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.

Relion advanced protection & control.
Protecting your electrical assets? today and tomorrow