Bridge Sub-module Research Articles

Comparative analysis of submodules and design of seventeen-level modular multilevel converter using a five-level submodule block

Abstract Multilevel inverters and various modulation techniques have been proven to be the best solutions to overcome all the limitations of conventional two- or three-level voltage source inverters. In recent years, among all topologies, modular multilevel converter (MMC) has advantages like low total harmonic distortion (THD), reduced filter requirement, fault-tolerant operation, scalability, modularity, transformerless operation, etc. The main application of MMC is found in high-voltage DC transmission (HVDC). This study describes and analyses the performance of two distinct MMC submodule (SM) topologies: the Half Bridge submodule (HBSM) and the Full Bridge submodule (FBSM). Switching pattern is simpler in HBSM topology but it has the disadvantage that it does not have DC fault current blocking capability. Whereas FBSM not only inherently can block the DC fault current but also has advantages like reduced volume of MMC and better performance. Further, having the capability to produce three voltage levels, it can be operated in boost mode. Also, by using a proper switching pattern, MMC can be designed to generate a various number of output levels by using the same five-level submodule block. Out of which 17 level MMC gives the best output. The MATLAB-Simulink model is used to simulate the circuit, and the findings are confirmed for a 17-level output voltage.

Real-time harmonics optimization of seven level converter for megawatt scale solar PV-utility integration

This paper presents a new single photovoltaic (PV) array fed cascaded H bridge (CHB) submodule based three-phase 10 MW solar photovoltaic (SPV) plant for medium voltage utility applications. Each submodule comprises of a single PV array, cascaded H bridge cells and line frequency transformers. A 10 MW seven level converter based solar PV plant is realised with four submodules feeding solar PV array power to 33 kV grid. In megawatt scale multilevel converters based configuration, semiconductor switches experience reduction in voltage stress and requirement of high switching frequency for gating for switches is eliminated. Moreover, an efficient single stage MPPT based decoupled controller is implemented with genetic algorithm inspired fundamental frequency switching. GA based optimization technique is used to calculate optimal switching angles in offline mode. Obtained switching angles are incorporated to achieve closed loop optimized decoupled control of SPV system. Lower order harmonics are eliminated and power is fed to grid at enhanced power quality. This large scale PV system is modelled in MATLAB-Simulink and simulated results are validated in real time.

A new hybrid multilevel thyristor-based DC-DC converter

The rapid growth in HVDC grids is becoming inevitable for long-distance power transmission. Therefore, the idea of interconnection between the point-to-point links becomes essential. However, these point-to-point connections face several challenges such as the requirement of DC fault blocking capability, interfacing of different grounding schemes, offering multi-vendor interoperability, and difficulty to achieve high DC voltage stepping. DC-DC converters are considered the optimum solution to tackle these challenges in DC grids interconnection. In this paper, a new hybrid modular DC-DC converter is proposed that achieves a low number of semiconductors, low losses, and cost in comparison to other DC-DC converters due to the utilization of thyristors. The new DC-DC converter consists of two hybrid MMC bridges connected through an isolating transformer. Each MMC bridge is comprised of half bridge submodules and bidirectional thyristors. Detailed mathematical analysis, design, and control are illustrated. A comparison is carried out between different topologies in terms of semiconductor count, power loss, and cost. Also, both simulation model and experimental test rig are built to validate the proposed hybrid modular DC-DC converter under different scenarios. Finally, another variant of the hybrid-thyristor based converter (version two) is proposed for multiport DC-Hub application to achieve DC fault blocking without turning off all connected bridges.

A Modular Multilevel Converter-Based Pulsed Electric Field Generator Design for Electroporation Applications

After a short historical background and mentioning common application areas and different clinical modalities of pulsed electric fields and the phenomena such as electro permeabilization and electroporation are introduced based on the concept of the basic mechanism. Subsequently, pulse generation (PG) and the adapted modular multilevel converter (MMC) topology that is to be covered are introduced including the operating principles and very basic analysis. Finally, performance of the proposed MMC with four half bridge submodules is tested on a prototype built in laboratory. A bidirectional voltage is produced with 80 V amplitude pulsating at 100 kHz operation frequency.

Exploration of the HBSM MMC Five-Level Inverter for D-STATCOM Application

This paper presents the concert exploration of a Modular Multilevel Converter (MMC) and its application for D-STATCOM. The Half Bridge (HB) sub-module is adopted in the MMC due to its simplicity and low conduction losses. An advanced modulation technique of the Phase Disposition (PD) scheme is applied for the control of the HB MMC switches. The converters are facing DC voltage balance problems under non-linear load conditions. The Proportional and Integral (PI) controller is adopted for the balance of DC voltage. Performance parameters such as apparent power, active power, reactive power, power factor, DC voltage balance, and Total Harmonics Distortion (THD) are presented and compared with the ones of the traditional two-level converter. The proposed scheme is simulated in MATLAB/Simulink.

A Multi-Port DC Power Flow Controller Integrated With MMC Stations for Offshore Meshed Multi-Terminal HVDC Grids

Multi-terminal dc (MTDC) systems have the potential to facilitate the integration of offshore wind farms. A dc power flow controller (DPFC) is necessary for meshed multi-terminal high voltage dc (HVDC) grids to regulate the power flow in dc lines. However, DPFCs based on modular multilevel converters (MMCs) require two dc-ac conversion stages and an extra ac transformer, which increases the cost and loss for offshore platforms. This paper proposes a multi-port DPFC that is integrated internally into the offshore MMC station, eliminating the need for an extra transformer and reducing dc voltage ripple. Additionally, the full bridge submodules (FBSMs) in the DPFC can further reduce the number of FBSMs required in the MMC station for dc fault blocking. The proposed DPFC is composed only of cascaded FBSMs connected to the arms of the offshore MMC station, which can exchange power directly with the MMC valve to achieve power balance. By using the MMC station, the proposed DPFC requires a small kVA rating, approximately 1%-5% of the total system power rating. The performance of the proposed DPFC is validated through simulation of a hybrid system of the IEEE 39 bus and the Cigre HVDC benchmark, and also through down-scale experimental testing.

Novel Distributed Control Platform and Algorithm for a Modular Multilevel Matrix Converter

The Modular Multilevel Matrix Converter (M3C) is an attractive topology for low-speed drives and DFIG applications. Both modularity and scalability make the topology attractive for high-power medium-voltage systems. One of the main challenges for the design and implementation of an M3C is the control platform, which has to handle a high number of submodules and measured values. This paper introduces a new control platform including a high-speed communication network between the distributed control units. The control platform and algorithms are implemented on a 15 kvar M3C test bench with 108 full bridge submodules.

Inter-cluster voltage balancing control of a delta connected modular multilevel cascaded converter under unbalanced grid voltage

A new inter-cluster DC capacitor voltage balancing scheme for a delta connected modular multilevel cascaded converter (MMCC)-based static synchronous compensator (STATCOM) is presented. A detailed power flow analysis of applying negative sequence current (NSC) and zero-sequence current (ZSC) injection methods in addressing the issue of inter-cluster DC voltage imbalance under unbalance grid voltage is carried out. A control scheme is proposed which integrates both inter-cluster methods using a quantification factor QF. This is used to achieve the sharing of the inter-cluster active power between the NSC and ZSC injection methods. An accurate method of determining the quantification factor is also presented. The proposed method offers better sub-module DC capacitor voltage balancing and prevents converter overcurrent. The influence of unbalanced grid voltage on the delta connected MMCC-based STATCOM rating using this integrated cluster balancing technique is investigated. The control scheme is verified with a 5 kV 1.2MVA MMCC-STATCOM using 3-level bridge sub-modules, and the results show the advantages of the proposed method over other inter-cluster methods.

Low Inductance Design for Symmetrical Submodules in Hybrid HVDC Circuit Breaker

The diode bridge submodule (SM) is one of the basic units of a hybrid high-voltage dc circuit breaker. This article works on the low inductance design of a single SM and series SMs. For the layout design of a single SM, this article proposes a simple and clear +-shaped SM layout, the low inductance design is simplified to the optimization of two laminated busbars with a simple structure. Based on the +-shaped SM layout, this article further proposes an SM structure with holes in diodes region, the stray inductance is further reduced, and only one integrated laminated busbar is needed in a single SM. For the layout design of series SMs, this article makes full use of the layout of a single SM and the mutual inductances between the SMs, series SMs are alternately connected in reverse. The single SM layout proposed in this article is effective in reducing the stray inductance. The loop stray inductance in stage II reduced from 148.4 to 72.3 nH, a 51.3% reduction. When the two parallel IGBTs turn off a 6 kA fault current in 2 μs, the voltage spike is reduced from 786.1 to 423.4 V, a 46.1% reduction. And the stray inductance difference of the bidirectional current path is controlled to about 10 nH. The layout of series SMs proposed in this article is effective in reducing the length of the series SMs, the length is reduced by 16.7%. The experimental results show that the SMs layout proposed in this article is effective.

Continuous operation of LVDC source/load under DC faults in MMC-DC distribution systems

With penetration and usage of renewable sources and loads based on power electronics increasing, the modular multi-level converter based direct current (MMC-DC) distribution system is getting broad attention. In MMC-DC system, DC/DC converters are employed to link the low voltage DC (LVDC) source/load and the medium-voltage DC (MVDC) distribution system. DC fault ride through capability at MVDC side is one of the technical difficulties, and has been the focus of previous studies. However, whether LVDC source/load could operate continuously under MVDC faults is still an open question. In this paper, key facilities, protection procedures and operating characteristics of LVDC source/load in two typical MMC-DC frameworks based on half bridge submodules (HB-MMC-DC) and full bridge submodules (FB-MMC-DC) are studied respectively. Analyses show LVDC source/load could operate continuously in HB-MMC-DC system with the help of DCCBs, but with voltage variation exceeding acceptable range, they fail to operate continuously in FB-MMC-DC systems. Therefore, a controllable storage unit is deployed at the LVDC side of the DC/DC converters to suppress voltage variation. The working principle and parameter design of the proposed solution are presented. Case studies are carried out in PSCAD/EMTDC for verification. Finally, comprehensive comparisons between the two MMC-DC systems are conducted.

Development of a Modular Educational Kit for Research and Teaching on Power Electronics and Multilevel Converters

This paper presents the detailed project of a modular educational kit to be used in research and teaching activities in the area of power electronics - with emphasis on applications in medium voltage multilevel converters. The half bridge topology has been chosen for the submodules, as they can be arranged together in a multitude of other topologies. A system composed by a mother board and daughter cards is presented. A DSP control card and a FPGA System-On-Module are inserted in the mother board in order to control the half bridge submodules. The targets of this project are applications in medium voltage - hence, the communication between the submodules and the FPGA/DSP is performed through optical fibers. Also a self power system is presented for the submodules, in order for them to operate at floating potentials. Experimental results have been presented for the most usual topologies of converters, including a Modular Multilevel Converter operating at 2000V.

Research on Design of Guangzhou LCC-HVDC Converter Station Retrofit for VSC-HVDC Converter Station

Introduction In order to avoid the commutation failures of receiving end convertor converter station in LCC-HVDC transmission systems， and to improve the voltage stability of receiving end converter station AC system， The research retrofitted Tianguang HVDC receiving end converter station for VSC-HVD， and expansion to 3 GW. Method Based on meeting the various operation modes of flexible DC transmission， facilitating capacity increase， adapting to the level of equipment research and development， and making full use of existing equipment and buildings to reduce the difficulty and cost of transformation. Research compared different converter valve topologies and submodule topologies， studied the setting of the start-up circuit， the electrical wiring of the DC field， and the type of interface transformer. Result Research and analysis show that the capacity-increasing transformation plan recommends the use of a 1.5 GW converter valve per pole， half bridge-full bridge submodule hybrid type， symmetrical bipolar wiring， start-up circuit set on the side of the valve， large transverse size connection transformer. Conclusion This research provides the ideas and methods for the transformation of existing of the existing LCC-HVDC converter station retrofit for VSC-HVDC converter station.

A New Grid Interactive 11-Level Hybrid Inverter Topology for Medium-Voltage Application

Multilevel inverter (MLI) has wide applications due to its numerous advantages. An improved configuration of eleven-level MLI suitable for medium-voltage range has been proposed in this article. The proposed topology contains two half-bridge submodules, one flying capacitor MLI, and one full bridge submodule. A simple method of capacitor voltage balancing has been demonstrated with less ripple. The study about the effectiveness of proposed MLI has been justified by loss calculation to find the efficiency and voltage ripple analysis of the capacitor. The comparative assessment in terms of switch count, driver count, capacitor requirement, device peak inverse voltage, total standing voltage (TSVp.u), cost function, etc. for the developed hybrid inverter (HI) topology has been presented to establish the utility of the proposed HI. This topology has been checked for its operation under various operating conditions, including frequency and modulation index variation, overmodulation range, and also with motor load. The grid connectivity option of the proposed HI has been verified to ensure the suitability for application with renewable energy sources.

A Hybrid-Arm Modular Multilevel Converters Topology With DC Low Voltage Operation and Fault Ride-Through Capability for Unidirectional HVDC Bulk Power Transmission

In this paper, a novel hybrid-arm modular multilevel converters (HA-MMCs) topology with DC low voltage operation and fault ride-through capability is proposed for unidirectional HVDC bulk power transmission. The HA-MMCs topology is a combination of two types of converter arms: the half-bridge sub-module based arm (HBSM-arm) and the diagonal bridge sub-module based arm (DBSM-arm). Replacing the traditional HBSM-MMC, the HA-MMCs are operated in current and reactive power control modes during normal conditions and can work as the rectifier or the inverter. Since the DBSM can provide bipolar voltages with unipolar currents, the HA-MMCs need not change its control strategy under DC low voltage conditions caused by the high-impedance faults. Under DC pole-to-pole or pole-to-ground faults, the HA-MMCs can both clear the DC fault current and work as the static synchronous compensator (STATCOM) to provide reactive power to support the AC grid. Compared with other MMC topologies with DC fault ride-through capability, the HA-MMCs topology is more economical for significantly decreasing the number of power devices. The operation states, control strategy and DC low voltage operation capability of the HA-MMCs are presented and simulation results of the LCC-HA-MMC hybrid system using PSCAD/EMTDC validate the characteristics and advantages of the HA-MMCs.

A new precharging strategy for hybrid MMC based on HBSM and FBSM

Precharging of the modular multilevel converter (MMC) is an important basis for ensuring the normal operation of MMC-HVDC system. The hybrid MMC based on full bridge sub-module (FBSM) and half bridge sub-module (HBSM) has reliable performance in DC fault ride-through capability and is receiving more and more attention. Due to the different charging characteristics of FBSM and HBSM, the sub-modules (SMs) capacitor voltages may be different at the end of the uncontrolled precharging process. This paper introduces conventional uncontrolled precharging strategy of the hybrid MMC and points out the problems in the process. Then a three-stage precharging strategy is proposed in this paper. This strategy can eliminate the capacitor voltages imbalance of different SMs and solve the problem of unsuccessful self-taking energy of HBSM during precharging process. Finally, simulation and analysis of the uncontrolled precharging process based on the proposed strategy are carried out.

A novel DC traction power supply system based on the modular multilevel converter suitable for energy feeding and de-icing

A novel DC traction power supply system suitable for energy feeding and de-icing is proposed in this paper for the urban rail transit catenary on the basis of the full bridge submodule (FBSM) modular multilevel converter (MMC). The FBSM-MMC is a novel kind of voltage source converter (VSC) and can directly control the output DC voltage and conduct bipolar currents, thus flexibly controlling the power flow of the urban rail transit catenary. The proposed topology can overcome the inherent disadvantages of the output voltage drop in the diode rectifier units, increase the power supply distance and reduce the number of traction substations. The flexible DC technology can coordinate multiple FBSM-MMCs in a wide area and jointly complete the bidirectional control of catenary power flow during the operation of the electric locomotive, so as to realize the local consumption and optimal utilization of the recovered braking energy of the trains. Besides, the FBSM-MMCs can also adjust the output current when the locomotive is out of service to prevent the catenary from icing in winter. The working modes of the proposed topology are illustrated in detail and the control strategy are specially designed for normal locomotive operation and catenary de-icing. Simulation cases conducted by PSCAD/EMTDC validate the proposed topology and its control strategy.

A Modular Multilevel Converter With Novel Double Reverse Blocking Sub-Modules for DC Fault Current Blocking Capability

The half bridge sub-module (HBSM)-based modular multilevel converter (MMC) is widely used in high voltage direct current (HVDC) transmission systems. However, it cannot block the dc fault current due to the uncontrolled freewheeling diodes in IGBT modules. The existing sub-modules with inherent dc fault current blocking capability still suffer from large power device amounts and complex control schemes. To solve these issues, this brief proposed a novel double reverse blocking sub-module (DRBSM), which has the fewer power devices than the existing sub-modules. With the same output voltage level, DRBSM has the same dc fault current blocking capability as full bridge sub-module (FBSM) and the same control logic circuit as HBSM. In order to further reduce the amount of power devices, a hybrid MMC based on HBSM and DRBSM is also proposed and the design principle is given in detail. Two study cases are built in simulation to verify the proposed DRBSM-based MMC and hybrid MMC well.

Imbalance Mechanism of Capacitor Voltage and Control Strategy for Hybrid Modular Multilevel Converter with Negative Output

In a flexible DC transmission system with a large-capacity overhead line, a hybrid modular multilevel converter (MMC) consisting of the half bridge submodule (HBSM) and the full bridge submodule(FBSM) has become a research hotspot due to its self-cleaning capability of DC short-circuit fault. Under the operating conditions of reducing the DC voltage, due to the different charging and discharging characteristics of the half bridge submodule and the full bridge submodule, a capacitor voltage imbalance may occur between the two submodules. This paper first studies the number configuration of half-bridge submodules and full-bridge sub-modules in modular multilevel converters. Then, the capacitor voltage imbalance mechanism of the hybrid MMC sub-module is analyzed in detail, and the optimal control strategy of the capacitor voltage balance is derived according to the configuration requirements of the two sub-modules. Finally, a hybrid MMC-based flexible DC transmission system is established in the PSCAD/EMTDC simulation environment, low DC voltage operation of the system are simulated to verify the correctness and effectiveness of the control strategy.

Effect of Loss Distributions on the Balance of Capacitor Voltages in MMC Using Full Bridge Sub-Modules

With the development of power electronics technology, modular multilevel converter (MMC) has become a popular converter due to its high modularity and good output characteristics. In recent years, half-bridge MMC has been widely studied. Compared with half-bridge MMC, MMC using full bridge sub-modules is more competitive because of its excellent DC short-circuit fault ride-through capability. Furthermore, the balance of capacitor voltages of sub-modules has a crucial influence on the working performance of MMC. To figure out the influence of non-ideal factors on the balance of capacitor voltages, this paper analyses the difference of the capacitor voltages and compares the unbalanced degree of capacitor voltages under different conditions. Firstly, in an ideal situation, the analytical expressions of bridge arm current and capacitor voltage increments are obtained, revealing the dynamic balance process of capacitor voltages. Secondly, expressions in a non-ideal condition reveal the effect of on-resistances of switching devices on the capacitor voltages. Finally, the influence of loss distributions in sub-modules on the balance of the capacitor voltages is analysed theoretically. Simulation models are built in PLECS to verify the theoretical analysis.

An Enhanced Modular Multilevel Converter With Flexible Interruption Capability of DC-Side Fault

The modular multilevel converter (MMC) has a promising prospect in high voltage direct current (HVDC) transmission, while the application of MMC is limited by lacking of dc fault interruption capability. Hence, an enhanced MMC topology with the flexible interruption capability of dc fault is proposed to handle this problem on the basis of the exiting dc fault blocking technology. The enhanced flexible interruption MMC (EFI-MMC) makes two improvements in topology compared with the traditional MMC, which adapts quasi-half bridge sub-module (QHBSM) and adds the bridge arm power consumption circuit. The QHBSM can interdict the infeed current from the ac system when dc-side fault occurs by combined with reverse voltage control strategy. Meanwhile, the residual energy of arm reactor and dc smoothing reactor can be absorbed by the arm power consumption circuit to suppress the voltage stress of reactors, which can reduce the technical requirements of arrester. Finally, a ±200kv EFI-MMC simulation system is built to verify the validity of the proposed topology.