Basler Electric BE1-87B - Manual (Page 20)

BE1-87B Flexibility
Because of the flexible wide range design of the BE1-87B, it is possible to apply the relay in situations
where the current sensing input circuit is less than ideal. It should be noted, however, that the possibility
of less sensitive settings, equipment overvoltage, or false operation could result. Careful review of the
following application notes is recommended:
•
It is possible to use a mixture of multi-ratio CTs, however, it is essential that the tapped value has the
same turns ratio as the other parallel CTs in the circuit. When taps are selected other than full ratio,
use the highest available tap setting that will allow all CTs in the scheme to have the same turns ratio.
Tap settings other than full ratio require a calculation of the peak voltage developed across the full
winding resulting from autotransformer action. The resulting voltage should not exceed the insulation
breakdown values of the connected equipment. The equation for this calculation is derived in the
paragraph on operating principles in this section and repeated in the paragraphs on Calculation of
Settings.
•
All CTs used in the differential circuit should have negligible leakage reactance on the connected
taps. Most, if not all, multi-ratio internal, bushing, and column type CTs made in the last 30 years
meet this requirement. All CTs wound on toroidally shaped cores meet this requirement if the
windings (on the tap used) are completely distributed around the core (consult your CT manufacturer
if you have questions). It may be possible to use CTs that do not meet this requirement if the leakage
reactance is known. The leakage reactance is added algebraically to the resistance of the CT circuit
in question. Less sensitive protection will occur as a result of a higher pickup setting.
•
It may be possible, although not recommended, to use the differential circuit CTs jointly for other
functions as long as an accurate impedance of the other function is known. The performance of the
system under these conditions can be calculated by algebraically adding the other impedance to the
CT winding and cable resistance. Less sensitive protection will occur as a result of a higher pickup
setting. Also, consideration must be given to the hazards of false operation due to extra connections
and errors in testing the added devices. To ensure proper relay setting, all cable and CT secondary
winding resistances should be evaluated before a decision is made to add other devices to the BE1-
87B CT circuits.
Bus Protection Application
Three single-phase BE1-87B relays or one three-phase BE1-87B relay and an auxiliary lockout relay (86),
provide a complete multi-phase and ground bus fault protection package. Typical external connections to
the relays are shown in Figures 2-1 and 2-2. The connections are illustrated for a bus with three circuits,
but the protection can easily be extended if more circuits are added to the bus. For additional circuits, it is
only necessary to connect the CTs associated with the added circuits to the respective junction points and
to connect the contacts of the lockout relay in the respective trip circuits. The relay voltage tap setting is
based on the maximum voltage that can be developed across the differential junction point during an
external fault. Calculation of the maximum voltage is easily made and methods for doing so are given in
the paragraph under calculation of settings. A sample calculation for a bus differential scheme is also
provided.
2-2 BE1-87B Application 9282300990 Rev P