If the panel meters show that the generator voltage is different from that of the bus, the voltage offset potentiometer may need adjustment. Make sure that the
panel meters are accurate, calibrated, and at operating temperature. Most panel meters are accurate to ±3% and are not temperature compensated.
In this case, a correct discrimination is achieved by means of an Iecos characteristic, as the
faulted feeder will have a large active component of residual current, whilst the healthy
feeder will have a small value.
For insulated earth applications, it is common to use the Iesin characteristic that can be
obtained by the setting of the characteristic angle to 90° or 270°.
2.5 Derived earth overcurrent (Ie_d>, P127 only)
The derived earth overcurrent element protection is used to cover applications such as
HTB/HTA transformers. The derived earth current (Ie_d>) is the vectorial summation:
IA + IB + IC:
IA + IB + IC = Ie_d>.
The derived earth overcurrent has two independent thresholds: Ie_d> and Ie_d>>.
The two thresholds can be set as definite delay time or inverse delay time using the IEC,
IEEE/ANSI, CO, RI and RECT curves. Their parameters are shown in the Technical Data
chapter of this Technical Guide.
IA
IB
IC
Ie = Tank Earth
IA+IB+IC = Ie>>>>
IA
IB
IC
Ie = Tank Earth
IA+IB+IC = Ie_d>
P3940ENb
As soon as Ie_d> threshold is running, the instantaneous output associated with this
threshold is activated. This output indicates that the protection element has detected an
earth fault and that the time delay associated with the threshold has started. This time delay
can be blocked via the logic input "Block Logic" associated with this threshold. If this blocking
input is activated by an output contact of a downstream relay, the logic that will lead to the
trip command is then blocked only if the relay that is the closest to the fault can see and
therefore eliminate the fault. This principle is known as «Blocking logic» or «Blocking». It is
described in more detail in this document.
