44x/EN AP/Hb6
-112 MiCOM P40 Agile
1 (minutes)
2 (minutes)
Limits
Oil-filled transformer 5 120 Rating 400 - 1600 kVA
The thermal time constants given in the above tables are typical only. Reference should
always be made to the plant manufacturer for accurate information.
4.10 Residual overvoltage protection
On a healthy three phase power system, the summation of all three phase to earth voltages
is normally zero, as it is the vector addition of three balanced vectors at 120° to one another.
However, when an earth (ground) fault occurs on the primary system this balance is upset
and a ‘residual’ voltage is produced.
Note: This condition causes a rise in the neutral voltage with respect to earth which is
commonly referred to as “neutral voltage displacement” or NVD.
The voltage setting applied to the elements is dependent upon the magnitude of residual
voltage that is expected to occur during the earth fault condition. This in turn is dependent
upon the method of system earthing employed and may be calculated by using the
formulae’s previously given in the above figures. It must also be ensured that the relay is set
above any standing level of residual voltage that is present on the healthy system.
Note: IDMT characteristics are selectable on the first stage of NVD and a time delay setting
is available on the second stage of NVD in order that elements located at various
points on the system may be time graded with one another.
The following figures show the residual voltages that are produced during earth fault
conditions occurring on a solid and impedance earthed power system respectively:
To Next Page
Loading...
Need help?
General
