MV design guide - Schneider Electric

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definition
Voltage transformer
Rated secondary voltage (Usr)
c For phase to phase VT the rated secondary voltage is 100 or 110 V.
c For single phase transformers intended to be connected in a phase to
earth arrangement, the rated secondary voltage must be divided by e.
E.g.: 100 V
e
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 = eUI in three-phase circuits)
c Standardised values are:
10 - 15 - 25 - 30 - 50 - 75 - 100 - 150 - 200 - 300 - 400 - 500 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 60 186
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 60 186
Classes 3P and 6P exist but in practice only class 3P is used.
c 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 between 2% between 5% Upr between 2%
and kT • Upr and 5% Upr and kT • Upr and 5% Upr
3P 3 6 120 240
6P 6 12 24 480
Upr = rated primary voltage
kT = voltage factor
phase shift = see explanation next page
62 Merlin Gerin MV design guide Schneider Electric