MV design guide - Schneider Electric

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