(1) The rated voltage is determined according to the voltage level at the current transformer unit (2) Determine ct one-time voltage rating based on Ie-Igmax/110% (3) According to the CT use of the transformer, determine its sub-group and wiring mode (4) 100kV line side setting differential, overcurrent, measurement of three sets of CT, connected to three-phase star10kV main variable outlet setting differential, metering, measurement and current protection three sets of CT, connected to a three-phase star shape10kV load outlet set up metering, measurement and current protection two sets of CT, connected to a two-phase star 5. Selected model, check thermal stability and dynamic stability according to short circuit conditions