IEEE Transactions on Power Apparatus and Systems, Vol. PAS-104, No. 7, July 1985 ENHANCEMENT OF THE TRANSIENT STABILITY OF INTEGRATED AC/DC SYSTEMS USING ACTIVE AND REACTIVE POWER MODULATION G. D. Galanos Power Systems Laboratory Electrical Engineering Department School of Engineering University of Patras Patras, Greece Abstract - Active power modulation has been successfully used to improve the dynamic performance of interconnected AC/DC systems. This, however, results in uncoordinated reactive power modulation which, in the case of weak systems, reduces the effectiveness of the method. This paper presents the basic design of a DC link controller which, in the presence of major disturbances in the AC system, modulates the active and reactive power of the convertors and improves the transient stability of the integrated, system. The proposed control. method was tested using dynamic simulation techniques and the results obtained are presented in the paper. These results show that the application of -the me,thod, under certain conditions, leads to signif icant improvement of the transient performance of the interconnected AC/DC system. INTRODUCTION Most- of the existing HVDC transmission systems have been, decided on.for purely technical and economic reasons and fall into one of three main groups: 1) Transmission of energy over long distances 2) Inter- connection of systems where a long cable circuit is necessary 3) Coupling between systems of different frequency or bulk transfer without increasing the short-circuit duty of the circuit breakers. .A very promising application of DC transmission is the improvement of the transient stability of the interconnected systems utilizing the fast electronic power flow control capability of the dc links [4] [5] [6J [7]-. Active power modulation can be implemented with fairly low cost control systems since no communication or synchronisation between the convertor controllers is necessary [2]. The .effectiveness of active power modulation, however, is limited by the consequent modulation, of the reactive power absorption of the convertors and the resulting changes of the ac voltages, [2]. These AC voltage changes are also responsible for the poor coupling between the DC link and the swinging generators, especially in the presence of voltage.sensitive loads [2]. A better, performance is obtained by combining active power modulation with constant reactive power control [7] [8]. To take full advantage of the inherent capabili- ties of the DC transmission for this particular appli- 84 SM 614-4 A paper recommended and approved by the IEEE Power System Engineering Committee of the IEEE Power Engineering Society for presentation at the IEEE/PES 1984 Summer Meeting, Seattle, Washington, July 15 - 20, 1984. Manuscript submit- ted February 3, 1984; made available for printing June 6, 1984. cation, both active and' reactive power must be modulated 1 2] [2]. This obviously leads to a more complex control system since communication between the converter controllers is necessary. In the following sections, a control system that modulates both the active. and reactive pQwer of the convertors, in order to improve the transient stability of the interconnected system, is developed.. The basic principles of the AC/DC transient stability program developed for the evaluation of the proposed method are discussed in the paper. The dynamic simulation results presented show the transient performance of an integrated AC/DC system subjected to a major disturbance, for the cases of- the CC-CEA control method, active power modulation only and the proposed control method. Comparison of these results shows that active and reactive power modulation imnproves the transient performance of the system studied. THE CONTROL SYSTEM The structure of the system The basic structure of the control system is shown in figure 1. The modulation controller monitors the Fig. 1. The structure of the control system state of the AC system, processes the information and produces the command for the change of the .active and reactive power of' the convertor, by hPc and AQc respectively. The DC link. controller monitors the state of the convertor, processes the inputs Oc and AQc and generates the changes of the DC current 'Id and the back voltages, of the convertors AVdl. and AVd2. The outputs of the DC link controller are fed to the convertor controllers which, in turn, generate the changes of the firing angles Aal and Aa2. With this arrangement, all the information required and the decision about the next state of the link are taken locally at the convertor bus of the system that suffered the disturbance and the only communication required is the transmission of the calculated changes of the DC current and back voltages to the convertor controller at the other end. Thus, the 0018-9510/85/0007-1696$01.00X1985 IEEE 1696 N. A. Vovos