GE UR Series L90, Instruction Manual, Page: 549 / 846

GE Multilin L90 Line Current Differential System 9-3
9 THEORY OF OPERATION 9.1 OVERVIEW
9
9.1.5 DISTURBANCE DETECTION
A disturbance detection algorithm is used to enhance security and to improve transient response. Conditions to detect a
disturbance include the magnitude of zero-sequence current, the magnitude of negative-sequence current, and changes in
positive, negative, or zero-sequence current. Normally, differential protection is performed using a full-cycle Fourier trans-
form. Continuous use of a full-cycle Fourier means that some pre-fault data is also used for computation – this may lead to
a slowdown in the operation of the differential function. To improve operating time, the window is resized to the half-cycle
Fourier once a disturbance is detected, thus removing pre-fault data.
9.1.6 FAULT DETECTION
Normally, the sum of the current phasors from all terminals is zero for each phase at every terminal. A fault is detected for a
phase when the sum of the current phasors from each terminal for that phase falls outside of a dynamic elliptical restraint
boundary for that phase. The severity of the fault is computed as follows for each phase.
The differential current is calculated as a sum of local and remote currents. The real part is expressed as:
(EQ 9.3)
The imaginary part is expressed as:
(EQ 9.4)
The differential current is squared for the severity equation:
(EQ 9.5)
The restraint current is composed from two distinctive terms: traditional and adaptive. Each relay calculates local portion of
the traditional and restraint current to be used locally and sent to remote peers for use with differential calculations. If more
than one CT are connected to the relay (breaker-and-the half applications), then a maximum of all (up to 4) currents is cho-
sen to be processed for traditional restraint:
The current chosen is expressed as:
(EQ 9.6)
This current is then processed with the slope ( S
1
and S
2
) and breakpoint (BP) settings to form a traditional part of the
restraint term for the local current as follows. For two-terminal systems, we have:
(EQ 9.7)
For three-terminal systems we have
(EQ 9.8)
The final restraint current sent to peers and used locally in differential calculations is as follows:
(EQ 9.9)
where: MULT
A
is a multiplier that increases restraint if CT saturation is detected (see CT Saturation Detection for details);
I
LOC_ADA_A
is an adaptive restraint term (see Online Estimate Of Measurement Error for details)
The squared restraining current is calculated as a sum of squared local and all remote restraints:
I
DIFF_RE_A
I
LOC_PHASOR_RE_A
I
REM1_PHASOR_RE_A
I
REM2_PHASOR_RE_A
+ + =
I
DIFF_IM_A
I
LOC_PHASOR_IM_A
I
REM1_PHASOR_IM_A
I
REM2_PHASOR_IM_A
+ + =
I
DIFF_A
 
2
I
DIFF_RE_A
 
2
I
DIFF_IM_A
 
2
+ =
I
LOC_TRAD_A
 
2
max I
1_MAG_A
 
2
I
2_MAG_A
 
2
I
3_MAG_A
 
2
I
4_MAG_A
 
2
I
q_MAG_A
 
2
     
=
If I
LOC_TRAD_A
 
2
BP
2

then I
LOC_REST_TRAD_A
 
2
2 S
1
I
LOC_TRAD_A
  
2
=
else I
LOC_REST_TRAD_A
 
2
2 S
2
I
LOC_TRAD_A
  
2
S
2
BP
  
2
–
 
2 S
1
BP
  
2
+ =
If I
LOC_TRAD_A
 
2
BP
2

then I
LOC_REST_TRAD_A
 
2
4
3
---
S
1
I
LOC_TRAD_A
  
2
=
else I
LOC_REST_TRAD_A
 
2
4
3
---
S
2
I
LOC_TRAD_A
  
2
S
2
BP
  
2
–
 
4
3
---
S
1
BP
  
2
+ =
I
LOC_RESTRAINT_A
I
LOC_REST_TRAD_A
 
2
MULT
A
I
LOC_ADA_A
 
2

+ =