IEEE Power Engineering Review

Harmonic distortion is not a new phenomenon. Concern over harmonic distortion emerged during the early history of AC power systems. Widespread applications of power electronic-based loads continue to increase concerns over harmonic distortion. Harmonic problems have sparked research that has led to much of the present-day understanding of power quality problems. The current-drawn by electronic loads can be made virtually distortion-free (i.e., perfectly sinusoidal), but the cost of doing this is significant and is debate between equipment manufacturers and electric utility companies in standards-making activities. This paper discusses the history of harmonics in power systems. It then defines harmonic distortion and discusses harmonic sources. System response characteristics to harmonics and the impact of harmonics are also discussed. Control of harmonics is also discussed.     

多馈入直流系统的谐波不稳定仿真研究

随着高压直流输电的发展及多馈入直流输电系统的逐步形成,多馈入直流系统的谐波不稳定问题日显突出。从谐波不稳定产生的机理出发,分析了导致谐波不稳定的交/直流系统频率互补谐振条件,以国际大电网会议(CIGRE)提出的高压直流输电标准测试系统为原型,在Matlab/Simulink仿真平台建立了多馈入直流系统的模型,并基于该模型对多馈入直流系统中各交直流网络间的谐波相互影响进行了仿真与分析。结果表明,该模型能对多馈入直流系统的谐波特性进行准确、有效的分析。     

浅谈风电场的电能质量

风电场的电能质量是风电场并入电网运行的关键指标。文章从风电场的电压谐波、三相电压不平衡度和频率偏差三个方面分析了风电场的电能质量,并引用广东惠来海湾石风电场的实测数据,说明风电场的电能质量能够满足公用电网电能质量的要求。     

Development of a novel point absorber in heave for wave energy conversion

This paper presents an advanced design methodology for electric power generation from the vast ocean wave energy. A novel single-buoy heaving device called wave energy converter (WEC) based on hydrostatic transmission (HST), or can be shortened as HSTWEC, is proposed to convert mechanical energy generated by ocean waves into electric energy. Modeling and simulations with both regular and irregular waves were then carried out to investigate working performances of the designed HSTWEC. The results showed that more than 78% of wave energy can be absorbed. In addition, an adaptive controller was designed to improve the performance of the suggested device. Effectiveness of the overall HSTWEC control system was finally proved by simulations.     

Comparison of a synergetic battery pack drive system to a pulsewidth modulated AC induction motor drive for an electric vehicle

A new battery configuration technique and accompanying control circuitry, termed a synergetic battery pack (SBP), is designed to work with lithium batteries, and can be used as both an inverter for an electric vehicle AC induction motor drive and as a battery charger. In this paper, the performance of a synergetic battery pack during motor drive operation is compared via computer simulation with a conventional motor drive which uses sinusoidal pulse width modulation (SPWM) to determine its effectiveness as a motor drive. The study showed that the drive efficiency was compatible with the conventional system, and offered a significant advantage in the lower frequency operating ranges. The voltage total harmonic distortion (THD) of the SBP was significantly lower than the PWM drive output, but the current THD was slightly higher due to the shape of the harmonic spectrum. In conclusion, the SBP is an effective alternative to a conventional drive, but the real advantage lies in its battery management capabilities and charger operation     

提高电力网谐波分析精度新方法的研究

在谐波测量实践中提出的以卷积定理为基础的新的加权窗,用以提高窗的消瓣效应和分辨能力,减少泄漏效应,提高频谱分析的精度以及总体的ＦＦＴ处理的效率,展现了新的加权窗函数在电力网络周期信号谐波分析中的应用前景．     

特高压直流输电系统触发角和输送功率对网侧谐波影响的仿真分析

针对直流输电系统运行方式和功率点多变导致谐波含有率有明显的不确定性问题,分析了+K168换流站特征谐波电流含有率,介绍了直流输电工程中常见的运行方式,利用PSCAD软件建立了特高压直流输电系统模型,仿真分析了触发角和功率变化对换流站网侧电流谐波含有率的影响。结果表明,换流器整流侧触发角增大时,交流网侧谐波含有率会逐渐上升;输送功率越大时,谐波含有率总体越小;输送功率越小时,谐波含有率总体越大。     

Electric fields effect on liftoff and blowoff of nonpremixed laminar jet flames in a coflow

The stabilization characteristics of liftoff and blowoff in nonpremixed laminar jet flames in a coflow have been investigated experimentally for propane fuel by applying AC and DC electric fields to the fuel nozzle with a single-electrode configuration. The liftoff and blowoff velocities have been measured by varying the applied voltage and frequency of AC and the voltage and the polarity of DC. The result showed that the AC electric fields extended the stabilization regime of nozzle-attached flame in terms of jet velocity. As the applied AC voltage increased, the nozzle-attached flame was maintained even over the blowout velocity without having electric fields. In such a case, a blowoff occurred directly without experiencing a lifted flame. While for the DC cases, the influence on liftoff was minimal. There existed three different regimes depending on the applied AC voltage. In the low voltage regime, the nozzle-detachment velocity of either liftoff or blowoff increased linearly with the applied voltage, while nonlinearly with the AC frequency. In the intermediate voltage regime, the detachment velocity decreased with the applied voltage and reasonably independent of the AC frequency. At the high voltage regime, the detachment was significantly influenced by the generation of discharges.     

Generator winding I2R losses and harmonic interferenceunder variable frequency operation of an HVDC unit-connectionscheme

Variable frequency operation of an HVDC unit-connection scheme may affect the generator winding I²R losses, and may cause harmonic interferences on both the AC and DC system. The purpose of this research is to determine the magnitudes of generator winding I² R losses under a certain frequency range so as to improve the efficiency of the hydro unit operation at nonrated head and output power. The generator winding additional I²R losses and harmonic interference, are investigated by using the EMTDC program and other subordinate programs. The overall study shows that the generator winding power losses will be increased as the operating frequency decreases under the condition of a fixed generator output. The magnitude of increased losses, though, will not exceed 8-10% of generator total winding I²R losses under normal system operating conditions. Harmonic interference, on the other hand, will become more severe in both the AC and DC systems, as compared to fixed frequency operation. Active or on line tuning harmonic filters may be considered for a particular variable frequency operation scheme being implemented     

Variable speed DFIG wind energy system for power generation and harmonic current mitigation

This paper presents a novel approach for simultaneous power generation and harmonic current mitigation using variable speed WECS with DFIG. A new control strategy is proposed to upgrade the DFIG control to achieve simultaneously a green active and reactive power source with active filtering capability. To ensure high filtering performance, we studied an improved harmonic isolator in the time-domain, based on a new high selectivity filter developed in our laboratory. We examined two solutions for harmonic current mitigation: first, by compensating the whole harmonic component of the grid currents or second, by selective isolation of the predominant harmonic currents to ensure active filtering of the 5th and 7th harmonics. Simulation results for a 3 MW WECS with DFIG confirm the effectiveness and the performance of the two proposed approaches.     

Steady-state performance of synchronous generators with AC and DCstator connections considering saturation

Synchronous generators with AC and DC stator connections are widely used in systems where simultaneous AC and DC power supplies are needed. For these systems the voltage harmonic distortion of AC voltage, DC voltage ripple and DC voltage varying range are of special interest. Because of the commutation of rectifier and the saturation of magnetic circuit, the accurate calculation of voltage harmonic distortion and DC voltage varying range is comparatively difficult. This paper describes a mathematical model on the basis of multiloop method and FEM, which aims at calculating the steady-state characteristics and the voltage waveforms of AC winding accurately. With this method, the full synergic effects of space harmonics resulting from the magnetic circuit geometry, saliency, saturation, winding layouts, and slotting are included. The calculated results are compared with those of experiments     

Stability of conducting viscous film flowing down an inclined plane with linear temperature variation in the presence of a uniform normal electric field

Weakly nonlinear stability analysis of a thin liquid film falling down a heated inclined plane with linear temperature variation in the presence of a uniform normal electric field has been investigated within the finite amplitude regime. A generalized kinematic equation for the development of free surface is derived by using long wave expansion method. A normal mode approach and the method of multiple scales are used to investigate the linear and weakly nonlinear stability analysis of film flow, respectively. It is found that both Marangoni and electric Weber numbers have destabilizing effect on the film flow. The study reveals that both supercritical stability and subcritical instability are possible for this type of film flow. It is interesting to note that both the Marangoni and electric Weber numbers have qualitatively same influence on the stability characteristics but the effect of Marangoni number is much stronger compare to the electric Weber number. Scrutinizing the effect of Marangoni and electric Weber numbers on the amplitude and speed of waves it is found that, in the supercritical region amplitude and speed of the nonlinear waves increases with the increase in Marangoni and electric Weber numbers, while in the subcritical region the threshold amplitude decreases with the increase in Marangoni and electric Weber numbers. Finally, we obtain that spatially uniform solution is side-band stable in the supercritical region for our considered parameter range.     

配电网中高次谐波对并联补偿电容器的影响及其限制措施

供配电系统通常接有并联补偿电容器,配电网中的高次谐波对电容的影响很大。本文从理论上分析了串联谐振和并联谐振引起的电容器过电流,阐述了无论哪类谐波源,只要参数配合适当,都可能出现电流放大现象或谐波谐振,使电容器过负荷,甚至损坏。并由此找出相应的抑制措施。     

PLANE HARMONIC WAVES IN A MICROPERIODIC LAYERED INFINITE THERMOELASTIC SOLID

A one-dimensional dynamical coupled refined averaged thermoelasticity for a microperiodic composite is used to study harmonic waves propagating in a layered infinite solid. In such a theory the waves are governed by an eighth-order-in-time partial differential equation in which the two intrinsic frequencies are present: a mechanical frequency Ω and a thermal frequency α; both these frequencies are high if the layering period l is small. The existence of harmonic waves of a given frequency ω propagating in a positive direction normal to the layering is established when (i) Ω→∞, α<∞ or (ii) Ω<∞, α→∞. It is shown that in each of the two cases there are two plane harmonic thermoelastic waves of a given frequency ω that are dispersive and attenuated. Also, a closed form of velocities and attenuation coefficients for the two waves are obtained. The velocities and attenuation coefficients, treated as the functions of ω, are illustrated graphically for a unit cell made of the two homogeneous isotropic thermoelastic layers: a zirconium oxide [Zr O₂] and a titanium alloy [Ti–6Al–4V].     

Performance monitoring results of the first grid-connected BIPV system in Colombia

This paper summarizes the operational performance results of the first grid-connected building integrated photovoltaic (BIPV) system installed in Colombia (in Bogotá, at 4°35′ latitude and 2.580 m altitude) after two years of monitoring. The performance monitoring was carried out with a sophisticated monitoring system, designed and implemented by us using the virtual instrumentation concept. The following parameters were measured: DC and AC power, inverter and system conversion efficiency, energy generated by the PV array, AC energy produced by the BIPV system, parameters to analyze power quality (%THD, harmonic components, frequency, voltage, flickers, power factor, active power, apparent power and reactive power), solar radiation in the inclination plane of the panels and environment temperature.The data obtained allowed to evaluate the general performance and the quality of the electric power generated by the photovoltaic plant. The results indicated that the power generated by the grid-connected BIPV plant fulfills the specifications demanded for such systems by National and International standards.     

A Liquid Crystal “Heat Switch”∗

A novel method of controlling heat flow from a surface is discussed. The method employs a nematic liquid crystal. A convective motion of the liquid crystal can be induced by subjecting it to either an AC or DC electric field. If the liquid crystal is used to transfer heat between two surfaces, the heat transfer rate can be controlled by varying the electric field strength. It is shown that the heat transfer rate through a thin layer of the liquid crystal can be increased by 25 limes when an electric field is present. This paper briefly discusses the physical phenomena that induce the convective motion in the liquid crystal due to the electric field. Experiments to determine the effects of field strength and gap distance on the enhancement of the heat transfer rate are discussed. An abrupt change in the rate of increase of the heat transfer rate with increasing voltage was found and is believed to be caused by a change in the fluid flow structure. This “heat switch” may find applications in aerospace and electronics cooling situations in which heat must be removed from a surface periodically while at other times the surface must remain insulated.     

The simultaneous impact of photovoltaic systems and plug-in electric vehicles on the daily load and voltage profiles and the harmonic voltage distortions in urban distribution systems

Photovoltaic systems (PVSs) and plug-in electric vehicles (PEVs) are becoming increasingly common and expected to be further integrated into electric power distribution systems (EDSs) in the near future. In this paper, the combined effects of PVSs and PEVs on the feeder and transformer loads, voltage profiles and harmonic distortions of an urban area EDS are investigated. The results indicate that the simultaneous connection of PVSs and PEVs, at the correct ratio, can impact load reduction and produce a reduction in voltage variations while potentially resulting in an increase the total harmonic distortion of voltage (THDV). In addition, this paper presents changes in the characteristic daily load and voltage profiles and changes in the THDV of the system caused by the simultaneous operation of PVSs and PEVs. The test EDS with 19 nodes is based on a real EDS. Two seasons, summer and winter, and several different degrees of penetration of PVSs and PEVs were studied. We used a stochastic approach to model the loads, PVSs and PEVs based on data measured in real systems and data found in the literature.     

Analysis of unstable solutions in a single-phase AC/DC converter

The study of harmonic instabilities in an AC/DC power system is presented in this letter. An existing frequency domain methodology is modified to assess the phenomena produced by this harmonic interaction. A set of analytical equations is derived to compute the commutation time taking place in the single-phase converter. Suitable guidelines are suggested for effective overlap angle initializations. Test results are presented that demonstrate the effectiveness of the proposed methodology. Instabilities are shown to take place for given operating conditions     

HALF-SPACE PROBLEM IN ONE-DIMENSIONAL GENERALIZED MAGNETOTHERMOELASTICITY

The theory of generalized thermoelasticity is used to solve the problem related to magnetothermoelastic waves produced by thermal shock in a half-space possessing finite electric conductivity. Eigenvalue approach is applied to obtain solution of the basic equations. Approximate small time solution is obtained by using Laplace transform method.