Development of a 630 kVA Three-Phase HTS ... - IEEE Xplore

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 17, NO. 2, JUNE 2007

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Development of a 630 kVA Three-Phase HTS Transformer With Amorphous Alloy Cores Yinshun Wang, Xiang Zhao, Junjie Han, Huidong Li, Ying Guan, Qing Bao, Liye Xiao, Liangzhen Lin, Xi Xu, Naihao Song, and Fengyuan Zhang

Abstract—This paper describes design and operation of a threephase HTS power transformer with capacity of 630 kVA operated in liquid nitrogen of 77 K for primary/secondary voltages of 10.5 kV/0.4 kV. The windings were wound by hermetic stainless steelenforced multifilamentary Bi2223/Ag tapes. The structures of primary and secondary windings are solenoid and double-pancake respectively. Cryostat is made from electrical insulating materials with room temperature bore for commercial amorphous alloy core. Fundamental characteristics of the transformer are obtained by standard short-circuit and no-load tests. The ac losses were calculated and measured by method of conventional transformer. The HTS power transformer successfully operated in a live power grid. Index Terms—AC losses, amorphous alloy core, Bi2223/Ag hermetic tapes, high temperature superconductor (HTS) transformer, windings.

I. INTRODUCTION

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INCE the discovery of high critical temperature superconductor (HTS) in 1986, HTS transformer has been expected to be one of the most promising technical applications. And it has a lot of advantages such as lighter weight, smaller volume, higher efficiency, oil-free and less environmental hazards. Particularly, the ability of superconductors to limit and withstand high currents is also interesting in the case of overload operation. With advance in production technology of high temperature (HTS) tapes, several transformers of various powers have been built [1], [2]. So as I know, only one three-phase HTS transformer developed by ABB had ever operated in live power grid for one year [3]. The important problems associated with its design are ac losses in windings in which the HTS tapes with high anisotropy carry ac transport current and are simultaneously exposed to the ac external magnetic field with different flux density and different orientations at different points. The effort to reduce them does not bring significant results yet. We designed and developed a three-phase 630 kVA HTS power transformer with hermetic stainless steel-reinforced multifilamentary Bi2223/Ag tapes fabricated by American superconductor Corporation (AMSC). The rated voltage and currents in primary and secondary windings of the transformer Manuscript received August 26, 2006. This work was supported in part by the Chinese Ministry of Science & Technology under Grant 2002AA306381, in part by TBEA, and in part by the 100 person project of CAS, China. Y. Wang, H. Li, Y. Guan, Q. Bao, L. Xiao, L. Lin, X. Xu, N. Song, and F. Zhang are with the Institute of Electrical Engineering, CAS, Beijing 100080, China (e-mail: [email protected]; [email protected]). X. Zhao and J. Han are with the Tebian Electric Apparatus Stock Co., Ltd., China (e-mail: [email protected]; [email protected]). Digital Object Identifier 10.1109/TASC.2007.898162

are 10.5 kV/0.4 kV and 34.64 A/909.33 A respectively. The structures of primary and secondary windings, assembled concentrically, are respectively solenoid and double-pancake. The core was made of commercial amorphous alloy and operated at room temperature; the toroidal cryostat was made from Glass Fiber Enforced Plastics (GFRP). Before operating in field serving a manufacture plant in Tebian Electric Apparatus Stock Co., Ltd. (TBEA), the largest transformer manufacturer in China, the three-phase 630 kVA HTS power transformer was tested by China National Transformer Quality Supervision Testing Center. In the paper, we present the design and fabrication of the transformer; ac losses are analysed and tested, the main test results of the HTS transformer are also presented at power frequency 50 Hz in liquid nitrogen of 77 K. II. DESIGN AND FABRICATION OF TRANSFORMER The three-phase HTS power transformer with capacity of 630 kVA in liquid nitrogen of 77 K for the primary and secondary voltage of 10.5 kV/0.4 kV was designed and fabricated by IEE (Institute of Electrical.Engineering, Chineses Academy of Sciences) in collaboration with TBEA industrial transformer group. The primary and secondary windings, wound with hermetic stainless steel-enforced multifilamentary Bi2223/Ag including tapes with average cross section of ), and insulation (bare average cross section of critical current with 115 A at self-field and 77 K, are solenoid and double pancake respectively. Primary winding is wound with 8 layers with cooling channels in longitudinal, each cross ; and secondary section of the cooling channel is winding is made of 23 double pancakes connected in parallel, each of them has 5 layers. The most of the HTS tapes in windings are exposed to external magnetic field which is mostly axial parallel to tape wide plane, but there is a significant radial component perpendicular to tape plane on both edge parts. All of windings are wound with a strand consisted of two parallel transposed multifilamentary tapes in order to prevent unbalanced current flowing because it may cause instability of the HTS windings as well as increase ac losses. Solenoid and double pancake windings are concentric cylindrical. And the double pancake windings are located coaxially outside the solenoid one [4]. The total length of used tape is 5.1 km. Specifications of windings are presented in Table I. Main parameters of the 630 kVA three-phase HTS transformer are summarized in Table II. The windings are cooled by liquid nitrogen in cryostat made from a vacuum insulated Fiberglass Reinforced Plastics with room temperature bores for commercial amorphous alloy core. There are 5 limbs in the magnetic circuit, Fig. 1 is schematic view of cryostats and

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TABLE I SPECIFICATIONS OF WINDINGS

Fig. 2. Overview of 630 kVA three-phase transformer.

TABLE II MAIN DESIGNED PARAMETERS OF THE TRANSFORMER

to secondary ones. So the current leads of secondary windings are only chosen as traditional gas-cooled current leads in order to reduce heat leakage into liquid nitrogen. Fig. 2 is an overview of developed 630 kVA three-phase HTS transformer installed in the GFRP cryostat. III. TESTS OF THE HTS TRANSFORMER The same items of the HTS transformer as conventional ones were tested by China National Transformer Quality Supervision Testing Center. The tests mainly include ac withstand voltage test, lighting impulse voltage test, noise level, temperature distributions of yoke core and current leads, insulation between primary and secondary windings in accordance with Chinese standards of test methods respectively. All of tests are in liquid nitrogen temperature of 77 K. A. No-Load Test

Fig. 1. Schematic associated view of cryostats and the amorphous alloy core with 5 limbs.

amorphous alloy core. Since the amorphous core has 5 limbs, the transformer needs a balanced-winding with an average diameter of 210 mm and 10 turns in each phase during operation. To our knowledge, the amorphous core is the largest in operated transformer in the world. Since the cross sections of secondary current leads are about 26 times larger than those of primary ones, heat-conduction in primary leads is very small compared

The usual characteristics of transformer were tested in nitrogen temperature of 77 K and power frequency 50 Hz so that steady characteristics were obtained in rated operation. In the test, the transformer was excited from secondary side at 400 V, the exciting current was 1.15% of rated level and the no-load loss was, attributed to iron core loss, 1090 W in conventional procedure. The magnetic flux density of iron core is 1.275 T at rated operation. The transformer ratio is 26.25. Main insulation which is between primary and secondary windings is 10 much higher than the national standard 1 . In the no-load test, the noise level was also tested to be 65 dB, lower than the Chinese national standard. The temperature distributions in core yokes and current leads were tested; Fig. 3 presents the results over 16 hours, which shows the temperatures are stable and safety. The main results of no-load tests for the HTS transformer are shown in Table III. B. Short Circuit Test Short-circuit test was performed from the primary side at rated current 34.64 A for 48 h, the impedance was estimated as 2.5% of rated value, ac loss of windings in rated operation was measured by a usual conventional electric method. The total ac loss in windings was 174.8 W including losses in current leads

WANG et al.: DEVELOPMENT OF A 630 kVA THREE-PHASE HTS TRANSFORMER WITH AMORPHOUS ALLOY CORES

Fig. 3. Temperature distributions of core yoke and current leads in no-load test. T , T , T and T are temperatures of amorphous core, high voltage lead; low voltage lead respectively. TABLE III MAIN RESULTS IN NO-LOAD AND SHORT CIRCUIT TEST FOR THE HTS TRANSFORMER

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Fig. 4. Temperature distributions of core yoke and current leads and load loss P in the short circuit test.

level wave. The sudden short circuit test was also tested with 25-time rated current in 0.2 s. And there is no demolishment in mechanical and superconductivity problems in the sudden short-circuit after testing [4]. D. Temperature Rise Tests

in room temperature in liquid nitrogen

and other trivial losses. The ac losses in HTS windings, mainly including hysteretic loss, coupling loss and eddy current loss, were estimated 110.7 W by subtracting those losses of current leads, joints and other trivial losses from the total losses, are good agreement with the calculated value 121.8 W [5]–[8]. Additionally, we performed an overload test of the transformer, where the current of the windings increased up to 120% of rated level for 2 hours, and also performed an over current of 150% of rated current about 30 seconds, the windings are very stable against thermal and mechanical disturbance. Finally temperature distributions in core yokes and current leads were also tested in the short circuit test. Fig. 4 presents the results more over 10 hours, which shows the temperatures are stable. The main results in short-circuit are also listed in Table III.

Since the windings operate in 77 K, there is no temperature change in windings during normal operation. But the temperature rise in amorphous core, current leads were difference during operating. According to the conventional transformer test, these temperatures should be tested in both no-load test and short-circuit test in longer time. We tested the temperature rise in both tests. Fig. 3 shows those temperature rises in core and current leads in no-load test. After operating 14 hours, the temperature in cores keeps constant in 52 . Since the secondary side opens, the temperature is as low as because of heat conduction with higher cross-section of low voltage current leads. And the temperature of high voltage current leads is same as the environment temperature since the small excited current and small heat conduction with small cross-section. Fig. 4 presents that the temperature rise in those components in short circuit test. The temperature reaches constant after operating 6 hours. The temperature in amorphous core decreases at the first several hours since the short-circuit test was performed just after no-load testing, so its temperature is higher than short-circuit test. When the short-circuit test going on, the temperature also reached balance temperature. All of the temperatures are reasonably higher than environment during the short-circuit test. In the period of the test, the load loss keeps constant, which means the transformer is stable. IV. OPERATION IN THE LIVE POWER GRID

C. Lightning Impulse and Sudden Short Circuit Tests Firstly, induced withstand voltage test was accomplished with 4-time power frequency (200 Hz) and 21 kV for 30 s. The secondary side was shorted and grounded with iron core. The lighting impulse test was performed with four full wave levels of impulse, 75 kV, 105 kV, 125 kV and 155 kV in a model, which has completely same structure as those of the present windings, the results show that there are no breakdown for 4 full

The HTS transformer operated in November, 2005 in a power grid after various testing and authorizing by China National Transformer Quality Supervision Testing Center in the city of Changji, Xinjiang Province, northwest of China, serving a TBEA cable manufacture plant. The operation scene is shown in Fig. 5. The several protection measures were taken, such as current breakers with 100 ms switch-off time, nitrogen level etc. external The open cooling cycle is supplied by volume of 6

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Fig. 5. The HTS transformer operation field serving cable manufacture plant of TBEA (Changji, Xinjiang, northwest of China).

The transformer losses including core loss, ac losses in the superconducting windings and currents leads are analysed theoretically and experimentally. The ac losses in windings are almost same magnitude of order with those in current leads. The efficiency of the transformer is estimated to be 98.5% considering those losses and coefficient of penalty in rated operation. In order to avoid the electric field focusing the boiling nitrogen gas bubble, the commercial HTS transformer should operate in sub-cooled liquid nitrogen under 77 K with closed cryocooler recycling in future instead of present cooling method. Based on the present HTS wire price, the HTS transformer has economically advantages compared with conventional transformer while its capacity is more than several MVA. Additionally, the dimensions of HTS tape present serious challenges for voltage levels great than 200 kV since the aspect ratio of the tape is too large (more than 10), meaning it is too thin and wide. As a result, the high electric field strength in the direction of the tape’s thickness can easily cause partial discharging which is considered to be the aging mechanism of electrical device dielectric, and will shorten the life of the insulation material, or even destroy it. Therefore the HTS transformer application also has some serious challenges for high capacity particularly the high voltage or super-high voltage in live grid. ACKNOWLEDGMENT The authors would like to thank J. Jin, Z. Xiao, H. Li, and H. Yu (Tebian Electric Apparatus Stock Co., Ltd.), J. Zhou, K. Chen, and F. Tian (China National Transformer Quality Supervision Testing Center) for their help in the HTS transformer test.

Fig. 6. Load profile of the transformer in period of 60 days.

REFERENCES liquid nitrogen container for compensating the evaporated nitrogen caused by ac losses of HTS transformer windings, heat-leakage due to GFRP cryostats and current leads. When switching on the transformer, there is an inrush current whose peak can reach values above 10 times the rated current with less than 0.2 seconds, there is no quench in HTS windings. The transformer safely worked in the power live grid. Fig. 6 shows the collected load profile of the HTS transformer in the first 60 days. During the operation, we had no transformer failure and the existing maintenance staff were completely able to handle this transformer same as a conventional ones. V. CONCLUSION A three-phase 630 kVA HTS transformer with room temperature amorphous core was designed and tested for operating characteristics. It successfully operated in a power live grid and has reliably ability of long-term operation.

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