Development of quench protection system for HTS coils by active power method

Development of quench protection system for HTS coils by active power method

Physica C 463–465 (2007) 1281–1284 www.elsevier.com/locate/physc Development of quench protection system for HTS coils by active power method N. Nana...

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Physica C 463–465 (2007) 1281–1284 www.elsevier.com/locate/physc

Development of quench protection system for HTS coils by active power method N. Nanato

a,*

, Y. Tsumiyama a, S.B. Kim a, S. Murase a, K.-C. Seong b, H.-J. Kim

b

a

b

Department of Electrical and Electronic Engineering, Faculty of Engineering, Okayama University, 3-1-1 Tsushima Naka, Okayama 700-8530, Japan Korea Electrotechnology Research Institute (KERI), 28-1, Seong-ju-dong, Changwon, 641-120 Gyeongnam, Republic of Korea Accepted 27 February 2007 Available online 2 June 2007

Abstract Recently, HTS coils have been developed for electric power apparatuses. In superconducting coils, local and excessive joule heating may give damage to the superconducting windings when a quench occurs and therefore it is essential that the quench is detected quickly and precisely so that the coils can be safely discharged. Resistive voltage measurement method is universally used for the quench detection, however, it is vulnerable to an electromagnetic noise which causes insufficient quench detection and at least needs a central voltage tap in windings. In a large superconducting coil, a lead-wire from the central voltage tap may cause a short-circuit when high voltage will be applied. In this paper, we present a quench protection system based on the active power method which detects a quench by measuring the instantaneous active power generated in a superconducting coil. The protection system based on this method is very strong against to the noise and no more needs a central voltage tap. The performance of system developed by us is confirmed by using test coil wound with Bi-2223 HTS tapes. Ó 2007 Elsevier B.V. All rights reserved. PACS: 74.72.Hs; 84.71.Ba Keywords: HTS coil; Quench detection; Active power method

1. Introduction Recently, HTS coils have been developed for electric power apparatuses. In superconducting coils, local and excessive joule heating may give damage to the superconducting windings when a quench occurs and therefore it is essential that the quench is detected quickly and precisely so that the coils can be safely discharged [1–5]. Resistive voltage measurement method is universally used for the quench detection. In this method, three voltage taps are installed on the center and both terminals of the supercon-

*

Corresponding author. Tel.: +81 86 251 8122; fax: +81 86 251 8259. E-mail address: [email protected] (N. Nanato).

0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.02.047

ducting coil. A resistive voltage can be measured by comparing the voltages across two halves of the coil. However, this is vulnerable to an electromagnetic noise which causes insufficient quench detection and a lead-wire from the central voltage tap may cause a short-circuit when high voltage will be applied. In this paper, we present a quench protection system based on the active power method [6,7] which detects a quench by measuring the instantaneous active power generated in a superconducting coil. The protection system consists of a few IGBTs, a quench detection circuit and a protection resistor, and can protect the coil in very short time. Fabricated quench detection system needs no central voltage tap. The performance of system developed by us is confirmed by using test coil wound with Bi-2223 HTS tapes.

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2. Experimental 2.1. Active power method In Fig. 1, vSC and vRC are the voltages across a superconducting coil after the quench and an air-cored transformer (or Rogowski coil), respectively. These voltages are shown as following equations: vSC ¼ ri þ L di=dt

ð1Þ

vRC ¼ M di=dt

ð2Þ

Fig. 2. A block diagram of the active power method.

where r is a resistance of windings and is zero in the superconducting state, L is an inductance of the superconducting coil, i is a transport current and M is a mutual inductance of the air-cored transformer [7]. From these equations, an active power P is calculated as follows: P ¼ ðvSC  kvRC Þi ¼ ri2

ðk ¼ L=MÞ

ð3Þ

In the superconducting state, P is zero and whereas in the normal conducting state it is not zero. Therefore the quench can be detected by measuring P. In general, conventional electric quench detection methods (e.g. resistive voltage measurement) are vulnerable to an electromagnetic noise. In the proposed method, LPF (low-pass filter) can eliminate the electromagnetic noise because P has a DC resistive component [7]. Therefore a quench signal can be detected regardless of the noise. And it is less susceptive to a high voltage because it is electrically insulated from the superconducting coil (except both terminal taps). Fig. 2 shows a block diagram of the proposed method. In this figure, a filtered P is defined as P 0 and when P 0 becomes larger than a specified threshold, the detector recognizes the quench occurrence. The threshold is determined so that the temperature of the normal region which generates after the quench occurrence i

r

vSC

Power Supply

Fig. 3. A quench protection circuit.

becomes lower than a specified temperature [8]. A quench detector fully consists of the analog circuits such as operation amplifiers and therefore calculation time (or time delay of the detection) is very short. 2.2. Quench protection system Fig. 3 shows a quench protection circuit, which consists of two sets of IGBTs switch, a quench detection circuit and a protection resistor. These IGBT switches are applicable for both of the direct and alternative transport current. In the superconducting state, IGBT gate signals G1 and G2 turn on and off, respectively. Then the switch 1 turns on and switch 2 turns off and the power source can provide the current i1 to the superconducting coil. Whereas in the normal conducting state (the quench occurrence), G1 (switch 1) and G2 (switch 2) turn off and on, respectively (G2 has to turn on earlier than G1 turns off in the quench detection in order to prevent a excess voltage occurrence in IGBTs) and the current i1 is cut-off and the magnetic energy stored in the coil is dissipated in the protection resistor (or i2 is decreased by the protection resistor) and then excessive joule heating (or temperature rise) is prevented.

L

3. Experimental results

vRC Fig. 1. An electric circuit for the active power method.

We made quench protection tests for test Bi-2223 HTS coil to investigate the feasibility of the proposed system. The HTS coil is a pancake coil with single layer and its critical current and self-inductance are 70 A (1 lV/cm, at 77 K, self field) and 340 lH, respectively. The tests are carried out for two patterns of the quench occurrence, one is to supply larger current than the critical current to the coil

N. Nanato et al. / Physica C 463–465 (2007) 1281–1284

cooled in LN2 on the assumption of charging to SMES coil and the other is the temperature rise by extracting in the atmosphere (the current is constant and less than IC). In these tests, cut-off frequency of LPF (see Fig. 2) is 30 Hz. Fig. 4 shows waveforms of the supply current i1, vSC, P 0 , i2, G1 and G2 in the over current test. In the test, a threshold of P 0 is 0.30 W. Before the quench occurrence, the current gradually increases with a constant slope and vSC is constant (=L di/dt). P 0 , i2, G1 and G2 are 0, 0, on and off, respectively. In other words, the HTS coil is in the superconducting state and therefore the power source can provide the current to the coil. After the current i1 becomes larger than IC, vSC and P 0 drastically increase because the resistive voltage generates in the HTS coil. Then P 0 reaches the threshold of 0.30 W at 3.85 s and gate signals G1 and G2 turn, respectively, off and on, and the current i1 is cut off and i2 flows in the protection resistor. That is to say, the quench is detected and the HTS coil is protected. From these results, the proposed quench protection system is useful for detecting the quench caused by the over current. Fig. 5 shows waveforms of the supply current i1, vSC, P 0 , i2, G1 and G2 in the temperature rise test. In the test, the current i1 is constant of 40 A (less than IC) and a threshold of P 0 is 0.45 W. This figure shows expanded waveforms before and after the quench detection and the left terminal of the time axis (0 s) is not the start time of the temperature rise. Before the quench detection, the vSC and P 0 gradually increase because the resistive voltage in the coil increases by the flux flow, the temperature rise and the normal zone propagation. P 0 is smaller than the threshold of 0.45 W and therefore and G1, G2 and i2 are on, off and 0, respectively. Whereas, when P 0 reaches the threshold at 2.95 s, gate signals G1 and G2 turn, respectively, off and on, and the current i1 is cut off and i2 flows in the protection resistor. In other words, the quench is detected and the energy

(a) (a) i11

20A/div

0A (b) (b) vvSC SC

5mV/div

(c) P P’’

0.1W/div

(d) (d) i22

0.5A/div 20 A/div

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(a) (a) ii11

20A/div

0A (b) (b) vvSC SC

10mV/div

(c) P’

0.1W/div

(d) (d) ii22

0.2A/div 80A/div

0mV

0W 0A

0V

0V

5V/div

(e) (e) G G11 (f) (f) G G22

0.0

5V/div

1.0

2.0

3.0 Time (s)

4.0

5.0

Fig. 5. Experimental results for the temperature rise test, (a) i1, (b) vSC, (c) P 0 , (d) i2, (e) G1 and (f) G2.

stored in HTS coil is dissipated in the protection resistor. From these results, it is shown that the proposed quench protection system is useful for detecting the quench caused by the temperature rise. Fig. 6 shows the relationship between di1/dt and i1q for a specified threshold in order to verify that the inductive voltage (=L di1/dt) is canceled by (1)–(3) for some current sweep rates. The vertical axis of i1q means the current i1 at quench detection time. For example, in Fig. 4, it is 83 A at 3.85 s and di1/dt is 28 A/s. When the inductive voltage is canceled and the threshold is constant, i1q is constant for any current sweep rate. In Fig. 6, i1q is almost constant of about 85 A for some sweep rates. From these results, it is expected that the proposed quench protection system can cancel the inductive voltage of the superconducting coil for any current sweep rate. Fig. 7 shows the relationship between the threshold Pth and vSCq in the quench detection tests for the over current (the current sweep rate is constant). The vSCq means the

0mV 0W

0A 0V

0V

5V/div

(e) (e) G G11 (f) (f) G G22

0.0

5V/div

1.0

2.0

3.0 Time (s)

4.0

5.0

Fig. 4. Experimental results for the over current test, (a) i1, (b) vSC, (c) P 0 , (d) i2, (e) G1 and (f) G2.

Fig. 6. Relationship between di1/dt and i1q for a specified threshold.

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Through the experimental results for a Bi-2223 HTS coil, features of the proposed system are summarized as follows: (I) The proposed system is useful for the quenches cased by flowing larger current than IC and the temperature rise. (II) The proposed system can detect the quench regardless of the existence of the inductive voltage. (III) The proposed system realizes earlier quench protection by the lower threshold.

References

Fig. 7. Relationship between the threshold Pth and vSCq for a specified current sweep rate.

voltage vSC at the quench detection time. In this figure, vSCq decreases with the decrease of Pth because the quench is detected earlier by the lower threshold.

4. Conclusions This paper presents a quench protection system based on the active power method for superconducting coils.

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