| Recently, the modular multilevel converter(MMC), with a group of cascaded submodules(SMs) to support high voltage, has been widely used in high voltage direct current transmission(HVDC) field due to its advantages in both structure and control. In order to realize the power exchange among DC lines of different voltage levels, forming dc transmission and distribution grid, high-voltage large-capacity DC/DC converter must be used. The isolated DC/DC MMC inherits the modular structure from the traditional AC MMC; meanwhile, is featured as bi-directional power flow, flexible DC voltage matching, and intrinsic DC fault isolation, thus is an attractive candidate for DC/DC conversion in DC grid. However, the researches on DC/DC MMC have just started; much work need be further done. This paper takes the mono-arrangement DC/DC MMC as the research object, studying on its modulation, voltage balance, modeling and so on. The main contents are described as follows:(1) Carried out the working principle analysis of DC/DC MMC. Focusing on analyzing the influence of each component of arm voltage on the DC-side current, system power flow and energy balance among upper and lower arm, a simple but effect control thought is then put forward. With this idea, the converter can operate well; besides, the computational burden of control circuit is lowered. Both the control relations and the control period are defined from qualitative respect.(2) Carried out the designing of modulation strategy. Considering the high-fundamental-frequency feature of DC/DC MMC, equal duty-cycle fundamental nearest level modulation(NLM) is designed. Each SM is strictly restricted to be switched on and off once a period, and the switching losses thus are reduced. Furthermore, combining with the equal duty-cycle feature, a new voltage balance method is proposed based on the SMs ripple voltage pre-calculation. With these two improvements, the computational burden of modulation system is greatly reduced.(3) Carried out the small-signal modelling and controller designing for the proposed modulation. Since the control system is executed with fundamental period, the circuit state variables can be expressed by Fourier series. Building the dynamic equations for each frequency components and applying small perturbations to these differential equations, the small signal model of DC/DC MMC is finally obtained. Proper controller is designed and the system is controlled well.In order to validate the effectiveness of above analysis and design, the simulation model and small-scale prototype are built in Matlab/Simulink and laboratory respectively. Both the simulation and experimental results prove the effectiveness of above design. |
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