Voltage Surge

Definition: Voltage surge is defined as the sudden rise in excessive voltage which damages the electrical equipment of an installation. The overvoltage in the lines occurs because of a rise in voltage between both phases and between phase and ground. The voltage surges are mainly classified under two heading; internal and external voltages.

Types of Voltage Surge

The overvoltage in the power station can be caused either by the internal disturbance or by the atmospheric eruption. On the basis of the generation of overvoltages the voltage surge are classified into two categories.These are

1. Internal Overvoltage
2. External Overvoltage

Internal Overvoltage

When the voltage in the system raises itself beyond the rated voltage, then such type of voltage is called internal overvoltage. The internal voltage may be transient, dynamic or stationary in nature. If the overvoltage wave is transient in nature, then there frequency is unrelated to the normal frequency, and it will persist few cycles only.

Transient overvoltage can be caused by the operation of circuit breakers when switching inductive or capacitive loads. These voltages can also be generated by interrupting very small current or by the sudden grounding of one phase of a system with an insulated neutral.

Dynamic overvoltage occurs at the normal frequency and persists only for a few seconds. These voltages may be generated by the disconnection of a generator or because of suddenly thrown off a large portion of the load.

Stationary overvoltage occurs at system frequency and remains for sometimes may be for an hour. Such type of voltages is generated when an earth fault on one line is continued for a long time. This voltage can also be caused when the neutral is grounded through an arc suppression coil, thereby leading to the overvoltage on the sound phase.

These voltages exceed three to five times the normal phase to neutral peak voltage of the system and relatively harmless for equipment having proper insulation.

The internal overvoltage mainly causes because of the following reasons;

• Switching Operation on Unloaded Line – During the switching operation, the line is connected to a source of voltage and travelling waves are set up which rapidly charge the line. These waves instantly attained the voltage of the magnitude not exceeding twice the supply voltage at the instant of disconnection.
• Sudden Opening  of Load Line – When the load on the line is suddenly opened a transient voltage of the value of e = i z₀ is set up, where i is the instantaneous value of the current at the instant of opening of the line and Z₀ is the natural or the surge impedance of the line. The transient overvoltage of the line does not depend on the line voltage and therefore low voltage transmission system is liable to overvoltage of the same magnitude as compared to high voltage system.
• Insulation Failure – The failure of insulation between the line and the earth is very frequent. When the insulation breakdown occurs the potential at fault suddenly falls from maximum to zero and therefore a negative voltage wave of very steep front in the form of surges are generated in both directions.

External Overvoltages

The overvoltage which is caused by the atmospheric discharge such as static discharge or lightning strokes such type of voltage is called external overvoltage. External  overvoltage can cause considerable stress on the insulation.The intensity of the voltages is varied in the case of the lightning.

The intensity of lightning depends on the how directly the line is struck, i.e., directly by the main discharge, directly by a branch or streamer, or by the induction due to a flash passing near to but not touching the line.

The installation in the power station is mainly classified into two types the one which is electrically exposed, resulting in the apparatus being subjected to overvoltages of the atmospheric origin and another which is electrically non-exposed and therefore not subject to this type of overvoltage.