Showing papers presented at "European Conference on Power Electronics and Applications in 2013"

Self-commissioning notch filter for active damping in three phase LCL-filter based grid converters

Abstract: LCL-filters are used to mitigate the harmonic current content in grid converters. The LCL-filter resonance must be damped in order to avoid stability problems in the current control. Active damping avoids resistors at the expense of increased control complexity. Large grid impedance variations can challenge the LCL-filter stability. Active damping by using a notch filter on the reference voltage for the modulator is simple to implement and does not require additional sensors. With the notch frequency tuned for the resonant frequency the voltage reference does not contain any component susceptible of exciting the LCL-filter. However, the notch filter tuning requires considerable design effort and the variations in the resonance frequency limit the LCL-filter robustness. This paper proposes a simple tuning procedure for the notch filter that results in proper robustness. In order to cope with the grid inductance variations it is proposed to estimate the resonance frequency by means of Fourier analysis. The Goertzel algorithm, instead of the FFT, is used to reduce the calculation and memory requirements. Thus, the proposed self-commissioning notch filter results robust and consumes little computational resources. Finally, the analysis is validated with both simulation and experiments.

Online Vce measurement method for wear-out monitoring of high power IGBT modules

Abstract: A simple Vce online monitoring circuit is presented in this paper. It allows an accurate wear out prediction of IGBT modules, in high-power applications, during normal converter operation. Bipolar measurement allows monitoring of both IGBT and antiparallel diode. The circuit uses two serial connected diodes to sense the Vce voltage with millivolt accuracy. One diode acts as a protection to block high DC voltage present on input terminals. When the device is conducting the voltage on the second diode is measured to compensate for the voltage drop on the protection diode thus eliminating voltage offset due to diodes' forward voltage temperature dependency. Using four diodes one can monitor voltages on all power devices in a converter leg.

Capacitance and inductance selection of the modular multilevel converter

Abstract: The paper presents a proposal for capacitance and inductance selection procedure of the modular multilevel converter. Two analysis criteria are taken into consideration, the circulating current and voltage ripple across submodule capacitors. Results are obtained numerically by using the converter averaged model based on state equations. In the modular multilevel converter operating under a direct modulation method, based on sinusoidal modulating signals, circulating currents flows through the converter arms. These currents are strongly dependant on the component parameters of the converter, such as: submodule DC-link capacitor capacitance and arm inductor inductance. Both components form a series resonance circuit in each converter arm. Resonance that occurs in the converter arm has to be avoided and therefore it is important to properly select component parameters while taking into account all possible resonances. Such components selection should be carried out at an early stage of the converter design.

Five level cross connected cell for cascaded converters

Abstract: Proposed here is an alternate Five-level four-quadrant cascaded multilevel converter cell configuration that compared to the other cell configurations, for dc fault current limitation, will be more compact and avoid the external dc breaker. Loss comparison on cells with dc fault blocking capability for the cascaded converter is also presented.

Future HVDC-grids employing modular multilevel converters and hybrid DC-breakers

Abstract: The necessary shift from fossil fuels to regenerative sources imposes a great challenge for future power transmission. A large, meshed HVDC-Grid is proposed by several experts as the best and most efficient solution. For meshed DC-Grids, however, new and essential requirements - especially regarding availability and reliability - have to be researched very thoroughly. Fast and reliable handling of DC-faults must be ensured without disturbing the energy flow in the whole grid. The following paper deals with the key components (Modular Multilevel Converters and DC-Breakers) in large meshed HVDC-Grids. A comparison between suitable submodule topologies with DC-fault handling abilities is presented alongside with a new topology for a hybrid DC-Breaker.

Tolerance-band modulation methods for modular multilevel converters

Abstract: Modular multilevel converters (M2C) are increasingly used in high voltage direct current (HVDC) systems. The efficiency of M2Cs is highly related to the modulation method which determines the switching frequency and capacitor voltage ripple in the converter station. A new approach to modulation of M2C is presented in this paper. Tolerance-band methods are employed to obtain switching instants, and also cell selection. The proposed methods overcome the modulation problem for converters with few numbers of cells and also reduce the sorting efforts for cell balancing purposes while maintaining the cell-capacitor voltage limits. The evaluation is done by time-domain simulation by which the performance of each method is studied in both steady-state and transient cases. It is observed that using tolerance band methods not only reduces the switching frequency but also allows for handling severe fault cases in a grid connected system. Use of this method can reduce the switching losses and also allow for reduction of the cell capacitor size.

Calculation of power losses for MMC-based VSC HVDC stations

Abstract: VSC technology is now well established in HVDC and is, in many respects, complementary to the older Line Commutated Converter (LCC) technology. Many advantages have been claimed for VSC technology in HVDC, including the smaller site footprint. On the other hand, VSC HVDC stations have higher power losses than LCC stations. Although the relative advantages and disadvantages are well known within the industry, there have been very few attempts to quantify these factors on an objective basis. In part, this is because of the lack of applicable standards for determining the power losses of a VSC station. No equivalent of IEC 61803 is today available for VSC-HVDC, but now a draft IEC standard (IEC 62751) is in preparation, which will permit objective comparisons to be made between power losses of LCC and VSC stations.

Control of grid-connected power converters based on a virtual admittance control loop

Abstract: The connection of electronic power converters to the electrical network is increasing mainly due to massive integration of renewable energy systems. However, the electrical dynamic performance of these converters does not match the behavior of the network, which is mainly formed by generation facilities based on big synchronous generation systems. Depending on the desired electrical operation mode different control structures can be implemented in the converters in order to get adapted with the grid conditions. However, changing between different control structures and operation is not an optimal solution, as the resulting system results complex and is not highly robust. As an alternative, this paper presents a new control technique for grid connected power converters based on the concept of virtual admittance. The proposed control permits to emulate the electrical performance of generation facilities based on classical synchronous generators with a power converter, with no need of implementing different control structures, giving rise to a system that provides a friendly and robust operation with the network.

A review on real time physical measurement techniques and their attempt to predict wear-out status of IGBT

Abstract: Insulated Gate Bipolar Transistors (IGBTs) are key component in power converters. Reliability of power converters depend on wear-out process of power modules. A physical parameter such as the on-state collector-emitter voltage (Vce) shows the status of degradation of the IGBT after a certain cycles of operation. However, the Vce mainly shows the wear-out of bond wire lift-off and solder degradation. The Vce is normally used to estimate the junction temperature in the module. The measurement of Vce is sensitive to the converter power level and fluctuations in the surrounding temperature. In spite of difficulties in the measurement, the offline and online Vce measurement topologies are implemented to study the reliability of the power converters. This paper presents a review in wear-out prediction methods of IGBT power modules and freewheeling diodes based on the real time Vce measurement. The measurement quality and some practical issues of those measurement techniques are discussed. Furthermore, the paper proposes the requirements for the measurement and prognostic approach to determine wear-out status of power modules in field applications. The online Vce measurement for a selected topology is also shown in the paper.

Experimental investigations of static and transient current sharing of parallel-connected silicon carbide MOSFETs

Abstract: An Experimental performance analysis of a parallel connection of two 1200/80 MΩ silicon carbide SiC MOSFETs is presented. Static parallel connection was found to be unproblematic. The switching performance of several pairs of parallel-connected MOSFETs is shown employing a common simple totem-pole driver. Good transient current sharing and high-speed switching waveforms with small oscillations are presented. To conclude this analysis, a dc/dc boost converter using parallel-connected SiC MOSFETs is designed for stepping up a voltage from 50 V to 560 V. It has been found that at high frequencies, a mismatch in switching losses results in thermal unbalance between the devices.

DC collector grid configurations for large photovoltaic parks

Abstract: This paper presents a theoretical efficiency comparison among the conventional ac collector grid configuration and two proposed dc collector grid configurations (one with string-pair-level maximum power point (MPP) tracking and one without) in large photovoltaic (PV) parks. A typical sunny day in September is used as a test case to determine the PV array yield. An already validated PV array performance model is used to estimate the expected yield of the PV array for the given environmental and connection conditions. Loss mechanisms in different grid components in the three configurations are discussed and simplified loss models are developed without compromising the accuracy. These components include the inverters, transformers, ac/dc cables and dc-dc converters. The chosen figure of merit is the total efficiency of the components lying between the PV array and the MV/HV transformer that feeds power in the HV grid. A comparison of the total losses helps to identify the configuration that has the potential to operate at higher efficiency and hence, generate more revenues.

A new HVDC-DC converter with inherent fault clearing capability

Abstract: For the upcoming need to transfer bulk power over long onshore distances, HVDC power transmission is the preferred choice. However, components like HVDC-DC converters and a protection concept for DC line faults present challenges, that still have to be solved. As up today, the sections of a segmented DC transmission corridor would still be linked via an AC connection, leading to high transformation losses, and high investment costs. With the newly presented HVDC-DC auto transformer it is possible to directly link two DC lines with different voltage levels. In this paper the ability of this HVDC-DC converter to interrupt DC faults is investigated and compared to the state of the art topology of linking two DC lines via an AC connection.

Optimal design of the modular multilevel converter for an energy storage system based on split batteries

Abstract: This paper presents the optimal design of a modular multilevel converter (MMC) for use in a standalone high power energy storage system based on split batteries (sBESS). The MMC allows for the sBESS to connect directly to the medium-voltage grid without the need for a line-transformer. A free parameter variation is performed to compare designs with different numbers of modules and different power semiconductors. Many commercially available IGBTs were found to be overdimensioned for the 5MW, 20kV target system, and better results were obtained with optimized assemblies. When the converter is designed with just a few more modules than absolutely necessary, the total size of the passive components can be reduced drastically, while the power losses increase only marginally. An attractive candidate system is given a closer look, for which a peak power-conversion efficiency of 99.3% is predicted (not including power losses in the line-filter and in the dc-dc converters to interface the batteries).

On energy variations in modular multilevel converters with full-bridge submodules for Ac-Dc and Ac-Ac applications

Abstract: The modular multilevel converter is a promising topology for high-voltage high-power applications. By the series-connection of submodules it can generate high-quality voltage waveforms with low harmonic distortion at low switching frequencies. The submodules are low-voltage converters with capacitive energy storages. These capacitive energy storages are a driving factor of the size, weight, and cost of the converter. For this reason it is important to ensure that the stored energy in the converter is distributed as evenly among the submodules as possible. In this paper it is found that during nominal operating conditions, a large amount of energy is moved back and forth between the upper and lower arms in the converter. These energy oscillations can, however, be reduced or even eliminated if an appropriate modulation index or voltage ratio is used. It is also found that an appropriate modulation index or voltage ratio can reduce the power rating of the semiconductors as well. The theoretical findings are validated by simulating two systems with different voltage ratios for ac-ac conversion.

Mission profile based multi-disciplinary analysis of power modules in single-phase transformerless photovoltaic inverters

Abstract: The popularity of transformerless photovoltaic (PV) inverters in Europe proves that these topologies can achieve higher efficiency (e.g., ≥ 98% has been reported). Along with the advanced power electronics technology and the booming development of PV power systems, a long service time (e.g. 25 years) has been set as a main target and an emerging demand from the customers, which imposes a new challenge on grid-connected transformerless inverters. In order to reduce maintenance cost, it is essential to predict the lifetime of the transformerless PV inverter and its components based on the mission profiles - solar irradiance and ambient temperature. In this paper, a mission profile based analysis approach is proposed and it is demonstrated by three main single-phase transformerless PV inverters - Full-Bridge (FB) with bipolar modulation scheme, the FB inverter with DC bypass (FB-DCBP) topology and the FB inverter with AC bypass leg (highly efficient and reliable inverter concept, HERIC inverter). Since the thermal stress is one of the most critical factors that induce failures, the junction temperatures on the power devices of the three topologies are analyzed and compared by considering the mission profiles. The lifetimes of these topologies are discussed according to the thermal performance and the power losses on the switching devices are also compared.

Online estimation of IGBT junction temperature (Tj) using gate-emitter voltage (Vge) at turn-off

Abstract: The paper presents a novel method for online estimation of the junction temperature (Tj) of semiconductor chips in IGBT modules, based on evaluating the gate-emitter voltage (Vge) during the IGBT switch off process. It is shown that the Miller plateau width (in the Vge waveform) depend linearly on the junction temperature of the IGBT chips. Hence, a method can be proposed for estimating the junction temperature even during converter operation - without the need of additional thermal sensors or complex Rth network models. A measurement circuit was implemented at gate level to measure the involved time duration and its functionality was demonstrated for different types of IGBT modules.

Transient robustness testing of silicon carbide (SiC) power MOSFETs

Abstract: This paper presents the development of a unified test set-up and experimental results of the robustness characterisation of new generation of silicon carbide (SiC) power MOSFETs. In particular, unclamped inductive switching (UIS) and short-circuit withstand capability (SC) are investigated, with the aim of assessing the actual limits of operation of the devices and highlighting the underlying physical mechanisms. An electro-thermal device model is used to support the experimental analysis and interpret the observations.

Current source modular multilevel converter for HVDC and FACTS

Abstract: A current source modular multilevel converter (MMC) is proposed for high voltage AC/DC power conversion applications, such as HVDC and FACTS. Current source converters possess the advantage of short-circuit fault tolerance, which is a pivotal feature for grid applications. By partially following the circuit duality transformations, the proposed converter is derived from the well-known voltage source MMC. Inductor-based current source cells are connected in parallel and form a current source arm that can synthesize a desired current waveform. By adding a reduced-energy capacitor in parallel to each current source arm, these arms can be further connected in series, thereby allowing voltage scaling. By using fully controllable switches, the converter is capable of providing full control on its active and reactive power. Protection schemes against open-circuit failures inside the inductor cells are also proposed. Simulation results show the operation of the current source MMC and its capability of DC fault tolerance.

Analysis of communication based distributed control of MMC for HVDC

Abstract: Modular Multilevel Converter (MMC) has gained a lot of interest in industry in the recent years due to its modular design and easy adaption for applications that require different power and voltage level. However, the control and operation of a real MMC consisting of large number of sub modules for high power and high voltage application is a very challenging task. For the reason that distributed control architecture could maintain the modularity of the MMC, this control architecture will be investigated and a distributed control system dedicated for MMC will be proposed in this paper. The suitable communication technologies, modulation and control techniques for the proposed distributed control system are discussed and compared. Based on the frequency domain modeling and analysis of the distributed control system, the controllers of the different control loops are designed by analytical methods and Matlab tools. Finally, sensitiveness of the distributed control system to modulation effect (phase-shifted PWM), communication delay, individual carrier frequency and sampling frequency is studied through simulations that are made in Matlab Simulink and PLECS.

DC transformer for DC/DC connection in HVDC network

Abstract: This paper presents the modeling and control of a multilevel DC/DC bidirectional converter suitable for medium voltage and power applications, with a special interest in wind-power applications. The proposed multilevel topology has a modular structure constituted by base DC/DC converter cells. The multilevel converter is consequently based on Dual Active Bridge (DAB). The overall control of the DC/DC converter is achieved by using a nonlinear control based in Lyapunov theory.

A three-phase 10 kVAC-750 VDC power electronic transformer for smart distribution grid

Abstract: In this paper, a power electronic transformer (PET) topology for distribution grid is presented based on modular multilevel converter (MMC). The presented topology is of three-phase type and have two dc voltage terminals, i.e. the high-voltage dc terminal and the low-voltage dc terminal. So, it can be connected directly to high-voltage direct current (HVDC) systems and renewable energy systems simultaneously. This feature makes it possible to perform as an “energy router” in smart grids. What is more important, the proposed PET can substantially reduce (by 44.4% for the studied case) the number of medium frequency transformers (MFTs), compared with the existing solutions. Computer simulation and experimental results on a 10 kV prototype show validity of this PET.

Energetic macroscopic representation and inversion based control of a modular multilevel converter

Abstract: This papers deals with the Modular Multilevel Converter (MMC). This structure is a real breakthrough which allows transmitting huge amount of power in DC link. In the last ten years, lots of papers have been written but most of them study some intuitive control algorithms. This paper proposes a formal analysis of MMC model which leads to the design of a control algorithm thanks to the inversion of the model. The Energetic Macroscopic Representation is used for achieving this goal. All the states variables are controlled to manage the energy of the system, avoid some instable operational points and determine clearly all the dynamics of the different loops of the system.

Integration of batteries into a modular multilevel converter

Abstract: In this paper the challenges of integrating batteries into the Modular Multilevel Converter are described. For this purpose, the technology of this converter topology is briefly introduced. An overview on already available products of battery energy storage systems is given. Based on that, a control concept for a DC-to-DC converter connecting a battery to a module of the Modular Multilevel Converter is presented. This concept contains an additional energy balancing algorithm. Several operating points of the whole system with integrated batteries are simulated. The paper is topped off with measurement results of a started-up hardware setup that is equipped with emulated DC-to-DC converters.

Design and optimization of medium frequency, medium voltage transformers

Abstract: Medium voltage, medium frequency transformers (MFTs) are much smaller in size and weight compared to conventional low frequency transformers. The MFTs are very attractive for applications where full control of the power flow and high power density are required, such as power electronic interface in smart grids and traction converter system. With reduced size, the optimal design of MFTs becomes more challenging due to the high isolation requirement and thermal stress. This paper presents a design and optimization methodology for MFTs with emphasis on thermal and insulation design. The improved thermal model for multi-layer windings consisting of litz-wire and the analytical calculation of maximum electric field in core window area are included in the optimization process. By using the methodology, the optimal designs of a 25kW/4 kHz MFT with two commonly used core materials are compared. The possibility of volume reduction by increasing the operating frequency is also investigated.

A sub-module capacitor voltage balancing scheme for the Alternate Arm Converter (AAC)

Abstract: A per-phase sub-module capacitor voltage balancing scheme for the Alternate Arm Converter (AAC) is proposed. Leg balancing regulates the sum of sub-module capacitor voltages in a leg to its set-point. Arm balancing regulates each sub-module capacitor voltage to the nominal sub-module capacitor voltage. Additional converter control requirements and the modulation scheme are also discussed. The control and modulation scheme has been verified by simulation. The leg balancing control loop was designed to be critically damped to primarily limit oscillations in the output variable caused by changes in AC-side power demands; the simulation results indicated this specification was met.

Possibility of energy saving by introducing energy conversion and energy storage technologies in traction power supply system

Abstract: After the 3.11 earthquake in Tohoku district in Japan, energy saving and renewable energy utilization become one of the important problems in Japan. East Japan Railway Company, as a public transportation company, tries to contribute to solve these problems by realizing eco-friendly railway transportation system. In this paper, three realized and under construction projects are summarized which enhance efficiency of traction power supply system. First project is installation of lithium ion battery at Haijima Substation on February 20th, 2013 to make use of regenerative power in d.c. traction power supply. Second project is the power conversion application to be installed at Ushiku sectioning post for a.c. traction power supply and will start operation in the end of 2013 fiscal year. The last project but not least one is “zero-emission station” realized at Hiraizumi Station on June 28th, 2013. The total electric power of the station is supplied from solar power even in the evening using lithium ion batteries. The detail of the operation data of “zero emission station” is shown in this paper.

Energy diverting converter topologies for HVDC transmission systems

Abstract: Grid codes imposed by utilities regulate the operation of Voltage Source Converter - High Voltage Direct Current (VSC-HVDC) interconnected offshore wind farms. Fault ride-through (FRT) specifications require the adoption of specific measures to avoid over-voltages of the HVDC link during faults in order to protect the HVDC equipment. Implementing Energy Diverting Converters (EDC), for instance Dynamic Braking Resistor (DBR) circuits, at the DC link is an established method to comply with the grid codes, where the excess energy of the wind farm is diverted into the parallel circuit during the fault. In this paper an evaluation of three different state-of-the-art DBR circuits is performed in order to establish the advantages and disadvantages of each circuit. The evaluation has shown that although the three solutions meet the FRT requirements, the modular topologies generate reduced slope current and voltage step changes during their operation, while being larger in size and requiring a higher number of semiconductors as compared to the traditional DC chopper circuit employing hard switched series connected semiconductor arrangements.

Characterization and comparison of 1.2 kV SiC power semiconductor devices

Abstract: This paper seeks to provide insight into state-of-the-art 1.2 kV Silicon Carbide (SiC) power semiconductor devices, including the MOSFET, BJT, SJT, and normally-on and normally-off JFET. Both commercial and sample devices from the semiconductor industry's well-known manufacturers; namely Cree, GE, ROHM, Fairchild, GeneSiC, Infineon, and SemiSouth, are evaluated in this study. To carry out this work, static characterization of each device is performed under increasing temperatures (25-200 °C). Dynamic characterization is also conducted through double-pulse tests. Accordingly, the paper describes the experimental setup used and the different measurements conducted, which comprise: threshold voltage, current gain, specific on-resistance, and the turn on and turn off switching energies. For the latter, the driving method used for each device is described in detail. Furthermore, for the devices that require on-state dc currents, driver losses are also taken into consideration. Key trends and observations are reported in an unbiased manner throughout the paper and summarized in the conclusion.

Encoderless model predictive control of back-to-back converter direct-drive permanent-magnet synchronous generator wind turbine systems

Abstract: This paper presents encoderless model predictive control scheme with time-varying sliding mode observer for a complete wind turbine system. The wind turbine system consists of a back-to-back converter (AC/DC/AC) and a direct-drive permanent-magnet synchronous generator (PMSG). We give a complete model of the system and present encoderless fixed-frequency model predictive direct torque control of the generator and finite-set model predictive direct power control of active and reactive power on the grid side. The sliding mode observer utilizes a time-varying switching gain and a time-varying cut-off frequency to estimate rotor position and rotor speed without chattering. The proposed strategy is illustrated by simulations as a first proof of concept. The simulation results show that the proposed strategy achieves fast torque control dynamics and highly decoupled control of active and reactive power.

Impact of absolute junction temperature on power cycling lifetime

Abstract: The influence of the absolute junction temperature Tj* on IGBT power module lifetime was systematically investigated by means of active power cycling tests. Both the impact on the wire bond lift-off and the chip solder degradation mechanism could be determined separately by applying the concept of separating failure modes. The test results not only prove that classical lifetime models overestimate the influence of Tj*, but also show that the two dominant failure mechanisms have to be treated differently. The wire bond lift-off failure mode is weakly affected by the absolute temperature level and possesses a very small activation energy of 0.069 eV. The solder degradation mode exhibits a significantly larger activation energy of 0.159 eV, which results in a massive decrease (factor 3) in power cycling capability when increasing Tj* by 85 K. For junction temperatures above 175°C SnAg-based solder joints are no longer suitable for reliable power module designs and advanced die attach technologies such as silver sintering have to be deployed. For the description of such advanced power modules specific lifetime curves are under development which exclusively represent the pure wire bond lifetime, as for common power cycling conditions the silver sintered die attach is not subjected to ageing.