Protection History

Author: Walter Schossig, Germany

Digital Protection: What happened in the 1990s?

Requirements regarding indications and operation
Already in 1984 at the German VDEW the working group “Integrated SCADA in Substations” was founded. Utilities, RTU specialists, SCADA vendors, protection engineers, switchyard specialists and IT experts have been talking together for the first time. As a result in 1990, a paper was presented that combines the wishes of users regarding digital protection.
There have been 2 goals: The utilities to have a common picture about the requirements for the new technology and the variety of types to be limited to allow an easier first implementation for vendors and users. The guideline contained:


  • Binary and analog information have to be visible and should be delivered via serial interface and binary interface
  • Important information have to be displayed without additional operation:
    • Protection events (red)
    • Internal faults and supervision (red)
    • Status information (e.g. recloser) white/ yellow
    • Live status (green)
    • Additional information should be in red as well because in most of the cases they indicate disturbances
    • Timer, triggered by general startup, reset by drop-off
    • The last value should be displayed for at least 24 hours without auxiliary power and stay after restoration of voltage
    • Additional useful values to be displayed could be fault impedance, short circuit current, measurements as primary values
    • Software release should be shown in display
  • Information on serial interface to be connected to PC and/ or SCADA:
    • Disturbance information
    • Analog measurements (current, voltage, time scale 1…2 ms)
    • Status information (recloser on/off)
    • Information about device errors and supervision
    • Operating values
    • Settings
    • Fault record (at least the 3 last faults, first in first out memory, duration 3 seconds in low voltage, 1.5 seconds in medium voltage, pre-fault time 100 ms)


  • Front panel interface as well as serial interface
  • Setup and check of settings must be possible without Users’ manual (sufficient help inside)
  • Password
  • Settings as secondary values
  • In case of operation via serial interface external devices should be used. For the different kinds of protection a common PC software and standard hardware should be used. The computer used for such applications shall be portable

 Supervision and Indication:
  Maximum self-supervision and indication of faults


  • Vendor independency to achieve compatibility
  • Control during operation of device should be possible

Status at the beginning of the 1990s
Figure 3 shows the development of devices related to the sales of protection relays. The number of digital devices increased dramatically since 1987, in 1991 for the first time there have been more digital than static devices. The complete change to digital devices was expected to happen in 1995 already (only small numbers of spare devices should stay).

Since 1985 in Germany the VDEW organized exchange of experiences in operation of digital protection. The experiences to be considered in the next release of the devices. Main issues have been about integrated functionalities, serial interfaces and user interface. In terms of communication the German standard DIN 19244 standardized the telegrams early. Even the content of information was standardized up to a certain level. 

The advantage for the users at this time was obvious - several protection functions could be combined (e.g. distance protection with auxiliary functions). As an example in SIEMENS 7SA511: 

  • Primary protection function
    • Distance protection with overcurrent startup or (under-) impedance startup
  • Additional protection functions:
    • Earth fault backup protection
    • Backup overcurrent protection
    • Earth fault detection (watt metric)
  • Additional functions (protection related):
    • Single phase or 3-phase autorecloser
    • Signal comparison
    • Power swing detection
  • Auxiliary functions:
    • Fault locator
    • Fault recorder
    • Breaker supervision
    • Detection of direction

The additional functions run on the same processor as the distance function. There have been no side effects or delays because the chosen 16-bit-processor was powerful enough and most of the additional functions have been complex but not real time. In software all functions have been separated.
Digital protection devices came with a permanent supervision (Figure 6), detecting errors in the device in most cases. In addition these devices supported manual tests such as tripping circuit breaker, starting direction detection and reading operating values. This increased the reliability of the protection devices.

Since the first relays have been delivered with hand terminals (Figure 5) the request for operating the device directly came up immediately. So the SIEMENS 7SA511 (distance) and 7SJ511/512 (overcurrent) came with a built-in user interface (Figure 1). On the back the serial interface to station control (SCADA) was realized- called VDEW6 interface what was the predecessor of IEC 60870-5-103 (Figure 2).

In 1995 digital machine protection 7UM516 was released- especially for huge block units (Figure 4).
The amount of built-in functionalities with impedance protection, power swing blocking, stator earth fault protection, out-of-step-protection, forward and reverse power protection, unbalanced load protection, trip supervision and others was huge at this time already. In the same year line differential 7SD502 (2 pilot wires) and 7 SD503 (3 pilot wires) was presented. Figure 7 shows the hardware structure using summation transformers.

For protection the infeed busbar or as backup protection integrated overcurrent was available. 2 devices could realize three-end pilot-wire scheme. The German Rail (16.7 Hz) introduced contact wire protection 7SA517 (Figure 8).
After the described SIPROTEC 3 series SIPROTEC 4 was released in 1998. The 7RW600 (Figure 11) can be used for de-coupling as well as load shedding. Also over excitation for generators and transformers was possible. Line differential 7SD52 (applicable for up to 6 ends, operating time less than one cycle) was introduced as well as busbar and breaker failure protection 7SS52 (Figure 12).

The German AEG introduced digital generator protection PG851 and PG871 (Figure 9) in 1995. The additional group PG811 came later (Figure10). As at other vendors the protection functions are distributed and redundant. The 90%-stator fault protection (67N) for example is integrated in PG851, the 10 % earth fault protection with 20-Hz-measurement system is included in PG871. Rotor earth fault protection SLG is available in PG811. The first application of these relays was the power station Porabka (Poland) and in Staßfurt (Germany). In this station three machines have been protected (each 50 MVA with 10/110-kV unit transformer. Also the largest pump storage power station in Germany (Goldisthal, 1060 MW, protection of 331-MVA-synchronous-and asynchronous motor-generators and 18/380-kV- unit transformers such protection is in use.

In 1995 AEG released distance protection PD521 and PD571 (Figure16), railway relay PD591 and differential PQ731 (Figure 13). PQ731 can be used as transformer, generator, motor and REF protection. ELIN’s DRS-COMPACT 1 and the entire DRS system came out in 1995. They are in use for instance in Suisse hydro power stations (SBB). 

ABB’s numerical busbar protection REB500 was released in 1996 (Figure 17, Figure 18). Figure 19 shows the parts of operating time. A single earth short circuit fault in a 380 kV switchgear could be switched off within 26 ms in February 1997. REL352 combined phase comparison line protection and was presented in 1996 followed by, REL356 line current differential in 1997.
In 1996, SEL released its first transformer differential protection relay- the SEL-587 Current Differential Relay-followed in 1997 by the SEL-387 Current Differential and Overcurrent Relay (Figure 22). SEL-387-The first multi-winding current differential and overcurrent relay with dual slope characteristics, harmonic restraint, and user programmable logic was released in 1997. Later, in 1998 - SEL-321 - the first relay to also include high-speed peer-to-peer serial protocol for teleportation applications (Figure 23) was available. Generator protection SEL-300G (Figures 21 and 25) with 100 % stator earth fault protection is available since 1998. SEL offers the first protective relays to also include synchronized phasor measurements (Synchrophasors) as a standard feature since 1999. Reyrolle (UK) started the ARGUS line in 1994 (Figure20).

These relays used microprocessor technology which enabled more features, such as communications, data storage and alphanumerical display to be provided. The relay incorporated an event recorder that time and date stamped the last 100 events and as a consequence was named Argus- a mythical creature with 100 eyes. The Argus family developed into six variants and in the latest versions the event log could hold up to 500 events. Digital overcurrent protection ARGUS 1 with IDMTL-, DTL-, NI-, VI-, EI- and LTI-functionality was produced since 1997 (Figure 24).

SIFANG (China) introduced the first digital line protection CSL101 for 220…500 kV in November 1996. Hardware and structure is shown in Figure 28 and Figure 14.
The variant CST100V (generator) and CST30A (block protection for huge generators) followed in 1999.

GEC ALSTHOM introduced distance relay EPAC (Figure 30) and LZFR (Figure 31) in 1995. In 1995 the Modulex3 Line was introduced (Figure 32).

AREVA’s MiCOM series was introduced in 1999. Derived from Optimho product line differential P54x (2 or 3 lines) and distance relay P44x was presented (Figures 15 and 29).

Basler’s BE1-24 Volts Per Hertz Relay is microprocessor based to provide a new standard in versatility and control in protecting generators, transformers, and iron core reactors from the adverse effects of over excitation in 1998 (Figure 26).

GE’s feeder management relay F30 and line current differential L90 (Figure 34) is available since 1998 (Figure 33). The F30 was the first relay to be UCA 2.0 (Utility Communications Architecture) compliant.  The history of communication protocols will be covered later.

At the end of the century, in 1999, GE presented the improved feeder management relay F60, transformer relay T60, distance relay D60 and line current phase comparison relay L60 (Figure 35).

In Hungary Protecta received a certificate for application in the Hungarian network for the digital frequency relay  FR90 (Figure 27), Frequency dependent load shedding device DFTK, Complex numerical motor protection (including overcurrent, asymmetry by negative sequence detection, motor starting logic, loss of load protection, thermal replica calculation) DMV (Figure 37) und Overcurrent protection for medium voltage network DTI+Io+ER (Figure 36)  was introduced in 1995. In 1996 a special protection of radial lines in a mashed network CVA was presented (Figure 38).

EAW presented the D2 series in 1995 (differential DS...Q2, directional overcurrent DSRZE2, distance DD2 and motor protection DM2). The distance relay types are:

  • DDS2 - distance with overcurrent startup
  • DD   - distance with additional under impedance startup (angle dependent)
  • DDSE2 - distance with additional watt metric earth fault direction detection.

The variants of the overcurrent were:

  • DSZ2 - overcurrent
  • DSRZ2 - overcurrent with direction
  • DSRZE2 - overcurrent with direction and additional watt metric earth fault direction detection

The Figure on page 70 shows the central unit DDS, and other devices.

The development of combined protection and control devices will be covered in the next issue.     

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