Induction motors protection against overcurrent taking into account nonlinear distortion of phase current

Authors:

O.H. Sereda, Cand. Sci. (Tech.), Associate Professor, National Technical University “Kharkiv Polytechnic Institute”, Senior Lecturer of the Department of Electrical apparatus, Kharkiv, Ukraine.

Abstract:

Purpose. Theoretical justification and engineering of induction motors heat protection method from overload currents considering nonlinear distortion of phase currents and its implementation as a microprocessor device functioning algorithm.

Methodology. The research involved the theory of the representing complex harmonic oscillations analog signals expansion into the oscillation spectrum forming elementary harmonic components in order to compare their properties by applying the theory of discrete signals and systems, as well as methods of spectral analysis and discrete signals filtering. The harmonic analysis versatility is that any periodic signal may be synthesized from harmonic oscillation certain amplitude, frequency and initial phase. A mathematical model for determining the phase current harmonic content of power supply networks with isolated neutral and non-linear loads types and, as a consequence, the distortion of sinusoidal phase currents change is developed by multiplying the analog current in time dependency on the grate delta-function with different sampling intervals, in which the use of simple and widely used in relay protection units, in particular circuit breakers microprocessor trip, mathematical operations of integration squares instantaneous current allows the most in harmony with the mathematical tools to build other network protection types.

Findings. The necessity to increase the sensitivity of the induction motors heat protection from overload currents taking into account nonlinear distortion of the phase currents was proved. By nonlinear distortion harmonic analysis of the phase currents the motor protection reliability increasing provided by taking into account the higher harmonic components of the phase currents, namely, the fifth harmonic current, which causes to additional losses and heating of the stator winding. It uses the simplest and widely used in protective relaying mathematical apparatus determining of most significant higher harmonics currents RMS.

Originality. A possibility of extending the implemented protection list of circuit breakers trip units based on the digital processing of signals from the current sensors is theoretical research by the harmonious analysis of phase currents spectrum with the nonlinear distortions presence. A new technical solution was proposed that allows in online sliding monitoring mode to form response time of circuit breakers for reliable protection of induction motors from overload currents taking into account nonlinear distortion of the phase currents.

Practical value. The microprocessor protection functioning algorithm of induction motors is designed against impermissible heating coils with overload currents taking into account nonlinear distortion of the phase currents.

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