The power conversion circuit of the grid-connected PV system using a Quasi Z-source inverter was analyzed in this paper. The MPPT (Maximum Power Point Tracking) control and inverter output current control of the Quasi Z-source inverter was realized using photovoltaic source simulator and real grid. And the method for power quality improvement is suggested and proved the performance through simulation and experiment.
The use of the renewable-energy generating system recently increased because of the exhaustion of fossil fuel and the influence of the environment. As such, the inverter, which converts variable energy sources like photovoltaic energy, wind, and fuel cell to acceptable electrical sources, has gained importance of late.
The traditional inverters, such as the voltage-source and the current-source inverter, cannot satisfy the requirements of the variable energy source mentioned above. As such, one more separate DC/DC converter was used. This cascaded arrangement of two power converters, however, increases not only the complexity of the circuitry and control but the cost and the space requirement as well. Moreover, the increased number of power switches brings about power loss.
The Z-source inverter and the Quasi Z-source inverter have a unique impedance network. Thus, the circuit configuration is changed from that of a voltage source to that of an impedance source. The Quasi Z-source inverter system is more suitable for PV system application in terms of structure and control. Since the input current is continuous the Quasi Z-source inverter system does not cause inconvenience to lifespan of a PV array. Moreover withstanding voltage of capacitor and diode is reduced. Since this inverter provides a common ground it is easy to manufacture the system.
The inverter for the grid-connected PV system must satisfy the following conditions. First, the inverter must drive MPPT (maximum power point tracking) control because of the PI or PV characteristic of the PV array. Second, it must be able to control the PCC (point of common coupling) current. In other words, the magnitude and phase of the inverter output current should be controlled. Moreover, the current wave form must be AC sinusoidal.
In this paper, the shoot-through state was used to control the MPPT, and the output current was controlled through DC link voltage regulation. With the Quasi Z-source inverter, the longer the shoot-through time, the higher the increase in the Quasi Z-source output voltage. In this paper, however, the output voltage was constantly controlled; as such, the boost effect did not occur. Otherwise, the inverter output power would have progressively increased. As such, the increase in the shoot-through time appeared to have increased the inverter output current, and this is equal to the requirement of the current from the PV array and occurred due to the decrease in the PV array output voltage due to the V-I characteristic of the PV array. Consequently, the increase in the shoot-through time caused the decrease in the PV array output voltage.
Therefore, the shoot-through time can be controlled by sensing the PV array output current and voltage using the P&O method.In case of capacitor voltage control, since the capacitor voltage of the Quasi Z-source impedance network is equal to the average of the DC link voltage, instead of the DC-link voltage, the capacitor voltage is used for control. If the voltage is more than the reference voltage, the difference between the voltages becomes the power reference. Based on this power reference, the current reference increases; thus, the output current also increases and the capacitor voltage decreases. Consequently, the capacitor voltage is regulated.
For higher efficiency of the system, if possible, the reference voltage of the capacitor should be set at a lower value. On the other hand, the reference voltage of capacitor has to be set at a higher value to have margin for a shoot-through reference and to inject a current into the grid. Although the capacitor voltage of an impedance network is controlled to be constant, the output voltage of an impedance network varies according to the shoot-through time ratio. This may result in an unstable operation. To overcome these matters, in this paper, a method to keep the output voltage of an impedance network constant is proposed. This results in an expansion of MPPT range and a higher efficiency of QZSI.
The impedance network parameters of the Quasi Z-source inverter system can be designed using a ripple ratio of voltage and current with other specifications of the system. Through the design, five modes which are caused by large ripple ratio of inductor current are simplified only two modes which are shoot-through mode and active mode. This simplified design makes the system control easy.
Through a LCL filter design, the THD of grid injected current can be reduced with compact volume of the system. Also using a resonant controller lower order harmonics of the grid are compensated. Through these power quality improvement methods, the requirement of grid connected inverter can be satisfied.