GE DGP Series, Instruction Manual

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DGP Digital Generator Protection System GE Power Management
1.3 PROTECTION FEATURES 1 PRODUCT DESCRIPTION
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1.3.5 ANTI-MOTORING (32)
On a total or partial loss of prime mover, if the power generated is less than no-load losses of the machine, real
power will start flowing into the generator. Typical motoring power of different kinds of prime movers are shown
in the table below. For a specific application, the minimum motoring power of the generator should be obtained
from the supplier of the unit.
The DGP system includes a reverse power function with adjustable time-delay. Either one or two (32-1 & 32-2)
independent setpoints are incorporated depending on the model number.
The 32-1 can be configured as a part of sequential tripping logic as shown in Figure 1–3: SIMPLE LOGIC DIA-
GRAM – 87G, 32, 27, 59, AND AE on page 1–12. If the sequential trip logic is used, 32-1 is enabled when clos-
ing of turbine inlet valves is indicated by digital input DI2 following a turbine trip. The trip sequence is then
continued when timer TL1 times out. The 32-2, if included, is not dependent on the DI2 and is primarily
intended to provide backup to the sequential trip. If the sequential trip is not enabled, the 32-1 can be used as
anti-motoring similar to 32-2.
1.3.6 TIME OVERCURRENT WITH VOLTAGE RESTRAINT 51V
A system must be protected against prolonged generator contribution to a fault. The DGP incorporates a time-
overcurrent function with voltage restraint (51V) to provide part of the system backup protection. As shown in
Figure 1–4: SIMPLE LOGIC DIAGRAM – 46, 40, AND 51V on page 1–13, this function is supervised by a fault
detector and VTFF. The VTFF supervision can be by an internal and/or external (DI6) VTFF function. See Sec-
tion 2.3.7: OVERCURRENT WITH VOLTAGE RESTRAINT (51V) on page 2–22 for the characteristic curves of
the 51V. Note that a separate algorithm is processed for each phase, with the restraint provided by correspond-
ing phase voltage. The restraint is proportional to the magnitude of the voltage and is independent of the phase
angle. A linear reset characteristic is incorporated for this function.
1.3.7 STATOR GROUND (64G/27TN)
This function consists of two overlapping zones (64G1 and 64G2/27TN) to detect stator ground faults in a high-
impedance-grounded generator system. The 64G1 is standard in all DGP models; however, the 64G2/27TN
function is provided in some models only. Together, the two zones cover 100% of the stator windings. See Fig-
ure 1–5: SIMPLE LOGIC DIAGRAM – 64G1, 64G2, 51GN, AND 24 on page 1–14.
Normally the generator-stator neutral has a potential close to ground. With the occurrence of a stator ground
fault, a potential increase will occur on the neutral for all faults except those near the neutral. 64G1 uses a fun-
damental-frequency neutral overvoltage to cover about 95% of the stator winding, depending on the pickup
voltage setting. Alternately, 64G1 can be used as a generator-bus ground detector in a high-impedance
grounded or an ungrounded system. For this application, the VN input must be a zero-sequence voltage
derived from the generator bus, and functions 64G2/27TN must be disabled.
Table 1–4: TYPICAL MOTORING POWER
TYPE OF PRIME
MOVERS MOTORING POWER IN %
OF UNIT RATING
Gas Turbine 10 to 100
Diesel 15 to 25
Hydraulic Turbine 2 to 100
Steam Turbine 0.5 to 4