Methods of Keeping Down Peaks on Power Purchased on a Peak Basis

The author mentions two general ways of reducing peaks, the first being to furnish the peak power from a separate source such as a local steam turbine, and the second to store some of the power furnished by the power company, which is given up later when the peak demand comes on. In the case of the plant described by the author a mixed pressure turbine was installed to take the peaks, this being selected on account of available exhaust steam from pumps, air compressors, etc. The turbine is direct-connected to two direct-current generators mounted on a common bed-plate. The first plan tried was to use a motor-driven rheostat operated with clutches which in turn were operated by solenoids energized by current controlled by a contact-making ammeter. After the installation of a maximum demand meter which integrates the one-minute peaks, the rheostat control was found to be too slow and a special peak-taking device was then installed. This device consists of a rod carrying points which successively make contact with mercury cups and which is operated by the moving element of the graphic meter. These mercury contacts close relay circuits which operate to short-circuit sections of resistance in the field rheostat of the turbo-generator, raising its voltage, and also cut in sections of resistance in the field rheostat of the motor-generator set, weakening its field and thus reducing its load.