ABB REG650 Technical Manual page 288

Section 9
Voltage protection
282
In normal non-faulted operation of the generating unit the neutral point voltage is close
to zero, and there is no zero sequence current flow in the generator. When a phase-to-
ground fault occurs the neutral point voltage will increase and there will be a current
flow through the neutral point resistor.
To detect a ground fault on the windings of a generating unit one may use a neutral
point overvoltage protection, a neutral point overcurrent protection, a zero sequence
overvoltage protection or a residual differential protection. These protections are
simple and have served well during many years. However, at best these simple
schemes protect only 95% of the stator winding. They leave 5% close to the neutral
end unprotected. Under unfavorable conditions the blind zone may extend up to 20%
from the neutral.
The 95% stator ground fault protection measures the fundamental frequency voltage
component in the generator star point and it operates when it exceeds the preset value.
By applying this principle approximately 95% of the stator winding can be protected.
In order to protect the last 5% of the stator winding close to the neutral end the 3rd
harmonic voltage measurement can be performed. In 100% Stator E/F 3rd harmonic
protection either the 3rd harmonic voltage differential principle, the neutral point 3rd
harmonic undervoltage principle or the terminal side 3rd harmonic overvoltage
principle can be applied. However, differential principle is strongly recommended.
Combination of these two measuring principles provides coverage for entire stator
winding against ground faults.
x E3 x E3
N N N
RN RN
uN uN
x
Samples of the
neutral voltage
from which the
fundamental and
3rd harmonic
Neutral point fundamental frequency
voltages are
over-voltage protection 5% - 100%
filtered out over- voltage protection 10% – 100%
3rd harmonic
Differential
differential
0% – 30%
0% - 30%
ANSI10000202 V1 EN
Figure 129: Protection principles for STEFPHIZ (59TD) function
stator winding stator winding
(1- x) E3 (1- x) E3
Rf Rf
1 - x 1 - x
1 or 100 %
1MRK 502 043-UUS B
CB 1 may not exist CB 1 may not exist
CB 1 CB 1
T T
Transformer
uT uT
Samples of the
terminal voltage
from which the
3rd harmonic
voltage is filtered
out
ANSI10000202-1-en.vsd
Technical Manual
CB 2 CB 2