MiCOM P632
General Equations
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The current transformer can be dimensioned
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either for the minimum required secondary accuracy limiting voltage acc. to IEC 60044‑1, 2.3.4:

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or for the minimum required rated accuracy limit factor acc. to IEC 60044‑1, 2.3.3, as follows:

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The relation between both methods is given as follows:

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The actual secondary connected burden Rb is given as follows:

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For phase-to-ground faults: graphics/mml_010.png

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For phase-to-phase faults: graphics/mml_011.png

The wire lead burden is calculated as:

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ρ = specific conductor resistance

(e.g. for copper 0.021 Ω mm²/m = 2.1⋅10-8 Ω m, at 75°C)

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l = wire length

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A = wire cross section

The MiCOM Px3x input CT burden Rrel is less than 20 mΩ, independent of the set nominal current (1A or 5A). Usually this relay burden can be neglected.

The rated knee point voltage Vk according to IEC 60044‑1-am1, 2.3.12 is lower than the secondary accuracy limiting voltage Vsal according to IEC 60044‑1, 2.3.4. It is not possible to give a general relation between Vk and Vsal, but for standard core material the following relations applies:

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VK≈0.85⋅Vsal for class 5P CTs, and

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VK≈0.75⋅Vsal for class 10P CTs, respectively.

Theoretically, the specifications of the current transformer could be calculated to avoid saturation by inserting its maximum value, instead of the required over-dimensioning factor Kd:

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However, this is not necessary. Instead, it is sufficient to consider an empirically determined dimensioning factor Kd=Kemp such that the appropriate operation of the protection function is ensured under the given conditions. This factor depends on application and relay type, as outlined in the following.