Earthing and return current conductors
Earthing conductors
Earthing conductors interconnect the GIS enclosure at numerous points with the earthing net (multipoint earthing). Being a component of the earthing system they effectively help to avoid dangerous touch voltages during a short circuit, and to keep HF transient overvoltages on a sufficiently low level during normal service. To meet these requirements they shall have low impedances at 50/60 Hz as well as in the HF range.
Return current conductors
The enclosures of all three phases of GIS type ELK are many times interconnected to each other and to the earthing net. In the so formed loops, the induced currents are circulating due to electromagnetic coupling with the phase conductors.The return current in the enclosure of each phase is shifted against the operating current by approx. 180°. This is the reason why these induced currents are called „return“ currents.
During normal service the return current conductors (crossing conductors) carry permanently the return currents, which can achieve up to 90% of the operating current, and balance return currents respectively. Accordingly, in case of external three-phase short circuit the return currents can achieve up to 90% of the short-circuit current. The return current conductors are dimensioned accordingly to these currents.
Overhead line connection
Depending on rated voltage and substation layout o switchgear there are four typical connecting possibilities for OHL connections. They are shown with corresponding layoutof the earthing and return current conductors in Fig below
Earthing of wall sealing
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