GE b30, Instruction Manual

Page: 534 / 632

8-6 B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
BIASED CHARACTERISTIC BREAKPOINTS CHAPTER 8: APPLICATION OF SETTINGS
8
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 Ω /m lead resistance and approximating the B30 input resistance for the 5A input CTs as 0.2 VA / (5 A)
2
or
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.
8.3.2 High breakpoint
As an external fault can happen on any of the connected circuits, threatening saturation of any of the CTs, take the
minimum value of the linear operation limit 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 B30 requires the breakpoints to be entered as ‘pu’ values. The relay uses the largest primary current of the CTs
bounding 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. 8-3
Table 8-3: Limits of linear operation of the CTS
The third and fourth columns of the table have the following significance.
If an external fault occurs on circuit C-1, then CT-1 carries the fault current. As the fault current is higher than any of the
other currents, the current supplied by CT-1 is used as the restraint signal. CT-1 is guaranteed to saturate if the current
exceeds 89.55 A secondary, or 17.9 times its rated current, or 8.96 pu of the bus differential zone. Consequently,
considering CT-1, the value of 8.96 pu is used as the higher breakpoint of the characteristic.
Considering CTs that can be connected (depending on the positions of the switches) to the North bus, the
HIGH BPNT
for the
North bus zone is selected as the minimum of (8.96, 9.13, 31.17, 22.88, 24.57, 31.17), or 8.96 pu.
Considering CTs that can be connected (depending on the positions of the switches) to the South bus, the
HIGH BPNT
for the
South bus zone is selected as the minimum of (9.13, 31.17, 22.88, 24.57, 31.17), or 9.13 pu.
8.3.3 Low breakpoint
The DC component in the primary current can saturate a given CT even with the AC current below the suggested value of
the higher breakpoint. The relay copes with this threat by using the Saturation Detector and applying a 2-out-of-2
operating principle upon detecting saturation.
The residual magnetism (remanence) left in the core of a CT can limit the linear operation of the CT significantly. It is
justified to assume that the residual flux can be as high as 80% of the saturation level leaving only 20% to accommodate
the flux component created by the primary current. This phenomenon can be reflected by reducing the saturation voltage
in the calculations by the factor of 100% / 20%, or 5. This, in turn, is equivalent to reducing the limit of linear operation by
the factor of 5, hence the last column in the Limits of Linear Operations of the CTs table.
CT R
s
(
Ω
) 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