关于轻型直流输电的部分英文文献及翻译.doc

精品文档英文文献原文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. 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 voltage 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 original 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 source 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 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 connected 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 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 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 deviation. 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 Asplund 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 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) 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 distortion, 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 sampling 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 modulation (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 generated 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. Double 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 VSC-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 high 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 strategy 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 verification 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 represented 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 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 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, , can 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 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 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 absorbed 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 Control 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), the 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 voltage 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.This method has such advantages as good dynamic response, suitable power quality under abrupt changes in active and reactive powers, a simple power estimation algorithm, sinusoidal line currents and also the unity power factor of the rectifier. Moreover, by this method no line voltage sensors are required. 英文翻译轻型直流输电系统的无传感器简单控制摘要：轻型直流输电（VSC-HVDC）是一种新的电力传输技术，适用于在中小功率传输。在本文中我们讨论一个新的基于空间矢量调制（SVM）的不含有电压线传感器的控制系统。使用直接功率控制（DPC）的SVM和一个新的双同步参照系锁相环（DSRF-PLL）的方法，控制系统可耐大部分线路电压扰动。此外，系统采用前馈功率解耦环而使响应加速。这种控制策略的运作在MATLAB 的SIMULINK仿真环境中可得到验证。为了验证这个控制系统，构造了一个5KVA的原型系统。相比原来的控制器，电流总谐波失真（THD）、有功和无功的偏差以及直流电压超调量分别降至2.5、6.2和8。整流器的功率因数在最坏的情况下为0.93，直流电压稳定时间为0.2秒。关键词：基于电压源换流器的高压直流输电（VSC-HVDC）；空间矢量调制（SVM）；直接功率控制（DPC）；轻型高压直流输电1 简介由脉宽度调制（PWM）控制的，基于电压源换流器的高压直流输电（VSC-HVDC），可向有源、无源电力系统提供功率。VSC和PWM引入，可以快速、灵活地进行潮流控制，更方便地操作电力系统。此外，与传统直流输电相比，这种进步大大地减轻了交流电压电流的谐波并提高了连接交流系统的功率因数（Li GK等，2005）。近年来，VSC-HVDC或者说HVDC Light已成功地在以下领域商业委托：向偏远的孤立负荷供电、授权于城市中心、连接分布式发电源、连接两个异步电力系统，提高电能质量，等等（Asplund，2000；Li 等， 2003）。 轻型直流输电系统的优点是：（1）只需要一个小滤波器过滤高频信号组件;（2）没有换相失败问题;（3）不需要无功补偿（4）对于多反馈没有限制;等等。 轻型直流输电系统有多种控制方法。张某等人（2002）用稳态模型的逆控制器跟踪运行点并采用双解耦控制回路以消除稳态误差。陈等人（2004）提出了一个基于dq0轴的稳态控制器设计方案。张等（2002）和陈等人（2004）假设两个轻型直流系统的终端已连接到一个无限大总线系统。但一个轻型直流输电系统终端可以连接到一台发电机（类似Asplund等1997年提出的），另一个轻型直流输电系统连接到电网发电机（如海上风力发电场）。这些方法重点在控制直流输电系统本身并没有考虑到交直流系统之间的相互作用。胡等人（2004）提出一个发电机励磁和轻型直流输电之间的最佳的协调控制策略，然而控制律的推导相当复杂。胡 等（2005）在确定控制器参数后用遗传算法（GA）优化之。Ooi、王（1991）、张和徐（2001）用一种相位幅值控制技术（PAC）应用于轻型直流输电。Li GI等（2005）提出了轻型高压直流输电系统的非线性控制。这些方法已使用了电压和电流传感器。直接功率控制（DPC）的方式是基于被称作VF-DPC的虚拟磁链，提供正弦线电流、低谐波失真、一个简单而抗噪的功率估计算法及良好的动态响应（拉赫马提等，2006）。然而，VF-DPC有以下知名的缺点（马林诺夫斯基等人，2001年；2004年）：（1）可变开关频率（LC输入滤波器设计的困难）；（2）需要高采样频率的数字实现迟滞比较器；（3）需要快速的微处理器和A / D转换的。因而，没有使VF-DPC工业化的倾向。但当应用空间矢量调制（SVM）代替交换表时上述弊端都可以消。DPC是基于瞬时直接有功和无功功率控制（马林诺夫斯基等人，2004年）的方法。在DPC中没有内部电流控制回路和PWM调制模块。此外，对静态开关转换器开关的命令产生自SVM。使用空间矢量调制会产生相对于正弦调制较低的电流谐波、较高的整定输出电压和更高的调制系数（马林诺夫斯基等人，2004年）。此外，它可以很容易地在基于DSP的系统中实现。基于VF的双同步参照系锁相环（DSRF-PLL）需要这个控制系统能耐大部分线路电压扰动。这保证了系统在异常和故障电网条件下仍能正常运作。本文提出了VSC-HVDC的一种新的控制策略。在这一方式中，我们对整流站中的无功功率和直流输出电压以及逆变站中的有、无功功率分别进行控制。此外，我们已有逆变器的DPC整流方程（马林诺夫斯基等人，2004年）。为了在大功率传输中能有更高的精度，二阶参数也包括在整流和逆变的方程中。有功和无功功率前馈解耦用于加快系统的响应。故DPC适用于V