Development of a high performance PV-Utility interface using phase angle control of PWM inverter

Abstract

The goal of this thesis has been to develop a ‘only phase angle based’ control strategy for single stage grid-connected photovoltaic (GCPV) system. Existing control strategies for single-stage and two-stage GCPV systems are presented. The literature review shows that the control strategies of the GCPV inverters involve adjustment of the phase angle and modulation index of the switching signal which determines the angle and amplitude of the inverter output voltage respectively. The phase angle dominantly controls the real power supplied by the inverter, whereas, the amplitude controls the reactive power. To inject current into the grid at unity power factor, the reactive power injected into grid is reduced to zero by adjusting the modulation index. Due to variation of modulation index, computation increases as the PWM pattern has to be determined each time the modulation index is adjusted. Lower value of modulation index increases harmonics and current THD in the system. To overcome the drawbacks, a single pre-calculated PWM pattern with highest modulation index is used to operate the inverter so as to track maximum power point (MPP) by adjusting the phase angle of the inverter output voltage with respect to the reference grid. The proposed ‘only phase angle based’ control scheme is simulated in matlab and simulink. Maximum power point tracking (MPPT) is performed by adjusting the phase angle of the inverter switching signal and employing common perturb and observe (PO) method. A modification is proposed in PO algorithm for setting a suitable initial phase angle and adaptive step-size to reduce the tracking-oscillations tradeoff problem associated with this popular MPPT technique. Another modified PO method based on gradient ascent technique with variable step-size is also dealt with for setting a range of the permissible step-size. The setting of range of permissible step-size is necessary since unstable condition or inefficient tracking at different irradiance levels may occur due to improper selection of the step-size. Simulation results show that harmonics and current THD are less with the proposed phase angle based control scheme. The validity of the proposed modifications in the PO method is established through investigation by simulation. The proposed scheme is simulated for stiff and weak grids. Weak grid is modeled assuming constant voltage at the high tension (HT) side of the distribution transformer. Grid impedance is considered sufficiently large at the low tension (LT) side. The effect of reactive power injection into grid with the proposed technique is analyzed. It is found that at higher irradiance reactive power supplied by the PV inverter is higher, whereas, it is lower at the point of common coupling (PCC) due to larger amount of reactive power is consumed in filter and coupling inductances at higher irradiance. In case of weak grids, voltage rise due to injection of PV is found to be high which is a desirable feature. The proposed scheme having such feature improves grid voltage profile. The performance of the proposed technique has also been investigated in a three-phase system. The range of phase angle adjustment for a wide range of variation in solar irradiance is investigated. The proposed scheme features stable operation since it avoids the feedback control of current. Along with modeling and analysis, detail simulation results are presented that demonstrates the effectiveness of the proposed technique.