FREQUENCY CONTROL STUDIES IN AC-DC SYSTEMS

Abstract

Isolated alternating current power systems may be interconnected for energy exchange by either alternating current or direct current lines. A dc tie line is asynchronous in that the power transfer over it is not inherently dependent upon the relationship between the frequencies at the two ends of the line. Auxiliary control is required to control the power flow over a dc line. This control can be made to initiate required power flow charges over the line in response t a number of system operating conditions including tie line power, tie line current, receiving and/or sending end frequency. The purpose of this thesis is to illustrate the application of some analytical techniques which can be used to study the small signal control characteristics of ac systems interconnected by a dc line, the power level of which is controlled as a function of sending and receiving end frequency deviations. The required analytical models are given for the sending and receiving end ac systems. The analytical model for the dc line and its controller are derived. These models are then expressed in transfer function and state variable form. The transfer function form is used in the analogue simulation of the system, and the state variable form is used for digital solution of the system. The results of stability and sensitivity analyses carried out on state variable models are given to illustrate the effects of the various adjustable control parameters in the composite ac-dc system. Analogue computer results are given also to illustrate the actual time responses of this system. The results obtained from the analytical computation and the sensitivity analyses are correlated. In general it was found that the dc system could be modeled using fairly conventional analytical techniques. It is felt that the preliminary results obtained with this model are realistic. Where possible the numerical constants chosen for the study for illustrative purposes were based on teh proposed Nelson River Project in the Manitoba Hydro Power System.