IMPACT OF BATTERY ENERGY STORAGE SYSTEM ON AC/DC HYBRID MICROGRID PROTECTION AND CONTROL

Abstract

The thesis investigates the impacts of battery on the control and protection of ac dc hybrid microgrid. The thesis focuses on local control schemes for different converters in the ac/dc hybrid microgrid in presence of two PV generators, one connected to each side of the microgrid and a BESS connected on the dc side. In grid-connected mode, both PVs are run on MPPT to feed the loads and export power to the main grid while BESS is charging. In this mode, the main converter or ILC (Inter-linking converter) runs on VQ control mode to maintain a stable voltage on the dc side and facilitate a smooth flow of power between two sides. The effect of change in solar irradiance has also been included in the study. In this thesis, a scheme is proposed to limit sudden change in power flow through the main converter due to the switching of dc loads. The scheme requires BESS to inject/absorb power into/from the dc bus as calculated from voltage imbalance in dc side and can also be employed to supply sudden increase in load for short time, in presence of high inertia DGs. In islanded mode, the BESS is run on voltage control mode while the PV of dc side is run on current control mode or on MPPT, based on loading conditions. The PV on the ac side of the microgrid always run on MPPT. BESS has been used to maintain the dc bus voltage at reference value while feeding the pulse loads. For this application, the voltage control scheme of a buck-boost bidirectional converter has been discussed. For maintaining ac bus voltage, the interlinking converter is controlled as a grid forming inverter on voltage control mode, facilitating the bidirectional flow of power. A 3 phase LCL filter's design procedure is also implemented for obtaining ac voltage without any significant harmonic content. LCL filter is selected over LC filter due to their increased ability to suppress the harmonics. The impact of BESS on fault detection has also been analysed. It has been discussed that a BESS works in different modes and may be modelled as a constant current source, constant current load, or a constant voltage source based on the mode it is working in. A fault detection scheme based on resistance estimation has been applied in different modes of the microgrid to verify if any mode of operation of BESS affects the accuracy of fault detection algorithms.