SIPROTEC 7SJ85 Relay: Overcurrent and Feeder Protection

Short answer

The 7SJ85 is the most complete overcurrent and feeder protection relay of the Siemens SIPROTEC 5 family. It protects feeders, lines and capacitor banks with directional and non-directional overcurrent, and its modular hardware lets one unit cover several bays.

What it is and where it is used

The SIPROTEC 7SJ85 is the overcurrent and feeder protection relay of Siemens’ SIPROTEC 5 family. Siemens presents it for the protection of feeders, lines and capacitor banks, and highlights that it can protect up to nine feeders with up to 40 analog inputs thanks to its modular hardware. See also Siemens 7SK82, 7SK85 and 7SK80 relays.

In practice it is found in distribution and subtransmission substations, in industrial plants with several medium voltage outgoing circuits, and as backup protection in high voltage bays. Its modularity allows, for example, protecting several outgoing circuits from one unit or combining protection and bay control. For the conceptual basis, read protective relays and radial feeder protection scheme.

Main functions

Function	ANSI	Comment
Non-directional phase and ground overcurrent	50/51, 50N/51N	Core of the unit (guide)
Directional overcurrent	67/67N	Rings, parallel feeders and networks with generation (directional elements)
Breaker failure	50BF	Local backup (guide)
Synchronism check	25	Safe closing between two sources (guide)
Automatic reclosing	79	Overhead networks (guide)
Voltage and frequency	27, 59, 81	Abnormal system conditions

The Siemens data sheet also mentions bay control functions, phasor measurement (PMU) and basic power quality functions. The exact set depends on the variant and the function packages ordered: two 7SJ85 units can have different capabilities even if they look the same.

Communications and IEC 61850

SIPROTEC 5 works with pluggable communication modules. According to Siemens’ protocol manuals, over Ethernet it supports IEC 61850, DNP3, IEC 60870-5-104, Modbus TCP and PROFINET IO, and through the serial modules IEC 60870-5-103 and serial DNP3. Siemens also offers PRP and HSR redundancy in the family. On this point, read System Configurator workflow with DIGSI 5.

One detail worth knowing: the manuals of recent versions warn that some Ethernet SCADA protocols cannot be activated at the same time on the same module. For that reason the communications architecture is defined before ordering the unit, not afterward. To review the protocols, read IEC 61850 MMS, GOOSE and PRP and HSR.

Events and oscillography

SIPROTEC 5 units store fault records with oscillography and a log of events and annunciations. They are downloaded with DIGSI 5 and analyzed with Siemens’ viewing tool or, independently, in COMTRADE format. For a serious analysis the oscillography alone is not enough: it has to be cross-checked against the event list and the state of the binary inputs; read how to read an oscillography record.

Modular hardware: what to choose when specifying it

According to the product page, the 7SJ85 is built on the SIPROTEC 5 modular system with flexible, expandable inputs and outputs:

1/6 expansion modules to add inputs and outputs without changing the case.
Large display, small display or no display, depending on the mounting: in medium voltage switchgear the front with keypad is usually chosen; in racks with remote control, the unit without a display is enough.
Conformal coating of the boards, optional, for environments with humidity, conductive dust or gases.
Up to nine feeders and 40 analog inputs in a single unit, which forces a decision on how many outgoing circuits to group in one relay and what happens to them if that relay is taken out for maintenance.

SIPROTEC 5’s online configuration tool lets you build the complete order reference before ordering. Keep that reference with the project documentation, because it is the only way to know later which modules and function packages each unit has.

Typical applications

Overhead feeders with reclosing. Inverse-time overcurrent coordinated with downstream fuses or reclosers; the case to prevent in the study is covered in fuse and recloser operate at the same time.
Capacitor banks. Siemens presents it for their protection; the particulars of inrush currents and harmonics are addressed in the bank design.
Rings and parallel feeders. This is where directional overcurrent comes in, whose behavior depends on polarization and on the grounding system.
Plants with generation. Current can reverse direction and faults fed by the generation change the short-circuit levels; read generator in parallel with the grid.

Points of attention in commissioning and maintenance

DIGSI 5 project as a controlled document. The project file used to configure the unit is part of the documentation and must be kept under version control; configuration management is covered in IED and SCD configuration management.
Firmware and DIGSI 5 versions. They are recorded in the commissioning report; a firmware update can change the behavior of functions and the availability of communication modules.
Bay assignments. With several bays in one unit, verifying each trip against its breaker, and each measurement against its transformer, is part of the functional test.
Protocol combinations. Check in the protocol manual for your version which combinations per module are valid before defining the links to SCADA.

Selectivity and starting data

A well-protected feeder starts with a short-circuit and coordination study, not with the relay. The minimum data are the maximum and minimum fault levels at the bus, the curves of the upstream and downstream devices (fuses, reclosers, other relays), the conductor capacity, and the inrush currents of transformers and starting currents of motors. With those data, the 7SJ85 is set to clear the closest fault and leave backup to the others; read what a protection coordination study needs and lack of selectivity: several feeders trip. When a plant changes its configuration, the study is reviewed before the relay settings are modified.

Frequently asked technical questions

Functions that do not appear: in SIPROTEC 5 the functions are instantiated inside function groups; if the function or the package is not in the unit, it cannot be set even if the hardware allows it.
Several bays in one unit: each function group must be tied to the correct current transformer and breaker. An assignment error makes the relay trip the wrong breaker.
The ground directional element does not operate: polarization, polarity of the instrument transformers, or a grounding system that requires a different method.
Differences between DIGSI 4 and DIGSI 5: whoever knows SIPROTEC 4 will find a different configuration logic in SIPROTEC 5.
Trips on energization: inrush current makes instantaneous overcurrent operate if there is no adequate restraint; read overcurrent trip on starting or energization.

How we help

At Tiza Group we configure and review SIPROTEC 4 and 5 relays remotely, worldwide: functions, logic, inputs and outputs, and IEC 61850 communication, working from the study and the approved settings. The scope is described under remote configuration of Siemens SIPROTEC relays. If the 7SJ85 tripped and the reason is unclear, send us the record and we quote the trip analysis, or write to us through contact.

References

IEEE C37.2, device function numbers.
IEC 60255-151, functional requirements for overcurrent protection.
IEC 61850 (series); IEC 62439-3, PRP and HSR.
IEEE C37.111 / IEC 60255-24, COMTRADE.
Siemens, SIPROTEC 7SJ85 product page and data sheet; SIPROTEC 5 communication protocols manual.

Frequently asked questions

What is the difference between the 7SJ85 and other SIPROTEC 5 overcurrent relays?

The 7SJ85 is the modular model of the range: it accepts I/O expansion modules and several communication modules, and Siemens states that it can protect up to nine feeders with up to 40 analog inputs. The smaller models of the family cover applications with fewer signals.

Which software configures the SIPROTEC 7SJ85?

DIGSI 5, Siemens' engineering tool for SIPROTEC 5. The functions, logic, inputs and outputs, and IEC 61850 communication of the unit are defined there.

Is the 7SJ85 suitable for a capacitor bank?

Yes. Siemens presents it for feeders, lines and capacitor banks. The specific functions activated for the bank depend on its connection and on the study.