Directional Element; Residual Voltage Polarisation; Figure 102: Idg Characteristic - GE P642 Technical Manual

Chapter 9 - Current Protection Functions
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Figure 102: IDG Characteristic

6.4

DIRECTIONAL ELEMENT

If Earth fault current can flow in both directions through a protected location, you will need to use a directional
overcurrent element to determine the direction of the fault. Typical systems that require such protection are
parallel feeders (both plain and transformer) and ring main systems, each of which are relatively common in
distribution networks.
A directional element is available for all of the Earth Fault stages for both Earth fault columns. These are found in
the direction setting cells for the relevant stage. They can be set to non-directional, directional forward, or
directional reverse.
The P642 has two neutral current inputs (TN1 and TN2). The P643 and P645 have three terminal current inputs
(TN1, TN2 and TN3). The earth fault overcurrent elements can be directionalised if the TN input is associated with
the winding to which the optional VT input has been connected and assigned.
For standard earth fault protection, two options are available for polarisation; Residual Voltage or Negative
Sequence.
6.4.1

RESIDUAL VOLTAGE POLARISATION

With earth fault protection, the polarising signal needs to be representative of the earth fault condition. As residual
voltage is generated during earth fault conditions, this quantity is commonly used to polarise directional earth
fault elements. This is known as Zero Sequence Voltage polarisation, Residual Voltage polarisation or Neutral
Displacement Voltage (NVD) polarisation.
Small levels of residual voltage could be present under normal system conditions due to system imbalances, VT
inaccuracies, device tolerances etc. For this reason, the device includes a user settable threshold (IN> VNPol set),
which must be exceeded in order for the DEF function to become operational. The residual voltage measurement
provided in the MEASUREMENTS 1 column of the menu may assist in determining the required threshold setting
during the commissioning stage, as this will indicate the level of standing residual voltage present.
Note:
Residual voltage is nominally 180° out of phase with residual current. Consequently, the DEF elements are polarised from the
"-Vres" quantity. This 180° phase shift is automatically introduced within the device.
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