Definitions of Terminology for Switching Phenomena in Power Systems for Electrical Engineers and Students-part 2

Lightning Impulse (LI):

Voltage pulse of a specified shape (assuming a lightning surge) applied during

dielectric tests with a virtual front duration of the order of 1 μs and a time to half

value of the order of 50 μs.

Note: The lightning impulse is defined by the two figures giving these durations

in microseconds; in particular, the standard lightning impulse is 1.2/50 μs.

Lightning Impulse Protection Level (LIPL):

Maximum permissible peak voltage value on the terminals of a surge protective device subjected to lightning impulses under specific conditions.

Typically, the protection voltage of surge arresters corresponds to the current

of 20 kA.

Switching Impulse (SI):

Voltage pulse of a specified shape (assuming a switching surge) applied during

dielectric tests, with a time to the crest of 100 μs to 300 μs and a time to half the value of a few milliseconds.

Note: The switching impulse is defined by the two figures giving these durations

in microseconds; in particular, the standard switching impulse is 250/2500 μs.

Switching Impulse Protection Level (SIPL) Maximum permissible peak voltage value on the terminals of a surge protective device subjected to switching impulses under specific conditions. Typically, the protection voltage of surge arresters corresponds to the current of 2 kA.

Very Fast Transient Overvoltage (VFTO):

Very fast transient overvoltage (VFTO) is often generated by the operation of a

disconnecting switch within GIS installations. It can reach up to 2.6 p.u. or even

more at frequencies in the MHz range.

Multi-Bundle Conductors:

Overhead power line consists of more than one conductor up to eight conductors are used in power transmission, especially for EHV and UHV levels.

DC Component of Current, X/R Ratio:

The initial value of the DC component of fault current is dependent on the exact time within a cycle at which the fault takes place and the value of current at that time. The DC component is equal to the value of the instantaneous AC current at

fault inception and with opposite polarity. In the worst case, the initial DC offset

will be √2 times the symmetrical short-circuit value (RMS). This initial DC

component decays over time, eventually reaching zero. The time constant τ is

dependent on the system reactance (X) and resistance (R). For a power frequency (f), it is given by:

τ=1/{(2πf)[X/R]}

Effectively Earthed Neutral System:

The system is earthed through a sufficiently low impedance such that for all system conditions the ratio of the zero-sequence reactance to the positive-sequence reactance (X0/X1) is positive and less than 3, and the ratio of the zero-sequence resistance to the positive-sequence reactance (R0/X1) is positive and less than 1. Normally such systems are solidly earthed (neutral) systems or low-impedance earthed (neutral) systems.

Non-effectively Earthed Neutral System:

The system does not correspond to the effectively earthed neutral system. Normally such systems are isolated neutral systems, high impedance earthed (neutral) systems, or resonant earthed (neutral) systems.

Zero-Sequence Impedance:

The ratio of the zero-sequence component of the voltage, assumed to be sinusoidal, supplied to a synchronous machine, and the zero-sequence component of the current at the same frequency.

Positive-Sequence Impedance:

The impedance offered by the system to the flow of positive-sequence current is

called positive-sequence impedance.

Negative-Sequence Impedance:

The impedance offered by the system to the flow of negative-sequence current is

called negative-sequence impedance.

Secondary Arc:

When a grounding fault occurs in the faulty phase of transmission lines, an arc is

generated from the faulty location to the ground. After a circuit breaker interrupts the fault current, a small secondary arc current continues to flow at the fault location due to electrostatic and electromagnetic induction. The secondary arc is extinguished when the insulation at the fault location is sufficiently recovered.

Source:

1-Switchgear Content Website

2- CIGRE Switching Equipment Green Book