This MMC (Modular Multilevel Converter) testbed is built mainly to test the proposed capacitor voltage balancing control scheme of MMC submodules. This control scheme is based on the 45-degree coordinates phase shift space vector modulation (SVM), which brings two advantages:
- On one hand, the basic vectors on the 45-degree coordinates locate at the integer grids, which simplifies the locating of the basic vectors synthesizing reference vectors and their dwell time calculation.
- On the other hand, the two-level phase shift SVM algorithm is adopted instead of the multi-level SVM algorithm, which reduces the computational burden produced by the multi-level SVM algorithm, including the calculation of the redundant switch state vectors and the selection of the switch state vectors.
By analyzing the reasons for the capacitor voltage imbalance of MMC submodules, it is found that the phase shift SVM algorithm causes the capacitor voltage of a submodule to be related to its position. In order to achieve the capacitor voltage balance control, the proposed method distributes the PWM signals in a cyclic way. Compared to other control schemes to balance the capacitor voltage of MMC submodules, the proposed scheme has no need to measure the arm current or submodule capacitor voltage and therefore, the hardware cost is reduced.
This MMC testbed can also be applied to investigate problems such as:
- Fault diagnosis and fault-tolerant techniques of the MMC submodule.
- Current circulation suppression scheme of the MMC.
- Switching frequency and switching loss analysis of the MMC under different modulation techniques.