The performance and behaviour of a non-linear resonant link for power systems

Abstract

The link is formed by a capacitor connected in series with a linear inductor and tuned to a state of resonance for normal load flow conditions so that the resulting impedance and voltage regulation are negligibly small. Under fault conditions the state of resonance is destroyed by the saturation of a reactor in parallel with the series capacitor hence the fault throughput current is severely limited to a few times full load current. The limitation occurring in the first half cycle. Certain sub-harmonic instabilities are investigated with a view to gaining insight into their cause and to remedy this response. A cause of the sub-harmonic response is put forward and a relationship between the sub-harmonic and the 50 Hz ferro-resonant response is suggested. An analysis of the circuit by two methods is given and digital integration of the system equations yields 1/3 sub-harmonic solutions. A double resonance link is introduced with load flow properties similar to those of a single resonance circuit, but which relies upon the high impedance of parallel resonance to limit the fault current to a little above full load value. It is shown that such a link actually reduces the fault level of a busbar to which other supplies are connected. Spectrum analysis of the current waveforms show that the harmonics and sub-harmonics have basically different circulation paths in the link. The possibility of protecting the link against internal faults by the application of standard commercial systems is demonstrated and a complete protective scheme is presented. Finally a discussion of previous papers is given and a summarised list of the salient points to emerge from the research.