Earth Fault Protection; Earth Fault Protection Elements - GE P24DM Technical Manual

P24xM
7

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.
7.1

EARTH FAULT PROTECTION ELEMENTS

Earth fault protection is implemented in the columns EARTH FAULT 1 and EARTH FAULT 2 of the relevant settings
group.
Each column contains an identical set of elements. EARTH FAULT 1 (EF1) is used for earth fault current that is
measured directly from the system (measured). EARTH FAULT 2 (EF2) 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
A range of User-defined curves
●
DT (Definite Time)
●
For the EF1 column, this is achieved using the cells:
IN1>(n) Function for the overcurrent operate characteristics
●
● IN1>(n) Reset Char for the overcurrent reset characteristic
IN1>(n) Usr RstChar for the reset characteristic for user-defined curves
●
For the EF2 column, this is achieved using the cells:
IN2>(n) Function for the overcurrent operate characteristics
●
IN2>(n) Reset Char for the overcurrent reset characteristic
●
IN2>(n) Usr RstChar for the reset characteristic for user-defined curves
●
where (n) is the number of the stage.
Stages 1 and 2 provide a Timer Hold facility. This is configured using the cells IN1>(n) tReset for EF1 and
IN2>(n) tReset for EF2.
Stages 3 and 4 can have definite time characteristics only.
The fact that both EF1 and EF2 elements may be enabled at the same time leads to a number of applications
advantages. For example, some applications may require directional earth fault protection for upstream
equipment and backup earth fault protection for downstream equipment. This can be achieved with a single IED,
rather than two.
P24xM-TM-EN-2.1
Chapter 6 - Current Protection Functions
109