Basler Electric BE1-87T Transformer Differential Relay

DESCRIPTION

BE1-87T Transformer Differential Relays provide primary protection for power transformers and are

available in either Single-Phase or Three-Phase configurations. The solid-state BE1-87T compares the

currents entering and leaving the protected transformer. If a fault is detected, the relay initiates a trip

signal to isolate the power transformer. This limits damage to the transformer and minimizes the impact

on the power system.

BE1-87T relays use three types of restraint:

• Percentage of through-current

• Second harmonic

• Fifth harmonic

Selectivity in differential relaying is based on the ability to distinguish between internal and external faults.

This is achieved by comparing the currents into and out of a power transformer. Comparing these

currents often requires more than two inputs. For example:

• Power transformers may have a significant portion (greater than 10 %) of the current flowing in a

third or tertiary winding.

• Power transformers can have multiple breakers for a given winding (e.g., ring bus or breaker

and-a-half bus).

BE1-87T relays are available with up to five restraint inputs for the single-phase unit and up to three

restraint inputs per phase for the three-phase unit.

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.

Three-phase BE1-87T relays use second-harmonic sharing. The second-harmonic content of all three

phases is summed together to derive the restraint for each phase. As a result, the second-harmonic

inhibit range and the associated factory setting, is higher than on single-phase relays.

Fifth-Harmonic Restraint

Power transformer overexcitation causes additional exciting current to flow into one set of terminals. This

presents an apparent differential (or operating) current not attributable to an internal fault. Although

potentially damaging, overexcitation is not an internal fault and, therefore, is not an appropriate condition

for transformer differential relay operation. One of the principal components in the complex waveform

produced during overexcitation is the fifth harmonic. BE1-87T relays use fifth-harmonic restraint to inhibit

the differential relay operation.

Unrestrained Trip

Severe internal transformer faults may cause CT saturation. Under such circumstances, harmonic

restraint transformer differential relays may fail to trip because of the extremely high harmonic content in

the waveform. Lack of operation can result in severe transformer damage.

BE1-87T relays provide an independent unrestrained tripping function. When set above the possible

inrush current magnitude, this function provides high-speed protection for the most severe internal faults.