These are hermetically sealed oil-filled distribution transformers used for converting medium voltage electrical energy (6 kV to 24 kV) into low voltage (400 V to 420 V).

The OLTC transformer is equipped with an ECOTAP VPD III on-load tap changer manufactured by Maschinenfabrik Reinhausen. The tap changer is designed to connect 9 taps, which allows for a 9-step regulation range, for example ± 4 × 2.5%. The tap changer consists of 3 parts: the switching part located under the tank cover and immersed in oil, the motor drive for changing taps mounted on the tank cover, and the controller, which is placed near the transformer, most often on the wall of the transformer room or under an electric pole in an additional box.

The tap changer is equipped with vacuum interrupter chambers for arc extinction during tap changes, ensuring long, trouble-free operation of the transformers.

The tap changer controller automatically changes taps to maintain a constant set voltage at the LV terminals of the transformer. The control system can be expanded so that the voltage at the transformer terminals depends on the transformer load or on the voltage in a remote LV network. Manual operation is also possible, where the operator decides which tap the transformer should operate on, as well as remote control operation via a SCADA system.