Ice NP900 Series, Инструкция по применению

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Application Guide - NP900 Series 325 (504
)
A996A
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).
Figure 1.1.1.5-157 Tap changer principle.
Generally tap changer means that the transformer transformation ratio can be adjusted in
order to receive nominal voltage more accurately to the secondary side of the transformer.
Reasons for voltage variations may be many for example heavy or light loading in the high
voltage side. In practice this means that if more or less voltage is needed in the secondary
side, more or less winding rounds in secondary side are utilized. This causes difference
into the nominal current condition which can be noticed as differential current in the relay.
Normally tap changer positions are presented as deviation steps of secondary voltage per
step into + and – direction from the center which gives nominal output voltage.