Power quality and harmonic distortion on distribution systems

With the increase of nonlinear loads on utility distribution systems, the voltage and current waveforms are becoming more distorted and the power quality is deteriorating. Since this is becoming a wide spread problem today, and new, more strict, distortion guidelines are under development, utility engineers are having to deal with analyzing and planning for the control of the distortion. This paper introduces some common harmonic analysis techniques and applies them to voltage waveforms recorded on a typical REA transmission and distribution system     

Power flow constrained harmonic analysis in AC-DC power systems

When calculating noncharacteristic harmonics generated by an HVDC converter, unbalance in the AC system at fundamental frequency and all harmonics must be taken into account. This entails that a full three-phase load flow be part of the solution, as there is considerable distortion interaction between the load flow and the converter. To date, this has been achieved by iterating sequentially between the load flow and a harmonic converter model, in a type of decoupled method which is likely to suffer poor convergence. The decoupled method is implemented here, and convergence compared with a unified Newton solution of the load flow and converter models. Finally, the effect of not modelling the distortion interaction between the two systems is investigated     

Power system harmonic resonance-a documented case

A case of power system harmonic resonance that resulted in failure of 13.8 kV switchgear and consequent plant losses is documented. Calculations of the power system natural frequencies are given for various power system configurations of supply reactance, motor reactance, and system shunt capacitance. Sources of harmonic current are identified, methods of measuring harmonic current and voltage levels throughout the system are described, and results of the power system harmonic current and voltage survey are reported. Also described is the corrective action taken to prevent a recurrence of harmonic resonance     

基于小波和短时傅里叶变换的电网谐波分析

为了测量电网中的波动谐波,将小波变换和短时傅里叶变换方法相结合用于电网谐波分析.通过小波变换设计出一组带通滤波器来分离出基波和各次谐波,并采用短时傅立叶变换计算出基波和各次谐波的幅值、频率和相位.仿真结果表明,当信号中存在高斯白噪声时该算法仍可准确检测出基波和2到63次谐波的幅值、频率和相位,且算法简单易于实现.     

基于小波和短时傅里叶变换的电网谐波分析

为了测量电网中的波动谐波,将小波变换和短时傅里叶变换方法相结合用于电网谐波分析。通过小波变换设计出一组带通滤波器来分离出基波和各次谐波,并采用短时傅立叶变换计算出基波和各次谐波的幅值、频率和相位。仿真结果表明,当信号中存在高斯白噪声时该算法仍可准确检测出基波和2到63次谐波的幅值、频率和相位,且算法简单易于实现。     

Coupling rectifier systems with harmonic cancelling reactors

This paper shows that double-diode rectifiers can generate 12-step voltages by using well-designed line-side reactors, although with relatively large self-inductances. The usage of three-level rectifiers can alleviate this disadvantage. The double three-level rectifier system proposed features the following advantages over conventional transformer-coupled systems: small in size, light in weight, and more economical; control capabilities of output DC voltage in a limited range; low current and voltage ratings required for additional circuits; and highly reliable operation due to 12-step operation capability without additional circuits     

基于RPROP神经网络的电力系统谐波分析

为了提高谐波分析的速度和精度,将RPROP( Resilient Propagation)神经网络应用于电力系统谐波分析.该网络利用加汉宁窗插值谐波分析算法获得其权值和阈值的初值,并在此基础上采用RPROP算法训练.与BP( BackPropagation)算法不同,该算法根据一阶偏导数的符号信息调整可变参数,避免了受对参教调整意义不大的一阶偏导数幅值信息的影响,且不存在参数选择问题,提高了谐波分析的收敛速度、精确度和实时性.通过变学习速率且加动量项的BP神经网络与RPROP神经网络的比较验证了分析结论的正确性.     

莱夫-文森特窗三谱线插值的电力谐波分析

为解决快速傅里叶变换(FFT)在电力谐波分析存在的问题,在分析莱夫-文森特窗和三谱线插值算法特点的基础上,提出了一种基于莱夫-文森特窗三谱线插值的谐波分析算法,用曲线拟合方法获得了修正公式,并基于这个修正公式对电力谐波进行了仿真。仿真结果表明,相同条件下,该算法在处理简单的谐波信号时的时间小于0.225 ms,幅值修正误差小于-1.4×10-6%,处理复杂信号时的时间小于0.997 ms,幅值修正误差小于-9.68×10-6%。该方法不仅具有较高的检测精度,而且具有较快的运算速度。     

多馈入直流输电系统谐波相互影响的研究

首先给出了交流系统各主要元件谐波频率下的数学模型,利用Ward多端等值法推导出多馈入直流输电系统各直流接入点在谐波频率下的节点导纳矩阵,进而可以得到各直流接入点的谐波电压。对上海电网500k V电压等级电网进行了等值并通过计算得到不同运行方式下各直流接入点的电压总谐波畸变率的范围,经分析可知上海地区多馈入直流输电系统谐波相互影响很小,各直流接入点电压的畸变主要受自身直流线路谐波的影响。     

Q'HARM-a harmonic power-flow program for small power systems

The salient features of a harmonic power-flow program called Q'HARM, are described. Two kinds of harmonic-producing components, the line-commutated six-pulse bridge rectifier and the gaseous discharge lighting load supplied from nonsinusoidal sources, are accurately modelled in this program. Case studies involving the use of Q'HARM for a small power system including converters and gaseous discharge lamps are described. The results include the effect of filters and the computed values of telephone influence indices. It is also shown that the maximum values of telephone influence indices for the different harmonics occur at different points on any transmission line in the system     

Measurement and analysis of harmonic distortion in power distribution systems

Power distribution systems often exhibit different harmonic behaviours at different locations. It is not usually possible to completely characterise the distortion at a particular site, however, due to insufficient data about the measured system and instrumentation inability to evaluate or register complete set of measurement quantities. In this paper existing power quality standards are analysed and new harmonic distortion indices are derived. A new index describing the total harmonic distortion in current measurements, TRD, is suggested in order to simplify and standardise harmonic measurements. Another index representing the increase of harmonic voltage distortion with TRD, equivalent harmonic impedance z_eq, is proposed as an efficient way of characterising a particular site. The framework described here can be used to predict future harmonic behaviour at any previously measured point. Finally, extensive harmonic measurements of a real distribution system were performed to prove the usefulness of the proposed indices.     

Hybrid-active filtering of harmonic currents in power systems

Use of nonlinear loads and devices in power systems is expected to grow rapidly. Examples are thyristor-controlled inductors for FACTS, converters for HVDC transmission and large adjustable speed drives. All of these loads inject harmonic currents into the power system. A novel hybrid filter topology and its control, to prevent such harmonic currents from entering the power system, are presented in this paper. Analysis shows that in the proposed filter, the power electronic converter, required to generate harmonic currents for compensation, has a rating of only 9% when compared to the converter rating in an active filter and approximately one-half of that in a series-hybrid filter discussed in the literature. The proposed topology enables practical implementation of active harmonic current filters. The control of this filter under transient conditions such as start-up and during steady state is demonstrated by means of simulations. Results from a scaled-down hardware prototype are presented to verify the simulations     

A torque-ripple compensation scheme for harmonic drive systems

This paper presents a scheme for controlling the output torque of a harmonic drive actuator equipped with a torque sensor. The proposed control is composed of a feedback control and a feedforward learning control, in which the feedback control shapes nominal system dynamics using the internal model control structure. The feedforward learning controller employs a disturbance observer (DOB) to evaluate compensation error of the feedforward control for the learning, so as to compensate for torque ripples induced by harmonic drives. Robust stability conditions of the proposed DOB-based learning control system are provided. Experimental results show the effectiveness of the proposed scheme in alleviating the major component of torque ripples whose frequency is twice the angular frequency of the input shaft.     

Power and energy systems planning

Some of the main factors influencing power system investment decisions are reviewed, and relevant aspects arising from energy systems analyses are considered.     

Power factor correction and harmonic mitigation in a thyristorcontrolled glass melter

Operation of a new all-electric glass melting furnace in a manufacturing plant caused severe disturbances to other plant functions. The problem was traced to the thyristor controllers, which regulated power to melter electrodes. Investigation showed severe distortion of the plant feeder voltage waveform due to harmonic currents. Corrective measures consisted of installing a set of series resonant harmonic traps and a high-pass filter on the 4160 V feeder. Harmonic distortion of the supply voltage was reduced from 5% to less than 1%, and the power factor was improved. Normal plant operations were resumed     

基于高倍过采样与加窗插值FFT的电力谐波分析

为提高谐波分析精度,分析了信号加窗引起的信噪比损失以及AD转换产生的量化误差,阐述了过采样技术提高信噪比的原理。在此基础上,提出了基于高倍过采样和加窗插值快速傅里叶变换(fast Fourier transform, FFT)的谐波分析方法。该方法充分利用AD转换器的潜力,以尽量高的采样速率进行AD采样,同时通过均值滤波避免高倍过采样引起的采样数据量激增问题。详细研究了所提谐波分析方法对信号中谐波分量幅值和相位的影响,并给出了简洁实用的谐波幅值和相位校正方法。仿真表明,所提方法可在不增加系统成本的前提下改善加窗插值FFT的抗噪声能力,提高谐波分析精度。     

基于小波包变换的电力系统谐波分析

由于新能源的广泛应用和电力电子设备的迅速增长,电力系统谐波越来越被关注,精确有效的谐波分析方法研究具有实际意义。基于小波包变换的良好局部性,小波包变换广泛应用于谐波分析,重点分析了小波函数的选取和分解层数的确定。通过仿真,得到了基于小波包变换谐波分析方法的最优小波函数与分解层数:选用小波函数sym35对谐波信号进行5层分解,得到的谐波分析结果通常是比较理想的。     

Stationary frame harmonic reference generation for active filter systems

Harmonic reference generators for active filter systems are commonly implemented in the synchronous reference frame, so that simple low- or high-pass filter functions can be used depending on the requirements of a particular application. High-pass filters in particular are used when the reference generator is required to produce an "everything but the fundamental" target waveform. This paper presents a stationary frame notch filter equivalent to a high-pass synchronous frame harmonic reference generator for such systems. The use of the stationary frame allows for quicker calculation of the reference generation in a discrete digital implementation, and also allows classical control techniques to be used to analyze the active filter system without requiring synchronous frame transformations of the outside system model. Both simulation (continuous and discrete) and experimental results showing the equivalency of the stationary and synchronous frame reference generation process are presented using both shift and delta-operator-based infinite-impulse response digital filters.     

高压直流输电系统的谐波分析方法综述

分析了用于高电压直流输电(HVDC)系统谐波分析的比较典型和有效的若干方法,如统一基波和特征谐波潮流算法、调制理论、谐波域分析法等,总结了它们的基本原理、实现算法、应用范围及各自的特点,指出了对换流器建模时需要注意的特殊问题,介绍了HVDC系统多谐波源谐波分析的调制迭代谐波分析法及周期时间序列分析法,并对HVDC系统谐波分析以后的发展方向作出展望。