Power Electronics and Wind Power 030711 - Semantic · PDF fileThe wind is a non-storable energy source, ... (software, design, automatic control, etc.) ... Power Electronics and Wind

Power Electronics and Wind Power

Lars Gertmar *,** * ABB, Corporate Research, Västerås, Sweden

Phone: +46 21 32 31 31 Fax: +46 21 32 32 64 ** Dep. of Industrial Electrical Engineering and Automation

LTH/IEA, Lund University, Sweden E-mail: [email protected] [email protected]

Keywords

Wind Energy, Wind Generator Systems, Adjustable Speed Generation Systems, Renewable Energy Systems, Power Transmission

Abstract

Renewable energy facilities will constitute a major contribution to electrical power in the future. The wind turbine will, together with biomass, form the major renewables parts up to year 2030 including erection of offshore wind farms. The wind is a non-storable energy source, whose electric energy needs priming into interchangeable energy to be competitive for large-scale power generation. An essential task is then to make wind power as commercially valuable and fungible as electric power produced by traditional, dispatched plants such as fossil fuel power plants, hydroelectric plants, nuclear plants and the like. Power electronics is a fundamental candidate when priming wind power. Another candidate is automation starting in the towers and ending in load management. Power electronics and automation shall be cost-effective and reliable. Power electronic hardware shall be common but in moderation because losses and investments must be paid back: by better trading on the power exchange, by availability, by reduced mechanical and/or civil engineering as well as by efficiency and reliability. An overview of wind power from different perspectives: collection & transmission (C&T), energy storage, R&D e.g. in EU and US, existing technology, wind’s future(s) business, etc. is comprised. EPE should be challenged to exploit wind power opportunities.

Introduction

Energy sources for power generation—year 2000 – 2030 – 2060

World electrical energy consumption is rising

by about two percent per year.

So far, fossil fuels have been the main electrical energy source,

but renewable sources are now taking off.

By 2060, more than half the world’s electrical energy

is expected to come from renewable sources.

Fig. 1: A stacked area diagram of today’s and long-term energy sources for power generation. [1] Slightly changed. Diagram also denoted Market Intelligence / Fuel / FU / 120 JW1 / 990531.

Power Electronics and Wind Power GERTMAR Lars

EPE 2003 - Toulouse ISBN : 90-75815-07-7 P.1

Conversion from storable and non-storable energy sources into electrical energy is a major issue for the world. Fig. 1 shows one forecast [1] of sources for power generation, originating from a former oil company, Shell—today it calls itself an energy company and has entered wind power business. The diagram shows a clear trend, that renewable energy facilities like wind energy will be included in the long-term, future electrical power production, taking off 2000 to 2030. But wind is a non-storable, non-dispatched energy source, it needs priming into fungible1 electrical power and thus most likely common but moderate amounts of power electronics in its energy conversion schemes.

The diagram in Fig. 1 shows a forecast of a worldwide, all types, electric power generation, which can be measured as a factor of 3 from 2000 to 2030 and another factor of 3 from 2030 to 2060, in total a factor of 3 during 60 years. This means a coarse figure of ln(3)/60 - 1 0.0183, a growth of 1.8 % per year. Similar but shorter forecasts can be found, e.g., on www.iea.org [2].

The diagram in Fig. 1 shows also that wind energy will contribute with 2000 TWh/year in 2030 or 15 % of today’s energy. 2000 h/year is a coarse annual wind turbine operation time; it means that

1000 GW wind power should be installed before 2030 to compare with today’s traditional power generation capacity amounting 3000 GW worldwide.

On the contents of the paper

The first section is a short introduction to energy sources for power generation and to electricity’s business and physical manifestations. Both business and physics are essential to get a basis for the next parts of the paper.

Quotes are culled from various sources that are rarely brought in front of the EPE audience but essential to evaluate in order to establish power electronic converters as efficient and reliable converters at suitable cost. Such quotes are given through almost all sections.

In the following paragraphs, overviews are there given of the wind power generation area and very briefly of power electronics for wind power, technically available today—intentionally excluding the general main circuits that are easily found elsewhere.

Large-scale renewable energy systems are thereafter covered together with other parts of distributed generation, mainly with aspects from a couple of power utilities point-of-view, in the following section of the paper.

Offshore wind power converters and their collection and transmission systems have been studied around the turn of the millennium. The works reviewed in this paper have mainly been carried out in various constellations at most parts of the North Sea (Nordmeer, Vesterhavet).

ABB’s role and objectives are described; as well as partnerships and activities in R&D; from sales of components to systems approaches. The intention is that renewable energy facilities will be utilised as supplementing electrical energy all over the world in regions with growing energy demands. This

1 fungible, adj. Law a) of goods or commodities; freely exchangeable for or replaceable by another of like nature or kind in the satisfaction of an obligation, http://www.cogsci.princeton.edu/cgi-bin/webwn/?stage=1&word=fungible b) (of goods etc. contracted for, when an individual specimen is not meant) that can serve for, or be replaced by, another answering to the same definition American. Heritage Dictionary c) “Fungible" comes from the Latin "fungibilis," which in turn came from the Latin phrase "fungi vice," meaning "to serve in place of." http://www.word-detective.com/110999.html#fungible



fits into ABB’s general strategy to focus on selected areas of electrical power engineering and on industrial automation as a whole.

Conclusions hold some thoughts on wind power’s need for power electronics and on EPE’s role in wind power generation on large scale.

On electricity’s business and physical manifestations

In order to establish wind power on large scale, one has thus to include business aspects like the payback by selling electrical energy to a market especially in time periods of electrical power utilities deregulation. One has also to include storage because a fungible [a law term for interchangeable] commodity business, like oil, gas, coal, food, metals, and the like, has normally a transport system and at least one storage capacity. It seems appropriate to quote H. Lee Willis2, ABB, Raleigh, NC, US, here: ”…. Modern power industry is particularly difficult to understand because of the dichotomy [= the division into two classes] between electricity’s business and physical manifestations. From the business perspective, electric power is a fungible commodity, something that can be traded much like oil, wheat, or coffee, and for which future markets and hedging systems can and do exist. But in its physical manifestation, electricity is quite unlike all other traded commodities. Perhaps the funda-mental difference is that it cannot be stored to any significant degree. This greatly affects how it must be managed as a business asset, and greatly constrains how its present and future market prices do or don’t interact, as compared to other commodities. In a large part due to its ‘storage-less’ nature ...”

Electricity’s storage capacity is normally located outside the plants as a hydro dam, an oil tank, a coal lot, a gas pipe, all in front of the power generation plant. The major exception is nuclear fuel, which has a very high energy density and is stored as an assembly inside the reactor tank to cover the production during a year. Electricity’s business and physical manifestation is connected by dispatch3.

The major part of the wind power generated is subsidised today. In a future large scale, it must be sold on a market. It can be sold through a monopoly but more and more electrical energy is traded in an open competition on a power exchange. So, wind power must be sold in competition via business manifestations.

EPE’s activities today are mainly dealing with those aspects of power electronics, which are clearly apparent to the sight (hardware) or to understanding by engineering models (software, design, automatic control, etc.) and by monitoring of physical quantities. The EPE activities and results can thus be seen as:

hidden to the public at large, associated to electricity’s physical manifestations to a high degree, very little connected to what could be called electricity’s business manifestations

but should most likely benefit from public visualization & business aspects in its curriculum.

2 Preface at page iii in M. Shahidehpour and M. Alomoush: Restructured Electrical Power Systems: Operation, Trading, and volatility, 2000, ISBN 0-8247-0620-X 3 Dispatch: Operating control of an integrated electric system involving operations such as assignment of levels of output to specific generating stations and other sources of supply; control of transmission lines, substations, and equipment; operation of principal interties and switching; and scheduling of energy transactions. US DOE: National Transmission Grid Study, May 2002, http://tis.eh.doe.gov/ntgs/gridstudy/main_screen.pdf



A wind power overview

Wind power is a business area with a considerable annual growth since several years. Research & Development is promoted by several European governments, which are aiming at new renewable energy facilities. Only a few countries have started manufacturing and installations in large scale. Denmark, Germany and the Netherlands made the choice to introduce wind power in their countries, decades ago. Spain came more recently. The wind turbine business has grown considerably during the years. It has been monitored annually with detailed figures like those in Fig. 2 and 3 during the years especially by Madsen [3], a wind power insider who several years ago was affiliated with Vestas. The worldwide power generation business which amounts to 80 G /year. The wind turbine business is today 10 % i.e. 8 G /year.

Fig. 2: The Top-10 list of suppliers 2002 [3]. GE Wind Energy, active in both Europe and US, was during a short period Enron Wind. GE’s German part was earlier Tacke.

Fig. 3: World Market Growth rates 1997-2002 [3].

An industrial dynamics perspective, focussed on wind power in Germany, in the Netherlands and in Sweden, was recently (spring 2002) finalised in a PhD-thesis by Bergek [4]. Her work holds in one of her papers [5] an overall description of the history when wind power became an alternative source to nuclear power in Denmark and in the Netherlands: “In the late 1970s and early 1980s, large firms such as MBB and SAAB entered the industry on the basis of government contracts to develop MW turbines. Most of these firms left in the 1980s or early 1990s, mainly because of the non-viability of MW turbines at that time. In parallel, many smaller firms entered, either as entrepreneurial start-ups or as a result of the diversification by mechanical engineering firms. These firms focused on smaller turbines and came from, e.g., Denmark, the Netherlands and Germany. In Denmark, as many as 26 firms had sold more than three turbines in the 1980s. In Germany, 13 firms were active in the 1980s. In the Netherlands, 15-20 firms entered the field in the late 1970s and early 1980s.”



Ms. Bergek was awarded an energy-related, pecuniary prize, the Engström Energy Prize in 2002. Her work increases the understanding of the mechanisms inducing and blocking capital goods industry development and the diffusion of renewable energy technology: “Four major observations can be made: (1) The Swedish capital goods industry has, so far, not been particularly successful in exploiting the opportunities and has not been able to shape the local innovation system to allow for the rapid diffusion of renewable energy technology in Sweden (with the possible exception of bioenergy). (2) Industry development and technology diffusion are dependent on the innovation system as a whole, and policy and strategy should, therefore, not be limited to the individual parts of the system. (3) A key objective in attending to the system as a whole should be to set self-sustaining virtuous circles in motion. (4) With respect to this, key issues for policy and strategy are to reduce uncertainty, increase the new technologies’ legitimacy and stimulate market formation.” Electrical energy certificates dealing with electricity from “new renewables” were recently introduced in Sweden, in the Netherlands, etc. Certificates could be seen as a type of market stimulation for installations but will hardly support a new wind turbine manufacturer in a specific country, e.g., in SE, NL,... Technology is mature what regards newcomers into business of making complete turbines.

Another perspective was recently (2002) published, also in Sweden, by Engström [6], a former governmental wind power research leader, nowadays an industrialist affiliated with Nordic Wind Power, Stockholm-Täby. Both his activities are thus within wind power R&D&D with experience from mid 1970s and on. His paper was published for a Swedish audience but some parts are well worth to quote (and translate) for those who do not read Swedish: ”The basic concept of wind turbines for electrical power production was not changed since the beginning of 1900s. One uses wind turbine rotors with horizontal shaft, two or three blades, high efficiency and low solidity (ca 3 %)” ”The first attempts in the 1970s to establish megawatt-range wind turbines were all commercial failures but in some cases a technical success like the Swedish-American Maglarp rated 3 MW. ... A restart from tens of kilowatts took place. All Danish manufacturers emanates from manufacturing agricultural equipment.” ”The most characteristic trend of wind power business is that the nominal ratings grow with 25 % per year since more than 20 years. This up-scaling is not without problems, as weight & cost grow more than power which grows as [length]2 while weight grows as [length]3.” ”Wind turbines rated 10 MW (turbine rotor diameter 150 m) are under discussion. ... the end ...” ”Adjustable-speed wind turbines were originally introduced to increase the energy yield. ... Losses in power electronics consume the increase. ... Adjustable-speed is nowadays used to cope with gusts and to damp torsion oscillations in turbine’s shaft and gearbox. A 3rd demand is controllability.” The author of this keynote paper knows that 15 to 20 MW turbines for installations offshore are discussed as EU R&D projects. This calls for more mechanical consideration in this introductive part of the paper and will be continued in next chapter.

Mechanical layouts are described in a comprehensive textbook written by Hau [7]. A very early paper on optimal selection of gearboxes was written by Thörnblad [8] in 1978 in the first MW-era. It is still well worth to quote two main findings in his work on 2.5 MW turbines although major parts of Thörnblad’s paper are reproduced in Hau’s book [7]. Quotes from the paper not explicitly found in the book read: ” 3-stage planetary gear / 1 alternator branch is the optimal gearbox from weight and cost” ”... coaxial arrangement of the whole system turbine–rotor–gear–alternator ... has its advantages, as it gives a fully symmetric weight distribution of the heavy components in the nacelle.”

Almost all gearless turbines are heavier than the geared for same ratings. Electrical machines as well as gearboxes are basically dimensioned by rated torque. This means that weight grows at least as



P Pi for constant blade tip speed. A small weight ratio between non-geared and geared turbines at hundreds of kilowatts could grow considerably at ten to twenty megawatts. Nacelle weights are a published subject for Germanischer Lloyd4 [9] as well as in a recent PhD-thesis by Jöckel [10], the latter is designing gearless turbine drives and comparing them with geared ones. The thesis deals also with power electronic converters and grid connections in combinations with gearless direct-drive generators. Dr. Jöckel has made a very comprehensive work, completed late 2002. Dr. Per Karlsson at LTH/IEA has studied faults handling as original power electronics-machines-power-systems works on mixed DC/AC-systems in his PhD-thesis [11], completed late 2002. Thomas Ackermann has published overviews of wind power [12] and is focussing economics in his works at KTH/EKC.

When reading this keynote at EPE 2003 in France, it is a pleasure to pass on the reference to “ÉTAT DE L’ART DANS LES AÉROGÉNÉRATEUR ÉLECTRIQUES” by Multon, et.al. It is available via Club CRIN Electronique de Puissance Energies Renouvelables. Keywords are “aérogénérateurs, énergie éolienne, technologie de générateur électrique, entraînements directs”.

Grid connections are described in a textbook written by Heier [13]. There are other general sources of grid or interconnection knowledge, e.g., IEEE’s Intertie Protection5 of Consumer-Owned Sources of Generation, 3 MVA or Less, as well as IEEE Standards Coordinating Committee SCC21/P1547 ongoing work on Interconnecting Distributed Resources6 (DR) with Electric Power Systems. IEEE’s publications go as deep as power quality and even ferroresonance, e.g., during an islanding condition. PQ and ferroresonance are also dealt with by Schneider Electric7 generally, not only wind power, in their “Cahier Technique” series. Single constant-/two-speed turbines without power electronics, traditionally erected close to or far from scattered houses form a challenge what regards flicker and visual aspects but is not the major issue here. Power electronics for wind farms way to act as power plants is the main issue in this paper.

Plans for wind power installations offshore are considerable, especially in Western Europe. They are dealt with later in this paper. USA has a focus on installations Mid West and to some extent on wind-diesel installations in Alaska. Denmark, especially Jutland and Funen, has large installations on land so that Eltra is most likely the region which comprises most distributed, and especially weather-bound, generation worldwide. More recently, Denmark, whose wind turbine manufacturers are mainly concentrated to Jutland, opened partly owned subsidiary branches abroad. Vestas, e.g., opened, established and then sold off Gamesa in Spain.

The investments in wind power, single-units as well as farms which start to be built offshore, are today all supported by governmental, utility-operated, EU-based, ... programmes. A major reason for this support is a scientific consensus on a climate change [14, 15]. Wind power cannot be dispatched in the same operational way as the traditional power plants with

embedded energy storage: fossil fuel plants, hydroelectric plants, nuclear plants and the like. There is very little power electronics embedded in today’s installed wind power generation seen as

a whole. Most new wind turbines comprise power electronics, among those, today’s major sales so-called Double-Fed Induction Generators, DFIGs, and so-called Direct Drives, DDs.

4 http://www.germanlloyd.de/mba/wind/publications.html 5 IEEE/PES, Power System Relaying Committee, http://www.pes-psrc.org/INTERTIE_PROT.ZIP, download June 13, 2003, Undated Report, most likely from 1989. 6 http://grouper.ieee.org/groups/scc21/1547/archives Examples of DR are Fuel Cells, Photovoltaics, Dispersed Generation, and Energy Storage. Distributed Energy Resources, DER, is used as a similar designation within the European Community. 7 Power Quality http://www.schneiderelectric.com.tr/ftp/literature/publications/ECT199-1.pdf Ferroresonance http://www.schneiderelectric.com.tr/ftp/literature/publications/ECT190.pdf



Power electronics for wind power–technically available today

Wiley Encyclopaedia of Electrical and Electronics Engineering [16], forms around 25 volumes on a book shelf. It is comfortably available by annual agreements as an on-line feature for anyone—the major Swedish universities have made such agreements, ABB, too. It holds various sections posted 1999-2000, e.g., on power electronics for adjustable [variable] speed motor drives, power generation plants, and power transmission & distribution systems—all with selected biography. For wind power, there are technology overviews and lists of references in the following sections:

♦ “Wind power” by Frede Blaabjerg, Aalborg University; Ned Mohan, University of Minnesota TOP OF ARTICLE for biography and overview POWER IN THE WIND POWER IN A WIND TURBINE POWER CONTROL METHODS SIMPLE WIND POWER SYSTEMS ADVANCED WIND POWER SYSTEMS

♦ “Wind turbines” by René Spée, Maxwell Technologies; Shibashis Bhowmik, Southwest Research Institute TOP OF ARTICLE BASIC WIND TURBINE CHARACTERISTICS for biography and overview CONVERSION SYSTEMS FOR WIND POWER GENERATION WIND TURBINE CONTROL SYSTEMS WIND TURBINE OPERATION OUTLOOK AND FURTHER INFORMATION

A very early paper on adjustable speed wind power drives was written by Lipo, already in 1970s. It is telling that the main circuits and active/reactive power characteristics are the same as for motors in adjustable speed drives, ASDs. Another early paper, even also on an ASD (DFIG), wind generator was written early 80s, in the first MW-era by Hinrichsen [17]. Both papers form still a basic truth although farm aspects, availability, cost-efficiency, fault handling and safety are essential aspects for generators’ drive-trains, often with embodiments that are different from the art of motor drives. Defective gearboxes have opened up for alternatives. Direct drive, geared drive and intermediate solutions were described in one paper at EWEC’97 [18]. Referring to this keynote paper’s Fig. 2, “The Top-10 list of suppliers 2002”, geared drives are today dominant in wind turbine business while Enercon, today’s second largest wind turbine manufacturer, originated with the product of direct drives [19] in the 1980s and quite a few have followed [20, 21] in various consortia. Intermediate solutions [22] with the first step out of the so-far traditional 3-step gear-box are still in their infancy but seem very promising from nacelle weight’s point-of-view [23]. Most geared drives of today—and all direct drives and intermediate solutions—are ASD power electronic drives. They comprise partly (DFIG, say 30 %) or fully (100 %) rated power electronic converters as well as control of blades for cost-effective torque-speed control.

Short & medium term issues for wind power to integrate wind farms

When a number of wind turbine generator systems, WTGSs, are combined to establish a large wind farm, the interconnection and its grid connection are often issues causing some difficulties. There are several reasons to this, ranging from cost factors via lack of knowledge to even misunderstanding between at least two parties: The wind side & The utility side. Maybe also other parties like consultants, electrical equipment manufacturers, investors, insurance companies, and the like. Misunderstandings and loose contacts between parties are more common than we really believe.



A modern wind turbine generator in the MW range is a power plant. No WTG manufacturer will ever doubt. But for a utility, it may sound ridiculous: To be accepted by a utility as a power plant, a wind farm with must act as a utility anticipates from a power plant (station). And today, wind power cannot. But of course, wind power cannot be like any other power plant, wind power have certain characteristics differing from characteristics of other types of power plants. Like all other types of power plants differ. There are potential innovations where wind power plants can be improved to come closer to the expectations of utilities, but there are also functions in wind power, which are superior to existing types of power plants.

Most stakeholders in wind participate in various works for standards and recommendations for best practice based on consensus as well as by sharing information at conferences, organized by EWEA, AWEA, and others.

This part of the paper deals with the design of a wind farm with respect to optimization. It is claimed that the communication problems between the parties can be minimized if the wind farm is seen from the grid as a power plant, having characteristics that are complementary to other types of power plants.

Fig. 4: Principal layout of offshore wind turbines, their collection and transmission (C&T) systems for interconnection to mains, i.e., for fungible power to the AC network and trading

Fig. 4 shows to the left symbols for the wind turbine generator systems, WTGS. The manufacturers of WTGSs combine products rated kilowatt to megawatt, mainly low-voltage technologies on components level and purchase these components from sub-suppliers. Manufacturers of electrical equipment supports wind power activities with power electronic converters, electrical machines (wind power generators, auxiliary motor drives), LV controls (switchgear, contactors, circuit-breakers, disconnectors, ...), turbine step-up transformers, and the like.

Standards and recommendations are published after comprehensive review procedures. International Electrotechnical Commission, IEC, TC-88 subcommittee covers international/European wind power standards. American Wind Energy Association, AWEA, is designated by the American National Standards Institute (ANSI) as the lead organization for the development and publication of industry consensus standards for wind energy equipment and services in the United States. AWEA participates through its representation in the IEC TC-88 Subcommittee. AWEA cooperates also with gearbox manufacturers for so-called AGMA/ANSI Standards within intra-structures. AGMA, ANSI, IEEE are natural bodies to represent different parties, from at least two sides: The Wind Side and the Utility Side. IEEE Standards Coordinating Committee SCC21/P1547 focuses as one body generally on electrical infra-structures under the IEEE headline “Interconnecting Distributed Resources (DR) with Electric Power Systems”. Wind Power exemplifies Dispersed Generation, DG8. Fuel Cells, Photovoltaics, and Energy Storage are other examples of DR/DG. In some rural areas co-generating units (“CHP”, heat and electricity from fossil fuels) are so small that these plants also must be regarded as DG8. Wind power is thus comprised within several more or less synonymous adjective descriptors in front of “generation” or “power generation”:

8 All four listed descriptors must be combined in order to carry out proper literature search on “DG” and the like.

Distributed (General term) Dispersed (IEEE)

Decentralized (Utility term) (also decentralised) Embedded (IEE)



In this paper, we are very little dealing with terminology and standards aspects. They are brought here to give a general fundament to describe the wind power generation area and other DG & DR areas to be well prepared to form a successor generation with best practice. This need for best practice is accentuated because power systems with wind farms will comprise substantially many more machines—at least an order of magnitude more—embedded in the power system than with today’s large traditional, dispatched plants, such as fossil fuel power plants, hydroelectric plants, nuclear plants and the like, which simply have less than a pocketful of electrical generators and some auxiliaries in a power plant.

Reactive power, filtering and the like are remedies, often with power electronic converters or capacitive VAr-compensation as actuators, seen from the “Wind side” of the matter. From the “Utility side”, it is seen as “Reactive supply and voltage control from generation sources” to use the terminology of FERC Order 888.

FERC, Federal Energy Regulatory Commission, www.ferc.fed.us, is an independent regulatory agency within the Department of Energy. FERC determined in Order 888, reproduced on DOE’s web9 as “History of Open Transmission Access”, that six ancillary services10 must be included in open-access tariffs; two of these services (“scheduling, system control, and dispatch”; “reactive supply and voltage control from generation sources”) must be purchased by transmission customers, because the transmitting utility is best suited to provide these services.

This indicates that a boundary—for our two main sides: The Wind Side and the Utility Side of a wind farm—could be the interconnection’s point-of-connection to the large-scale power grid. This boundary is a starting point in this chapter headed “Short and medium term issues ...” but will be questioned rather soon and further discussed in next chapter headed “Long term issues ...”.

The interconnections for wind energy collection & transmission are of a main interest to power electronics. Interconnections will most likely need to have a voltage quality standard, which is more or less the same as in traditional electric power distribution network, because auxiliary power is needed in all units forming the intra-/infra-structure. This is an economic reality, which will be under discussion both regarding traditional single turbines close to scattered houses and large farms in areas where there is a weak electrical infrastructure until—and maybe even after—the farm is established.

An existing long overhead transmission line can—as in a recent (July 2003) study by the Norwegian Sintef, published [24] as a demonstration of reactive power’s impact—be loaded to its thermal limits with at least one reactive source in order to enable increased wind power penetration, Fig. 5. One or two reactive sources, like SVC (SVC Light®, AdvSVC=STATCOM) or a (tuned, not blank) stepwise controllable capacitor bank, so-called thyristor switched capacitor, TSC, are needed in the grid. The most advantageous layout varies from grid to grid and from type of wind turbines.

Wind turbines with single-/two-speed induction generator drive-trains and fixed power factor correction, PFC, need approximately the same reactive power compensation SVC in its interconnection grid as its rated active power.

Wind turbines with embedded continuously (cos φ=1) variable reactive power compensation in the towers need naturally considerably less reactive power compensation SVC in the interconnection, either they are DFIGs, with partly (say 30 %) rated power electronics, or they are ASDs with fully (100 %) rated power electronics.

As will be seen soon, SVC provides less mechanical stress in the drive-trains and less electrical stress in the electrical power systems. SVC+DFIG form a better solution compared to fixed/stepwise PFC.

9 http://www.eia.doe.gov/cneaf/solar.renewables/rea_issues/html/appendix.html 10 Ancillary Services: Interconnected Operations Services (IOS) identified as necessary to effect a transfer of electricity between purchasing and selling entities and which a transmission provider must include in an open-access transmission tariff. US DOE: National Transmission Grid Study, May 2002, http://tis.eh.doe.gov/ntgs/gridstudy/main_screen.pdf



Fully compensated wind turbines, “modern solution”:

Local voltage control at the wind farm ob-tained by placing an SVC at BUS7, controlling the voltage at BUS7.

One SVC at BUS5 equipped with a secondary controller to limit reactive power exchange with main grid.

Fig. 5: Examples of localizations of SVCs to penetrate a wind farm into an existing 132 kV overhead

line distribution network to the line’s full thermal capacity [24]

AC cables longer than 50 to 100 km, depending on load, are not practical, Fig. 6. The critical length of a cable is related to the capacitive charging current for the cable at the conditions, when the charging current has maximum value. So if it is possible to connect a dynamic reactive source/sink, i.e. an SVC, at a location in a mid point of the cable, the critical length will increase. HVAC interconnection cable range will then be extended considerably and be prosperous in coastal areas where right-of-ways for overhead transmission lines might be difficult to accomplish.

Fig. 6: Examples of efficiencies for AC sea cables, Source: AC and HVDC Cables for Offshore Wind Farms by Olle Tollerz, ABB, at IBC Offshore Wind Conference, London, Febr. 18 - 19, 2003

Fig. 7: Example of a fixed-PFC wind farm grid to study fault recovery with and without SVC for a wind turbine farm. Source: Per Halvarsson, ABB Power Systems; See also Fig. 9



An SVC will have dual purpose for a wind farm: Advantages for the turbines as well as advantages for the grid. When wind turbines are to remain connected to the grid during a fault in the grid, Fig. 7, the SVC will minimize the stresses on the turbine during the fault, Fig. 8. Fig. 7 shows arrangement for simulation of a three-phase short circuit and ground fault close to a wind farm. The duration of fault is 0.2 seconds, and, after clearing, a successful auto reclosure of the line is performed. Wind turbine generators are of the induction type with squirrel cage rotor. Results of two SIMPOW simulations are shown in Fig. 8 with similar axis values. x-axis show time 0 to 5 seconds and y-axis show flux values from 0 to 1.2 per unit and speed values from 0.98 to 1.12 per unit. The upper curves show flux in stator and rotor. The lower curves show wind turbine and generator speeds. Diagram a) shows an obvious low-damped oscillation behaviour during a long time period in case without SVC. Diagram b) shows a well-damped recovery with SVC within less than 2 seconds.

a) Oscillating fault recovery without SVC

b) Damped fault recovery with SVC

Fig. 8: Damping effect of an SVC for a wind turbine compared to no SVC. Source: Per Halvarsson, ABB Power Systems, SVC presentation at Meeting with utility SEAS on island Gotland, April 10, 2003



From the grid side, the SVC can assure there is no lack of reactive power and eliminate oscillations when the fault is disconnected. Most failures of grid operation are of reactive nature. The faults have so to say used the reactive power in the grid and the SVC can assist in the restoration process. This is of course of highest importance when the turbines use induction generators, which draw reactive power from outside the generator terminals. The SVC will prevent voltage collapse, that is a severe voltage depression without inherent recovery. An SVC stabilizes also the power flow in existing lines and cables. The damping function seen on the turbine can dampen power oscillations and thus allow power lines to be operated at higher load.

HVDC, High-voltage direct current transmission systems, e.g., HVDC Light®, will be used for large offshore plants, where the distance is long and the rating is high. HVAC will remain as a main choice, Fig. 6, for the offshore locations nearest in time, e.g. as a tradition started with Horns Reef and Rødsand in Denmark. SVC is judged to be the most likely, preferred solution to stabilize turbines, Fig. 7 and 8, as well as interconnections and their neighbouring power systems, with disregards for HVDC and for UPFC, Unified Power Flow Controllers.

Utilities’ demands are likely to be severe for the turbines in areas where wind power can be expected to have a high share of the total generated power. The demands are first of all that turbines should stay connected to the grid during contingencies so power delivery can continue and even take share in the recovery process. Secondly the turbines should contribute to the short circuit level of the grid during the fault and finally, the utilities wish turbines to take share of the spinning reserve.

The first demand can be solved today, even for severe faults. Dynamic compensation is a part of the solution. The second demand is in the process of being solved and the third demand have been solved to a great extent: Figures from Denmark—with 20 % of the produced energy from wind power—indicate that wind farms produce to some extent 85 % of the time. There is furthermore a market price for regulating power and with the right pricing it can be equally interesting for the investor to operate at 90 % of possible capacity than full load. Further parts of the Danish solution is strong interconnections to neighbour countries and a domestic electricity production with several types of power plants—despite there is neither hydro nor nuclear power plants. Wind power control is really fast, a fact, which can be utilized by utilities for area control.

Power utilities and/or transmission system operators, TSOs, are always among the main stakeholders for large wind farms, at least at the high-voltage end of the interconnection to the main power transmission system. Overhead-line poles are marked as associating symbols Fig. 4 right for the point-of-connection and area-of-connection to the standard frequency AC power grid, which is of main interest.

One will avoid sub-optimization from only concentrating on the components, based on experiences from a growing number of examples from sales, deliveries, commissioning, and even operation, to turbine manufacturers as well as to wind farms, optimization with sub-systems on a full system basis.

Initiatives between WTGS manufacturers and electrical equipment manufacturers will show to be cost-effective, especially for large-scale farms, where the interconnections are proprietary, as for hydro. Cost-effectiveness will be the fact for electrical components like generators and transformers, low, medium and high voltage switchgear and reactive compensation but deals also with different solutions for the components. Further the system approach includes Supervisory Control and Data Acquisition, SCADA, systems and the optimization of the investment with a combination of minimizing investments and losses and maximizing availability related to the system layout.

A power transformer may be equipped with a controllable tap changer, on-line or off-line, for minor adjustments of its voltage ratio. It might be configured to change phase-angles as well as the number of phases, e.g., from standard 3-phase to two sets of 3-phase with 30 degrees phase shift, also called 2*3-phase, suitable for power semiconductor converters with lower harmonic stress on the power grid. A power transformer is usually used between large rotating electrical machines like a power generator and a standard frequency AC power grid. Transformer-less connections of wind power generators as well as wind power transformers can be built with a cable-based insulation system. This



allows for, up until now, undreamed-of possibilities regarding cost-effective short-circuit reactance values for power transformers as well as an improved ability to withstand transient short-circuit forces on the windings in the machines and stability in the power systems.

As infrastructures, one must establish prefabricated repetitive products to deliver substations cost-effectively. These products comprise step-up transformers from low-voltage and/or medium-voltage to (sub)-transmission voltage levels, for interconnections. Furthermore, repetitive easy-to-adapt-and-deliver products, like cables, VAr-compensators, and other products, like overhead lines & cables, are utilized to interconnect wind power for medium-sized farms with equipment from different power equipment manufacturers. Flexible AC Transmission Systems, FACTS, especially SVC, will be used for larger farms coordinated with power electronics in embedded reactive compensation.

To sum up, a main issue to get a wind farm to be a power plant is thus to establish a limited number of variants of collection & transmission, interconnection systems. Stability, fault handling, efficiency, availability and reliability are essential. Electrical equipment manufacturers will cooperate with turbine manufacturers as well as with utilities in combinations with consultants, investors, insurance companies, and the like.

Further one need to discuss the long-term issues of building the wind farms with defined power plant characteristics at least comparable to other types of power plants and thus head-lighting advantages for utilities by accommodating an increasing amount of wind power plants prosperously connected to the grid. This is expanded under the headline long-term issues in next section of this paper.

Long term issues for wind power to improve wind farms on large scale

The wind is basically a rough, kinetic, non-storable bulk energy source. Its energy contents need not only conversion for direct use—historical, or nearby, small-scale energy storage with applications in: Grain mills (historical) Water pumps (historical & modern) Desalination (islands & deserts)

DC Electricity to Hydrogen (1890s & modern) Combined Wind-Diesel Electricity Compressed Air or Pumped Hydro

For large-scale electrical power production, wind energy needs to be cost-effective and behave like traditional electrical energy to be traded. There is also a power quality, PQ, demand. Wind’s PQ has not only traditional voltage aspects but also availability aspects—from “transportation and storage” of the wind’s rough “bulk energy”—expanded below. One should avoid dedicated wind “electricity storage” and “hydrogen as a carrier” to be cost-effective and—when needed—prefer to use optimally located, supplemental prime movers, e.g., in coactive converters to get fungible wind power [25].

It seems also appropriate to remind of the earlier mentioned quote originally written by H. Lee Willis, ABB: “... in its physical manifestation, electricity is quite unlike all other traded commodities. Perhaps the fundamental difference is that it cannot be stored to any significant degree. ...”

The major part of the wind power generated is subsidized today. In a future large scale, it must be transmitted on a large-scale power system and be sold on a market. Wind power can be sold through a monopoly but more and more electrical energy is traded in an open competition on a power exchange. In order to establish wind power on large scale, one has to include business aspects like investors’ payback from selling electrical energy on a market—especially in time periods of electrical power utilities deregulation. One has also to include some sort of dispatch/storage function because a fungible [law for interchangeable] commodity business, like oil, gas, coal, food, metals, and the like, has normally a transport system and at least one storage capacity.

The wind’s rough “bulk energy” needs therefore priming into fully interchangeable, electrical energy11 to supplement traditional large-scale generation and participate in worldwide reduction of CO2-emisson/climate-change and to be used with a good power-quality, PQ, for large-scale industrial

11 All electric power generation, transmission and consumption (conversion into mechanical power, heat, light, etc.) influence—like almost all human activities—our environment via so-called externalities.



manufacturing like in Fig. 9. The most essential parts of a wind turbine nacelle (left) and an overview of a pulp-&-paper factory (right) are depicted..

Fig. 9: A long-term vision: Adjustable-speed drive, ASD, power electronic wind turbine generators to provide fungible power to a pulp-&-paper factory. Pictures originate from www.vestas.dk and from 2000 ABB Americas IT Conference (June 7-9, 2000, Raleigh, NC).

A long-term task for all stakeholders in electric power generation, transmission & distribution is to make wind power as reliable, commercially valuable and fungible in utilization as electric power produced by the traditional (via schedule) dispatched plants. Among those plants are fossil fuel power plants, hydroelectric plants, nuclear plants and the like. Wind energy conversion will be established as a supplement to oil, gas and coal energy conversion for power generation, thus reducing dependency on those traditional fossil fuel sources and emerging biomass fuel.

With carefully prepared intra- & infrastructures, e.g., principally like in Fig. 10, there is very little need for physical electro-chemical storage—like accumulator batteries or fuel-cells—to prime rough wind power into fungible electrical power in the long term. Interconnections, communication, automation (IndustrialIT, Appendix 1, and the like) as well as business agreements will form the most cost-effective solutions, [25]. WTG manufacturers and power utilities should jointly think in the way: “A large wind farm is a power plant”.

A main long-term issue is then to develop intra-structures, like the mechanical drive-trains, in the wind turbines, WTs, and to develop matching infra-structures like AC/DC interconnections for C&T, collection and transmission, from the electrical generators to the mains. Business agreements on balancing are thus also needed to avoid physical “electricity storage”. Such agreements will be mutually beneficial when wind power grows in size and develops in stature.

The interconnection’s point-of-connection and area-of-connection to the standard frequency AC power grid Fig. 4 right, is of main interest to sum up on infra-structures.

The drive-trains and their connections of the wind turbine generator systems to the inter-connection are of main interest to sum up on intra-structures.

There should not be a strict boundary for intra- & infra-structures associated to the two main sides: The Wind Side and the Utility Side of a wind farm. There should be a smooth transfer around interconnection’s point-of-connection to the large-scale power grid.

Power electronics systems like an SVC will have dual purpose for a wind farm: Advantages for the turbines as well as advantages for the grid. Less mechanical stress in the drive-trains and less electrical stress in the electrical power systems.



Intra- and infrastructures—Drive-trains and interconnections

Electrical generators and power electronics are essential in converting kinetic wind energy and in priming wind power, Fig. 10. Another item is automation in a long chain, from nacelles to load management.

Generators are optionally direct-drive or geared with a single-step-gear ("intermediate solutions") [18 – 22]. Today's standard drives with a multi-step-gear [8] will remain as a choice, which also needs further development to be appropriate for increased ratings and new localizations. Major stakeholders of drive-trains combine more than one solution of components or complete systems as options to their customers.

What regards interconnections: AC & VAr-compensation are essential; DC is pending.

Several companies are partners and actors on systems level because cooperation procreates compre-hensive knowledge of components as well as of systems for drives as well as for T&D in general.

Priming EnergySource

VirtualEnergy Storage

InterconnectionC&T grid

The power grid A transnational grid

Sub-station

Sub-station

OptionalCoActiveConverter

To optional loads

Renew.SourceUnit 1

Renew.SourceUnit K

A CoActive Converterhere a sub-station with

a CPU, a prime mover P.M. drivinga rotating machine xMand optional converters

(dotted)

The PowerGrid

Transmissionlines

3-φ

xMP.M.C

Inter-connection

“The C&T grid”

Systemfor collection

of electr. energyfrom renewableslike wind, solar,

and the like

Optional loads

Loads

Fig. 10: Two ways to demonstrate integration of wind energy to get fungible electrical power [25]. Power electronics is a natural option in the intra- and infra-structures for AC/DC- & DC/AC-conversion as well as for voltage/power quality and the like.



Fig. 11: An electrical “ glimpse” of roles and areas in wind power business

A primary—obvious—aspect of the roles and areas for the electrical and automation engineering actors in wind power business is to compete by offering the electrical components for a wind turbine generator system, WTGS, shown to the left in Fig. 11. The WTGS is the start to convert kinetic wind energy into fungible electrical power, whereby:

1. There is a fluctuating torque in a huge shaft driven at low-speed by the wind turbine’s blade assembly and a hub, whereby the rotating machine, electro-mechanical, generating drive-train subsystem is the essential.

— but one must consider the drive-train’s interaction with the characteristics of a) interconnection12, b) point of connection and c) power balancing and priming. Point “c)” to handle the wind’s stochastic character, [25].

2. A growing issue is to establish a number of variants of collection and transmission ac and/or dc systems, to the right of the centre in Fig. 11, whereby fault handling, efficiency, availabil-ity and reliability are essential.

3. Wind power control is really fast and should be utilized by the utilities as power system stabilizers. Balancing should be performed via transmission capacity, Fig. 10, and preferably with hydro power plants—via dam storage facilities—for most cost-efficient control but also with fossil fuel plants.

4. A fourth issue is to establish priming into fungible electrical power, i.e., to make the wind energy fully available for power trading. It is an integration issue of distributed generation (DG) systems dealt with in the next chapter, Fig. 15 & 16.

Items 3 – 4 were also shown middle right in Fig. 11 but are more obviously depicted in Fig. 10, 15 & 16. Balancing, priming, stability, etc are easily forgotten—technically & commercially—in the renewables debate. All four items, 1. – 4. above, are needed in order to make wind power cost-effective for large-scale power generation.

This means that the farm-utility boundary, earlier put at the point-of-connection PoC, is not distinct. SVC could be localized on both sides of PoC and be beneficial12 on both sides of PoC what regards turbines and grid.

12 Interconnection length, voltage and type, overhead line or underground cable or both; with/without SVC: • The available length and damping characteristics of an HVAC interconnection is related to the capacitive charging current for the line/cable at the conditions, when the charging current have maximum value, so, if it is possible to connect a dynamic reactive source/sink, i.e. an SVC, near mid point of the line/cable, the available length will increase. • Recall also an SVC’s damping capabilities inside the wind turbines, Fig. 7 and 8.



Offshore wind power converters An essential part of future wind turbine installations will be offshore. Most wind power plants will be based on land or be coastal/near-shore. As mentioned before AC will be in favour unless there are severe restrictions, e.g., interconnections through national parks (North Sea Coast Line, Wattenmeer) [26], via cables on long distance and the like. There are some studies completed around offshore wind power conversion, collection & transmission: IEA-CADDET, Electricity from Offshore Wind [27], DOWEC, Dutch Offshore Wind Energy Converters [28], CA-OWEE, Concerted action on Offshore Wind Energy in Europe [29]. Summary (on page 2 top) in the latter report13 reads: ”Variable speed: there is a tendency towards variable-speed designs. Wide range variable speed has a further advantage in the ability to avoid damaging resonances, important for offshore turbine structures, where the resonant frequencies have proved difficult to predict accurately, and may also change over the lifetime of the structure. It remains somewhat unclear whether power electronic converters can be made reliable enough at suitable cost.” It is expanded, CA-OWEE’s page 2-43. The sentence in bold is the background to volunteer for a keynote presentation to EPE.

Horns Reef (www.hornsrev.dk) was built and commissioned in 2002. It is the world’s largest offshore wind farm and located at the Danish west coast. The wind farm is sited 14-20 km into the North Sea, west of Blåvands Huk, and represents the first phase of a large-scale Danish effort to produce non-polluting electricity from offshore wind turbines. With the Horns Rev project it will be possible to determine whether or not the Danish Government’s ambitious energy plan is feasible. Under the plan, wind turbines with a total capacity of 4000 MW must be established in Danish waters before 2030.

Power electronics is there. Geared double-fed induction generators are used. The diagram of the main circuits seems not to be available on the web but it is likely the principal one in Fig. 12. The wind turbines are rated 2 MW each and quite similar to those shown in Fig. 9 left. To Elsam14: “wind power is an attractive business area – nationally as well as internationally. It is therefore Elsam’s ambition to build, own and operate land-based and offshore wind farms. In addition to the Horns Rev project, Elsam is actively involved in a number of Danish and international wind energy projects. These projects benefit from the expertise and experience that Elsam has accumulated over the years. Today Elsam operates some 500 wind turbines in Denmark alone.”

Fig. 12: Left: A grid-connection scheme for an offshore wind farm [27c]. Right: Actual installations owned by Elsam A/S – an international player in wind energy. Source: www.hornsrev.dk

13 http://www.offshorewindenergy.org/ca-owee/indexpages/downloads/CA-OWEE_Complete.pdf 14 http://www.hornsrev.dk/Engelsk/Projektet/uk-elsam_og_vindmoeller.htm



Large-scale Renewable energy systems—Distributed Generation in general

Technologies15 for Distributed16 Generation Technology Typical “available” size per module “Traditional combustion” Combined cycle gas turbines 35 MW – 400 MW Internal combustion engines 5 kW – 10 MW Combustion turbines 1 MW – 250 MW Micro-Turbines 35 kW – 1 MW “Small-scale combustion” Stirling engine 2 kW – 10 kW “Renewable” Small hydro 1 MW – 100 MW Micro hydro 25 kW – 1 MW Wind turbines 200 W – 3 MW Photovoltaic arrays 20 W – 100 kW Solar thermal, central receiver 1 MW – 10 MW Solar thermal, Lutz system 10 MW – 80 MW Biomass, e.g. based on gasification 100 kW – 20 MW Geothermal 5 MW – 100 MW Ocean energy 100 kW – 1 MW “Electricity Storage—Fuel cells and Batteries” Fuel cells, phosacid 200 kW – 2 MW Fuel cells, molten carbonate 250 kW – 2 MW Fuel cells, proton exchange 1 kW – 250 kW Fuel cells, solid oxide 250 kW – 5 MW Battery storage 500 kW – 5 MW

Hydro, wind, solar, biomass, and geothermal are under various steps of R&D [30]. Regarding all renewable energy systems, benefits from improved power control, e.g., with power electronic and electro-magnetic/electro-mechanic converters and with automation should be stressed empathically in high-level research and education.

Power electronics and automation will almost always be a basis for electrical power generation based on renewable energy facilities. Those facilities will, for Europe-wide, large-scale generation, need reliable power collection, transmission and distribution grids and priming measures to provide electrical energy: to large-scale industrial manufacturing, e.g. as shown in Fig. 13 left; to transportation; and, to commercial facilities; i.e., not only to households. This observation is in a clear contrast to pictures like, Fig. 13 right, where it could be misunderstood that the Europe-wide traditional high-voltage-grid with today’s electrical energy flow via electrical power transmission poles in the countryside will be eliminated and taken away. It could furthermore be misunderstood that wind power plants will be located in densely populated areas either in the vicinity of commercial buildings or villages. Integration of wind power in the transmission grids has started, especially in the Danish transmission system operator’s, Eltra, grid at Jutland and on Funen, Fig. 12 and in the North-western Germany, especially in the former Preussen-Elektra grid, today a part of E.ON Netz.

15 The table is a revision of Table 1 in Electric Power Systems Research 57 (2001) 195–204: Distributed generation: a definition, by Thomas Ackermann, Göran Andersson, Lennart Söder 16 There are more than one denoting of DG in English: distributed, decentralized (decentralised), dispersed, embedded. They mean more or less the same. Search on Internet and in databases ought to combine all of them.



Fig. 13: Today’s “centralized utility” and Tomorrow’s “distributed utility” [1], the electrical power transmission poles are missing in the pictures but will exist.

E.ON Energie AG17, Excerpts from Annual Report for 2000 ”Thanks to the considerable technological progress made in recent years, the production of energy in small units will take over more and more of the power market. The so-called decentralized production is today an interesting supplement to large power plants. This is true particularly for wind energy. Under current subsidy regulations, the generation of electricity from wind is a competitive proposition. E.ON Energie has resolved to further increase the share of wind energy in its power generation. We want to participate in the erection of offshore wind farms in Germany; ... Without the innovation in load management implemented by E.ON Energie, it would not have been possible for the wind industry to input so much of its highly fluctuating power production into the German grid: in Germany alone, wind facilities feed some 3,800 MW into our network. Under the trademark Naturpower E.ON Vertrieb offers environmentally conscious customers certified electricity from renewable energies.” ”The Renewable Energies Act requires network operators to accept electricity supplied from renewable energy sources and sets the prices they have to pay. In most instances the rates are above those contained in the old law. This means a further increase in costs for the power utilities and their customers. However, unlike the previous Electricity Supply Act, the Renewable Energies Act does call for the electric power from renewable sources of energy to be passed on to the transmission network operators and for both the accepted quantities of energy and the burden of payment to be uniformly balanced nationwide. As a result, the additional cost will be shared equally among all energy utilities supplying electric power to end consumers.” ”The capital expenditure program of E.ON Netz in the year 2000 was to a large extent dominated by wind energy. The high payment for electric power from wind generators is resulting in large wind parks being planned on land and offshore, making it necessary for E.ON Netz to analyze capacity requirements in the pertinent areas. It appears that the network will need to be greatly expanded if the wind generator projects currently in the planning phase are actually built. For a wind power investor E.ON Netz has built a 380 kV transformer station in Rhede that is the first in Germany to be used exclusively for wind energy.”

17 E.ON Energy AG was founded out of the merger of former Preussen Electra and Bayernwerk. It is shareholder, partly respectively wholly, of a couple of different subsidiaries first of all located in Germany but also with subsidiaries whole over Europe. Business areas are beside the electricity business natural gas market, fresh water, sewage and supporting business fields like IT service and engineering services.



Dr. W. Woyke, E.ON Energie AG, Neue Technologien, regenerative Energieumwandlung, IERE (International Electric Research Exchange), Zürich, June 12, 2002, ”Up to now large scale power plants feed into the electrical grid with base-load and peak-load power in a manner which is working in a stable status since several decades. Due to liberalization of electricity market and public support of regenerative and combined heat and power generation (CHP) the amount of uncontrollable power generation is increasing dramatically. Further on there is a clear trend to distributed generation units to feed into the 400 V grid, which results in new effects in the 400 V grid and new issues of grid control systems. New structures of energy management especially concerning distributed generation have to be developed within the next years to hold the usual high level of availability and quality in electrical power supply.” The transition to utilise electric power generation based on renewables started obviously in Germany.

Eltra has distributed power generation (partly owned by Elsam, Fig. 12) in its grid on Jutland and Funen. Eltra issued Specifications for Connecting Wind Farms to the Transmission Network [31]: ”In connection with the preparations for the future large offshore wind farms specifications have been worked out for such offshore wind farms. These specifications are a counterpart to the power station specifications for land-based plant - except wind turbines. The most important new requirement is that the offshore wind farms - like other major production plants - should not lose stability or trip at short-circuits in the network disconnected by the primary network protection. Said in a popular way, the turbines must be able to survive a short dead time (~100 milliseconds) and resume production when the fault has been disconnected and the voltage starts to return. The above-mentioned wishes result in requirements on the regulation ability of the wind farm that the farm should be able to reduce its production from full load to a level between 0 and 20 per cent in a few seconds.” The so-called wind turbine ride-through demand is apparent also in several other grids, e.g., as an E.ON demand, and has roots 30 years back in Nordel’s (issued mid 1970s) and others’ demands on large power plants. Power generation based on renewables started obviously in Denmark, too.

Thomas A. Wind, a US consultant, is technical advisor within ISEP, Iowa Stored Energy Plant, a project, which aims to combine wind and Compressed Air Energy Storage (CAES). It is a 200 MW power plant. Air is compressed and then stored 1200 feet below ground in a 50 feet thick underground aquifer. When power is needed, the high pressure air is heated with natural gas and released through a special combustion turbine. It comprises an 85 MW wind farm. They are operated together to become a fully dispatchable 200 MW intermediate load power plant. It is proposed by municipal utilities in Iowa for completion in 2007. Its basic design is a CAES Power Plant. It takes 0.83 kWh of off-peak power plus 4,300 BTU of natural gas as input energy at this site to produce 1 kWh of output power on peak. It is integrating wind power generation to fungible power via CAES. At night, wind generation would be supplemented with off-peak energy purchases from the grid to compress air to 500 psi for storage in the underground aquifer 1200 feet below ground level. During the daytime, the compressed air will be released and heated to 1600ºF for the power turbine to generate power. Any wind generation during the daytime would be used to Supplement the CAES generator output. The result is that the CAES generator converts off-peak purchases and wind energy into more valuable on-peak firm power. Because the plant will generate about half of the time, it will be an intermediate load power plant.

Construction cost for the 200 MW CAES plant will be about $126 million and about $77 million for the wind farm. Total cost will be about $200 million, or $1,000 per firm kW. Marginal energy cost will be about 2.4 ¢ per kWh, based on $4.00 / MMBTU natural gas prices and $15 / MWh off peak energy purchases. Wind generation counted at zero fuel cost. “All-In” delivered cost of power will be about 4.5 ¢ per kWh at 50% capacity factor. There will be additional value from the sale of ancillary services, scheduling flexibility, and green tags, which will reduce the net costs shown above.



Wind generation provides about 1/3 of total energy. A larger wind farm would provide a larger fraction. CAES Plant uses standard compressor equipment used for petroleum refining and air separa-tion businesses: Equipment is conservatively designed to be robust and highly reliable. Combustion power turbine expander is conservatively designed (1600 vs. 2200ºF for other combustion turbines). Provides ability to be cycled once or twice a day. Generator can be ramped up and down very quickly and operated down to 10 % of rating with very modest loss of efficiency.

Thomas A. Wind presented ISEP at American Wind Energy Associations Conference, May 18-21, 2003 in Austin, TX, [32], from which the text above is gotten. His presentation comprised a successor generation view for Iowa, USA. He gave the audience both a proposed technical generation principle, and a view of climate change for a successor generation, Fig. 14.

Fig. 14: Top: Principal weekly operation scheme of ISEP [32]; Bottom: Tom Wind’s personal view of carbon-dioxide emissions. The diagram of carbon-dioxide and temperature variation origi-nates in [33], is downloadable at http://carto.eu.org/article2481.html with graphic design by Philippe Rekacewich. Similar diagrams are found in [34] EPE’2003; [14, 15] UN’s IPCC



Construction cost for the 200 MW CAES plant Fig. 15 will be about $126 million and about $77 million for the wind farm. Total cost will be about $200 million, or $1 per firm kW. Marginal energy cost will be about 2.4 ¢ per kWh, based on $4.00 / MMBTU natural gas prices and $15 / MWh off peak energy purchases. (Wind generation counted at zero fuel cost.) “All-In” delivered cost of power will be about 4.5 ¢ per kWh at 50 % capacity factor. There will be additional value from the sale of ancillary services, scheduling flexibility, and green tags, which will reduce the net costs shown above. Wind generation provides about 1/3 of total energy. A larger wind farm would provide a larger fraction. The proposed principle can be further developed to use the annual harvest from the farms as fuel to become 100 % renewable. Similar principles are shown in Fig. 16 [25].

Fig. 15: ISEP as a way to show integration of renewables without short-time weather forecasting [32]. The compressed fossil natural gas bubble can be replaced via gasification of annual harvest.

Fig. 16: Two ways to show integration of renewables with meteorology especially short-time weather forecasting to get fungible electrical power out of renewables [25].



Roles: ABB’s present stakes and earlier experiences

An overall objective for ABB’s partnerships and activities in R&D is not only to provide components to be used inside the wind turbines and inside the collection and transmission, C&T, grid. There is also an objective to provide remedies which enhance the commercial value of electric power produced by wind turbine facilities and from other renewable facilities so as to make that electric power as commercially valuable and fungible as electric power produced by dispatched traditional power plants such as fossil fuel plants, hydro-electric plants, nuclear plants and the like.

Renewables energy solutions are intended to be used as supplementing electrical energy all over the world in regions with growing energy demands, Fig. 17, even though most examples in this presentation are chosen from the countries in Scandinavia and from the Netherlands and the north-western Germany, where only approximately 3 % of the world’s electrical energy production is consumed. The primary energy distribution measured all over the world is mainly changing from oil to gas as shown in Fig. 17, while renewable sources are negligible among all primary sources.

Fig. 17: a) Energy consumption (stacked area diagram). b) The world primary energy distribution (100 % stacked area). a) & b) with the prognosis starting from 1990 [35, 36]

A need for different types of technology development and of cooperation areas was described and is summarized in Fig. 18. ABB’s main long-term issues to establish wind power are:

to develop intra-structures, like the electro-mechanical drive-trains, in the wind turbine generators, WTGs, with and without power electronics; and

to develop matching infra-structures like SVC-stabilized AC interconnections as well as HVDC systems for collection and transmission, from the generators to the mains;

in order to be cost-effective and avoid physical “electricity storage” & hydrogen.

ABB sold recently its nuclear company to British Nuclear Fuel Ltd, BNFL, who operates it under the well-known company name Westinghouse. ABB’s large-scale traditional power generation business was sold to Alstom, a few years after the invention of the high-voltage generator, Powerformer®.

ABB is nevertheless active within R&D&D (research & development & demonstration) and/or sales of components/subsystems in electrical power generation with CHP (combined heat and power), wind power, micro-turbines, etc. ABB is still a major sub-supplier of electrical power generators, up to 70 MW mainly for geared gas-turbines; but partly also for standby & mobile “gen-sets”, for rural electrification and for remote power in various partnerships. ABB Denmark commenced, already in the 1970’s, its sales of constant-speed, wind power generators and other electrical components, like low-voltage apparatus and small power transformers, as well as automation technology products, today’s IndustrialIT, on a continuously growing scale.



Fig. 18: An overall “ glimpse” of intra- and infrastructural aspects in wind power business Renewable energy facilities like wind turbines need technology development and cooperation.

Fig. 19: A detailed “ glimpse” of intra- and infrastructural aspects in wind power business Renewable energy facilities like wind turbines is multi-discipline and need cooperation.

ABB has today an extensive sub-system wind power portfolio, 37, 38, 39, 40]. During the decades, it was filled with adjustable-speed generator drives, e.g., double-fed induction generators, DFIGs, direct-drives and “intermediate solutions” as intra-structures, [38, 39]. As infra-structures, there are productified substations, like PS-1 [41, ], with step-up transformers from low-voltage and/or medium-voltage to (sub)-transmission voltage levels, for interconnections, “up and running in half the time”; as well as with cables, high-voltage direct current transmission systems & Flexible AC Trans-mission Systems, Fig. 11; HVDC & FACTS, [40].



Fig. 20: An SVC Light valve to be used for PQ control and its associated control system [42]

Industrial IT is overviewed in Appendix 1. It comes in both intra- & infra-structures and enables ABB’s products for "wind automation” to match needs for utility automation from distribution network operators and transmission system operators. It is Industrial IT that connects areas left to the areas right in Fig. 18, i.e., ABB’s division AT connected to division PT. ABB with its Industrial IT will provide wind power solutions for connectivity, integration and optimization.

Voltage and power quality, PQ, control in wind power is a nearby priming issue. It was described recently in a conference paper from ABB, as an overview of components, sub-systems and control [42, ].

Parts of this keynote paper were previously published at CWEA’s Wind Power Asia 2003, Beijing, and at AWEA’s Windpower 2003, Austin, TX, both during the spring 2003. The author expresses his gratitude for various contributions from his industrial colleagues working with wind power within ABB Europe, USA and China, resulting in several patent applications and so far few papers on the way to new products.

Parts of ABB staff participate in various academic duties especially in Scandinavian universities. An ABB colleague Dr. Stefan Johansson was recently a member of the examination committee on “Modelling of Wind Turbines for Power System Studies” by Dr. Thomas Petru in Gothenburg [43]. A work where power electronics meet power systems with different time scales in modelling.

The author wants to express his gratitude to academic colleagues for kind invitations: to be censor, e.g., for Vladislav Akhmatov, at Lyngby, Denmark, focussing large scale wind

power generation and collection & transmission; he is completing his PhD in connections with the Danish utility NESA during 2003, [44];

to be a guest lecturer, opponent, member of examination committee in Gothenburg, Stockholm and Trondheim, today mainly focussing renewables and themes like solar, wind, fungible, ... [25, 45] on various levels of knowledge;

to have gotten the privilege to be a part-time professor within power electronic drives at Lund University since 12 years this summer. The engagement has recently resulted in introducing Power Generation as a new part of a compulsory course, [34, 46] as well as in a recent PhD-thesis on faults handling in power electronics-machines-power-systems with mixed DC/AC-systems for renewable energy systems [11].



Conclusions—Wind power’s need for power electronics

Wind turbine's intra-structure is an obvious issue: Power Factor Compensation, PFC, of induction generators is essential Generators’ operation is characterized by phasors, shafts in drive-trains, etc Adjustable-Speed Drives, ASDs, were originally introduced to increase the energy yield and

then used to cope with gusts and to damp torsion oscillations in turbine’s shaft and gearbox. A 3rd demand is today controllability to produce and deliver a specified—sold—power level

Double-Fed Induction Generator, DFIG, turbines suffer far too often from defective gearboxes Direct Drive, DD, turbines suffer all from extreme nacelle weight

Infra-structure from wind turbines/farms as interconnections to AC mains is a hidden but major issue: AC-only vs. AC+DC interconnections: HVAC with embedded SVC will remain as a main

choice for wind farms with disregards for HVDC and UPFC Interconnections for wind power collection & transmission differ considerably from power

transmission & distribution Power collection grids are energized from no-load at all or extremely low load,

via idling generators at low wind to extremely high power collection from generators at high wind

Mixed power collection & distribution is not trivial; e.g., fault handling; flicker, ...

Active power handling in both intra- and infra-structures: Electrical energy generated is not as simple and solely as the time integral of (active) power (of

stochastic shape), when it is discussed multi-disciplinarily as professionals, neither in society nor in industry and academia [34]

Driving thrust in wind turbines differs considerably from motor load in industrial drives

Reactive power handling in both intra- and infrastructures: SVC is necessary for high wind power penetration and efficient fault handling, impact in grid

and drive-trains DFIG’s embedded, controllable PFC/SVC is efficient but not enough for long and weak HVAC

interconnections SVC localisation/utilisation in interconnection grid is cross-bordering farm to utility

Simultaneous cooperation within intra- & infrastructures are necessary to procreate an entity out of traditionals and renewables within electrical power generation.

EPE's stakeholders should visualise power electronics with machines, power systems and automation for automatically controlled and protected, prosperous, commoditised subsystems in wind

power’s commercialisation in cooperation with wind turbine—and electrical equipment—manufacturers, utilities, ... for cost-efficient & sustainable electric power generation.

New structures of distributed generation with power electronics and automation have to be developed within the next years to hold the usual high level of availability and quality in electrical power supply and to overcome doubts like those on power electronics’ costs and reliability expressed by CA-OWEE [29].

Wind energy will contribute with 2000 TWh/year in 2030 or 15 % of today’s energy. Wind power will support mankind's need for reduced CO2-emissions but globally only up to what will be lost as 10-20 % in worldwide use of CO2-sequestration. Wind power cannot be established on large scale without a considerable amount of power electronics.



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Appendix 1: Industrial IT, what is it? by Thomas Mandery, ABB Inc., New Berlin, WI

Straight from ABB materials — Industrial IT is an ABB concept. It is both the vision behind ABB’s business platform and a strategy for future growth. It is also a process of standardization in which the entire range of ABB products is gradually becoming compliant with the requirements and standards of Industrial IT certification. Simply put, Industrial IT is a real-time information architecture that enables all of ABB’s products and services to work together seamlessly.

Industrial IT brings a common language to plant automation, asset management and collaborative commerce. Its scope is such that it can integrate a company’s entire business and production processes into a single system, bringing under one umbrella its automation and control systems, its production and manufacturing processes, and its design, sales, marketing, finance and administrative systems. It can also link that company with its suppliers and customers to form a seamless web of communication in which information essential to their relations with one another is made accessible instantly and in real time.

We should keep in mind that ABB has determined four different levels of Industrial IT certification with each level corresponding to a certain level of integrated functionality and three of these levels have been discussed repeatedly throughout this paper, level 1 to level 3. Industrial IT is what connects the left to the right in Fig. 18, i.e., ABB’s division AT connected to division PT. ABB with its Industrial IT will provide wind power solutions for connectivity, integration and optimization.

Level 0: Information Means that the product can be handled in an Industrial IT environment. Demands for Level 0 certifi-cation concerning product aspects:

Identification, eg ABB ID-numbers, product names Documentation, eg manuals CAD data, electrical and mechanical Technical data and product classification, eg test reports Environmental data

Level 1: Connectivity Means that the product can be connected to and will work together in an Industrial IT system. Overview of demands:

HW can be physically connected via defined and approved interfaces SW can be installed and handled in a consistent manner Basic data can be exchanged via defined protocols The product does not introduce disturbances in the environment it is inserted into

Level 2: Integration Means that the product integrates well in an Industrial IT system. Overview of demands:

Extended data (status, maintenance etc) can be exchanged via defined protocols Functionality is available as Aspect Systems on integration level 2

Level 3: Optimization Means that the product when integrated in an Industrial IT system, will make it possible to use all Industrial IT functionality (that is supported by other components in the system). Overview of demands:

Aspect Object Types are provided with the Extended set of Aspects Functionality is available as Aspect Systems on integration level 4 The product is handled in a consistent manner throughout the whole life cycle and value chain



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