Basler Electric BE1-87B - Manual (Page 34)

RT2 = [1 + 0.00385 (50-25)] 0.450
= (1.096) (0.450) = 0.493
Ω
Substituting the various quantities in Equation 5 yields:
V
DIFF
= 1.25 [.524 + 2 (0.493)] 12500/240
= 98.31 volts
Since 98.31 volts is not an exact equal to one of the V
DIFF
setting, select the next higher available setting,
which is 100 volts.
V
DIFF
= 100 volts
Since the CTs are all used on the full winding (suggested practice), there is no need to check that
excessive voltages will be produced in the CT circuits. Now that V
DIFF
setting has been selected, the
sensitivity may be calculated following the procedure outlined in the section under Minimum Fault to Trip.
From Figure 2-6, the knee point coordinates E
S
and Ie, are 290 volts and 0.06 ampere. From Equation 7:
V = (7) (E
S
) = (7) (290) = 2030 volts
I = (5) (Ie) = (5) (0.06) = 0.30 ampere
Plot this point (V, I) on the graph of Figure 2-6 and draw the lines A-B and C-D. This gives the modified
secondary excitation characteristics. Calculate the voltage V
S
using Equation 8.
V
S
= (V
DIFF
)
= 2.83 (100)
= 283 volts
From the modified curve, the current Ie corresponding to V
S
= 283 volts is 0.05 ampere.
The relay current from Equation 9 is:
5000
) (V (2)
I
DIFF
R
=
amperes 0.04
5000
(100) (2)
I
R
= =
The sensitivity of the relay voltage element or the minimum fault level of the voltage element from
Equation 6 is:
I
min
= [(5) (.05) + 0.04] (240) = 70 amperes.
With the relay set at 0.25 amperes sensitivity, 60 amperes of primary current are required to produce 0.25
ampere secondary from the 1200/5 CTs. Therefore, the minimum current sensitivity of the relay is 60
amperes primary. If a higher minimum current sensitivity is used, the minimum current required for pickup
will be correspondingly higher. For example, with a 1200/5 CT ratio, 120 amperes of primary current are
required to produce 0.50 ampere secondary with the relay set at 0.50 ampere sensitivity.
CT Test Circuit Calculations
General
Operating parameters for the CT Test Circuit are defined by the user application. That is, the Pickup
Voltage setting for the specific bus protection application is required before operating parameters for the
test circuit can be determined. The Alarm Voltage should be set to 10% to minimize the voltage
magnitude required from the external CT Diagnostic Test Source.
Definition of Terms
•
V DIFF – symmetrical rms voltage setting of the relay.
•
V ALARM – unbalance differential Voltage setting of the relay (V DIFF X 10%).
•
V TEST- external test source voltage for shorted CT test circuit (V ALARM X 1.5). The multiple of 1.5
compensates for voltage drop across R TEST and guarantees operation of V ALARM during test of a
Healthy CT circuit. The multiplier was derived through the iterative process.
•
Ie - secondary excitation current from the CT excitation curve.
•
Z CT –V TEST/Ie.
2-16 BE1-87B Application 9282300990 Rev P