| Nowadays,the shortage of traditional fossil resources,environmental pollution and global warming caused by the burning of fossil fuels have become one of the public crises of human society.Vigorously developing new types of clean energy and advocating energy interconnection to achieve efficient use of energy has become an inevitable trend of electric power development,in which the grid-connected technologies of new energy and large-capacity long-distance transmission technologies play an important role.The development of advanced power electronics technology has created favorable conditions for the development of high-voltage high-power inverters and frequency conversion devices.Power transmission has shifted from a fixed power frequency to different frequencies to reduce investment in power transmission systems and improve power system operation indicators.The research on low-frequency(multi-frequency,frequency-divided,and direct-current)power transmission systems has become an important direction for the development of power in the new century.Compared to traditional two-level or three-level converters,the modular multilevel matrix converter(M3C)topology family can apply low-voltage level power devices in high-voltage and high-power applications with a cascaded configuration,and possesses merits as low input/output harmonic contents,modular design,easy production and maintenance,improved operational reliability and low switching frequency.Therefore,M3C topology family are becoming popular in flexible DC transmission,low-frequency transmission,high power motor drives and power electronics transformers,and so on.At present,the modular multilevel converter(MMC)control technology for HVDC is becoming mature and has accumulated a lot of engineering application experience,but its control under unbalanced grid condition is still rather complex.The modular multilevel matrix converter(M3C)can achieve direct ac/ac conversion,and has received extensive attention in the field of motor drive,but its research in the low-frequency ac transmission field is still in its infancy.The full-bridge modular multilevel converter(FMMC)can be used in power electronic transformer,in ice-melting and to solve the DC fault ride-through in HVDC.What's more,it can also be applied in single-phase low frequency ac transmission field.Currently,there is not any report about single phase low frequency ac transmission.Therefore,the research of M3C in low frequency ac transmission to achieve safe and stable operation under normal and abnormal conditions of the power grid,the explore of FMMC in single phase low frequency ac transmission,and the simplification of control strategy for MMC-HVDC under unbalanced grid condition have very important theoretical research value and engineering practical value.This paper takes three types of topologies in the modular multilevel matrix converter family as the research objectives,i.e.,3×3 M3C(M3C),full-bridge based 3×2 M3C(FMMC),half-bridge based 3×2 M3C(MMC),and makes a study on the key technologies when applying these three types of topologies in,low frequency ac transmission(LFAC),single phase low frequency ac transmission(SLFAC)and high voltage dc transmission(HVDC),including:(1)This paper studies the general small signal modeling and controller design method of M3C topology family.A unified small-signal model of arm with N cascades full-bridge and half-bridge modules is established.A generalized hierarchical capacitor voltage control strategy is proposed for M3C topology family.Taking M3C as an example,the voltage and current cascade control system is established:the inner loop applies arm current direct control,which can simultaneously control the ac side currents and the internal circulating current,removing the need of independent circulating current controller.The outer loop applies hierarchical control to achieve balanced control of the capacitor voltage.Universal parameter tuning methods are given for arm current controller and capacitor voltage controllers.A nearest level modulation method(NLM)is proposed for cascaded full-bridge modules.By selecting appropriate zero-switching state within single cycle or two adjacent cycles of arm voltage,the frequencies of four switching of a sub-module is balanced.(2)This paper gives the filter design method of control loop for M3C.By deducing the capacitor voltage ripples of each submodule(SM),the frequency of capacitor voltage ripples and the relationship among all ripple frequencies are obtained.Based on this,a moving-average filter suitable for the 50/3Hz low-fre-quency transmission is proposed,which can completely remove the sub-module capacitance voltage ripple,thereby improving the steady-state performance of the entire control system.By further analyzing the frequency relationship of the capacitance voltages,the following conclusions are drawn:From arm unit phase unit? the entire M3C.the capacitance voltage ripples change from complicated to simple.According to the law and combined with the hierarchical capacitor voltage control strategy,a hierarchical filter scheme is proposed.This scheme can not only ensure the steady-state performance of the capacitor voltage control,but also inprove the dynamic performance of the system.(3)This paper studies the control strategy for MkC under unbalanced grid condition,and gives two comprehensive control strategies based on,respectively,double circulating currents and single circulating current together with neutral-point voltage.The both control strategies can automatically adjust the internal power of M,C and ensure the currents of both sides symmetrical under unbalanced grid and unbalanced output power of power frequency side.Besides,they both do not need a circulating current suppressing controller.Arm current controller is composed of a proportional regulator and the feedforward steady-state duty cycle,which possesses excellent performance under unbalanced grid condition too and greatly simplifies the circulating current control and parameter setting.(4)This paper studies the single-phase low frequency ac transmission(SLFAC)technology based on FMMC.The infeasibility analysis of applying the sinusoidal waveform as the voltage waveform of the low frequency side is done.Then a quasi-square wave suitable for single-phase low frequency ac transmission is proposed.The waveform can not only reduce the power fluctuations of low frequency side and the capacitor voltage ripple,but also make the capacitor voltage ripple independent of the frequency of low-frequency side,therefore the SLFAC can achieve the transmission with 1Hz or even lower frequency.A power balance strategy suitable for single-phase low-frequency ac transmission is proposed.On the basis of ensuring capacitor voltage balance,the power fluctuations of low frequqncy side are absorbed by capacitors absolutely and do not flow into the power grid.The capacitor voltage ripple caused by the power fluctuation of low-frequency side is calculated.The time range of the quasi-square wave switching process is obtained when the capacitor voltage fluctuation limitation is satisfied.The voltage divarication between the upper and lower arms of the FMMC-SLFAC is deduced,and it concludes that the voltage divarication of the upper and lower arms can be eliminated by selecting frequency of low frequency side.The frequency range and the comprehensive control strategy of FMMC-SLFAC are given.(5)The control strategy of the modular multilevel converter under unbalanced grid condition is studied.By analyzing the active power.two control strategies are proposed for MMC under unbalanced grid condition:The first one superimposes an an current symmetrical controller;the second one selects the indirect control variables.The both control strategies can automatically adjust the active power output to dc bus and active the symmetrical ac current.The arm current direct control is adopted,which controls the ac side current,dc bus current and the circulating current simultaneously,removing the need of three-sequence ac side current controller and the three-sequence circulating current suppressing controller under unbalanced grid condition.A zero-sequence current suppressor is proposed to add in arm current reference,which can eliminate the fundamental zero-sequence current which is introduced into the dc bus by the asymmetrical loss of the arms. |
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