Application of Flexible AC Transmission System Devices in Wind Energy Conversion Systems

Overview of Wind Energy Conversion Systems and Flexible AC Transmission Systems

Abu-Siada et al.*

Electrical and Computer Engineering Department Curtin University, Australia

Abstract

Flexible AC transmission system (FACTS) is a technology that consists of a variety of power electronic devices which was developed with the aim of controlling both power and voltage at certain locations of the electricity grids during disturbances, improving the existing transmission line capacity and providing a controllable power flow for a selected transmission direction. This chapter provides a general overview for variable FACTS devices, concepts and topologies. It also provides brief information about various wind energy conversion systems.

Keywords: Flexible AC transmission systems, Wind energy conversion systems.

* Corresponding author A. Abu-Siada: Electrical and Computer Engineering Department Curtin University, Australia; Tel/Fax: ??????????; E-mail[email protected]

This chapter has been derived from author’s Thesis entitled Application of Unified Power Flow Controller to Improve the Performance of Wind Energy Conversion System by Yasser Alharbi

Introduction

The increase in human population in the last few decades has been associated with concerns as to the corresponding rise in demand for life-supporting resources such as water, food and electrical power [1]. As for electrical power, the unparalleled industrial and technology advances are other factors that call for increasing demand in electrical consumption [2]. Globally, the power generation sector is facing significant challenges to meet the increasing demand for power. To date, conventional energy sources including oil, gas and coal are the world’s main sources of energy. Unfortunately, these fossil fuel resources are associated with emissions that can severely harm the environment, with the symptoms being as air pollution, climate change, oil spills and acid rain [3]. Interest in harnessing the benefits of renewable energy has been increasing steadily due to its advantages, which include sustainability, environmental friendly nature and affordable cost. Solar, geothermal and wind resources are among the most promising renewable energy alternatives [4, 5].

As a natural energy source, the radiation delivered by the sun is a promising renewable energy source of electrical power [6]. Nowadays, solar energy is one of the favourable energy alternatives. Photovoltaic (PV) technology is used for converting solar energy to electrical energy [7]. Globally, PV installation has contributed to about 177 GW of electrical power in 2014, and it is expected to deliver more than 1% of the total global electricity demand by the end of 2017

[8]. The geothermal power has the advantage of using fewer infrastructure elements for electrical power generation when compared with other energy sources such as coal or nuclear power [9]. In 2015, the world’s installed capacity of geothermal power reached 12.635 MW, and this is expected to reach about 21.441 MW by 2020 [10]. Since the early stages of using natural energy resources, wind power has been considered as a main renewable energy source. And nowadays, there is a rapid increase in the utilization of wind energy [11], which has led to significant advancement in wind energy technology, including wind turbine design and sizing, and integration of wind turbines with existing electricity grids.

Wind Energy System

Wind energy has become one of the most popular renewable energy sources worldwide. In 2014, there was an additional of 51,473 MW of new wind power capacity that was brought into service [12]. Fig. (1.1) illustrates the top 10 installed wind power capacity worldwide during the period from January to December 2014. The diagram shows that China has the highest installed wind power capacity with 23,196 MW generation, followed by Germany at 5,279 MW and USA at 4,279 MW [12].

Fig. (1.1))

Distribution of the top 10 installed wind capacity in 2014.

Figure (1.2) shows the magnitude of the globally installed wind- capacity between 1996 and 2014. It can be seen from Fig. (2.2) that the capacity increased from 197,943 MW 369,597 MW over this period.

Fig. (1.2))

Global production of wind power between 1996 and 2014.

The installed wind power capacity in Australia reached 3,806 MW by the end of 2014, and the installed wind power capacity in the year 2014 was 13% less than that installed in 2013 [12]. Despite the decrease in the newly installed capacity in 2014, the wind energy market is leading Australia towards its goal of using renewable energy to supply 20% of the power requirements by the year 2020 [12].

Wind Turbine

Wind turbines capacity ranges from a few kilowatts for standalone units for houses to several megawatts in a wind farm. Small wind turbines are usually rated below 300kW and have the capability to be combined with other energy sources as generation system at farms and houses to support the need for electrical power. However, the integration of small wind turbine with existing grids is difficult and costly [13]. The wind turbine size and rating have been increasing gradually since 1980 as shown in Fig. (1.3); increasing the size of the wind turbine rotor increases the amount of energy harvested by the wind turbine. In the early stage of wind turbine manufacturing, wind turbine power rating started with 50 kW and a size of 15 m rotor radius but nowadays wind turbines are designed to produce up to 7.5 MW with up to 126 m rotor diameter. A higher rating of 10 MW and associated 160 m rotor diameter is available nowadays as well [13].

Wind Energy Conversion Systems

Both fixed speed and variable speed generator can be used in wind energy conversion systems (WECS). In the early stages of the design of (WECS), wind turbines used to function at fixed speeds. Nowadays, with the new concept of generators and power electronics, variable speed wind turbines dominate the wind turbine market. Fig. (1.4) illustrates different configurations of wind energy conversion systems. A review of the two types of fixed and variable speed systems is given below.

Fig. (1.3))

Evolution of wind turbine size.

Fig. (1.4))

Different Configurations of WECS.

Fixed Speed Wind Turbine

A fixed speed wind turbine (Fig. 1.5) comprises a generator that is directly coupled to the power network and connected to the wind turbine through low-speed shaft, gearbox and high-speed shaft [14].

Fig. (1.5))

Fixed speed WECS configuration.

Fixed speed WECS has the advantages of being simple, inexpensive in addition to the fact that it does not call for a power electronic interface. However, fixed speed wind turbines suffer from the limitation of controlling the power quality, uncontrollable reactive power compensation and high mechanical stress on the shaft sections [14]. The oldest wind energy conversion system topology employed a fixed speed generator (e.g. synchronous generator) that is coupled directly to the AC network, including mechanical dampers in the drive train. Modern fixed speed wind energy conversion systems use induction generators [15]. Fig. (1.6) shows a conceptual scheme of the first fixed speed wind energy conversion system.

Fig. (1.6))

Typical configuration of the first generation of WECS.

Variable Speed Wind Turbine

Technology advancement has driven the wind turbine operation from being of fixed speed mode to variable speed. A variable speed wind turbine consists of a generator driven by a power converter, which facilitates the variable speed operation mode and aids in improving the WECS dynamic performance [14]. Owing to its variable speed mode, more power can be captured, enhanced power quality can be achieved and reduced mechanical stress on the drive train can be accomplished by wind power generators [14-16]. Comparison between the fixed speed wind turbine and variable speed wind turbine is summarized in Table 1.1.

Table 1.1 Comparison of fixed and variable speed-based WECS.

Partly Variable Speed Wind Turbine

Partly variable speed wind turbines (Fig. 1.7) or the so-called type B wind turbines operate in a limited variable speed mode. In this concept, the generator is directly coupled to the AC network and the generator rotor is connected to a variable resistance to control the generator speed. Depending on the variable resistance size, the slip can be increased by up to 10% that allows operation at a partly variable speed in the super synchronous range (i.e. up to 10% above the rated speed). The Danish manufacturer, Vestas, used this design feature in limited variable speed wind turbines since the mid1990’s [14, 15].

Fig. (1.7))

Typical configuration of partly variable speed WECS.

Full Converter Variable Speed Wind Turbine

A full converter variable speed wind turbine that based on a multi-pole synchronous generator is shown in Fig. (1.8). In this type, the generator is coupled to the AC network through a full-scale converter station that facilitates the variable speed operation of the wind turbine. The converter station is a combination of grid side converter and generator side converter connected back to back via a dc link. The generator’s electrical frequency changes with the change in the wind speed, whereas the power network frequency remains unaffected [14].

Fig. (1.8))

Typical configuration of full converter variable speed WECS.

Doubly Fed Induction Generator Wind Turbine

Typical configuration of a doubly fed induction generator (DFIG) wind turbine is shown in Fig. (1.9). Among the variable speed wind turbine generators, DFIG is the most popular technology currently dominating the market of wind turbines, because of its superior advantages over other wind turbine technologies [17]. In this concept, the stator circuit is coupled directly with the power network through a coupling transformer while a back-to-back partial-scale voltage source converter (VSC) connects to the rotor circuit to the grid via the coupling transformer. The VSC facilitates a decoupled control of the generator’s active and reactive power [15].