1. Field of the Invention
This invention relates to a system of direct current power supplies for an electrical transit system employing electrically powered vehicles for transportation and, more particularly, to a system of power supplies for an electrical transit system which is capable of handling normal demand, surge demand, and continuous peak demand power requirements.
2. Description of Prior Art
An electrical transit power supply system is generally comprised of several interconnected direct current power supplies which are dispersed throughout the service area. A network of either rails or overhead cables is energized by the dispersed power supplies and the rails or cables are used as feeding lines to transfer the power to electrically energized vehicles. A system for supplying power to the feeding lines must overcome several problems. First, due to the distances involved, consideration must be given to overcoming the large voltage drops and losses incurred due to the resistance of the feeding lines. Generally, the mid-distance point between the two farthest apart power supplies is the point of highest impedance. Therefore, the highest loss and voltage drop occurs at this mid-point and effectively dictates the design of the entire system. Second, the system has to be responsive to three distinct power requirements of different characteristic, namely: normal demand, surge demand, and continuous peak demand.
Normal demand is the normal load created by a fixed number of vehicles during normal operation after start-up and on a substantially level grade. It is generally of a fairly constant amperage. Surge demand is created each time a vehicle starts its motion from rest and develops momentum. Such surge demand may generally last for several seconds or tens of seconds during which the system's voltage drops dramatically, especially at locations remote from the primary power supplies. A similar load and surge demand will be developed when a vehicle negotiates an up grade. Continuous peak demand is associated with transportation systems in which additional vehicles are added to the system during "rush hour." Such continuous peak demand may last up to several hours at a time. Since either of the above mentioned demands may occur at any point along the transit system where a vehicle is located, the power supply system must be able to deliver the required power at any point along the transit system. These factors dictate that the power supplies themselves, both in size and power output, and the distribution conductors be as much as four times the size that would be required for meeting a normal demand exclusively.
In order to minimize the losses associated with the resistance of the cables or rails, separate direct current power supplies are usually spaced apart throughout the length of the system and at a distance typically no more than two miles apart. In addition, in order to minimize the losses and voltage drops which may occur in the conductors extending between the power supplies, some systems utilize large conductors in an overhead catenary configuration or a "third" power rail. In situations where the size of the primary feeding line is limited by aesthetic or other practical considerations, a "parallel feeding line" is employed.
The parallel feeding line is typically buried in the earth and is a large diameter line of much lower resistance than the primary feeding line. Consequently, a large amount of power may be transmitted by the parallel line without significant loss. This parallel line is connected to the primary line every block or so, thereby reducing the need for power being fed through the primary line. The installation of a parallel feeding line generally involves the largest amount of civil work, disruption, and the longest time element. It entails digging of a large trench for the entire length of the transit system along which the parallel feeding line is buried. Another problem encountered with transit power supply systems designed according to the prior art is the power supplies interference with other users of the utility line. The surge demands and continuous peak demands significantly strain the capability of a utility power line. Such demands interfere with other users, which problem intensifies in a densely populated area where, in order to avoid power disturbances to residents, a dedicated utility line may be needed. Furthermore, because the transit power systems of the prior art satisfy the severe demand by drawing on the utility power, utility demand charges become a material part of the operational cost of the transit systems. Large, high capability generative devices are typically required in the prior art power supply systems.