The paper shows that the vacuum circuit breaker is well suited for the duties that a circuit breaker must perform. Reasonable care must be taken under certain circumstances when applying these breakers. Such applications are identified and simple corrective measures are proposed.

A Guide to the Application of Vacuum Circuit Breakers

This paper covers the derivation of two sets of short line fault equations for the maximum rate of rise of recovery voltage (RRRV) characteristic of gas blast interrupters. The assumption that the electrical conductivity is proportional to the heat flux potential leads to a Cassie integral equation for post-arc current, a critical thermal time constant and the first set of RRRV equations. The assumption that the electrical conductivity varies exponentially with the heat flux potential leiads to the Mayr equation and the second set of RRRV equations. For both sets of equations it is shown that the maximum RRRV is (a) approximately proportional to the gas pressure; (b) proportional to the effective nozzle arc length at current zero and the number of series breaks; (c) proportional to temperature dependent coefficients which are functions of gas properties; and (d) inversely proportional to (dI/dt)m where m=3/2 and 1 for the "Cassie" and "Mayr" RRRV equations, respectively. These equations are complementary and serve as "first-order" limits on the interrupting capability of gas blast interrupters.

Arc Cooling and Short Line Fault Interruption

Power systems are subjected to many forms of transient phenomena brought about essentially by sudden changes in the steady state values of voltages or currents. Such changes may be the result of a lightning stroke, some malfunction of the system or be brought about by the switching of a circuit either to clear a fault or as a normal operational procedure. To be practical the scope of the review has had to be restricted. Nevertheless it has been found possible to cover a wide range of transient voltages from those which pose problems in transmission systems operating at the highest levels of system voltage down to those which can appear in the domestic situation. Apart from mentioning nonlinear elements in systems, the review is, in the main, restricted to phenomena which are essentially linear in nature and no attempt has been made to include the more complicated phenomena associated with ferroresonance or the production and propagation of harmonic voltages and currents. The review does not claim to be exhaustive but it is hoped that the major causes of transient overvoltages have been considered. In addition to dealing with the causes of transient overvoltages, the review indicates the range of analytical methods that are now available for their analysis and assessment.

Transient overvoltages on power systems

The short-line fault interrupting ability of gas-blast circuit breakers is limited by arc energy balance processes very close to current zero. Practical calculation of the energy balance limit in a given circuit requires mathematical modeling of the arc as a circuit element. Using the combined Cassie-Mayr model, useful relations are derived for treating the limiting cases of the short-line fault both with and without capacitance shunting. Included are methods for approximate analysis and extrapolation of data from actual and simulated short-line fault tests with and without RRRV controlling resistance and capacitance shunts. In these analyses the arc is described by only two model parameters: a near-current-zero arc voltage Eo and an arc time constant 0, both definable as simple functions of the circuit di/dt. Examples are given of such analyses of high di/dt interrupting tests on actual and model SF6-blast interrupters.

Practical Modeling of the Circuit Breaker ARC as a Short Line Fault Interrupter

Representative unmodified transient recovery voltages (TRVs) of power systems have been selected to derive the recovery voltage ratings for power circuit breakers. Mathematical expressions have been derived that combine fundamental system parameters such as load current, line impedancee, voltage regulation, and transformer reactance, together with the rated voltage and circuit interrupting capabilities of the breaker, to describe the TRV withstand requirements for power circuit breakers. Two points in a volt-time plane are developed to describe the TRV withstand requirements of power breakers, and numerical values are listed for these proposed rating values. The TRV ratings for fault currents less than rated currents and for kilometric short line faults are also developed. Since the expressions developed describe system recovery voltages, they may also be used to calculate the TRV of actual power systems.

C. H. Titus

Papers

Severe Rates of Rise of Recovery Voltage Associated with Transmission Line Short Circuits