ON DYNAMIC STABILITY OF EHV AC-DC POWER SYSTEMS

Abstract

The work pertains to the study of dynamic stability of extra high voltage (EHV) AC-DC power systems. The various types of power system configurations considered are (i) Synchronous system (ii) Asynchronous AC-DC system with 3-machines (iii) Multi-machine AC-DC system (iv) Multiterminal HVDC 3system embedded in multi-machine network. A mathematical model of the power system, amenable to computer solution, is developed for first three cases in such a manner that D-partition technique can be applied for parametric stability study without actually obtaining the characteristic equation of the system and in fourth case state-space technique is used for modal control of the system. Appropriate models for long EHV AC-DC transmission lines and detailed modelling of machines inclusive of automatic voltage regulator and electrohydraulic governor are used in first three cases. For fourth case as the study is aimed at ascer taining the effectiveness of the feedback of different states for designing the stabilizing loops, it is an established practice to use a simplified model of the power system. There fore in this study simple mode<jl of machines and lines are used for the last case. Previous study have reported dynamic performance of a parallel AC-DC system using simple structures of regulators.In this study an attempt is made to improve the dynamic perfor mance of integrated AC-DC system using regulators consisting of

proportional, first and second derivative gains. In case (i) two studies are made * (a) Single machine connected to infinite bus through 50 % compensated AC line in parallel with DC line along with local loads and (b) Single machine connected to infinite bus through AC line of variable compensation in parallel with DC line along with local loads with an aim to study in detail the SSR phenomenon. In case i(a) optimization of governor regulator para meters and then that of excitation regulator parameters using D-partition technique for maximum system damping is done. These parameters of excitation regulator are then optimized further for better transient response using newly developed computer-oriented method of obtaining transient response from real frequency response characteristic of the system for a step input. The parameters corresponding to maximum damping point as obtained by D-partition technique do not necessarily correspond to best transient response and hence for best transient response, transient responses of points around this maximum damping point are obtained utilizing the newly developed method and compared. Best transient response, almost monotonic in nature, with a settling time of 1 second order has been obtained by coordinating the stabilizing (derivative) current gain parameters of excitation regulator. Sub-synchronous oscillations in power systems with series compensated lines can be amplified and sustained due to inter action between the electrical system and the turbine generator mechanical system. It has been found that an HVDC link also may have similar effect. Previous studies have reported results for parallel AC-DC system using HVDC simulator. The common way to

study Sub Synchronous Resonance (SSR) phenomenon is to use digital programs. In this way it is possible to obtain a detailed repre sentation of turbine generator shafts, the synchronous machines and other ordinary AC network components. In course of discussion of SSR study performed on ASEA'S HVDC simulator which includes models of synchronous machines with torsional dynamic shaft repre sentation, it was observed that digital computer programs can be quite accurate for SSR phenomenon and a very effective study tool. The present work concerns a study of the SSR phenomenon in a para llel EHV series compensated AC-DC transmission system using digital representation of HVDC converters along with their associated equipments. In case 1(b),the minimum essential value of K (the to speed proportional signal gain of DC current regulator) is found out for different degree of AC line compensation to avoid SSR due to torsion interactions using digital computer. In case (ii) the study of SSR phenomenon is extended to • AC-DC asynchronous system involving three machines. Herein again two cases are considered (a) With speed feedback from the rotor of synchronous machine connected at the rectifier end to the DC current controller at the rectifier end and (b) With speed feed back from the rotor of both synchronous machines connected at two ends of the AC link to DC current controller at the rectifier station in opposite direction. In each of these cases a minimum desired value of K^ is found out for different degree of compen sation of AC line to avoid torsional oscillations. Effect of variation of load and its quality in AC line is also studied.

A number of earlier work studied the dynamic stability of multi-machine AC-DC system using eigenvalue approach. The model ling considered detailed representation of the generators and DC links with their associated controllers. However SSR phenomenon study due to torsional interaction for multi-machine system incorporating an HVDC link has not been reported so far. Therefore in case (iii) of multi-machine system optimum value of K^ to give suitable first quadrant area in plane of excitation regulator stabilizing gains is found out first. With this value of K^, the excitation regulator stabilizing gains are optimized using D-partition technique for maximum damping. Thereafter torsional dynamic shaft representation is included in one of the generators and optimum value of K to avoid torsional interaction is deter mined for the case of uncompensated AC line and for the case of 50 '/. compensated AC line in parallel with DC line. All studies carried out so far in the present thesis concern point-to-point (2-terminal) DC transmission. In the case of dynamic stability of multi-terminal HVDC-AC systems results earlier reported in literature considers e contro ller which modulated the DC power or current setting of the converters using signals proportional to the speed and terminal voltages and their derivatives. These signals were obtained by a combination of output feedback and modal control so that the closed loop system had a prescribed set of eigen values for super ior transient response. The modal controller required speed and its derivative, bus-voltage and its derivative from all converter

stations. It thus required the feedback of all the states and this imposed restrictions on the measurement of all the states or on the construction of a state observer. Therefore, the approach utilized here in case (iv) is to design a propor tional-plus-derivative type controller using the system out puts,, only, but at the same time the dominant eigenvalues are shifted to the prescribed locations using modal control. In this study the effect of DC line inductance neglected previous ly is also taken into account and the method of obtaining the modal feedback control matrix is simplified. The modal control of the power orders of either rectifier or inverter or of both (rectifier and inverter) have been applied to shift the dominant eigenvalues of the system to prescribed locations. Transient responses of some of the important states of the system for the modal control of the power orders of either rectifier or inverter or of both (rectifier and inverter) have been compared. The work concludes with inferences drawn on the basis of the studies presented in the thesis.