Application - ABB RELION REX640 Technical Manual

Section 4
Protection functions
4.2.8.6
514
Timer
Once activated, Timer activates the START output. The Timer characteristic is
according to DT. When the operation timer has reached the value set by Operate delay
time, the OPERATE output is activated. If the fault disappears before the module
operates, the reset timer is activated. If the reset timer reaches the value set by Reset
delay time, the operation timer resets and the START output is deactivated.
Timer calculates the start duration START_DUR value, which indicates the
percentage ratio of the start situation and the set operation 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 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.
1MRS759142 C
REX640
Technical Manual