Submodule Voltage Research Articles

Circulating Current Suppression Combined with APF Current Control for the Suppression of MMC Voltage Fluctuations

Modular Multilevel Converters (MMCs) are widely used in HV and MVDC transmission. However, their application causes the voltage level to increase, and the number of sub-modules also increases. Problems such as circulating currents and sub-module voltage fluctuations should not be neglected. Considering the coupling relationship between the circulating current and the sub-module capacitor voltage fluctuation, the circulating current suppressing controller (CCSC) and the active power filter (APF) techniques are used to solve the two problems mentioned above simultaneously. Firstly, in order to reduce the influence of clutter on the tracking of the target components of the CCSC, a Second-Order Generalized Integrator (SOGI) is added to accurately lock the main 2nd and 4th harmonic components in the circulating current. Secondly, an APF is added on top of the circulating current suppression, and the two methods can be mutually reinforcing in their roles. The APF applies the strategy of current inner-loop dominant and voltage outer-loop bias control. It is regarded as a whole for absorbing the voltage fluctuations of the sub-module and also eliminates the error caused by the inductive voltage. Finally, the effectiveness of the above method is verified in MATLAB/Simulink, which demonstrates that the proposed method provides better suppression of both circulating current and sub-module voltage fluctuations compared to the conventional MMC that only incorporates APF.

Research on a New Topology and Reactive Power Compensation Application of Reinjection Multilevel Voltage Source Converter

Based on the principles of reinjection converter technology and submodule operation, a novel topology for a reinjection multilevel voltage source converter (RMVSC) is proposed. In this topology, submodules are dynamically connected in series as the reinjection circuit. This new topology retains all the functions of the RMVSC while offering flexible control of the reinjection circuit and ease of modularization, making it highly suitable for high-voltage, high-power applications. A circulating drive pulse sequence was developed for the 7-level serial submodule RMVSC, ensuring dynamic stability of the submodule voltage and maintaining low harmonic distortion when interfaced with the grid. Building on this, a double-loop control strategy—comprising an outer power loop and an inner current loop—was proposed for the static synchronous compensator (STATCOM) of RMVSC. A simulation model was constructed in PSCAD/EMTDC, and the simulation results confirm the excellent performance of the proposed topology and the effectiveness of both the modulation strategy and the double-loop control strategy. The RMVSC-STATCOM demonstrates continuous and bidirectional reactive power regulation, with high control accuracy and superior compensation current quality.

Research on Asymmetrical Operation of Multilevel Converter-Type Solid-State Transformers Based on High-Frequency Link Interconnection

The large size of the sub-module (SM) capacitor is a typical problem in traditional modular multilevel converter-type solid-state transformers (MMC-SSTs). The MMC-SST based on high-frequency link interconnection is an effective solution for achieving lightweight capacitance. This structure can help to eliminate the symmetric SM fluctuating power, thereby reducing the SM capacitance. In a three-phase interconnected MMC-SST with low capacitance, potential risks may arise during transient processes, especially in cases of three-phase voltage asymmetry, such as large fluctuations in the SM voltage and unstable DC bus voltage. Aiming to solve this problem, this article re-analyzes the internal power characteristics of the MMC-SST under asymmetric operation and re-derives the SM capacitance constraint suitable for different degrees of three-phase voltage asymmetry. The new SM capacitance constraint enhances the asymmetric voltage ride-through capability of the MMC-SST. The new capacitance constraint is higher than that in symmetric operation, but it still has significant advantages in capacitance compared with the traditional MMC-SST.

An optimized Closed-loop Precharging Strategy for Modular Multilevel Converters Based on Power Balancing Control under start or fault restart conditions

Modular multilevel converter (MMC)-based high-voltage DC (HVDC) systems have found rapid growth in power grids. To reduce the inrush current, it is necessary to precharge the submodule (SM) capacitors to the rated voltage before normal operation. In this paper, an optimized pre-charging strategy for MMC is proposed based on power balancing . First, the charging power is obtained by a designed closed-loop control of the SM capacitors. Then the total power released by the AC or DC side could be calculated by power conservation and feedback by controlling the charging current. At the same time, the power difference between the upper and lower bridge arm of each phase and the internal voltage balancing of bridge arms are also considered by the proposed bridge arm and SM voltage algorithms. By comparing with conventional closed-loop pre-charging strategies, the pre-charging speed is greatly enhanced as the achievement of synchronous charging of upper and lower bridge arm SMs. Moreover, as the same voltage and current double closed-loop control applied in both MMC pre-charging and normal state, the state transition time could be reduced. Finally, a series of simulations and prototype experiments are conducted to verify the feasibility of the proposed pre-charging strategy.

Improved Sequential Model Predictive Control of Cascaded H‐Bridge Multilevel Converter

The cascaded H‐bridge (CHB) converter can achieve rich functions, such as active rectification, active power filtering, distributed energy storage, etc. The control of the CHB converter is a multiobjective optimization problem. Finite control set model predictive control (FCS‐MPC) is an effective method developed in recent years to solve this problem. However, because of the large amount of calculation, traditional FCS‐MPC cannot be directly used to control the multilevel CHB converter. It is also difficult to tune the weighting factors in traditional FCS‐MPC. To solve these problems, this paper proposes an improved sequential MPC (IS‐MPC) method for the CHB converter. The proposed method divides the control objectives of the CHB converter into different layers according to their priorities and the control degrees of freedom of each layer can be adjusted according to the system state. Simulation and hardware‐in‐the‐loop (HIL) implementation on a three‐phase 7‐level CHB converter shows that the proposed method can effectively control the ac current and submodule voltage of the CHB converter and reduce the switching loss. Moreover, it significantly reduced the calculation burden of traditional FCS‐MPC, and the use of the weighting factors is also avoided. © 2024 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

Optimal Design of a Submodule Capacitor in a Modular Multilevel Converter for Medium Voltage Motor Drives

This paper proposes an algorithm for determining the optimal capacitance by utilizing a mathematical model of a submodule (SM) capacitor in a modular multilevel converter (MMC) specifically for medium voltage motor drives (MVDs). By approximating the voltage fluctuation of the SM capacitor during low-frequency operation, it is feasible to ascertain the minimum capacitance required for the SM capacitor, ensuring that its voltage fluctuations remain within an acceptable limit that is predefined as a specified value. Moreover, the study considered the injection of both a high-frequency common-mode voltage (CMV) and a circulating current to alleviate the SM voltage fluctuation during the acceleration of motor drives. The effectiveness of the proposed method is validated through verification using time-domain simulation results obtained using the MATLAB/SIMULINK software and real-time simulation results acquired using the OPAL-RT simulator platform.

A comprehensive circuit and modulation design of mixed frequency conversion cascaded power electronic transformer

The power electronic transformer (PET) is one of the promising equipment of the energy Internet. In a distributed network, cascaded (cascaded full-bridge, CFB)-type PETs can withstand high-voltage stress and contain a low-voltage DC (LVDC) port, which provides access for distributed generation (DG). However, the hardware cost of the traditional CFB-type topologies remains high, and structure compactness can still be improved. The recently presented mixed-frequency conversion cascade PET (MFCC-PET) is one of the promising solutions. In this study, a comprehensive circuit design of MFCC-PET, a low-voltage distortion nearest-level pulse width modulation (NL-PWM), and a sub-module (SM) voltage balancing strategy for the proposed modulation method under mixed-frequency conversion are presented. A 100 V/2,000 W MFCC-PET prototype is built based on the above analysis and successfully placed into operation.

Control tuning methodology for modular multilevel converter‐based STATCOM

SummaryThe application of modular multilevel converters (MMC) in medium voltage static synchronous compensators (STATCOMs) has been investigated in recent years. A huge challenge for MMC‐STATCOM is to compute the gain of the controllers. Therefore, the contribution of this paper is the proposal of a control tuning methodology for MMC‐STATCOM. For this purpose, the control and tuning strategies are presented. Since MMC‐STATCOM can employ different modulation strategies, two distinct modulation strategies are considered: nearest‐level control, with variable switching frequency; and phase‐shifted carrier pulse‐width modulation, with fixed switching frequency. Moreover, the proposal was validated through simulation and an experimental platform. The results indicate that the power response follows the reference, besides supplying the internal losses of the converter. The 10% ripple tolerance of the submodule voltages is respected in steady state for both modulation strategies. Moreover, the 5% total demand distortion (TDD) limit recommended by the Institute of Electrical and Electronics Engineers (IEEE) standard is attended. These results demonstrate the effectiveness of the controls employed.

Coprime Polynomial Based Dual-Circulant Modulation for Inherent Submodule Voltage Balancing in MMDC

The modular multilevel dc-dc converters (MMDCs) have attracted much interest in medium voltage dc applications, but have to face the main issue of balancing submodule (SM) capacitor voltages. This paper proposes a dual-circulant modulation and proves it possessing the inherent balancing capability for arbitrary operation cases. High-speed communication for real-time sensor data transfer could be avoided, which reduces implementation costs and also enhances the system reliability. Two sets of circular switching patterns are preset and combined to complete the full constrains on SM voltages. The associated polynomial of circulant matrix is introduced and the coprime of polynomials demonstrates the full-rank feature of the extended switching matrix that promises the inherent balancing for any operation cases. Then the switching patterns are reallocated and optimized to keep the SM uniformity and reduce the capacitor voltage ripple at the same time. Full-scale simulations on MMDC models and down-scaled experiments on prototypes are both presented, which validates the inherent voltage balancing capability and the optimization of SM capacitor voltage ripple with the proposed dual-circulant modulation.

Delta Configured Multi Busbar Sub-Module Modular Multilevel STATCOM With Partially Rated Energy Storage for Frequency Support

This paper presents a delta-configured, modular multilevel, STATCOM with integrated partially-rated energy storage (ES) and submodules (SM) based on the Multi-Busbar Sub-module (MBSM) topology. The soft-paralleling of SM capacitors using the nature of diodes leads to lower SM voltage deviation and lower circulating current to be used for SM balancing and improves the overall operation of the converter. Partially rated ES are distributedly integrated in SMs to provide ancillary service, e.g. frequency support, in addition to the classic reactive power provision. The study uses the analytical formulation of the MBSM to derive the control system and the optimal phase angle of the circulating current required for SM balancing. A comparison of different operating modes (MBSM vs emulated full-bridge SM) is presented and confirmed by simulation results. Analysis indicates that the MBSM improves the power efficiency (30.5% total losses reduction at the 1 pu reactive power set-point and 3.56% reduction at 1 pu active power set-point) compared with the conventional full-bridge SM, at the cost of doubling the number of semiconductor devices; but not the number of sub-modules.

Variable Sampling Frequency-Based SM Power Loss Balancing Control for MMCs With Bypassed Faulty SMs

The modular multilevel converter (MMC) is one of the most appropriate topologies for high power applications due to its high modularity and low harmonic distortion. As faulty submodules (SMs) are bypassed, the capacitor voltage and switching frequency of healthy SMs in faulty arm will rise, which causes unbalanced power losses distribution among the arms of MMCs and affects the reliability of MMCs. This paper proposes a variable sampling frequency based power losses balancing control (VSF-PLBC), which can balance power losses distribution among the arms of MMCs through adjusting sampling frequency for arm inserted SM number to modify switching losses in power devices in the MMC with bypassed SMs. The proposed VSF-PLBC can effectively improve the reliability of the MMC with the balanced power losses distribution among the arms in the MMC with bypassed SMs. The proposed VSF-PLBC is simulated with the professional tool PSCAD/EMTDC and is validated on a down-scale MMC prototype. The simulation and experimental results confirm the effectiveness of the proposed VSF-PLBC.

Optimal Switching Sequence Model Predictive Control for Modular Multilevel Converter

This paper proposes a new optimal switching sequence model predictive control method for modular multilevel converter (MMC). In this method, the common mode voltage (CMV) is cosidered, which makes the model more accurate. The proposed method realizes the optimization of multiple control objectives of output current tracking, circulating current suppression (CCS), and submodule voltage balancing. The sequence that allows the output current to track its reference value is calculated in the <inline-formula><tex-math notation="LaTeX">$\alpha -\beta$</tex-math></inline-formula> coordinate system. And the virtual vectors aim to suppress the circulating current without influencing the output current are inserted. Capacitor voltage balancing is achieved by redundant vectors selection and tranditional selection algorithm. These control objectives are calculated independently, thus weighting factor is not required. At the same time, the computation burden is reduced by restricting the voltage mutation. This method has a satisfactory dynamic response and steady-state performance, the simulation and experimental results are given to verify the effectiveness of the method.

Space vector pulse width modulation strategy for modular multilevel converters in power system

AbstractAs a superior modulation strategy, space vector pulse width modulation (SVPWM) provides redundant voltage vectors and adjustable action time, which can achieve multi‐objective control of modular multilevel converter (MMC). An SVPWM strategy suitable for MMC is proposed. The strategy is divided into three stages. In the first stage, the appropriate voltage vector, the action time and the basic sub‐module (SM) input number are quickly calculated to ensure the output quality by equating MMC as a 2‐level inverter. In the second stage, a finite set of the circulating current suppression is established on the basis of the basic SM input number. The optimal SM input number is selected through rolling optimisation. In the last stage, according to the SM voltage sorting and the optimal SM input number, the optimal switching state is determined to realise the SM voltage balance control. The proposed control strategy simplifies the design of the control system, reduces the computational burden and can be easily extended to MMC with any SM number. The simulation and experimental results show that the proposed SVPWM strategy can reduce the circulating current and balance the SM capacitor voltage while ensuring the output quality.

Median valve submodule voltage control of MMC VSC for energy management application

The Modular Multilevel Converter (MMC) is the prevailing topology used for high-voltage high-power converters. The energy management of the valves is key for the availability and controllability of the converter station. Moreover, managing the energy ensures the stress on the power electronics is kept within acceptable limits. Conventionally, the valve energy is controlled through the regulation of the mean submodules (SM) capacitor voltage. In this paper, a novel approach of controlling a selected ordered statistic, in particular the statistical median voltage, of the SM capacitor is presented. The simulation results of a full onshoreoffshore HVDC scheme verifies the feasibility of using the median voltage of the SM capacitor voltage as alternative to the mean voltage for MMC VSC energy management.

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.

Mechanism analysis and protection strategy of flexible DC converter power module with high switching frequency

There are a large number of sub modules in mmc-hvdc project. NLM modulation algorithm is widely used in the project. The voltage sharing sequence of sub modules is carried out in combination with the current direction of bridge arm and the voltage of sub modules. Based on the existing modulation and voltage sharing strategy, it is inevitable that some sub modules will have high switching frequency or even ultra-high switching frequency in the process of project implementation. The phenomenon and mechanism of high frequency in engineering are analyzed, and the influence of high frequency on converter valve sub module is analyzed; Then combined with the highfrequency withstand capability of power devices and the reliability of protection, a two-stage inverse time limit sub module high-frequency protection strategy is proposed, and the protection setting method and basis are established. The strategy has good real-time performance and reliability, and has no impact on the normal operation of the system. Finally, it will be applied to improve the reliability of mmc-hvdc project in operation.

A Novel Fault Ride-Through Topology With High Efficiency and Fast Fault Clearing Capability for MVdc PV System

The multiport dc/dc converter with input independent output series structure is simple and efficient for photovoltaic (PV) applications. Through the cascade of sub-modules (SM), the converter has a high step-up ratio to connect MVdc bus. However, it has many ports which increase the risk of short-circuit faults. Therefore, this article proposes a fault ride-through (FRT) topology for multiport converter. When low-voltage ports are short-circuited, they can be isolated by blocking the switches of their own. The FRT topology can support the output voltage of faulty SMs to avoid the disconnection from MVdc bus. Other healthy ports can still generate power to MVdc bus. When the medium-voltage port is short-circuited, the FRT topology can quickly clear the fault current to prevent devices from being damaged. Due to the zero voltage switching, the proposed topology has higher efficiency. For medium-voltage fault, the FRT topology has faster fault clearing speed because more capacitors are reused. Finally, the proposed topology is verified by simulation and experiment.

Optimal design of improved H-infinity controller for MMC

ABSTRACT In MMC, the variations in capacitor voltage of sub-modules produce the circulating current. This research work presented intends to develop the MMC-HVDC using an the most effective 3 H-infinity regulator for solar energy systems. Initially, the unstable currents are observed using the new sensor and are compared to the current controller, which produces an erroneous current flowing. The current controller is set to zero. Since variations amongst error circulating current adds reference mistakes was expressed as a non-linear meaningful purpose, it is thought to be the primary factor in lowering the total harmonic distortion (THD). Using the Self-Adaptive Cuckoo search algorithm (SA-CS) for signal control, the gain is maximised in the H-infinity controller. Regulate evaluation, conversion performance monitoring, and battery voltage analysis are also used in the performance calculation of the proposed model in addition to THD and circulating current analysis. The results of the simulation show how well the proposed control strategy operates.

Circulating Current Generating Mechanism and Suppression Control of HMMC

Hexagonal modular multilevel converter (HMMC) is a full-bridge ac/ac converter, and it can achieve the interconnection between the ac grid and the low frequency system. However, the coupling of input and output on the branches causes the large fluctuation both in dc voltage of submodules (SMs) and branch currents. Moreover, the internal circulating current in branch current comprises large dc component and ac components, which causes an increase in current stress. The traditional circulating current control of HMMC adopts circulating current injection for dc voltage stability, which cannot decrease the current stress. Therefore, basing on dc voltage fluctuation analysis, the circulating current generating mechanism is analyzed. According to the influence mechanism of circulating current, a circulating current suppression control (CCSC) is proposed to decrease the current stress. Eventually, the CCSC is verified in the real-time (RT)-lab platform.

A High Voltage Gain Capable MMC for Offshore Wind Farms: Frequency Component Analysis and Minimization of Capacitor Voltage Ripple

Modular multilevel converter (MMC) and HVdc technology together provide an indispensable solution to integrate offshore wind farms with onshore ac grid. However, the bulky size of MMC and voltage boosting transformers (VBTs) pose a big challenge to keep the footprint and cost of the onshore-platform low. With this background, a full-bridge submodule (SM) modified MMC and a suitably modified switching function are proposed, which utilize the negative SM voltage to achieve ac side voltage boosting. How much negative SM voltages are to be applied across the arms is determined by the group submodule ratio (GSR) parameter, introduced in the switching function. The acquired boosting ability not only shrinks the voltage boosting requirement of VBTs but also reduces the semiconductor switches (upto 10%) and/or the SM count (8%–30%). Analytical solutions derived for the SM capacitor sizing are also presented. The analysis is further utilized to obtain a novel circulating current injection scheme (CCIS), which utilizes phasor analysis to obtain the values of circulating current control parameters. The CCIS suppresses SM capacitor voltage ripple, especially fundamental, and reduces the capacitance requirement by 10%–15% for a wide range of GSR. Representative simulation results are showcased, and the theoretical claims are validated by hardware experiment.