Buck型动态电容器无功和负序电流的综合补偿

不平衡感性负载产生的无功和负序电流不仅会增加线路损耗,而且会引起电网中以负序分量启动的继电保护装置误动作,进而威胁电力系统的安全运行,因此补偿无功和负序电流尤为重要。针对三相三线负载不平衡系统,首先研究了Buck型动态电容器(D-CAP)进行功率因数校正和不平衡抑制之间的关系。然后分析了D-CAP补偿无功和负序电流的原理并确定了其补偿负序电流的能力。最后针对D-CAP提出了补偿无功和负序电流的优化方法和控制策略,在负载负序电流超过其补偿范围时,该优化方法和控制策略可以实现负序电流的最优补偿。仿真结果验证了理论分析的正确性和控制策略的有效性。     

基于对称分量法的电弧炉无功补偿技术

电弧炉具有低电压、大电流而且负荷非线性的特点,在其运行过程中产生大量的负序电流,导致电网不稳定。通过对称分量法提取出负序分量,进而应用单片机控制投切补偿电纳,进行无功功率补偿,从而有效地抑制了负序电流,使电网稳定运行。计算机仿真表明,提出的无功补偿技术可以有效地补偿负序电流,使电弧炉运行在三相对称状态。     

含不平衡负载的微电网中三相微源逆变器的VSG控制策略

在离网模式下,微电网中虚拟同步发电机的输出电压易受不平衡负载影响。针对此问题,文章基于一阶全通滤波器(All-Pass Filter,APF)的电压电流正、负序分离方法,利用正序功率和正序电流建立了改进VSG控制模型,改善了VSG输出电压参考。采用比例积分(Proportional Integral,PI)+准比例谐振(Quasi Proportional Resonant,QPR)电压调节器对VSG输出负序电压分量进行控制,论证了PI+QPR调节器抑制负序电压分量的优良性能。最后,仿真结果验证了该控制策略的有效性,该方法有效地改善了三相微源逆变器输出电压的对称性。     

基于正弦幅值积分器的全电流谐波检测方法

基于瞬时无功功率理论的i_p-i_q谐波检测法,环节繁多且结构复杂,同时低通滤波器(low pass filter,LPF)的存在又影响了谐波检测的快速性。为了准确且快速检测出电网谐波电流,实现对电网的补偿,该文提出一种基于正弦幅值积分器(sinusoidal amplitude integrator,SAI)的全电流谐波检测法,该方法在i_p-i_q谐波检测法的基础上省去负载电流有功分量和无功分量的计算环节,利用SAI的正序基波提取结构替换原来检测法中的LPF,提取出正序基波电流经过运算得到谐波指令电流,从而对电网进行补偿。在平衡电网下对所提方法进行仿真与实验,验证了该方法的可行性和有效性。     

电网正、负序电压基波分量的提取算法在并网型逆变器控制中的应用

提出一种在电网三相电压不平衡的情况下,通过三角函数运算的方法解耦不平衡电网基波正、负序电压分量。由于电网负序电压分量送入锁相环后会造成锁相环的震荡,导致锁相环输出误差大无法精确锁相。根据正序分量与负序分量之间的相角关系,推导出提取正序分量的公式,把该公式进行数字化处理,将其应用于dq数字锁相环,提高了dq锁相环在电网三相电压不平衡条件下的有效性和准确性。提出了网压前馈对逆变器控制的重要性,分析了利用正序电压瞬时值进行前馈控制的优点。并通过实验验证了公式的有效性和控制的稳定性、可靠性。     

针对不平衡电网的谐波检测新方法

为解决不平衡电网中电压相位偏移及电流中存在不平衡分量对谐波检测造成的影响,利用二阶广义积分原理设计了一种适用于电网电压不平衡且可抗直流分量干扰的锁相环;其次,在传统d-q变换谐波检测法的基础上做出改进,利用对称分量法提取负序基波电流,经Park变换得到负序基波电流在正序坐标系下的交流分量,从而消除传统谐波检测方法在三相不平衡工况下对正序基波电流检测时的误差。经仿真验证分析,该方法能有效检测出不平衡电网中的谐波电流。     

电力系统谐波、负序与无功电流复合补偿策略的研究

基于瞬时无功功率理论,提出了一种适用于三相三线制和三相四线制电力系统谐波、负序以及无功电流的复合补偿策略。数值仿真结果表明,该复合补偿策略的投入使用,可使电力系统综合补偿装置运行在有源电力滤波器和静止无功发生器功能相结合的复合工作模式,从而实现具有不对称非线性负载的电力系统的谐波抑制、无功补偿以及负载不平衡抑制等综合控制任务。     

直驱风电机组低电压穿越改进控制方法仿真研究

采用改进的瞬时对称分量法对电网电压瞬时值进行对称分量分解,提出了电网电压不对称跌落时D-PMSG的低电压穿越控制策略,按照电网电压正序分量和额定电压的比值减小发电机功率,并在解耦控制中分别控制正序和负序分量,正序通道完成能量的传输,负序通道产生和电网负序电压相等的负序电压,从而保证网侧逆变器电流中无负序分量,避免了逆变器非全相过负荷,充分利用其容量。仿真结果研究表明,提出的改进控制策略实现了不对称故障下的低电压穿越,并且保持了逆变器三相电流对称。     

适用于弱电网中大功率整流器次同步振荡抑制的新型控制策略研究

在弱电网中，由于电网的阻抗特性较大，线路中阻抗参数改变了整流器控制的关键参数，影响了整流器的整体性能，可能会导致整流器输出不稳定，使输出呈现次同步振荡特性。为此，在不增加硬件补偿系统的基础上，提出了一种能够有效抑制次同步振荡的控制策略，该抑制策略从数学模型角度出发，通过低频滤波器提取出次同步振荡交流电流分量，进行d/q变换后，将该次同步振荡的d/q电流分量进行PI闭环控制，次同步振荡分量的PI调节器输出值进行解耦补偿后乘以补偿系数得到电压的d/q补偿分量，对原电流内环的输出进行补偿。仿真和试验结果表明，该抑制策略有效、可行，能够有效抑制次同步分量。     

基于电压补偿的定直流电压及无功控制的光伏系统

在光伏并网运行中,当电网电压不平衡时会导致电压、电流等的波动,从而影响电网电能质量及系统的稳定性。结合电压补偿控制等环节,提出了一种基于电压补偿控制的定直流电压、无功控制策略,将采集到的正、负序电压经电压补偿控制环节得到电压的不平衡系数,对电网电压的不平衡度进行补偿,并引入负序电流抑制环节。在PSCAD中分别设计搭建了改进前后的仿真模型,并对光伏并网发电系统的动态变化特性进行研究。仿真结果表明,在光照强度和温度阶跃扰动下,系统能够迅速做出响应并工作于最大功率点处,电压及电流波动幅度减小,电网电能质量得到明显改善,验证了MPPT算法及改进后控制策略的准确性及有效性。     

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.     

Compensation of network voltage unbalance using doubly fed induction generator-based wind farms

A control strategy for compensating AC network voltage unbalance using doubly fed induction generator (DFIG)-based wind farms is presented. A complete DFIG dynamic model containing both the rotor and grid side converters is used to accurately describe the average and ripple components of active/reactive power, electromagnetic torque and DC bus voltage, under unbalanced conditions. The principle of using DFIG systems to compensate grid voltage unbalance by injecting negative sequence current into the AC system is described. The injected negative sequence current can be provided by either the grid side or the rotor side converters. Various methods for coordinating these two converters are discussed and their respective impacts on power and torque oscillations are described. The validity of the proposed control strategy is demonstrated by simulations on a 30 MW DFIG-based wind farm using Matlab/Simulink during 2 and 4% voltage unbalances. The proposed compensation strategy can not only ensure reliable operation of the wind generators by restricting torque, DC link voltage and power oscillations, but also enable DFIG-based wind farms to contribute to rebalancing the connected network.     

Load sharing and power quality enhanced operation of a distributed microgrid

Control methods for proper load sharing between parallel converters connected in a microgrid and supplied by distributed generators (DGs) are described. It is assumed that the microgrid spans a large area and it supplies loads in both in grid-connected and islanded modes. A control strategy is proposed to improve power quality and proper load sharing in both islanded and grid-connected modes. It is assumed that each of the DGs has a local load connected to it which can be unbalanced and/or non-linear. The DGs compensate the effects of unbalance and non-linearity of the local loads. Common loads are also connected to the microgrid, which are supplied by the utility grid under normal conditions. However, during islanding, each of the DGs supplies its local load and shares the common load through droop characteristics. Both impedance and motor loads are considered to verify the system response. The efficacy of the controller has been validated through simulation for various operating conditions using PSCAD. It has been found through simulation that the total harmonic distortion (THD) of the microgrid voltage is ~10% and the negative and zero sequence components are ~20% of the positive sequence component before compensation. After compensation, the THD remains <0.5%, whereas negative and zero sequence components of the voltages remain <0.02% of the positive sequence component.     

PEM fuel cell supported distribution static compensator for power quality enhancement in three-phase four-wire distribution system

One of the major problems in electrical power system is the lack of quality of power due to the rapid growth of nonlinear load and unbalanced load utilization in three-phase four-wire distribution system. In this paper, PEM (Proton Exchange Membrane) fuel cell supported four-leg Distribution Static Compensator (DSTATCOM) is modelled to mitigate harmonics, neutral current and load balancing under nonlinear load and unbalanced load conditions in three-phase four-wire distribution system. The instantaneous reactive power (IRP) theory control algorithm is proposed for four-leg DSTATCOM. The Real coded Genetic Algorithm (RGA) optimized Proportional Integral (PI) controller and Adaptive Neuro Fuzzy Inference System (ANFIS) controller are used for regulating the DC link voltage of DSTATCOM. This paper also investigates the performance of ANFIS based DSTATCOM with conventional method. The proposed system is modelled and its performance is analyzed in MATLAB/SIMULINK.     

不平衡电压下双馈变速风电机组的控制策略

提出了不平衡电网电压下双馈发电机的控制策略,并建立了双馈发电机在正、反旋转坐标系下的数学模型。在此基础上推导和分析了电网电压不平衡条件下双馈发电机输出的瞬时有功、无功功率的组成。提出了4种可供选择的不平衡电压控制方案,并给出了不同控制目标下转子的正、负序电流目标值的计算原则。通过MATLAB/SIMULINK仿真验证了控制方案的有效性。     

Coordinated control for unbalanced operation of stand‐alone doubly fed induction generator

This paper proposes a coordinated control scheme of a stand‐alone doubly fed induction generator (DFIG)‐based wind energy conversion system to improve the operation performance under unbalanced load conditions. To provide excellent voltage profile for load, a direct stator flux control scheme based on auto‐disturbance rejection control (ADRC) is applied, and less current sensors are required. Due to the virtues of ADRC, the controller has good disturbance rejection capability and is robust to parameter variation. In the case of unbalanced loads, the electromagnetic torque pulsations at double synchronous frequency will exist. To eliminate the undesired effect, the stator‐side converter (SSC) is used to provide the negative sequence current components for the unbalanced load. Usually, proportional integral controllers in a synchronous reference frame are used to control SSC. To simplify the algorithm, an improved proportional resonant (PR) control is proposed and used in the current loop without involving positive and negative sequence decomposition. The improved PR provides more degree of freedom which could be used to improve the performance. The effectiveness of the proposed control scheme has been validated by the simulation and experimental results. Copyright © 2012 John Wiley & Sons, Ltd.     

基于双序前馈解耦的光伏逆变器并网控制策略

为了消除三相不平衡LCL型光伏并网逆变器在同步旋转坐标下耦合项对电流控制性能的影响,该文提出适用于逆变侧电流反馈的双序前馈解耦的方法。通过双旋转剔除耦合项中负序直流量,使用正序直流量进行前馈解耦,实现电网电压和变换耦合项对控制环路的零干扰。在PI控制的基础上加入重复控制,抑制死区效应,降低并网电流谐波含量。通过仿真和实验,验证所提控制策略的有效性。     

Three-phase four-wire system power redistribution using a powerelectronic converter

This letter presents a scheme to balance the active power taken by an unbalanced load in a three-phase four-wire systems. It is shown that a power electronic converter, which operates under a control strategy based on the generalized instantaneous power theory, can redistribute active power between phases. The power source, therefore, only supplies balanced power, the negative and zero sequence components generated by the unbalanced load are prevented from flowing into the power system