SEL SEL-787 Transformer Protection Relay

differential operate current levels exceed the 87U pickup

threshold that is set depending on the application.

Typical 87U pickup level settings are between 8 and 10

per unit of operate current.

Restricted Earth Fault (REF) Protection

Apply the REF protection feature to provide sensitive

detection of internal ground faults on grounded wye

connected transformer windings and autotransformers.

Order the SEL-787 with the Slot E card containing the

1A or 5A current input for REF protection. The single

phase 1A or 5A CT provided as a Slot E ordering option

is used for introduction of neutral CT operating current.

Polarizing current is derived from the residual current

calculated for the protected winding (Winding1.

Winding2. or both windings). A sensitive directional

element determines whether the fault is internal or

external. Zero-sequence current thresholds and selectable

CT saturation logic supervise tripping.

Overcurrent Protection

The SEL-787 provides complete overcurrent protection

for a two-winding transformer. Phase overcurrent

protection is provided for both three-phase winding

inputs. The following overcurrent elements are provided.

Instantaneous Overcurrent Elements

The following instantaneous overcurrent elements are

provided in the SEL-787 base configuration. All

instantaneous overcurrent elements provide torque control.

➤Four instantaneous phase overcurrent (50P) elements

for Windings 1 and 2 (eight total). These phase ele

ments operate on the maximum of the phase cur

rents. Level1 phase overcurrent elements also

include per-phase elements. Peak detection algo

rithms are used to enhance element sensitivity during

high fault current conditions, where severe CT satura

tion may occur.

➤Two instantaneous negative-sequence overcurrent

(50Q) elements for Windings 1 and 2 (four total).

These phase elements operate on negative-sequence

content for each three-phase winding.

➤Two residual overcurrent (50G) elements for wind

ings 1 and 2 (four total). These elements use calcu

lated residual (3I0) current levels from each winding

for ground fault detection.

When ordered with the optional current channel in

SlotE, the SEL-787 also provides two instantaneous

neutral-overcurrent elements (50N) with two levels of

neutral-overcurrent detection.

Time-Overcurrent Elements

The SEL-787 base configuration provides the following

time-overcurrent elements. The time-overcurrent

elements support the IEC and US (IEEE) time

overcurrent characteristics shown in Table1.

Electromechanical disc reset capabilities are provided for

all time-overcurrent elements.

➤Eight phase time-overcurrent (51P) elements are pro

vided. These phase elements operate on the individ

ual phases as well as the maximum of the phase

currents for each winding.

➤Two negative-sequence time-overcurrent (51Q) ele

ments are provided. These elements operate on the

calculated negative-sequence current for each set of

three-phase winding inputs of the transformer.

➤Two residual overcurrent (51G) elements are pro

vided. These elements use calculated residual (3I0) cur

rent levels from each winding for ground fault detection.

When ordered with the optional current channel in

SlotE, the SEL-787 also provides one neutral time

overcurrent element (51N)