Jordningar- - Tillämpad fysik och elektronik - Umeå universitet

Abstract
The purpose of this thesis work is to analyze groundings, and their ability to function in case of
lightning. In Sweden the main focus of grounding validation have been on the resistance to ground,
this works for currents around 50 Hz. The standards do not pay attention to the fact that the lightning
current is a transient, containing frequencies up to the MHz area.
Today the validation of groundings are made regarding the groundings resistance to ground, this may
give a false comfort that the grounding is going to work in case of lightning. An insufficient grounding
can result in damaged electronics, fires, and personal injuries.
A cause to the focus on the resistance to ground is that there have been no commercial
methods to measure the impedance to ground. This thesis work analyzes two new measuring
methods, called tree pole sweep and the high frequency method, who both measures
resistance to ground. Large amounts of data from these methods are analyzed.
At high frequencies the inductive properties of groundings will have a big influence. A long ground
electrode will often have low resistance to ground, but in case of lightning it´s impedance will increase
dramatically, and the grounding will probably not work. For longer copper wires there will also be
reflections of the lightning current inside the wire.
There is a massive increase of the amount of wind turbines in Sweden and many parts of the world. A
wind turbine is basically a big lightning collector, there are suspicions that the rotating blades may
trigger lightnings. To make things worse many wind turbines are built in sizes approaching 200 m.
Wind turbines can also be damaged by indirect lightnings, whose consequences increase with the size
of the wind farm. Due to the fact that wind turbines and stations are linked by cables and copper wires.
Only considering the resistance to ground, may be a serious mistake. That may lead to big financial
losses due to damaged wind turbines and lost electrical generation.
This thesis work also analyzes how wind turbines should be grounded, regarding their
impedance to ground and the lightning current at different frequencies. A method of
optimization have been developed, whose goal is to minimize the construction cost of
groundings and maximize their ability to function in case of lightning. To evaluate groundings
a new function is introduced, which is named the groundings evaluation function.