Ing. Pavel Kobrle, Ph.D.

This paper presents an assessment of Solid State Transformer (SST) topologies from the usability in energy applications point of view. SSTs are nowadays at the center of researchers’ attention mainly due to reducing size and weight of traction transformers in railway vehicles. Our contribution aims to use the SST as a cornerstone of an Energy Router that should replace a typical distribution substation with a classical transformer. After a general overview of SST topologies classification an overview of design guidelines for key SST components (ferrite cores for medium frequency (MF) transformers, semicon-ductors and high voltage capacitors) is presented for two topologies that seem to be promising for energy applications: classical three-stage SST and two-stage SST with a low voltage DC link and galvanic insulation in the first stage. These two topologies are then compared from an economical point of view based on acquisition cost of key components in order to evaluate whether the technology has matured to be used commercially.

The current and torque ripple of inverter-fed induction motor drives is an inherent problem of control strategies working with switching frequencies in the range of multiple kilohertz, such as direct torque and, more recently, predictive torque control. If the drive operates in a wide-speed and wide-torque range and is equipped with a machine with an accessible terminal block whose winding is nominally connected in delta, then the current and torque ripple can be reduced by utilizing the delta-star winding changeover technique. When the winding configuration is switched from delta to star, the instantaneous motor phase voltage peak is lowered, and its total harmonic distortion is reduced. However, the control strategy must be adjusted according to the actual winding topology, mainly due to the difference in the coordinate transformations of the measured currents and the difference between the phase voltage vectors obtained from the inverter. This paper proposes a predictive torque control of an induction motor drive with a switchable delta-star winding configuration. The paper is supported by theoretical background, and the key idea is verified by simulations in MATLAB/Simulink and experiments conducted on a dSPACE-controlled 5.5-kW laboratory drive. The simulations validated the presented equations and show the effects of not respecting the actual winding topology. The experiments mainly focused on analyzing the total harmonic distortion of the currents and consumed electrical power in multiple operating points.

This paper presents a new, computationally modest on-line identification method for the simultaneous estimation of the rotor resistance and magnetizing inductance of an induction machine suitable for electric drives that use an indirect field-oriented control strategy (IFOC), and their control hardware is equipped with a resource-constrained microcontroller. Such drives can be found both in the manufacturing industry and railway traction vehicles in the thousands, having either older control hardware that cannot cope with computationally excessive identification methods or being in cost-sensitive applications, thus being equipped with a low-cost microcontroller. IFOC is a very common control strategy for such drives due to its good dynamic properties and comparatively simple implementation. However, it is sensitive to inaccuracies of rotor resistance and magnetizing inductance. These two parameters change during the operation of the drive, being influenced by the temperature, frequency, and saturation of the magnetic circuit. Improper values of parameters in the controller can degrade the performance of IFOC, resulting in slower acceleration or unnecessary oversaturation of the machine. Respecting these changes can therefore bring significant benefits such as the good dynamic properties of the drive, which can shorten operations in the manufacturing industry or travel times of vehicles. A number of on-line identification methods for monitoring the parameter changes have been published so far, but the majority of them are demanding on microcontroller time or its memory. The proposed method, on the contrary, is comparatively simple and thus easy for implementation with low requirements to the microcontroller.

The conventional distributed super capacitor energy storage system (DSCESS) based on the modular multilevel converter (MMC), using dispersed energy storage units, inconvenient assembly and maintenance, and complex system control. In this paper, an improved modular multilevel converter (IMMC) based symmetrical super capacitor energy storage system (SSCESS) is proposed by adding two DC buses to simplify system control complexity and enhance the dynamic response performance. The operation principles and power transmission of IMMCSSCESS are analyzed firstly. Based on mathematical modeling analysis, the sub-modules capacitor voltage control and super capacitor state of charge (SOC) balancing control are studied in detail. Finally, the comparison between IMMC-SSCESS and conventional MMC-DSCESS is validated by simulation. The results show that IMMC-SSCESS can quickly realize SOC balance and effective power transmission under corresponding control strategy, and shows good industrial application prospects.

Energy storage system (ESS) is generally applied in electrical energy router (EER) for flexible interfacing with renewable-energy sources. However, the complex structure and power flows of EER-ESS raise new requirement that coordinated power control is supposed to be adopted. In this paper, a modified EER (MEER) with two-level super capacitor ESS (SCESS) is proposed for energy interconnection and power buffering in medium and low voltage distribution network. The three-stage configuration and corresponding control strategies of MEER are introduced in detail. After analysis of the typical power flow situations, a coordinated power control strategy is proposed to achieve flexible power management between SCESS and renewable-energy sources under different operation conditions for the system stability. The effectiveness of the proposed coordinated power control strategy is finally verified by the simulation and experimental results.

For reducing backflow power and improving dynamic performance of the input-series output-parallel dual-active-bridge (ISOP-DAB) converter in the electric energy router application, a hybrid control strategy based on the extended-phase-shift (EPS) output voltage model predictive control and gradient descent algorithm (MPC-GDA) is proposed. Firstly, the backflow power models under single-phase-shift (SPS) and EPS control are established, and the output voltage state space averaging equation under EPS control is also derived. Then the output voltage model predictive control is obtained, and the gradient descent algorithm for backflow power optimization is proposed. Finally, the overall control block diagram of the ISOP-DAB based on MPC-GDA control is derived. The simulation models of ISOP-DAB under MPC-GDA and SPS control are built and compared. It is verified that the MPC-GDA control shows better dynamic response and backflow power optimization compared with the traditional SPS control.

One of the main challenges of current electric grid not only in the Czech republic is the integration of an increasing number of small and often unpredictable power sources, especially renewable ones. Transition to so called smart grids which should be an answer to this problem includes almost all components of electrical grid. This means that in the future there will be also some smart replacement of a distribution transformer. A device called energy router contains a power part - so called solid state transformer (SST) which could be such a replacement of the distribution transformer. This paper presents how such a device could be carried out and how control of electric power in the SST could be performed.

In this paper, modular multilevel converter (MMC) based super capacitor energy storage system (SCESS) is employed as the front end of electrical energy router (EER) to access medium voltage distribution network. Utilization of energy storage capacity is limited by the state of charge (SOC) of each energy sub-module (ESM). To solve SOC balancing control issues, the concept of SOC equalization factor is introduced into the mathematical model. Then a modified SOC equalization control strategy among phase legs, upper and lower arms and ESMs in the same arm is obtained. Simulation results show that the proposed control strategy improves robustness and dynamic performance of the system, especially suitable for EER applications in future energy system.

Energy storage system becomes one of key components in the medium voltage grid with the ever-increasing development of renewable energy resources. This paper proposes an improved modular multilevel converter(IMMC) where symmetrical super capacitor energy storage banks are interfaced to the three-terminal power unit through a Buck/Boost converter. Six typical operation modes are taken into account and the control algorithms such as state of charge(SOC) equalization and sub-module capacitor balance based on carrier phase shift sine pulse width modulation(CPS-SPWM) are developed. Finally., the analysis and control strategies are verified through the MATLAB/Simulink simulation results and down-scaled prototype experimental results.

MMC is popular converter topology due to benefits such as low harmonic content, absence of filter or low capacity of the capacitors needed, to be mentioned among others. This paper presents general workflow for calculating the optimal value of MMC submodule capacity. The analysis is done for basic harmonic component based on energy flow.

An improved modular multilevel converter with symmetrical integrated super capacitor energy storage system (IMMC-SSCESS) is proposed for interfacing energy interconnection and power buffering in electrical energy routers. Compared with traditional modular multilevel converter based distributed super capacitor energy storage system (MMC-DSCESS), the proposed IMMC-SSCESS increases the overall system rapidity and stability, reduces system control complexity and is suitable for energy storage system (ESS) management. Based on analysis of operation principles, the control algorithms are developed for both the sub-modules (SMs) capacitor voltage control and super capacitor state of charge (SOC) balancing control. Feasibility of proposed topology and control strategies are verified by simulation and experiment results.

The simplest mathematical models of the induction motor (IM) use the presumption of the machine’s linear magnetising characteristics. However, this may hold only in a limited operating range of the electric drive. It is well known that the magnetising inductance saturates as a function of the magnetising current due to the nonlinear properties of the magnetic circuit. However, this is not the only type of saturation. Depending on the design of the rotor, the machine’s magnetising and leakage inductances may also saturate as a function of the rotor current. A new experimental method is proposed in this paper. It identifies the dependence of the T equivalent circuit magnetising inductance on the rotor flux and torque producing current component using parallel operation of the so-called current and voltage model of the IM.

The paper describes the procedure and the result of analysis of mechanical vibrations of mining machines at the Palašer mine in the Russian Federation. The cause of these vibrations was the change in the magnitude of the magnetic flux in the air gap. The simulations showed the possibility of eliminating these changes in magnetic flux by injecting suitable voltages into the stator windings. The implementation of this injection is further described by torque variable component feed-direct compensation. Finally, oscillography records are presented before and after injection.

The paper describes the procedure and the result of the analysis of mechanical vibrations of mining machines at the Palashermine in the Russian Federation. The cause of these vibrations was the change in the magnitude of the magnetic flux in the air gap. The simulations showed the possibility of eliminating these changes in magnetic flux by injecting suitable voltages into thestator windings. The implementation of this injection is further described by torque variable component feed-direct compensation. Finally, oscillography records are presented before and after injection.

The device rated capacity, system loss, and cost of a modular multilevel converter are proportional to the circulating current value. In order to enhance suppressing the second order harmonic component of circulating current and improve the dynamic response of the system, a novel circulating current suppressing strategy based on virtual impedance sliding mode control (VI-SMC-CCS) is proposed. This paper first analyzes the essential CCS mechanism, then presents a circulating current decoupling model using a state feedback linearization method, improving the system accuracy. Moreover, the sliding mode control and virtual impedance loop are introduced to strengthen system robustness. Different circulating current suppressing strategies are compared by simulation, the results demonstrate the correctness of the VI-SMC-CCS, which not only effectively reduces the second order harmonic component of the circulating current, but also improves the stability, reliability, and rapidity of the system.

This paper explains the term energy router with help of the similarity between the energy network and internet. Their main differences are discussed as well. Furteher, all main parts of energy router - power electronics module, communication module and intelligent network module - are described. The solution of the power electronics module (Solid State Transformer) is described in detail. The other part of the paper deals with the power flow control. The components of the intelligent network are introduced in the end of the paper.

This paper concerns a modular pulse width modulation (PWM) modulator that is suitable for polyphase multilevel inverters. The modulator is implemented in a field programmable gate array (FPGA) with a softcore microprocessor controlling it. Further it presents experiments with a five level flying capacitor multilevel inverter and the proposed modulator.

Many buzzwords concerning the problematic of energy production and its effective (smart) utilization in industry as well as in households – like Smart Grid, Big Data, Industry 4.0, Energy Router, Green Energy etc. – are often used in many different meanings or even only for making one’s idea more attractive. Nevertheless, their definitions aren’t united or they don’t express the essence in their entirely. The aim of this paper isn’t to define some of the buzzwords; however, it tries to explain what is really a so called energy router and why is it spoken about in the context of existing and to the near future planned system of electrification in the central Europe. Some kinds of energy storage are integral to the modern grid. The short overview of energy storage devices is presented at the end. This text should make the understanding of the problematic in wider extent easier.

This paper carries out analysis of possible switching processes for voltage balancing of flying capacitors of the flying capacitor multilevel inverter. After short description of the FCMI principle, state diagrams and two stage modulation algorithm for generating the switching pulses are explained. Then a theoretical analysis of switching with 1C, 2C and 3C transitions is given with its effect to voltage balancing of flying capacitors. Conclusions of this analysis are then confirmed with measurements made on a laboratory model of five-level flying capacitor multilevel inverter.

The presented article discusses the possibility of using the multilevel inverters (MI) as the active filters. It is explained both the principle of an active filter as well as the principle of three types of the MI – diode-clamped MI, flying capacitor MI and cascaded H-bridge MI. Further, the usually used semiconductor devices for medium voltage area (IGCT and HVIGBT) are mentioned. Next part discusses several specific requirements that should be taken into consideration for medium voltage active filters – their topology, semiconductor devices, switching frequency and the DC link voltage that depend on an order of a filtered harmonic.

This paper deals with historical evolution of power electronic devices and converters from the very first beginnings of the mercury-arc rectifiers to the multilevel converters as the most recent technology. Various devices used for converters are described, both vacuum and semiconductor. Paper mentions not only historical dates and periods but also parameters and properties specific for the devices as well as their applications.

This paper aims to summarize the development of the devices with the rectifying properties and devices usable for power converters. Except the history, there are described also the parameters and their specific properties, these properties are also discussed from the time point of view. Further, the semiconductor converters are mentioned. The historical overview is focused on the multilevel converters in more details because the multilevel converters belong to very progressive converters topologies nowadays and they are the only one suitable for high power applications.

Multilevel converters are widely used in different high voltage applications. The are used in branch of electric drives as well as for HVDC transmission and renewable energy sources. This paper follows the similar one from XXXII. conference - there was described the development of a model of three-phase five-level flying capacitors inverter. In this paper, several demands and aspects of the design of multilevel converters for non-standard application - high voltage active power filter. Further, there is performed the suggestion of a filter control and results of simulation.

This contribution deals with the problematic of using a medium voltage multilevel inverter as an active filter. There are described the differences between low voltage and medium voltage active filters connections as well as the demands on inverters in the paper. Also the topologies of multilevel inverters and methods of control are encapsulated.

This paper discusses the possibility of medium voltage multilevel inverter use as active filter. The differences between low voltage and medium voltage circuits as well as the requirements on inverters are given. Further, the possible topologies of multilevel inverters and ways of the control are summarized.

This paper deals with the analysis of unsprung masses of electric vehicle with the in-wheel drive, from dynamics point of view. The system safety and comfort are evaluated. This proceeding also brings together the modern possibilities of use of the propulsive components of the in-wheel drive - the basic types of electronic commutated motors. The technical note presents the ROMO vehicle - the project of DLR (German Aerospace Center).

This paper describes the model of the five level flying capacitors inverter (FCI) that was designed and realized at CTU in Prague, FEE - Department of Electric Drives and Traction. Some aspects of FCI control algorithm which influenced the design are mentioned. The first experimental results are also presented. Further the possibility of medium voltage multilevel inverter use as active filter is discussed. The requirements on inverters are given.

Onlzý a few poublications deal with a contruction of multilevel converters. Most of them describes their usage, modulations a and contrl strategies. In the first part of this paper is decsribed topology and principle of three-phase five-level flying capacitors inverter. Further, there is decsribed the design of a laboratory prototype (power of 4 kW), which is realized at the Departement of Electric Drives. At the end are mentioned the possible ways of flying capacitors precharging.

Only a couple of publications describe the hardware of the multilevel converters, most of them deal with their utilization, modulations and control strategies. This paper briefly summarizes the development and the design of the model of the three phase five level flying capacitors converter. There are described the partial main items of the power part of the converter in this article: driver board with the transistors (IGBT), voltage and current measurement boards, control and hardware protections board. Further, there is mentioned the intended control strategies in this paper, which should be implemented and tested on this model.

The presented paper deals with control strategy of flying capacitors multilevel inverters. The main issue is the analysis of the permissible switching states, especially the possibility of the multiple commutations.

In introduction of this paper is shortly described topology and principal function of the five-level flying capacitor inverter. Further, in the light of following description of the control strategy are summarized the control requirements. These requirements are captured in the state diagram. Then follows self explanation of the control strategy in consideration of the flying capacitors stabilization. In conclusion are presented and described attained simulation results.

To investigate machines' static and dynamic features, simulating programs working with mathematical models are used. There are many mathematical models, but they differ in counting speed and stability and in the extent, how the results correspond to the reality. On a synchronous machine example, this paper shows several possible improvements which enable the model to work more accurately and stable.

In this paper is solved the problem of mathematical stability of synchronous generator model. Making of simulation equations goes from common electrical engine equation system, where is looked at magnetic parts of machine nonlinearity as well as at magnetic coupling between exciting winding and rotor winding of the direct axis. Thank this consideration has been the equation system more difficult but better quantitative results are achieved. For the solvation is necessary making a suitable choice of the state values and suitable form of equations to obtain the mathematical stability of this model.

In this paper is solved the problem of mathematical stability of synchronous motor model. Making of simulation equations goes from common electrical engine equation system, where is looked at magnetic parts of machine nonlinearity. This nonlinearity is expressed by magnetizaton curves in suitable forms, what are fitted to implementation to synchronous motor model. This model looks as well as at magnetic coupling of rotor windings. Thank this consideration has been the equation system more difficult but better quantitative results are achieved. For the solvation is necessary making a suitable choice of the state values and suitable form of equations to obtain the mathematical stability of this model.