Basler Electric BE1-87T - Manual (Page 14)

1-2 BE1-87T General Information 9171300990 Rev V
Application
In general, power transformers have different values of current flowing through their primary, secondary,
and tertiary windings. These currents have specific phase relationships depending upon the connections
of the individual windings (e.g., wye/delta). As inputs to a differential relay, these currents must be
compensated or scaled so that the relay can compare the inputs and determine when an unbalance
exists. Under ideal operating conditions, the scaled vector sum of these currents is zero.
Because it is practically impossible to match the magnitudes of these detected currents from the various
power windings using standard CT ratios, the currents are matched within the relay by scaling each of the
applied currents by an appropriate factor called a Tap Setting. This is set by means of the front panel
INPUT dials. By selecting suitable tap ratios, the applied currents are scaled within the relay to achieve
the desired balance for normal operating conditions. BE1-87T relays offer a range of available tap settings
for inputs between 0.4 A to 1.78 A (Sensing Input Range Options 2 and 4 for a 1 A CT) or 2.0 to 8.9 A
(Sensing Input Range Options 1 and 3 for a 5 A CT). These settings are independently adjustable in
increments of 0.02 A for Sensing Input Range Options 2 and 4, or 0.1 A for Sensing Input Range Options
1 and 3. These small increments allow more precise scaling of the applied currents and usually eliminate
the need for installing auxiliary ratio-matching CTs.
Single-Phase
BE1-87T Single-Phase relays require phase angle compensation to be accomplished externally by proper
connection of the system CT secondaries. A wye/delta transformer requires that the CT secondaries be
connected in delta for the wye winding and in wye for the delta winding. This type of connection also
eliminates the zero-sequence component of current which could cause a false trip (operation) during
external ground fault conditions on the wye system.
Three-Phase
BE1-87T Three-Phase relays can provide zero-sequence filtering and compensation for phase shifts
introduced by the connections of the power transformer. This 30° Phase Shift compensation (either ±30°
or no compensation) is field selectable. Additionally, this feature allows sharing the transformer differential
relay CTs with other relays or instrumentation.
BE1-87T relays use the highest input current (in per unit values) to operate on maximum restraint. The
relay does not have a conventional operate winding in the internal magnetics. Operating current is
developed within the electronics of the relay.
Percentage Restraint
A primary concern in differential relay applications is security against high current levels caused by faults
outside the protected zone. Inevitable differences in the saturation characteristics between current
transformers require a compensating decrease in relay sensitivity. It is also necessary to be able to adjust
the sensitivity to compensate for transformer voltage taps or CT mismatches. This is accomplished by
providing a restraint factor proportional to the current flowing through the protected zone (through-
current).
BE1-87T relays maintain sensitivity at a specified ratio of trip current to through current. This ratio,
generally referred to as slope, is front-panel adjustable in 5% increments from 15 to 60%.
Second-Harmonic Restraint
Magnetizing inrush current presents another problem unique to transformer differential relays. Relays
must be capable of detecting the small differences in current caused by the shorting of a limited number
of turns, yet remain secure against the occurrence of magnetizing currents many times the transformer
rating (as seen at one set of terminals).
Although magnetizing inrush is usually associated with the energizing of the transformer, any abrupt
change in the energizing voltage may produce this phenomenon. Common causes are the transients
generated during the onset, evolution, and removal of external faults. Desensitizing the relay only during
energization is therefore insufficient.
Magnetizing inrush produces an offset sine wave rich in all harmonics. BE1-87T relays use the second
harmonic to restrain operation because it predominates and because it does not occur in significant
magnitude or duration at other times.