MV design guide - Schneider Electric

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Switchgear definition Current transformer Differential protection Many manufacturers of differential protection relays recommend class X CT's. c Class X is often requested in the form of: Vk ≤ a . If (Rct + Rb + Rr) The exact equation is given by the relay manufacturer. Values characterising the CT Vk : Knee-point voltage in volts a : asymmetry coefficient Rct : max. resistance in the secondary winding in Ohms Rb : loop resistance (feed/return line) in Ohms Rr : resistance of relays not located in the differential part of the circuit in Ohms If : maximum fault current seen by the CT in the secondary circuit for a fault outside of the zone to be protected If = Isc Kn Isc : primary short-circuit current Kn : CT transformation ratio What values should If be given to determine Vk? c The short-circuit current is chosen as a function of the application: v generator set differential v motor differential v transformer differential v busbar differential. c For a generator set differential: v if Isc is known: Isc short-circuit current for the generator set on its own If = Isc Kn relay CT G CT v if the Ir gen is known: we will take If = 7 • Ir gen Kn v if the Ir gen is unknown: we will take If = 7 • Isr (CT) Isr(CT) = 1 or 5 A c For motor differential: v if the start-up current is known: we will take Isc = I start-up relay CT M CT If = Isc Kn v if the Ir motor is known: we will take If = 7 • Ir Kn v if the Ir motor is not known: we will take If = 7 • Isr (CT) Isr(TC) = 1 or 5 A Reminder Ir : rated current Schneider Electric Merlin Gerin MV design guide 59
Switchgear definition Current transformer CT CT relay c For a transformer differential The Isc to take is that flowing through the CT's for a current consumer side fault. In all cases, the fault current value If is less than 20 Isr(CT). v if we do not know the exact value, we will take: If = 20 Isr(CT) c For busbar differential v the Isc to take is the switchboard Ith If = Ith Kn c For a line differential The Isc to take is the Isc calculated at the other end of the line, therefore limited by the cable impedance. If the impedance of the cable is not known, we will take the switchboard Ith. 60 Merlin Gerin MV design guide Schneider Electric
Page 1 and 2:
Merlin Gerin technical guide Medium
Page 3 and 4:
General contents MV design guide Pr
Page 5 and 6:
Presentation Metal-enclosed, factor
Page 7 and 8: Presentation Metal-enclosed, factor
Page 9 and 10: Presentation Metal-enclosed, factor
Page 11 and 12: Design rules Short-circuit currents
Page 21 and 22: Design rules Busbar calculation Tem
Page 23 and 24: Design rules Busbar calculation Let
Page 25 and 26: Design rules Busbar calculation For
Page 27 and 28: Design rules Busbar calculation Mec
Page 29 and 30: Design rules Busbar calculation Her
Page 31 and 32: Design rules Busbar calculation c C
Page 33 and 34: Design rules Busbar calculation c L
Page 35 and 36: Design rules Busbar calculation Mec
Page 37 and 38: Design rules Dielectric withstand A
Page 39 and 40: Design rules Dielectric withstand U
Page 41 and 42: Design rules Protection index Item
Page 43 and 44: Design rules Protection index The v
Page 45 and 46: Switchgear definition Medium voltag
Page 53 and 54: Switchgear definition Current trans
Page 55 and 56: Switchgear definition Current trans
Page 57: Switchgear definition Current trans
Page 61 and 62: Switchgear definition Voltage trans
Page 63 and 64: Switchgear definition Derating Intr
Page 65 and 66: Units of measure Names and symbols
Page 67 and 68: Units of measure Names and symbols
Page 69 and 70: Standards IEC - ANSI comparison The
Page 71 and 72: Standards IEC - ANSI comparison Pea
Page 73 and 74: Standards IEC - ANSI comparison Acc
Page 75 and 76: Standards IEC- ANSI comparison Dera
Page 77 and 78: References Reference to Schneider E
Page 79 and 80: Index Alphabetical Index Denominati
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MV design guide - Schneider Electric

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