Earth Fault Protection - GE MiCOM P40 Agile Technical Manual

Chapter 13 - Current Protection Functions
4

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.
4.1 EARTH FAULT PROTECTION IMPLEMENTATION
Earth fault protection is implemented in the EARTH FAULT column of the relevant settings group. The element uses
quantities derived internally from summing the three-phase currents.
The product provides four stages of Earth Fault protection with independent time delay characteristics, for each
EARTH FAULT column.
Stages 1 and 2 provide a choice of operate and reset characteristics, where you can select between:
A range of 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
●
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.
Earth fault Overcurrent IN> can be set to:
Permanently disabled
●
Permanently enabled
●
● Enabled only if VT fuse/MCB fails
Enabled only if protection communication channel fails
●
Enabled if VT fuse/MCB or protection communication channel fail
●
Enabled if VT fuse/MCB and protection communication channel fail
●
Each stage can be individually inhibited with a DDB signal Inhibit IN>(n), where n is the stage number.
4.2 IDG CURVE
The IDG curve is commonly used for time delayed earth fault protection in the Swedish market. This curve is
available in stage 1 of the Earth Fault protection.
The IDG curve is represented by the following equation:
t 5 8 1 35 . . log
= −
op
where:
372
I
 
 
e
IN Setting
>
 
P543i/P545i
P54x1i-TM-EN-1