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Functions

2.5 Restricted Earth Fault Protection (optional)

SIPROTEC, 7SD610, Manual

C53000-G1176-C145-6, Release date 02.2011

2.5.2 Functional Description

Measuring principle

During normal operation, no starpoint current I

flows through the starpoint lead. The sum of the phase cur-

rents 3I

= I

+ I

approximates zero.

When an earth fault occurs in the protected zone, a starpoint current I

will flow; depending on the earthing

conditions of the power system a further earth current may be recognized in the residual current path of the

phase current transformers (dashed arrow in Figure 2-27), which is, however, more or less in phase with the

starpoint current. The current direction into the protected object is defined as positive.

Figure 2-27 Example for an earth fault in a transformer with current distribution

When an earth fault occurs outside the protected zone (Figure 2-28), a starpoint current I

will also flow. But

an equally large current 3 I

must then flow through the phase current transformers. Since the current direction

into the protected object is defined as positive, this current is in phase opposition with I

Figure 2-28 Example for an earth fault outside a transformer with current distribution

When an external non-earthed fault causes heavy currents to flow through the protected zone, differences in

the magnetic characteristics of the phase current transformers under conditions of saturation may cause a sig-

nificant summation current which could resemble an earth current flowing into the protected zone. Measures

must be taken to prevent this current from causing a trip. For this, the restricted earth fault protection provides

stabilization methods which differ strongly from the usual stabilization methods of differential protection

schemes since it considers both the magnitude of the measured currents and their direction (phase relation-

ship).

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