Introduction to the History of Selective Protection

Author: Walter Schossig, Germany

Generations of Protection

Relay technology has evolved from the simple electromechanical or moving-coil relays that were introduced in 1940, to electronic or static relay modules in the 1960s and ultimately towards the more sophisticated microprocessor-based protection devices in the 1980s. Technology was exploited to avoid unwanted tripping and provide more discriminating, stable and reliable performance through the addition of time-delays, fault direction and impedance measurements to the relay's tripping criteria.  

It is incredulous to modern protection engineers that at one time the timing elements on protection relays were running bowls as shown in figure 6. Such devices are perhaps the origin of the phrase "relay run times".


Figure 6 Time Relays 1891, DRP 59 192

Protection sensitivity increased with the use of new measuring technologies in instrument transformers and relays. Relay self supervision also led to increased reliability. The development of voltage memory for three-pole close-up faults helped to avoid unwanted relay operations ("dead zone").

Difficulties in detecting faults that had short-circuit values below the relay pick-up levels, for example when large machines are operating at low load times such as on Sunday and at night, caused researchers to develop protection that would operate under such fault criteria. BBC developed the crossed-coil instruments for protection purposes and patented this in 1928.  These electromechanical relays are no longer manufactured but many remain in successful service worldwide demonstrating the effectiveness of the design.

Other developments in technology now mean that engineers no longer have to go to the substations and look for physical indications of faults, e.g. broken pieces of porcelain from power system equipment, fallen lines and towers etc. Modern PAC equipment aids engineers in identifying the nature and location of the fault.

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