Nathalie Holtsmark

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

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.

Matrix converter efficiency in a high frequency link offshore WECS

An offshore wind park layout of series connected turbines with HVDC transmission is presented in this paper. The Wind Energy Conversion System (WECS) features a matrix converter as full converter interface. The efficiency of the matrix converter is the object of this paper and a preliminary loss investigation is conducted to determine which matrix converter topology and switch configuration are the most efficient.The two different topologies of the matrix converter that are investigated by simulation are the conventional and indirect matrix converter. Furthermore the losses in the matrix converter will be computed for different implementations of the switches, using both RB-IGBTs connected in anti-parallel and IGBTs in anti-series with diodes in anti-parallel. RB-IGBTs are not available at high ratings and series and parallel connections of small rating devices are necessary. High rating IGBTs and diodes are available. However they present poorer switching behavior than small rating devices. Thus the losses are computed both for high rating IGBTs and diodes and series and parallel connections of low rating IGBTs and diodes.

Reactive power compensation using an indirectly space vector-modulated matrix converter

This paper investigates the implementation of a matrix converter for shunt reactive power compensation. The input of the matrix converter can be connected to the power grid and the output to a Permanent Magnet Synchronous Machine (PMSM). By controlling the input displacement angle Φi, the reactive power flowing into the grid can be controlled.

Speed regulation of a wind turbine with current source or matrix converter: Tuning procedure

In DC series-connection of wind turbines the common variable for the turbines is the DC current and thus the simplest control approach is to use current source type converter to perform the speed control of the generators for maximum power extraction. When using current source type converters like the Current Source or the matrix converter, LC filters are needed at the input of the generator to remove switching harmonics. Thus resonance can be an issue and special care needs to be taken when designing controllers. In this article a wind farm topology with series connected turbines is presented along with the wind energy conversion system inside in which a matrix converter performs speed control of the generator. To counteract resonance, three nested loops controlling respectively the turbine rotational speed, the stator current and the filter capacitor voltage are implemented. The tuning should be performed carefully so that the control is fast, stable and tracking the maximum power point. The system model is analyzed in the dq reference frame to find appropriate controllers for each loop and a method is presented to tune each controller in a systematic manner for a satisfactory system response. Simulation results are presented.

Reactive power compensation capability of a matrix converter-based FACTS device

In this paper a shunt FACTS device composed of a matrix converter and a permanent magnet machine is suggested for reactive power compensation at the point of connection to the grid. The device can either provide pure reactive power compensation or reactive power compensation in combination with active power control. The conventional indirect space vector modulation (ISVM) scheme was first chosen to modulate the matrix converter because of its simplicity. The modulation is described and its impact on the reactive power range of the FACTS device is analyzed. The ISVM presents some serious limitations for the reactive compensation range at low matrix converter output power factors and the FACTS cannot provide pure reactive power compensation with this modulation. The use of hybrid modulation, based on ISVM, is suggested to remedy these issues and the new reactive compensation range is calculated. A simulation model of the FACTS device was implemented and the results of the simulation study are confronted with the analytical results.

Matrix converter modulation for series-connected wind turbines with high frequency link

In this paper a matrix converter modulation adapted for the wind energy conversion system of series-connected turbines is presented. The considered conversion system is composed of a permanent magnet synchronous generator, a matrix converter, a high frequency transformer and a diode bridge rectifier for the AC-DC conversion. This conversion system topology allows for a modification, introduced in this paper, of the indirect space vector modulation that increases the current ratio of the matrix converter and thus enhances its advantage for this wind park configuration. The modulation can be further modified into a square wave modulation for high frequency operation of the transformer. An efficiency analysis of the suggested modulation and its high frequency version is performed. Also some attention is given to the flux balance issue in the transformer and a possible solution for a modulation yielding balanced flux is given.

Loss comparison of matrix and back-to-back converters for offshore WECS

In this paper is presented a wind park layout of series-connected wind turbines. The wind energy conversion system (WECS) inside the nacelle is described and the main technical challenges presented by this wind park topology are briefly discussed. The WECS requires a full scale AC-AC converter as the generator is a Permanent Magnet (PM) synchronous machine. The matrix converter is an interesting candidate for offshore applications because it is compact and reliable as it does not feature the DC link capacitor which is required by the conventional back-to-back (B2B) converter. Performance of the matrix converter in terms of efficiency is the object of this paper. Losses of both direct and indirect matrix converter are assessed by simulation and compared with those of the B2B converter showing that the matrix converter performs better. Different switch implementations are also included in the comparison: single high rating IGBTs and diodes or series and parallel connections of low rating IGBTs and diodes and Reverse Blocking IGBTs. The series and parallel connection of low rating IGBTs and diodes is found to perform superiorly.

Thermal analysis of matrix and Back-To-Back converters for series-connected wind turbines

The efficiency and thermal performance of AC-AC converters in series-connected DC wind turbines is the object of this paper. The considered conversion system is composed of a permanent magnet synchronous generator, an AC-AC converter, a high frequency transformer and a diode bridge rectifier. Two different converter topologies are compared: the Back-to-Back converter and matrix converter, in terms of efficiency and thermal performance. The modulations used for each converter are presented. The control for each converter is described as well as the method used for loss and temperature calculations. The results show that the matrix converter is more efficient than the back-to-back converter, however in terms of thermal performance and temperature fluctuation, the back-to-back is slightly superior.

Comparative study of the efficiency and power density of offshore WECS with three-phase AC-link

In this paper three converter topologies are compared for the AC-AC conversion stage of an offshore wind turbine with permanent magnet synchronous generator. The WECS (Wind Energy Conversion System) is based on four stages: an input filter, an AC-AC converter, a three-phase high-frequency transformer and a full-bridge diode rectifier. The conventional back-to-back converter, the matrix converter and the indirect matrix converter are the three selected topologies. Analytical models and numerical simulations are considered in order to evaluate the WECS efficiency and power density as a function of the operational frequency of the transformer. It was found that for range of transformer frequencies from 100[Hz] to 10[KHz], the direct matrix converter topology was the most efficient topology and had the highest power density. In general, the back-to-back converter topology presents the lowest efficiency and power density in the range of frequencies considered.