44x/EN AP/Hb6
-114 MiCOM P40 Agile
Figure 88: Residual voltage, resistance earthed system
As shown in the figure above, a resistance earthed system will always generate a relatively
large degree of residual voltage, as the zero sequence source impedance now includes the
earthing impedance. It follows then, that the residual voltage generated by an earth fault on
an insulated system will be the highest possible value (3 x phase-neutral voltage), as the
zero sequence source impedance is infinite.
From the above information it can be seen that the detection of a residual overvoltage
condition is an alternative means of earth fault detection, which does not require any
measurement of zero sequence current. This may be particularly advantageous at a tee
terminal where the infeed is from a delta winding of a transformer (and the delta acts as a
zero sequence current trap).
Where residual overvoltage protection is applied, such a voltage will be generated for a fault
occurring anywhere on that section of the system and hence the NVD protection must co-
ordinate with other earth/ground fault protection.
4.11 Undercurrent protection
The undercurrent protection is activated when the current lower than a threshold. It uses
definite delay time undercurrent protection.
The undercurrent protection included within the P442 and P444 relays consists of two
independent stages. These stages may be selected or disabled within the “I<1 Status” and
“I<2 Status” cells.
Two stages are included to provide both alarm and trip stages, where required.
Alternatively, different time settings may be required depending upon the severity of the
current dip.
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