GE DGP Series, Instruction Manual

Page: 19 / 278

GE Power Management DGP Digital Generator Protection System 1-
7
1 PRODUCT DESCRIPTION 1.3 PROTECTION FEATURES
1
1.3.2 STATOR DIFFERENTIAL (87G)
This function provides high-speed protection of the generator stator during internal phase-to-phase and three-
phase faults. It uses a product-restraint algorithm with dual-slope characteristic described in Section 2.3.2:
STATOR DIFFERENTIAL 87G on page 2–13. Refer to Figure 1–3: SIMPLE LOGIC DIAGRAM – 87G, 32, 27,
59, AND AE on page 1–12 for the logic diagram of this function.
Function 87G will not operate for turn-to-turn faults in the machine windings.
It will also not operate for single-phase-to-ground faults if the system is ungrounded or high-impedance
grounded. Phase-to-ground protection by this function requires that the neutral of the machine (or another
machine operating in parallel) be grounded. A small portion of the winding next to the neutral will not be pro-
tected, the amount being determined by the voltage necessary to cause minimum pickup current to flow
through the neutral-to-ground impedance. Current-limiting devices in the neutral-ground circuit increase this
impedance and will decrease the ground-fault-protection coverage of this function.
1.3.3 CURRENT UNBALANCE (46T)
There are several causes of generator unbalance. Some of these include unbalanced loads, unbalanced sys-
tem faults, and/or open circuits. The negative-sequence component (
I
2
) of stator current is directly related to
this unbalance and sets up a counter-rotating flux field in the machine. This in turn causes local heating in the
rotor iron. The capability of machines to withstand heating caused by unbalance currents is typically exper-
essed in terms of an
constant, and is supplied by the manufacturer of the machine.
The current unbalance trip function (46T) of the DGP provides operating-time characteristics expressed as
I
2
2
T = K, as shown in Figure 2–6: TIME CURRENT CHARACTERISTIC OF 46T FUNCTION on page 2–19. A
linear reset characteristic is incorporated to approximate the machine cooling following an intermittent current-
unbalance condition. In addition to 46T, the DGP system also includes a current-unbalance alarm function,
46A, which is operated by the negative-sequence component (I2) with an adjustable pickup and time delay.
See Figure 1–4: SIMPLE LOGIC DIAGRAM – 46, 40, AND 51V on page 1–13 for the logic diagram.
1.3.4 LOSS OF EXCITATION (40)
This function is used to detect loss of excitation on synchronous machines. It includes two mho characteristics
looking into the machine, each with adjustable reach, offset, and time delay. Logic is provided to block this
function by presence of a negative-sequence voltage (indicating a voltage transformer fuse failure VTFF condi-
tion) and/or an external VTFF Digital Input DI6 (see Figure 1–4: SIMPLE LOGIC DIAGRAM – 46, 40, AND 51V
on page 1–13).
Excitation can be lost due to inadvertent tripping of the field breaker, open or short circuit on the field winding,
regulator failure, or loss of the source to the field winding. Loss of excitation can be damaging to the machine
and/or detrimental to the operation of the system. When a synchronous generator loses excitation, it will tend
to act as an induction generator: it will run above normal speed, operate at reduced power and receive its exci-
tation (VARS) from the system. The impedance seen by a relay looking into a generator will depend on the
machine characteristics, the load flow prior to the loss of excitation, and the type of excitation failure.
Studies indicates that first zone mho function (40-1) can be set to detect severe cases of excitation failure with
a shorter time delay, whereas the second zone (40-2) can be set to detect all the excitation failure cases. A
longer time delay setting is required for the 40-2 function for security during stable power system swing condi-
tions. Figure 2–7: MHO CHARACTERISTICS FOR 40-1 & 40-2 FUNCTIONS on page 2–21 shows the charac-
teristics of this function.
I
2
2
T