This paper presents the utilization of harmony search algorithm (HSA) to optimally design the proportional-integral (PI) controllers of a grid-side voltage source cascaded converter with two additional loops for smooth transition of islanding and resynchronization operations in a distributed generation (DG) system. The first loop is the frequency control loop which is superimposed on the real power set point of the cascaded controller of voltage source converter to minimize the frequency variation during the transition from the grid mode to islanding mode. The second loop is the resynchronization loop which reduces the phase shift of the AC voltages of the DG with the utility grid AC voltages during islanding operation leading to a successful grid reconnection event. The response surface methodology (RSM) is used to build the mathematical model of the system dynamic responses in terms of PI controllers' parameters. The effectiveness of the proposed PI control scheme optimized by the HSA is then compared to that optimization by both genetic algorithm and conventional generalized reduced gradient techniques. The HSA code is built using MATLAB software program. The validity of the proposed system is verified by the simulation results which are performed using PSCAD/EMTDC.
In this paper, an ac-dc hybrid micro-grid system including a centralized power control scheme is proposed. Multiple ac-dc bidirectional converters connected in parallel are considered in the system instead of a single converter that connects ac and dc buses. The proposed control scheme is basically coordination of two modes-control of power through the converters and selection of converter units. The power through the converter is controlled because the load unbalance between ac and dc buses should be mitigated. Selection of converter units is also important because each small unit can deal partial amounts of power of the entire micro-grid system and therefore control of parallel operation of multiple converter units should be considered. If any converter fails to operate in the system then the alternate converters come into operation, so that the micro-grid system will not be totally disconnected and thus reliability of the system is ensured. This will also increase the efficiency of the system during low power transmission condition. The system operation is investigated under three different conditions to show the effectiveness of the proposed control scheme.
Abstract-This paper presents a generic centralized supervisory control scheme for the power management of multiple power converters based hybrid micro-grid system. The system consists of wind generators, photovoltaic system, multiple parallel connected power converters, utility grid, ac and dc loads. Power management of the micro-grid is performed under two cases: grid mode and local mode. Central supervisory unit (CSU) generates command signal to ensure the power management during the two modes. In local mode, the dc loads in the ac-dc hybrid system can be controlled. In the case of grid mode operation, power flow between the utility grid and micro-grid is controlled. A novel feature of this paper is the incorporation of the multiple power converters. The generated command signal from the CSU can also control the operation of the multiple power converters in both grid and local modes. An additional feature is the incorporation of sodium sulfur battery energy storage system (NAS BESS) which is used to smooth the output power fluctuation of the wind farm.The effectiveness of the control scheme is also verified using real time load pattern. The simulation is performed in PSCAD/EMTDC.
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