Variable carrier hybrid modulation method suitable for modular multilevel converter

Abstract

The invention provides a variable carrier hybrid modulation method suitable for a modular multi-level converter. The method comprises the following steps: (1) generating a step wave signal which has the same period as a sinusoidal signal and has the same change trend as the sinusoidal signal by rounding a sinusoidal modulation voltage signal through downward approximation by adopting the nearestlevel approximation principle; (2) subtracting the step wave signal generated in the step (1) from a sinusoidal modulation signal to obtain a new modulation signal; (3) according to the control principle of SPWM, in combination with the new modulation signal in the step (2), comparing a triangular carrier signal with variable frequency with the new modulation signal to obtain a series of PWM pulsesignals; and (4) superposing the step wave obtained in the step (1) and the PWM pulse signals obtained in the step (3) to obtain a final modulation signal, and applying the final modulation signal tothe sub-module switching control of the modular multilevel converter. The switching frequency is reduced to a certain extent, the power loss is reduced, and the service life of a switching device isprolonged.

Description

technical field [0001] The invention designs a mixed modulation strategy of variable carrier frequency of a modular multilevel converter, which belongs to the field of power electronics. Background technique [0002] Due to the symmetry of the internal structure of the modular multilevel converter, and the characteristics of easy expansion of sub-modules, strong fault tolerance, stability and reliability, flexible control, and high operating efficiency, it is used in flexible direct current transmission and energy Internet key equipment - solid-state transformers, etc. Applications in the energy field are becoming more and more extensive. Different from the traditional two-level and three-level VSC converters, under the control of the modulation strategy, the modular multi-level converter with N sub-modules in each bridge arm can output N+1-type power levels, The output voltage waveform is closer to the sinusoidal modulation signal. [0003] The modulation strategy of the ...

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Problems solved by technology

The traditional modulation strategies mainly include carrier phase shift pulse width modulation (CPS-PWM), nearest level approximation (NLM), space vector pulse width modulation (SVPWM), etc. At present, carrier phase shift is mainly used in engineering applications. The pulse width modulation strategy and the nearest level approach strategy. Among them, the nearest level approach strategy is mainly used in fields with higher voltage levels, more sub-modules, and more levels; The wide modulation strategy is widely used, however, the switching frequency of the carrier phase-shift pulse width modulation strategy is high, resulting in large power loss and short service life of switching devices and other practical problems

Although the traditional hybrid modulation strategy combines the characteristics of the recent level modulation strategy and carrier phase-shift pulse width modulation strategy, the corresponding PWM pulse signal is obtained by comparing the ordinary carrier signal with the modulation signal, but the modulation signal The amplitude will change, and there are still some errors in the modulation result formed by the superposition of the corresponding PWM signal and the ladder wave

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