Operation Mode And Control Stratery Of Energy Storage System In Microgrid

With the increasing accident of power system and energy depletion, microgrid is becoming the focus of power system research as a new form of networking in recent years. Due to the smaller capacity and inertia, microgrid is vulnerable to the fluctuations of load and distributed power such as solar power or wind energy, so it’s faced with many problems such as the intermittency of the power output and stability control. But the addition of energy storage technology to microgrid has solved these problems. Now energy storage has become the important part of microgrid.Battery Energy Storage System is more and more applied in microgrid because of its long history, matured technology and low cost. In recent years, the total installed capacity of sodium-sulfur cell which is pollution-free ranks first in global BESS. Taken Battery Energy Storage System as the research object, its role to support microgrid in conversion between grid mode and island mode or in island mode was studied in this paper.First of all battery equivalent circuit model was established, compositions of BESS were analyzed, and the function as well as the importance of each part were studied. Battery Management System can ensure reliable operation of storage and the measurement of residual capacity is the most important part of BMS. So a simple method regarding judging the residual capacity via battery voltage was studied.Power Conversion System is the important part of energy exchange between battery and outside, which can provide two-way power for system. On the basis of comparative analysis, charging and discharging circuit of three-phase voltage source PWM converter was determined, operation mode was analyzed, and the mathematical models of the voltage source PWM converter in three-phase static coordinates and in rotary two-phase coordinates were established. Space Vector Pulse Width Modulation was introduced in the switch control to get a better control performance.In order to realize the supporting action of BESS in microgrid, microgrid comprised of large energy storage system was established. In this storage system, one storage device adopts V/f control(island mode) or PQ control(grid mode) as a key source; other devices always adopt PQ control as a secondary source to maintain maximum power output. Getting accurate information of voltage phase was the precondition to realize two kinds of control. After basic phase-locked loop was studied, the three-phase software PLL in application of control was analyzed. The corresponding control model of V/f and PQ were established.Power balance is a decisive factor of the stability of the system in microgrid. In order to verify the validity of large energy storage system, microgrid model comprised of energy storage and photovoltaic system was established, which aims at important load of city and remote areas. In the city, on the basis of lack of photovoltaic energy, whether the energy storage system can effectively adjust the balance of power in microgrid or not in conversion between grid mode and island mode or in island mode was studied. In remote areas, on the basis of sufficient photovoltaic energy, microgrid simulation model comprised of energy storage and photovoltaic was established. The results of simulation show that, whether running mode or load in island mode is changed, the large energy storage system can effectively adjust the balance of power in microgrid and keep voltage and frequency constant.