GE UR Series L90 Instruction Manual page 352

5.6 GROUPED ELEMENTS
The equation above brings an advantage of generating the restraining signal of twice the external ground fault current,
while reducing the restraint below the internal ground fault current. The negative-sequence component of the restraining
signal (IR2) is meant to provide maximum restraint during external phase-to-phase faults and is calculated as follows:
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

2 If I_1
3 else IR1
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:
5
where k represents a present sample, k – 1 represents the previous sample, and  is a factory constant (  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 logic diagram below.
5-214
IR2 I_2
=
of phase CT, then
 
I_0 , then IR1 3 I_1
= –
0
=
Igr k   max Irest k    Igr k 1 
=
Figure 5–109: RESTRICTED GROUND FAULT SCHEME LOGIC
L90 Line Current Differential System

or IR2 3 I_2
=

I_0
    
–
5 SETTINGS
(EQ 5.27)
(EQ 5.28)
GE Multilin