The utility PEPCO in Washington DC, USA has awarded Siemens Energy Austria with the delivery of three 220/138/13.8kV, 280MVA transformers. A challenge here is that, according to statements by established tap-changer suppliers, conventional tap-changer solutions for switching from voltage-ratio control (VRC) to phase-angle control (PAC) cannot cope with the high voltages which occur during the operation and testing of these transformers.
Therefore, a special single-column rotating design DEETAP® DU with modifications between the switching contact levels was initially considered to be the best tap-changer solution. A below-tank-cover reconnecting system was also considered as an alternative. Finally, engineers from MR and Siemens Energy Austria were able to identify the linear de-energized tap-changer CAPT SPT 030 K B 2 170.KC800 with 12 columns as the first choice since this solution allowed the team to keep the dimensions within the customer’s needs.
Not only the compact size was a contributing factor in favor of the CAPT SPT 030, but also the more secure and safer operation provided by the integration of two CAPT motor drives which are synchronized and interlocked when in service. In the end, a safe and sound solution for the lifetime of the units was achieved thanks to the good collaboration between PEPCO, Siemens, MR and CAPT.