Showing papers presented at "European Conference on Power Electronics and Applications in 2016"

A galvanically isolated gate driver with low coupling capacitance for medium voltage SiC MOSFETs

Abstract: This paper presents a galvanically isolated gate driver system for medium voltage SiC-MOSFETs. A low common mode coupling capacity of 1 pF and good electrical insulation of the gate driver power supply are achieved by using a current-loop AC-bus power supply. The power semiconductor is protected against unintentional self-turn-on by a low resistance gate path that is active while the gate driver is not powered.

Virtual inertia with PV inverters using DC-link capacitors

Abstract: Soon, virtual inertia for grid control must be covered by photovoltaic inverters. It is suggested to use DC link capacitors for this task. This requires 5 W, 50 J and a capacitor size of about 200 cm3 per installed kW, corresponding to the size of single phase DC link capacitors. It is shown that the additional power ripple (and thus current ripple) is in the order of 0.1% and the voltage ripple of the intermediate voltage will typically remain between +/−3.6%. The related control can be easily extended by adding a voltage signal to the control voltage, which is proportional to the frequency deviation. Then, the existing controller inherently sets the required additional power fluctuation required for the virtual inertia function.

Comparison of modular multilevel converter based HV DC-DC-converters

Abstract: Efficient HV DC-DC converters are one key component of a future HVDC backbone grid. In this paper three promising MMC based HV DC-DC converter topologies, namely the front-to-front converter, the HVDC auto transformer and the modular multilevel DC converter are compared in terms of functionality, conversion efficiency and topology effort.

A multi-site real-time co-simulation platform for the testing of control strategies of distributed storage and V2G in distribution networks

Abstract: This paper presents a real-time co-simulation platform aimed to test control strategies for the management of the interaction between a smart grid and active prosumers. The main feature of the proposed framework relies on the multi-site approach that allows the decoupling between the network model and the system under test. This allows separate testing with the exchange of a limited amount of information between the two systems, helping to preserve the confidentiality of data belonging to different parties. As an example the paper addresses the development and testing of a distributed storage and vehicle-to-grid management system connected to a real distribution network model.

A new dc breaker with reduced need for semiconductors

Abstract: A renewed interest for dc breakers has evolved owing to the visions of connecting several HVDC links into a HVDC grid. Several breaker concepts have been presented and some also have been implemented and tested. A new approach is proposed in this paper. The fundamental principle is to take advantage of the fast recovery of the voltage withstand capability in a Vacuum Interrupter (VI) when the current is temporarily forced to zero using a low-voltage power electronic converter. The latter excites an oscillating current in a passive resonant circuit during the time interval needed to mechanically separate the contacts in the interrupter. The sum of the line current (to be interrupted) and the oscillating current passes through the vacuum interrupter and experiences zero-crossings when the amplitude of the oscillating current exceeds the line current. The most important benefit of the proposed concept is that a favorable cost can be achieved, because excitation of the oscillating current only requires a low output voltage (compared to the breaking voltage across the VI terminals) from the converter. The total voltage rating of the semiconductors in the latter therefore is only a fraction of that needed in a hybrid HVDC breaker and, additionally, they are not requested to turn off the main current but rather to commutate close to current zero-crossings. Experimental verification of the concept has been performed with current up to 1.6 kA against 1.2 kV.

Batteries 2020 — Lithium-ion battery first and second life ageing, validated battery models, lifetime modelling and ageing assessment of thermal parameters

Abstract: The European Project “Batteries 2020” unites nine partners jointly working on research and the development of competitive European automotive batteries. The project aims at increasing both the energy density and lifetime of large format pouch lithium-ion batteries towards the goals targeted for automotive batteries (250 Wh/kg at cell level, over 4000 cycles at 80% depth of discharge). Three parallel strategies are followed in order to achieve those targets: (i) Highly focused materials development; two improved generations of NMC cathode materials allows to improve the performance, stability and cyclability of state of the art battery cells. (ii) Better understanding of the ageing phenomena; a robust and realistic testing methodology has been developed and was carried out. Combined accelerated, real driving cycle tests, real field data, post-mortem analysis, modelling and validation with real driving profiles was used to obtain a thorough understanding of the degradation processes occurring in the battery cells. (iii) Reduction of battery cost; a way to reduce costs, increase battery residual value and improve sustainability is to consider second life uses of batteries used in electric vehicle application. These batteries are still operational and suitable to less restrictive conditions, such as those for stationary and renewable energy application. Therefore, possible second life opportunities have been identified and further assessed. In this paper, the main ageing effects of lithium ion batteries are explained. Next, an overview of different validated battery models will be discussed. Finally, a methodology for assessing the performance of the battery cells in a second life application is presented.

Optimization tool for direct water cooling system of high power IGBT modules

Abstract: Thermal management of power electronic devices is essential for reliable system performance especially at high power levels. Since even the most efficient electronic circuit becomes hot because of ohmic losses, it is clear that cooling is needed in electronics and even more as the power increases. One of the most important activities in the thermal management and reliability improvement is the cooling system design. As industries are developing smaller power devices with higher power densities, optimized design of cooling systems with minimum thermal resistance and pressure drop become important issue for thermal design engineers. This paper aims to present a user friendly optimization tool for direct water cooling system of a high power module which enables the cooling system designer to identify the optimized solution depending on customer load profiles and available pump power. CFD simulations are implemented to find best solution for each scenario.

A new modular multilevel converter for medium voltage high power oil & gas motor drive applications

Abstract: This paper presents the first commercially available water-cooled MV motor drive based on the Modular Multilevel Topology (M2C). The motor drive specific features and HW are described. Measurement results from a 12 MW back-to-back test is included, e.g. operation at low motor frequencies down to zero speed at high torque.

SiC MOSFETs for future motor drive applications

Abstract: This paper investigates the switching performance of six-pack SiC MOSFET and Si IGBT modules for motor drive applications. Both the modules have same packaging and voltage rating (1.2 kV). The three bridge legs of the modules are paralleled forming a single half-bridge configuration for achieving higher output power. Turn-on and turn-off switching energy losses are measured using a standard double pulse methodology. The conduction losses from the datasheet and the switching energy losses obtained from the laboratory measurements are used as a look up table input when simulating the detailed inverter losses in a three-phase motor drive inverter. The total inverter loss is plotted for different switching frequencies in order to illustrate the performance improvement that SiC MOSFETs can bring over Si IGBTs for a motor drive inverter from the efficiency point of view. The overall analysis gives an insight into how SiC MOSFET outperforms Si IGBT over all switching frequency ranges with the advantages becoming more pronounced at higher frequencies and temperatures.

Grid-connected PV power plant induced power quality problems — Experimental evidence

Abstract: This paper presents new findings on phenomena contributing to flicker and voltage variations caused by grid-connected photovoltaic (PV) inverters. The voltage variations caused by two different 6 kW single-phase grid-connected PV inverters were studied during climatic variations by varying their grid-coupling impedance. Two different methods for characterizing the PV-plant induced voltage variations were studied: the short-term flicker index (Pst) and the 10 minute very-short voltage variation value (VSV). The results clearly indicate that PV inverter power fluctuations induced by cloud shading and enhancement have a significant effect on the VSV value, but not on Pst. PV inverters have a clear effect on the Pst as well, but the main contributors are related to the inverter design rather than the power fluctuations caused by clouds. The main contributor in the elevated Pst values could be traced back to the poor design of the maximum power point tracking (MPPT) of the inverters. The MPPT caused subharmonic current variations at a frequency of approximately 8 Hz which is close to the most sensitive frequency of human eye. Another factor causing rapid voltage variations in low irradiance conditions was the current transients related to the inverter start-up and shut-down. Harmonic current distortion is also a potential PV inverter related power quality (PQ) issue. This study indicates that although the current total harmonic distortion (THD) may be very large at low power levels the total demand distortion (TDD) of the PV inverters is almost constant regardless of the output power and the harmonic current had only a very limited effect on the voltage quality even at the weakest network having a short-circuit current of Isc=250 A. Thus, voltage variations caused by the PV inverters were the main PQ issue in the studied networks. The investigations also clearly show that a part of the power quality problems found in the PV plants are caused by the poor design of the PV inverters.

AC/AC modular multilevel converters in wind energy applications: Design considerations

Abstract: Modular multilevel topologies are possible candidates for medium-voltage wind turbines because of their scalability and possible redundancy. This paper concentrates on their design and investigates the influence of generator-to-grid voltage ratio, generator nominal frequency, module-capacitance minimising control approaches, third harmonic injection and fault ride-through capability requirement on the main design parameters (installed switching power, energy variation at module capacitors and efficiency). For different design scenarios, different topologies are concluded as being optimal.

Externally-fed auxiliary power supply of MMC converter cells

Abstract: We designed an auxiliary power supply for medium voltage MMC-converters with system voltages up to 36 kV rms . The system supplies up to 250 W power and is based on a two-stage design. The first stage is an inductive power transfer system that isolates against the system voltage. It is connected to a low voltage DC-supply on ground potential, and shows DC-DC efficiency between 90–95%. Two distinct insulation concepts for the coupled coil design have been designed and passed partial discharge tests. The second stage supplies the various gate and control units of each MMC power cell. Its insulation requirement is given by the cell voltage. Functional and insulation tests as well as a sensitivity analysis are included. The full auxiliary power supply shows an efficiency of 86%.

Protection of hybrid transformers in the distribution grid

Abstract: Due to the increasing integration of renewable energy sources and power electronic loads into the distribution grid, a deterioration of the grid power quality is expected. Consisting of a low frequency transformer and a fractionally rated power electronic converter, the hybrid transformer can be applied to ensure a high power quality by controlling voltage, current, active and reactive power dynamically. For the application in grids with conventional grid protection infrastructure, hybrid transformers have to withstand considerable overvoltage and -current stresses during voltage surges or grid short circuits. Since the semiconductors are less robust than low frequency transformers with respect to these stresses, the effects of possible fault scenarios and a protection concept are studied in this paper.

Impact and opportunities of medium-voltage DC grids in urban railway systems

Abstract: Due to the increasing number of power-electronic components connected to the grid, medium-voltage dc (MVDC) grids are a promising alternative to established ac distribution grids. Within this work, the advantages and additional flexibility of substations for (urban) railways connected to future MVDC distribution grids are investigated. Possible topologies are presented and the efficiencies are compared.

Insulation coordination for a modular multilevel converter prototype

Abstract: This paper provides a detailed overview of the dielectric design and insulation coordination applied to a medium voltage modular multilevel converter prototype. The complete system has ratings of 0.5 MVA and is designed for connection to a 10kVd c supply with a system voltage of 6.6kV ac . The choice of air as insulating and cooling medium requires careful considerations regarding the clearance and creepage distances with respect to the selected materials. The design considerations from the submodule to the cabinet level are presented in the paper, considering safety standards and their requirements for a selected over-voltage category and pollution degree. The final dielectric design is verified experimentally with dielectric ac withstand test and partial discharge measurements.

Rugged MMC converter cell for high power applications

Abstract: A converter cell for a Modular Multilevel Converter (MMC) suitable for the Chain-Link Converter topology is presented. IGCT semiconductors are being used as the main switch technology thanks to features like • inherent stable Short Circuit Failure Mode (SCFM) capability • very low overall losses • superior load cycling capabilities The cell makes use of a single thyristor switch to protect the cell against catastrophic damages in all relevant failure modes. The same thyristor enables continued operation of the chain-link converter after a single cell level failure. A comparison of losses in a STACOM application between IGBT plastic modules and hermetically sealed RC-IGCT press pack devices is presented.

Optimized design of a hybrid-MMC and evaluation of different MMC topologies

Abstract: This paper investigates the combined use of basic cell types (half and full bridges) in a Modular Multilevel Converter in order to achieve an improved MMC performance. Based on an MMC without any additional features (only considering the power transmission capability), an optimal design is presented for additional capabilities such as DC fault ride-through capability or safe operation during reduced DC voltage. These different MMC configurations are compared to a Half and a Full Bridge MMC with regard to the total cost per transmitted power, which includes semiconductor losses and basic power electronic costs.

Multi-objective optimization and comparison of multi-level DC-DC converters using convex optimization methods

Abstract: Multi-level flying-capacitor converters are a promising topology for low-power applications. Conventional buck, 3- and 4-level converters have been modeled as a geometric program and optimized for loss and volume. Results show that 3- and 4-level converters are both smaller and more efficient over a wide range of design parameters.

Silicon carbide MOSFETs in more electric aircraft power converters: The performance and reliability benefits over silicon IGBTs for a specified flight mission profile

Abstract: This paper describes the design, construction and performance of a 5 kVA aviation power module containing silicon carbide MOSFETs The function and control of this module within a commercial aviation power electrical control unit (ECU) application is explained and the power dissipation benefits from the use of these MOSFETs instead of silicon IGBTs when driving an electrical motor controlling an aileron are presented The paper shows the calculated reliability figures for the power module in this application and an application-specific reliability test to verify 150,000 flight hours of module operation is introduced Performance test results from a prototype unit are also presented

Online impedance spectroscopy estimation of a battery

Abstract: Electrochemical impedance spectroscopy (EIS) is a helpful tool to understand how a battery is behaving and how it degrades One of the disadvantages is that it is typically an “off-line” process This paper investigates an alternative method of looking at impedance spectroscopy of a battery system while it is on-line and operational by manipulating the switching pattern of the dc-dc converter to generate low frequency harmonics in conjunction with the normal high frequency switching pattern to determine impedance in real time However, this adds extra ripple on the inductor which needs to be included in the design calculations The paper describes the methodology and presents some experimental results in conjunction with EIS results to illustrate the concept

Optimum semiconductor voltage level for MMC submodules in HVDC applications

Abstract: This paper presents a comparison of the achievable submodule utilization, losses and other basic figures of merit for the design of the modular multilevel converter half bridge submodules for transmission applications. Based on those figures of merit, a method to find out the optimum semiconductor voltage level for MMC submodules in HVDC Applications for a given power and boundary condition is elucidated. Based on the boundary conditions used for the examples of this paper, 4.5 kV blocking voltage is the optimum voltage level for semiconductors at transmission power of less than 900 MW. For higher transmission power the better performance was achieved by the submodule based on the 6.5 kV IGBT.

Modular multilevel battery (M2B) for electric vehicles

Abstract: Electromobility is expected to successfully compete with conventional propulsion technology in the near future and thus creates new challenges for the automotive industry. Major issues with state of the art electric vehicles (EVs) include limited range due to insufficient energy density and heavy weight of existing battery storage systems, unsatisfactory charging duration as well as comparatively high costs. A leap in power electronics technology might meet these challenges, as the aforementioned characteristics are predominantly determined by existing EV system architectures and power converters. This paper presents a novel modular multilevel parallel converter-based split battery system for electric vehicles, enabling dynamic switching of battery cells in parallel and in series. Each individual battery cell may be interconnected to its neighbours according to operational needs, e.g. to provide optimum source resistance, lowest state-of-charge (SOC) cycling, and balanced aging, rendering separate battery management systems (BMS) unnecessary. Applying the proposed technology in EVs may fundamentally change existing powertrain architectures and charger topologies, as it merges the battery storage system and the power converter. This forms the basis for a highly integrated power electronics unit that includes traction converter, battery charger, and BMS.

Balancing of modular multilevel converters with unbalanced integration of energy storage devices

Abstract: In the currently predominant AC-Grid, energy reserves are provided by the inertia of rotating synchronous generators. In a potential DC-Grid, there is no such inherent energy reserve and thus additional energy storage (ES) is required to ensure grid stability. The Modular Multilevel Converter (MMC) is an emerging technology for HVDC-Grid applications and offers a convenient way of connecting ES to the grid. Short strings of batteries or supercapacitors can be integrated directly into the MMC submodules, creating an ES enabled MMC, which reduces the susceptibility of the system to an individual ES cell failure. Since connecting ES to the submodule cells requires additional electrical devices (switches and/or inductors), reducing the number of submodules with integrated ES can significantly reduce the additional cost for ES integration. In most research conducted on this topic so far, the existence of ES in all MMC modules is assumed. However, the amount of ES in an ES enabled MMC can be reduced by utilizing circular currents to achieve energy balance between submodules with and without integrated ES. By inducing circular currents, energy can be transferred between any phase arms, and phase arms with ES can supply those without. The focus of this examination is the additional semiconductor load caused by the circular currents. It is shown that there is only a very small increase in the semiconductor load, when only a few arms supply the whole output power.

Improved control strategy of the modular multilevel converter for high power drive applications in low frequency operation

Abstract: Modular Multilevel Converters (M2C) are considered an attractive solution for high power drives. However, its operation at low rotational speeds can produce undesired voltage fluctuations in the M2C capacitors. In this paper, two methodologies to improve the converter performance in this speed range are analysed and tested. The first strategy proposes the control of the inner converter currents combining a synchronous dq rotating frame and resonant controllers to improve the current tracking and to reduce the voltage fluctuations. The second strategy achieves the reduction of the voltage fluctuations by adjusting the DC Port voltage as a function of the machine frequency. Both methods are validated by simulation and experimental work, where a 18 cell M2C prototype is applied to drive an induction machine.

Design approach and interoperability analysis of wireless power transfer systems for vehicular applications

Abstract: This article gives an overview of the analysis and modelling for 22 kW wireless power transfer (WPT) for Light Duty (250V traction battery) and Heavy Duty (700V) electric vehicles (EV). The system under investigation is the series/series (SS) WPT compensation topology, for which an iterative design approach is considered to select the suitable final system parameters. A simulation study is performed for a selected WPT design for different power electronics topologies, differing in the converter type at secondary side and EV battery voltage. The study focusses on the design and system evaluation of two proposed WPT vehicle architectures and their control for the different secondary systems that are supplied by the same and identically controlled grid system, to achieve interoperable WPT systems. Stable control at high efficiency (>90%) is achieved for both studied topologies, one with a boost DC/DC converter at secondary side and charging a 700V battery, the other with buck DC/DC converter to 250V.

Application of GaN power transistors in a 2.5 MHz LLC DC/DC converter for compact and efficient power conversion

Abstract: In this work a demonstrator with a switching frequency up to 2.5 MHz is shown. These comparatively high frequencies reduce the weight and the system costs of the resonant 3-kW-DC-DC-converter. The electrical properties of the implemented Gallium Nitride (GaN) power transistors enable high switching frequencies while maintaining high efficiency. The presented converter has a power density of approximately 3 W/cm3. The total efficiency of the converter is higher than 90 % for all operation points above 1/5 of the nominal load. Exceptionally high efficiency of 94.5 % can be reached at half of the nominal load and a switching frequency of 2 MHz. Possible improvements are defined and could be reached by adding a continuous dead time control as well as adapting the driver circuit of the synchronous rectification.

Efficiency analysis of a high frequency PS-ZVS isolated unidirectional full-bridge DC-DC converter based on SiC MOSFETs

Abstract: In this work, the static and dynamic characteristics of a new double-trench SiC MOSFET are experimentally investigated and compared to two different available commercial SiC MOSFETs. The SiC devices are applied in a ZVZCS Phase-Shifted Full-Bridge converter which uses the parasitic leakage inductance of the transformer to achieve soft switching. A practical method to break down the converter losses based on separating the measured semiconductor and magnetic losses is proposed and compared to an analytical loss model. The experimental results show several advantages of using SiC MOSFETs as well as ZVZCS in the converter. The main results can be summarized as elimination of the freewheeling circulating current mode, clamping the overshoot voltage across the rectifier diodes and achieving soft switching in a wide power load range without additional auxiliary circuits or resonant inductor. Experimental results for a 10kW, (100–250) kHz prototype show an efficiency of up to 98,1% for the whole converter.

A custom, high-performance real time measurement and control system for arbitrary power electronic systems in academic research and education

Abstract: In high-quality education, topics concerning the behavior and control of electrical machines and power electronics have to be taught not only theoretically but also in a practical manner Hard- and software tools are necessary to fulfil this obligation Nevertheless, commercial systems partially lack of functionality or full accessibility to implement custom solutions, which is obligatory in the research and education domain Therefore, a digital signal processing system is presented, that allows full modification of every used device, both in hardware and in software

Catenary free operation of light rail vehicles — Topology and operational concept

Abstract: Difficult building conditions but also the impairment of the cityscape through overhead lines can lead to high cost and lack of acceptance from citizens and town planners. In these cases the use of a tram with an energy storage (ES) which enables for catenary free operation (CFO) could be the more cost-effective solution and could create the necessary acceptance. This article presents the operational concept of catenary free operation of a tram and the integration of a Lithium-Ion (Li-Ion) energy storage into the propulsion system of a BOMBARDIER∗ FLEXITY∗ 2 tram. The first application of this propulsion system was carried out for the newly built tram line of the Chinese metropolis of Nanjing. The trams are successfully in passenger operation since the youth Olympic Games in August 2014, running without catenaries on 90% of the line.

The Series Bridge Converter (SBC): A hybrid modular multilevel converter for HVDC applications

Abstract: This paper presents a novel hybrid modular multilevel voltage source converter suitable for HVDC applications. It has the advantages of other modular multilevel topologies and can be made more compact making it attractive for offshore stations and city infeed applications. The Operating principle of the converter and internal energy management are discussed with simulation results from a scaled medium voltage demonstrator presented to validate the concepts.