Honghua Xu

Control strategy for autonomous operation of three-phase inverters dominated microgrid under different line impedance

The traditional power droop controllers are commonly used in distributed generation (DG) units to share the loads among the parallel inverters. However, the parameters of the power transmission and distribution line in microgrid are small and generally presented high resistance-to-inductance (R/X) ratio which will seriously influence the performance of the traditional power droop controller, even leading to instability. In this paper, the control strategies for parallel inverters under the inductive and resistance line impedance are researched contrastively based on the small signal models of the power loop. The employed droop scheme can maintain the load sharing capability under different line impedance. These theoretical findings are eventually verified experimentally on laboratory prototypes.

Voltage and frequency control of inverters connected in parallel forming a micro-grid

The “communication mechanism” to realize the power share of grid-forming inverters is analyzed and a droop control method through active and reactive current is proposed in this paper. The control strategies of inverters connected in parallel forming a micro-grid are described in detail. The droop controller is divided into three units that are current (power) decoupling, the first-order inertia and droop control. From the point of synchronous generator, the design principles of first-order inertia constant and the droop coefficients are analyzed. Finally, simulation models are established in Simulink/Matlab and two prototypes with the capacity of 10kVA are established in laboratory. The simulation and experiment results verify the feasibility of the proposed method.

An MPC-Based ESS Control Method for PV Power Smoothing Applications

Random fluctuation in photovoltaic (PV) power plants is becoming a serious problem affecting the power quality and stability of the grid along with the increasing penetration of PVs. In order to solve this problem, by the adding of energy storage systems (ESS), a grid-connected microgrid system can be performed. To make this system feasible, this paper proposes a model predictive control (MPC) based on power/voltage smoothing strategy. With the receding horizon optimization performed by MPC, the system parameters can be estimated with high accuracy, and at the same time the optimal ESS power reference is obtained. The critical parameters, such as state of charge, are also taken into account in order to ensure the health and stability of the ESSs. In this proposed control strategy, communication between PVs and ESS is not needed, since control command can be calculated with local measured data. At the same time, MPC can make a great contribution to the accuracy and timeliness of the control. Finally, experimental results from a grid-connected lab-scale microgrid system are presented to prove effectiveness and robustness of the proposed approach.

Design of energy storage control strategy to improve the PV system power quality

Random fluctuation of PV power is becoming a more and more serious problem affecting the power quality and stability of grid as the PV penetration keeps increasing recent years. Aiming at this problem, this paper proposed a control strategy of energy storage system based on Model Predictive Control (MPC). By the continuous optimizing of MPC, we can obtain the system parameters accurately, and then calculate the energy storage power. At the same time, this control took state of charge (SOC) and other parameters into account to ensure the health and stability of the energy storage units. Finally, simulation results proved the proposed control strategy had good performance and robustness.

Research on the control technique of low voltage ride-through for grid-connected photovoltaic inverter

According to the new requirements of large-capacity MW solar power station, this paper establishes a mathematical model of the inverter, analyzes the drawbacks of synchronous reference frame phase-locked loop in low voltage ride through, improves the performance of decoupled dual synchronous reference frame phase locked loop and designs the proportional integral controller parameters. It uses the feed-forward decoupling, grid voltage feed-forward control and coordinated control in current control loop. The experiment results show that the proposed control techniques can make the three-phase photovoltaic grid-connected inverter operate stably and reliably under low voltage ride through condition, and make the inverter meet the relevant standard requirements. This proves the validity and feasibility of the proposed control technologies.

Novel Islanding Detection Method for Distributed PV Systems with Multi-Inverters

This study proposes a novel islanding detection method for distributed photovoltaic (PV) systems with multi-inverters based on a combination of the power line carrier communication and Sandia frequency shift islanding detection methods. A parameter design method is provided for the novel scheme. On the basis of the designed parameters, the effect of frequency measurement errors and grid line impedance on the islanding detection performance of PV systems is analyzed. Experimental results show that the theoretical analysis is correct and that the novel method with the designed parameters has little effect on the power quality of the inverter output current. Non-detection zones are not observed, and a high degree of reliability is achieved. Moreover, the proposed islanding detection method is suitable for distributed PV systems with multi-inverters.