从图12(a)可以了解：t1时刻,a相相电压跌落至20%,交流电流在网压发生跌落后逐渐增加并始终保持对称状态,说明相间功率均衡控制策略未对交流输出产生影响。图12(b)分别为直流电流和直流电压波形,此时直流侧功率为零,且直流电压受调制策略影响,基本稳定在650 V。从图12(c)(d)可知,上、下桥臂电流对称,验证了前文关于桥臂电流中不存在偶次谐波分量的理论分析。而在t2时刻,分别向各相环流注入直流分量,桥臂电流和环流发生相应地偏置。图12(e)波形表明,SOC因电池充放电也呈现出基频的波动。由于t1时刻交流侧不平衡的产生,SOC将出现发散现象,其发散速度与交流侧不平衡程度有关。随着t2时刻,相间功率均衡策略的投入,各相SOC发散停止,说明此时各相电池吸纳释放功率速度相同。
图12的仿真结果表明,交流电压的不平衡会引起MMC-ESS电池SOC的不一致。而相间功率均衡控制策略投入后,环流发生相应偏置,子模块电池SOC发散现象停止,证明了通过注入直流环流控制相间功率均衡的有效性。
4.2 功率均衡的实验验证
实验样机外观如图13所示,实验设备使用的器件为三菱PS21765,控制器由DSP和FPGA构成。由于实验条件限制,子模块电池电压选取为15 V,交流电压线电压有效值为24 V。实验时a相跌落至额定值的20%。
图14为网压不平衡时的桥臂电流和环流变化波形。t时刻投入功率均衡控制策略,各相桥臂电流和环流发生一定偏移。其中由于交流侧变压器的缘故,a相、c相环流均向下偏移约250 mA,b相环流向上偏移约500 mA,各相环流偏移量之和基
图13 实验平台
Fig. 13 Physical experiment platform
图14 桥臂电流及环流
Fig. 14 The arm current and the circulating current
本为零,与前文分析一致,说明本文提出的功率均衡控制不会造成额外功率的输入输出。总体来说,实验结果与仿真结果一致,能够验证提出的功率均衡控制算法的有效性。
5 结论
本文针对网压不平衡下模块化多电平储能系统的相间功率均衡控制策略进行了研究：
1)利用对称工况下开关函数与子模块电压,对桥臂电流进行分析,证明了储能电池直接并联在SM直流侧的拓扑不存在环流。
2)再利用不对称工况下的各类电气量,详细推导了电池SOC表达式,说明了交流功率不对称引起了SOC的不一致。
3)利用环流传递能量的原理,通过向桥臂电流中注入直流环流,使各相电池输出功率实现均衡,该方法可以避免因重构调制波造成的过调制问题,同时也能够最大限度保证实现相间功率均衡;通过分析环流控制调节器参数,结果也表明调节器参数使系统始终保持稳定,可适当修改参数以确保系统获得更好的动态响应;同时,利用三维图分析电压跌落深度对桥臂电流变化的影响情况,重新确定器件保护参数。
4)通过仿真和实验对该控制策略进行了验证,结果表明该相间功率均衡控制策略的有效性和可行性。
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