SEL-787 Transformer Relay: 787-2, -3, -4 and 787Z

Short answer

The SEL-787 is the transformer protective relay from Schweitzer Engineering Laboratories. The current SEL-787-2, -3 and -4 line protects two-, three- and four-winding transformers with dual-slope differential, negative-sequence differential, restricted earth fault, backup overcurrent and breaker failure. The SEL-787Z is a high-impedance differential relay, intended mainly for buses and restricted earth fault.

What it is and where it is used

The SEL-787 is the transformer protective relay from Schweitzer Engineering Laboratories (SEL), part of the 700 family. It is used on distribution and subtransmission substation transformers, industrial transformers, generator step-up transformers and variable-frequency applications. For the principle behind the protection, see transformer differential protection 87T.

The current line is the SEL-787-2, -3 and -4, which SEL recommends in place of the original SEL-787. The SEL-787Z is a different device: a high-impedance differential.

SEL-787, 787-2, 787-3, 787-4 and 787Z variants

Model	Application	Main difference
SEL-787-2X, -21, -2E	Two-winding transformers	-2E adds voltage inputs (V/Hz, voltage, frequency)
SEL-787-3E, -3S	Three-winding transformers	-3S adds a synchronism or battery channel
SEL-787-4X	Four-winding transformers	Four sets of current inputs
SEL-787 (original)	Two-winding transformers	Predecessor; SEL recommends the -2/-3/-4 line
SEL-787Z	Buses and restricted earth fault	High-impedance differential, with optional SEL-HZM module
SEL-787L	Lines	Line differential; not a transformer relay

How the model is built: current and voltage cards

In the data sheet for the -2/-3/-4 line, SEL defines each model by the analog input cards in its slots, and that combination is what limits the application:

Model	Differential current terminals	Additional inputs
SEL-787-2X	2	None
SEL-787-21	2	1 neutral current input
SEL-787-2E	2	Neutral and 3 phase voltage inputs
SEL-787-3E	3	Neutral and 3 phase voltage inputs
SEL-787-3S	3	3 phase voltage inputs and 1 synchronism or battery input (VS/VBAT)
SEL-787-4X	4	No voltage inputs

Two practical consequences. First, the number of windings the relay can protect is fixed by the current cards ordered: it cannot be extended later through settings. Second, overexcitation protection and the voltage elements exist only in models with voltage inputs; an installed -2X, -21 or -4X does not gain them without changing the card or the device. SEL also offers, as an option, low-level analog inputs for voltage sensors (up to 8 V rms) and for Rogowski coils or low-power current transformers (LPCT), with gain adjusted automatically according to rated current.

How the SEL-787 differential operates

It helps to understand what the relay calculates before interpreting a trip:

Three restrained elements (87R), with operate and restraint quantities calculated from up to four winding currents, adjustable with a single slope or dual slope. The slope prevents misoperation caused by CT mismatch from the tap changer and by the error difference between the CTs on each side.
Harmonic blocking and restraint, selectable or combined. Even harmonics (second and fourth) give security during energization; the fifth gives security against overexcitation. The three thresholds are set separately, and there is a timed fifth-harmonic alarm, useful for transformers near generating plants. See transformer trip on energization and overexcitation protection 24.
Three unrestrained elements (87U), which operate with no delay when the operate current exceeds their threshold. SEL gives 8 to 10 times the rated operate current as typical values.
Negative-sequence element (87Q), with a single slope because it is blocked when an external fault is detected; its purpose is low-magnitude faults, such as turn-to-turn faults.
Restricted earth fault with a directional element that decides whether the fault is internal or external, supervised by zero-sequence current thresholds.

Records, monitoring and local operation

Event reports: up to 5 reports of 180 cycles, 18 of 64 cycles or 50 of 15 cycles in nonvolatile memory, with 1/4-cycle or 1/32-cycle resolution and COMTRADE format (see COMTRADE files). The Sequential Events Recorder (SER) keeps the last 1,024 entries.
Through faults: the relay records the magnitude, duration and date of every external fault passing through the transformer and accumulates an I²t per phase, with an alarm on excess. SEL proposes this for scheduling maintenance, because those currents can displace the windings and accelerate thermal wear.
Temperature: up to 12 thermal elements with trip and alarm, by RTD or 4 to 20 mA inputs (ambient, tap changer tank and oil), plus 12 external RTDs with the SEL-2600 module. See transformer temperature indicators.

Main functions

According to the SEL data sheets:

Function	ANSI	What it provides for the transformer
Dual-slope differential with harmonic blocking and restraint, up to four windings	87T	Main protection (guide)
Negative-sequence differential	87Q	Low-magnitude faults, such as turn-to-turn
Restricted earth fault (up to three elements)	87N	Ground faults on grounded-wye windings (guide)
Phase, negative-sequence and ground overcurrent	50/51	Backup (guide)
Breaker failure, up to four breakers	50BF	Backup if a breaker fails to open (guide)
Overexcitation (models -2E, -3E, -3S)	24	Frequency tracking from 15 to 70 Hz
Under- and overvoltage, frequency	27, 59, 81	Load shedding and control
High-impedance differential (SEL-787Z)	87Z	Buses and restricted earth fault (87B)

The SEL-787 also includes through-fault monitoring of the transformer and optional 4–20 mA or RTD inputs for oil, tap changer and ambient temperature.

Communications and integration

SEL lists as options for the SEL-787-2/-3/-4 and the SEL-787Z: IEC 61850 Edition 2, Modbus TCP, SNTP, IEEE 1588 PTP, EtherNet/IP, serial and network DNP3, IEC 60870-5-103, RSTP and PRP. Modbus RTU, Mirrored Bits and IRIG-B input come as standard. Settings are made with AcSELerator QuickSet and logic with SELogic. See IEC 61850 GOOSE.

SEL-787Z: what the high-impedance circuit includes

The high-impedance principle, with a large series resistor that provides stability against CT saturation on a through fault, is covered in high-impedance differential protection. What is specific to the SEL-787Z, according to its data sheet:

Optional SEL-HZM: a module that brings together the stabilizing resistors, the varistors (MOV) and an optional bypass relay. It can be ordered prewired with the SEL-787Z or separately, to integrate into existing installations. It comes in panel, rack or surface mounting.
2000 Ω resistors, enough to provide security against CT saturation on through faults, and MOVs in parallel that limit the voltage below 850 V during a bus fault.
Circuit supervision: MOV energy elements that alarm when the absorbed energy exceeds a threshold and, optionally, supervision of the bypass control inputs (with the 8 AI card). SEL asks for these components to be supervised continuously because they are critical during the fault.
Voltage and current elements: per phase, two elements on fundamental voltages, two on instantaneous voltages, two on fundamental currents and two on instantaneous currents, with two levels: level 1 for bus protection and level 2 for alarm.
CT open circuit: a low-setting voltage differential element detects open CTs.

On transformers with both sides in grounded wye, SEL indicates using one element per side. This function covers only phase-to-ground faults: for phase-to-phase faults and external faults at the terminals, it must be combined with instantaneous and time overcurrent.

If your device is no longer manufactured

If you have an original SEL-787, SEL points to the SEL-787-2, -3 and -4 line as its replacement. The change requires choosing the right model by number of windings and inputs, translating settings, reviewing the logic and testing the differential with secondary injection before energizing. We do it with remote configuration and testing.

Frequently asked technical questions

87T trip on energization or with no internal fault: harmonic restraint, vector group compensation or CT polarity. See 87T trip with no internal fault.
Vector group compensation: the relay corrects the phase shift according to the settings; a configuration error produces differential current under load.
SEL-787Z misapplied: a high-impedance differential needs CTs with suitable characteristics and calculated stabilizing resistors.
Through-fault monitoring: the SEL-787 accumulates the energy of external faults passing through the transformer, useful data for maintenance.
Low-level inputs: current models accept low-level voltage sensors and Rogowski coils or LPCTs as an option.
What the 87T covers and what it does not: the differential protects the zone between the CTs of all windings. An external fault with high current produces imbalance through saturation; that is why the slope, the restraint and, in the 87U, a high threshold are used. Low-magnitude phase-to-ground faults on grounded wye are the domain of restricted earth fault (guide).

How we help

At Tiza Group we configure SEL relays remotely, with a backup before and after the change. See remote configuration of SEL relays and remote relay configuration. If the transformer has already tripped and the cause is unclear, we quote the event analysis in trip review, or write to us via contact.

References

IEEE C37.2, standard for electrical power system device function numbers.
IEEE C37.91, guide for protecting power transformers (2021 edition, revision of the 2008 one).
Schweitzer Engineering Laboratories, data sheets for the SEL-787-2/-3/-4, SEL-787 and SEL-787Z.

Frequently asked questions

What is the difference between the SEL-787 and the SEL-787-3?

SEL now recommends the SEL-787-2, -3 and -4 line in place of the original SEL-787. The number tells how many windings the relay protects: two, three or four. The -3E, -3S and -2E models add voltage inputs for overexcitation (V/Hz), voltage and frequency; the -3S also allows synchronism check or battery monitoring.

Can the SEL-787Z protect a transformer?

The SEL-787Z is a high-impedance differential: SEL designs it mainly for buses and also for restricted earth fault on transformers with a grounded-wye winding. For full transformer protection with a percentage differential, SEL points to the SEL-787-2, -3, -4 or the SEL-387, with the SEL-787Z added if needed.

Is the SEL-787L a transformer relay?

No. The SEL-787L is an SEL line differential relay, not a transformer relay. It shares its number with the SEL-787, but its application is different.