Siemens 7UT82, 7UT85, 7UT86, 7UT87 (SIPROTEC 5) Relays

Short answer

The Siemens SIPROTEC 5 7UT82, 7UT85, 7UT86 and 7UT87 are the transformer differential protection relays of Siemens' current platform. They differ in the number of windings and measuring points: the 7UT82 and 7UT85 for two windings, the 7UT86 for three, and the 7UT87 for transformers with many measuring points. They are configured with DIGSI 5 and integrated over IEC 61850.

What they are and where they are used

The Siemens 7UT82, 7UT85, 7UT86 and 7UT87 are the transformer differential protection relays of the SIPROTEC 5 platform. They protect power transformers in medium-, high- and extra-high-voltage substations and also in industry and generation, where they cover unit transformers, station service transformers and plant interconnection transformers. See also SEL-787 transformer relay.

Their main function is transformer differential protection (87T): they compare the currents on all sides of the transformer and trip when the difference indicates an internal fault. Around that function, each model adds overcurrent backup, restricted earth fault and, depending on the configuration, voltage and frequency functions. For the complete scheme, read the transformer protection scheme guide.

Variants: 7UT82, 7UT85, 7UT86 and 7UT87

Model	Application	Main difference
7UT82	Two-winding transformer	Entry device of the family; Siemens states that its backup functions can cover downstream cables, lines and reactors
7UT85	Two-winding transformer	Modular: expandable with input, output and communication modules
7UT86	Three-winding transformer	Modular, for three sides
7UT87	Transformers with many measuring points	Several differential and earth fault functions in one device; Siemens aims it at phase-shifting transformers, FACTS and converter transformers

Siemens also states that the 7UT85, 7UT86 and 7UT87 can carry the line differential function, useful when the transformer is connected to a cable or short line. The exact number of differential functions of the 7UT87 changes with the firmware version: confirm the one for your device in the documentation of that version.

Main functions

Function	ANSI	What it adds
Transformer differential	87T	Main protection against internal faults (guide)
Restricted earth fault	87N	Sensitivity to earth faults near the neutral (guide)
Phase and ground overcurrent	50/51, 50N/51N	Backup for the transformer and the network (guide)
Overexcitation (with voltage inputs)	24	Protects the core against high voltage or low frequency (guide)
Thermal image	49	Sustained overload (guide)
Breaker failure	50BF	Backup if a breaker does not open (guide)

The differential is stabilized against inrush current and overexcitation. When that stabilization is not well adapted to the actual transformer, the trips on energization described in transformer trip on energization appear.

Communications and integration

SIPROTEC 5 works with pluggable communication modules. According to Siemens’ protocol manuals, over Ethernet it supports IEC 61850, DNP3, IEC 60870-5-104 and Modbus TCP, and through serial modules IEC 60870-5-103 and serial DNP3. The family offers PRP and HSR redundancy. With IEC 61850, trips and interlocks between devices travel over GOOSE. On this point, read System Configurator workflow with DIGSI 5.

Configuration, settings, CFC logic and IEC 61850 files are handled with DIGSI 5 (DIGSI software). Oscillography is exported in COMTRADE.

What Siemens says about each model

Siemens describes the 7UT82 as designed specifically for two-winding transformers; in its SIPROTEC 5 documentation it is listed as a non-modular device whose number of inputs and outputs cannot be expanded, unlike the 7UT85, 7UT86 and 7UT87 series. A typical Siemens tender text for this family mentions phase-shifting transformer differential, overload and an earth fault differential that improves sensitivity near the neutral.

7UT85: for two windings; Siemens highlights its modular architecture and integration with DIGSI 5. Earlier manufacturer descriptions mentioned a differential function with additional stabilization and up to two restricted earth fault functions.
7UT86: for three windings, expandable to four, with backup for cables, lines and shunt reactors. Siemens states compatibility with IEC 61850 editions 1, 2.0 and 2.1, network access authenticated by IEEE 802.1X, role-based access control, security event logging and signed firmware.
7UT87: for transformers of up to five sides, with up to three stabilized differential functions and up to five differential earth fault functions. It supports up to 20 three-phase current measuring points and up to 20 three-phase voltage points, up to four communication modules and PRP and HSR redundancy. Siemens aims it at phase-shifting, FACTS, converter, arc furnace and HVDC transformers.

The electronic boards of the 7UT86 and 7UT87 carry a protective conformal coating for harsh environments. Because the number of functions changes with the firmware version, the reference is always the catalog and the documentation of the installed version.

Standard and differential principle

The reference standard for the performance of this type of protection is IEC 60255-187-1, edition 1 of 2021, “Restrained and unrestrained differential protection of motors, generators and transformers”. It sets minimum requirements for evaluating longitudinal differential protection of motors, generators and transformers and for documenting test results, and together with parts 187-2 and 187-3 it replaces IEC 60255-13. Its scope expressly excludes, among others, phase-shifting transformer differential, directional restricted earth fault and converter transformers and reactors, although its principles can serve as guidance in those cases. That is why manufacturers treat these applications with their own functions and specific settings, and why the study of the application weighs more than any typical value. To go further: protection of loaded transformer tertiary windings.

Points of attention in commissioning and maintenance

Ratio and group adaptation: the relay compensates the ratio and phase shift of the sides; the power transformer data (rating, voltage, vector group, neutral grounding) must be those of the nameplate and the test report.
CTs on each side: saturation and class differences between the current transformers on the sides are a frequent cause of misoperation; see excitation tests on current transformers.
Energization: stabilization against inrush current is reviewed with the records of a real energization; related topic: transformer inrush current.
Versions: the 7UT87 and other models of the platform change functions with firmware versions, so DIGSI 5, the firmware and the manual must match.

Frequent technical questions

7UT85 or 7UT86 manual: Siemens publishes a common manual for the SIPROTEC 5 transformer differential family; the manual version must match the firmware version of the device.
Drivers and connection: the 7UT85 connects to DIGSI 5 through the front USB port or over the network. If DIGSI does not recognize the device, the usual cause is a software or driver version that does not match.
The 87T trips on energization or with an external fault: it is usually due to the vector group, the current transformer ratios or harmonic stabilization. We cover it in 87T trip without an internal fault.
7UT85 or 7UT86: the choice depends on the number of windings and measuring points, not on voltage.
Migrating from a 7UT612, 7UT613 or 7UT63: these are SIPROTEC 4 devices worked with DIGSI 4; their files do not carry over as is to DIGSI 5, so the settings and logic are translated and tested again. This is complemented by Siemens 7UT612, 7UT613 and 7UT63 relays.

How we can help

At Tiza Group we configure and review SIPROTEC 5 relays remotely: we start from the approved settings and the protection study, leave a backup before and after the change and document what was done. The scope is described under remote configuration of Siemens SIPROTEC relays. If the 87T has already tripped and the reason is not clear, send us the record and the configuration for a trip analysis. To replace an old relay with a 7UT8, look at engineering design and studies, or contact us.

References

IEEE C37.2, device function numbers for power systems.
IEEE C37.91, guide for protecting power transformers.
IEC 61850 (series), communication networks and systems for power utility automation.
Siemens, product pages and profiles of the SIPROTEC 7UT82, 7UT85, 7UT86 and 7UT87.

Frequently asked questions

What is the difference between the 7UT85 and the 7UT86?

Both are modular SIPROTEC 5 transformer differential relays. Siemens presents the 7UT85 for two-winding transformers and the 7UT86 for three-winding transformers (three sides). Backup functions and hardware are chosen through configuration and the order code.

Where do I find the manual or drivers for the 7UT85?

Siemens publishes the manual of the SIPROTEC 5 transformer differential family and the description of each device on its support portal. The device is connected and configured with DIGSI 5; the DIGSI and firmware versions must match those of the installed relay.

Does the 7UT8 replace the 7UT612, 7UT613 and 7UT63?

Siemens' current transformer differential line is SIPROTEC 5 (7UT82, 7UT85, 7UT86 and 7UT87), while the 7UT6 are SIPROTEC 4 and several are being retired. The change is not direct: settings and logic have to be translated, inputs and outputs reviewed and the SCADA integration redone.