Mixed Technology Modular Multilevel Converter Cell - A Cost/Efficiency Analysis

The widely used Line Commutated Converter (LCC) suffers from commutation failure which might cause severe economic and secure consequence. The recently proposed Modular Multilevel Converter (MMC) HVDC is expected to be an alternative solution. However, the half bridge MMC (HBMMC) cannot interrupt the short current caused by DC transmission line fault, which have stunted the engineering practice of MMC in the long-distance overhead transmission line situation. A hybrid LCC-FBMCC system was proposed in this paper. Firstly, based on the parameter of a practical ±500kV HVDC project, the electromagnetic simulation model of this LCC-FBMCC system was established in PSCAD/EMTDC. Secondly, the electromagnetic transient process under DC line fault was simulated. The transient over-voltage characteristics of the LCC-FBMMC system were compared with that of the LCC-HBMMC system. Finally, the DC fault ride ability and the impact of DC short fault on the power transmission was discussed, which can present reference to the development and application of MMC in remote and bulk power transmission area.

An open-set fault diagnosis framework for MMCs based on optimized temporal convolutional network

The expansion of renewable energy systems has led to the widespread use of large-capacity converters, and the application of modular multilevel converters (MMC) is expanding. In large-capacity MMCs, the carrier frequency is limited by the switching device characteristics. Therefore, in steady-state response, the characteristics of MMCs output a smooth sinusoidal waveforms, but there is still a problem with the response to load fractuation. In this paper, a new space vector PWM (SVPWM) method that improves the transient response by 1MHz sampling between carriers and compensating the output vector in real time. The output vector is recalculated every 1us by using an FPGA based hardware controller, and the recalculation is immediately reflected to the current vector, which improves the output characteristics of MM Cs with low carrier frequency. The effectiveness of the proposed method is verified.

Dual three-phase (asymmetrical six-phase) induction motors have received significant attention in high power medium voltage drive applications. Typically, two three-phase voltage source converters are needed to feed such a drive system. In medium voltage applications, multilevel inverters are preferred as they provide a stepped output voltage waveform that reduces dv/dt stresses and provides a better output current waveform, which reflects positively on the machine developed torque and overall drive performance. Among different multilevel inverters, the Modular Multilevel Converter (MMC) has recently shown promise in medium and high voltage applications. For a dual three-phase machine, two MMCs with 6 arms each, are needed. This paper proposes a nine-arm MMC (9A-MMC) to drive such a motor. The proposed approach reduces the number of involved arms by 25%, i.e. reduces the number of required dc capacitors, semiconductor devices and their gate drive circuits, which reduces system complexity and cost. A complete analysis of the proposed system is presented and a simulation model is used to verify the proposed concept.

The paper presents a discrete-time controller for high voltage direct current (HVDC) based on modular multilevel converter. Voltage source converter HVDC (VSC-HVDC) systems are mainly built with pulse-width modulation converter technology. As it operates at high frequency, its high switching losses is an obstacle to its progress. Modular multilevel converter is another kind of converter built with serial-connected half-bridges which only need to switch at line frequency. Therefore it does not need to exert high voltage on power switch and can be easy to enlarge, and modular multilevel converter has been applied to HVDC recently. The control system of modular multilevel converter HVDC can be cascade control like other VSC-HVDC. For its low switching frequency, its sampling period has to be long which would result in slow dynamic response. This paper proposes a high dynamic controller based on a discrete-time model. In the control the reference for the inner loop has taken account of the transition ...

This paper proposes a new strategy for balancing the Capacitor Voltages (CVs) for Modular Multilevel Converters (MMCs). The balancing is one of the main challenges in MMC applications and it is usually solved by adopting a global arm control approach. For performing such an approach, a sorted list of the SubModules (SMs) according to their capacitor voltages is required. A common way to accomplish this task is to implement a sorting algorithm in the same controller used for the modulation technique. However, the execution time and the computational efforts of these kinds of algorithms increase very rapidly when the number of SMs grows. A novel idea is presented in this paper by using a mapping strategy that directly stores in a ranked list the SMs according to the measured CVs. Avoiding the use of sorting algorithms leads to a considerable reduction of the execution time as well as the computational efforts.

The Multi-terminal HVdc systems is a key technology for bulk power transmission. The stability analysis of such power electronics-dominated power systems is a new challenge. This paper focuses on the analysis based on impedance representation of a modular multilevel converter. Therefore, a small signal input impedance modeling method is described for high and low frequencies. A comparative evaluation with the impedance obtained via simulations was performed by the application of a voltage perturbation to the DC side. In addition, the stability of the interconnection of two MMCs' in a Back-to-Back configuration was analyzed by means of Nyquist criteria.

Although the application of modular multilevel converter (MMC) is becoming more and more widespread, due to its working mode, there will be low-frequency ripple voltage in the sub-module (SM) capacitors. The most significant ones are the fundamental frequency ripple and the second-order frequency ripple, which will increase the volume of the SM capacitors and affect the application of MMC. This paper intends to change the traditional MMC SM topology to two capacitors with symmetrical structure, let the ripple voltage in the symmetrical structure capacitor the same in magnitude and opposite in direction. Then, the voltage ripple of the SM can be completely eliminated, and the capacitance value used can be reduced. Experimental results prove the validity of the method.

The modular multilevel converter (MMC) is one of the most promising topologies for high- and medium-voltage applications. However, the conventional MMC relies on large arm inductance and complicated feedback control to maintain submodule (SM) capacitor voltages balanced, which requires a large number of voltage sensors and complicated arm current control loops. In contrast to the conventional perspective, the MMC can be regarded as a derivative from the general multilevel topology based on switched-capacitor (SC) structure. The SC structure endows excellent properties to the MMC, such as minimum or no arm inductor and automatic SM capacitor voltage balancing capability. This MMC with minimum or no arm inductor is named as SC-MMC. Since the arm inductances are very small, the SM capacitor voltages can be balanced automatically. Small stray/parasitic inductance is enough to limit inrush current during SM's switching instants to the rated current level. To unleash the automatic SM capacitor voltage balancing capability of the SC-MMC, a Y-matrix modulation (YMM) has been proposed, which does not require closed loop control or any sensor, has no limitations on the number of SMs, and works for both half-bridge and full-bridge SMs. An unresolved issue of the emerging YMM is that, in the published literature, the existence of the Y matrix with full rank has not been proved theoretically beyond the observation for the SC-MMC when the number of SMs is large. In this paper, an analytical proof of the existence of the full rank Y matrix is provided, which implies the automatic voltage balance of SMs for the MMC with arbitrary number of SMs in each arm. Simulation results using the YMM on a 21-level SC-MMC with minimum arm inductance are provided to verify the voltage balance theory/observation of the SC-MMC using the YMM.

Modular multilevel converter (MMC) has been widely used in high-voltage direct current (HVDC) transmission system due to its advantages of modularity, low switching frequency and excellent output voltage waveforms. Proper operation of MMC is based on capacitor voltage balancing. Conventional MMC capacitor voltage balancing control needs a large number of voltage sensors and sorting algorithm which increases computing burden on the processor. In this paper, a capacitor voltage self-balancing MMC with DC fault ride-through capability is proposed. Capacitor voltage balancing control of this proposed MMC is significantly simple due to its self-balancing auxiliary circuit. Sorting algorithm is not needed, each phase leg only needs one voltage sensor for top SM. So control complexity and computing burden are reduced greatly. Furthermore, DC fault ride-through capability is also reserved. Simulation results validates capacitor voltage of this new MMC topology can be well balanced and DC fault ride-through capability is effective.

THIS dissertation addresses the technical challenges associated with the operation and control of high-power modular multilevel converters. To improve the performance of modular multilevel converter (MMC), a generalized three-phase mathematical model with common-mode voltage (CMV) is proposed. By using the proposed mathematical model, the magnitude of circulating currents, capacitors voltage ripple, and the ripple in DC-link current during balanced and unbalanced operating conditions can be minimized. The modulation scheme and switching frequency are directly affected the output power quality and the performance of the converter and control method. In this dissertation, a novel sampled average and space vector modulation scheme is proposed. These modulation schemes are suitable to control the MMC with any number of submodules (without modifications), operates at low switching frequency, minimizes the ripple in output current and voltage harmonic distortion, and reduces the output filter size. For reliable operation of MMC, the voltage balancing among submodules is mandatory. This dissertation proposes a generalized single-stage balancing approach with reduced current sensors to control the MMC. The proposed balancing approach is suitable to implement with both phase-shifted and level-shifted pulse width modulation schemes. With the proposed approach, it is also possible to control the MMC with half-bridge and three level flying capacitor submodules. Also, an improved balancing approach often referred as the dual-stage balancing approach is proposed to minimize the voltage harmonic distortion and device power losses. This dissertation also proposes a direct model predictive control (D-MPC) approach to minimize the ripple in submodule capacitors voltage. To implement D-MPC approach, a discrete-time model of MMC with CMV is proposed. With the use of proposed model, the D-MPC approach does not require a cost function to minimize the circulating currents. The computational complexity is one of the major issues in the implementation of D-MPC approach for MMC. In this dissertation, a novel reduced computational MPC approaches named as dual-stage D-MPC and indirect model predictive control (I-MPC) approach are proposed. These approaches significantly minimize the computational complexity and, improve the voltage and current waveform quality while operating at the low switching frequency. Finally, the simulation and experimental studies are presented to validate the dynamic and steady-state performance of proposed methodologies. Index Terms • Modular Multilevel Converters. • Capacitors Voltage Balancing. • Pulse Width Modulation Schemes. • Circulating Currents. • Capacitors Voltage Ripple • Direct Model Predictive Control. • Dual-Stage Direct Model Predictive Control. • Indirect Model Predictive Control. • Total Harmonic Distortion.

The modular multilevel converter (MMC) is a highly efficient multilevel converter topology suitable for high and medium voltage applications. However, one of the negative traits of MMC is the existence of circulating current. The detrimental effects of circulating currents include distortion in arm voltages, converter power loss and high switching stresses on semiconductor devices. This paper aims to focus on different circulating current mitigation techniques and their comparative analysis. This paper also proposes a circulating current suppressing controller (CCSC) which depends upon the repetitive control (RC) to minimize the inner circulating current in an MMC. The proposed model is simulated in MATLAB/Simulink and the results are quite in proximity with theoretical analysis.

Online DMD-based Kalman filter for current sensor fault-tolerant control of MMC-HVDC transmission systems

Modular multilevel converter (MMC) is a recently emerged multilevel topology for high-voltage high-power applications. However, in wind power application the performance of the MMC has not been deeply investigated. In this paper the application of MMC in wind energy systems is studied. The converters used to connect the wind turbine to the grid having the rated active powers of 2MW and 10MW are designed and investigated. Electrical losses and thermal loading of the power devices in the proposed converter solutions are analyzed. The efficiency of the MMC converter under different P/Q boundaries defined by grid codes is investigated and compared with two-level and three-level NPC converters. It is concluded that it is possible to use the MMC in wind power application and the losses are distributed evenly between the sub-modules of the MMC converter. However, inside a sub-module the losses of the power devices are not equal that can lead to the de-rating of the converter.

Submodule losses for Modular Multilevel Converter (MMC) are frequently inferred based only on the number of components. This procedure lacks support because Submodule losses also depend on other conditions. In this work, the simulation of different Submodules with various pulse-wide modulation (PWM) techniques for MMC was performed. A new approach to fairly compare Submodule losses is proposed considering Clamp Single (CS), Clamp Double (CD), Full Bridge (FB), and Cross Connected (CC) submodule topologies. The PWM techniques applied were Phase Shifted (PS), Level Shifted Phase Disposition (LSPD), and Space Vector (SV). In addition, IGBT and SiC MOSFET devices were also considered, and DC sources were used instead of Submodule Capacitors to avoid balancing algorithms. A 9-level Three-phase MMC was developed for simulation purposes. The results show that FB, CS, and CC based MMCs present the same losses when LSPD PWM or SV PWM are applied. CD based MMC shows the overall lowest losses, except when PS PWM and IGBT are used becoming the CC based MMC the lower losses submodule in that case. These simulation results support the importance of taking a holistic approach when comparing Submodules for MMC applications.