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A New Submodule for Capacitor Voltage Ripple Reduction with Fewer Semiconductor Components in Modular Multilevel Converters
Abstract
The size of energy storage resources has a vital role in the cost, size, and weight of power electronics converters. Multilevel converters usually operate at low switching frequencies to prevent high switching loss. Since modular multilevel converters are constructed of a series connection of some submodules, a lower switching frequency causes a higher capacitor voltage ripple. To deal with this issue, the size of converter capacitors has to be increased. This paper presents a new submodule (SM) that enables the parallel connection of capacitors. This SM decreases the ripple of the capacitors and offers three-level voltage at its terminals. In addition, it uses fewer switching devices and gate driver circuits in comparison with other submodules presented in the literature. Also, the number of capacitor voltage sensors is reduced by half due to the parallel connection of SM capacitors. The simulation and experimental results verify the performance of the proposed submodule.
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The sub-modules of the modular multilevel converter (MMC) in practical engineering mainly adopt half bridge sub module (HBSM), which does not have the capability to clear DC fault current. Based on the analysis of the DC fault current path of traditional HBSM, this paper proposes a novel MMC sub-module With DC fault clearance capability. It adds a bidirectional switch and two diodes to the HBSM. When the DC fault occurs, the bypass absorption circuit is introduced through the diodes and the freewheeling effect of diodes is eliminated by the sub-module capacitor voltage. Thereby, the fault current can be extinguished rapidly and the requirement for the consistency of the trigger pulse is reduced. By comparing with the operation principle of HBSM, it can quickly block the DC fault current without changing the original control and modulation strategy. Then, the fault clearance principles of the sub-module and the voltage stress of the key power switches are analyzed. The simulation results in MATLAB/SIMLINK have verified the validity and feasibility of the proposed sub-module topology in DC fault current clearance.
Modular Multilevel Converters for HVDC applications normally adopt a distributed control system, where a large number of local controllers are employed, to manage considerable sub-modules in the system. The manufacturing tolerance of crystal oscillators for local controllers introduces controller clock discrepancy and eventually leads to the asynchrony of triangular carriers implemented in individual sub-module for phase-shifted PWM scheme. Considering the tolerance of oscillators having a normal distribution, the deviation of the triangular carrier frequency is modeled based on the probability theory in this letter. The switching function model taking such asynchrony into account is proposed allowing quantitive analysis and evaluation of the switching harmonic contents. Good agreements of the harmonic contents have been achieved between the theoretical calculation and experimental results. The calculated switching harmonics can be used to determine the interval of the sub-module synchronization scheme according to the system THD or harmonic content requirement.
This paper proposes a new high‐voltage pulse generator (PG) fed from a low‐voltage DC supply Vs, which charges one arm of N series‐connected full‐bridge (FB) modular multilevel converter (MMC) sub‐module (SM) capacitors sequentially, through a resistive‐inductive branch. By utilising FB‐SMs, the proposed PG is able to generate bipolar rectangular pulses of peak NVs and unipolar rectangular pulses of either polarity, at high repetition rates. Asymmetrical pulses are also possible. The proposed topology is assessed via simulation and scaled‐down experimentation, which establish the viability of the topology for water treatment applications.
A new highly compact topology for solid-state transformers is presented. It is based on modular multilevel converters and, therefore, has their usual advantages such as superior voltage shape, high reliability, and scalability. It utilises arm inductors for the isolated energy transfer between two converters and thereby allows for a more compact build compared to the design that uses separate transformers.
In this work, a decentralized PI passivity-based controller (PI-PBC) is applied to Modular Multilevel Converters (MMCs) to ensure global stability of a multi-terminal HVDC system. For the derivation of the controller, an appropriate model with constant steady-state solutions is obtained via a multifrequency orthogonal coordinates transformation. The control design is next completed using passivity arguments and performance guarantees are established by a small-signal analysis. The obtained results are validated by means of detailed timedomain simulations both on a single-terminal and a four-terminal benchmark.
The Modular Multilevel Converter (MMC) is a promising topology for high power drive applications. However , large voltage fluctuations are produced in the floating capacitors when the machine is operating with high stator currents at low rotational speed. To compensate these oscillations , relatively large mitigation currents are required to keep the capacitor voltages within an acceptable range. In this paper, a new integrated control scheme is discussed to regulate the voltage fluctuations. The strategy is based on closed-loop vector-control of the voltage fluctuations, maintaining them inside a pre-defined threshold. The proposed control system is also augmented using flux weakening operation of the machine at low rotational speeds. An experimental prototype composed of eighteen power cells, feeding a vector-controlled induction machine in the whole speed range, is used to validate the effectiveness and feasibility of the proposed control strategies. Index Terms-Modular Multilevel Converter, variable speed drive, voltage balancing, flux-weakening operation.
Conventionally, only one out of the two capacitors in a three-level submodule is involved in the arm current path to generate the first non-zero voltage level (E) in modular multilevel converters (MMCs). While two capacitors are connected in series to obtain the second non-zero voltage level (2E). In this paper and in order to generate the first non-zero voltage level (E); parallel-connection of the two capacitors is proposed which in turn forces the capacitors voltages to be equal, as a result the parallel-connection supports employing a single voltage sensor per the proposed module instead of using two voltage sensors which reduces the hardware and software complexity of the control system. A new switched capacitor submodule (SCSM) is proposed to enable this feature. A detailed illustration of the operational concept of the proposed architecture is presented in this work. The simulation results for an MMC with the proposed SCSM is presented to validate the proposed concept.
Modular multilevel converters (MMCs) have become one of the most promising topologies for high-voltage dc-ac conversion. DC-side fault blocking capability of the MMC has prompted significant research in recent years. In this paper, a new switched capacitor submodule (SCSM) for MMCs is proposed which provides operation with DC fault blocking capability. In addition, successful voltage balancing is achieved with half the number of voltage sensors used with existing MMC cells. Generally, conventional sensor-based balancing techniques require a significant amount of measurements, 2m(N-1) voltage sensors, and 2m current sensors for an N-level m-phase MMC. The proposed cell will thus aid in reducing the complexity of the control system. A detailed illustration of the operational concept of the proposed architecture is presented in this paper. A comparison between the proposed SCSM and other existing MMC cells has been included to highlight the benefits of the proposed SCSM. A simulation model for an MMC-based HVDC system along with the proposed cell has been built to test its performance during normal as well as abnormal operating conditions. The simulation results show the effectiveness of the proposed architecture.
The modular multilevel converter is a promising topology for high-voltage and high-power applications. By using submodules equipped with dc-capacitors excellent output voltage waveforms can be obtained at low switching frequencies. The rated energy storage of the submodule capacitors is a driving factor of the size, cost, and weight of the submodules. Although the modular multilevel converter has been thoroughly investigated in the literature, a more detailed analysis of the energy-storage requirements will provide an important contribution for dimensioning and analysis of modular multilevel converters. Such an analysis is presented in this paper. The analysis relates the power transfer capability to the stored energy in the converter and the findings are validated by both simulations and experimental results. The required size of the submodule capacitors in a 4.5 MW grid-connected converter is first calculated and the calculated operating range is then compared with simulation results. The experimental results show that if the average capacitor voltage is allowed to increase 10% above the nominal value an energy storage to power transfer ratio of 21 J/kW can be achieved. It is concluded that the presented theory can relate the power transfer capability to the energy storage in the converter and is thus a valuable tool in the design and analysis of modular multilevel converters.
The internal control of a modular multilevel con- verter aims to equalize and stabilize the submodule capacitor voltages independent of the loading conditions. It has been shown that a submodule selection mechanism, included in the modulator, can provide voltage sharing inside the converter arm. Several procedures for controlling the total stored energy in each converter arm exist. A new approach is described in this paper. It is based on estimation of the stored energy in the arms by combining the converter electromotive force reference, the measured alternat- ing output current, and the known direct voltage. No feedback controllers are used. Experimental verification on a three-phase 10 kVA prototype is presented along with the description of the new procedure. Index Terms—Energy balance, modular multilevel converter (M2C), modulation, open-loop control, prototype.
In the switched-capacitor based modular multilevel converter-solid state transformer (MMC-SST) scheme, the switched-capacitor circuit plays a key role in power transfer, electric isolation, voltage clamping, and low-frequency voltage ripple decoupling. However, the operational performances of switched-capacitor circuits are depended on the distribution parameters of synchronous transformers. To explore the parameter constraints, the mathematical model is established, the relationship between the active power transmission capacity and the transformer time constant is studied, the dynamic inrush current stress is also given. The analysis results show that the distribution resistance should be minimally designed to reduce the voltage clamping error, however, a large distribution resistance is good for suppressing the inrush current. Moreover, if the distribution resistance is large and the leakage inductance is small, the time constant is small, which benefits the power transfer. So, there are some contradictions in the transformer design, which needs to make a trade-off. For coordinating the parameter optimization, the design criteria and influence factors of distribution parameters for the synchronous transformer are analyzed. Then the transformer optimization design flow considering distribution parameter constraints is proposed. Finally, the toroidal transformer prototype and the switched-capacitor experimental platform are developed to verify the correctness of the proposed scheme.
Modular multilevel converter (MMC) is a modern and preferred topology among the voltage source converters (VSC) in high voltage direct current (HVDC) transmission. However, the traditional half-bridge sub-module can’t block the DC fault current. Hence, the novel sub-module topology with DC fault ride-through capability is introduced. This paper proposes an improved diode-clamp sub-module (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 -
u
c
, 0,
u
c
, and 2
u
c
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 paper adopts an equivalent half-bridge modulation (EHBM) strategy that can modulate all the MMC sub-module 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.
Under grid distortions, Modular Multilevel Converters (MMC) must adopt proper control strategies to fulfill the power system requirements and ensure a stable operation. An inappropriate control under such conditions may lead to energy unbalances between the MMC legs, inaccurate current injection, and failure in the synchronization process. In this context, sequence extraction methods play a critical role in enhancing the performance of the control, firstly, by aiding the Phase-Locked Loop (PLL) to maintain the synchronization with the AC grid by following the positive sequence fundamental component of the voltage, secondly, by allowing accurate active and reactive currents injection via the decoupled Voltage Oriented Control (dVOC), thirdly, by properly managing the internal energy of the MMC through the circulating current control. In prior researches, some sequence extraction methods have been used for MMC STATCOM. However, the sequence extraction was not the core of the performed studies and their impact on the system behavior has not been highlighted or tested in several grid conditions. This work fills this gap by first assessing the performance of a Single Delta Bridge Cell MMC (SDBC-MMC) STATCOM with four well-known sequence extraction methods (i.e., Decoupled Double Synchronous Reference Frame (DDSRF), Dual Second Order Generalized Integrator (DSOGI), Improved DSOGI, and Fortescue matrix-based (FMB) filter under normal and abnormal grid conditions, then, finding the most suitable one in terms of stability, dynamics, and functionalities.
This paper proposes a modified topology of modular multilevel converter (MMC) for medium-voltage applications, which can reduce the ACvoltage fluctuation in the submodule (SM) capacitor at low-frequency operation. A novel structure of middle SM is inserted between the upper and lower arms, which has lesser count of components compared with the existing ones, leading to the reduction in cost and power losses. At the low operating frequency, furthermore, the middle SM can attenuate the power fluctuation in arms by distributing the high-frequency power components. Therefore, the SM capacitor voltage fluctuation is significantly alleviated without imposing the common-mode voltage on the AC side. The effectiveness of the proposed topology has been verified by experimental results.
Conventional half bridge modular multilevel converter (MMC) system will suffer great over-current when there is a dc side fault. Therefore, topology study in sub-modules with dc side fault blocking capability have become one of the hotspots in MMC field. However, fault blocking topologies using conventional IGBT devices must employ more semiconductors than half bridge topology. The extra semiconductor devices employed in the topology will lead to the increase of cost and conduction loss. To reduce the conduction loss of the fault blocking topology, a novel sub-module topology is proposed in this paper. The proposed topology employs reverse blocking devices to form the low conduction loss path. Conduction loss analysis is conducted in this paper to show the priority of the proposed topology. Simulation and experiment results verified the dc fault blocking capability of the proposed topology.
This paper presents a modified modular multilevel converter (MMC) equipped with the middle-cells and control algorithm for medium voltage applications. It is intended to reduce the sub-module (SM) capacitor voltage ripples by the third-order harmonic voltage injection (THVI) and second-order harmonic current injection (SHCI) without the common-mode voltage injected on the AC-side. On the one hand, using the proposed algorithm, the independent power degrees of freedom can be produced to eliminate the dominant fundamental part in arm powers, which can dramatically reduce the SM capacitor voltage ripples. On the other hand, the smaller amplitude of the SHCI can be obtained, which decreases the power losses. Meanwhile, by properly controlling the proposed middle-cells of the modified MMC, the compensated parts can be produced to eliminate the common-mode voltages caused by the THVI. Additionally, the arm power analysis of the modified MMC under different power factors is conducted, and the obtained results are discussed. The mathematical models of the SM capacitor voltages for the half-bridge based SMs and the proposed middle-cells in the modified MMC are also analyzed. Finally, the effectiveness of the theoretical analysis of the modified MMC is verified by simulation and experimental results.
This paper presents the interaction analysis of external disturbances and internal dynamics of MMC-HVDC (Modular Multilevel Converter High Voltage Direct Current) systems considering sub-module (SM) capacitor voltage ripples, and proposes an enhanced control strategy for disturbance suppression. The disturbance equation indicating the interaction of the external MMC output currents and internal SM capacitor voltage ripples are firstly established. Then based on the disturbance equation, the dynamic analytical expressions of the differential-mode capacitor ripple voltages and the converter currents are derived in the
d
-
q
reference frame, which explains that the resultant current term
i
0
can be regarded as an equivalent disturbance that will lead to transients in the capacitor voltages during external perturbations. Next, the disturbance observer is introduced to track the resultant disturbance current term and an enhanced control for the MMC based on disturbance suppression strategy is proposed to improve the system dynamic response during external transients. Furthermore, system stability characteristics under the enhanced control with different parameters are explored through theoretical analysis. Finally, a benchmark single-terminal MMC-HVDC simulation model is established in PSCAD/EMTDC to verify the proposed control.
Modular multilevel converters (MMCs) are frequently featured to their modular structure, which results in fault-tolerant operation and easy redundancy realization. Nevertheless, in order to reach a given redundancy factor, more cells must be included in the converter, which directly affects the costs. This paper discusses the inherent redundancy of a modular multilevel converter (MMC) based static synchronous compensator (STATCOM) operating in the overmodulation region. Analytical expressions for the limits of the converter linear region were developed in order to define the minimum required dc-link voltage. Moreover, sensitivity analyses were implemented in order to show the effects of grid voltage variations, different output impedances, power factor and injected currents. The results indicated that the operation in overmodulation region has significant inherent redundancy. For a MMC based STATCOM with 26 cells, the converter can ride through 4 failures per arm without significantly increasing the output THD or reducing the injected current into the grid.
This paper proposes a modular multilevel converter (MMC)-based switched reluctance motor (SRM) drive with decentralized battery energy storage system (BESS) for hybrid electric vehicle (HEV) applications. In the proposed drive, a battery cell and a half-bridge converter is connected as a submodule (SM), and multiple SMs are connected together for the MMC. The modular full-bridge converter is employed to drive the motor. Flexible charging and discharging functions for each SM are obtained by controlling switches in SMs. Multiple working modes and functions are achieved. Compared to conventional and existing SRM drives, there are several advantages for the proposed topology. A lower dc bus voltage can be flexibly achieved by selecting SM operation states, which can dramatically reduce the voltage stress on the switches. Multilevel phase voltage is obtained to improve the torque capability. Battery state-of-charge (SOC) balance can be achieved by independently controlling each SM. Flexible fault-tolerance ability for battery cells is equipped. The battery can be flexibly charged in both running and standstill conditions. Furthermore, completely modular structure is achieved by using standard half-bridge modules, which is beneficial for market mass production. Experiments carried out on a three-phase 12/8 SRM confirm the effectiveness of the proposed SRM drive.
This paper presents a modified modular multilevel converter(MMC) with the three-terminal middle-cells inserted between arms and ac-side for medium voltage applications. It is intended to reduce the sub-module(SM) capacitor voltage ripples by controlling the high frequency powers flowing among arm branches for low-frequency output operations without common-mode voltage injected in ac-side. Due to the physical connection among the upper arm, the lower arm and ac-side, the middle-cell can be regarded as the excess degree of freedom to modify the dc-side and ac-side characteristics. Therefore, the common-mode voltage in ac-side can be eliminated by the inverse common-mode voltage produced by the middle-cell in ac-side. Moreover, owing to the unique structure of the middle-cell, the middle-cell capacitor voltage also results in a small ripple due to the high frequency current flowing into it by proper control. The operation principle and control strategy of the middle-cell has been proposed in detail. Simulation and experimental results have verified the effectiveness of the proposed topology.
Design, Control and Application of Modular Multilevel Converters for HVDC Transmission Systems is a comprehensive guide to semiconductor technologies applicable for MMC design, component sizing control, modulation, and application of the MMC technology for HVDC transmission. Separated into three distinct parts, the first offers an overview of MMC technology, including information on converter component sizing, Control and Communication, Protection and Fault Management, and Generic Modelling and Simulation. The second covers the applications of MMC in offshore WPP, including planning, technical and economic requirements and optimization options, fault management, dynamic and transient stability. Finally, the third chapter explores the applications of MMC in HVDC transmission and Multi Terminal configurations, including Supergrids. Key features: Unique coverage of the offshore application and optimization of MMC-HVDC schemes for the export of offshore wind energy to the mainland. Comprehensive explanation of MMC application in HVDC and MTDC transmission technology. Detailed description of MMC components, control and modulation, different modeling approaches, converter dynamics under steady-state and fault contingencies including application and housing of MMC in HVDC schemes for onshore and offshore. Analysis of DC fault detection and protection technologies, system studies required for the integration of HVDC terminals to offshore wind power plants, and commissioning procedures for onshore and offshore HVDC terminals. A set of self-explanatory simulation models for HVDC test cases is available to download from the companion website. This book provides essential reading for graduate students and researchers, as well as field engineers and professionals who require an in-depth understanding of MMC technology.
This paper evaluates the performance of modular multilevel converters with integrated battery cells when used as traction drives for battery electric vehicles. In this topology, individual battery cells are connected to the dc link of the converter submodules, allowing the highest flexibility for the discharge and recharge. The traditional battery management system of battery electric vehicles is replaced by the control of the converter, which individually balances all the cells. The performance of the converter as a traction drive is assessed in terms of torque-speed characteristic and power loss for the full frequency range, including field weakening. Conduction and switching losses for the modular multilevel converter are calculated using a simplified model, based on the datasheet of power devices. The performance of the modular multilevel converter is then compared with a traditional two-level converter. The loss model of the modular multilevel converter is finally validated by experimental tests on a small-scale prototype of traction drive.
The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted.
This paper presents a modulation strategy for the modular multilevel converter (MMC) which provides the voltage balancing of the capacitors of different submodules comprising the converter. Not only is this modulation applicable to MMCs constructed with classic two level (2L) submodules, but it is also effective for arrangements with different submodule concepts, for instance, topologies like neutral point clamped (NPC), flying capacitors (FCs), neutral point piloted (NPP), etc. Therefore, firstly, the general modulation philosophy is explained applied to the two level (2L) submodule concept, and secondly, the extension to 3L-NPC and 3L-FC submodule topologies is analyzed. After that, the validation of the studied modulation strategy is carried out by means of successful simulation results, at different switching frequencies and number of submodules. The corresponding experimental results are shown in Part II of this paper after having implemented the aforementioned modulation strategy in a real test bench. In addition to this, a comparison based on thermal analysis and sizing of elements among the four studied submodule topologies is also included.
Demanding future applications in power transmission and drives like solar-thermic power plants and off-shore wind power require advanced converter systems. Modular Multilevel Converters are well adopted to these needs, owing to industrial scalability, high efficiency and fast dynamic controllability under transient and fault conditions including DC-Side faults. The respective features of the established concepts for the high power range (M2C) and of new concepts for the low power range (MHF) are explained.
A submodule implementation for parallel connection of capacitors in modular multilevel converters
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