This paper reviews the trends in wind turbine generator systems. After discussing some important requirements and basic relations, it describes the currently used systems: the constant speed system with squirrel-cage induction generator, and the three variable speed systems with doubly fed induction generator (DFIG), with gearbox and fully rated converter, and direct drive (DD). Then, possible future generator systems are reviewed. Hydraulic transmissions are significantly lighter than gearboxes and enable continuously variable transmission, but their efficiency is lower. A brushless DFIG is a medium speed generator without brushes and with improved low-voltage ride-through characteristics compared with the DFIG. Magnetic pseudo DDs are smaller and lighter than DD generators, but need a sufficiently low and stable magnet price to be successful. In addition, superconducting generators can be smaller and lighter than normal DD generators, but both cost and reliability need experimental demonstration. In power electronics, there is a trend toward reliable modular multilevel topologies.

Trends in Wind Turbine Generator Systems

It is well known that single-phase pulse width modulation rectifiers have second-order harmonic currents and corresponding ripple voltages on the dc bus. The low-frequency harmonic current is normally filtered using a bulk capacitor in the bus, which results in low power density. However, pursuing high power density in converter design is a very important goal in the aerospace applications. This paper studies methods for reducing the energy storage capacitor for single-phase rectifiers. The minimum ripple energy storage requirement is derived independently of a specific topology. Based on the minimum ripple energy requirement, the feasibility of the active capacitor's reduction schemes is verified. Then, we propose a bidirectional buck-boost converter as the ripple energy storage circuit, which can effectively reduce the energy storage capacitance. The analysis and design are validated by simulation and experimental results.

A High Power Density Single-Phase PWM Rectifier With Active Ripple Energy Storage

Interleaving technique is used in some applications due to its advantages regarding filter reduction, dynamic response, and power management. In dual battery system vehicles, the bidirectional dc-dc converter takes advantage of this technique using three-to-five paralleled buck stages. In this paper, we propose the use of a much higher number of phases in parallel together with digital control. It will be shown that this approach opens new possibilities since changes in the technology are possible. Thus, two 1000-W prototypes have been designed using surface mount technology devices (SO-8 transistors). An additional important feature is that due to the accuracy of the digital device [field-programmable gate array (FPGA)], current loops have been eliminated, greatly simplifying the implementation of the control stage

Automotive DC-DC bidirectional converter made with many interleaved buck stages

Power density of power electronic converters in different applications has roughly doubled every 10 years since 1970. Behind this trajectory was the continuous advancement of power semiconductor device technology allowing an increase of converter switching frequencies by a factor of 10 every decade. However, today's cooling concepts, and passive components and wire bond interconnection technologies could be major barriers for a continuation of this trend. For identifying and quantifying such technological barriers this paper investigates the volume of the cooling system and of the main passive components for the basic forms of power electronics energy conversion in dependency of the switching frequency and determines switching frequencies minimizing the total volume. The analysis is for 5 kW rated output power, high performance air cooling, advanced power semiconductors, and single systems in all cases. A power density limit of 28 kW/dm3@300 kHz is calculated for an isolated DC-DC converter considering only transformer, output inductor and heat sink volume. For single-phase AC-DC conversion a general limit of 35 kW/dm3 results from the DC link capacitor required for buffering the power fluctuating with twice the mains frequency. For a three-phase unity power factor PWM rectifier the limit is 45 kW/dm3@810 kHz just taking into account EMI filter and cooling system. For the sparse matrix converter the limiting components are the input EMI filter and the common mode output inductor; the power density limit is 71 kW/dm3@50 kHz when not considering the cooling system. The calculated power density limits highlight the major importance of broadening the scope of research in power electronics from traditional areas like converter topologies, and modulation and control concepts to cooling systems, high frequency electromagnetics, interconnection technology, multi-functional integration, packaging and multi-domain modeling and simulation to ensure further advancement of the field along the power density trajectory.

https://www.hpe.ee.ethz.ch/uploads/tx_ethpublications/IEEJ_PowerDensity_Paper_FinalRevised_03.pdf

PWM Converter Power Density Barriers

There is a growing interest on plug-in hybrid electric vehicles (PHEV's) due to energy security and green house gas emission issues, as well as the low electricity fuel cost. As battery capacity and all-electric range of PHEV's are improved, and potentially some PHEV's or EV's need fast charging, there is increased demand to build high power off-board charging infrastructures. A charge station architecture for municipal parking decks has been proposed, which has a DC microgrid to interface with multiple DC-DC chargers, distributed renewable power generations and energy storage, and provides functionalities of normal and rapid charging, grid support such as reactive and real power injection (including V2G), current harmonic filtering and load balance. Several non-isolated bidirectional DC-DC converters suited for charge station applications have been reviewed and compared, as the major focus of this paper. Half bridge converter is a good candidate but it is difficult to maintain high efficiency in wide battery pack voltage range. A variable frequency pulse width modulation (VFPWM) scheme is proposed to mitigate this issue. Finally three-level bi-directional DC-DC converter is suggested to be employed in this application. A 10kW prototype verifies that 95.1–97.9% full load efficiency can be achieved in charging mode with 180–360V battery pack voltage. In addition, the inductor size is only one third of the half bridge counterpart, which is a great advantage for high power converters.

Review of non-isolated bi-directional DC-DC converters for plug-in hybrid electric vehicle charge station application at municipal parking decks

Interleaving in synchronous rectifiers can lead to reduced losses in both the active and passive components. However, this depends on selecting the correct number of phases and the correct phase inductance for a particular application and requirements. In this paper optimizing the number of phases and the phase inductance is considered to maximize the interleaved synchronous rectifiers efficiency over the desired operating range. To do this, the RMS currents and losses in the bus capacitors, the phase inductances and the switching devices as a function of the number of phases and duty cycle are considered. Generic equations are presented and used to predict the RMS currents in the passive components with some non-intuitive results especially concerning the bus capacitors. It is shown in the paper, that the optimum number of phases is dependent on the converter parameters such as the phase inductance and operating requirements. Practical results are presented confirming the synchronous rectifier loss model.

Interleaving optimization in synchronous rectified DC/DC converters

Generally, passive components and inductors in particular, contribute greatly to the overall volume of power electronic converters. These components are normally packaged individually with little concern for the overall system. For high density switching power supplies it is imperative to minimise the volume to as great an extent as possible. which implies that the passive component volume usage needs to be improved. This can be accomplished by applying suitable packaging and cooling techniques to these components. In this paper, two inductor structures finding application in a 2.1 kW synchronous automotive converter are described. The airgap placement, losses, cooling methods and thermal profiles are analysed and verified experimentally with an inductor designed for operation at 85/spl deg/C ambient.

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A very high density, heatsink mounted inductor for automotive applications

In this paper, a very high power density DC/DC converter module for automotive applications is investigated. The 14-V/42-V converter is specified to operate at a power level of 2.1 kW with a water cooled heat sink at 85/spl deg/C. The design and implementation of very high density passive components are discussed. Using the results of the passive component design, a prototype converter is built, achieving a final power density of 170W/in/sup 3/. The thermal performance of the passive components and the converter module under different electrical and thermal excitations is investigated and recorded. Results are presented over the full excitation range.

/pdf/high-density-packaging-of-the-passive-components-in-an-5giguj6brn.pdf

High density packaging of the passive components in an automotive DC/DC converter

The integration of an electrical machine and power electronics promises many benefits, the largest being an increase in power density. The use of system integration may lead to an increase in loss density and local or component temperatures. For this reason much attention needs to be paid to the thermal design aspects of the system. In the aerospace industry weight and reliability are of significant importance. In this paper the integration of a permanent magnet machine and power electronics into a single structure with joint thermal management is investigated. An overall goal of 2 kW/kg is set for system power density. Additionally in this paper fault tolerance is introduced into the design. This will allow the system to continue functioning should a single electrical fault occur. Principle faults are identified and discussed. A 50 kW, 6 phase, 8 pole permanent magnet machine is presented with a rotor speed of 50 krpm. A symmetrical phase shifted full bridge is used to drive the permanent magnet machine, aiding the functionality of fault tolerance and enabling the use of soft switching. A mathematical model for both machine and power electronics is created and used to calculate phase currents and losses in the machine and power electronics. Finally, spatial integration and thermal management concepts are presented and analysed.

A 50kW integrated fault tolerant permanent magnet machine and motor drive

Power electronic systems that are implemented in the automotive environment are expected to operate under harsh electrical and thermal conditions while achieving high power densities. One example of such a power electronic system is the bi-directional 14/42 V DC/DC converter that is utilized in the dual voltage networks of ultra modern vehicles. This paper presents the design and implementation of such an automotive converter as an integrated system module (ISM). The presented prototype achieves a high power density (120 W/in/sup 3/) within the automotive environment while using the engine liquid coolant having a maximum temperature of up to 125 /spl deg/C for cooling. The complete power conversion system is implemented in the ISM requiring dedicated thermal management structures for all the components, both active and passive. An overview of the design and evaluation of the automotive ISM, as well as the implementation of the novel thermal management concept required for the passive components is presented and supported with practical results.

M. Gerber

Papers

Interleaving optimization in synchronous rectified DC/DC converters

A very high density, heatsink mounted inductor for automotive applications

High density packaging of the passive components in an automotive DC/DC converter

A 50kW integrated fault tolerant permanent magnet machine and motor drive

Design and evaluation of an automotive integrated system module