1、 外文原文 Design Study of Doubly-Fed Induction Generators for a 2MW Wind Turbine ABSTRACT A design study for a 2 MW commercial wind turbine is presented to illustrate two connection methods for a standard doubly-fed induction machine which can extend the low speed range down to 80% slip without an incre
2、ase in the rating of the power electronic converter. This far exceeds the normal 30% lower limit. The low speed connection is known as induction generator mode and the machine is operated with a short circuited stator winding with all power flow being through the rotor circuit. A two loop cascaded P
3、I control scheme has been designed and tuned for each mode. The purpose of this paper is to present simulation results which illustrate the dynamic performance of the controller for both doubly-fed induction generator connection methods for a 2 MW wind turbine. A simple analysis of the rotor voltage
4、 for the doubly-fed connection method is included as this demonstrates the dominant components that need to be considered when designing such advanced control strategies. Keywords: Doubly-fed, Induction generator, Wind turbine LIST OF IMPORTANT SYMBOLS vrdq Direct and quadrature rotor voltage irdq D
5、irect and quadrature rotor current sdq Direct and quadrature stator flux linkage Ps Stator real power Qs Stator reactive power pfs Stator power factor Te Torque p Differential operator Lm Magnetising reactance Rr Rotor resistance Lr Rotor reactance Total leakage inductance sf Slip frequency s Stator
6、 referred s Rotor referred * Reference value 1. INTRODUCTION There is continuing interest in wind turbines, especially those with a rated power of many megawatts.This popularity is largely driven by both environmental concerns and also the availability of fossil fuels. Legislation to encourage the r
7、eduction of the so called carbon footprint is currently in place and so interest in renewables is currently high. Wind turbines are still viewed as a well established technology that has developed from fixed speed wind turbines to the now popular variable speed technology based on doubly-fed inducti
8、on generators (DFIGs). A DFIG wind turbine is variable speed with the rotor converter being controlled so that the rotor voltage phase and magnitude is adjusted to maintain the optimum torque and the necessary stator power factor 1, 2, 3. DFIG technology is currently well developed and is commonly u
9、sed in wind turbines. The stator of a DFIG is directly connected to the grid with a power electronic rotor converter utilised between the rotor winding and the grid. The variable speed range is proportional to the rating of the rotor converter and so by limiting the speed range to 30% 4, 5, 6, 7 the
10、 rotor converter need only be rated for 30% of the total DFIG power whilst enabling full control over the full generator output power. This can result in significant cost savings for the rotor converter 4. The slip ring connection to the rotor winding however must be maintained for reliable performa
11、nce. The power generator speed characteristic shown in figure 1 is fora commercial 2 MWwind turbine. The generator speed varies with wind speed however this relation is set for a specific location. As wind speed, and therefore machine speed, falls the power output of the generator reduces until the
12、wind turbine is switched off when the power extracted from the wind is less than the losses of the generator and converter. An operating mode has been proposed by a wind turbine manufacturer that is claimed to extend the speed range so that at lower speed the power extracted from the wind is greater
13、 than the losses in the system and so the system can remain connected. This proposed that the standard doubly-fed (DF) connection is used over the normal DF speed range and the so-called induction generator (IG) mode is used to extend the low speed operation. Previous work has illustrated that IG mo
14、de enables the DFIG to operate down to 80% slip 8. This change in operation is achieved by disconnecting the stator from the grid in DF mode and then short circuiting the stator to enable IG operation. All of the generator power flows through the rotor converter in IG mode. The IG curve is identical
15、 to the DF curve for 30% slip. The estimated IG power extracted from the wind at low speeds is obtained by extrapolating the curve for the DF mode. The reference torque required by both controllers (DF and IG mode) can easily be derived from this curve. The torque speed data can then be stored in a
16、look-up table so the reference torque is automatically varied with speed. The capability of modern DF wind turbines to vary the reactive power absorbed or generated 6, 9, 10 allows a wind turbine to participate in the reactive power balance of the grid. The reactive power at the grid connection cons
17、idered in this work is described, for the UK, by the Connection Conditions Section CC.6.3.2 11 available from the National Grid. The reactive power requirement for a wind farm is defined by figure 2. Point A - MVAr equivalent for 0.95 leading power factor at rated MW Point B - MVAr equivalent for 0.
18、95 lagging power factor at rated MW Point C - MVAr -5 % of rated MW Point D - MVAr 5 % of rated MW Point E - MVAr -12 % of rated MW The objective of this paper is to investigate the controller performance of DF and IG mode for a 2MW, 690V, 4-pole DFIG using machine parameters provided by the manufac
19、turer. This is further research building on a previous paper which demonstrated the steady-state performance of the two modes of operation, DF and IG mode 8. In 8 the authors discussed the steady-state efficiency for both connections. The steady-state performance work illustrated that there were ben
20、efits to operating the machine in one connection method as opposed to the other. This paper examines the controllability (i.e. transient performance) of the 2 MW wind turbine. Results of the full dynamic controller (current regulation, decoupling equations and vector control) in both DF mode and IG
21、mode are shown. A detailed analysis of thecomponents that form the rotor voltage over the full operating range in DFIG mode is presented as this enables the dominant control components to be identified. This is particularly important when designing advanced control schemes as an overview over the fu
22、ll operating range can be identified. Simulation models, which have been validated against a 7.5kW laboratory rig 12, are applied to a realistic 2 MW wind turbine to enable conclusions to be made regarding the proposed use of IG mode in a real wind turbine 2. CONNECTION METHODS Doubly-fed induction
23、machines are commonly connected as shown in figure 3. The grid side inverter (GSI) is controlled to maintain a fixed dc link voltage with a given power factor at the grid (in our case unity). The rotor side inverter (RSI) is controlled so the maximum energy is extracted from the kinetic energy of th
24、e wind whilst enabling the stator power factor to be controlled within the limits of the grid requirements though unity power factor is often desirable. An alternative connection method for a doubly-fed machine is shown in figure 4, here called the induction generator (IG) connection. The stator is
25、disconnected from the grid and is short-circuited. The rotor circuit is unchanged from figure 3. The GSI is controlled as in DF mode. The objective of the RSI is to control the stator flux linkage while extracting the maximum power from the kinetic wind energy. 3. CONTROLLER PERFORMANCE A closed loo
26、p controller for both DF mode and IG mode has been discussed in prior work 12 but only for a 7.5 kW laboratory test rig. The dynamics of a 2 MW system are somewhat different and are investigated in this paper. The performance of the dynamic controller for both DF and IG mode are shown in this sectio
27、n for a 2 MW wind turbine. 3.1. DFIG Mode (T and Q Control) The reference values for the controller in DF mode are torque (see figure 1) and stator reactive power to enable the grid code requirement 11 to be achieved, figure 2. Two speeds are investigated in this section to enable the performance of
28、 the controller to be shown both above and below the 20% of rated power limit from the grid code requirements. A nominal generated power of 320 kW is achieved at 1150 rpm (less than 20% of rated power) and a nominal power of 1.25 MW is achieved at 1550 rpm (greater than 20% of the rated power). The reference and actual torque, Te, and stator reactive power, Qs, are shown for both speeds in figure 5. The value of reference torque, Te*, for both speeds is the specific nominal torque for a given speed calculated from
