The many different protection methods may be considered for that mentioned line and the best way to choose main protection system is related to the length of line and purposed protection system reliability. The longitudinal differential relay and low or high impedance differential relay may be applied for this purpose.
In one of the projects, the substation contractor proposes a new economical opinion to reduce protection system costs. The representative of contractor says that protection zone of power plant transformer can be extended to within the high voltage substation to cover the transmission line; because the length of line is very short.
The related power transformer protective relay is a standard low impedance differential relay with three analog input currents which can support one electrical zone with three branches. Two input currents of relay is specified to low voltage side of main power transformer (low voltage side contains generator and unit transformer branches) and the other is usually connected to high voltage BCT of main transformer.
In deed, the technical proposal of contractor is replacement of connection of relay current input from main transformer BCT to substation current transformers. In the other hand, the primary scheme of substation is "one and half "; therefore the input of relay shall be fed by two similar sets of substation current transformers. The representative of client answers that it is not possible; because just one input can be specified for high voltage side connection.
But the representative of the contractor says; we can sum the secondary currents of two quite similar sets of current transformers in one nod then apply their summation currents to the differential relay.
Is the technical proposal of contractor correct? Is applying secondary current summations to the relay suitable solution in this case?