Protection History

Author: Walter Schossig, Germany

Protection Relays ? Continued

This article is continuation from the March 2014 issue and covers the development of the first digital relays by several additional manufacturers based on information provided by the them. AEG, Basler, BBC/ABB, EAW, ELIN, GEC/ALSTOM have been covered in the last issue. Further developments of the different vendors will be covered in the next issues. Any comments and additional remarks are most welcome. The vendors appear in alphabetic order.

Reyrolle: Reyrolle presented their first CMOS numeric protection and control relay using lithium cells in 1981. The first numeric transformer differential relay with CTs modelled in software - the DUOBIAS-M was also released in this year. (see Figures 3a and b).

SEL: Schweitzer Engineering Laboratories, Inc. (SEL), introduced the world’s first commercially available, all-digital protective relay in 1984. The SEL-21 offered all the traditional distance protection of electromechanical relays and also provided several ground-breaking innovations that have become standard features of many protective relays. These included fault location, event reporting, self-testing, and multiple levels of password protection (Figure 2).
At the time, Dr. Schweitzer liked to say, “This relay is 1/8 the size, 1/10 the weight, and 1/3 the price of existing protective relays.” Because the technology in the SEL-21 was so new, the relay was initially used only as a fault locator. As utilities gained confidence in the SEL-21, they began using its protection features as well. With the SEL-21’s fault location technology, utilities saw a substantial increase in system availability due to faster power restoration times (Figures 1/2).

Electromechanical relays provide only rudimentary indications of involved phase and zone targets. At the time, more detailed oscillography was very expensive. With SEL’s introduction of event reports, utility operators had a computerized method of analyzing power system events. The self-testing capability introduced in the SEL-21 allowed the relay to diagnose itself and alarm operators if the power supply was going out of tolerance or if there was something wrong, such as with onboard memory or the analog channels drifting or developing an offset.
SEL designed security into its first relay. The SEL-21 offered two levels of password protection: one let the user view the relay’s information, such as the settings, metering, and events, and the second allowed a user to make changes, for example to enter new settings or operate the breaker.
In 1985, SEL released the SEL-PRTU Protective Relay Terminal Unit (Figure 4). The SEL-PRTU was the first device to provide a way to communicate to multiple relays and set the foundation for the development of a range of automation products including the Real-Time Automation Controller (RTAC). More than just a gateway to get information, today’s RTAC from SEL provides real-time control for advanced power system applications.

SEL was the first to include IRIG-B time code synchronization in protective relays in 1988. With precise time, engineers can align event reports from various relay locations to determine if events are related. The availability of precise time in a relay allowed SEL to develop synchrophasor technology in relays in 1999 as a standard feature, allowing utilities to see the power system state in real time, not just as an off-line calculation or estimate.
In 1989, SEL introduced multiple settings groups in the SEL-121B relay (Figure 4). The initial application was for a bus-tie breaker position.
The accompanying SEL-121B (with six separate settings groups) was preprogrammed with the unique settings for the line relays/breakers, and it would be switched into service during line relay/breaker maintenance. Other protection innovations have been the introduction of negative-sequence overcurrent protection (for faster, more sensitive phase-to-phase fault clearing) in the SEL-151 distribution relay in 1991 and load encroachment (to prevent tripping on load) in the SEL-321 distance relay in 1992.

Siemens: Microprocessor relays developments started early. At the end of the 1970s, the INTEL 8086 was the first 16-bit universal processor to come on the market. This brought within reach the possibility to develop relays with more complex protective functions such as distance protection. Siemens tested the first prototypes from 1979 to 1981. The development of digital frequency relay 7RP22 (page 70) and 7RP72 (Figure 9) started at SIEMENS in 1980.
The decisive break-through came with the upgraded 16-bit microprocessor version 80186 at the beginning of the 1980s. At the same time, the project "LSA" for novel substation automation with integrated protection and control was started in Siemens. In this completely digital secondary system technology, the protection was now defined as self-contained component with a serial communication link to the control system.

At the Hannover Fair in 1985, Siemens introduced the world’s first integrated protection and control system, LSA678, and also the first multifunctional numerical relay 7SA502 (distance relay) for high voltage as commercial product.
Figure 7 shows the relay with the hand-held operating device. It already had the distinguishing features of modern numerical relays:

  • High sampling rate (1 kHz)
  • Fully numeric processing, and
  • Serial communication and menu-guided setting

It further contained a number of add-on functions for protection, measuring and fault recording, and it was already widely self-monitored. It came out, that the German utilities have not been looking for digital protection to be used in high voltage grids but in medium voltage. So in 1986 the digital distance protection 7SA500/501/502 as well as overcurrent 7SJ52 was released. Figure 6 shows a medium voltage switchgear 8DA20 with built in distance relay 7SA500 in substation UW Königsee (OTEV, Germany). Protection related functions as:

  • Fault locator
  • Measurements for disturbance recordings
  • Measurements as U,I,P,Q
    could be transmitted with serial link or to the handheld.

TOSHIBA: The advent of the microcomputer began to influence protection relaying and in the 1970s a digital relay that would use a microcomputer was actively researched by Toshiba. In order to gain experience with this new technology field tests were arranged in 1968 using a current differential relay for application on long-haul multi-terminal transmission lines. Digital data transmission using high-quality, high-accuracy pulse code modulation was employed in this relaying system as a means for transmitting the current waveform from one terminal to the other; however the demodulated analogue waveform was used as the relay input quantity. Although the relay was extensively tested in the field it did not directly result in the practical use of the system, however the tests did provide valuable experience for Toshiba to aid further development of the digital relay.

In 1973 Toshiba commenced with the full-scale development of digital relays using a new element; the microprocessor. Two approaches were adopted for the digital relaying algorithm; one was based on the fundamental wave and the other on travelling wave phenomena. Extensive simulation studies were also conducted to develop an algorithm which had both high computation efficiency and excellent protection capability.
In terms of hardware the development of a high speed microcomputer using bit slice type bipolar microprocessors was adopted because of the requirement for high speed computation. Around this time optical data transmission technology was introduced with its immunity to interference.

Based on these developments in July 1977 a field test of a current differential relay for EHV transmission lines commenced and in 1978 the field test was extended to include a digital distance protection.
The first Toshiba digital relays went into service in the early 1980s; see Figure 12 when a PCM current differential relay was applied to a 275kV transmission line and a digital distance protection was applied at 66kV. Figure 8 shows the basic configuration of digital protective relay.

The use of digital relays increased and in the mid-1980s protections using 16-bit multiprocessor designs with sampling rates of 600/720Hz for 50/60Hz devices became available. Digital transformer protections was commercially available and in 1990 a Toshiba decentralized digital busbar protection entered service. The busbar protection employed distributed bay units from multiple vendors and featured a countermeasure for CT saturation. The current data and binary status information was retrieved over a dedicated fiber-optic LAN using IEEE 802.4 (Token Bus), a level of interoperability being achieved by unifying the data format and the characteristics of the analog filters used by the various vendors.

Introduction of the fault recording function to digital protection relays was made in 1989. Today this function is making a major contribution to the analysis of relay operations and is implemented in IEDs the world over.
The next step in technology came in the mid-1990s with the move to high performance microprocessors such as 32-bit devices along with the adoption of 16-bit A/D converters. Higher sampling rates were introduced typically 4800/5760Hz for 50/60Hz devices.
Toshiba presented the first microprocessor based transformer relay in 1985. The world's first microprocessor based power system stabilizing relay applied to TEPCO's metropolitan underground cable network was released in 1984. Figure 10 and 11 show the relay equipment configuration of digital protective relay and the block diagram.
Japan was the country with proposals for solutions utilizing microprocessors in protection relays in 1979 already (TEPCO/ TOSHIBA). Figure 13 shows the schematic diagram.

VEIKI/Protecta: To develop digital relays with microprocessors started in 1985. According a development of László Epersesi VEIKI produced a device for automatic reclosing in 1988. It could operate single pole and three pole, long and short reclosing periods (Figure 16).

Distance relay DTVA was produced in 1995 (Figure 17). It was a complex bay unit for medium voltage network (including SCADA functionality).

Westinghouse: The Westinghouse - (now ABB (US)) microprocessor relay development started very early- with the papers described in the last issue of the magazine and the computer based distance protection on P2000- the Prodar in 1973.
The Multi set point Under/Over frequency MDF followed in 1975. The WESPAC (minicomputer substation terminal with line distance, busbar, transformer) was released in 1985. The MVH (Volts/Hertz Overexcitation) as well as MPS Motor Protection System came out in 1986. Also MMCO (Multi phase overcurrent, 1987) and MDAR (REL300, numerical line distance terminal, 1988) have to be mentioned. (Figures 14, 15).

Further developments of these vendors and other manufacturers will be covered in the next issue.


Walter Schossig (VDE) was born in Arnsdorf (now Czech Republic) in 1941. He studied electrical engineering in Zittau (Germany), and joined a utility in the former Eastern Germany.  After the German reunion the utility was renamed as TEAG, Thueringer Energie AG in Erfurt. There he received his Masters degree and worked as a protection engineer until his retirement. He was a member of many study groups and associations. He is an active member of the working group “Medium Voltage Relaying” at the German VDE. He is the author of several papers, guidelines and the book “Netzschutztechnik
[Power System Protection]”. He works on a chronicle about the history of electricity supply, with emphasis on protection and control.

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