多储能逆变器并联系统在微网孤岛条件下的稳定性分析及其控制策略

Stability Analysis and Control Strategy of Parallel Storage Inverter System Working Under the Micro-grid's Island Condition

多台储能逆变器在微网孤岛条件下并联运行时,需要为整个微网系统提供稳定的电压频率支撑,但逆变器等效输出阻抗和线路阻抗的差异会造成功率分配不均以及环流过大等问题,从而导致整个微网系统的不稳定。为了解决上述问题,可以在传统P-U、Q-f(调整有功功率来稳定微网电压、调整无功功率来稳定微网频率)下垂控制策略的基础上采用虚拟阻抗技术,通过对虚拟阻抗的设计将所有逆变器的等效输出阻抗设计为阻性,从而实现负荷功率的均分。从多储能逆变器并联系统的拓扑结构入手,分析了储能逆变器并联系统的功率流动特性并建立其输出阻抗模型;对整个系统的控制策略进行详细的介绍,包括引入虚拟阻抗的下垂控制策略以及储能逆变器的双闭环控制策略;根据阻抗稳定性分析法,分析了逆变器滤波参数和控制参数对整个系统稳定性能的影响,基于该工况可以发现当滤波电感L增加到5 m H时,逆变器并联系统趋于不稳定;虚拟阻抗系数kL增大到3时,系统阻抗比乃奎斯特曲线越过拒绝域,同时也会使系统的等效输出阻抗由偏阻容性变成感性,不利于高次谐波的抑制;而虚拟阻抗系数RD增大可以加强功率均分效果且对系统的稳定性影响较小。仿真结果说明,在该工况条件下,通过合理的设计逆变器输出阻抗,可以使多逆变器间的环流最大值由30 A降低到3 A以内,从而保证光储微网在孤岛条件下的稳定运行。

When the parallel battery inverters work under the micro-grid island mode, the parallel system needs to provide a stable voltage and frequency support for the micro-grid, but the differences of inverters＇ equivalent output impedance and the line impedance may cause the problem of power sharing and circulating current. In order to solve these problems, on the basis of the conventional P-U and Q-f droop control strategy, the virtual impedance is proposed to redesign the converters＇ equivalent output impedance as pure resistance and then the power sharing performance can be improved. Be- ginning with the multi-inverter parallel system＇s topology, we analyzed the power flow characteristics of the system and established its output impedance model; The entire system＇s control strategy was introduced in detail including the droop control with the introduction of virtual impedance and the inverter＇s double-loop control; The influence of the filter and controller＇s parameter was analyzed according to the impedance stability analysis method. Based on the working condi- tion in the article, we can find that when the filter inductor increases to 5 mH, the inverter parallel system tends to be unstable; When the virtual impedance coefficient kL increases to 3, the nyquist curve of system＇s impedance ratio will go cross the rejection region, and the system＇s equivalent output impedance turns into inductive one against high-frequency harmonic suppression; The increase of virtual impedance coefficient RD can strengthen the effect of power split and slightly affects the system＇s stability. Based on the working condition in the article, the simulation results indicate that the proposed control strategy can decrease the maximum circulation current between the multiple inverter from 30 A to 3 A ,which can ensure the stability of the PV/battery micro-grid system.