Francois D. Martzloff

Power quality site surveys: facts, fiction, and fallacies

Site surveys are generally initiated to evaluate the quality of the power available at a specific location with the aim of avoiding equipment disturbances in a planned installation or of explaining (and correcting) disturbances in an existing installation. Monitoring disturbances of the power supply has been an objective of various site surveys, but results often appear to be instrument-dependent or site-dependent, making comparisons difficult. After a review of the origins and types of disturbances, the authors describe the types of monitoring instruments. A summary of nine published surveys reported in the last 20 years is presented, and a close examination of underlying assumptions is carried out allowing meaningful comparisons which can reconcile some of the differences. Finally, the authors appeal for improved definitions and applications in the use of monitoring instruments. >

The Propagation and Attenuation of Surge Voltages and Surge Currents in Low-VOltage AC Circuits

Examples are given showing the propagation of voltage and current surges in low-voltage wiring systems. The difference between surge impedance (characteristic impedance) of a transmission line and the impedance to the surge of wire runs is pointed out and illustrated. A comparison is made between the propagation of a surge through an isolating transformer and a ferro-resonant line conditioner. The effect of connection options are shown for one or several surge protective devices connected at the end of a 3-wire line.

Surge Voltages in Residential and Industrial Power Circuits

Special instrumentation was developed for monitoring the magnitude and frequency of occurrence of surge voltages in residential and industrial circuits. Over a period of 2 years, more than 400 locations in 20 cities were surveyed. Monitoring was accomplished by automatic recording cathode-ray oscilloscopes and simple surge counters. In residential circuits, two significant sources of surge voltages were identified: load switching within the house, and external surges, most likely associated with lightning, coming through the service drop. In industrial circuits, the levels of surges are lower than in residential circuits. However, switching surges on the load side of the switch can be severe. Internally generated surges as high as 2500 volts were recorded during this test program, and surges due to lightning reaching 5600 volts have been recorded on a 120-volt overhead distribution line.

Coordination of Surge Protectors in Low-Voltage AC Power Circuits

Surge protectors can be installed in low-voltage ac power systems to limit overvoltages imposed on sensitive loads. Available devices offer a range of voltage-clamping levels and energy-handling capability, with the usual economic trade-off limitations. Coordination is possible between low- clamping-voltage devices having limited energy capability and high-clamping- voltage devices having high energy capability. The paper gives two examples of coordination, as well as additional experimental results on surge propagation.

Matching Surge Protective Devices to Their Environment

A method is described for the rational selection of metal oxide varistors as surge protective devices in low-voltage applications, combining data on the surge environment and device characteristics. Fuses that may be connected in series with the varistors will be exposed to current surges resulting from the varistor operation. Experimental results on the effect of repetitive current surges on such fuses are described, and some guidance is provided on the necessary derating of fuses for series applications with varistors.

Coupling, propagation, and side effects of surges in an industrial building wiring system

Measurements were made in an industrial building to determine the propagation characteristics of surges in the AC power wiring of the facility. The surges, of the unidirectional type or the ring-wave type described in ANSI/IEEE C62.41-1980, were injected at one point of the system, and the resulting surges arriving at other points were measured. The results show how unidirectional surges couple through transformers and produce a ring-wave component in the response of the system. An unexpected side effect of these surges, applied to the power lines only, was apparent damage suffered by the data-line input components of some computer-driven printers.<<ETX>>

Varistor versus Environment: Winning the Rematch

An unusual case of difficult application of surge protective devices was solved by field measurements with retrofit of protective devices suitable for the particular environment. Onsite measurements indicated that capacitor switching transients were causing excessive current surges in the varistors and fuses protecting the input to a thyristor motor drive. Knowledge of the environment gained by the measurements allowed understanding of the problem and specification of matching surge protective devices.

Electrical fast transient tests: applications and limitations

The IEC Electrical Fast Transient (EFT) test was performed on two line configurations selected as typical of low-voltage power lines: a three-conductor line in a steel conduit and a three-conductor cable with a nonmetallic jacket. An attenuation model that provides a tool for understanding the significance of the line parameters and extends the usefulness of results to general cases is proposed. The two line configurations and their models yield similar results for measurements and for computations. Some differences might be expected between the quasi-coaxial configuration of a conduit-enclosed line and the open-line configuration with unshielded conductors of a nonmetallic jacket. The effects of propagation of an EFT pulse in the lines are a reduction in the amplitude, a decrease in the steepness of the front, and an increase in the duration of the pulse. The first two effects reduce the severity of a pulse arriving at the victim equipment; the third effect is less significant for victim equipment sensitive to rate-of-change disturbances. It is concluded that equipment likely to be installed close to sources of EFT pulses would benefit from immunity demonstration.<<ETX>>Recent proposals advocate fast-transient tests demonstrating external overvoltage interference immunity. The test waveforms represent electrostatic discharge effects of reignitions occurring during switching sequences. The rationale for requiring the tests is based on the assumption that interfering transients generated by power circuit switching will couple into adjacent power or signal lines, and will then propagate toward susceptible equipment. The authors describe specific measurements conducted for two typical low-voltage power line configurations, namely, a three-conductor line in a steel conduit and a three-conductor cable with a nonmetallic jacket. An attenuation model is proposed that provides a tool for understanding the significance of the line parameters and extends the usefulness of results to general cases. The two line configurations and their models yield similar results for measurements and for computations.<<ETX>>

Validating surge test standards by field experience: high-energy tests and varistor performance

Proposed IEEE and IEC high-energy surges tests and surge-test modeling are reviewed. The results of modeling the application of a surge test to a family of commonly used varistor sizes (14, 20 and 32 mm diameter) are discussed. For each varistor size, the computations were performed for three levels of manufacturing tolerances on the varistor: nominal value, -10%, and +10%. The results were obtained by quantifying the current through the varistor and the corresponding energy deposited in the variator. The computed results are compared to the published device ratings to predict the likelihood of failure. This likelihood is then compared to the available information from field experience on failure rates.<<ETX>>

Transient control levels

Failure and circuit upset of electronic equipment due to transients is a problem now and is one which has promise of becoming more of a problem in the future as trends continue toward miniaturization and circuit complexity. Protection methods are used more or less extensively and often haphazardly. At present, there does not appear to be a clear approach toward achieving compatibility between the transient withstand capability of devices and the transients to which such devices are exposed. A more scientific approach is needed to guide manufacturers and users of equipment. The purpose of this paper is to promote a concept of transient coordination for electronic and other low-voltage equipment through the establishment of a system of Transient Control Levels, similar to the concept of Basic Insulation Levels so successfully used for many years in the electric power industry. Specific suggestions for possible Transient Control Levels and standard test wave shapes are made, in order to promote wide discussion as to whether these waveforms and levels are the best that can be developed toward good transient coordination for the electronic industry.