Electric Power Systems Research, 7 (1984) 271 - 277 271 Damping Effect of Load on the Ferroresonance Phenomenon in Power Networks M. M. SAIED*, H. M. ABDALLAH and A. S. ABDALLAH Electrical Engineering Department, Faculty of Engineering, Assiut University, Assiut (Egypt) (Received December 5, 1983) SUMMARY A generalized approach to the ferroreso- nance phenomenon, occasionally resulting from power transformers connected to transmission systems, is given. The effect of the transformer load current on the ferroreso- nance phenomenon at different load power factors is studied. A second-order equation determining the value of the critical load impedance at any load power factor is de- rived. Also, the effect of the transmission line parameters and the saturation degree of the transformer magnetization characteristic on the value of the critical load current and hence on the ferroresonance phenomenon is shown. 1. INTRODUCTION As power transmission distances have increased, new problems, both technical and economic, have been encountered. As trans- mission distances continue to increase, it is expected that, in addition to problems which have already been exposed, new ones may arise which previously have not required consideration. One of the most serious problems in electrical power systems is the possibility of overvoltages resulting from a ferroresonant condition between the system capacitances and the nonlinear inductance of the trans- former magnetization characteristic. This phenomenon has been studied extensively by many investigators in the last 50 years [1 - 6]. From these papers, the ferroresonance phe- nomenon can be defined as a special case of jump resonance in which the nonlinearity is the transformer iron core magnetization *Present address: Department of Electrical and Computer Engineering, College of Engineering and Petroleum, Kuwait University, P.O. Box 5969, Kuwait. curve. Jump resonance refers to circuits comprising a nonlinear inductor and a linear capacitor with or without other elements present. Investigators have used different approaches in studying the ferroresonance phenomenon, such as the incremental describ- ing function (IDF) method, which is used by Maklad and Zaky [3], or by studying the phenomenon in the G_-l(jco) plane, which was also clone by Pmsty and Sanayal [4 - 6]. In all the previous papers [1 6], the authors studied the ferroresonance on a general nonlinear reactor [ 1] or an unloaded transformer [2 - 6]. As in the actual power systems, the power transformers are loaded most of the time. So, it is important to study the effect of the load on this phenomenon; this is the main object of this work. Here, the load has been considered with regard to predicting the possibility of the ferroreso- nance phenomenon in the power system under load conditions. The load is considered by studying the circuit of Fig. l(a). It illus- trates a transmission line represented by its exact equivalent 1r-section connected to a loaded power transformer, with all param- eters referred to its high voltage side. Also, a new generalized second-order equation giving the critical load at any power factor for any power system circuit parameters is derived. The effect of the line parameters and the Rt %It ,~t cl :! .•L! it I1 i2 R2 L2 i,-Td ,oT J fl tilt i~ |. ! I i3 R 3 L 3 Voltagej T.L. ' _ .j_ Transformer _L Load Sourc. l - T T Fig. l(a). Equivalent circuit of system considered, with CI = C~ + Ci and R e = rc/(Ggrc + 1). 0378-7796/84/$3.00 © Elsevier Sequoia/Printed in The Netherlands