Study Of Control Strategy Of Battery Energy Storage System In Micro-grid

In the micro-grid with a large number of renewable energy generations such asPV generations and wind power generations, the battery energy storage system (BESS)is one of its most important parts. In order to improve the capability of power grid tointegrate more renewable energy generations and to enhance the power grid reliability,control strategies of BESS both in grid-connected mode and in islanded mode weredeeply studied in this thesis. Simulation results revealed that the proposed controlstrategies were helpful to smooth the output power fluctuations of the renewableenergy generations and to improve the power supply reliability. Main work is asfollowing:(1) Due to the large-fluctuation and strong-randomness characteristics ofrenewable energy generation power, a new battery energy storage system controlmethod based on real-time state-of-charge (SOC) and variable filter time constant wasfirst proposed with considering certain constraints such as the battery life cycle. Thismethod could compensate the fluctuation composition of a particular frequency bandof the renewable energy generation power. At the same time, the filter time constantwas adjusted in order to stabilize the value of batteries’ SOC in a certain range, whilethe over-discharge and over-charge of the batteries could be avoided.(2) A new energy management method for hybrid battery/ultracapacitor storagesystem based on smoothing control was proposed. In this method, batteries andultracapacitors were coordinated to better compensate the low-and high-frequencycomponents of the renewable energy generation power fluctuation respectively.Simultaneously, on the basis of traditional threshold energy management method,ultracapacitor terminal voltage pre-control was presented according to thecharging/discharging modes and state-of-charge (SOC) values of ultracapacitors, andthen the output power references of batteries and ultracapacitors were modified toprevent ultracapacitors from violating terminal voltage limit.(3) Influence of feeder impedance on the BESS droop control method wasanalyzed and an improved droop control method was proposed. Line voltage sag errorwas adopted in the method to create the compensation control signal. According to thefeeder line impedance parameters, the measured active and reactive power values of the inverter output, the proposed method could eliminate the load sharing control errorby compensating the line voltage sag error caused by unequal feeder impedance.(4) At last, small signal model of a micro-grid composed of inverters, networkand load was derived and eigenvalue analysis was adopted to determine the key factoraffecting its stability. Mechanism of BESS inverters to provide positive damping wasclarified and an improved droop control method based on additional damping wasproposed. It was according to the analysis of dynamic frequency-power characteristicand phase/magnitude conditions. And additional control was introduced to thetraditional droop loop. It was found that the method could effectively increase thesystem’s damping to suppress the output power oscillation of BESS inverters andimprove the system stability without changing the steady state frequency and outputpower of the inverter.