Protection History

Author: Walter Schossig, Germany

The Development of Digital Protection

In power stations electronic calculators and computers have been in use for control already in the 1960s. In several power stations computers have been used for data acquisition, supervision, logging of events, starting and shutdown of plants and for optimizing of power. Especially in nuclear power stations the supervision of reactors was very expensive and caused huge efforts.

For transmission systems the usage of controllers was not developing that fast as in power stations. In stable conditions there was no real need for controlling, of course transient phenomena would demand quick response and reaction.
In early days, many research works and prototype development are mainly carried out in many science and industry universities or special research institutes.

In 1967 I.F.Morrison, University of New South Wales, Kensington (Australia) investigated the usage of computers for "online-tasks" in grids and substations.
The first single-chip-microprocessor produced in series was presented by Intel (US) in 1971. The 4-bit µP contained 2300 transistors and 256 Byte ROM with 32 Bit RAM.

First stage (1966-1974)
IEEE's Relays Committee published a "Supplement to recent practices and trends in protective relaying" in 1964. In this paper we find a chapter "use of digital computers" and the following statement: "Because of the complexity of power systems, it is imperative that relay engineers use every available tool to analyze the quantities which affect relaying. Analog computers have been used extensively for many years to determine currents and voltages which occur during short circuit or other abnormal conditions. However, because of the time required to make and analyze such studies, only a limited number of conditions could be investigated. The advent of the high-speed digital computer has made it possible to increase the amount of data available for the relaying engineer under essentially all types of system operation. Load flow, short circuit, and stability studies can usually made more rapidly and accurately and the result printed out in much more detail. As an example, at least one utility has expanded a short-circuit computer program to calculate and print out the impedances seen by relays located near the assumed fault. Such information greatly assists the engineer in selection, application, and setting of system relays."

First papers describing how to realize protection utilizing computers have been published at the end of 1960s, e.g. "The use of real-time digital computers for system protection" was advocated in 1966 by F.H.Last and A.Stalewski. Continuing rapid advances in digital computer technology have prompted a re-evaluation of protective devices and techniques. In 1967, George, D., Rockefeller, Westinghouse, discussed the use of on-line digital computers for substation protection. Bary J. Mann and I.F.Morrison demonstrated the feasibility of digital computer applications to transmission line protection in laboratory environment. A digital computer was subsequently applied to protect the Tesla-Ballot 230 kV line of the Pacific Gas and Electric Company.

The paper "Generator differential protection using a hybrid computer" (Mohindar S. Sachdev and D.W. Wind, was published in 1972 and presented a hybrid, analog-digital, technique suitable for differential protection of generator stators. This was a project undertaken at the University of Saskatchewan in Canada to demonstrate the feasibility of the real-time digital computer techniques for protecting generating station equipment.

As the main paper describing digital protection very often George, D. Rockefeller's, Westinghouse, paper "Fault Protection with a Digital Computer“(1968) is mentioned. The abstract is worth to be cited here: "A fundamental basis has been developed for the use of a time-shared stored-program digital computer to perform many of the electrical power-system protective-relay functions in a substation. Logic operations are given to detect a fault, locate it, and initiate the opening of the appropriate circuit breakers, whether the fault is in the station or on lines radiating from the station.

The instantaneous values of the station voltages and currents are sampled at a 0.5-ms rate, converted to digital form, and stored for computer main-frame use. Operating times are compatible with the 25-ms breaker trip capability of modem two-cycle breakers. Computer speed in initiating tripping is a maximum of 4 ms for severe faults and a maximum of 10 ms for moderate or distant faults. Little attention has been given to hardware or programming aspects; instead this treatment represents the viewpoint of a protective-relay engineer who is attempting to answer the question: can it be done and what is involved? However, major emphasis was placed on minimizing computer main-frame duty.” Figure 2 shows the "overall station differential" as published in Rockefeller's paper. The distance is shown in Figure 4.

Rockefeller's "Phase comparison relaying network" was patented in 1971 (United States Patent 3590324 (Figure, page 70). It describes a protective relay apparatus especially useful in protecting a line section of a transmission line in which the line section has at least first and second terminals connected through first and second breaker switches to busses at first and second switching stations and in which operating information of the line section is transmitted between the first and the second stations (page 70).

Early field experiments have been described in papers of Gilcrest, Rockefeller and Udren in 1972. Because of the cost of the computers in those times, a single high-cost minicomputer was proposed by Rockefeller to perform multiple relaying calculations in the substation. In addition to having high costs and high power requirements, early minicomputers systems were slow in comparison with modern computer systems and could only perform simple calculations. The well-founded belief, that computers would get smaller, faster and cheaper combined with expectations of benefits of computer relaying kept the field moving. The third IEEE tutorial on microprocessor protection held by Sachdev in 1997 lists more than 1100 publications in the area since 1970. Nearly two thirds of the papers are devoted to developing and comparing algorithms. It is not clear this trend should continue. Issues beyond algorithms should receive more attention in the future. But let's go back to the early papers.

We mentioned the pioneer work of J.Barry, B.J. Mann and I.F.Morrison coming from the University of N.S.W. Kensington (Australia) already. Important publications from M.Ramamoorty (India, 1971) and R.Poncelet in 1972 should be mentioned.

In addition to the field tests of Gilcrest, Udren and Rockefeller (Westinghouse, 1972) A.G.Phadke, T.Hlibka and M. lbrahirn. presented a digital computer system for EHV substations with analysis and field tests at IEEE in 1976. The paper describes the research and development being done at the AEP Service Corp. on digital computer applications in EHV substations. relaying of transmission lines and other power system equipment, station alarm monitoring and data logging, oscillography and supervisory control functions are considered for possible computer implementations already. In addition a new algorithm for computing the impedance as seen from the terminals of a faulted transmission line was presented already. Figure 5 shows the setup used for field tests.

Mann and Morrison proposed a method of distance-type protection suitable for on-line digital computer protection of transmission line in 1971. The basic principle is the predictive calculation of peak fault current and voltage from a small number of sampled values.

This article was extended to describe the protection of a three-phase transmission line in the same year. The programme detected the presence of a disturbance, classified the fault into one of six fault types and determined the modulus and phase of the impedance of the faulted line (Figure 8). The already discussed "computer protected 230-kV-line Tesla-Bellota is shown in Figure 3 and described in Gilcrest's, Rockefeller's and Udren's paper " High-Speed Distance Relaying Using a Digital Computer" in 1971. Figure 6 shows the used system- PRODAR 70.

Typical for the first stage have been the usage if small and medium sized process computers as central automation device of a substation. The classic protection criteria have been realized with more simple algorithms. The systems never have been widely used but there have been first implementations as the described Tesla-Bellota project in 1972 with the functionalities:

  • Distance protection
  • Over current protection
  • Reclosing
  • Fault recording
  • Control

The used hardware was described in PACWorld's guru section interviewing Rockefeller (March 2011).

The joint Pacific Gas & Electric-Westinghouse project evaluated the field performance of an experimental hardware/software computer system functioning as one terminal of transmission-line fault protection. In practice, dedicated, parallel operating hardware now performs this function of detecting and locating short circuits and grounds (i.e., faults); usually decisions require less than 50 ms.

Adolf Kolar, of Sprecher & Schuh (CH), proposed in 1970, to use computers instead of protection devices to increase reliability of backup protection and to solve time grading issues. First studies about the possibility of high-speed protection utilizing travelling waves have been published by AEP (US) and ASEA (Sweden) between 1971 and 1974.
The first system RALDA was presented by ASEA in 1976- fault detection time between 2 and 4 ms (Figure 9).

Stage 2 (1972-1981)
The goal was to get cheaper single solutions for certain applications (retrofit, field tests, investigations on reliability...) So the tasks have been divided and could be done on smaller computers. Typical for this stage have been:

Usage of mini- and small computers for special protection solutions, mainly for line distance protection and differential protection of transformers and generators

Special investigations on power of digital technology for the calculation considering none sinusoidal values (harmonics, DC components). This demanded detailed investigation and discussions of algorithms. Signal theory, line theory, functional relations and correlation analysis became important

So typical examples are the papers of B.J.Mann and I.F.Morrison ("Digital Calculation of Impedance for Transmission Line Protection", 1971); G.S.Hoppe and V.S.Umamaheswaran ("Sampling for computer protection of transmission lines", 1974); P.G. Mc Laren and A.Redfern ("Fourier-series techniques applied to distance protection", 1975); A.M.Ranjbar, and B.J.Cory ("Algorithms for distance protection", 1975) and T.Lobos and H.-J.Koglin ("Investigation of fast algorithms for digital line protection", 1978).

Several developments have been reported from Japan –Y.Miki; Y.Sano and J.Makino presented their study of high speed distance relay using microcomputer in 1977. The usage of microprocessor based digital relays application was presented in 1981 by TEPCO and TOSHIBA.
TEPCO used "current differential carrier relaying system (CDCR)" realizing line differential (Figure 7) and "balance and directional distance relaying system (BDDR)" (Figure 1).

M.M.Chen, W.D.Breingan and T.F.Gallen presented their "Field experience with a digital system for transmission line protection" in 1979. In a joint research project, an experimental transmission line relying system using digital techniques was installed in the field for one year at the terminals of a 116 km 500 kV line. Figure 10 shows the system, Figure 15 the field interface equipment and Figure 12 the zone characteristics.
Other laboratory investigations for protection of transmission lines have been described by the authors W.D.Breingan, .M.M.Chen and T.F.Gallen in 1979 as well.

Introducing microprocessor technology for simple protection purposes in 1975 showed new developments and features very quickly as:

  • Reducing the number of elements (=increasing reliability)
  • Self-supervision with the goal to reduce the efforts for routine testing
  • Possibility to combine several protection tasks in a single device

This started the revolution in protection.

Since 1975 8-bit-µP have been used for frequency protection, over current and overload protection. In 1980 microprocessors became part of control systems as well. First steps to combine protection and control are reported. Between protection and control serial interfaces have been used.

Stage 3 (late 70s and early 80s)
With microprocessors and other integrated circuits cheap elements have been available to start a new generation. Main characteristics:

  • Usage of microprocessors, micro-computers and systems with new algorithms, data storage capabilities and huge bandwidth for data
  • Fiber for communication
  • Re-taking the ideas of stage 1, but now as self-sufficient subsystems communicating with industrial bus systems
  • Additional protection possibilities

Very important papers became the theoretical base for digital protection in Germany in the early 1980s- Jürgen Schlabbach and Rainer Speh (Darmstadt) and in Zittau Hans-Joachim Herrmann and Klaus Rothe. At this time also field investigations have been performed.

After comprehensive laboratory tests and a two years trial period in a HV-substation, the protection computer went finally into operation on April 28, 1977 as the sole main protection in the substation Bad Kissingen. A detailed report was delivered at the CIRED conference in 1979 (Figure 13).
The experiences in Japan have been described already in the last chapter- so the different stages have been mixing.

In Germany the positive experience gained during operation was good enough reason for the utility Überlandwerke Unterfranken AG to introduce central computer systems on a larger scale. In the years from 1982 to 1987 further 10 installations went into operation with the meanwhile innovated process computer R30. Functions for the substation control were also integrated. A decentralized system structure was already being used for some tasks, such as switchgear interlocking.

After 1985, the new microprocessor based feeder protection relays 7SA502 and remote terminal units SINAUT FW 1024 were connected to the central computer via serial interface. Please note, that the different vendors shall be described in detail in the next issues of PACWorld magazine.

Protection systems based on µP consisted of some units only. Functionalities as:

  • Distance protection with auxiliary function
  • Signal protection
  • Auto recloser
  • Syncrocheck

Modular software could be used to realize also protection related functionalities , such as:

  • Fault locator
  • Measurement
  • Fault recording

New 16 bit microprocessors (as Intel 80186) opened the door to new and challenging tasks such as distance and differential protection- first devices have been available in 1985.

Pierre Bornad and Jean-Claude Bastide, EDF (France) presented a prototype of multiprocessor based distance Relay in 1982 (Figure 17). The relay is made of 8 INTEL SBC's format boards: 4 CPUs, 2 boards for the 3 arithmetic units, one for the analog filters and one for data sampling and acquisition. The input isolation amplifiers and the output relays are on the back of the device, for evident dielectric reasons.
Phadke presented a microprocessor based three phase transformer differential relay in 1982. (Figure 16 shows the flow chart )

A "symmetrical component distance relay (SCDR) was proposed and tested by Phadke and others in 1981. It was used to protect a line of 151 miles (765 kV) between Kammer and Marisville, AEP (US, Figure 11).
Also in Eastern Germany developments took place. The institute for power, the company "Energiebau" and the utility EKK developed an integrated system for process control. The substation UW Frankenberg (110/10-kV) was put into operation in 1987. It based on a system ursadat (KEAW) and the computer system K1520 of Robotron. It consisted of control, measurement, voltage control, interlockings, automation logic in case of failures in assets and over current protection (single pole, multi pole, 2 stages)- Figures 18 &14


As mentioned already this paper tries to collect the most important papers for the base of later developed digital protection systems. The vendors input and view will be described in the next issues. If somebody notices that we missed something- please feel free to send us an e-mail.


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