Petersen Coil Earthed Systems; Figure 50: Directional Tripping Characteristic - GE P24DM Technical Manual

Chapter 6 - Current Protection Functions
The current sensitivity of the relay should be approximately 30% of Ö2 times the charging current for the rest of
the system (3 x steady state value). The relay connections to give a defined direction for operation are shown on
the relay connection diagram.
The timer setting of this element is not critical because minimal damage results from the first fault. However, for
subsequent faults fast tripping is required.
Note:
Similar comments regarding the VT requirements for the P24xM relay when directionalizing earth fault elements (R.C.A = relay
characteristic angle) can be seen in the Insulated system section.
V00783

Figure 50: Directional tripping characteristic

7.5.8

PETERSEN COIL EARTHED SYSTEMS

Principle
Power systems are usually earthed to limit transient overvoltages during arcing faults and to assist with detection
and clearance of earth faults. Impedance earthing has the advantage of limiting damage incurred by plant during
earth fault conditions. It also limits the risk of explosive failure of switchgear, which is a danger to personnel. In
addition, it limits touch and step potentials at a substation or in the vicinity of an earth fault.
If a high impedance device is used for earthing the system, or the system is unearthed, the earth fault current is
reduced. However, the steady state and transient overvoltages on the sound phases can be very high. Therefore,
high impedance earthing is only used in low or medium voltage networks where the necessary insulation against
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Tripping Zone
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