These settings will give a characteristic suitable for most applications. It leaves only the Phase Is1 setting to bedecided by you. The value of this setting should be in excess of any mismatch between line ends. It should alsoaccount for line charging current, where necessary.By considering the circuit shown below, the settings for the phase current differential element can be established.P54xV02620P54xDigital communication linkProtected line275 kV 275 kV400/5400/545 kmEnd A End B30 kmCh 1 Ch 2P54x10 km275 kVEnd CCh 2Ch 1 Ch 2Ch 1Steady state charging current = 0.58 A/km - overhead lineDigital communication link Digital communication link1200/5Figure 49: Typical three-terminal plain feeder circuitIf there are no VT inputs connected, the setting Phase Is1 must be 2.5 times the steady state charging currentsuitably corrected for the different CT ratios as presented below in the calculation of charging currentcompensated values. For an overhead line circuit with a charging current of 0.58A/km the charging current will be:Ich = 0.58 A ( 45 + 30 + 10 ) = 49.3 AIf VT inputs are connected, there is a facility to overcome the effect of charging current. To do this you need toenter the positive sequence capacitive susceptance value.Considering the charging current on the circuit shown in the figure above, the following calculation applies:Ich = 0.58 A ( 45 + 30 + 10 ) = 49.3 ASusceptance =wC = Ich/VB = 49.3 A/( 275/Ö3) kV primaryB = 0.31 x 10-3 S primary.The CT ratios on the three ends are different, so it is necessary to apply a correction factor to ensure secondarycurrents balance for all conditions:To calculate the correction factor (CF), the same primary current must be used even if this current is not theexpected load transfer for every branch. This will ensure secondary current balance for all conditions.CT ratio correction input data is shown in the table below:Setting End A End B End CPhase CT Primary 4000A 4000A 1200APh CT Corr’tion 4000/1200 = 3.33 4000/1200 = 3.33 1 (reference)We recommend the following settings:Phase Is1 = 0.2 p.u. = 0.2*1200A = 240APhase Is2 = 2 p.u. = 2*1200A = 2400AChapter 6 - Current Differential Protection P543i/P545i130 P54x1i-TM-EN-1
