Parę słów o… ODC. 216 –⚡️DLACZEGO NA PKP WYBRANO PRĄD STAŁY 3kV?⚡️

Summary

The video explains why Polish State Railways (PKP) adopted 3 kV direct current (DC) for railway electrification, why the system remained in use for decades, and why Poland is now also introducing 25 kV alternating current (AC).

Why Poland chose 3 kV DC

• Work on choosing an electrification system began in 1921. The choice of 3 kV DC was formalized in the 1930s, alongside the electrification of the Warsaw railway junction.
• Poland had limited resources and no dedicated railway power plants. With 3 kV DC, railways could draw standard 50 Hz three-phase power from the public grid and convert it at traction substations.
• Some nearby countries used AC systems at unusual frequencies, which required dedicated power plants and transmission networks—an expensive option for Poland at the time.
• The Warsaw cross-city tunnel also influenced the decision. A higher-voltage AC system would have required greater insulation clearances, making the tunnel larger and more costly to build.
• DC motors of the period offered strong starting torque and were well suited to heavy railway vehicles. Contemporary AC motors were more complex, heavier, and less reliable.
• Choosing 3 kV rather than the 1.5 kV DC used in some other countries meant substations could be spaced farther apart, reducing infrastructure costs and energy losses.

Why the system continued after the war

• The 3 kV network was considered a success and became the basis for Poland’s large-scale railway electrification from the 1950s through the 1980s.
• In 1956, officials considered switching new lines to 25 kV, 50 Hz AC. The proposal was dropped because Poland already had a 3 kV network, British equipment and locomotive designs had been acquired, and domestic industry could produce DC components. A change would have required expensive new technology and risked creating disconnected sections of the network.
• DC technology also suited the resource constraints of the period. Locomotives and multiple units such as the EU07, ET21, ET2, and EN57 used relatively simple equipment that domestic workshops could maintain and repair.
• Electrification helped replace steam locomotives, improve the movement of heavy freight—especially coal—and support frequent suburban services. The video also notes that improvements in traction-substation rectifiers helped make the system cheaper to operate.

The present-day transition

• The video says that roughly 12,000 of 19,000 km of track currently operate at 3 kV DC.
• As speeds and power demands rise, 3 kV DC is increasingly limiting, particularly for high-speed operation. The video presents 25 kV AC as necessary for new high-speed lines and cites EU interoperability requirements for lines above 250 km/h.
• A complete conversion would be extremely costly: existing overhead equipment and substations would need major replacement, and many trains designed only for 3 kV DC would also need to be replaced or modified.
• Instead, the two systems are expected to coexist. New lines may use 25 kV AC, while the existing DC network remains in service.
• Multi-system trains, including the Pendolino mentioned in the video, can operate under both voltages. At a transition point, they pass through a neutral section and switch between power systems without stopping.
• Connecting the networks will require careful engineering to address incompatible equipment, stray currents, electromagnetic interference, and grounding at system interfaces.

Overall

The video presents 3 kV DC as a practical, economical choice for Poland’s circumstances in the interwar years and a good fit for postwar electrification. It argues that the system is now approaching its limits, so the challenge is to integrate new 25 kV AC routes with the established DC network rather than replace the entire system at once.

Speakers and sources mentioned

• Speaker: One narrator or presenter, not identified by name in the subtitles. The channel credits Mirosław Romański for the video’s realization, research, and editing.
• People, institutions, and examples mentioned: Professor Roman Podowski; the Railway Electrification Project Office; the Ministry of Communications; the Railway Institute; British railway experience and equipment; and the EU interoperability requirements discussed in the video.
• No interviewees or separate quoted speakers appear in the subtitles.