Xiangzhen Yang

Study on microgrid operation control and black start

Along with the increasing distributed generation (DG) technologies, the development of the power market and the shock of serious blackout, more and more people focus research on microgrid (MG) technologies. Firstly under normal operating conditions, MG operation control is the most important problem for MG stable work. Based on current state of the development of MG, this paper analyses the technologies of MG operation control on two dimensions: local-level distributed generation control and system-level operation control. Then, in order to deal with black start and islanded operation following a general or local blackout, an emergency operation mode of MG is introduced. It includes the conditions of microgrid with black start capability, the sequence of actions, and the relationship between MG black start and electric power system black start. Furthermore, this paper presents a framework of comprehensive testbed of MG with black start capability including key technologies of hardware and software design. This testbed of MG in lab is composed of renewable energy generation systems (wind turbines, photovoltaic (PV) arrays, biomass et al.), clean energy generation systems such as micro gas turbines, grid-connected inverters with storage units, local loads (some of them interruptible), and static var compensators et al. The MG system adopts supervisory operation control mode. It consists of MG central controller (MGCC), microsource controller, communication system, and smart switches which apply to plug and play of DG.

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.

A coordinated DC voltage control strategy for H-bridge cascaded STATCOM

Keeping the equalization of the DC capacitor voltage is one of the critical problems for the reliable operation of the cascaded H-bridge multilevel STATCOM. In this paper, a novel coordinated DC voltage control strategy for H-bridge cascaded STATCOM is proposed to solve this above problem without extra infrastructures. This control strategy is a two-level hierarchy which consists of a predicted central controller and an amending distributed controller. This predicted central controller is to obtain the reference of equivalent output voltage of STATCOM by a dual-loop DC voltage and AC instantaneous current controller. The result of distributed DC voltage controller is functioned to just amend the above average voltage reference slightly while the implement of predict ac output reference as a feed forward component. The simulation platform has been built on Matlab/Simulink and PLECS. The simulation results show that the proposed control strategy has excellent robustness and fast-response characteristic in a large step change.

An optimal secondary voltage control strategy for islanded microgrid

In this paper, an optimal secondary voltage control (SVC) strategy has been proposed for an islanded microgrid(MG).The proposed strategy consists of multi-objective optimal function, power flow equality constraints and three sets of inequality constraints. The multi-objective function aims to minimize voltage errors of selected buses and reactive circulating currents among voltage modulation units by taking full advantage of both droop-controlled and PQ-controlled DER units connected in the MG. Power flow equations for the islanded MG were considered as equality constraints. Limits of DER capacities, bus voltages and power flows on distribution lines, must be met during the process of solving this proposed control strategy iteratively using interior-point algorithm. Finally, simulation results have been presented validating the proposed SVC scheme, illustrating good voltage regulation while minimizing reactive circulating current in islanded MG.

Microgrid secondary frequency control method based on life optimization of energy storage units

In order to deal with frequency restoration and protect the energy storage units, this paper analyses the control method of microgrid based on hierarchical control, and proposes an innovative secondary frequency control method which optimize the output of energy storage units in island mode. The simulation results show that the algorithm can make microgrid provide high stability and power quality. Meanwhile, this algorithm calculates the life model of energy storage units to assign power load reasonably so as to make the utility of energy storage units with high cost-effective, avoid frequent battery charging, extend the operational life and make microgrids more reliable.

A decentralized multi-framed droop-controller for improving harmonic power sharing in an islanded microgrid

The nonlinear loads in microgrid not only distort the voltage but also degrade the harmonic power distribution in a conventional droop-controlled microgrid. This paper starts with analyzing the trade-off between voltage total harmonic distortion (VTHD) and harmonic sharing effectiveness. A multi-framed droop controller is proposed to improve the harmonic power sharing without deteriorating VTHD by adjusting the equivalent impedance. The inherent output impedance of inverter is reduced by multi selected-frequency resonant (R) controllers in dq frame whereas the influence of the mismatching impedance is minored by moving-average-filtered complex impedance in αβ frame. The simulation results demonstrate the effectiveness of the proposed method.

Impedance-phase reshaping of LCL-filtered grid-connected inverters to improve the stability in a weak grid

The stability of grid-connected voltage-source inverters (VSI) is deteriorated in a weak grid due to the interaction between the increasing grid impedance and the inherent inverter impedance. To improve the stability of LCL-filtered VSI, an impedance-phase reshaping method is proposed in paper. With this output current feedback compensator, the phase margin at the impedance intersection can be improved dramatically. A straightforward parameter design method is also introduced to simplify the design procedure. Simulation and experimental results valid the effectiveness of the proposed the method and design methodology.

A review on the small signal stability of microgrid

Small signal stability is the key issue in the operation and control of a microgrid. For the small signal stability of an inverter-based microgrid, existing analysis methods mainly contain eigenvalue analysis using state-space models, impedance analysis based on impedance models and other nonlinear analysis methods. This paper reviews these analytical methods as well as their respective advantages and disadvantages. Finally, for the improvement of the small signal stability, this paper summarizes the research situation in terms of the optimization of controller parameters, the improvement of droop control and the optimized design of hierarchical control strategy.

Improved virtual oscillator control for parallel operation inverter systems

An improved virtual oscillator control for wireless parallel connected inverters was proposed. The inverters with virtual oscillator control in parallel system are equal to the oscillator itself. Oscillator model is given by digital controller. Optimize the topology and control function of the oscillator, achieved the purpose of synchronization. Calculate the output power of oscillator circuit, and the state of power sharing decide compensation amount for load current disturbance, increases the quality of output voltage. Simulation results verify feasibility of the proposed control method.