MV design guide - Schneider Electric
MV design guide - Schneider Electric
MV design guide - Schneider Electric
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Presentation<br />
Metal-enclosed,<br />
factory-built equipment<br />
Voltage<br />
Operating voltage U (kV)<br />
This is applied across the equipment terminals.<br />
Rated voltage Ur (kV)<br />
Previously known as nominal voltage, this is the maximum rms.<br />
(root mean square) value of the voltage that the equipment can withstand<br />
under normal operating conditions.<br />
The rated voltage is always greater than the operating voltage and,<br />
is associated with an insulation level.<br />
Insulation level Ud (kV rms. 1 mn) and Up (kV peak)<br />
This defines the dielectric withstand of equipment to switching operation<br />
overvoltages and lightning impulse.<br />
c Ud: overvoltages of internal origin, accompany all changes in the circuit:<br />
opening or closing a circuit, breakdown or shorting across an insulator,<br />
etc…<br />
It is simulated in a laboratory by the rated power-frequency withstand<br />
voltage<br />
for one minute.<br />
c Up: overvoltages of external origin or atmospheric origin occur when<br />
lightning falls on or near a line. The voltage wave that results is simulated<br />
in a laboratory and is called the rated lightning impulse withstand voltage.<br />
N.B.: IEC 694, article 4 sets the various voltage values together with, in article 6,<br />
the dielectric testing conditions.<br />
Example:<br />
c Operating voltage: 20 kV<br />
c Rated voltage: 24 kV<br />
c Power frequency withstand voltage<br />
50 Hz 1 mn: 50 kV rms.<br />
c Impulse withstand voltage<br />
1.2/50 µs: 125 kV peak.<br />
6 Merlin Gerin <strong>MV</strong> <strong>design</strong> <strong>guide</strong> <strong>Schneider</strong> <strong>Electric</strong>
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