Checkerboard Hill

Hong Kong MTR uses electric trains, powered by two different technologies – 1500 V DC on the ‘urban’ rail lines, and 25 kV AC for the former KCR network. Today we look at 25 kV AC railway electrification across Hong Kong.

Powering the trains

Each train has a pantograph.

Or maybe many.

To collect power from the overhead wires.

Fed by a handful of substations.

Which have transfomers to convert 132 kV power from the CLP Group grid to the 25 kV power used by the railway system.

The substation at Fanling looking very much a product of the KCR in the 1980s.

Google Stret View

While that at Tai Wai looks like any other mosiac tile clad building in Hong Kong.

Google Stret View

Taking a deep dive

A total of six substations provide the 25 kV AC traction power to the MTR network, with fifteen 132 kV to 25 kV transformers between them – ten of which are active and five on standby.

East Rail, four active transformers in three substations:

• Tai Wai: 1x 38MVA active, 1x 38MVA standby
• Fanling: 2x 26.5 MVA active, 1x 26.5 MVA standby
• Ho Man Tin: 1x 26.5 MVA

West Rail, five active transformers in three substations:

• Ho Man Tin: 1x 26.5 MVA
• Kwai Fong: 2x 26.5 MVA active, 1x 26.5 MVA standby
• Tin Shui Wai: 2x 26.5 MVA, 1x 26.5 MVA standby

Ma On Shan Rail, one active transformer:

• Tai Wai: 1x 38MVA
• Wu Kai Sha: 1x 38MVA standby

Standby transformers are in place to ensure that if an active transformer fails, train service can be maintained by switching in a standby transformer, restoring the 25 kV AC supply to the overhead wires. Without them, neighbouring transformers can be switched in to supply the section, but the additional electrical load means train services need to be reduced.

Due to the nature of AC power, the power supply from each transformer needs to be separated by neutral section is required to prevent a train from short circuiting the different electrical phases.

On East Rail neutral sections include:

• Lo Wu, at the connection to the Guangshen railway
• Fanling, at traction substation
• Tai Po Kau, at sectioning cabin
• Tai Wai, at traction substation
• Ho Man Tin, at traction substation

On West Rail:

• Tin Shui Wai, at traction substation
• Ho Pui, at sectioning cabin
• Kwai Fong, at traction substation
• Yau Ma Tei, at sectioning cabin

And on Ma On Shan Rail.

• City One station

On the approach to each neutral section a magnet on the tracks indicates to passing trains to begin coasting through the section, and run onboard systems from battery power, until a second magnet after the section will indicate it is safe to switch back to overhead power.

Clear as mud? Dr. C.T. Tse from the The Hong Kong Polytechnic University included a diagram of the complete network in a presentation titled ‘Tackling Power Quality problems in Railway Systems’.

Presentation by Dr. C.T. Tse

A brief history of 25kV railway electrification in Hong Kong.

• 1981: Commissioning of Tai Wai KCR substation, the new East Rail Line was supplied by two transformers.
• 1989: Commissioning of Fanling KCR substation with two transformers. East Rail now supplied by four Tx in two s/s.
• 2003: Commissioning of Tin Shui Wai West Rail substation, supplying power to the new line.
• 2003: Commissioning of Kwai Fong West Rail substation, power was then supplied by four Tx in two s/s.
• 2004: Commissioning of Ho Man Tin KCR substation, transformer HMTN replaced transformer TWS, and TWS was then at standby. ERL power was then supplied by four Tx at three s/s.
• 2004: East Rail was extended to East Tsim Sha Tsui, power supplied by transformer HMTS. ERL power was then supplied by five Tx in three s/s.
• 2005: Commissioning of new Ma On Shan Rail, power was supplied by one Tx of MOS.
• 2009: Kowloon Southern Link was completed and East Tsim Sha Tsui extension became part of West Rail. Since then, West Rail power was supplied by five Tx in three s/s, and East Rail by four Tx in three s/s.

Something incredibly obscure

Dr. C.T. Tse also has a presentation titled Impact of Imbalance due to Single-Phase Traction Load that details the impact that the unbalnaced 25 kV AC traction network places on Hong Kong’s power grid.

AC traction is of single phase, and imbalance to 3-phase supply is inevitable.

To minimize the imbalance, an engineer should evenly shares load to each phase at design stage.

The imbalance for 3 or 3-multiple sections is zero. The imbalance for non-3-multiple sections decreases with more sections.

In AC traction design, the traction load of each line section depends on the line length, number of sections, the train headway (peak or off-peak), the number of cars in each train; all information are ready at design stage.

East Rail is the dominant line in KCR with three times transformer loading to those in other lines.

However since 2004 the East Rail transformers are only connected to two phases, resulting in a current imbalance of 25.5% between the phases.

This is worse than the 25% imbalance in the late 1980s when there were only 4 sections.

Sources

• Tackling Power Quality problems in Railway Systems by Dr. C.T. Tse
• Reliability, Security, Best Price : A Consumer’s View Presentation by KCRC
• Power Supplies at the HK Rail Engineering Centre (Chinese language)
• Overhead Supply System at the HK Rail Engineering Centre (Chinese language)
• Neutral zones at The Encyclopedia of Rail Transport in Hong Kong (Chinese language)