GE G60 Instruction Manual page 193

5 SETTINGS
The multiplier of 1 is used by the relay for first two cycles following complete de-energization of the winding (all three phase
currents below 5% of nominal for at least five cycles). The multiplier of 3 is used during normal operation; that is, two cycles
after the winding has been energized. The lower multiplier is used to ensure better sensitivity when energizing a faulty
winding.
The positive-sequence component of the restraining signal (IR1) is meant to provide restraint during symmetrical condi-
tions, either symmetrical faults or load, and is calculated according to the following algorithm:
>
1 If I_1 1.5 pu of phase CT, then
>
2 If I_1 I_0 , then
3 else IR1 0
=
I_1 8 ⁄
4 else IR1
=
Under load-level currents (below 150% of nominal), the positive-sequence restraint is set to 1/8th of the positive-sequence
current (Line 4). This is to ensure maximum sensitivity during low-current faults under full load conditions. Under fault-level
currents (above 150% of nominal), the positive-sequence restraint is removed if the zero-sequence component is greater
than the positive-sequence (Line 3), or set at the net difference of the two (Line 2).
The raw restraining signal (Irest) is further post-filtered for better performance during external faults with heavy CT satura-
tion and for better switch-off transient control:
where k represents a present sample, k – 1 represents the previous sample, and α is a factory constant (α < 1). The equa-
tion above introduces a decaying memory to the restraining signal. Should the raw restraining signal (Irest) disappear or
drop significantly, such as when an external fault gets cleared or a CT saturates heavily, the actual restraining signal (Igr(k))
will not reduce instantly but will keep decaying decreasing its value by 50% each 15.5 power system cycles.
Having the differential and restraining signals developed, the element applies a single slope differential characteristic with a
minimum pickup as shown in the Restricted Ground Fault Scheme Logic diagram.
SETTING
RESTD GND FT1
FUNCTION:
Disabled=0
Enabled=1
SETTING
AND
RESTD GND FT1
BLOCK:
Off=0
SETTING
RESTD GND FT1
SOURCE:
G
Differential
N
_0
Restraining
_1
Currents
_2
Figure 5–67: RESTRICTED GROUND FAULT SCHEME LOGIC
GE Multilin
IR2 I_2 or IR2
=
× ( )
IR1 3 I_1 I_0
= –
Igr k ( ) max Irest k ( ) α Igr k 1 (
=
SETTING
RESTD GND FT1
PICKUP:
RUN
gd > PICKUP
SETTING
RESTD GND FT1
SLOPE:
RUN
gd > SLOPE * gr
and
ACTUAL VALUES
RGF 1 gd Mag
RGF 1 gr Mag
G60 Generator Management Relay
×
3 I_2
=
, × ( ) )
–
SETTINGS
RESTD GND FT1 PICKUP
DELAY:
RESTD GND FT1 RESET
DELAY:
t PKP
AND
t RST
5.5 GROUPED ELEMENTS
(EQ 5.21)
(EQ 5.22)
FLEXLOGIC OPERANDS
RESTD GND FT1 PKP
RESTD GND FT1 DPO
RESTD GND FT1 OP
828002A2.CDR
5-111
5