模块化多电平换流器型直流输电若干问题研究
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摘要
模块化多电平换流器(MMC)是电压源换流器的一种多电平拓扑。由于MMC不需要将IGBT直接串联来提高阀的耐压等级,从而避免了大功率器件开关过程中的动态均压问题,因此非常适合在高压大功率的柔性直流输电场合应用。由于MMC-HVDC大大降低了柔性直流换流阀设备制造商的技术门槛,因此目前成为了全世界工业界和学术界的研究热点之一。虽然最近几年有大量关于MMC的研究成果公开,但相对于二电平VSC-HVDC,对MMC-HVDC的研究还处于起步阶段。本文针对子模块类型为半H桥的MMC-HVDC系统,主要研究了以下几个方面的问题：
(1)在MMC低频连续数学模型的基础上,建立了MMC的交直流侧解耦等效电路。从理论上证明了二电平VSC-HVDC的交流侧外环功率控制和内环电流控制策略完全适用于MMC-HVDC系统。同时从内部电动势取值范围的角度,分析了MMC额定运行点的选取方法和稳态运行范围的确定原则。
(2)对MMC现有的几种调制方式进行了比较,提出了适用于MMC的改进载波相移调制方式和降开关频率的电容电压均衡控制算法。将该方法与最近电平控制方式相比较,分别从谐波水平、基波跟踪误差两个方面分析了各自的优缺点。
(3)研究了MMC内部环流的形成机理,从数学角度证明了环流的存在性。然后针对环流中的主要分量——二次谐波分量,利用二倍频负序旋转坐标变换,建立了dq坐标下的环流模型,并设计了相应的环流抑制控制器(CCSC),将交流环流分解为两个直流分量,并分别加以抑制。CCSC可以在不影响交流侧电流控制的情况下,消除MMC的内部环流,同时还可以减小子模块电容电压的波动范围。
(4)在分析MMC换流阀损耗构成的基础上,用曲线拟合方法对厂家的IGBT模块参数进行提取。根据IGBT的热电路模型,将结温作为反馈变量,提出了考虑结温变化的MMC损耗计算方法。最后用该方法编写了基于仿真模型的损耗计算模块,对MMC子模块各部分的损耗和半导体器件的结温进行了计算。针对较多子模块和较少子模块两种情况,进行了损耗评估,结果表明在较多子模块的情况下,可以选择较低的载波频率,并结合提出的降开关频率电容电压均衡控制算法,使MMC的开关损耗大幅下降。
(5)针对MMC-HVDC交流系统不对称故障时,直流电压中存在二次谐波的问题,设计了一种直流电压波动抑制控制器(DCVRSC)。该控制器可以在不影响交流侧负序电流控制,保持交流侧电流对称的情况下,同时消除直流侧二次谐波电压。还针对直流电压控制站交流系统故障时直流电压失控的问题,将二电平VSC-HVDC系统的低压限流单元(VDCOL)引入到MMC-HVDC系统中,并分别分析了不同类型换流站交流侧故障情况下VDCOL的工作原理。最后通过仿真算例验证,说明VDCOL可以在直流电压控制站交流侧故障情况下,保持MMC-HVDC系统直流电压的稳定和功率的连续传输,增强了系统的故障穿越能力。
Modular multilevel converter (MMC) is an attractive multilevel topology of voltage-sourced converter based HVDC (VSC-HVDC). Since there is no requirement of converter valves with direct-series connection of IGBTs, the difficulty of valve manufacture technology is reduced. As a result, MMC is very suitable in high-voltage and high-power applications. Recently, MMC-HVDC has attracted many engineers' and scientists'attention. A large number of references are published these years. However, MMC topology is still not well understood compared with2-level VSC topology. The dissertation makes researches on several important issues in basic control and operation of MMC-HVDC system.
The dissertation is organized as follows:
(1)An ac-dc decoupled model of MMC is presented based on the continues model. This model demonstrates that the outer power control loop and inner current control schemes developed for2-level VSCs are also suitable for MMCs. The range of inner EMF is analyzed, the nominal operation point, active and reactive power operation range are discussed.
(2)Based on the comparison of different modulation schemes of MMC, an improved phase-shifted carrier PWM method is introduced. To reduce the switching losses of MMC, a reduced switching-frequency voltage balancing algorithm (RSFVBA) is investigated. The presented PSC-PWM and the nearest level control (NLC) method are compared from the aspects of harmonics and modulation errors.
(3)The characteristic of the circulating current is investigated based on an analytical model. With the double line-frequency, negative-sequence rotational frame, the three-phase alternative circulating currents are decomposed into two dc components and are minimized by a dedicated circuiting current suppressing controller (CCSC). The alternative current control is not affected at the same time. The CCSC can also reduce the capacitor voltage fluctuations.
(4)To evaluate the power loss of MMC, the manufacture data of IGBT modules is used with the curve fitting method. The power loss is calculated with the real-time simulation voltages and currents. Based on the thermal circuit model, the variation of junction temperature is considered and a feedback method is implemented to calculate the power loss more accurately.
(5)There are dc voltage ripples when MMC-HVDC operates under unbalanced grid conditions. To eliminate the second order harmonic voltage in the dc bus, a dc voltage ripple suppressing controller (DCVRSC) is introduced. The controller is incorporated with the negative-sequence current controller, which is widely used in2-level VSCs. Thus, the dc voltage ripple is eliminated as well as the ac current are balanced. To enhance the low voltage ride through (LVRT) capability of MMC-HVDC system, a dc voltage dependent current order limiter (VDCOL) is investigated. The VDCOL changes the active current reference according to the dc link voltage and keep the active power balanced in the dc system. Simulation results demonstrate the dc voltage is stable with the VDCOL, when the ac voltage of dc voltage control (DCVC) station is unbalanced.
(1)在MMC低频连续数学模型的基础上,建立了MMC的交直流侧解耦等效电路。从理论上证明了二电平VSC-HVDC的交流侧外环功率控制和内环电流控制策略完全适用于MMC-HVDC系统。同时从内部电动势取值范围的角度,分析了MMC额定运行点的选取方法和稳态运行范围的确定原则。
(2)对MMC现有的几种调制方式进行了比较,提出了适用于MMC的改进载波相移调制方式和降开关频率的电容电压均衡控制算法。将该方法与最近电平控制方式相比较,分别从谐波水平、基波跟踪误差两个方面分析了各自的优缺点。
(3)研究了MMC内部环流的形成机理,从数学角度证明了环流的存在性。然后针对环流中的主要分量——二次谐波分量,利用二倍频负序旋转坐标变换,建立了dq坐标下的环流模型,并设计了相应的环流抑制控制器(CCSC),将交流环流分解为两个直流分量,并分别加以抑制。CCSC可以在不影响交流侧电流控制的情况下,消除MMC的内部环流,同时还可以减小子模块电容电压的波动范围。
(4)在分析MMC换流阀损耗构成的基础上,用曲线拟合方法对厂家的IGBT模块参数进行提取。根据IGBT的热电路模型,将结温作为反馈变量,提出了考虑结温变化的MMC损耗计算方法。最后用该方法编写了基于仿真模型的损耗计算模块,对MMC子模块各部分的损耗和半导体器件的结温进行了计算。针对较多子模块和较少子模块两种情况,进行了损耗评估,结果表明在较多子模块的情况下,可以选择较低的载波频率,并结合提出的降开关频率电容电压均衡控制算法,使MMC的开关损耗大幅下降。
(5)针对MMC-HVDC交流系统不对称故障时,直流电压中存在二次谐波的问题,设计了一种直流电压波动抑制控制器(DCVRSC)。该控制器可以在不影响交流侧负序电流控制,保持交流侧电流对称的情况下,同时消除直流侧二次谐波电压。还针对直流电压控制站交流系统故障时直流电压失控的问题,将二电平VSC-HVDC系统的低压限流单元(VDCOL)引入到MMC-HVDC系统中,并分别分析了不同类型换流站交流侧故障情况下VDCOL的工作原理。最后通过仿真算例验证,说明VDCOL可以在直流电压控制站交流侧故障情况下,保持MMC-HVDC系统直流电压的稳定和功率的连续传输,增强了系统的故障穿越能力。
Modular multilevel converter (MMC) is an attractive multilevel topology of voltage-sourced converter based HVDC (VSC-HVDC). Since there is no requirement of converter valves with direct-series connection of IGBTs, the difficulty of valve manufacture technology is reduced. As a result, MMC is very suitable in high-voltage and high-power applications. Recently, MMC-HVDC has attracted many engineers' and scientists'attention. A large number of references are published these years. However, MMC topology is still not well understood compared with2-level VSC topology. The dissertation makes researches on several important issues in basic control and operation of MMC-HVDC system.
The dissertation is organized as follows:
(1)An ac-dc decoupled model of MMC is presented based on the continues model. This model demonstrates that the outer power control loop and inner current control schemes developed for2-level VSCs are also suitable for MMCs. The range of inner EMF is analyzed, the nominal operation point, active and reactive power operation range are discussed.
(2)Based on the comparison of different modulation schemes of MMC, an improved phase-shifted carrier PWM method is introduced. To reduce the switching losses of MMC, a reduced switching-frequency voltage balancing algorithm (RSFVBA) is investigated. The presented PSC-PWM and the nearest level control (NLC) method are compared from the aspects of harmonics and modulation errors.
(3)The characteristic of the circulating current is investigated based on an analytical model. With the double line-frequency, negative-sequence rotational frame, the three-phase alternative circulating currents are decomposed into two dc components and are minimized by a dedicated circuiting current suppressing controller (CCSC). The alternative current control is not affected at the same time. The CCSC can also reduce the capacitor voltage fluctuations.
(4)To evaluate the power loss of MMC, the manufacture data of IGBT modules is used with the curve fitting method. The power loss is calculated with the real-time simulation voltages and currents. Based on the thermal circuit model, the variation of junction temperature is considered and a feedback method is implemented to calculate the power loss more accurately.
(5)There are dc voltage ripples when MMC-HVDC operates under unbalanced grid conditions. To eliminate the second order harmonic voltage in the dc bus, a dc voltage ripple suppressing controller (DCVRSC) is introduced. The controller is incorporated with the negative-sequence current controller, which is widely used in2-level VSCs. Thus, the dc voltage ripple is eliminated as well as the ac current are balanced. To enhance the low voltage ride through (LVRT) capability of MMC-HVDC system, a dc voltage dependent current order limiter (VDCOL) is investigated. The VDCOL changes the active current reference according to the dc link voltage and keep the active power balanced in the dc system. Simulation results demonstrate the dc voltage is stable with the VDCOL, when the ac voltage of dc voltage control (DCVC) station is unbalanced.
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