One difficulty with this solution is that the current transformer ratio is normally so
large so that the secondary residual current will be very small. The false residual
current, due to dif
ference between the three phase current transformers, can be in the
same range as the secondary ground fault current. Thus if physically possible, cable CT
is recommended for such applications in order to measure 3I
0
correct.
As indicated above, there will be very small neutral voltage or residual current if the
stator ground fault is situated close to the generator neutral. The probability for this
fault is quite small but not zero. For small generators the risk of not detecting the stator
ground fault, close to the neutral, can be accepted. For medium and large generator it is
however often a requirement that also theses faults have to be detected. Therefore, a
special neutral end ground fault protection STEFPHIZ (59THD) is required.
STEFPHIZ (59THD) can be realized in different ways. The two main principles are:
•
3
rd
harmonic voltage detection
• Neutral point voltage injection
The 3
rd
harmonic voltage detection is based on the fact that the generator generates
some degree of 3
rd
harmonic voltages. These voltages have the same phase angle in the
three phases. This means that there will be a harmonic voltage in the generator neutral
during normal operation. This component is used for detection of ground faults in the
generator, close to the neutral.
If the 3
rd
harmonic voltage generated in the generator is less than 0.8 V RMS
secondary, the 3
rd
harmonic based protection cannot be used.
In this protection function, a 3
rd
harmonic voltage differential principle is used.
10.6.3 Setting guidelines
SEMOD155847-4 v6
The 100% Stator ground fault protection, 3rd harmonic based (STEFPHIZ,
59THD)protection is using the 3
rd
harmonic voltage generated by the generator itself.
To assure reliable function of the protection it is necessary that the 3
rd
harmonic
voltage generation is at least 1% of the generator rated voltage.
Adaptive frequency tracking must be properly configured and set for
the Signal Matrix for analog inputs (SMAI) preprocessing blocks in
order to ensure proper operation of the generator differential protection
function during varying frequency conditions.
Operation
: The parameter Operation is used to set the function /EnabledDisabled.
Section 10 1MRK 502 071-UUS A
Voltage protection
468 Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062
Application manual
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