A novel droop control method to achieve maximum power output of photovoltaic for parallel inverter system

Abstract

In general, the output power of PV inverter should be coordinated with the load. In the initial stage of the design, when the PV inverter operating in islanded mode, the load power is supposed to be no more than the maximum PV output power with the purpose to stabilize the system. However, it may waste PV cells' energy. Consequently, more and more solar panels are linked to other energy sources in order to create a parallel inverter system, which makes it feasible to fit the power output of each source. The power distribution of parallel inverter is achieved by the use of droop control in microgrid system. For such a parallel inverter system consisted of PV inverters and non-regeneration energy sources inverters without energy storage devices in islanded mode, if the shared load power is no more than the available maximum PV inverter output power, there is a power waste for PV inverter; Moreover, due to the intermittency of PV sources, the system may become unstable if the shared load power is more than the available maximum output power of PV inverter. However, the maximum utilization of the PV output energy cannot be achieved automatically with the traditional methods to add virtual impedance or adjust the droop coefficients. Thus, in order to avoid power waste and potential instability caused by insufficient PV power by traditional droop control, this paper recommends an improved droop control scheme to maximize the power output of PV units. Required by the load, the remaining power is composed of the other inverters, which could effectively improve the utilization rating of renewable energy sources and system stability. At the same time, according to the system stability analysis based on the small signal modeling, it has been designed about the droop coefficients of the improved droop control loop. In the end, the experimental results have showed that the suggested scheme has a varied validity and robustness.