Yajuan Guan

A New Way of Controlling Parallel-Connected Inverters by Using Synchronous-Reference-Frame Virtual Impedance Loop—Part I: Control Principle

A novel simple and effective autonomous current-sharing controller for parallel three-phase inverters is proposed in this paper. The proposed controller provides faster response and better accuracy in contrast to the conventional droop control, since this novel approach does not require any active or reactive power calculations. Instead, a synchronous-reference-frame (SRF) virtual impedance loop and an SRF-based phase-locked loop are used. Stationary analysis is provided in order to identify the inherent mechanism of the direct and quadrature output currents in relation to the voltage amplitude and frequency with different line impedances by means of the system transfer functions. Comparison experiments from two parallel inverters are presented to compare the control performance of the conventional droop control and the proposed control with different line impedances. In addition, experimental results from a setup with three parallel 2.2-kW inverters verify the effectiveness of the proposed control strategy in different scenarios.

Frequency Stability of Hierarchically Controlled Hybrid Photovoltaic-Battery-Hydropower Microgrids

Hybrid photovoltaic (PV)-battery-hydropower microgrids (MGs) can be considered to enhance electricity accessibility and availability in remote areas. However, the coexistence of different renewable-energy sources with different inertias and control strategies may affect system stability. In this paper, a hierarchical controller for a hybrid PV-battery-hydropower MG is proposed in order to achieve the parallel operation of the hydropower and PV-battery system with different rates and to guarantee power sharing performance among PV voltage-controlled inverters, while the required power to the hydropower-based local grid is supplied. In this case, the PV-battery system will operate as a PQ bus to inject the desired active and reactive powers to the local grid, while the hydropower station will act as a slack bus which maintains its voltage amplitude and frequency. An integrated small-signal state-space model is derived to analyze the system stability of the hybrid MG. The simulation results show system frequency and voltage stability for a hybrid MG demonstration which includes the 2-MWp PV installations, a 15.2-MWh battery system, and a 12.8-MVA hydropower plant. The experimental results on a small-scale laboratory prototype verify the validity of the theoretical analysis and proposed control strategy.

Active Power Quality Improvement Strategy for Grid-Connected Microgrid Based on Hierarchical Control

When connected to a distorted grid utility, droop-controlled grid-connected microgrids (DCGC-MGs) exhibit low equivalent impedance. The harmonic and unbalanced voltage at the point of common coupling (PCC) deteriorates the power quality of the grid-connected current (GCC) of DCGC-MG. This paper proposes an active, unbalanced, and harmonic GCC suppression strategy based on hierarchical theory. The voltage error between the bus of the DCGC-MG and the grid’s PCC was transformed to the dq frame. On the basis of the grid, an additional compensator, which consists of multiple resonant voltage regulators, was then added to the original secondary control to generate the negative fundamental and unbalanced harmonic voltage reference. Proportional integral and multiple resonant controllers were adopted as voltage controller at the original primary level to improve the voltage tracking performance of the inverter. Consequently, the voltage difference between the PCC and the system bus decreased. In addition, we established a system model for parameter margin and stability analyses. Finally, the simulation and experiment results from a scaled-down laboratory prototype were presented to verify the validity of the proposed control strategy.

Secondary Coordinated Control of Islanded Microgrids Based on Consensus Algorithms

This paper proposes a decentralized secondary control for islanded microgrids based on consensus algorithms. In a microgrid, the secondary control is implemented in order to eliminate the frequency changes caused by the primary control when coordinating renewable energy sources and energy storage systems. Nevertheless, the conventional decentralized secondary control, although does not need to be implemented in a microgrid central controller (MGCC), it has the limitation that all decentralized controllers must be mutually synchronized. In a clear cut contrast, the proposed secondary control requires only a more simplified communication protocol and a sparse communication network. Moreover, the proposed approach based on dynamic consensus algorithms is able to achieve the coordinated secondary performance even when all units are initially out-of-synchronism. The control algorithm implemented in an islanded microgrid system is tested in different scenarios by means of hardware-in-the-loop results.

A Dynamic Consensus Algorithm to Adjust Virtual Impedance Loops for Discharge Rate Balancing of AC Microgrid Energy Storage Units

A dynamic consensus algorithm (DCA)-based coordinated secondary control with an autonomous current-sharing control strategy is proposed in this paper for balancing the discharge rate of energy storage systems (ESSs) in an islanded ac microgrid. The DCA is applied for information sharing between distributed generation (DG) units to regulate the output power of DGs according to the ESS capacities and state-of-charge (SoC). Power regulation is achieved by adjusting the virtual resistances of voltage-controlled inverters with an autonomous current-sharing controller. Compared with existing methods, the proposed approach can provide higher system reliability, expandability, and flexibility due to its distributed control architecture. The proposed controller can effectively prevent operation failure caused by over-current and unintentional outage of DGs by means of balanced discharge rate control. It can also provide fast response and accurate current sharing performance. A generalizable linearized state-space model for

Small-signal modeling, analysis and testing of parallel three-phase-inverters with a novel autonomous current sharing controller

{n}

Frequency stability of hierarchically controlled hybrid photovoltaic-battery-hydropower microgrids

-DG network in the

Comparison of a synchronous reference frame virtual impedance-based autonomous current sharing control with conventional droop control for parallel-connected inverters

{z}

Coordinated secondary control for balanced discharge rate of energy storage system in islanded microgrids

-domain is also derived and proposed in this paper; the model includes electrical, controller, and communication parts. The system stability and parameter sensitivity have been analyzed based on this model. To verify the effectiveness of the proposed control approach, this paper presents simulation results from a ten-node network and a comparison between experimental results obtained from the conventional power sharing control and the DCA-based SoC coordinated control in a setup with three 2.2 kW DG units.

A simple autonomous current-sharing control strategy for fast dynamic response of parallel inverters in islanded microgrids

A novel simple and effective autonomous current-sharing controller for parallel three-phase inverters is employed in this paper. The novel controller is able to endow to the system high speed response and precision in contrast to the conventional droop control as it does not require calculating any active or reactive power, instead it uses a virtual impedance loop and a SFR phase-locked loop. The small-signal model of the system was developed for the autonomous operation of inverter-based microgrid with the proposed controller. The developed model shows large stability margin and fast transient response of the system. This model can help identifying the origin of each of the modes and possible feedback signals for design of controllers to improve the system stability. Experimental results from two parallel 2.2 kVA inverters verify the effectiveness of the novel control approach.