Research On Grid-Connected Photovoltaic Inverter System Based On Z-Source Network

The grid-connected photovoltaic (PV) system power generation is the most important way in the solar energy application area, and has broad prospects. In PV system, the grid-connected inverter is a critical component responsible for the control of electricity flow between the PV module and power grid. Nowadays, the topology and control strategy of PV inverter has become the research hotpot. Z-source inverter is a single-stage system. It has some advantages such as simple structure, high reliability of the inverter for avoiding influence of the shoot-through by EMI, and small output waveform distortion. Especially, this topology applies to the occasions of large rang input voltage such as PV power generation, etc.In this paper, the topology and control strategy of the single-phase Z-source grid-connected PV inverter was analyzed in detailed. The whole paper is organized as followed:The topology, operating principle, commutation process and current and voltage stresses of the power switches was analyzed in this paper. And the power losses of the single-phase Z-source grid-connected PV inverter were studied, which can provide important guidance of the thermal design, devices and switching frequency selection.This paper presented small-signal modeling of the Z-source network. The output response characteristics had been analyzed with change of Z-source network parameters. The double-loop control strategy was adopted in this system. By regulating shoot-through duty cycle and modulation index, maximum power point tracking (MPPT) control and grid-connected current control was achieved, respectively. This paper proposed the one-cycle capacitor voltage-change adjustment method to control ac line current amplitude, combined with the characteristics of the single-phase Z-source PV grid-connected inverter. The proposed method can achieve Z-source capacitor voltage regulation within one ac mains cycle, and thus improve the power quality of the grid and minimize the injected ac line current variation.Due to the small size, high efficiency and low cost, the transformerless PV grid-connected inverter was widely used in low-power PV grid-connected systems. The paper analyzed the generating mechanism of common-mode currents in the Z-source inverter and full-bridge inverter. The shortcoming of common-mode voltage of Z-source inverter and full-bridge inverter with unipolar modulation is the high-frequency PWM pulse voltage. In order to overcome the generated common-mode current, the common method is to add transformer. While bipolar PWM modulation, no changes appear in the common-mode voltage and no common-mode current is generated. However, this modulation mode could not prevent the injection of DC current into the grid. A dc current fed from the customer's side into the grid can shift the transformers operating point and might cause saturation. This would result in high primary current peaks, which might trip the input fuse and thus cause a power outage to that specific section of the grid. Furtherly, it would cause increased harmonics. Therefore, a new control algorithm was proposed in this paper. The algorithm uses two compensation links to suppress the DC components, which were caused by pulse width imbalance in PWM process and the error in the actual current measurement. Without adding auxiliary circuit, the algorithm occupied a few control chip resources.In the case of low irradiation, the Constant Voltage (CV) method has a high efficiency, and the Perturb and Observe (PO) method has a high efficiency under high irradiation. Therefore, in order to improve the efficiency of solar panel effectively, a new MPPT method is proposed combined with the characteristics of Z-source inverter. In order to reduce frequency disturbance and improve the stability of the inverter, CV method is adopted during the startup process. The proposed scheme can track the maximum power point with high efficiency at both low and high irradiation, which combines the CV method with variable shoot-through duty cycle PO method.Islanding operations occurs when power supply from the main grid is interrupted due to fault and intentional disconnection of the line for servicing but the Distributed Generation (DG) keeps supplying power into distribution networks. Inadvertent islanding causes serious problems. For these reasons islanding detection is an indispensable feature for DG-systems. The over/under voltage, over/under frequency method and slip-mode frequency shift (SMS) method was discussed. And the non-detection zones of above two methods were analyzed. In order to overcome the disadvantage of SMS, an adaptive sliding-mode frequency shift method is proposed. This method sets the starting angle. When the measured frequency deviation is beyond the error range, the maximum phase angle and starting angle will be increased in the positive feedback manner. Thus, the frequency deviation of point of common coupling is accelerated, and the time of islanding detection is shortened.A 1.5kW experimental device of Z-source PV grid-connected inverter was built after the above research. By using TMS320F2812, the proposed methods were realized on this platform. The experimental tests showed the maximum efficiency and power factor reaches above 94% and 0.99, respectively. The experimental results verified the proposed control algorithms.