MV design guide - Schneider Electric

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Switchgear definition Voltage transformer Transformation ratio (Kn) Upr Kn = = for a TT Usr N1 N2 Voltage ratio error This is the error that the transformer introduces into the voltage measurement. voltage error (%) = (kn Usr - Upr)•100 Upr Kn = transformation ratio Phase error or phase-shift error This is the phase difference between the primary voltage Upr and the secondary voltage Usr. IT is expressed in minutes of angle. The thermal power limit or rated continuous power This is the apparent power that the transformer can supply in steady state at its rated secondary voltage without exceeding the temperature rise limits set by the standards. Schneider Electric Merlin Gerin MV design guide 63
Switchgear 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
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Merlin Gerin technical guide Medium
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General contents MV design guide Pr
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Presentation Metal-enclosed, factor
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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 61: Switchgear definition Voltage trans
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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