Research Of Control Technology In Output Power Quality For Off-grid Photovoltaic Parallel Inverters

Off-grid PV system can be divided into centralized PV power system andhousehold PV power system, which supply power as a voltage source. According to theadvantages and disadvantages of existing AC parallel structures, off-grid PV systemuses a centralized parallel structure to increase the capacity of power supply. In order tomeet the requirements of the output power quality for load: the quality of the voltagewaveform, the transient characteristics of power and the effect of current sharingbetween parallel modules. In this paper, several measures based on main circuit andcontrol system are proposed to improve the output power quality, which are tested andverified in the experiment.①In order to get the attenuation of output harmonic voltage and high frequencycirculation, T-type filter is introduced to improve the output power quality. Theharmonic of the inverter output voltage at switching frequency can’t be suppressed byclosed-loop control, while it’s only suppressed by the main circuit. Through thecomparative analysis of the T-type filter and LC filter transfer function and the inverteroutput impedance, it can be drawn that T-type filter has the characteristics of higherattenuation and higher output impedance at switching frequency. The characteristics notonly help to attenuate harmonic voltage at the switching frequency, but also to suppressthe high-frequency circulation. The experiments verify the validity of the T-type filter.②Contra the problem that conventional current regulator tracks power frequencyinstruction with poor accuracy, robust model following controller is proposed toimprove performance of current regulator. Robust model following controller haslow-frequency dual integral characteristic, which tracks the current reference moreaccurately comparing with PI controller. Embedded reference model makes RMFcontroller have little influence to distributed parameter difference.③Contra the problem that limited bandwidth of PI controller lead to voltagewaveform distortion with non-linear load, improved-repetitive controller based onvoltage regulator is proposed to tracking a cycle reference signal with no static error. Byanalysising the compensated Bode when inner current loop is active, it’s obvisous thatinner current has non-ignored impact for repetitive control object. In the process ofdesigning parameters, the lead-lag part takes the place of the traditional notch to finishthe phase and amplitude correction. Considering repetitive controller lags a fundamental cycle, the quasi-proportion resonance controller is used as a feed-forward channel forreference signal in this paper, which makes the quasi-resonant controller regulateroughly and repetitive controller regulate precisely. Then, with the help of simulation,the effect of main circuit and control parameters are analyzed by simulation, proving thecorrectness of theoretical analysis and parameter design.④Experimental platform consists of two parallel inverter modules, and controlalgorithm achieves through the DSP2812. Experimental results show that parallelsystem based on control technology proposed in this papaer improve the output powerquality when load is linear or nonlinear,which has good voltage waveform, goodtransient response performance and limited circulation. The proposed controltechnology to improve the output power quality is verified to apply for off-gridphotovoltaic system.