5.3.19 100 % stator earth fault protection (U03rd>; 64S)
When using conventional neutral overvoltage protection function (U0>; 59N) even in the best case
scenario at least 5 % of the stator remains outside the function's range. The 100 % stator earth fault
protection function (also known as neutral 3rd harmonic overvoltage protection) is used to detect earth
faults near the neutral point. When an Earth fault occurs near the neutral point, it does not produce
enough neutral voltage for a conventional neutral overvoltage protection to detect the fault. Generators
induce some of the 3
rd
harmonic neutral voltage; when there is an earth fault near the neutral point,
this function detects the drop in the 3
rd
harmonic neutral voltage and trips the breaker. If the
3
rd
harmonic neutral voltage induced by the generator is high enough, the whole stator can be
protected by combining the neutral overvoltage and the 100 % stator earth fault protection functions.
The figure below demonstrates the overlapping range of the neutral overvoltage and the 100 % stator
earth fault protection function.
Figure. 5.3.19 - 168. Overlapping demonstration.
The function can operate on instant or time-delayed mode. In the time-delayed mode the operation can
be selected for definite time (DT) or for inverse definite minimum time (IDMT).
The inputs for the function are the following:
• operating mode selections
• setting parameters
• digital inputs and logic signals
• measured and pre-processed voltage and current magnitudes.
The function outputs the START, TRIP and BLOCKED signals which can be used for direct I/O
controlling and user logic programming. The function generates general time-stamped ON/OFF events
to the common event buffer from each of the three (3) output signals. In instant operating mode the
function outputs START and TRIP events simultaneously with an equivalent time stamp. The time stamp
resolution is 1 ms. The function also provides a resettable cumulative counter for the START, TRIP and
BLOCKED events.
The following figure presents a simplified function block diagram of the 100 % stator earth fault
protection function.
A
AQ
Q-G215
-G215
Instruction manual
Version: 2.04
242