Differential relaying with overcurrent relays requires connecting current transformers in each phase of each circuit in parallel with an overcurrent relay for that phase. (See Fig 1, illustrating the connections for 1 phase of a 3-phase system.)
While it is permissible to utilize auxiliary current transformers to match ratios, it is most desirable for all current transformers to have the same ratio on the tap used so that auxiliary current transformers are not required.
Ground differential relaying, with an overcurrent relay for bus ground faults only, has been applied where current transformers are not available to dedicate to bus protection and where the bus construction minimizes the possibility of phase faults. In this case only the current transformer residual current circuits are connected, as shown in Fig 1.
The usual precautions concerning burden, etc, apply. Where bus selection flexibility is provided, the system can be switched. Switching of residual current is less hazardous than switching phase currents.
When applying overcurrent relays in differential schemes, special consideration should be given to the current transformer saturation problem. Based on an assumed value of residual flux, calculations can be made to estimate the extent of the error due to dc saturation of the current transformer core.
In general, this type of protection should be limited to locations that are electrically remote from generating stations which can produce large dc offset fault currents with long time constants.
To minimize possible incorrect operations, the relay may be set less sensitive, time delay may be increased, and inverse time induction-type overcurrent relays with short time characteristics may be used.
The induction principle and design makes these relays less sensitive to the dc and harmonic components of the differential current.