This tutorial paper presents the derivation of a simple, low order System Frequency Response (SFR) model that can be used for estimating the frequency behavior of a large power system, o r islanded portion thereof, t o sudden load disturbances. The SFR model is a simplification of other models used for this purpose, but it is believed to include the essential system dynamics. The SFR model is based on neglecting nonlinearities and all but the largest time constants in the equations of the generating units of the power system, with the added assumption that the generation is dominated by reheat steam turbine generators. This means that the generating unit inertia and reheat time constants predominate the system average frequency response. Moreover, since only two time constants predominate, the resulting system response can be computed in closed form, thereby providing a simple, but fairly accurate, method of estimating the essential characteristics of the system frequency response.

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C. Calculated Results 63 1. Effect of the length of untransposed transmission line on network unbalances 63 2. Effect of system loading on network unbalances 3. Effect of power coupling on power flows 4. Effects of load unbalances 5. Comparisons between the 3-4> load flow program and system reduction method D. Conclusions IV. TRANSIENT ANALYSIS OF UNBALANCED THREE-PHASE TRANSMISSION SYSTEM 99 A. Introduction 99 B. Transition Point on Three-Phase Systems 99 C. Summary and Discussion 110 V. RESULTS OF THE ELECTROMAGNETIC TRANSIENT ANALYSIS 113 A. Introduction 113 B. Study of the. 24-Bus EHV Test System 113 1. Effects of untransposed transmission lines and unbalanced loads on the transient overvoltages 114 2. Evaluation of the accuracy of using balanced initial conditions 115 C. Conclusions