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Now this basic sensitivity is taking account the starting situation with no load to Turnpoint 1 in the
characteristics. Next thing is to decide where to set the Turnpoint 1. In most of differential relays this
point is either xed or automatically de ned based on base sensitivity and slope 1. In this IED type
differential relay it is settable. For high sensitivity it may be set to 1 xIn since the calculated base
sensitivity takes into account already the tap changer effect and all of the other normal operating
caused differential current sources.
For coarse settings this Turnpoint 1 can be set to for example 0.5xIn or even 0.01xIn. How to
determine this limit is the sum of the protection principle wanted. Smaller value leads to conservative
and stabile operation, while bigger means highly sensitive and possibly unstable protection.
Also there if Turnpoint 1 is set to 0.01xIn, the Slope 1 will start directly from the setting. In such case
there is no unbiased sensitive section available. This could be used in case when the tap changer
effect is not wanted to be taken into account for basic sensitivity and the effect is wanted to be taken
into account in the rst slope directly. This can lead to optimal sensitivity and stabile settings for
differential relay even there is no non biased sensitive section in the characteristics.
In this case the base sensitivity setting should be set as follows:
Now the Slope 1 settings, this part presents the relay restraint characteristics over the load current
range of the transformer. This slope should be effective up to the maximum possible loading of the
transformer. Normally for power transformer this value should be about 1.0 – 2.0xIn (for large power
transformers) when typical value would be 1.5xIn. The functionality purpose is to compensate
measurement errors caused by relative high current with the tap changer effect included. Slope 1 is
calculated by using the transformer and CT nominal values in the maximum full load (Turnpoint 2) of
the transformer with highest possible differential current causing tap position. Generally Slope 1 setting
is calculated as below:
Now the calculation of the maximum differential current in the Turnpoint 2 includes before calculated
correction factors for HV and LV side CTs.
Also is needed the corrected transformation ratio effect due to the tap changer position on maximum
voltage position (usually this generates the highest differential current).
AQ-T257
Instruction manual
Version: 2.00
155
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