In this paper, the concept of an all-dc wind park with series-connected turbines is investigated as an alternative to the classical ac parallel or radial wind park. This paper presents a literature overview of all-dc wind park concepts with series connection. A three-phase conversion system with permanent magnet machine, ac-ac converter, high-frequency transformer, and diode bridge rectifier is suggested in this paper for the series connection of dc turbines. The dc series park with the suggested conversion system is compared in terms of losses, cost, and reliability to the state-of-the-art park configuration which is the ac radial park with HVDC transmission. It is found that the dc series park becomes comparable with the ac radial design for high ratings of the dc turbines. Furthermore, the comparison shows that emphasis must be put on reducing the losses in the conversion system of the dc turbine and, particularly, the ac-ac converter. Therefore, the efficiency of the ac-ac converter is compared for three different topologies: the direct matrix converter, the indirect matrix converter, and the conventional back-to-back converter. The direct matrix converter is found to be the most efficient, suitable for the suggested conversion system.

An All-DC Offshore Wind Farm With Series-Connected Turbines: An Alternative to the Classical Parallel AC Model?

Reduced matrix converter (RMC) is a convenient topology for offshore wind farm due to its potential to reduce the size and weight of the converter, to improve the reliability by removing the electrolytic capacitor, and to increase the efficiency inherent to less stages of conversion. Moreover, it is a very flexible topology which permits different types of operation with a simpler modulation compared with conventional three-phase matrix converter. This paper investigates different modulation strategies applied to RMC for offshore wind farms, focused on efficiency improvement of the entire convention system. Simulation results using a detailed loss model for high-power level are presented. Four cases are investigated according to the modulation strategies (space vector modulation and carrier-based modulation) and the operation principle (current source converter or voltage source converter). Losses in the clamp circuit are also calculated. Different wind velocities are considered in the simulations. Results show that current source operation with space vector modulation presents minimum losses at nominal wind velocity. This operation is suitable for series connection of offshore wind farms which has been reported as the most efficient alternative from the grid losses point of view.

A Study of Efficiency in a Reduced Matrix Converter for Offshore Wind Farms

The Matrix Converter (MC) evolved from the forced commutated cycloconverters and has been extensively investigated for more than thirty years. In this publication, the milestones in research on MCs in academia and industry are reviewed and presented chronologically and thematically ordered. The major contributions in the fundamental topic areas such as the development of the topology, topological extensions, commutation, modulation, loss calculation, control, or filtering and EMC are compiled and then expanded with examples of the latest activities in the corresponding field of research. In addition, an overview of the publicly reported research on MCs in industry is provided and the development of the commercialized MCs is briefly summarized. This review concludes with a comparison of the MC with the Voltage DC-Link Back-to-Back Converter (V-BBC) and a discussion of the current status of the MC technology and its future potential. The key contributions in the development of the MC can be summarized as follows: the investigation of the basic MC concept by Venturini in 1980 [19], the development of the multi-step commutation by Burany and Oyama et al in 1989 [64,65] to solve the commutation problem of the bidirectional switches, and finally the space vector representation of the modulation by Huber and Boroyevich in [80] to enable a consistent mathematical description of the converter system from the source to the load. With these fundamental features, the MC (CMC) is operational and can be used as a direct ac-ac converter with variable voltage and frequency transformation capability. The world’s first commercial MC was presented by the Japanese drive manufacturer Yaskawa in 2005 with the product name “Varispeed AC”. This converter series is based on the CMC topology and is implemented with RB-IGBTs. One year later in 2006, Fuji Electric announced also a new MC product, the Frenic-MX, which, however, is currently not anymore part of Fuji’s product portfolio. Meanwhile, Yaskawa extended their matrix converter product line with a medium voltage MC series. Despite intensive research for the last three decades, MCs have until now only achieved low market penetration with Yaskawa Electric as the only drive manufacturer currently offering MCs as commercial products. The reason for the low usage of the MC technology in industry is mainly due to the intrinsic, physical limitations given by the MC concept, such Fig. 1. Basic three-phase Matrix Converter topologies: (a) single-stage half-bridge or Conventional (direct) Matrix Converter (CMC) and (b) two-stage Indirect Matrix Converter (IMC)

Milestones in Matrix Converter Research

An MC is a direct AC-AC converter that has benefits of high reliability, small size, and high efficiency over a conventional PWM converter-inverter system. This article introduced the Yaskawa MC product specifications, applications, and techniques that helped commercialization of a low-voltage and medium-voltage MC. The 9-114 kVA (200 V and 400 V classes) for a low voltage MC and 200-6,000 kVA (3.3 kV, 6.6 kV) for a medium-voltage MC have thus far been commercialized. Three typical applications of an MC were also introduced along with the related technology that helped to achieve desired results for each application. The application of an MC is still expanding to other fields in industry. Authors are continuing their efforts in improving performance and expanding the product lineup.

Development of MCs and its Applications in Industry [Industry Forum]

A power conditioning system consisting of a Super-conducting Magnetic Energy Storage (SMES) which can be used to smooth out differences between output power of a generator and power loads is analyzed in this paper. The SMES is connected in shunt with the distribution lines through a converter. The converter is a voltage source converter type with a DC-chopper. The system is controlled by three regulators; they are working on the DC-link voltage, the power to and from the SMES and the AC-currents in the inverter. The regulators controlling the power flow and the AC-currents are cascaded and the DC-link voltage controller is situated in an independent loop with its voltage kept constant using the SMES. Simulations have been carried out using the software PSCAD/EMTDC.

Superconducting Magnetic Energy Storage (SMES) in power systems with renewable energy sources

In this paper, simple simulation methods to calculate power semiconductor loss and instantaneous junction - case temperature difference in a power module of a matrix converter are proposed. The validity of the proposed simulation method for loss calculation is confirmed through experiment using a 22 kW test set-up of the matrix converter. By using the simulation method for temperature calculating, the influence of the output frequency on the junction - case temperature difference is investigated. Moreover, the effect of using a novel IGBT (RB-IGBT) that has reverse blocking capability is discussed. It is shown that the efficiency of a matrix converter using the RB-IGBT is higher by 1.3 points than that of a conventional PWM rectifier and inverter system.

H. Umida

Papers

Analysis of loss and junction temperature in power semiconductors of the matrix converter using simple simulation methods