Hybrid Modular Multilevel Converter Research Articles

An Optimized Fault-Ride-Through Control Strategy of Hybrid MMC with Fewer FBSMs

The modular multilevel converter (MMC) has many advantages of low switching losses, good harmonic performance and high modularity structure in state-of-the-art HVDC applications. The full-bridge submodules (FBSMs) of the hybrid MMC can inherently output negative voltage to absorb fault currents, and consequently the hybrid MMC can ride through severe DC faults without blocking. During the DC fault-ride-through process, the submodule capacitor voltage and arm current of the MMC will be temporarily increased. These characteristics limit the proportion of the FBSMs, which should not be too low and thus increase the cost and operating losses of the hybrid MMC. In this paper, an improved sorting algorithm of SM capacitor voltage is established, and a novel virtual damping control strategy is proposed that can effectively suppress the increase in submodule capacitor voltage and arm current of the hybrid MMC during the DC fault-ride-through process. By adopting this optimization control, the proportion of FBSMs can be reduced significantly without deteriorating the fault-ride-through capability or safety of the MMC. The effectiveness of the proposed control is verified by careful theoretical analysis and detailed simulation results.

Modeling and Control of a Hybrid Modular Multilevel Converter for High-AC/Low-DC Medium-Voltage Applications

Modeling and Control of a Hybrid Modular Multilevel Converter for High-AC/Low-DC Medium-Voltage Applications

A simplified control parameter optimisation method of the hybrid modular multilevel converter in the medium‐voltage DC distribution network for improved stability under a weak AC system

AbstractTo improve the stability of the hybrid modular multilevel converter (MMC), a simplified dominant mode‐based control parameter optimisation method of the hybrid MMC system is proposed. Firstly, in the medium‐voltage DC distribution network, the small‐signal model of the hybrid MMC is established. Secondly, the influence of a weak AC system on stability is analysed through eigenvalue analysis. Finally, a simplified objective function is designed for eigenvalues of the dominant mode by considering only real parts, and improved small‐signal stability can be achieved by control parameters optimisation. The proposed method optimises all control parameters at the same time, which further reduces the number of algorithm iterations. Simulation results show that by the proposed control parameter optimisation method, the hybrid MMC has better transient performance and reduced disturbance under SCR variation, indicating a significantly improved system stability, and the dynamic response time can be reduced.

Local outlier factor based condition monitoring for sub-module capacitors in modular multilevel converters

Modular multilevel converters (MMC) as a key technology for future power grids is rapidly developing. The failure rate of sub-module (SM) capacitors is only lower than that of power devices in MMC, so accurate evaluation of the reliability of capacitors is crucial for the safe operation of MMC. Both the increase of equivalent series resistance (ESR) and decrease of equivalent series capacitance (ESC) are features for capacitor deteriorating. ESC or ESR of the capacitor reaching to limited values indicates that the end of its lifespan and it must be replaced. Most of contemporary researches only focus on ESC or ESR. This article considering the characteristics of both ESC and ESR, proposes a method for monitoring the status of SM capacitors based on machine learning algorithm: local outlier factor (LOF). By utilizing the existing SM switching function and capacitor voltage signals within a fundamental frequency cycle in the control system of MMC, the proposed strategy can not only monitor the faulty capacitors, but also can sort the degrees of abnormal states of capacitors, and the strategy is applicable to MMC with single and multiple types of SM topologies. The proposed method is verified by the hybrid MMC model on Matlab/Simulink.

Suppression of the Capacitor Voltage ripple in Modular Multilevel Converter for Variable-Speed Drive Applications

The modular multilevel converters (MMC) utilization has brought about a transformative impact on high voltage direct current (HVDC) transmission relying on voltage-sourced converters (VSC). However, their application in medium-voltage (MV) variable-speed motor drives has not achieved broad acceptance due to the substantial voltage fluctuation in the capacitor, especially at lower frequencies. The present study introduces a hybrid MMC (HMMC) aimed at markedly limiting capacitor voltage fluctuations, particularly during low motor speeds. Vector control was used to achieve the required motor speed. The proposed HMMC validity was confirmed through the MATLAB/ SIMULINK environment. The results were compared with conventional MMC from the standpoint of the capacitor voltage fluctuation at low frequencies.

Capacitor voltage balancing method for hybrid modular multilevel converters based on second-harmonic voltage injection

Capacitor voltage balancing method for hybrid modular multilevel converters based on second-harmonic voltage injection

A Novel Hybrid Modular Multilevel Converter with Three-Phase Coupled High-Frequency Modules for Multi-Index Optimization

A Novel Hybrid Modular Multilevel Converter with Three-Phase Coupled High-Frequency Modules for Multi-Index Optimization

Analysis and Design of a Novel Hybrid Modular Multilevel Converter With Time-Sharing Alternative Arm Converter

This paper proposes a time-sharing principle-based modular multilevel converter (TS-MMC) for medium- to high-voltage power transmission applications, which is designed with a lightweight running feature. The TS-MMC consists of cascaded switch stacks (CSSs) and alternative arm converters (AACs), integrating the merits of both the MMC and the two-level voltage source converter (VSC). In TS-MMC, the AACs shape the required multilevel arm waveforms, and the CSSs arrange the AACs operating at different intervals for producing the desirable output waveforms. By applying a “time-sharing” principle for the AACs, a dramatic reduction of the sub-module (SM) number is achieved. Additionally, the TS-MMC does not require any arm inductors, whereas only three inductors are equipped in AC side. Moreover, featured by the specific structure, the DC fault blocking capability of the TS-MMC is also enabled. Furthermore, to explore the performance of the TS-MMC, mathematical model and control strategy of the TS-MMC are provided. Finally, the simulation and experimental results are carried out to verify the feasibility of the TS-MMC.

Continuous Control Set Model Predictive Control of a Hybrid Modular Multilevel Converter for Wind Energy Applications

Continuous Control Set Model Predictive Control of a Hybrid Modular Multilevel Converter for Wind Energy Applications

A hybrid DC-DC modular multilevel converter with capacitors parallel connectivity for arm energy balancing

The integration of renewable energy sources in Medium-/High-voltage DC grids has called up continuous research to develop DC-DC conversion systems. This paper presents a hybrid modular multilevel DC-DC converter topology which successfully overcomes the issue of capacitors voltage imbalance during DC-DC conversion. In the proposed topology, single MMC leg is employed with half-bridge submodules in the upper arm and full-bridge submodules in the lower arm along with soft-switched high-voltage valves. The proposed hybrid modular multilevel converter has inherited arm energy equalization with the help of the parallel connectivity of involved arms’ capacitors. During the equalization period, the series-connected upper arm capacitors are connected in parallel to the series-connected lower arm capacitors to transfer the energy between the involved arms which results in operating with balanced capacitors voltages. The proposed modular DC-DC converter is a bidirectional converter which allows power flow from high-voltage side to low-voltage side and vice versa, i.e., it can be operated successfully as a DC transformer. The operational concept of the proposed configuration, control strategy, and parameters design are presented. Simulation results are presented along with experimental results of a scaled-down prototype to validate the proposed approach. Simulation and experimental results show the viability of the suggested approach.

A Lightweight Hybrid Modular Multilevel Converter Topology for DC Fault Blocking

A Lightweight Hybrid Modular Multilevel Converter Topology for DC Fault Blocking

Analysis and Suppression of Capacitor Voltage Ripple for Hybrid MMCs Under Boosted AC Voltage Conditions

Capacitor voltage ripple suppression is significant for hybrid modular multilevel converters (MMCs) to reduce the capacitance, footprint and cost. However, the existing voltage ripple suppression methods are mainly intended for half-bridge submodule (HBSM)-based MMCs, and not applicable to the hybrid MMC due to its special configuration of mixed half-bridge and full-bridge submodules (FBSMs). Therefore, this article aims to reduce the capacitor voltage ripple for hybrid MMCs under boosted ac voltage conditions. The analysis of charging and discharging behaviors of FBSMs and HBSMs with boosted ac voltage is conducted first to obtain the maximum energy ripple in SM capacitors, which is equivalent to peak-to-peak value of voltage ripple. Then, the effect of modulation index and power factor angle on the capacitor voltage ripple is investigated in detail to find the possible ways to suppress the fluctuation. Consequently, the optimal modulation index that minimizes the capacitor voltage ripple is derived. Furthermore, this article proposes a circulating current injection method to suppress the capacitor voltage ripple under various operating points. In this method, the capacitor voltage ripple amplitude is taken as the direct suppression object to ensure the effect of voltage ripple minimization. Finally, the validity of the proposed method is proved under different operating conditions by simulation and experimental results.

Power quality improvement of a distribution system integrating a large scale solar farm using hybrid modular multilevel converter with ZSV control

Power quality improvement of a distribution system integrating a large scale solar farm using hybrid modular multilevel converter with ZSV control

A review of system topologies, key operation and control technologies for offshore wind power transmission based on HVDC

AbstractOffshore wind farm (OWF) is considered as a perfect zero‐carbon energy source for the future power system. However, the growing offshore distance and water depth of OWF make the OWF HVDC transmission technique a more promising solution than HVAC due to higher cost‐efficiency and reliability. In this paper, the current situation of OWF‐HVDC projects is introduced at first. Then, novel converter topologies with the higher power density and cost‐efficiency are presented, including the hybrid modular multilevel converter (MMC), alternative arm converter (AAC), and diode rectifier (DR). Next, several OWF HVDC transmission system topologies are introduced, including terminal‐hybrid, station‐hybrid and all‐DC delivered system. Furthermore, the key technologies for OWF HVDC operation and control are summarized, including grid‐forming control strategy for offshore wind turbines, stability analysis method, corresponding stability enhancement measures and frequency support control strategies. Additionally, the fault ride‐through and protection strategies for different fault locations have been presented. Finally, the main conclusions and prospects for OWF HVDC are summarized.

Polarity of current area based active injection protection scheme for hybrid MMC-HVDC grids

With the massive increase of power electronic devices to facilitate renewable integration, the system’s fault characteristics will be dramatically changed due to the different control strategies adopted by converters. As a result, the dependability and security of the traditional protection can be severely compromised. Considering the high controllability of power electronic converters, this paper presents an active injection protection scheme to effectively identify the fault. By analyzing the traveling wave (TW) refraction and reflection characteristics under different fault conditions, the variation of the measured terminal current caused by the TW is revealed. Then, a rectangular wave is injected by the hybrid modular multilevel converters (MMC) for a short time after the fault ride-through process. By introducing the concept of the polarity of the current area, the proposed protection scheme only needs to measure the terminal current without any complex signal extracting and processing algorithm to identify the internal and external faults. The feasibility and effectiveness of the proposed protection scheme are verified by realistic case studies with simulation conducted in the PSCAD platform.

A protection method for LCC-VSC hybrid HVDC system based on boundary transient power direction

In a line-commuted converter–voltage source converter (LCC-VSC) system, in the hybrid multi-terminal high-voltage direct-current (HVDC) sending end, a line-commuted converter (LCC) converter is used, while the receiving end uses a full-half hybrid modular multi-level converter (MMC), with a fault self-clearing capability. The DC line faults can be self-cleared within 1–2 ms. Therefore, there is an inevitable problem of short availability of faulty information since only 1-ms complete fault data are available to identify faults. Thus, it is necessary to develop a fast and reliable protection scheme for an LCC-VSC system. To address this need, this paper proposes a DC line protection method based on the boundary transient power direction. First, the characteristics of boundary transient power direction of the internal and external faults are analyzed. For an internal fault, the boundary transient power direction is positive, whereas for an external fault, this direction is negative. Therefore, this paper uses the sum of the boundary transient power directions to structure internal- and external-fault identification elements and a pole detection element. The proposed method is validated by real-time digital simulations using Kun-Liu-Long LCC-VSC three-terminal hybrid HVDC–RTDS test system. Numerous tests have demonstrated that the proposed protection method is effective in detecting different fault types under various fault distances, fault transition resistance values, and signal-to-noise ratios in the period of 1 ms. Compared with the existing methods, the proposed protection principle has a better performance in withstanding fault transition resistance (600 Ω) and noise interference (10 dB).

A hybrid arm‐multiplexing MMC for DC fault ride‐through without blocking

AbstractConsidering both the DC fault ride‐through (FRT) and lightweight requirements of modular multilevel converter (MMC), this paper proposes a hybrid arm‐multiplexing modular multilevel converter (HAM‐MMC). The phase leg of the proposed MMC topology is divided into upper, middle, and lower arms, configured with full‐bridge submodules (FBSMs), half‐bridge submodules (HBSMs), and FBSMs, respectively. The time‐division multiplexing of middle arms between upper and lower arms is achieved by introducing arm selection switches, which considerably enhances the submodule utilization and thus minimizes the submodule number. The structure and zero voltage switching strategy of arm selection switches are presented, and a modified sorting algorithm is developed for capacitor voltage balance. The negative level output capability of FBSMs in upper and lower arms allows the DC voltage to be reduced in response to DC faults. Compared with the conventional hybrid MMC composed of HBSMs and FBSMs in a 1:1 ratio, the HAM‐MMC has the same power quality and DC FRT capability, but uses 25% fewer capacitors for lightweight. Simulation results demonstrate that the proposed HAM‐MMC can smoothly ride through DC voltage dip without interrupting power transmission, and can also rapidly resume normal operation after a zero‐voltage disturbance.

A Flying Capacitor Hybrid Modular Multilevel Converter With Reduced Number of Submodules and Power Losses

This paper presents a novel flying capacitor type hybrid modular multilevel converter (FC-HMMC) as an alternative to the traditional modular multilevel converter (MMC) for medium voltage (MV) applications. Combining the high voltage line-frequency switches with the low voltage high frequency submodule (SM) based chain-link, the proposed hybrid solution can balance the benefits from conventional multilevel converter and MMC. Compared to the HB-MMC, FC-HMMC can save 50% SMs, 25% devices, 37% capacitor and 20% conduction losses, hence improving the power density and power efficiency a lot. Besides, the dc and ac side is not coupled for this topology, which means the modulation index is not fixed. Another advantage is that the high voltage switch operates in soft-switching naturally at any modulation index or power factor, thereby reducing the power losses. Based on this characteristic, a unidirectional FC type hybrid modular multilevel rectifier (FC-HMMR) with only HV diodes is proposed as well. Finally, the feasibility of the proposed topology is validated by both simulation and experiments.

Low-Cost and Compact Asymmetrical Unidirectional-Current Modular Multilevel Converters

Although the modular multilevel converters (MMCs) that contain submodules (SMs) with negative voltage capability have various advantages, they usually demand many more semiconductors than the conventional half-bridge SM-based MMC (HB-MMC). This study proposes a unidirectional-current clamp-double submodule (UC-CDSM) by combining two unidirectional-current full-bridge SMs (UC-FBSMs) using a shared switching device. The sharing design enables the UC-CDSM-based MMC (UC-CD-MMC) to have 25% fewer switching devices compared with the UC-FBSM-based MMC. The quantity of switching devices is rather similar to that in a conventional HB-MMC while the UC-CD-MMC still retains the advantages, such as low capacitor usage, dc fault clearing capability, and wide-range dc voltage adjustability. Moreover, a unidirectional-current hybrid MMC composed of UC-CDSMs and UC-FBSMs (UC-HYB-MMC) is presented to further enlarge the adjustable range of dc voltage. Detailed comparisons indicate that the UC-CD- and UC-HYB-MMCs can reduce the valve costs by 32% and 25%, respectively, and volumes by 39% and 34%, respectively, compared with HB-MMCs. Simulation and experimental results verify the steady-state and dc-fault clearance of the proposed topologies, and that the capacitor voltages in the UC-FBSMs and UC-CDSMs are well maintained and balanced in the UC-HYB-MMC.

Analysis of Asymmetric Hybrid Modular Multilevel Topology for Medium-Voltage Front-End Converter Applications

Modular multilevel converters (MMCs) have been conceived as an alternative in front-end converter applications to enhance the converter system’s reliability, minimize total harmonic distortion, and improve power quality. These converters utilize several DC-link capacitors and power electronic switches, along with switches operating with high switching frequencies, to attain the desired characteristics. Thereby, this paper systematically proposes a novel three-phase asymmetric hybrid modular multilevel converter (AHMMC) for front-end converters used in lower-medium-voltage applications. The AHMMC configuration is based on a three-phase converter connected to a per-phase series arrangement with a cascaded converter module (CCM). The study investigates the AHMMC and proposes a control scheme, which minimizes the voltage range on switches and maintains the current to its reference value. Furthermore, the study also introduces an active balancing of voltage across DC-link capacitors based on the phase opposition disposition PWM (POD-PWM) method. Our new configuration has features such as low switching loss, reduced DC-link voltage, a wider modulation range for the unity power factor (PF), and low voltage and current harmonic distortion. The simulation results are added to verify the performance of the new AHMMC topology and the usefulness of the modular control scheme. In addition, a low-voltage laboratory prototype based on customized control and power boards is built to validate the proposed converter and its control scheme in practice.