Physical Cause of Autoparametric Resonance in Longitudinal Power System

Abstract

There is a possibility that a low-frequency oscillation gets unstable due to autoparametric resonance in a longitudinal power system. This paper clarifies that the physical cause of the resonance is load. First, we superimpose two oscillation modes, and examine changes in voltage and consumed power at load buses. The changes in comsumed power almost directly lead to changes in generator output, and change the force applied to the low-frequency oscillation. Next, we show through simulations and the Mathieu diagram that the strength of the resonance conspicuously changes with quantity of load. Furthermore, the quantity of allowable load is inversely proportional to the length of system, and does not depends on the number of generators. Namely, the length of system automatically determines the quantity of allowable load. The length of system is essentially an electrical distance related with reactance of transmission line. Lastly, we show that construction of a new system with same capacity as existing one is equivalent to electrically halving its length. On the other hand, if we divide a system into two short subsystems, then each subsystem can carry double load, and the system can carry quadruple load in all.