Voltage Surge & Its Classification | Power System | Electrical Engineering

By voltage surge it is meant any sudden excessive rise in voltage that may be dangerous to the electrical equipment of an installation. Among such surges are those whose value approaches the voltage at which the equipment in the installation has been tested. Over voltages may occur both as a rise in voltages between phases and between a phase and ground.

All voltage surges in high-voltage installations are broadly classed under two headings:

1. Internal and

2. External voltages.

1. Internal Over-Voltages:

They originate in the system itself and may be transient, dynamic or stationary. Those of a transient nature will have a frequency unrelated to the normal system frequency and will persist a few cycles only. They can be caused by the operation of circuit breakers when switching inductive or capacitive loads, “current chopping” when interrupting very small currents or by the sudden grounding of one phase of a system operating with an insulated neutral.

Dynamic over-voltages occur at normal system frequency and persist only for a few seconds. They may be caused, for example, by the disconnection of a generator which over-speeds, or when a large portion of the load is suddenly thrown off.

Stationary over-voltages also occur at system frequency but they may persist for some time, perhaps hours. Such a condition can arise when an earth fault on one line is sustained, as indeed it may intentionally be when the neutral is grounded through an arc-suppression coil, thereby leading to the over-voltages on the sound phases.

Over-voltages described above rarely exceed three to five times the normal phase-to-neutral peak voltage of the system and subject to apparatus having an adequate insulation level, should be relatively harmless.

2. External Over-Voltages:

They are caused by atmospheric discharges such as static charges or lightning strokes and are therefore not related to the system. They are often of such magnitude as to cause considerable stress on the insulation and, in the case of lightning, will vary in intensity depending on how directly the line is struck, i.e., directly by the main discharge, directly by a branch or streamer, or by induction due to a flash passing near to but not touching the line.

Switchgear specifications describe two classifications for an installation namely one which is “electrically exposed” resulting in the apparatus being subject to over-voltages of atmospheric origin, and another which is “electrically non-exposed” and therefore not subject to this type of overvoltage.

The first of these is generally recognized as an installation connected directly to overhead transmission lines, e.g., outdoor switchgear or transformers but it can apply to indoor gear connected to overhead lines through outdoor to indoor through bushings or by a short length of cable. The second classification is one usually associated with underground cable networks and the switchgear will normally be of the indoor type.

Because of the stresses due to external voltages, it is now required that certain switchgear and transformers shall be impulse tested, and this is the case for circuit breakers of the outdoor type for service voltages of 22 kV and above.

It is also intimated that suitable protective devices be installed to ensure that the amplitude of the surges imposed on the terminals of the outdoor apparatus is limited to a value not exceeding 80 per cent of the impulse test level.

A surge is the movement of a charge that is suddenly released in a conductor and which travels along that conductor in the form of a wave. The potential of a conductor being a measure of charge density, it follows that the potential wave must be accompanied by a current wave.

The shape of the potential wave is affected by a number of factors, such as inductance and capacitance. Thus, on a line where the impedance is purely inductive, the front of the wave could be almost vertical, while if the impedance is purely capacitive then the wave front would slope steeply away from the vertical. From this it is clear that the wave-shape is affected by the relation between L and C values, this relationship being expressed as the surge impedance,

where Z₀ is surge impedance in ohms, L is inductance in henries and C is capacitance in farads.

As the wave travels along the line its front is modified by the inductance of the line and the distributed capacitances to earth. It may be further modified by the capacitances of bushings, insulators, etc., which the wave encounters on its journey, thus reducing the steepness of the wave front.

A further point has also to be considered, namely that when the travelling wave reaches a point where there is a change in surge impedance of the line, reflection of the wave will occur, reflection being whole or partial depending on the amount of change in surge impedance.

In the case, for example, of an open-ended feeder, the change in surge impedance is infinite and a travelling wave would be totally reflected, with the result that the pressure would be practically doubled at the point of reflection due to the front of the wave being reversed and adding itself to the remainder of the wave still travelling forward; the duration of doubled value will depend on the length of the wave tail.

In other cases, such as one where an overhead line is joined to an underground cable, the change in surge impedance is not infinite and the reflection will depend on the relative values of the impedances of the overhead line and the cable.

It is obvious that surges when travelling along a line will, at some point reach terminal apparatus such as cable boxes, transformers, or switchgear, and, unless some means is provided to release the surge, there is a danger of a breakdown of the insulation of the terminal apparatus, unless the insulation level of the latter is high enough to be invulnerable. On smaller systems it could be an expensive solution and, therefore, it becomes necessary to install lightning arresters or surge diverters.