电子电路英文文献及翻译A sensorless and simple controller for VSC based HVDC systems.docx

1、电子电路英文文献及翻译A sensorless and simple controller for VSC based HVDC systems英文文献原文A sensorless and simple controller for VSC based HVDC systemsAbstract: Voltage source converter high-voltage direct current (VSC-HVDC) is a new power transmission technology preferable in small or medium power transmission

2、. In this paper we discuss a new control system based on space vector modulation (SVM) without any voltage line sensors. Using direct power control (DPC) SVM and a new double synchronous reference frame phase-locked loop (DSRF-PLL) approach, the control system is resistant to the majority of line vo

3、ltage disturbances. Also, the system response has accelerated by using a feed forward power decoupled loop. The operation of this control strategy was verified in a SIMULINK/MATLAB simulation environment. To validate this control system, a 5 kVA prototype system was constructed. Compared to the orig

4、inal controllers, the current total harmonic distortion (THD), the active and reactive deviations and the DC voltage overshoot were lowered by 2.5%, 6.2% and 8%, respectively. The rectifier power factor in the worst condition was 0.93 and the DC voltage settling time was 0.2 s.Key words: Voltage sou

5、rce converter high-voltage direct current (VSC-HVDC), Space vector modulation (SVM), Direct power control (DPC), HVDC Light1 INTRODUCTIONVoltage source converter high-voltage direct current (VSC-HVDC), controlled by pulse width modulation (PWM), can supply power to both active and passive electrical

6、 systems. The introduction of VSC and PWM makes possible fast and flexible control of power flow and more convenient operation of power systems. Besides, this advancement, compared with conventional HVDC, mitigates harmonics in AC current and AC voltage greatly and improves power factors of the conn

7、ected AC systems (Li GK et al., 2005). VSC-HVDC or HVDC Light, in recent years, have successfully been commercially commissioned in such fields as supplying power to remote isolated loads, empowering urban centers, connecting distributed generation sources, linking two asynchronous electrical power

8、systems, improving power quality, and so on (Asplund, 2000; Li et al., 2003).The advantages of a VSC based HVDC system are (Asplund, 2000): (1) only a small filter is required to filter high frequency signal components; (2) there is no commutation failure problem; (3) reactive power compensation is

9、not required; (4) there is no restriction on multiple in-feeds; etc.There are various control methods for VSC based HVDC systems. Zhang et al.(2002) used the inverse steady state model controller to trace the operating point and adopted two decoupled controlling loops to eliminate the steady state d

10、eviation. Chen et al.(2004) proposed a steady-state controller design scheme based on dq0-axis. Zhang et al.(2002) and Chen et al.(2004) assumed that the two terminals of VSC-HVDC have been connected to an infinite bus system. But one terminal of VSC-HVDC may be connected to a generator and, as in A

11、splund et al. (1997), an HVDC Light system connects the generator (such as an offshore wind farm) to the grid. These strategies focus on control of the HVDC system itself and do not consider the interaction between AC and DC systems. Hu et al.(2004) presented an optimal coordinated control strategy

12、between the generator excitation and VSC-HVDC, whereas the derivation of control law is complicated. Hu et al. (2005) applied a genetic algorithm (GA) to optimize parameters of the controller after determining them. Ooi and Wang (1991) and Zhang and Xu (2001) used a phase and amplitude control (PAC)

13、 technique for VSC based HVDC applications. Li GI et al.(2005) proposed a nonlinear control for an HVDC Light system. These methods have used voltage and current sensors.A direct power control (DPC) strategy based on virtual flux, called VF-DPC, provides sinusoidal line current, lower harmonic disto

14、rtion, a simple and noise-robust power estimation algorithm and good dynamic response (Rahmati et al., 2006). However, the VF-DPC scheme has the following well-known disadvantages (Malinowski et al., 2001; 2004): (1) variable switching frequency (difficulties of LC input filter design), (2) high sam

15、pling frequency needed for digital implementation of hysteresis comparators, (3) necessity for a fast microprocessor and A/D converters.Therefore, there is no tendency to implement VF-DPC in industry. All the above drawbacks can be eliminated when, instead of the switching table, space vector modula

16、tion (SVM) is applied. DPC is a method based on instantaneous direct active and reactive power control (Malinowski et al., 2004). In DPC there are no internal current control loops and no PWM modulator block. Moreover, the turn-on and turn-off commands of the static switches of the converters are ge

17、nerated by SVM. Use of space vector modulation causes lower current harmonics, relatively high regulation and stability of output voltage and obtains a higher modulation factor relative to sinusoidal modulation (Malinowski et al., 2004). Also, it can easily be implemented in a DSP based system. Doub

18、le synchronous reference frame phase-locked loop (DSRF-PLL) based on VF causes this control system to be resistant to the majority of line voltage disturbances. This assures proper operation of the system for abnormal and failure grid conditions.In this paper a new control strategy is proposed for V

19、SC-HVDC. In this strategy, the reactive power and output DC voltage in the rectifier station and the reactive and active powers in the inverter station are controlled, separately. Also, the DPC rectifier equations (Malinowski et al., 2004) have been developed for the inverter. For more accuracy in h

20、igh power, the second order parameter is included in the rectifier and the inverter equations. Active and reactive power feed forward decoupling are used for accelerating the system response. Finally, DPC is applied to the rectifier and inverter stations of VSC-HVDC.The operation of this control str

21、ategy is verified in a SIMULINK/MATLAB simulation environment for steady state, active and reactive power variations, single-line-to-ground faults and unbalanced sources at the rectifier and the inverter stations. Also, this control strategy is applied to a 5 kVA prototype system which is verificati

22、on that this control strategy has a fast response and strong stability.2 CONTROL of VSC BASED HVDC SYSTEM2.1 VSC based HVDC systemVSC-HVDC involves two voltage source converters with the same configuration, linking with a dc transmission line or cable (Fig.1). There are four control variables repres

23、ented by, ,andfor this system. In this paper, a rectifier station is chosen to control DC-bus output voltage of rectifier (). Also, reactive power () and inverter station are set to control active power () and also reactive power (). Rc is the equivalent resistance of the transmission cable and can

24、be practically neglected. Thus we may write .Fig.1 A physical model for a VSC based HVDC system2.2 Virtual-flux estimator for rectifier and inverterFrom the economical point of view, and for simplicity, more reliability and separation of power stage and control, AC line voltage sensors are replaced

25、by a flux estimator (Malinowski et al., 2004).The basic model of a VSC station is shown in Fig.2. If Da, Db, and Dc are the duty cycles of Sa, Sb, and Sc signals, respectively, Udc is the converter DC voltage, and uL and uL are line voltage in - coordinates, then the related flux of AC voltage, , ca

26、n be written as (Malinowski et al., 2004) (1)Also, the converter voltage equations in - coordinates are: (2) (3)Fig.2 Basic model of a voltage source converter2.3 Direct power controlActive and reactive power in the rectifier and the inverter stations are estimated using the line current vectorsand

27、estimated virtual flux in - coordinates (Malinowski et al., 2004): (4) (5)2.4 Rectifier control designThe full control algorithm of the proposed control system is presented in Fig.3. The DPC-SVM uses closed-loop power control. In the rectifier station, reference reactive power (qrefr) is set to zero

28、 for unity-power-factor operation. In an ideal case, the active power in the rectifier station and the active power in the inverter station are equal, and no storage elements are needed. Nevertheless, in real systems differences between these active powers are inevitable, and these differences are a

29、bsorbed by the DC link capacitor and are reflected in fluctuations of the DC link voltage. Thus, the reference active power (pref r) at the side of the rectifier is the sum of the outer proportional-integral (PI) dc voltage controller and estimated active power in the inverter station (pi).Fig.3 Con

30、trol scheme for a VSC based HVDC system with the rectifier and inverter stationsAccording to the current direction, the line voltage uLr can be expressed as the sum of the inductor voltage uIr, the resistor voltage uRr and the rectifier voltage uSr (Rahmati et al., 2006): (6)By considering Eq.(1), t

31、he estimated virtual fluxes are: (7) (8)2.5 Inverter control designIn the inverter station, reference reactive power (qref i) and reference active power (pref i) are set to network demand. According to the current direction, the inverter voltage uSi can be expressed as the sum of the inductor voltag

32、e uIi, the resistor voltage uRi and the line voltage uLi at the side of the inverter. The estimated virtual fluxes are (Rahmati et al., 2006): (9) (10)3 CONCLUSIONThis paper proposes a new method for controlling a VSC based HVDC system which has been connected between two distribution systems with different frequencies. This method is effective in damping system oscillations quickly, and enhances power quality when power flow is reversed. VF and DSRF-PLL cause this control system to be resistant to the majority of line voltage disturbances