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 1500 V DC railway electrification across Hong Kong.

Powering the trains

Each train has a pantograph.

To collect power from the overhead wires.

To minimise voltage loss, multiple substations supply traction power – each one converting incoming 33 kV power to 1500 V with a transformer, then from AC to DC with rectifiers.

The 1500 V DC feeders are connected via a network of circuit breakers.

So that in the event of a traction substation fault, other adjacent substations can supply the portion of line affected.

Different styles of overhead wire

The bulk of the MTR uses standard overhead wires, suspended from the tunnel roof, with double contact wire to reduce electrical losses.

The bulk of elevated viaducts support the wires with cantilevered arms.

The Disneyland Resort line is a little different – “Victorian” style overhead structures to fit in with the Disney resort theme.

Which transition to “modern” themed overhead structures at the Sunny Bay end.

Newer lines such as the South Island line feature noise barrier ‘tunnels’, which are put to use supporting the overhead.

These recent project have also used solid overhead conductor rails inside tunnels, like these on the West Island line extension.

And these on the South Island line.

Electrical sections

Each traction substation feeds a separate electrical section, with an insulated gap in the overhead wires between them. When the pantograph of a train crosses this gap, there is a momentary surge of power between the two electrical sections, which isn’t normally an issue. But if the train stops there, the pantograph will complete the circuit, so lineside signs warn train captains that the pantograph of their train is in a position of danger.

But thanks to the different models of train having their pantographs in different positions, the Lantau Airport Railway has a second type of sign – ‘K’ prefixed for the small number of K-stock trains in use on the line.

Failure to follow these signs can cripple an entire railway. Melbourne, Australia also uses 1500 volt DC for traction power, and in 2008 a train stopped with a pantograph across an insulated gap – an electric arc resulted, burning through the overhead wiring and then falling onto the train roof, shutting down traction power for the entire line.

So why a 33 kV distribution grid?

While 1500 V DC is a suitable voltage to power electric trains, it is subject to high losses over long distances. Meanwhile Hong Kong’s 132 kV AC power grid is subject to far lower voltage loses, but requires larger switchgear and insulation for safety.

So it was decided to provide an internal 33 kV AC distribution grid for the MTR system, supplied by a handful of ‘infeed’ traction substations which use transformers to convert 132kV power from the CLP Group grid to 33 kV.

This 1986 paper describes the power supplies available to the MTR network and the desire for reliability.

Two separate power utilities, the Hongkong Electric Company (HEC) and China Light and Power (CLP), provide electricity supply for the island and mainland territory of Hong Kong. Supply had to be taken from each utility. A single failure reliability criterion was adopted, i.e. that the loss of a single item of plant or equipment should not lead to a reduction of the full service.

The choice of 1500 V DC traction supply.

The traction supply voltage selected was 1500 V DC. This was considered a more efficient and cost effective voltage for distributing power over the required track section lengths than the 3000 V and 750 V options. Studies were carried out on the basis of a 90 second headway between trains. These identified a requirement for traction sub-stations of about 8 MVA rating at spacing of 2.5-3 km.

And how the use of a 33 kV grid enabled a highly available power supply.

Power supply from the utilities could be taken either at each passenger station or traction substation, or from a small number of bulk infeed points with distribution to load centres being carried out by the MTRC. The latter option was favoured because:

(a) connection to the highly secure 132 kV grids of each utility could be negotiated
(b) the MTRC’s internal distribution could be designed and operated to meet the MTRC’s need alone, as it is independent of any other consumers.

It was decided to take supply from each utility company at two points with autochangeover of loads in the event of one supply infeed failure. The subtransmission voltage of 33 kV was chosen to allow load transfer over the 30 km of planned railway.

Reliability through diversity

Diversity in supply is provided at the 132 kV level by Hong Kong’s two electrical utilities.

A network of four infeed substations supplying the initial three line MTR system with 33 kV power.

The modified initial system was designed with two 50 MVA 132/33 kV infeeds from China Light and Power at Kowloon Bay.

A second infeed in the modified initial system was provided by two 40 MVA 132/33 kV infeeds at Admiralty from the Hongkong Electric Company.

The Tseun Wan extension used a second infeed from China Light and Power at Kwai Fong with two 60 MVA 132/33 kV transformers. China Light and Power agreed to supply this infeed from a 132 kV network which was electrically independent of the first China Light and Power infeed at Kowloon Bay.

A similar arrangement was made with the Hongkong Electric Company when the second Island infeed was required at Chai Wan for the Island Line development.

Each of the four infeeds comprise two 100% rated 132/33 kV transformers.

The 33 kV supply from the infeed substations forms a mesh network via a series of circuit breakers.

This diverse network allows trains to keep running, despite the failure of multiple infeeds.

A number of predetermined strategies are automatically initiated to cover infeed failures.

First-order contingencies, which involve the loss of one infeed, will see the 33 kV bus-section circuit breaker close automatically so that
the other source can supply the complete system. The total loss of supply from one power company would appear as two simultaneous non-interactive single-infeed failures, and would result in two simultaneous interconnections.

Second-order contingencies, of much lower probability, involve two interactive single-infeed failures, one from each power company. They
are dealt with by tripping the cross-harbour circuits and closing some interconnector circuit breakers which are normally specifically
prohibited by interlocking, such that the systems on each side of the harbour are fed from the remaining power supply on each side.

Third-order contingencies involve loss of three infeeds. This requires the whole system to be run solid, which again means defeating otherwise vital interlocks, and feeding from the sole remaining supply.

The strategies for second- and third-order contingencies generally will involve swinging large blocks of power from one power company to the other, at a time when one of them may well be in difficulties of its own. The reconnection is therefore not initiated automatically.

In all instances except the three-infeed failure, it is possible to continue normal (but not rush-hour) service of the railway; for the third-order contingency, some reduction of service would be required.

In 1994 this 33 kV network was supplying 18 traction substations, powering 35 kilometres of route across the three MTR lines.

As the network has expanded, additional infeed substations have been provided.

• Lantau Island Railway: Tai Kok Tsui in Kowloon and Sham Shui Kok on Lantau Island (1998);
• Tseung Kwan O Line: Tseung Kwan O (CLP Group) and Quarry Bay (Hongkong Electric Company) (2002);
• West Island Line: additional 50MVA gas insulated transformer at Admiralty (2013);
• South Island Line: two 30MVA gas insulated transformers at Heung Yip Road (2013)
• Island Line upgrade: two replacement 50MVA gas insulated transformers at Admiralty (2015 and 2016)

Which continues to provide a highly available power supply system resilient to upstream issues.

Footnote: station power

A 11 kV ring distribution system also covers the MTR network, fed by the 33 kV network. Substations connected to the 11 kV network provide 415 V power to critical loads such as the Central Control Room at Kowloon Bay, passenger stations, and tunnel ventilation fans. Depot and workshop loads are also fed from the 11 kV distribution system

Sources

• Hong Kong mass transit railway power supply system by P. Lawton and F.J. Murphy
• Design for safety for the Hong Kong Mass Transit Railway by R. Edgley
• Harmonic Simulation of Traction System by Lai Tsz Ming Terence
• Hong Kong Subway Systems, West Island Line by Masahiro Tobita
• Tackling Power Quality problems in Railway Systems by Dr. C.T. Tse
• Power Supplies at the HK Rail Engineering Centre (Chinese language)
• Overhead Supply System at the HK Rail Engineering Centre (Chinese language)
• Tseung Kwan O Extension – smooth sailing on a voyage of discovery by Angela Tam
• Power Assets Holdings Limited Annual Report 2010