MV design guide - Schneider Electric
MV design guide - Schneider Electric
MV design guide - Schneider Electric
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Design rules<br />
Short-circuit currents<br />
G<br />
Synchronous generators<br />
(alternators and motors)<br />
Calculating the short-circuit current across the terminals of a<br />
synchronous generator is very complicated because the internal<br />
impedance of the latter varies according to time.<br />
c When the power gradually increases, the current reduces passing<br />
through three characteristic periods:<br />
v sub-transient (enabling determination of the closing capacity of circuit<br />
breakers and electrodynamic contraints), average duration, 10 ms<br />
v transient (sets the equipment's thermal contraints),<br />
average duration 250 ms<br />
v permanent (this is the value of the short-circuit current in steady state).<br />
c The short-circuit current is calculated in the same way as for<br />
transformers but the different states must be taken account of.<br />
Example:<br />
Calculation method for an alternator<br />
or a synchronous motor<br />
c Alternator 15 <strong>MV</strong>A<br />
c Voltage U = 10 kV<br />
c X'd = 20 %<br />
Sr<br />
Ir = =<br />
15<br />
= 870 A<br />
e • U e • 10 000<br />
Isc =<br />
Ir 870<br />
= = 4 350 A = 4.35 kA<br />
Xcc trans. 20/100<br />
fault<br />
appears<br />
Ir<br />
healthy<br />
state<br />
courant<br />
subtransient<br />
state<br />
transient<br />
state<br />
Isc<br />
time<br />
permanent<br />
state<br />
short-circuit<br />
c The short-circuit current is given by the following equation:<br />
Isc =<br />
Ir<br />
Xsc<br />
Xsc : short-circuit reactance c/c<br />
c The most common values for a synchronous generator are:<br />
State Sub-transient X''d Transient X'd Permanent Xd<br />
Xsc 10 - 20 % 15 - 25 % 200 - 350 %<br />
M<br />
Asynchronous motor<br />
c For asynchronous motors<br />
v the short-circuit current across the terminals equals the start-up current<br />
Isc z 5 at 8 Ir<br />
v the contribution of the motors (current feedback) to the short-circuit<br />
current is equal to:<br />
I z 3 ∑ Ir<br />
The coefficient of 3, takes account of motors when stopped and the<br />
impedance to go right through to the fault.<br />
14 Merlin Gerin <strong>MV</strong> <strong>design</strong> <strong>guide</strong> <strong>Schneider</strong> <strong>Electric</strong>