HTS Transformers.pdf

Research Communication

Research Communication P2.1 Vol. 1, No. 1, December 2007

Development and Technology of HTS Transformers

Xiaoyuan Chen and Jianxun Jin Center of Applied Superconductivity and Electrical Engineering

School of Automation Engineering University of Electronic Science & Technology of China

Chengdu, Sichuan Province, 610054, China

Abstract--With the improvement of high temperature superconductor (HTS) practical performance, research and application development of HTS transformers have been progressed actively in the world. The current focus on the applied superconducting technology for HTS transformers has been moving to the industrial preparations from its laboratory research stage, and the technology has been well verified for practical applications from small to large scales. This paper provides a comprehensive and analytic summary on the development of HTS transformers with regard to various HTS power transformers and HTS traction transformers. Common configurations and main benefits of HTS transformers are also presented in the paper.

Index Terms--High temperature superconductor (HTS),

HTS power transformer, HTS traction transformer, HTS winding, AC loss.

I. INTRODUCTION

With the discovery of HTS with higher temperature, electrical applications of HTS have received more attention than low temperature superconductor (LTS) [1-6], particularly for the benefits in commercial operations. HTS transformer has been expected to be one of the most promising HTS device applications. Recently research and development concerning application of HTS transformer have been progressed actively in the world. The long-term market for HTS transformers will become larger and larger with the continued development of HTS transformers.

II. DEVELOPMENT STATUS

Development of HTS transformers that mainly achieved in the United States, Japan, Europe, China, Korea and some other countries currently have two main directions, namely power transformers and traction transformers. The development status to HTS transformers is summarized as follows.

A. Power Transformer

1). United States The United States development program – Supercond-

uctivity Partnership Initiative (SPI) project has a subproject of HTS power transformer project, with a purpose to establish HTS transformers of medium-to-large (>10MVA) ratings through three stages: 1 MVA to 5/10 MVA to 30/60 MVA [7].

A single-phase 1 MVA (13.8kV/6.9kV) HTS power transformer using Bi-2212 coated conductor and a three-phase 5/10 MVA (24.9kV/4.2kV) HTS power transformer using Bi-2223 coated conductor has been developed in 1998 and in 2004, respectively [8-9]. The

main specifications of the 5/10MVA transformer are shown in Table I [10]. Assembly of the 5/10 MVA prototype transformer was complete and tests have been completed as shown in Fig.1 [11].

TABLE I

MAIN SPECIFICATIONS OF THE 5/10 MVA HTS TRANSFORMER Phase 3

Capacity 5/10 MVA

Voltage (prim./second.) 24.9 kV / 4.2 kV

Current (prim./second.) 67 A / 694 A

Cooling method Cryocoolers (below 30K)

Wire Bi-2223

Fig.1. A 5/10 MVA HTS power transformer

An innovative commercial HTS transformer design that

operates at transmission-level voltages of at or above 138 kV has been investigated. Its principal goals are to adapt the design for 2nd generation YBCO HTS wires and use more conventional transformer manufacturing technology. The ultimate goal is to fabricate and test a pre-commercial beta prototype HTS transformer. As the costs of producing HTS wire continue to fall, plans are made for the construction of a 30/60 MVA HTS transformer incorporating with YBCO wires in near future [12-13].

2). Japan In Japan, Kyushu University group has developed a

single-phase 500 kVA (6.6kV/3.3kV) HTS transformer in 1996 [14] and a single-phase 1 MVA (22kV/6.9kV) HTS transformer in 2001 [15].



Recently, a single-phase 2 MVA (66kV/6.9kV) HTS transformer has been manufactured and tested since 2004, so that the essential technology of 10 MVA (66kV/6.9kV) class HTS power transformer could be developed in advance. Its main specifications are shown in Table II, and the completed 2 MVA HTS transformer is shown in Fig.2 [16]. It was confirmed that all test items which had been shown in JEC-2200 (The standard of the Japanese Electrotechnical Committee JEC) were satisfied. Test results indicate that essential, high voltage and large current technologies were established for the development of a 10 MVA (66kV/6.9kV) class HTS power transformer.

TABLE II MAIN SPECIFICATIONS OF THE 2 MVA HTS TRANSFORMER

Phase 1

Capacity 2 MVA

Voltage (prim./second) 66 kV/6.9 kV

Current (prim./second) 30.3 A/290 A

Cooling method Sub-cooled LN2 (66 K)

Wire Bi-2223

Winding Layer

Fig.2. A 2 MVA HTS power transformer

In order to improve the total efficiency, controllability and stability in the future power system, a superconducting fault current limiting transformer (SFCLT) was proposed. The SFCLT has the functions of HTS transformer since it can reduce leakage impedance of HTS transformer and improve the system stability and capacity in normal operating condition. In the fault condition, It serves as superconducting fault current limiter (SFCL) because the limiting impedance induced by the quench of SFCLT winding will reduce the fault current and bring about the improved dynamic stability of the power system [17-19]. So, the longer-term benefits resulting from HTS SFCLT include simple configuration for elimination of the need for other current limiting devices, low leakage impedance for the enhancement of power system stability and increase reactive power availability from generators.

Japan has already designed and fabricated a HTS

SFCLT in 2004 and verified fundamental characteristics as a transformer and a FCL. The main specifications of the HTS SFCLT are shown in Table III [20].

TABLE III

MAIN SPECIFICATIONS OF THE HTS-SFCLT Phase 1(3)

Capacity 2.08 kVA (6.25 kVA)

Voltage (prim./second) 159 V/61 V (275 V/105 V)

Ic (77K, self field, 1 µV/cm) 57 A (coil A), 63A (coil B)

Cooling method LN2 (77 K)

Wire (prim./second) Bi2212/Copper

In 1991, a single-phase 2.5kVA (300V/150V) air-core

autotransformer (SCACAT) using LTS technology is fabricated [21]. After successful load and no-load measurements [22-24], the theoretical analysis of the 3-phase air-core superconducting transformer has been researched [25]. However, it seems that the research is currently suspended. It is expected to develop air-core transformer by using HTS technology in the coming research.

3). Europe In Switzerland, ASEA-Brown-Boveri (ABB) has built a

three-phase 630 kVA (13.72kV/0.42kV) HTS transformer using multifilamentary BSCCO-2223 wire produced by American Superconductor Corp. [26]. The completed HTS transformer was taken into operation in the power grid at the spring of 1997 and operated successfully for one year.

Initially, ABB, American Superconductor, Los Alamos National Laboratory, Southern California Edison and American Electric Power had an intention to develop a 10 MVA transformer [27]. The 10 MVA transformer was to serve as the prototype to validate the design of a 100 MVA transformer. But this project was finally abandoned because the expected return on investment could not be met based on the cost of HTS wire. Eventually, ABB plans to create a 100 MVA transformer. The main specifications describing this transformer are shown in Table IV.

TABLE IV

MAIN SPECIFICATIONS OF THE 100 MVA HTS TRANSFORMER Capacity 100 MVA

Voltage (prim./second.) 225 kV/20 kV

Cooling method Closed cycle

Volume savings 5%

Weight savings 20%

Load loss savings 80%

Up-front cost 150%

Life-cycle cost 90%

Some other countries like Italy, France, Hungary and Spain also start to research possible applications of HTS transformers in recent years. In Italy, a single-phase 10 kVA (1000V/231V) HTS transformer has been manufactured, using copper for primary windings and Bi2223 HTS wires for secondary windings [28-29]. In France, a single-phase 41 kVA (2050V/410V) HTS



transformer has been designed and built [30]. In Hungary, a small-size self-limiting transformers with YBCO wires were designed, built and tested in Budapest University of Technology and Economics [31]. In Spain, a solenoidal air-core HTS prototype transformer based on Bi2223 wires was constructed [32-33].

4). China The Tebian Electric Company (TBEA) HTS power

transformer project which is supported by the Chinese “863” program and TBEA started in 2001, in order to develop a three-phase 630 kVA (10.5kV/400V) HTS power transformer. A three-phase 26 kVA (400V/16V) HTS transformer [34] and a single-phase 45 kVA (2400V/160V) prototype HTS transformer were firstly designed and fabricated [35]. And after that, a three-phase 630 kVA HTS power transformer with amorphous alloy cores was completed and tested in November 2005 [36-37]. The main specifications of the three-phase 630 kVA HTS transformer are summarized in Table V. The completed HTS transformer is shown in Fig.3.

TABLE V

MAIN SPECIFICATIONS OF THE 630 KVA HTS TRANSFORMER Phase 3

Capacity 630 kVA

Voltage (prim./second) 10.5 kV/0.4 kV

Current (prim./second) 34.64 A/909.33 A

Cooling method LN2 (77 K)

Wire Bi2223

Winding (prim./second) Solenoid/Double pancake

Fig.3. A 630 kVA HTS power transformer

It successfully operated in live power grid and has

reliably ability of long-term operation [36]. So, it is the second three-phase HTS transformer which has reliably ability of long-term operation in live power grid since only one three-phase HTS transformer developed by ABB had ever operated in live power grid for one year. Also, it represents the first HTS transformer with amorphous alloy cores in the world.

5). Korea In Korea, a single-phase 1 MVA (22.9kV/6.6kV) HTS

transformer was fabricated and tested in 2004 [38]. Its main specifications are shown in Table VI.

TABLE VI MAIN SPECIFICATIONS OF THE 1 MVA HTS TRANSFORMER Phase 1

Capacity 1 MVA

Voltage (prim./second) 22.9 kV/6.6 kV

Current (prim./second) 44 A/152 A


Wire Bi2223

Winding Double pancake

21st century frontier research and development activity

is taking place with the goal of developing the three-phase 100 MVA HTS transformer in 2011. Since 60 MVA (154kV/23kV) transformer is the standard one for the distribution power systems in Korea, a 60 MVA (154kV/23kV) HTS transformer using YBCO coated conductors was designed in 2004 [39]. Its primary winding and second winding are double pancake type and solenoid type, respectively. And in 2006, s single-phase 33 MVA (154kV/22.9kV) HTS transformer with on load tap changer (OLTC) was designed [40], for 154 kV class three-phase power transformer is composed of 3 individual single-phase ones and single-phase transformer can be directly applied to the three-phase one. As for its windings wound with YBCO coated conductors, primary winding is continuous disk type, secondary winding and tertiary winding are layer type.

Recently, the Korean power company starts to design a HTS transformer on a common magnetic core, because it can be a possible and effective solution for the increment of capacity without new construction of substations. This year, a three-phase 100 MVA (154kV/22.9kV) HTS transformer on a common magnetic core was conceptually designed [41].

B. Traction Transformer 1). Germany Siemens and GEC Alsthom LINDE cooperated and

began to develop 10 MVA HTS electric locomotive traction transformers in October 1996, with an aim to apply in high-speed rail system in Germany. Its goal is to reduce the weight of traction transformer from 12 t to 7.7 t by using HTS technology. Siemens companies claimed that, if HTS technology is applied, the efficiency would increase from 94% to 99% and the volume would reduce by 30~40% as compared to traditional traction transformers.

A nominal single-phase 100 kVA (5.5kV/1.1kV) transformer was firstly constructed in 1999 [42]. After that, a single-phase 1 MVA (25kV/1.4kV) transformer was completed in 2001 and run a variety of characteristic tests [43]. Siemens then planned on manufacturing a full-size prototype that could handle several MVAs. However, it seems that development is currently suspended.



2). Japan With the cooperation of Kyushu University, Fuji

Electric Systems, and Taiyo Nippon Sanso, the Railway Technical Research Institute (RTRI) began to develop HTS transformers that could be installed on bullet trains in the later half of the 1990’s. Recently, a floor type single-phase 4 MVA (25kV/1.2kV) HTS traction transformer that was designed for Shinkansen rolling stock was fabricated and tested in 2004 [44-47]. The specifications are shown in Table VII and the completed HTS traction transformer is shown in Fig.4 [48].

TABLE VII MAIN SPECIFICATIONS OF THE 4 MVA HTS TRANSFORMER

Primary winding 4 MVA, 25 kV, 160 A

Secondary winding 3.6 MVA, 1.2 kV, 750 A *4 windings

Tertiary winding 400 kVA, 440 V, 909 A

Cooling method Subcooled LN2 (66 K)

Wire Bi-2223

Winding Solenoid

Fig.4. A 4 MVA HTS traction transformer

The maximum output of the 4 MVA traction transformer

maintaining the superconductivity was approximately 3.5 MVA, with the value of AC loss is about 6.2 kW. It is able to allow 750 A flowing to the secondary winding corresponding to a total output of 4.0 MVA, and the calculated AC loss is 7.9 kW at 4 MVA output. The HTS wires could not decrease AC loss, AC loss therefore became larger and was anticipated to be over 6 kW. Also, the rated capacity is equivalent to 3.5 MVA in the superconducting state, which is some what lower than the designed value, because it was unable to use high-performance HTS wires as presumed at the design stage [48]. So, whether the designed value could be achieved or not is still unknown and should be verified in the coming research. Additionally, the floor type was chose at the first research stage to realize some possible problems of under-floor type and the ultimate design goal should be a transformer of under-floor type.

III. ELEMENTARY CONFIGURATION

Generally, a HTS transformer is primarily consisted of a winding, a magnetic core, a cryostat and a refrigerator. Its common configuration is shown in Fig.5 as follows.

Fig.5. Principle structure of a HTS transformer

A. Windings HTS wires which are commonly used in high voltage

power transformer can be divided into two types: the 1st generation Bismuth Strontium Copper Oxides (BSCCO) HTS wires, and the 2nd generation Yttrium-Barium- Copper Oxide (YBCO) HTS wires [49-50]. For BSCCO, it is mainly applied in two forms: Bi2Sr2CaCu2O (Bi2212) and Bi2Sr2Ca2Cu3O (Bi2223). At present, Bi2223 has been more applied than Bi2212 since its critical temperature is 20 K higher than Bi2212.

To deal with high voltage, the superconducting power transformer needs windings which have hundreds of turn [51]. A common windings configuration in HTS transformer is shown in Fig.6. BSCCO wires have been used to make the HTS windings. HTS winding with YBCO wires begin to be considered because YBCO wires have higher current density and better current magnetic field characteristics than BSCCO wires [52-54].

Fig.6. A winding and core configuration for a HTS transformer

Solenoidal winding, pancake winding and layer winding are commonly used ones in HTS transformers. In Korea, a new winding that can be called continuous disk winding (CDW) was proposed for HTS transformers [55]. The CDW has advantages such as good insulation and limitation of voltage stress and low ac losses compared with the layer winding and double pancake winding.



B. Core

A suitable magnetic core has also been studied to reduce the HTS transformer losses. In China, the TBEA HTS power transformer project has developed the first HTS transformer with amorphous alloy cores in the world [36]. In Spain, a straight solenoidal geometry is now considered and studied in an air-core transformer [32-33]. In Korean, the Korean power company has considered the possibility of a HTS transformer on a common magnetic core since it can be a solution for the increment of capacity without new construction of substations [41].

C. Cryostat

Cryostat is commonly manufactured by fiber reinforced plastic (FRP) in shape of toroid that core can go thorough in midway and have vacuum and super-insulation layer to minimize thermal invasion. Also, it acts as insulator between core and winding. Its configuration is shown in Fig.7 [41].

Fig.7. Cryostat configuration

D. Refrigerator

HTS wires, in which the critical temperatures are above 77 K, enable the cooling system to be simplified. This aspect makes it very advantageous when compared with the LTS system.

However, the critical current of HTS materials decreases steeply as the magnetic field, especially the perpendicular component of the field, is applied. To overcome this weakness, the HTS materials can be cooled down below 77 K to increase the critical current. However, the operation in low temperature makes the cooling system more complicated.

Recently the subcooled system is used worldwide and simply shown in Fig. 8. It cools down the temperature to about 65 K so that the HTS critical current at the zero applied field may be up to about 1.5 times of the critical current in 77 K. In addition, the subcooled nitrogen is superior over the liquid nitrogen in electrical insulating properties so that it may be easier to design the high voltage HTS power machines [56].

Fig.8. Refrigerator configuration

Cooling system for a three-phase 100 MVA class HTS transformer is designed in Korea [57]. The operation temperature is set at 67 K to increase the critical current and reduce the amount of HTS tape usage and the volume. However, the size of current cooling system is not small and the cost of cooling system is more expensive than the HTS winding. So, the current cooling system is not good enough to satisfy the temperature requirement and need to be improved in the coming research.

IV. PRINCIPAL BENIFITS

One big difference between HTS transformers and conventional transformers lies in the use of HTS wires including BSCOO coated conductor and YBCO coated conductor, rather than traditional copper or aluminum. Meanwhile, HTS transformers differ from conventional transformers in many ways due to different operating temperature of windings. The main differences are summarized in Table VIII.

TABLE VIII DIFFERENCES BETWEEN HTS TRANS. AND CONVENT. TRANS.

Item HTS Trans. Conventional Trans.

Wire HTS wires Copper or aluminum

Core Common core or low Tc core

Common core

Cooling method LN2 or refrigerator Oil or air

Insulating material LN2 or low Tc insulating material

Oil or common insulating material

Cryostat FRP N/A

The advantages of HTS transformers compared to

transformers with normal conductivity are well known and are described in detail in [58] and [59]. A number of benefits over conventional one include lighter and smaller units, higher efficiency and substantial energy savings, lower life-cycle costs, fewer hazards due to the reduced fire risk and reduction of environmental pollution.

In this section, a practical design for HTS transformer in Korea was chose to testify varied benefits of HTS trans -formers. In 2004, a three-phase 60 MVA (154kV/23kV) HTS power transformer was designed with YBCO coated conductor in Korea [39]. Its main specifications were shown in Table IX.



TABLE IX MAIN SPECIFICATIONS OF 60MVA HTS TRANSFORMER

Phase 3

Capacity 60 MVA

Voltage (prim./second) 154 kV/23 kV

Current (prim./second) 225 A/1506 A


Wire YBCO

Winding (prim./second) Double pancake/Solenoid

The HTS transformers were designed according to

voltage per turn (V/T) and they were compared with 60 MVA (154kV/23kV) conventional transformer on the basis of core and coil assembly. The comparison results are shown in Table X.

TABLE X

COMPARISON OF HTS TRANS. WITH CONVENTIONAL TRANS. (HTS TRANS./CONVENTIONAL TRANS.* 100 %)

V/T Width Thick. Height Vol. Area Weigh

59.8156 74.68 88.52 104.63 69.21 66.17 46.07

69.8898 77.38 92.22 98.92 70.59 71.41 49.64

79.9943 79.85 95.37 95.21 72.52 76.17 53.88

89.7234 82.16 98.15 93.01 75.00 80.69 58.81

100.5989 84.55 101.11 91.31 78.03 85.45 64.17

The V/T for the HTS transformer to be superior above

the conventional one in all items was found in the range from 69.9 to 89.7. When the V/T is 69.9 and 89.7, the required HTS wire is about 55.5 km and 47 km, respectively. The cost gap between them will be about $51000 in 2010, when the price of HTS YBCO coated conductor wire is expected to $30/kAm [60].

TABLE XI

COMPARISON OF HTS TRANS. WITH CONVENTIONAL TRANS. Item 60MVA HTS trans. 60MVA conventional trans. Weight 16.6 tons (include

cryostats, without LN2)27.2 tons (without insulation oil)

Core size 2674 mm *2429 mm 3150 mm * 2590 mm Copper loss N/A 100 kW Core loss 28.7 kW 33 kW Refrig. loss 12 kW N/A Total ac loss 40.7 kW 133 kW Efficiency 99.93% 99.3% Impedance 16.43% 19.83%

In the economic sense, the large V/T is desirable. So,

89.7 as the V/T of HTS transformer for electromagnetic field analysis was selected in practical design. After successful design and tests, the 60 MVA HTS transformer was compared with a 60 MVA conventional transformer that is standard one for the distribution power systems, in terms of weight, volume, AC loss, efficiency and percent impedance. The results are shown in Table XI.

V. CONCLUSION After the discovery of HTS and the availability of the

first HTS wires for use in windings, different groups all over the world began working on the development of HTS transformers. What all these projects have in common is that they are aiming at commercial applications. At present the focus on the applied superconducting technology for HTS transformer has been moving to the industrial preparations from laboratory research stage, and the technology has been well verified for practical applications from small to large scales. There is a reason to believe that the foregoing market penetration analysis is credible, and to see the benefits of HTS transformer in the near future can be expected.

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