不平衡负载下三相离网逆变器的序阻抗建模及特性分析

Modeling and characteristic analysis of sequential impedance of three-phase off-grid inverter under unbalanced load

孤岛微电网中三相负载不平衡是引起微电网电压不对称的主要原因。为了清晰直观地分析和改善电压不对称问题,从序阻抗角度,采用一种谐波线性化方法,建立了不平衡负载下三相离网逆变器采用无正负序分离电压控制模式的正负序阻抗模型,在不同PI控制参数、不同的运行工况下对三相离网逆变器的正负序阻抗特性进行了对比分析。分析表明PI控制参数对正负序阻抗特性影响较大,运行工况产生的影响较小。然后在三种不同运行工况下,对单序dq控制模式和无正负序分离电压控制模式进行了仿真对比分析。得到的序阻抗特性曲线和输出波形表明:采用单序dq电压控制模式时离网逆变器的输出电压幅值不平衡度高,而采用无正负序分离电压控制模式时,由于三相离网逆变器的负序等效输出阻抗在基频下较小,不平衡负载引起的负序电流流过阻抗产生的压降也较小,使逆变器的输出电压幅值基本保持平衡。最后,仿真验证了无正负序分离电压控制模式下的序阻抗模型的有效性。

In order to analyze and improve the voltage asymmetry problem clearly and intuitively,this paper adopts a harmonic linearization method from the order impedance angle to establish a three-phase off-grid inverter with unbalanced load and phase separation voltage control mode under unbalanced load.The positive and negative sequence impedance models are compared and analyzed under different PI control parameters and different operating conditions.The analysis shows that the PI control parameters have a great influence on the positive and negative sequence impedance characteristics,and the impact of the operating conditions is small.Then,under three different operating conditions,the single-sequence dq control mode and the non-positive-negative-sequence separation voltage control mode are simulated and compared.The obtained sequence impedance characteristic curve and output waveform show that when the single-sequence dq voltage control mode is adopted,the output voltage amplitude of the off-grid inverter is unbalanced.When the positive and negative sequence separation voltage control mode is adopted,since the negative-sequence equivalent output impedance of the three-phase off-grid inverter is small at the fundamental frequency,the voltage drop caused by the negative sequence current flowing through the balanced load is also small.The output voltage amplitude of the inverter is basically balanced.Finally,the simulation verifies the effectiveness of the sequence impedance model without positive and negative sequence separation voltage control mode.