Offshore and Onshore Wind Turbines

30.5.4 Offshore and Onshore Wind Turbines

• Development of high efficiency/high quality voltage source AC/DC/AC converter for a main connection of One of the main trends in wind turbine technology is offshore variable wind turbines, operating with either a permanent installations. There are great wind resources at sea for installing magnet, a synchronous or an asynchronous generator.

wind turbines in many areas where the depth of the sea is rel- • Operation at a power factor around one with higher- atively shallow. There are several demonstration plants that harmonic voltage distortion less than international stan- have had extremely positive results, so interest has increased dards.

in installing offshore wind farms, because of the develop- • The power quality of the electrical output of the wind ment of large commercial power MW wind turbines. Offshore farms may be improved by the use of advanced static wind turbines may have slightly more favorable energy balance var compensators STATCOM or active power filters using than onshore turbines, depending on local wind conditions. power semiconductors like IGBTs, IGCTs, or GTOs. These In places where onshore wind turbines are typically placed on kind of power conditioning systems are a new emerging flat terrain, offshore wind turbines will generally yield some family of FACTS (flexible AC transmission system) con- 50% more energy than a turbine placed on a nearby onshore verters, which allow improved utilization of the power site. The reason is the low roughness of the sea surface. On the network. These systems will allow wind farms to reduce other hand, the construction and installation of a foundation voltage drops and electrical losses in the network without requires 50% more energy than onshore turbines. It should the possibility of transient over voltage at islanding due

be remembered, however, that offshore wind turbines have a to self-excitation of wind generators. Moreover, power longer life expectancy, around 25 to 30 years, than onshore tur- conditioning systems equipment with different control bines. The reason is that the low turbulence at sea gives lower algorithm can be used to control the network voltage, fatigue loads on the wind turbine.

30 Wind Turbine Applications 819 From a power electronics point of view, offshore wind tur-

Desired stator current. bines are interesting because, under certain circumstances, they i rg ,i sg ,i tg

ref ref i ref

R s ,i S s ,i T s

Public grid phase currents. become desirable to transmit the generated power to the load

Desired magnetizing current. center over DC transmission lines (HVDC). This alternative i m

i ref

Magnetizing current.

becomes economically attractive versus AC transmission when i dsm ,i qsm Instantaneous values of the direct and quadrature-

a large amount of power is to be transmitted over a long axis magnetizing stator current components, distance from a remote wind farm to the load center [8].

respectively, and expressed in the magnetizing Moreover, the transient stability and the dynamic damping

current-oriented reference frame. of the electrical system oscillations can be improved by HVDC J

Total inertia of the system referred to the high transmissions.

speed shaft.

k ws ,k wr

Winding factors of rotor and stator.

Coupled inductance. L AC Inductance of inductors at the AC side of the inverter.

L S ,L r

Stator and rotor windings inductances.

Nomenclature

L DC Inductance of the step-up chopper.

Turns ratio of the machine. n =k ws ·n s /k wr ·n r

Number of turns of each rotor and stator phase. C dc DC link capacitor.

n s ,n r

C (λ, β) Power coefficient at tip speed ratio λ and pitch β. N 1 Angular speed of the magnetic field (synchronous p

e rg ,e Instantaneous values of grid voltages. speed) expressed in rpm. sg ,e tg e α ,e β

Grid voltages expressed in an orthogonal reference

Angular speed of the generator rotor expressed in frame.

rpm.

f Excitation frequency (the same as the grid fre-

Number of pole pairs. 1

quency) in hertz. p 1 ,p 2 Number of pole pairs of machine number 1 and f 2 Frequency of the voltage supplied to the rotor of

machine 2 in hertz. P 1 Electrical power in the stator of principal machine G Gearbox ratio.

number 1.

i dc DC inductor current of the step-up converter. P 2 Electrical power in the stator of auxiliary machine ref

number 2.

i dc Desired DC inductor current of the step-up P e Electrical generated power. converter.

Mechanical power in the low speed shaft. i dse ,i qse

Instantaneous values of the direct and quadrature-

Generated maximum power. axis stator current components, respectively, and

P max

P rate

Generator rate power.

Active and reactive power through the stator. ref i ref ,i Desired instantaneous values of the direct

expressed in rotor-flux-oriented reference frame.

Desired active and reactive power through the and quadrature-axis stator current components,

P S ,Q S

stator.

respectively, and expressed in rotor-flux-oriented

P slip

Slip power.

Available wind power. i dsr ,i qsr

reference frame.

Desired real power and desired reactive power on reference frame. ref ref

Stator current components established in the rotor

p ref ,q ref

the grid side.

i dsr ,i qsr Desired stator current components established in Q e Electromagnetic torque of the machine. the rotor reference frame.

ref

i ,i qs Instantaneous values of the direct and quadrature-

ds e Desired electromagnetic torque of the machine.

axis stator current components, respectively, and

Torque in the low speed shaft. expressed in the rotor reference phase.

Generator rate torque. i Ds ,i Qs

Q rate

Instantaneous values of the direct and quadrature-

Torque in the high speed shaft. axis stator current components, respectively, and

Rotor radius.

Stator and rotor winding resistors. ref i ref

expressed in the stator reference frame.

R S ,R r

Ds ,i Qs Desired direct and quadrature-axis stator cur- rent components expressed in the stator reference

Slip.

s 1 =ω 1 −ω r /ω 1 Slips of machine principal number 1. frame.

s 2 =ω 2 −ω r /ω 2 Slips of machine principal number 2. i

Stator and rotor time constants, respectively. exc

Synchronous generator excitation current.

T s ,T r

i Desired synchronous generator excitation current.

ref

Desired direct and quadrature-axis stator volt- exc

ref ref ref age expressed in the rotor-flux-oriented reference

i R g ,i S g ,i T g Desired stator current.

frame.

Desired direct and quadrature-axis stator voltage i R s ,i S s ,i T s

r ,i S r ,i T r Rotor current.

Stator current. expressed in the stator reference frame.

820 J. M. Carrasco et al. u R ,u S ,u T

Rotor voltage. ω r min r r r Minimum angular speed of the synchronous gen- u R s ,u S s ,u T s

Stator voltage.

erator.

ref ref u ref ,u ,u Desired stator voltage.

R s S Desired angular speed for the torque controller.

ω ref

u ref r ,i r ,λ r Rotor voltage, current, and flux, respectively, ω β Desired angular speed for the pitch controller. referred to a reference frame that rotates with the

Rated value of the rotor speed. rotor.

ω rate r

ω e Electrical angular speed of the magnetizing cur- u ′ r ,i ′ r ,λ r ′

Rotor voltage, current, and flux, respectively,

rent reference frame.

referred to a reference frame fixed with the stator. u s ,i s ,λ s

Stator voltage, current, and flux, respectively, referred to a reference frame fixed with the stator.

u ds ,u qs Direct and quadrature-axis stator voltage expressed in the magnetizing current reference frame.

References

v rat Rated wind speed. v p max

Maximum power wind speed. 1. S. Heier, “Grid Integration of Wind Energy Conversion Systems”. v start

Start wind speed. Chichester, Sussex (UK): John Wiley & Sons, 1998. v stop

Stop wind speed. 2. G.L. Johnson, “Wind Energy Systems”. Englewood Cliffs, NJ (US): v w

Wind speed.

Prentice-Hall, INC., 1985.

V dc DC-Link capacitor voltage. 3. P. Vas, “Vector Control of AC Machines”, NY (US): Oxford Clarendon V ref dc Desired DC-Link capacitor voltage.

Press, 1990.

4. V. Subrahmanyam, “Electric Drives. Concepts and Applications”. NY V st max Maximum RMS stator voltage of the synchronous

(US): MacGraw-Hill, 1996.

5. M. Alatalo, M. Sc, and T. Svensson, “Variable Speed Direct-Driven V st

generator. min

Maximum RMS stator voltage of the synchronous PM-Generator With a PWM Controlled Current Source Inverter”. generator.

European Community Wind Energy Conference. March 1993. Lùbeck- β

Pitch angle.

Travemùnde, Germany.

β ref Desired pitch angle. 6. P. Vas, “Sensorless Vector and Direct Torque Control”. NY (US): δ

Load angle.

Oxford University Press, 1998.

λ = ω · R/V v Tip speed ratio. 7. Chee-Mun Ong. “Dynamic Simulation of Electric Machinery Using λ opt

Optimal tip speed ratio. Matlab/Simulink”. Prentice Hall PTR, 1998. λ m

Magnetizing flux linkage vector. 8. N. Mohan, T.M. Undeland, and W.P. Robbins, “Power Electronics, λ md ,λ mq

Instantaneous values of the direct and quadra- Converters, Applications, and Design”. Second edition, John Wiley & ture axis magnetizing flux linkage components

Sons, INC., 1995.

expressed in the rotor reference frame. 9. R.E. Tarter. “Solid-State Power Conversion Handbook”. John Wiley & |λ m |

Estimated modulus of magnetizing flux linkage

Sons, Inc., 1993.

vector. 10. B.K. Bose, “Power Electronics and Variable Frequency Drives. Tech- ref

nology and Applications”. IEEE Press, 1997. λ m

Desired modulus of magnetizing flux linkage 11. S.A. Papathanassiou and M.P. Papadopoulos, “A Comparison of Vari- vector.

able Speed Wind Turbine Configurations”. Wind Energy Conference. η

Electrical performance.

March 1999, France.

η r Phase angle of the rotor flux linkage space pha- 12. D.S. Zinger and E. Muljadi. “Annualized Wind Energy Improvement sor with respect to the direct axis of the stator

Using Variable Speeds”. IEEE Transactions on Industry Applications, reference frame.

Vol. 33, No. 6, Nov–Dec 1997.

θ e Magnetizing current angle. 13. K. Pierce, “Control Method for Improved Energy Capture below θ sl

Angle corresponding to the angular slip frequency. Rated Power”. Third edition ASME/JSME Joint Fluids Engineering θ r

Rotor angle. Conference. July, 1999. San Francisco, California. ρ

Air density. 14. Theory Manual. E.A Bossanyi, “Bladed for Windows”. Garrad Hassan ω 1 = 2πf 1 /p 1 Angular speed of the rotating magnetic flux pro-

and Partners Limited. September 1997.

duced in the stator of machine 1 relative to the 15. E. Muljadi, K. Pierce, and P. Migliore, “Control Strategy for Variable- stator.

Speed, Stall-Regulated Wind Turbines”. American Controls Conference. ω 2 = 2πf 2 /p 2 Angular speed of the rotating magnetic flux pro-

June 1998. Philadelphia, NREL.

duced in the rotor of machine 2 relative to the 16. A.D. Simmons, L.L. Freris, and J.A.M. Bleijs, “Comparison of Energy rotor.

Capture and Structural Implementaions of Various Policies of Con- ω L

Low-speed shaft angular speed. trolling Wind Turbines”. Wind Energy: Technology and Implementa- ω r

Angular speed of the generator rotor.

tion. 1991. (Amsterdam EWEC’91).

ω ref r Reference rotor speed. 17. W.E. Leithead, S. de la Salle, and D. Reardon, “Wind Turbine ω max r

Maximum angular speed of the synchronous Control Objectives and Design”. European Community Wind Energy generator.

Conference. September 1990. Madrid, Spain.

30 Wind Turbine Applications 821 18. M.H. Rashid, Power Electronics. Circuits, Devices, and Applica-

35. G. Escobar, J. Leyva-Ramos, J. M. Carrasco, E. Galvan, R. Portillo, tions. Second edition. Englewood Cliffs, NJ(US): Prentice Hall,

M.M. Prats, and L.G. Franquelo, “Control of a Three Level Converter 1993.

Used as a Synchronous Rectifier”. 2004 IEEE 35th Annual Power Elec- 19. H. Akagi, A. Nabae, and S. Atoh, “Control Strategy of Active

tronics Specialists Conference, PESC’04, Vol. 5, pp. 3458–3464, June Power Filters Using Multiple Voltage-Source PWM Converters”. IEEE

Trans. Ind. Applications, Vol. IA-22, No. 3, pp. 460–465, May–June 36. M.M. Prats, L.G. Franquelo, R. Portillo, J.I. León, E. Galván, and 1986.

J.M. Carrasco, “A Three Dimensional Space Vector Modulation Gen- 20. S. Bhowmik, R. Spée, and J.H.R. Enslin, “Performance Optimization

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Letters, Vol. 1, pp. 110–114, 2003.

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Stations”. Power Engineering Journal [see also Power Engineer], Vol. 3, tor Control to the Cascaded Induction Machine for Wind Power

No. 3, pp. 145–154, May 1989.

Generation Schemes”. 7th IEE European on Power Electronics, EPE’97. 38. S.J. Haslam, P.A. Crossley, and N. Jenkins, “Design and Evaluation September 1997. Trondheim (Norway).

of a Wind Farm Protection Relay”. Generation, Transmission and 22. P.O. 12.3 “Propuesta sobre requisitos de respuesta frente a huecos

Distribution, IEE Proceedings, Vol. 146, No. 1, pp. 37–44, January de tensión de las instalaciones eólica”. Red Eléctrica de España, S.A.

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39. M.P. Palsson, T. Toftevaag, K. Uhlen, and J.O.G. Tande, “Large-scale 23. C. Rasmussen, P. Jorgensen, and J. Havsager, “Integration of Wind

Wind Power Integration and Voltage Stability Limits in Regional Power in the Grid in Eastern Denmark”. In Proc 4th International

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25. M. Johan and de Hann Sjoerd W.H. “Ridethrough of Wind Turbines 41. H. Grüning, B. Ødegård, J. Rees, A. Weber, E. Carroll, and with Doubly-fed Induction Generator During a Voltage Dip”. IEEE

S. Eicher, “High Power Hard-Driven GTO Module for 4.5kV/3kA Transaction on Energy Conversion, Vol. 20, No. 2. June 2005.

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Trends”. European Power Electronic Conference EPE’97. Trondheim, 27. International Electrotechnical Commission. Draft IEC 61400-21:

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Converters”. IEEE Transactions on Industry Application, Vol. 32, No. Clamped PWM Inverter”. IEEE Transactions on Industry Appli-

3. May–June 1996.

cations, Vol. IA-17, No.

49. M. Sugeno, “An Introductory Survey of Fuzzy Control”. Information 1981.

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34. G. Escobar, J. Leyva, J.M. Carrasco, E. Galvan, R. Portillo, M.M. Prats, 50. E. Galván, A. Torralba, F. Barrero, M.A. Aguirre, and L.G. Franquelo, and L.G. Franquelo, “Modeling of a Three Level Converter Used in

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“A Robust Speed Control of AC Motor Drives based on Fuzzy

822 J. M. Carrasco et al. Reasoning”. Industry Application Society (IAS). Toronto (Canada),

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Generation System”. IEEE Transaction on Power Electronics, Vol. 12, Selftuning System for Induction Motor Controllers”. IEE European

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IFSA’99.

(USA), November 1995.

HVDC Transmission

Vijay K. Sood, Ph.D.

31.1 Introduction .......................................................................................... 823

Hydro-Quebec (IREQ), 1800 Lionel 31.1.1 Comparison of AC–DC Transmission • 31.1.2 Evaluation of Reliability and Availability Boulet, Varennes, Quebec,

Costs • 31.1.3 Applications of DC Transmission • 31.1.4 Types of HVDC Systems Canada

31.2 Main Components of HVDC Converter Station ........................................... 829

31.2.1 Converter Unit • 31.2.2 Converter Transformer • 31.2.3 Filters • 31.2.4 Reactive Power Source • 31.2.5 DC Smoothing Reactor • 31.2.6 DC Switchgear • 31.2.7 DC Cables

31.3 Analysis of Converter Bridge ..................................................................... 832

31.4 Controls and Protection ........................................................................... 832

31.4.1 Basics of Control for a Two-terminal DC Link • 31.4.2 Control Implementation • 31.4.3 Control Loops • 31.4.4 Hierarchy of DC Controls • 31.4.5 Monitoring of Signals

• 31.4.6 Protection against Overcurrents • 31.4.7 Protection against Overvoltages

31.5 MTDC Operation ................................................................................... 840

31.5.1 Series Tap • 31.5.2 Parallel Tap • 31.5.3 Control of MTDC System

31.6 Application ............................................................................................ 841

31.6.1 HVDC Interconnection at Gurun (Malaysia)

31.7 Modern Trends ....................................................................................... 843

31.7.1 Converter Station Design of the 2000s

31.8 HVDC System Simulation Techniques ........................................................ 846

31.8.1 DC Simulators and TNAs • 31.8.2 Digital Computer Analysis

31.9 Concluding Remarks ............................................................................... 848 References ............................................................................................. 849

31.1 Introduction

31.1.1 Comparison of AC–DC Transmission

Making a planning selection between either ac or dc transmis- High voltage direct current (HVDC) transmission [1–3] is sion is based on an evaluation of transmission costs, technical

a major user of power electronics technology. The HVDC considerations, and the reliability/availability offered by the technology first made its mark in the early undersea cable

two power transmission alternatives.

interconnections of Gotland (1954) and Sardinia (1967), and then in long distance transmission with the Pacific Intertie (1970) and Nelson River (1973) schemes using mercury arc

31.1.1.1 Evaluation of Transmission Costs

valves. A significant milestone development occurred in 1972 The cost of a transmission line comprises of the capital invest- with the first back-to-back (BB) asynchronous interconnection ment required for the actual infrastructure (i.e. right-of-way at Eel River between Quebec and New Brunswick; this instal- (ROW), towers, conductors, insulators, and terminal equip- lation also marked the introduction of thyristor valves to the ment) and costs incurred for operational requirements (i.e. technology and replaced the earlier mercury arc valves.

losses). Assuming similar insulation requirements for peak Until 2005, a total transmission capacity of 70,000 MW voltage levels for both ac and dc lines, a dc line can carry HVDC is installed in some 95 projects all over the as much power, with two conductors (having positive/negative world. To understand the rapid growth of dc transmission polarities with respect to ground), as an ac line with three con- (Table 31.1) [4] in the past 50 years, it is first necessary to ductors of the same size. Therefore, for a given power level, a compare it to conventional ac transmission.

dc line requires a smaller ROW, simpler and cheaper towers

824 V. K. Sood

TABLE 31.1 Listing of HVDC installations HVDC link

Location Gotland I #

Supplier

Year

Power (MW)

DC voltage (kV)

Length (km)

96 Sweden English channel

A 1954

64 England–France Volgograd–Donbass ∗

Unknown Russian

New Zealand Konti-Skan I

Denmark–Sweden Sakuma

Japan Sardinia

Italy Vancouver I

69 Canada Pacific intertie

U.S.A. Pacific intertie

U.S.A Nelson River I ∗∗

Canada Kingsnorth

82 England Gotland

96 Sweden Eel River

A 1970

Canada Skagerrak I

Norway–Denmark Skagerrak II

Norway–Denmark Skagerrak III

Norway–Denmark Vancouver II

77 Canada Shin-Shinano

Japan Shin-Shinano

Japan Square Butte

U.S.A. David A. Hamil

U.S.A. Cahora Bassa

Mozambique-S. Africa Nelson River II

Canada Nelson River II

Canada CU Project

U.S.A. Hokkaido–Honshu

Paraguay Vyborg

Russia (tie w/Finland) Vyborg

Austria Gotland II

Duernrohr J

Sweden Gotland III

Sweden Eddy County

U.S.A. Chateauguay

Canada Oklaunion

U.S.A. Itaipu

Brazil Itaipu

Brazil Itaipu

Brazil Inga-Shaba

DR Congo Pacific Intertie upgrade

U.S.A. Blackwater

U.S.A. Highgate

U.S.A. Madawaska

Canada Miles City

U.S.A. Broken Hill

Australia Intermountain power

U.S.A. project Cross-channel: (Les Mandarins)

72 France–England (Sellindge)

Des Cantons-Comerford

Canada-U.S.A. Sacoi ##

Corsica Island, Italy Sacoi ###

H 1986

H 1992

31 HVDC Transmission 825

TABLE 31.1 —Contd HVDC link

Location Itaipu II

Supplier

Year

Power (MW)

DC Voltage (kV)

Length (km)

Brazil Sidney (Virginia Smith)

U.S.A. Gezhouba–Shanghai

Sweden–Denmark Vindhyachal

Konti-Skan II

India Pacific Intertie Exp.

U.S.A. McNeill

Canada Fenno-Skan

Finland–Sweden Sileru–Barsoor

India Quebec-New England

Canada-U.S.A. Nicolet Tap

Canada DC Hybrid Link

A 1992

New Zealand Etzenricht

Germany (tie w/Czech) Vienna-South east

Austria (tie w/Hungary) Haenam–Cheju

South Korea Baltic Cable Project

Sweden-Germany Welch–Monticello

U.S.A. Kontek Interconnection

Denmark–Germany Scotland–N. Ireland

United Kingdom Chandrapur–Ramagundum

India Chandrapur–Padghe

India Greece–Italy

Italy Gazuwaka–Jeypore

India Leyte-Luzun

Philippines Cahora Bassa

Mozambique–S.Africa TSQ-Beijao

China Thailand–Malaysia

Thailand–Malaysia Moyle

64 Ireland–Scotland East-South Intercon.

India Rapid City DC tie

S.Dakota, U.S.A. Three Gorges-Changzhou

China Three Gorges-Quangdong

China Guizhou-Guangdong

China Celilo Conv. station

U.S.A. Nelson River Bipole II

Canada Basslink

Australia–Tasmania Lamar

Colorado, U.S.A. Vizag II

India Estlink

Estonia – Finland Three Gorges-Shanghai

HVDC Light

China NorNed

Norway – Netherlands Valhall offshore

Norway A – ASEA; B – Brown Boveri; C – General Electric; D – Toshiba; E – Hitachi; F – Russian; G – Siemens; H – CGEE Alsthom; I – GEC (Formerly English Electric);

AB 2009

HVDC Light

J – HVDC Working Group. (AEG, BBC, Gmens); K – (Independent); AB – ABB (ASEA Brown Boveri); JV – Joint Venture (GE and ASEA); ∗ two valve groups replaced with thyristors in 1977; ∗∗ two valve groups in Pole 1 replaced with thyristors by GEC in 1991; ∗∗∗ Back-to-back HVDC System; ∗∗∗∗ Multiterminal system. Largest terminal is rated 2250 MW; # Retired from service; ##

50 MW thyristor tap; ### Uprated w/thyristor valves.

826 V. K. Sood Right-of-Way

Typical DC and AC Transmission Line Structures for approx. 2000 MW

ROW: 60 m

ROW: 85 m

ROW: 100 m

FIGURE 31.1 Comparison of ROW for ac and dc transmission systems.

and reduced conductor and insulator costs. As an example, economical than dc for distances less than the “breakeven dis- Fig. 31.1 shows the comparative case of ac and dc systems tance” but is more expensive for longer distances. This is due to carrying 2000 MW.

a combination of the terminal equipment costs and line costs With the dc option, since there are only two conductors for the two types of transmission. The breakeven distances can (with the same current capacity of three ac conductors), the vary from about 500 to 800 km in overhead lines depending power transmission losses are also reduced to about two-thirds on the per unit line costs. With a cable system, this breakeven of the comparable ac system. The absence of skin effect with distance approaches 50 km.

dc is also beneficial in reducing power losses marginally, and the dielectric losses, in case of power cables is also very much

31.1.1.2 Evaluation of Technical Considerations

less for dc transmission. Corona effects tend to be less significant on dc than for Due to its fast controllability, a dc transmission system has full

ac conductors. The other factors that influence line costs are control over transmitted power, an ability to enhance transient the costs of compensation and terminal equipment. DC lines and dynamic stability in associated ac networks and can limit do not require reactive power compensation but the terminal fault currents in the dc lines. Furthermore, dc transmission equipment costs are increased due to the presence of converters overcomes some of the following problems associated with ac and filters.

transmission:

Figure 31.2 shows the variation of infrastructure costs with

distance for ac and dc transmission. AC tends to be more Stability limits

The power transfer in an ac line is dependent on the angular difference between the voltage phasors at the two line ends. For

a given power transfer level, this angle increases with distance. The maximum power transfer is limited by the considera- breakeven distance

Costs

AC tions of steady state and transient stability. The power carrying capability of an ac line is inversely proportional to transmis-

sion distance whereas the power carrying ability of dc lines is

DC line costs

unaffected by the distance of transmission.