Mohammad S. Golsorkhi

Decentralized Method for Load Sharing and Power Management in a PV/Battery Hybrid Source Islanded Microgrid

This paper proposes a new decentralized power management and load sharing method for a photovoltaic based islanded microgrid consisting of various photovoltaic (PV) units, battery units and hybrid PV/battery units. Unlike the previous methods in the literature, there is no need to communication among the units and the proposed method is not limited to the systems with separate PV and battery units or systems with only one hybrid unit. The proposed method takes into account the available PV power and battery conditions of the units to share the load among them. To cover all possible conditions of the microgrid, the operation of each unit is divided into five states and modified active power–frequency droop functions are used according to operating states. The frequency level is used as trigger for switching between the states. Efficacy of the proposed method under different load, PV generation, and battery conditions is validated experimentally in a microgrid lab prototype consisting of three units.

A Decentralized Control Method for Islanded Microgrids Under Unbalanced Conditions

Unbalanced load currents not only give rise to unbalanced voltages but also adversely affect the performance of the conventional current-limiting mechanisms. The latter might result in overcurrent stress on the distributed energy resources (DERs) or current harmonics. In this paper, a novel decentralized control method is proposed to improve the power quality and protect DERs from overload. The proposed controller makes use of the model predictive control (MPC) technique to minimize the voltage unbalance, improve current limiting, and prevent active power overload. The MPC is combined with the V- I droop method to realize coordinated operation with fast dynamic response. The proposed method is tested on the CIGRE benchmark microgrid. Simulation results demonstrate that the proposed method improves power quality but also allows for operation close to the maximum load capacity without imposing DERs to overload.

A Root-Locus Design Methodology Derived From the Impedance/Admittance Stability Formulation and Its Application for LCL Grid-Connected Converters in Wind Turbines

This paper presents a systematic methodology for the design and the tuning of the current controller in LCL grid-connected converters for wind turbine applications. The design target is formulated as a minimization of the current loop dominant time constant, which is in accordance with standard design guidelines for wind turbine controllers (fast time response and high stability margins). The proposed approach is derived from the impedance/admittance stability formulation, which, on one hand, has been proved to be suitable for the controller design when the active damping is implemented and, on the other hand, has also been proved to be very suitable for system-level studies in applications with a high penetration of renewable energy resources. The tuning methodology is as follows: first, the physical system is modeled in terms of the converter admittance and its equivalent grid impedance; then, a sensitivity transfer function is derived, from which the closed-loop eigenvalues can be calculated; finally, the set of control gains that minimize the dominant time constant are obtained by direct search optimization. A case study that models the target system in a low-power scale is provided, and experimental verification validates the theoretical analysis. More specifically, it has been found that the solution that solves the minimization of the current controller time constant (wind turbine controller target) also corresponds to a highly damped electrical response (robustness provided by the active damping).

A Distributed Control Framework for Integrated Photovoltaic-Battery-Based Islanded Microgrids

This paper proposes a new cooperative control framework for coordination of energy storage units (ESUs), photovoltaic (PV) panels, and controllable load units in single-phase low voltage microgrids (MGs). The control objectives are defined and acted upon using a two level structure; primary and secondary control. Unlike conventional methods, a V–I droop mechanism is utilized in the primary control level. A distributed strategy is introduced for the secondary control level to regulate the MG voltage and manage state of charge (SoC) and power among the ESUs. The distributed secondary controllers are coordinated based on a leader-follower framework, where the leader restores the MG voltage to the rated value and the followers manage the sharing of power between the ESUs so as to balance the SoCs. Once the ESUs reach the minimum charge level, the information state increases above a positive critical value, at which point load control units perform load shedding. Similarly, fair PV curtailment is conducted in case the ESUs reach the maximum charge level. Experimental results are presented to demonstrate the efficacy of the proposed method.

A GPS-Based Decentralized Control Method for Islanded Microgrids

Coordinated control of distributed energy resources (DER) is essential for the operation of islanded microgrids (MGs). Conventionally, such coordination is achieved by drooping the frequency of the reference voltage versus active (or reactive) power. The conventional droop method ensures synchronized operation and even power sharing without any communication link. However, that method produces unwanted frequency fluctuations, which degrade the power quality. In order to improve the power quality of islanded MGs, a novel decentralized control method is proposed in this paper. In this method, the GPS timing technology is utilized to synchronize the DERs to a common reference frame, rotating at a nominal frequency. In addition, an adaptive Q-f droop controller is introduced as a backup to ensure stable operation during GPS signal interruptions. In the context of the common reference frame, even sharing of active (id) and reactive (iq) components of the current are achieved based on vd - id and vq - iq droop characteristics. The method has been tested on a laboratory-scale MG. Experimental results demonstrate the efficacy of the proposed method in terms of dynamics, power quality, and robustness with respect to GPS interruptions.

A decentralized negative sequence compensation method for islanded mirogrids

Single phase loads, which constitute the majority of low voltage consumers, give rise to unbalanced load currents in microgrids. As a consequence the voltage unbalance might rise beyond the permissible range. The voltage unbalance can be reduced by injecting a negative sequence (NS) compensation signal into the DERs reference voltage. However, such compensation might cause circulating currents between the DERs and also excessive current in the heavily loaded phases of some DERs. In this paper, a novel NS compensation method is proposed to reduce the circulating currents and eliminate the excess current in inverter-based microgrids. In this method, the NS voltages of the DERs are adjusted according to again scheduled NS droop method. The compensating signal is then computed by solving an optimization problem, which aims at tracking the NS voltage with minimum compensating action. Simulation results are proposed to verify the efficacy of the method.

H∞ structured design of a cascaded voltage/current controller for electronically interfaced distributed energy resources

In this paper a new controller design method for inverter-based distributed energy resources in standalone microgrids is proposed. The objective of the design is to exploit the flexibility and fast dynamics of the voltage inverter. The proposed control method is composed of droop, voltage and current controllers. The droop controller is based on V-I droop characteristics, and exhibits significantly faster dynamics compared to the conventional droop method. The voltage and current controllers utilize the structured H∞ PI regulation strategy to track the reference voltage and current. The PI controller parameters are tuned using Hinfstruct optimization function from the MATLAB® Robust Control Toolbox. The performance of the closed-loop system is analyzed to illustrate the capabilities of the new technique. Simulation results are presented to prove validity and verify the efficiency of the proposed control.

A GPS-based control method for load sharing and power quality improvement in microgrids

This paper proposes a novel control method for accurate sharing of load current among the Distributed Energy Resources (DER) and high power quality operating in islanded ac microgrids. This control scheme is based on hierarchical structure comprising of decentralized primary controllers and a centralized secondary controller. The controllers in the primary level use GPS timing technology to synchronize their local time with a common time reference. In this context, proportional current sharing is achieved by adjusting the reference voltage of each DER unit according to a voltage-current (V-I) droop characteristic. The droop coefficient, which acts as a virtual resistance, is adaptively changed as a function of peak current. This strategy not only simplifies the control design but also enables faster dynamics and higher accuracy of current sharing especially at high loading conditions. The secondary controller produces compensation signals at fundamental and dominant harmonics to improve the voltage quality at a sensitive load bus. Experimental results are presented to validate the efficacy of the proposed method.

A GPS-Based Control Framework for Accurate Current Sharing and Power Quality Improvement in Microgrids

This paper proposes a novel hierarchical control strategy for improvement of load sharing and power quality in ac microgrids. This control framework is composed of a droop-based controller at the primary level, and a combination of distributed power sharing and voltage conditioning schemes at the secondary level. The controllers in the primary level use GPS timing technology to synchronize the local reference angles. The voltage reference of each distributed generation is adjusted according to a voltage–current (V–I) droop characteristic to enable proper current and power sharing with a fast dynamic response. The droop coefficient, which acts as a virtual resistance, is adaptively changed as a function of the peak current. This strategy not only simplifies the control design but also improves the current sharing accuracy at high loading conditions. The distributed power sharing scheme uses consensus protocol to ensure proportional sharing of average power. The voltage conditioning scheme produces compensation signals at fundamental and dominant harmonics to improve the voltage quality at a sensitive load bus. Experimental results are presented to validate the efficacy of the proposed method.

Voltage and frequency control of wind-powered islanded microgrids based on induction generator and STATCOM

This paper presents a comprehensive modeling of a three-phase cage induction machine used as a self-excited squirrel-cage induction generator (SEIG), and discusses the regulation of the voltage and frequency of a self-excited SEIG based on the action of the static synchronous Compensator (STATCOM). The STATCOM with the proposed controller consists of a three-phase voltage-sourced inverter and a DC voltage. The compensator can provide the active and reactive powers and regulate AC system bus voltage and the frequency, but also may enhance the load stability. Moreover, a feed forward control method for the STATCOM is introduced and applied for controlling the SEIGs terminal voltage using a two-degree of freedom RST controller. Simulation results for the steady-state operating condition and transient operating conditions for the system subjected to a wind reference step change, and a step load change are presented to demonstrate the effectiveness of the proposed controller.