9-4
B90 Low Impedance Bus Differential System
GE Multilin
9.3 BIASED CHARACTERISTIC BREAKPOINTS
9 APPLICATION OF SETTINGS
9
9.3BIASED CHARACTERISTIC BREAKPOINTS
9.3.1 DESCRIPTION
The limits of linear operation of the CTs need to be found in order to set the breakpoints of the biased differential character-
istic. The settings for the North and South bus relays are analyzed simultaneously from this point on as the two differential
zones share some CTs and the results of computations apply to both the relays.
For microprocessor-based relays it is justified to assume the burden of the CTs to be resistive. The limits of the linear oper-
ation of a CT, neglecting the effects of the DC component and residual magnetism, can be approximated as follows:
(EQ 9.1)
where:
I
max
is the maximum secondary current transformed without saturation (AC component only, no
residual magnetism),
R
s
is the total burden resistance,
V
sat
is the saturation voltage of the CT.
The total burden resistance depends on both the fault type and connection of the CTs. For single-line-to-ground faults and
CTs connected in Wye, the burden resistance is calculated as:
(EQ 9.2)
where:
R
lead
is the lead resistance (one way, hence the factor of 2)
R
CTsec
is the secondary CT resistance
R
relay
is the relay input resistance.
Assuming 0.003
/m lead resistance and approximating the B90 input resistance for the 5A input CTs as 0.2 VA / (5 A)
2
or
0.008
, the limits of the linear operation of the CTs have been calculated and presented in the Limits of Linear Operations
of the CTs table.
9.3.2 HIGH BREAKPOINT
As an external fault may happen on any of the connected circuits, threatening saturation of any of the CTs, the minimum
value of the linear operation limit should be taken as the
HIGH BPNT
setting. The limit of linear operation that neglects both
the residual magnetism and the effect of the DC component should be the base for setting the higher breakpoint of the
biased differential characteristic.
The B90 requires the breakpoints to be entered as ‘pu’ values. The relay uses the largest primary current of the CTs bound-
ing the bus differential zone as a base for the pu settings. Both the North and South buses have the largest primary current
of the CTs of 1200 A (CT-7 and CT-8), thus upon configuration of the relays, 1200 A is automatically selected as base for
the pu quantities. With a given
I
base
current, the limits of linear operation have been recalculated to pu values as follows:
(EQ 9.3)
The third and fourth columns of the above table have the following significance.
Table 9–3: LIMITS OF LINEAR OPERATIONS OF THE CTS
CT
R
S
(
)
I
MAX
(A SEC)
I
MAX
(PU)
(NO REMANENCE)
I
MAX
(PU)
(80% REMANENCE)
CT-1
1.61
89.55
8.96
1.79
CT-2
1.58
91.25
9.13
1.83
CT-3
1.85
155.84
31.17
6.23
CT-4
1.75
137.30
22.88
4.58
CT-5, CT-6
1.63
147.42
24.57
4.91
CT-7, CT-8
1.85
155.84
31.17
6.23
I
max
V
sat
R
s
----------
=
R
s
2
R
lead
R
CTsec
R
relay
+
+
=
I
max pu
I
max secondary
I
base
-----------------------------------
CT ratio
=
Summary of Contents for B90
Page 10: ...x B90 Low Impedance Bus Differential System GE Multilin TABLE OF CONTENTS ...
Page 284: ...5 166 B90 Low Impedance Bus Differential System GE Multilin 5 8 TESTING 5 SETTINGS 5 ...
Page 334: ...10 8 B90 Low Impedance Bus Differential System GE Multilin 10 2 BATTERIES 10 MAINTENANCE 10 ...
Page 338: ...A 4 B90 Low Impedance Bus Differential System GE Multilin A 1 PARAMETER LISTS APPENDIX A A ...
Page 460: ...C 30 B90 Low Impedance Bus Differential System GE Multilin C 7 LOGICAL NODES APPENDIX C C ...
Page 476: ...E 10 B90 Low Impedance Bus Differential System GE Multilin E 1 IEC 60870 5 104 APPENDIX E E ...
Page 502: ...viii B90 Low Impedance Bus Differential System GE Multilin INDEX ...
