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NO COMBINED CHARACTERISTIC

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According to the operation of differential relays, the differential of each phase is calculated separately

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and based on the differential characteristic, the relay will either trip or not trip. In single-phase faults

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you will notice that the differential and bias currents in the tested phase are twice those of the other two phases

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In such conditions, the relay essentially operates with two characteristics. The first
characteristic is the one set on the relay

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and the second characteristic is obtained by
multiplying the Idiff and Ibias of the set characteristic by 2

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In differential characteristics with two slopes, by plotting the two mentioned
characteristics, the resulting differential characteristics are obtained as shown

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To derive the practical characteristic from these two characteristics, for each Ibias

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you need to find the lowest Idiff value that causes the relay to operate

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which typically refers
to lower differential currents and is obtained as shown

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The following explanation can help clarify this concept

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In single-phase fault types, in a section of the characteristic (the green region shown in the figure below), the test point in question falls in the NO trip region

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However, if you check the differential and bias currents of the other two phases, you'll see that they are in the TRIP region, and the relay will trip

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With the mentioned modifications, the differential characteristic is adjusted accordingly

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That is, changes are made so that in this part of the characteristic, the selected point falls within the Tripping region

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For relays that can filter out high differential currents in healthy phases, such as the ABB RET670

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to deactivate the use of combined characteristics, select the "No combined characteristic" option in the Protection Device tab