Phasor model of Modular Multilevel Converter with circulating current suppression control

Modular Multilevel Converter (MMC) bridge arm circulating current increases the MMC bridge arm current distortion and bridge arm current stress, whereas it also increases converter losses and capacitor voltage fluctuations. In this paper, based on the average value of the switching function of MMC, the mathematical relationship between the second harmonic component of the Bridge Arm Common-Mode Voltage (BACMV) and the Circulating Current Suppressing Controller (CCSC) is derived in the d <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> q <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> rotating coordinate system. It proposed an MMC's CCSC based on BACMV. The proposed controller consists of two PI controllers, namely the outer controller and the inner controller. The inner controller suppresses the common-mode voltage, and the outer controller eliminates the second harmonic component of the bridge arm common-mode voltage. The MMC-HVDC simulation model is established, and PI-CCSC, PR-CCSC, and BACMV-CCSC are used for comparative study. The results show that the overshoot of PRCCSC is the largest and that of BACMV-CCSC is the smallest in response transient process. After entering the steady-state, the fluctuation of BACMV-CCSC is the smallest, whereas that of PI-CCSC and PR-CCSC is larger. The harmonic component of the secondary current of BACMV-CCSC is 0.08% and THD content is 0.39%, which is significantly lower than the other two suppression controllers. When the load changes abruptly, compared with the dynamic simulation, BACMV-CCSC has a smaller overshoot in the transient response process and has less disturbance after reaching the steady-state, which improves the steady-state performance of the system and the circulating current suppression controller effect, and provides a new idea about MMC circulating current suppression controller.

The system of multi-parallel inverters connected with modular multilevel converter is the very suitable for the transmission and utilization of the renewable energy. The harmonic stability is the foundation for the operation of the modular multilevel converter based AC system connected with multi-paralleled inverters. In this paper the harmonic stability criterion for the modular multilevel converter based AC system connected with multi-paralleled inverters is proposed based on global admittance. The different discharge numbers of the submodules correspond to different impedance states. The state impedance model of modular multilevel converter for harmonic stability analysis is proposed. Based on the state impedance model, the impedance model of the modular multilevel converter based AC system connected with multi-paralleled inverters is obtained. Then the harmonic stability for the system is analyzed by the global admittance criterion. An application example is used to verify the effectiveness of the state impedance model of modular multilevel converter and the harmonic stability for the modular multilevel converter based AC system connected with multi-paralleled inverters.

This paper validates the modular multilevel converter (MMC) models, which are developed by two different institutes using PSCAD/EMTDC. The MMC uses a large number of power-electronic switching devices that operate independently. Modeling such converters with individual switches in an electromagnetic transient (EMT) simulation program leads to extremely slow runs, and hence seems to be impractical. To overcome this issue, MMC can be modeled as a detailed equivalent circuit. In this method, each MMC arm is represented by a Thévenin equivalent branch whose voltage and resistance are calculated at each time step, thus reducing the computational burden. The models have been developed by Yonsei University in South Korea and the Manitoba HVDC Research Centre in Canada based on this technique. The presented models include the detailed equivalent representation of each converter arm, corresponding controls, waveform generation, and cell capacitor balancing strategies. To validate overall performance and transient response of the model, a 7-level MMC-HVDC is modeled with individual switches and through the detailed equivalent circuit. As an example for a realistic system, a 201-level MMC-HVDC system is also simulated. The results demonstrate validity and compatibility of two MMC models.

In this study, we propose a wind power generation system model for operating modular multilevel converter (MMC) in a hardware-in-the-loop simulation (HILS) application. The application of the MMC is a system that connects wind power to a grid through high-voltage direct current (HVDC) in the form of back-to-back connected MMCs, whereas a HILS is a system used to test or develop hardware or a software algorithm with real time. A real-time operation model of the MMC is required to conduct a HILS experiment. Although some studies have introduced the HILS model of MMCs for grid connection using PSCAD/EMTDC, it is difficult to find a study in the literature on the model using Matlab/Simulink, which is widely used for power electronic simulation. Hence, in this paper, we propose a real-time implementation model employing a detailed equivalent model (DEM) using MATLAB/Simulink. The equivalent model of both wind power generation system and MMC are presented in this paper. In addition, we describe how to implement components such as a variable resistor that is not provided in the Simulink’s library. The feasibility of the proposed model is demonstrated with real-time operation of a wind power generation system.

This paper introduces an enhanced equivalent model of the half-bridge symmetrical monopole Modular Multilevel Converter (MMC) topology for the analyses of the electromagnetic transients. As compared with the existing MMC models, the proposed model reduces the simulation computational time without compromising accuracy, and contrary to the existing models, it provides accurate representation of the MMC behavior with respect to the DC-side fault. This paper also evaluates and verifies the accuracy of the proposed equivalent model under various operating conditions, i.e., start-up, DC-side pole-to-pole fault, and AC-side three-phase to ground fault. The study results indicate that: 1) in contrast to the existing equivalent models, the proposed enhanced model accurately represents the MMC under various operating conditions; 2) the implemented simplifications to the model do not adversely affect the accuracy of the simulation results; 3) a reduced-level approximated model of the MMC is also adequately accurate for protection studies.

Modular multilevel converter (MMC) is a promising topology for medium and high power applications. The MMC system consists a large number of sub-modules (SMs), which take long time to simulate. In a real-time simulator, the MMC model is optimized to have a fast simulation speed for real-time applications, such as hardware-in-the-loop (HIL) tests. The methodology to evaluate model fidelity is important. In this paper, a methodology is proposed to validate an MMC model capable of real time simulation and implemented in central processing unit (CPU) and field-programmable gate array (FPGA). The model is validated in both open-loop and closed-loop control in a simple MMC test system, and compared to a reference detailed model made with SimPowerSystems (SPS) blocks. The effect of the model sampling time on the accuracy is also studied. The results indicate the MMC model optimized for real time simulation with a time step of 25 μs is as accurate as an SPS model with a time step of 0.2 μs.

Presently the global wind energy capacity is increasing rapidly and it has become the fastest developing renewable energy technology. The early technology used in wind turbines was based on squirrel cage induction generators directly connected to the grid through normal converters. In this paper, a new modular multilevel converter based integrated wind energy system for reducing harmonics with different pulse width modulation techniques is proposed. Modular multilevel converter is the state of art technology and hence this converter is integrated with wind energy system. In this paper, harmonics, voltage and current responses at different loads at different wind conditions by using new proposed integrated wind energy system connected to a modular multilevel converter through multi winding transformer are compared. The proposed model of modular multilevel converter integrated to the wind energy system is developed in MATLAB/SIMULINK.

To address the limited speed and scale of the simulation of modular multilevel converter (MMC) in electromagnetic transient simulation program (EMTP), this paper put forward a MMC fast electromagnetic transient model based on rotation transformation. The crucial advantage of the rotation transformation is to reduce the fundamental frequency of the original signal via the rotation transformation of coordinate system. With proposed method implemented in the EMTP of MMC, the simulation step size can be relatively increased, and the fast electromagnetic transient simulation can be achieved. In this paper, the equivalent model of MMC switch function model expressed by the state equation is derived. Based on this, the novel MMC model based on rotation transformation is further derived combined with the concept of rotation transformation. Then, the state equation of proposed novel MMC model is calculated in Matlab, and the simulation results and simulation time-consuming are compared with which of 51-level topological MMC electromagnetic transient model established in PSCAD/EMTDC. The results of comparative analysis show that the simulation duration of the MMC model based on rotation transformation is far less than the traditional MMC electromagnetic transient model without sacrificing accuracy.

Modelling of Modular Multilevel Converter (MMC) for electromagnetic transient type (EMT-type) simulations is challenging due to numerous IGBTs/diode devices. Detailed models of HVDC-MMC transmission systems require long computation times. Simplified models have been proposed in the past to overcome this problem. However, some of these types of model require a hard-coding development which depends on the EMT software used. This article presents a simple and efficient approach for modelling these types of MMC using only generic EMT blocks (i.e. power circuits and control components) that can be found in most EMT software libraries. Half- and Full-bridge submodules are considered. The models developed include the blocking state of submodule. A realistic HVDC-MMC link is used as a test case for validating the MMC models proposed.

This paper presents impedance modeling of modular multilevel converters (MMC) for stability and resonance analysis of HVDC transmission, renewable energy, STATCOM and other systems that increasingly rely on MMC as the hardware building block. The basis of the modeling method is harmonic linearization, which has been applied to other types of converters and converter systems, but a new formulation is presented to allow the inclusion of harmonic effects. This generalization, called multi-harmonic linearization, is necessary for MMC because of the significant second harmonic in the arm currents, module capacitor voltages, and control signals. To accommodate multiple harmonics in the linearization process, a matrix formulation is also introduced and used to model both the converter and its control. The developed impedance models are validated in the frequency-domain by point-by-point simulation of detailed converter circuit models, and used to characterize MMC impedance under different conditions.

The high-capacity modular multilevel converter (MMC) is becoming the most popular type of voltage-source converters for high voltage direct current (HVDC) transmission. However, the nonlinearity of the loss characteristics of the switching devices and the complexity of the switching patterns impose great challenge on the loss modeling of MMC. The present methods still have some flaws in modulation methods and switching loss calculation. This paper proposes a simplified MMC loss mathematical model based on the RMS and average value of the arm current. The analytical expressions of this simplified model are also presented. Simulation results of a ±350 kV/1000 MW MMC using PSCAD/EMTDC verify the correctness and the accuracy of the proposed model.

The model of a modular multilevel converter (MMC) determines the extent of critical circuit information that electromagnetic transient simulations can reveal. In this paper, two MMC models are proposed for efficient real-time hardware-in-the-loop (HIL) emulation on the field-programmable-gate arrays (FPGA). The nonlinear switch-based model employing the insulated-gate bipolar transistor (IGBT) dynamic curve-fitting model considers factors affecting its transient performance so that device-level behavior such as power loss and junction temperature can be reproduced accurately in the electromagnetic-thermal simulation of a power converter for its design evaluation. Meanwhile, regarding the MMC submodule as a transmission line stub achieves faster computation speed and enables the formation of a hybrid arm to save FPGA hardware resources. As the large network that the MMC presents is burdensome for real-time execution with a small time-step, circuit simplification based on partitioning and merging is conducted. Hardware implementation of a three-terminal high-voltage direct-current (HVDC) system containing an MMC-based DC-DC converter is carried out and the efficacy of proposed models is validated by comparing HIL emulation results with the off-line simulation tool PSCAD/EMTDC.

Modular Multilevel Converter (MMC) is a mainstream technology in the field of flexible DC transmission engineering. The establishment of electromagnetic transient simulation model of MMC is the basis for theoretical research and engineering application. The Thévenin’s equivalent-based electromagnetic transient model of MMC has high simulation accuracy and less simulation time, which is widely used in the simulation system of MMC with high level number. However, the model is difficult to simulate the internal electromagnetic transient process of the submodule, such as the inability to effectively measure the current stress of the IGBT and the continuity diode when testing and analyzing the equivalent experimental stress, as well as the inability to simulate and analyze the internal faults of the submodule. In order to solve this problem, this paper proposes a highly accurate and efficient electromagnetic transient simulation model of MMC converter based on PSCAD/EMTDC. Specifically, the internal sub-modules of the MMC model are divided into two categories: special sub-modules and general sub-modules. For the special sub-modules, the detailed models are established respectively; for the general sub-modules, the Thévenin’s equivalent model is established. The model has high simulation speed and accuracy, while the interface is generalized and preserves the station-level and valve-level control strategies. The simulation is carried out by PSCAD/ EMTDC, and the experimental results verify the correctness and effectiveness of the proposed modeling method.

This article presents three dynamic linear state space models of Modular Multilevel Converter (MMC) which are suitable for small-signal dynamic studies and controller design. The three models differ by the number of states (two, six and ten) and therefore are suitable for different applications based on the required accuracy. The 2nd and 6th order models ignore dynamics of the second harmonics and circulating current suppression control. The main challenges of dynamic analytical modelling of MMC are the non-linear multiplication terms in equations for modulating oscillating signals. The multiplication non-linear terms is therefore considered directly in the rotating DQ frame. This requires simultaneous modeling in zero sequence, fundamental frequency DQ and double fundamental frequency DQ2 frames. The proposed linear analytical models are implemented in state-space in MATLAB. The validity and accuracy of the models are verified against detailed 401 level MMC model in PSCAD/EMTDC in both: time and frequency domains. The results show very good accuracy for the 10th order model and decreasing accuracy for the lower order models.

The calculation of the short-circuit current is an important basis for fault detection and equipment selection in the DC distribution system. This paper proposes a linearized model for modular multilevel converter (MMC) considering different grounding methods and different failure scenarios. This model can be used in different fault conditions before MMC’s block. Under different fault forms, the model has different manifestations. This paper analyzes and models the DC distribution network with two types of faults: inter-pole short circuit and single-pole grounding short circuit. Among them, the modeling and analysis of single-pole grounding short-circuit uses the method of common- and differential-mode (CDM) transformation. To solve such a model, an analytical calculation method is proposed. As a mean of evaluating the effectiveness and accuracy of the proposed model, the analytical calculation solution is compared to the solution produced by PSCAD/EMTDC. A comparison of the results reveals the efficacy of the proposed model.