Operation and control of DC microgrids with integration of renewable energy sources and energy storages
Date of Issue2015
School of Electrical and Electronic Engineering
With high penetration of DC-compatible energy sources, storages and loads, DC microgrids with minimized number of DC/AC/DC power conversions are becoming more popularized. Proper control algorithm for autonomous operation of system interfacing various system units stands as a challenge. Centralized energy management system with communication link between the central controller and individual system unit is usually implemented to realize systematic coordination. However, low system reliability due to the need for communication link and long response time due to communication delay are the main disadvantages of the centralized control. DC bus signaling, in which DC bus voltage is regarded as the global indicator for system power balance, is implemented for distributed control to enhance system reliability. Therefore, system units can schedule their operating modes and output power based on local bus voltage variations in distributed manner so that to eliminate the communication link. In multiple modes DC microgrid control, the allowable bus voltage range is divided into several regions. The operating modes of system units in different regions are assigned based on their utilization priorities. However, lacking of active power sharing among slack terminals and significant bus voltage variation during operating region transitions are the main drawbacks of multiple mode operation. Combination of multiple modes operation and voltage/power (V/P) droop control is proposed in multiple-slack-terminal DC microgrid control. Droop relationships are imposed for system units operating in voltage regulation mode to generate the reference voltage based on actual output power. It helps active power sharing among slack terminals based on their power capacities and reducing the bus voltage fluctuations during the voltage region transitions. To equalize system bus voltage/load power droop relationship in different regions, capacity based bus voltage partition is proposed to determine the threshold voltages. Online droop coefficient tuning in distributed manner is also implemented to prevent bus voltage discontinuity due to reduction of system units’ power capacities. Due to the transmission line impedance and droop relationships imposed, power tracking errors and bus voltage deviation exist in multiple-slack-terminal DC microgrid. To enhance system control accuracy while retaining the reliability, multi-level energy management system is proposed. Multiple-slack-terminal DC microgrid distributed operation for bus voltage regulation and power sharing is scheduled as the primary control. Bus voltage restoration is implemented in secondary control to eliminate the steady state bus voltage deviations. In tertiary control, the power references of slack terminals are generated based on the comparison of their marginal costs. The power tracking errors are minimized with power sharing compensation. In case of communication failure, system operation can still be retained with distributed control, but the control accuracy is degraded. Energy storages (ESs) are widely installed to compensate the intermittency of renewable energy generation and uncertainty of load consumption. ESs be categorized based on their energy density, power density, ramp rates, etc. Hybridization of ESs with difference characteristics is an effective and economic solution to comply with system design specifications. Hybrid energy storage system (HESS) with centralized control for system net power decomposition and ESs power dispatch is normally implemented. Low speed of response and low system reliability limit its effectiveness for real-time operation. Hierarchical control of HESS, comprised of both centralized and distributed controls, is proposed to ensure both system reliability and control accuracy. System operates with HESS centralized control in which iteration based power dispatch is applied to maximize the utilization of ESs ramp rates. In case of communication failure, system net power decomposition and ESs power dispatch are realized with localized low pass filters in distributed control. Reassignment of slack terminal in case of slack terminal outage and ESs operating mode change in case of communication failure are the main disadvantages of HESS hierarchical control. Multi-level energy management system is thus proposed for HESS control. In primary control, all ESs are scheduled to operate in voltage regulation mode with droop control. System net power decomposition and ESs power dispatch are realized with localized low pass filters applied to ESs with low ramp rate so that they only respond to low frequency net power changes. Bus voltage restoration and power sharing compensation are implemented in secondary control to enhance system control accuracy. Autonomous state of charge recovery of ESs with high ramp rates is applied to generate their voltage references based on the real-time stored energy. In case of communication failure, system operation can still be retained without ESs operating mode change. In grid-tied DC microgrid with bidirectional interlinking converter (BIC) for power exchange between the utility grid and DC microgrid, the BIC is usually scheduled to regulate DC bus voltage in grid-tied state. The rest of system units are configured as power terminals to track respective power reference based on systematic optimization. In case of utility grid outage, DC microgrid islanded state will be activated by disabling BIC power electronic switches to isolate the utility grid disturbance. At the same time, localized energy storage is scheduled to regulate system bus voltage. Operating mode changes of system units during the transitions induces significant bus voltage fluctuations. Multiple-slack-terminal DC microgrid, in which both BIC and localized ESs operate in voltage regulation mode, is applied to eliminate the operating mode changes of system units. Bus voltage restoration and power sharing compensation are implemented in multi-level energy management system to enhance system control accuracy. Detailed procedures for transitions between grid-tied and islanded states are proposed to minimize bus voltage fluctuations and inrush current during the transitions. Various system control algorithms for operations of DC microgrid and HESS have been verified with MATLAB simulation. Besides, a lab-scale DC microgrid with integration of various energy sources, storages and loads has been developed in the Water and Energy Research Laboratory (WERL), School of Electrical and Electronic Engineering, Nanyang Technological University. Experimental case studies have been carried out to verify the feasibility and effectiveness of the proposed control algorithms.
DRNTU::Engineering::Electrical and electronic engineering::Electric power::Production, transmission and distribution