KESS is designed to be configured to provide voltage regulation and support, reduction of peak power demands, recapture of braking energy, and short-term energy sink. Current and planned installations range from single UPT Kess 50kW units to multiple systems with a capacity of 2.4MW. The technology has been selected by a number of operators including New York City Transit (NYCT), Sytral in Lyon, France, Paris Transport Authority (RATP), and NDK, Japan, on behalf of JR East.
At the heart of the UPT Kess is a patented, high-speed composite flywheel. The modular unit stores 14MJ of energy, of which up to 11MJ can be used usefully, depending on the application. The design life of the rotor and beatings is 20 years or 10 million discharges. The flywheel and bearings are maintenance free, with annual maintenance checks required of the vacuum pump oil and cooling systems.
Kess is differentiated from many other forms of energy storage such as large battery banks, by being able to respond rapidly and regularly to cyclic loads and hence provide a power management capability. In such applications, a control algorithm calculates the required energy flow (in both direction and magnitude) based on the dc bus voltage. It contains three distinct control regions: discharge, recovery and charge. The recovery region can be used to realign the rotor speed to any pre-defined level, depending on the application. This allows the system to act as both a current source and sink, smoothing out variations in supply or load demand.
Each Kess is connected directly to a dc bus and can be operated individually or in groups. But for system flexibility and enhanced reliability, and to cater for possible future expansion, each Kess operates as a single system, independent of other units. This ensures that all power transfer decisions are made solely from the value of the dc bus voltage, which is common to all units. Load demand is automatically shared between all units, giving n+1 reliability.
A 1MW system has been installed on NYCT's Far Rockaway units, test track. The primary aim was to support track voltage, with an additional benefit of energy saving. The ten 100kW machines have been in operation for more than 10,000 hours, reinforcing the voltage of both the test track during testing of new trains being supplied to NYCT, and the adjacent revenue line during normal operation.
The voltage, without Kess support, dropped typically from a high of 687V, when a train was braking, to 625V when the train was accelerating. 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 train acceleration trials when a normal train passed the test track.
When Kess was in operation, and a test train accelerated from a standing start, the voltage never dropped below 625V. This was still the case even under the worst conditions when a revenue train passed the test track site at the same time.
In view of the recent major power cuts experienced in eastern Canada and northeastern United States, Italy, and London, a test was conducted. It involved switching off all power to the test track to see how far the UPT Kess could make the train travel on kinetic energy alone. The train reached a speed of 45km/h travelling for a period of 1.7 minutes, covering 1.2km.
Now that the system has proved its capability, there are plans to upgrade it to an optimum 2MW using 10 of UPT's latest 200kW units.
Since its inception, the Lyon metro has always regenerated energy during braking with accelerating trains absorbing the energy sent back to the line by braking trains. If there are insufficient trains available to absorb the energy, over-voltage occurs, which can cause wear and damage to the equipment on board the rolling stock. This issue of excess regenerated energy is greatest on the rack Line C, which has a gradient of 18% on a 500m section. Trains re-inject braking energy during their descent at Hotel de Ville station.
In this instance, unlike Far Rockaway, which was set up to give both voltage support and a limited level of energy recovery, the machines were configured to give the maximum level of energy recovery. UPT says the system installed at Hotel de Ville substation is working well and will be extended to a second phase installation at Sans Souci, on Line D.
Installation has commenced of four 200kW UPT Kess units at the Fort d'Aubervilliers substation on Line 7 of the Paris metro. These will provide voltage support to the line while equipment is replaced in an adjacent substation.
Undertaking routine maintenance and necessary repair of substations and related equipment has become increasingly difficult on congested metros. Increased ridership and demands for shorter headways make it very difficult to isolate substations whilst providing adequate voltage support and efficient operation of the system. It is a growing problem; a traditional substation replacement is not a viable option because of time and cost
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