Functions
2.28 90-%-Stator Earth Fault Protection (ANSI 59N, 64G, 67G)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
207
Earth Current Detection (Earth Differential Protection with Displacment Voltage as the Pickup Criterion)
In the industrial sector, busbar systems are designed with high or low resistance, switchable starpoint resis-
tances. For earth-fault detection, the starpoint current and the total current are detected via toroidal current
transformers and transmitted to the protective device as current difference. In this way, the earth current por-
tions derived both from the starpoint resistance and from the power system contribute to the total earth current.
In order to exclude an unwanted operation due to transformer faults, the displacement voltage is used for trip-
ping (see the following figure).
The protection feature detects a machine earth fault if the following two criteria are fulfilled:
• Displacement voltage greater than set value U0>,
• Earth fault current difference ΔI
E
larger than setting value 3I0>,
Figure 2-88 Earth Current Differential Protection with Busbar Connection
Determination of the Faulty Phase
In addition to this, a supplementary function serves to determine the faulty phase. As the phase-earth-voltage
in the faulty phase is less than in the two remaining phases and as the voltage even increases in the latter ones,
the faulty phase can be determined by determining the smallest phase-earth voltage in order to generate a cor-
responding result as fault message.
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