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1 Abstract The power transmission utility in Uruguay UTE has commissioned an HVDC station at the Melo substation to provide a link to the transmission system in Brazil Uruguay s indigenous hydro power resources have been almost fully exploited and can be an un predictable source due to prolonged dry seasons By linking to the much larger Brazilian power system Uruguay will be able to have access to large energy resources and thus provide a medium term solution to their generation issues ensuring security of supply to their customers An existing HVDC converter station which was built in the late 1990 s at Rivera in Uruguay inter connects the power networks of the two countries However this station has limited capacity of only 70MW and can not meet the future energy requirements of UTE This paper will describe the complete AC and DC inter connector project between the San Carlos 50Hz substation in Uruguay via a 325 km 500kV AC transmission line to the Melo converter station and a 116 km 525kV AC transmission line to the Candiota 60Hz substation in Brazil where some of the 60 Hz side harmonic filtering is installed This is a full turnkey project with a new 500kV sub station on the 50Hz side and a new 525kV substation on the 60Hz side to supply the converter station At the San Carlos sub station AREVA is responsible for the design and supply of the 500kV equipment to connect the transmission line to the substation At the Candiota sub station the harmonic filters and the associated equipment to connect the transmission line to the substation are being provided by AREVA Although equipped with local and remote control facilities the Melo station is designed such that it can be un manned in normal operation The project which was awarded in early 2009 is due to be completed in a 31 month period Index Terms Frequency converter Back to back HVDC Nomenclature I INTRODUCTION HE electric power system in Uruguay has an estimated peak power demand of 1 7GW in 2010 and has an anticipated growth rate of 4 to 5 per annum With an installed power capacity of 2 4GW of which more than 60 is produced by hydroelectric power there is an urgent need to C Horwill N M Macleod and R E Bonchang are with ALSTOM Grid UK Ltd St Leonards Avenue Stafford ST17 4LX UK e mail chris horwill areva norman macleod areva and rafael bonchang areva D Castagna M Artenstein and M Croce are with UTE Palacio de la Luz Paraguay 2431 11800 Montevideo Uruguay e mail dcastagna uy martenstein uy and mcroce uy expand the power available in order to be able to supply the demand during dry seasons Clearly this system faces strong challenges in the near future The transmission system operator UTE has studied a number of options to secure the energy supplies for Uruguay One of the alternative solutions is to import power from its much bigger neighbors Argentina and Brazil with the additional difficulty that Uruguay shares the same rated frequency with Argentina 50 Hz but not with Brazil 60 Hz At present the Uruguayan electric system is already strongly connected to the neighboring Argentinean system but it has only a very weak connection of 70 MW capacity with the Brazilian system This is a HVDC back to back converter station at the Rivera sub station supplied by ALSTOM in 1999 The transmission operators in Uruguay and Brazil set up a joint task force to consider the possible methods of inter connecting their EHV networks This group performed a number of planning studies in 2005 and 2006 to consider the options available These studies showed that at as first stage a 500 MW interconnection was feasible both from the technical and economic point of view The planning studies also recommended a future expansion of the interconnection to 1000MW These studies took into account the possibilities of expanding both the transmission capacity at both sides in the medium term and the present and future generation capacity of the South Brazil system During the planning studies one of the alternatives explored was to connect the North East Uruguayan System with the EHV Brazilian system near the city of Porto Alegre through a 1000 km HVDC line The solution selected on technical and economic grounds was to implement the interconnection link through an HVDC back to back Frequency Converter Station to be installed on the Uruguayan side near the Uruguay Brazil border The choice of a back to back solution was favored both for economic reasons and for the operational advantage of expanding the existing EHV AC networks in order to supply in the future new loads both in South Brazil and North East Uruguay The overall project will comprise a new transmission link which will connect the existing San Carlos 500 kV substation in the South of Uruguay to the new Candiota 525 230 kV substation on the Brazilian side In its initial phase of operation the connection point with the existing Brazilian system will be the neighboring Presidente M dici 230 kV substation However in the near future a new substation will be built at Candiota on the Brazilian side of the border and a new 525 kV line will connect the frequency converter station to the Candiota substation and the existing 525 kV Brazilian system A New 500MW Frequency Converter Station to Exchange Power between Uruguay and Brazil C Horwill Senior Member IEEE N M Macleod Member IEEE R E Bonchang Non Member IEEE D Castagna Non Member IEEE M Artenstein Senior Member IEEE and M Croce Non Member IEEE T 978 1 61284 788 7 11 26 00 2011 IEEE 2 The Frequency Converter Station will be constructed near the Uruguayan city of Melo 60km from the Uruguay Brazil border Fig 1 shows the new 500kV transmission line from San Carlos to Melo in red and the 525kV transmission line from Melo to Candiota in purple Fig 1 Geographical location II REACTIVE POWER INTERCHANGE A key feature of an HVDC station is its reactive power interchange with the AC systems on both sides of the station During the planning stage a set of steady state studies was performed in order to evaluate the acceptable limits of reactive power interchange of the HVDC Converter Station with the Uruguayan and Brazilian systems These load flow studies were performed for the Uruguayan side using different scenarios of load and generation for the 2009 and 2012 planned networks Cases were run both in n and n 1 condition and with different scenarios of magnitude and direction of power through the HVDC converter The acceptable limits were defined in order to respect the acceptable 500 kV voltage tolerances both in n and n 1 conditions These tolerances are stated by the Uruguayan Grid Code 3 A more simplified approach was taken when evaluating the Brazilian system In this case a reactive power interchange limit of 70 Mvar was defined at the point of interconnection with the Brazilian grid In this way any possible future voltage limit violation inside the Brazilian grid for not having a 0Mvar interchange could be solved by the installation of a single capacitor and or reactor bank of 70Mvar at Candiota The 70Mvar limit was defined taking into account that the switching on and off of such a possible reactor or capacitor bank will not provoke an excessive voltage step in the Brazilian system The main result of these studies was a set of reactive power interchange curves as a function of the transferred power through the converter as shown in Fig 2 Fig 2 Limits of reactive power interchange Uruguay side Brazil to Uruguay transfer Note In this diagram reactive power is taken positive if the converter absorbs reactive power from the grid A complete set of reactive power limit curves was included in the technical specification as a design requirement for the HVDC station III SCHEME DESIGN The Back to Back frequency converter station connects the 500kV 50Hz system in Uruguay to the 525kV 60Hz system in Brazil The converter station has been designed to have a continuous rating of 500MW which is defined at the inverter terminal at Melo and an overload rating of 550MW at the same point at a maximum ambient temperature of 20 C The scheme is fully bi directional designed to allow power flow from Uruguay to Brazil or in the opposite direction The converter is a monopole design with an earth at the centre point of the thyristor valve on the 50 Hz side giving a DC voltage of 79 3 kV nominal The centre point on the 60 Hz side is earthed through a surge arrester By earthing the converter at the centre point the DC stress on the valve winding of the converter transformers is reduced thus giving the most economic design for the thyristor valve The valves are connected in 12 pulse configuration universally used for HVDC schemes For a 500MW scheme an economic design of converter transformers can be achieved using single phase three winding units These have a primary winding connected in wye by means of buswork outside the building and two valve windings that are connected in wye and delta respectively by connections within the valve hall The valve winding bushings 3 protrude horizontally into the valve hall thus avoiding any external DC insulation A key feature of the design if the station is that the transformers on the two sides of the converter are of a common design Thus they can be operated at 500kV 50Hz and 525kV 60Hz The tap changer has an extended tapping range which covers 50Hz and 60Hz operation although different parts of the tapping range are not used dependent on the side to which the transformer is connected By using a common design only one spare unit is required which provides a considerable economic advantage to the owner Single circuit overhead transmission lines connect to the two AC systems to the converter station One connects San Carlos substation in the south east of Uruguay to Melo and the other connecting Candiota substation on Eletrosul s system in the south of Brazil to Melo The transmission line from San Carlos to Melo is 325km long and the line from Melo to Candiota is 116km long approximately half being in Uruguay Each transmission line has shunt reactors at both ends Partly because of the length of the lines the minimum short circuit level on both sides of the converter is relatively low The minimum short circuit level on the 50 Hz side is 1250 MVA with an X R ratio of 4 and on the 60 Hz side it is 1400 MVA with an X R ratio of 9 For most of the time the short circuit level is expected to be close to the lower limit The substations at Melo are both breaker and a half designs and both have room for expansion to feed a second converter pole The 50Hz substation has a switched shunt reactor for voltage control The harmonic distortion produced on the AC systems by the converter station is subject to limits imposed by the transmission operators on both sides of the station In order to comply with these limits there are four harmonic filters on each side of the converter three being required for operation of the converter The fourth filter on the 50Hz side is redundant and the filter on the 60Hz side is normally redundant unless the bus voltage is too low in which case it is switched in to increase the voltage The filters are described in detail elsewhere in this paper To meet the harmonic requirements at Candiota two filters are installed at the substation One filter has to be energized before the converter is energized and stays in service during converter operation The second filter is redundant During the initial start up of the converter station in order to meet reactive power and harmonic distortion limits one filter on each side is energized before the converter is de blocked and the second and third filters on each side are switched on at preset power levels as the power demand is increased As is normal practice for Alstom Grid even for non seismic zones the thyristor valves are suspended from the ceiling in the valve hall and incorporate 125 mm thyristors which are rated at 8 5kV each The valve has a current rating of 3150A DC Each valve has 24 thyristor levels of which two are redundant The single circuit cooling system uses a mixture of pure de ionized water and mono ethylene glycol for operation in ambient temperatures below 0 C 32 F A fully redundant digital control system will be installed comprising two independent pole control suites of cubicles one for each side and a station control suite The converter is designed to be controlled from the remote control centre in Maldonado close to Montevideo In normal operation the station is designed to be unmanned Control is also possible from a control point in a building close to the converter The auxiliary supplies are derived from a neighboring substation and also from a secondary winding on the line reactor on the 50Hz side at Melo All of the auxiliary plant is run at 50Hz There is also provision for a standby diesel generator in the event of failure of both normal supplies The cooling plant is fed from a UPS which can supply the plant for the time taken to start and load up the standby generator The main sources of audible noise associated with the converter station are the transformers and associated coolers the harmonic filter reactors and capacitors and the fans on the radiators associated with the valve cooling plant There are also some oil filled reactors in the substation Alstom Grid carried out a detailed acoustic noise study to ensure that the defined limit of 50 dB A is met at the property boundary The study showed that the noise limit is most difficult to meet directly opposite the converter station By suitable location of the harmonic filters and the installation of noise barriers round the transformers it was possible to achieve the noise limit at the station boundary The compliance with the noise limits was made easier to achieve because the property boundary is some distance outside the substation In the early planning stages for this station UTE took the decision to purchase additional land to mitigate any compliance issues with regards to audible noise The switching of the thyristor valves has the potential to create high frequency electromagnetic interference both radiated from the valves and conducted through the AC connections UTE specified limits for both conducted and radiated interference from the station Alstom Grid performed studies to ensure that these limits are met The valve hall is constructed as a Faraday cage to reduce the RF emissions from the valves and the CVTs which are connected to the filter busbars have tuning units to reduce the conducted interference from the converter IV SINGLE LINE DIAGRAM The single line diagram of the Melo HVDC converter station is shown in Fig 3 There are four identical switched filter banks shown as F on the 50Hz side and two sets of two identical switched filter banks shown as F on the 60Hz side Details of the filter design are given in section V 4 Fig 3 Melo single line diagram As the point of common coupling on the 60Hz system at which harmonic performance is assessed is at the end of a dedicated 525kV line it was decided to install part of the harmonic filters at the Candiota sub station As shown in Fig 4 there are two identical switched filter banks shown as F Details of the filter design are given in section V Fig 4 Candiota single line diagram V HARMONIC FILTERS The harmonic filters in an HVDC station need to be designed to meet a number of design criteria maintain harmonic performance at the Point of Common Coupling PCC on both the 50Hz and 60Hz to levels within the customer s declared limits to minimize reactive power imbalance with the two AC networks to minimize fundamental frequency losses whilst providing harmonic damping i e harmonic losses to meet the performance limits The harmonic performance for the station was defined in the UTE Technical Specification in terms of the following factors 1 Individual distortion limits 50Hz and 60Hz sides see Table 1 2 Total harmonic distortion THD less than 3 0 3 Telephone Influence Factor TIF less than 40 4 Telephone weighted current IT less than 130 000 These limits apply at the Melo 50Hz busbar and the Candiota 230kV or 525kV busbars 5 Non integer harmonic distortion caused by the cross modulation of 50Hz and 60Hz harmonic components was limited to the minimum of the closest integer harmonic from Table 1 TABLE 1 INDIVIDUAL HARMONIC DISTORTION LIMITS Order Order Order 5 2 0 3 2 0 2 1 0 7 2 0 9 1 0 4 1 0 11 1 5 15 0 3 6 0 5 13 1 5 21 0 2 8 0 4 17 1 0 21 0 2 10 0 4 19 1 0 12 0 2 23 0 7 12 0 2 25 0 7 25 0 5 A key issue in the design of the harmonic filters is the modelling of the harmonic impedance of the AC systems Impedance sector models were provided by UTE for the 50Hz and 60Hz both 230kV and 525kV systems The impedance sector diagram is shown in Fig 5 The parameters for the impedance sectors were defined for each harmonic Fig 5 Impedance sector diagram 50Hz system As shown in Fig 1 on the 60Hz side of the station there is a dedicated 116km long 525kV transmission line to the Candiota station At Candiota there is a 75Mvar switched reactor and a 525 230kV transformer to connect to the 230kV transmission system The filter solution for the project used the novel approach of installing part of the filters on the 60Hz side at the converter station and part at the remote Candiota sub station which was the PCC The complete filter design was 50Hz side Melo 4 off 110Mvar triple frequency tuned filters 11th 14th 24th 5 60Hz side Melo 2 off 96Mvar triple frequency damped filters 13th 24th 36th 2 off 96Mvar triple frequency damped filters 3rd 13th 47th 60Hz side Candiota 2 off 65Mvar double frequency damped filters 3rd 11th During the filter design studies the potential for a low order harmonic resonance between the filters and the AC system was identified on the 60Hz side This required the inclusion of 3rd harmonic filtering which was conveniently included within multiple tuned filters No such low order resonance was identified on the 50Hz side This filter solution achieves the required