Hybrid Predictive Control with Simple Linear Control Based Circulating Current Suppression for Modular Multilevel Converters

The modular multilevel converter(MMC)has become a promising topology for widespread power converter applications.However,an evident circulating current flowing between the phases will increase system losses and complicate the heatsink design.This paper proposes a novel hybrid model predictive control method for MMCs.This method utilizes an indirect structure MPC and a sorting algorithm to implement current tracking and capacitor voltages balancing,considerably resulting in reduced calculation burden.In addition,different from the conventional MPC solutions,we add a simple proportional-integral(PI)controller to suppress circulating current through modifying the submodule(SM)inserted number,which is parallel to the MPC loop.This hybrid control solution combines both advantages of MPC and linear control,evidently resulting in improved performance of circulating current.Finally,the MATLAB/Simulink results of an 11-level MMC system verify the effectiveness of the proposed solution.

GENERATOR VIBRATION FAULT DIAGNOSIS METHOD BASED ON ROTOR VIBRATION AND STATOR WINDING PARALLEL BRANCHES CIRCULATING CURRENT CHARACTERISTICS

Rotor vibration characteristics are first analyzed, which are that the rotor vibration of fundamental frequency will increase due to rotor winding inter-turn short circuit fault, air-gap dynamic eccentricity fault, or imbalance fault, and the vibration of the second frequency will increase when the air-gap static eccentricity fault occurs. Next, the characteristics of the stator winding parallel branches circulating current are analyzed, which are that the second harmonics circulating current will increase when the rotor winding inter-turn short circuit fault occurs, and the fundamental circulating current will increase when the air-gap eccentricity fault occurs, neither being strongly affected by the imbalance fault. Considering the differences of the rotor vibration and circulating current characteristics caused by different rotor faults, a method of generator vibration fault diagnosis, based on rotor vibration and circulating current characteristics, is developed. Finally, the rotor vibration and circulating current of a type SDF-9 generator is measured in the laboratory to verify the theoretical analysis presented above.

The Circulating Current Analysis and Parameter Design of Interphase-Reactor of HT-7U Toroidal Power Supply

This paper analyzes the circulating current which is produced by HT-7Usuperconducting toroidal power supply-two sets of two-reverse-star converter with aninterphase-reactor in parallel running on the basis of the output voltage mathematical equation ofthree-phase semi-wave converter circuit. A new idea of omitting interphase-reactor between twoconverters is proposed, and the parameter design of interphase-reactor of HT-7 toroidal power supplyis presented. Simulated results demonstrate the validity of this new project.

Circulating current suppression for ground tied variable frequency to constant frequency converter system in aircraft

This paper presents a solution to the circulating current fault of aircraft power supply.The DC-link type Variable Frequency to Constant Frequency(VFCF)converter system is the preferred scheme to feed the constant 400 Hz load in an aircraft with a variable frequency power supply.Due to the requirement of aircraft standards,both grounds of the rectification and inversion stage are tied to the metal frame of the aircraft.With such a tied ground,the DC bus voltage rises greatly,and a large circulating current appears in the casing as the ground,which leads to equipment failure and potential safety hazards.According to the existing methods of circulating current fault suppression,this paper analyzes the causes of the above faults and the harmonic components of circulating current and points out the limitations of the existing methods.Therefore,a Common-Mode(CM)choke-based method is proposed to provide a high impedance in the path of the CM circulating current.By doing so,the circulating current can be suppressed without the additional burden of the hardware and control algorithm,which is quite friendly for quality control of mass-production aircraft.Moreover,a simplified mathematic model of the VFCF converter system is derived to calculate the minimum inductance value reference of the CM choke,which saves the weight of passive devices to the greatest extent.Finally,simulation and experimental results are studied to verify the effectiveness of the proposed method.

Circulating Current Suppression Method with Adaptive Virtual Impedance for Multi-bidirectional Power Converters Under Unbalanced Conditions

Multi-paralleled bidirectional power converters(BPCs)are commonly used to improve the power capacity and reliability in an AC/DC hybrid microgrid.However,circulating current through multi-BPCs has been one of the challenges and it can be aggravated in the presence of non-ideal operating conditions,such as unbalanced AC voltages,and the mismatch of hardware parameters.In order to suppress the circulating current,this paper proposes a distributed method based on adaptive virtual impedance,which also employs positive sequence power droop control and voltage deviation compensation control.The traditional positive sequence power droop control is adopted to only regulate the positive components of the BPCs output voltage.The negative sequence power term is fed to an adaptive virtual impedance generator to modify the damping characteristics of the BPCs.Also,an adaptive virtual impedance-based voltage deviation compensation method is proposed to recover the fluctuated output voltage of the BPCs due to droop action and the power fluctuations.The fully distributed regulation of adaptive virtual impedance enables the load power to be shared accurately among BPC modules and thus the circulating current can be effectively suppressed.The proposed control strategy does not require an additional communication system and the precise parameters of hardware equipment and line impedance.Furthermore,the effectiveness of the proposed method is verified by the experimental results.

Circulating Current Suppression for MMC-HVDC Systems with Asymmetric Arm Impedance

The modular multilevel converter(MMC)has been a highly promising topology in the high-voltage direct-current(HVDC)transmission area,where each arm of the MMC may consist of hundreds of series-connected submodules and an inductor.Due to its parameter inaccuracy,component aging,and so on,the component parameter in different arms of the MMC may be different,which may cause circulating current in the MMC-HVDC transmission system,and result in current deterioration,power losses,and electromagnetic interference,etc.In this paper,the circulating current suppressing(CCS)in the MMC-HVDC system,due to asymmetric arm impedance,is analyzed.Based on the mathematical analysis,a method of using an auxiliary circuit is proposed for the MMC to realize the CCS and improve the performance of the MMC-HVDC system.Simulation studies are conducted with PSCAD/EMTDC in the HVDC system,which confirms the feasibility of the proposed method.

Harmonic Circulating Current Injection Based Power Loss Optimization Control of Bottom Switch/Diodes for Modular Multilevel Converters

Power loss management is one of the most significant challenges for reliability improvement of modular multilevel converters(MMCs).In the MMC,the bottom switch/diode in each submodule(SM)normally takes the maximum power loss.In this paper,a power loss optimization control(PLOC)for MMCs is proposed,where the maximum power losses in the bottom switch/diode of each SM can be effectively reduced through injecting optimum second-order harmonic current into the circulating current of MMCs,and accordingly the reliability of MMCs can be improved by the proposed PLOC.Simulation results with PSCAD software and experimental results with a 1 kW MMC platform are provided to confirm the validity of the proposed PLOC for MMCs.

Analysis on Circulating Current and Split Capacitor Voltage Balance for Modular Multilevel Converter Based Three-phase Four-wire Split Capacitor DSTATCOM

We propose a modular multilevel converter(MMC)based three-phase four-wire(3P4W)split capacitor distribution static synchronous compensator(DSTATCOM),aiming at compensating unbalanced and reactive load currents.Due to the zero-sequence current compensation,the circulating current char-acteristics of 3P4W MMC-DSTATCOM are different from conventional MMCs.Moreover,the distinct working principle of IMMC would affect the features of split capacitor voltage.The decoupled positive-,negative-and zero-sequence second-order and DC components of the circulating current are deduced explicitly.Two proportional-integral controllers with dual dq transformation are employed to suppress the positive-and negative-sequence components of second-order circulating current,while quasi proportional-resonance controller is designed to eliminate the zero-sequence component.Besides,the phenomenon of the unbalanced split capacitor voltages is revealed,and fast-tracking balancing method by controlling zero-sequence current flowing through the split capacitors is provided.Digital simulation results verify the accuracy of the analysis and the feasibility of the suppression methods.

A Review of Medium Voltage Single-Core Cable Armouring, Induced Currents and Losses

Insulated underground cables have the potential to reduce power outages, maintenance costs, and transmission losses compared to overhead lines. On the other hand, they are exposed to several risks and physical damages, since they are buried in the ground. Though the cables are armoured in order to provide mechanical protection and achieve tensile strength, and also to provide effective conductance of earth fault currents. The main purpose of this paper is to introduce insulated underground cables, armouring process, and to analyze the induced currents in metallic parts such as sheath and armour that causeohmic losses which are categorized mainly in two groups as circulating current and eddy current. This paper presents a review on analytical techniques used to analyze the effect of magnetic fields, andcalculate the losses in the armour of the cables, besides providing the strategies and solutions used for armour loss reduction.

Multilevel Current Source Converters for High Power Medium Voltage Applications

In medium voltage high power applications,multi-level current source converters(CSCs)are good candidate to increase system power region,reliability,and the quality of output waveforms.Compared with widely researched voltage source multi-level converters(MLCs),the current source MLCs have the advantages of inherent short-circuit protection,high power capability and high quality of output current waveforms.The main features of MLCs include reduced harmonics,lower switching frequency and reduced current stress on each device which is a particularly important for high power application with low voltage and high current requirements.This paper conducts a general review of the current research about MLCs in higher power medium voltage application.The different types of parallel structure based MLCs and the modulation methodologies will be introduced and compared.Specifically,the circuit analysis of the common-mode(CM)loop for parallel structures will be conducted,the common-mode voltage(CMV)and circulating current suppression methods developed on the base of multilevel modulations will be addressed.

Progress of studies in China from July 2010 to May 2015 on the influence of the Kuroshio on neighboring Chinese seas and the Ryukyu Current

The influence of the Kuroshio on neighboring Chinese seas and the Ryukyu Current is a very important subject of interest in physical oceanography. To deeply explain the research progress made by Chinese scientists from July2010 to May 2015, the following three aspects are reviewed in this paper. The first concerns the Kuroshio intrusion into the South China Sea（SCS） and its circulation around the Luzon Strait. There are two very important points to be explained： the seasonal and inter-annual variation of the Kuroshio intrusion and the mechanisms of the Kuroshio intrusion and the influence of the Kuroshio on currents in the Luzon Strait and circulation in the northern SCS. The second concerns the variability of the Kuroshio and its interaction with the East China Sea（ECS）. There are following four interesting topics to be explained： an overview of studies on the Kuroshio in the ECS; the Kuroshio intrusion into the ECS, water exchange, and dynamic impacts; the downstream increase of nutrient transport by the Kuroshio; and the application of satellite remote sensing on terrestrial material transport by the Kuroshio intrusion into the ECS. Third, the interaction between the Ryukyu Current and Kuroshio in the ECS are also discussed. Finally, the main results are summarized and areas of further study are simply discussed.

Low-frequency variability of the shallow meridional overturning circulation in the South China Sea

The low-frequency variability of the shallow meridional overturning circulation（MOC） in the South China Sea（SCS） is investigated using a Simple Ocean Data Assimilation（SODA） product for the period of 1900-2010. A dynamical decomposition method is used in which the MOC is decomposed into the Ekman, external mode, and vertical shear components. Results show that all the three dynamical components contribute to the formation of the seasonal and annual mean shallow MOC in the SCS. The shallow MOC in the SCS consists of two cells： a clockwise cell in the south and an anticlockwise cell in the north; the former is controlled by the Ekman flow and the latter is dominated by the external barotropic flow, with the contribution of the vertical shear being to reduce the magnitude of both cells. In addition, the strength of the MOC in the south is found to have a falling trend over the past century, due mainly to a weakening of the Luzon Strait transport（LST） that reduces the transport of the external component. Further analysis suggests that the weakening of the LST is closely related to a weakening of the westerly wind anomalies over the equatorial Pacific, which leads to a southward shift of the North Equatorial Current（NEC） bifurcation and thus a stronger transport of the Kuroshio east of Luzon.

A modification to the Munk wind-driven ocean circulation theory

In order to fulfill the no-slip condition at the western and eastern boundaries of the ocean basin, introduced ＂effective wind stress＂, which has much larger spatial variations towards the boundaries than in the ocean interior. The effective wind stress can thus be decomposed into spatially slow-varying and fast varying components. Careful scale analysis on the classical Munk winddriven ocean circulation theory, which consists of the interior Sverdrup flow and the western boundary current but of no eastern boundary current, shows that the wind stress curl appearing in the Sverdrup equation must have negligible spatial variations. In the present model the spatially slow-varying component of the wind stress appears in the Sverdrup equation, and the spatially fastvarying component becomes the forcing term of the boundary equations. As a result, in addition to the classical Munk solution the present model has an extra term at the western boundary which （Northern Hemisphere） increases the northward transport as well as the southward return transport, and has a term at the eastern boundary corresponding to the eastern boundary current.

Study on the mesoscale eddies around the Ryukyu Islands

Results of the Ocean General Circulation Model for the Earth Simulator（OFES） from January 1977 to December2006 are used to investigate mesoscale eddies near the Ryukyu Islands. The results show that：（1） Larger eddies are mainly east of Taiwan, above the Ryukyu Trench and south of the Shikoku Island. These three sea areas are all in the vicinity of the Ryukyu Current.（2） Eddies in the area of the Ryukyu Current are mainly anticyclonic, and conducive to that current. The transport of water east of the Ryukyu Islands is mainly toward the northeast.（3）The Ryukyu Current is significantly affected by the eddies. The lower the latitude, the greater these effects.However, the Kuroshio is relatively stable, and the effect of mesoscale eddies is not significant.（4） A warm eddy south of the Shikoku Island break away from the Kuroshio and move southwest, and is clearly affected by the Ryukyu Current and Kuroshio. Relationships between the mesoscale eddies, Kuroshio meanders, and Ryukyu Current are discussed.

Characteristics of the Current Sales and Circulation System in China

5 Former circulation enterprises finding new market position Those large-sized auto circulation enterprises, which used to lead the circulation system, have been suffering the twist and turns from the new system oriented by automakers. Their market shares shapely fell down and have emerged into chaos. Parts of them that are powerful enough have established new sales companies controlled by the

Characteristics of the Current Sales and Circulation System in China

1. The sales and circulation system, taking by automakers in the lead, basically formed. The jointly managed and joint companies, established in the mid-1980%, have become gradually dominant element in the system. Along with the steady growth of auto production in the coming years, these companies will be getting stronger and stronger, with their sales and circulation networks being further perfected.

FPGA-based Digital Implementation of Flexible Power Control for Three-phase to Single-phase MMC-based Advanced Co-phase Traction Power Supply System

A three-phase to single-phase modular multilevel converter based advanced co-phase traction power supply(MMC-ACTPS) system is an effective structure to address the concerns of phase splitting and poor power quality of the conventional electrified railway. Due to the large number of MMCACTPS system modules, I/O resources and computing speed have high requirements on processors. Moreover, the module capacitor balance is challenging because the sorting time is too long when the traditional sorting algorithm for voltage balance is used. To solve the above issues, a digital implementation scheme of flexible power control strategy for three-phase to single-phase MMC-ACTPS system based on field programmable gate array(FPGA), which has sufficient I/O resources, has been proposed. Due to the parallel execution characteristics of the FPGA, the execution time of the controller and the modulator can be greatly reduced compared with a digital signal processor(DSP) + FPGA or DSpace. In addition, an improved sorting algorithm is proposed to reduce the sorting time and the implementation steps are analyzed. Finally, simulation and experimental results are presented to demonstrate the effectiveness and correctness of the proposed control strategy.

Overview on Submodule Topologies,Modeling,Modulation,Control Schemes,Fault Diagnosis,and Tolerant Control Strategies of Modular Multilevel Converters

In the present scenario,modular multilevel converters(MMCs)are considered to be one of the most promising and effective topologies in the family of high-power converters because of their modular design and good scalability;MMCs are extensively used in high-voltage and high-power applications.Based on their unique advantages,MMCs have attracted increasing attention from academic circles over the past years.Several studies have focused on different aspects of MMCs,including submodule topologies,modeling schemes,modulation strategies,control schemes for voltage balancing and circulating currents,fault diagnoses,and fault-tolerant control strategies.To summarize the current research status of MMCs,all the aforementioned research issues with representative research approaches,results and characteristics are systematically overviewed.In the final section,the current research status of MMCs and their future trends are emphasized.

Control Strategy of MMC Battery Energy Storage System under Asymmetrical Grid Voltage Condition

A battery energy storage system using modular multilevel converter(MMC)as the interfacing converter could have several inherent advantages when compared with battery energy storage systems based on two-level inverter or cascaded H-bridge converter.It can manage the state-of-charges(SOCs)of all batteries to be equal to avoid the overcharge or over discharge of single battery.The inherent power exchange characteristics using circulating currents could increase the control flexibility and consequently improve the output quality and the internal regulation capability.This paper investigates the operational principle and control strategy of MMC battery energy storage system under unbalanced grid voltage condition,where key issues of system model and corresponding control methods are very different from those under the normal grid voltage condition.The injected de current control,SOC balancing control and circulating current control methods are fully discussed in this paper.Finally,the theoretical findings were verified through Matlab simulation and a scaled down experimental prototype.