CN106452142B - 一种适用于模块化多电平变换器的改进调制策略 - Google Patents

一种适用于模块化多电平变换器的改进调制策略 Download PDF

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CN106452142B
CN106452142B CN201610892326.9A CN201610892326A CN106452142B CN 106452142 B CN106452142 B CN 106452142B CN 201610892326 A CN201610892326 A CN 201610892326A CN 106452142 B CN106452142 B CN 106452142B
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converter
bridge arm
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modulation strategy
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CN106452142A (zh
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周求宽
刘衍
姚骏
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Chongqing University
State Grid Corp of China SGCC
Electric Power Research Institute of State Grid Jiangxi Electric Power Co Ltd
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Chongqing University
State Grid Corp of China SGCC
Electric Power Research Institute of State Grid Jiangxi Electric Power Co Ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H02M7/4835—Converters with outputs that each can have more than two voltages levels comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/08—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters
    • H02M1/088—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters for the simultaneous control of series or parallel connected semiconductor devices
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/12—Arrangements for reducing harmonics from ac input or output
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
    • H02M7/5387—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration

Abstract

一种适用于模块化多电平变换器(modular multilevel converter，MMC)的改进调制策略，该方法通过在原有的两个电平之间增加一个过渡状态将变换器的输出电平数由N+1提高至2N+1。本发明通过提高输出电平数，能够大大改善阶梯波对正弦调制波的逼近效果，有效降低了变换器输出电压的谐波含量，提高了变换器输出性能。同时，所提调制策略还能够降低子模块电容电压的波动，有利于延长电容器的使用寿命。

Description

一种适用于模块化多电平变换器的改进调制策略
技术领域
本发明涉及一种适用于模块化多电平变换器的改进调制策略，属电力电子技术领域。
背景技术
模块化多电平变换器拥有易于扩展、设计灵活等诸多优点，在电力系统中得到越来越多的应用。而作为模块化多电平变换器(modular multilevel converter，MMC)的核心技术之一，调制算法根据调制波得到开关器件的驱动触发信号，以保证变换器输出目标电压，对变换器的电能转换性能有着十分重要的影响。
目前，针对模块化多电平变换器的调制策略中已有下列公开文献：
[1]Hagiwara M,Akagi H.PWM control and experiment of modularmultilevel converters[C].IEEE Power Electronics SpecialistsConference.Rhodes,Greece:IEEE,2008:154-162。
[2]孙世贤,田杰.适合MMC型直流输电的灵活逼近调制策略[J].中国电机工程学报,2012,32(28):62-68。
[3]Li Z X,Wang P,Zhu H B,et al.An improved pulse width modulationmethod for chopper-cell-based modular multilevel converters[J].IEEETransactions on Power Electronics,2012,27(8):3472-3481。
文献[1]中采用载波移相调制(Carrier Phase Shifted,CPS)，其优点在于可以利用较低的开关频率实现较高的等效开关频率，但CPS需要为每个子模块辅以专门的三角载波信号，对于一个三相全桥MMC，变换器每增加一个电平数，就需要增加6路载波信号，背靠背系统则需要增加12路载波信号，这就增加了系统的扩展难度。此外，为稳定子模块电容电压，需要为每个子模块配备专门的电压平均控制器和均衡控制器，进一步增加了系统的复杂程度。文献[2]提出最近电平逼近调制(Nearest Level Modulation,NLM)算法，NLM不再依赖于载波信号，不再需要控制脉冲宽度，实现方法更为简单，因此更加容易适应MMC电平数的扩展。但当子模块数目较少时，用阶梯波逼近正弦调制波将带来较大的误差。文献[3]提出一种将PWM调制与最近NLM调制相结合的改进调制策略，有效提高了MMC的输出电平数，降低了变换器输出电压的谐波含量，但由于引入了PWM调制，增大了策略的复杂度。
发明内容
本发明的目的是，针对模块化多电平变换器(MMC)包含子模块数较少时，最近电平逼近调制策略存在较大误差的问题，本发明根据MMC变换器及其子模块基本工作状态，提出了一种改进调制策略。
本发明的技术方案如下：一种适用于模块化多电平变换器的改进调制策略，通过在原有变换器的两个电平之间增加一个过渡电平，将变换器的输出电平数由N+1提高至2N+1。
所述改进调制策略步骤如下：
(1)采集MMC母线电压Udc，按以下公式求得子模块投入到变换器各上下桥臂中的数目，其中表示需要变换器输出的交流侧参考电压信号，N为桥臂包含子模块数；Npi和Nni为子模块数目，P代表上桥臂，n代表下桥臂，i代表(a、b、c)；
(2)根据(1)计算出的Npi和Nni生成六个桥臂的触发脉冲来控制各个子模块电力电子开关的开通与关断；
(3)根据(2)生成的触发脉冲，判断上桥臂和下桥臂子模块是否需要切除或投入；
(4)当判断出需要上桥臂切除1个子模块、下桥臂投入1个子模块时，发出改进调制标志信号；
(5)根据(4)中改进控制标志信号，在下桥臂投入子模块的同时保持上桥臂中子模块状态不变，并生成相应的六个桥臂的触发脉冲，以得到原有两电平之间的过渡电平，使子模块数为N的MMC的输出电平为2N+1。
所述变换器相单元包括上下两个桥臂，每个桥臂由N个半桥型子模块和一个桥臂电感Larm串联而成。
相比现有技术，本发明具有如下有益效果：本发明实现方法简单，在不需要增加额外的硬件设施的前提下可将变换器的输出电平数由N+1提高至2N+1。通过提高输出电平数，所提策略能够大大改善阶梯波对正弦调制波的逼近效果，有效降低变换器输出电压中的谐波含量，并能在一定程度上减小子模块电容压波动幅值，降低电容器的损耗，延长电容器的使用寿命。
附图说明
图1为MMC改进调制策略流程图；
图2为MMC相单元及子模块拓扑结构图；
图3为交流侧输出电压波形，
图3(a)为采用传统NLM调制策略的仿真波形，
图3(b)为采用改进NLM调制策略的仿真波形；
图4为交流侧输出电压FFT分析结果，
图4(a)为采用传统NLM调制策略的仿真波形，
图4(b)为采用改进NLM调制策略的仿真波形；
图5为子模块电容电压波形，
图5(a)为采用传统NLM调制策略的仿真波形，
图5(b)为采用改进NLM调制策略的仿真波形
图5(c)为采用本发明方法时a相单元上下桥臂中4个子模块的电容电压波形；
图5(d)为采用本发明方法时a相单元上下桥臂中4个子模块的电容电压波形。
具体实施方式
以下结合附图对本发明的具体实施方案作详细描述。
图1所示为MMC改进调制策略流程图，改进调制算法具体实施如下：
(1)利用电压霍尔采集MMC母线电压Udc，按最近电平逼近调制(Nearest LevelModulation,NLM)求得子模块投入到变换器各上下桥臂中的数目，其公式如下，
其中表示需要变换器输出的交流侧参考电压信号，N为桥臂包含子模块数，Npi和Nni为子模块数目，P代表上桥臂，n代表下桥臂，i代表(a、b、c)；
(2)根据步骤(1)计算出的Npi和Nni生成六个桥臂的触发信号A1来控制各个子模块电力电子开关的开通与关断；
(3)根据步骤(2)生成的触发信号判断上桥臂和下桥臂子模块是否需要切除或投入；
(4)当判断出需要上桥臂切除1个子模块、下桥臂投入1个子模块时，发出改进调制触发信号A2；
(5)根据步骤(4)中的改进调制触发信号，在下桥臂投入子模块的同时保持上桥臂中子模块状态不变，并生成相应的六个桥臂的触发脉冲，以使原有的两个电平之间多出一个过渡状态，变换器工作在过渡状态时的输出电平介于原有的两个输出电平之间，由此将变换器的输出电平由的N+1提升至2N+1。
以子模块数为4的MMC为例，表1为采用传统最近电平逼近调制的MMC工作状态：
表1 包含4个子模块的MMC工作状态表
表2为采用本方法的MMC工作状态，由表可知，MMC电平数由5个增加为9个，表中序号2、4、6、8为采用本方法时增加的过渡电平：
表2 包含4个子模块的MMC改进工作状态表
序号 Nin_top Nin_low 交流电压uac
1 4 0 Udc/2
2 4 1 Udc/2～Udc/4
3 3 1 Udc/4
4 3 2 (Udc/4)～0
5 2 2 0
6 2 3 0～(-Udc/4)
7 1 3 -Udc/4
8 1 4 (-Udc/4)～(-Udc/2)
9 0 4 -Udc/2
图3(a)和(b)分别为采用传统NLM调制策略和本发明所提改进调制策略时MMC变换器交流侧输出电压波形，仿真中MMC子模块数N＝4，由仿真结果可以看出，采用改进型调制策略后，变换器交流侧输出电压波形由5个电平增加到9个电平，电平数提高了近一倍，从而有效减小了阶梯波与正弦调制波之间的误差，调制策略的逼近效果得到明显改善。
图4(a)和(b)分别为采用传统NLM调制和本发明所提改进调制策略时，变换器输出电压的FFT分析结果。对比图4(a)(b)波形可知，采用改进型调制策略后，由于变换器交流侧输出电压电平数增加，变换器包括3、5次在内的输出电压谐波含量明显减小，变换器的输出性能得到显著提升。
图5(a)(b)和(c)(d)分别为采用传统调制策略和本方法时a相单元上下桥臂中4个子模块的电容电压波形。由图可见，两种调制策略下的子模块电容电压均维持稳定且所有子模块电容电压保持均衡。但是由于投入子模块数增多，采用本发明的子模块电容电压波动幅值较小，更小的电容电压波动幅值有利于延长电容器的使用寿命，这也进一步体现了本发明的优势。

Claims (2)

1.一种适用于模块化多电平变换器的改进调制策略，其特征在于，所述改进调制策略通过在原有变换器的两个电平之间增加一个过渡电平，将变换器的输出电平数由N+1提高至2N+1，具体步骤如下：
(1)采集MMC母线电压Udc，按以下公式求得子模块投入到变换器各上下桥臂中的数目，其中表示需要变换器输出的交流侧参考电压信号，N为桥臂包含子模块数；Npi和Nni为子模块数目；
(2)根据步骤(1)计算出的Npi和Nni生成六个桥臂的触发脉冲来控制各个子模块电力电子开关的开通与关断；
(3)根据步骤(2)生成的触发脉冲，判断上桥臂和下桥臂子模块是否需要切除或投入；
(4)当判断出需要上桥臂切除1个子模块、下桥臂投入1个子模块时，发出改进调制标志信号；
(5)根据步骤(4)中改进控制标志信号，在下桥臂投入子模块的同时保持上桥臂中子模块状态不变，并生成相应的六个桥臂的触发脉冲，以得到原有两电平之间的过渡电平，使子模块数为N的MMC的输出电平为2N+1。
2.根据权利要求1所述一种适用于模块化多电平变换器的改进调制策略，其特征在于，所述变换器相单元包括上下两个桥臂，每个桥臂由N个半桥型子模块和一个桥臂电感Larm串联而成。
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