Influence of non-sinusoidality of the mains voltage on the operation threshold of the voltage unbalance protection device
An analytical substantiation of the principle of operation of the device for the protection of asynchronous electric motors from voltage asymmetry, made on the basis of RC filters of symmetric components of the positive and negative phase sequences, has been carried out. A technique for determining the resistances of resistors and capacitances of capacitors included in the filter circuits is described, and also an assessment of the influence of non-sinusoidal voltage in the supplying electric motors of the electric network on the threshold of operation of the protection device was made. When assessing, they were limited by the presence in the curve of the phase voltages of the network of the values of the third and fifth harmonic components at their different initial phases. When assessing the effect, the value and the initial phase of the symmetrical component of the zero phase sequence voltage, which does not affect the operation threshold of the protection device with a sinusoidal voltage, were also taken into account. A method for determining the threshold of operation of the protection device for any of the possible combinations of these parameters is proposed. As an example, to assess the degree of deviation of the response threshold rc when the voltage is not sinusoidal from the response threshold ry of this protection device with sinusoidal voltage, rc was calculated according to the above method using the universal system of mathematical calculations MathCAD. As a result of the performed theoretical calculations, it was established that the nonlinearity of the mains voltage has a significant effect on the response threshold of the considered protection device of asynchronous electric motors based on R-C filters of the direct and reverse phase sequences, which responds to the value of the mains voltage asymmetry coefficient. The presence of harmonics in the voltage spectrum increases the sensitivity of the protection to the symmetrical voltage component of the negative phase sequence. The maximum (albeit unlikely) decrease in the response threshold for this component reaches 72% of the response threshold for sinusoidal voltage. This can be considered quite appropriate, since both asymmetry and nonlinearity of voltage have a more negative effect on the protected object.