Energy Storage Requirement Optimization of Hybrid Modular Multilevel Converter With Circulating Current Injection

Experimental Validation of a Nested Control System to Balance the Cell Capacitor Voltages in Hybrid MMCs

“…The capacitor voltage imbalance problems produced in a hybrid MMC operating with a high modulation index m have already been reported in [9], [22]- [24]. For completeness they are briefly discussed here.…”

“…The main drawback of the methods discussed in [13], [21], [23], [24] is that the feed-forward circulating currents are calculated off-line and are imposed without considering the degree of imbalance between the capacitor voltages of the FBSMs and HBSMs, i.e., there is no closed-loop control of the FBSM-HBSM capacitor voltage imbalance, and the system lacks the capability to compensate for changes in the operating point and/or variations in the parameters of the hybrid MMC. Consequently, the off-line calculated feedforward currents may not be totally effective in dealing with the imbalance.…”

“…The compensating currents are calculated offline using a methodology based on an optimising algorithm and, therefore, the injected current is the minimum that ensures voltage balance between the FBSMs and HBSMs. The main contributions of this work are: 1) Unlike previous works [9], [23], [24] the proposed strategy implements a nested closed loop control system to regulate the FBSM-HBSM capacitor voltage imbalance. An outer control loop acts on the voltage imbalance FBSM-HBSM and an inner control loop regulates the compensating current in the MMC (based on either the CLC-I or CLC-II strategy).…”

Optimal Second-Harmonic Current Injection for Capacitor Voltage Fluctuation Reduction in Hybrid MMCs Under Grid-Side SLG Faults

“…The HB-SM and FB-SM based hybrid MMC combines the merits of the two types of MMCs [15]- [16]: 1) The hybrid MMC has DC fault-ride-through capability; 2) The hybrid MMC has a wider modulation index range compared to HB-MMC, which provides more space to improve the suppression of capacitor voltage fluctuation. Under steady states, the modulation index of 1.414 can significantly reduce the fundamental frequency component of the SM capacitor voltage, and at the same time, suppress the fluctuation of the SM capacitor voltage [19], [20]. In [19], an optimal second-harmonic current injection of amplitude and phase has been developed based on the defined fluctuation function of SM capacitor voltage.…”

“…Under steady states, the modulation index of 1.414 can significantly reduce the fundamental frequency component of the SM capacitor voltage, and at the same time, suppress the fluctuation of the SM capacitor voltage [19], [20]. In [19], an optimal second-harmonic current injection of amplitude and phase has been developed based on the defined fluctuation function of SM capacitor voltage. An optimal circulating current injection method considering the limitation of the current rating of semiconductor (switch) has been studied in [20].…”