| Three-level converter is widely used in medium voltage and high-power occasions,including electric vehicle high-power charging,reactive power compensation,locomotive medium voltage traction system,etc.Compared with the two-level converter,it has the advantages of low device stress,small current ripple,low filter volume weight and high power density.As a kind of three-level converter,the active neutral point clamped(ANPC)three-level converter has the advantages of traditional three-level converter,and can realize bidirectional energy flow,compared with the neutral point clamped(NPC)three-level converter has a more flexible switching mode,so it can realize the relative temperature balance between different switches under high power and high switching frequency.The fluctuation of neutral point voltage is an inherent problem of three-level converter.The large fluctuation of neutral point voltage leads to the increase of capacitor voltage stress and device stress,and at the same time,the introduction of AC current harmonics in the AC side.At the same time,under light load,different modulation methods will lead to different AC side current ripple,so under light load,larger current ripple will deteriorate the quality of waveform.This paper will analyze the above problems and put forward corresponding improvement methods.This paper introduces the working principle and mathematical model of ANPC converter,analyzes the operation characteristics under different working modes,compares the different switching modes of ANPC converter,and qualitatively analyzes the heating of switching tubes under different switching modes.The modulation mode can be divided into carrier modulation and space vector modulation(SVPWM).This paper discusses the realization principle of carrier modulation and space vector modulation,and analyzes the neutral point voltage fluctuation characteristics of carrier modulation,space vector modulation and intermittent modulation strategy based on zero sequence injection.At the same time,the influence of the DC component of the neutral point voltage and the third harmonic on the input current under different modulation modes is analyzed,and the capability of the unbalanced load under different modulation modes is analyzed.Finally,the three modulation strategies of SVPWM,dpwm1 and SPWM are simulated and compared to verify the correctness of the analysis.In order to solve the problem of neutral point potential fluctuation,this paper presents a hybrid intermittent pulse width modulation strategy(hybrid)for high power ANPC converter DPWM)is realized by space vector modulation.In each switching cycle,one phase switch does not act.In the range of full power factor,the low-frequency fluctuation of neutral point potential is eliminated.The number of switching actions in each switching cycle is 4-6,so the switch loss is significantly reduced while the neutral point potential balance is ensured.Due to the complexity of the algorithm,a carrier based neutral point potential balance algorithm is proposed,which can simplify the implementation process.Compared with the traditional modulation strategies such as SVPWM,SPWM,dpwm1,and so on,the simulation and experiment prove the effectiveness of the proposed modulation strategy in the neutral point potential balance.Under light load,the current ripple will worsen the current quality.At this time,the switching loss is not the main factor.Firstly,this paper deduces the calculation method of peak value of ripple,and makes a comparative analysis of the ripple under different modulation modes.The intermittent modulation mode has the characteristics of large current ripple.Under the same conditions,the current ripple is twice that of SVPWM.Therefore,according to the carrier modulation and The consistency of space vector modulation proposes a carrier modulation strategy under light load,which can reduce the peak value of current ripple without increasing the carrier frequency.Finally,the simulation of the proposed modulation strategy are carried out to prove the effectiveness of the proposed modulation strategy in reducing ripple under light load. |
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