Edition 1.02023-02
INTERNATIONAL STANDARD
NORMEINTERNATIONALE
Power transformersPart 25:Neutral grounding resistors
TransformateursdepuissancePartie 25:Resistances de mise a la terre du neutre
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ELECTROTECHNICAL INTERNATIONALCOMMISSION
COMMISSIONELECTROTECHNIQUEINTERNATIONALE
INTERNATIONAL STANDARD
NORMEINTERNATIONALE
Power transformersPart 25:Neutral grounding resistors
Transformateurs de puissance-Partie25:Resistancesdemisealaterreduneutre
ISBN 978-2-8322-6431-7
CONTENTS
FOREWORD. 41 Scope. ..62 3 Normative references. Terms and definitions ..64 Service conditions ..95 Ratings. ..95.1 Rated ambient temperature. .95.2 5.3 Rated resistance (R) Maximum resistance variation... 6 ..95.4 Rated time .95.5 Rated voltage (U) .95.7 5.6 Rated insulation level. Rated short time current 10 .105.8 Rated continuous current 105.9 Rated frequency .106 6.1 Rating plates (Nameplates) General. ...11 .116.2 Minimum information to be provided. .117 Design and construction . ...117.1 7.2 Resistive elements. General. 11 117.3 Resistive banks.. .127.4 Electrical connections.... ..127.5 7.5.1 IN and OUT terminals General ...1.2 127.5.2 IN terminal. .127.5.3 Insulators. OUT terminal ..13 127.6 7.6.1 General ..137.6.2 Insulators between resistive elements .137.6.3 7.6.4 Creepage distance. Insulators between resistor banks and enclosure. 137.7 Enclosure ...137.7.1 Protection against contact with live parts ingress of solid foreign bodies and water ..137.7.2 Design.. ...1.37.7.3 Protection against corrosion.. ..1448 8.1 Tests General. ..14 .148.2 Test classifications 148.2.1 Routine tests Type tests.. 148.2.2 8.2.3 Special tests.. ..15 148.3 Test documentation ..58.4 8.4.1 Routine tests. Visual inspection and dimensional verification .1515
8.4.2 8.4.3 Rated resistance measurement. Power frequency withstand voltage test. .15 .158.4.4 Insulation resistance. 168.5 Temperature rise tests. 178.5.1 8.5.2 Continuous current duty... General .17 .178.5.3 Rated time duty ..178.5.4 Acceptance criteria. Tolerances .188.6 8.5.5 Special tests .18 .188.6.1 Inductance measurement... ...1.88.6.2 8.6.3 Lightning impulse test. Mechanical design verification. ..18 188.6.4 Protection degree of enclosure . 19Annex A (informative) Resistance variation.. 20A.2 A.1 Material properties. General. .20 20A.3 Effect of NGR resistance variation . 20A.4 Rate of variation . .21A.5 A.6 Consecutive faults Protection coordination. 22 22A.7 System capacitive charging current. 22Bibliography.. A.8 Conclusion. 22.23
Figure A.1 - Ground fault protection settings example. .21
Table 1 - Rated insulation levels ..10Table 2 Maximum temperature rises above ambient for resistive elements. Table 3 - Tests classifications . 12Table A.1 - Resistance and current change with temperature rise . .14 21
