Modular Multilevel Converter Submodules Research Articles

Novel data-driven open-circuit fault diagnosis method for modular multilevel converter submodules based on optimized deep learning

Novel data-driven open-circuit fault diagnosis method for modular multilevel converter submodules based on optimized deep learning

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.

A Novel Mixture-Devices-Based Submodule for MMC by Using Low on-State Voltage IGCT and High di/dt Ability IGBT

At present, insulated-gate bipolar transistor (IGBT) is widely used as the converter component in the submodule of modular multilevel converter (MMC) for its excellent performance in driving circuit and switching speed, but the further application is restricted by its high conduction loss and cost rate. Integrated gate-commutated thyristor (IGCT) is another converter device used in MMC with low conduction loss and cost. However, the d <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</i> /d <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> capability of IGCT is limited so that a snubber circuit is required in the submodule for protection, which generates unwanted power loss for MMC. For better performance and reliability, this paper proposes a mixture-devices-based submodule (MS) for MMC based on the cooperation of parallel IGCT and IGBT devices. Double-pulse switching experiments for the MS are conducted to verify its effectiveness, and to get the data for the follow-up switching power loss calculation. The power loss of MMC based on IGBT, IGCT and mixture devices are calculated based on the model established in this paper. Furthermore, the comprehensive comparison for power loss among IGBT-based, IGCT-based and mixture-device-based MMCs are performed, proving the effectiveness of the mixture topology in terms of power loss reduction. Meanwhile, the proposed MS is cost-competitive.

Optimal Busbar Design for the Press-Packed IGBT-Based Modular Multilevel Converter Submodule Considering Both Normal and Fault Ride-Through Conditions

Optimal Busbar Design for the Press-Packed IGBT-Based Modular Multilevel Converter Submodule Considering Both Normal and Fault Ride-Through Conditions

Power Mismatches Elimination Strategy for MMC-Based Photovoltaic System and Lightweight Design

For the modular multilevel converter (MMC) based photovoltaic system (MMC-PV), when the sub-modules (SMs) of MMC are connected to distributed photovoltaic (PV), power mismatches between legs and arms is the inherent problem. To eliminate the power mismatches, this article proposes a lightweight MMC-based grid-connected energy router (MGER) integrated PV and energy storage (ES). Large-scale PV and centralized ES are connected to horizontal three SMs through four-terminal dual-active-bridges (FDAB). The primary three full-bridges of which adopts synchronous switching to achieve mismatch power elimination between horizontal SMs, so legs power balance can be realized. In addition, the SMs three-phase symmetrical ripple-power can also cancels each other out under synchronous switching of primary FDAB, so as to achieve low capacitance and lightweight of MGER, and the MMC arms 2nd-order circulating current can be also eliminated from root cause. In this article, the power mismatches analysis, MGER configuration, power mismatches elimination, control method, and lightweight implementation are discussed in detail. Finally, the correctness and effectiveness of proposed scheme are verified by simulation and experiment.

Comprehensive Analysis and Experiment of Extreme Faults in MMC Based on IGCT

High voltage direct current voltage source converter (HVDC-VSC) technology is one of most important solutions for transmission of large-scale renewable sources. Modular multilevel converter (MMC) is one of the most popular topologies for HVDC-VSC system. The extreme faults affect the safety and reliability of MMC greatly. This paper gives comprehensive analysis and experiment of extreme faults in MMC based on integrated gate commutated thyristor (IGCT). Causes and characterization of the extreme faults are analyzed in detail. A new protection solution based on IGCT with controlled punch-through design (CP-IGCT) and fast recovery diode with high surge current capability (HS-FRD) is proposed. The surge current capability of HS-FRD is validated via experiment successfully. Theoretical calculation shows that the protection solution based on CP-IGCT and HS-FRD can help reduce the maximum fault current below 50% when the anode inductor is 0.8 μH in the extreme shoot-through fault. Test results show that the protection solution can realize the discharge of the DC capacitor below 4.5 kV in the extreme shoot-through fault. The maximum fault current in MMC based on IGCT is only about 600 kA while the maximum fault current in MMC based on insulated gate bipolar transistor (IGBT) is over 1100 kA. Explosion proof of CP-IGCT is validated during the tests, which is similar to that of the protection thyristor. However, IGBT can't guarantee the stability of the package and may cause severe safety problem. The failed CP-IGCT has the short circuit failure mode (SCFM) and can be used for bypass of the faulty MMC sub-module (MMC-SM). Via comparison, the proposed protection solution has lowest cost and complexity, as well as the highest safety.

Gate modulation drive and modeling of IGBT in MMC converters

The energy surplus caused by the alternating current (AC) system’s failure results in a severe overvoltage problem in Bai hetan-Jiangsu ±800 kV hybrid cascade direct current (DC) project. However, the insulated gate bipolar transistor’s (IGBT) off-state voltage is the crucial reason for the insufficient voltage capacity of modular multilevel converter (MMC). Therefore, analysis and suppression of the off-state overvoltage are of vital significance. The off-state model of IGBT in an MMC converter is established, and the process of IGBT’s turning off is analyzed. Then, based on the characteristics of IGBT’s off-state and traditional resistance switching strategy, the gate modulation drive strategy is proposed, and the equivalent topology of the MMC submodule with the strategy is established. Finally, MMC submodule models with gate modulation drive and resistance switching strategy, respectively, are built in simulation software, and two groups of data are compared under multiple groups of the control cycle and reference current slope given. The simulation results show that the gate modulation drive supresses the off-state overvoltage on IGBT better than the resistance switching strategy. Moreover, the shorter the control cycle and the less reference current slope, the better the overvoltage suppression.

Research on life prediction control of MMC submodule based on fiber sensing

With the rapid development of energy, power electronic equipment plays a crucial role in renewable energy. Modular multilevel converter (MMC) has turned into cutting-edge inverter topology in flexible HVDC and offshore wind power situations. However, under the conditions of high humidity and high voltage, the internal submodules lifetime of MMC might be affected. The system reliability mainly depends on good running of the submodules, so stable performance of the submodules serious threat to the normal operation of the inverter. Therefore, this paper proposes a model based on Peck accelerated life predictable model, and model predictive control (MPC) strategy is improved, at the same time without affecting the external characteristics can better control the submodule voltage fluctuations. What is more, real time monitoring of the working environment of the converter through the optical fiber sensing module. Finally, the experimental and simulative outcomes show that the proposed control strategy can effectively reduce the voltage fluctuations in the small module, and the life parameters of the submodules can be obtained in real time by bringing the results into the proposed life prediction model. By comparing the results, the results can be obtained the proposed control strategy can effectively increase the reliability of the submodule.

Optimization-Based Methodology to Design the MMC's Sub-Module Capacitors

This paper proposes an optimization-based size reduction methodology for Modular Multilevel Converters (MMC), focusing on minimizing the converter's sub-module capacitor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$C_{SM}$</tex-math></inline-formula> . The analysis is performed considering both the converter's current and voltage limitations and the Transmission System Operator (TSO) Fault Ride Through (FRT) requirements. By means of a steady-state analysis, the time-domain expressions of the converter's arms energies are obtained and their behavior throughout the MMC's operating range is shown. Based on these expressions, the optimization-based problem to reduce the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$C_{SM}$</tex-math></inline-formula> size is developed and its constraints are imposed to ensure that the converter's voltage and current levels are within its design limitations. The suggested method is compared with different approaches for distinct active and reactive power set-points, where it is shown that the SM capacitor size can be reduced up to 24% in comparison with the method with worst performance and up to 7% regarding the best method used for comparison purposes. Furthermore, time-domain simulations of the MMC considering several AC voltage sags are performed in order to demonstrate that the dynamics of the SM capacitor and the arm applied voltages are within acceptable margins during the different operations.

Research on MMC Improved Sub-Module Topology With DC Fault Ride-Through and Negative Level Output Capability

Modular multilevel converter (MMC) is a modern and preferred topology among the voltage source converters (VSCs) in high voltage direct current (HVdc) transmission. However, the traditional half-bridge sub-module (SM) cannot block the dc fault current. Hence, the novel SM topology with dc fault ride-through capability is introduced. This article proposes an improved diode-clamp SM (IDCSM) structure. Based on the DCSM topology, the two capacitors’ connection in the IDCSM is modified to operate independently and generate output voltage levels of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- u_{c}$ </tex-math></inline-formula> , 0, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$u_{c}$ </tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2u_{c}$ </tex-math></inline-formula> based on the additional switching devices. The IDCSM with dc fault ride-through and negative level output capability can increase the number of output levels and significantly reduce hardware cost. In addition, this article adopts an equivalent half-bridge modulation (EHBM) strategy that can modulate all the MMC SM topologies with the comparison algorithm, simplifying the complicated logic calculation process. Then the IDCSM power loss, cost, and dc fault ride-through characteristics are illustrated in detail. Finally, the simulation and experimental results verify the feasibility of the proposed IDCSM topology and EHBM.

Development of a MMC demonstrator for nuclear fusion devices power supplies

The modular multilevel converter (MMC) has become one of the most attractive converters for high-power applications such as high voltage DC (HVDC) converters, but also for fusion devices power supplies. The combination of this technology with a high power density energy storages such as supercapacitors (SC) represents a promising alternative to power the toroidal field (TF) magnets of the ASDEX (Axially Symmetric Divertor Experiment) Upgrade experiment operated at the Max-Planck-Institute for Plasma Physics (IPP) research center. After a first feasibility study, a single MMC submodule (SM) composed by four insulated-gate bipolar transistors (IGBT) forming a 4QC (4-quadrant chopper), a SC module and a power stage filter has been developed and successfully tested. Therefore, three additional identical modules were built to test their series and parallel operation in order to prove their capability to be scaled up.This work shows the step-by-step development of the MMC demonstrator, highlighting the results of the SM synchronized operation which is fundamental for the scalability of the system. The outcome of these experiments is relevant not only for the specific application of ASDEX Upgrade TF coils, but also for many other applications due to the flexible four-quadrant operation of the converter.

An Asymmetrical Power Module Design for Modular Multilevel Converter With Unidirectional Power Flow

Modular multilevel converters (MMCs) are widely used in grid scale power electronics. Due to the presence of a dc component in the arm current, the MMC submodules (SMs) show an unbalanced current loading profile. This may lead to some semiconductor chips being more stressed than others, significantly affecting the overall reliability. To level the junction temperatures and improve the SM reliability, this article proposes an asymmetrical power module design in line with the loading profile, allocating more chips to higher current parts. This is shown to be realistic if the power flow is unidirectional. The concept is investigated using simulation backed by experiment. It is shown that the designed power module can significantly reduce the maximum temperature and the total power loss can be reduced by 5%–8%. The chip utilization rate is also improved. The proposed module is applicable in a certain range of power factor and modulation index adjustment. This article provides a guidance for optimum design of the power module for the SMs in an MMC.

A modified submodule of modular multilevel converter using active power decoupling method for reducing capacitor voltage ripple under low‐frequency operation

AbstractIn this paper, a modified submodule (MSM) method is proposed to reduce capacitor voltage fluctuation by integrating a bidirectional buck‐boost type active power decoupling (APD) circuit into each SM for the low‐frequency operation of a converter system. The capacitor voltage fluctuation of the proposed MSM‐MMC can be significantly reduced compared to conventional SMs with the same capacitance by transferring the low‐frequency ripple power components to the auxiliary energy storage capacitor, resulting in a lower voltage fluctuation of the main capacitor in the SM, which requires a lower total SM capacitance. Additionally, a closed‐loop controller is proposed to control the APD capacitor voltages by inheriting the advantages of the repetitive controller and proportional‐integral (PI) controller to enhance the performance of SM capacitor voltage ripple suppression. The method can improve the start‐up performance of medium voltage drives without injecting high‐frequency components, which can avoid the common‐mode voltage imposed on the AC side of the system. The operating principle, auxiliary capacitor voltage control, and capacitor voltage ripple suppression control of the proposed MSM‐MMC method are analyzed in detail. The feasibility of the proposed method has been confirmed by simulation results based on case studies using MATLAB/SIMULINK software and further verified with hardware‐in‐the‐loop simulation (HILS) using the real‐time simulator OPAL‐RT platform.

Mission Profile Emulation for Flexible Number of Submodules in Modular Multilevel Converters With Nearest Level Modulation

Mission profile emulation (MPE) for submodules (SMs) in the modular multilevel converter (MMC) is a crucial step before a full-scale system is put into field operation. Existing methods suffer from limitations of the testing accuracy due to the staircase-like arm voltages that result in distorted loading current and require large filter inductance. This is especially the case when a limited number of SMs are under test. In this article, an efficient MPE method is proposed to emulate the loading behaviors of multiple SMs in MMC. The testing circuit allows two arms of MMC with flexible SM number, operating in inverting and rectifying mode under nearest level modulation, to be tested simultaneously with reduced dc supply voltage. A common H-bridge circuit is introduced in the testing circuit, with each arm of the H-bridge circuit functioning as the voltage compensator by using feedforward control. In this way, the current distortion can be significantly suppressed, and testing accuracy can be guaranteed in a cost-efficient manner even though no large inductor is included in the testing circuit. The proposed method allows SMs under test to be loaded as in the actual back-to-back MMC system. Simulations and experimental measurements are given to validate the proposed method.

Studies on Alternative Schemes for the MMC-Based Acceleration Grid Power Supply of DEMO Neutral Beam Injector

The European DEMO will make use of a significant additional heating power, which could be partly provided by neutral beam injectors (NBIs), which accelerate negative ions by means of grids placed at increasing potentials. These grids will be fed by the acceleration grid power supply (AGPS), divided in a number of stages, which has to provide an overall dc voltage down to −1 MV and currents in the order of tens of Amperes. The AGPS will have to satisfy a set of static and dynamic requirements, mainly in terms of ripple, accuracy, and rise time. In addition, during the NBI operation, frequent losses of insulation between the grids, called breakdowns (BDs), are expected. The AGPS will be able to handle such events by nullifying the output currents as fast as possible in order to limit the energy discharged onto the grids. Adopting the modular multilevel converter (MMC) technology for the AGPS of DEMO and future tokamaks seems promising, due to its intrinsic properties of modularity, high efficiency, fast dynamic response and small energy transferred to the arc in case of BD. Since the converter will be air-insulated, one of the main drawbacks is the large volume occupied. This can be partly reduced by adopting alternative MMC schemes, to minimize the number of components and optimize the counter-voltage applied by the converter at BD. In this article, alternative topologies for the MMC submodules (SMs) or combinations of different schemes [full-bridge (FB), half-bridge (HB)] are investigated. After a preliminary design of the converter, the results of numerical simulations carried out with circuit models are shown, with control schemes customized for the NBI operation. The performance of the different solutions in steady-state, dynamic and anomalous conditions are discussed, with particular focus on BD events. Finally, thermal analyses on the converter are carried out, to verify whether natural convection of air can be a suitable cooling method for the power components.

Discrete Time Adaptive Observer for Capacitor Voltage Estimation of MMC

Modular multilevel converters (MMC) are increasingly gaining its importance in HVDC, drive applications due to several attributes such as modularity, scalability, low device stress, excellent output voltage waveform. For MMC operations, it is important to accurately measure the voltages across the individual capacitors of submodules (SMs). These voltage sensors, while upgrading for high voltage applications, may cause a few concerns in terms of cost, complexity, and data acquisition system. To avoid all these, a discrete time adaptive observer (DTAO) is proposed in this article to observe the voltages across the capacitors of the SMs and use the observed values for closed-loop operation of the MMC. The proposed observer is designed by considering the parametric uncertainty in the MMC and its convergence is proved mathematically. Moreover, the DTAO is compared with existing observation techniques for MMC SM capacitor voltages. The efficacy of the proposed observer is shown through simulation and validated using an experimental setup.

A Line-Frequency Commutated HV VSC Embedded Modular Multilevel Converter With DC Fault Blocking Capability

Modular multilevel converter (MMC) has become one of the most favored topologies in medium-voltage and high-voltage (HV) applications. However, it still suffers from some drawbacks, including a lot of sizeable capacitors, high conduction losses, and no dc-fault ride-through capability. To circumvent these issues, this article presents a modified MMC, namely hybrid-leg MMC (HL-MMC). HL-MMC integrates a pair of three-phase line-frequency commutated HV voltage source converters (VSC) to replace up to 40% MMC submodules (SMs), leading to lower cost and conduction losses. Due to the ripple energy cancellation in the three-phase HV VSCs, the total energy storage requirement in HL-MMC could be also reduced, leading to up to 30% energy storage capacitance reduction compared with MMC. In addition, by combing the dc hybrid circuit breaker, the HV VSCs in the proposed HL-MMC also help provides the dc fault blocking capability, enabling a dc-fault tolerant HL-MMC without using the full-bridge (FB) MMC SMs. Therefore, a significant device and loss saving can be attained compared with the traditional MMC with FB SMs. The detailed converter operational analysis, control method, high-voltage direct current simulation model, and scale-down prototype are provided to validate the effectiveness of proposed topology.

A modified CPS-PWM for capacitor voltage reduction of MMC based variable speed drive

&lt;span&gt;In recent years, modular multilevel converters (MMC) have become one of the most popular multilevel converter topologies. Despite its growing popularity, MMC is still less widespread in variable speed drives (VSDs) applications. The reason for this is that the voltage ripple of the MMC submodules (SM) increases during the low-speed constant torque operation. In this paper, carrier phase shift-pulse width modulation (CPS-PWM) is modified to reduce the average voltage of SM and thus accommodate more ripples without exceeding the maximum SM voltage. The proposed method does not involve the injection of circulating current components, thereby resulting in less power loss. In addition to that, the necessity for increasing the SM capacitor rating during low-speed operation is not required. Extensive simulation has been conducted under various speed commands to validate the effectiveness of this method.&lt;/span&gt;

Unified Strategy for Fault-Tolerant Operation of MMC with Multiple SMs Failure Based on SMs Grouping Management

Modular multilevel converter (MMC) is distinguished by its modularity. In order to improve its reliability and avoid unscheduled maintenance, it requires the MMC to continue operating even though some of its submodules (SMs) have failed. In this paper, a grouping management method of SMs in MMC is first presented where every six SMs are conceptually grouped into a virtual subunit (SU), and on this basis, a simplified space vector modulation (SVM) implementation is developed. Then, the fault-tolerant solutions are proposed by investigating the post-fault operation of the MMC based on the virtual SU concept. It is analyzed that the SU failures can be categorized into three basic types, according to the phase where the failed SMs are located, and also multiple SM failures can be decomposed into one or multiple basic fault types. In this way, the fault-tolerant solutions are unified regardless of the number of faulty SMs. Further, the sorting capacitor voltage control is combined into the proposed methods to balance all of the SM capacitor voltages. With the proposed fault-tolerant method, the amplitude and THD of the line voltages are almost unchanged under fault conditions when compared with normal operations. The simulation and experiment verify the propositions.

Fault diagnosis and location of independent sub-module of three-phase MMC based on the optimal deep BiD-LSTM networks

Modular multilevel converter (MMC), which is highly modular, easy to be expanded, and has good output voltage waveform, is especially suitable for medium and high voltage power system applications. The MMC bridge arm inductance fault and the independent sub-module fault have the greatest influence on the transmission quality of transmission systems. However, there is few research on fault diagnosis and location for multiple sub-modules(SM) in a single arm at present. A deep bidirectional Long Short Term Memory (Bid-LSTM) network is proposed in this paper, which is used to diagnose and locate the faults of different sub-modules. which can deal well with the prediction problem with sequence dependence. In addition, because the Bid-LSTM model contains many hyperparameters, which seriously affects the prediction performance of BiD-LSTM, an optimal fruit fly optimization (FOA) algorithm based on the composite chaotic mapping of 3-dimensional Logistic-Sine (3d-CCFOA), which searches the optimal value of related hyperparameters to ensure the accuracy of classification. 3d-CCFOA has good chaos characteristics, which enables FOA to avoid falling into local optimal solution effectively and obtains an effective global optimal solution. Then, the MMC simulation system of RT-TAB is used to simulate the open circuit faults of different independent sub-modules of one bridge arm and collect relevant fault data. The analysis process and experimental results adequately indicate that this method can effectively and accurately diagnose and locate the faults of independent sub-modules. Meanwhile, this method also provides a reference diagnosis strategy for multi-bridge arm fault diagnosis and multi-sub-module location.