Biased Characteristic Breakpoints; Description; High Breakpoint - GE B30 Instruction Manual

10 APPLICATION OF SETTINGS

10.3BIASED CHARACTERISTIC BREAKPOINTS

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:
where: I is the maximum secondary current transformed without saturation (AC component only, no
max
residual magnetism),
R is the total burden resistance,
s
V is the saturation voltage of the CT.
sat
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:
where: R is the lead resistance (one way, hence the factor of 2)
lead
R is the secondary CT resistance
CTsec
R is the relay input resistance.
relay
Assuming 0.003 Ω/m lead resistance and approximating the B30 input resistance for the 5A input CTs as 0.2 VA / (5 A)
0.008 Ω, the limits of the linear operation of the CTs have been calculated and presented in the Limits of Linear Operations
of the CTs table.
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
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 B30 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
Table 10–3: LIMITS OF LINEAR OPERATIONS OF THE CTS
CT
CT-1
CT-2
CT-3
CT-4
CT-5, CT-6
CT-7, CT-8
The third and fourth columns of the above table have the following significance.
GE Multilin
V
----------
I
=
max
R
R 2R R
= +
s lead CTsec
HIGH BPNT
current, the limits of linear operation have been recalculated to pu values as follows:
base
I
max secondary (
-----------------------------------
I
=
max pu ( )
I
base
R (Ω) I (A SEC)
S MAX
1.61 89.55
1.58 91.25
1.85 155.84
1.75 137.30
1.63 147.42
1.85 155.84
B30 Bus Differential System
10.3 BIASED CHARACTERISTIC BREAKPOINTS
sat
s
R
+
relay
setting. The limit of linear operation that neglects both
)
×
CT ratio
I (PU)
MAX
(NO REMANENCE)
8.96
9.13
31.17
22.88
24.57
31.17

10.3.1 DESCRIPTION

(EQ 10.1)
(EQ 10.2)
2
or

10.3.2 HIGH BREAKPOINT

(EQ 10.3)
I (PU)
MAX
(80% REMANENCE)
1.79
1.83
6.23
4.58
4.91
6.23
10-5
10