A robust direct current control of DFIG wind turbine with low current THD based predictive approach

This paper presents a simple and robust direct current control based predictive approach for rotor side converter (RSC) of the doubly fed induction generator (DFIG), which operates at a constant switching frequency and has a fast dynamic response. First, sector of required rotor voltage vector is predicted in this strategy, and according to this predicted sector, two active vectors and two zero vectors are elected in each switching period. Derivatives of rotor current in the synchronous frame are determined for each predicted voltage vector in every period. These derivatives are used to compute the duration of the vectors in such a way that the current error at the end of the switching period gets minimized. The accuracy of the proposed control strategy under variation of rotor speed is evaluated in Matlab/Simulink environment for a 2 MW DFIG. Moreover, the impact of parameter variations on the system is examined for this suggested technique. Furthermore, the dynamic response and stator current total harmonic distortion (THD) of proposed strategy is compared with traditional vector control (VC), direct power control (DPC) and predictive direct power control (PDPC) methods. Finally, the performance of the proposed method is evaluated under disturbance voltage. The results demonstrate that suggested control technique has the lowest stator current THD and operates perfectly near the synchronous speed and under grid voltage dip. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

SMC-DPC based active and reactive power control of grid-tied three phase inverter for PV systems

In this paper, sliding mode control (SMC) – direct power controller (DPC) based active and reactive power controller for three-phase grid-tied photovoltaic (PV) system is proposed. The proposed system consists of two main controllers: the DC/DC boost converter to track the possible maximum power from the PV panels and the grid-tied three-phase inverter. The Perturb and Observe (P&O) algorithm is used to transfer the maximum power from the PV panels. Control of the active and reactive powers is performed using the SMC-DPC strategy without any rotating coordinate transformations or phase angle tracking of the grid voltage. In addition, extra current control cycles are not used to simplify the system design and to increase transient performance. The fixed switching frequency is obtained by using space vector modulation (SVM). The proposed system provides very good results both in transient and steady states with the simple algorithms of P&O and SMC-DPC methods. Moreover, the results are evaluated by comparing the SMC-DPC method developed for MPPT and the traditional PI control method. The proposed controller method is achieved with TMS320F28335 DSP processor and the system is experimentally tested for 12 kW PV generation systems.     

Performance investigation of artificial intelligence based controller for three phase four leg shunt active filter

In this paper, the choice of power quality compensator is a DSTATCOM which constitutes a three phase four leg voltage source converter (VSC) with a DC capacitor. The control strategy proposed for the DSTATCOM is a neural network based one cycle control (OCC). This control strategy involves neural network block, digital circuits and linear elements, which eliminates the sensors required for sensing the load current and coupling inductor current in addition to the multiplier employed in the conventional method. The calculation of harmonic and reactive currents for the reference current generation is also eliminated, thus minimizing the complexity in the control strategy. The control strategy mitigates harmonic/reactive currents, ensures balanced and sinusoidal source current from the supply mains that are nearly in phase with the supply voltage, compensates neutral current, and maintains voltage across the capacitor under unbalanced source and load conditions. The performance of the DSTATCOM with the proposed artificial neural network (ANN) controllers is validated and investigated through simulations using Matlab software. The simulation results prove the efficacy of the proposed neural network based control strategy under varying source and load conditions.     

Direct Power Control of DFIG With Constant Switching Frequency and Improved Transient Performance

This paper proposes a new direct power control (DPC) strategy for a doubly fed induction generator (DFIG)-based wind turbine system. The required rotor control voltage, which eliminates active and reactive power errors within each fixed time period, is directly calculated based on stator flux, rotor position, and active and reactive powers and their corresponding errors. No extra power or current control loops are required, simplifying the system design, and improving transient performance. Constant converter switching frequency is achieved that eases the design of the power converter and the ac harmonic filter. Rotor voltage limit during transients is investigated, and a scheme is proposed that prioritizes the active and reactive power control such that one remains fully controlled while the error of the other is reduced. The impact of machine parameter variations on system performance is investigated and found negligible. Simulation results for a 2 MW DFIG system demonstrate the effectiveness and robustness of the proposed control strategy during variations of active and reactive power, machine parameters, and wind speed     

Power quality improvement of solar photovoltaic transformer-less grid-connected system with maximum power point tracking control

This paper presents a transformer-less single-stage grid-connected solar photovoltaic (PV) system with active reactive power control. In the absence of active input power, grid-tied voltage source converter (VSC) is operated in the reactive power generation mode, which powers control circuitry and maintains regulated DC voltage. Control scheme has been implemented so that the grid-connected converter continuously serves local load. A data-based maximum power point tracking (MPPT) has been implemented at maximum power which performs power quality control by reducing total harmonic distortion (THD) in grid-injected current under varying environmental conditions. Standards (IEEE-519/1547) stipulates that current with THD greater than 5% cannot be injected into the grid by any distributed generation (DG) source. MPPT tracks actual variable DC link voltage while deriving maximum power from PV array and maintains DC link voltage constant by changing the converter modulation index. Simulation results with the PV model and MPPT technique validations demonstrate effectiveness of the proposed system.     

Direct power control of wind‐turbine‐driven DFIG during transient grid voltage unbalance

In this paper, a direct power control (DPC) of a wind‐turbine‐driven doubly fed induction generators (DFIGs) under unbalanced network voltage conditions is studied. Variations of the stator output active, reactive and electromagnetic powers are fully deduced in the presence of negative sequence supply voltage. The rotor side converter is controlled on the basis of DPC to eliminate the electromagnetic torque oscillations at double supply frequency under unbalanced stator supply. The rotor voltage references estimation requires only simple calculations without any integral operation. The proposed control scheme removes rotor current regulators and the decomposition processing of positive and negative sequence rotor currents. The performance of the proposed and conventional DPC schemes is compared under the same operating conditions. Simulation results using Matlab/Simulink are carried out for a 1.5 MW DFIG wind generation system to show the validity of the proposed scheme during unbalanced voltage supply. Copyright © 2013 John Wiley & Sons, Ltd.     

内置式永磁同步电机12扇区直接转矩控制方法

以电压矢量的最大利用率为基础引入12扇区控制方法,该方法细化矢量选择和扇区划分,增加可供选择的电压矢量数目,有效发挥了电压矢量对磁链和转矩的控制优势;研究变化的内滞环带对转矩脉动的影响,仿真结果表明相对于传统的6扇区直接转矩控制方式,该文引入的12扇区控制方法可明显减小转矩和磁链脉动,同时保留了传统的直接转矩控制中转矩动态响应迅速的特点;在不同操作条件(转速和转矩不同)下,进一步优化12扇区的控制效果。仿真结果验证了理论分析的正确性和所提出方法的可行性。     

不平衡电网电压下T型三电平光伏并网逆变器的有限集模型预测控制

为实现电网电压不平衡时对T型三电平光伏并网系统输出功率和电流质量的控制,以达到入网功率平稳或电流正弦为控制目标,结合光伏阵列输出功率前馈,在两相静止坐标系下提出一种直流母线电压外环PI控制、并网电流内环有限集模型预测控制的控制策略,并在电压外环中引入2倍频陷波器以获得平滑的入网功率参考值。仿真结果表明:当电网电压不对称时,采用所提控制策略能够实现对入网有功、无功功率2倍频脉动及负序电流的分别抑制或协调控制,且并网电流谐波畸变小、入网电能质量高,同时实现T型三电平逆变器的中点电位平衡。     

Power management in fuel cell based hybrid systems

This paper presents modeling, design and analysis of a Grid-connected Hybrid Photovoltaic Fuel Cell System (HPVFCS) with a reactive power compensation feature. A hydrogen based fuel cell is a proven technology and its use along with the photovoltaic system (PV) can lead to energy stability in grid-connected or standalone systems. In this paper, the Voltage Source Converter (VSC) is connected between the DC output of HPVFCS and an AC grid. The control strategy employed guarantees the maximum utilization of the PV array and the optimum use of an FC. The active and reactive power of VSC can be controlled independently using P-Q control theory. The additional function of the reactive power compensation using P-Q control theory can enhance the performance of the distribution systems where HPVFCS system is connected. Its applicability is verified by the test bench created with MATLAB/Simulink^®     

Accurate power sharing among multi-DG resources in a microgrid

In this paper, a sliding mode (SM)-based direct active and reactive power control for the distributed generations (DGs) in microgrid is presented. The grid-connected microgrid contains two three-phase DGs that are photovoltaic (PV) units, and three single-phase DGs consisting of fuel cell (FC), PV, and battery. In the proposed strategy, controlling of the active and reactive powers is carried out for the single-phase and three-phase DGs without any phase angle tracking of the network voltage or synchronization transformations. The proposed robust control strategy improves power sharing and regulates power components injected by the DGs, and it is tested under balanced and unbalanced loads.     

Direct active and reactive power control of DFIG for wind energy generation

This paper presents a new direct power control (DPC) strategy for a doubly fed induction generator (DFIG)-based wind energy generation system. The strategy is based on the direct control of stator active and reactive power by selecting appropriate voltage vectors on the rotor side. It is found that the initial rotor flux has no impact on the changes of the stator active and reactive power. The proposed method only utilizes the estimated stator flux so as to remove the difficulties associated with rotor flux estimation. The principles of this method are described in detail in this paper. The only machine parameter required by the proposed DPC method is the stator resistance whose impact on the system performance is found to be negligible. Simulation results on a 2 MW DFIG system are provided to demonstrate the effectiveness and robustness of the proposed control strategy during variations of active and reactive power, rotor speed, machine parameters, and converter dc link voltage.     

基于PR控制和谐波补偿的三相光伏并网系统

将电网电压定向矢量控制与比例谐振(PR)控制相结合的控制策略应用于三相光伏并网系统中,实现逆变器的并网控制。该控制策略比传统PI调节器的双闭环电网电压定向控制更简便,不需要复杂的坐标旋转变换和前馈解耦控制。同时,PR控制器可以很方便地实现并网电流低次谐波的补偿。仿真结果验证了该系统的结构和控制策略的有效性,可提高电网的电能质量。     

Fault-tolerant control of an open-winding brushless doubly-fed wind power generator system with dual three-level converter

To improve the fault redundancy capability for the high reliability requirement of a brushless doubly-fed generation system applied to large offshore wind farms, the control winding of a brushless doubly-fed reluctance generator is designed as an open-winding structure. Consequently, the two ends of the control winding are connected via dual three-phase converters for the emerging open-winding structure. Therefore, a novel fault-tolerant control strategy based on the direct power control scheme is brought to focus in this paper. Based on the direct power control (DPC) strategy, the post-fault voltage vector selection method is explained in detail according to the fault types of the dual converters. The fault-tolerant control strategy proposed enables the open-winding brushless doubly-fed reluctance generator (BDFRG) system to operate normally in one, two, or three switches fault of the converter, simultaneously achieving power tracking control. The presented results verify the feasibility and validity of the scheme proposed.     

Closed loop torque SVM-DTC based on robust super twisting speed controller for induction motor drive with efficiency optimization

This paper presents a space vector modulation (SVM) based Direct Torque Control strategy (DTC) for induction motor (IM) in order to overcome the drawbacks of the classical DTC. SVM can reduce the high torque and flux ripples by preserving a fixed switching frequency. This technique is known by the closed loop torque SVM-DTC. Moreover, the control scheme performance is improved by inserting a second order sliding mode super twisting controller in the outer loop for speed regulation. This nonlinear technique ensures a good dynamic and high robustness against external disturbance. Furthermore, the IM energy optimization is treated in the second objective of this paper. A proposed model based loss minimization strategy is presented for efficiency optimization. This strategy chooses an optimal flux magnitude for each applied load torque. The proposed optimized SVM-DTC algorithm will be investigated by simulation and real time implementation using Matlab/Simulink with real time interface based on dSpace 1104 signal card.     

Artificial intelligence based active and reactive power control method for single-phase grid connected hydrogen fuel cell systems

In grid-connected power generation systems, power factor fluctuations caused by non-linear power circuits used between the grid and source should be controlled with the help of voltage source inverters (VSI). In addition, in order to improve the quality of the electrical energy injected into the grid and to prevent possible electrical faults, the total harmonic distortion (THD) in the grid current should be controlled in accordance with IEEE-519 standards. In other respects, uncontrolled injected energy may causes losses, excessive electrical energy demand and overloading. In this study, an artificial neural network based active and reactive power control method is proposed for grid-connected single-phase Proton Exchange Membrane Hydrogen Fuel Cell (PEMFC). The aim of the proposed control structure is to have low harmonic distortion, high power factor performance as well as an easy and understandable structure. The proposed method was applied to a 6 kW prototype. Five different scenarios and nine different activation function were tried to verify the performance of the proposed control method. As a result of these processes, the power factor was measured as unity (>0.99) and the total harmonic distortion (THD) of the grid current, under all operation states, is <2%. In addition, it has been observed that the success rate of artificial neural networks is >97%.     

基于虚拟阻抗的储能微网VSG控制策略研究

目的  为了使储能微网并网变流器（Power Conversion System, PCS）具备同步发电机一样的旋转、励磁特性,提出一种在储能微网并网PCS控制系统中采用虚拟同步机（Virtual Synchronous Generation, VSG）控制策略。 方法  针对传统VSG不具备低电压穿越（Low Voltage Ride Through, LVRT）能力,文章构建VSG虚拟阻抗无功控制环节,实时修正电压控制输入信号,提高系统暂态压降支撑能力;为了得到虚拟阻抗的幅值及阻感比,将LVRT工况分为线路对称和不对称短路障,研究正、负序电压、电流,得出虚拟阻抗参数。 结果  为了验证所提控制策略的性能,通过Matlab/Simulink进行仿真,设置仿真工况为：储能微网外接输电线路分别发生三相接地和单相接地故障。 结论  仿真结果表明：将虚拟阻抗引入VSG控制系统中,可以提高短路情况下储能微网并网PCS输出电压,起到LVRT期间电压暂态支撑作用,使储能微网具备LVRT能力,可为实际应用提供指导。     

直驱型风力发电变流器低压穿越控制策略研究

研究了直驱型风力发电变流器系统低压穿越控制策略。首先提出了一种对三相电量进行快速准确的正负序分离软件锁相环。在此基础上,为消除直流电压的二次谐波,采用正、负序双电流内环控制不对称运行控制策略。正负序分离软件锁相环采用了正负序级联延时信号消除法,能够实现对三相电压电流基波正负序分量在同步旋转坐标下的快速提取,并且通过选择不同的参数,可以滤除任何次数谐波的干扰。该方法无需采用滤波器,从而同时具备了稳态精确性和动态快速性。现场实验结果表明,该软件锁相环为三相并网型风力发电变流器在电网发生跌落及谐波畸变时提供了良好运行控制提供保障,正负序双电流内环不对称运行的控制策略保证了在电网电压不对称跌落时的正负序分离控制,消除了直流电压的二次谐波。     

Comparative study of various artificial intelligence approaches applied to direct torque control of induction motor drives

In this paper, three intelligent approaches were proposed, applied to direct torque control (DTC) of induction motor drive to replace conventional hysteresis comparators and selection table, namely fuzzy logic, artificial neural network and adaptive neuro-fuzzy inference system (ANFIS). The simulated results obtained demonstrate the feasibility of the adaptive network-based fuzzy inference system based direct torque control (ANFIS-DTC). Compared with the classical direct torque control, fuzzy logic based direct torque control (FL-DTC), and neural networks based direct torque control (NN-DTC), the proposed ANFIS-based scheme optimizes the electromagnetic torque and stator flux ripples, and incurs much shorter execution times and hence the errors caused by control time delays are minimized. The validity of the proposed methods is confirmed by simulation results.     

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.