Abstract：The power transmission and circulating current characteristics of a parallel-inverter system are firstly analyzed in this paper. It reveals that the difference of the line impedance and the deviation of the output voltages between two parallel units both have significant effects on the power sharing accuracy of reactive power under the condition of the inductive line impedance. An adaptive virtual impedance method is proposed to reduce the difference of the equivalent connection impedance of the parallel system to some extent, andthe power sharing accuracy will be obviously improved by keeping the equivalent connection line impedance inductive. Additionally, an improved droop control strategy, which forms the equivalent droop control curve like a parabolic shape, is further put forward by using the feedback of AC bus voltage, realizing the trade-off between power sharing accuracy and output voltage sag effectively. With the proposed control strategy, the current circulation can be drastically suppressed and the system overall performance is greatly improved. Finally, the validity of the theoretical analysis and the proposed strategy is proved by simulation and experimental results.
[1] 鲍微, 胡学浩, 李光辉, 等. 独立型微电网中基于虚拟阻抗的改进下垂控制[J]. 电力系统保护与控制, 2013, 41(16): 7-13. Bao Wei, Hu Xuehao, Li Guanghui, et al.An improved droop control strategy based on virtual impedance in islanded micro-grid[J]. Power System Protection and Control, 2013,41(16): 7-13. [2] 林燎源, 林钊, 刘伟,等. 基于阻性下垂的逆变器无线并联均流控制[J]. 电工技术学报, 2016, 31(8): 43-50. Lin Liaoyuan, Lin Zhao, Liu Wei, et al.Wireless current sharing for parallel operation of inverters using resistive output impedance[J]. Transactions of China Electrotechnical Society, 2016, 31(8): 43-50. [3] 张庆海, 罗安, 陈燕东, 等. 并联逆变器输出阻抗分及电压控制策略[J]. 电工技术学报, 2014,29(6): 98-105. Zhang Qinghai, Luo An, Chen Yandong, et al.Analysis of output impedance for parallel invertersand voltage controlstrategy[J]. Transactions of ChinaElectrotechnical Society, 2014, 29(6): 98-105. [4] 徐玉琴, 马焕均. 基于改进下垂控制的逆变器并联运行技术[J]. 电力系统保护与控制, 2015, 43(7): 103-107. XuYuqin, Ma Huanjun. Parallel operation technology of inverters based on improved droop control[J]. Power System Protection and Control, 2015, 43(7): 103-107. [5] 王逸超, 罗安, 金国彬. 微网逆变器的改进鲁棒下垂多环控制[J]. 电工技术学报, 2015, 30(22): 116-123. Wang Yichao, LuoAn, Jin Guobin. Improved robust droop multiple loop control for parallel inverters in microgrid[J]. Transactions of China Electrotechnical Society, 2015, 30(22): 116-123. [6] BrabandereK D,Bolsens B, Keybus JV D, et al. A voltage and frequency droop control method for parallel inverters[J].IEEETransactions onPowerElectronics,2007,22(4):1107-1115. [7] GuerreroJ MMatasJ, VicuñaL G de, et al.Decentralized control for parallel operation of distributed generation inverters using resistive output impedance[J]. IEEE Transactionon Industrial Electronics, 2007, 54(2):994-1004. [8] MatasJ, CastillaM, Vicuna DLG, et al. Virtual impedance loop for droop-controlled single-phase parallel inverters using a second-order general-integrator scheme[J].IEEE Transactions on Power Electronics, 2010, 25(12):2993-3002. [9] ZhangYu, YuMi, LiuFangrui, et al. Instantaneous current-sharing control strategy for parallel operation of UPS modules using virtual impedance[J]. IEEE Transactions on Power Electronics, 2013, 28(1):432-440. [10] TabatabaeeS,KarshenasHR,BakhshaiA,et al. Investigation of droop characteristics and X/R ratio on small-signal stability of autonomous microgrid[C]//Power Electronics,Drive Systems and Technotogies Conference, 2011: 223-228. [11] ZhangXuan, LiuJinjun, YouZhiyuan, et al. Small-signal analysis and modeling of parallel inverters based on the droop control method in micro-grid[C]//Energy Conversion Congress and Exposition(ECCE), 2013: 4580-4586. [12] 鄂志君, 应迪生, 陈家荣, 等. 动态相量法在电力系统仿真中的应用[J]. 中国电机工程学报, 2008,28(31): 42-47. EZhijun, Ying Disheng,Chen Jiarong, et al. Application of dynamic phasor in power system simulation[J]. Proceedings of the CSEE, 2008, 28(31): 42-47. [13] Wang L, Guo X Q, Gu H R, et al.Precise modeling based on dynamic phasors for droop-controlled parallel-connected inverters[C]//IEEE International Symposium on Industrial Electronics (ISIE),2012: 475-480. [14] 林永朋, 陶顺, 肖湘宁. 基于补偿电流的并联逆变器环流抑制方法[J]. 现代电力,2015, 32(2):29-35. Lin Yongpeng, Tao Shun, Xiao Xiangning.A method of suppressing circulating current for parallel inverters based on the compensating current[J].Modern Electric Power, 2015, 32(2): 29-35. [15] Shafiee Q, GuerreroJM, VasquezJC. Distributed secondary control for islanded microgrids-anovel approach[J]. IEEE Transactions on Power Electronics, 2014,29(2): 1018-1031. [16] ZhangX, Liu J, LiuT,et al. A novel power distribution strategy for parallel invertersin islanded mode microgrid[C]//2010 Twenty-Fifth Annual IEEE Applied Power Conference and Exposition (APEC), California, 2010: 2116-2120. [17] Vasquez J C, Guerrero J M.Adaptive droop control appliedto voltage-source inverters operating in grid-connected and islandedmodes[J]. IEEE Transactions on Power System, 2009,56(10):4088-4096. [18] 张尧, 马皓, 雷彪, 等. 基于下垂特性控制的无互联线逆变器并联动态性能分析[J].中国电机工程学报, 2009, 29(3): 42-48. Zhang Yao, MaHao, Lei Biao,et al. Analysis of dynamic performance for parallel operation of inverterswithout wire interconnections[J]. Proceedings of the CSEE,2009, 29(3): 42-48. [19] 陈杰, 钮博文, 张俊文, 等. 微电网运行模式平滑转换的混合控制策略研究[J]. 中国电机工程学报, 2015, 35(17): 4379-4387. Chen Jie, Niu Bowen, Zhang Junwen, et al.Research on the hybrid control strategy for the seamless transfer of microgrids[J]. Proceedings of the CSEE, 2015, 35(17): 4379-4387. [20] 张庆海, 彭楚武, 陈燕东, 等. 一种微电网多逆变器并联运行控制策略[J]. 中国电机工程学报, 2014, 34(25): 126-132. Zhang Qinghai, PengChuwu, ChenYandong, et al. A control strategy for parallel operation of multi-inverters in microgrid[J]. Proceedings of the CSEE, 2014, 34(25): 126-132. [21] 张俊文. 基于下垂控制的微网逆变器控制策略研究[D].南京:南京航空航天大学,2015.
2018, Vol. 33 
