Several small factories manufacturing electrical appliances, electric motors and transformers merge into the Vasil Kolarov High Voltage Equipment Plant.
Manufacturing of On-Load Tap Changers (OLTC) in Bulgaria starts in 1952. The first (YR) series of Tap Changers is based on licenses from the established Austrian company ELIN-Union.
The RS series is developed with the latest models in the series still in production today. The RS established the main trends in the development of OLTC production in Bulgaria. The RS3 is the first OLTC successfully exported abroad.
Manufacturing of C, D and F series starts in 1963 with Motor Drive Units (MDU) of the type MA 7 and MA 4 licensed from Maschinenfabrik Reinhausen, Germany.
A new RS4 series is developed for 200 A, 400 A and 630 A currents and voltage from 35 kV to 220 kV. RS4 is a significant leap in the development and production of OLTC and Bulgaria becomes the second largest manufacturer of OLTC in the world.
Development of new RS series of tap changers for general and specialized use: RS5 (1983), RS9 (1984), RS6 and RS7 (1988). Improved variants of these types are still in production today.
R&D at Elprom continues with the adoption of the series RS12 and RS16. RS12 quickly becomes one of the most preferred Elprom OLTC together with the RS9.
Development and adoption of De-energized Tap Changers.
Hyundai Heavy Industries Co. becomes the controlling shareholder of the company after a privatization procedure from the Bulgarian state and the integration process of Elprom into the Hyundai corporation begins. Elprom becomes Hyundai Heavy Industries Co.
A new series of Hyundai Bulgaria OLTC is introduced – the RS21 designed to operate in SF6 gas medium according to requirements of the Korean Electric Power Corporation (KEPCO).
The first Hyundai Bulgaria OLTCs with vacuum interrupters are put into production – the RSV9 series, a vacuum interrupter variant of the universal RS9 oil-based OLTC.
The prototype for a new vacuum type OLTC is tested and put into production – the RSV12, based on the single column design of the RS12 oil type series. Soon it turns into one of Hyundai Bulgaria most popular series of OLTC.
As part of a large restructuring effort the newly named Hyundai Electric & Energy Systems Co., Ltd. divests of a number of overseas assets, including Hyundai Bulgaria. The new owner of the company is BEZ International Group, based in Central Europe.