Modelling of wind energy‐based microgrid system implementing MMC

Nowadays, renewable energy systems have come up with more potential in power generation so as to meet the power demand. Among all the renewable systems, the wind energy generating system is believed to be at the peak. However, the wind energy‐based microgrid system is associated with many problems such as fluctuations in output voltage due to the fluctuated wind speeds and harmonics generations in the system. To address these issues, this article proposes a new method in order to achieve harmonic mitigation across its output by maintaining constant voltage. Nevertheless, particular attention has been given to the form and function of modular multilevel converter with multi‐winding transformer connected to the grid. Modular multilevel converter has been implemented with an advanced voltage controller tuned to control the voltage at its output. Also, a new system topology has been introduced with two wind turbines that are interconnected to multi‐winding transformer through asynchronous generators. The proposed system has been implemented with constant and variable wind speeds, and their respective results have also been analysed. The proposed scheme shows its effectiveness by theoretical calculations, verified by simulation and experimental results. Copyright © 2016 John Wiley & Sons, Ltd.     

Harmonic control using matrix converter of unbalanced input voltage supplied from solar power system

Most of the renewable energy sources are not distributed directly to consumer. It gone through several section to get optimum output voltage and current with the almost zero distortion. Direct Matrix converters are well known of its better power quality and low power distortion to the reactive loads. Conversely, multilevel H bridge inverters are broadly used to attain a sinusoidal output voltage in solar photovoltaic application. Reactive power is subject to high importance for the operation of alternating current (AC) power systems. Moreover there is always a challenge to obtain the balance current control from an unbalanced supply voltage with minimum harmonic distortion for reactive load application from solar power system. The main objective of the proposed system is to maximize the power usage by maintaining the current and power factor nearby unity. There are many different optimization techniques are used so far in matrix converter, a novel method of optimization BAT algorithm is proposed for the power quality to get the best result. This work mainly deals with the compensation of reactive power used in Solar Photovoltaic Power System. Bidirectional energy flow is possible with matrix converter for battery charging. Simulation results are presented to confirm the appropriate operation of the system under different of operating conditions. Also, in the proposed method the percentage of the higher order harmonics can reduced less than 9% of matrix converter for solar photovoltaic application.     

Rapid transaction to load variations of active filter supplied by PV system

This paper deals with the analysis and control of a photovoltaic (PV) system connected to the main supply through a Boost converter and shunt active filter supplied by a PV system providing continuous supply of nonlinear load in variation. A robust control of a PV system connected to the grid while feeding a variable nonlinear load is developed and highlighted. This development is based on the control of the Boost converter to extract the maximum power from the PV system using the Perturb and Observe (P and O) algorithm in the presence of temperature and illumination. The proposed modeling and control strategy provide power to the variable nonlinear load and facilitates the transfer of power from solar panel to the grid while improving the quality of energy (harmonic currents compensation, power factor compensation and dc bus voltage regulation). Validation of the developed model and control strategy is conducted using power system simulator Sim-Power System Blockset Matlab/Simulink. To demonstrate the effectiveness of the shunt active filter to load changes, the method of instantaneous power (pq theory) is used to identify harmonic currents. The obtained results show an accurate extraction of harmonic currents and perfect compensation of both reactive power and harmonic currents with a lower THD and in accordance with the IEEE-519 standard.     

A multi-objective control strategy for grid connection of DG (distributed generation) resources

This paper presents a flexible control technique for connection of DG (distributed generation) resources to distribution networks, especially during ride-through on faulty grid. This strategy is derived from the abc/αβ and αβ/dq transformations of the ac system variables. The active and reactive currents injected by the DG source are controlled in the synchronously rotating orthogonal dq reference frame. The transformed variables are used to control the VSI (voltage source inverter) which connects the DG to the distribution network. Using a P.L.L. (phase locked loop) in circuit of proposed control technique, the angle of positive sequence has been detected, in order to synchronize the currents to the distribution network. The proposed control technique has the capability of providing active and reactive powers and harmonic currents to nonlinear loads with a fast dynamic response. Simulation results and mathematical analysis have been completed in order to achieve a reduced THD (total harmonic distortion), increased power factor and compensated load’s active and reactive powers. The analyses show the high performance of this control strategy in DG applications in comparison with other existing strategies.     

直驱永磁风力发电系统电网侧变换器的模糊功率控制

针对基于储能的并网永磁直驱风力发电系统的运行特点,提出了一种对电网侧变换器的模糊功率控制策略,当电网发生电压瞬时跌落时使电网侧变换器运行在模糊功率控制模式,依据电网电压跌落深度及风机输出功率大小通过模糊控制器来确定发出无功电流的大小,从而使风电系统能够适度地向电网提供一定的无功功率来支撑电网电压。仿真结果表明,所提出的控制策略正确、有效。     

THD reduction with reactive power compensation for fuzzy logic DVR based solar PV grid connected system

Dynamic voltage restorer (DVR) is used to protect sensitive loads from voltage disturbances of the distribution generation (DG) system. In this paper, a new control approach for the 200 kW solar photovoltaic grid connected system with perturb and observe maximum power point tracking (MPPT) technique is implemented. Power quality improvement with comparison is conducted during fault with proportional integral (PI) and artificial intelligence-based fuzzy logic controlled DVR. MPPT tracks the actual variable DC link voltage while deriving the maximum power from a photovoltaic array and maintains DC link voltage constant by changing modulation index of the converter. Simulation results during fault show that the fuzzy logic based DVR scheme demonstrates simultaneous exchange of active and reactive power with less total harmonic distortion (THD) present in voltage source converter (VSC) current and grid current with fast tracking of optimum operating point at unity power factor. Standards (IEEE-519/1547), stipulates that the current with THD greater than 5% cannot be injected into the grid by any distributed generation source. Simulation results and validations of MPPT technique and operation of fuzzy logic controlled DVR demonstrate the effectiveness of the proposed control schemes.     

Comparative study of power converter topologies and control strategies for the harmonic performance of variable-speed wind turbine generator systems

Power converters play a vital role in the integration of wind power into the electrical grid. Variable-speed wind turbine generator systems have a considerable interest of application for grid connection at constant frequency. In this paper, comprehensive simulation studies are carried out with three power converter topologies: matrix, two-level and multilevel. A fractional-order control strategy is studied for the variable-speed operation of wind turbine generator systems. The studies are in order to compare power converter topologies and control strategies. The studies reveal that the multilevel converter and the proposed fractional-order control strategy enable an improvement in the power quality, in comparison with the other power converters using a classical integer-order control strategy.     

Coordinate control strategy for stability operation of offshore wind farm integrated with Diode-rectifier HVDC

Due to low investment cost and high reliability, a new scheme called DR-HVDC (Diode Rectifier based HVDC) transmission was recently proposed for grid integration of large offshore wind farms. However, in this scheme, the application of conventional control strategies for stability operation face several challenges due to the uncontrollability of the DR. In this paper, a coordinated control strategy of offshore wind farms using the DR-HVDC transmission technology to connect with the onshore grid, is investigated. A novel coordinated control strategy for DR-HVDC is proposed based on the analysis of the DC current control ability of the full-bridge-based modular multilevel converter (FB-MMC) at the onshore station and the input and output characteristics of the diode rectifier at the offshore. Considering the characteristics of operation stability and decoupling between reactive power and active power, a simplified design based on double-loop droop control for offshore AC voltage is proposed after power flow and voltage–current (I–V) characteristics of the offshore wind farm being analyzed. Furthermore, the impact of onshore AC fault to offshore wind farm is analyzed, and a fast fault detection and protection strategy without relying on communication is proposed. Case studies carried out by PSCAD/EMTDC verify the effectiveness of the proposed control strategy for the start up, power fluctuation, and onshore and offshore fault conditions.     

Combined operation of DC isolated distribution and PV systems for supplying unbalanced AC loads

This paper describes a DC isolated network which is fed by distributed generation (DG) from photovoltaic (PV) renewable sources to supply unbalanced AC loads. The battery energy storage bank has been connected to the DC network via DC/DC converter called storage converter to control the network voltage and optimize the operation of the PV generation units. The PV units are connected to the DC network via its own DC/DC converter called PV converter to ensure the required power flow. The unbalanced AC loads are connected to the DC network via its own DC/AC converter called load converter without transformer. This paper proposes a novel control strategy for storage converter which has a DC voltage droop regulator. Also a novel control system based on Clarke and Park rotating frame has been proposed for load converters. In this paper, the proposed operation method is demonstrated by simulation of power transfer between PV units, unbalanced AC loads and battery units. The simulation results based on PSCAD/EMTDC software show that DC isolated distribution system including PV units can provide the balanced voltages to supply unbalanced AC loads.     

Power quality investigation of a solar PV transformer-less grid-connected system fed DVR

This paper presents a single stage transformer-less grid-connected solar photovoltaic (PV) system with an active and reactive power control. In the absence of active input power, the grid-tied voltage source converter (VSC) is operated in a reactive power generation mode, which powers the control circuitry, and maintains a regulated DC voltage to the VSC. A data-based maximum power point tracking (MPPT) control scheme which performs power quality control at a maximum power by reducing the total harmonic distortion (THD) in grid injected current as per IEEE-519/1547 standards is implemented. A proportional-integral (PI) controller based dynamic voltage restorer (DVR) control scheme is implemented which controls the grid side converter during single-phase to ground fault. The analysis includes the grid current THD along with the corresponding variation of the active and reactive power during the fault condition. The MPPT tracks the actual variable DC link voltage while deriving the maximum power from the solar PV array, and maintains the DC link voltage constant by changing the modulation index of the VSC. Simulation results using Matlab/Simulink are presented to demonstrate the feasibility and validations of the proposed novel MPPT and DVR control systems under different environmental conditions.     

A variable speed wind energy conversion scheme for connection toweak AC systems

A three level control system for a variable speed wind energy conversion scheme (VSWECS) supplying a weak AC power system is presented. The objective of the control strategy is to maximize energy capture and simultaneously to support the voltage of the bus where the VSWECS is connected. Using an insulated gate bipolar transistor (IGBT) inverter, both control of active and reactive power supplied to the grid and reduction of harmonic distortion can be achieved. The response of the proposed scheme has been tested and evaluated in a test system using a developed computer program simulating in detail the system operation     

虚拟同步发电机技术控制的MMC型固态变压器

针对在大容量直流负荷和高渗透率分布式电源接入固态变压器低压侧时,高压并网接口易呈惯量低、阻尼特性差的问题,提出一种虚拟同步发电机技术控制的模块化多电平换流器（MMC）型固态变压器。首先,分析虚拟同步发电机原理并推导MMC与虚拟同步发电机的等效数学模型,并将虚拟同步发电机技术融入到输入级的控制中,使并网接口的惯性与阻尼增强,在输出端功率变化时对上级电网呈现出友好的柔性缓冲能力。其次,为提升MMC型固态变压器对上级电网的频率支撑能力,在低压直流环节配置储能装置,通过改变充放电功率主动响应一次调频。然后,通过输入级的无功控制环节,验证其具备一定的调压能力。最后建立仿真模型验证了所提控制策略的有效性与可行性。     

Grid power quality with variable speed wind turbines

Grid connection of renewable energy sources is essential if they are to be effectively exploited, but grid connection brings problems of voltage fluctuation and harmonic distortion. In the paper, appropriate modeling and simulation techniques are discussed for studying the voltage fluctuation and harmonic distortion in a network to which variable speed wind turbines are connected. Case studies on a distribution network show that the voltage fluctuation and harmonic problems can he minimized with the proposed power electronics interface and control system while the wind energy conversion system captures the maximum power from the wind as wind speed varies. The studies have also demonstrated the ability of the advanced converter to assist the system voltage regulation     

基于反演控制的DC-MMC双闭环解耦控制策略研究

随着大量分布式电源投入电网,分压式模块化多电平直流变换器（DC-MMC）得到广泛应用。针对模块化多电平变换器控制系统多输入多输出、强耦合非线性的特点,推导变换器拓扑数学模型,建立此类拓扑的电流和能量模型,提出一种基于反演控制的双闭环解耦控制方法。通过消除系统之间的耦合,采用模型反演理论对所建解耦模型进行控制,实现桥臂电流保持稳定并交流分量最小化,增强系统的抗扰动能力。在Matlab/Simulink中搭建DC-MMC双闭环模型,分析表明闭环控制模型具有良好的跟踪效果和调控特性,验证了提出的控制策略控制桥臂电流及高低压侧电流的可行性和有效性,为解耦控制的应用提供了参考依据。     

Control strategies for single-phase grid integration of small-scale renewable energy sources: A review

Small-scale renewable energy sources, such as small hydro turbines, roof-mounted photovoltaic and wind generation systems, and commercially available fuel cells are usually connected to the single-phase distribution grid through a voltage source converter. To regulate the power exchange with the single-phase grid, and at the same time, reduce the harmonic distortions in the ac current, different current control structures have already been proposed, among which the current hysteresis control, the voltage oriented control, and the proportional-resonant based control have found more attentions. This paper provides an overview of the main characteristics of these control strategies. Also, some implementation aspects such as the fictitious signal generation and the single-phase grid synchronization techniques are discussed. Finally, through extensive simulations a comparative study of the presented control strategies is presented. The simulations are supported by experiments.     

Optimal operation of DC smart house system by controllable loads based on smart grid topology

From the perspective of global warming mitigation and depletion of energy resources, renewable energy such as wind generation (WG) and photovoltaic generation (PV) are getting attention in distribution systems. Additionally, all-electric apartment houses or residence such as DC smart houses are increasing. However, due to the fluctuating power from renewable energy sources and loads, supply-demand balancing of power system becomes problematic. Smart grid is a solution to this problem. This paper presents a methodology for optimal operation of a smart grid to minimize the interconnection point power flow fluctuation. To achieve the proposed optimal operation, we use distributed controllable loads such as battery and heat pump. By minimizing the interconnection point power flow fluctuation, it is possible to reduce the electric power consumption and the cost of electricity. This system consists of photovoltaic generator, heat pump, battery, solar collector, and load. To verify the effectiveness of the proposed system, results are used in simulation presented.     

Improving voltage harmonic compensation of a single phase inverted-based PEM fuel cell for stand-alone applications

Fuel cell (FC) is an efficient energy conversion technology that is growing rapidly and will have a significant role to play in a number of energy end-use sectors, from small-scale applications to large-scale power plants. In this paper, two new methods for voltage harmonic compensation of a stand-alone single phase inverted-based FC are presented and evaluated. The stand-alone power system under study consists of: 1) Proton-Exchange-Membrane (PEM) FC with unidirectional DC/DC converter, which converts the DC voltage delivered by the FC to the DC bus voltage; 2) single-phase pulse width modulated (PWM) inverter; 3) transformer; 4) passive filter; and 5) linear and non-linear loads. The dynamic model of this system and the non-sinusoidal output-based controls applied to the PWM inverter for voltage regulation and harmonic compensation are detailed in this paper, and evaluated by simulation under different linear and non-linear loads. Simulation results show the effectiveness of the two purposed methods for voltage harmonic compensation to acceptable levels defined in grid codes.     

Research on control strategy of energy storage converter based on super capacitor

为了对储能逆变器在不平衡与非线性负载时的输出电压的谐波进行抑制,本文针对双级式逆变器拓扑建立数学模型,基于模型分析了直流电容中点电压波动对逆变器输出的影响,以及不平衡输出电流和谐波电流对直流电容中点电压影响的机理,进而提出了该拓扑逆变器在不平衡和非线性负载情况下抑制交流输出电压不平衡和零序谐波的抑制策略。最后建立了MATLAB仿真模型对该控制策略进行了理论验证,并在80 kW样机上进行了实验验证,仿真和实验结果都表明了该控制策略的有效性。     

Crowbar优化控制策略下的双馈风机无功调节能力研究

在分析双馈风机(DFIG)无功调节原理的基础上,根据最新的低电压穿越要求,建立优化的Crowbar控制策略,进而提出在电网严重故障期间内,Crowbar投入时由网侧变流器充当STATCOM为电网提供无功,Crowbar退出时无功输出继续由转子侧变流器励磁调节控制,推导出DFIG网侧及定子输出无功功率极限的表达式,结合优化的Crowbar控制策略研究DFIG的无功调节能力,最后利用RTDS平台进行仿真验证。结果表明,DFIG的无功调节能力与理论分析一致,在电网故障期间,应用此控制策略的DFIG可连续提供最大无功支持,且能帮助恢复电网电压。     

Dead-beat instantaneous power control for off-board charger of electric vehicle for V2G application

Dead-beat instantaneous power control strategy for electric vehicle off-board V2G charger is presented in this paper, to suppress harmonic pollution to power grid and realize bi-directional flow of electric energy. The charger consists of three-phase voltage source PWM grid-side converter and bi-directional dc/dc converter. A double closed-loop for three-phase voltage source PWM grid-side converter is designed firstly. And the outer voltage loop is used to keep dc bus voltage constant. The expected switching voltage components are achieved by substituting predicted instantaneous power with its instruction value, to form the inner dead-beat instantaneous power loop. Then a voltage–current double closed-loop for bi-directional dc/dc converter is proposed to implement two-stage charging, combining with constant-current charging and constant-voltage charging. Finally, the effectiveness and feasibility of the proposed control strategy are verified on the power-hardware-in-loop-simulation platform.