GE P642 Technical Manual page 230

Chapter 9 - Current Protection Functions
Under system fault conditions, the fault current vector will lag its nominal phase voltage by an angle dependent
upon the system X/R ratio. Therefore the device must operate with maximum sensitivity for currents lying in this
region. This is achieved by means of the characteristic angle (RCA) setting, which defines the angle by which the
applied current must be displaced from the applied voltage in order to obtain maximum sensitivity. This is set in
cell I>Char Angle. You can set characteristic angles anywhere in the range from –95° to +95°.
In the P642, the standard single-phase VT input cam be complemented by an optional single-phase VT that can be
used to directionalise some of the overcurrent protection elements. This connection is shown on the wiring
diagrams and labelled as 'V
overcurrent elements are the same as those for the overfluxing input and are described by the Aux' VT Location,
Aux' VT Primary and Aux' VT Sec'y settings in the CT AND VT RATIOS column.
In the P643 and P645, the standard single-phase VT input can be complemented by an optional three-phase VT
input that can be used to directionalise some of the overcurrent protection elements. This connection is shown on
the wiring diagrams and labelled as 'OPTIONAL". The settings for the optional VT input that can be used to
directionalise overcurrent elements are those described by the 'Main VT Location, Main VT Primary and Main VT
Sec'y settings settings in the CT AND VT RATIOS column.
The overcurrent elements that can be directionalised are those associated with the windings to which the optional
VTs are assigned. You can only directionalise overcurrent elements associated with the winding to which the
optional input is assigned.
So, to provide directional overcurrent you need:
● To have the appropriate VT option
Connect the VT input to the winding that you want directional protection for AND assign the optional VT
●
input to that winding in the CT AND VT RATIOS column
Set the overcurrent to directional (either forward or reverse).
●
As well as directionalising a particular winding current, for models with more than two winding inputs, you can
directionalise individual inputs if they are associated with that winding.
For close up three-phase faults, all three voltages will collapse to zero and no healthy phase voltages will be
present. For this reason, the device includes a synchronous polarisation feature that stores the pre-fault voltage
information and continues to apply this to the directional overcurrent elements for a time period of a few seconds.
This ensures that either instantaneous or time-delayed directional overcurrent elements will be allowed to operate,
even with a three-phase voltage collapse.
206
'. The settings for the optional VT input that can be used to directionalise
bc OPTIONAL
P64x
P64x-TM-EN-1.3