• Electrical India
  • Apr 5, 2017

An Overview of Superconducting Transformer Technology

Electrical energy is a major source of the entire process of evolution and modern lifestyle. This paper highlights the developments of the high temperature superconducting (HTSting) power transmission and distribution equipments such as transformer. Demand of energy, electricity generation and power transmission and distribution system require energy efficient equipments and machines... 

- Hambir Singh,
Muvendra Kumar Singh


 Worldwide, demand of fossil fuels and electrical energy for the growing human luxurious life style and economic growth are increasing with 2% annual growth rate. Due to the technical and non-technical reasons, in India, electrical power losses due to theft and technical losses are about 26% compared to international norms of 6-7%. Technical losses include power transmission and distribution (T&D) and end user which need more energy efficient electricity generation machines and demand side efficiency. In the power utility, transformer is the most important devices in T&D network. From power plants, power of high voltages is transported into the grid and to the end user, where electrical appliances operate at lower voltages (100-200 volts). So, transformer must be required for voltage conversions where at each conversion point, electrical energy is wasted in the form of heat in the Cu and Al wires/coil, iron core and cooling tank. After the discovery of high temperature superconductors (HTSrs), commercial applications of 1st, 2nd and 3rd generation (G) of HTSrs are being demonstrated and implemented all over the world; where high temperature superconducting (HTSting) equipments and machines (such as lossless transformers, generators, motors, pumps, cables, bearings etc) offer many benefits (Such as energy efficiency improvement, low pollution, low losses, ecological balance etc) over conventional electrical equipments and machines. HTSrs provide highly efficient transformer with halved size and weight, elimination of hazard oil, reduce leakage reactance, possible fault current limiting feature for improved system performance, reliability and safety. In 1996, world's first HTSting transformer was established in Geneva (Switzerland) by ABB. HTSting materials at and above liquid nitrogen (LN2, 77 K) temperature have the ability to transport electricity with zero resistance. Now, new HTSting materials (Such as Tl7Sn2Ba2TiCu10O20+ with Tc ~65 °C; (Tl5Pb2) Ba2Mg2Cu9O17+ with Tc ~ 28 °C etc) are also being tested to conduct electricity at and above the room temperature so called room temperature superconductors which can carry many times more current than HTSting wires. Losses in HTSrs wires and cables are also significantly lower than the losses in conventional power T&D lines. HTSrs are being operated at 77 K promise to replace the Cu winding of transformers to reduce losses by as much as 70-80%. Technical losses in conductors and equipment (e.g., transformers) of power T&D system are responsible for inefficiency of systems. Here, in this paper author highlights the comparison of the efficiency of HTSting (from 2nd and 3rd G HTSrs) and conventional transformers. Today, 1G (BSCCO) and 2 G (344-YBCO) and 3G (YBCO- multi-filament) of HTSrs have been developed for long length wires and power cables with higher current carrying density Jc, (>100-300 times) than Cu wire (Table 1).

HTSr in the Developments of Transformer

  Developments of superconducting transformers began in the early 1960s when liquid helium (LHe) cooling based low temperature superconductors (LTSrs) NbTi and Nb3Sn were discovered and were available for the commercial and power applications (Table 1). But, uneconomical power devices of LTSrs and high cost of LHe (4.2 K) cooling systems were more difficult to afford for all nations. But, ray of hope was seen in 1986, when La and Cu oxide materials came into existence with the high transition temperature (>35-90 K) and high critical density (Jc) at and above 77 K which have provided the opportunity to reduce refrigeration costs compared to LHe cryogenic systems. Thereafter, many types of HTSting electrical equipments and machines came into existence in the electricity generation and power T&D systems. One of them is power transformer. When transformers are under a loaded condition, Joule heating (I2R losses) of the conventional conductor Cu coil gives more energy losses causing fossil fuels resources depletion and emissions of the pollutants especially greenhouse gases.

  Since the discovery of HTSrs, technology of HTSting transformer has been investigated for reduction of energy losses. The four major techniques used to manufacture HTSting conductors are: (1) powder in tube method, (2) dip coating and other ceramic coating methods, (3) deposition of bi-axially textured thin films on textured buffer layers or substrates, and (4) bulk growth techniques. Now, 2nd and 3rd G of HTSrs (Bi,Pb)2Sr2Ca2Cu3Ox (Bi2223) ;Bi2Sr2CaCu2Ox Bi2212); (Tl,Pb) (Ba,Sr)2Ca2Cu3Ox (Tl1223) and YBa2Cu3Ox (Y123) are being used for all large-scale power applications (Table 2 and 3). 3G HTSting multi-filament (round) wire technology realise a 5 to 10 times decrease in cost, weight and size of electrical machines & equipment and yet can transmit power at room temperature like Cu and Al (Source: HTS10, AMSC USA, etc). The annual energy loss of a transformer in power T&D system depends on the load and load-dependent efficiency of transformer. In a conventional power transformer, load losses are about 80% of total losses. Of this load loss, 80% are I2R losses which are due to the resistance of winding wire. The remaining 20% consists of stray and eddy current losses. A study of US has estimated that annual energy losses are 839.7 MWh for the conventional and 160.3 MWh for the HTSting transformer. It would save about 679.4 MWh of energy per year. The use of HTSrs windings in transformers promise to reduce transformer losses (12R). Total life of transformer is estimated by the life and characteristic conditions of electrical insulation, which depends on the operating temperature and service conditions. Because of the LN2 cooling based HTSrs winding, modern HTSting transformers have very long life if it always operates at cryogenic temperature, 77K even during overload conditions. 

Environmental and Economic Benefits of HTSr Transformers

  Generally, transformers are installed in the population dense city and town, which restrict the use of oil-filled hazardous transformers and numerous accidents have been reported all over the world. But, HTSrs eliminate hazards of oil-filled devices by the LN2 based HTSting technology because nitrogen is an inert and environmental friendly gas, possess no fire hazards and no threat to the environmental conditions comparable to flammable oils and toxic chemicals such as PCBs and paint. Scientists have estimated that increasing energy efficiency of the power generation and electrical machines by HTSrs could reduce energy consumption as much as 20% by 2020 and will save million KW of power (Table 4). And economic benefits for consumers and businesses will increase many fold by the 2020 (Fig.1). Scientific studies have reported that positive impacts of HTSrs include an additional 35% saving in total owning cost over conductor and LTSrs based designs. And life-cycle cost analysis indicates that cost of HTSting transformer may be 30-60% less of conventional transformer. IEA estimates that if HTSrs devices are used in USA then the emissions reduction of carbon, SOx and NOx would be 1.64 million metric tons, 16891 metric tons and 8351 metric tons respectively. For an example: the 50MVA HTSting transformer is expected to be about half the size and weight of a similar rated conventional transformer. It is capable of carrying 50% overload if cooled to 70K by providing extra cooling and it could carry 100% overload if operated at 66K. A comparable rating conventional transformer 50MVA has volume W 2.29m x L 6.40m x H 4.57m. HTSting transformer width and length are about 1.26m x 3.31 m; and height is about 3.26m (~30% less) of the conventional transformer; weight is about 26 tonnes which is about 1/3rd of the conventional transformer (Source: A general cable superconductors white paper, Kalsi Green Power System, 2009). HTSting transformers provide several other benefits compared to a conventional transformer:

Fig 1: Future market comparison of HTSrs and conventional conductor (Source:  IEEE/CSC&ESAS European superconductivity news forum, No.4, April 2008).

• Smaller size transformers are more beneficial in congested city substations, high rise buildings, wind turbine nacelles and railway network.
• Lighter weight is easy for transportation, installation and maintenance.
• Higher efficiency and other conditions save operating cost by reducing feed fuels in power generation, accessories, transportation load, greenhouse gas emissions and other pollution load.
• HTSrs transformers are free from oil cooling and can be installed inside multistory buildings.
• More compact and lighter HTSrs transformers save natural resources (like fossil fuels, underground water, forest wood etc) and curb greenhouse gases emissions generated during manufacturing of equipments and its accessories.

Conclusion

  Progress in the HTSting technologies has developed many energy efficient electrical machines and power equipments. Higher level of Jc of HTSrs promises lower cost and better manufacturability of the electrical machines. Operation of transformer at 77 K gives system performance improvement, lower life-cycle cost, acceptable conditions and mechanical tolerance. Now, 2nd and 3rd G HTSting machines and equipments have achieved higher levels of durability and reliability into a power utility and electricity generation system. However, HTSrs is valuable for natural resource & energy conservation and environmental pollution reduction (especially GHGs emissions). Many scientific results show that HTSting technology can raise the efficiency of power transformer much higher than conventional Cu technology. Further research on the commercial application of room temperature superconductors will change the scenario of power generation and utilisation in future.


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