Submodule Voltage Research Articles

Modeling and Mitigation Control of the Submodule-Capacitor Voltage Ripple of a Modular Multilevel Converter under Unbalanced Grid Conditions

A modular multilevel converter’s (MMC’s) submodule (SM)-capacitor voltage will increase under unbalanced grid conditions. Depending on the imbalance level, the voltage ripple can be considerably high, and it can exceed the pre-defined safe limits. If this occurs, the converter will trip, which can lead to serious stability problems for the grid. This paper first proposes an analytical solution for deriving the three-phase imbalanced SM ripple of an MMC under an unbalanced grid. With this analytical tool, the imbalance mechanism of the SM voltage ripple can be easily understood. What is more, the symmetrical component method is first applied to analyze the three-phase SM capacitor ripple, and the positive-/negative-/zero-sequence components of the three-phase SM voltage ripple are easily identified by the proposed analytical method. Then, based on this powerful analytical tool, the proper circulating-current profile to be injected can be obtained, allowing for the right compensation of the voltage ripple. Based on this approach, two new voltage ripple compensation methods are proposed in this paper. Simulations were carried out to validate the analytical description of the submodule-capacitor voltage ripple proposed in this paper. Moreover, simulation and experimental results are provided to validate the new compensation techniques introduced in this paper.

Comparative Evaluation of Voltage Source Converters With Silicon Carbide Semiconductor Devices for High-Voltage Direct Current Transmission

Recent advancements in silicon carbide (SiC) power semiconductor technology enable developments in the high-power sector, e.g., high-voltage-direct-current (HVdc) converters for transmission, where today silicon (Si) devices are state-of-the-art. New submodule (SM) topologies for modular multilevel converters offer benefits in combination with these new SiC semiconductors. This article reviews developments in both fields, SiC power semiconductor devices and SM topologies, and evaluates their combined performance in relation to core requirements for HVdc converters: grid code compliance, reliability, and cost. A detailed comparison of SM topologies regarding their structural properties, design and control complexity, voltage capability, losses, and fault handling is given. Alternatives to state-of-the-art SMs with Si insulated-gate bipolar transistors (IGBTs) are proposed, and several promising design approaches are discussed. Most advantages can be gained from three technology features. First, SM bipolar capability enables dc fault handling and reduced the energy storage requirements. Second, SM topologies with parallel conduction paths in combination with SiC metal-oxide-semiconductor field-effect transistors offer reduced losses. Third, a higher SM voltage enabled by a higher blocking voltage of SiC devices results in a reduced converter complexity. For the latter, ultrahigh-voltage bipolar devices, such as SiC IGBTs and SiC gate turn-off thyristors, are envisioned.

Inherent SM Voltage Balance for Multilevel Circulant Modulation in Modular Multilevel DC-DC Converters

The modularity of a modular multilevel dc converter (MMDC) makes it attractive for medium-voltage distribution systems. Inherent balance of submodule (SM) capacitor voltages is considered as an ideal property, which avoids a complex sorting process based on many measurements thereby reducing costs and enhancing reliability. This article extends the inherent balance concept previously shown for square-wave modulation to a multilevel version for MMDCs. A switching duty matrix dU is introduced: it is a circulant matrix of preset multilevel switching patterns with multiple stages and multiple durations. Inherent voltage balance is ensured with a full-rank dU . Circulant matrix theory shows that this is equivalent to a simplified common factor criterion. A nonfull rank dU causes clusters of SM voltage rather than a single common value, with the clusters indicated by the kernel of the matrix. A generalized coprime criterion is developed into several deductions that serve as practical guidance for design of multilevel circulant modulation. The theoretical development is verified through full-scale simulations and downscaled experiments. The effectiveness of the proposed circulant modulation in achieving SM voltage balance in an MMDC is demonstrated.

Feed-forward Modulation Technique for more Accurate Operation of Modular Multilevel Converters

Modular multilevel converters have become the prominent topology for medium- and high-voltage applications. The performance of these converters highly depends on the accuracy of the used modulation approach, for which the capacitor voltage of submodules (SM) are usually assumed to be equal. This article exhibits that ignoring the capacitor voltage differences among SMs adversely affects the system performance. This becomes more obvious the larger the capacitor voltage differences are. Hence, this article proposes a more accurate feedforward modulation approach that takes into account either the instantaneous capacitor voltage value and the real output voltage in the modulation stage. As a result, in applications where larger SM voltage differences are expected, the current distortion and control performance are improved. Particularly, switching–saving approaches benefit from this method as it enables their operation with reduced switching losses without the downsides of increased distortion due to capacitor voltage differences. The proposed approach is analyzed and compared with the nearest-level modulation and with the level-shift PWM. Simulations and experimental validation are presented to confirm the effectiveness of the proposed technique.

Control and Design for Mission Profile Emulator of Individual Submodule in CPS-PWM-Based Modular Multilevel Converter

Modular multilevel converter (MMC) may contain a large number of submodules (SMs). The overall reliability of MMC strongly relies on the comprehensive testing and validation of SMs. However, building a relatively complete MMC system/arm for the testing of SMs is time-consuming and costly. In this article, mission profile emulator (MPE) for individual SM of MMC is presented along with the design and control methods. By the proposed schemes, the complete electrical characteristics including the load current and capacitor voltage of SM under test can behave closely to the actual SM in an MMC system. Moreover, the current ripple, which is an important loading factor for SM, can be accurately designed and restrained. Both the simulation and experimental results are provided to validate the effectiveness of the proposed methods.

Optimum Design of MMC-Based ES-STATCOM Systems: The Role of the Submodule Reference Voltage

In recent years, the combination of battery energy storage (ES) systems and static synchronous compensators (STATCOM) in a single device, known as ES-STATCOM, has been studied by both academia and industry. The modular multilevel converter is an interesting candidate for ES-STATCOM realization. The integration of batteries based on dc/dc converter presents advantages in terms of design flexibility. However, there is a gap in the literature in terms of design and cost optimization of ES-STATCOM systems. To fill this void, this article provides a methodology to support engineers to design and perform cost optimization of ES-STATCOM systems. Both capital expenditures and operational expenditures due to energy losses and battery replacement are taken into account. The methodology identifies what is the optimum submodule voltage reference, which minimizes the ES-STATCOM total cost for a given target of operating time. A lifetime model is employed to predict if battery replacement is required. This model computes the battery capacity fade caused by the operation mission profile considering both calendar and cycling aging mechanisms. The case study is based on a 25 MVA/75 MWh ES-STATCOM connected to a 34.5 kV grid, which provides ancillary services for a photovoltaic power plant. Different rated blocking voltages are assumed for the semiconductor devices. Moreover, different maximum and minimum allowed state-of-charge are adopted in the converter design. The proposed methodology can also be used to find the optimum combination of the battery and insulated gate bipolar transistor (IGBT) part-numbers based on a given initial set.

Improved post‐fault operation strategy for a cascaded H‐bridge based STATCOM

This study presents a multi-fault tolerant strategy for a cascaded H-bridge (CHB) based static synchronous compensator (STATCOM) with star configuration. A new method for post-fault operation of CHB-STATCOM is proposed, where after switch failures, the full-bridge (FB) submodules are used as two different types of half-bridge submodules named as positive half-bridge (P-HB) and negative half-bridge (N-HB). Using a modified level shift pulse-width modulation technique, a combination of FB, P-HB and N-HB submodules is employed in three legs of the converter to generate the maximum attainable ac side voltage in post-fault condition. In order to restore the nominal capacity of CHB-STATCOM, the capacitor voltage of submodules is increased based on the type and number of switch failures. Furthermore, by utilising a voltage balancing strategy, the voltages of submodules’ capacitors are kept balanced and regulated at the desired value in post-fault condition. The feasibility and effectiveness of the proposed strategy is validated on a scaled-down 9-level CHB-STATCOM laboratory prototype.

Modular Multilevel Converter Based Topology for High-Speed, Low-Voltage Electric Drives

The increasing demand for higher power density and more efficient electric drives, in particular, from the automotive industry, has pushed engineers investigating new solutions. Recently, wide bandgap devices have been proposed to improve performances of classical ac/dc converters, mainly in terms of power conversion efficiency. In this article, an alternative approach is proposed, aiming to obtain the desired improvements in the overall drive performances using a more complex power conversion system, based on standard silicon devices. In particular, a modification of the modular multilevel converter (MMC) is proposed in order to eliminate the submodule voltage ripple that would otherwise limit the MMC applicability in low voltage, variable frequency electric drives. This article introduces the design procedure and the analytical modeling of this novel topology together with the validation of the theoretical claims via simulations and experimental tests.

Analysis and Investigation of Internal AC Frequency to Minimize AC Current Magnitude and Reactive Power Circulation in Chain-Link Modular Multilevel Direct DC–DC Converters

Chain-link modular multilevel direct dc-dc converters (CLMMCs) have attracted much interest recently in for dc power systems because they achieve higher device utilization, lower power losses and they are physically more compact than the alternative front-to-front modular multilevel dc-ac-dc converters (FFMMCs). The CLMMCs rely on circulating an internal ac current to manage energy balance of the sub-module (SM) stacks but this current inevitably contributes to extra current stresses for circuit components and can also lead to excess reactive power circulation within the converter. This paper presents a circuit analysis that there exists a frequency value for the internal ac components that may minimize the current stresses and avoid excessive reactive power circulation. For illustration, the circuit analysis is applied to one of the base formats, the buck-boost CLMMC as an example. The key relationship between the CLMMC and the standard dc-ac modular multilevel converter is explored and established. The equivalent circuits for their dc and ac components with consideration of SM capacitance and SM voltage ripples are created and analyzed in detail, and a full derivation is provided for the specific ac frequency. From this example, this analytical method is extended and applied to other base formats of CLMMCs. The theoretical analysis and results are verified by a set of full-scale simulation examples and down-scaled experiments on a laboratory prototype.

Modular Multilevel Converter (MMC) Modeling Considering Submodule Voltage Sensor Noise

The modular multilevel converter (MMC) is a popular topology in medium- and high-voltage applications, and many efforts have been spent on MMC modeling. However, the impact of submodule voltage sensor noise (SVSN), which becomes more severe due to increasing switching speed of power semiconductors and compact submodule design, has not been considered in conventional models. In this letter, the SVSN is introduced by coupling capacitances between the sensor and power stage in an MMC switching model. Furthermore, the SVSN impact is considered in an MMC average model based on derivation of the relationship between the SVSN and the duty cycle. The proposed MMC switching model and average model considering the SVSN are validated by comparing simulations with experimental results in an MMC prototype using 10-kV SiC MOSFETS.

A Fast Finite-Level-State Model Predictive Control Strategy for Sensorless Modular Multilevel Converter

In this article, we propose a fast finite-level-state model predictive control (FFLS-MPC) strategy for solving the well-known challenges in the predictive control-regulated modular multilevel converter (MMC), which aims to overcome the high computational complexity and enhance the reliability of the control system. First, the presentable output voltage level is determined from solving the Diophantine equations' point of view, which has a good potential to avoid the online optimization process and the selection of weighting factors compared with the conventional finite control-set model predictive control (FCS-MPC) approach. In this sense, the overall computational effort is significantly decreased while achieving satisfactory control performance. Alternatively, in order to enhance the system reliability under sensor failures (e.g., arm current sensor and submodule (SM) voltage sensor), a novel control strategy for sensorless MMC is presented by combining an adaptive linear-neuron-based SM voltage estimation scheme with a currentless sorting-based capacitor-voltage-balancing approach to serve this purpose. We contribute two main points to the relevant literature. The first one is the improvement of computationally efficient by employing the proposed FFLS-MPC methodology. The second one is the elimination of the arm current sensor and SM voltage sensor while guaranteeing adaptability to different conditions. Finally, compared with the state-of-the-art FCS-MPC strategies, comprehensive simulation studies and experiments are presented to demonstrate the effectiveness and feasibility of the proposed methodology for MMC.

A Reference Submodule Based Capacitor Condition Monitoring Method for Modular Multilevel Converters

This letter proposes a novel capacitor condition monitoring method for modular multilevel converters (MMCs) based on reference submodules (SMs). It significantly enhances the capacitor monitoring accuracy by making full use of the SM voltage sensor measurement range. Accuracy comparison between the existing method and the proposed method is conducted to quantify the improvement of accuracy. Moreover, its operation principle and practical implementation considerations are presented. Finally, a three-phase MMC platform is built to experimentally verify the effectiveness of the proposed method.

A Novel Converter Topology for a Primary-Side Controlled Wireless EV Charger With a Wide Operation Range

The full-bridge commonly employed in the primary of an SAE J2954 compliant inductive power transfer (IPT) based wireless electric vehicle (EV) charger possesses many challenges. To alleviate these issues, expensive switches, and extra components are employed together with secondary-side regulation. As an alternative, this article proposes a new topology, termed integrated boost multilevel converter (IBMC). An IBMC consists of series-connected half-bridge submodules (SMs) in each arm, and the dc supply is connected to each arm through a dc inductor. When an IBMC is used as the primary converter, its ability to generate a boosted multilevel output voltage, enables it to regulate the power flow efficiently over a wide load/coupling range without requiring a secondary-side regulator. This article focuses on the operating principles of this new IBMC under steady-state conditions and discusses a modulation scheme that can be used to regulate the power flow while balancing the SM voltages. The benefits of the IBMC is experimentally validated using an SAE J2954 WPT2/Z2 compliant prototype. This prototype used an IBMC built with low-cost switching devices on the primary-side to regulate the power flow to 7.7 kW as the coupling factor changed between 0.11 and 0.25 while maintaining the system efficiency between 90.4% and 91.9%.

Capacitor Condition Monitoring Based on the DC-Side Start-Up of Modular Multilevel Converters

This letter proposes a new submodule capacitor condition monitoring method for modular multilevel converters (MMCs) based on the dc-side start-up procedure. During this process, the MMC can be simplified into an RC charging circuit, and the submodule capacitance can be estimated by investigating its relationship with the phase current and submodule voltage. Several practical issues related to the implementation, advantages, and limitations of the proposed method are presented as well. In order to validate the method, a series of experiments with different submodule capacitances are conducted based on a three-phase MMC platform. Experimental results confirm the effectiveness and feasibility of the proposed method.

A Step-Up Nonisolated Modular Multilevel DC–DC Converter With Self-Voltage Balancing and Soft Switching

Evolving from the popular modular multilevel ac-dc converter, the single-stage nonisolated modular multilevel dc-dc converter (MMDC) is advantageous for medium- and high-voltage applications. However, exploiting ac circulating power to balance the submodule energy, when utilized for high step ratio applications, existing MMDC topologies suffer from circulating current through the arms and large filter inductor at the low-voltage side. To overcome these issues, this article presents a new power transfer mechanism to balance the submodule energy automatically by reconstructing the half-bridge submodule into a quasi-resonant circuit. Based on this submodule structure, a new MMDC topology for step-up applications is proposed. Compared to the existing MMDCs, the proposed one offers the following advantages. First, the common-mode circulating current through the lower and upper arms is avoided. Second, the self-balancing of the capacitor voltages is guaranteed by the proposed modulation method to insert and bypass adjacent submodules in a complementary manner. Third, the soft-switching operation is achieved for the majority of the switches to alleviate switching losses. Fourth, the voltage stress across the input side inductor is limited to the submodule voltage, thereby reducing the size of the inductor. Simulation analysis and experimental results verify the performance of the proposed MMDC.

Fault Localization Strategy for Modular Multilevel Converters in Rectifier Mode Under Submodule Switch Open-Circuit Failure

The modular multilevel converter (MMC) is a promising candidate for the high-voltage and high-power applications. Fault localization is an important issue for the reliable operation of the MMC. The localization strategies of switch open-circuit failures for the MMCs in the inverter mode have been widely reported in the existing literature. This brief proposed a complete fault localization strategy for the MMC in the rectifier mode. In this brief, the differences of the fault characteristics between the inverter mode and rectifier mode are analyzed in detail. Based on that, the switch signal is used as the criterion for the fault localization. After the faulty submodule (SM) is identified, combining the switch signal and the capacitor voltage of the faulty SM can further locate the faulty switch. The proposed strategy can simplify the localization and identify three kinds of switch failures in SMs. In addition, the validity of the proposed strategy is verified by the simulation results.

Arm-Sensorless Sub-Module Voltage Estimation and Balancing of Modular Multilevel Converters

Modular multilevel converters (MMCs) have established their position as the choice of next-generation high voltage converters. These converters, however, require a bulk number of sensors in each phase for sub-modules (SMs) voltage balancing. A limited number of studies are found in the literature where the number of sensors is reduced by incorporating observers to estimate the SM voltages. Nonetheless, most of these methods require accurate measurements of the arm voltage. Thus, the dependence on sensors is not completely eliminated. In this paper, a Kalman-filter-based estimation of SM voltages is presented, which does not require any voltage sensor to measure the arm voltage. Therefore, this method considerably simplifies the implementation of MMCs. Extensive simulations and experimental test cases are conducted to validate the performance of the proposed SM voltage estimation method. The performance assessments verify the accuracy and robustness of the proposed method for various test scenarios performed over a wide range of sampling frequencies.

A Solution of Implicit Model of Series-Parallel Photovoltaic Arrays by Using Deterministic and Metaheuristic Global Optimization Algorithms

The implicit model of photovoltaic (PV) arrays in series-parallel (SP) configuration does not require the LambertW function, since it uses the single-diode model, to represent each submodule, and the implicit current-voltage relationship to construct systems of nonlinear equations that describe the electrical behavior of a PV generator. However, the implicit model does not analyze different solution methods to reduce computation time. This paper formulates the solution of the implicit model of SP arrays as an optimization problem with restrictions for all the variables, i.e., submodules voltages, blocking diode voltage, and strings currents. Such an optimization problem is solved by using two deterministic (Trust-Region Dogleg and Levenberg Marquard) and two metaheuristics (Weighted Differential Evolution and Symbiotic Organism Search) optimization algorithms to reproduce the current–voltage (I–V) curves of small, medium, and large generators operating under homogeneous and non-homogeneous conditions. The performance of all optimization algorithms is evaluated with simulations and experiments. Simulation results indicate that both deterministic optimization algorithms correctly reproduce I–V curves in all the cases; nevertheless, the two metaheuristic optimization methods only reproduce the I–V curves for small generators, but not for medium and large generators. Finally, experimental results confirm the simulation results for small arrays and validate the reference model used in the simulations.

Model analysis, simplified control and sensitivity verification of modular multilevel DC–DC converter with parallel branches

In view of the modular multilevel topology with its uses in medium-voltage DC vessel integrated power systems, this paper deals with typical modular multilevel DC–DC converter (MMDC) topologies and AC control methods. In terms of DC control, a brief introduction is first made about a MMDC topology with parallel branches and its static characteristics. Then, an analysis is focused on the modeling of the MMDC and its model simplification. An optimal control strategy is proposed for the simplified model. Finally, the sensitivity of this system to model and parameter uncertainties is verified both theoretically and experimentally. The obtained results show that the proposed MMDC consisting of a heterogeneous full-bridge submodule and a parallel-branch structure can make the submodule voltage self-balancing and the range of output voltage wide. The use of an optimal control strategy based on a simplified model can lead the system to achieve good static and dynamic performance and robustness.

AC/AC grid connection of six-phase wind power generator based on enneagon MMC converter

In this paper, an enneagon multi-level converter (E-MMC) is proposed, which can directly connect six-phase wind power generator to AC grid. It has the advantages of high efficiency, small fluctuation of capacitor voltage of sub-modules (SMs), and no need of step-up transformer. The working principle of the converter is analyzed, and the optimal phase shifting angle between the two windings of the six-phase wind turbine is determined under different boost ratios. The voltage stability control strategy of the converter and the current tracking control strategy based on proportional resonance (PR) are proposed. The simulation and experiment verify the effectiveness of the proposed system and its control strategy from both steady and transient aspects.