M.D. Cox

Practical definitions for powers in systems with nonsinusoidal waveforms and unbalanced loads: a discussion

Existing definitions for power terms in alternating current systems work well for single-phase and three-phase systems where both voltages and currents are sinusoidal with respect to time. This paper clarifies and proposes definitions for power terms that are practical and effective when voltage and/or currents are distorted and/or unbalanced. It also suggests definitions for measurable values that may be used to indicate the level of distortion and unbalance.

Discrete wavelet analysis of power system transients

Wavelet analysis is a new method for studying power system transients. Through wavelet analysis, transients are decomposed into a series of wavelet components, each of which is a time-domain signal that covers a specific octave frequency band. This paper presents the basic ideas of discrete wavelet analysis. A variety of actual and simulated transient signals are then analyzed using the discrete wavelet transform that help demonstrate the power of wavelet analysis.

Frequency characteristics of wavelets

Wavelets detect and locate time of disturbances successfully, but for measurement of power/energy they also have to estimate and classify them accurately. This paper investigates the factors on choice of a certain wavelet function and qualitatively shows how the number of coefficients of the wavelets is an important number that affects output decomposition and energy distribution leakage. Wavelets provide an output in terms of the time-frequency scale. The frequency bandwidth characteristics of these individual wavelet levels provide better understanding of the wavelets. The sampling frequency and the number of data points are important parameters and must be carefully selected to avoid the frequency of interest falling into the end regions.

A New Static Var Compensator for an Arc Furnace

This paper presents the design of an electronic compensator that can compensate the reactive current drawn by an arc furnace. Consisting of three independent single-phase pulse-width modulated (PWM) inverters, the proposed compensator responds quickly to any sudden load changes and compensates both the fundamental displacement current and the harmonic distortion current drawn by the load. Unlike conventional static var compensators, the electronic compensator can also supply the active current demanded by the load if an energy source independent of the utility is available. Simulations are conducted, including the modelling of the furnace current and voltage waveforms during the early stage of scrap melting, in order to determine the effectiveness of several suggested detection methods that can be used to separate the active and reactive components of the furnace current. Actual waveforms observed on a local furnace are included that corroborate the computer modelling. Experimental results indicate that the electronic compensator equipped with a suitable detection circuit can accurately compensate an arc furnace.

Survey of harmonic levels on the Southwestern Electric Power Company system

A survey of current and voltage distortion levels measured at 76 different sites is presented. Over 5000 pairs of current and voltage waveforms were recorded and analyzed for this study. In addition to summarizing the harmonic levels at the various industrial, commercial, and substation sites, the authors use data at a particular nonlinear load site to show how US utilities may be failing to collect the correct revenues from power factor penalty clauses. The instrumentation used to collect the field data is briefly described. The results of the field measurements are discussed, with particular emphasis on the problem sites identified during the project. >

Induction VArhour and solid-state VArhour meters performances on nonlinear loads

Experimental measurements of solid-state and induction VArhour meters operating in the presence of steady-state nonsinusoidal currents are presented. A computer-based data-acquisition and analysis system is used to determine the various electrical quantities of some common nonlinear three-phase loads. The basic operating principles of the VArhour meters are reviewed along with the nature of reactive power and energy for nonsinusoidal waveforms. Test results confirm that for nonlinear loads driven by sinusoidal voltages, both types of VArhour meters register the fundamental displacement voltampere hours only and do not respond to distortion voltampere hours. >

Temperature rise of small oil-filled distribution transformers supplying nonsinusoidal load currents

Temperature rise tests on standard single-phase, oil-filled distribution transformers are compared for sinusoidal and nonsinusoidal current conditions. In addition to a stock 50 kVA unit, two 25 kVA transformers were specifically constructed with embedded thermocouples that permitted the true hot-spot temperatures to be measured under load. Test results show that when subjected to full-load currents having low-order harmonic distortion values up to 40 percent THD, the average winding temperature rise is 2 to 7/spl deg/C hotter than for full-load sinusoidal currents. The greatest difference recorded between the average winding temperature and the hot-spot temperature was 7/spl deg/C, significantly less than the 15/spl deg/C allowance given in the standards. This study indicates that the present standard for sizing small distribution transformers supplying nonsinusoidal currents appears to provide good estimations of load capability.

Near-field coupling between broadband over power line (BPL) and high-frequency communication systems

Electromagnetic coupling between a broadband over power line (BPL) system and an HF communication system is discussed in this paper. Numerical analyses using the method of moments were performed to determine the near-field coupling between the BPL and communication systems at 3.5, 7, 14, and 28 MHz. Half-wave dipole and quarter-wave vertical antennas were positioned at 1-m intervals along the power line to find the maximum and minimum signal levels caused by the BPL system. Signal levels induced into the BPL system by a 100-W HF transmitter are also presented

Vibration and audible noise of capacitors subjected to nonsinusoidal waveforms

Vibration and audible noise generated by power capacitors subjected to nonsinusoidal waveforms are discussed. A study of the relation between the vibration (audible noise generated) and the capacitor current (and voltage) is presented. Capacitor data obtained in the laboratory and in a distribution substation are compared. This investigation confirms that a bank of power capacitors can generate significant noise if the capacitor current contains high frequency harmonics. >

Calculations and measurements of the Unity Plus three-phase induction motor

The Unity Plus induction motor configuration for three-phase motors is described and analyzed. In the Unity Plus winding method only one set of windings, called the power winding, is connected directly to the source. The second set of windings is connected to capacitors and is coupled to the power winding through transformer action. The equivalent circuit of the Unity Plus motor for steady-state operation is obtained through standard induction motor analysis methods. Laboratory experiments on a 10 hp three-phase induction motor, both as a conventional winding configuration and as a Unity Plus configuration after rewinding by Unity Plus representatives, verified the accuracy of the circuit model and showed that the Unity Plus winding configuration did not result in higher efficiencies than that obtainable from conventional winding methods. >