Fault Current Contributions from Wind Plants

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

Fault Response of Type III and Type IV Wind-Turbine Generators

f wind-turbine generators.  He developed frequency-domain models of several wind-turbine generator types for use in protection system analysis and design.  Through this work, Dr. Howard gained extensive experience in the transient simulation, hardware implementation, and control of electric drives and power electronics.
Dr. Howard joined GE Energy Consulting in 2013.  Since joining Energy Consulting, his focus has been on modeling electric machines, motor drives, and power electronics for harmonic and sub-synchronous torsional interaction studies. 

Reigh Walling is the principal of Walling Energy Systems Consulting, LLC, where he provides his expertise in transmission, Type III Wind-Turbine Generator Fault Response:  For the Type III WTG, severe faults cause excessive voltage to be induced onto the machine’s rotor, which are, in turn, imposed on the power converter. It is not economical to design the converter to withstand the voltages and currents imposed by the most severe faults. Thus, a crowbar function is used in practice to divert the induced rotor current. There are various approaches to achieving this crowbar functionality, including:

  • A shorting device (typically using thyristors) connected in shunt between the machine’s rotor and the rotor-side power converter. The crowbar may include some impedance in the shorting path. This option is illustrated in Figure 1
  • Shorting of the rotor via switching of the rotor-side power converter
  • A chopper circuit on the converter’s dc bus, to limit dc bus voltage by diversion of some or all of the current coming from the rotor is more often used in newer designs

While the crowbar function is engaged, the Type III DFG generator effectively becomes an induction generator, but with practical differences introduced by the substantially different pre-fault conditions possible. Unlike a Type I or II generator, which operate with a relatively small slip, a Type III generator can operate with a large slip (typically up to +/- 30%). Following application of the crowbar, the potentially large slip can create significant rotor-current-induced frequency components in the stator windings, producing sinusoidal fault current contributions that are not at the fundamental frequency. This condition can be observed in the first few cycles of fault current in Figure 6.

In the crowbarred state, the fault behavior is defined by the flux equations of the physical machine. When the crowbar is not engaged, however, the machine operates according to its control design. Unlike induction machine fault current performance, which is established by the physics of the machine, there is a wide range of possibilities in the design and objectives of Type III generator controls. Variations can be wide between different manufacturers, and even different models from the same manufacturer.  Control design practices evolve over time, in response to changing grid requirements and equipment capabilities.

In addition to the variations in controlled behavior, the criteria for applying and removing the crowbar function can also vary widely. Different measures may be used for the crowbar threshold, such as rotor AC current or DC bus voltage, as well as different magnitude thresholds for each of these measures. In older designs, once a machine was crowbarred, it was tripped. In more modern designs, the machine may switch in and out of the crowbarred state, sometimes repeatedly.

In summary, there are basically three different regimes of fault current behavior for Type III DFG wind turbines, depending on fault severity:

  • Very severe faults where the crowbar is applied and not removed, thus providing the fault current performance of a simple induction machine
  • Faults of insufficient severity to cause crowbar operation, for which injected currents are con-trolled and performance is very similar to a Type IV (full conversion) wind turbine
  • Faults of intermediate severity where the nonlinearities of crowbar operation are critical, resulting in complex behaviors

Type IV Wind-Turbine Generator Fault Response:  The fault response of Type IV WTGs is fundamentally determined by the control strategy implemented in the full-scale frequency converter, which varies significantly among manufacturers.
Often state-of-the art Type IV WTGs only inject positive-sequence current under all operating conditions including balanced and unbalanced faults. Hence, the negative and zero sequence components of the current during a fault are non-existent.  However, it should be noted that the technology allows Type IV WTGs to contribute negative-sequence current, if required.

In most cases the inverter is controlled for constant power output with current limiting functionality. The current limiting function in many WTG technologies is often set close to the rated inverter value, e.g. 1.1 per unit, but can be higher depending on cooling and the rating of the converter.
The current limit may also be dynamic, allowing higher currents for a short period of time and then reducing the limit to stay within equipment capabilities.  This value is then equivalent to the maximum current contribution of the WTG.

However, other markets might specify different current injection behavior during faults such as no current injection, or maintenance of the pre-fault current. If the fault response is not specified by a grid code or the utility, the manufacturer will define what the response will be. 
Hence, no general statement with regards to the fault contribution of Type IV WTGs can be made.  Figure 7 provides an example of current contribution from a Type IV WTG for a three phase fault on the collector network.  In this example, the current limit is ramped over approximately three cycles from approximately 1.5 to 1.1 p. u.

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