John A. Orr

A survey of harmonic voltages and currents at the customer's bus

Seven day-measurements of harmonic current and voltage made at the main buses of industrial, commercial, and residential customers are reported. The results are for a large residential load, an accounting operations building, a welded pipes plant, a passing and machining plant, a large supermarket, and a retail store. The results are representative of low-voltage buses in the northeast US. At all the locations it was found that 99% of the time the voltage total harmonic distortion was much smaller than 5% limit recommended by IEEE Std 519. The data provide the initial core of documentation for future reference and follow-up of the trends in harmonic levels at end users. >

High impedance fault arcing on sandy soil in 15 kV distribution feeders: contributions to the evaluation of the low frequency spectrum

The measured values of current harmonics at a staged high-impedance ground fault on sandy soil are presented. The measured low-frequency spectrum is compared with current harmonics recorded continuously for one week at the substation. This comparison was carried out to determine to what extent 120 Hz and 180 Hz components can be used to help detect a high-impedance fault. The field measurements are supported by a simple theoretical model and laboratory measurements. It is concluded that, for the studied feeder, detection of high-impedance arcing faults may be possible by monitoring of the second-harmonic current. >

Applications of Tensor Theory to Object Recognition and Orientation Determination

A method is developed by which images resulting from orthogonal projection of rigid planar-patch objects arbitrarily oriented in three-dimensional (3-D) space may be used to form systems of linear equations which are solved for the affine transform relating the images. The technique is applicable to complete images and to unlabeled feature sets derived from images, and with small modification may be used to transform images of unknown objects such that they represent images of those objects from a known orientation, for use in object identification. No knowledge of point correspondence between images is required. Theoretical development of the method and experimental results are presented. The method is shown to be computationally efficient, requiring O(N) multiplications and additions where, depending on the computation algorithm, N may equal the number of object or edge picture elements.

A survey of harmonic voltages and currents at distribution substations

The authors report seven day measurements of harmonic current and voltage made at the sending out of five feeders. The comprehensive survey monitored voltage and current distortion at eleven locations: five distribution feeders, one large apartment building, three industrial customers, and two commercial. The major goals are to present different ways of recording and displaying harmonics, to explain the results, and to compare them with existing engineering recommendations. >

Current Harmonics, Voltage Distortion, and Powers Associated with Electric Vehicle Battery Chargers Distributed on the Residential Power System

The harmonic performance of the networks of several types of electric vehicle (EV) battery chargers is documented. Cumulative effects at the substation level for random distributions of each of five different charger types are reported. Chargers with and without current- smoothing inductors and with and without controlled rectifiers for maintenance of constant current are included. Results are reported as magnitudes of expected harmonic current; active power; apparent, reactive, and distortive volt amperes; and power factor hour by hour over typical daily recharge cycles for the network of chargers. Results regarding comparisons among chargers demonstrate the desirability of including a current-smoothing inductor in the charging circuit and indicate that constant-current type chargers using controlled rectifiers generate significantly more harmonic current than the simple noncontrolled taper-current chargers. Typical third harmonic current values of 15 A per charger on the 120-V side and 20 A per phase on the 12.8-kV side for a network of chargers (at ten percent penetration of chargers into the residential distribution network) indicate the possibility for harmful effects to customer and utility equipment and for interference into communications circuits. The results reported here should be useful in both predicting harmful effects at various densities of EV chargers on the residential network and in designing chargers to minimize those effects. supported in part by the New England Electric System.

Summary of innovations in electrical engineering curricula

Two aspects of innovation in electrical engineering education are summarized: the membership and goals of the NSF Engineering Education Coalitions, and innovations implemented at individual schools. The latter summary results from responses to a survey of electrical engineering department heads. Responses from 35 schools were received; they are organized in this report under the categories of major program/curriculum revisions, curricular revisions, first-year experience, undergraduate engineering design experience, course innovations, and innovations in graduate education. Also reported are issues in electrical engineering education that merit attention, as reported by the department heads responding to the survey. >

Current Harmonics Measurement by Means of Current Transformers

This paper analyzes the accuracy of current transformers (CT) when excited with nonsinusoidal currents. It uses a nonlinear model that accounts for the minor hysteresis loop of the core. It was concluded that instrument transformers with accuracy class 0.6 or better provide reasonably accurate measurements of current harmonics magnitudes, however the phase angle error may lead to unacceptable errors when the current transformers are used to measure active powers. If such measurements are used to determine the power flow direction, i.e., if a load or consumer pollutes or is polluted, even high accuracy class CTs may yield unsatisfactory results.

Current Harmonics, Voltage Distortion, and Powers Associated with Battery Chargers Part I: Comparisons Among Different Types of Chargers

This report compares the performance of five types of battery chargers for use with electric vehicles. Chargers with and without controlled rectifiers for maintenance of constant charging current and with and without current-smoothing inductors are analyzed. Based on a typical electric vehicle battery pack and an 8 hour recharge cycle, current and voltage waveshapes, current harmonic levels, power, reactive and distortive volt-amperes, voltage distortion factor, and power factor are reported for each charger type.

On the need for strict second harmonic limits

Even harmonics, especially the second harmonic, cause waveform distortions characterized by asymmetries between the positive and the negative half-cycles. Certain single-phase converter topologies, when supplied with asymmetrically distorted voltage wave-forms, respond by injecting significant levels of direct current, even when the mean value of the voltage is zero. In turn the DC will bias transformers causing a host of undesirable effects. This paper concludes that the existing recommendations for permissible levels of the second harmonic must be stricter and proposes a ban on half-controlled three-phase converters.

Current and voltage harmonic measurements and modeling at the Gardner photovoltaic project

Measurements on harmonics taken at the Gardner photovoltaic (PV) project during the summer of 1987 are reported. The project includes 28 PV homes, each with a roof-mounted 2 kW PV system, connected to one phase of a 13.8 kV three-phase feeder. The measurements discussed pertain to voltage and current distortion and power flow. Theoretical calculations were carried out in order to study the effects of PV inverters other than the type installed for this study. Computer modeling of the feeder in Gardner indicates, and measurements confirm, that small increases (0.2%) in voltage total harmonic distortion will occur on this feeder with the indicated penetration of PV systems using high-quality, forced-commutation inverters. >