Control And Islanding Detection Methods Of Grid-connected Photovoltaic Systems With LCL Filters

The development of Distributed generation and smart grid has provided a new opportunity for renewable energy generation. Under the background of power grid upgrade and energy structure adjustment, renewable energy generation, especially grid-connected ones, is facing a new challenge in coping with the problems of global energy shortage and environmental pollution. The paper is focused on grid-connected photovoltaic (PV) systems, which is part of renewable energy generation family. The main tasks of the paper are as follows:(1) In order to introduce the more prevailing LCL filter in mid/high-power systems, special effort should be made to suppress the resonant peak resulted from LCL's third-order characteristic. To solve the related problems, considering the dual-mode operation demand and the relative cost of PV systems, a dual-loop control strategy based on filter capacitor voltage inner loop is proposed. The proposing procedure, along with the control parameters design and the performance analysis compose three aspects of the strategy, whose feasibility is verified by simulation and experiment at last.(2) The dual-loop control strategy based on filter capacitor voltage inner loop needs derivative controller to guarantee system stability. The paper provides a method to reduce the sensibility of derivation in high frequency by adding a first-order low-pass filter in the inner loop feedback. The influences to active damping of the feedback caused by low-pass filter are analyzed, and the calculation method of filter parameter's value range is presented, with all verified by Simulation and experiment results.(3) The application of the proposed dual-loop control strategy is extended to system dual-mode operation. The strategy adopted in this paper for dual-mode operation uses the same controller at different modes to reduce current surge at the mode-transfer moment. Only feedback quantity and its reference are needed to be changed when transfer happens. Simulation results demonstrate the successful operation and seamless transfer of different modes, which verifies the effectiveness of the strategy.(4)In grid-connected PV systems, fast and reliable islanding detection strategies are always needed whether to avoid harms or take advantage of islanding. After estimating major schemes of passive and active islanding detection methods, the paper selects one based on single harmonic extraction for application in single/three-phase PV systems. Software and hardware design of the selected method is made under practical circumstances, and system block diagram and program flow chart are presented as well. Finally, experiment results validate the simplicity, effectiveness and low hardware requirements for sampling and calculation of the method.