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.
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