Considering DC and AC Transmission Options

June 1, 2012
The final design of the Madeira transmission system reflects a thorough planning process.

The Santo Antônio and Jirau Dams on the Madeira River in Brazil were the first large hydroelectric plants to be constructed in the Amazon. Located near the borders of Bolivia and Peru, these two plants have a combined generating capacity of 6,450 MW. This generation project was conceived to supply the small local load with the energy surplus being transmitted a long distance to the load centers in Brazil's Southeast region.

A transmission system with two ±600-kV direct-current (DC) parallel lines over a distance of 2,375 km (1,476 miles) was planned to interconnect Madeira River power plants with the load centers. The distance and load-transfer capacity of the proposed high-voltage DC (HVDC) transmission system, routed through different regions of the country, imposed a unique and challenging proposition for the system design team at Empresa de Pesquisa Energética (EPE).

Alternative Solutions

In the early planning stages, analyses were undertaken to consider the steady-state operation, dynamic performance and economic evaluation of the project. For 2012/2013, the first stage of the Madeira transmission system would comprise a small number of generators. By the final stage, in 2017, the system would have all generators at the Santo Antônio (44 units) and Jirau (44 units) hydro plants in commission.

Several alternative schemes were investigated, including several circuit topologies, voltage levels and different technologies such as HVDC and high-voltage alternating-current (HVAC) transmission, and combinations of both. The final conclusion of these studies recommended the best technical solution and economic performance was to adopt an HVDC transmission system. Therefore, EPE selected a transmission system with two HVDC ±600-kV lines, each having a transfer capability of 3,150 MW, and two back-to-back devices (400 MW each) for the connection to a 230-kV transmission system to feed local loads.

In addition to the long-distance point-to-point connection, new reinforcements were planned to integrate the Porto Velho local load with the main Brazilian transmission system. Typical ±600-kV DC transmission line guyed towers were chosen for the long-distance transmission.

At this stage of the transmission planning process — in view of the large capital investment for such a huge transmission system for the first time in Brazil and to encourage competition — the Brazilian authority recommended at least three different technical alternatives be considered for the Madeira transmission system bidding process. Following additional studies, three alternatives were recommended:

  • HVDC, as initially selected

  • Hybrid alternative, with HVDC ±600-kV bipole (3,150-MW transfer capability) and two HVAC 500-kV parallel-compensated lines (series and shunt)

  • HVAC alternative, with three HVAC 765-kV parallel-compensated lines (series and shunt).

Complementary and Detailed Analysis

As with all new transmission systems planned in Brazil, the following stage of analysis required the recommended feasible solution to be submitted to complementary and detailed evaluation. This was to ensure it fulfilled all technical requirements to be eligible for the competitive bidding process.

The analysis included transient switching simulation resulting from circuit energization, reclosing, load rejection and short circuit, using electromagnetic transients program-alternative transients program (EMTP-ATP) and EMTDC programs. For the alternatives with HVDC bipoles, power recovery after short circuits, bipole blocking and overvoltages on the HVDC side following faults on the HVDC line were undertaken.

One of the main concerns of these studies, as a result of fault applications on the interconnected HVAC transmission system, was to simulate the accelerating torque submitted to the Madeira low-inertia, bulb-type turbo generators. These analytical studies confirmed the HVDC and alternative hybrid schemes performed satisfactorily for all of the system disturbances simulated.

However, when the HVAC 765-kV alternative scheme was simulated, the energization and reclosure of the 765-kV transmission lines resulted in overvoltages and arrester energy below acceptable limits. Nevertheless, for load rejection, the final stage — with all generators in circuit and the simultaneous opening of the three 765-kV parallel lines — resulted in temporary high overvoltage and, consequently, a high level of energy at the arresters. This imposed the need for a prohibitive number of arrester columns, even when the isolation of the transmission lines at the opposite terminal was an operation strategy.

As a result of this analysis, the redesign of the HVAC alternative scheme was recommended once typical solutions to mitigate the problems identified were not applied. Very long HVAC radial transmission systems above 1,500 km (932 miles) have restrictions in supporting full load rejection. Therefore, the HVAC alternative was discarded as a feasible solution because of the inherent risks.

The Bidding Process

In November 2008, the HVDC and alternative hybrid schemes competed in a bidding process promoted by the National Agency of Electrical Energy (ANEEL), the Brazilian regulatory agency, which resulted in the HVDC scheme being the winner, with annual revenues smaller (7.15% on average) than the established ceiling. Note, the Brazilian regulatory model considers a ceiling annual payment for each new transmission element submitted for auction.

To improve competition, ANEEL split the transmission system in sections to be offered in the auction for the HVDC scheme into seven different lots, or contracts, including the necessary reinforcement to accommodate the main transmission system interconnection to the national network.

The bidding process resulted with different vendors being assigned for each pair of converters (rectifier and inverter) and the HVDC lines. Also, each one of the converter pairs was awarded to different manufacturers. Since then, the integration of these two HVDC bipoles, two back-to-back devices and a master control has become a challenge.

Final Design

The final planning activity required a demonstration by the new owners to ensure the proposed transmission system would fulfill all of the technical requirements previously established. In a complex transmission system, like the Madeira project, a potential number of details had to be discussed with the manufacturers and new owners to ensure the final design conceived by the planners would be ready to be implemented without any constraints.

However, in accordance with the bidding process rules, auction winners have the freedom to make design changes, provided the performance is equal to or superior to the originally selected solution.

As a result, the final solution proposed by the new owners of the Madeira transmission system resulted in some changes:

  • Operation schedule anticipation to match with the generation hydro plants schedule

  • Capacitor commutated converter (CCC)-type back-to-back converters instead of conventional-type converters for local load, with no need for synchronous compensation in Porto Velho

  • HVDC transmission line bipoles provided with all aluminum conductors instead of aluminum conductor steel-reinforced conductors

  • HVDC transmission lines divided into three sections — with a distinct design — in accordance with the local environment, as the long lines cross each region in the country to take into account wind velocity and lightning levels

  • Change in the converter filters' layout, reinforcing the islanding concept

  • One of the converter stations provided with three-winding transformers instead of two-winding transformers.

Completion of the Planning Process

The planning process for the Madeira transmission system — to meet the need for a large power-transfer capability to be transmitted over a long distance, crossing different regions of Brazil — was a huge challenge. In addition, the need to select three transmission system alternative schemes, instead of a single solution, increased the simulation and detailed analysis required.

Even though the feasibility studies indicated the HVDC alternative was the most efficient and economic solution, the complementary and detailed analysis reinforced this indication and recommended a necessity to redesign the HVAC alternative, which was discarded. Redesign would have increased the costs of an already costly alternative.

A new transmission system can reduce transmission costs, as indicated by the competitive bidding results of the Madeira transmission system, with annual revenues for the new Madeira transmission owners averaging 7.15% less than the established ceiling value. And, even considering a reference solution selected by the planner agent, which was EPE, the new transmission owners had the possibility to optimize this solution and, consequently, improve its efficiency, keeping the same or better technical performance than the original solution.

The final design of the Madeira transmission system reflects an efficient planning process, in which the best technical and economical alternatives were selected by the planners and reinforced by the transmission agents and their financial supporters.

Paulo Cesar Vaz Esmeraldo ([email protected]) has a BSEE degree from Rio de Janeiro Federal University and a master's degree in power systems from UNIFEI, Brazil. He joined the Empresa de Pesquisa Energética in 2005 and since then he is the Transmission Planning Superintendent. He has more than 35 years of experience in the power transmission system planning and engineering studies, mainly with FURNAS. He also is part of the IEEE Fellow class of 2000 and was named for the CIGRÉ Technical Committee Award.

Edna Maria A. Araujo ([email protected]) received her BSEE degree from the Catholic University of Belo Horizonte and works for the Empresa de Pesquisa Energética (EPE) at the Transmission Planning Superintendence since 2005. She has more than 30 years of experience in the power and energy industry, mostly in the planning of transmission systems, including Companhia Energética de Minas Gerais. In EPE, she coordinated the transmission studies for the Madeira River hydroelectric project and also is a CIGRÉ member.

Dourival S. Carvalho Jr. ([email protected]) joined the Transmission Planning Superintendence of Empresa de Pesquisa Energética in 2007 and has more than 30 years of experience in the power and energy sector, mostly in consulting companies. He has BSEE degree from Catholic University of Rio de Janeiro, a master's degree in power systems from the Federal University of Rio de Janeiro and a MBA degree from Catholic University of Rio de Janeiro. He also is a member of CIGRÉ.

Impacts of Large Contract Numbers

The manner in which the Madeira transmission project was decided — by auction with tenderers bidding for a large number of contracts — has provoked some interest in the international power system community as it has had an impact on financial, technical and other matters.

From the perspective of the owners and bidders who were awarded contracts, there are advantages and disadvantages of the auction process that Empresa de Pesquisa Energética had to consider.

However, from the consumer's perspective, the advantages seem to justify the auction process. Keep in mind, the Brazilian national grid is an open-access network, and its centralized planning is focused on the consumers, with the main emphasis on low tariffs. All transmission assets integrated into the grid have no owners but rather temporary concessionaires

With the Madeira transmission project, the main potential disadvantages were related to integration difficulties, and different project conception justifies the main advantage of cost reduction and, consequently, smaller tariffs.

Disadvantages and Advantages of a Large Number of Contracts from the Owner's Perspective

Disadvantages Advantages Increases competition with specialized competitors, resulting in price reduction Investment diversity with different projects Difficulties in the integration with other owners From previous specialization and experience in project implementation and operation Potential difficulties sharing substation operations Difficulties establishing interference responsibilities, like harmonics

Disadvantages and Advantages of a Large Number of Contracts from the Contractor's Perspective

Disadvantages Advantages Price reduction in negotiation with owners Value in specialization and experience, during project development and implementation Adjust project conception and schedule with remaining lots Competitive advantage when focus on its technology or cost differentials Difficulties during project integration

Disadvantages and Advantages of a Large Number of Contracts from the Consumer's Perspective

Disadvantages Advantages Difficulties in the integration of different suppliers Potential to attract a number of competitors and improve competition Different project conceptions for similar equipment and systems (for example, filters and converters) Potential to attract qualified competitors, as the lots are separated by type of installation (that is, transmission lines, converters) Closer follow-up during project development, as each lot has limited responsibilities for the entire project Cost reduction per project Precise bidding documents to fulfill all necessary responsibilities for the good of the complete project Low tariffs for consumers

Company mentioned:

Empresa de Pesquisa Energética |

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