Jidong Lai

An Optimal Secondary Voltage Control Strategy for an Islanded Multibus Microgrid

In this paper, an optimal secondary voltage control (SVC) strategy has been proposed for an islanded multibus microgrid (MG). The proposed strategy consists of a multiobjective function, power flow equality constraints, and four sets of inequality constraints. The multiobjective function aims to achieve a compromise between multibus voltage regulation and reactive power distribution among voltage modulation units by taking full advantage of both droop-controlled and PQ-controlled distributed energy resource (DER) units in the MG. Power flow equations for an islanded MG with or without secondary frequency control were considered as equality constraints. The limits of DER capacities, bus voltages, power flows on distribution lines, and system frequency must be accommodated during the process of solving the proposed SVC control strategy iteratively using the primal-dual interior-point method. Finally, simulation results have been presented to validate the feasibility of the proposed optimal SVC strategy.

Pulse energy modulation of a single-phase transformer-less inverter with active decoupling

Single-phase inverters suffer from the double line frequency ripple problem. This paper presents a novel decoupling power control method that is applied to single-phase grid-connected photovoltaic systems. Based on pulse energy modulation, the proposed is direct control of the output and decoupling energy in every switching period rather than indirect control of current or voltage to eliminate the power difference between the input and output port of single-phase inverters. In this control method, there is no dc current sampling. Decoupling capacitor and the input voltage are independent of other system voltages. Decoupling capacitance reduction makes it possible to employ long lifespan film capacitor. The simulation results verify the valid of the proposed decoupling power control strategy.

Research on frequency control strategy for islanding microgrid

This paper mainly research on frequency control strategy for a microgrid operating in islanding mode which consists of inverters controlled in traditional P-f droop method. Through modeling and analysis of microgrid system as well as equivalent and simplification of transfer function, a parameter optimal equation of controller for frequency regulation has derived. While under different situations, such as distribution ratios altering, the access number of inverters connected to the microgrid varying, or the output impedance changing, the controller can adjust its parameters online to make frequency of the system quickly restore stability. Experiments have verified the effectiveness and feasibility of the method.

A mixed decoupling power method for single-phase grid-connected inverters

This paper presents a novel decoupling power method that is applied to single-phase grid-connected photovoltaic systems. Based pulse energy modulation, the proposed method conditions ripple power by active and passive decoupling respectively in the different intervals, which not only reduces the required decoupling capacitance and the switching losses at the same time but also is independent of the specific inverter topologies. That means mixed decoupling can be used in most of single-phase inverters. Capacitance requirements and the impact of switching losses reduction ratio on other system variations is discussed. The simulation results verify the valid of the proposed decoupling power control strategy.

Research on control method for flywheel battery energy storage system

This paper presents a new control method for the flywheel battery energy storage (FBES) system. The proposed method adopts a double closed-loop control structure, which is based on an outer DC bus voltage loop cascaded with an inner current loop, and has an additional speed control loop. It can achieve charge and discharge process of the flywheel battery through regulating the flywheel speed without changing the outer loop control object. During the charge process, the DC bus voltage is completely determined by power system and the speed control loop limits the flywheel speed to avoid overcharge. Accordingly, the stability of the DC bus voltage is maintained by the flywheel battery during the discharge process. A FBES system is established with Matlab/Simulink to complete the charge and discharge control simulation and the sudden loading simulation during discharge. Meanwhile correlative experiments on a flywheel battery prototype are conducted. Simulation and experimental results verify the effectiveness of the proposed method.

Research on control strategy and stability of common DC bus solar pumper cluster system

The solar pumper cluster control system based on the common DC bus is applicable for the large scale solar water supply system which can enhance the efficiency. The structure of the system is introduced in detail. The drop control method and reference voltage compensation method is proposed to achieve the current sharing and array maximum power point tracking. Numbers of the paralleled inverter and the nonlinear characteristics of photovoltaic array impact on system stability are studied by establishing the mathematical model of multi-paralleled inverters. Finally result of experiment confirms the validity of the proposed control system.

Study on digital implementation of full-order flux observer for induction motor

Sensorless vector control system based on the full-order flux observer is widely applied benefiting from development of the digital signal processor (DSP). An essential step in the digital implementation is discretizing the continuous observer into the expression which can be implemented by DSP. Forward Euler method (FEM) is simple and easy to work out. However, the discrete full-order flux observer discretized by the FEM losses its performance, especially of precision and convergence, because this discretization method has characters of low precision which will result in poor convergence. In this paper, error and convergence properties of the FEM are analyzed firstly, and then an improved method for the implementation of the discrete full-order flux observer in DSP is proposed. It increases iteration frequency of the observer within a switching period and the iteration step size is equivalent to be decreased. Consequently, precision and convergence of the observed states are improved. Finally, simulation and experimental results verify the effectiveness of the proposed method.