Doubly Fed Induction Generator in an Offshore Wind Power Plant Operated at Rated V/Hz Preprint Eduard Muljadi, Mohit Singh, and Vahan Gevorgian
To be presented at the IEEE Energy Conversion Congress and Exhibition Raleigh, North Carolina September 15-20, 2012
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Doubly Fed Induction Generator in an Offshore Wind Power Plant Operated at Rated V/Hz
E. Muljadi, Fellow, IEEE, M. Singh, Member, IEEE, and V. Gevorgian, Member, IEEE
Abstract—This paper introduces the concept of constant constant. In this paper, the performance of DFIG at rated volt/hertz operation of offshore wind power plants (WPPs). The volt/hertz operation and the size of the power converter deployment of offshore WPPs requires power transmission from needed for rated volt/hertz operation is explored in the plant to the load center inland. Because this power transmission requires submarine cables, there is a need to use comparison to conventional DFIGs operated at 60 Hz. Results high-voltage direct current (HVDC) transmission, which is from dynamic simulations performed to illustrate the control economical for distances greater than 50 km. In the concept algorithms proposed for HVDC, electrical generator and presented here, the onshore substation was operated at 60 Hz power converter, and aerodynamic controls are shown. synced with the grid, and the offshore substation was operated at In this analysis, we assumed that the wind speed was variable frequency and voltage, allowing the WPP to be operated relatively uniform over an offshore wind plant, compared to at constant volt/hertz. In this paper, a variable frequency at rated volt/hertz operation was applied to a Type 3 doubly fed induction an onshore plant, because of elimination of surface roughness generator (DFIG) wind turbine generator. The size of the power and terrain effects. Wake may affect the validity of this converter at the turbine can be significantly reduced from 30% assumption, particularly when the wind is directed along the of the rated power output in a conventional Type 3 turbine to 5% turbine rows. Observed power losses due solely to wake can of the rated power. The DFIG allows each turbine to vary its be up to 10% for closely spaced wind plants [9]. In [10], the operating speed with respect to the other turbines. Thus, small authors presented an analysis of variable frequency (though wind diversity within the WPP can be accommodated by the DFIG, and the collector system frequency can be controlled by not constant volt/hertz) offshore plant operation and included HVDC to follow large variations in average wind speed. a discussion of wake effects as well. In our analysis, we considered that the average wind speed measurements had Index Terms—wind turbine generator, variable speed, already incorporated the wake-linked reductions. Wake effects induction generator, HVDC, HVAC, renewable energy may also increase the diversity of wind speed within the plant, but the 5% power converter discussed here is able to I. INTRODUCTION accommodate the 10% speed variation between turbines he use of high-voltage direct current voltage-sourced because of wake effects. Tconverters (HVDC-VSC) has been proposed for offshore Because of the decoupling of the plant from the grid by the power systems, including renewable energy resources [1]–[3], HVDC link, the HVDC grid side converter assumed the because these systems become more economical than responsibilities of ensuring that low-voltage and fault ride- alternating current systems for transmission over distances through criteria were met. Fault and low-voltage operation is greater than 50 km in an offshore environment. Although the not examined in this paper. dynamics of the high-voltage direct current (HVDC) itself are In this paper, the following sequence occurs. In section II, important [4], [5], we focused our investigation on the wind the doubly fed induction generator is explored based on steady turbine and plant operation. The use of HVDC confers another state characteristics, and the size of the power converter advantage in that it is able to operate at variable frequency: needed for rated volt/hertz operation is explored in several studies have proposed different control algorithms comparison to conventional DFIG operated at 60 Hz. Section with the wind power plant (WPP) interconnected with HVDC- III describes the dynamic simulation we performed to VSC [6], [7]. illustrate the control algorithms proposed for HVDC, electrical Figure 1 illustrates the layout diagram of the offshore WPP. In generator and power converter, and aerodynamic controls. our proposed control strategy, the HVDC connecting the offshore WPP substation was operated at variable frequency at Individual WTGs constant volt/hertz, and the onshore substation was operated at 60 Hz connected to the grid inland. The collector system frequency and voltage followed the average wind speed. In previous work [8], we reported on constant volt/hertz HVDC Link operation of fixed-speed turbines; in this paper, we Grid concentrate on Type 3 doubly fed induction generator (DFIG) turbines. At rated frequency, the collector system voltage level Substation platform and collector system was 34.5 kV, 60 Hz. At the turbine level, the pad mounted transformer converted the voltage to the generator voltage Anemometer level, for example, 4160 V, 60 Hz at rated frequency. The masts operating frequency and the voltage at the collector system changed, but the ratio of the voltage to the frequency was Figure 1. Offshore WPP connected via HVDC to the grid
1
II. TYPE III WIND TURBINE—DOUBLY FED INDUCTION The useful active power at the stator side can be computed as GENERATOR WITH A PARTIAL RATING POWER CONVERTER [4] Recently, the use of wound rotor induction machines has Re been revived in the form of an induction generator for wind The real power at the rotor side can be computed as turbine applications. With the power converter connected to the rotor winding, the induction generator can be operated at [5] Re variable speed at partial load. A physical connection of the DFIG system is shown in Figure 2 below. By substitution, we can get the following equation