Handbook of Electrical Installation Practice - BeKnowledge

Protective Systems 633
the heavy current rig is the only recourse but, even in this case, care must be taken
that saturation of the various current limiting reactors or impedance changing transformers
does not distort the current feeding the primary of the CT. This is particularly
important in high impedance schemes where, for an internal fault, the CT is
virtually unloaded and thus the impedance reflected into the primary, although
extremely small, may still be significant if the heavy current machine is not carefully
matched to the CT and the relay combination. It should also be appreciated that with
a distorted waveshape the ammeter is unlikely to have the same relative response as
the relay, so that the only true measurement is when the current is completely sinusoidal
because relay and ammeter are both calibrated using sine-wave current.
In some critical relays, the harmonic content in the driving voltage can cause
incorrect measurement and this must then be reduced by using inductance to
provide a current source simulating the power system conditions more accurately
than the more commonly used resistance current limiting.
Another common problem is the determination of direction of current. This is
important in a relative sense for establishing the residual connection of CTs, and in
a fundamental sense for setting up the operate direction of directional relays.
Fig. 22.21 Ratio and directional tests with neutral CT.
Fig. 22.22 Ratio and directional tests without neutral CT.