10-12-2012, 12:35 PM
Evaluation of the DFIG Wind Turbine Built-in Model in PSS/E
Evaluation of the DFIG Wind.pdf (Size: 887.38 KB / Downloads: 131)
Abstract
Due to growth of environmental concern, more electricity must be generated from renewable energy sources. One of the most cost efficient alternatives is wind energy. Large wind farms with several hundred megawatts of rated power have been connected to grid. Nowadays, many transmission system operators require wind farms to maintain zero reactive power exchange with the grid during normal operation. Furthermore, like other conventional power plants wind turbines must be able to remain connected to the grid during grid faults. In order to study impact of wind power generation on the power system valid models of wind turbines are needed.
The purpose of this study is to evaluate DFIG wind turbine built-in model in PSS/E. In this report, the responses of the model subjected to grid disturbances are investigated. At first, the typical dynamic model of a DFIG wind turbine is introduced. Secondly the built-in dynamic model for DFIG wind turbine in PSS/E is illustrated and it is used to study the dynamic behavior of DFIG subjected to a symmetrical short circuit at the point of common coupling of a large wind farm. The load flow study for a network where a wind farm consisting 67 numbers of DFIG wind turbines and each one with 1.5MW rated power connected to the grid are presented and the output results from load flow study is used to perform dynamic simulation. The voltage, current, output power, speed and pitch angle profiles are presented in the dynamic study.
Introduction
The utilization of wind turbine to produce electricity is increasing rapidly in different parts of the world. It has become one of the main alternatives for non pollutant and environmentally friendly type for power generation in Sweden and a large amount of wind power are connected to the Swedish national grid and some others will be connected in next coming years.
Not until recently, the contribution of wind power generation on the system stability was considered to be small. However with increasing in the wind farm capacity it is clear that disconnecting a large wind farm will result in losing of a big part of power generation in grid, which can aggravate instability problems. Due to increasing portion of wind power, wind turbines have to contribute in reactive power support during steady state as well as during transient conditions. Transmission system operators (TSO) in countries where the amount of wind power generation is significant have established a special regulation and added to the grid codes that specify a number of requirements for wind farms to stay connected to the grid during disturbances.
Wind Turbine
The wind turbines are divided into four main types: fixed speed wind turbine with induction generator, variable-speed wind turbine with variable rotor resistance, variable speed wind turbine with doubly fed induction generator (DFIG) and variable speed wind turbine with full converter (FCWT) [3].
Fixed-Speed Wind Turbine
In fixed speed wind turbine, the generator is a squirrel-cage induction generator which is directly connected to the grid as in Figure 3.1. The rotor of a fixed-speed wind turbine rotates at a fixed speed determined by the frequency of the grid, the gear ratio and the pole pairs of generator. A fixed-speed wind turbine is connected to the grid through a soft-starter. The induction generator absorbs reactive power from the grid, so capacitor bank is necessary to provide reactive power compensation. A gear box is used to transform power from the turbine with lower-rotational speed to the generator rotor with high-rotational speed. The generator terminal voltage is increased with a step-up unit transformer to a medium voltage level.
This type of wind turbines has the advantage of being simple, robust and more cost-efficient compared to the other wind turbine types. However, the reactive power consumption cannot be controlled. Another drawback with the fixed speed wind turbine is that wind speed fluctuation is transmitted into the mechanical torque and it is finally transferred to the electrical power on the grid. The fluctuation in the delivered power to the grid can lead to large voltage fluctuation where the wind farm connected to a weak grid [4].
Wind Turbine with Variable Rotor Resistance
In wind turbine with variable rotor resistance the generator is an induction machine with
wound rotor and it is directly connected to the grid. Like a fixed-speed wind turbine, a
capacitor bank is necessary to provide reactive power for induction machine. By changing the
rotor resistance magnitude, the rotor speed can be regulated in a short range up to 10% higher
than synchronous speed [5]. The structure of this type of wind turbine is shown in Figure 3.2.
Variable-Speed Wind Turbine with Full Converter
In variable-speed wind turbine with full converter, the generator can be a squirrel-cage induction
or a synchronous generator which is connected to the grid via a power electronic converter as
shown in Figure 3.4. The whole power output from generator goes through the converter and
therefore the converter is rated at full power. The voltage level and the reactive power can be
regulated by using power electronic converters.
Wind Turbine with Doubly Fed Induction Generator (DFIG)
The objective of this chapter is to provide an overview on the dynamic model of different
parts in a DFIG wind turbine. At first, the main mathematic equations, that describe the
relationship between voltage and fluxes in an induction machine and they are basic equations
for establish dynamic model, are presented with a short description of the other electrical
parts. Model of aerodynamic and mechanical parts are also presented. Later, the simplified
model which is used in PSS/E software is introduced and the mechanism for controlling
active and reactive power in this model is illustrated.
DFIG and Converter Dynamic Model
An induction generator with power supply on the rotor is a main part of the DFIG wind
turbine. The DFIG is modeled by means of an equivalent circuit model. The main components
of a DFIG wind turbine are shown in Figure 3.1. In DFIG wind turbine, the stator directly
connected to the grid, while the rotor circuit is connected to a power converter by means of
slip rings and brushes. The rotor current is regulated by the power converter to control the
electromagnetic torque and field current and thus the stator output voltage. The DFIG can
operate in either sub-synchronous or in super-synchronous operation modes due to capability
of converter to operate in bi-directional operation power mode.