Chapter 6 - Current Protection Functions
The action of setting the PN> threshold to zero would effectively disable the wattmetric function and the device
would operate as a basic, sensitive directional earth fault element. However, if this is required, then the SEF option
can be selected from the SEF/REF Options cell in the menu.
The residual power setting, PN>, is scaled by the programmed Transformer ratios.
A further point to note is that when a power threshold other than zero is selected, a slight alteration is made to the
angular boundaries of the directional characteristic. Rather than being ±90° from the RCA, they are made slightly
narrower at ±85°.
The directional check criteria is as follows:
Directional forward: -85° < (angle(IN) - angle(VN + 180°) - RCA) < 85°
Directional reverse: -85° > (angle(IN) - angle(VN + 180°) - RCA) > 85°
ICOS PHI / ISIN PHI CHARACTERISTIC
In some applications, the residual current on the healthy feeder can lie just inside the operating boundary
following a fault condition. The residual current for the faulted feeder lies close to the operating boundary.
Figure 63: Operating characteristic for Icos
The diagram illustrates the method of discrimination when the real (cosf ) component is considered. Faults close to
the polarising voltage will have a higher magnitude than those close to the operating boundary. In the diagram,
we assume that the current magnitude I is in both the faulted and non-faulted feeders.
For the active component Icos, the criterion for operation is: Icosf > ISEF>(n) current
For the reactive component Isin, the criterion for operation is: Isinf > ISEF>(n) current
Where ISEF>(n) current is the sensitive earth fault current setting for the stage in question
If any stage is set to non-directional, the element reverts back to normal operation based on current magnitude I
with no directional decision. In this case, correct discrimination is achieved by means of an Icos characteristic as
the faulted feeder will have a large active component of residual current, whilst the healthy feeder will have a