The production of the On Load Tap Changers started in 1952 with the creating of the first Tap Changers of the series having modern construction developed on the grounds of the constructions of the Austrian organization ELIN-Union. These On Load Tap Changers were determined for the demands of the Bulgarian transformer manufacturers.
The series RS was developed in the period 1962-1963. Its later improved series RS 2 - 1964 and RS 3 - 1967 established the main trends in the development of the Tap Changers production in Bulgaria. The acquired during the years theoretical and experimental experience together with the experience in manufacturing and putting into exploitation created preconditions for a leap in quality and quantity. The series RS 3 significantly increased the export of On Load Tap Changers.
In 1963-1966 the production of C, D and F series completed with Motor Drive Units of the type MA 7 and MA 4 basing on the license of MR Germany. In 1973 samples of these apparatuses were made.
In the period 1973-1975, basing on the acquired experience, a new RS 4 series was developed for nominal current of 200 A, 400 A and 630 A and sequent voltage from 35 kV to 220 kV. These new Tap Changers marked a significant leap in the development and production of these devices and Bulgaria became the second largest world manufacturer of On Load Tap Changers.
Development and adoption of new series for general and specialized us: RS 5 (1983), RS 6 and RS 7 (1988).
Although RS 4 and RS 10 marked high technical, economical and competitive level, their improvement and innovation continued and in 1984 the new series RS 9 was adopted.
Since 1989 the main production of ELPROM TRAFO has used only the RS 9 type of On Load Tap Changers completed with MDU4 and their design and construction has been continuously improved and modernized.
Development and adoption of the series: RS 12 (1990), RS 16 (1992).
Development and adoption of Off Circuit Tap Changers.
Development and improvement of the On Load Tap Changer series RS 9 and RS 9 1250 A.
Development and manufacturing of the On Load Tap Changer series RS 21 prototypes designed to operate in gas medium and introduction in regular production.
Development and manufacturing of the On Load Tap Changer series RSV 9.3 I 400 A prototypes and introduction in regular production.
Development and manufacturing of the On Load Tap Changer series RSV 9.3 I 550 A prototypes and introduction in regular production.
Development and manufacturing of prototypes for: RSV 9.3 III 400 A; RSV 9.3 III 550 A; RSV 9.3 I 1200 A. Introduction in regular production.