Power quality problems are manifested in voltage, current or frequency deviations causing malfunction of sensitive equipment. Integration of inverter connected PV and wind power plants, and rampant rise in nonlinear loads have led to harmonic problem in power system. Nonlinear loads and switched devices energized by sinusoidal sources or linear loads and switched devices with non-sinusoidal sources, produce harmonics in distribution system. Academic harmonic analysis study consists of modeling nonlinear loads to develop Norton and Thevenin equivalent circuits of devices for integration into harmonic analysis software. Experimental researchers often use harmonic analyzers to measure the harmonics in real systems to evaluate suitable mitigation alternatives. The distortive power losses force utilities to increase apparent power to maintain reliable and uniform power supply. Harmonic analyzers use data acquisition hardware and inbuilt software algorithms to perform onsite measurements. Harmonic analyzers help find true power factor, total harmonic distortions, reactive and distortive power losses. Use of shunt capacitance at unity power factor worsens the situation instead of supplying distortive power compensation. Active power factor correction techniques, using smart algorithm to cancel the distortive power, have been reviewed for further research. Nonlinear physics of harmonic phenomenon is described to explore its applications. Harmonic mitigation technologies have been compared, current state of the art technology reviewed and demonstrated by designing a harmonic filter. Measurement of harmonics, waveform distortions, and true power factor (TPF) of single and three phase electronic loads is carried out to test their compliance to harmonic standard limits. Energy conservation concept requires reduction of harmonics in distribution networks. This study found 60±10% reduction in power factor and more than 2% increase in line losses due to widespread use of nonlinear loads. Utility apparent power demand increases due to consumers’ inadvertent violation of IEC Standard 61000-3-2 and IEEE Standard 519–1992.

Review of harmonic analysis, modeling and mitigation techniques

The increased use of power electronic controlled equipment, such as variable speed drives, automated production lines, personal computers and non-linear electronic devices in power systems has given rise to a type of voltage and current waveform distortion called as ‘harmonics’. Harmonic can be defined as the undesirable components of a distorted periodic waveform whose frequencies are the integer multiples (non-integer multiples in case of inter-harmonics, and the frequency less than fundamental frequency in case of sub-harmonics) of the fundamental frequency. Presence of these harmonics results in increased losses, equipment heating and loss-of-life, and interference with protection, control and communication circuits as well as customer loads. The research has been underway since very beginning for control of power system harmonics and to supply consumers with reliable and ‘clean’ fundamental-frequency sinusoidal electric power that does not represent a damaging threat to their equipment. This paper, therefore, reviews the progress made in power system harmonics research and development since its inception. Attempts are also made to highlight the current and future issues involved in the development of quality and reliable electric power technology for future applications. A list of 145 research publications on the subject is also appended for a quick reference. Copyright © 2007 John Wiley & Sons, Ltd.

Power system harmonics research: a survey

The interharmonics, which are the same as integer harmonics, have serious impacts on the design of harmonics compensation equipment. Therefore, it is important to measure interharmonics accurately. The interharmonics cannot be obtained accurately by using fast Fourier transform (FFT) for an unsynchronized sample sequence analysis because of large errors in such cases. To reduce the errors caused by using FFT for an unsynchronized sample sequence analysis and to improve the measuring precision of interharmonics in electric power systems, an interharmonics estimation method based on the interpolation FFT algorithm is proposed. An algorithm for estimating interharmonics parameters based on the Blackman-Harris window, Rife-Vincent (I) window, and Rife-Vincent (III) window is given. Simulation results demonstrate that by using the proposed algorithm for the unsynchronized sample sequence analysis, the estimated interharmonics frequency, amplitude, and phase are highly accurate

Interharmonics Analysis Based on Interpolating Windowed FFT Algorithm

IEC-61000-2-1 defines interharmonic as follows: between the harmonics of the power frequency voltage and current, further frequencies can be observed which are not an integer of the fundamental. They can appear as discrete frequencies or as a wideband spectrum. Harmonics and interharmonics of a waveform can be defined in terms of its spectral components in the quasisteady state over a range of frequencies. The term sub-harmonic does not have any official definition but is simply a special case of interharmonic for frequency components less than the power system frequency. Use of the term subsynchronous frequency component is preferred, as it is more descriptive of the phenomena. This paper discusses the sources of interharmonics, chief among which is the cycloconverter. The impacts of interharmonics and the measurement of interharmonics are also discussed.

Interharmonics in power systems

A cycloconverter is a power electronic device used to convert constant voltage constant frequency AC power to adjustable voltage adjustable frequency AC power without a DC link. Cycloconverters inject significant harmonics and nonstandard frequency components (i.e., noninteger multiples of the power frequency) into power systems. The impact of cycloconverters on power quality is studied, and the relationships between power quality indices and cycloconverter control strategies are developed. Control strategies are proposed to minimize the power quality impact of the cycloconverters. An innovative group frequency concept is introduced. This is a technique whereby many frequency components in the cycloconverter voltage and current spectra are collapsed into several group frequencies, and the power quality impact of the group frequencies is the same as that of the full spectrum of the cycloconverter signals. Some concepts are illustrated by simulation, and some by measurement at a twelve pulse cycloconverter load.

The power quality impact of cycloconverter control strategies

Findings from a study on the harmonics generated by a 15 MVA three-phase 13.2 kV 60 Hz to single-phase 24/12 kV 25 Hz static frequency converter (SFC) or cycloconverter are summarized. At light loads, some of the characteristic harmonics (e.g. 10 Hz, 110 Hz) have magnitudes that exceed the fundamental frequency. Since the three-phase components at these frequencies do not necessarily nullify each other, many frequency components, in addition to zero-sequence components, can be measured in the neutral (residual) current. Because of low system impedance at harmonic frequencies, the voltage at the 13.2 kV substation supplying the SFC is not severely affected. A worst-case scenario for the voltage spectrum is presented. The corresponding distortion factor is calculated to be 4.12%. The power factor of an assumed parallel load is calculated using this scenario, and the effect of the SFC harmonics on the power factor is found to be insignificant. However, depending on the system impedances, the shunt capacitors for power factor compensation at the assumed load may be ineffective due to harmonic resonance. >

J.J. Burns

Papers

Impact of cycloconverter harmonics