Studies On Application Of Distributed Energy Storage In Microgrid

Under the energy and environmental crisis, it is necessary for social and economicdevelopment to improve energy efficiency, develop new energy sources and enhance theuse of renewable energy sources. Microgrid, an effective access form of distributed energysource, performs as a single controllable unit for the external large power grid, can operateindependently as well as be connected to the large power grid, meeting users’ requirementson power quality, reliability and security of power supply, in which the energy storage playsa vital role. Therefore, the application of Energy Storage System (ESS) in micrgrid is takenas the topic of this thesis with important theoretical and practical significance.This paper studies the control strategy for the ESS, aiming at optimizing the powerflow at the point of PCC. There are two different control schemes for the power flowcontrol at the point of PCC, one is keeping a constant power value and the other is keepingpower values within limitations. Simulations are carried out on PSASP and they verify thecontrol strategy. Meanwhile, both two control schemes are compared under the followingproperties: the network loss, the voltage in the microgrid and the necessary capacities of thestorage devices. The result shows that the latter scheme is more efficient and economical.Due to the sensitivity of the microgrid to load/generation changes, it requires a storagesystem with high energy density, high power density and fast response. While none of thecurrent available ESS technologies satisfies these features, it is necessary to combine two ormore ESSs to create a Hybrid Energy Storage System (HESS). This paper establishes thebattery/supercapacitor HESS control model, consisting of two parts: a single storage systemmodel and an energy management system. It is proposed that the supercapacitor acts priorto the battery in the frequency and voltage-controlled HESS. While the energy managementsystem divides the reference power into high frequency part and low frequency part whenthe HESS is controlled by active and reactive power (PQ control). The validity of thefrequency and voltage-controlled HESS model is verified in the operation condition when the microgrid changes from grid-connected mode to island mode. Finally, a windspeed-power model for PMSG is established on PSASP and the simulation result indicatesthe PQ control model of the HESS can smooth the wind power fluctuations effectively.