Colloquium HVDC and Power Electronics to Boost Network Performance October 2-3, 2013 Brasilia - Brazil Possible HVDC Power Transmission Solutions for Remote Hydroelectric Plants in Challenging Environments Andre Balzi ABB Ltda. Brazil Abhay Kumar ABB AB, HVDC Sweden SUMMARY India, China and Brazil are well advanced in the development of remote hydroelectric power plants. Brazil is now considering ever longer transmission distances, with the planning of hydroelectric development in the Amazon region. There are also plans to develop large hydroelectric projects with very long transmission distances in Africa. This increase in transmission distances has led to the use of even higher voltages for HVDC transmission, with ± 800 kv already being used for projects in China and India and several projects are now already in commercial operation. This paper addresses the advantages of using HVDC for bulk power transmission at voltage levels including ± 800 kv and discusses optimum line and converter ratings taking into account the relative size of the receiving network and staged development of the plant. The use of multi-terminal configurations is considered, both as a collector system and to allow separation of converter stations in the receiving network. This permits a reduced number of transmission lines, resulting in reduced rightof-way and consequent reduction in environmental impact. Brazil already has a good history of the use of HVDC to connect remote hydroelectric plants with the Itaipu ± 600 kv, 6300 MW transmission, now in operation for more than 30 years and the very long HVDC transmission from the Rio Madeira complex due to enter into service this year. India is constructing the first ±800 kv Multi-terminal UHVDC link, collecting power from several hydro power stations from its Northeast region, eastern region and, its neighbour Bhutan and transporting it in one bipolar line to the major load centre at Agra, passing though the chicken neck area. KEYWORDS HVDC, Bulk Power Transmission, ±800 kv, Multi-Terminal, UHVDC 1
1. Introduction In recent years countries around the world are expanding its sources of hydro-electricity to remote location where they can still construct new hydro plants. Countries like Brazil, India and China, have explored, in the last 50 years, theirs hydro potential close to the big cities where the major of energy is consumed, and now have to move to those remote locations where the infrastructure is not at the same level as close to the cities. Therefore, a number of challenges arise from this necessity. Constructing the hydro plant is the first one, where a huge number of employees are needed, as well all the necessary steel and concrete. Roads need to be built to transport turbines and generators. The second phase is to transmit the power to the load centres that can be more than 2000 km away. Combining massive power generation, from 4000 MW up to 10.000 MW, coupled with long distances pushes the use of higehr transmission voltage, in order to lower the current and thereby reducing transmission losses. From this need arise the solution of Ultra High Voltage Direct Current (UHVDC), applying ±800 kv to the long distance transmission. 2. Hydro Generation Potential 2.1. Brazil Brazil is the 5 th biggest country in the world and has developed through its 500 years of history; different regions with different industry level and population density. In the South and Southeast there is population concentration and major industry parks. In this area, most of the hydro-generation potential has already been explored and they supply the local load. Itaipu HVDC project, for example, has only 800 km of transmission line, which was, at the time considered to be, a very big distance between the Itaipu damn and Sao Paulo area. The North region, on the other side, have Amazon forest on its heart and large infrastructure projects in this area suffer a lot of difficulties, which arise from their own location and environmental challenges, as well from governmental department that have the mission to give permission for such projects, and require a lot of studies and impact assessment before a permit can be released. Figure 1 illustrates this disparity between regions in Brazil, where it can be noted the agglutination of power plants in the south-southeast region. According to ANEEL [1], Brazilian Energy Regulator, Brazil has more than 100.000 MW of potential hydropower generation, and 70% of it is located on the North region. Two major recent projects under construction are the Santo Antonio and Jirau Damns, located on the Madeira River. Both plants can generate together more than 6.500 MW, and its power will be dispatched via two ±600 kv Bipole HVDC links, 3150 MW each, and two HVDC Back-to-Back, 400 MW each. Another recent project is the Belo Monte Hydro Plant, which is under construction, and will generate more than 11.000 MW. To dispatch its power, two ±800 kv UHVDC (referred hereafter as NS800) links are planned to be constructed, to bring its power to the Southeast region, the inverters being at two different locations. 2
Cartographic Conventions Installed Power per State (kw) Power (kw) Federal Capital Capital State Division 0 a 1.000.000 1.000.001 to 5.000.000 5.000.001 to 10.000.000 Over 10.000.001 Figure 1 - Hydropower Potential in Brazil Until 100.000 100.001 to 1.000.000 1.000.001 to 4.000.000 Over4.000.001 2.2. India The North Eastern region of India has an abundance of hydro power resources, reaching up more than 65.000 MW. The resources are scattered over a large area, whereas load centres are located several hundreds kilometres away. The power has to pass through the so called chicken neck area, a very narrow patch of land in the state of West Bengal having borders with Nepal on one side and Bangladesh on the other side. Also India s neighbour Bhutan has a vast potential for development of hydro power. To transmit part of this power, a ±800 kv UHVDC Multi Terminal System (referred hereafter as North-East Agra or NEA800) is being constructed [2], with 6.000 MW as its rated capacity and with a continuous overload capacity of further 33%. This is the first UHVDC Multi Terminal system in world, and it has two rectifier stations; one located in the North Eastern region in the state of Assam (Biswanath Chariali), and second one 432 km apart from this in Eastern region in the state of West 3
Bengal (Alipurduar). The inverter station with two terminals in parallel is located in the Agra region, distant almost 1300 km from the nearest rectifier station. Each 800 kv converter is configured as single 12-pulse converter. Figure 2 illustrate the North East Agra UHVDC project. ~432 km ~1296 km 3000 MW 3000 MW +800 kv 3000 MW 3000 MW 400 kv 400 kv 400 kv Pole 1 Pole 3 Pole 1 Pole 3 Pole 2 Pole 4 Pole 2 Pole 4 Biswanath Chariali Bipole 1 Alipurduar Bipole 2-800 kv Bipole 1 Agra Bipole 2 Figure 2 - The NEA800 Project The NEA800 transmission system is designed to operate in bipolar, monopolar (ground return/metallic return) and hybrid mode of operation giving a wide flexibility to operate in case of outage of different HVDC line sections and/or different converters. A Master Control facility is provided for the system which shall control and co-ordinate the power flow. In case one converter is tripped, for any reason, during this operation mode, the other pole will automatically compensate, within its capability, for the lost power. In master control mode the compensation will primary affect the other pole within the same station; e.g. in case of a trip of pole 4 in Alipurduar, see Figure 2, the power will be shifted from pole 4 to pole 3 within the capability of pole 3 in Alipurduar. The remaining power will be compensated by the bipole in Biswanath Chariali, provided that Biswanath Chariali is in operation at the occasion. Considering high industrial pollution close to city of Agra, 800 kv equipment in inverter station will be located in indoor DC hall. 3. HVDC Technology Developing long transmission schemes always causes the questioning on what would be the right voltage and power for each application. As the major cost of a UHVDC is on the transmission line, the optimum solution for it is the driver to select the voltage and power level. The evaluation among different option takes into account the capital investment of the converters and transmission line and also the capital related to the losses in the transmission line. Previous studies [3] have shown that for a 2000 km HVDC transmission scheme, the 800 kv solutions is the cheapest one, especially at higher prices for energy losses. Figure 3 illustrates this evaluation. 4
Figure 3 - Evaluation of UHVDC options 3.1. 800 kv optimum line Selecting the best option to the transmission line, after decision of voltage and power, requires evaluation of different cables/conductor and bundle sizes. The main driver at the end is the losses generated by the resistance of the line. In the Brazilian NS800 evaluation [4], 3 different cable/conductor configurations have been studied. The overall evaluation showed that the configurations had very similar investment, with a difference of just 1,1% among them. Figure 4 show the evaluation calculations. Figure 4 NS800 UHVDC alternatives Even though option 1 had the lowest overall investment, the selected option is option 2, using Lapwig (1590 MCM) cables, as it presented the lowest level of losses. 5
3.2. 800 kv Schemes and its Challenges To achieve the desired power, when constructing bulk power UHVDC transmission systems, thyristors had to evolve to achieve a greater level of voltage and current, in order to allow for the best cost/benefit solution when constructing thyristors valves. Today, in UHVDC schemes, the thyristor in use is of up to 6 size [5], which has capacity for 8.5 kv of Peak Continuous Applied Voltage (PCAV) and 4.500 A of current rating. Taking ±800kV UHVDC as example, it has already reached 7.200 MW on a single bipole in Jingping Sunan UHVDC Project in China which is already in commercial operation. Another important decisive parameter is to decide how the converter will be configured and how many numbers of converters per pole will be used in the project. With regards to physical arrangement of thyristor valves whether these will be arranged in bi-valves (also called double valves) or quadricvalves schemes; the main driver for this decision is not the valve, but the transformer as these basically decides the length of valve halls. 3.2.1. Converter Station configurations The converter station can be arranged in a single group configuration, or in series and parallel connected groups [6]. Figure 5 illustrated these possibilities. It can be noted that the series and parallel arrangements have higher availability than a single 12-pulse converter per pole, because in case of failure of one of the converters, the system can still run with the other converter and half of the rated power be still transmitted. This kind of configuration was already used in the past in the ±600 kv Itaipu project, where in case of failure of one converter, the system will transmit power with half of the rated voltage, in this case, 300 kv. But it is important to note that this may not be the most effective solution in every case. In the NS800 project, the performance criteria has been loss of pole or bipole, and thus increasing the number of converters per pole will not help to achieve this criteria, although the overall energy availability will be higher. Figure 5 Possible different converter configuration arrangements 3.2.2. Converter Transformer When constructing transmission systems of high power level, the biggest single equipment will be the converter transformer, which will need to be transported to site. As the site likely will be on a remote 6
location, one can expect that the road infrastructure may limit the weight and transport profile of the transformer. In this case, the system will need to be configured considering the possible maximum weight and size for transport of the transformer. Alternatively site assembly may also be considered as one possible option [7]. The recent Brazilian Rio Madeira project has its rectifier in the Amazon forest region, and its inverter located in the Southeast area. Even though Amazon region have very big restriction to transportation, this project received the biggest ever built HVDC converter transformer (single-phase three winding), shipped to the rectifier station. Due to its location close to one of the major rivers of Amazon, it was possible to take the 400 tons transformer from Ludvika, Sweden to Porto Velho station first by rail and then by ship and ferryboat (figure 6), until unloaded in dry land and transported by 5 trucks to its final destination. Figure 6 - Ferryboat transportation of converter transformer in Brazil On the other hand, the inverter station of Rio Madeira HVDC project, although located in the load centre, have road limitation for transportation and the transformers built had to be of single-phase two winding configuration. The valves were arranged in a bi-valve configuration, while the valves at Porto Velho were arranged in quadric-valves structures. 3.2.3. Multi-Terminal configuration The AC systems connected to an UHVDC transmission system have to be strong enough to withstand a loss of converter, pole and if required by system planning criteria even a bipole. The NEA800 project is conceived with its rectifier stations split by 432 km (figure 2) and serves as pooling stations for several power generation plants in the area. Each converter can convert 1.500 MW, during normal operation, and can withstand 33% overload continuously, allowing the system to compensate for loss of any converter and still transmit rated 6.000 MW. The location of converter stations apart contributes directly for the reduction of environmental impact, placing the stations closely to the group of generation sites, therefore reducing the number and length of AC transmission lines, resulting in reduced right-of-way. On the other hand, some application may require different inverter stations. This is the case of future NS800 project, where the inverter location are separated 300 km from each other, one close to the São Paulo load centre, and the other close to Rio de Janeiro load centre, connected at different locations of the 500 kv AC network. The rectifier stations will normally operate independently, sending power to each inverter location, but they can be connected in parallel due to the various switches in the DC yard which allow for paralleling of poles and/or transmission lines, giving full flexibility to the System Operator to choose the best configuration. This application will be similar to the Rio Madeira HVDC transmission system [8], as described in table 1 and figure 7. 7
Table 1 - Rio Madeira HVDC Operating Configurations Figure 7 - Rio Madeira ±600kV HVDC system The inverter stations of NS800 will be constructed on different times (Figure 8) to match the time schedule of the Belo Monte dam, and also the increase of loads according to the load forecasting of the National Operator. Planning ahead, the most cost effective solution can be chosen, not only on its amount but also on its cash flow. 8
4. Conclusions Figure 8 - NS800 Construction Schedule The development of countries and its demand for increasing energy consumption have generated many power plants projects close to the load centres in past. To provide more and more energy to serve further increased demand of industries and citizens, remote sites have to be selected, especially hydro power plants providing renewable energy. UHVDC plays an essential increasing important role in transmitting bulk power from those remote stations. The use of higher voltages allows concentrating more power on a single transmission line, reducing the necessary number of lines and its right-of-way, contributing directly to the reduction of environmental impact as well as reduces transmission losses. The right design is fundamental to achieve the best solution that fits the demands imposed by challenging environments. Multiple converters can be used on a single UHVDC pole line in order to reduce the size of the transformers and allow them to transport to sites that lack the proper infrastructure for transportation of heavy equipments. The use of multi-terminals UHVDC schemes permits that different sources or load centres be connected in the same transmission scheme, optimizing the solution and allowing full flexibility for system operators. As several UHVDC transmission projects are in commercial operation and in execution in China and India, ±800 kv is already a well established voltage for UHVDC transmission schemes, whereas R&D for even higher voltages are being actively pursued. 9
BIBLIOGRAPHY [1] Agência Nacional de Energia Elétrica ANEEL, Atlas da Energia Elétrica do Brasil, Brasilia: Aneel, 2008. [2] www.abb.com/hvdc, North-East Agra a ±800 kv transmission superhighway / Multi-terminal system with 8,000 MW converter capacity, Ludvika. [3] J.F. Graham, A. Kumar, and G. Biledt, HVDC Power Transmission for Remote Hydroelectric Plants, CIGRE SC B4 Colloquium 2005, Bangalore. [4] EPE, Expansão das Interligações Norte-Sudeste e Norte-Nordeste Relatório R1 - EPE-DEE- RE-063/2012-rev0, Rio de Janeiro: EPE, 2012. [5] V. Botan, J. Waldmeyer, M. Kunow, and K. Akurati, Six Inch thyristors for UHVDC Transmission, International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management 2010, Nuremberg [6] G. Ett, G. Y. Saiki, J. A. Jardini, J. G. Tannuri, L. B. Reis, M. Masuda, M. L. Santos, R. L. V. Arnez, R. P. Casolari, S.O. Frontin, T. Souza, Alternativas não convencionais para a transmissão de energia elétrica estado da arte, Brasilia: Teixeira, 2011. [7] L. Zehong, G. Liying, W. Zuli, Y. Jun, Z. Jin, L. Licheng, R&D Progress of ±1100 kv UHVDC Technology, paper B4-201 Cigré Session 2012 Paris, August 2012 [8] J.F. Graham, T. Holmgren, P. Fischer, and N.L. Shore, The Rio Madeira HVDC System Design aspects of Bipole 1 and the connector to Acre-Rondônia, CIGRE Session 2012, Paris, August 2012 10