به هنگام اتصال دو یا چند ماشین به یکدیگر نوسان منحنی توان تنها بین بار و مولد اتفاق نمی افتد بلکه بین ژنراتن.ورها نیز اتفاق می افتد. بنابراین تحلیل شرایط پایداری هر ژنراتور در این حالت با محاسبات دستی بسیار مشکل است و لازم است از محاسبات کامپیوتری و نرم افزارهای اختصاصی در این زمینه استفاده کرد. فصل 16-8 کتاب مرجع استیونسن به این موضوع پرداخته است.
16.8 MULTIMACHINE STABILIlY STUDIES:
The equal-area criterion cannot be used directly in systems where three or more machines are represented. Although the physical phenomena observed in the two-machine problems are basically the same as in the multimachine case, nonetheless, the complexity of the numerical computations increases with the n umber of machines considered in a transient stability study. When a multimachine system operates under electromechanical transient conditions, intermachine oscillations occur through the medium of the transmission system connecting the machines. If any one machine could be considered to act alone as the single oscillating source, it would send into the interconnected system an el ectromechanical oscillation determined by its inertia and synchronizing power.
A typical frequency of such an oscillation is of the order of 1 -2 Hz, and this is superimposed upon the nominal 60-Hz frequency of the system. When many machine rotors are simultaneously undergoing transient oscillation, the swing curves reflect the combined presence of many such oscillations. Therefore, the transmission system frequency is not usuduly perturbed from nominal frequency, and the assumption is made that the 60-Hz network parameters are still applicable. To ease the complexity of system modeling, and thereby the computational burden, the following additional assumptions are commonly made In transient stability studies:
1. The mechanical power input to each machine remains constant during the entire period of the swing curve computation.
2. Damping power is negligible.
3. Each machine may be represented by a constant transient reactance in series with a constant transient internal voltage.
4. The mechanical rotor angle o f each machine coincides with 0 , the electrical phase angle of the transient internal voltage.
5. Al l loads may be considered as shunt impedances to ground with values determined by conditions prevailing immediately prior to the transient conditions.
The system stability model based on these assumptions is called the classical stability model, and studies which use this model are called classical stability studies. These assumptions, w h i c h we shall adopt, are in addition to the fundamental assumptions set forth in Sec. 16.1 for all stability studies. Of course, detailed computer programs with more sophisticated machine and load models are available to modify one or more of assumptions 1 to 5. Throughout this chapter, however, the classical model is used to study system disturbances originating from three-phase faults.