THE GARABI 2000 MW INTERCONNECTION BACK-TO-BACK HVDC TO CONNECT WEAK AC SYSTEMS

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1 THE GARABI 2000 MW INTERCONNECTION BACK-TO-BACK HVDC TO CONNECT WEAK AC SYSTEMS John Graham Birger Jonsson R.S. Moni Brazil Sweden India ABB Utilities AB, SE Ludvika, Sweden SUMMARY The Argentina - Brasil interconnection first phase of 1000 MW, reported elsewhere, [1] has recently been increased to 2000 MW total power delivery. The second phase is very similar to the first, also employing a back-to-back HVDC converter at Garabi, in Brasil, close to the border with Argentina. A back-to-back HVDC converter can be used when two asynchronous AC systems need to be interconnected for bulk power transmission or for AC system stabilization reasons. In an HVDC back-to-back station there are no overhead lines or cables separating the rectifier and the inverter, hence the DC current can be kept high and the DC voltage low. The low DC voltage means that the air clearance requirement is low, which is in favor of a compact design of the valve housings. This enabled the modular back-to-back HVDC concept to be developed. The back-to-back HVDC converter stations utilized at Garabi are of the modular type with Capacitor Commutated Converters (CCC). The CCC is used to ensure satisfactory performance at low short circuit levels and to reduce the impact of reactive power variations. 1 THE GARABI PROJECT 1.1 Introduction In April 1997, the governments of Argentina and Brazil signed an agreement to facilitate crossborder energy trading between the two countries. On May 5, 1998, the Brazilian Ministry of Mines and Energy, through Eletrobrás, Furnas and Gerasul, and the Argentinean government, signed a 20-year contract with CIEN, Companhia de Interconexão Energética, to import 1000 MW from the wholesale energy market in Argentina. CIEN is a company of the ENDESA group. As the Argentinean system operates at 50 Hz and Brazil's operates at 60 Hz, the inter-connection is made through HVDC frequency converters in back-to-back configuration, located at the Garabi Station in Brazil, close to the border with Argentina. Both phases link the 500 kv networks of Brazil and Argentina through 500 kv transmission lines, each nearly 500 km in length. This first phase of the Garabi Project entered operation in June 2000 and has been reported elsewhere [1]. In August of this year the second phase was put into commercial operation, raising the total transfer capacity to 2000 MW. The second phase is very similar to the first, employing the same modular back-to-back converters, with Capacitor Commutated Converters (CCC). Both phases link the 500 kv networks of Brazil and Argentina by 500 kv transmission lines, nearly 500 km in length, running from the Rincón de Santa Maria substation near Yacyretá in Argentina, to the Itá substation in Brazil. The Argentinean system operates at 50 Hz and Brazil's operates at 60 Hz, and the back-to-back frequency converters for both phases are integrated into the Garabi Station in Brazil, close to the border with Argentina. A simple one line diagram for the first phase in given in fig (1) below:

2 Figure (1): Garabi HVDC Back-to-back station, Phase 1, Simplified One Line Diagram It can be seen that the first phase has a connection with the 230 kv system in southern Brazil via a transformer station at Santo Ângelo. This was not foreseen at the time of initiating the work on the interconnection and was installed by Eletrosul to reinforce the system in the State of Rio Grande do Sul. The phase two interconnection does not connect at Santo Ângelo, although physically passing close in order to facilitate a future connection. Like the Garabi 1 project, the supply of the second 1100 MW phase also is based on a full EPC (Engineering, Procurement and Construction) contract including 500 kv transmission lines and the substation extensions in both countries. Engineering included all studies to satisfy requirements of national agencies as well as engineering studies to ensure that the system works satisfactorily under the wide range of operating conditions. Guarantees on the electrical performance include availability greater than 97%. This was possible because both contracts were based on technical specifications with true functionality, concentrating principally on the values of power to be transmitted and the physical location of the substations. This meant that each supplier could bid his most cost effective technical solution as long as the functional requirements were met and necessary guarantees were sufficient to fulfill the clients criteria. 1.2 System aspects The two electric systems are rather large, yet they are connected at relatively weak points within their networks. The Argentinean integrated system (SADI) has an installed capacity of about MW. However, the Rincón de Santa Maria substation is at the northeastern extreme, and although adjoining the Yaciretá generating station, may have a relatively low short circuit capacity. This can vary between 9000 and 3000 MVA, depending on the number of connected generators at Yaciretá. This has remained virtually unchanged between the two phases of the interconnection. The infeed point to Brazilian S/SE interconnected system was defined at the Itá 500 kv substation for the power purchase. The integrated Brazilian system has an installed capacity of MW, but the Itá substation is on the southern extension, remote from major generation, although a power plant of 1450 MW is coming online with one 290 MW generator now in operation. At the time set for commercial operation of the phase one, the short circuit capacity at Itá was calculated to be about 6000 MVA, dropping to 3500 MVA under contingency conditions, however now at entry of phase two the level is MVA, dropping to 5000 MVA under the worst considered contingencies. The location of the converter station, close to the proposed Garabi Hydroelectric power plant, was also defined in the power purchase documents for phase one and so set conditions for the complete interconnection. This effectively

3 defined the 50 Hz connection point as Rincón de Santa Maria, giving a transmission line length of 136 km, duplicated with an identical parallel line for phase two. However, in the case of the line to Itá, which has a length of 354 km, the first phase has a connection at Santo Ângelo, mentioned above, while the second goes directly, although the system is designed to permit the connection to be made. In addition the station at Garabi is designed to operate with the phase one and phase two converters either connected in parallel or operating individually. This gives a set of rather challenging operating conditions for a converter station where there is a guaranteed delivery of two times 1000 MW into a rather weak network. In order to fulfill all required operating situations, systems studies were carried out by ABB together with CEPEL (Centro de Pesquisas de Energia Electrica) to check compliance with the requirements of the system operators, both in Brazil and Argentina [2]. These studies included load flows over the full operating range at the given system voltage limits, verification of reactive interchange at the connection points and 500 kv / 50 Hz load rejection overvoltages. For the 50 Hz side, with the relatively short transmission line this presented little problem. The long 60 Hz line was a considerable challenge to avoid compensation at an intermediate point. For the chosen transmission line parameters, the short circuit capacity at the Garabi phase two 60 Hz side is about 2000 MVA, dropping to about 1600 MVA under contingency conditions. This was met by using Capacitor Commutated Converters(CCC), with minimum-sized ConTune harmonic filters. In this way fixed line reactors are used and the CCC allows that the converter has characteristics to absorb or supply reactive power as required by the system. The converter acts like a static compensator, giving smooth continuous control of voltage and power flow. The minimum size of the ConTune filters help to keep load rejection overvoltages within limits. However for the phase two expansion additional switched shunt reactors are needed on the 60 Hz side to cover all operating conditions. The resulting system configuration for the final arrangement with both phases in parallel is shown in figure kv / 60 Hz Block 4 Block 3 Garabí 2 Line 2 Tie-breakers Line 1 Shunt reactor Line 2 Tie-breakers Line 1 Block 2 Garabí 1 Block 1 AC filter Figure (2) Integrated Bus Arrangement The above diagram appears to be quite different from the arrangement used for the first phase, however in fact the two parts are identical with

4 respect to converter equipment and associated performance. The apparent difference comes from the change to a double breaker scheme for the 500 kv switchyards, compared to the modified ring bus used in phase one. This gives greater flexibility when the two phases are integrated. The second phase maintains the philosophy of having a filter bank associated with each converter and using a spare switched phase to be available for either of the converters. This allows the use of minimum sized ConTune filters, which alleviates the effects associated with line energisation and load rejection when large shunt capacitance is present. The filter banks have a total of four branches, two ConTune for 11 th and 13 th harmonics, plus two passive high pass tuned at 24 th and 36 th. The inductances of the tunable branches are adjusted continuously to compensate for temperature and frequency deviations The total fundamental frequency rating for the filter bank is 85 Mvar per 550 MW converter. With this rather low value the studies also have to take into account the value of the PLC noise filters connected on the lines from the station. These have fundamental ratings of 9 Mvar on the 50 Hz side and 22 Mvar on the 60 Hz. The CCC capacitors, as all converter design, are identical for the two phases, with values of 190 Mvar per 550 MW converter on the 50 Hz side and 322 Mvar on the 60 Hz side. The larger value on the 60 Hz side permits supply of reactive power to the system as required at full load. Thus it can be seen that the CCC capacitors not only fulfill the role of ensuring satisfactory commutation at low short circuit levels, but also satisfy steady state requirements of reactive power. This is illustrated in figures (3) and (4), and further discussed in item 2 below. U d CCC γ = constant Conventional γ = constant Qdc [MVAr] Qmax Qmin I d [pu I dn ] Pdc [MW] Figure (3) Ud vs Id Characteristics Figure (4) Reactive Power, 60 Hz Side 1.3 Integration As noted in the earlier paper the Garabi Project employed many new features: Commutated Converters (CCC) required to permit operation with low short circuit levels at the inverter Self tuning AC filters (ConTune) One redundant AC filter phase Modular out-door valve enclosures A new PC based control system, MACH2 AC breakers with optical CTs (Compact)

5 These features aided the integration of the two phases, especially the MACH2 control system and the 500 kv HPL Compact switching module. In the first case the powerful computers used for the control system permit the rearrangement of the control and protection systems as determined by the new layout. Further the integrated system can be tested in the factory to ensure that the software changes to the existing equipment will be satisfactory. This test was carried out prior to shipment of the actual phase two control equipment. These principles of the MACH2, namely extremely powerful computers, fully integrated software for all protection, control and monitoring functions, plus a comprehensive factory system test, are not unique to the modular back-to-back concept, but are utilized in all HVDC project now supplied by ABB [4]. Secondly the modular switchgear, designed primarily for ease of installation, can also be moved rather easily. The HPL Compact switching module includes all functions performed by a traditional circuit breaker bay; HPL circuit breaker, pantograph disconnectors, earthing switches, digital optical current transducer, common base frame and pre-wired control cubicle. The module is mounted on a single foundation for each phase. This results in a substantial reduction of required space and facilitates easy and fast maintenance of the breaker bay. This has resulted in rather short outage times being required for the necessary relocation of the switching modules from the ring bus in Garabi 1 to the double breaker scheme used for the integrated station. Figure 5 below shows Garabi 1 on the left with the original ring bus and Garabi 2 to the right with the double breaker scheme. Figure (5): Garabi 500kV yards and ConTune Filter Area The 500 kv transmission lines and the first converter block associated with phase two went into service in April this year (2002). The complete phase two started commercial operation in August following extensive system tests. This is via connection directly to Itá and the full integration of the two phases has yet to be completed. However the integration is a complex process involving both scheduled outage times and system testing. This process is on going and programmed for completion only next year.

6 2 MODULAR BACK-TO-BACK CONCEPTS 2.1 The CCC/ConTune Combination The converter used in the CCC concept is characterized by the use of commutation capacitors inserted in series between the converter transformers and the converter valves, see Figure 6. below. It makes it possible to operate HVDC in very weak networks and eliminates the need for synchronous compensators. Fig. (6): Single line diagram of a monopolar station with CCC and ConTune filter. This capacitor commutated converter is coupled with the ConTune filter in order to give the maximum benefits from a low shunt connected capacitance and a controllable reactive power source in the converter. As described in [3], the approach offers better electrical performance than a conventional converter by virtue of the following factors: Switchable shunt filter banks are no longer required for reactive compensation and are replaced by a series capacitor bank in which the reactive generation is a function of the load current flowing through the capacitor. Small fixed filter banks, optimized for filter performance, can be used providing lower load rejection overvoltages. Figure 7 indicates the reactive power balance variation for conventional and CCC based converters. The CCC based converters give a smooth variation avoiding voltage steps. The series capacitor provides a more positive resistance characteristic at the inverter as shown in Figure 3. In a conventional inverter, an increase in direct current results in a reduction in inverter commutating voltage due largely to a reduction in AC network voltage. With a series capacitor at the inverter, increasing current results in an increasing voltage across the capacitor, offsetting the drop in the AC network voltage. It is this factor that permits stable operation with low short circuit ratios and better commutation failure immunity. Another manifestation of this factor is the increased maximum power curve [3]. The series capacitor s impedance (dxc) is typically several times greater than the transformer s impedance (dxl), increasing the total secondary side impedance and reducing transformer and valve currents during DC side short circuits. Even though higher valve voltage ratings are required, the lower current stresses allow optimization of the converter transformer and valve. The series capacitor inserts a leading phase shift between the transformer secondary and the valves, compensating the naturally lagging characteristic of the converter. The converter s power factor, seen from the transformer s primary side, can be kept close to unity, or even become leading, allowing the converter to generate reactive power as may be noted in Figure 4. In Figure 4 the Qmin curve indicates reactive power generation, while the Qmax curve refers to absorption. Fig. (7): Reactive power conditions for a typical conventional converter and for a CCC.

2.2 Modular Construction The converter valves are in modular housings, factory assembled and tested, and were shipped to the site ready for reassembly and operation.

In order to optimize the costs of the thyristor valves, as well as the DC side equipment, the DC current is kept as high as the standard thyristor rating and the valve cooling system can handle

7 2.2 Modular Construction The converter valves are in modular housings, factory assembled and tested, and were shipped to the site ready for reassembly and operation. The control equipment and auxiliaries were also factory assembled and tested to reduce the installation and commissioning time. In order to optimize the costs of the thyristor valves, as well as the DC side equipment, the DC current is kept as high as the standard thyristor rating and the valve cooling system can handle safely. This means that the DC voltage can be kept fairly low, and is thus chosen to meet the rated DC power required. The low DC voltage means that the air clearance requirement is low which favors a compact design of the valve housings. This has enabled the modular back-toback HVDC concept to be developed. The thyristor valves are air insulated at atmospheric pressure and installed in modular valve housings. In the case of Garabi each valve module contains two single valves, which means up to six valve modules per twelve-pulse converter. The modules are positioned in stacks of two, giving a quadri-valve type arrangement. This gives a convenient layout for the use of single phase, three winding converter transformer, as can be seen in figures 8 and 9. Figure(8): Converter Area The thyristor valves are suspended within the modular housing and are easily accessible for the maintenance crew. The housings also contain the surge arresters connected across the valves. The valve control units can be seen external to the valve modules, besides the access door. Figure 8 also shows the valve cooling modules, each one close to the converter block. This houses the water pumps and water treatment system. The water-to-air closed circuit coolers are located immediately behind the valve cooling module. This modular housing approach is used for all auxiliary and control equipment. The principle of factory assembly and testing has been used extensively to ensure complete and reliable delivery as well as fast erection times. In addition a weatherproof location with a controlled environment is ensured Figure (9): Valve Modules The modular concept has also been used for all converter bus breakers as described above, employing compact design, with breaker, disconnects and optical current transformer integrated into one unit. The modular concept used in the station, extended to converters, auxiliaries and switchgear has the considerable advantage of simplifying civil works, the design process being now restricted essentially to foundations, without the need for complex interfaces between buildings and equipment. This results in reduced costs for civil works and reduced construction times. The complete station, with converter capacity of 2200 MW, is constructed on an area of 367m by 670m, giving a very high utilization of land. In environmentally sensitive areas this is also considered a great advantage from a permitting

8 point of view. The ISO certification states that the design of the equipment and the construction at the site are performed in an environmentally acceptable way. 3. CONCLUSIONS The Argentina - Brasil interconnection, having now entered into commercial operation of the two 1100 MW phases, demonstrates the advantages of the modular back-to-back concept. The HVDC converters at Garabi are notable for two main characteristics, use of the CCC/ConTune combination and the construction using the modular design concept. The use of the CCC/ConTune combination permits secure operation at low short circuit levels and improves performance with respect to reactive power control. The modular design concept simplifies the civil design and ensures a safe and environmentally acceptable solution, as well as permitting reduced delivery times. REFERENCES 1. J. Graham, G. Biledt, Mauro Baleeiro, José Ramón Diago, Fco Javier Bugallo; Garabi the Argentina Brasil 1000 MW Interconnection; VIII Erlac, Cidade del Este, Paraguay, May John Graham, Don Menzies, Geir Biledt, Antônio Ricardo Carvalho, Wo Wei Ping, Acacio Wey; Electrical System Considerations for the Argentina-Brazil 1000 MW Interconnection; CIGRE Biennial, Paris, Don Menzies, Hans Eriksson, Fabiano Uchoas Ribeiro, Commissioning The Garabi I Back-to-Back Converter Station, Erlac Modern Control and Protection System for HVDC, Torbjörn Karlsson, Mats Hyttinen, Lars Carlsson and Hans Björklund 5. Capacitor Commutated Converters for HVDC, Tomas Jonsson, Per-Erik Bjorklund, IEEE Power Tech Conference, Stockholm, 1995.

REFURBISHMENT OF SECONDARY SYSTEMS IN HIGH VOLTAGE SUBSTATIONS LESSONS LEARNED IN VENEZUELA

21, rue d'artois, F-75008 Paris http://www.cigre.org B3-110 Session 2004 CIGRÉ REFURBISHMENT OF SECONDARY SYSTEMS IN HIGH VOLTAGE SUBSTATIONS LESSONS LEARNED IN VENEZUELA by E. PADILLA * L. CEDEÑO E. PELAYO