Cryo-MMC: a Modular Multilevel Converter with Superconducting Arm Coils

Fractional frequency transmission system (FFTS) is a promising solution to offshore wind power integration. The back-to-back modular multilevel converter (MMC) and modular multilevel matrix converter (M3C) are universally regarded as two promising ac/ac converter candidates of the next generation. This paper performs a detailed comparison between back-to-back MMC and M3C with special emphasis on their total capacitor and semiconductor ratings. The arm current and capacitor ripples are first analyzed, then comparison results of a 220 kV, 500 MW case is given. In terms of peak-current-based and RMS-current-based total semiconductor ratings the M3C is 42.56 % and 22.97 % larger respectively, while its total capacitor rating is merely 33.15% of the back-to-back MMC. The better candidate can be determined according to the quantitative comparison results and the latest prices of semiconductors and capacitors.

This paper presents a new converter topology for voltage source converter based high voltage dc application, named modular embedded multilevel converter (MEMC). MEMC is based on a three level structure and consists of series-connected IGBT bridge stacks, like the popular modular multilevel converter (MMC) and series-connected thyristor stacks. The IGBT bridge stacks are leveraged to generate the output the voltage waveforms and to commute the thyristors. The basic operation principle and the methods to commutate the thyristors are explained in detail. Due to the three level structure, the total number of IGBT stacks are reduced by half compared with MMC, leading to much lower energy storage, weight, volume and system complexity. Also by replacing part of the IGBT stack in MMC with thyristor stack, both of the system conduction loss and switching loss can be further reduced. The simulation results proved the analysis and verified the feasibility and benefits of the proposed converter topology.

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.

Power flow controllers and circuit breakers are important devices in a DC grid, and combining them and expanding their ports can save many components. This paper proposes an integrated multiport DC power flow controller with a DC circuit breaker that can achieve power flow control and fault removal on multiple DC lines during steady and fault states, respectively. First, the device's topology based on modular multilevel converters is proposed, and the principles of power flow control and self-power balance based on bridge arm voltage are introduced. Second, it is improved by adding a fault removal capability. The equivalent circuit and fault currents in different time sequences are analyzed, and the component parameter selection method is designed. Third, in the PSCAD/EMTDC, the power flow control and fault removal capabilities are verified using three conditions: flow control, line fault, and bus fault. Finally, the number of components and steady-state losses are analyzed. Overall, the proposed device has power flow control and fault removal functions as well as a certain limiting effect on the fault current, which greatly reduces the system construction cost.

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.

Modular multilevel converters (MMCs) have become one of the most promising topologies for medium- and high-power applications given their characteristics, such as flexibility. An MMC is composed of several submodules (SMs). Hence, the diagnosis of faulty SMs is important to guarantee proper operation. In this paper, we propose an effective fault detection and localization approach for MMCs, which is based on the Fast Fourier Transform (FFT). Specifically, detection and phase fault method relies on the analysis of the MMC ac characteristics. To verify the feasibility and effectiveness of the proposed fault diagnosis approach, we performed MMC simulations under normal and open-circuit fault conditions using RT-LAB. The results show that the proposed approach can quickly and easily identify the fault condition and locate the faulty phase to allow a stable operation of the MMC.

The small-signal impedance modeling of a modular multilevel converter (MMC) is the key for analyzing resonance and stability of MMC-based power electronic systems. The MMC is a power converter with a multifrequency response due to its significant steady-state harmonic components in the arm currents and capacitor voltages. These internal harmonic dynamics may have great influence on the terminal characteristics of the MMC, which, therefore, are essential to be considered in the MMC impedance modeling. In this paper, the harmonic state-space (HSS) modeling approach is first introduced to characterize the multiharmonic coupling behavior of the MMC. On this basis, the small-signal impedance models of the MMC are then developed based on the proposed HSS model of the MMC, which are able to include all the internal harmonics within the MMC, leading to accurate impedance models. Besides, different control schemes for the MMC, such as open-loop control, ac voltage closed-loop control, and circulating current closed-loop control, have also been considered during the modeling process, which further reveals the impact of the MMC internal dynamics and control dynamics on the MMC impedance. Furthermore, an impedance-based stability analysis of the MMC-high-voltage direct current connected wind farm has been carried out to show how the HSS-based MMC impedance model can be used in practical system analysis. Finally, the proposed impedance models are validated by both simulation and experimental measurements.

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.

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).

The arm inductors utilized in a modular multilevel converter (MMC) can suppress the MMC circulating current, fault current, and output current, and circulating current ripples. This paper presents a four-step design and optimization method for the MMC arm inductor. The first step is the define the requirements and specifications of the arm inductor based on the MMC ratings and detailed modeling, in order to define the core material and select the appropriate winding wire as a second step. The third step is to develop a multi-objective optimization for the core design of the arm inductor, while the considered constraints are area product, maximum magnetic flux density, and window area, which is here done based on a Non-dominated Sorted Genetic Algorithm (NSGA-II). The objective functions are inductor total mass, total volume, total power losses, total core length, and air gap length. Finally, using the average ranking (AR) method, the best optimal solution is selected from the Pareto solution provided by NSGA-II.

A quench protection system for a high-temperature superconducting (HTS) coil using Cu tape co-wound with an HTS tape was studied by numerical analysis using a thermal model of a coil winding pack. In normal operation, the voltage across the resistive zone Vs in the HTS coil was monitored by measuring the voltage difference between the HTS coil and the co-wound Cu coil. When Vs exceeded the quench detection voltage, the HTS coil was disconnected from the power supply and the energy stored in the HTS coil was dumped into a dump resistor. At the same time, the terminals of the co-wound coil were connected to another resistor, and some of the current in the HTS coil was quickly transferred to the co-wound Cu tape coil due to the tight magnetic coupling of both coils. This is expected to decrease the hot-spot temperature due to the quick decrease of HTS coil current and to protect the coil from damage caused by overheating at the hot-spot. The analysis investigated the effectiveness of this method.

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 emerged as the most favourable multilevel converter topology for medium to high-voltage applications. Reliability is one of the major concerns in an MMC due to high fragile component count. Due to low-cost and high energy density, electrolytic capacitors (ECs) are usually preferred as floating capacitors in the MMC. However, the ECs are gradually degraded over the time due to inherent chemical processes, which results in a decrease in capacitance value. This results in an increase in voltage ripple across the SM capacitors and would distort the output waveforms. Moreover, the prolonged use of these aged capacitors could disrupt the MMC operation. Therefore, monitoring and failure detection of submodule (SM) capacitors in an MMC are pivotal to enhance the reliability. This paper presents an SM capacitance estimation strategy for an MMC using nearest level modulation (NLM). A modified voltage balancing control structure with NLM is presented to reduce the unbalance in the capacitor voltages. The proposed approach uses available hardware for converter control with easy implementation in the same controller. Extensive studies are conducted on a three-phase MMC in PLECS simulation platform. Furthermore, experimental results are presented to substantiate the effectiveness of the proposed approach.

Small‐signal modelling of modular multilevel converters (MMC) has drawn lots of attention recently. Existing research mainly studies MMC terminal impedances by injecting corresponding AC‐side and DC‐side external small‐signal perturbations. This paper focuses on the MMC internal small‐signal perturbations, that is, perturbations in control signal and their effect. There are two kinds, that is, differential‐mode and common‐mode, of internal small‐signal perturbations, which influence MMC external and internal state variables, respectively. Then, the existing MMC small‐signal model is improved. With its help, accurate MMC control‐signal to state‐variables transfer functions are established, which can aid precise MMC controller analysis and performance optimization. In this paper, an analysis of MMC phase capacitor voltage regulation control is demonstrated as an example. The stability can be precisely ascertained based on the obtained transfer function. The effect of parameter variation of the proportional‐integral controller is also studied. Moreover, an additional compensator is introduced in the existing control loop to attenuate resonance peaks in the transfer function, which mitigates the large voltage overshoot and makes the transient state much smoother. Simulation and experimental results support and well confirm the analysis of this paper.