A Bipolar Voltage Modulation Spectrum Analysis

Abstract

Features of power grid based on renewable energy sources are described. The necessity of isolated power converters in the power grid is shown. Low harmonics negative impact in modulated AC voltage based on unipolar pulse width modulation for transformer weight of isolated invertors is underlined. An approach of bipolar voltage generation with inverting every second modulation pulse that significantly decreases the low harmonics amplitude and allows to decrease the transformer weight in several times is described. A double Fourier series implementation for bipolar voltage analysis that allows expressing a signal spectrum characteristic in compact form for any multiplicity parameter value is proposed. Representation of modulated signal based on two variables of carrier and modulation functions is considered. Two different cases of modulated voltage generation depending on multiplicity parameter value are described. Bipolar voltage signal representation based on the sum of two unipolar signals with two times lower carrier function frequency is proposed. Bipolar and unipolar modulated signals spectrum components of double Fourier series are calculated. Spectrum characteristic of bipolar voltage modulation is analyzed. Transmission of the significant energy amount of bipolar voltage by harmonics with frequencies near two times lower value than carrier function frequency is proved. A case with multiplicity parameter which is multiplied to number four is noted as more desirable due to decreasing the first harmonic value. A proposition about subsequent improvements of bipolar voltage modulation is given