Application - ABB 615 series Technical Manual

Section 4
Protection functions
4.2.7.5
428
timer is activated. If the reset timer reaches the value set by Reset delay time, the operation
timer resets and the PICKUP output is deactivated.
The Timer calculates the pickup duration PICKUP_DUR value, which indicates the
percentage ratio of the pickup situation and the set trip time. The value is available in the
Monitored data view.
The binary input BLOCK can be used to block the function. The activation of the BLOCK
input deactivates all outputs and resets internal timers.

Application

Mechanical and thermal stress deteriorates stator winding insulation, which can
eventually cause a ground fault between the winding and stator core.
The fault current magnitude in case of stator ground fault depends on the grounding type.
Common practice in most countries is to ground the generator neutral side through a
resistor. The resistor is selected such as to limit the maximum ground-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 ground, and with an equivalent resistor on the secondary side
of the transformer.
In a normal operating condition, that is, when there is no ground fault, the residual voltage
is close to zero with no zero-sequence current flowing in the generator. When a phase-to-
ground fault occurs, the residual voltage increases and the current flows through the
neutral. The simplest way to protect the stator winding against a ground 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.
A ground 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 ground fault
near the neutral point is bypassing the high-impedance grounding, and then another
ground 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 a ground 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 ground-fault protection, two protection functions should
always run in parallel.
1MAC059074-MB A
615 series ANSI
Technical Manual