Unbalanced-Voltage Control Strategy of Fractional Order PI-Quasi Proportional Resonant Combined with Virtual Impedance

doi:10.12204/j.issn.1000-7229.2021.05.014

Abstract

Abstract:

In order to solve the problem of voltage imbalance in three-phase three-leg off-grid inverter, a fractional order proportional integral (PI) quasi proportional resonant voltage control strategy with virtual impedance is proposed. In the d-q rotating coordinates, the fractional order PI tracking control is designed for the positive sequence component of the fundamental wave, and the quasi-proportional resonant tracking control is designed for the negative sequence component of the fundamental wave. Because of the difference of bandwidth between fractional order PI and quasi proportional resonant controller, the compound voltage controller does not need the separation of positive and negative sequence of voltage, which avoids the complexity of program implementation and the problem of delay and precision degradation caused by separation. Considering the influence of unbalanced voltage drop on the line impedance on the common bus voltage, the negative-sequence virtual impedance is designed to reduce the total negative sequence impedance, thus realizing the three-phase voltage balance at the point of common coupling. Finally, the effectiveness of the proposed control strategy is verified by simulation and experiment.

Key words: three-phase three-leg, voltage imbalance, fractional order proportional integral (PI), quasi proportional resonance, virtual impedance

CLC Number:

TM464

CHENG Yong, LUO Changqing, CHEN Liuxin. Unbalanced-Voltage Control Strategy of Fractional Order PI-Quasi Proportional Resonant Combined with Virtual Impedance[J]. ELECTRIC POWER CONSTRUCTION, 2021, 42(5): 130-137.

Figures/Tables 17

Fig.1 Structure of off-grid inverter system

Fig.2 Block diagram of voltage double closed-loop control

Fig.3 Bode diagram of FOPI and QPR

Fig.4 Realization block diagram of double-sequence virtual impedances

Fig.5 Structure of inverter control system

Table 1 Simulation parameters

Fig.6 Output voltage response in simulation 1

Fig.7 Output voltage response in simulation 2

Fig.8 Output voltage imbalance degree in simulation 2

Fig.9 Experiment hardware

Table 2 Experiment parameters

Fig.10 Three-phase output voltage waveform of unbalanced load input under balanced load

Fig.11 Three-phase output voltage waveform with QPR controller

Fig.12 Three-phase output voltage waveform with negative-sequence virtual impedance

Fig.13 Three-phase voltage amplitude under different working conditions

Table 3 Voltage imbalance under different working conditions

Fig.14 Three-phase voltage and current waveform at PCC side

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