Micro-source Inverter Research Articles

Current‐Limiting Strategy for Unbalanced Low‐Voltage Ride Through of the SMSI‐MG Based on Coordinated Control of the Generator Subunits

Unlike the inverters in the traditional alternating‐current (AC) microgrid, those in a microgrid with series microsource inverters (SMSI‐MG) are connected to the power grid after being cascaded. The authors of this study first divide the control sections according to the degree of grid voltage dips and formulate a coordinated scheme to suppress fluctuations in the output powers of the SMSI‐MG. For the section in which the degree of unbalanced grid voltage dips is relatively low, a current‐limiting strategy that reduces the output power of the SMSI‐MG through the coordinated control of the generator subunits (CCGU) is proposed. More active power can be provided by the SMSI‐MG when the proposed strategy is used, than in the strategy that is based on changing the reference power, and the output reactive power of the SMSI‐MG can be automatically changed with the degrees of dip and unbalance of the grid voltage. The Light Gradient‐Boosting Machine (LightGBM) is used to establish a mapping relationship between the parameters characterizing overcurrent and the reduction quantity in output active power of the SMSI‐MG to implement the CCGU‐based current‐limiting strategy. The complex collaborative control is simplified to improve the low‐voltage ride through (LVRT) capability of the SMSI‐MG.

Retracted: Research on Improved Droop Control Strategy of Microsource Inverter Based on Internet of Things

Retracted: Research on Improved Droop Control Strategy of Microsource Inverter Based on Internet of Things

MPC-based three-phase unbalanced power coordination control method for microgrid clusters

The microgrid cluster group-level bus voltage is high, the capacity is large, and the interconnection bus is connected to various microgrid subsystems. It is challenging for the microsource inverters in the subsystems to directly participate in the imbalance regulation of the group-level bus voltage, so it is currently difficult to solve the three-phase imbalance of the group-level bus voltage. In this paper, a particular structure of a series microgrid is applied to the cluster system, and a three-phase unbalanced control strategy based on model predictive control for the cluster system is proposed. First, the grid-connected current prediction control model of the series microgrid inverter using an LCL filter is established, a medium-voltage high-capacity three-level neutral point clamped (NPC) power electronic converter is introduced into the system and used as the central inverter of the DC microgrid, and the model prediction control method is applied to this central inverter. Second, considering possible load asymmetry operating conditions of the group bus, the p-q-o method is used to calculate the load imbalance current component that must be compensated, which is used as a part of the output current of the series microgrid or DC microgrid, based on which the three-phase unbalanced power coordinated control method between two heterogeneous microgrids is studied. The simulation results show that the series microgrid and DC microgrid exhibit good three-phase unbalance suppression under load unbalanced conditions, and the coordinated control between them can effectively improve the three-phase unbalance suppression capability of the system, which confirms that the proposed control strategy can achieve high-quality voltage output of the microgrid cluster system in multiple scenarios.

Research on Improved Droop Control Strategy of Microsource Inverter Based on Internet of Things

Microgrid connects the distributed power supply with the assistance of power electronic devices. Power electronic devices, especially in the inversion link, play a crucial role in the access of distributed power to microgrid. Whether in grid-connected mode or island mode, the control method of inverters is related to the stable operation of distributed power supply and plays an important role in the control strategy of microgrid. In this paper, by adding the drop control of controllable virtual impedance, the power coupling problem caused by resistive line impedance is reduced, and virtual impedance key points such as voltage feedback and frequency compensation are added. By optimizing the power reference value, the parallel operation stability of the control strategy is improved. The experimental results show that the proposed method improves not only the stability of the system and the power quality but also the accuracy of reactive power distribution.

A Study of Over-modulation Impact on the MSI Series Micro-Grid Output Voltage

In order to analyse the over-modulation technique impact on the micro-source inverter (MSI) series micro grids (SMGs) output voltage, firstly Double Fourier method applied to derive the single inverter output voltage characteristics under over-modulation condition; Secondly, based on the characteristics of the MSI series micro-grids, a new formula of the system output voltage is derived. Thirdly, the impact on the system output voltage fundamental component and harmonic component is analysed in detail, the results show that the output voltage fundamental component increases with the increasing modulation while keeps constant after a certain over-modulation depth. Finally, a simulation was carried out and the simulation verified the theoretical analysis.

Enhanced control and parameter analysis for virtual synchronous generators without PLL

Currently, virtual synchronous generator (VSG) technology is proposed to be a solution to stability controlling of micro-source inverter. However, the deviation from phase locked loop (PLL) influences the reference frequency calculating which is important to the VSG control system. This paper focuses on the virtual inertia controller in the VSG control system, an enhanced controller is achieved for avoiding the effect of PLL on the VSG dynamics. For analysing dynamic characteristics of the enhanced VSG control system, the linearised small-signal model of VSG in isolated operation is well established, including electrical system, enhanced virtual inertia controller, voltage and current controller, virtual impedance, and droop controller. By small-signal stability analysis, the influence rules of the main parameters (load, line, virtual inertia and proportional-Integral gain) on VSG system impacts are provided, which helps identifying possible feedback signals for controller design to improve the system stability. Results obtained from time-domain small-signal model are compared with those from the electrical simulation to verify its accuracy.

Small Signal Stability Analysis of Micro-source Inverter System in Grid-connected Series Micro-grid

In order to accurately analyze the small signal stability of micro-source inverter system in grid-connected series micro-grid, the paper establishes a small signal model of grid-connected series micro-grid system, including the series connection model, power controller model, phase locked loop model, power and current controller model, load and network model. Based on the model, the step response of the state variable of the system is calculated when the active load is increased, and the feasibility of the model is validated by MATLAB/Simulink software. Meanwhile, we can calculate the initial value of the system and the eigenvalues of the coefficient matrix of the equivalent resistance change, we further analyzes the small signal stability of micro-source inverter system in grid-connected series micro-grid. At the same time, the correctness of the theory is verified by simulation.

A Comprehensive Strategy for Power Quality Improvement of Multi-Inverter-Based Microgrid With Mixed Loads

In order to solve the influence of nonlinear and unbalanced mixed loads on output voltage of microsource inverters in microgrid, this paper proposes a comprehensive strategy which can be used to accurate power distribution, harmonic suppression, negative-sequence voltage component suppression, and stability improvement. On one hand, a fundamental control strategy is proposed upon the conventional droop control; the problem of accurate reactive power distribution is solved by introducing virtual impedance to inverters, while considering the aggravating problem of stability because of introducing virtual impedance, a secondary power balance controller is added to improve the stability of voltage and frequency. On the other hand, the fractional frequency harmonic control strategy and negative-sequence voltage control strategy are proposed to solve the influence of mixed loads, which focus on eliminating specific harmonics caused by the nonlinear loads and the negative sequence component of the voltage. The power quality of microgrid can be improved effectively. Finally, small signal analysis is used to analyze the stability of the multiconverter parallel system after introducing the whole control strategy. The simulation results show that the strategy proposed in this paper has a great performance on distributing reactive power, regulating and eliminating harmonic components, eliminating negative-sequence components and stabilizing output voltage of inverters and frequency, and improving the power quality of multiinverter-based microgrid.

Control principles of micro-source inverters used in microgrid

Since micro-sources are mostly interfaced to microgrid by power inverters, this paper gives an insight of the control methods of the micro-source inverters by reviewing some recent documents. Firstly, the basic principles of different inverter control methods are illustrated by analyzing the electrical circuits and control loops. Then, the main problems and some typical improved schemes of the ωU-droop grid-supporting inverter are presented. In results and discussion part, the comparison of different kinds of inverters is presented and some notable research points is discussed. It is concluded that the most promising control method should be the ωU-droop control, and it is meaningful to study the performance improvement methods under realistic operation conditions in the future work.

A multi-mode operation control strategy for flexible microgrid based on sliding-mode direct voltage and hierarchical controls

Multi-mode operation and transient stability are two problems that significantly affect flexible microgrid (MG). This paper proposes a multi-mode operation control strategy for flexible MG based on a three-layer hierarchical structure. The proposed structure is composed of autonomous, cooperative, and scheduling controllers. Autonomous controller is utilized to control the performance of the single micro-source inverter. An adaptive sliding-mode direct voltage loop and an improved droop power loop based on virtual negative impedance are presented respectively to enhance the system disturbance-rejection performance and the power sharing accuracy. Cooperative controller, which is composed of secondary voltage/frequency control and phase synchronization control, is designed to eliminate the voltage/frequency deviations produced by the autonomous controller and prepare for grid connection. Scheduling controller manages the power flow between the MG and the grid. The MG with the improved hierarchical control scheme can achieve seamless transitions from islanded to grid-connected mode and have a good transient performance. In addition the presented work can also optimize the power quality issues and improve the load power sharing accuracy between parallel VSIs. Finally, the transient performance and effectiveness of the proposed control scheme are evaluated by theoretical analysis and simulation results.

마이크로그리드 시스템의 안정도에 관한 기초 연구

Micro-grid consists of micro-sources which adopt environmentally friendly and reliable power sources such as Fuel-Cell and Micro-Turbines with independent real and reactive power control capability for providing premium power quality. This paper deals with the basic aspect of dynamic modeling and the stability analysis of the micro-grid system. The fundamental frequency model of the micro-source inverters are considered to form a dynamic model of the micro-grid system Stability analysis is performed based on the linearized dynamic model of the micro-grid system Case study results show the parameters affecting the stability of the micro-grid.