Protection Features - GE DGP Instruction Manual

1 PRODUCT DESCRIPTION

1.3 PROTECTION FEATURES

64G2 is based on the percentage of third-harmonic voltage at the generator neutral (VN3) compared to the
total third-harmonic voltage generated. This function is designed to cover 15% of the neutral end of the stator
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windings, and is supervised by fundamental and third-harmonic voltage thresholds. These thresholds are fixed
at 30 and 0.5 volts respectively. The third-harmonic comparator method eliminates the need to know the gener-
ator harmonic characteristic to use or set this function. Note that wye-connected VTs are required for
proper operation of 64G2.
27TN is the third-harmonic neutral undervoltage function with a forward power supervision and can be used
with either wye or delta connected VTs. The percentage of stator windings covered by this function depends on
its threshold setting as well as the VN3 generated by the machine at the time of the fault. The magnitude of
VN3 under normal conditions is a function of several factors, such as type of generator, load current, load
power factor, system status, etc. It can be very small (nearly zero) under some conditions. To enhance security
during low VN3 voltage conditions, this function can be inhibited by a settable window of forward power. How-
ever, it should be noted that other conditions influencing the VN3 voltage may make 27TN insecure. In these
cases, function 64G2 (available in some models; see the DGP nomenclature guide) or some other means
should be considered.
Digital input DI1 can be configured to block 64G2/27TN when the generator is off-line. This provision is made
to enhance security of the functions under conditions such as static start of a gas turbine generator. Temporary
ungrounding of generator neutral during the static start can look like a ground fault near the neutral.
1.3.8 GROUND OVERCURRENT (51GN)
51GN is an inverse overcurrent function available in some models. It can be used to detect stator ground faults
in a high or low resistance grounded generator system. See Figure 1–5: SIMPLE LOGIC DIAGRAM – 64G1,
64G2, 51GN, AND 24 on page 1–14 for simplified logic diagram and Figure 2–16: 51GN TIME-CURRENT
CHARACTERISTICS on page 2–39 for the inverse time-current characteristics.
This function uses current INR which can be derived by residual connection or by using a generator neutral CT
as noted in Figures 1–9: ELEMENTARY DIAGRAM WITH TEST BLOCKS, WYE VTs and 1–12: ELEMEN-
TARY DIAGRAM WITHOUT TEST BLOCKS, DELTA VTs.
Since this function is independent of the phase current inputs, it can alternately be connected to a CT in the
neutral of the generator step-up transformer.
1.3.9 OVEREXCITATION (24)
Overexcitation can be caused by regulator failure, load rejection, or an excessive excitation when the genera-
tor is off-line. It can also result from decreasing speed while the regulator or an operator attempts to maintain
rated stator voltage. The Volts/Hertz quantity is proportional to magnetic flux in the generator and step-up
transformer cores, and is used to detect the overexcitation condition. See Figure 1–5: SIMPLE LOGIC DIA-
GRAM – 64G1, 64G2, 51GN, AND 24 for details.
The overexcitation protection includes trip (24T) and alarm (24A) functions. 24T consists of an inverse function
and an instantaneous function with time-delay characteristics. The combination of these two characteristics
allows the 24T setting to closely follow the generator and/or step-up transformer V/Hz limit curve. Both 24A
and 24T are computed for each of the three phase voltages (see Table 2–3: 24A VOLTAGES on page 2–30).
Function 24T can be configured to operate different output relays for generator on-line and off-line conditions.
This function incorporates a user-settable linear reset characteristic to mimic machine cooling. The figures in
Section 2.3.12: OVEREXCITATION TRIP (VOLTS/HERTZ: 24T) show the characteristics of this function.
GE Power Management DGP Digital Generator Protection System 1-
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