Basler Electric BE1-87B - Manual (Page 31)

Bus Protection
Two methods will be outlined for evaluating Equation 5 in order to determine an appropriate relay voltage
tap setting.
Method 1. The first method offers a simplified conservative approach to the problem and requires that
equation 5 be evaluated only once. With this method, it is assumed that a single-phase-to-
ground fault with a current magnitude equal to the maximum interrupting rating of the breaker
occurs on the feeder associated with the CT having the longest cable run from the differential
junction point. Under these assumptions, the effect of the fault current, I
F
, is maximized, and
so is the effect of cable resistance, because the highest value of resistance is used and P is
set equal to 2. Thus, the highest possible value of V
DIFF
will be obtained.
Method 2. The second method offers an exact approach but requires that Equation 5 be evaluated a
number of times in order to obtain the maximum V
DIFF
. With this method, calculations must be
made for the maximum single-phase-to-ground fault and the maximum three-phase fault just
off each of the n feeders on the bus. Therefore, Equation 5 must be evaluated 2n times using
the associated value of cable resistance and P = 1 or P = 2, as required.
In general, Method 2 will produce a lower voltage tap setting than Method 1, but Method 1 is simpler to
utilize. The user should begin with Method 1. If the voltage setting resulting from the use of this method
results in adequate sensitivity, a unique advantage is realized in that the setting does not require
recalculation following future changes in the power system that result in higher fault current magnitudes. If
the sensitivity resulting from the use of Method 1 does not prove adequate, then Method 2 should be
used. Each method is outlined below.
Method 1 (Simplified Conservative Approach)
a. Use the maximum interrupting rating of the circuit breaker as the maximum single-phase-to-ground
symmetrical fault current (I
F
).
b. R
L
is based on the distance from the differential junction point to the most distant CT.
c. Calculate V
DIFF
substituting the values of current and resistance from a. and b. and set P = 2.
d. Select the highest available voltage setting that just accommodates the voltage calculated in c.
above.
Method 2 (Exact Approach)
a. Determine the maximum three-phase and single-phase-to-ground fault currents for faults just off each
of the n breakers on the bus.
b. R
L
is the one-way dc resistance of the cable from the associated CT to the differential junction point.
c. For each breaker in turn, calculate V
DIFF
separately, utilizing the associated maximum external three-
phase symmetrical fault current in the fault CT, with P = 1 and the maximum external single-phase-to-
ground symmetrical fault current in the fault CT, with p = 2.
d. Use the highest V
DIFF
resulting from the calculations and select the next highest available voltage
setting that just accommodates this value.
Shunt Reactor Protection
Depending on the type of protection required, shunt reactors may be protected by the BE1-87B relay in
one of two ways (see Figures 2-3 and 2-4). Since the shunt reactors contribute no current to an external
fault, Equation 5 should be evaluated using the highest magnitude of current that can flow in the reactor
under any system condition, exclusive of a fault in the reactor. If the differential junction point is located
near the reactors, the resistance of the CT connecting cables can probably be ignored, and Equation 5
need only be evaluated using the CT resistance and the maximum expected current. If the cable
resistance cannot be ignored, use the maximum expected reactor current and P = 2. After a value of
VDIFF has been calculated, select the next higher available voltage setting that just accommodates this
voltage.
Application with Mixed Multi-Ratio CTs
Where CTs are used on other than their full windings, (not recommended) the application should be
evaluated after a voltage setting has been selected to determine that excessive voltages are not
developed across the full windings of these CTs as a result of autotransformer action. It is desirable to
limit the peak value of the voltage to less than the insulation breakdown of the connected equipment.
9282300990 Rev P BE1-87B Application 2-13