单相变压器直流偏磁试验与仿真

6台单相变压器组成三相组式变压器，分别对变压器开路、低功率三相四线制对称负载、低功率三相四线制不对称负载、低功率三相三线制不对称负载和额定功率下三相四线制对称负载5种情况下的变压器一次绕组电流、一次和二次绕组电压就不同直流入侵情况下进行了测量。研究变压器以上5种工作状态下，直流入侵导致的励磁电流和一次绕组电流畸变和谐波分布、一次绕组电流峰值和各次谐波与直流电流的关系曲线、不同运行方式对直流偏磁的影响。试验发现，变压器直流偏磁将导致空载电流畸变，空载电流峰值及各次谐波随直流电流的增加直线上升。谐波阶次越低，增加速度越快。变压器负载情况下，功率越大，奇次谐波随直流的变化增加速度越慢，偶次谐波增长速度基本不受负载的影响。利用非线性磁滞回线与励磁电流波形的关系，对试验模型进行了仿真计算，同时得到了变压器铁心无直流偏磁和有直流偏磁下的磁滞回线。计算模型建立在Jiles-Atherton理论基础上，计算与试验吻合较好，验证了变压器非线性模型的有效性和正确性。     

A new three-phase harmonic-free rectification scheme based on zero-sequence current injection

This paper proposes a new three-phase harmonic-free rectification scheme based on a zero-sequence injection technique. The proposed scheme incorporates two half-bridge inverters and two single-phase transformers to actively and individually shape the positive and negative dc rail currents of the rectifier. The shaped zero-sequence harmonic currents are then circulated through the ac side of the rectifier via a zigzag transformer, resulting in pure sinusoidal input currents in the three-phase diode rectifier. A design example along with component ratings calculation are detailed. Experimental results on a 1.5-kVA prototype are provided to validate the proposed technique.     

三相多驱动系统带移相电流控制的谐波消除方法

目前,许多三相可调速驱动器仍采用二极管整流器(DR)或半可控硅整流器(SCR)作为前端整流。但是,这种AC-DC整流会带来大量谐波电流,使电网电流畸变。现有的谐波消除策略因为总成本高和复杂性难以运用到多驱动系统中。因此,提出了移相电流控制和直流调制相结合的新方法来减轻多驱动系统中谐波电流。并且分析了并联驱动器之间不相等负载的影响,运用负载自适应方案确保各个整流器从电网汲取的电流幅值相等,从而增强了移相电流控制谐波消除效果。仿真和实验结果证明了该方案在三相多驱动系统中谐波消除的有效性。     

Novel schemes used for estimation of power system harmonics and their elimination in a three-phase distribution system

Estimation of power system harmonics and their elimination is an interdisciplinary area of interest for many researchers. This paper presents Variable Step Size Least Mean Square (VSS-LMS) approach for harmonics estimation and Shunt Active Power Filter (SAPF) with two-level Hysteresis Current Control (HCC) technique for their elimination in a three-phase distribution system. In the estimation process, the weight is updated using VSS-LMS algorithm. Harmonics components are estimated from the updated weights. In order to mitigate harmonics produced by the nonlinear load connected in a three-phase distribution system, SAPF with two-level HCC is proposed. A three-phase insulated gate bipolar transistor (IGBT) based current controlled voltage source inverter (CC VSI) with a dc bus capacitor is used as an active power filter. The first step is to calculate SAPF reference currents from the sensed nonlinear load currents by applying the synchronous detection method and then the reference currents are fed to the proposed controller for generation of switching signals. The nonlinear load consists of one three-phase and one single-phase diode rectifier feeding R–L load, so that the effectiveness of the two-level HCC scheme to compensate for unbalanced nonlinear load can be tested. Various simulation results are presented to verify the good behavior of the SAPF with proposed two levels HCC.     

Harmonics reduction of three-phase boost rectifier by modulating duty ratio

Line currents of three-phase diode bridge rectifier suffer from high THD. This THD is higher than IEEE standards. Injection of six harmonic components from the DC bus back to the duty ratio of the boost converter connected to the three-phase diode bridge rectifier reduces the THD of the utility line current. In this study a sine wave and a triangle wave with six times the fundamental frequency is presented. The analysis has been carried out to reduce THD of the utility line current of three-phase boost rectifier. The THD of the utility line currents depends on the amplitude and phase-shift of the six-order component injected to the duty ratio of the boost converter. A complete mathematical and simulation analysis of the injection technique has been carried out. The mathematical analysis and simulation results of 6 kW prototype system show the superiority of the proposed injection technique.     

Improvement of a double-connected diode rectifier with a smoothing capacitor by the use of an interphase transformer

A novel method to reduce harmonics involved in ac line currents of a diode rectifier circuit with a smoothing capacitor inside the dc output is proposed. This circuit has only passive rectifier devices. It is applied to the conventional double-connected diode rectifier, 12-phase rectifier, and is added to an auxiliary circuit comprising an interphase transformer with a secondary winding and a single-phase diode rectifier. Input voltage waveforms of the proposed circuit are quasi 24-step voltages that correspond to those of the conventional 24-phase rectifier. In this paper, circuit performances of the proposed method are explored by analyzing the input voltage waveforms. The effects on harmonic reductions can be examined by theoretical and experimental results; the ratings of system components are investigated. © 1997 Scripta Technica, Inc. Electr Eng Jpn. 118 (4): 46–57, 1997     

A Novel Stand-Alone Induction Generator System for AC and DC Power Applications

This paper presents a novel dynamic model in the stationary reference frame for a diode bridge rectifier with a dc-link filter directly connected to an induction generator (IG). Moreover, a hybrid excitation system consisting of a capacitor bank with a small-scale active power filter (APF) regulates the stand-alone IG systems output voltages by controlling its reactive current component and cancels the harmonic currents generated by a nonlinear diode rectifier load. A deadbeat current control strategy for the small-scale APF is used to enhance the operating performances of the stand-alone IG. This power generating system can be used for small hydro and wind energy applications, where its generated electrical power is supplied to different load types, i.e., dc and ac loads. The measurement results validate the proposed power generating system with the deadbeat current controller as a good application for the IG to reduce the total system cost and the required number of sensors.     

基于平衡变压器的三相-单相对称供电条件研究及参数匹配

提出一种在平衡变压器三相变两相的基础上,根据移相原理匹配以无功补偿元件,实现三相系统对单相负荷平衡供电的三相-单相对称供电方式。该供电方式利用平衡变压器输出两相电压的正交特性,从而简化了匹配元件的计算;讨论了其三相-单相变换的拓扑结构,针对纯阻性、阻感性、阻容性3种不同性质的负荷提出了3种不同的接线方式及其匹配元件参数的计算公式;并对由匹配元件参数的误差和负荷在一定范围内变化所引起的三相侧电流负序分量增大情况进行了分析,给出了抑制此负序电流的一种解决办法;最后在一台2 kVA的平衡变压器样机上验证了所推计算方法的正确性和实用性。     

Compensation characteristics of shunt active and series active power filters

Active power filters have been used in practice to suppress the harmonic interference in power systems. To compensate for harmonic currents of loads, active power filters usually are connected to power systems in parallel with the loads. These filters, which are called shunt active filters here, are very effective for loads that can be considered as current sources, such as thyristor rectifiers with large dc reactances. Many papers have covered the shunt active filters applied to these current-source loads, however, none has discussed characteristics of the shunt active filters when they are applied to voltage-source loads. On the other hand, since more and more diode rectifiers with capacitive dc filters have recently been used, harmonics generated by them have become an issue. The diode rectifier with capacitive dc filters behaves as a voltage source rather than a current source. When a shunt active filter is applied to such a diode rectifier, the current injected from the shunt active filter may flow into the diode rectifier. As a result, harmonics of the source current cannot be reduced effectively, and harmonic current flowing into the diode rectifier increases greatly. This paper presents the aforementioned problem of shunt active filters analytically and experimentally. Then a series active filter is proposed to suppress the harmonic current of the diode rectifiers. The features, operating conditions, and considerations of shunt active filters and series active filters are described analytically and demonstrated experimentally. Taking a diode rectifier with capacitive dc filter as a typical voltage-source load, compensation characteristics of shunt active filters and series active filters are discussed by experiment and simulation. The validity of the series active filters is illustrated experimentally.     

Closed-form analysis of adjustable-speed drive performance under input-voltage unbalance and sag conditions

Voltage unbalance or sag conditions generated by the line excitation can cause the input rectifier stage of an adjustable-speed drive (ASD) to enter single-phase rectifier operation. This degradation of the input power quality can have a significant negative impact on the induction-machine performance characteristics. This paper provides an approximate closed-form analysis of the impact of line-voltage sags and unbalance on the induction-machine phase voltages, currents, and torque pulsations for a general-purpose ASD consisting of a three-phase diode bridge rectifier, a dc link, and a pulsewidth modulation (PWM) inverter delivering constant volts-per-hertz excitation. Attention is focused on the impact of the dominant second harmonic of the line frequency, which appears in the dc link voltage during the sag/unbalance conditions, neglecting the impact of the other higher order harmonics. In addition to the closed-form analytical results that assume constant rotor speed, both simulation and experimental results are presented, which confirm the key analytical results, including the dominance of the second harmonic in the resulting torque pulsations. The analytical results can be used as a valuable design tool to rapidly evaluate the approximate impact of unbalance/sag conditions on ASD machine performance.     

Single-phase full-wave rectifier study with experimental measurements

This paper studies experimentally the harmonic absorbed currents by single-phase rectifiers under sinusoidal supply voltage conditions. Laboratory tests have been developed with a rectifier supplied with non-distorted voltages to characterize its absorbed harmonic currents (magnitude and phase angle). From these results, a database that characterizes rectifier harmonic behavior experimentally and allows the rectifier currents to be easily calculated is proposed. The database applicability is experimentally studied in three laboratory tests.     

基于诺顿等效电路的多谐波源责任划分研究

明确区分多个非线性负载对电网公共连接点(PCC)处的谐波贡献是实现多谐波源责任划分的重要问题。首先采用波动量法将单相不控整流谐波源等效为诺顿等效模型,并通过谐波电流源值与单相不控整流电路的各次谐波电流值近似相等证明了诺顿等效模型的有效性。然后搭建了含有三个谐波源的等效电路,并等效出各自的诺顿等效模型。最后利用等效阻抗计算出各非线性负载各次谐波电压值,利用电压表示的谐波责任指标因子进行多谐波源电压责任分摊。文中方法简单明了且有效,有利于实际配电网含多谐波源责任划分的工程应用。     

Harmonic analysis of a single-phase rectifier with a DC filter capacitor

A single-phase diode bridge rectifier with a filter capacitor on the dc side is often employed to convert ac input into a dc voltage. The input current of the rectifier contains harmonic currents which cause undesirable power line effects. Recently, a method using the time domain analysis has been proposed to calculate the harmonic currents of rectifier considering noninfinite capacitance, i.e., non-zero dc side impedance. This method is very accurate, but it requires a long computing time and a complicated algorithm. This paper proposes a new method that makes it possible to easily calculate the harmonic currents taking into account the effects of the ac and dc side impedances of rectifier. The proposed method, which is based on the frequency domain method, can be executed only with the algebraic computation, and its accuracy is quite high. The validity of the proposed method is also demonstrated by comparison with the results of time simulation.     

一种使用无源谐波抑制方法的多脉波整流器

提出一种使用直流侧无源谐波抑制方法的多脉波整流器及其设计方法,该方法采用一组单相整流桥产生环流,调节多脉波整流器的输入线电流。单相整流桥的输入端与平衡电抗器的二次侧相连,其输出端与负载相连,将吸收的谐波能量回馈给负载。分析单相整流桥的几种运行模态,设计平衡电抗器的最佳匝比。所提出的多脉波整流器是一种24脉波整流器,其输入线电流的THD理论上约为7.6%。仿真和实验结果表明,该多脉波整流器输入线电流的THD低于5%。此外,该多脉波整流器具有易于实现且谐波抑制成本低的优点。     

Neural network-based voltage regulator for an isolated asynchronous generator supplying three-phase four-wire loads

This paper deals with a neural network-based solid state voltage controller for an isolated asynchronous generator (IAG) driven by constant speed prime mover like diesel engine, bio-gas or gasoline engine and supplying three-phase four-wire loads. The proposed control scheme uses an indirect current control and a fast adaptive linear element (adaline) based neural network reference current extractor, which extracts the real positive sequence current component without any phase shift. The neutral current of the source is also compensated by using three single-phase bridge configuration of IGBT (insulated gate bipolar junction transistor) based voltage source converter (VSC) along-with single-phase transformer having self-supported dc bus. The proposed controller provides the functions as a voltage regulator, a harmonic eliminator, a neutral current compensator, and a load balancer. The proposed isolated electrical system with its controller is modeled and simulated in MATLAB along with Simulink and PSB (Power System Block set) toolboxes. The simulated results are presented to demonstrate the capability of an isolated asynchronous generating system driven by a constant speed prime mover for feeding three-phase four-wire loads.     

直流侧采用C滤波的改进型低谐波输入三相整流器

针对传统三相不控整流器接容性负载时网侧电流严重畸变,提出了一种直流侧采用C滤波的改进型低谐波输入三相整流器,该整流器由交流侧电感、三相不控整流桥、整流桥上桥臂并联电容以及直流侧滤波电容组成.分析了该整流器的工作原理,和原有的直流侧采用LC滤波的整流器进行了比较,并针对这两种整流器分别给出了其参数设计.实验结果表明通过增加交流侧辅助电感和电容的办法,可有效改善网侧电流波形;且直流侧采用C滤波的电路结构更加简洁、辅助元件取值较低,因而具有较优的综合性能,提高了该类型整流器的应用潜力.     

单相分布式发电接入台区电网的谐波补偿控制

非线性负荷不断增加导致台区电网的谐波问题愈加严峻,而越来越多分布式发电(DG)接入在实现新能源就地并网的同时,也为台区电网的谐波治理提供了一个有效途径。以含单相DG的台区电网为对象,考虑三相不对称负荷的接线方式差异,提出了一种利用单相DG的分相谐波补偿控制策略。首先对含单相DG的台区电网接线方式和典型台区谐波源负荷情况进行分析,将不对称负荷等值成星形有中性线、星形无中性线和三角形接线方式,分别讨论了台区电网的谐波电流分布,进而推导了单台和两台单相DG接入时的谐波电流补偿函数。据此,采用基于延时环节和同步坐标变换的谐波电流检测方法,提取各相负荷电流和线路电流的谐波分量,建立不同接线方式下单相DG的谐波补偿控制方案。最后,利用Matlab/Simulink验证了所提的分相谐波补偿控制对台区电网谐波的治理效果。     

一种新型的中性线电流补偿器

针对目前非线性负荷造成低压配电网中性线过负荷的问题,提出一种新型的中性线电流补偿器.该新型补偿器将逆变器接入曲折变压器中性点和中线之间,逆变器输出的电流通过曲折变压器提供的低阻抗的零序电流通道注入系统,进行中性线电流的补偿.曲折变压器的二次侧输出利用全控整流桥整流,为逆变器提供直流电源,实现了能量的双向流动,有效地解决了采用不控整流模块时,在补偿器吸收有功功率的情况下所造成的直流电压升高,易造成开关器件过压损坏的问题.通过建立模型对提出的补偿器进行仿真研究,结果表明该新型中性线电流补偿器能够有效地补偿中性线中的谐波及不平衡电流,逆变器的直流侧电压保持稳定.     

一种改进的无功及谐波电流检测方法

有源电力滤波器的工作性能很大程度上取决于是否能够准确、实时地检测无功及谐波电流。针对单相系统瞬时无功与谐波检测算法复杂的问题,提出了一种改进的检测方法。不需要锁相环(PLL)和复杂的硬件电路,利用正交波形特性检测出电压相位,不仅减少了计算量,并且可根据需求分别检测出总谐波电流或特定次谐波的大小。该算法还能直接推广应用于三相三线制、三相四线制系统无功及谐波电流检测。     

检测单相系统谐波电流和无功电流的一种新方法

针对单相系统谐波电流和无功电流检测中存在的问题,提出一种新的检测方法:基于瞬时无功功率的虚拟三相法.通过理论分析和仿真实验论证,表明这种算法能有效解决单相系统和三相不平衡系统中谐波电流和无功电流检测中存在的问题,且检测精度高,动态响应快.