Hua Han

Review of Power Sharing Control Strategies for Islanding Operation of AC Microgrids

Microgrid is a new concept for future energy distribution system that enables renewable energy integration. It generally consists of multiple distributed generators that are usually interfaced to the grid through power inverters. For the islanding operation of ac microgrids, two important tasks are to share the load demand among multiple parallel connected inverters proportionately, and maintain the voltage and frequency stabilities. This paper reviews and categorizes various approaches of power sharing control principles. Simultaneously, the control schemes are graphically illustrated. Moreover, various control approaches are compared in terms of their respective advantages and disadvantages. Finally, this paper presents the future trends.

An Improved Droop Control Strategy for Reactive Power Sharing in Islanded Microgrid

For microgrid in islanded operation, due to the effects of mismatched line impedance, the reactive power could not be shared accurately with the conventional droop method. To improve the reactive power sharing accuracy, this paper proposes an improved droop control method. The proposed method mainly includes two important operations: error reduction operation and voltage recovery operation. The sharing accuracy is improved by the sharing error reduction operation, which is activated by the low-bandwidth synchronization signals. However, the error reduction operation will result in a decrease in output voltage amplitude. Therefore, the voltage recovery operation is proposed to compensate the decrease. The needed communication in this method is very simple, and the plug-and-play is reserved. Simulations and experimental results show that the improved droop controller can share load active and reactive power, enhance the power quality of the microgrid, and also have good dynamic performance.

New Perspectives on Droop Control in AC Microgrid

Virtual impedance, angle droop, and frequency droop control play important roles in maintaining system stability, and load sharing among distributed generators (DGs) in microgrid. These approaches have been developed into three totally independent concepts, but a strong correlation exists. In this letter, their similarities and differences are revealed. Some new findings are established as follows: 1) the angle droop control is intrinsically a virtual inductance method; 2) virtual inductance method can also be regarded as a special frequency droop control with a power derivative feedback; and 3) the combination of virtual inductance method and frequency droop control is equivalent to the proportional–derivative type frequency droop, which is introduced to enhance the power oscillation damping. These relationships provide new insights into the design of the control methods for DGs in microgrid.

Stability Analysis and Stabilization Methods of DC Microgrid With Multiple Parallel-Connected DC–DC Converters Loaded by CPLs

Constant power loads may yield instability due to the well-known negative impedance characteristic. This paper analyzes the factors that cause instability of a dc microgrid with multiple dc–dc converters. Two stabilization methods are presented for two operation modes: 1) constant voltage source mode; and 2) droop mode, and sufficient conditions for the stability of the dc microgrid are obtained by identifying the eigenvalues of the Jacobian matrix. The key is to transform the eigenvalue problem to a quadratic eigenvalue problem. When applying the methods in practical engineering, the salient feature is that the stability parameter domains can be estimated by the available constraints, such as the values of capacities, inductances, maximum load power, and distances of the cables. Compared with some classical methods, the proposed methods have wider stability region. The simulation results based on MATLAB/simulink platform verify the feasibility of the methods.

An f-P/Q Droop Control in Cascaded-Type Microgrid

In cascaded-type microgrid, the synchronization and power balance of distributed generators become two new issues that needs to be addressed urgently. To that end, an f-P/Q droop control is proposed in this letter, and its stability is analyzed as well. This proposed droop control is capable to achieve power balance under both resistive-inductive and resistive-capacitive loads autonomously. Compared with the inverse power factor droop control, an obvious advantage consists in extending the scope of application. Finally, the feasibility of the proposed method is verified by simulation results.

Stability analysis of DC microgrids with constant power load under distributed control methods

Abstract Constant power loads (CPLs) often cause instability due to its negative impedance characteristics. In this study, the stability of a DC microgrid with CPLs under a distributed control that aims at current sharing and voltage recovery is analyzed. The effect of the negative impedance on the behavior of distributed controller is investigated. The small-signal model is established to predict the system qualitative behavior around equilibrium. The stability conditions of the system with time delay are derived based on the equivalent linearized model. Additionally, eigenvalue analysis based on inertia theorem provides analytical sufficient conditions as a function of the system parameters, and thus it leads to a design guideline to build reliable microgrids. Simulations are performed to confirm the effectiveness and validity of the proposed method.

A Single-Phase Current-Source Bidirectional Converter for V2G Applications

Bidirectional power flow, reactive power compensation capability and high efficiency are essential for vehicle to grid (V2G) systems. In this paper, a single-phase/single-stage bidirectional AC/DC converter for V2G application is presented, which consists of a line frequency commutated unfolding bridge and an interleaved buck-boost stage. In addition to sinusoidal input current, bidirectional power flow, high efficiency and reduced battery current ripple, the proposed topology uses the direct power control and PWM modulation to provide wide output voltage range and excellent reactive power compensation capability. The topology structure and operating principles of the proposed converter are analyzed in detail. The feasibility of the converter is validated and tested by MATLAB simulation.

Improvement of transient stability in inverter-based AC microgrid via adaptive virtual inertia

Unlike centralized synchronous generator (SG) in conventional power system, distributed micro-sources are parallel connected to neighbors by inverters, which are less-inertia. To improve the transient stability, inverter-based distributed generators (DG) can draw an analogy between the storage and rotor rotational kinetic energy of SG in inertia function. Instead of emulating a fixed virtual inertia, this study proposes adaptive virtual inertia to alleviate the frequency response. In the transient process, a large inertia is implemented when the frequency will deviate from the normal value, and a low inertia is adopted when its necessary to recover the frequency. The proposed method can enhance the anti-interference and over-load capacity, so that frequency stability is strengthened. Lyapunov stability analysis is utilized to prove the convergence. Simulation results are presented to verify the effectiveness of the method.

A novel operation mode for PV-storage independent microgrids with MPPT based droop control

In microgrids, photovoltaic (PV) and storage are integrated as a droop-controlled ideal source, which is not practical and challenges the system efficiency. Instead, PV-storage can be allocated separately. This PV-storage independent system is more flexible and easier to optimize. But, the conventional PQ-controlled PVs rely on master voltage-controlled storage sources. In this paper, a novel operation mode is proposed for the PV-storage independent microgrids. PVs operate as current controlled voltage sources (CCVS). A maximum power point tracking (MPPT) based droop control scheme is introduced to explain the proposed operation mode. Thus, this mode enables PVs to supply an auxiliary service of frequency regulation and a maximum utilization of renewable energy. The small signal stability of entire system is analyzed to design the physical and control parameters. Finally, simulations are presented to verify the system effectiveness.

A unified SoC balancing method with low-bandwidth distributed communication in island microgrid

Due to different initial state of charge (SoC) of distribution energy storage system (DESS) and mismatched line impedance, the SoC of DESS is unbalanced in microgrid. The conventional P-f droop, used to obtain accuracy load sharing, cannot balance the SoC. To overcome the drawback, this paper proposes a unified SoC based droop control method with distributed communication lines. In this scheme, the droop coefficient is changed by the correction information in real-time, which is obtained by the distributed control. Moreover, regardless of charging mode or discharging mode, the unified SoC balancing is able to be guaranteed. Meanwhile, load demand sharing is adjusted by this control method. The model of SoC based droop control is established, and the stability is verified through small signal analysis of the complete system. Finally, the effectiveness of SoC balancing and transient response are verified through simulations.