本文選題：電壓源型換流器 + 弱交流電網(wǎng)��； 參考：《華北電力大學(xué)(北京)》2017年博士論文

【摘要】：電壓源換流器直流輸電(Voltage Source Converter based High Voltage Direct Current,VSC-HVDC)又稱柔性直流輸電,采用全控型電力電子器件,不會發(fā)生換相失敗,并且可以對有功功率與無功功率進行獨立調(diào)節(jié),因此相比于傳統(tǒng)直流輸電技術(shù)更適用于向弱交流電網(wǎng)供電。一般而言,評價交流電網(wǎng)強度的指標(biāo)主要有:交流電網(wǎng)短路比(Short Circuit Ratio,SCR)和交流電網(wǎng)慣性時間常數(shù)(Inertia Constant)。當(dāng)交流電網(wǎng)強度較弱時,工程中普遍采用的矢量電流控制器(Vector Current Control)有可能導(dǎo)致柔性直流輸電系統(tǒng)失穩(wěn),并且隨著交流系統(tǒng)變?nèi)踹@種趨勢更加明顯。因此,為保證柔性直流系統(tǒng)的阻尼特性、穩(wěn)定運行能力以及系統(tǒng)快速響應(yīng)等實際工程需求,非常有必要研究聯(lián)接弱交流電網(wǎng)的柔性直流輸電系統(tǒng)小信號穩(wěn)定性問題,揭示影響系統(tǒng)穩(wěn)定運行的機理,提出改善系統(tǒng)阻尼特性的控制策略。為此,本文將著重開展如下研究,以得到具有重要理論和工程應(yīng)用價值的成果。(1)VSC-HVDC系統(tǒng)數(shù)學(xué)模型與控制策略為了給本文提供重要的數(shù)學(xué)模型支撐,首先建立了聯(lián)接弱交流電網(wǎng)VSC-HVDC系統(tǒng)非線性數(shù)學(xué)模型。其中,交流電網(wǎng)采用等值同步發(fā)電機串聯(lián)線路等效阻抗來模擬,可以考慮交流電網(wǎng)的頻率特性;電壓源換流器分別采用兩電平VSC與MMC拓?fù)?控制系統(tǒng)均采用經(jīng)典的矢量電流控制;同時為了構(gòu)建完整的柔性直流輸電系統(tǒng)小信號模型,開發(fā)了不同狀態(tài)空間方程之間的接口,通過對比PSCAD/EMTDC中的系統(tǒng)時域仿真模型以及MATLAB中搭建的非線性數(shù)學(xué)模型的計算結(jié)果,驗證了非線性數(shù)學(xué)模型的正確性。(2)聯(lián)接弱交流電網(wǎng)VSC-HVDC系統(tǒng)阻尼特性研究基于開發(fā)的聯(lián)接弱交流電網(wǎng)VSC-HVDC系統(tǒng)數(shù)學(xué)模型,研究了交流電網(wǎng)SCR與慣性時間常數(shù)對系統(tǒng)阻尼特性的影響,結(jié)果表明系統(tǒng)小信號穩(wěn)定裕度會隨著交流電網(wǎng)SCR或慣性時間常數(shù)的降低而減小。相比于交流電網(wǎng)SCR,慣性時間常數(shù)的影響較小,只有當(dāng)SCR較低時減小交流電網(wǎng)的慣性時間常數(shù)才會導(dǎo)致系統(tǒng)小信號失穩(wěn)。研究了控制器參數(shù)對弱交流電網(wǎng)VSC-HVDC系統(tǒng)小信號穩(wěn)定裕度的影響,其中影響較大的有鎖相環(huán)(Phase-Locked-Loop,PLL)參數(shù)和外環(huán)控制器參數(shù)。同時,等值同步發(fā)電機的勵磁控制器參數(shù)對系統(tǒng)的阻尼特性也有較大影響。對比了交流電網(wǎng)采用等值同步發(fā)電機模型串聯(lián)線路等效阻抗的形式以及理想電壓源串聯(lián)線路等效阻抗的形式,結(jié)果表明由于采用理想電壓源串聯(lián)線路等效阻抗的形式忽略了交流電網(wǎng)的頻率特性,對聯(lián)接弱交流電網(wǎng)VSC系統(tǒng)的小信號穩(wěn)定域評估會產(chǎn)生較大的影響,傾向于得到系統(tǒng)小信號穩(wěn)定的結(jié)果。分別研究了采用兩電平VSC與MMC時直流系統(tǒng)小信號穩(wěn)定域,發(fā)現(xiàn)當(dāng)MMC子模塊電容較小,電容電壓波動過大時,MMC內(nèi)部動態(tài)過程與PLL之間會產(chǎn)生較強的相互作用進而發(fā)生高頻振蕩,嚴(yán)重影響系統(tǒng)的穩(wěn)定運行;在增大MMC子模塊電容容值后,MMC與兩電平VSC在聯(lián)接弱交流電網(wǎng)時的小信號穩(wěn)定域基本一致。(3)聯(lián)接弱交流電網(wǎng)VSC-HVDC系統(tǒng)先進控制器設(shè)計提出了利用虛擬電阻增強聯(lián)接弱交流電網(wǎng)VSC-HVDC系統(tǒng)小信號穩(wěn)定性的方法。對已有功率同步控制方法(Power Synchronization Control,PSC)進行改進,通過附加虛擬電阻控制策略,可以等效增大換流閥側(cè)電阻,從而抑制換流器的高頻振蕩。同時,研究表明由于PSC控制器本身類似于間接電流控制,當(dāng)交流電網(wǎng)較強時該控制器并不能保證換流器的響應(yīng)速度。同時,提出了附加阻尼環(huán)節(jié)的新型鎖相環(huán)來增強聯(lián)接弱交流電網(wǎng)VSC-HVDC系統(tǒng)小信號穩(wěn)定性的方法,通過在PLL中增加阻尼環(huán)節(jié)來增強系統(tǒng)的小信號穩(wěn)定性,該方法通過降低控制器的響應(yīng)速度來增強系統(tǒng)阻尼特性,從而提高系統(tǒng)的穩(wěn)定運行能力�？紤]到交流電網(wǎng)強度通常是變化的,綜合考慮控制器的阻尼特性與快速響應(yīng)特性,提出了交流電網(wǎng)強度自適應(yīng)虛擬阻抗控制器,該控制器通過改變換流器電氣量的測量點來虛擬地改變換流器閥側(cè)阻抗與交流電網(wǎng)阻抗之間的關(guān)系。具體而言,當(dāng)電網(wǎng)較弱時,將一部分虛擬阻抗轉(zhuǎn)移到換流器閥側(cè)來虛擬改變系統(tǒng)短路比,增強系統(tǒng)阻尼特性;當(dāng)交流電網(wǎng)較強時,減小虛擬阻抗,從而在保證系統(tǒng)小信號穩(wěn)定運行的前提下提高換流器的響應(yīng)速度。(4)聯(lián)接弱交流電網(wǎng)雙饋入VSC-HVDC系統(tǒng)穩(wěn)定性分析研究了聯(lián)接弱交流電網(wǎng)雙饋入VSC-HVDC系統(tǒng)的穩(wěn)定性問題,重點關(guān)注了雙饋入VSC之間的電氣距離對系統(tǒng)小信號穩(wěn)定性的影響。研究表明,當(dāng)雙饋入VSC之間的電氣距離過短時,雙饋入VSC的控制器會激發(fā)線路電抗與無功補償電容之間的諧振,從而引發(fā)系統(tǒng)的高頻振蕩,但是可以通過在控制器中增加虛擬電阻控制環(huán)節(jié)改變控制器的諧振頻率來抑制高頻振蕩;當(dāng)雙饋入VSC之間的電氣距離過長時,在換流器輸出的無功功率達到系統(tǒng)穩(wěn)態(tài)運行電流約束條件后,無功類控制器將從定交流電壓控制轉(zhuǎn)變?yōu)槎o功功率控制,此時公共連接點交流電壓會在一定程度下降,從而降低系統(tǒng)的小信號穩(wěn)定裕度,但是可以通過采用虛擬阻抗控制器來改變換流器控制參數(shù)的測量點,以增強系統(tǒng)小信號穩(wěn)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.

【學(xué)位授予單位】：華北電力大學(xué)(北京)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2017
【分類號】：TM721.1

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