Showing papers presented at "European Conference on Power Electronics and Applications in 2015"

Development and test of a 200kV full-bridge based hybrid HVDC breaker

Abstract: Recent development in DC technologies and renewable energy increase the requirement in building large HVDC networks. The world first Zhoushan five-terminal HVDC project constructed in China has been put into operation. To increase the operation reliability, a HVDC circuit breaker will be required. However the existing HVDC breaker developed in recent years still can't deal with severe fault current of a high rise of rate in this project. Considering the availability in a real HVDC network, this paper proposed a new full-bridge based hybrid HVDC breaker. The hybrid HVDC breaker has significant technical advantage compared with other options especially in terms of current breaking ability, and the compact modular integration of the prototype greatly reduce the demand of the space. Safely test procedures is developed to verify the performance of the hybrid HVDC breaker. The test results show that the full-bridge hybrid HVDC breaker could be used in the Zhoushan five-terminal project.

Quasi-Y-Source Boost DC–DC Converter

Abstract: In this letter, a new topology called “quasi-Y-source dc–dc converter” is presented. It inherits all the advantages of the original Y-source converter. In addition, the new topology draws a continuous current from the source, which is definitely more appropriate for most renewable sources. It also has dc-current-blocking capacitors, which will definitely help to prevent the coupled inductor core from saturation. Experimental testing has proven the validity of the proposed network and its application as a high boost dc–dc converter.

Short-circuit evaluation and overcurrent protection for SiC power MOSFETs

Abstract: In this paper, the short-circuit (SC) performance of two different SiC MOSFETs is experimentally investigated for different input voltages, biasing voltages and case temperatures. The measurement results are compared to simulations, and a good agreement is achieved. For fault handling, two different overcurrent protection (OCP) circuits are designed and applied to the SiC MOSFETs. The desaturation method is successfully tested with a hardware solution substituting the blanking time delay. The second method is based on sensing the voltage drop across the parasitic inductance at the source pin. The experimental and simulation results show that both OCP methods have the capability to detect a short circuit condition in the SiC MOSFET within safe SC time avoiding device failure.

Analysis of modular multilevel converters with DC short circuit fault blocking capability in bipolar HVDC transmission systems

Abstract: This paper analyzes the station-internal phase-to-ground fault in bipolar HVDC transmission systems. An overvoltage problem due to the existence of the bipolar cells in the modular multilevel converter (MMC) arms closer to the grounding pole are presented. Consequently, a new hybrid arm MMC is proposed to overcome the overvoltage problem while providing the benefits of: a lower number of cells, fewer switching devices and lower conduction losses. Guidelines are developed and confirmed by simulation results to determine the required number of cells to block the DC side fault.

Evaluation of topologies and optimal design of a hybrid distribution transformer

Abstract: Due to the increasing integration of distributed generators, the grid underlies conceptual challenges. To ensure an efficient and reliable operation, more controllable assets are expected in the future grid. The transformer as one of the most important components possesses only limited control capabilities today. By combining a conventional transformer with a converter, a hybrid transformer with comprehensive controllability is obtained. In this paper, suitable concepts are investigated and an optimum design is determined to compare its potential to conventional and solid state transformers.

Prototype of smart energy router for distribution DC grid

Abstract: Smart energy router (SER), also called as power electronics transformer (PET) or solid state transformer (SST), will serve as a critical component in the next-generation electric power system. Taking advantage of modular multilevel converter (MMC) and input series output parallel (ISOP) topologies, new type of smart energy router has been developed. Presented architecture using two stages with AC/DC conversion in the medium-voltage (MV) side, and ISOP type DC/DC conversion in the low-voltage (LV) side, thus allows interaction with DC grid and local renewable sources, etc. Compared to existing topologies, proposed SER uses less power switches and high-frequency transformers. The details related to the 2-MVA SER prototype, including the electrical design of the circuits and control system is presented in this paper. Simulation and experimental results on this prototype with distribution DC grid show validity of this SER.

Recent advancements in IGCT technologies for high power electronics applications

Abstract: In this paper, we review the progress made recently for further developing the Integrated Gate Commutated Thyristor (IGCT) device concept for high power electronics applications. A wide range of newly introduced IGCT technologies are discussed and recent prototype experimental results as well as novel structures and future trends of the IGCT technology are presented. This will provide system designers with a comprehensive overview of the potentials possible with this device concept.

Auxiliary power supply for medium-voltage modular multilevel converters

Abstract: Design considerations for a modularized auxiliary power supply suitable for variable voltage modular multilevel converters, in terms of electrical performance and isolation against high voltages, are presented. In the presented auxiliary supply, an LLC resonant converter along with several individual floated voltage sources containing high-frequency and high-voltage-isolation transformers are employed. The design process of the individual floating voltage sources is based on circuit and FEM simulations. Partial discharge measurements are also shown for the proposed high-voltage transformer design. Last but not least, electrical measurements performed on a down-scaled laboratory prototype reveal the stable and proper operation of the system at various operating points.

A review of failure mechanisms in wind turbine generator systems

Abstract: Reliability is a critical consideration for wind turbine generator systems as failures contribute directly to operation and maintenance costs and hence the cost of energy. Improving reliability hinges on an understanding of the mechanisms of failures that occur. This paper discusses the major failure mechanisms in the WT generator system, particularly the converter and the generator, from the point of view of their use in wind turbines. It also looks at the opportunities for improving reliability.

Direct torque control with feedback linearization for induction motor drives

Abstract: This paper describes a Direct Torque Controlled (DTC) Induction Machine (IM) drive that employs feedback linearization and sliding-mode control. A feedback linearization approach is investigated, which yields a decoupled linear IM model with two state variables: torque and stator flux magnitude. This intuitive linear model is used to implement a DTC type controller that preserves all DTC advantages and eliminates its main drawback, the flux and torque ripple. Robust, fast, and ripple-free control is achieved by using Variable Structure Control (VSC) with proportional control in the vicinity of the sliding surface. The VSC component assures robustness as in DTC, while the proportional component eliminates the torque and flux ripple. The torque time response is similar to DTC and the proposed solution is flexible and highly tunable due to the proportional controller. The controller design and its robust stability analysis are presented. The sliding controller is compared with a linear DTC scheme, and experimental results for a sensorless IM drive validate the proposed solution.

Optimum efficiency control of interior permanent magnet synchronous motors in drive trains of electric and hybrid vehicles

Abstract: In automotive traction applications the interior permanent magnet synchronous motor (IPMSM) is preferentially chosen as traction drive due to its high torque and power densities. In drive trains of electrical vehicles (EV) and most hybrid electrical vehicles (HEV) the traction motor is operated in torque controlled mode. In a field-oriented control scheme an operation point selection strategy is required to choose appropriate current setpoints to generate the requested torque with high precision and optimal efficiency. In this paper the potential to improve the efficiency by operating a motor with minimum overall losses instead of minimum current per torque is investigated. The interaction between the motors loss characteristic, the impact of the voltage limit and the relevance of operation points for vehicle operation is explored theoretically and based on loss measurements. It is shown, that the losses can potentially be reduced at medium to high motor speeds with moderate torque requirements. This operation range corresponds to traveling with constant medium to high speed and slight acceleration and deceleration in electric vehicles. As well most operation points of the New European Driving Cycle (NEDC) and the Worldwide harmonized Light vehicles Test Procedure (WLTP) are in that operation range due to the moderate accelerations and decelerations in both driving cycles.

6.5kV FREEDM-Pair: Ideal high power switch capitalizing on Si and SiC

Abstract: 6.5kV Si IGBTs have been used widely in median voltage drives, HVDC, FACTs and traction systems. However, the large switching losses of the Si IGBT limit its switching frequency to only 100Hz to 1kHz. On the other hand, wide bandgap (WBG)power devices such as Silicon Carbide (SiC) MOSFET or JFET have demonstrated their superior advantages over Si IGBT, especially in terms of significantly reduced switching losses. A major issue facing large scale adoption of SiC power devices is still the much higher cost. This paper proposes that the FREEDM-Pair, a Si/SiC hybrid switch, should be an ideal and cost effective switch for high power applications. In the proposed FREEDM-Pair, a SiC MOSFET is connected in parallel with Si IGBT to combine the advantages of IGBT and MOSFET. A 6.5 kV FREEDM-Pair is developed as an example to demonstrate its superior cost/performance. Experimental results demonstrated 70% switching loss reduction and the FREEDM-Pair cost is estimated to be only 50% higher than 6.5 kV Si IGBT.

Comparison of the power losses in 1700V Si IGBT and SiC MOSFET modules including reverse conduction

Abstract: The paper presents a study of the power losses in 1700V rated half-bridge power modules applied in a 250kVA three-phase converter. Two types of the modules with comparable parameters (1700V/300A) are analyzed: the first one is based on Si IGBT and the second is built with SiC MOSFETs and Schottky diodes. A special focus of this paper is a reverse conduction of SiC MOSFETs. This phenomenon is analyzed by means of Saber simulations and new, corrected equations describing conduction power losses of the diodes and transistors are provided. Then, combined electro-thermal calculations are conducted using datasheet parameters of the compared modules. The collected data suggest that the Si IGBT module show lower conduction power losses while SiC MOSFETs provides much better switching performance. Thus, both modules are comparable at low switching frequencies but an advantage of the SiC MOSFET module is more visible with the switching frequency increase. When the three-phase converter is operating in an active rectifier mode the conduction power losses are strongly reduced due to the reverse conduction of SiC transistors. In consequence, the SiC MOSFET module shows lower conduction losses and total power losses than Si IGBT.

Ultra-fast charging station for electric vehicles with integrated split grid storage

Abstract: This paper presents the detailed analysis, optimisation and hardware realisation of an ultra-fast charging station with a split grid storage battery which enables to recharge electric vehicles (EVs) in less than 10 minutes. The station consists of a T-type AC-DC grid interface connected to the 400 V low voltage AC grid, a DC-DC dual active bridge (DAB) isolation stage, a stationary storage battery and a high power multi-phase interleaved buck converter for charging operation. The operating principle of the converter systems is briefly explained and the analytical loss models of the components are derived which are used in an optimisation procedure to evaluate the pareto front limits in terms of efficiency and power density for the DAB isolation stage and the high power charger. By introducing a split storage battery, beneficial conditions for the semiconductor devices are achieved so that the losses of the high power charger can be reduced by approximately 35 % with respect to a standard solution without a split as will be shown in the paper.

A new topology of fast solid-state HVDC circuit breaker for offshore wind integration applications

Abstract: Emerging new voltage source converter based HVDC applications demand fast short-circuit fault current interruption. Fast dc circuit breakers are identified as the feasible solution to handle the dc fault current. Switching overvoltage across the dc circuit breakers is destructive for the interrupter device and also for the other components of the system. A new topology of fast solid-state circuit breaker for HVDC applications is proposed in this paper. Instead of conventional approaches, a pre-charged capacitor is used for soft switching. Different modes of operation of proposed circuit breaker are analysed and also design process of circuit parameters are described. Finally, simulation results are presented.

Design and identification of a lumped-parameter thermal network for permanent magnet synchronous motors based on heat transfer theory and particle swarm optimisation

Abstract: A lumped-parameter thermal network (LPTN) for a permanent magnet motor (PMSM) is developed to estimate the most crucial motor temperatures. In this contribution a 60 kW PMSM prototype designed for automotive traction drives is used as the investigation basis. Aiming at real-time motor monitoring well-known analytic equations from the heat transfer theory are used to model the dominant heat paths. Based on a three-dimensional approach in cylindrical coordinates a differential-algebraic state-space model with varying parameters (LPV) is proposed. Due to the chosen level of model abstraction as well as motor material data uncertainties significant estimation errors between the LPTN and experimental test bench measurements result. To improve the estimation accuracy particle swarm optimisation (PSO) is applied for strategic fitting of uncertain model parameters with respect to a maximum likelihood cost function. To avoid converging in suboptimal local minima, which is a typical problem of gradient-based standard optimisation methods, the meta-heuristic PSO is utilised for the resulting multi-variable, non-linear and constrained optimisation problem. For the identification process experimental training data is used which is statistically independent from the (cross-)validation profiles. As a result the maximum estimation error (worst-case) regarding all considered motor component temperatures can be drastically reduced to 8 °C.

Evaluation of on-state voltage V CE(ON) and threshold voltage V th for real-time health monitoring of IGBT power modules

Abstract: This paper investigates by experiment and simulation the use of the real-time measurements of on-state voltage V CE(ON) and threshold voltage V th for real-time health monitoring of IGBT power modules. A study of the dependencies of each parameter on temperature, wear-out mechanisms and operating conditions is presented. Online measurement circuits are developed to obtain these two parameters during the normal operation of power converters. Junction temperature estimation for health monitoring is implemented using V th which is a thermo-sensitive electrical parameter. The data of V CE(ON) , V th and T J are combined in a residual-based health monitoring framework which allows the discrimination between two dominant failure mechanisms of power modules: wire-bond lift-off and solder fatigue.

Integrated inductor for interleaved operation of two parallel three-phase voltage source converters

Abstract: This paper presents an integrated inductor for two parallel interleaved Voltage Source Converters (VSCs). Interleaving of the carrier signals leads to improvement in the harmonic quality of the resultant output voltage and the line current filtering requirements can then be reduced. However, the instantaneous potential difference, caused by the interleaved carriers, may drive large circulating current between the parallel VSCs and an additional inductor is often placed in the circulating current path to suppress the current to an acceptable limit. Integration of both line filter inductor and circulating current filter inductor is proposed. The flux in the magnetic structure is analyzed and the values of the line filter inductance and circulating current filter inductance are derived. Steady-state and the transient performance of the system has been verified by means of simulation and experimental results.

Systematic comparison of modular multilevel converter topologies for battery energy storage systems based on split batteries

Abstract: The advent of the modular multilevel converter has spawned a new breed of battery energy storage systems, able to connect directly to the medium voltage grid without a mains transformer. This paper compares the three most prominent variants: The single-delta bridge-cell (SDBC), the single-star bridge-cell (SSBC) and the double-star chopper-cell (DSCC). The comparison is based on a pareto optimal design approach, respecting the trade-off between power conversion efficiency and size of the passive components, which present important driving factors of the overall system costs. To make the assessment fair, the total semiconductor die area is equalized among all candidates and the switching frequency is controlled such that the average switched power is equal for all three converters.

Dynamic control and dead-time compensation method of an isolated dual-active-bridge DC-DC converter

Abstract: This paper presents dynamic control and a dead-time compensation method of an isolated dual-active-bridge (DAB) dc-dc converter. Conventional phase-shift control methods for the DAB converter may cause dc offsets in both inductor current and transformer magnetic flux density in transient states. The dynamic control method in this paper independently controls the diagonal switches in each H-bridge converter to modify the duty ratios in transient states. This method enables both inductor current and transformer magnetic flux density to be settled within a half switching period without any dc offsets. Moreover, the phase-shift error caused by the dead time is analyzed in terms of the switching angle. This analysis reveals that the dead time causes the switching-angle error in only one of the two H-bridge converters and may cause dc offsets. A new dead-time compensation method based on this analysis is proposed to eliminate the dc offsets and to achieve a good dynamic response. A 5-kW experimental system verifies the validity of the proposed control method.

Performance of a new fast switching DC-Breaker for meshed HVDC-Grids

Abstract: For the future demand of electrical energy, both a reliable and an environmental friendly solution is of high importance. The integration of many widely dispersed regenerative energy sources imposes a great challenge for the existing HVAC grids. A meshed, multiterminal HVDC-SuperGrid is considered to be the best and most efficient solution. This concept would be realized as an overlay grid alongside existing AC-grids, which enables large and controllable energy exchanges over long distances. Extremely high reliability and availability of this HVDC-Grid will become a key point in future, obviously. Fast acting, robust DC-Breakers will be necessary for this purpose, too. This paper presents a new thyristor based DC-Breaker of high switching speed and robustness. The generic requirements for HVDC-Breakers are discussed alongside with known proposed concepts. Afterwards, the characteristics and the dimensioning of the new DC-Breaker is presented and investigated in detail. Measuring results from a test setup show the capabilities and advantages of the new topology.

Accurate and computationally efficient modeling of flyback transformer parasitics and their influence on converter losses

Abstract: Emerging renewable energy applications, such as PV micro inverters, demand for high step-up isolated DC-DC converters with high reliability and low cost, at high efficiency. Thanks to its low part-count the flyback converter is an optimal candidate for such applications. To achieve high efficiency over a wide load range, a decent transformer design must be performed, considering also the effects of the transformer parasitics. Therefore this work analyzes the influence of the transformer parasitic capacitances and leakage inductance in such a way, that it presents a complete tool to consider the transformer parasitics in the flyback-converter design process. A loss-analysis is performed for all three operation modes of the flyback-converter and methods for modeling the parasitic elements are discussed. To model the frequency dependence of the leakage inductance a new method is proposed. The applied models are explained in-depth and verified with measurements on prototype transformers.

Power decoupling with autonomous reference generation for single-phase differential inverters

Abstract: The second-harmonic power ripple in single-phase inverter may introduce the issue of low reliability and low power density. In order to replace the bulky dc-link capacitor, an alternative approach is to use active power decoupling so that the ripple power can be diverted into other energy storages. However, the performance of existing active power decoupling methods depends heavily on certain control references, which unfortunately are parameter dependent. In this paper an autonomous reference generation technique is proposed for single phase differential inverter without relying on the system parameters. A unified average switching model is firstly derived for Buck, Buck-Boost and Boost differential inverter, which is apparently have nonlinear characteristics. Then, dual-loop cascading control and a feedback linearization method is employed to design the inner- and outer loop controllers, which are used for realizing an improved power decoupling control, capacitor voltage and inductor current regulation. By substituting the corresponding parameter into unified model, the proposed control loop can be applied to different inverter types (Buck, Buck-Boost and Boost). Finally, detailed laboratory prototype experimental results have been done to verify the effectiveness of this power decoupling method.

A four-level π-type converter for low-voltage applications

Abstract: This paper has introduced a four-level π-type converter for low-voltage applications which has a simple structure with six switches per phase leg. The line output voltage has seven levels and the output harmonics is much lower than the conventional two-level converter. The switching states and their associated output voltage levels have been analyzed. A simple carrier-based modulation method with zero-sequence signal injection has been devised to modulate the converter and regulate dc-link neutral points' voltages. The two neutral points' voltages can be well controlled with a back to back configuration even under high modulation index and high power factor. Simulation and experimental results have validated the topology, modulation and control strategy for the four-level π-type converter.

Wide-bandwidth Identification of small-signal dq impedances of ac power systems via single-phase series voltage injection

Abstract: The injection of single-phase wide-bandwidth signals into three-phase ac power systems significantly reduces measurement time using the hardware with minimized number of components. Interleaved transformer-less H-bridge converter is optimized to operate as a series voltage injector, increasing the low frequency injection range by avoiding transformer saturation problems. The decoupling control is implemented, providing the effective way to balance dc capacitor voltages and regulate the series injection voltage. The modular and scalable single-phase impedance measurement unit (IMU) is designed and constructed to inject chirp, multi-tone and sinusoidal signals in series with the three-phase ac power systems. The measurement system is designed to inject all three types of signals in the full frequency range, offering a trade-off between measurement time, identification process precision, and number of identification points. The effectiveness of the proposed identification approach is verified with the online estimation of source and load impedances of actively controlled programmable voltage source and a three-phase resistive load.

Multiagent based distributed control for operation cost minimization of droop controlled AC microgrid using incremental cost consensus

Abstract: Microgrid, as a promising technology to integrate renewable energy resources in the distribution system, is gaining increasing research interests recently. Although many previous works have been done based on the droop control in a microgrid, they mainly focus on achieving proportional power sharing based on the power rating. With various types of distributed generator (DG) units in the system, factors that closely related to the operation cost, such as fuel cost and efficiencies of the generator should be taken into account in order to improve the efficiency of the whole system. In this paper, a multiagent based distributed method is proposed to minimize operation cost of the AC microgrid. Each DG is acting as an agent which regulates the power individually using proposed frequency scheduling method. Optimal power command is obtained through carefully designed consensus algorithm with only light communication between neighboring agents. Case studies verified that the proposed control strategy can effectively reduce the operation cost.

A converter control field bus protocol for power electronic systems with a synchronization accuracy of ±5ns

Abstract: This paper is presenting a field bus protocol for modular converter systems. It is optimized for minimal cycle times and synchronisation of the converter modules to ±5ns. The principle of operation is shown in detail, implemented on an FPGA based prototype system and validated by multiple measurements.

1-MW solar power inverter with boost converter using all SiC power module

Abstract: Recently, the market of the PV power plant is growing up in the Asian market. In the PV power plant, typically, an inverter which has the rated power of few hundreds kVA is applied to feed the power to the grid. A 1-MW solar power inverter which employs all SiC Power Modules has been developed. The developed solar power inverter consists of two conversion stages, first stage is a boost converter and second stage is a T-type NPC inverter. A chopper module in the boost converter is configured with SiC-based MOSFETs and Schottky Barrier Diodes, and 48 chopper modules are used in parallel. Each chopper module is controlled individually. The T-type NPC inverter stage is configured with Si-based IGBTs and RB-IGBTs. In this paper, the circuit configurations of the developed solar power inverter, employed SiC-based power devices, and the control scheme are described in detail. In the end, the total efficiencies for the minimum, nominal, and maximum DC voltages are experimentally measured. The measured efficiency at the rated output power varies from 98 % to 98.6 % depending on the values of DC input voltage. The maximum efficiency of 98.8 % is achieved in the case of maximum DC input voltage.

A new package with kelvin source connection for increasing power density in power electronics design

Abstract: Nowadays, there is a high demand from the power electronics market in having systems more efficient and compact. Packaging has become one of the focus factors in power electronics. With more sophisticated products being developed today, in term of Silicon device like Power MOSFETs and IGBTs, there are higher loads and stresses being put on the packaging of these semiconductor materials. Today the silicon technology of a power transistor has reached very high integration, having day by day a smaller die size (smaller dimensions and increased performances). So the impact of the parasitic phenomena starts to become more significant than before. In this paper, the parasitic effect of the internal source connection between the die and the package of IGBT will be analyzed. A new Four-lead TO-247 package for the Field stop II IGBT has been will be introduced. The advantage of the kelvin connection will be validated against the normal TO-247 in a 4kW Double-switch-forward-converter prototype. A comparison will be shown between a conventional TO-247 and an innovative one that allows withdrawing the effect of the parasitic source inductance.

Configurable Modular Multilevel Converter (CMMC) for flexible EV

Abstract: Today, there is a long-term trend towards bi-directional and flexible charger functions for not only drawing current from the grid but also feeding excess energy back into it. The Configurable Modular Multilevel Converter (CMMC) offers a large flexibility in order to handle the different voltage levels and current intensities in electric vehicle's battery charging and discharging. Therefore, this paper is devoted to demonstrate the feasibility of the CMMC for Flexible Electric Vehicle (Flex-EV). It is a smart method to achieve a high power density and a compact power conversion for electric vehicles. The concept allows increasing the electric vehicle (EV) fault ride-through capabilities under damaged battery cells. It allows interfacing EV to worldwide charging infrastructures, meaning from standard household single phase socket to direct current (DC) ultra-fast charging stations. This concept is based on integration of the motor subsystem, the battery management subsystem as well as the universal and flexible charging subsystem. In fact, the power conversion consists of a modular multilevel converter (MMC) with split integrated storage (SIS) based on battery modules.