Earting System, Low and medium Voltage
In balanced operating conditions these three voltages are phase shifted by 120° and have the value: U / (3)1/2 U being the phase-to-phase voltage measured between phases.
From a physical point of view, the neutral is the common point of three star-connected windings. It may or may not be accessible, may or may not be distributed and may or may not be earthed, which is why we refer to the earthing system.
The neutral may be connected to earth either directly or via a resistor or reactor. In the first case, we say that the neutral is solidly (or directly) earthed and, in the second case, we say that the neutral is impedance-earthed.
** The earting System plays in very important role in a network. On ocurrence of an insolation fault or a phase accidentlly earthed, the values taken by the fault currents, touch voltages and overvoltages are closely related to the type of neutral earting connection.
A solidly earthed neutral helps to limit overvoltages. However, it generates very high fault currents. On the other hand, an isolated or unearthed neutral limits fault currents to very low values but encourages the occurrence of high overvoltages.
In any installation, service continuity in the presence of an insulation fault also depends on the earthing system. An unearthed neutral allows continuity of service in medium voltage, as long as the security of persons is respected. On the other hand, a solydly earthed neutral , or low impedance-earthed neutral, requires tripping to take place on occurrence of the first insulation fault.
Different earthing systems
The different types of neutral point connection to earth are shown in the follow list: We can make a distinction between:
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the solidly (or directly) earthed neutral,
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the unearthed neutral, or high impedance-earthed neutral,
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resistance earthing
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reactance earthing
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Petersen coil earthin
References:
1.- WebPage:electrical-engineering-portal.com/download-center/books-and-guides/schneider-electric/earthing-systems
2.- Document: Earthing-systems-in-MV-LV-electrical-networks.pdf