05 overvoltages and insulation coordination

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393 V 3 3 V 2 2 V 1 1 earth fault V E 1 V 2E V 3E V Neutral Z Neutral V 2 V 3 Neutral V 2E V 3E V 1 V 1E 0 V 1 ,V 2 ,V 3 : phase-neutral voltages V 1 E ,V 2 E ,V 3 E : phase-earth voltages Z Neutral : earthing impedance (Z Neutral = ∞ for an unearthed neutral) Figure 5-1: overvoltage on an unearthed or impedance earthed network on occurrence of a phase-to-earth fault Note 1 : for an impedance earthed neutral, the value of Z Neutral is much greater than the value of the transformer and cable impedances and the fault resistance, which is why VNeutral = − V1 . Note 2 : in overhead public distribution networks, there are highly resistive faults (several kΩ), having a value close to or higher than the earthing impedance. In this case, a highly resistive fault will cause an overvoltage lower than 3V n . Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services. The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent. Industrial electrical network design guide T & D 6 883 427/AE
394 solidly earthed neutral (HV or MV) On occurrence of an earth fault on one network phase, a high current is generated which circulates in the circuit formed by the fault phase, earth and neutral earth electrode (see fig. 5-2). At the fault point, the three-phase voltage system is disturbed. The fault phase voltage in relation to earth is almost zero if we neglect the fault resistance. The voltages of the other two phases in relation to earth are higher than the single-phase voltage, while remaining lower than the phase-to-phase voltage. V 3 Z T Z C V 2 Z T Z C V 1 Z T Z C fault V 1E V 2E V 3E R e R f V 1 ,V 2 ,V 3 : single-phase voltages Z T : transformer impedance Z C : cable impedance R e : neutral earth electrode resistance R f : fault resistance Figure 5-2: equivalent diagram of a phase-earth fault when the neutral is solidly earthed Thus, we can define an earth fault factor k characterising the phase-earth overvoltage occurring on the healthy phases: V2E = V3E = kVn V n : nominal single-phase voltage The symmetrical component calculation method (see § 4.2.2. of the Protection guide) can be used to determine the value of k in relation to the positive, negative and zero-sequence impedances: k = 1 − 2 Z + a Z + aZ (1) ( 2) ( 0) (1) ( 2) ( 0) R f Z + Z + Z + 3 Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services. The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent. Industrial electrical network design guide T & D 6 883 427/AE
Page 1 and 2: 390 5. OVERVOLTAGES AND INSULATION
Page 3: 392 - disturbance of control, monit
Page 7 and 8: 396 we can determine: R( 1 ) = RT +
Page 9 and 10: 398 For a solid fault (R f =0 ) at
Page 11 and 12: 400 For a transformer or a cable in
Page 13 and 14: 402 The exposed conductive part of
Page 15 and 16: 404 5.1.2.2. Resonance and ferro-re
Page 17 and 18: 406 The voltage U is common to the
Page 19 and 20: 408 In high voltage, the substation
Page 21 and 22: 410 Numerical application: Let us t
Page 23 and 24: 412 This equation can be graphicall
Page 25 and 26: 414 series ferro-resonance (see fi
Page 27 and 28: 416 example of parallel ferro-reso
Page 29 and 30: 418 In the case of (see fig. 5-19).
Page 31 and 32: 420 example of series ferro-resona
Page 33 and 34: 422 By applying formula (1), we fin
Page 35 and 36: 424 R L I V C V A I V V A t L, R :
Page 37 and 38: 426 The phenomena observed are illu
Page 39 and 40: 428 multiple restrike If we consid
Page 41 and 42: 430 The current is then interrupted
Page 43 and 44: 432 When the supply voltage reverse
Page 45 and 46: 434 If the bank is energized shortl
Page 47 and 48: 436 5.1.3.3. Circuit-breaker cleara
Page 49: 438 High resistance earthing with r
Page 57 and 58:
440 5.1.4. Atmospheric overvoltages
Page 59 and 60:
442 Similarly, the steepness of the
Page 61 and 62:
444 By applying an electro-geometri
Page 63 and 64:
446 direct lightning strike (on ph
Page 65 and 66:
448 indirect lightning strikes (li
Page 67 and 68:
450 The induced voltage is given in
Page 69 and 70:
452 The values of C t are generally
Page 71 and 72:
454 Upstream of M , we have: ' v1 =
Page 73 and 74:
456 5.2.2. Spark-gaps operation Th
Page 75 and 76:
458 5.2.3. Surge arresters To overc
Page 77 and 78:
460 connectingspindle flange (alumi
Page 79 and 80:
462 choice of zinc oxide surge arr
Page 81 and 82:
464 nominal discharge current I nd
Page 83 and 84:
466 Numerical application: Let us c
Page 85 and 86:
468 LV surge arrester installation
Page 87 and 88:
470 electricalswitchboard disconnec
Page 89 and 90:
472 The clearance is directly relat
Page 91 and 92:
474 Table 5-5: standard withstand v
Page 93 and 94:
476 symbols In the following parag
Page 95 and 96:
478 TN − b , TT − b and IT −
Page 97 and 98:
480 TT − a and IT − b earthing
Page 99 and 100:
482 recapitulative table of touch
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