Experimental and theoretical methods for determining the surge impedance of towers are presented. It is demonstrated that the values so obtained adequately describe the response of the tower when a transmission line is struck by lightning. A right cone representation for conventional double-circuit steel towers is proposed, and a theoretical analysis using field theory yields the surge impedance as Z= 60 ln (2/S) where S is the sine of the half-angle of the cone.

Tower Surge Impedance

Power transformers can fail from dielectric stresses caused by electromagnetic transients. In this paper, we focus on a special phenomenon where excessive overvoltages arise due to resonance. This situation can take place when a transformer on the high-voltage side is connected to a cable and the low-voltage side is unloaded. Very high overvoltages can then result on the low-voltage side from transient events that cause a weakly attenuated overvoltage on the cable with a dominant frequency matching a resonance peak in the transformer voltage ratio. Laboratory tests on a 11-kV/230 V distribution transformer show that a step voltage excitation on a 27-m cable produces 24-p.u. overvoltage on the open low-voltage side. The voltage waveforms are accurately reproduced by a black-box model obtained from frequency sweep measurements. Simulations show that overvoltages as high as 43 p.u. could occur with the most unfavorable cable length. It is further shown that the following situations can lead to high overvoltages on an unloaded transformer low-voltage side: 1) ground fault initiation at the far cable end, 2) cable energization from a busbar with several other cables connected, 3) cable energization from another cable with the same length, and 4) capacitor bank energization at the far cable end.

Study of Transformer Resonant Overvoltages Caused by Cable-Transformer High-Frequency Interaction

In prediction of HV and EHV lightning outage rates, the tower surge response consists of two parameters, the tower impedance and the tower travel time. Of the two, tower travel time has a larger effect on the predictions. A new model for tower travel time is developed based on the relationship between tower capacitance and tower surface area. Effects of tower crossarms and other features are modelled correctly by the new theory. When the new model is used, improved lightning outage rate prediction accuracy is obtained for 500kV single circuit lines.

Travel Time of Transmission Towers

This paper presents results of field testing on towers whose heights vary from 26 to 214 meters according to the author's new measuring method. The data of each tower were consistently in agreement. Surge characteristics of the towers were made clear by these measurements.

Studies of the Surge Response on a Transmission Line Tower

An analysis is made of the potential for magnified transient voltages at customer buses during capacitor switching on the primary distribution system. The various factors affecting this phenomena are analyzed in detail through extensive sensitivity analysis simulations. These factors include the switched capacitor size, short-circuit capacity at the switched capacitor, customer step-down transformer size, low-voltage power factor correction, and customer load characteristics. The impact of these transients on customer equipment are described, and possible solutions to control the transients are presented. >

Impact of utility switched capacitors on customer systems-magnification at low voltage capacitors

MAGNIFICATION of transient voltages in one part of an electric power system may occur in another part of that system, possibly at a different voltage level. Experiences such as these have resulted in system flashovers or equipment damage. The degree of voltage magnification does not necessarily involve circuit-breaker restriking or prestriking, although these factors may increase the severity of the disturbance.

Magnification of Switching Surges

Some Fundamentals on Capacitance Switching [includes discussion]

Lightning Performance of 138-Kv Twin-Circuit Transmission Lines of Commonwealth Edison Company - Operating Experience and Field Studies

Undesired voltages often accompanied by phase reversal have been experienced1,2 on distribution systems after the opening of 1- or 2-phase conductors. The mere opening of phase conductors on a 3-phase system will not necessarily result in undesirable voltages and phase reversal, but certain combinations of open conductors and unbalanced circuit impedances3 or open conductors and balanced impedances containing inductive and capacitive reactance4,5 can result in undesirable voltage conditions. In the latter, previous studies have not included simultaneously the effect of system load and of the transformer non-linear magnetizing characteristic upon these abnormal voltages. The results in this paper include the combined effect and are therefore more applicable to a wider range of system conditions. Specifically, these phenomena were investigated with regard to the grounded-wye or 4-wire distribution system containing floating wye or delta-connected capacitor banks for kilovar supply, because it is the most common circuit arrangement occurring in practice. However, it is recognized that many other configurations may result in the same phenomena. It is the purpose of this paper to define the nature of the phenomena and under what conditions they may be obtained, including the effect of capacitor size, transformer magnetizing characteristics, magnitude of load, and load power factor. This study was made largely on systems in miniature on the transient network analyzer.

Abnormal Voltage Conditions Produced by Open Conductors on 3-Phase Circuits Using Shunt Capacitors [includes discussion]

Although power circuit breakers are designed primarily to interrupt heavy inductive short-circuit currents, system growth in recent years has produced a greatly increased requirement of interrupting relatively lighter currents which are associated with the switching of capacitive kilovolt-amperes. This capacitive kilovolt-ampere requirement has manifested itself in long-distance high-voltage transmission lines, in relatively long high-voltage cables, and in large size shunt capacitor banks which are becoming a common fundamental unit in system design and operation.1-3 There is not necessarily a relationship between the ability of a circuit breaker to interrupt short circuit currents and its ability to switch capacitance currents. It is widely recognized that high-frequency voltage and current oscillations may be produced which, if uncontrolled, may result in damage to apparatus or system outages. In general, the problem is the severity of the system disturbances produced by the switching operation, either opening or closing, rather than the failure of the circuit breaker to interrupt the current or to close the circuit. It is the purpose of this paper to present: 1. Typical circuit breaker performance in switching capacitive kilovolt-amperes. 2. Circuit behavior and circuit breaker performance considerations from test laboratory and transient analyzer studies. 3. Criteria of acceptable circuit breaker performance in capacitive switching.

N. R. Schultz

Papers

Magnification of Switching Surges

Some Fundamentals on Capacitance Switching [includes discussion]

Lightning Performance of 138-Kv Twin-Circuit Transmission Lines of Commonwealth Edison Company - Operating Experience and Field Studies

Abnormal Voltage Conditions Produced by Open Conductors on 3-Phase Circuits Using Shunt Capacitors [includes discussion]

Switching Capacitive Kilovolt-Amperes with Power Circuit Breakers [includes discussion]