Earth Fault Protection; Earth Fault Protection Elements - GE P642 Technical Manual

Chapter 9 - Current Protection Functions
6

EARTH FAULT PROTECTION

Earth faults are overcurrent faults where the fault current flows to earth. Earth faults are the most common type of
fault.
Earth faults can be measured directly from the system by means of:
● A separate current Transformer (CT) located in a power system earth connection
A separate Core Balance Current Transformer (CBCT), usually connected to the SEF transformer input
●
A residual connection of the three line CTs, where the Earth faults can be derived mathematically by
●
summing the three measured phase currents.
Depending on the device model, it will provide one or more of the above means for Earth fault protection.
6.1

EARTH FAULT PROTECTION ELEMENTS

The product provides three earth fault elements for backup earth fault protection. Each element provides four-
stages of non-directional or directional three-phase overcurrent protection with independent time delay
characteristics. You can select the overcurrent element operating quantity for each of the elements with the
setting cells Earth Fault 1, Earth Fault2 and Earth Fault3 in the EARTH FAULT column of the relevant settings
group. You can set each element as T1, T2, T3, T4, T5, HV winding, LV winding or TV winding.
You can either measure the Earth Fault directly or derive it by summing the phase currents. This depends on the
settings EF 1 Input, EF 2 Input, EF 3 Input, which you can set to Measured or Derived. For the derived element,
the HV, LV and TV windings comprise the vector sums of the CT inputs associated with a particular winding. For the
measured elements TN1, TN2 and TN3 are used.
Each earth fault element provides four stages of Earth Fault protection with independent time delay
characteristics.
Stages 1 and 2 provide a choice of operate and reset characteristics, where you can select between:
A range of standard IDMT (Inverse Definite Minimum Time) curves
●
DT (Definite Time)
●
This is achieved using the cells:
● IN>(n) Function for the overcurrent operate characteristics
IN>(n) Reset Char for the overcurrent reset characteristic (IEEE only)
●
where (n) is the number of the stage.
Stages 1 and 2 provide a Timer Hold facility. This is configured using the cells IN>(n) tReset.
Stages 3 and 4 can have definite time characteristics only.
216
P64x
P64x-TM-EN-1.3