SELF-EXCITED OSCILLATIONS OF SYNCHRONOUS MACHINES CONNECTED TO SERIES-COMPENSATED TRANSMISSION SYSTEMS

Abstract

The work pertains to the problem of self-excited oscillations of conventional as well as the divided-winding - rotor (d.w.r.) synchronous generators connected to seriescompensated transmission systems. Studies have also been made for the multi-machine system. Synchronous generators feeding power through seriescompensated transmission lines, under certain conditions, are found to exhibit self-excited oscillations. The loss of stability may be due to either the sustained low-frequency self-excited oscillations that can occur when the ratio of line resistance to reactance is higher than a particular value or the high-frequency self-excited oscillations likely to occur when the circuit resistance falls below a certain value as it could be experienced in a seriescompensated system using bundled conductors. The instabi lity can also occur as a result of the bilateral coupling of the shaft torsional modes with the electrical resonance modes, at frequencies close to that of the torsional mode. The consideration of torsional dynamics of the turbinegenerator system results in an additional region of insta bility at frequencies close to that of torsional mode, on the stability boundary separating the zone of highfrequency self-excited oscillations, plotted in the X -R plane. The subject of self-excited oscillations has

received great attention rfter the incident of severe generator damage at Mohave in 1970. Most of the previous studies reported in literature have considered a series R-L-C representation of the transmission line. Although some work pertaining to the phenomenon in regulated system has already been reported in literature, the effect of different types of excitation controllers, and governor on the self-excited oscillations needs a further examinat ion. The present work takes into consideration the dis tributed nature of the series inductance and shunt capaci tance of the transmission line. Some of the results so obtained have been compared with those obtained by using R-L-C representation of the line. Sine, the characteristic equation of the system with distributed parameters is transcendental, the methods like the Routh's criterion etc. can not be applied for stability analysis. The analysis can, however, be done by the Frequency-Scanning method which is applicable to both the lumped, and distri buted parameter systems. An alternative approach has been suggested in this work, to interpret the frequency traject ories of different shapes obtained by the Frequency- Scanning method so that the decision about the stability of the system can be taken unambiguously. Further, the stability analysis in this work has been done by the D-decomposition method which has been found to be

comparatively less laborious. Point check for stability in the possible stable zone has, however, been done by the Frequency-Scanning method. The phenomenon pertaining to conventional syn chronous generators connected to series-compensated trans mission system has been initially studied by assuming the macnine inertia to be infinite.so as to confine the studies to electrical self-excitation. The zones of self-excited oscillations of the generator with finite moment of inertia, have been determined, taking into consideration the torsional dynamics of the turbine generator system. Both the unregulated and regulated systems have been considered and the effect of damper winding has been taken into account The results have been presented in the form of stability boundary in the XQ-R plane. The frequencies at which the oscillations occur, have been indicated at chosen points along the stability boundary. An electrohydraulic governor has been found to be effective in extending the stable region in the neigh bourhood of the boundary of additional region of instability created as a result of the ineraction of the shaft torsional mode, and the electrical resonance mode. The effect has been examined for the system with and without excitation controllers. Parameter plane analysis has been done to find suit able gains of different types of excitation controllers, and

electro-hydraulic governor,to improve the stable zone for a particular degree of series-compensation. The effect of such gains has been examined on the entire stability bound ary in the Xc~R plane. The work also deals with the study of the phenomenon of self-excited oscillations of divided-winding-rotor synchronous generators connected to series compensated transmission systems. The previous studies reported do not seem to have considered the distributed nature of series inductance, and shunt capacitance of the line, the torsional dynamics of the turbine-generator system, and the effect of governor. The present work takes into account all these aspects in addition to the effect of different types of excitation controllers on the self-excited oscillations, which needs a further examination, because of the proposed model. Both the unregulated and regulated systems have been considered, and the stability boundary has been plotted in the X -R plane. As in the case of conventional synchronous generators, an electrohydraulic governor has been found to be effective in suppressing the torsional frequency oscillations of d.w.r. generators. Suitable gains of excitation controllers, and electrohydraulic governor, to improve the stable zone for a parti cular degree of series-compensation, have been found, using V parameter-plane technique. The effect of such gains have been examined for other degrees of series compensation and results presented as a stability boundary in the X -R plane. It has been indicated in literature that seriescompensation of the line fails to limit the voltage rises along the line caused by the considerable capacitive power of the line itself. It is, however, well recognised that the neutralisation of these voltage rises can be affected by means of shunt compensation, using shunt reactors. It is, therefore, of interest to study the pheno menon of self-excited oscillations in the presence of shunt reactors, which does not seem to have been examined earlier. The present work deals with the study of the phenomenon of self-excited oscillations of both the conventional, and d.w.r. synchronous generators, connected to systems with series and shunt compensations of transmission lines. The work has been extended to the multi-machine system, taking into account the torsional dynamics of the turbine-generator system, and the effect of governor and different types of excitation controllers.