Study on fully direct current integrated offshore wind farm

Introduction to HVDC transmission

Power transmission is an important factor in the increase of cost of offshore wind farms. HVDC transmission is the only alternative when the distance from the wind farm to shore is beyond 50-80 km, but the converter station costs are very high. Power losses in the distribution and transmission lines have become a concern for wind farm owners and a reduction of a few tenths of a percentage point becomes a mayor source of revenue and a significant increase in the profitability of the wind farms.

Power transformers are usually one of the bulkiest and most expensive components in power converters and they introduce a power loss of around 1 % in the wind turbines and the HVDC converters. Suitable power transformers are seldom available as standard components and they must be specified and designed during the converter design process.

There is a significant increase in the interest of high frequency transformers and DC grids in distribution systems. The opportunities for size, cost and loss reduction with the introduction of high frequency in wind turbines are clear but it is still technologically very challenging to build this type of transformer in the MW range.

New technologies must be developed in the following fields:

• Connection between high voltage static submarine cables and floating platforms or vessels;
• HVDC system cost reduction;
• Cable installation at sea bed depths beyond 1000 m;
• Direct drive of Very High Voltage generators from the DC bus in HVDC VSC systems;
• Low power converters fed from high or medium voltage DC lines.

This thesis aims to introduce high frequency power transformers in the power stage of wind turbines to reduce the size, power loss and cost of the power stage.

This thesis presents a fully DC integrated system for the elimination of power stage redundancies and an overall reduction of power loss and cost of the wind farm system.

The document describes of a SWHFR DC/C converter using high frequency transformer for direct connection to DC lines. The document explains the use of the leakage inductance of the transformer as an inherent element of the converter to shape the current waveforms. Interleaved operation of series connected SWHFR is introduced to achieve low output current and voltage ripple.

Last, the thesis presents the operation of a controlled three phase rectifier fed from a square wave three phase inverter through a transformer (CSWHFR). The effect of the transformer leakage inductance and the resulting operating modes have been described in detail, as well as the operation in continuous and discontinuous mode. A combination of SWHFR and CSWHFR can be used to obtain very efficient DC/DC conversion for connection to medium and high voltage lines.

While most of the power is delivered through very efficient diode rectifiers, input DC bus voltage control can be obtained by means of a low voltage rating CSWHFR converter.

Objectives of the study

This work intends to study the transmission system of offshore wind farms and study alternatives for the reduction of cost of DC transmission and distribution architectures.

This main goal is further broke down in the following objectives:

• Study the state of the art of wind turbine power conversion systems.
• Analysis of the existing alternatives for transport of offshore electric power.
• Study the state of the art of DC transmission and distribution systems.
• Introduction of high frequency transformers for cost size and loss reduction in offshore wind farms.
• Evaluation of the possibility of eliminating the offshore VSC Power Converter in conventional HVDC VSC systems.

Contents of the thesis

This thesis consists of 7 chapters and 2 annexes. The technology used in offshore wind farms is studied, with emphasis in the transport of electrical power. Besides this introductory chapter, the contents of the document are as follows.

Chapter 2 describes briefly the technology of the power stage of wind turbines, with a discussion on present and future trends as conclusions.

Chapter 3 introduces the available technology for power transmission of offshore wind farms. The main technologies (HVAC and HVDC VSC) are described, including a detailed study of the accessories and additional equipment such as cable and platform technology. Although HVDC LCC has not been used in offshore transmission systems, a brief introduction has been included.

The chapter includes a review of the state of the art in HVDC offshore power transmission.

The next chapters describe an alternative for fully DC wind farms with high frequency transformers.

Chapter 4 presents the behaviour of a Square Wave High Frequency converter. The voltage and current waveforms are described in detail, and the relation between input and output voltage dependency on converter parameters is studied. The chapter also includes the description of the operation of series interleaved SWHFR connection for output voltage and current reduction.

Chapter 5 describes the power losses of the proposed architecture. The reduction in power loss of direct connection of wind turbines to DC transmission lines, or even the use of intermediate wind farm DC grids is studied.

Chapter 7 briefly summarizes the results of the previous chapters, lists the published papers based on this thesis and discusses further field of study derived from this work.

Annex A illustrates the method for equalizing diodes and thyristors connected in series for high output voltage rectifiers and annex B describes the main design parameters of the high frequency transformers.