MV design guide - Schneider Electric
MV design guide - Schneider Electric
MV design guide - Schneider Electric
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Switchgear<br />
definition<br />
Current transformer<br />
Secondary circuit's characteristics<br />
according to IEC standards<br />
Rated secondary current (Isr) 5 or 1 A?<br />
c General case:<br />
v for local use Isr = 5 A<br />
v for remote use Isr = 1 A<br />
c Special case:<br />
v for local use Isr = 1 A<br />
N.B.: Using 5 A for a remote application is not forbidden but leads to an increase in transformer<br />
dimensions and cable section, (line loss: P = R I 2 ).<br />
Accuracy class (cl)<br />
c Metering: class 0.5<br />
c Switchboard metering: class 1<br />
c Overcurrent protection: class 10P sometimes 5P<br />
c Differential protection: class X<br />
c Zero-sequence protection: class 5P.<br />
Example:<br />
c Cable section: 2.5 mm 2<br />
c Cable length<br />
(feed/return):<br />
5.8 m<br />
Real power that the TC must provide in VA<br />
This is the sum of the consumption of the cabling and that of each device<br />
connected to the TC secondary circuit.<br />
c Consumption of copper cabling (line losses of the cabling),<br />
knowing that: P = R.I 2 and R = ρ.L/S then:<br />
(VA)<br />
L<br />
= k •<br />
S<br />
c Consumed power<br />
by the cabling:<br />
1 VA<br />
k = 0.44 : if Isr = 5 A<br />
k = 0.0176 :<br />
if Isr = 1 A<br />
L : length in metres of link conductors<br />
(feed/return)<br />
S : cabling section in mm 2<br />
c Consumption of metering or protection devices.<br />
Consumption of various devices are given in the manufacturer's technical<br />
data sheet.<br />
Rated output<br />
Take the standardised value immediately above<br />
the real power that the CT must provide.<br />
c The standardised values of rated output are:<br />
2.5 - 5 - 10 - 15 - 30 VA.<br />
Safety factor (SF)<br />
c Protection of metering devices in the case of a fault is defined by the safety<br />
factor SF. The value of SF will be chosen according to the current consumer's<br />
short-time withstand current: 5 ≤ SF ≤ 10. SF is the ratio between the limit of<br />
rated primary current (Ipl) and the rated primary current (Ipr).<br />
SF =<br />
Ipl<br />
Ipr<br />
c Ipl is the value of primary current for which the error in secondary<br />
current = 10 %.<br />
<strong>Schneider</strong> <strong>Electric</strong><br />
Merlin Gerin <strong>MV</strong> <strong>design</strong> <strong>guide</strong><br />
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