Author: Walter Schossig, Germany
Overriding an operating current with a timing element caused unacceptable delays. A bypass was used to eliminate the timer in case of the startup of an additional overcurrent protection used in that case. When transformers with granular transformer magnetic sheet steel were introduced, the high operating currents did not allow this bypassing anymore. In 1943 AEG provided a fast protection device consisting of two complete differential relays. One relay was used as the starting relay of another one (Fig. 2). This configuration was based on two patents of Gutmann (DE 889031 and DE 896676). The differential current flows through the starting elements. D2 is bypassed in this first step and it trips with a time delay. It has been shown that the third peak of the current was only 40%. The K-Relays (dashed line), which could trip independently, was set at 0.5 s. This configuration was quite stable, but was a huge effort. This effort could be decreased in 1950 with a simple auxiliary circuit instead of the second differential relay. This auxiliary circuit uses the differential relay two times. In 1944 AEG developed the high-speed differential relay QS2 (Fig. 5).
Blocking with second harmonics (harmonic-restraint) was invented by Kenneddy,L.F. and Hayward,C.D.in 1938. In the differential relays provided by BBC in 1961 (Fig. 1) stabilization was reached with a contact system mechanically tuned to the frequency of the operating current. The curve is distorted (Fig. 4). The force of the current on the iron system of the relays is with the nominal frequency, while the force of the sinusoidal alternating current is according to the doubled nominal frequency. The contact system vibrates due to the impact of the sinusoidal current. The impulses are too short to activate the contactor, but in case of a fault the contact will be closed immediately, even in case of switch onto fault. It trips after 0.1 to 0.15 seconds. Three-winding transformers used the DM2s. The difference (Fig. 3) of two currents has been compared with the current of winding 3. Only when two systems tripped it was a fault in a transformer. Because these devices were single-pole devices, six were used for one three-winding transformer.