Xialin Li

Energy Management System for Stand-Alone Wind-Powered-Desalination Microgrid

An energy management system for stand-alone microgrid consisting of a wind turbine (WT) generator, a diesel generator, an energy storage system (ESS), and a sea water desalination system is proposed in this paper. The coordinated control of the distributed generations and ESS is researched with two operation modes. Then, a real-time rolling horizon energy management method is presented based on hour-ahead wind speed forecast. The operation mode of the microgrid system and the reference output power of WT generator are determined according to the forecasted wind speed and state of charge of the ESS, which can achieve the goal of maximizing utilization of wind energy and minimizing utilization of diesel generator on the basis of system stable operation. The proposed energy management method has been tested on the real-time digital simulator system. The results clearly verify the effectiveness of the proposed method.

Stability Analysis and Damping Enhancement Based on Frequency Dependent Virtual Impedance for DC Microgrids

This paper focuses on the stability analysis and damping performance improvement of dc microgrids. The small-signal model of a dc microgrid has been derived. Eigenvalue analysis results reveal the relationship between the system stability and different factors of dc microgrids, including types of dc load, the droop coefficient, line parameters, etc. It shows that the poorly damped LC circuits in dc microgrids reduce the system damping and bring in high frequency oscillations. To improve the damping performance, a frequency-dependent virtual impedance approach is proposed, which can effectively shape the high frequency impedance of the dc bus voltage control units, improve the system stability, and mitigate the oscillations. Detailed design guidelines on virtual impedance to achieve good stability and transient performance are also provided. Simulation and experimental results are obtained to confirm the validity of the proposed approach.

Coordinated control of battery storage system and diesel generators in AC island microgrid

This paper proposes a coordinated control method of the diesel generators (DGs) and battery storage system (BSS) in AC island microgrid. When the DGs run as the main power source, an auxiliary power control signal is added to the conventional droop control for BSS to prevent system collapses caused by long-time over-current of the DGs, which can improve the stability of the microgrid. To avoid short-term outage when the main power source in the micro-grid changes from the DGs to BSS or from the BSS to the DGs, a seamless transition control strategy for the DGs and BSS is presented. Finally, the control methods previously mentioned had been validated with the PSCAD simulation software.

Control Strategies for a Hybrid PV/Battery System with Grid-Connected and Island Mode

This paper has developed a hybrid PV/Battery system with the ability of grid-connected and island operating control, the objectives are to improve controllable grid-connected ability of PVs, reduce PVs output fluctuation due to changes of environmental factors such as solar intensity, regulate isolated network voltage and frequency with the participation of PVs. The converter consists of three DC/DC modules and one three-phase inverter module, and each module can achieve two-way flow of energy. This paper also proposes a control strategy for DC/DC converter with soft-start function, in order to solve the DC bus voltage control under no-load and light-load conditions. Firstly, energy flow of PV/Battery system in both grid-connected mode and stand-alone mode is analyzed, then control strategies for each module under different operation modes is proposed, and the control strategies is verified by actual devices.

Stability analysis of a DC microgrid with master-slave control structure

Nowadays there is an increasing interest on dc microgrid for its higher energy efficiency and higher reliability as compared to the ac system. This paper addresses the stability problem in a dc microgrid with master-slave control structure. Considering the discrete sampling and control algorithm in a digital system, the small-signal models of this dc microgrid have been derived in the z plane. Eigenvalue analysis results show that the system stability and transient response will be affected by the variation of the dc load parameters, the number of slave distributed generations (DGs), and the type of dc load. A centralized oscillation suppression controller, which is composed of a proportional compensator followed by a bandpass filter, is proposed to overcome the unstable oscillations caused by different eigenvalues. The validity of the proposed approach is confirmed by simulations.

Observer-Based DC Voltage Droop and Current Feed-Forward Control of a DC Microgrid

An observer-based dc voltage droop and current feed-forward control for a dc microgrid has been proposed in this paper. With the proposed control scheme, dynamic response of dc voltage control can be improved effectively through using an observer. Moreover, the feedback current for dc voltage droop and the feed-forward current are both obtained from the observer without additional current measurement. Furthermore, system stability analysis shows that, the propose method has enhanced robustness and stability when compared to the traditional droop method under variations of system parameters, such as loads, cable impedances, and droop control gains. In a droop controlled dc microgrid, there exists a trade-off between the load demand sharing accuracy (which needs a high droop gain) and stability (which limits the droop gain). With the above mentioned advantages, the proposed method can effectively address this trade-off in such a system. The effectiveness of the proposed method is verified by experiments in a laboratory dc microgrid with two boost dc–dc converters based distributed generations and a buck type constant power dc load.

Robust and autonomous dc bus voltage control and stability analysis for a dc microgrid

In this paper, a novel approach for robust and autonomous dc bus voltage control in a dc microgrid has been proposed. A nonlinear disturbance observer (NDO) only with local information is designed firstly to track power disturbances caused by load changing or the volatility of the renewable sources in the dc microgrid. With the observer, voltage droop method has been adopted in achieving power sharing and autonomous operation of a dc microgrid. Furthermore, an improved current feedforward is developed to suppress wide fluctuations of the dc bus voltage during transients. In addition, a notch filter has been used to effectively eliminate the second harmonic ripple components in the inductor currents of the droop control units to improve the power quality. The effectiveness of the proposed control scheme is verified on a laboratory prototype including two bidirectional dc-dc converters supplied by two programmable dc power supply systems, a dc load with a resistor connected to the dc bus via a buck type dc-dc converter and a three-phase unbalanced ac load.

Wireless based real-time power coordinated control strategy for a DC microgrid

In a dc microgrid, the bidirectional dc-ac converters are usually operated as a utility interface to control the dc bus voltage and ensure the power balance within the system. In the case of grid failure, power limiting or malfunction condition for dc-ac converters, the dc microgrid will lose its power balance unit. At that time, the distributed generations in dc system will change their operation mode to maintain the dc bus voltage through energy management control. However, this kind of control strategy may not be able to implement realtime power balance due to communication delay and may lead to system collapses in some extreme cases. This paper proposes a wireless based real-time power coordinated control strategy, which is a decentralized control strategy without extra communication link and makes it a viable option for plug and play. In this strategy, each unit in dc microgrid not only can realize smooth transition between different operation modes upon the dc bus voltage signal, but also can adjust its droop curve adaptively according to operation condition. Therefore, the voltage regulation, power balance and power sharing among different sources can be guaranteed. Finally, the validity of the proposed control strategy is confirmed by experiments based on a typical low voltage dc microgrid with simulated dc source, dc-ac interface converters and dc load.

A Unified Control for the DC-AC Interlinking Converters in Hybrid AC/DC Microgrids

A novel unified control of the dc–ac interlinking converters (ICs) for autonomous operation of hybrid ac/dc microgrids (MGs) has been proposed in this paper. When the slack terminals in the ac and dc MGs are available, the ICs will operate in autonomous control of interlinking power between the ac and dc subgrids, with the total load demand proportionally shared among the existing ac and dc slack terminals. With a flexible control variable added in power control loop, design of the interlinking power control, and droop features of ac and dc MGs can be decoupled. Moreover, this control variable can be tuned flexibly according to different power control objectives, such as proportional power sharing in terms of capacity (which is considered in this paper), interlinking power dispatch, and other optimal power dispatch algorithms, ensuring a well-designed flexibility and compatibility. Furthermore, if the dc MG or the ac MG loses dc voltage control or ac voltage and frequency control capability due to failures of operation of its slack terminals, the ICs can automatically and seamlessly transfer to dc MG support or ac MG support control modes without operation mode detection, communication, control scheme switching, and control saturation. In order to enhance the stability of the proposed unified control in different modes with different control plants, a phase compensation transfer function has been added in the power control loop. After thorough theoretical analysis and discussions, detailed simulation verifications based on PSCAD/EMTDC and experimental results based on a hardware experimental MG platform have been presented.

A nonlinear disturbance observer based DC bus voltage control for a hybrid AC/DC microgrid

DC-bus voltage control is an important task in the operation of a dc or a hybrid ac/dc microgrid system. To improve the dc-bus voltage control dynamics, traditional approaches attempt to measure and feedforward the load or source power in the dc-bus control scheme. However, in a microgrid system with distributed dc sources and loads, the traditional feedforward-based methods need remote measurement with communications. In this paper, a nonlinear disturbance observer (NDO) based dc-bus voltage control is proposed, which does not need the remote measurement and enables the important “plug-and-play” feature. Based on this observer, a novel dc-bus voltage control scheme is developed to suppress the transient fluctuations of dc-bus voltage and improve the power quality in such a microgrid system. Details on the design of the observer, the dc-bus controller and the pulsewidth-modulation (PWM) dead-time compensation are provided in this paper. The effects of possible dc-bus capacitance variation are also considered. The performance of the proposed control strategy has been successfully verified in a 30 kVA hybrid microgrid including ac/dc buses, battery energy storage system, and photovoltaic (PV) power generation system.