Figure. 5.4.18. - 80. Natural differential current sources.
Differential current sources in normal operation:
Primary side CT measurement accuracy (CTE pri)
Secondary side CT measurement accuracy (CTE sec)
Relay measurement accuracy (primary and secondary) (REm)
Tap changer on load side (TCE)
Possible auxiliary transformer or auxiliary winding, which currents are not measured separately
(AUTE)
Transformer core magnetizing current (TME)
Safety margin (SME)
Now how to get the base sensitivity setting limit , errors have to be calculated part by part.
CTEpri: In this example case primary side CT:s are class 10P which means 10% of measurement error.
CTEsec: In this example case secondary side CT:s are class 5P which means 5% of measurement
error.
REm: Relay measurement error is below 0.5% and with optional accuracy below 0.2% per
measurement channel, so this value for both sides combined is either 1% or 0.4%.
TCE: In this example transformer there is tap changer with rating of +/- 5 x 2.5% which means that
from nominal center position the secondary side windings can be set to + 5 x 2.5% or -5 x 2.5%
position causing deviation max of 5 x 2.5% from the nominal conditions. So therefore TCE is in this
case 12.5%. (Note that the tap position is not always necessarily nominal in center position, check
from your application and calculate the maximum effect to worst side border).
AQ-T257
Instruction manual
Version: 2.00
© Arcteq Relays Ltd
152
