Chapter A General rules of electrical installation design

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B8
B - Connection to the MV public
distribution network
Earth faults on medium-voltage systems
can produce dangerous voltage levels on
LV installations. LV consumers (and substation
operating personnel) can be safeguarded
against this danger by:
b Restricting the magnitude of MV earth-fault
currents
b Reducing the substation earthing resistance
to the lowest possible value
b Creating equipotential conditions at the
substation and at the consumer’s installation
Fault
HV LV
I f
I f
R s
Fig. B9 : Transferred potential
V= I f R s
Consumer
(1) The others being unearthed. A particular case of earth-fault
current limitation is by means of a Petersen coil.
1
2
3
N
Supply of power at medium
voltage
Earthing systems
Earthing and equipment-bonding earth connections require careful consideration,
particularly regarding safety of the LV consumer during the occurrence of a shortcircuit
to earth on the MV system.
Earth electrodes
In general, it is preferable, where physically possible, to separate the electrode
provided for earthing exposed conductive parts of MV equipment from the electrode
intended for earthing the LV neutral conductor. This is commonly practised in rural
systems where the LV neutral-conductor earth electrode is installed at one or two
spans of LV distribution line away from the substation.
In most cases, the limited space available in urban substations precludes this
practice, i.e. there is no possibility of separating a MV electrode sufficiently from
a LV electrode to avoid the transference of (possibly dangerous) voltages to the
LV system.
Earth-fault current
Earth-fault current levels at medium voltage are generally (unless deliberately
restricted) comparable to those of a 3-phase short-circuit.
Such currents passing through an earth electrode will raise its voltage to a medium
value with respect to “remote earth” (the earth surrounding the electrode will be
raised to a medium potential; “remote earth” is at zero potential).
For example, 10,000 A of earth-fault current passing through an electrode with an
(unusually low) resistance of 0.5 ohms will raise its voltage to 5,000 V.
Providing that all exposed metal in the substation is “bonded” (connected together)
and then connected to the earth electrode, and the electrode is in the form of (or is
connected to) a grid of conductors under the floor of the substation, then there is no
danger to personnel, since this arrangement forms an equipotential “cage” in which
all conductive material, including personnel, is raised to the same potential.
Transferred potential
A danger exists however from the problem known as Transferred Potential. It will be
seen in Figure B9 that the neutral point of the LV winding of the MV/LV transformer
is also connected to the common substation earth electrode, so that the neutral
conductor, the LV phase windings and all phase conductors are also raised to the
electrode potential.
Low-voltage distribution cables leaving the substation will transfer this potential to
consumers installations. It may be noted that there will be no LV insulation failure
between phases or from phase to neutral since they are all at the same potential. It is
probable, however, that the insulation between phase and earth of a cable or some
part of an installation would fail.
Solutions
A first step in minimizing the obvious dangers of transferred potentials is to reduce
the magnitude of MV earth-fault currents. This is commonly achieved by earthing the
MV system through resistors or reactors at the star points of selected transformers (1) ,
located at bulk-supply substations.
A relatively medium transferred potential cannot be entirely avoided by this means,
however, and so the following strategy has been adopted in some countries.
The equipotential earthing installation at a consumer’s premises represents a remote
earth, i.e. at zero potential. However, if this earthing installation were to be connected
by a low-impedance conductor to the earth electrode at the substation, then the
equipotential conditions existing in the substation would also exist at the consumer’s
installation.
Low-impedance interconnection
This low-impedance interconnection is achieved simply by connecting the neutral
conductor to the consumer’s equipotential installation, and the result is recognized as
the TN earthing system (IEC 60364) as shown in diagram A of Figure B 0 next page.
The TN system is generally associated with a Protective Multiple Earthing (PME)
scheme, in which the neutral conductor is earthed at intervals along its length (every
3rd or 4th pole on a LV overhead-line distributor) and at each consumer’s service
position. It can be seen that the network of neutral conductors radiating from a
substation, each of which is earthed at regular intervals, constitutes, together with
the substation earthing, a very effective low-resistance earth electrode.
Schneider Electric - Electrical installation guide 2008