Accommodating High Levels of Variable Generation - NERC

Characteristics of Power Systems & Variable Generation
speed) dictated by the speed-torque characteristic of the induction generator. As wind speed
varies up and down, the electrical power output also varies up and down per the speed-torque
characteristic of the induction generator. In its simplest form, this technology has a fixed pitch
and is aerodynamically designed to stall (i.e. naturally limit their maximum output). The
primary advantage of Type 1 induction generators is their simplicity and low cost. A major
disadvantage is the significant variation in real and reactive power output correlated to wind
speed changes. Simple induction generators consume reactive power primarily dependent on the
active power production. Type 1 wind turbines generally incorporate reactive compensation in
the form of staged shunt capacitors to correct power factor.
Type 2 Variable-slip Induction Generator - The variable-slip induction generator is similar to
the Type 1, except the generator includes a wound rotor and a mechanism to quickly control the
current in the rotor. Known as “Type 2,” this generator has operating characteristics similar to
the Type 1, except the rotor-current control scheme enables a degree of fast torque control,
which improves the response to fast dynamic events and can damp torque oscillations within the
drive train. Type 1 and 2 wind turbines have limited performance capability. However, their
performance can be enhanced to meet more stringent interconnection performance requirements
through the addition of suitable terminal equipment such as Static VAR Compensator (SVC) or
STATCOM in order to control or support power system voltage.
Type 3 Double-fed induction (asynchronous) generator (DFG) - Power electronic applications
have led to a new generation of wind generating technologies with utility interface characteristics
which can make a large contribution to overall power system performance and provide for
improved operation and system reliability than earlier technologies. The double-fed induction
(asynchronous) generator (DFG), or Type 3 wind turbine-generator, includes a mechanism that
produces a variable-frequency current in the rotor circuit. This enables the wind turbinegenerator
to operate at a variable speed (typically about 2:1 range from max to min speed),
which improves the power conversion efficiency and controllability of the wind turbinegenerator.
The AC-DC-AC power converters need only be rated to carry a fraction, typically
30%, of the total wind turbine-generator power output. Although the original incentive for this
scheme was variable speed power conversion, the power converters have since evolved to
perform reactive power control, which, in some cases, can be effectively used to dynamically
control voltages similar to conventional thermal and hydro power plants. Further, DFGs have a
light overall weight which is important during construction. The fast response of the converters
also enables improved fast voltage recovery and voltage ride-through capability. Advanced
features include governor-type functions (for speed control in Type 3 and 4) and, in some cases,
dynamic reactive power can be supplied when the wind turbine is not generating real power.
Type 4 Wind Turbine-Generator (full conversion) - The Type 4 wind turbine-generator (full
conversion), passes all turbine power output through an AC-DC-AC power electronic converter
system. It has many similar operating characteristics to the DFG (Type 3) system, including
variable speed, reactive power control, pitch control, and fast control of power output. Type 4
Accommodating High Levels of Variable Generation 17