Basler Electric BE1-87B - Manual (Page 30)

speed operation minimizes potential damage to the protected equipment. Refer to Figures 1-2 and 1-3 in
Section 1, General Information for illustrations of response times.
Hold Timer
The BE1-87B has a hold timer (200 milliseconds, fixed) that prevents the Trip output relay contacts from
opening prematurely.
Calculation of Settings
The BE1-87B relay is set based on the maximum possible voltage that can be produced in the differential
circuit as a result of a fault external to the zone of protection. Determination of the maximum voltage for
this condition is subject to simple calculations, and thus, the relay setting is easily determined. The relay
has a setting range 50 to 400 volts rms in 50-volt steps.
It is first necessary to calculate the maximum voltage that can be produced in the differential circuit for an
external fault. Once that value is determined, the appropriate voltage setting can be selected. If a mixture
of multi-ratio CTs is used (not recommended) or if the CTs are applied on taps other than full ratio,
calculations must be performed to determine if excessive voltages will be produced across the full winding
of the CT. Last, the minimum internal fault for which the relay will just operate will be calculated.
Calculation of Voltage Differential Settings
The minimum acceptable differential voltage setting can be determined using the following equation.
N
I
) PR (R 1.25 V
F
L S DIFF
+ =
(EQUATION 5)
V
DIFF
= minimum acceptable voltage tap setting. Since V
DIFF
in general will not come out exactly equal to
one of the available settings, the next higher setting should be used. The available voltage settings are 50
to 400 volts rms in 50-volt increments.
R
S
= dc resistance of fault CT secondary windings and leads to the CT makeup box (at maximum
expected operating temperature).
R
L
= single conductor dc resistance of the current circuit cable for a one-way run from the differential
junction point to the fault CT makeup box (at maximum expected operating temperature).
P = one (1.0) for three-phase faults and (2.0) for single-phase to ground faults.
I
F
= maximum external fault current in the fault CT in primary symmetrical rms amperes.
N = CT ratio.
1.25 = margin for safety.
The following comments may be made with respect to the evaluation of Equation 5.
•
It is only necessary to calculate three-phase and single-phase-to-ground faults. If the results yield a
satisfactory application, the application will also be satisfactory for multi-phase faults.
•
For single-phase-to-ground faults, the differential circuit is such that the CT secondary fault current
will flow through both of the fault CT cables; thus the multiplier P must be set equal to two. On the
other hand, the CT secondary currents during a balanced three-phase fault will result in 0 current in
the return cable; thus only the one-way cable resistance is involved, and P is set equal to one.
•
If the single-phase-to-ground fault current at a given location is greater than or equal to the three-
phase fault current, the calculations need only be made for the single-phase-to-ground faults.
•
The resistance of the CTs and connecting cables will increase with increasing temperature; therefore,
if adequate margin is to be maintained at all times, Equation 5 should be evaluated using resistance
values corresponding to the maximum expected operating temperature (see sample calculation in this
section).
The methods to be used in calculating the voltage tap setting using Equation 5 will to some extent be
dependent on the type of application. The following paragraphs discuss different areas in which the BE1-
87B relay may be applied.
2-12 BE1-87B Application 9282300990 Rev P