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Trends for future HVDC Applications

W. Breuer, D. Povh*, D. Retzmann, E. Teltsch Siemens, Germany

ABSTRACT

During their development, power systems become more and more interconnected and heavily loaded. With the increasing size and complexity of systems and as the result of the liberalization of the electrical markets, needs for innovative applications and technical improvements of the grids will further increase. HVDC plays an important role for these tasks.

Commercial applications of HVDC started in the 1950ies. In the meantime, HVDC became a reliable and economically important alternative for AC transmission, offering advantages in the operation of power systems in addition to the power transfer.

The paper discusses expected present and future HVDC applications. Integration of HVDC into AC systems will be used more frequently as it can simplify the system configuration, control load flow and, at the same time, it improves the dynamic system performance and increases the system reliability. For the interconnection of large power systems HVDC offers technical and economical advantages, especially if the interconnection is weak. The connection of remote power stations to the system, e.g. off-shore wind generation, can be effectively put into practice by means of HVDC. Advantages of these applications will be discussed and demonstrated by implemented projects.

KEY WORDS:

Power system development, use of HVDC, types of HVDC transmissions, integration of HVDC into AC systems, project examples

1. INTRODUCTION

The development of electric power supply began more than one hundred years ago. The growth and extension of AC systems and consequently the introduction of higher voltage levels have been driven by the fast growth of power demand over the decades. It has been followed by the development of new technologies in the field of high voltages and by innovations in design and manufacturing of the equipment [1]. Increasingly higher voltages have been used, first at 110 and 220 kV levels in Europe, then 287 kV in USA and 380 kV in Europe. Finally, the 735 kV level in Canada and 765 kV level in other countries have been established. A transmission project with 1150 kV level has been built-up for testing purposes in the former Soviet Union.

By applying interconnections to the neighboring systems, power systems have been extended to achieve technical and economical advantages. Regional systems have been built-up towards national grids and later to interconnected systems with the neighboring countries. In industrialized countries,

*dusan.povh.ext@siemens.com

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large systems came into existence, covering parts of or even whole continents. Additionally, the liberalization of the power industry supports interconnections to enable the exchange of power between the regions or countries, and it helps to transport suitable energy cheaper and more ecologically over long distances to the load centers. Maximum reasonable distances to transmit power still economically are in the range of up to 3000 km. However, the situation can change in the future if ecological and political terms or the present cost conditions alternate.

In the second half of last century, high power HVDC transmission technology has been introduced, offering new dimensions for long distance transmission. This development started with the transmission of power in an order of magnitude of a few hundred MW and was continuously increased to transmission ratings up to 3 - 4 GW over long distances by just one bipolar line.

By these developments, HVDC became a mature and reliable technology. Up to now, almost 55 GW HVDC transmission capacities have been installed worldwide, ref. to Fig. 1 [2]. Transmission distances over 1,000 to 2,000 km or even more are possible with overhead lines. Transmission power of up to 600 - 800 MW over distances of about 300 km has already been implemented using submarine cable, and cable transmission lengths of up to about 1,300 km are in the planning stage.

Bulk Power UHV AC and DC transmission schemes over distances of more than 2000 km are currently under planning for the connection of various large hydropower stations in China [11]. Ultra high DC voltage (up to 800 kV) and ultra high AC (1000 kV) are the preferred voltage levels for these applications to keep the transmission losses as low as possible. In India, there are similar prospects for UHV DC as in China due to the large extension of the grid [12]. AC, however, will be implemented in India by EHV levels of up to 800 kV.

2. TYPES OF HVDC TRANSMISSIONS

During the development of HVDC, different kinds of applications have been carried out. They are shown schematically in Fig. 2. First commercial applications were HVDC cable transmissions to transmit power over long distances. AC long distance cable transmission is not feasible due to reactive

Fig. 1: Worldwide installed Capacity of HVDC Links

1970 1980 1990 2000 2010

60

50

40

30

20

10

0

GW Worldwide installed HVDC “Capacity”: 55 GW in 2005Worldwide installed HVDC “Capacity”: 55 GW in 2005

Sources: IEEE T&D Committee 2000 - Cigre WG B4-04 2003

This is 1.4 % of the Worldwide installed Generation CapacityThis is 1.4 % of the Worldwide installed Generation Capacity

An additional 48 GW are expected from Chinaalone until 2020 !

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power limitations. Then, long distance HVDC transmissions with overhead lines have been built because they were more economical than transmission with AC lines [3, 4].

To interconnect systems operating at different frequencies, Back-to-Back (B2B) schemes have also been applied (Fig. 2a). B2B schemes can also be connected to long AC lines.

A further and for the future very important application of HVDC transmission is its integration into the complex interconnected system (Fig. 2c). The reasons for these transmissions are mainly lower transmission costs and the advantage to bypass heavily loaded AC systems [5]. This will be shown in the paper by study results and by the demonstration of the already implemented projects. A multiterminal system using three or more converter stations can interconnect several locations in the surrounding AC network and is at least partly integrated into the power system.

In the past, the synchronous operation of power systems was the common way to build an interconnected system. The best example of this is the development of the UCTE system in Western

Fig. 2: Types of HVDC Transmissions

Can be connected to long AC Linesa)a)

b)b)

c)c) b) HVDC Long Distance Transmissiona) Back-to-Back Solution

c) Integration of HVDC into the AC System

High Costs for System Adjustments (Frequency Controls, GenerationReserve)

Need for close Coordination of joint System Operation

AC Interconnections normally weak at the beginning (additional Lines needed or dynamic Problems expected)

Bottlenecks in the System because of uncontrolled Load Flow

Spinning Reserve in the System to be transmitted over long Distances (additional Loading and Cost increase)

Reduction of economic Advantages because of large TransmissionDistances at relatively low Transmission Voltage (e.g. 400-500 kV)

In liberalized Markets Disadvantages and higher Costs when transmittingPower through a number of Systems and involvement of a number ofPartners

Fig. 3: Problems of large complex interconnected Systems

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Europe which has gradually been extended to today’s very complex configuration.

However, technical problems occur in large power systems due to meshed structures on one hand, and problems of long distance AC transmission on the other hand [6]. To avoid these problems, additional improvements of the system are necessary; the operation of joint systems becomes very complex and is less reliable [7]. The technical limitations of very large interconnected systems have also an impact on the cost benefits of the interconnection. The reasons for these limitations are listed in Fig. 3 [3].

3. LONG DISTANCE TRANSMISSION

Fig. 4 shows both alternatives of long distance transmission, HVAC and HVDC. For HVDC, one +/- 500 kV bipolar line and for AC three 550 kV, alternatively up to two 765kV lines have been assumed. These transmission configurations are nearly equivalent, also with respect to the reliability. The costs in Euro-Cents/kWh, including the cumulated costs of losses, have been evaluated for 2000 MW transmission power over a distance of 900 km. It can be seen that the DC alternative offers much lower costs. Results of other similar studies show that in general the HVDC interconnection is the cheapest solution for transmission distances of about 700 km at a rating of 1000 MW and above (break-even distance). This is also valid if the energy is transmitted through the AC interconnected system. In these cases, however, mostly HVDC offers additional cost advantages. That means that in total the integration of an HVDC transmission into the interconnected system is more economical than the power transmission through the AC system over long distances.

4. STUDY ON INTEGRATION OF HVDC INTO THE AC SYSTEM

4.1. System Representation The advantages of HVDC integrated into an interconnected system have been studied with a network simulation model which can be used as a benchmark system for generic analysis [1]. The reference case of the interconnected system model is shown in Fig. 5. The configuration is similar to existing interconnected AC systems in different areas of the world. The network model consists of 8

Fig. 4: Cost Comparison between AC and DC Long Distance Transmission

HVDC

ACSystem

ACSystem

HVDC

ACSystem

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HVAC

ACSystem

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HVAC

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Transmission Distance 900 kmTransmission Power 2000 MWTransmission Distance 900 kmTransmission Power 2000 MW

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costs

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3 Lines: for Redundancy

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subsystems which are interconnected. The total peak power of the fully interconnected system is in the range of 140 GW. The transmission voltage level of the systems is 400 kV.

The geographical size of each of the subsystems is assumed to be in the range of about 300-500 km in diameter. The peak power of the subsystems differs from about 10 GW (subsystems D, F and G), 20 GW (subsystems B, C, and E) to 50 GW (subsystems A and H). Subsystem H has been assumed to be in the position to deliver additional power to the other subsystems. Each subsystem is represented by a simplified network consisting of lines, loads and power plants as indicated in Fig. 5b). The power generation in the subsystems consists of about 1/3 gas, 1/3 hydro, and 1/3 coal or nuclear power stations. Generator parameters in the calculation correspond to the type of the power plants in use. The subsystems have dynamic features similar to real systems. The data for the reduced systems have been established according to the experiences gained by the reduction of different real systems using network reduction method. The subsystems are interconnected by AC overhead lines of about 250 km length. Power exchange between the subsystems is about 7 % of the total installed capacity, as depicted in the reference scenario (Fig. 5).

The main feature of the operational base case is that the subsystem C is importing power of about 4 GW (corresponding to 20 % of the subsystem peak power) delivered by subsystems A, E, and H. The study on n-1 contingencies in the system shows that some of the line interconnections are loaded close to the transmission limit at an outage of one line in the system. An outage of a second line, especially between the subsystems A, C, and F could, however, lead to full disconnection (blackout) of the subsystem C if no load shedding is applied. These problems could arise not only in the subsystem where the fault occurs, but also in other parts of the system. The developed system model is, according to the n-1 criterion, just secure, however, close to the operational limit.

4.2. Power Import of additional 2000 MW from Subsystem H to Subsystem C

Based on a long term contract, an additional power of 2000 MW should be transmitted from subsystem H to subsystem C. Without investments into the network, transmission of this additional power is not possible as a number of lines in the existing system would be overloaded. The detailed study has shown that additional lines, as presented in blue in Fig. 6, are needed to achieve similar

Fig. 5: Load Flow for the Reference Case a) and Equivalent of a Subsystem b)

526 MW

a) b)

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reliability as in the reference case (Fig. 5). At least three additional lines between subsystems H and G, double lines between subsystems F and C and inside the subsystem F, as well as additional single lines between subsystems A and C and D and C are required.

An alternative to the transmission of additional power through the AC system is the point-to-point 2000 MW HVDC transmission between the subsystem H and the subsystem C, as shown in Fig. 7. In this case, load-flow conditions in the AC interconnected system remain practically unchanged.

The comparison of both alternatives shows that the HVDC alternative is economically the much more favorable solution. Fig. 8 is the result of a classical planning procedure where investments and capitalized cost of losses are evaluated. In total, the AC alternative asks for additional 950 km double and 750 km single lines at 400 kV and corresponding switchyards. For the HVDC alternative, two DC converter stations rated at 2000 MW and one 950 km bipolar DC overhead-line are needed. Using average line costs for moderate climatic conditions, the investment costs of the AC alternative are about 1,100 million EUR, and 765 million EUR for the DC alternative respectively. A further important topic in the comparison of the alternatives is the evaluation of losses. Additional transmission of 2000 MW through the AC system means a loss increase of 285 MW. The losses of the HVDC alternative are, however, only 114 MW in total. If investment costs and costs of losses are taken into account, the AC alternative is about 650 Mio EUR (70 %) more expensive than the DC alternative. In addition to that, HVDC offers important advantages by controlling load flow and damping possible system oscillations.

Under liberalized market conditions, the AC alternative would have further disadvantages as the additional lines needed are located in different subsystems which are not directly involved in the delivery contract to transmit power from subsystem H to C. It would be very difficult and time

Fig. 6: Load Flow of the System strengthened to transmit an additional 2000 MW from Subsystem H to Subsystem C

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consuming to implement such a project. Also, system charges through the subsystems as applied in the liberalized market would be higher than the costs of the HVDC alternative.

5. BENEFITS OF HVDC INTEGRATION INTO A SYNCHRONOUS AC SYSTEM

HVDC can be integrated into a synchronous AC network to reinforce the interconnection of different parts of the system when an increase of power exchange without overloading of weak links or bottlenecks in the existing grid is required. Such a situation is e.g. expected in the German network,

Fig. 7: Load Flow of the Reference System with an additional 2000 MW HVDC Transmission from Subsystem H to Subsystem C

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Fig. 8: Investment Costs and Losses of the Alternatives for additional Transmission of 2000 MW from Subsystem H to Subsystem C.

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when large amounts of renewable energy sources, e.g. wind parks, are to be connected to the northern parts of the grid, ref. to [8]. In 2003, a total amount of about 12 GW wind power has already been installed in Germany. A further increase of 30-50 GW wind power capacities can be expected in the upcoming decades, from which about 50 % will be generated by off-shore wind parks in the north- and east-sea areas.

Fig. 9 shows a typical example of the present conditions. Plotted are wind power infeed and the regional network load during a week of maximum load in the E.ON control area. The relation between consumption and supply in this control area is illustrated in the figure. In this part of the German power system, the transmission capacity is already at its limits, especially during times with low load and high wind power generation. If the northern generation surplus should be transmitted to the southern parts of the grid, network overloading is identified [6].

Power output of wind generation can vary fast in a wide range, depending on the weather conditions. Therefore, a sufficiently large amount of controlling power from the network is required to substitute the positive or negative deviation of actual wind power infeed to the scheduled wind power amount.

The expected controlling power and the installed wind power are depicted in Fig. 10. It can be seen that even at present the need for controlling power is more than 2000 MW.

Both tasks, the transmission of power surplus out of the northern wind generation area and the provision of the controlling power from the generation in central and southern grid parts would additionally load the existing network and lead to bottlenecks in the transmission system. An upgrade of the high voltage transmission system will be essential. As an even more efficient alternative, the integration of an HVDC long distance transmission link into the synchronous AC system is schematically shown in Fig. 11.

Fig. 9: Network Load and aggregated Wind Power Generation during a Week of maximum Load in the E.ON Grid

Additional Reserve Capacity is requiredAdditional Reserve Capacity is required

This will be a strong Issue in the German Grid Development

Problems with Wind Power Generation:o Wind Generation varies stronglyo It can not follow the Load Requirements

Source: E.ON - 2003

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6. EXAMPLES OF IMPLEMENTED PROJECTS

6.1. Gui-Guang HVDC Project – China The 3000 MW +/-500kV bipolar Gui-Guang HVDC system (Fig. 12) with a transmission distance of 980 km was build to increase the transmission capacity from west to east [10-12]. It is integrated into

Share in installed wind energy of 12,223 MW

E. ON Netz: 48 %Vattenfall Europe Transmission: 37 %RWE Net: 14 %EnBW Transportnetze: 1 %

Source: E.ON - 2003

E. ON Netz: 48 %

Share in installed Wind Energy of 12,223 MW

Vattenfall Europe Transmission: 37 %

RWE Transportnetz Strom: 14 %EnBW Transportnetze: 1 %

E. ON Netz: 48 %E. ON Netz: 48 %

Share in installed Wind Energy of 12,223 MW

Vattenfall Europe Transmission: 37 %Vattenfall Europe Transmission: 37 %

RWE Transportnetz Strom: 14 %RWE Transportnetz Strom: 14 %EnBW Transportnetze: 1 %EnBW Transportnetze: 1 %

Benefits of such a Solution:o Load Sharingo Generation Reserve Sharing

Vattenfall Europe Transmission

Long-term: 30 - 50 GWLong-term: 30 - 50 GW

Installed Generation Capacity: 120 GW (2006)Installed Generation Capacity: 120 GW (2006)

Fig. 11: Integration of HVDC Transmission into a synchronous AC System

Fig. 10: Forecast of installed Wind Generation Capacity and required Controlling Power

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the large AC interconnected system. In the same system there is also an already existing HVDC scheme in operation. Both DC systems operate in parallel with AC transmission in this grid.

In addition to that, Fixed Series Compensation (FSC) and Thyristor Controlled Series Compensation have been used in the system. Due to long transmission distances, the system experiences severe power oscillations after faults, close to the stability limits. With its ability to damp power oscillations, HVDC essentially contributes to reliable operation of the system [3, 11].

6.2. HVDC Project Neptune - USA After the 2003 blackout in the United States, new projects are smoothly coming up in order to enhance the system security.

One example is the Neptune HVDC project. Siemens PTD has been awarded a contract by Neptune Regional Transmission System LLC (RTS) in Fairfield, Connecticut, to construct an HVDC transmission link between Sayreville, New Jersey and Long Island, New York. Because new overhead lines can not be built in this high density populated area, power should directly be brought to Long Island by HVDC cable transmission, by-passing the AC sub-transmission network.

Neptune RTS was established to develop and commercially operate power supply projects in the United States. By delivering a complete package of supply, installation, service and operation from one single source, Siemens is providing seamless coverage of the customer’s needs. The availability of this combined expertise fulfills the prerequisites for financing these kinds of complex supply projects through the free investment market.

Siemens and Neptune RTS developed the project over three years to prepare it for implementation. In addition to providing technological expertise, studies, and engineering services, Siemens also supported its customer in the project’s approval process.

In Fig. 13, highlights of this innovative project that are typical for future integration of HVDC into a complex synchronous AC system are depicted

Rating: 3000 MWVoltage: ± 500 kV

Contract: Nov. 1, 2001Project terminated 6 Months ahead of Schedule by Sept. 2004

Thyristor: 5" LTT with integrated Overvoltage Protection

View of the Thyristor-Module

Project completed 6 Months ahead of Schedule by Sept. 2004Project completed 6 Months ahead of Schedule by Sept. 2004

2004

Fig.12: Geographic Location and main Data of Gui-Guang HVDC Project - China

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6.3. East-South Interconnector - India The grid in India has been developed to regional power systems which were operating asynchronously [9]. Later interconnections between regional systems have been made by AC and Back-to-Back HVDC. The first HVDC long distance transmission was Rihand-Delhi which is integrated into the 400 kV AC system. Fig. 14: Geographic Map and main Data of Indian East-South Interconnector

Kolar

Talcher

Kolar

Talcher

2003

Fig. 13: Geographical Location and main Data of Neptune HVDC - USA

Customer:

End User:

Location:

Project

Development:

Supplier:

Transmission:

Power Rating:

Transmission Dist.:

Neptune RTS

Long Island Power

Authority (LIPA)

New Jersey: Sayreville

Long Island: Duffy Avenue

NTP-Date: 07/2005

PAC: 07/2007

Consortium

Siemens / Prysmian

Sea Cable

600/660 MW monopolar

82 km DC Sea Cable

23 km Land Cable

Customer:

End User:

Location:

Project

Development:

Supplier:

Transmission:

Power Rating:

Transmission Dist.:

Neptune RTS

Long Island Power

Authority (LIPA)

New Jersey: Sayreville

Long Island: Duffy Avenue

NTP-Date: 07/2005

PAC: 07/2007

Consortium

Siemens / Prysmian

Sea Cable

600/660 MW monopolar

82 km DC Sea Cable

23 km Land Cable

Ed Stern, President of Neptune RTS: “High-Voltage Direct-Current Transmission will play an increasingly important Role, especially as it becomes necessary to tap Energy Reserves whose Sources are far away from the Point of Consumption”

Ed Stern, President of Neptune RTS: “High-Voltage Direct-Current Transmission will play an increasingly important Role, especially as it becomes necessary to tap Energy Reserves whose Sources are far away from the Point of Consumption”

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The HVDC East-South interconnection (commercial operation in 2003) uses both advantages, the avoidance of transmission of additional power through the AC system and the interconnection of power areas which can not be operated synchronously. Fig. 14 shows the geographical location of the DC Interconnector and its main data. A view of the HVDC southern terminal in the industry region of Bangalore is given in Fig. 15. In April 2006, Siemens has been awarded an order by Powergrid Corporation of India to increase the transmission capacity of the East-South DC transmission from 2000 MW to 2500 MW. After the upgrade is completed, it will be possible to make maximum use of the system’s overload capacity. To increase the capacity of the link, the Siemens experts have developed a solution known as Relative Aging Indication and Load Factor Limitation (RAI & LFL). By these means, it will be possible to utilize the overload capacity of the system more effectively without having to install additional thyristors.

7. CONCLUSIONS Deregulation and privatization is posing new challenges on high voltage transmission systems. System elements are going to be loaded up to their thermal limits, and wide-area power trading with fast varying load patterns will contribute to an increasing congestion.

Environmental constraints will also play an important role. The loading of existing power systems will further increase, leading to bottlenecks and reliability problems. As a consequence of “lessons learned” from the large blackouts in 2003, HVDC will play an important role for the system developments, leading to “Smart Grids” with better controllability of the power flows.

Fig. 15: Site View of Indian East-South Interconnector – DC Station Kolar, close to Bangalore

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HVDC provides the necessary features to avoid technical problems in the power systems; it increases the transmission capacity and system stability very efficiently, and assists in prevention of cascading disturbances.

UHV DC transmission will be applied in emerging countries like India and China to serve their booming energy demands.

The given review of different implemented projects shows examples of hybrid solutions with HVDC transmission integrated into the synchronous AC system. 8. BIOGRAPHY [1] D. Povh, et al.: “Advantages of Large AC/DC System Interconnections” Report B4-304, CIGRE Session 2006, Paris

[2] W. Breuer, et al.: “Application of HVDC for large Power System Interconnections” Report B4-106, CIGRE Session 2004, Paris

[3] W. Breuer, et al.: “Solutions for large Power System Interconnections” 15th CEPSI, October 18-22, 2004, Shanghai, China

[4] Economic Assessment of HVDC Links, CIGRE Brochure Nr.186 (Final Report of WG 14-20) [5] R. L. Lee, et al.: “DC System Support to Southern California during Pacific AC Intertie Failures of December 22, 1982” Proceedings of the Symposium on Urban Applications of HVDC Power Transmission, Oct. 24- 26, 1983, Philadelphia, USA

[6] Load-Flow Analysis with Respect to a possible synchronous Interconnection of Networks of UCTE and IPS/UPS UCTE Study Report, May 8, 2003, Brussels [7] F. Vandenberghe: “State of UCTE Studies on the Interconnection between the UCTE System and CIS & Baltic States” Cigré Conference, September 17-19, 2003, St.-Petersburg, Russia

[8] M. Luther, U. Radtke, “Betrieb und Planung von Netzen mit hoher Windenergieeinspeisung” ETG Kongress, October 23-24, 2001, Nürnberg, Germany

[9] J. D. Wheeler, et al.: “Building India’s Grid: an Examination of the Infrastructure. Benefits of HVDC Transmission” Report 14-114, CIGRE Session 2002, Paris

[10] Li Wengi: “Power Grid development and HVDC Transmission in China” ICPS, September 3-5, 2001, Wuhan, China

[11] U. Armonies, M. Häusler, D. Retzmann: “Technology Issues for Bulk Power EHV and UHV Transmission” HVDC Congress 2006 – Meeting the Power Challenges of the Future using HVDC Technology Solutions, July 12-14, 2006, Durban, Republic of South Africa

[12] W. Breuer, M. Lemes, D. Retzmann, “Perspectives of HVDC and FACTS for System Interconnection and Grid Enhancement”, Brazil-China-India Summit Meeting on HVDC and Hybrid Systems – Planning and Engineering Issues, July 16-18, 2006, Rio de Janeiro, Brazil