| As the demand of power electronic converter is increasing in the medium or high voltage high power application fields, the combination of traditional power converter and multilevel technology has become a research focus. Compared to the traditional two or three level converter, low voltage power devices are used in medium or high voltage applications via modular multilevel converter (MMC) with low harmonic and dU/dt. MMC technology has accumulated a lot of experience in technology development and engineering application. However, there are some limitations for MMC, for instance, the sub module (SM) structure needs to be changed for the DC fault problem in MMC in order to improve its fault clearing ability, and in the low-frequency fields, MMC is underutilized in medium or high voltage power applications because extra power injection method has disadvantage of voltage current margin limit. A cascade H bridge modular multilevel matrix converter (M3C or MMMC) provides a new solution, where DC fault rides through capability is acquired in the H bridge structure, the number of arms can be saved 1/4 compared to the back-to-back MMC in AC-AC power conversion, and problems in the low-frequency control fields can be solved merely by the internal power exchange in M3C.M3C has the advantages of both the traditional matrix converter (MC) and MMC. The output of any arbitrary amplitude, frequency and phase can be realized. M3C can realize four-quadrant operation with any input power factor, and can be applied to different voltage levels by increasing or decreasing the SM number. Short circuit risk and control constraints of traditional M3C never exist in the M3C because each arm of M3C has an inductance in series, consequently, each arm has relative independence as a single phase cascade inverter. The MMC, cascade STATCOM and MMC-STATCOM can provide a certain common technical support as special cases of M3C. M3C can meet the requirements of power grid development for advanced power electronic converter, and provide an effective technical support for various FACTS devices. Therefore, M3C has a broad application prospect.This paper takes M3C as the research object, and the research work is carried out on some key problems, such as the M3C capacitor voltage fluctuation mechanism, the system modeling and control design, the whole frequency section control scheme, the DC-side unbalance mechanism analysis and the control of special 3X3 M3C, and main contents are as follows:(1) A simplified equivalent circuit model for M3C is established. The mechanism of capacitor voltage fluctuation in steady state is studied, and the general rule of voltage fluctuation is achieved by the power analysis and simulation calculation. When frequencies of the input and output are close to each other or one of them approximates to zero frequency, ripple voltage will tend to infinity and block the normal operation of the system. Taking the symmetry of the M3C into account, general conditions, low-frequency conditions and similar input-output frequency conditions are regarded as three M3C research conditions. The small signal model for M3C is established in accordance with the basic modeling sequence from single SM to N SMs of each arm, and the transfer function of each arm is derived and simplified for closed loop design.(2) Under general conditions, a cascade control system for hierarchical capacitor voltage balancing and arm current control is established. From the active balance point of view, the current control variables are selected, and circulating currents are constrained to flow among arms. According to active conservation between AC- and DC-side of each arm, voltage controller design is simplified and unified. The inner loop control of arm currents can realize the indirect decoupling control of the input and output current avoiding the complex decoupling transformation. At the same time, circulating currents can be effectively suppressed as a controllable component.(3) Under low-frequency conditions, a low-frequency envelope control method based on instantaneous power tracking is proposed in order to make M3C cross the frequency range when the output frequency is close to 0 Hz. According to common power exchange rules of output frequency-doubled power and active power among three phase units, the output frequency-doubled ripple voltage is put into the closed-loop feedback control as a useful component, which adopts the mixed feedback model of the output frequency-doubled ripple voltage and capacitor voltage DC bias among phase units, and outputs the circulating current tracking power fluctuation. This method can realize a unified control of output frequency-doubled power and active power among phase units. A comprehensive control framework of voltage and current for M3C under low-frequency conditions is proposed.(4) In order to make M3C cross the frequency range when frequency ratio tends to 1/3 and enhance the ability of low-frequency continuous operation for M3C, a general low-frequency envelope method based on circulating currents reconstruction is proposed, and the basic reason of voltage fluctuation suggests that the positive component of circulating currents used for inter-phase balancing excites the huge low-frequency ripple voltage. The method to eliminate positive current directly by closed loop is proposed, which is based on the dislocation symmetric distribution characteristics of positive and negative sequence currents in M3C, and positive sequence current are removed as zero sequence component among special arm combinations, and finally negative sequence current is extracted indirectly. According to the equivalent principle of positive and negative sequence currents, to reserve and double negative sequence current amplitude compensates the active contribution of positive sequence current. Therefore, the circulation reconstruction method does not affect the inter-phase balancing.(5) When the input frequency approaches output frequency, it is revealed that the effective decoupling of the difference frequency power is crucial to the suppression of the large difference frequency voltage fluctuation. The high-frequency circulation injection method and the symmetrical frequency circulation injection method are proposed, and both methods are implemented in the balancing control among arms.1) High-frequency circulation injection method is realized according to the common power exchange characteristics of the difference frequency power and active power among arms. The difference frequency power is incorporated into the active balancing control loop among arms. According to the hybrid feedback control of the capacitor voltage DC bias and difference frequency ripple voltage, high-frequency circulating current is output by close loop and helpful to realize the effective decoupling control of difference frequency power by additional power injection.2) Symmetrical frequency circulation injection method. According to the conclusion that the circulation power is crucial to effective decoupling control of difference frequency power, circulating currents from conventional input and output active current injection method are optimized. The circulation power is suppressed actively and the cycle coupling of circulation power is blocked essentially with injecting symmetric frequency circulation synchronously. This method can limit the peak value of arm current.(6) A simplified equivalent circuit model of special 3×3 M3C is established. It is concluded that the reactive power bias between arms is crucial to maintain the stability of the capacitor voltage average value. According to the common power exchange characteristics of the difference frequency power, active and reactive power between arms, the above power is incorporated into the same inner-arm balancing control. Inheriting the hybrid feedback control model of multi-frequency power, high-frequency circulation can be output by the unified control loop. Under the single circulation path, the input and output frequency circulating currents are used to adjust the non-zero sequence active power deviation and the inter-phase power difference independently.(7) Two kinds of semi-physical hardware-in-loop analog platforms are designed for low-voltage physical experiment of MMC:1) The platform based on operational amplifier is designed by the equivalent external characteristic of the actual power converter, which is established by system modeling, signal conditioning and implementation.2) The platform based on Rtlab is designed by calling system model library, which is interconnected with the actual controller as controlled object to realize analog functions. |
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