Differential Current Pickup Setting Calculation

Many resources across the field of power system protection suggests a differential current pickup of 0.2 to 0.3 of the TAP setting. However, the calculation behind this recommendation is quite confusing.

A good resource presented in the SEL-387A Instruction Manual provides a straightforward calculation of the differential current pickup. The approach is particularly based on the CT errors and voltage ratio variation of the power transformer load tap-changer.

Calculation Considerations

Let e be the current transformer error equal to ±3%; a be the voltage ratio variation equal to ±10%. In per unit quantities,

Going back to the previous discussion on transformer differential protection, operate quantity is calculated using

It is important to note that for SEL differential relays, the differential current pickup is termed as the operate quantity. This terms are used interchangeably through the article.

In a through-fault condition, transformer input currents and output currents must be equal and out of phase by 180°, thus

The worst-case theoretical differential current occurs when the input current is measured with maximum positive CT error, that is,

The output current, on the other hand, is measured with maximum negative CT error in addition to current offset due to maximum load tap-changer variation.

Using these considerations, the differential current pickup setting can simplified to

In per-unit of the through-fault current, the differential current pickup setting can be calculated using

Using the same equation, with e = 0.10

The differential current pickup for SEL-387A relays can be set from 0.1 to 1.0. SEL recommends a setting of 0.3. In any case, it is very important to note that the choice of the setting should be at a minimum for increased sensitivity but high enough to avoid misoperations.

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Reference:

SEL-387A Instruction Manual. Available at https://selinc.com