Surge protection for energy systems
Very large surge voltages are caused mainly by lightning strikes on or close to energy systems. Even from several hundred metres away, lightning currents can also cause impermissible surge voltages in conductor loops, through either capacitive, inductive or galvanic coupling.
Large surge voltages are coupled over a radius of up to 2 km. Switching operations involving inductive loads create dangerous surge voltages in the medium and low-voltage power networks.
1. Lightning discharges
(LEMP: Lightning Electro Magnetic Impulse)
The international lightning protection standard IEC 62305 describes how direct lightning strikes of up to 200 kA are safely arrested. The current is coupled into the earthing system and, due to the voltage drop at the earthing resistor, half of the lightning current is coupled into the internal installation.
The partial lightning current then divides itself among the power lines entering the building (number of cores of power line entering building), while around 5% enters data cables.
Example split between earth/installation:
50% – 50%
i = 50 kA; R = 1 Ohm
U = i × R = 50,000 A × 1 Ohm = 50,000 V
- U – Surge voltage
- i – Surge current
- R – Earthing resistance
The voltage resistance of the components is exceeded and uncontrolled arcing occurs. Only surge arresters can safely arrest these dangerous voltages.
The biggest surge voltages are caused by lightning strikes. According to IEC 62305 (VDE 0185-305), lightning strikes are simulated with lightning surge currents of up to 200 kA (10/350 μs).
Table 1 – Typical distribution of lightning current
|1||Lightning strike||100%||Iimp = max 200 kA (IEC 62305)|
|2||Earthing system||~ 50%||I = 100 kA (50%)|
|3||Electrical installation||~ 50%||I = 100 kA (50%)|
|4||Data cable||~ 50%||I = 5 kA (5%)|
1.1 Switching operations
(SEMP: Switching electromagnetic pulse)
Switching operations occur due to the switching of large inductive and capacitive loads, short circuits, and interruptions to the power system. They are the most common cause of surge voltages.
2. Electrostatic discharge (ESD)
Electrostatic discharges are caused by friction. When a person walks on a carpet, charge separation occurs – in this instance it is however harmless to humans. However, it can interfere with and destroy electronic components.
Equipotential bonding is necessary here to avoid this charge separation.
|Title:||Design tips for lightning and surge protection systems – OBO Betetrmann|
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