Use of Measurement Transformers

Increase in demand on power and transmission of energy over long distances was the reason to build bigger generators or to operate generators and grids in parallel. This required the use of measuring transformers. It seems that the first voltage transformer had been used in the "Californian Light Company" in San Francisco in 1879. The current transformer was invented by Benischke,G. in 1898. And since 1900 relays were designed to use current transformers.

Emmet,W.L.R. und Hewlett,H.E., GEC, built in 1901 oil circuit breakers with two tripping coils, directly connected to the current transformers to open the locking mechanism. Figure 16 shows an "automatic machine tripping device" with an overcurrent relay supplied by current transformers (used by AEG in power stations in 1905). The figure shows the solution with auxiliary power supply. But this breaker was manufactured with tripping coils supplied by two parallel operating current transformers.

It was a logical conclusion to change the measuring devices in such a manner that the indicator also operates the contact. Nevertheless this solution was unsatisfying. The breaking capacity was poor (6 VA at 220 V) and the electric arc held the contacts together.

Since the same measuring principles have been used the first relays have been quite similar to measuring devices (especially the round form Figure 19).

The Swedish Company ASEA delivered the first induction relay for a water station of the Swedish Rail (16 2/3 Hz) in 1912. To cut one's own path was the decision of Voigt & Haeffner (V&H). Vogelsang, M. developed an overvoltage relay with timing element in 1902. They decided to do it like that because they did not produce current transformers themselves. Additionally Vogelsang developed an "oil circuit breaker with fuse". To change the fuse it was at first necessary to switch off the breaker before opening the cap.

The history of measuring transformers will be covered in a later issue of PACWorld.