Basler Electric BE1-87B - Manual (Page 32)

Refer to Equation 4 under Operating Principles for information on how to calculate the peak voltage
across the full winding.
See Figure 2-8 for an illustration of terms from Equation 4. If V
F
is less than the insulation breakdown, and
if the current rating of the CT is not exceeded, the application is permissible. Equation (4) should be
evaluated for the CT having the highest N1/N2 ratio. If the condition of Equation (4) is met for this CT,
then it will also be met for the remaining CTs.
Current Element Setting
The setting of the current element is based upon four factors.
1. The current setting needs to be set so that the relay will operate at minimum fault levels. The main
application where this will be of concern will be when one wishes to ensure operation of the relay for
ground faults on impedance grounded systems. This matter is discussed in the Minimum Fault to Trip
sub-section.
2. The current setting should be set high when there are surge arresters in the zone of protection. This
factor was discussed in the Application with Lightning Arresters sub-section.
3. The third factor is hard to quantify. It is possible for noise to be induced on the bus differential CT
circuit by the magnetic fields generated by out-of-zone fault currents. This includes magnetic fields
generated by both the primary fault currents and by secondary fault currents where CT leads are in
the same conduit at the differential relay CT leads. Due to the high impedance of the bus differential
circuit, the induced voltage can be high enough to cause the relay voltage element to transiently pick
up. However, this induced voltage cannot carry any appreciable current after the BE1-87B SCRs are
turned on. If the BE1-87B voltage element operates but current in the CT string remains low (less
than the BE1-87B current element setting) after the SCRs turn on, the relay will not trip. Hence, the
current element is set at some level that will prevent induced pickup of the relay for this condition. A
typical setting for this purpose is 0.5 amperes.
4. The current should be set high enough so that if the BE1-87B CT Test feature is used (to test for CT
short-circuits), the current that is induced in the relay by the test will be less than the current setting of
the relay by a comfortable margin. This matter is discussed in the CT Test Circuit Calculations sub-
section.
Minimum Fault to Trip (Voltage Element)
NOTE
In the following sensitivity analysis, relay impedance is rounded to 5000 ohms
for simplicity and the algebraic addition of current magnitudes rather than a
more exact phasor addition of currents is used. A comparison of the simplified
calculation approach to the more exact calculation approach results in a
minimum fault sensitivity value that is higher than when the more exact
approach is used. Hence, the simplified approach is a more conservative
method for finding minimum sensitivity.
After the differential voltage setting has been established, a check should be made to determine the
minimum internal fault current that will just cause the voltage element of the relay to operate. This current
level should be compared to the current element setting. The greater of these two quantities determines
the relay minimum fault-to-trip. The minimum fault-to-trip should be less than the bus minimum fault duty.
This will be an issue mainly with impedance-grounded systems. The following expression can be used to
determine the minimum internal fault current required for a particular tap setting.
N I (I)x I
R
n
1 x
MIN 







+ =
∑
=
(EQUATION 6)
I
min
= minimum rms symmetrical internal fault current required to operate the BE1-87B relay
n = number of CTs (number of circuits)
I = secondary excitation current of individual CT at a voltage equal to (VDIFF)
I
R
= current in the relay at pickup setting
N = CT ratio on tap used
2-14 BE1-87B Application 9282300990 Rev P