Definitions of Terminology for Switching Phenomena in Power Systems for Electrical Engineers and Students-part 1

A circuit breaker should be capable of making, carrying, and interrupting the current under both normal and abnormal conditions, especially in case of a short circuit or fault occurrence. Several short-circuit conditions including a single-phase grounded,three-phase grounded, and phase-to-phase ungrounded fault are expected to occur in power systems. When a circuit breaker interrupts the short-circuit current, different switching phenomena are observed depending on the conditions.

In this glossary, we have tried to define the most recent and most useful terms used for the phenomenon of switching in high and medium-voltage networks.

Recovery Voltage:

The power frequency voltage which appears across the terminals of a pole of a

switching equipment after the current interruption.

Transient Recovery Voltage (TRV):

A transient recovery voltage for circuit breakers is the voltage that appears across the terminals immediately after the current interruption. It is a critical parameter for fault interruption by a circuit breaker; its amplitude and rate of rise of TRV are dependent on the characteristics of the system connected on both terminals of the circuit breaker and on the type of fault that this circuit breaker has to interrupt.

Thermal Interruption/Thermal Interrupting Region:

Thermal interrupting period of a circuit breaker within an interval of less than a

the quarter cycle of power frequency after an interruption at current zero, where a very small post-arc current (residual current) is still flowing and putting energy into the high ohmic resistance of the vanishing arc plasma. When a circuit breaker cannot provide sufficient fresh gas and cooling, thermal reignition may happen.

Dielectric Interruption/Dielectric Interrupting Region:

Dielectric interrupting period of a circuit breaker within an interval of a quarter cycle of power frequency or longer after interruption at current zero, where the transient recovery voltage (TRV) is applied between the contacts. The dielectric strength across the contacts increases with the contact gap. When the TRV exceeds the dielectric strength across the contacts at any moment during the dielectric interrupting region, dielectric breakdown named restrike will happen.

Overvoltage:

Any voltage between one phase and the earth or between phases having a peak value or values exceeding the corresponding peak of the highest voltage for equipment.

Short-Circuit Current

An overcurrent resulting from a short circuit due to a fault or an incorrect connection in an electric circuit.

Out-Of-Phase Conditions:

Abnormal circuit conditions of loss or lack of synchronism between the parts of an electrical system on either side of a circuit breaker in which, at the instant of operation of the circuit breaker, the phase angle between rotating vectors,

representing the generated voltages on either side exceed the normal value.

Short-Line Fault (SLF):

A short-line fault refers to a fault that occurs on a line a few hundred meters to

several kilometers down the line from the circuit-breaker terminal. When a circuit

breaker clears the SLF, TRV with a steep rate of rise is observed due to high-frequency oscillation generated by the propagating waves that iterate traveling on

the line and reflections between the circuit-breaker terminal and the fault point.

Peak Factor (of the Line Transient Voltage):

The ratio between the maximum excursion and the initial value of the line transient voltage to earth of a phase of an overhead line after the interruption of a short-line fault current. The initial value of the transient voltage corresponds to the instant of arc extinction in the pole considered.

First-Pole-To-Clear Factor (in a Three-Phase System):

When interrupting any symmetrical three-phase current, the first-pole-to-clear factor is the ratio of the power frequency voltage across the first interrupting pole before current interruption in the other poles, to the power frequency voltage occurring across the pole or the poles after interruption in all three poles.

Amplitude Factor:

The ratio between the maximum excursions of the transient recovery voltage to the crest value of the power frequency recovery voltage of that pole.

Minimum Clearing Time:

Sum of the minimum opening time, minimum relay time (0.5 cycles), and the shortest arcing time of a minor loop interruption in the phase with intermediate asymmetry that starts with a minor loop at short-circuit current initiation.

Note 1: This definition is applicable only for the determination of the test

parameters during short-circuit breaking tests according to test duty T100a.

Note 2: For testing purposes the minimum arcing time found during test duty

T100 s is used.

Bus Terminal Fault Interruption:

Fault current interruption when a fault has occurred close to a circuit breaker (bus terminal).

Short-Line Fault Interruption:

Fault current interruption when a fault occurs from a transmission line at a

distance that ranges from 100 m to a few kilometers from a circuit breaker.

Long-Line Fault (LLF) Interruption:

Fault current interruption when a fault occurs from a transmission line at a

distance longer than a few kilometers up to a few hundred kilometers from a circuit breaker.

Transformer Limited Fault Interruption:

Fault current interruption when a fault occurs immediately after a power

transformer. The fault location and the circuit breaker are on the same side of the transformer or on opposite sides.

Small Inductive Current Switching:

Current switching in case of a circuit breaker connected to an inductive load.

Small Capacitive Current Switching Current switching in case of a circuit breaker connected to a capacitive load.

Insulation Coordination:

Selection of the electric strength (LIWV and SIWV requirements) of equipment in

relation to the voltages which can impose on the system for which the equipment is intended, taking into account the service environment and the characteristics of the available protective devices such as MOSA arrangements.

Source:

1-Switchgear Content Website

2- CIGRE Switching Equipment Green Book