电气工程学报, 2015, 10(8): 80-85 doi:

工程技术

MMC型STATCOM子模块电容电压的自适应优化平衡控制

邹积勇, 黄秋燕, 杨志, 杨立军

威凡智能电气高科技有限公司 镇江 212132

Self-Adaptive Control of the Sub-Module Voltage for STATCOM Based on Modular Multilevel Converter

Zou Jiyong, Huang Qiuyan, Yang Zhi, Yang Lijun

Weifan Intelligent Electrical High-Tech Co. Ltd Zhenjiang 212132 China

责任编辑: 李小平

收稿日期: 2015-05-10   网络出版日期: 2015-08-25

Received: 2015-05-10   Online: 2015-08-25

作者简介 About authors

邹积勇 男 1980年生,博士,高级工程师,研究方向为电力电子功率拓扑,电能质量优化治理装置系统研发。

黄秋燕 女 1986年生,硕士,工程师,研究方向为电力系统仿真,电力电子控制技术。

摘要

对模块化多电平变换器(Modular Multilevel Converter,MMC) 子单元模块电容电压的谐波特性进行理论分析,并给出电容值选型的计算方法。针对传统的基于CSPWM分级电容电压平衡控制和基于NLM排序电容电压平衡控制,提出了一种适合MMC型STATCOM的自适应电容电压优化平衡控制策略,将平衡控制的重点放在如何选择最优控制器上。使用Matlab/Simulink对自适应优化控制策略进行仿真,同时搭建3kvar的实验平台,仿真及实验结果一致,结果均表明该优化平衡控制策略与传统方法相比,可以在不增加电容电压波动的前提下,降低器件的开关频率,并显著提高电容电压的响应速度。

关键词: 模块化多电平变换器 ; STATCOM ; 自适应优化平衡控制

Abstract

The harmonic characteristic of sub-module capacitor voltage in modular multilevel converter is analyzed. Based on the result of theoretical analysis, the principle and calculation method for the selection of sub-module capacitance are given. In order to solve the problem of hierarchical control strategy based on carrier shifted PWM modulation and the capacitor voltage sorting control based on nearest level modulation, a novel self-adaptive capacitor voltage balancing algorithm for STATCOM have been presented. The proposed control strategy is verified by simulation Matlab/Simulink, and the results show that this balancing algorithm can achieve low switching losses, fast DC voltage response speed and good dynamic performance.

Keywords: Modular multilevel converter ; STATCOM ; self-adaptive control

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本文引用格式

邹积勇, 黄秋燕, 杨志, 杨立军. MMC型STATCOM子模块电容电压的自适应优化平衡控制. 电气工程学报[J], 2015, 10(8): 80-85 doi:

Zou Jiyong. Self-Adaptive Control of the Sub-Module Voltage for STATCOM Based on Modular Multilevel Converter. Journal of Electrical Engineering[J], 2015, 10(8): 80-85 doi:

1 引言

随着电力电子装置在电力系统的广泛应用,电力系统的电能质量问题也随之日益突出,与此同时,精密设备制造、生产过程自动化等均对电能质量提出了更高的要求,因此,改善电能质量已成为电力电子在电力系统中的研究热点。模块多电平变换器(Modular Multilevel Converter,MMC)采用模块化设计,通过调整子模块的串联个数可以实现电压及功率等级的灵活变化,并且可以扩展到任意电平输出,减小了电磁干扰和输出电压的谐波含量,输出电压非常平滑且接近理想正弦波形,因此在网侧不需要大容量交流滤波器;开关器件的开关频率低,开关损耗也相应减少;由于MMC拓扑将能量分散存储在桥臂的各个子模块电容中,提高了故障穿越能力。因此MMC型静止同步补偿器(以下简称M-STATCOM)也已成为近年来国内外研究的热点[1,2,3]。M-STATCOM采用半桥型模块化结构,电容电压不均衡会导致装置输出电压和电流THD变大,不平衡工况下该问题会直接影响系统的故障穿越能力,严重时危害其他装置的正常安全运行。文献[4]将电容均压策略与载波相移调制技术结合,仅在PWM波电平变换点进行投切操作,称为基于载波相移的子模块均压优化技术。每次投切操作根据电流方向,仅改变电容电压最高和最低子模块的工作状态,以起到降低开关频率的目的。但该方法对均压控制器的控制频率要求较高,实现有一定难度。文献[5]将重点放在电容电压越限的子模块上,对电容电压未越限的子模块,通过引入保持因子增加保持原来状态的概率,以降低开关损耗,该方法为基于保持因子的子模块均压优化技术。文献[6,7,8]在原有子模块调制信号基础上叠加相应控制电容均压的修正信号,从而改变子模块吸收的有功功率实现直流电容电压平衡。

本文首先分析了M-STATCOM子单元模块电容电压的谐波特性,并给出了电容值的选取方法,同时结合三相MMC电容均压问题的产生及控制机理,提出了一种适合MMC型STATCOM系统的自适应电容电压优化平衡控制策略,能够显著降低器件的开关频率,减少开关损耗,提高电容电压的响应速度。最后,通过Matlab/Simulink仿真验证了本文所提出的优化策略的效果。

2 MMC电容电压谐波特性及容值计算

图1为三相模块化多电平变换器主电路拓扑示意图,其中,usausbusc为电网侧三相交流电压,Ls为线路等效电感,Rs为线路电阻,ipjiNjj = a,b,c)为三相上、下桥臂电流,P、N分别为直流支撑电压的正、负母线,母线电压为Vdc,o为假想直流母线电压的中点,即VPo = - VNo = 1/2Vdc

图1

图1   三相MMC主电路拓扑示意图

Fig.1   Three phase MMC circuit topology diagram


MMC变换器有6个桥臂,每个桥臂都是由n个相同的子模块和一个桥臂电抗L串联而成,每个子模块包含两个IGBT 和续流二极管组和一个储能电容[9],通过控制子模块的投入与切出拟合出期望的交流输出电压。三相调制波相差120°,保证交流输出电压三相对称,一个相单元每一时刻投入的子模块数固定,上下桥臂共投入n个子模块,维持直流母线电压恒定。

为了保持MMC子模块电容电压的平衡,进入变换器的有功功率应等于送出的有功功率与变换器损耗的有功功率之和,否则多余或不足的能量引起子模块电容储能的变化将造成电容电压上升或下降。同时,输入变换器的无功功率也会引起子模块电容产生能量波动。

2.1 MMC电容电压交流谐波特性

MMC变换器6个桥臂的工作规律相同,以A相桥臂为例进行分析。图2为MMC 单相等效电路图,其中,箭头方向为各电气量的正方向,usa为电网侧交流输出电压、电流,iPaiNa为A相上、下桥臂电流,VPVN为上、下桥臂等效受控电压源,Vdcidc为直流母线电压、直流母线电流。

图2

图2   MMC 单相等效电路图

Fig.2   MMC single phase equivalent circuit


UI分别为瞬时输出电压、电流的有效值或直流分量,设A相输出电压为 ,A相输出电流为 ,ω为角频率,$\varphi$为电流相对于电压的角度。A相桥臂的瞬时输入总能量为[5]

由于任意时刻投入n个模块,故可假设能量在n个功率单元中平均分配,每个功率单元的瞬时能量为

可知,功率单元直流电容电压瞬时值为

从式(3)可知,在含有有功功率,或者说M-STATCOM进行三相平衡补偿时,输出电流中为无功分量时,功率单元直流电容电压主要含有2次和1次脉动。

2.2 MMC电容电压容值计算

忽略直流母线电压和电流的波动,则A相上桥臂电压和电流的表达式为

可得,A相上桥臂充电功率为

式中,电压调制比 ;电流调制比

iap = 0时,上桥臂模块总电容电压达到极限,分别对应时刻Φ1Φ2。当Φ1< ωt<Φ2时,上桥臂模块电容的能量变化为

设A相上桥臂子模块电容电压最大值、最小值分别为vap(t1) = VCa(1 + ε),van(t2) = VCa(1 - ε),则A相上桥臂单个子模块电容的存储能量变化为

根据ΔWap = nΔWC可得到电容容量的计算公式为

3 电容均压控制策略

目前适用于模块化多电平变换器的电容均压控制策略主要包括基于最近电平逼近调制(Nearest Level Modulation,NLM)的排序控制和基于载波移相脉宽调制(Carrier Shifted PWM,CSPWM)的分级控制,前者为针对MMC拓扑的自身特点提出,后者衍生于级联多电平电容均压策略。其中基于NLM的排序均压控制策略物理概念明确,其原理是使用最近的电压矢量或电平瞬时逼近调制波,通过NLM算法给出每一时刻系统中投入和切除子模块的个数,而电容电压排序控制解决具体哪些子模块投入和切除的问题。基于NLM的排序均压控制策略根据电容电压的大小、输出电平数以及桥臂电流方向,选择相应的子模块触发导通。

基于CSPWM的分级控制策略,将子模块电容均压的方法分为平均电压均衡控制(average balancing control)、独立电压均衡控制(individual balancing control)以及桥臂电压均衡控制(arm balancing control)三部分[8],通过在调制信号上叠加平均电压控制信号、独立电压以及桥臂电压平衡控制以保证各子模块电容电压在三相之间、各相上、下桥臂之间以及各桥臂内部子模块之间平均分配。

上述的控制策略在具体实现过程中存在硬件需求较高、开关损耗较大、开关频率不一致,其实际效果不太理想。如果可根据子模块电容电压情况自动选择合适的控制器,便可在开关频率和均压效果之间折中,具有很强的实用意义。按照这种思想,在电容电压额定值附近设定一组电压上、下限,将平衡控制的重点放在电压越限的子模块电容上。若电容最高电压大于设定上限,或者是最低电压小于设定下限,模式切换控制器自动选择基于NLM的排序均压控制策略,若电容电压均未超出上、下限,则选择基于载波移相的分级均压控制策略,该方法称为自适应子模块电容均压优化控制策略,其具体控制框图如图3所示。

图3

图3   自适应子模块电容均压优化策略设计

Fig.3   Adaptive submodule capacitor voltage balance


4 仿真研究

为了验证M-STATCOM自适应均压优化控制策略的正确性及有效性,本文以三相M-STATCOM在三相三线系统的应用为例进行了仿真验证,系统电压等级为6.6kV,连接电感Ls = 5mH,等效电阻R = 1Ω,每相8个模块,上下桥臂各4个,桥臂串联电感L = 4mH,电容电压设定为1kV,其余仿真参数见表1

表1   三相1MW仿真模型参数

Tab.1  Three phase 1MW simulation parameters

参 数数 值
额定输出功率P /MW1
直流电容容值C /μF2 500
三角载波频率fc /Hz1 000
公共母线电压(Vdc = 2Vd) /V12 000
电容电压初始值VC_Init /V12 000/4 = 3 000

新窗口打开| 下载CSV


为验证本文的控制策略,考察系统输出无功功率对设定值阶跃扰动的跟踪性能,设定在t = 0.1s时无功指令值阶跃到1Mvar,在t = 0.2s时无功反向,由1Mvar变成 -1Mvar。系统在额定参数下,系统无功功率在上述指令值阶跃响应时的仿真波形如图4~图7所示,图4中两曲线分别为MMC装置A相输出电压和电流,在t = 0.1s前电流为0;在t = 0.1~0.2s,补偿电流滞后电压90°,可等效为电感;在t = 0.1~0.2s,补偿电流超前电压90°,等效为电容。补偿电流的波形失真度小,谐波THD分别为4.0%和3.2%。

图4

图4   M-STATCOM输出补偿电流

Fig.4   M-STATCOM compensation current


图5

图5   M-STATCOM输出电压

Fig.5   M-STATCOM compensation voltage


图6

图6   M-STATCOM的A相环流

Fig.6   M-STATCOM A phase circulation


图7

图7   M-STATCOM的A相电容电压

Fig.7   M-STATCOM A phase capacitor voltage


图5为三相M-STATCOM交流侧输出电压波形,由于MMC单相子模块数2n = 8,相电压可实现9电平输出,其电平数高,使得谐波THD含量降低。图6为A相环流,频率为100Hz,即2倍工频波动的交流量,幅值为12A,约为补偿相电流87A的10%,故可认为其被桥臂串联电抗限制到足够小;图7为M-STATCOM的A相上下桥臂各8个电容电压波形,其波动趋势相反, 是由于上下桥臂模块投入和切除状态互补造成的,在整个动态响应过程中,电压波动在设定值的±10%范围内,并且在无功功率大范围变化时,电容电压波动不大,表明了自适应优化控制策略的有效性。

5 实验波形

把本文提出的控制策略在一台±3kvar的试验样机上进行验证,直流侧电容电压为350V,试验平台参数如表2所示。试验平台的测试波形如图8图9所示。

表2   三相MMC-STATCOM试验平台参数

Tab.2  The parameters of three phase
Tab.2 Three phase MMC-STATCOM experimental parameter

参 数数 值
三相电网电压V /V220V/50Hz
额定功率P /kvar±3
桥臂串联电感L /mH8
子模块直流侧电容C /μF470
三角载波频率f /kHz5

新窗口打开| 下载CSV


图8

图8   A相各模块电容电压

Fig.8   A phase modules capacitor voltage


图9

图9   MMC-STATCOM A相输出补偿电流

Fig.9   MMC-STATCOM compensation current of phase A


从上述试验波形可以看出,本文提出的控制策略在MMC-STATCOM试验平台上得到了正确的验证,证明了本文所提的理论的正确性。

6 结论

本文提出了一种适用于MMC型STATCOM系统的自适应电容均压优化控制策略,针对传统的基于CSPWM分级电容均压控制和基于NLM排序电容均压控制,将平衡控制的重点放在自动选择合适的控制器上,并分析了M-STATCOM子单元模块电容电压的谐波特性,给出了子模块电容选型的计算方法。最后,通过Matlab仿真和搭建实验平台测试,结果表明该优化平衡控制策略可以在基本不增加电容电压波动的前提下,降低器件的开关频率,并显著的提高了电容电压的响应速度。

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介绍模块化多电平换流器(modular multilevel converter,MMC)电容电压平衡的原理以及子模块(sub-module,SM)电容的充放电过程。指出传统的电容电压平衡控制下,子模块投切较频繁,器件开关频率较高,会造成较大的开关损耗。针对传统方法的问题,提出一种适合MMC型直流输电系统的电容电压优化平衡控制策略,将平衡控制的重点放在电容电压越限的子模块上。对电容电压未越限的子模块,优化策略通过引入保持因子使具有一定的保持原来投切状态的能力,以降低开关器件的开关频率。使用PSCAD/EMTDC对优化控制策略进行仿真,结果表明优化平衡控制策略与传统方法相比,可以在基本不增加电容电压波动的前提下,显著降低器件的开关频率。

Makoto Hagiwara, Hirofumi Akagi .

Control and experiment of pulse width modulated modular multilevel converters

[J]. IEEE Transactions on Power Electronics, 2009,24(7):1737-1746.

DOI:10.1109/TPEL.2009.2014236      URL     [本文引用: 1]

Hagiwara M, Maeda R, Akagi H .

Theoretical analysis and control of the modular multilevel cascade converter based on double-star chopper-cells (MMCC-DSCC)

[C]. International Power Electronics Conference (IPEC), 2010: 2029-2036.

[本文引用: 1]

Hagiwara M, Maeda R, Akagi H .

Control and analysis of the modular multilevel cascade converter based on double-star chopper-cells (MMCC-DSCC)

[J]. IEEE Transactions on Power Electronics, 2011, 26(6)(s):1649-1658.

DOI:10.1109/TPEL.2010.2089065      URL     [本文引用: 2]

This paper presents the modular multilevel cascade converter based on double-star chopper-cells, which is intended for grid connection to medium-voltage power systems without using line-frequency transformers. The converter is characterized by a modular arm structure consisting of cascade connection of multiple bidirectional pulsewidth modulation chopper-cells with floating dc capacitors. This arm structure requires voltage-balancing control of all the dc capacitors. However, the voltage control combining an averaging control with an individual-balancing control imposes certain limitations on operating conditions. This paper proposes an arm-balancing control to achieve voltage balancing under all the operating conditions. The validity of the arm-balancing control as well as the theory developed in this paper is confirmed by computer simulation and experiment.

Rodriguez J, Moran L .

A vector control technique for medium-voltage multilevel inverters

[J]. IEEE Transactions on Industrial Electronics, 2002,49(4):882-888.

DOI:10.1109/TIE.2002.801235      URL     [本文引用: 1]

管敏渊, 徐政, 潘伟勇 , .

最近电平逼近调制的基波谐波特性解析计算

[J]. 高电压技术, 2010,36(5):1327-1331.

Guan Minyuan, Xu Zheng, Pan Weiyong , et al.

Analytical calculation of fundamental wave and harmonic characteristics for nearest level modulation

[J]. High Voltage Engineering, 2010,36(5):1327-1331.

王姗姗, 周孝信, 汤广福 , .

模块化多电平HVDC输电系统子模块电容值的选取和计算

[J]. 电网技术, 2011,35(1):26-32.

URL    

The mathematical relation between the value of sub-module capacitance and operational characteristics of HVDC system is analyzed in four aspects, i.e., the steady-state energy conversion process, dynamic response process of active power control and transient energy conversion process in modular multi-level converter HVDC power transmission system as well as the requirement to bridge arm protection during bipolar short-circuit fault, is researched. Based on the result of theoretical analysis, the principle and calculation method for the selection of modular multi-level sub-module capacitance are given. Usually, the value of the sub-module capacitance is calculated according to the voltage fluctuation and the requirement to dynamic response characteristic of active power control, and the calculation result is verified by transient voltage fluctuation of sub-model and the requirement to the bridge arm protection. Utilizing the model based on electromagnetic transient simulation software PSCAD, a practical design case is simulated, and simulation results show that the proposed selection principle and calculation method are reasonable and feasible.

Wang Shanshan, Zhou Xiaoxin, Tang Guangfu , et al.

Selection and caclation for sub-module capacitance in modular multi-level converter HVDC power transmission system

[J]. Power System Technology, 2011,35(1):26-32.

URL    

The mathematical relation between the value of sub-module capacitance and operational characteristics of HVDC system is analyzed in four aspects, i.e., the steady-state energy conversion process, dynamic response process of active power control and transient energy conversion process in modular multi-level converter HVDC power transmission system as well as the requirement to bridge arm protection during bipolar short-circuit fault, is researched. Based on the result of theoretical analysis, the principle and calculation method for the selection of modular multi-level sub-module capacitance are given. Usually, the value of the sub-module capacitance is calculated according to the voltage fluctuation and the requirement to dynamic response characteristic of active power control, and the calculation result is verified by transient voltage fluctuation of sub-model and the requirement to the bridge arm protection. Utilizing the model based on electromagnetic transient simulation software PSCAD, a practical design case is simulated, and simulation results show that the proposed selection principle and calculation method are reasonable and feasible.

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