【摘要】：相比較于傳統(tǒng)的兩電平或三電平換流器,模塊化多電平換流器(Modular Multilevel Converter, MMC)具有輸出電壓波形質(zhì)量高、擴(kuò)展性好、低損耗和開關(guān)頻率低等優(yōu)點(diǎn),已成為柔性直流輸電領(lǐng)域的主流拓?fù)�。早期MMC的橋臂由大量半橋子模塊(HBSM)級(jí)聯(lián)構(gòu)成,無法自主切斷直流故障電流,難以擴(kuò)展到架空線領(lǐng)域。采用箝位雙子模塊的MMC(簡(jiǎn)稱C-MMC)是最新的換流器改進(jìn)拓?fù)?具有直流故障自清除能力,非常適合于架空線輸電,可大大擴(kuò)展柔性直流系統(tǒng)的應(yīng)用場(chǎng)合。然而,目前涉及至C-MMC的文獻(xiàn)很少,因此對(duì)其進(jìn)行深入系統(tǒng)的研究具有工程意義。本文主要圍繞C-MMC運(yùn)行特性及其在大容量架空線輸電場(chǎng)合應(yīng)用的關(guān)鍵問題展開。主要工作如下： (1)研究了C-MMC阻抗頻率特性和直流故障穿越機(jī)理。建立了穩(wěn)態(tài)運(yùn)行下MMC連續(xù)數(shù)學(xué)模型和狀態(tài)空間方程,證明了系統(tǒng)在運(yùn)行平衡點(diǎn)具有漸進(jìn)穩(wěn)定性。推導(dǎo)了C-MMC交直流側(cè)阻抗的等效表達(dá)公式,設(shè)計(jì)了基于測(cè)試信號(hào)法的阻抗計(jì)算方法和詳細(xì)實(shí)現(xiàn)流程,結(jié)果表明：交流側(cè)阻抗因非線性調(diào)制作用呈現(xiàn)波動(dòng)特性,直流側(cè)阻抗可等效為單調(diào)諧濾波器；運(yùn)行工況和控制模式對(duì)交直流側(cè)阻抗影響均很小,環(huán)流抑制會(huì)影響直流側(cè)阻抗。建立了C-MMC直流故障下的等值電路,揭示了其直流故障閉鎖機(jī)理本質(zhì)。為提高C-MMC穩(wěn)態(tài)運(yùn)行效益和故障暫態(tài)特性,提出了兩點(diǎn)改進(jìn)措施：a)橋臂可由HBSM和CDSM混合而成,b)在子模塊內(nèi)部箝位二極管處串聯(lián)阻尼電阻。 (2)構(gòu)建了C-MMC子模塊故障多層次冗余保護(hù)體系。將子模塊劃分為三個(gè)保護(hù)區(qū)域,引入了雙晶閘管保護(hù)方案；設(shè)計(jì)了子模塊內(nèi)部容錯(cuò)控制流程,以處理不同區(qū)域內(nèi)元件失效問題。在換流器層次,設(shè)計(jì)了改進(jìn)型最近電平調(diào)制和電容電壓平衡策略。該方法動(dòng)態(tài)調(diào)節(jié)電容電壓參考值,通過引入故障因子對(duì)電容電壓測(cè)量值進(jìn)行修正,以防止故障元件參與投切。提出了動(dòng)態(tài)備用和實(shí)時(shí)安全裕度的概念,并采取功率調(diào)整、擋位動(dòng)作等措施擴(kuò)大動(dòng)態(tài)備用元件數(shù)量。提出了抑制直流電流波動(dòng)的電壓補(bǔ)償策略,在故障橋臂電壓參考波上附加補(bǔ)償電壓分量,使交流相電流在上下橋臂近似平均分配。 (3)設(shè)計(jì)了C-MMC-HVDC的正常啟動(dòng)和故障后重啟動(dòng)策略。將正常自勵(lì)充電過程分為兩個(gè)基本階段即靜態(tài)充電階段和動(dòng)態(tài)充電階段。利用電路原理方法化簡(jiǎn)了靜態(tài)充電階段的系統(tǒng)等值電路,推導(dǎo)了最大充電電流的估算公式和限流電阻的選取原則。提出了分組可控有序充電方法,在該方法中模塊電容分組依次充電,以保證每個(gè)電容獲取足夠的能量。設(shè)計(jì)了直流故障后的系統(tǒng)自動(dòng)重啟動(dòng)控制時(shí)序。 (4)研究了C-MMC串并聯(lián)容量擴(kuò)展技術(shù)和單元投退策略。為實(shí)現(xiàn)高電壓大容量輸電,設(shè)計(jì)了采用C-MMC組合式換流器的雙極柔性直流拓?fù)?每極由若干換流單元串并聯(lián)構(gòu)成,接地極線從上下正負(fù)極結(jié)構(gòu)中間引出。將換流單元投退過程劃分為并聯(lián)類和串聯(lián)類,分別設(shè)計(jì)了詳細(xì)的投退控制流程。提出了基于投旁通對(duì)的串聯(lián)類單元投退控制策略,實(shí)現(xiàn)了待投退單元的快速旁路。 (5)研究了LCC-C-MMC混合直流穩(wěn)態(tài)運(yùn)行控制和直流低電壓穿越策略�；旌现绷髂孀儌�(cè)采用組合式C-MMC改進(jìn)型拓?fù)?以達(dá)到與傳統(tǒng)直流相匹配的大容量要求。為解決混合直流系統(tǒng)因整流側(cè)交流電網(wǎng)故障引起的直流低電壓穿越問題,提出了帶投旁通對(duì)的后備定電流和后備定電壓兩種控制策略。二者分別以逆變側(cè)和整流側(cè)為控制主導(dǎo)站,通過引入三次諧波注入調(diào)制和無功功率動(dòng)態(tài)調(diào)整策略,擴(kuò)展了系統(tǒng)的穩(wěn)態(tài)和暫態(tài)調(diào)節(jié)范圍；故障嚴(yán)重時(shí),系統(tǒng)借助旁通對(duì)快速旁路高位端的換流單元,進(jìn)入半壓運(yùn)行模式,剩余健全的換流單元維持功率的持續(xù)輸送。 (6)提出了一種MMC閥損耗計(jì)算通用方法,可統(tǒng)一分析不同子模塊結(jié)構(gòu)：HBSM、 CDSM和全橋子模塊(FBSM)。該方法基本步驟為：第一步,基于系統(tǒng)運(yùn)行參數(shù)和調(diào)制控制策略計(jì)算各子模塊元件的電流電壓時(shí)域變化波形；第二步,利用廠商提供的特性曲線對(duì)半導(dǎo)體器件特性參數(shù)進(jìn)行擬合；第三步,結(jié)合器件電流、電壓波形和開斷次數(shù)計(jì)算其損耗和結(jié)溫。本方法能夠計(jì)及優(yōu)化電容電壓附加控制,便于編程實(shí)現(xiàn),可快速計(jì)算各種工況下的換流器功率損耗分布。
[Abstract]:Compared with traditional two-level or three-level converters, modular multilevel converters (MMCs) have become the mainstream topology in the field of flexible HVDC because of their high output voltage waveform quality, good scalability, low loss and low switching frequency. The MMC (C-MMC) with clamped double sub-modules is the latest converter improved topology with DC fault self-cleaning capability. It is very suitable for overhead line transmission and can greatly expand the application of flexible DC systems. In this paper, the operation characteristics of C-MMC and its application in large-capacity overhead line transmission are discussed. The main work is as follows:
(1) The impedance frequency characteristics and DC fault traversing mechanism of C-MMC are studied. The continuous mathematical model and state-space equation of MMC under steady-state operation are established, and the asymptotic stability of the system is proved. The equivalent expression formula of AC-DC impedance of C-MMC is deduced, and the impedance calculation method based on test signal method is designed. The results show that the AC-side impedance fluctuates due to nonlinear modulation, and the DC-side impedance can be equivalent to a single tuned filter. The operating conditions and control modes have little effect on the AC-DC side impedance, and the circulating current suppression will affect the DC-side impedance. In order to improve the steady-state operation benefit and transient characteristics of C-MMC, two improvements are proposed: a) the arm can be made up of a mixture of HBSM and C DSM, and b) the damping resistance in series at the diode clamped inside the sub-module.
(2) A multi-level redundancy protection system for C-MMC sub-module faults is constructed. The sub-module is divided into three protection zones and a double thyristor protection scheme is introduced. The internal fault-tolerant control flow of sub-module is designed to deal with component failure in different zones. The method dynamically adjusts the reference value of capacitor voltage and corrects the measured value of capacitor voltage by introducing fault factors to prevent the faulty components from participating in switching. The fluctuating voltage compensation strategy adds a compensation voltage component to the voltage reference wave of the fault leg, so that the AC phase current is approximately equally distributed between the upper and lower arm.
(3) The normal start-up and restart strategy of C-MMC-HVD C is designed. The normal self-excited charging process is divided into two basic stages: static charging stage and dynamic charging stage. Principle. A grouped controllable ordered charging method is proposed, in which the module capacitors are grouped and charged sequentially to ensure that each capacitor gets enough energy.
(4) The C-MMC series-parallel capacity expansion technology and unit switching-back strategy are studied. To realize high voltage and large capacity transmission, a bipolar flexible DC topology using C-MMC combined converter is designed. Each pole is composed of several commutation units in series and parallel, and the grounding poles are drawn from the upper and lower positive poles. Detailed control flow is designed for the cascade and cascade classes respectively, and a cascade control strategy based on the cast-by-pass pair is proposed to realize the fast bypass of the cascade.
(5) The LCC-C-MMC hybrid DC steady-state operation control and DC low-voltage traversal strategy are studied. A combined C-MMC topology is adopted in the hybrid DC inverter side to meet the large capacity requirement matched with the traditional DC. To solve the DC low-voltage traversal problem caused by rectifier-side AC power grid fault in the hybrid DC system, the band is proposed. The backup constant current control strategy and the backup constant voltage control strategy with bypass pair are adopted. The inverter side and the rectifier side are used as the control dominant stations respectively. By introducing the third harmonic injection modulation and reactive power dynamic adjustment strategy, the steady-state and transient regulation range of the system is extended. The converter unit enters the half pressure operation mode, and the remaining sound converter unit maintains the continuous transmission of power.
(6) A general method for calculating the loss of MMC valves is proposed, which can analyze different sub-module structures: HBSM, CDSM and FBSM. The basic steps of this method are as follows: First, calculating the time-domain variation waveforms of current and voltage of each sub-module component based on system operating parameters and modulation control strategy; second, using the special features provided by manufacturers. In the third step, the loss and junction temperature are calculated by combining the device current, voltage waveform and the number of interruptions.
【學(xué)位授予單位】：浙江大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2014
【分類號(hào)】：TM721.1

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