Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)

Corresponding author: S. Ito, satoshi@okubo.nuee.nagoya-u.ac.jp

Self-recovery Characteristics of High-Tc Superconducting Fault Current Limiting Transformer (HTc-SFCLT)

with 2G Coated Conductors

Satoshi Ito1, Hiroki Kojima2, Naoki Hayakawa1,2 Fumihiro Endo2 Mathias Noe3 and Hitoshi Okubo2

1. Department of Electrical Engineering and Computer Science, Nagoya University, Nagoya, Japan

2. EcoTopia Sience Institute, Nagoya University, Nagoya, Japan 3. Forschungszentrum Karlsruhe, Germany

Abstract: We have been developing a 3-phase, 100 kVA, 6600V/210V high temperature Supercon-

ducting Fault Current Limiting Transformer (HTc-SFCLT) with functions of both superconducting trans-former and fault current limiter. The development HTc-SFCLT is characterized by the application of 2G coated conductors with the higher performance and flexibility on current limitation than those of the pre-vious HTc-SFCLT with 1G Bi2212/CuNi composite bulk material. This paper focuses on the recovery characteristics after current limitation of the developed 2G HTc-SFCLT. Keywords: 2G coated conductor, superconducting transformer, superconducting fault current limiter.

1. INTRODUCTION We have been developing Superconducting Fault Cur-

rent Limiting Transformer (SFCLT) with both functions of superconducting transformer and superconducting fault current limiter. As the Phase-IV of the SFCLT project, in this paper, we designed, fabricated and tested “2G HTc-SFCLT” with YBCO coated conductors to be oper-ated in liquid nitrogen at 77K [1]. The ratings of the 2G HTc-SFCLT are 3-phase, 100 kVA, 6600 V / 210 V. The 2G HTc-SFCLT is characterized by the combination of different kinds of YBCO coated conductors, in order to make the current limiting characteristics flexible and controllable to meet the desired performances.

The no-load and short-circuit tests of the 2G HTc-SFCLT have been already carried out and verified the design parameters as a superconducting transformer. The current limiting tests have also been conducted and confirmed the effective current limitation as a supercon-ducting fault current limiter. In this paper, the recovery

characteristics into superconducting state after the current limitation are discussed for different load and fault condi-tions. 2. DESIGN AND FABRICATION OF 2G HTc-SFCLT

The specifications and constraction of 2G HTc-SFCLT are shown in Table 1 and Fig.1. We designed 3-phase 2G HTc-SFCLT with the ratings of 100 kVA, 6600V/210V, 8.7A/275A. As a single phase of 2G HTc-SFCLT, we fabricated 33.3 kVA, 3810V/210V, 8.7A/159A (Y-D) HTc-SFCLT. Low voltage coil consists of the 2G tapes, and high voltage coil is composed of copper wire with 1.4 mm diameter, both of which are immersed in liquid nitrogen at 77K together with the iron core.

In the case of 1G HTc-SFCLT [2], Bi2212/CuNi bulk coil of the low voltage HTS coil was used as both the transformer coil and the fault current limiter coil. There-fore, the limiting resistance as the fault current limiter was restricted by the transformer design, i.e. the size and stiffness of Bi2212/CuNi bulk coil. Here, in the case of 2G HTc-SFCLT, the transformer coil of the low voltage

Table 1. Specifications of 2G HTc-SFCLT. Phase 3

Frequency 60Hz

Capacity 100kVA

Rated voltage 6600V/210V

Rated current 8.7A/275A

Turn ratio 1344/74

Magnetic flux density 1.7T

Leakage impedance 4.6%

LV(I) YBCO

LV(II) YBCO/Cu stabilized Material

HV Copper

LV (I) :Tr/FCL windingIc = 131A×2 layers

FRP bobbin295

780

LV winding (YBCO tape)

HV winding (Copper wire)

570

LV （II)：Tr windingIc = 71A×4 layers

Iron core

Copper wire

LV HV

100

Iron core

LV (I) :Tr/FCL windingIc = 131A×2 layers

FRP bobbin295

780

LV winding (YBCO tape)

HV winding (Copper wire)

570

LV （II)：Tr windingIc = 71A×4 layers

Iron core

Copper wire

LV HV

100

Iron core

Fig.1. Construction of 2G HTc-SFCLT.

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Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)

HTS coil is divided into 2 parts; limiting coil with current limitation function (Tr/FCL winding in Fig.1) and non-limiting coil without current limitation function (Tr winding in Fig.1). Such a hybrid construction of HTS coils has the merit that 2G HTc-SFCLT can be relieved from the above design constraint and obtain higher flexi-bility for the transformer and fault current limiter designs. With the variation of the ratio between the limiting Tr/FCL coil and the non-limiting Tr coil, 2G HTc-SFCLT can perform the desirable current limiting characteristics as well as transformer functions, e.g. the higher ratio of limiting Tr/FCL coil would bring about the more effective current limitation, which should be coordinated with the other design factors such as dimension constraints. 3. RECOVERY TEST RESULTS

Figure 2 presents an example of low-voltage side cur-rent (ILV) waveform of 2G HTc-SFCLT in the current limitation and recovery tests, where prospective current (IPRO) is 980Apeak, load current before fault (ILV1) is 112Apeak, and fault duration is 5 cycles (60Hz). The fault current was limited to 516Apeak (52.6% of IPRO) at the first peak and 330Apeak (33.7% of IPRO) at the 10th peak, re-spectively, after the fault. In this case, the load current after the fault clearance (ILV2) was equal to ILV1, which means that the HTS coil at the low-voltage side recovered into superconducting state immediately after the fault clearance.

Figure 3 shows the fault current limitation characteris-tics of 2G HTc-SFCLT, where the broken line presents IPRO, and the solid curve displays ILV as the limited fault current at the first peak after fault. The fault current limi-tation appears at IPRO > 300Apeak, and ILV is limited to 58% of IPRO at IPRO = 610Apeak and 42% at IPRO = 1220Apeak. These results verify that the 2G HTc-SFCLT exhibits the excellent current limiting function as a su-perconducting fault current limiter.

Figure 4 shows the recovery characteristics of 2G HTc-SFCLT as parameters of IPRO and ILV1. The recovery into superconducting state was defined as ILV2/ILV1 > 99% in this experiment. The symbols “○” and “×” in Fig.4 denote the recovery case and non-recovery case, respec-tively. A broken curve can be found between the recovery and non-recovery cases, which can be regarded as the

criterion or the limit of recovery for the 2G HTc-SFCLT. In other words, when the load current (ILV1) and the pro-spective fault current (IPRO) were lower than those on the critical curve in Fig.4, the 2G HTc-SFCLT would recover into superconducting state immediately after the fault clearance.

4. CONCLUSIONS We developed the 2G HTc-SFCLT with coated conduc-

tors, and carry out the current limitation and recovery tests. The main results are summarized as follows: 1) The current limiting characteristics of 2G

HTc-SFCLT exhibited the excellent current limiting function as a superconducting fault current limiter.

2) The recovery characteristics into superconducting state after the current limitation were discussed, and a criterion of the recovery was quantified for differ-ent load and fault conditions.

REFERENCES 1. H.Okubo, C.Kurupakorn, S.Ito, H.Kojima, N.Hayakawa,

F.Endo and M.Noe, Applied Superconductivity Conference 2006, ILW04 (2006)

2. C.Kurupakorn, H.Kojima, N.Hayakawa, F.Endo, N.Kashima, S.Nagaya, M.Noe and H.Okubo, Journal of Physics: Con-ference Series, Vol.43, pp.950-953, 2006

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Pros

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curre

nt: I

PRO (A

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140120100806040200Load current before fault: ILV1 (Apeak)

recovery casenon-recovery case

Fig. 4. Recovery characteristics of 2G HTc-SFCLT

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Fault Current

Fig. 3. Fault current limitation characteristics of 2G HTc-SFCLT

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0200400600800

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tage

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cur

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: ILV

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0.300.250.200.150.100.050.00-0.05 Time (s)

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Fig.2. Current waveform of recovery-test. (IPRO = 980 Apeak, ILV = 112 Apeak)

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