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A flexible active and reactive power control strategy for a variable speed constant

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A flexible active and reactive power control strategy for a variable speed constant 412 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. LO, NO. 4, JULY 1995 A Flexible Active and Reactive Power Control Strategy for a Variable Speed Constant Frequency Generating System Yifan Tang, Member, IEEE, and Longya Xu, Senior Member, IEEE Abstract...

A flexible active and reactive power control strategy for a variable speed constant
412 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. LO, NO. 4, JULY 1995 A Flexible Active and Reactive Power Control Strategy for a Variable Speed Constant Frequency Generating System Yifan Tang, Member, IEEE, and Longya Xu, Senior Member, IEEE Abstract- Variable-speed constant-frequency genkrating sys- tems are used in wind power, hydro power, aerospace, and naval power generations to enhance efficiency and reduce friction. In these applications, an attractive candidate is the slip power recovery system comprising of doubly excited induction machine or doubly excited brushless reluctance machine and PWM con- verters with a dc link. In this paper, a flexible active and reactive power control strategy is developed, such that the optimal torque- speed profile of the turbine can be followed and overall reactive power can be controlled, while the machine copper losses have been minimized. At the same time, harmonics injected into the power network has also been minimized. In this manner, the system can function as both a high-efficient power generator and a flexible reactive power compensator. I. INTRODUCTION ARIABLE-SPEED constant-frequency (VSCF) power V generating is desirable in many situations. A salient example is the wind power generation, where turbine speed should be able to vary according to the wind speed, such that energy efficiency can Be improved with a reduced torsional stress and windage friction on the wind mill blades. While in variable speed, the system output voltage should be maintained at a constant frequency to interface with the power system. Other applications of VSCF include hydro power generation, aerospace and naval power generation. A promising VSCF generating concept is the slip power recovery system composing of a doubly excited induction machine and power converters. In recent years, renewed interest has led to the progress of this concept. [l) proposed a decoupled active and reactive power control strategy for doubly excited induction machines. A cycloconverter was used in the rotor circuit, resulting in control simplicity while restricting control flexibility. The overall system power flow problem was not studied, as only the output power from the stator side is controlled. [2] studied the overall power flow of a self-cascaded induction generator, for which a synchronous condenser is needed to provide the necessary reactive power for field excitation. Since only simple thyristor inverters were employed in the system, the system’s ability to handle reactive power is disabled. [3 ] studied power regeneration of a typical Manuscript received February 14, 1994; revised March 13, 1995. Y. Tang is with U.S. Electrical Motors, Emerson Motor Technology Center, L. Xu is with the Department of Electrical Engineering, The Ohio State IEEE Log Number 9412188. St. Louis, MO 63136 USA. University, Columbus, OH 43210 USA. Fig. 1. Power flow of Slip Power Recovery System. singly-excited adjustable speed drive by simple thyristor recti- fier using an innovative dc reactor circuit. It is a good example of cost-reduction for power regeneration of high-power drives. For a generating system, however, this scheme would not be a wise choice if reactive power control is required. [4] had developed a stator field oriented control for the doubly-excited induction generator, in which both the active and reactive power of the stator are controlled by a PWM regulated current in the rotor circuit. It had also been shown that the concept in [4] is applicable to the doubly-excited brushless reluctance machine [5]. Conventional slip power recovery systems had been ex- tensively researched, such as in [6]-[8]. It appears that the high-performance type, dual PWM power converter (Fig. 1), slip power recovery VSCF system has not been fully studied, such as regarding the control of both active and reactive power of the overall system, stability problem as especially associated with the dc link voltage and with the lack of rotor damping circuit, control coordination between the two converters, etc. In this paper, power control of VSCF slip power recovery generating system are discussed. A closed- loop control strategy is developed to coordinate the dual PWM converters in the rotor circuit. Flexible and stable control of overall active and reactive power is obtained, while the machine copper losses are minimized. It is shown that with the control strategy proposed, the VSCF system can actually function both as a power generating system and as a reactive power compensator. Application in wind power generation is simulated to verify the proposed control strategy. 11. CONTROL STRATEGY DEVELOPMENT A schematic slip power recovery system is shown in Fig. 1 , with the reference directions of active and reactive power indicated. As a generating system, obviously in most situations 0885-8993/95$04.00 0 1995 IEEE TANG AND XU: FLEXIBLE ACTIVE AND REACTIVE POWER CONTROL STRATEGY 413 Ps < 0. When the machine in variable speed operates below synchronous speed, slip power P, > 0; when the machine operates above synchronous speed, P, < 0. Note that doubly-excited generators are inherently capable of super- synchronous speed operation. To ensure sub-synchronous and super-synchronous speed range operation, the requirement lies in the configuration of the power converter. For most of the configurations with cycloconverters, natu- rally or line commutated converters and low-frequency forced commutated thyristor converters, harmonic distortion, and poor power factor are the major shortcomings, along with limited control flexibility. To realize advanced control, such as the field oriented control of the variable speed generator, and to achieve overall active and reactive power control and harmonic reduction, the dual PWM converter structure with a dc link is an attractive candidate. As a result, new control issues arise and of most importance is the coordination between the two PWM converters. For convenience, in the following analysis the two converters are termed rotor side converter and line side converter, respectively. The PWM converter has seldom been applied in slip power recovery systems, which have traditionally been used in high power, narrow speed range applications without high- performance requirement. The PWM converters inevitably bring higher switching losses, and the losses increase at higher switching frequencies and power levels. However, on the one hand the power converters handles only the slip power, and their rating is only a portion of the machine rating depending on the speed range, and on the other hand the rapid technology advance in solid-state power devices is making higher power switching feasible at an ever reducing cost. A. Field Oriented Control Through Rotor Side Converter In the VSCF generating system, control schemes for the doubly-excited induction machine are expected to achieve the following objectives: 1) The induction generator is required to track a prescribed torque-speed curve, for maximum power capturing; 2 ) The stator output voltage frequency must be constant; 3) Flexible reactive power control is. achievable. These control objectives must be achieved with adequate stability of the system which also includes the power converter and the dc link. The stator field orientation control is based on the stator field dq model, where the reference frame rotates synchronously with respect to the stator flux (linkage), with the d-axis of the reference frame instantaneously overlaps the axis of the stator flux. In short, w = we and A,, = 0. In such a reference frame, the machine dynamical equations can be written as [4] d A d s v d s = rsids + - d t Since the d-axis of the reference frame is the instant axis of the stator winding flux, the phase angle of the stator line voltage is generally not a constant in the reference frame, although its frequency and magnitude are constants constrained by the power system. The electromagnetic torque and stator active power can be derived as In the doubly excited machine, the level of the stator flux remains approximately unchanged, restricted by the constant magnitude and frequency of the line voltage. Therefore, as can be observed from (6), the torque control can be achieved by controlling the rotor current component orthogonal to the stator winding flux. Then from (7), stator active power is subsequently controlled. The reactive power at the terminal of the stator winding can be derived as or, from (1) and (2), with the stator flux remaining unchanged As ( 3 ) indicates, i d s is controllable by id,, with Ads un- changed. Therefore, the d-axis component of the rotor current, i d , can be controlled to regulate the stator reactive power. As a result, the control of stator active power P, via i,, and the control of stator reactive power Q, via zdr are essentially decoupled, and a decoupler is not necessary to implement field orientation control for the slip power recovery system. The flux control i s generally unnecessary since it maintains a constant level, while the control of reactive power becomes possible. B. Minimization of Machine Copper Losses It is well known that the slip power recovery configura- tion has improved energy efficiency. Field oriented control described above enhances this improvement by permitting variable speed operation with reactive power control. There is, however, still another improvement possible. By controlling the reactive power circulation of the system, the copper losses can be minimized. This is analyzed in this subsection. The machine overall copper losses can be expressed as 3 . 3 . Pcu = T(z:, + &)rS + 5 ( z : ~ + i;,)r,. (10) By using (1) to (4), (10) can be derived as In (1 I), i,, has been used to control torque or active power, and Ads remains approximately unchanged as described above, then the amount of machine copper losses is a function of id,. 414 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 10, NO. 4, JULY 1995 It can be shown that for P,, to achieve the minimum, it is necessary that (12) However, in the field oriented control, as already discussed, i d s controls the stator reactive power. Therefore, it is sufficient to conclude that the stator reactive power flow determines the level of copper losses. From (9) and (12), the optimal stator reactive power flow can be shown as L s T r x d s i d s = T, L& + rr L:. (13) 3 P L . J J $ , & - - - w e ' - 2 2 rsL& + r,.L:' C. Control of Line Side Converter Through field oriented control of the rotor side converter, the optimal torque-speed profile can be tracked and the stator output reactive power can be separately controlled. The dc link capacitor provides dc voltage to the machine side converter and any attempt to store active power in the capacitor would raise its voltage level, and vice versa. Thus to ensure stability of the dc link, the power flow of the converters should attempt to meet the following control objective: Pl - Pwl = Pr + Pw2 where Pwl and Pw2 are the losses of the line side converter and the rotor side converter, respectively, with the same direction Fig. *. Since vds 0, uqs ?lm, pl and Ql can be controlled by iql and &, respectively. In the Same reference frame as determined by the machine stator flux, i , ~ and idl are also field oriented currents, produced by the line side current regulated (14) PWM converter. 111. IMPLEMENTATION AND SIMULATION of the power flow as indicated in Fig. 1. , The dc link dynamical equation can be written as dV1 . c- .dt = a 1 - i 2 in which Vd is the dc bus voltage and c is the capacitance. The dc fink currents i l and "2 as indicated in Fig. 1 can be derived as From (15) through (17), as long as (14) is satisfied, the dc link voltage maintains stable, though small ripples may exist due to the instantaneous inequality between i l and i 2 and a small variation may occur during transients as a result of energy transferring. As can be seen from Fig. 1, another result of (14) is that the overall generated active power equals to the electromagnetic power minus the system losses, i.e., Reactive power flow constitutes another control objective: where &* is the overall reactive power command required by the power network. In the stator flux dq reference frame, A. Implementation Scheme of Closed Loop System Based on the control strategy discussed above, Fig. 2 shows an implementation of the overall control system, which enables the slip power recovery system to function as both a VSCF generating system (or a high performance variable speed drive system) and a reactive power compensator. Individual control of the rotor side converter and of the line side converter and related feedback between the two converters are shown. For speed control purpose, a speed control loop generates the torque command. A current-regulated pulse-width-modulation (CRPWM) voltage source converter provides the field oriented currents iqr and i d r to the rotor circuit, controlling the electromagnetic torque and the stator reactive power, respectively. The torque command is given by the turbine optimal torque-speed profile and the reactive power command is calculated to minimize the machine copper losses. Overall active power generated is directly related to the torque, as indicated by (6) and (18). Another CRPWM voltage source converter is used to inter- face with the power network. In the same dq reference frame as determined by the machine stator flux, its currents i,l and i d l are also field oriented, controlling Pl and Q1, respectively. As discussed earlier, Pl is controlled through i,l to stabilize the dc bus voltage and & 1 is controlled through i d l to meet the overall reactive power command. One way to maintain a stable dc bus voltage is to satisfy (14) to (17). Another way is to directly use the dc bus voltage feedback and the ideal reference to generate i,l by PID regulation. Depending upon different performance requirements and engineering considerations such as cost, simplicity and re- liability, the implementation structure shown in Fig. 2 can TANG AND XU: FLEXIBLE ACTIVE AND REACTIVE POWER CONTROL STRATEGY 475 TABLE I PARAMETERS OF SIMUIATED MACHINE [I Hrated) I 50hv 1 time (sec) time (sec) (a) (b) wind-mill tque ___.------- ___.--- 0.5 1 1.5 2 2.5 3 3.5 4 time (sec) (C) (aHc) Turbine speed tracking. Fig. 3. be simplified to a varying degree. As an example, the sta- tor current measurement can be eliminated if
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