Introduction to the History of Selective Protection

Author: Walter Schossig, Germany

The Birth of the Protection Relay

The first relay for protection purposes was proposed by Stillwell at 1900 although its application was limited since there were no reliable switching devices for the interruption of short-circuit currents at that time.



Figure 3 Lewis B. Stillwell (IEEE)

In future articles we will consider the development from simple overcurrent relays through to directional and distance/differential protection devices. We will discuss the different generations of relays as the technology progressed from the initial basic designs towards the current generation of adaptive digital relays.

The aim will be to inform the reader about key aspects of the different technology employed for protection purposes. It is interesting to note at this stage that one of the earliest cases of the application of a line differential relay was in England in 1912 and of a distance protection relay in Germany in 1921. Records show that both types were being used in the United States shortly thereafter.


Figure 4 First Protection Relays

The physical aspects of relays such as the shape, size, dimensions, casing and color have evolved and it can be noted that the physical appearance of relays have changed dramatically.

In the early years they looked like measuring instruments and changed as shown in figure 4.

One of the earliest connection of two separate grids (19th of November 1887), namely two power stations (Mark­grafenstrasse and Mauerstrasse in Berlin, Germany), was synchronized using the synchronizing-bright method / synchronizing-dark method or sometimes referred to as dark-lamp synchronizing. An incandescent lamp is used by engineers to estimate the synchronism of the two grids by using to the brightness or otherwise of the light to estimate the voltage difference (magnitude and angle).

The lamp is switched between the same phases of each generator/ system. When the engineer uses the dark method to synchronize the systems the lamp is initially brightly lit and this reflects the fact that there is a voltage from one generator and no voltage from the other. The excitation of the second generator is adjusted and the voltage changes. Consequently the brightness of the lamp changes as the phase angle or the rotation speed of the generator changes. The lamp is completely dark when both generators/systems are in a fully synchronized state and it is safe to switch in. The bright-method works in the opposite way, i.e. the lamp is bright under synchronous conditions.

This synchronizing method was published by Brown Boveri Company (i.e. BBC but now known as ABB) in October 1928. The paper describing this method was published by Stoecklin but it is not clear whether he was the inventor. The author was unable to obtain any further information concerning the patent for this method.

Nowadays almost half the continents are interconnected and operating in parallel using WAMS and GPS systems. Such interconnection would barely have been imagined a hundred years ago. The 220kV grids of France, Germany and Switzerland ("the star of Laufenburg") were interconnected in 1958. The Western-European UCPTE-grid and the East-European CENTREL/VEAG/BEWAG-grid with parts of the Romanian and Ukrainian grid were inadvertently connected one year before the "electrical reunion" of Germany. Obviously consumers remained unaware of this fact. Such interconnection was made possible by the effective operation of synchronizing devices.

Trip times of early PAC equipment were often dubious and dependent on inaccurate timing mechanisms that could only be set to operate in 2 seconds or in the case of generator protection 10 seconds or even up to 30 seconds. The tripping schedule published in 1922 (see figure 5) shows the settings of an overcurrent protection relay (i.e. 12 seconds). The relay operated on lines 5 and 6 of the Zschornewitz - Lichterfelde substation in Berlin and its operating time upon detection of a fault was set at 10 seconds. Such relay time delay settings are unimaginable by today's standards.



Figure 5 110-kV-Tripping-Schedule 1922 (Elektrowerke AG, Berlin)

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