1. Field of the Invention
This invention relates to systems for transmitting power between interconnected, but otherwise independent, power generating networks and particularly to an asynchronous tie between such power generating networks which utilizes concatenated brushless wound rotor machines.
2. Description of the Prior Art
Electrical power is transmitted over great distances from one power system to another by means of high voltage transmission lines. The interconnection of large independent power systems has made possible the efficient allocation of available power over wide areas. For stability reasons, such interconnected systems are synchronized in time and frequency of generation and thus are said to be "synchronously tied."
Power flow in synchronous to synchronous transmission systems is determined by total system adjustments, including load frequency control, which accomodate a surplus or a deficiency of interconnection generation with respect to the load to be supplied. These control systems have generally proven satisfactory for use with systems having large power reserves; however, there has been continuing and rapid growth of electrical loads which has cut power reserve margins to a critical value. Furthermore, the synchronously tied system with load frequency control is inherently unstable since it will allow an individual system to contribute indefinitely to an external disturbance. Thus, there is an increasing risk that the synchronously interconnected system, operating with marginal reserves, will become unstable upon the occurrence of a remote load disturbance and possibly cause a major power failure.
Certain alternatives to the synchronous tie method of interconnecting independent power systems have been considered over the years. There has been some interest in a high voltage DC transmission system which has the advantage of avoiding synchronous to synchronous stability limitations since such a system inherently provides a non-synchronous tie. While some proposals have been made in this direction, progress in this direction has been slow because of the practical difficulty in generating the high voltage DC levels required and the added expense of converting the DC so that it may be utilized by equipment designed for a 60 Hz AC power supply.
An asynchronous tie which is compatible with existing AC transmission and distribution systems has been disclosed by Alexanderson U.S. Pat. No. 2,213,945. According to the teachings of Alexanderson, asynchronous power transfer is achieved in an induction generator which is driven by a prime mover at a range of speeds extending below and above synchronous operation. As is well known, high slip operation is encountered when the prime mover shaft speed varies, and it is accompanied by corresponding high rotor losses and thus low conversion efficiency. The complex control circuitry which has been suggested to permit high efficiency operation regardless of slip results in a relatively low power factor. A further limitation of such prior art schemes has been the requirement for slip rings and brushes to permit connection to the rotor windings. Such a connection is not well suited for large machines running at high speeds and drawing large currents.
Because of the disadvantage of the prior art as described above, an improved system tie was sought which would be compatible with existing power systems and transmission lines, which would control the power flow between the systems even under transient load conditions, and which would improve the operating stability of the interconnected systems. The present invention permits the efficient use of concatenated brushless wound rotor induction machines to satisfy these requirements.
By way or addition background, reference is made to the fact that the prior art shows some combinations of apparatus that include concatenated brushless wound rotor induction machines. For example, see B. H. Smith, "Synchronous Behavior of Doubly Fed Twin Stator Induction Machine," IEEE Transactions on Power Apparatus and Systems, Volume PAS-86, No. 10, October 1967, pages 1237-2136, in which an induction machine is fitted with two independent sets of stator windings and two interconnected rotor windings is synchronously excited from two systems to permit variable speed operation. It is additionally acknowledged that cycloconverters have been used to generate low frequency AC for variable speed motor drives, for example as disclosed in B. R. Pelly, "Thyristor Phase-Controlled Converters And Cycloconverters," New York, Wyley-Interscience, 1971, pages 18 et seq.