Medium Voltage technical guide - Schneider Electric

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Switchgear defi nition Voltage transformer Rated secondary voltage (Usr) b For phase to phase VT the rated secondary voltage is 100 or 110 V. b For single phase transformers intended to be connected in a phase to earth arrangement, the rated secondary voltage must be divided by 3. 100 V E.g.: 3 Rated output Expressed in VA, this is the apparent power that a voltage transformer can provide the secondary circuit when connected at its rated primary voltage and connected to the nominal load. It must not introduce any error exceeding the values guaranteed by the accuracy class (S = 3 UI in three-phase circuits). b Standardised values are: 10 - 15 - 25 - 30 - 50 - 75 - 100 VA. Accuracy class This defines the limits of errors guaranteed in terms of transformation ratio and phase under the specified conditions of both power and voltage. Measurement according to IEC 60044-2 Classes 0.5 and 1 are suitable for most cases, class 3 is very little used. Application Accuracy class Not used industrially 0.1 Precise metering 0.2 Everyday metering 0.5 Statistical and/or instrument metering 1 Metering not requiring great accuracy 3 Protection according to IEC 60044-2 Classes 3P and 6P exist but in practice only class 3P is used. b The accuracy class is guaranteed for values: v of voltage of between 5% of the primary voltage and the maximum value of this voltage which is the product of the primary voltage and the rated voltage factor (kT x Upr) v for a secondary load of between 25% and 100% of the rated output with a power factor of 0.8 inductive. Accuracy class Voltage error as ± % Phase shift in minutes Between 5% Upr and kT • Upr Between 2% and 5% Upr Between 5% Upr and kT • Upr 3P 3 6 120 240 6P 6 12 24 480 Upr = rated primary voltage kT = voltage factor Phase shift = see explanation next page Between 2% and 5% Upr 68 AMTED300014EN.indd
Switchgear defi nition Voltage transformer AMTED300014EN.indd Transformation ratio (Kn) Kn = Upr Usr Voltage ratio error N1 = for a VT N2 This is the error that the transformer introduces into the voltage measurement. (Kn Usr – Upr) • 100 Voltage error % = 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. 69
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Technical collection Medium Voltage
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Medium Voltage technical guide AMTE
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Presentation Prefabricated metal-en
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Design rules Short-circuit power DE
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Design rules Short-circuit currents
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Medium Voltage technical guide - Schneider Electric

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