Published by Electrical Installation Wiki, Chapter J. Overvoltage protection – Overvoltage of atmospheric origin
Overvoltage definitions
Overvoltage (in a system) any voltage between one phase conductor and earth or between phase conductors having a peak value exceeding the corresponding peak of the highest voltage for equipment.
Definition from the International Electrotechnical Vocabulary (IEV 604-03-09); available on [1]
Various types of overvoltage
An overvoltage is a voltage pulse or wave which is superimposed on the rated voltage of the network (see Fig. J1).
This type of overvoltage is characterized by (see Fig. J2):
• the rise time tf (in μs);
• the gradient S (in kV/μs).
An overvoltage disturbs equipment and produces electromagnetic radiation. Moreover, the duration of the overvoltage (T) causes an energy peak in the electric circuits which could destroy equipment.
Four types of overvoltage can disturb electrical installations and loads:
• Switching surges: high-frequency overvoltages or burst disturbance (see Fig. J1) caused by a change in the steady state in an electrical network (during operation of switchgear).
• Power-frequency overvoltages: overvoltages of the same frequency as the network (50, 60 or 400 Hz) caused by a permanent change of state in the network (following a fault: insulation fault, breakdown of neutral conductor, etc.).
• Overvoltages caused by electrostatic discharge: very short overvoltages (a few nanoseconds) of very high frequency caused by the discharge of accumulated electric charges (for example, a person walking on a carpet with insulating soles is electrically charged with a voltage of several kilovolts).
• Overvoltages of atmospheric origin.
Overvoltage characteristics of atmospheric origin
Lightning strokes in a few figures: Lightning flashes produce an extremely large quantity of pulsed electrical energy (see Figure J4)
• of several thousand amperes (and several thousand volts),
• of high frequency (approximately 1 megahertz),
• of short duration (from a microsecond to a millisecond).
Between 2000 and 5000 storms are constantly undergoing formation throughout the world. These storms are accompanied by lightning strokes which represent a serious hazard for persons and equipment. Lightning flashes hit the ground at an average of 30 to 100 strokes per second, i.e. 3 billion lightning strokes each year.
The table in Figure J3 shows some lightning strike values with their related probability. As can be seen, 50% of lightning strokes have a current exceeding 35 kA and 5% a current exceeding 100 kA. The energy conveyed by the lightning stroke is therefore very high.
Lightning also causes a large number of fires, mostly in agricultural areas (destroying houses or making them unfit for use). High-rise buildings are especially prone to lightning strokes.
Effects on electrical installations
Lightning damages electrical and electronic systems in particular: transformers, electricity meters and electrical appliances on both residential and industrial premises.
The cost of repairing the damage caused by lightning is very high. But it is very hard to assess the consequences of:
• disturbances caused to computers and telecommunication networks;
• faults generated in the running of programmable logic controller programs and control systems.
Moreover, the cost of operating losses may be far higher than the value of the equipment destroyed.
Lightning stroke impacts
Lightning is a high-frequency electrical phenomenon which causes overvoltages on all conductive items, especially on electrical cabling and equipment.
Lightning strokes can affect the electrical (and/or electronic) systems of a building in two ways:
• by direct impact of the lightning stroke on the building (see Fig. J5 a);
• by indirect impact of the lightning stroke on the building:
– A lightning stroke can fall on an overhead electric power line supplying a building (see Fig. J5 b). The overcurrent and overvoltage can spread several kilometres from the point of impact.
– A lightning stroke can fall near an electric power line (see Fig. J5 c). It is the electromagnetic radiation of the lightning current that produces a high current and an overvoltage on the electric power supply network. In the latter two cases, the hazardous currents and voltages are transmitted by the power supply network.
– A lightning stroke can fall near a building (see Fig. J5 d). The earth potential around the point of impact rises dangerously.
In all cases, the consequences for electrical installations and loads can be dramatic.
The various modes of propagation
Common mode
Common-mode overvoltages appear between live conductors and earth: phase-to-earth or neutral-to-earth (see Fig. J7 ). They are dangerous especially for appliances whose frame is connected to earth due to risks of dielectric breakdown.
Differential mode
Differential-mode overvoltages appear between live conductors:
phase-to-phase or phase-to-neutral (see Fig. J8). They are especially dangerous for electronic equipment, sensitive hardware such as computer systems, etc.
Characterization of the lightning wave
Analysis of the phenomena allows definition of the types of lightning current and voltage waves.
• 2 types of current wave are considered by the IEC standards:
– 10/350 µs wave: to characterize the current waves from a direct lightning stroke (see Fig. J9);
• 8/20 µs wave: to characterize the current waves from an indirect lightning stroke (see Fig. J10).
These two types of lightning current wave are used to define tests on SPDs (IEC standard 61643-11) and equipment immunity to lightning currents.
The peak value of the current wave characterizes the intensity of the lightning stroke.
• The overvoltages created by lightning strokes are characterized by a 1.2/50 µs voltage wave (see Fig. J11).
This type of voltage wave is used to verify equipment’s withstand to overvoltages of atmospheric origin (impulse voltage as per IEC 61000-4-5).
Source URL: https://www.electrical-installation.org/enwiki/Overvoltage_of_atmospheric_origin
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