基于模糊控制的微网平滑切换控制策略的研究

Research on Control Strategy of Smooth Switching Between Microgrid and Distribution Network Based on Fuzzy Principle

目前的微电网与配电网平滑切换控制策略均是以检测微电网为孤岛运行为前提设计的,此种控制策略在微电网并离网转换的暂态过程中会失去频率、电压的参考值,严重影响微电网在此期间的稳定。为实现并离网的平稳过渡,分别对电压、频率的下垂控制(调差率控制)特性进行了改进,使传统的U/f控制方式在并网时具有P/Q特性。分别引入电压、频率变化率,结合微电网的电压、频率偏差值,采用模糊控制方法,使传统U/f的单一下垂特性变为三折线的变斜率下垂控制特性。采用改进后的控制策略,不用检测并网孤岛状态,可实现并离网的平滑切换。仿真结果表明,在并网时,系统发生超越国标规定的-5%^+7%电压偏移时,微网可提供无功功率稳定电压;在并离网转换时,可维持微网频率波动不超过±0.2 Hz、电压波动不超过-5%^+7%额定电压,并可作为采用其它控制方式的微电源频率、电压的参考值。

At present, strategy on the microgrid smooth switching from distribution network is usually designed on pre- mise of microgrid operating in the island state detected. Adopting this strategy will enforce microgrid losing frequency and voltage reference during the transient process between grid-connected and off-grid, which seriously influenced the stability of the micro power grid. In order to achieve a smooth transition from the network, the U/f Droop character of control scheme is improved, and it turns into a P/Q character of control scheme while the mirogrid is grid-connected.By introducing the traditional single droop into a tri-linear curve of variable slope droop, and reasonably combining voltage deviation and voltage change rate （frequency deviation and frequency change rate as well） based on fuzzy control method which needs not detect the state of microgrid, the microgrid can switch smoothly from parallel operation to islanding. The simulation results show that the microgrid can provide positive reactive power to stabilize voltage when the large voltage deviation of the system occurs over national criterion -5%-＋7%; On-and-off grid conversion, the grid frequency fluctua- tion within ±0.2 Hz, voltage fluctuation within -5%-＋7%, and other control modes can be used as a micro power frequency and voltage reference.