Coördinated Operation of Hydro and Steam Plants: Some Operating and Economic Features That Should Receive Careful Attention
Authors: Allner F. A. 
Published: January 2024

Due to the low generating costs of modern steam plants the margin of saving offered by a hydro development is frequently quite small. The utmost utilization of the combined hydro-steam source of power has become of greater economic importance in recent years. The economic service of hydro power is usually expressed as replacement value of the equivalent amount of steam power. This comprises two different elements: one, the steam investment cost for the hydro demand service; the other, the steam generating costs for the hydro energy delivered to the load system.

A hydro development that is not equipped with seasonal storage can render the greatest demand service to a given system load if, on the days of greatest steam demand on the system coinciding with minimum flow, the hydro plant supplies the peaks and the steam plant the base of the system load. The greatest energy contribution from the hydro plant is obtained by letting it carry the base portion of the system load during high flow and having the peak portion produced by steam.

General and specific methods are explained by which these economic principles are put into practice. Particulars are given of a system of load signaling by means of small changes in frequency above or below normal which has been found very useful for a more perfect distribution of load during certain hours when it is otherwise difficult to secure the desired division of generation.

Hydro-steam parallel operation may produce a favorable operating combination as these two sources of power can supplement each other to mutual advantage in a number of desirable service features. Hydro units are generally more reliable than steam units. They can be brought up to full load from standstill in a small fraction of the time that is required for safe operation of steam units. Troubles at hydro stations usually give sufficient forewarning to permit calling in steam assistance to make up for threatened capacity reductions. Inherent limitations which prevent too short a governor traversing time on hydro units are explained, with oscillograms of “load on” and “load off” tests recording speed, gate travel, and pressure in wheel pit. Steam units give better frequency regulation for sudden changes in load. Thus a load system that is served by mixed hydro-steam supply can obtain during a great part of the time certain service advantages of both sources of power without being subjected to the imperfections of either. The hydro-steam combination may at times offer an opportunity of better localizing the disturbing effect of city distribution trouble and of securing a more positive selective relay action than if power were supplied only from one source.

Particulars are given of a proposed ideal hydro-steam parallel operation where the steam station will be located immediately adjacent to the hydro plant. The generating units of both plants will be operated from the same control board. Circulating water for the condensers will be drawn from and returned to the forebay of the hydro plant. During frazil-ice attacks this arrangement is expected to minimize capacity reductions at the hydro plant. There is a natural seasonal diversity in the time of overhauling work at the two types of plants. It will be possible to keep the same skilled labor force employed more steadily throughout the year and to use the same shop facilities for the repair work of both plants.