Fault Current Contributions from Wind Plants

Authors: Dustin F. Howard, GE Energy Consulting, USA, and Reigh Walling, Walling Energy Systems Consulting, USA

Short-Circuit Modeling of Type III and Type IV Wind-Turbine Generators

Type III Fault Modeling:  The fault behavior of a Type III WTG is complicated by the inherent discontinuous behavior between the normal and crowbarred states. And, in the non-crowbarred state, the behavior is substantially the product of control designs based on a wide range of possible design philosophies and equipment capabilities. Thus, it is not possible to describe a generic short-circuit model for Type III wind turbine generators providing accuracy over the range of possible fault severities.

Fortunately, the maximum fault current results from the crowbarred state, and the short-circuit behavior when crowbarred can usually be calculated using existing short-circuit analysis software and the generator’s physical parameters. This maximum current can be calculated using the generators sub-transient reactance, typically on the order of 0.2 per unit on generator rating.  Maximum current is the limiting condition for purposes such as determining equipment fault current withstand.
Protective relaying and fusing must be coordinated over the full range of operating conditions. Because a wind plant may have any number of its wind turbines operating at a given time, the short-circuit contribution varies from zero (with no wind turbines in operation) to the maximum current with all turbines operating and in the crowbar condition for a close-in transmission fault.

Also, fault current contributions from Type III wind plants, particularly when operated in the controlled state, tend to be dwarfed by the typically much larger contributions from other sources in the transmission grid.  Thus, detailed and highly accurate models of Type III wind turbines in the controlled (non-crowbarred) state may not be routinely needed.
Where highly accurate short-circuit modeling is necessary, phasor-domain short circuit analysis tools do not have sufficient capability, and the only recourse is detailed electromagnetic transient (EMT) simulation.  EMT programs are fully capable of modeling wind turbines in great detail, sufficient to perform any needed short-circuit current analysis.  However, most protection engineers do not have the skill set required to perform such analyses nor possess the proprietary control details of the WTG controls required to model the WTG in a meaningful way.

Type IV Fault Modeling: As described in the previous sections, the fault contribution of Type IV WTGs depends on the inverter control strategy implemented by a specific manufacturer and the applicable grid code. In this context, highly accurate short-circuit modeling may be quite difficult to achieve with the only practical alternative a compromise between accuracy and complexity of the wind turbine model.

For short circuit studies, Type IV WTGs act as a controlled current source, with current limited to protect the converter electronic devices. The operating point of a Type IV wind turbine may in principle have any value between zero and the converter maximum current. That provides a boundary of the WTG contribution to fault current.
A rough calculation of the minimum and maximum system short circuit power is then possible by taking the extreme cases: no current injection during fault and injection of maximum inverter reactive current.  If the simulation software provides only the classical model of a voltage source behind impedance, usual for traditional generators, the calculation may be iterated to impose the desired current.

A better approximation may be obtained by considering the applicable grid code requirement regarding fault ride through. A typical requirement is the injection of reactive current as a function of residual voltage.  But also in this case some assumptions will probably be necessary as most grid codes do not specify values for active current injection during faults. The amplitude and angle of the fault current is therefore defined by each manufacturer in order to optimize the operation of the full-scale converter.
Further accuracy can only be achieved using proprietary time-domain models provided by the WTG’s manufacturer.

Conclusion: The prevalent use of power electronics in modern wind turbines cause power plants of this type to respond to faults in a fundamentally different way than a conventional power plant with a synchronous generator.  Therefore, additional considerations are required when performing protective relaying studies and establishing equipment rating, and the established tools used for performing these studies should be used with caution.

For more detail regarding the fault response of Type III and Type IV WTGs, along with the other WTG types, please reference the full joint-working group report from the IEEE PSRC web site at:
www.pes-psrc.org/Reports/Fault%20Current%20Contributions%20from%20Wind%20Plants.pdf    

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BeijingSifang June 2016