Introduction to the History of Differential Protection

Author: Walter Schossig, Germany


Forerunners of Differential Protection

Already at the end of the 19th century some engineers talked about differential protection- it was the comparison of voltages and currents in single phases. A first proposal in 1896 compared the voltages of the two phases of a DC system with the neutral phase. A balance beam relay with coils on both sides was balanced as long as the voltages were equal (German patent DRP 92442 Figure 1)

According to a proposal of Elektrizitäts-AG Schuckert, DRP 113521, in three-phase systems the currents of different phases are compared (one relay and two coils for each phase). Each coil moves a side of the beam. During symmetrical conditions the currents are equal. This scheme was used for the detection of unsymmetrical short-circuits and of breaks of a line.

Schuckert proposed in 1900 another differential relay using balanced beams. Now the current of one phase operates one side; the other two phases on the other side. A change of load in a single phase trips the relay (DRP 122503). So far this relay ca be described as a phase balance relay, it utilizes the zero sequence system. AEG had already published in 1899 a paper regarding a differential relay in the German magazine "Elektrotechnische Rundschau".

In 1898 Kallmann described in the magazine ETZ, his idea for a "differential scheme for direct indication of deviation of currents" . This system was used for announcement only.

Differential Protection as Selective Protection

A differential protection that compares the currents on both sides of power system equipment was patented by Charles Hestermann Merz and Bernhard Price (England). ?Improvements in the Method of and Means for Protecting

Apparatus on Alternating Current Systems" was patented in the UK (patent 3896) on 16 February 1904 and on 31 May 1904 in Germany (DRP 166224)- the year 1904 became the birth year of differential protection. On both sides of a line or a transformer now current transformers are installed. The secondary windings are connected in a manner that during normal operation the current is zero. The connection of the transformers is realized with pilot wires. For the protection of lines, relays were installed on both sides, each tripping a circuit breaker in the case of a fault (Figure 2) For the protection of transformers only one relay was necessary that trips both sides. In case of a fault between the measuring transformers, the currents on both sides became different and the relay trips. A disadvantage of this system is that when the transformers operate under no-load condition; this causes a high voltage in the circuit. To solve this, a bridge circuit was used (Figure 3).

The cumulative circuit of the current transformers and relays in the diagonal connection was introduced by Kuhlmann, AEG in 1904.

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