PAPER Title Dry-type transformer innovation: HiDry 72 for subtransmission lines Registration Nº: (Abstract) Authors of the paper Name Country e-mail Martin Carlen Switzerland martin.carlen@ch.abb.com Mariano Berrogain Spain mariano.berrogain@es.abb.com Alfonso Quilez Spain alfonso.quilez@es.abb.com Key words dry-type power transformer, subtransmission, indoor substation, GIS, fire-safety Summary HiDry 72 is the safe and environmentally friendly power transformer solution in drytype technology for 72.5 kv voltage class subtransmission lines. The transformer is available for power ratings of up to 63 MVA and can be delivered with an on-load tap changer. Substations containing dry-type power transformers means superior fire and explosion safety for people and property and allows simple installations in buildings and underground locations. These advantages make the dry power transformer a product perfectly suitable for applications in urban substations, in substations located near to or in public buildings, or in caverns and in power plants. Since the transformers do not contain any liquids, they are also the preferred choice for substations located in water-protection areas. The transformers are suitable for indoor or outdoor installations. In this paper we present the basic technology of the HiDry 72 transformer and its technical characteristics. Further we report on two recent substation installations in Europe and the Americas and show the set-up of the respective substations. In the first installation two 25 MVA, 69 kv HiDry 72 transformers are installed in a Brazilian soccer stadium where games of the 2014 FIFA World Cup have taken place. The second case is a substation containing two 12 MVA, 66 kv HiDry 72 transformers supplying an industrial plant in Spain.
Introduction Dry-type transformers do not use any inflammable liquids for insulation or cooling purpose. Their big advantage compared to liquid-immersed transformers is therefore the excellent safety. This makes them the preferred choice for all applications were safety and the potential consequences of a transformer fire or explosion are of primary concern like installations in or close to buildings. In the past the use of dry-type transformers was only possible in the distribution grid since there were no products available for voltages >36 kv. Dry-type transformers for the 52 kv voltage class were introduced about a decade ago. They are meanwhile used in a number of utility and industrial installations and proved their high reliability. Another step in voltage has recently been done when ABB introduced the HiDry 72 transformers. These are dry-type transformers for the 72.5 kv subtransmission voltage level. CIGRE has published in 2013 the brochure Guide for transformer fire safety practices applying to power transformers rated >10 MVA and 66 kv [1]. A survey on transformer failures concluded that about 10% of all transformer failures results in a fire and that the annual fire rate is about 0.1%, i.e. one out of 1000 power transformers catches fire. Assuming 50 years lifetime, one out of 20 transformers catches therefore fire during its life. This fire risk is the main reason why power transformers are rarely installed in public, commercial or industrial buildings and transformer substations are mostly placed outdoor. In case of indoor installations with liquid-immersed power transformers quite expensive means for fire protection have to be installed. With having dry-type power transformers available, this changes and compact indoor substation installations can be made when combined with GIS or hybrid switchgear equipment. Expensive space in buildings can be minimized and used for other purpose. Underground substations in downtown areas allow to provide higher power into the city centers and to reduce transmission losses. In general, installation and application of dry-type transformers is simple and fast. The transformers have high strength against short-circuit or other mechanical loads and don t require maintenance. No risk for oil spillage and contamination of soil or water and no risk of failure due to leaking transformers exists. This of course also applies for HiDry 72. Another important advantage of the dry-type transformer technology is the lack of a tank and bushings containing liquids, which might break under application of vibrations or strong acceleration. In regions with high risk for earthquakes or for demanding applications like wind turbine tower installations, the mechanical structure of HiDry 72 can easily be additionally reinforced. Dry-type transformers use air as main insulation medium. The dielectric strength of air is considerably less than the dielectric strength of transformer oil and dry-type transformers require therefore larger insulation distances, resulting in a larger and more costly active part. In case of indoor installation this higher cost of the transformer is mostly easily compensated if a cost comparison is made for the whole substation installation. In this paper we outline the main features of the HiDry 72 dry power transformer technology. We present two examples of HiDry 72 indoor transformer installations, the first in the Salvador di Bahia Fonte Nova Arena, Brazil, one of the FIFA 2014 Soccer World Cup stadiums, and the second in an industrial plant in Spain. In both cases the transformers are equipped with on-load tap changer (OLTC), a component necessary for most installations at subtransmission voltage level. HiDry 72 dry-type power transformer technology The base for the development of the 72.5 kv dry power transformers were the well-established ABB medium voltage transformers in VCC or RESIBLOC technology. But the higher voltage, a higher rated power and an increased range for voltage regulation lead to several technical challenges. An excellent understanding of the underlying physics and intensive use of FEM based simulations in combination with experimental testing were required for materializing a reliable new product. The dielectric stress within a dry-type transformer is taken up by solid and air insulation. Due to the higher permittivity of the solid insulation, the electric field in the solid material is reduced and the main insulation comes from the air. The dielectric strength of air is therefore the limiting factor and defines maximum electric fields and minimum insulation distances. In order to keep dielectric clearance distances as small as possible, we have introduced a number of new concepts for dry-type transformers like shielding rings in the windings, shielding of core parts, and the application of multiple barrier concepts and barrier arrangements. 2 / 6
Magnetic stray fields can cause eddy currents in the windings, which can become very significant with increased power rating and growing cross section of the conductors, and create additional eddy losses and hot spots at unexpected locations. These eddy losses strongly depend on the winding design. When the OLTC is connected at its minimum position and part of the winding turns are deactivated, the magnetic stray fields and eddy currents are especially strong and local hot spots can easily be generated. Figure 1: Development of 72.5 kv dry-type power transformer: electric field simulation with electric stream lines, stray losses in structural components and busbars, and testing beyond the limits Although there is presently no IEC or IEEE standard for 72.5 kv dry-type transformers, our ambition was to fulfill the same demanding requirements for partial discharge as the standard requires for transformers up to 36 kv, i.e. a maximum level of 10 pc at 1.3 U n. This guaranties that no degradation of the insulation due to PD is taking place and that the transformer will have a long lifetime. HiDry 72 dry-type power transformer characteristics The HiDry 72 transformers offer identical functionality as 72.5 kv liquid-immersed power transformers. The design of the transformer can be adapted over a large range in order to match the specific customer requirements, like e.g. the short-circuit impedance or the change of the impedance with tap position. The latter is especially important in case the transformer is connected in parallel to existing liquid-immersed power transformers. The key characteristics are shown in Table 1. Rated power up to 63 MVA Partial discharge <10 pc Primary voltage up to 72.5 kv Insulation class F (155 C) or H (180 C) Lightning impulse voltage Short duration AC withstand voltage 325 kv for IEC 350 kv for ANSI/IEEE 140 kv for IEC 140 kv for ANSI/IEEE Environmental class Climatic class Secondary voltage up to 36 kv Fire class F1 Connection group Y or D Cooling AN, ANAF, AFAF, AFWF Tapings Table 1: 17 positions, + 8 x 1.25% Enclosure Characteristics of 72.5 kv dry-type power transformer HiDry72 E2 C2 IP00 (none) IP21 (indoor) to IP54 or IPX4D (outdoor) In case of indoor installation, the transformer does not need to be protected by an enclosure. It is sufficient to install some means to make sure that nobody is able to touch the energized transformer. For outdoor installations, enclosures of different protection degree can be used, depending on the specific installation requirements. For applications where a high amount of dust is present like e.g. in mining, the use of a high IP enclosure together with an air-to-air or an air-to-water heat exchanger is also possible. HiDry 72 dry-type power transformer installations A number of HiDry transformer indoor and outdoor substation installations have been realized and are in operation. In the following we present two installations in more detail. Further information is presented in [2-4]. 3 / 6
Indoor 69 kv substation in soccer stadium For the 2014 FIFA Soccer World Cup in Brazil a new stadium, the Fonte Nova Arena, has been erected in Salvador da Bahia (Figure 2). The stadium offers 55 000 seats and is located in the center of the city, which is inhabited by 2.7 million people. It was inaugurated in April 2013, well in time for the 2013 Soccer Confederation Cup. The stadium is supplied by a 69 kv cable line and has an integrated HV/MV substation (Figure 3). Safety and reliability are key features for this substation where tens of thousands of people might be affected in case of an incident. This safety aspect, together with cost considerations, made the local utility Coelba decide to install two 69 kv dry-type transformers, each rated 25 MVA. Figure 2: Fonte Nova Soccer Arena, Salvador da Bahia, Brazil, equipped with 69 kv/25 MVA drytype transformers The incoming 69 kv feeder connects to a 72.5 kv hybrid gas-insulated switchgear (PASS). The substation has a redundant configuration, with two transformers and HV switchgear. The transformer secondary voltage is adjustable to two voltage levels: 11.95 kv or 13.8 kv and connects to the MV switchgear. Cable ducts are located below the transformer and allow a compact overall arrangement. The VCC coils are of the demanding E2 environmental class for dry-type transformers and guarantee safe long-term performance also in the hot and humid Brazilian climate. The transformer is cooled by natural convection (AN). It is designed and tested for a lightning impulse voltage of 350 kv. The primary transformer coils are equipped with dry-type on-load tap changers (OLTC), offering a regulation range of +4/-12 x 1.25%. For each coil a single-phase OLTC device is placed in front of the coil. The OLTC uses vacuum interrupters for switching. Figure 3: 69 kv substation integrated in the stadium and 25 MVA, 69 kv dry-type power transformer with OLTC. The PASS switchgear is located on the right side of the picture and connected via open busbars to the transformer HV coils. The transformer is installed without an enclosure. Placing a simple fence is sufficient to prevent unintentional contact of service personal or other people with the energized transformer. Requirements for maintenance of transformer and OTLC are minimal. The experience gained so far with this substation is very positive, installation was simple and fast and operation is unproblematic. Indoor 66 kv substation in industrial plant ABB operates in Cordoba, Spain, a 22 000 m 2 factory for the production of large pwer transformers (Figure 4). The factory was originally established in 1930. Transformers for voltages up to 765 kv are produced. A specialty of the factory is the production of large shell-type power transformers which are exported to the whole world. The factory is located close to downtown Cordoba and the railway line, 10 minutes from the high speed train station. 4 / 6
Figure 4: Factory for production of oil-immersed Power Transformers in Cordoba, Spain. The 66 kv substation containing the dry-type power transformers for the supply of the factory and the test lab are placed in the building located at the top center of the picture. The factory is supplied by a 66 kv cable line and has its own substation. For both, technical and safety reasons, a full upgrade of the entire electrical installation of the factory (HV, MV and LV) became necessary. The project included a complete 66 kv gas insulated switchgear (GIS) substation, 2x12 MVA HiDry 72 transformers, ten primary MV switchgear (ZX0.2) with 2x8500 kva, 20/5 kv drytype transformers; and six secondary MV switchgear with 6x800 kva, 20/0.4 kv dry-type transformers. The 8.5 MVA transformers supply the test lab, the 800 kva transformers the different parts of the factory and compressors. There are additional feeders for external 3 rd party supply. A sketch of the layout is shown in Figure 5. The planning started in 2011, the HiDry 72 transformers were produced and tested in 2012, installation of all equipment took place in 2013/14 and the final permission for operation of the new substation was received in spring 2014. Figure 5: Layout of 66 kv/20 kv/5 kv/0.4 kv substation for electrical supply of factory The new substation was placed in an existing building, formerly used for storage purpose. An additional floor was put up in order to have about 1 m of space to pass all cables below the floor. An older shed located along the building was replaced by two new simple cabins for installation of the transformers. No oil pits, fire walls or fire protection equipment are needed. The dimensions of the cabins are 9x8.6 m 2 and leave plenty of space to easily walk around transformer and OLTC. Cables are entered via a cable duct placed in the floor and passing between transformer and OLTC. The 72.5 kv GIS consists of three bays, one bay connecting to the incoming line and two bays connecting to the HiDry 72 transformers (Figure 6). Each bay contains a circuit breaker, a disconnecting/earthing switch, a current transformer and an additional earthing switch. The incoming feeder bay and the common GIS busbar contain additionally voltage transformers. Figure 6: Substation room with 72.5 kv GIS on the right side and 20 kv switchgear in the background 5 / 6
Selection of dry-type transformers for this application allows to place all transformers directly into or at the factory without having any fire or explosion risk. Secondary side cables can be kept short and the required amount of cables and the related losses are reduced. The HiDry 72 transformers have a power rating of 12 MVA and a voltage of 66/20 kv. Installation of two transformers guarantees redundancy. The power rating of one transformer is sufficient to supply the factory and the test lab. The transformer has low load losses and the ratio of load loss to no-load loss is 1.9. From a poor economic point of view it is therefore more beneficial to only connect one transformer, also if the transformer operates under full nominal load. For environmental reasons, the European Union (EU) has recently decided to introduce a Regulation for transformer losses. Starting from July 2015, all new transformers must fulfill the requirements. For power transformers a minimum efficiency is required, corresponding to the peak efficiency value (PEI) in the efficiency versus loading curve. For a 12 MVA dry-type transformer this value is PEI=99.36%. The installed transformers already exceed this requirement. Cooling of the transformer is by natural convection (AN). The transformers are equipped with an OLTC giving a total of 17 steps of 1.25% for voltage regulation. Figure 7: Installation of dry-type transformer: unloading of 12 MVA dry-type power transformer from truck (left), placing of transformer for moving into transformer room (middle) and transformer and OLTC ready for getting connected (right) Figure 7 shows the unloading of the transformer after transportation on a low-bed trailer truck from the dry-type transformer factory in Zaragoza to Cordoba. The experience made during the transformer installation was very positive. The transformers were unloaded by a truck crane, and placed in front of the transformer cabin. The OLTC was transported separated from the transformer. The final positioning of the transformer and its connection on primary and secondary side and to the OLTC were simple and fast. Figure 8 shows how the primary side of the transformer is connected to the 66kV cable. Figure 8: Connection of the dry power transformer to the 66 kv cable Bibliography [1] A. Petersen et al, Working Group A2.33 CIGRE, Guide for transformer fire safety practices, CIGRE brochure 537, June 2013 [2] M. Berrogain, M. Carlen, Dry-type transformers for subtransmission, paper 1140, CIRED, Stockholm, June 2013 [3] M. Carlen, M. Berrogain, Dry-type subtransmission transformer installations and potential grid interactions, CIGRE SC A2 & C4 Joint Colloquium 2013, Zürich, Sep. 2013 [4] M. Carlen, M. Berrogain, R. Cameroni, M. Spiranelli, Dry-type subtransmission transformer: compact and safe indoor substations, CIGRE, Paris, Aug 2014 6 / 6