ABB RELION REX640 Technical Manual page 546

Protection functions
4.2.8.6 Application
Mechanical and thermal stress deteriorates stator winding insulation, which can
eventually cause an earth fault between the winding and stator core.
The fault current magnitude in case of stator earth fault depends on the grounding
type. Common practice in most countries is to earth the generator neutral side
through a resistor. The resistor is selected such as to limit the maximum earth-fault
current in the range of 5...10 A. The same can be done by connecting a single phase
voltage transformer between the neutral side and earth, and with an equivalent
resistor on the secondary side of the transformer.
In a normal operating condition, that is, when there is no earth fault, the residual
voltage is close to zero with no zero-sequence current flowing in the generator.
When a phase-to-earth fault occurs, the residual voltage increases and the current
flows through the neutral. The simplest way to protect the stator winding against
an earth fault is by providing residual overvoltage protection (or residual/neutral
overcurrent protection). However, at best these simple schemes can protect only
95% of the stator winding, leaving 5% of the neutral end unprotected. This is
because the voltage generated in the faulted winding decreases as the fault point
becomes closer to the neutral point and it is not enough to drive the protection.
Under certain unfavorable conditions, the blind zone may extend up to 20% from the
neutral point.
An earth fault close to the neutral point is not dangerous, but an undetected fault
may develop into an interturn fault or phase-to-phase fault. Also an undetected
earth fault near the neutral point is bypassing the high-impedance grounding, and
then another earth fault at the terminal results in a catastrophic situation.
Therefore, it is important to extend the protection to full 100%. The third harmonic
voltage-based protection is one such protection which provides effective protection
during an earth fault at the neutral point, and at least in the range up to 15...20%
from the neutral point along the stator winding.
To achieve a complete stator earth-fault protection, two protection functions
should always run in parallel.
• Fundamental frequency-based residual overvoltage protection ROVPTOV
• Third harmonic-based stator earth-fault protection H3EFPSEF
Figure 314: Complete stator earth-fault protection
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1MRS759142 F
REX640
Technical Manual