A Grid Voltage Feed-forward Control Strategy Based on The Virtual Impedance

Abstract

Abstract. This paper initially investigates the conventional LCL filter grid-connected inverts system, which introduces the control strategy with inverter-side current feedback, and analyzes the deficiencies within the strategy. The stability of the controller can be improved by adopting the active damping scheme.However, the output current of the inverter is severely influenced by the external grid voltage distortion and the grid low-order harmonic voltage. In order to reduce this impact and gain high-quality power, a novel control strategy is proposed in this paper, in which the virtual complex impedance is introduced to feed grid voltage forward. By using the proposed control strategy, the total harmonic distortion can be greatly decreased, allowing the grid current to synchronize the reference current well. In addition, the modulation of the virtual complex impedance by controller can also quantificationally regulate the equivalent output impedance, which brings great convenience for inverters system designers to calculating and devising. Simulation results are presented to prove the validity and improvements achieved by the proposed controller. Ⅰ IntroductionWith the acceleration of the industrialization process, the pollution problems caused by the use of fossil fuels and problems of the energy shortage have attracted people's attention, which also promotes the development of renewable energy technologies, such as photovoltaic power generation technology[1-2], wind power generation technology[3-4]and tidal power generation technology [5][6]. In order to meet the demands of users, the power generated by these renewable energy power generation devices needs to be connected into grid through inverters [7]. Hence, stable operation of the inverter can greatly improve the overall capacity and the stability of the grid.In the case of grid connection, the total harmonic distortion(THD) of the grid current and the power factor of the generation are two important technical indicators [8].Under the condition in which inverters are required to work both independently and corporately, the inverter normally adopts an LCL filter to inhibit harmonics and control the THD of the grid current in a reasonable range. The distortion of the inverter can also meet the distortion limits as recommend in IEEE Std [9]. However, the LCL filter increases the order of the grid-connected inverter, thereby reducing the system's stability. To address this problem, many kinds of strategies are presented, including double-closed loop control strategy with active damping [10][11][12][13],closed-loop control in inverter-side current [14],Split capacitance in closed-loop control [15][16]. In active damping control schemes, the closed-loop control in inverter-side current and the split capacitance in closed-loop control are the most widely used, and compared with other control strategies, the power factor of grid is lower. However, these two strategies have both a stable system and only single