a typical sync gen and therefor has an impact on reducing contribution from a DFIG due to inverter control with rotor?

Appreciate any insight to help my understanding.

 

[waross]

curious about how fault current is limited on the DGIG type gens.

Are you comparing fault current levels to grid fault levels or to synchronous generator fault current levels?
Individual generators typically have fault current capability far less than grid fault currents.
Before the widespread application of Permanent Magnet Generator excitation, many islanded plants struggled to provide enough fault current to activate instantaneous protection.
It was common to see current boost circuits to increase the fault current.
The trend from self excitation to PMGs has raised fault current levels somewhat but levels are still much less than grid scale.
You may be seeing an effect common to smaller (Less than grid scale) generators.

 

[Gr8blu]

One of the overlooked factors in wind power is the duration of a fault. Until the rotor comes to a complete stop - regardless of whether the breaker opens to isolate the stator from the grid - the fault will continue to be fed from the moving rotor. Most modern DFIG gens use a "power take off" approach, where the rotor is powered from an inverter which is tied to the stator output - BEFORE it gets to the inverter which turns it into line frequency for the grid. Thus, if the fault is internal (i.e. gen) side of the main inverter, it stays "live" until the rotor stops. If the fault is between the main inverter and the grid, it can usually be of limited duration due to the action of a breaker.

PMGs are similar - there is simply no way to turn the magnet off, so as long as it spins, it feeds the winding to which it is connected - which may or may not be the winding that is faulted. Either way, the fault continues to be "live" until movement stops.

Of course, enough damage to the winding and an open circuit occurs, which would effectively snuff out the fault. But the winding (and probably the core iron) will be finished.

 

[waross]

PMGs are similar - there is simply no way to turn the magnet off,

Either the inverter circuit or the output of the PMG may be switched off.
I agree that a fault internal to a PMG can not be turned off, but the output of the PMG and both the input and the output of an inverter may be switched off by protection circuits.
And "Fault current contribution" generally refers to external faults.

 

[electricuwe]

Look for the keywords "low voltage ride through" or "grid code". These regulation define how wind turbines have to behave during a fault. In general they have to stay online for a defined duration and feed in a little more than nominal current during that time.

Power from the turbine has to be fed into a dynamic brake resistor or a crowbar. This is independent of the kind of generator, mainly due to the fact that the DFIG (doubly fed induction generator) is more a synchronous generator with a three-phase winding than usual induction generator.

Unless there is sufficient time to change the blade pitch, power of the generator has to be dissipated to avoid increase in speed.

 

[wcaseyharman]

I’m working on relay settings for a type 3 wind farm. According to the manufacturers literature, the rotor inverter controls the fault current of the generator unless the voltage across the electronics get too high, then a crowbar operates to short the rotor circuit and protect the rotor electronics. The fault current is about 4 times higher when the unit is crowbarred in my Aspen model.
The documentation suggests the crowbar typically operates for close-in faults on the medium voltage system.

 

[electricuwe]

For type 3 the stator winding is able to provide a significant SC current as described by wcaseyharman. However, this is not mandatory in most areas and it creates significant stress to the rotor side inverter, making its design challenging.