7.2.4.5 Restricted earth fault protection
For solidly earthed systems a restricted earth fault protection REFPDIF is often
provided as a complement to the normal transformer differential function. The
advantage with the restricted earth fault functions is the high sensitivity for internal
earth faults in the transformer winding. Sensitivities of 2-8% can be achieved whereas
the normal differential function will have sensitivities of 20-40%. The sensitivity for
high impedance restricted earth fault function is mostly dependent on the current
transformers magnetizing currents.
The connection of a restricted earth fault function is shown in Figure
65. It is
connected across each directly or low impedance earthed transformer winding.
IEC05000177 V2 EN
Figure 65: Application of HZPDIF function as a restricted earth fault protection
for a star connected winding of an YNd transformer
Setting example
It is strongly recommended to use the highest tap of the CT whenever
high impedance protection is used. This helps in utilizing maximum
CT capability, minimize the current, thereby reducing the stability
voltage limit. Another factor is that during internal faults, the voltage
developed across the selected tap is limited by the non-linear resistor
but in the unused taps, owing to auto-transformer action, voltages
much higher than design limits might be induced.
Basic data:
Transformer rated current on HV winding: 250 A
Current transformer ratio: 300/1 A (Note: Must be the same at all locations)
CT Class: 10 VA 5P20
Table continues on next page
1MRK 502 071-UEN - Section 7
Differential protection
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 165
Application manual
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