Colin Tarrant, Technical Director, Urenco Power Technologies Ltd
FLYWHEEL energy storage is an old concept, but it is only in recent years that technology used by the nuclear industry in high-speed gas centrifuges has been adapted to produce a commercially available high-speed flywheel suitable for rail applications.
RATP in Paris is the latest operator to use a Kinetic Energy Storage System (KESS) supplied by Urenco Power Technologies. Located at Fort d'Aubervilliers substation on Line 7, it will be used to support the line voltage while equipment in the substation is renewed.
A series of trials by London Underground took place in 2000-01 on a 2·8 km test track between South Ealing and Northfields. Three 100 kW KESS units connected in parallel were installed at Northfields substation, and three 1996 Jubilee Line cars with regenerative braking were used for the trials. The three KESS units could absorb braking energy and reduce power requirements during acceleration (RG 4.01 p241). The trials also proved that KESS performance could be accurately modelled.
KESS stores kinetic energy in a rotor consisting of a carbon-glass composite cylinder packed with neodymium-iron-boron powder that forms a permanent magnet (Fig 1). This spins at up to 37800rev/min, and a 100 kW KESS can store 11 MJ of re-usable energy. With a conventional IGBT voltage source inverter between the DC busbars and the AC motor/generator, a conversion efficiency of 95% between electrical and kinetic energy can be achieved in either direction.
Subsequently, a 200 kW KESS module has been developed with a capacity well suited to the requirements of mass transit systems. Greater capacities up to 2·4MW (so far) are created by connecting several modules in parallel.
When configured with the appropriate traction power interface and control logic, KESS forms the integrated UPT Trackside Energy Management System (TEMS), which has achieved commercial success in North America and Europe. Bespoke TEMS have also been developed for an experimental trolleybus route in Hong Kong, and to provided uninterrupted power at stations in Japan.
Unlike other energy storage systems such as super-capacitors, TEMS can assist traction power engineers in a number of ways by providing:
- voltage regulation and support;
- a power upgrade for new trains;
- reduction of peak power demand;
- a constant energy sink;
- efficient capture of braking energy.
- temporary power during maintenance or replacement of equipment.
KESS can operate effectively under repeated cycling conditions without any deterioration in its performance over a design life exceeding 20 years. It is compact, and can be easily installed in an existing building or portable enclosure. Current and planned installations range from single 50 kW units through to multiples totaling 2·4MW.
NYCT sought voltage stability
New York City Transit was faced with two problems as it introduced AC-motored cars with regenerative braking capability: the need to reduce power demand peaks, and to recover a higher proportion of braking energy.
NYCT's Maintenance of Way Department created the Far Rockaway test track in 2001 to undertake acceleration and braking tests on new cars before putting them into revenue service. Candidate technologies for ironing out voltage fluctuations included batteries, super-capacitors and flywheel energy storage. The UPT TEMS was preferred because of its frequent cycling capability. Following successful installation, extensive trials met all NYCT's objectives and demonstrated the functionality and compatibility of KESS technology.
A 1MW TEMS with ten 100 kW KESS units has been operational since January 2002, working consistently and reliably in a real-time environment. It reinforces the voltage of the test track during testing of the new trains as well as providing voltage support to the adjacent line carrying trains in revenue service.
Far Rockaway test results
Voltage regulation at the Far Rockaway test track was poor. Even with the no-load voltage set at 680V, the voltage dropped below 600V during acceleration trials when a revenue train was passing the test track. With KESS operational, and a test train accelerating from a standing start, the voltage never dropped below 625V even with a revenue train passing at the same time.
One interesting test that has become more pertinent since the US Northeast, including New York, suffered a widespread power failure last year, involved switching off power to the test track to see how far KESS could propel a train.
From a standing start, the train reached 45 km/h and travelled 1·2 km in just over 100sec. Fig 2 shows that the power drawn peaked at 850 kW before declining to 200 kW, at which point the KESS disconnected. Now that the system has proved its capability, there are plans to upgrade it to an optimum 2MW using 10 of the latest UPT tr200 KESS modules.
London suffered a major power failure on August 28 2003 which saw much of the Underground closed for several hours. Much of Italy was similarly afflicted around the same period. With KESS in place, it should be possible to move stalled trains into a station for evacuation should such circumstances recur.
Lyon seeks energy sink
Since its inception, Lyon's metro has regenerated energy with accelerating trains absorbing the energy sent back to the line by braking trains. If there are insufficient trains available, mechanical braking has to be used, which causes wear to the brake shoes, resulting in a heavy maintenance burden.
Excess regenerated energy is greatest on the rack-and-pinion Line C, where trains inject braking energy during their descent to Hotel de Ville station on a gradient of 18% for some 500m. In this instance a 600 kW TEMS was configured to give maximum energy recovery. Installed at the H“tel de Ville substation, it is working well. Metro operator SLTC has decided to install a second 600 kW TEMS at Sans Souci on Line D.
Maintenance support
In Paris, the 800 kW TEMS at Fort d'Aubervilliers on Line 7 has already been mentioned. Undertaking routine maintenance of substations and related equipment has become increasingly difficult on congested metros. Greater ridership and shorter headways make it very difficult to isolate substations while continuing to provide adequate voltage support and efficient operation.
Faced with this dilemma, RATP chose KESS technology because of its ability to provide the required voltage support and its compact size. The four tr200 modules will be installed alongside existing equipment and will continue to provide supplementary voltage support when the refurbishment project is completed. The availability required by the contract for this installation is 99·8%.
RATP's decision to use KESS was based on a review of alternative technologies and evaluation of the successful 1MW installation for NYCT.
Uninterrupted power
Each 100 kW KESS has a maximum useable energy content of 3 kWh, and can be configured to give a range of power outputs of various durations. This is the feature that attracted NDK (which supplies power conditioning equipment to JR Group operators in Japan).
JR operators have diesel generators of varying types and ages which provide emergency power to escalators and ticket gates at stations. To ensure that there is sufficient time for more than one attempt to cold-start the engines, JR requires 50 kW for 3 min.
NDK initially undertook a nine-month technical evaluation of KESS prior to embarking on a phased deployment across the network in 2004.