A Novel Detection Method for Open-Circuit Fault in Modular Multilevel Converters

This paper presents the detailed description of DC/DC operation for reliable grid integration of renewable energy into HVDC grid, under normal operation and different fault conditions. The DC/DC uses the isolated dual-active-bridge DC/DC converter based on MMC (Modular Multilevel Converter) technology. Within the MMC DC/DC converter, an optimal efficiency control strategy to achieve minimal inner current magnitude and fast control for the full power range is proposed. At grid level, the 3-level cascaded current controller design achieves reliable power transfer and grid voltage control. Under normal grid operating conditions, the tie line DC/DC converter enables flexible power exchange between interconnected HVDC links. During fault conditions, the proposed DC/DC cascaded current controller limits the converter currents. The DC/DC converter will therefore function differently, either in power control mode or voltage control mode, for a proper post-fault grid management without grid-wide communication. The validity of the proposed DC/DC control is verified using a 4-terminal DC system with a large wind farm on a detailed PSCAD model for both normal and fault conditions.

Condition Monitoring of Submodule Capacitors in Modular Multilevel Converters—A Review

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.

Grid emulators provide efficient manners to test grid-connected distributed power generators. They test the power generator such as photovoltaics, wind turbines without hardware damages. Recent studies investigated conventional voltage source inverters such as a two-level voltage source inverter and a three-level voltage source inverter as grid emulators. However, they have limited voltage capabilities and weak transient responses. This paper presents a technique for the control of a modular multilevel converter as a grid emulator in fault scenarios, where voltage sags, harmonics distortions, and frequency deviations are emulated. The steady-state performance in these scenarios is analyzed and compared with that of a two-level voltage source inverter. Energy balancing between the sub-modules in a modular multilevel converter is always a crucial topic in terms of stability, particularly under fault conditions. In this paper, the transient response of the point of common coupling voltage and the sub-module energy in the modular multilevel converter with a delta-wye configuration is analyzed and compared with a two-level voltage source inverter. Simulations of a 24 sub-modules modular multilevel converter verify the control method with S70 V at the dc-link and a 400 V line-to-line ac voltage at the point of common coupling. The modular multilevel converter grid emulator with a delta-wye transformer configuration shows better steady-state performances than a two-level voltage source inverter. The energy stored in each sub-module is well balanced during the fault scenarios, which indicates strong stability.

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).

The direct chain-link modular multilevel dc converter has raised great interest recently in medium/high-voltage dc-dc conversion due to the high power device utilization and lower power losses than the front-to-front configurations of modular multilevel converter. This paper introduces a single-phase chain-link modular multilevel buck-boost converter for medium voltage dc applications and presents an analysis methodology on its internal circulating current frequency. Its connection and comparison with the classic dc-ac modular multilevel converter are given first, and its dc and ac components are analyzed in the respective equivalent circuits. Then, the derivation methodology for the proper circulating current frequency with lowest internal reactive power is provided, which could minimize the current stress and decrease the power losses. Also, this method can be directly applied in the derivative topologies and further configurations to satisfy various conversion requirements. The theoretical analysis is verified by a set of full-scaled simulations and further verified against experimental tests on a down-scaled prototype.

The modular multilevel converter (MMC) is attractive for medium-/high-voltage and high-power applications because of the advantages of its high modularity, availability, and high power quality. Due to the chemical process, aging effect, etc., the capacitor in the submodule (SM) of the MMC would gradually deteriorate and the capacitance would drop, which would cause unbalanced SM power losses distribution in the same arm of the MMC and affect the reliability of the MMC. This paper proposed an equivalent-reference (ER) control method, which can effectively realize balanced SM power losses distribution in the same arm of the MMC through the voltage-balancing control for the virtual capacitor voltages (VCVs) in the MMC under the capacitor deterioration. The proposed ER control can effectively improve the reliability of the MMC with the balanced SM power losses distribution in the MMC under capacitor deterioration. The simulation studies with the time-domain professional tool PSCAD/EMTDC are conducted, and a down-scale MMC prototype is also tested with the proposed control strategy. The study results confirm the effectiveness of the proposed control strategy.

The modular multilevel converter (MMC) has become one of the most promising candidates for medium-/high-power applications, specifically for high-voltage direct current transmission systems. This paper proposes a control strategy for the MMC based on the sum-of-squares decomposition method. A bilinear mathematical discrete model of the MMC is derived in the rotating reference frame by including the dynamics of ac-side currents, circulating currents, dc-side current, and stored energy of the converter. The coordinates of the developed model are transformed to the origin, and the steady-state operating point of the MMC is calculated based on the desired reference values of the states. A quadratic Lyapunov function is defined and by converting the Lyapunov difference inequality to an SOS problem, a stabilizing controller is designed. The designed controller not only guarantees the Lyapunov stability of the states, but also controls the external and internal dynamics of the MMC with only one controller, instead of using several proportional-integral controllers. To improve the region of stability of the MMC, an optimization problem is formulated and solved, so the controller can stabilize the MMC from an initial operating point far from the steady-state operating point. The performance of the proposed strategy for a 20-level MMC is evaluated based on the time-domain simulation studies and is compared with the existing control methods.

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.

This paper presents the techniques for regulation of system balancing in modular multilevel converter (MMC) based HVDC system under the AC grid double line fault condition. Due to the special structure of MMC, the balancing of the cell capacitor voltage is crucial for any operating circumstance of the system. Also, the responses against the grid faults of HVDC system based on MMC are different to those based on the conventional voltage sourced converters. Therefore, even under the nonpermanent fault in AC grid, the balancing of the cell capacitor energy of MMC has to be guaranteed continuously. Among AC grid fault modes, the double line fault causes one of the most severe transients in MMC. In this paper, a balancing control method of MMC based on an offset voltage injection strategy in the output reference voltages is proposed. By the virtue of the proposed method, the HVDC system based on MMC can have the enhanced fault ride-through capability against the double line fault in AC grid. The validity of the proposed method is verified by simulation results of a ±200kV, 400MVA point-to-point HVDC system.

The development of fully controlled power semiconductor devices has paved way the path for Voltage Source Converters (VSC) based High Voltage Direct Voltage (HVDC) transmission systems. For these high voltage applications, Modular Multilevel Converter (MMC) is a good candidate which utilizes interconnection of several Sub-Modules (SM). Furthermore, it provides several advantages including flexibility, scalability, better voltage quality etc. However, the MMC can be vulnerable to the DC short circuit faults. Different Sub-Module (SM) topology based MMCs exists which can suppress the DC short circuit current. The MMCs can be modelled with different modelling technics. This paper presents a comparative study of a point to point (PTP) MMC based HVDC system with feed-forward current control based on detailed and averaging modelling technics. Furthermore, the DC short circuit handling capabilities of Half Bridge SM (HBSM) and Full Bridge SM (FBSM) topologies are examined. Real time simulation results are provided to demonstrate the results of the study on FPGA based MMC support unit from Real Time Digital Simulator (RTDS).

Modular multilevel converters (MMCs) have become one of the most foremost power converter topology for medium and high power applications in recent years. In this paper, single-phase traditional and hybrid MMC are designed, simulated and compared under various voltage levels. Output voltage and current waveform are analyzed in terms of total harmonic distortion by changing modulation index at operated 7-level, 19-1evel, 31-level and 43-level traditional and hybrid converter systems. Both types of MMCs are investigated under different conditions. At the end of the study, simulation results are deeply presented with different cases and evaluated to show the performances of the designed circuit topologies.

The modular multilevel converter (MMC) is the most promising converter topology for medium- and high-power applications. One of the main concerns in the operation of the MMC, particularly for high-power applications, is its efficiency, which should be maximized. Silicon Carbide (SiC)-based devices have the potential to provide significant efficiency improvement compared with silicon devices. However, the possibility and impact of using SiC-based devices instead of silicon devices for high-power conversion have not been thoroughly explored. This paper reports on the results obtained from a detailed study to evaluate the performance of MMCs based on medium-voltage SiC MOSFETs and diodes with hybrid MMCs that employ silicon IGBTs and SiC diodes. The results are based on detailed circuit simulations that use simple physics-based circuit models. The study suggests the potential for significant efficiency gain for MMCs based on SiC power devices.

Offshore wind farms (OWFs) are often connected to onshore systems via a modular multilevel converter (MMC)-high voltage dc (HVDC) line. However, for the tie line between the OWF and the MMC, the unique fault behaviors will threaten the correct operation of conventional distance relays. Therefore, a coordination scheme between protection and control is proposed in this article. The proposed method can make the distance relay on the OWF terminal be triggered within 0.3 s. After that, the zone I distance relay on the MMC terminal can safeguard the entire line, allowing the fault to be switched off in less than 0.7 s. The proposed method can make the OWF-MMC system compatible with the traditional distance relays, while the overvoltage can be suppressed. Furthermore, the typical fault ride-through (FRT) demands such as current limitation and reactive current injection are still satisfied. In addition, the suggested approach possesses a robust capacity to tolerate fault resistance. These advantages avoid investment in modifying the protection algorithm while effectively protecting the inverter. PSCAD simulation and hardware-in-the-loop (HIL) testing validate its performance under various fault conditions.

The Modular Multilevel Converter (MMC) is a consolidated attractive technology taking great importance in medium / high power applications compared to other topologies of multilevel converters. One of the main lines of research is on voltage modulation techniques and its influence on the converter performance. Specifically for a relatively high number of submodules (SMs) that may depend on the specific application type. In this paper, a brief overview of the MMC basic operation is presented and the modulation techniques based on switching frequency as the main parameter are studied. Also, three different modulation techniques are studied to later be performed in Matlab/Simulink, in simulation is considering two cases of MMC systems with 5 and 20 SMs/arm, in order to comparing the impact of modulation technique on MMC performance.