Protection History

Author: Walter Schossig, Germany

The First Digital Protection Relays

The last issue of the magazine described the usage of computers in substations for protection. The central computer solution was driven by economic reasons. Since powerful microprocessors became available decentralized devices to be used in feeders could be realized. This started in 1975.
The article describes the implementation of the first digital relays at different vendors. The data is based on information provided by the vendors. Any comments and additional remarks are most welcome. Because of the huge amount of data missing details are possible. The author apologizes for any issues. The vendors are presented in alphabetic order. The series will be continued.
First devices have been realized as frequency- or motor-protection. Typical examples are BBC's frequency relay FC95 and motor protection MC91 (Figures 3 and 4).

In the 1960s papers about "protection computers" with distance protection have been published for the first time. Later analog-digital hybrid solutions followed. The measurement acquisition was realized with analog electronics. Time estimation and tripping logic was realized digital. Later full-blown-digital-relays came out in the 1980s.

Auxiliary functions as auto-recloser, signal comparison, under-impedance startup and synchrocheck have been added as well as functions like:

  • Self-supervision
  • Measurement supervision
  • Fault locator
  • Measurement and visualization
  • Fault recording

The solutions on the way to digital protection are shown in Table 1. They are based on powerful 16-bit-microprocessors performing also self-supervision and plausibility check of input values.

  • Modern digital protection devices consist of some functional elements only. The main parts are:
  • Process periphery (current, voltage)
  • Outputs
  • Inputs
  • Core (8 bit, 16 bit processors, ROM and RAM
  • Communication
  • Auxiliary power (DC/DC converter, stabilization)
  • Local control
  • Mechanics (housing)

Two terms are used and are a part of IEC 60050 International Electrotechnical Vocabulary since 1995:

  • IEV447-01-10 digital relay
    static relay whose operating function is achieved principally by digital signal processing (usage e.g. in Germany and Austria)
  • IEV447-01-11 numerical relay
    digital relay whose operating function is achieved by algorithmic computation (usage e.g. in Switzerland)

AEG:AEG: The digital frequency relays SFT10-30 was introduced in 1981. The frequency or change of frequency is estimated from period of line-line-voltage. The calculation was done in 8-bit-microprocessors. The cycle time of the processor was 2.5 𝜇s what is en equivalent in frequency of 400 kHz. To start frequency stage the startup value has to be reached in at least 4 periods. So at operating frequency of 50 Hz the minimal time needed was 80 ms.


The digital frequency relays SFT10-30 was introduced in 1981. The frequency or change of frequency is estimated from period of line-line-voltage. The calculation was done in 8-bit-microprocessors. The cycle time of the processor was 2.5 𝜇s what is en equivalent in frequency of 400 kHz. To start frequency stage the startup value has to be reached in at least 4 periods. So at operating frequency of 50 Hz the minimal time needed was 80 ms.

AEG: The digital frequency relays SFT10-30 was introduced in 1981. The frequency or change of frequency is estimated from period of line-line-voltage. The calculation was done in 8-bit-microprocessors. The cycle time of the processor was 2.5 𝜇s what is en equivalent in frequency of 400 kHz. To start frequency stage the startup value has to be reached in at least 4 periods. So at operating frequency of 50 Hz the minimal time needed was 80 ms.


To calculate df/dt was possible with the time difference of period. Cyclic supervision checked the content of memory in microprocessor. This repeated every 40 seconds. In case of problems a message is issued every 4 seconds. The supervision could be tested with a button (Figure 2).
It was obvious, that multi-processor setups with a common bus for information exchange have been very useful for complex protection. Already the partly-digital distance protection SD36, to be used in medium voltage grids and launched in 1985 was using several 8-bit-processors for data acquisition and operation. Title picture shows the schematic of SD36 and Figure 1 the device SD36G.

The digital protection PD551 was introduced in 1990 (digital distance protection). The overcurrent relays PM481 and PS451 followed later as well as differential protection PQ721. The PD551 was developed especially for high voltage applications and contains an additional intermediate earth fault relay for selective direction of earth fault detection.
The popular PD531 to be used in medium voltage followed one year later (Figure 5).

BASLER: Basler Electric introduced for sale the BE1-51 family of microprocessor based multifunction protective relays in early 1983. The BE1-51 Protective Relay was one of the first commercially available microprocessor relays in the industry, and is still available for sale. The BE1 -51 families consists of the BE1-51 Overcurrent Relay, BE1-51/27C Overcurrent Relay with Voltage Control, and the BE1-51/27R Overcurrent Relay with Voltage Restraint. All models are available in single phase, three-phase, and three-phase with neutral configurations.

The BE1-51 family was innovative at the time of introduction, since it was not common to have multi-phase protection in one device, and that each element and phase of the device exhibited independent phase measurement, pickup detection, phase timing, and targeting.
Due to the limited mathematical capabilities of the single chip microprocessors of the time, a unique log-base-2 algorithm was developed to allow high speed timing calculations without utilizing direct multiplication or division math functions.

The BE1-51C was was released in August of 1990. It was a three-phase and neutral overcurrent relay with three-phase voltage sensing and designed for easy incorporation into a computer managed power system. . The BE1-51C provided for the incorporation of an optional Communications board to interface with external devices and included both local RS-232 and remote network communications. When fully implemented, it allowed remote monitoring of historical data and remote control of operating parameters.

The relay Basler Electric Company submitted for patent #5,309,312 was Overcurrent Protective Relay with Communications in September, 1990. This patent was granted in May, 1994. The patent was based upon the Basler BE1-51C Microprocessor Overcurrent Relay with Communications.

The Basler BE1-BPR Breaker Protection Relay was introduced in 1994. It incorporates a powerful means of programming internal relay logic to satisfy a wide range of user requirements without making any relay hardware changes. Microprocessor based design provides the basic features of a programmable logic controller (PLC) combined with an Instantaneous Overcurrent module. Built-in diagnostics and monitoring features provide information from the relay and the health of the breaker being monitored. Breaker diagnostics include a timing\ diagnostic log, breaker contact duty monitoring, breaker resistor protection, and breaker arc detection. Other monitoring features include oscillographic fault records and fault summary logs.
Figure 9 illustrates Basler’s devices.

BBC/ABB: Microprocessor operator frequency relay FC95 (Figure 6) was introduced in 1984. Also the motor protection MC91 (Figure 7) as high impedance protection SU91 followed. The name of the company at this time was BBC.
In frequency relay FC95 the measurement value is used several times in 4 independent measurement stages (over- and underfrequency). The underfrequency stage can be used with a gradient of frequency stage for release. The defined frequency is shown digitally and supervised permanently. This causes high reliability with high accuracy.

In 1986 the first fully numerical multi-function line distance protection terminal was presented, the RELZ100 (Figure 8) came out in1990. In Modures-series numerical line protection 316LZ (later RL316) and transformer protection RET316 have been presented.
Figure10 and Figure12 show a card of REL316 and 316LZ

EAW (Eastern Germany): The development of digital motor protection started in 1984. Following the international trends this was forced in 1990. The reunion in Germany allowed the access to previously restricted digital parts. The first series ("Baureihe 1") was tested in the grid successfully in 1992. Two devices have been launched- overcurrent relay with 19" racks (DSZ1) and one device transformer operated (DSZW1).

To trip circuit breaker capacitor release was included (Figure 11). A device with directional element (DSRZ1 and DSRZW1) followed.

ELIN (Austria): Microprocessors can be used for special tasks in protection. The first solution in Austria was stability protection for hydro protections at Österreichischen Donaukraftwerke AG. The machines are mechanically very sensitive if the voltage comes back after a short circuit. Grid's voltage can't be measured in case of short circuit, so it has to be estimated.

This is possible with microprocessors only. The Austrian industry realized DoKW's concept with first usage of microprocessors for protection in 1979. ELIN realized stability protection MSTAB in 1981. It was realized with single card process computers made by DEC.
The developers have been Jörg Strobl and Klaus Schreiber zu nennen. The realization proposed in 1996 is shown in Figure 13.

GEC/ALSTOM: The YTG distance relay came out in the late 1970's and was in use in Malaysia (Figure 16).
GEC technology center in Stafford developed microprocessor distance protection in 1980. The matrix could be programmed freely in SHNB Micromho subcycle distance protection. The certificate issued 2nd of February 1983 shows, that the multicharacteristic microcomputer controlled overcurrent relay for phase and earth faults MCGG (Figure 15) was developed and tested successfully. The functionality of this relay was intended for applications where inverse time, definite time, and instantaneous overcurrent relays are normally applied. It also gives selective phase and earth fault overcurrent protection for time graded systems. Figure 18 shows the details.

The fully digital current differential protection LCFB was released by GEC in 1986. It could be used in all voltage levels and was the leading device in this time. It was working with MUX comms for the first time (Figure 21).

The GEC St Leonard's site in Stafford, UK, was active in a number of areas mixing the technology generations. GEC developed hybrid digital-static subcycle distance protection in 1980, marketed as the SHNB Micromho (Figure 17). The protection was implemented using analog comparators for speed, but all logic control of opto inputs, output contacts and distance aided schemes was digital. Watchdog self-supervision was introduced for the first time, at that time with a supervision interval of a few hours.

Overcurrent and ground fault relays went fully-digital for the first time, with the MCGG launched in 1982 (Figures 14 and 18), and being the first relay to claim full execution of a numerical program. In fact many IDMT relays today still mimic the exact same methodology, whereby a number of sequential "up-counts" are required in order to achieve a tripping threshold, with the size of each up count increment varying according to the point on the inverse curve.

Interestingly, at the approval meeting with the Central Electricity Generating Board, the minutes record the only questions being about the radio frequency immunity of the relay, and the security of sourcing the Intel 8022 microprocessor (with on-board A-D converter).
In 1986, the first L-series modular GEC relay was born, the LFCB line current differential (Figure 20), being the first relay in the family to include multiplexed end-end teleprotection communications, measured values, and settings and measurements from a PC using VT100 terminal emulation. Many of the numerical algorithms and communications supervision techniques still find acceptance today, now executed within Alstom's MiCOM P54x relays.

The Optimho from 1989 (Figure 19) closes this part of the history chapter in the same way it started in Stafford, with a hybrid device.

Here, digital implementation of analogue phase comparison techniques was done for the distance protection elements, supplemented by a fully-numerical on-board fault locator and measurements communication board.
The Stafford relays mentioned all had manufactured lives of over 20 years, until replaced by Alstom's MiCOM successors - indicating the relative stability and reliability attained by digital technology.

In the September 2014 issue we will continue with the history of microprocessor based protection at SEL, Siemens, Toshiba, VEIKI/Protecta, Westinghouse. For the companies that have not yet submitted their history - we encourage you to do so. This way we will be able to help our readers get a more complete picture about the development and implementation of this revolutionary technology that changed our industry.


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