低压微电网中，采用虚拟同步发电机（virtual synchronous generator，VSG）策略控制多台逆变器并联运行时，由于线路阻抗差异较大，无法实现输出功率的按容量精确分配。针对这一问题，文章提出一种改进的VSG控制技术，在无功电压控制环中引入公共点电压反馈和积分环节，消除线路阻抗对无功分配的影响；并在虚拟阻抗环引入无功功率反馈，根据系统运行情况实时调整虚拟阻抗的阻值。在Matlab/Simulink环境下搭建了仿真模型，仿真结果表明，所提改进控制策略实现了有功和无功功率的精确分配，降低了逆变器输出电压幅值跌落，并且具有较强的鲁棒性。

In a low-voltage microgrid， the virtual synchronous generator （VSG） strategy is adopted to control the parallel operation of multiple inverters. Due to the difference in line impedance， the accurate allocation of output power according to capacity cannot be realized. In view of this problem， this paper proposes an improved VSG control strategy， in which feedback and integration of voltage of common coupling point are introduced into the reactive voltage control loop to eliminate the influence of line impedance on reactive power distribution. The reactive power feedback is introduced into the virtual impedance loop to adjust the resistance value of the virtual impedance in real time according to the operation of the system. The simulation which results in Matlab/Simulink show that， the improved control strategy can realize the accurate distribution of active and reactive power， also can reduce the amplitude drop of inverter output voltage， and has strong robustness.

在低压微电网中,由于受到线路阻抗参数呈阻性以及阻抗值不匹配的影响,常规下垂控制方法往往存在功率耦合和稳态无功功率分配不均的问题。针对上述问题提出一种自适应系数的改进下垂控制方法。该策略通过引入基准虚拟电抗将等效输出阻抗调节为感性,削弱线路阻性成分带来的耦合问题,从而能够应用感性下垂控制;其次,引入低带宽通信,根据功率均分需求自适应调节下垂系数,消除线路阻抗不匹配问题,从而实现无功功率精确均分。相比于传统下垂控制方法,该方法适用任意线路阻抗条件下的微电网控制,具有良好的动、稳态性能。最后,通过Matlab/Simulink仿真证明了该方法的正确性与有效性。

In the low-voltage microgrid, due to the influence of the impedance of the line and the impedance mismatch, the conventional droop control often has the problems of power coupling and uneven distribution of steady-state reactive power. Aiming at the above problems, an improved droop control method with adaptive coefficients is proposed. This strategy adjusts the equivalent line impedance to inductive by introducing the reference virtual reactance, weakening the coupling problem caused by the line resistive component, so that inductive droop control can be applied; secondly, low-bandwidth communication is introduced, and the droop is adaptively adjusted according to the power sharing requirements. Coefficient is adjusted to eliminate the problem of line impedance mismatch, so as to achieve accurate and even distribution of reactive power. Compared with traditional droop control, this method is suitable for microgrid control under any line impedance condition, and has good dynamic and steady-state performance. The simulation results in Matlab/Simulink prove the correctness and effectiveness of the method.

LCL滤波器因其良好的滤波性能被广泛应用于并网逆变器系统中。然而,LCL滤波器的固有谐振特性以及数字控制延时会严重危及系统稳定性。为提高系统鲁棒性,工程上通常采用无源阻尼方法以抑制滤波器谐振。由于滤波器谐振问题受多种因素影响,阻尼电阻的选取标准通常是模糊的。为准确、高效地设计阻尼电阻,文章结合阻尼电阻与滤波电容串联和与滤波电容并联两种典型无源阻尼方法,推导分析了“临界阻尼”指标。当系统阻尼大于临界阻尼时,LCL滤波器幅频特性曲线上的谐振峰被完全抑制,同时以最少的无源阻尼损耗为代价,充分保证了基于LCL滤波器的并网逆变器系统稳定性。此外,文章在PLECS软件中进行三相并网逆变器系统仿真,仿真结果验证了理论分析的有效性和正确性。

LCL filter is widely used in grid-connected inverter system for its advanced filtering performance. However, the inherent resonance characteristic of LCL filter and digital control delay will seriously endanger system stability. To increase the system robustness, passive damping method is usually applied to suppress the filter resonance in engineering. Since the filter resonance problem is affected by many factors, the selection standard of damping resistor is unclear. In order to design the damping resistor accurately and efficiently, this paper introduces the index of“critical damping factor”and analyzes two typical passive damping methods: damping resistor in series with filter capacitor or in parallel with filter capacitor. When system damping factor is larger than the critical damping factor, the resonant peak of the amplitude frequency characteristic curve can be completely suppressed and the stability of grid-connected inverter system is fully guaranteed with the minimum cost of passive damping losses. Moreover, the simulation of three-phase grid-connected inverter is carried out in PLECS and the simulation results verify the effectiveness and correctness of the theoretical analysis.