Simulation Of Modular Multilevel Converter Research Articles

Modular muti-level converter grid-connected oscillation suppression measures based on virtual impedance

Modular multilevel converters(MMC) are widely used in power systems. However, relevant studies and reports show that MMC may cause oscillation problems by connecting with the grid. The second-harmonic circulating current is caused for the energy exchange between MMC phases under the current conversion process. The unreasonable parameter design of converter will lead to the natural frequency of the MMC approaching the frequency-doubled of grid. Farther, it may cause the bridge arm current oscillation. When the bridge arm resistance is large enough, the operating conditions will be stable for the overdamping in the system. Virtual damping control strategy is applied to stability study, which can improve the damping of the system without increasing the system loss and suppress the oscillation. Therefore, a stability control strategy base on virtual damping is designed to improve the stability margin of the system. In order to verify the effectiveness of the method, the main circuit model of MMC is established by using the form of matrix. Besides, it considers the virtual damping control. In order to verifie the effectiveness of the virtual damping control, the MMC simulation model is established under the Matlab/Simulink . The results show that the proposed virtual damping control can effectively suppress the MMC oscillation problem.

Inverse Application of Artificial Intelligence for the Control of Power Converters

This article proposes a novel application method, inverse application of artificial intelligence (IAAI) for the control of power electronic converter systems. The proposed method can give the desired control coefficients/references in a simple way because, compared to conventional methods, IAAI only relies on a data-driven process with no need for an optimization process or substantial derivations. Noting that the IAAI approach uses artificial intelligence to provide feasible coefficients/references for the power converter control, rather than building a new controller. After illustrating the IAAI concept, a conventional application method of artificial neural network is discussed, an optimization-based design. Then, a two-source-converter microgrid case is studied to choose the best droop coefficients via the optimization-based approach. After that, the proposed IAAI method is employed for the same microgrid case to quickly find good droop coefficients. Furthermore, the IAAI method is applied to a modular multilevel converter (MMC) case, extending the MMC operation region under unbalanced grid faults. In the MMC case, both simulation and experimental online tests validate the operation, feasibility, and practicality of IAAI.

Harmonic Current Emission Estimation of Modular Multilevel Converters in MV Charging Stations

This paper proposes a novel framework for modular multilevel converter (MMC) current harmonic emission estimation from medium voltage (MV) charging stations. The framework is enlightened by wave decomposition, exploiting an analytical derivation of the emission, without a time-domain model. Accordingly, voltage harmonic distribution in grids can be estimated with a frequency-domain model, using the estimated voltage-dependent current emission in the iterations. Compared to conventional time-domain simulation, such method not only enables analytical harmonic estimation integrated with a large load-flow model, but also benefits from the potential of distributed computation. A 10.5 kV grid-tied single-phase half-bridge MMC simulation case on PLECS has been implemented and investigated for comparison. Corresponding results revealed that the estimated harmonic series kept the absolute average difference to the PLECS model lower than 0.02% and a correlation ratio higher than 99.990% with the simulated ones. The effectiveness of the proposed estimation framework is successfully verified.

Optimal Suppression Strategy for Capacitor Voltage Ripples of Hybrid MMCs Under Unbalanced Grid Voltages

Submodule (SM) capacitor voltage ripples in modular multilevel converters (MMCs) can be suppressed by harmonic injection strategy, which can reduce the volume and cost of MMCs. The existing methods using the dual harmonic injection strategy for hybrid MMCs only consider steady-state conditions. Harmonic injection strategies for unbalanced grid voltage conditions only use single harmonic injection and can be applied for single-phase faults. To address the above issues, this paper proposes an optimal dual harmonic injection of second-order harmonic current and third-order harmonic voltage strategy for hybrid MMCs. Firstly, the mathematical model of MMC under unbalanced grid voltages is established. Then the ripple characteristics of SM capacitor voltage are analyzed to reveal the relationship between the capacitor voltage and the arm power ripples. Based on the developed instantaneous arm power model, optimal harmonic injection parameters are calculated. A hybrid MMC simulation model is built in PSCAD/EMTDC to verify the effectiveness of the proposed strategy. The results show the excellent performance of the proposed strategy in suppressing the voltage ripples under unbalanced voltage conditions.

Inertial imitation method of MMC with hybrid topology for VSC-HVDC.

A new virtual synchronous generator (VSG) control strategy was researched and proposed for a VSC-HVDC (High Voltage Direct Current Based on Voltage Source Converter) transmission system. It can be applied to half-bridge or full-half-bridge hybrid topology modular multi-level converter (MMC) to improve the stability and reliability of the system. First, it is proposed that the energy stored in the equivalent capacitor of MMC power module was used to imitate the rotor inertial of synchronous generator. It can buffer transient power fluctuations and synchronize autonomously with the grid. Then the impedance characteristics of the proposed control method have been deduced and analyzed. The results show that the VSG control loop mainly improves the low frequency characteristics of the converter. Secondly, the ability to suppress transient fault current is weak. So, a method, that the given values of inner current loop are calculated by grid impedance matrix, was used. A double closed loop control structure composed by a power outer loop based on VSG control and a current inner loop is obtained. The simulation results show that it can effectively improve the current control capability during the transient process for systems with a 1:2 ratio of converter capacity to grid capacity (The grid short-circuit capacity is 60MW and the MMC is 30 MW). Finally, a hybrid MMC simulation model was built based on PSCAD and the steady-state and transient fault ride-through simulations were performed. The power adjustment time of MMC under the proposed VSG control is about 1s, while the adjustment time under the conventional control strategy is greater than 4s.

Fault diagnosis and location of independent sub-module of three-phase MMC based on the optimal deep BiD-LSTM networks

Modular multilevel converter (MMC), which is highly modular, easy to be expanded, and has good output voltage waveform, is especially suitable for medium and high voltage power system applications. The MMC bridge arm inductance fault and the independent sub-module fault have the greatest influence on the transmission quality of transmission systems. However, there is few research on fault diagnosis and location for multiple sub-modules(SM) in a single arm at present. A deep bidirectional Long Short Term Memory (Bid-LSTM) network is proposed in this paper, which is used to diagnose and locate the faults of different sub-modules. which can deal well with the prediction problem with sequence dependence. In addition, because the Bid-LSTM model contains many hyperparameters, which seriously affects the prediction performance of BiD-LSTM, an optimal fruit fly optimization (FOA) algorithm based on the composite chaotic mapping of 3-dimensional Logistic-Sine (3d-CCFOA), which searches the optimal value of related hyperparameters to ensure the accuracy of classification. 3d-CCFOA has good chaos characteristics, which enables FOA to avoid falling into local optimal solution effectively and obtains an effective global optimal solution. Then, the MMC simulation system of RT-TAB is used to simulate the open circuit faults of different independent sub-modules of one bridge arm and collect relevant fault data. The analysis process and experimental results adequately indicate that this method can effectively and accurately diagnose and locate the faults of independent sub-modules. Meanwhile, this method also provides a reference diagnosis strategy for multi-bridge arm fault diagnosis and multi-sub-module location.

FPGA-Based Real-Time Simulation of Dual-Port Submodule MMC–HVDC System

Aiming at the problems of the high switch numbers, complex working mechanisms, and complicated real-time simulation of modular multilevel converters (MMCs) composed of dual-port submodules, in this study, we designed a unified equivalent model of the multiple submodule network by analyzing the combination of parallel submodules in the bridge arm. The proposed model decouples the submodules that do not affect each other in the subnetwork calculation process, thereby reducing the number of prestored parameters in the subnetwork simulation. In the Xilinx Virtex-7 FPGA VC709 (Xilinx Corporation, San Jose, CA, USA) development board, we replaced the inline computation combined with the prestorage of parameters with the proposed equivalent model to optimize the execution unit structure and redesigned the FPGA-Based Real-Time Digital Solver (FRTDS). Taking the P-FBSM-based MMC–HVDC system as the simulation object, we performed a real-time simulation with a step size of 10 μs, which verified the effectiveness of the proposed model and the improvement in the hardware. We compared the results with the offline MATLAB/Simulink simulation results to verify the accuracy of the simulation.

FPGA algorithm implementation of hybrid MMC harmonic injection strategy

Capacitor submodule occupies a large volume in Modular multi-level Converter (MMC) and costs a lot of money. Harmonic injection strategy can often achieve the effect of reducing the voltage fluctuation of the sub-module capacitors. Therefore, it can reduce reduce the construction cost of the HVDC transmission system. On the basis of introducing the basic principle of hybrid MMC harmonic injection and simulation verification on PSCAD platform, this paper introduces the physical platform of four-terminal flexible power grid, including its hardware display and monitoring and testing interface, showing the diversity of real-time data acquisition methods. Finally, through the experimental research on the implementation of the hybrid MMC harmonic injection strategy by FPGA, the modulation wave, the conduction number of the sub-module, the capacitance voltage of the sub-module and other physical quantities before and after the second harmonic injection are compared and analyzed. It is verified that the second harmonic injection can change the shape of the voltage modulation wave. At the same time, the second harmonic injection can reduce the voltage fluctuation of the capacitor of the sub-module and realize the lightening from the perspective of reducing the capacitor demand.

Autonomous Control Based on Capacitor Energy Storage of Converter for DC Distribution System

The converter valve is the core equipment of the DC distribution systems. This paper proposes an autonomous control strategy for grid-connected and islanded operation of hybrid topology modular multilevel converter (MMC). The overall control structure is designed, including the outer loop for autonomous control and the inner current loop. Firstly, for the outer loop, it is proposed to use the equivalent capacitance storage energy of MMC power module to simulate the inertia of synchronous generator rotor to achieve the purpose of buffer power fluctuation. So, the outer loop provides system inertia. Second, the inner loop is designed for current tracking control and suppresses transient fault current effectively. It is proposed that the current inner loop reference value can be directly calculated through the grid-connected impedance matrix and voltage across it. And the small signal model of autonomous control for a hybrid MMC is established. Then the influence of the proposed control structure and control parameters on the impedance characteristics of the system is analyzed in detail. Finally, a hybrid MMC simulation model for DC distribution system is built based on PSCAD, simulating island operation, grid-connected operation mode and transient faults respectively for verifying the proposed strategies.

Fault diagnosis of modular multilevel converter based on principal component analysis and support vector machine

Modular multilevel converter (MMC) is widely used in DC transmission, new energy grid-connected power generation, transmission, reactive power compensation, power flow control and other fields. When the sub-module fails, detecting and locating the fault quickly and accurately is the key to improving the operational reliability of the converter. Principal component analysis (PCA) obtains the feature space of reduced dimension by extracting the principal components of the fault sample set, which is conducive to the extraction of fault features. Support vector machine (SVM) has good classification performance when applied to fault diagnosis. Combining the advantages of both, this paper takes modular multi-level converter as the research object, extracts the fault characteristics of MMC, and uses PCA algorithm to reduce dimensionality. Then, the SVM classifier is constructed, the processed fault samples are used for training, and the trained SVM classifier is used to perform fault diagnosis. Finally, a three-phase eleven-level MMC simulation model is built to simulate the method used. The results show that this method can effectively improve the diagnosis speed and accuracy of MMC fault diagnosis, and provides a reference for the application of the PCA-SVM method in the actual engineering of MMC fault diagnosis.

Research of Modulation and Control Strategy of Modular Multilevel Inverter

As a new multilevel converter structure, modular multilevel converter (MMC) has many outstanding advantages, such as high degree of modularization and easy cascade expansion, which greatly promotes the development and application of flexible DC transmission technology. In this paper, the modular multilevel inverter is the main research object, its topology and multilevel generation mechanism are analysed, and the capacitor voltage of sub modules is balanced based on the carrier phase shifted PWM strategy. The MMC simulation model is built on the MATLAB/Simulink software platform. According to the simulation results, the feasibility of the modulation and control strategy in reducing the harmonic component of output voltage and maintaining the stability of capacitor voltage is verified.

A capacitor overvoltage elimination strategy for reduced-voltage-sensor-based MMC

As modular multilevel converter(MMC) requires a large number of sensors to measure capacitor voltages, reducing the number of sensors for MMC has become an emerging topic. One of the technical challenges for reduced-voltage-sensor-based MMC operated with NLM is to eliminate the overvoltage that occurred on submodules. In order to solve this problem, a capacitor overvoltage elimination strategy for reduced-voltage-sensor-based MMC is proposed in this paper. The overvoltage elimination is consists of pre-measurement and in-process measurement of capacitor voltage, which is to obtain the actual value of submodule capacitor voltage so that to sort submodule capacitor voltages in the correct sequence. In this way, the submodules with excessively high voltage or with overvoltage trends can be selected for discharging until their voltages reach the normal level. Therefore, the overvoltage that occurred on submodules can be eliminated. A 31-level single-phase MMC simulation model is performed in MATLAB/SIMULINK, and a 9-level one is constructed in an experimental laboratory. Both the simulation and experimental results verify the effectiveness of the proposal even in the situation of capacitance parameter deviation.

Circulating Current Suppression Strategy of Modular Multilevel Converter Based on Model Predictive Control

Modular Multilevel Converter (MMC) has the characteristics of high voltage level and low switching frequency. The traditional modular multilevel converter circulating current control strategy mostly adopts the PI control principle, and the parameter setting is complicated and the accuracy is not high, and the control process is more difficult. Model predictive control strategy is the optimal control method based on the model in the existing time domain. This paper proposes a Model Predictive Control (MPC) method based on carrier phase-shifted pulse width modulation (PSC-PWM) to suppress the circulating current, and output the optimal modulation wave through model prediction. Compared with the traditional control strategy, this strategy is simple to implement, does not require complex tuning calculations, and combines with the traditional capacitor voltage equalization strategy to obtain the output modulation wave. A 7-level MMC simulation control system is built in MATLAB / SIMLINK to verify the theory, comparing with existing control methods, it can be concluded that the proposed method has high calculation efficiency, good control accuracy and strong robustness.

On the Modeling and Design of Modular Multilevel Converters With Parametric and Model-Form Uncertainty Quantification

Modeling and design with parametric and model-form uncertainty quantification is an alternative to conventional model-based design approaches as it improves the existing modeling practice and validates the model used in the design of power converters. However, in the case of modular multilevel converters (MMCs), uncertainty quantification, as the main step in this design methodology, becomes challenging due to the inherent complexity and sheer size of such units. In this article, these limitations are discussed and a systematic study for developing a simplified testbed for uncertainty quantification of an MMC is presented. To this end, first sensitivity analysis is conducted to identify the key parameters whose tolerances contribute the most to the parametric uncertainty of the selected design variables. Second, the effect of increasing the number of power cells in each arm on the estimated total uncertainty, and thus the predictive capability of the MMC simulation models for medium- and high-voltage applications is studied. A simplified testbed for model validation of the power cell in the design of an MMC is developed accordingly. The development of this simplified testbed allows validating the models used and estimating uncertainties in the design with less computational cost and hardware prototyping.

Active Power Decoupling for a Modified Modular Multilevel Converter to Decrease Submodule Capacitor Voltage Ripples and Power Losses

The usage of too many sizeable capacitors significantly increases the volume and cost of the modular multilevel converter (MMC), and limits its spread in medium- and high-voltage applications. By adding a buck-type active power filter (APF) circuit into a particular MMC topology [of which each phase multiplexes one submodule (SM)], a modified MMC (APF-M3C) topology, which inherently has the ability to suppress capacitor voltage fluctuations and decrease the power losses compared with conventional MMC topology, is proposed in this article. Both the SM capacitor voltage ripples, which can be reduced up to 50% of original value and are related to the SM capacitance, and the arm currents, which are related to SM power losses, are calculated in detail. Its topology, and operating principle is presented in detail, and a novel modulation method for the APF-M3C topology are proposed. In addition, an improved control method for the voltage balancing is also recommended to further reduce system cost and power losses. Finally, the APF-M3C topology and traditional MMC simulation and experimental platforms were built. Simulations and experimental results verify the feasibility and effectiveness of the APF-M3C topology and control strategy by comparing with the conventional MMC.

Virtual Synchronous Generator Control of VSC-HVDC System Based on MMC of Hybrid Topology

For a VSC-HVDC transmission system based on a hybrid topology converter of full-bridge and half-bridge, a kind of virtual synchronous generator (VSG) control strategy which can be applied to modular multilevel converter (MMC) grid-connected structure was researched and proposed. First, based on the conventional VSG control strategy, the energy stored in the equivalent capacitor of MMC power module was used to imitate the rotor inertia of synchronous generator. The characteristics of generator can be simulated during transient frequency fluctuations and it can help relieve the power fluctuations. Secondly, with respect to the structural characteristics of the direct grid connection through the reactor on the AC side of the MMC, which is unlike the microgrid inverter, there are no additional filter capacitors. So, the existing commonly used VSG control strategy of the microgrid inverter and double-closed-loop structure composed of filter capacitor voltage and AC current cannot be directly applied. For this, a method where the given values of inner current loop are calculated by grid impedance matrix was used. So, a double-closed-loop control structure composed of a power outer loop based on VSG control and a current inner loop is obtained. It can effectively improve the current control capability during the transient process. Finally, a hybrid MMC simulation model was built based on PSCAD to verify the correctness and effectiveness of the proposed methods.

Research on Power Interface Algorithm of Power Hardware-in-the-loop Simulation for Isolated Operation of Modular Multilevel Converters

A current source type power interface algorithm is proposed for the power hardware-in-the-loop (PHIL) simulation of modular multi-level converters (MMC) operating with an isolated gird such as a wind farm. To improve the stability and accuracy of hybrid simulation system, a virtual impedance and the running method is proposed for MMCs. A simulation model based on the proposed interface algorithm is established in MATLAB. The test results show that the hybrid simulation system maintains high stability and accuracy. At the power interface, the relative error of voltage is less than 2%, and the relative error of active power is less than 2%. The proposed power interface algorithm is applicable to MMCs operating with an isolated grid.

Sliding Mode Observer Based Robust Fault Reconstruction for Modular Multilevel Converter with Actuator and Sensor Fault

In this paper, a design method of sliding mode observer (SMO) is proposed to solve the problem of robust fault reconstruction for modular multilevel converter (MMC) with actuator and sensor fault. A state space model of MMC system is established to consider simultaneously actuator fault, sensor fault and uncertainty. Based on the obtained system model, a SMO is introduced to reconstruct the fault and an augmented system is obtained. Especially, the fault can be detected and the fault dynamics can be reconstructed by controlling the sliding mode motion with the equivalent output. Moreover, the SMO gain can be designed via the semi-definite program method. Finally, the effectiveness and feasibility of this method can be verified by using a MMC simulation example.

Combining Detailed Equivalent Model With Switching-Function-Based Average Value Model for Fast and Accurate Simulation of MMCs

Modeling and simulation play a vital role in the design and testing of modular multilevel converter (MMC) high voltage direct current (HVDC) systems. Detailed equivalent model (DEM) and switching-function-based average value model (SFB-AVM) are two major types of accurate and efficient models to represent the dynamic response of the MMCs. However, the DEM and the SFB-AVM possess unique benefits depending on the purpose of the simulation studies. The DEM provides a detailed representation of submodule (SM) switching events and individual capacitor ripples. The SFB-AVM provides faster simulation speed by using arm equivalent capacitance. Combining both models in a universal arm equivalent circuit gives the users the choice of selecting the most appropriate modeling method during dynamic simulation. This paper proposes a universal modeling framework combining the DEM with the SFB-AVM which allows the DEM and the SFB-AVM smoothly switch from one to the other during dynamic simulation. The proposed SFB-AVM can accurately represent the MMCs with different SM types. The proposed models are validated in offline and real-time simulation studies which demonstrate the improved simulation speeds of the proposed SFB-AVM over the DEM especially for large numbers of SMs.

Concurrent fault diagnosis of modular multilevel converter with Kalman filter and optimized support vector machine

ABSTRACTIn this paper, concurrent fault diagnosis problem of modular multilevel converter (MMC) with Kalman filter and optimized support vector machine (SVM) is investigated. The state space model by synthesizing the circulating current and the output current is first established. Recurring to the Kalman filtering theory, the estimation on circulating and output current is realized, the residual is achieved by using the innovation which involved the predicted and measured current. Based on the obtained residual, the residual evaluation function and its threshold are constructed. Then, the fault can be detected according to the proposed fault detection strategy. Once the fault is detected, the fault localization unit is triggered and the residual data is adopted as data set. By employing the optimized SVM with genetic algorithm, the concurrent and intermittent fault localization of MMC can be accomplished. Finally, an 11-level MMC simulation systems with concurrent fault and intermittent fault are set up in MATLAB/Simulink, and the effectiveness of the proposed fault detection and localization method is verified.