Xiaoqing Lu

Droop-Based Distributed Cooperative Control for Microgrids With Time-Varying Delays

This paper develops a droop-based distributed cooperative control scheme for microgrids under a switching communication network with non-uniform time-varying delays. We first design a pinning-based frequency/voltage controller containing a distributed voltage observer and then design a consensus-based active/reactive power controller, which are employed into the secondary control stage to generate the nominal set points used in the primary control stage for different distributed generators (DGs). By this approach, the frequencies and the weighted average value of all DGs’ voltages can be pinned to the desired values while maintaining the precise active and reactive power sharing. With the proposed scheme, each DG only needs to communicate with its neighbors intermittently, even if their communication networks are local and time-varying, and their variant delays may be non-uniform. Sufficient conditions on the requirements for the network connectivity and the delay upper bound that guarantee the stability and reliability of the microgrid are presented. The effectiveness of the proposed control scheme is verified by the simulation of a microgrid test system.

Distributed Secondary Voltage and Frequency Control for Islanded Microgrids With Uncertain Communication Links

This paper presents a robust distributed secondary control (DSC) scheme for inverter-based microgrids (MGs) in a distribution sparse network with uncertain communication links. By using the iterative learning mechanics, two discrete-time DSC controllers are designed, which enable all the distributed energy resources (DERs) in an MG to achieve the voltage/frequency restoration and active power sharing accuracy, respectively. In special, the secondary control inputs are merely updated at the end of each round of iteration, and thus, each DER only needs to share information with its neighbors intermittently in a low-bandwidth communication manner. This way, the communication costs are greatly reduced, and some sufficient conditions on the system stability and robustness to the uncertainties are also derived by using the tools of Lyapunov stability theory, algebraic graph theory, and matrix inequality theory. The proposed controllers are implemented on local DERs, and thus, no central controller is required. Moreover, the desired control objective can also be guaranteed even if all DERs are subject to internal uncertainties and external noises including initial voltage and/or frequency resetting errors and measurement disturbances, which then improves the system reliability and robustness. The effectiveness of the proposed DSC scheme is verified by the simulation of an islanded MG in MATLAB/SimPowerSystems.

A Novel Distributed Secondary Coordination Control Approach for Islanded Microgrids

This paper develops a new distributed secondary cooperative control scheme to coordinate distributed generators (DGs) in islanded microgrids (MGs). A finite time frequency regulation strategy containing a consensus-based distributed active power regulator is presented, which can not only guarantee the active power sharing but also enable all DGs’ frequencies to converge to the reference value within a finite time. This enables the frequency and voltage control designs to be separated. Then an observer-based distributed voltage regulator involving certain reactive power sharing constraints is proposed, which allows different set points for different DGs and, thus, accounts for the line impedance effects. The steady-state performance analysis shows that the voltage regulator can accurately address the issue of global voltage regulation and accurate reactive power sharing. Moreover, all the distributed controllers are equipped with bounded control inputs to suppress the transient overshoot, and they are implemented through sparse communication networks. The effectiveness of the control in case of load variation, plug-and-play capability, communication topology change, link failure, time delays, and data drop-out are verified by the simulation of an islanded MG in MATLAB/SimPowerSystems.

Distributed power control for DERs based on networked multiagent systems with communication delays

This paper develops a distributed secondary cooperative scheme for the current-controlled PWM inverters (CC-PWMIs) based on networked multiagent systems that can guarantee the accurate power sharing among the distributed energy resource (DER) units in microgrids. With the assumption that the communication networks are local, time-varying, and with low bandwidth, two distributed secondary controllers are designed such that all DERs in the microgrid can share their active and reactive powers accurately on the conditions of both fixed and switching topologies with time-varying communication delays. Moreover, delay-dependent sufficient conditions are obtained by using the Lyapunov-Krasovskii stable analysis method. The proposed controllers, allowing each DER unit to receive the current and active/reactive load information intermittently from its neighboring DERs, are then fully distributed. The effectiveness of the proposed control methodology is verified by the simulation of a low-voltage microgrid test system in the MATLAB/SimPowerSystems Toolbox.

Synchronization of Hybrid Microgrids with Communication Latency

A distributed cooperative control scheme is proposed in order to implement a distributed secondary control for hybrid lossy microgrids. The designed distributed control is able to synchronize the frequency of inverse-based distributed generators (DGs) and minisynchronous generators (MSGs/SGs) to the desired state with a virtual leader DG/SG (reference value) in a distribution switching network under the existence of time-varying communication delays. The secondary control stage selects suitable frequencies of each DG/SG such that they can be synchronized at the desired set point. Using the proposed algorithm, each DG/SG only needs to communicate with its neighboring DGs/SGs intermittently even if the communication networks are local, the topology is time-varying, and the communication delays may exist. Therefore, the failure of a single DG/SG will not produce the failing down of the whole system. Sufficient conditions on the requirements for the network connectivity and the delays boundedness which guarantees the stability and synchronization of the controlled hybrid lossy microgrid power systems are presented. The feasibility of the proposed control methodology is verified by the simulation of a given lossy microgrid test system.

Finite-Time Control for Robust Tracking Consensus in MASs With an Uncertain Leader

This paper investigates the finite-time control for robust tracking consensus problems of multiagent systems with an uncertain leader for situations where the state of the considered active leader may not be measured and the directed network topology is time-varying. Based on the neighbor-based state-estimation rule and a new Lyapunov stability analysis method, a continuous and nonlinear distributed tracking protocol using only relative position information is designed, under which each agent can follow the leader in finite time if the input (acceleration) of the leader is known, and the tracking errors can converge to a bounded region in finite time if the input of the leader is unknown. In particular, a special continuous distributed tracking protocol with bounded control inputs is introduced to track the active leader in finite time. Numerical simulations are also given to illustrate the effectiveness of the theoretic results.

Distributed Coordination of Islanded Microgrid Clusters Using a Two-Layer Intermittent Communication Network

This paper proposes a distributed hierarchical cooperative control strategy for a cluster of islanded microgrids (MGs) with intermittent communication, which can regulate the frequency/voltage of all distributed generators (DGs) within each MG as well as ensure the active/reactive power sharing among MGs. A droop-based distributed secondary control scheme and a distributed tertiary control scheme are presented based on the iterative learning mechanics, by which the control inputs are merely updated at the end of each round of iteration, and thus, each DG only needs to share information with its neighbors intermittently in a low-bandwidth communication manner. A two-layer sparse communication network is modeled by pinning one or some DGs (pinned DGs) from the lower network of each MG to constitute an upper network. Under this control framework, the tertiary level generates the frequency/voltage references based on the active/reactive power mismatch among MGs while the pinned DGs propagate these references to their neighbors in the secondary level, and the frequency/voltage nominal set points for each DG in the primary level can be finally adjusted based on the frequency/voltage errors. Stability analysis of the two-layer control system is given, and sufficient conditions on the upper bound of the sampling period ratio of the tertiary layer to the secondary layer are also derived. The proposed controllers are distributed, and thus, allow different numbers of heterogeneous DGs in each MG. The effectiveness of the proposed control methodology is verified by the simulation of an ac MG cluster in Simulink/SimPower Systems.

Networked-based distributed cooperative voltage control for power electronics interfaced microgrids

This paper develops a distributed cooperative voltage control scheme involving primary and secondary voltage control for an autonomous microgrid (MG). The proposed distributed control algorithms are able to implement the fully local communication and control function among the inverter-based distributed energy resources (DERs) at the secondary stage. Moreover, it will not only regulate the voltage of DERs to the desired values but also achieve reactive proportional power sharing via a robust communication network with time-varying delays. The criteria for the stability analysis and delays boundedness of autonomous microgrids are derived. Steady-state performance analysis via the simulation of a autonomous microgrid test system shows that the proposed distributed voltage control algorithms not only are viable to accurately deal with the global voltage regulation and proportional reactive power sharing, but also is very robust with respect to communication delays and switching network topologies.

Real-time implementation of affine nonlinear optimal control for SMIB system

This paper presents the simulation and the experimental validation of the designed robust power system controller to stabilise a linearised uncertain power system using affine nonlinear optimal control (ANOC) loop shaping design procedure. Guidance for setting the feedback configuration for loop shaping, weighing functions selection and synthesis are also presented. The stability and effectiveness of the designed controller is simulated using MATLAB/Simulink and tested by implementing on real-time environment using iNetCon-PC104 work stations and the control circuit designed. Meanwhile, the hardware in the loop (HITL) simulation results of single-machine infinite-bus (SMIB) transient stability are compared with the off-line digital simulation results based on MATLAB/Simulink, thus validating the simulation results with the experimental results. Also, justification of robustness is presented by considering with or without controller based on affine nonlinear algorithm when large disturbances (faults) happen, which shows the effectiveness of the ANOC presented in the power system.