Fault Current Contributions from Wind Plants

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

Wind-Turbine Generator Types
Wind-turbine generators (WTGs) are typically classified into five types. Type I is a squirrel-cage induction generator connected directly to the grid. 

Similarly, Type II is directly connected to the grid and consists of a wound-rotor-induction generator with an adjustable resistor in the rotor circuit. Type III is an asynchronous wound-rotor generator that has a three phase AC field applied to the rotor from a partially-rated power-electronics converter.
Type IV is an AC generator in which the stator windings are connected to the power system through a fully-rated power-electronics converter. 

Type V WTGs use a variable speed / variable ratio hydraulic transmission to drive a conventional synchronous generator.   This type has seen very little application, and will not be discussed further in this article.

Both Type I and II generators are commonly configured with shunt power-factor-correction capacitors on the generator terminals, as shown in Figure 2 and Figure 3. Both Type III and IV generators, as shown in Figure 4 and Figure 5, are able to produce real power and reactive power through the control of their power electronics. 
The vast majority of transmission-connected WTGs installed today are Type III or Type IV.  A brief extract from the joint working group report on the fault behavior of these WTG types is given below.  In the context of the following sections, the term “fault” refers to under voltage conditions, unless otherwise indicated.

Type III Wind-Turbine Generator:  Type III double-fed asynchronous generators (DFG) are used in wind generation to provide variable speed operation over a wide range (typically ±30% of synchronous speed), and highly responsive reactive power and AC voltage regulation capabilities. Closed-loop regulation of Type III machine output has a typical response time of two or three cycles or less. These machines are also commonly called double-fed “induction” generators. However, their performance capabilities and fundamentals of operation are substantially different from any induction machine. The topology of a typical DFG wind generator is shown in Figure1.

A DFG can appear similar to a synchronous machine, because its rotor’s flux rotates at synchronous speed. However, the operational behavior is quite different. The inherent characteristics of the DFG machine provide fast control of the real and reactive power output of the wind turbine.

These characteristics are the controllability of the voltage-source converters used in the machine, and the fact that AC excitation of the rotor necessitates a laminated rotor design.  A laminated rotor results in very short rotor flux time constants; far shorter than those of a synchronous generator.  In practice, these factors yield an approximately constant source of real power and voltage regulation response that is much faster than the response of a synchronous generator.

Because the power converter responsiveness and the very short time constants of the laminated rotor allows extremely fast control of the applied rotor excitation, the real and reactive power of a DFG machine can be precisely controlled at high bandwidth. The control of real power plays a critical role in mitigating mechanical loads imposed on the wind turbine. The fast control of reactive power provides voltage regulation capability approaching that of a STATCOM, which is instrumental in achieving stringent low-voltage ride-through requirements imposed by most grid codes today.

Because of these LVRT requirements, and the aerodynamic efficiency advantages of variable speed operation, Type III and Type IV (full conversion, described later) wind turbine technologies have largely supplanted the Type I and Type II induction generator technologies in North America and other markets.

Type IV:  The Type IV WTG is composed of an electrical machine interconnected to the collector system through a full-scale back-to-back frequency converter. The electrical machine of this wind turbine type may use a synchronous machine excited either by permanent magnets or electrically, or an induction machine.  As described above and in contrast to the other WTG technologies, the generator of Type IV WTGs is completely decoupled from the grid, so a gearbox may not be required. If an induction generator is used, a gearbox is often included in the design.  The electrical output is completely defined by power electronics, i.e. the full-scale converter, and not the inherent behavior of the generator.
This design allows Type IV WTGs to rotate at an optimal aerodynamic speed providing extreme flexibility in generation in combination with excellent grid integration characteristics such as flexible reactive power capabilities and a wide voltage and frequency operating range.

The full-scale, i.e. back-to-back, frequency converter is composed of a rectifying bridge, typically in the nacelle, DC link, and inverter either in the nacelle or at ground level within the WTG tower.  The DC link provides the inverter the ability to be controlled and deliver output power independent of the input power of the machine within manufacturer specified voltage ranges of the DC link. The inverter is controlled to synchronize its output with the collector system frequency.

Type IV WTGs can dynamically inject / absorb reactive power to / from the grid over a wide active power output range. Furthermore, most WTGs of this type can be designed to provide STATCOM-like characteristics, i.e. dynamically provide the full amount of reactive power over the entire active power range.  Under and over voltage ride through capabilities of Type IV WTGs are not limited to remaining in operation and connected to the grid during contingency conditions.
Due to the power electronics, which may also include a bypass resistance to transform electric energy into heat during contingency conditions, the active and reactive current injection behavior during a fault can be controlled and does not depend on the inherent behavior of the electric generator.

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.

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    

Biographies

Dustin Howard was born in Cairo, GA, USA.  He received a B.S. and Ph.D. in Electrical Engineering in 2008 and 2013, respectively, from Georgia Institute of Technology in Atlanta, GA.  Dr. Howard’s Ph.D. work focused on the short-circuit modeling odistribution, renewable energy, and distributed generation technologies to utilities, developers and research institutions.  Prior to founding this form in 2012, he was with GE Energy Consulting for 31 years.
Mr. Walling received his B.S. and M.Eng., in Electric Power Engineering from Rensselaer Polytechnic Institute, and is a registered Professional Engineer (Minnesota).   He is a member of CIGRE, Fellow of the IEEE, has authored over ninety papers and articles, and has been awarded thirteen patents

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