The positive sequence line charging current is calculated according to equation 12.arg11123 3Ch eU UIXCf Cp=××=×EQUATION1417 V2 EN-US (Equation 12)where:U is system voltageXC1 is capacitive positive sequence reactance of the linef is system frequencyC1 is positive sequence line capacitanceIf the charging current compensation is enabled, the setting of IdMin must be:IdMin ≥ 1.2 × ICharge, concidering some margin in the setting. If the chargingcurrent compensation is disabled, the setting of IdMin must be IdMin ≥ 2.5 ×ICharge. In many cases, the charging current is quite small, which makes the lowerlimit of the setting range, that is 20% of IBase the practical limit of sensitivity.When a power transformer is included in the protected zone, the setting of IdMinshall be the highest of recommendations considering charging current as describedabove and 0.3 × IBase.IdMinHigh M12541-165 v5The IdMinHigh setting is a multiple of IBase and used to temporarily decreasesensitivity in situations when the line is energized.Energizing a line can cause transient charging currents to appear. These currentsare pure differential currents, but as they are rich in harmonics, they can partly bemeasured by the differential protection, which in this case measures the Fourier-filtered differential current. Desensitizing the differential protection in this situationby using IdMinHigh instead of IdMin is a safety precaution, and a setting of 1.00 ×IBase should be suitable in most cases.If there is a power transformer included in the protected zone, energizing the linemeans that the transformer is energized at the same time. If the transformernominal current is more than 50% of IBase, IdMinHigh is recommended to be setat 2.00 × IBase, otherwise it can be kept at 1.00 × IBase.Switching of a transformer inside the protected zone does not normally occur. If thetransformer is equipped with a breaker on the HV side, it would most probably notbe included in the protected zone. However, tap transformers are sometimesconnected with a disconnector on the HV side, and the normal procedure is then toenergize the transformer with the disconnector. In such cases, where the tap (shunt)power transformer's power rating is relatively small in comparison to the normalload of the circuit, connecting the tap power transformer to the voltage source, that1MRK 505 393-UEN B Section 6Differential protectionLine differential protection RED650 2.2 IEC 89Application manual
