transformer windings or the current transformers will be in breakers that are part ofthe bus, such as a breaker-and-a-half or a ring bus scheme. For current transformerswith primaries in series with the power transformer winding, the currenttransformer primary current for external faults will be limited by the transformerimpedance. When the current transformers are part of the bus scheme, as in thebreaker-and-a-half or the ring bus scheme, the current transformer primary currentis not limited by the power transformer impedance. High primary currents may beexpected. In either case, any deficiency of current output caused by saturation ofone current transformer that is not matched by a similar deficiency of anothercurrent transformer will cause a false differential current to appear. Differentialprotection can overcome this problem if the bias is obtained separately from eachset of current transformer circuits. It is therefore important to avoid paralleling oftwo or more current transformers for connection to a single restraint input. Eachcurrent connected to the IED is available for biasing the differential protectionfunction.The unrestrained operation level has a default value of IdUnre = 10pu, which istypically acceptable for most of the standard power transformer applications. In thefollowing case, this setting need to be changed accordingly:• When CT from "T-connection" are connected to IED, as in the breaker-and-a-half or the ring bus scheme, special care shall be taken in order to preventunwanted operation of transformer differential IED for through-faults due todifferent CT saturation of "T-connected" CTs. Thus if such uneven saturationis a possibility it is typically required to increase unrestrained operational levelto IdUnre = 20-25pu• For differential applications on HV shunt reactors, due to the fact that there isno heavy through-fault condition, the unrestrained differential operation levelcan be set to IdUnre = 1.75puThe overall operating characteristic of the transformer differential protection isshown in figure 30.Section 6 1MRK 504 169-UEN ADifferential protection78 Transformer protection RET650 2.2 IECApplication manual
