Question 1.2-7 A dc transmission line running through a forest terrain is vulnerable to forest fires. If it is a bipolar line then both poles can be affected at the same time. What steps can be taken to minimize this risk? Is the answer to run each pole on a separate tower with sufficient distance between two poles? Are there any concerns about continuous earth return operation for a long time? This issue is of particular importance when considering the increasing number of long distance transmission schemes under consideration or under construction in China and India, respectively mentioned in Papers 110 and 117. Is there any data available on the failure rate of bipolar HVDC transmission lines? What data is available on the types of dc transmission line faults? Should AGB4-04 be collecting this data? Question 1.2-8 The converter losses of HVDC VSC systems can be almost 3 times as that of HVDC Line Commutated Converters, as mentioned in Paper 109. How are the other HVDC VSC advantages capitalized against the higher converter losses? Is there more detailed comparative analysis between VSC and LCC technologies to be reported by authors of the above four papers, from Paper 104 or from other contributors? What are presently the main reasons for selecting/rejecting VSC transmission? In the application shown in Paper 116, were there any issues with ESCR requirements in Sicily and Sardinia and for power reversal to North Africa? To contribute with the questions 1.2-7 and 1.2-8 an introduction to the Madeira Transmission System is a must. The Madeira River Power Plants represent the beginning of the Amazon Basin exploitation and are located about 2500 km from the major load centers of the Brazilian Interconnected System. The transmission studies recently finished indicated the DC technology as the solution of minimum investment and less overall costs. The solution recommended by these studies was 2 bipoles of 3,150 MW each, + 600 kv, 4 x 2312 MCM, 2375 km, to supply the loads in the Brazilian Southeast region, as shown in Figure 1. To local reinforcements (Acre and Rondônia), 2 back-to-back converters of 400 MW each are provided, to connect with the existing system of 230 kv, in parallel with the planned HVDC trunk. The reason for using such devices is to avoid overload conditons in the 230 kv system in parallel with the two bipoles and prevent torsional effects on local thermal plants.
Rio Branco 160 km 305 km S.Antônio Jirau Coletora 30km Back-to-back 2x400MW 41km 150km 165km Samuel Ariquemes 118km 2 x 3150 MW - 2375 km Pimenta Bueno 160km 354km Vilhena Jauru Cuiabá +600 kv 230 kv 335km Ribeirãozinho 364km 242km 200km Rio Verde Itumbiara SOUTHEAST REGION Atibaia 350 km N. Iguaçu 3 x 1250 440 kv 250 km 345 kv 440 kv 138 kv Figure 1: DC Alternative According to the Brazilian regulatory system, the process to implement new transmission facilities is done by auctions, and considering the significant investments associated to the Madeira Project, the Ministry of Energy deemed strategic to bid out, in addition to the alternative of lower cost (DC), also the technology of alternating current (AC), in order to increase competitiveness, once in the recent AC new installations expressive discounts were observed. For the distances involved, however, a purely transmission would demand 4 to 5 circuits and would lead to a very high amount of ohmic losses, around 17% of the transmitted power. So, the AC option to be auctioned would then be the 765 kv, and for that the more promising alternative to that level of voltage was rescued, which would be composed by 3 circuits of 6 954 MCM conductor bundle per phase each. In the evolution of technical studies, however, sustained overvoltage problems and risk of industrial frequency resonance appeared. The lack of time for resizing this alternative, and its cost, already well above that of DC alternative, led to the prudent decision to discard it. Finally, in order to meet the strategy of having two technologies at the auction, to stimulate competition and to grant bigger discounts, a hybrid (HB) DC-AC alternative came into play. This alternative provides the flow of approximately 50% of the power to Southeast using DC technology (1 x 3150 MW bipole - equivalent to 1 of the 2 bipoles of the DC alternative) and 50% in alternating current (2 lines of, 6 x 1033 MCM, single circuit, 70% series compensated).
The supply to the existing regional 230 kv system is accomplished by 3 x 500/230 kv transformers in. The alternative also provides for step down transformations in Jauru, Cuiabá and Água Vermelha substations. The configuration proposed for this alternative is shown in Figure 2. R.Branco Jirau 3300MW 305 km S.Antônio 3150MW Coletora Porto Velho 160 km 3 X 300 30km 41km Samuel Ariquemes 300 km C.Oeste P.Bueno Vilhena 1 x 3150 MW - 2375 km 335 km 3x954MCM Jauru Cuiabá Ribeirãozinho +600 kv 230 kv 200km 380 km Rio Verde 400 km Rio Araguaia A. Vermelha 5 km A. Vermelha existente Itumbiara SOUTHEAST REGION 250 km Atibaia 350 km N. Iguaçu 3 x 1500 440 kv 345 kv 440 kv 138 kv Figure 2: HB Alternative With the experience gained in the preparation of these studies, the Brazilian contributions on the issues raised in WG - B4 are listed: 1. Steps to minimize the risks of forest fires and concerns about earth return operation for a long time: a. To minimize the risk of problems due to fires and winds, a minimum distance of 10 km between bipoles was established; b. The possibility to put converters in parallel, in case of losing one line, together with the capacity of the remaining line of twice the rated current in emergency, increases the reliability as well as reduces the time of system operation with earth return;
c. The possibility to operate lines in parallel, in case of losing one converter, not only reduces losses but also keeps the lines always energized, minimizing the risk of theft. 2. Definition of converters overloading capacity The converters capability to sustain overload is not necessary in alternative HB, since, during the first 30 minutes after any failure in the DC side, the AC system accommodates this overload. After that, re-dispatching becomes necessary, due to the loading limitations of the series capacitor banks. In the DC alternative, a transient overload capability (about 5 seconds) of 50% is essential to prevent frequencies higher than 66 Hz. In a period of 30 minutes, an overload capacity of 10% in the remaining converters is sufficient to ensure the fulfillment of the N-1 criterion in any scenario studied. In the worst of them, losing a pole during the wet period (5 months per year) and with light load in Acre/Rondônia (about 9 hours a day) and with all the 88 generators fully dispatched, the need arises to redistribute a generation amount of no more than 1000 MW along the system. This is accomplished by the machine controllers (speed governors), at the Madeira complex as well as in the rest of interconnected system. According to the above results, and considering that currently the system coexists with losses of generation even larger than 1000 MW, the choice of an overload capacity of about 10% could be an option. However, to ensure some operating security margin to the system, the System Operator suggested as a strategy the requirement of 33 %overcapacity for the DC alternative converters. 3. Use of other technologies For reinforcements in the local Acre/Rondônia system, two back-to-back converters of 400 MW each were considered. It was not found necessary to provide overload capacity for these equipments. Due to the low short-circuit ratio verified at 230 kv busbar, it was necessary to recommend the installation of 3 synchronous compensators of 100 Mvar each. For each of the above, other control or technology related solutions to improve the performance of the CA system of 230 kv can be adopted, provided that they clearly have performance equal to or superior than those obtained in the analyses already prepared. 4. - Definition of voltage transmission According to the characteristics of the Brazilian Interconnected System at the time of the Madeira River power plants implementation (year 2012), a sudden loss of a single ±800 kv bipole carrying close to 6000 MW would certainly imply stability problems, leading to the collapse of the same Interconnected System. Even though it was not part of the planning criteria, the possibility of loss of a bipole was considered as an additional security measure to select two bipoles, regardless of the system voltage.
The choice of the 600 kv level was based on the comparison of the total costs of the ±500, ±600 and ±800 kv transmissions. Figure 3 shows the percentage of investments associated with the construction of 2 bipoles at each voltage level. One can see, as expected, that the investments in lower voltages are lower. The losses, however, have opposite behavior, and an analysis of the overall transmission costs (investment and losses) shown in Figure 4 points out that the transmission at ±500 or ±800 kv has costs approximately 10% higher than those at 600 kv. Comparação de investimentos não comuns % 120,0 100,0 80,0 60,0 40,0 20,0 0,0 2CC5 00 2CC600 2CC800 % 100,0 107,4 114,1 Figure 3 Investm ent (%) 120,0 100,0 80,0 60,0 40,0 20,0 0,0 2CC500 2CC600 2CC800 % 110,5 100,0 108,7 Figure 4 Overal Cost (%) It is interesting to note that, if a single bipole were acceptable, the level of ±800 kv would have greater economic benefits and would become the alternative with the lower total costs. For the assumptions used, therefore, the conclusion is that the alternative of ±600 kv is the one with lower overall costs.