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A hybrid energy system for propelling an off-highway vehicle includes an engine rated at a first power capacity, and a power converter driven by the engine for providing primary electric power. A traction motor system receives the primary electric power, the traction motor system propelling the off-

A hybrid energy system for propelling an off-highway vehicle includes an engine rated at a first power capacity, and a power converter driven by the engine for providing primary electric power. A traction motor system receives the primary electric power, the traction motor system propelling the off-highway vehicle in response to the primary electric power, and the traction motor system further including a dynamic braking mode of operation. An energy storage medium captures electrical energy generated by the traction motor system in the dynamic braking mode of operation, the energy storage medium transferring a portion of the captured energy to the traction motor system to augment the primary electric power, wherein the traction motor system is rated at a second power capacity exceeding the first power capacity.

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The invention claimed is: 1. A hybrid energy system for propelling an off-highway vehicle, comprising: an engine, said engine rated at a first power capacity, the engine not being mechanically coupleable to drive wheels of the vehicle; a power converter configured to be driven by said engine for p

The invention claimed is: 1. A hybrid energy system for propelling an off-highway vehicle, comprising: an engine, said engine rated at a first power capacity, the engine not being mechanically coupleable to drive wheels of the vehicle; a power converter configured to be driven by said engine for providing primary electric power; a traction motor system configured for receiving said primary electric power and for propelling the off-highway vehicle in response to said primary electric power, and said traction motor system further including a dynamic braking mode of operation; and an energy storage medium configured for capturing electrical energy generated by said traction motor system in said dynamic braking mode of operation, said energy storage medium transferring a portion of said captured energy to said traction motor system to augment said primary electric power; wherein said traction motor system is rated at a second power capacity exceeding said first power capacity. 2. The hybrid energy system of claim 1, wherein said second power capacity corresponds to a load carrying capacity of the off-highway vehicle. 3. The hybrid energy system of claim 2, wherein said power converter is also rated at said second power capacity. 4. The hybrid energy system of claim 3, wherein said second power capacity has a first value in a motoring mode of operation and a second value in said dynamic braking mode of operation. 5. The hybrid energy system of claim 4, wherein said energy storage medium is rated at a third capacity, said third capacity being sufficient to make up the difference between said first power capacity and said first value of said second power capacity. 6. The hybrid energy system of claim 4, further comprising a dynamic braking grid system configured for dissipating electrical energy generated by said traction motor system not captured by said energy storage medium, said dynamic braking system further being rated at a lower power dissipation capacity with respect to said second value of said second power capacity. 7. The hybrid energy system of claim 6, wherein said dynamic braking grid system with lower power dissipation capacity is configured to dissipate energy generated by said traction motor system in excess of said third capacity of said energy storage medium. 8. A method for operating a hybrid energy system configured for propelling an off-highway vehicle, the method comprising: receiving a traction input command; determining whether a resulting power demand on a traction motor system of the off-highway vehicle exceeds a first power capacity of an engine included in the hybrid energy system, said engine supplying primary electric power to said traction motor system, the engine not being mechanically coupleable to drive wheels of the vehicle; and supplementing said primary electric power with captured electrical energy stored in an energy storage medium whenever said power demand exceeds said first power capacity. 9. The method of claim 8, wherein said captured electrical energy stored in said energy storage medium is generated by at least one of: said traction motor system during a dynamic braking mode of operation and said engine. 10. The method of claim 9, wherein said traction motor system is rated at a second power capacity exceeding said first power capacity, said second power capacity corresponding to a load carrying capacity of the off-highway vehicle. 11. The method of claim 10, wherein said energy storage medium is rated at a third capacity, said third capacity being sufficient to make up the difference between said first power capacity and said second power capacity. 12. The method of claim 9, further comprising: determining whether dynamic braking energy generated during said dynamic braking mode of operation exceeds an energy dissipation capacity of a dynamic braking grid system included within the hybrid energy system; and storing dynamic braking energy in excess of said energy dissipation capacity of said dynamic braking grid system within said energy storage medium. 13. The method of claim 12, wherein said energy dissipation capacity of said dynamic braking grid system is selected to be less than an energy generation capacity of said traction motor system during said dynamic braking mode of operation. 14. A hybrid energy system for propelling a vehicle powered by both an external electrical power source and an on-board engine, comprising: an external electrical power source; a connector configured for coupling the external electric power to a traction motor system; said traction motor system configured for propelling the vehicle in response to said primary electric power, and said traction motor system further including a dynamic braking mode of operation; an engine on-board the vehicle, said engine rated at a first power capacity, the engine not being mechanically coupleable to drive wheels of the vehicle; a power converter configured to be driven by said engine, said power converter for providing secondary electric power to said traction motor system; and an energy storage medium configured for capturing electrical energy, said energy storage medium further configured for transferring a portion of said captured energy to said traction motor system to augment one of said primary electric power and said secondary electric power; wherein said traction motor system is rated at a second power capacity exceeding said first power capacity. 15. The hybrid energy system of claim 14, wherein said captured electrical energy is generated by at least one of: said traction motor system in said dynamic braking mode of operation, said engine, and said trolley line. 16. The hybrid energy system of claim 15, wherein said second power capacity corresponds to a load carrying capacity of the trolley connected vehicle. 17. The hybrid energy system of claim 16, wherein said power converter is also rated at said second power capacity. 18. The hybrid energy system of claim 17, wherein said second power capacity has a first value in a motoring mode of operation and a second value in said dynamic braking mode of operation. 19. The hybrid energy system of claim 18, further comprising a dynamic braking grid system configured for dissipating electrical energy generated by said traction motor system not captured by said energy storage medium, said dynamic braking system further being rated at a lower power dissipation capacity with respect to said second value of said second power capacity. 20. The hybrid energy system of claim 17, wherein said dynamic braking grid system with said lower power dissipation capacity is configured to dissipate energy generated by said traction motor system in excess of said third capacity of said energy storage medium. 21. The hybrid energy system of claim 17, wherein said energy storage medium is rated at a third capacity, said third capacity being sufficient to make up the difference between said first power capacity and said first value of said second power capacity. 22. A method for operating a hybrid energy system configured for propelling a trolley connected, off-highway vehicle, the method comprising: determining the availability of a trolley line for providing primary electric power to a traction motor system of the trolley connected vehicle; determining whether a power demand on said traction motor system exceeds the capacity of an energy storage medium of the system; activating an engine for providing secondary electric power to said traction motor system if said primary electric power is not available and said power demand on said traction motor system exceeds the capacity of said energy storage medium, the engine not being mechanically coupleable to drive wheels of the vehicle; determining whether a power demand on said traction motor system exceeds the capacity of primary power available from said trolley line if said engine is not activated; determining whether said power demand on said traction motor system exceeds a first power capacity of said engine if said engine is activated to supply secondary electric power to said traction motor system; and supplementing said primary or said secondary electric power with captured electrical energy stored in said energy storage medium. 23. The method of claim 22, further comprising utilizing said energy storage medium for supplying tractive power if said trolley line is unavailable and said power demand on said traction motor system does not exceed the capacity of said energy storage medium. 24. The method of claim 22, wherein said captured electrical energy is generated by at least one of: said traction motor system in a dynamic braking mode of operation, said engine, and said trolley line. 25. The method of claim 22, wherein said traction motor system is rated at a second power capacity exceeding said first power capacity, said second power capacity corresponding to a load carrying capacity of the off-highway vehicle. 26. The method of claim 25, wherein said energy storage medium is rated at a third capacity, said third capacity being sufficient to make up the difference between said first power capacity and said second power capacity. 27. The method of claim 22, further comprising: storing dynamic braking energy within said energy storage medium up to said capacity thereof; determining the availability of said trolley line to receive dynamic braking energy, in excess of said storage medium capacity, generated during a dynamic braking mode of operation; directing said excess dynamic braking energy onto said trolley line to the extent said trolley line is capable of receiving said excess dynamic braking energy; and dissipating dynamic braking energy in excess of said trolley line capability within a dynamic braking grid system. 28. The method of claim 22, further comprising: determining whether dynamic braking energy generated during a dynamic braking mode of operation exceeds an energy storage capacity of a energy storage medium included within the hybrid energy system; and dissipating dynamic braking energy in excess of said energy storage medium capacity of said hybrid energy system within a dynamic braking grid system. 29. The method of claim 28, wherein said energy dissipation capacity of said dynamic braking grid system is selected to be less than an energy generation capacity of said traction motor system during said dynamic braking mode of operation. 30. An energy system for propelling an off-highway vehicle, comprising: an engine configured to generate electrical power, the engine not being mechanically coupleable to drive wheels of the vehicle; an energy storage medium configured for receiving electrical power from the engine and providing primary electric power; and a traction motor system configured for receiving said primary electric power and for propelling the off-highway vehicle in response to said primary electric power, and said traction motor system further including a dynamic braking mode of operation; wherein said energy storage medium is further configured for capturing electrical energy generated by said traction motor system in said dynamic braking mode of operation.