Research On A Coordinated Control Strategy Of Microgrid Based On State Transition

This thesis was supported by Natural Science Foundation of Chongqing (CSTC 2009BB6190) and independent research project of State Key Laboratory of Power Transmission Equipment & System Security and New Technology(2007DA1051270 9208).As an independent controllable unit, microgrid comprises Distributed generation (DG), local loads and energy storage device. There are two categories of DGs in microgrid according to their power characteristics, intermittent DGs and continuous DGs. Since the output power of the former is greatly affected by physical conditions such as weather, their fluctuation and intermittence are distinct. On the other hand, the continuous DGs are capable of outputting comparative secure power and are with better power control ability. In order to satisfy the local customers'diversified demands and to realize the seamless operation mode switches, a proper Distributed Storage (DS) device is required in microgrid. For a microgrid contains several DGs, the key of a secure and stable operation is how to control the DG, DS, local loads and switches coordinately and effectively so as to meet the local customers'demands as much as possible when the microgrid structure changes. Furthermore, the coordinated control is also a necessary for an economical operation of microgrid. So the study of coordinated control strategy of a microgrid with multi-energy generation systems is significant for applications and research of microgrid. This paper designs a coordinated control strategy of microgrid based on state transition, which targets the hierarchical microgrid containing DGs, local loads and energy storage devices.â‘ According to their power characteristics, DGs in microgrid are divided into three categories, intermittent DGs, continuous DGs and DSs. Simulation models are established to study the dynamic response characteristics of the DGs. Maximum Power Point Tracking (MPPT) is implemented in intermittent DGs, which, in turn, does not participate in the active power regulation, in order to obtain the maximum utilization rate of renewable energy. As a primary energy source in microgrid, continuous DGs are adept at energy and active power regulation, but some continuous DGs, namely micro turbine and fuel cell, are with relatively slow regulation rate. Consequently, they are unable to response promptly to rapid changes in loads unless it cooperates with a proper DS device. â‘¡Microgrid central controller (MGCC) and local controller of a two-stage hierarchical control microgrid is studied. The upper structure, MGCC, aims to determine set value of active and reactive power through collected load information, fuel data of DG and DS, operation state and external electricity market information in order to achieve optimal power allocation as well as microgrid mode switches. The infrastructure includes microsource controller (MC) and load controller (LC), which are local controller units to carry out the operational command from MGCC. Three control methods used in local controllers of power-electronics interfaced DG are analyzed, including PQ control, Droop control and V/f control. And then the power management unit and synchronization unit of MGCC are studied after the expatiation of structure and functions of MGCC.â‘¢This paper proposed a coordinated control strategy of microgrid based on state transition. An allowable effective operation state is obtained through simplified combination of the possible operation modes in accordance with local power demands. By taking current operation state and trigger events as input variables of MGCC and the control methods of components as outputs, a real-time adjustment scheme of microgrid operation state is achieved. For the uninterruptible power supply of sensitive loads and with the consideration of no-deviation regulation for frequency under all operation states and the capacity limitation of energy storage devices, the corresponding control methods and trigger events of all components are proposed under the defined operation states in this paper.â‘£Simulation model is established in PSCAD/EMTDC to analyze dynamic response in the process of typical state transition. The feasibility of the proposed coordination control strategy is verified. According to the proposed control strategy, three typical state transition progresses are simulated and analyzed in detail, namely from continuity connection state to continuity support state, from comprehensive support state to comprehensive connection state and from comprehensive standby state to intermittent standby state. The simulation results show that the coordinated control strategy is able to achieve smooth state transition and no-deviation regulation for frequency, to ensure a uninterruptible power supply and good power quality of sensitive loads and to extend the life of energy storage devices as much as possible.