SEL SEL-787 Transformer Protection Relay

SEL-787 Transformer Protection Relay

Major Features and Benefits

The SEL-787 Transformer Protection Relay provides unsurpassed protection, integration, and control features in a

flexible, compact, and cost-effective package.

➤ Standard Protection Features. Two-winding dual-slope differential protection with harmonic blocking and

restraint, and phase, negative-sequence, residual-ground, and neutral-ground overcurrent elements provide backup

protection. Breaker failure protection is included for two three-pole breakers.

➤ Optional Protection Features. Use the SEL-787 with optional voltage inputs to provide volts/hertz protection with

frequency tracking from 20 to 70 Hz for generator step-up and other variable frequency applications. Implement

load shedding and other control schemes with over- and underfrequency and over- and undervoltage elements. In

addition, use Sensitive Restricted Earth Fault (REF) protection for grounded-wye transformers.

➤ Transformer Monitoring. Use transformer through-fault monitoring to measure accumulated through-fault levels

of the transformer. Monitor ambient, load tap-changer (LTC), tank, and transformer oil temperature through the use

of optional 4–20 mA or RTD thermal inputs.

➤ Operator Controls. Four programmable front-panel pushbuttons each with two programmable LEDs allow for a

wide variety of uses including easy trip and close control and status indications for a breaker. Implement local and

remote operator control schemes through use of 32 local and 32 remote control bits.

➤ Relay and Logic Settings Software. ACSELERATOR QuickSet® SEL-5030 Software reduces engineering costs for

relay settings and logic programming. Tools in QuickSet make it easy to develop SELOGIC® control equations. Use

the built-in phasor display to verify proper CT polarity and phasing.

➤ Metering and Reporting. Built-in metering functions eliminate separately mounted metering devices. Analyze

Sequential Events Recorder (SER) reports and oscillographic event reports for rapid commissioning, testing, and

post-fault diagnostics. Unsolicited SER protocol allows station-wide collection of binary SER messages.

➤ Additional Standard Features. Includes Modbus RTU, Event Messenger support, MIRRORED BITS® communications,

load profile, support for 12 external RTDs (SEL-2600 series module), IRIG-B input, advanced SELOGIC, configurable labels,

IEEE C37.118-compliant synchrophasor protocol, and SEL-2812 compatible ST connectors fiber-optic serial port.

➤ Optional Features. Select from a wide offering of optional features, including IEC 61850. Modbus TCP/IP, Simple

Network Time Protocol (SNTP), DNP3 LAN/WAN, DNP3 Serial, 10 internal RTDs, expanded digital/analog I/O,

voltage inputs, additional EIA-232 or EIA-485 communications ports, and single or dual copper wire or fiber-optic

Ethernet ports.

Protection Features

The SEL-787 provides the same dual-slope differential

characteristic used in the popular SEL-387 and SEL-587

series of transformer differential relays. The SEL-787

includes a complete set of phase, negative-sequence, and

residual overcurrent elements for each winding, as well

as REF and neutral-overcurrent elements for grounded

wye transformers.

Use as many as 12 independent RTD driven thermal

elements with trip and alarm levels to monitor ambient

and equipment temperatures throughout the substation.

For the optional volts/hertz element, you can add three

phase voltage inputs that give the SEL-787 volts/hertz

protection with definite and time-delay characteristics,

along with directional power, over- and underfrequency and

voltage elements with two independent pickup levels.

Transformer Differential

The SEL-787 has three restrained differential elements

(87R). These elements use operate and restraint

quantities calculated from the winding input currents. Set

the differential elements with either single- or dual-slope

percentage differential characteristics. Figure2

illustrates a dual-slope setting. The percent-slope

characteristic helps prevent unwanted relay operation

because of possible unbalance between CTs during

external faults. CT unbalance can result from tap

changing in the power transformer and error difference

between the CTs on either side of a power transformer.

The relay allows you to choose harmonic blocking,

harmonic restraint, or both, providing a reliable

differential protection during transformer inrush

conditions. Even-numbered harmonics (second and

fourth) provide security during energization, while fifth

harmonic blocking provides security for overexcitation

conditions. Set second-, fourth-, and fifth-harmonic

thresholds independently.

An additional alarm function for the fifth-harmonic

current employs a separate threshold and an adjustable

timer to warn of overexcitation. This may be useful for

transformer applications in or near generating stations. A

set of unrestrained differential current elements simply

compares the differential operating current quantity to a

setting value, typically about 10 times the TAP setting.

This pickup setting is only exceeded for internal faults.

The three independent unrestrained differential elements

(87U) provide rapid assertion without delay when

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)