MV design guide - Schneider Electric
MV design guide - Schneider Electric
MV design guide - Schneider Electric
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Switchgear<br />
definition<br />
Derating<br />
Introduction<br />
The various standards or recommendations impose validity limits on<br />
device characteristics.<br />
Normal conditions of use are described in the "Medium voltage circuit<br />
breaker" chapter.<br />
Beyond these limits, it is necessary to reduce certain values, in other<br />
words to derate the device.<br />
c Derating must be considered:<br />
v in terms of the insulation level, for altitudes of over 1 000 metres<br />
v in terms of the rated current, when the ambient temperature exceeds<br />
40°C and for a protection index of over IP3X,<br />
(see chapter on "Protection indices").<br />
These different types of derating can be accumulated if necessary.<br />
N.B.: there are no standards specifically dealing with derating.<br />
However, table V § 442 of IEC 60 694 deals with temperature rises and gives limit temperature<br />
values not to be exceeded according to the type of device, the materials and the dielectric<br />
used.<br />
Example of application:<br />
Can equipment with a rated voltage<br />
of 24 kV be installed at 2500 metres?<br />
The impulse withstand voltage required is<br />
125 kV.<br />
The power frequency withstand 50 Hz is<br />
50 kV. 1 mn.<br />
c For 2500 m:<br />
v k is equal to 0.85<br />
v the impulse withstand must be<br />
125/0.85 = 147.05 kV<br />
v the power frequency withstand 50 Hz<br />
must be 50/0.85 = 58.8 kV<br />
c No, the equipment that must be<br />
installed is:<br />
v rated voltage = 36 kV<br />
v impulse withstand = 170 kV<br />
v withstand at 50 Hz = 70 kV<br />
Insulation derating according<br />
to altitude<br />
Standards give a derating for all equipment installed at an altitude<br />
greater than 1 000 metres.<br />
As a general rule, we have to derate by 1.25 % U peak every 100 metres<br />
above 1 000 metres.<br />
This applies for the lightning impulse withstand voltage and the power<br />
frequency withstand voltage 50 Hz - 1 mn. Altitude has no effect on the<br />
dielectric withstand of circuit breakers in SF6 or vacuum, because they<br />
are within a sealed enclosure. Derating, however, must be taken account<br />
of when the circuit breaker is installed in cubicles. In this case, insulation<br />
is in air.<br />
c Merlin Gerin uses correction coefficients:<br />
v for circuit breakers outside of a cubicle, use the graph below<br />
v for circuit breakers in a cubicle, refer to the cubicle selection <strong>guide</strong><br />
(derating depends on the cubicle <strong>design</strong>).<br />
Exception of the Mexican market: derating starts from zero metres<br />
(cf. dotted line on the graph below).<br />
N.B.:<br />
if you do not want to supply 36 kV equipment,<br />
we must have the appropriate test certificates<br />
proving that our equipment complies with the<br />
request.<br />
1<br />
0.9<br />
0.8<br />
0.7<br />
0.6<br />
Correctilon coefficient k<br />
0.5<br />
0 1000 2000 3000 4000 5000<br />
altitude in m<br />
64 Merlin Gerin <strong>MV</strong> <strong>design</strong> <strong>guide</strong> <strong>Schneider</strong> <strong>Electric</strong>