MV design guide - Schneider Electric
MV design guide - Schneider Electric
MV design guide - Schneider Electric
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Switchgear<br />
definition<br />
Voltage transformer<br />
We can leave a<br />
voltage transformer in an<br />
open circuit without any danger<br />
but it must never be<br />
short-circuited.<br />
The voltage transformer is intended to provide the secondary circuit<br />
with a secondary voltage that is proportional to that applied to the<br />
primary circuit.<br />
N.B.: IEC standard 60 186 defines the conditions which voltage transformers must meet.<br />
It comprises a primary winding, a magnetic core, one or several<br />
secondary windings, all of which is encapsulated in an insulating resin.<br />
Characteristics<br />
The rated voltage factor (KT)<br />
The rated voltage factor is the factor by which the rated primary voltage<br />
has to be multiplied in order to determine the maximum voltage for which<br />
the transformer must comply with the specified temperature rise and<br />
accuracy recommendations. According to the network's earthing<br />
arrangement, the voltage transformer must be able to withstand this<br />
maximum voltage for the time that is required to eliminate the fault.<br />
Normal values of the rated voltage factor<br />
Rated voltage Rated Primary winding connection mode<br />
factor duration and network earthing arrangement<br />
1.2 continuous phase to phase on any network<br />
neutral point to earth for star connected<br />
transformers in any network<br />
1.2 continuous phase to earth in an earthed<br />
neutral network<br />
1.5 30 s<br />
1.2 continuous phase to earth in a network<br />
without an earthed neutral with<br />
1.9 30 s automatic elimination of earthing faults<br />
1.2 continuous phase to earth in an isolated neutral network<br />
without automatic elimination of earthing faults,<br />
1.9 8 h or in a compensated network with an extinction coil<br />
without automatic elimination of the earthing fault<br />
N.B.: lower rated durations are possible when agreed to by the manufacturer and the user.<br />
Generally, voltage transformer manufacturers comply with the following<br />
values: VT phase/earth 1.9 for 8 h and VT phase/phase 1.2 continuous.<br />
Rated primary voltage (Upr)<br />
c According to their <strong>design</strong>, voltage transformers will be connected:<br />
v either phase to earth<br />
v or phase to phase<br />
3000 V<br />
e<br />
/<br />
100 V<br />
e<br />
Upr =<br />
U<br />
e<br />
3000 V / 100 V<br />
Upr = U<br />
<strong>Schneider</strong> <strong>Electric</strong><br />
Merlin Gerin <strong>MV</strong> <strong>design</strong> <strong>guide</strong><br />
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