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Balanced Three-Phase Grid Supply for Traction Power
- 1. Dinesh Bansal Chief Electrical Engineer (Ret.) Indian Railways Presented by Karl Berger, P.E DCM, Inc. Joint Rail Conference 2012 Philadelphia
- 2. Indian annual GDP growth at 7% for the last decade. Transport & allied sectors growth 15% last year. IR annual revenue US$21B mainly by bulk commodities like coal, iron ore, cement and fertilizer. 64,215km route miles, 33% electrified. 4,000 electric locomotives carry 63% freight & 51% passengers. Heavier freight & longer passenger trains are needed to haul readily available business. 2
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- 4. Design goals for traction power system: ◦ Low cost, ◦ Energy efficient, ◦ Reliable Minimize the unbalance imposed on the three-phase utility grid by the single-phase traction load. Isolate load harmonics from the utility supply. 4
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- 7. Unbalance at individual substation drawing power from two phase wires can be 100% of load (15 minutes) for multiple stations. Power factor typically 0.7 lagging with rectifier locomotives. Harmonics could be 20-30% mainly 3rd, 5th and 7th, causing distortion PF and higher losses. Demand charges and low PF penalty can be 10 20% of energy bill; Harmonics can be reduced but not eliminated with LC filters. Large peak loads drive up utility demand charges. 7
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- 10. Synchronous power supply at 25kV 50Hz with paralleled feed has no need for phase breaks. Relies on proven technology used on 15kV 16.7Hz European systems and 12kV 25Hz Amtrak system. Regenerative braking power feed back to 3-phase grid possible through dc link at traction substation. Improved locomotive and Head End Power reliability through elimination of shut downs at Phase Breaks. Load spread over multiple parallel substations reduces peak demand charges. Possibility for selling reactive power to the utility. 25kV break-free parallel feed could permit 40% higher loading of existing infrastructure. Parallel feed improves operational reliability during substation outage. 10
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- 14. Load factor improvement due to parallel load sharing (double-end feed). Reduced filter requirements at TSS with dc buffer to grid. Balanced unity power factor load on grid. Reactive power to and from grid to balance grid requirement. Voltage drop and energy consumption reduced. EHV additional cost 25% for 73% increased power. 14
- 15. Additional Capital Investment in two 20MVA converters with magnetic devices at TSS may cost about Rs 75M($1.5m). May cost less for bulk supplies. Power factor surcharge: Rs 15M($.3m) per year (based on 10% of annual energy charges) may be saved annually. Contract demand charges: Rs 10M($.2m) may be saved every year by reduction in contract demand from 20MVA to 15MW, by leveling of peak demand by parallel feed from adjacent TSS and unity power factor by 4 Quadrant control of converters. Elimination of a TSS due to half Voltage drop by feeding from both ends. Saving Rs 100M ($2m) capital investment for new construction projects. Reduced energy charges due to improved load factor and balanced power drawn at each TSS, would need to be negotiated with the SEB. Estimated savings may be Rs 10M($.2m). The TSS dc links located 30miles apart may be used to stabilize the grid by meeting reactive power requirement. Grid authority/ SEB could pay for this service. Increased section capacity due to reduced locomotive failures, can not be valued now. These are estimates and actual savings may vary based on SEB tariff and load pattern in the system. A payout period of a 3-5 years may be expected. 15
- 16. 132 KV 20MVA magnetics Rs. 25M ($.5m). Power Electronics Rs.40 M ($.8m). $1.5M limit should be doable for a 2X20MVA SS. India has rich experience in magnetics; there are many national and international companies. Semiconductor manufacturers around the world would be willing to sell ready made strings of duly protected devices to make up Power Modules. Academic institutes & industry could develop, if funded, for materials and manpower. 16
- 17. THANK YOU BansalDinesh@hotmail.com Dinesh Bansal, Indian Railways Karl.Berger@dcm-va.com Karl Berger, PE, DCM, Inc. 17
Hinweis der Redaktion
- Productivity of 25kV 50Hz electric traction is high and it has been widely accepted during the last 60 years. The overhead wires are connected to 25kV phase supply while the return current is led to the substation through the rails. The feeder wire is charged at 25kV in 50kV TSS system.
- Present fleet is mainly 4500kW rectifier locomotives. 9000kW locomotives for 25t axle load bulk trains are under development to make full use of 600m (2000’) available loop length and increase throughput by over-powering.
- 4500kW 25kV Silicon Rectifier Electric locomotive haul 20-24 coaches at 65mph. OHE is inexpensive without BTRC at Ernakulam. Domestic production.
- As with any system design the primary goals are to achieve the end results with the lowest capital and operating costs. Two particular design challenges from the earliest days of electrification to the present are keep the electric utility happy by balancing the single-phase load and minimizing harmonics.
- (Simplified schematic. Switchgear not shown.) When the catenary employs the same frequency as the utility grid, single-phase transformers are used to feed line segments up to 30km in length. Since each segment is energized by a different phase a Phase Break or Neutral Section must be used to prevent bridging of one section with another during the passage of the pantograph.
- UK neutral Sections are 4m fiberglass rods. Note red highlighting and magnetic detectors in track for automatic propulsion power off. Other railroads requiring manual power off experience locomotive failures and
- Use technology developed for frequency converter stations to decouple the traction load from the utility grid. Frequency and phase of each station is synchronized by SCADA (Supervisory control and data acquisition). Phase Brake is eliminated. Simpler Sectionalizer may be used that allows bridging of circuits. In fact, the Section Break can be shunted with a normally closed switch as seen on Track 2. This helps with voltage regulation and is a major benefit of this proposal.
- Double-end feed reduces voltage drop by half in a simple catenary feeding system. Other voltage regulation techniques such as autotransformer feed are also enhanced by double-end feed. Earth return currents are also halved to improve EMC.
- Most Loco failures occur at phase breaks. Break-free supply similar to 15kV 16.7Hz in Europe and 25Hz on Amtrak has been practiced for long. 15% energy saving on coaching trains and up to 30% on stopping suburban trains may be feasible.
- Amtrak 12kV Section Break. Note arc horns.
- Connection to 3-phase grid in the rear. Semiconductor bay uses IGCT Integrated Gate CommutatedThyristors) in 3-level converters. Water cooled. Phase and anti-phase connections on front.
- 15MVA 20kV 3 phase 50Hz Input 110kV 16.7Hz single phase output schematic. Eight in service for 5 years on DB Limburg SS. Efficiency 97% and reliability 99.9%.
- Some of the utility companies do not agree to arithmetic sum of leading and laggingkVAr over a month and expensive Dynamic PF compensator equipment have to be installed to save PF penalty. Not required with Static converters.
- 132kV EHV three phase conductors could be carried on the existing standard structures at most places. Scott connected transformers in Western region and Tuned LC and other filters can be dispensed with.
- The estimated cost is more or less a guess work based on wild extrapolations; however power electronics for large converters has been in industrial use for the last 20 years for transmitting power at +- 500kV in India.