A New Automatic Load Control for Turbine Generators Η. B. R U U D S. B. F A R N H A M

M E M B E R A I E E

IN AN interconnected power system, load changes sufficient to affect system frequency are shared by all

the generators on the system in proportion to their ratings and their governor characteristics. While this is desirable in most instances, it may present a problem where large fluctuating loads are remote from major sources of genera­tion. Such a problem exists in the case of a new aluminum rolling mill near Davenport, Iowa, whose electrical re­quirements are supplied by the Iowa-Illinois Gas and Electric Company. The mill load swings are expected to vary between 5,000 and 8,000 kw above and below the mean load (16,000 kw maximum total swing) and vary at a rate of between 2,000 and 5,000 kw per second.

In order to confine these swings to the local generators, rather than letting them flow over the 161-kv transmission line which links the Iowa-Illinois Gas and Electric Com­pany system with other utilities in Iowa and Missouri, a new type of automatic load-regulating equipment is being installed on the turbine-generator sets at the River­side Station, which is adjacent to the aluminum mill. This equipment will cause the controlled machines to vary their power output rapidly in response to the demands of the mill, and in anticipation of any change in system frequency. Thus, the remote machines on the inter­connected system will be unaware of the mill load changes, and this fluctuating load will be confined locally to the controlled machines. In operation, the load control func­tion can be assigned to any one of four machines in the Riverside Station, or divided among them in any desired proportion. While a controlled machine can also carry system base load, sufficient margin must be left to permit its picking up its assigned share of the mill load swings as they occur. This in general will militate toward the use of the older, less efficient machines for load regulation, leaving the newer machines for base-load operation.

The controlling quantity is derived from instrument

RIVERSIOE 13,800 VOLT GENERATOR BUS

OC FIELO TURBINE GENERATOR

SYNCHRONIZING MOTOR

FEEDERS TO ALCOA MILL

F i g u r e 1. Simplif ied d iagram of automatic control c ircuit

transformers in the mill feeders, and is introduced into a. bridge circuit which can be balanced at any predetermined value of generator base load. Variations in the mill load,, by upsetting the balance of the bridge, produce a voltage-at the terminals of a continuously driven amplidyne generator. This voltage operates the main generator synchronizing motor, which is somewhat oversized to-afford the necessary high speed of response, and thereby causes more or less steam to be admitted to the turbine a& is required to handle the changed load. Balance of the bridge is re-established at the new load level through a follow-up system which introduces into the bridge circuit a voltage proportional to the turbine valve position, and hence to kilowatt load. Refinements in the circuit permit operation of the synchronizing motor at conventional speed as required for manual synchronizing and for operation in conjunction with the usual tie-line bias control equipment.

It is to be noted in Figure 1 that the reference voltages and the follow-up voltages are supplied from a common source. Not only must these voltages be in phase with one another, but also they must be derived from the same phase in which the current transformers for the Alcoa load are connected.

Other somewhat similar load control arrangements: have had the common characteristic that when the auto­matic load-regulating feature was in service, the speed governor was rendered inoperative, except that on over-speed it could take precedence and shut down the machine. Conversely, when the machine was under the control of its speed governor, the load-control feature was necessarily disconnected. Never before has it been possible to operate a machine under both types of control simultaneously. Such a limitation has not been detrimental in past installa­tions since they concerned relatively small machines serving industrial power systems which were interconnected with larger utility systems. In these instances, it was not ex­pected that the industries' generators would help support the frequency of the interconnected systems.

The requirements at Riverside, however, in common, with any similar station on a utility system, are such that in the event of an emergency each machine must be able to contribute its full capabilities toward maintaining: system frequency. That is, its speed governor must at alL times be fully operative to help support system frequency. The new load control equipment is believed to be unique in its ability to do this while simultaneously performing-its special function of confining the aluminum mill load swings to the local generation.

Digest of paper 49-289, "A New Automatic Load Control for Turbine Generators,'*' recommended by the AIEE Power Generation Committee and approved by the AIEE Technical Program Committee for presentation at the AIEE Fall General Meetings Cincinnati, Ohio, October 17-21, 1949. Scheduled for publication in AIEE Trans­actions, volume 68, 1949.

Η. B. Ruud is with the Iowa-Illinois Gas and Electric Company, Rock Island, 111.;. S. B. Farnham is with the General Electric Company, Schenectady, Ν. Y.

396 Ruud, Farnham—New Automatic Load Control for Turbine Generators ELECTRICAL E N G I N E E R I N G