Output Voltage Regulation for Harmonic Compensation under Islanded Mode of Microgrid

Abstract

This study examines a P+multi resonant-based voltage control for voltage harmonics compensation under the islanded mode of a microgrid. In islanded mode, the inverter is defined as a voltage source to supply the full local load demand without the connection to the grid. On the other hand, the output voltage waveform is distorted by the negative and zero sequence components and current harmonics due to the unbalanced and nonlinear loads. In this paper, the P+multi resonant controller is used to compensate for the voltage harmonics. The gain tuning method is assessed by the tendency analysis of the controller as the variation of gain. In addition, this study analyzes the slight voltage magnitude drop due to the practical form of the P+multi resonant and proposes a counter method to solve this problem by adding the PI-based voltage restoration method. The proposed P+multi resonant controller to compensate for the voltage harmonics is verified through the PSIM simulation and experimental results.

Seamless Grid Synchronization of a Proportional+Resonant Control-Based Voltage Controller Considering Non-Linear Loads under Islanded Mode

“…In this case, the proportional current loop causes a significant phase shift at the operating frequency. Therefore, the large voltage loop controller gain is essential to compensate for the phase shift and to guarantee a negligible steady state error [4,12]. Figure 2 shows the configuration of the P+MR voltage controller, which includes the inner proportional current control loop.…”

“…Although droop control is used to share the output power of the inverters by changing the voltage and frequency intentionally, this changeable frequency range is within the stable boundary by a constant voltage and frequency. Therefore, in islanded mode, the practical PR control form with ωn.cut, as shown in Equation (1), is sufficient to meet the stable output voltage regulation for both transient and steady-state of islanded mode [12] (note that [23,24] investigated the adaptive frequency control mainly for a steady state operation. In the present study, however, it is not considered because these authors only considered the transient dynamics under islanded mode of which the operating frequency is fixed at 60 Hz).…”

“…In the event of any grid faults or intentional switchover, the inverter has to operate as a voltage source to supply power to the local load under islanded mode [2].However, the transient performance of the inverter output voltage control is limited due to the characteristics of the non-zero output impedance of the inverter, the accurate voltage regulation becomes more difficult when the nonlinear or unbalanced load is connected to the inverter [2][3][4][5]. The design of a multi loop voltage controller has been studied conventionally to improve the dynamic response of the inverter [6][7][8][9][10][11][12][13][14][15][16][17], and different types of controllers for harmonics compensation, such as Proportional+Integral (PI), Proportional+Resonant (PR) and repetitive based methods, etc., have been proposed [4][5][6][7][8][9][10][11][12][13]. Among these controllers, the harmonic compensation method using the Proportional+Multi Resonant (P+MR) controller has attracted attention because it is simpler to implement than the PI-based harmonics compensation method and the positive and negative sequence components can be controlled simultaneously by one controller [13,14].…”

“…Among these controllers, the harmonic compensation method using the Proportional+Multi Resonant (P+MR) controller has attracted attention because it is simpler to implement than the PI-based harmonics compensation method and the positive and negative sequence components can be controlled simultaneously by one controller [13,14]. In [2,4,12], the durability of the P+MR controller was improved by modifying it as a quasi-PR control form, which has an extremely large gain at the target frequency. Therefore, the allowable band of the control frequency was extended, and it was possible to compensate for the specific order of harmonics with high stability in spite of the frequency change due to sudden load changes or limited component tolerance [13,14].…”

“…Because this PI control was based on the fundamental frequency, it could not compensate the harmonics properly. The addition of PI-based harmonics compensation terms was not also preferred due to the complexity of implementation [12]. A PR-based grid synchronization method was proposed in [11], but it considered only the linear resistive local load.…”