Hybrid Modular Multilevel Converter with Reduced Energy Storage for Medium Voltage Variable Speed Drives

The second-harmonic injection strategy for modular multilevel converters (MMCs) can effectively suppress the capacitor voltage fluctuation in steady states. However, the capacitor voltage fluctuation under grid-side single-line-to-ground (SLG) faults will be more severe. Moreover, there is no research on the mitigation of capacitor voltage fluctuation under grid-side SLG faults for the hybrid MMC with mixed half-bridge and full-bridge sub-modules. To address this issue for hybrid MMCs, an optimal second-harmonic current injection scheme for mitigating the capacitor voltage fluctuation in the hybrid MMC under grid-side SLG faults is proposed in this paper. The mathematical models of arm power fluctuations of the hybrid MMC are firstly developed under both steady states and grid-side SLG faults. Then, the principle and optimal strategy of the proposed capacitor voltage fluctuation strategy are presented. The capacitor voltage fluctuation under grid-side SLG faults can be reduced by suppressing the three-phase arm power fluctuations. A hybrid MMC model with the proposed scheme has been established in PSCAD/EMTDC to verify its effectiveness and feasibility. Simulation results show that the average voltage fluctuation reduction rate can reach 55.77% with the proposed strategy.

A single-phase-to-ground (SPG) fault in a neutral non-effectively grounded AC distribution network may cause the arc at the fault point and a power supply interruption, permeating the zero-sequence voltage into the DC network and the nonfault AC-side of AC/DC hybrid distribution networks (HDNs). This paper proposes a flexible control method for a modified hybrid modular multilevel converter (HMMC), incorporating an additional full-bridge submodules (FBSM) cluster at its DC side, to deal with SPG faults in HDN. The HMMC includes a modified modular multilevel converter with a zero-sequence current flow path to realize active ground fault arc suppression (AGAS) and an FBSM cluster in its DC side to block the zero-sequence component to the DC grid and the non-fault circuits of HDN. The operation principles of the HMMC that achieves AGAS and zero-sequence voltage transmission suppression (ZVTS) are presented. The HMMC can realize AGAS when there is an SPG fault on the AC side, and ZVTS can be realized on the DC side by utilizing the FBSM cluster. The flexible control method proposed herein can ensure the normal operation of the HMMC under SPG fault conditions, achieve power transfer during fault current suppression, and realize ZVTS during arc suppression. Simulation and experimental results verify the feasibility and validity of the proposed HMMC and its flexible control method.

Hybrid modular multilevel converters (HMMC) address the DC-fault blocking limitations of the half-bridge submodules (HB-SMs) of the standard MMC by combining HB-SMs with full-bridge submodules (FB-SMs). In order to improve the overall conversion efficiency, this article proposes two lowloss configurations for the HMMC. The main improvement in the proposed HMMCs is achieved by the combination of low-loss unipolar and low-loss bipolar SMs in the same arm of the HMMC. It is shown that the simplest structure (LLH1) can reduce the SM losses (switching and conduction losses in an SM) by 30% compared to the HMMC at the cost of limited fault blocking capabilities. The fully controlled structure (LLH2) reduces SM losses by 10% compared to the HMMC, and 30% compared to the FBSM-based MMC. These results represent an increase of 20% over the typical HBSM-based MMC for a converter (LLH2) that can provide fault-blocking capabilities. Assessment of efficiency in both HMMCs is provided over a range of operating conditions both in inverter and rectifier mode from an 800-MVA HVDC system model.

The conventional half-bridge sub-module (HB-SM) based modular multilevel converter (MMC) fails to provide the DC fault handling capability. Nevertheless, this drawback can be overcome by replacing the HB-SMs with the full-bridge (FB) SMs since the FB SMs can generate bipolar voltages thus handling the DC fault condition. But the drawback of using FB-SMs arises as it employs double the number of switches used that in case of HB-SM-based MMC, thus increasing the switching losses due to an increase in device count. Then, the Hybrid MMC, which employs HB SMs in addition to FB SMs, came into the picture and mitigated the drawbacks mentioned earlier significantly. This paper presents two cases for 21-level Hybrid MMC, wherein the first case implements the hybridization of 50% conventional SMs and 50% FB-SMs, and case two implements the same with 33.33% conventional SMs and 66.66% FB-SMs. The reduced switching frequency (RSF) capacitor voltage balancing approach is employed to balance the capacitor voltages of HB-SMs and FB-SMs for the first combination (50% HB and 50% FB-SMs); whereas the modified voltage balancing approach has been applied to the second case (33.33% HB and 66.66% FB-SMs). To evaluate their performances, a comparative analysis of 21-level conventional HB-SM-based MMC, FB-SM-based MMC, and Hybrid MMC with both mentioned combinations, is carried out in MATLAB simulation for the corresponding voltage balancing techniques used. The impact of voltage balancing approach on switching transitions is also presented.

This paper presents a new single-phase Hybrid Modular Multilevel Converter (HMMC) topology with reduced number of sub-modules and its application in grid integration of Photovoltaic (PV) systems. This HMMC has reduced switch counts as compared to other existing multilevel topologies and so has potential advantages like reduction in converter losses and size, high efficiency, fewer circuit complexities, and the cost. The modified ripple correlation control (RCC) based maximum power point tracking (MPPT) is used to extract maximum power from the PV systems. A control strategy has been proposed for the control of the injected active and reactive power to the grid. The overall system has been simulated using MATLAB/Simulink platform, and the results are presented to support the theoretical concepts about HMMC with the control scheme proposed for the specific application.

The hybrid modular multilevel converter (MMC) consists of half-bridge sub-modules and full-bridge sub-modules, and for the application of PV power DC collection system, the system soft startup is studied in this paper. The capacitor filters on PV side converter affect the full-bridge sub-module and the half-bridge sub-module of the hybrid MMC on the grid-connected side, that the voltage of the half-bridge module and the full-bridge module can't rise synchronously. During the startup stage, the DC bus voltage is too low when the hybrid MMC under the uncontrolled charging process. So this paper proposes a stage soft start control strategy, which allows the system to eventually establish a smooth DC bus voltage with a soft manner. A hybrid MMC model for PV DC collection system has been built by MATLAB/Simulink, which verifies the correctness and effectiveness of the proposed soft start control strategy.

This paper presents a finite-control-set predictive current control (FCS-PCC) scheme for the recently proposed hybrid modular multilevel converter (HMMC)-based grid connected system. The HMMC has several advantages over the conventional modular multilevel converter (MMC) as well as other multilevel topologies. This topology has reduced number of switch counts compared to the conventional MMC, eliminates the problem of circulating current and having higher efficiency. The required number of sub-modules (SM) is half of the conventional MMC, along with an H bridge circuit per phase. This paper analyses the real and reactive power control through FCS-PCC and addresses the AC side grid unbalance by third harmonics injection method. During grid side voltage unbalance, there is a problem of the unequal amount of power exchange between per-phase converter and grid, which needs to be taken care of with an appropriate control method. This study proposed a method to add or subtract an amount of third harmonic component with the reference current signal given to the predictive controller to maintain the grid current at the desired level. The whole concept has been presented along with a simulation study in MATLAB/Simulink environment and the results are presented in support of the described concept.

Hybrid modular multilevel converter (Hyb-MMC) consisting of a mix of half bridge (HB) and full bridge (FB) submodules (SMs) has emerged as a topology capable of handling reduced dc voltage conditions without causing large increase in capital cost and losses. However, successful application of Hyb-MMC requires suitable modulation schemes that can establish stable operation. The challenge intensifies in cases of reduced dc voltage operation where the modulation index reaches values much higher than unity. This work proposes a method in which the arm voltage reference is divided between the HB and FB SMs such that the energy balance for both the groups (HB and FB) are maintained independently. The mathematical basis of the method of division of the arm voltage reference has been derived and the upper limit of modulation index for stable operation with the proposed strategy is computed. The method is tested by applying it on a Hyb-MMC simulation in EMTDC/PSCAD platform.

Hybrid modular multilevel converter (Hyb-MMC), which consists of both half-bridge (HB) and full-bridge (FB) submodules (SMs), has the capability to withstand wide variations in dc-link voltage ranging from nominal to dc short circuit conditions. With around 50% FBSMs, the Hyb-MMC can block the dc short-circuit current while exchanging reactive power with its ac grid. Other than faults, often, the high voltage direct current system may have to operate at reduced dc voltage levels with rated dc-link current, which results in arm currents becoming unipolar. This phenomenon affects the cyclic charge/discharge of the HBSM capacitors. To overcome this problem, it is proposed to inject second harmonic circulating current (SHCC) for making the arm currents bipolar and, hence, ensuring capacitor voltage balance. However, the choice of suitable amplitude and phase angle of the SHCCs is crucial to avoid increase of losses in the converter. Hence, a method to optimize the amplitude and phase angle of the SHCCs is also presented. The proposed method is verified through simulations performed on PSCAD/EMTDC platform followed by experiments on laboratory scale prototype.

Hybrid modular multilevel converters (MMCs), which combine submodule chain links and device series switches, offer advantages such as lower costs and smaller volumes compared with MMCs. However, the hybrid MMCs only operate at a fixed modulation ratio, potentially compromising system adjustment ability. This paper presents a half-wave phase shift modulation (HPSM) strategy aimed at extending the operation range of a hybrid MMC. First, the commutation angle is introduced as a control variable to change the fixed voltage modulation ratio. The energy balance of the converter is completed by adjusting the commutation angle. Then, the operation performance for the half-wave alternating multilevel converter (HAMC) with the proposed HPSM strategy is analyzed. Finally, the full-scale simulations are carried out to verify the theoretical analysis and the feasibility of the proposed control strategy. Compared to the third-order harmonic current injection (THCI) strategy, HPSM reduces operating losses by 50% and demonstrates superior control performance.

The Modular Multilevel Converter (MMC) topology is getting popularized for the medium to high voltage applications after its inception. This paper presents a modified topology of the conventional MMC, named as Hybrid MMC (HMMC). This topology has reduced number of switch counts than the conventional MMC. Therefore, it is more efficient and has less complex circuitry than other existing multi-level topologies. In this paper a modified phase-shifted carrier pulse width modulation scheme (PS-PWM) is used for the switching of HMMC. The three-phase circuit is simulated in the Matlab/Simulink for different load conditions and the results are provided to validate the concepts behind the topology.

In a hybrid modular multilevel converter (MMC), capacitor voltage balance between the Full-Bridge Sub-Modules (FBSMs) and Half-Bridge Sub-Modules (HBSMs) is only possible when the arm currents are bipolar. For a grid-connected MMC, operating at unity power factor, this is typically only achievable when the modulation index is less than 2. Previous control methodologies, based on open-loop feed-forward compensating currents, have been proposed to operate an MMC with a higher modulation index. However, these solutions do not minimize the compensating currents; they cannot compensate entirely for both the variations in the operating conditions and the parameters typically encountered in a real implementation; and they do not consider the actual capacitor voltage imbalance between the FBSM and HBSMs. In this paper, a new nested closed-loop control algorithm based on an outer voltage control loop with an inner current loop is proposed and experimentally validated. Feed-forward currents are still utilised in the inner loop, but they are calculated using a new optimising algorithm which minimises the required compensating currents. Moreover, to the best of our knowledge, this is the first work where explicit algebraic equations to calculate these compensating currents are provided. Experimental results to validate the approach, obtained with an 18-cell hybrid MMC, are presented and discussed in the paper.

This article proposes the partially signed binary voltage control (PSBVC) method for a hybrid modular multilevel converter (MMC) (HMMC), which has a mixture of half-bridge submodules (SMs) (HB-SMs) and full-bridge SMs (FB-SMs). With the proposed PSBVC method, the HMMC can generate a large number of voltage levels with much fewer SMs than the MMC and enhanced MMC (EMMC) by having different voltages for each FB-SM. That is, all HB-SMs have the same voltage and their sum in an arm is the same as the dc-link voltage. On the contrary, the voltages of FB-SM are sequentially decreased by half from HB-SM voltage. As the voltage level becomes more sophisticated, the circulating current decreases effectively because the voltages of HB-SMs are tightly regulated. As the result, the HMMC with the proposed PSBVC method is superior in not only harmonic performance but also circulating current. To verify the effectiveness of the HMMC with the proposed PSBVC method, its controller is firstly designed by the electromagnetic transients program (EMTP). Then, it is experimentally verified with the hardware prototype of HMMC. Finally, the main advantages of HMMC with the proposed PSBVC method are emphasized by comparing it with MMC and EMMC in detail.

Modular multilevel converters (MMCs) have become one of the most foremost power converter topology for medium and high power applications in recent years. In this paper, single-phase traditional and hybrid MMC are designed, simulated and compared under various voltage levels. Output voltage and current waveform are analyzed in terms of total harmonic distortion by changing modulation index at operated 7-level, 19-1evel, 31-level and 43-level traditional and hybrid converter systems. Both types of MMCs are investigated under different conditions. At the end of the study, simulation results are deeply presented with different cases and evaluated to show the performances of the designed circuit topologies.

The modular multilevel converter (MMC) is the most promising converter topology for medium- and high-power applications. One of the main concerns in the operation of the MMC, particularly for high-power applications, is its efficiency, which should be maximized. Silicon Carbide (SiC)-based devices have the potential to provide significant efficiency improvement compared with silicon devices. However, the possibility and impact of using SiC-based devices instead of silicon devices for high-power conversion have not been thoroughly explored. This paper reports on the results obtained from a detailed study to evaluate the performance of MMCs based on medium-voltage SiC MOSFETs and diodes with hybrid MMCs that employ silicon IGBTs and SiC diodes. The results are based on detailed circuit simulations that use simple physics-based circuit models. The study suggests the potential for significant efficiency gain for MMCs based on SiC power devices.