联接弱交流电网的柔性直流输电系统小信号稳定性研究

发布时间：2018-04-23 04:34

本文选题：电压源型换流器 + 弱交流电网　；参考：《华北电力大学(北京)》2017年博士论文

【摘要】：电压源换流器直流输电(Voltage Source Converter based High Voltage Direct Current,VSC-HVDC)又称柔性直流输电,采用全控型电力电子器件,不会发生换相失败,并且可以对有功功率与无功功率进行独立调节,因此相比于传统直流输电技术更适用于向弱交流电网供电。一般而言,评价交流电网强度的指标主要有:交流电网短路比(Short Circuit Ratio,SCR)和交流电网惯性时间常数(Inertia Constant)。当交流电网强度较弱时,工程中普遍采用的矢量电流控制器(Vector Current Control)有可能导致柔性直流输电系统失稳,并且随着交流系统变弱这种趋势更加明显。因此,为保证柔性直流系统的阻尼特性、稳定运行能力以及系统快速响应等实际工程需求,非常有必要研究联接弱交流电网的柔性直流输电系统小信号稳定性问题,揭示影响系统稳定运行的机理,提出改善系统阻尼特性的控制策略。为此,本文将着重开展如下研究,以得到具有重要理论和工程应用价值的成果。(1)VSC-HVDC系统数学模型与控制策略为了给本文提供重要的数学模型支撑,首先建立了联接弱交流电网VSC-HVDC系统非线性数学模型。其中,交流电网采用等值同步发电机串联线路等效阻抗来模拟,可以考虑交流电网的频率特性;电压源换流器分别采用两电平VSC与MMC拓扑,控制系统均采用经典的矢量电流控制;同时为了构建完整的柔性直流输电系统小信号模型,开发了不同状态空间方程之间的接口,通过对比PSCAD/EMTDC中的系统时域仿真模型以及MATLAB中搭建的非线性数学模型的计算结果,验证了非线性数学模型的正确性。(2)联接弱交流电网VSC-HVDC系统阻尼特性研究基于开发的联接弱交流电网VSC-HVDC系统数学模型,研究了交流电网SCR与惯性时间常数对系统阻尼特性的影响,结果表明系统小信号稳定裕度会随着交流电网SCR或惯性时间常数的降低而减小。相比于交流电网SCR,惯性时间常数的影响较小,只有当SCR较低时减小交流电网的惯性时间常数才会导致系统小信号失稳。研究了控制器参数对弱交流电网VSC-HVDC系统小信号稳定裕度的影响,其中影响较大的有锁相环(Phase-Locked-Loop,PLL)参数和外环控制器参数。同时,等值同步发电机的励磁控制器参数对系统的阻尼特性也有较大影响。对比了交流电网采用等值同步发电机模型串联线路等效阻抗的形式以及理想电压源串联线路等效阻抗的形式,结果表明由于采用理想电压源串联线路等效阻抗的形式忽略了交流电网的频率特性,对联接弱交流电网VSC系统的小信号稳定域评估会产生较大的影响,倾向于得到系统小信号稳定的结果。分别研究了采用两电平VSC与MMC时直流系统小信号稳定域,发现当MMC子模块电容较小,电容电压波动过大时,MMC内部动态过程与PLL之间会产生较强的相互作用进而发生高频振荡,严重影响系统的稳定运行;在增大MMC子模块电容容值后,MMC与两电平VSC在联接弱交流电网时的小信号稳定域基本一致。(3)联接弱交流电网VSC-HVDC系统先进控制器设计提出了利用虚拟电阻增强联接弱交流电网VSC-HVDC系统小信号稳定性的方法。对已有功率同步控制方法(Power Synchronization Control,PSC)进行改进,通过附加虚拟电阻控制策略,可以等效增大换流阀侧电阻,从而抑制换流器的高频振荡。同时,研究表明由于PSC控制器本身类似于间接电流控制,当交流电网较强时该控制器并不能保证换流器的响应速度。同时,提出了附加阻尼环节的新型锁相环来增强联接弱交流电网VSC-HVDC系统小信号稳定性的方法,通过在PLL中增加阻尼环节来增强系统的小信号稳定性,该方法通过降低控制器的响应速度来增强系统阻尼特性,从而提高系统的稳定运行能力。考虑到交流电网强度通常是变化的,综合考虑控制器的阻尼特性与快速响应特性,提出了交流电网强度自适应虚拟阻抗控制器,该控制器通过改变换流器电气量的测量点来虚拟地改变换流器阀侧阻抗与交流电网阻抗之间的关系。具体而言,当电网较弱时,将一部分虚拟阻抗转移到换流器阀侧来虚拟改变系统短路比,增强系统阻尼特性;当交流电网较强时,减小虚拟阻抗,从而在保证系统小信号稳定运行的前提下提高换流器的响应速度。(4)联接弱交流电网双馈入VSC-HVDC系统稳定性分析研究了联接弱交流电网双馈入VSC-HVDC系统的稳定性问题,重点关注了双馈入VSC之间的电气距离对系统小信号稳定性的影响。研究表明,当双馈入VSC之间的电气距离过短时,双馈入VSC的控制器会激发线路电抗与无功补偿电容之间的谐振,从而引发系统的高频振荡,但是可以通过在控制器中增加虚拟电阻控制环节改变控制器的谐振频率来抑制高频振荡;当双馈入VSC之间的电气距离过长时,在换流器输出的无功功率达到系统稳态运行电流约束条件后,无功类控制器将从定交流电压控制转变为定无功功率控制,此时公共连接点交流电压会在一定程度下降,从而降低系统的小信号稳定裕度,但是可以通过采用虚拟阻抗控制器来改变换流器控制参数的测量点,以增强系统小信号稳定性。
[Abstract]:Voltage source converter DC transmission (Voltage Source Converter based High Voltage Direct Current, VSC-HVDC), also known as flexible DC transmission, uses fully controlled power electronic devices, will not have commutation failure, and can adjust the active power and reactive power independently, so it is more suitable to the conventional direct current transmission technology than the conventional direct current transmission technology. Weak AC power grid power supply. Generally, the index of evaluating the strength of AC power grid is mainly: AC network short circuit ratio (Short Circuit Ratio, SCR) and AC grid inertia time constant (Inertia Constant). When the intensity of AC power grid is weak, the vector current controller (Vector Current Control) commonly used in the project may lead to flexibility. As the DC transmission system is unstable, the trend is more obvious with the weakening of the AC system. Therefore, it is very necessary to study the small signal stability of the flexible DC transmission system to ensure the damping characteristics of the flexible DC system, the stable operation capacity and the rapid response of the system, and to reveal the problem of the small signal stability of the flexible DC transmission system connected to the weak AC power grid. The mechanism of the stable operation of the sound system and the control strategy for improving the damping characteristics of the system are put forward. This paper will focus on the following research to get the results of important theory and engineering application value. (1) the mathematical model and control strategy of the VSC-HVDC system, in order to provide the important support of the mathematical model in this paper, first established the weak communication of connection. The nonlinear mathematical model of the VSC-HVDC system in the power grid is used to simulate the equivalent impedance of the synchronous generator in series with the equivalent impedance of the synchronous generator. The frequency characteristics of the AC power grid can be considered. The voltage source converter uses two level VSC and MMC topology respectively, the control system uses the classical vector current control, and the complete flexibility is built in order to construct the control system. The small signal model of the DC transmission system has developed the interface between different state space equations. By comparing the system time domain simulation model in PSCAD/EMTDC and the calculation results of the nonlinear mathematical model built in MATLAB, the correctness of the nonlinear mathematical model is verified. (2) the research foundation of the damping characteristic of the weak AC power grid VSC-HVDC system is connected. The influence of the SCR and the inertia time constant on the damping characteristics of the system is studied in the developed VSC-HVDC system. The results show that the stability margin of the system decreases with the decrease of the SCR or the inertia time constant of the AC grid. The inertia time constant is less affected than the SCR of the AC current grid. The effect of the controller parameters on the small signal stability margin of the weak AC power grid VSC-HVDC system is studied only when the inertia time constant of the AC grid is reduced only when the SCR is low. The influence of the controller parameters on the small signal stability margin of the weak AC power grid is studied, in which the Phase-Locked-Loop (PLL) parameters and the outer loop controller parameters are greatly affected. The parameters of the excitation controller also have great influence on the damping characteristic of the system. The equivalent impedance of the series line with the equivalent synchronous generator model and the equivalent impedance of the ideal voltage source series line are compared. The result shows that the AC network is ignored in the form of the equivalent impedance of an ideal voltage source in series line. The frequency characteristics of the weak AC power grid VSC system will have a greater impact on the small signal stability area assessment, and tend to get the result of the system small signal stability. The small signal stable region of the DC system with two level VSC and MMC is studied respectively. It is found that when the capacitance of the MMC sub module is small and the capacitance voltage fluctuates too much, the internal dynamic of the MMC is dynamic. There will be a strong interaction between the process and the PLL, and the high frequency oscillation will occur, which seriously affects the stable operation of the system. After increasing the capacitance value of the MMC sub module, the small signal stable region of the MMC and two level VSC in the weak AC grid is basically the same. (3) the advanced controller design of the weak AC network VSC-HVDC system is used for the use of the advanced controller. The virtual resistor strengthens the small signal stability of the weak AC power grid VSC-HVDC system. The existing power synchronization control method (Power Synchronization Control, PSC) is improved. By adding the virtual resistance control strategy, the side resistance of the converter valve can be increased equivalent and the high frequency oscillation of the converter is suppressed. At the same time, the research shows that the P is due to the high frequency oscillation of the converter. The SC controller itself is similar to the indirect current control. When the AC power grid is strong, the controller can not guarantee the response speed of the converter. At the same time, a new phase locked loop with additional damping link is proposed to enhance the small signal stability of the weak AC power grid VSC-HVDC system. The damping link is added to the PLL to enhance the small system. In order to enhance the damping characteristic of the system by reducing the response speed of the controller, the method improves the stability of the system and improves the stability of the system. Considering that the strength of the AC power grid is usually changed, the adaptive virtual impedance controller of the AC current power network is proposed, which takes into consideration the damping characteristic and fast response characteristic of the controller. By changing the measuring point of the electric flux of the converter, the system changes the relationship between the valve side impedance of the converter and the impedance of the AC grid. In particular, when the power grid is weak, a part of the virtual impedance is transferred to the valve side of the converter to virtual change the system short circuit ratio and enhance the damping characteristic of the system; when the power grid is stronger, the virtual power is reduced. The response speed of the converter is improved on the premise of ensuring the stable operation of the small signal in the system. (4) the stability analysis of the dual infeed VSC-HVDC system connecting the weak AC power grid is studied. The stability of the double fed VSC-HVDC system is connected with the weak AC power grid, and the stability of the electrical distance between the double fed VSC is focused on the stability of the system small signal stability. The study shows that when the electrical distance between the two feed VSC is too short, the dual feed VSC controller will stimulate the resonance between the line reactance and the reactive compensation capacitance, thus triggering the high frequency oscillation of the system, but the resonant frequency of the controller can be changed by adding the virtual resistance control link in the controller to suppress the high frequency vibration. When the electrical distance between the two feed VSC is too long, the reactive power controller will change from the constant AC voltage control to the constant reactive power control after the reactive power of the converter output reaches the steady state current constraint. At this time the AC voltage of the common connection point will decrease at a certain degree, thus reducing the stability of the small signal of the system. However, the measurement of the control parameters of the converter can be changed by using the virtual impedance controller to enhance the small signal stability of the system.

【学位授予单位】：华北电力大学(北京)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TM721.1

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