Modular Multilevel Converter Converter Research Articles

DC protection coordination for multi‐terminal HB‐MMC‐based HVDC grids

AbstractFast progress of High Voltage Direct Current (HVDC) technology has been considerable over the past decades. Emerging Modular Multilevel Converter (MMC) provides power systems with promising solutions to integrate large‐scale renewable energy sources, as well as multiple asynchronous power grids. Consequently, implementation of a multi‐terminal HVDC transmission system has been easier using MMC converters. However, protection of multi‐terminal MMC‐based HVDC grids under DC fault conditions is a challenge for the industry. This study sheds light on concept of system‐level DC protection coordination for multi‐terminal HVDC grids. Although, there are different protection methods for HVDC transmission systems including multi‐terminal ones, there are no well‐defined protection coordination strategies for transmission links protection and belonging HVDC converters under DC fault conditions. The proposed protection coordination prevents unnecessary tripping of MMCs in the multi‐area HVDC grids and supports the system with minimum effect on healthy parts. For the first time, different HVDC Converter Tripping (HCT) profiles are presented based on a fully selective protection strategy and their characteristics are discussed comprehensively. Simulation results demonstrate the effectiveness of the proposed strategy for multi‐terminal MMC‐based HVDC grids. The proposed coordination can be applied to DC grid code requirements of meshed HVDC grids in the future.

Bilinear Quadratic Feedback Control of Modular Multilevel Converters

The present paper introduces the formulation and development of a bilinear quadratic control algorithm for Modular Multilevel Converter (MMCs), with a specific emphasis on achieving internal energy stabilization and balance within the converter. A bilinear average model of the MMC is employed, enabling the separation between the DC voltage and the voltage generated by submodules. The algorithm proposed in this study is formulated using bilinear theory and is founded on quadratic feedback control principles. The stability of the suggested controller is scrutinized using a meticulous mathematical approach based on Lyapunov theory. Subsequently, the theoretical findings are assessed using a comprehensive MMC switching model implemented in Matlab Simscape Electrical. The utilization of a phase-shift PWM technique, accompanied by a sorting algorithm, is considered in the study. Additionally, a comparison between the proposed bilinear controller and a standard PI controller is conducted. The outcomes demonstrate that the proposed controller effectively facilitates the regulation of circulating and AC currents, along with managing the internal energy of MMCs. Consequently, this achievement makes a noteworthy contribution to the field, as it introduces an innovative bilinear control approach capable of stabilizing all the state variables of MMCs converters using a single control law.

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.

Submodule Switching-State Based EMI Modeling and Mixed-Mode EMI Phenomenon in MMC

Modular multilevel converter (MMC) is widely used in high-voltage occasions for its good power qualities and flexible controls. However, the large number of semiconductor switches would lead to an extremely complicated electromagnetic environment. To clarify the detailed conducted electromagnetic interference (EMI) mechanisms of MMC, this article builds an EMI model of MMC considering different submodule switching states. Based on this model, the EMI paths of submodule switching- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> and switching- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> processes are analyzed. Then, this article analyzes the effects of submodule heatsink connecting patterns on EMI, and explains the mechanisms of mixed-mode (MM) EMI phenomenon that common-mode component brings in extra differential-mode component. Simulation and experiment results verify the proposed EMI mechanisms and MM phenomenon. The conclusions of this article have great reference significance for the EMI research of MMC and other high-voltage converters.

Studies on high voltage dc cable connection to supply the acceleration grids of the Neutral Beam Injector for DTT

The Divertor Tokamak Test facility (DTT) is the new tokamak under construction in Frascati (Italy), with the main aim of testing different divertor magnetic configurations and technologies in view of DEMO. The Heating and Current Drive function will be provided also by one Neutral Beam Injector (NBI), accelerating negative Deuterium ions with an energy of 510 keV, giving an output power of 10 MW maximum. The Acceleration Grid Power Supply (AGPS), with 3 stages rated 167 kV each, will feed the NBI accelerator. For optical reasons, the insulating gap among the acceleration grids is at the limit of the voltage holding in vacuum, therefore frequent arc breakdowns (BD) among the grids will occur, requiring the quick switch-off of the AGPS to limit the arc energy. For the AGPS, beside the ITER-like scheme including gas-insulated HV components, an innovative approach based on the Modular Multilevel Converter (MMC) technology is being considered. This would include air-insulated HV converters, requiring a huge building volume, which at present is not available in close proximity to the tokamak building. The alternative of installing the MMC converters in a new building to be erected in an available area located far from the tokamak, and connecting the AGPS to the load with HV coaxial cables, is under study. This paper presents the studies carried out to verify the impact of the cables length on the arc energy, in case of breakdown among the grids, and to estimate the magnitude of the voltage oscillations due to resonance effects. These oscillations can occur not only between each hot pole and the return conductor, endangering the cable insulation, but also between the return conductor and local ground, with potential consequences on the laying method, the human safety and the electromagnetic compatibility with sensitive apparatus nearby, which have all been evaluated.

Unbalanced Active Power Distribution of Cascaded Multilevel Converter-Based Battery Energy Storage Systems

In a battery energy storage system (BESS) based on the cascaded H-bridge (CHB) converter or modular multilevel converter (MMC), the active power distribution among the submodules (SMs) can be highly unbalanced, especially when batteries with different power ratings are used. Meanwhile, there are certain limits for the unbalanced power distribution among the SMs. It is a challenge to determine whether the active power distribution is viable for cascaded multilevel converters with unequal capacitor voltages because the complexity grows exponentially as the number of SMs increases. This article proposes a generalized analytic solution to this problem, which reduces the complexity significantly and is readily applicable to converters with large number of SMs. The validity of this method is proved theoretically and verified with simulation and experimental results on both the MMC and CHB converter.

Novel travelling wave directional pilot protection approach for LCC-MMC-MTDC overhead transmission line

Currently, line commutated converter (LCC) and modular multi-level converter (MMC) based hybrid high voltage direct current (HVDC) transmission technology is utilized in actual power grid. Aiming to acquire large transmission capacity and avoid commutation failure, LCC and MMC converters are applied in rectifier and inverter stations in multi-terminal direct current (MTDC) topology structure, respectively. Generally, LCC-MMC-HVDC transmission line adopts travelling wave (TW) protection principle based on one-ended signals. However, it cannot identify fault direction and exists threshold issue. Besides, the different structures of LCC and MMC stations have not been considered. In order to overcome these shortcomings, the paper proposes a novel TW directional pilot backup protection for hybrid LCC-MMC-MTDC transmission line. First, TW propagation process in hybrid multi-terminal transmission system is analysed. Afterwards, fault direction identification criterion is proposed. Eventually, using PSCAD/EMTDC, a ±400 kV hybrid three-terminal LCC-MMC-MTDC simulation model is established. And the simulation results prove that the novel protection is not affected by different fault locations, different fault resistances and different fault types.

Comparative Evaluation of Silicon and Silicon-Carbide Device-Based MMC and NPC Converter for Medium-Voltage Applications

The neutral-point-clamped (NPC) converter and the modular multilevel converter (MMC) are among the most popular converter topologies for medium- and high-voltage applications. This article investigates the impact of silicon-carbide (SiC) switches on the performance of MMC and NPC converters for medium-voltage (MV) applications. Furthermore, two different cases of the circulating current control schemes are considered for the MMC configuration. The experimental prototypes of three-level configurations for both of these topologies are developed to compare and analyze their performances. Furthermore, the double pulse test (DPT) setup is constructed to obtain the device loss characteristics of the switching devices. These characteristics are then used to compute the device losses using the derived loss models. Moreover, in order to generalize the results for wide power range of operation, two sets of devices are considered for the loss analysis of the considered converter configurations. The obtained results for conduction and switching losses are compared for both the converter configurations at various loading conditions. These results provide the operating range for the considered converters to achieve reduced converter losses. Finally, a comparative analysis for the selection of converter configuration based on the obtained results is presented.

Application of Resistance SFCL in MTDC Grid With Reclosing Protection Strategy

Resistive superconducting current limiter (RSFCL) is an effective technical means to solve the short-circuit current problem of DC transmission system. Aiming at the problem of large fault current and fast rising speed in multi terminal DC system (MTDC) with modular multilevel converters (MMC), a new protection strategy based on RSFCL is proposed to isolate the fault area quickly. Through analyzing the quench and recovery characteristics of RSFCL, the quench resistance meeting the reclosing requirements is determined, of which recovers to superconducting in 300 ms. The coordination principle and operation sequence of RSFCL, DC circuit breaker (DCCB) and MMC converter are analyzed for setting up the protection strategy. Taking the bipolar short circuit fault of overhead line in a three terminal flexible DC project as an example, the rationality of the proposed protection strategy is verified in PSCAD/EMTDC software. The simulation results show that the coordination strategy can help to quickly isolate the fault area, carry out fault ride through of non-fault station, accelerate the restart of fault pole converter, and reduce the outage time of MTDC grid.

System Level Real-Time Simulation and Hardware-in-the-Loop Testing of MMCs

In this paper we present an approach for real-time simulation and Hardware-in-the-Loop (HIL) testing of Modular Multilevel Converters (MMCs) that rely on switching models while supporting system level analysis. Using the Latency Based Linear Multistep Compound (LB-LMC) approach, we achieved a 50 ns simulation time step for systems composed of several MMC converters and for converters of various complexity. To facilitate system level testing, we introduce the use of a serial communication-based (Aurora) interface for HIL testing of MMC converters and we analyzed the effect that communication latency has on the accuracy of the HIL test. The simulation and HIL results are validated against an MMC laboratory prototype.

Research on MMC Voltage Equalization Control Strategy Based on Optimal Sorting Algorithm

The voltage sharing control strategy based on sorting algorithm is widely used in modular multilevel converter Converter (MMC) in practical engineering, aiming at the problems of frequent switching states, high switching frequency and large loss caused by traditional scheduling algorithm, a voltage sharing control strategy based on optimal sequencing is proposed by introducing the average value of capacitor voltage and the maximum deviation threshold under the premise of considering the switching state of sub modules at the last moment. Finally, the simulation system of modular multilevel converter is built in PSCAD / EMTDC for verification. The simulation results show that the proposed optimal sequence voltage sharing control strategy significantly reduces the switching frequency of the devices and improves the operation performance of the system.

Application of modified triangular injection technique on MMC to decrease ARM current

Abstract In the area of the converter, the main focus of research is going on to improve the performance of Modular Multilevel Converter (MMC). This paper discusses the triangular injection modulation structure as well as the capacitor voltage balancing approach of the submodule (SM) for MMC converters to maintain a minimum harmonic profile as well as to achieve a lower arm current. To identify the value of Total Harmonic Distortion (THD) and arm current as a minimum in comparison with some well-known modulation methods from literature, a converter structure was developed in MATLAB Simulink environment, and the simulation results showed that the value of arm current and THD profile of MMC in case of this method was lower in comparison with typical modulations. A small prototype of the converter driven by the triangular injection method has been presented to confirm this method's validity.

Interactions between a half‐bridge MMC and a hybrid DC breaker during fault current interruption compared to a full‐bridge MMC

The large-scale integration of remote renewable energy resources into existing AC grids is one of the main drivers for the construction of multi-terminal direct current (MTDC) systems. Today, a few MTDC systems are already in execution stage or even in operation. However, with a growing number of extended MTDC systems, the probability of high-voltage DC (HVDC) insulation faults is likely to increase and therefore, reliable solutions for handling of such disturbances are crucial in order to maintain high availability. Since most HVDC converters are realised by modular multilevel converters (MMC) converters are based on half-bridge (HB) modules, they lack an inherent capability to control DC fault currents. Therefore, additional equipment like HVDC breakers needs to be installed as well. Among the various breaker types, the so-called ‘hybrid’ DC circuit breaker (DCCB) is considered most popular. This contribution provides a more holistic view on the overall system behaviour, represented AC grid, MMC HVDC converter, DC reactor and DCCB altogether. It discusses the physical interactions of these main components in MTDC systems. In particular, the influence of the electrical parameters of the hybrid DCCB and its required DC reactors on the MMC converter's internal current and voltage conditions is outlined.

Analytical Derivation of Intersubmodule Active Power Disparity Limits in Modular Multilevel Converter-Based Battery Energy Storage Systems

Due to a dramatic increase in grid-connected renewable energy resources, energy storage systems are interesting and important for future power systems, among which the modular multilevel converter (MMC)-based battery energy storage systems (BESSs) are one of the most modular, efficient, and flexible topologies. Uneven active power distribution among submodules (SMs) in the arms of an MMC-based BESS is necessary for certain applications. The main contribution of this article is to present a general analysis of the inter-SM active power disparity problem which incorporates the inherent operational constraints of the MMC converter. An analytical method to derive inter-SM active power disparity limits is introduced. The proposed analysis can help facilitate the design of MMC-based BESS for applications such as recycled batteries and hybrid battery chemistries, which can both require significant inter-SM active power disparity. The analysis formulates a criteria vector and criterion value that describes whether an MMC-based BESS is capable of supplying demanded output powers while subject to inter-SM active power disparity. Simulation and experimental results are obtained on a single-phase system with varying numbers of SMs per arm, which verifies the feasibility and generality of the proposed analytical method.

Predictive control of a permanent magnet synchronous generator connected to an MMC converter in an oscillating water column based power plant

Modular Multilevel Converters (MMC) are traditionally used in high-power applications although their field of application has lately widened to lower powers. In medium and low-power applications such as electric drives, a fast control system is usually required and model-based predictive control approaches (MPC) are highly suitable to obtain superior performance. Unfortunately, although MPC is widely used with two-level three-phase power converters, typically in motor and generator control applications, it has been barely developed for MMC. Recent studies discuss MPC approaches for MMC but they are primarily focused on improving the internal operation of MMC. In contrast, this paper presents an MPC which is designed to control the torque and the magnetic field of a surface permanent magnet synchronous generator (SPMSG) by means of an MMC. The proposed approach features a new and fast method to find the location of the reference voltage vector among the large number of multilevel vectors present in typical MMCs. The performance of the new control system has been demonstrated for an oscillating water column (OWC) based power plant simulation. The control system stands out for its fast response, low THD currents and reasonably low computing time which enables its easy implementation in microcontrollers (MCU).

Positive-Negative Sequence Current Controller for LVRT Improvement of Wind Farms Integrated MMC-HVDC Network

Positive and negative sequence based current control strategy is developed for the modular multilevel converter (MMC), which complies with the grid code requirements during the balanced and unbalanced faults at the point of common coupling of AC grids. This research proposes an efficient and simplified current limiting technique to keep the inverter individual phase current at a predefined threshold during faults. In this work, the current controller of the MMC converter is designed based on stationary reference frame while positive sequence synchronous reference frame is used to design the current limiter. The proposed work considers the complete dynamics of permanent magnet synchronous generator (PMSG) and doubly fed induction generator (DFIG) based wind energy. The MMC aggregate model is developed and simulated in Matlab/Simulink. The MMC model is also developed using real-time digital simulation (RTDS). A comparative performance analysis between the proposed and the conventional current limiter is provided where the proposed sequence-based current controller with the current limiter shows promising performance under severe disturbances. Besides grid compliance, the results with the proposed control scheme demonstrate the improvement of low voltage ride-through capability without any violation of converter current and HVDC link voltage limits.

Coupling Effect Investigation of Series-Connected Converters in MMC-Based Ultra-HVDC Station

The modular multilevel converter (MMC) technology is being utilized in ultrahigh voltage direct current (UHVDC) transmission for much larger power transmission and a series of multiple MMC converters are necessary to achieve ultrahigh voltage. Based on the scheme of the Wudongde multiterminal UHVDC project in China, this article develops a small-signal model (SSM) for MMC-based UHVDC station under constant active power-control mode. The dynamics of the high-end converter (HEC) and low-end converter (LEC) are modeled independently considering the internal harmonics. Then, the dynamic model of an MMC-based UHVDC station is developed by interfacing the models of HEC and LEC, considering the characteristics of parallel connection at the ac side and series connection at the dc side. In addition, a voltage-balancing control approach, which is necessary to balance the voltage between HEC and LEC, is also included in the dynamic model. After verifying the accuracy of the derived SSM, this article studies the coupling effect between HEC and LEC. The results show that the vector current controllers of HEC and LEC closely interact with each other, and the voltage-balancing controller makes the coupling between HEC and LEC more complex. Moreover, the circulating current suppressing controllers (CCSCs) of HEC and LEC are relatively independent.

Comparative study of two potential recuperating converters in DC railway electrification system for harmonic mitigation

&lt;span&gt;The regenerative braking energy produced by Light-Rail-Transit (LRT) train is commonly transferred back to power grid via a conventional three-phase inverter (recuperating converter). Although this is a cost saving solution but the ac grid current and voltage waveforms were distorted. Hence passive filters are integrated to mitigate the harmonics. This paper proposed to replace the conventional inverter system with a multilevel converter. Cascaded H-Bridge (CHB) converter and Modular Multilevel Converter (MMC) are selected to be evaluated in this paper due to their modularity structures. The aim of this study is to determine the most potential multilevel converter to be implemented without additional passive filters. Nine-level CHB and nine-level MMC converters are modeled with MATLAB/Simulink simulation tool. Both converters are modulated with Level-Shifted Pulse Width Modulation technique. The output voltage and current waveforms generated by CHB and MMC are presented with full analysis. It is concluded that MMC converter is more suitable to be used as a recuperating converter. It produces a clean voltage and current waveforms. The voltage and current Total Harmonic Distortion (THD) indexes are found approximate to 8% and 3%.&lt;/span&gt;

Virtual Submodule Predictive Control method for Power Electronic Transformer based on MMC Structure

In order to achieve flexible and efficient operation of intelligent power distribution, solving the problems of traditional distribution transformer such as large volume and weight, easy to generate harmonics when overload, and need supporting protection equipment to protect it when failure, etc. We propose a power electronic transformer structure based on modular multilevel converter (MMC). Firstly, we consider the multi-dimensional control target of MMC converter to establish a mathematical model. Then a virtual submodule predictive control method is proposed. The method introduces the concept of virtual submodule to realize the optimal switching state rapid mapping and reduce the switching loss of MMC. Finally, the experimental results show that the mmc-based power electronic transformer has excellent dynamic steady-state performance and can effectively overcome the high loss of traditional predictive control.

Optimal Design of High-Power Modular Multilevel Active Front-End Converter Using an Innovative Analytical Model

Optimal design and selection of arm inductances have been as a challenging subject in the field of modular multilevel converter (MMC). The average steady-state model is generally used as the analytical circuit model that neglects the switching frequency and harmonic values. Also, most of the researchers neglect the saturation effect of inductor core to simplify the analytical model. Contrary to the transformers, choosing the maximum flux density of inductance core is a sensitive issue, in order to design and minimize the inductors. Increasing the flux density reduces the inductor size, but getting close to the saturation region might alter the performance of the converter. This paper presents a systematic optimization approach to minimize high-power MMCs with saturable arm inductance considering technical, thermal, and manufacturing constraints. An accurate steady-state analytical model of MMC converter has been proposed and verified. A combination of converter circuit model and inductance electromagnetic model is employed to find the optimal arm inductances and capacitor values. The effect of nonideal inductance core on converter outputs has been investigated. A dimensioning model of inductor consisting of electromagnetic and thermal models is presented. To compute the optimal inductor size, a novel hybrid optimization loop is proposed including the analytical model of the converter and the inductor in which circuit, electromagnetic, and thermal properties are taken into consideration. In order to increase the accuracy of the dimensioning model, an internal verification loop is employed to verify and correct the analytical model using finite-element analysis. The proposed optimization loop aims to find the minimum inductor size considering technical and manufacturing constraints. Finally, the converter mass sensitivity of MMC converter versus some important constraints, such as temperature rise and capacitor ripple, has been investigated.