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The European development of a full scale switching unit for the ITER switching and discharging networks
Fusion Engineering and Design 75–79 (2005) 193–200
The European development of a full scale switching unit for the ITER switching and discharging networks T. Bonicelli a,∗ , A. De Lorenzi b , D. Hrabal c , R. Piovan b , E. Sachs c , E. Salpietro a , S.R. Shaw d a
EFDA-CSU, Boltzmannstr. 2, 85748 Garching (D), Germany b Consorzio RFX, Corso Stati Uniti 4, 35127 Padova, Italy c FEAG, Guenther-Scharowskystr. 2, 91058 Erlangen, Germany d UKAEA, Culham Science Centre, OX14 3EA, Abindgdon, UK Available online 26 July 2005
Abstract The European Fusion Programme included since the mid-1990s, the development of a full scale, full rating current commutating unit (CCU) to be used as a centre-piece of the ITER magnet protective circuits. The CCU is mainly composed of a by-pass switch (BPS) in parallel to a vacuum circuit breaker (VCB). Both full scale switches were subjected to extensive testing in conditions simulating the actual ITER operation. The operation in absence of the saturable reactor series connected in the ITER reference design was successfully assessed. Interruption tests at low current and full counter-pulse capacitor voltage, were also satisfactorily performed. The tests included an extensive and novel characterisation of the interrupting capability of the vacuum circuit breaker in the presence of an external magnetic field as in the actual location of installation in ITER. © 2005 The European Commission. Published by Elsevier B.V. All rights reserved. Keywords: DC circuit breakers; ITER; Superconductive magnet protection
1. Introduction Large superconductive coils, like the ITER PF and TF magnets, require an extremely reliable system to quickly discharge the magnetic energy in case a coil quench is detected (for the TF magnet, 40.2 GJ stored at full current must be discharged with 11 s time constant). ∗ Corresponding author. Tel.: +49 89 3299 4260; fax: +49 89 3299 4198. E-mail address: [email protected] (T. Bonicelli).
In ITER, the discharge is performed with the current commutating unit (CCU) of Fig. 1. A by-pass switch (BPS), carrying the steady state current but with very limited current interruption capability, is parallel connected with a vacuum circuit breaker (VCB) suitable for the interruption of the current. The scheme, proposed by the European Home Team (EU HT), was adopted in the ITER reference design [1]. After the initiation of a fast discharge, the BPS is opened and the current is fully transferred to the closely located VCB which is then opened. The commutation
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which could be directly used for the ITER procurement packages. Two phases were foreseen: the first phase took place from 1995 to 1999 and was aimed at the separate development of the full scale VCB and BPS [5–7]; the second phase took place from 2000 to 2004 and was aimed at:
Fig. 1. Current commutating unit: a simplified diagram.
of the current to the discharge resistors (DR) is achieved by means of the discharge of a counter-pulse capacitor (C) that creates an artificial current zero in the VCB arc chamber. A series saturable reactor (LS) decreases the current derivative around the current zero-crossing to enhance the possibility of de-ionisation and facilitate interruption. To increase the reliability of the CCU, two vacuum interrupters, each able to interrupt the nominal current and to sustain the full voltage, are series connected. Finally, a single action circuit breaker (pyrobreaker, PB) is installed in series to the CCU and is operated in the unlikely event of complete failure of the unit. Other CCUs are also foreseen in ITER circuits for generating the loop voltage required at plasma breakdown and current start-up. These units operate at every pulse and could be in principle based on the same type of circuits [1]. A total of 200 fast discharges are specified for the magnet systems [2] during the ITER life and 50 fast discharges (10 actual quenches) are actually considered for the TF system [3]. A lifetime of 250 operations was therefore, considered adequate for the main components.
2. The EU HT development of a CCU As a part of the EU HT R&D efforts towards ITER [4], the development of a full scale CCU (Fig. 2), was undertaken with the aim to develop components
- adapting the design to the new ITER requirements (2001), in particular the increase of rated TF coil current from 60 kA to 68 kA; - testing the combined operation of BPS and VCB; - testing the interruption performances in presence of external B field; - testing the interruption performances at low current (e.g., 5 kA) but with fully charged counter-pulse capacitor; - testing the interruption performances in absence of the series saturable reactor. The BPS switch manufactured and tested in the first phase will be identified as BPS60 and in the last phase as BPS70.
3. The by-pass switch The BPS, whose main specifications are in Table 1, is based on industrial high current/low voltage (around 1 kV) switches used in electrochemistry plants. The main features of the BPS70 are: - 12 main poles parallel connected, arranged on two rows of six each; - each main contact has a size of 120 mm × 150 mm, made of silver plated copper; - each fixed main contact is provided with four copper–silver–beryllium alloy multicontact strips; Table 1 By-pass switch main specifications Rated voltage Rated DC current Peak withstand current Opening time Jitter on opening Mechanical lifetime Main contacts lifetime Arc contacts lifetime Cooling
17.5 kV 70 kA BPS70/(60 kA BPS60) 250 kA <500 ms ±20 ms 2000 cycles 1000 openings 100 openings Natural circulation of air
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Fig. 2. Complete CCU: on the left, the VCB and its actuator; on the right, the BPS.
- each main contact is provided with an arc contact made of high tungsten graded–copper alloy (75%–25%) soldered on a copper base. The qualification of the BPS was carried out both during phase 1 (BPS60) and phase 2 (BPS70). Besides standard factory tests, the following tests were performed: -
contact resistance measurements; extended mechanical endurance test; temperature rise test; peak withstand current test; commutation qualification tests to the parallel connected VCB; - life commutation tests. The average resistance in cold conditions of each main contact is about 6 and the total resistance measured at the BPS terminals is 0.47 (BPS70). The resistance of the parallel path with the VCB is instead 62 and this implies that only less than 1% of the current flows in steady state in the VCB.
The extended mechanical endurance test was performed on BPS60 at the nominal operating pressure of 6 bar. Up to 2024 operating cycles were successfully carried out without need of any maintenance. In particular, the jitter on opening was typically only ±2 ms over the full test cycles. The switch also passed successfully the peak current test performed with an AC current of 250 kA lasting 300 ms. 3.1. BPS heat run tests The heat run test was carried out at 75 kA, about 10% higher than the maximum ITER CCU current (68 kA). Several thermocouples were installed to measure the temperature reached on different parts of the switch. The VCB was always kept closed during the test. The admissible temperatures are [8]: - Contact material silver coated in air: 105 ◦ C - Connections, like Cu or Al busbars in air: 90 ◦ C A continuous current of 75 kA was maintained for a total of about 7.5 h at an ambient air temperature of
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Fig. 3. BPS heat run test: maximum and minimum main contact temperature.
31–32 ◦ C. Fig. 3 shows the temperatures of the coldest and hottest main contacts. The voltage drop across the BPS was 46.4 mV (3480 W at 75 kA). The temperature of the water connections was “adjusted” by varying the water flow (0–24,330 s, 100 l/h; 24,330–26,500 s, 500 l/h; 26,500–27,100 s, stopped). The test current (75 kA) produced a power dissipation 15% higher than at the BPS rated current (70 kA) and 21% higher than at the maximum circuit current (68 kA). If an approximate linear behaviour of the temperature increase T with the power is assumed, the maximum T on the contacts would be 59 K at 70 kA. The test showed that the environment and cooling conditions of the actual ITER installation are decisive for the operation of the BPS within the safe temperature limits. In particular, the cooling of the connecting busbars affects the temperature of the switch. When Tair = 35 ◦ C (ITER maximum average indoor temperature over 4 h) and water cooled connections at 45 ◦ C, the absolute maximum contact temperature would be 94 ◦ C. Considering the 105 ◦ C limit, a higher temperature could be allowed on the external connections. To reduce the need of active cooling, additional measures could be considered, e.g., black painted busbars and connections with increased dissipating surfaces. Furthermore, the design of the BPS allows the number of
poles of the BPS to be increased without any major problem. In case of loss of cooling, the time constant of the temperature increase is of the order of a few hundreds seconds, allowing enough time for taking remedial actions or to activate the relevant protections. 3.2. BPS commutation and contact life tests The BPS60, with the VCB parallel connected and kept closed, was tested at the JET Joint Undertaking for more than 1000 operating cycles at 66 kA aimed at characterizing its commutation features and assessing its life [7]. Having established the reliability and endurance of the switch, a reduced number of pulses (about 300) was performed on BPS70 to characterize its commutation features up to 75 kA. Each of the 12 arc contacts was provided with a Rogowski coil. The opening times (from command to a separation of 27–28 mm of the main contacts) were from 357 to 367 ms and after a few operations, the jitter stabilised at ±2 ms. The commutation time from the first main contact separation to the full transfer of current to the VCB was 11 ms at 70 kA. At the end of the pulses, the main contacts were in almost pristine conditions. No maintenance or
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intervention was performed during the execution of the test campaign.
4. The vacuum circuit breaker The development and experimental assessment of the vacuum circuit breakers was based, beside on the general know-how on VCBs for AC applications, on the earlier development work for a 50 kA DC VCB (interrupter type D10) carried out in mid-1980s, in particular, for the RFX experiment [9]. The ITER main ratings for the VCB are: • • • • • • • •
Rated voltage: 17.5 Kv Insulation test voltage: 38 kV rms Breaking current: 70 kA, 2001/(66 kA, 1998) Recovery voltage: 24 kV I2 t before opening: 2.3 × 109 A2 s (2001)* Total opening time: 30 ms ±2 ms Lifetime (original 1995): 2000 cycles Lifetime for CCU (revised 2001): 250 cycles
*A value of 8.15 × 109 A2 s was originally specified. The development work was aimed at using as much as possible components coming from standard AC VCB, to ensure basic industrial reliability and reduce costs. The VCB current produces an axial magnetic field (AMF, ca. 3–4 mT/kA) by means of an appropriate shaping of the contacts. The AMF maintains a diffuse arc and reduces the erosion of contacts. In view of the higher current, a vacuum interrupter was produced with larger contacts (Fig. 4, 120 mm diameter) than for previous projects (100 mm, D10). These VCBs are derived from the circuit breakers protecting the output of large generators (3 kA rated current, 80 kA breaking current) and are characterized by: contact separation time <12 ms, speed ca. 1 m/s, distance between contacts 11 mm and jitter ±2 ms. The contacts are reinforced to limit deformations produced by the heat generated during interruption. The VCB is composed of two series connected interrupters, each rated to sustain the re-applied full voltage. It is, in fact, known that occasionally (once over several hundreds of pulses), a single interrupter may fail to extinguish the current (re-strike). The series connec-
Fig. 4. X-Ray image of a D12 vacuum interrupter.
tion of two interrupters make the complete failure of the unit extremely unlikely. The VCB underwent extensive life testing at Consorzio RFX in Padua. The interruption and life tests on the VCB were carried out during two distinct campaigns, the first one in 1997 and the second one in 2003–2004. Some of the VCB manufacturing procedures changed between the two test phases. The “standard” testing conditions were: - Maximum current: 66 kA (1997)–70 kA (2003– 2004); - Re-applied voltage: 24 kV; - Maximum I2 t before interruption: 2.3 × 109 A2 s; - dI/dt at interruption: ca. 200 A/s; - dV/dt after interruption: ≤200 V/s; - Arcing time: ca. 7 ms. The operation of the test circuit reproduced very closely the actual ITER operating conditions. In particular, after 7 ms of arcing time (arc voltage 30–40 V) the counter-pulse capacitor banks were discharged so producing an artificial zero current and, consequently, the current extinction. The current is transferred to the parallel discharge resistor and a 24 kV recovery voltage is applied to the VCB. In the RFX tests, the VCB reapplied voltage decays with a time constant of ca. 20 ms. The arcing time was chosen as a compromise in order to guarantee enough contact gap at the arc extinction and to limit the energy dissipation due to arc.
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During the first phase (1997), an extensive life test was done on the interrupter, with more than 1600 interruptions at 66 kA at 24 kV; only very few re-strikes were recorded, in agreement with the expected statistic. One of the most important findings was a stringent limitation of the allowable pre-arcing I2 t before opening. This was a new effect which had not been evident in previous experiments and is unknown from AC applications. A successful interruption was found to be strongly dependant upon a limitation of the prearcing I2 t at less than 2.3 × 109 A2 s. When the I2 t went above this threshold, the number of failed interruptions increased beyond an acceptable limit possibly due to excessive vaporized material from the contacts. The limit I2 t corresponds to a time of 470 ms at I = 70 kA and is adequate taking into account the commutation times from the BPS to the VCB. It was originally advised to reverse the direction of the current by rotating the tubes every few hundreds shots in order to equalize the wearing of the contacts (the anode is normally more eroded than the cathode). However, it was experienced that after the inversion, the interruption performances could be seriously degraded, possibly due to some mechanical realignment of the contacts. The “inversion” procedure was therefore, soon dismissed. The tests were performed on six nominally identical interrupters identified as tubes 5–10. Besides the standard conditions, further tests were performed also: - in the presence of an external magnetic field; - in the absence of the series saturable reactor; - at low current (e.g., 5 kA) but at full counter-pulse capacitor charging voltage. During the second test campaign, life tests were carried out over 250 pulses and the combined operation of the BPS and VCB was normally used fully reproducing the ITER operating conditions of the CCU (Fig. 5). 4.1. Tests with an external B field The AMF produced by the VCB (250–300 mT at 70 kA) plays an essential role for the creation of a diffuse arc and for the breaking capability of the interrupter. In ITER, an external B field of 25 mT, essentially axial, is expected in the location of installation of the CCUs.
Fig. 5. Combined operation of BPS and VCB. (a) BPS and VCB currents (in kA). (b) Total recovery voltage across VCB and on each interrupters (in kV).
External fields may distort or decrease the AMF. However, in the VCB configuration adopted for ITER, a vertical field would produce a reduction of the AMF in one tube but an enhancement of the AMF of the other tube. There was very little experience about the behaviour at interruption of VCBs with substantial external B fields (>10 mT). The shielding necessary to decrease the field to a safe level (e.g., 2.5 mT) would be quite massive and the presence of ferromagnetic material could affect the interruption. An extensive campaign was therefore performed to assess the interruption capabilities with B field of approximately 40 mT generated by a current circulating in two turns placed around the tubes. Tests at different current levels, from 10 to 70 kA, were successfully executed. In addition, a full life test (250 shots on interrupters 7 and 8) was also carried out with only one single tube re-strike while the interruption was always successful. The mechanical operability of the VCB, including actuator, was also tested in the presence of external magnetic fields. Correct operation was possible for B = 25 mT and in general no malfunction was recorded up to ca. 50 mT.
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4.2. Tests without saturable reactor and at low current and full charged counter-pulse capacitor The same interrupters were also subjected to 40 tests at full current and in the presence of magnetic external field (ca. 40 mT) after excluding the series saturable reactor from the circuit. The current derivative at interruption increased to ca. 200 A/s from ca. 100 A/s in presence of the saturable reactor. Also this test campaign was successful and no re-strike was recorded. These results prove therefore that the saturable reactor is not essential for the interrupting capabilities of the VCB [10]. The removal of the reactors from the circuit could yield some cost and space saving advantages. A CCU may be required to intervene and discharge the magnet at any moment. This means that the current to be interrupted cannot be anticipated. The counter-pulse capacitor will always be charged at the voltage suited to the maximum current and therefore, not optimised for interrupting lower currents (larger negative recovery voltage). More than 40 pulses in this operating condition (down to 5 kA) were successfully performed without any abnormal result. 4.3. A pending issue When the two last tubes available for testing (numbers 9 and 10) were tested in ‘standard conditions’, after a few successful pulses (seven interruptions at increasing currents from 8 to 50 kA), the first attempt to interrupt 70 kA failed. While single ‘restrikes’ can be occasionally experienced, it has never happened, over thousands of pulses, that two interrupters operated within their nominal ratings, failed to interrupt simultaneously. The occurrence of the failure at the first attempt to interrupt 70 kA rules out the possibility of a ‘statistical blip’. The detailed investigation of the interrupters 9 and 10 did not reveal anything abnormal either mechanical or electrical and the metallographic inspection of the contacts showed that the copper/chromium matrix was identical to the one of all the other successful interrupters. The only identified difference was the sequence of pulses to which the interrupters had been subjected. In fact, the successful interrupters had per-
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formed several tens of operations at lower currents before operating at full performances. Interrupters 9 and 10 instead executed only seven pulses before the first 70 kA interruption was unsuccessfully attempted. It could be concluded therefore that the contacts may need some training, or conditioning, before reaching their full interrupting capabilities. Further investigations are being carried out to verify this thesis.
5. Life assessment of the CCU The VCB performed well in excess of the ITER requirements for the CCUs. Interrupters 7 and 8 performed about 600 successful pulses, of which more than 500 at 70 kA, in different operating conditions (“standard”, with external magnetic field, without saturable reactor) with only one single tube re-strike. The final visual inspection showed that the contacts were in good conditions and had some further operation life. By the end of the tests, the BPS70 had operated without maintenance (the arc contacts were replaced only once) for about 1300 pulses. No malfunctioning was ever recorded and the main contacts were in almost pristine conditions. The arc contacts were still in good operating conditions.
6. Conclusions An extensive and exhaustive R&D programme has been completed by the EU HT and the development of a full scale, full rating switching unit for ITER was achieved. Both the BPS and the VCB have been successfully developed and thoroughly tested in conditions closely reproducing the ITER ones, fulfilling the ITER requirements. Some statistics: - in total, about 3000 interruptions have been carried out, 80% of them at currents of 66 kA and 70 kA; - the BPS60 and BPS70 were operated more than 6000 times (4000 operations purely mechanical); - about 1500 pulses, 800 of which at full current, were performed with the combined operation of BPS and VCB.
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A few important constraints in the operation conditions have been identified, in particular:
References
- the limitation of the pre-arcing I2 t at 2.3 × 109 A2 s for the VCB; - the importance to define clearly the cooling conditions of the external connections to the BPS terminals.
[1] ITER Design Description Document, DDD 4.1, Pulsed Power Supplies, 2001. [2] ITER Design Requirements and Guidelines Level 2, DRG2, Part 2: Magnet System. [3] ITER Design Description Document, DDD 1.1, Magnet Engineering, 2001. [4] P.L. Mondino, T. Bonicelli, V. Kuchinskiy, A. Roshal, Coil power supply components, Fusion Eng. Des. 55 (2001) 325–330. [5] F. Bellina, Design, construction and operation of the 66 kA life test facility for the ITER magnet protection vacuum circuit breakers, in: Proceedings of the 17th IEEE/NPSS Symposium on Fusion Engineering, San Diego, CA, 1997, pp. 1137– 1140. [6] T. Bonicelli, The development and testing of a 66 kA by-pass switch with arc commutation capability for the ITER coils power supply system, in: Proceedings of the 17th IEEE/NPSS Symposium on Fusion Engineering, San Diego, CA, 1997, pp. 1129–1132. [7] S.R. Shaw, T. Bonicelli, M. Huart, C. Lescure, The JET DC high power test facility and life testing of a high current ITER switch, in: Proceedings of the 21st Symposium on Fusion Technology, Madrid, Spain, 2000, pp. 99–103. [8] IEC 60694, Common Specifications for High-Voltage Switchgear and Controlgear Standards, 2002. [9] I. Benfatto, Life tests on vacuum switches breaking 50 kA unidirectional current, IEEE Trans. Power Deliv. 6 (2) (1991) 824–832. [10] P. Bettini, A. De Lorenzi, A. Maschio, Parametric analysis of current extinction in counter-pulse system, Plasma Dev. Operat. 5 (4) (1998).
A successful test campaign proved that the VCB can safely interrupt ‘low’ currents also when the counterpulse capacitor is fully charged. Several pulses, all successful, were carried out without series saturable reactor, which could yield advantages under the cost and layout viewpoints. The capability of the VCB to operate in presence of an external magnetic field without any shielding was proved up to ca. 40 mT, in excess of the ITER requirements.
Acknowledgments The authors would like to thank J. Neuser (Ritter), R. Renz (Siemens AG), E. Gaio, V. Toigo, F. Milani, L. Zanotto (Consorzio RFX), C. Lescure (UKAEA) and B. Bareyt (ITER IT) for their contributions. The continuous support and help from P.L. Mondino (previously with the ITER IT now with EFDA-CSU Garching) is gratefully acknowledged.
The European development of a full scale switching unit for the ITER switching and discharging networks T. Bonicelli a,∗ , A. De Lorenzi b , D. Hrabal c , R. Piovan b , E. Sachs c , E. Salpietro a , S.R. Shaw d a
EFDA-CSU, Boltzmannstr. 2, 85748 Garching (D), Germany b Consorzio RFX, Corso Stati Uniti 4, 35127 Padova, Italy c FEAG, Guenther-Scharowskystr. 2, 91058 Erlangen, Germany d UKAEA, Culham Science Centre, OX14 3EA, Abindgdon, UK Available online 26 July 2005
Abstract The European Fusion Programme included since the mid-1990s, the development of a full scale, full rating current commutating unit (CCU) to be used as a centre-piece of the ITER magnet protective circuits. The CCU is mainly composed of a by-pass switch (BPS) in parallel to a vacuum circuit breaker (VCB). Both full scale switches were subjected to extensive testing in conditions simulating the actual ITER operation. The operation in absence of the saturable reactor series connected in the ITER reference design was successfully assessed. Interruption tests at low current and full counter-pulse capacitor voltage, were also satisfactorily performed. The tests included an extensive and novel characterisation of the interrupting capability of the vacuum circuit breaker in the presence of an external magnetic field as in the actual location of installation in ITER. © 2005 The European Commission. Published by Elsevier B.V. All rights reserved. Keywords: DC circuit breakers; ITER; Superconductive magnet protection
1. Introduction Large superconductive coils, like the ITER PF and TF magnets, require an extremely reliable system to quickly discharge the magnetic energy in case a coil quench is detected (for the TF magnet, 40.2 GJ stored at full current must be discharged with 11 s time constant). ∗ Corresponding author. Tel.: +49 89 3299 4260; fax: +49 89 3299 4198. E-mail address: [email protected] (T. Bonicelli).
In ITER, the discharge is performed with the current commutating unit (CCU) of Fig. 1. A by-pass switch (BPS), carrying the steady state current but with very limited current interruption capability, is parallel connected with a vacuum circuit breaker (VCB) suitable for the interruption of the current. The scheme, proposed by the European Home Team (EU HT), was adopted in the ITER reference design [1]. After the initiation of a fast discharge, the BPS is opened and the current is fully transferred to the closely located VCB which is then opened. The commutation
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which could be directly used for the ITER procurement packages. Two phases were foreseen: the first phase took place from 1995 to 1999 and was aimed at the separate development of the full scale VCB and BPS [5–7]; the second phase took place from 2000 to 2004 and was aimed at:
Fig. 1. Current commutating unit: a simplified diagram.
of the current to the discharge resistors (DR) is achieved by means of the discharge of a counter-pulse capacitor (C) that creates an artificial current zero in the VCB arc chamber. A series saturable reactor (LS) decreases the current derivative around the current zero-crossing to enhance the possibility of de-ionisation and facilitate interruption. To increase the reliability of the CCU, two vacuum interrupters, each able to interrupt the nominal current and to sustain the full voltage, are series connected. Finally, a single action circuit breaker (pyrobreaker, PB) is installed in series to the CCU and is operated in the unlikely event of complete failure of the unit. Other CCUs are also foreseen in ITER circuits for generating the loop voltage required at plasma breakdown and current start-up. These units operate at every pulse and could be in principle based on the same type of circuits [1]. A total of 200 fast discharges are specified for the magnet systems [2] during the ITER life and 50 fast discharges (10 actual quenches) are actually considered for the TF system [3]. A lifetime of 250 operations was therefore, considered adequate for the main components.
2. The EU HT development of a CCU As a part of the EU HT R&D efforts towards ITER [4], the development of a full scale CCU (Fig. 2), was undertaken with the aim to develop components
- adapting the design to the new ITER requirements (2001), in particular the increase of rated TF coil current from 60 kA to 68 kA; - testing the combined operation of BPS and VCB; - testing the interruption performances in presence of external B field; - testing the interruption performances at low current (e.g., 5 kA) but with fully charged counter-pulse capacitor; - testing the interruption performances in absence of the series saturable reactor. The BPS switch manufactured and tested in the first phase will be identified as BPS60 and in the last phase as BPS70.
3. The by-pass switch The BPS, whose main specifications are in Table 1, is based on industrial high current/low voltage (around 1 kV) switches used in electrochemistry plants. The main features of the BPS70 are: - 12 main poles parallel connected, arranged on two rows of six each; - each main contact has a size of 120 mm × 150 mm, made of silver plated copper; - each fixed main contact is provided with four copper–silver–beryllium alloy multicontact strips; Table 1 By-pass switch main specifications Rated voltage Rated DC current Peak withstand current Opening time Jitter on opening Mechanical lifetime Main contacts lifetime Arc contacts lifetime Cooling
17.5 kV 70 kA BPS70/(60 kA BPS60) 250 kA <500 ms ±20 ms 2000 cycles 1000 openings 100 openings Natural circulation of air
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Fig. 2. Complete CCU: on the left, the VCB and its actuator; on the right, the BPS.
- each main contact is provided with an arc contact made of high tungsten graded–copper alloy (75%–25%) soldered on a copper base. The qualification of the BPS was carried out both during phase 1 (BPS60) and phase 2 (BPS70). Besides standard factory tests, the following tests were performed: -
contact resistance measurements; extended mechanical endurance test; temperature rise test; peak withstand current test; commutation qualification tests to the parallel connected VCB; - life commutation tests. The average resistance in cold conditions of each main contact is about 6 and the total resistance measured at the BPS terminals is 0.47 (BPS70). The resistance of the parallel path with the VCB is instead 62 and this implies that only less than 1% of the current flows in steady state in the VCB.
The extended mechanical endurance test was performed on BPS60 at the nominal operating pressure of 6 bar. Up to 2024 operating cycles were successfully carried out without need of any maintenance. In particular, the jitter on opening was typically only ±2 ms over the full test cycles. The switch also passed successfully the peak current test performed with an AC current of 250 kA lasting 300 ms. 3.1. BPS heat run tests The heat run test was carried out at 75 kA, about 10% higher than the maximum ITER CCU current (68 kA). Several thermocouples were installed to measure the temperature reached on different parts of the switch. The VCB was always kept closed during the test. The admissible temperatures are [8]: - Contact material silver coated in air: 105 ◦ C - Connections, like Cu or Al busbars in air: 90 ◦ C A continuous current of 75 kA was maintained for a total of about 7.5 h at an ambient air temperature of
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Fig. 3. BPS heat run test: maximum and minimum main contact temperature.
31–32 ◦ C. Fig. 3 shows the temperatures of the coldest and hottest main contacts. The voltage drop across the BPS was 46.4 mV (3480 W at 75 kA). The temperature of the water connections was “adjusted” by varying the water flow (0–24,330 s, 100 l/h; 24,330–26,500 s, 500 l/h; 26,500–27,100 s, stopped). The test current (75 kA) produced a power dissipation 15% higher than at the BPS rated current (70 kA) and 21% higher than at the maximum circuit current (68 kA). If an approximate linear behaviour of the temperature increase T with the power is assumed, the maximum T on the contacts would be 59 K at 70 kA. The test showed that the environment and cooling conditions of the actual ITER installation are decisive for the operation of the BPS within the safe temperature limits. In particular, the cooling of the connecting busbars affects the temperature of the switch. When Tair = 35 ◦ C (ITER maximum average indoor temperature over 4 h) and water cooled connections at 45 ◦ C, the absolute maximum contact temperature would be 94 ◦ C. Considering the 105 ◦ C limit, a higher temperature could be allowed on the external connections. To reduce the need of active cooling, additional measures could be considered, e.g., black painted busbars and connections with increased dissipating surfaces. Furthermore, the design of the BPS allows the number of
poles of the BPS to be increased without any major problem. In case of loss of cooling, the time constant of the temperature increase is of the order of a few hundreds seconds, allowing enough time for taking remedial actions or to activate the relevant protections. 3.2. BPS commutation and contact life tests The BPS60, with the VCB parallel connected and kept closed, was tested at the JET Joint Undertaking for more than 1000 operating cycles at 66 kA aimed at characterizing its commutation features and assessing its life [7]. Having established the reliability and endurance of the switch, a reduced number of pulses (about 300) was performed on BPS70 to characterize its commutation features up to 75 kA. Each of the 12 arc contacts was provided with a Rogowski coil. The opening times (from command to a separation of 27–28 mm of the main contacts) were from 357 to 367 ms and after a few operations, the jitter stabilised at ±2 ms. The commutation time from the first main contact separation to the full transfer of current to the VCB was 11 ms at 70 kA. At the end of the pulses, the main contacts were in almost pristine conditions. No maintenance or
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intervention was performed during the execution of the test campaign.
4. The vacuum circuit breaker The development and experimental assessment of the vacuum circuit breakers was based, beside on the general know-how on VCBs for AC applications, on the earlier development work for a 50 kA DC VCB (interrupter type D10) carried out in mid-1980s, in particular, for the RFX experiment [9]. The ITER main ratings for the VCB are: • • • • • • • •
Rated voltage: 17.5 Kv Insulation test voltage: 38 kV rms Breaking current: 70 kA, 2001/(66 kA, 1998) Recovery voltage: 24 kV I2 t before opening: 2.3 × 109 A2 s (2001)* Total opening time: 30 ms ±2 ms Lifetime (original 1995): 2000 cycles Lifetime for CCU (revised 2001): 250 cycles
*A value of 8.15 × 109 A2 s was originally specified. The development work was aimed at using as much as possible components coming from standard AC VCB, to ensure basic industrial reliability and reduce costs. The VCB current produces an axial magnetic field (AMF, ca. 3–4 mT/kA) by means of an appropriate shaping of the contacts. The AMF maintains a diffuse arc and reduces the erosion of contacts. In view of the higher current, a vacuum interrupter was produced with larger contacts (Fig. 4, 120 mm diameter) than for previous projects (100 mm, D10). These VCBs are derived from the circuit breakers protecting the output of large generators (3 kA rated current, 80 kA breaking current) and are characterized by: contact separation time <12 ms, speed ca. 1 m/s, distance between contacts 11 mm and jitter ±2 ms. The contacts are reinforced to limit deformations produced by the heat generated during interruption. The VCB is composed of two series connected interrupters, each rated to sustain the re-applied full voltage. It is, in fact, known that occasionally (once over several hundreds of pulses), a single interrupter may fail to extinguish the current (re-strike). The series connec-
Fig. 4. X-Ray image of a D12 vacuum interrupter.
tion of two interrupters make the complete failure of the unit extremely unlikely. The VCB underwent extensive life testing at Consorzio RFX in Padua. The interruption and life tests on the VCB were carried out during two distinct campaigns, the first one in 1997 and the second one in 2003–2004. Some of the VCB manufacturing procedures changed between the two test phases. The “standard” testing conditions were: - Maximum current: 66 kA (1997)–70 kA (2003– 2004); - Re-applied voltage: 24 kV; - Maximum I2 t before interruption: 2.3 × 109 A2 s; - dI/dt at interruption: ca. 200 A/s; - dV/dt after interruption: ≤200 V/s; - Arcing time: ca. 7 ms. The operation of the test circuit reproduced very closely the actual ITER operating conditions. In particular, after 7 ms of arcing time (arc voltage 30–40 V) the counter-pulse capacitor banks were discharged so producing an artificial zero current and, consequently, the current extinction. The current is transferred to the parallel discharge resistor and a 24 kV recovery voltage is applied to the VCB. In the RFX tests, the VCB reapplied voltage decays with a time constant of ca. 20 ms. The arcing time was chosen as a compromise in order to guarantee enough contact gap at the arc extinction and to limit the energy dissipation due to arc.
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During the first phase (1997), an extensive life test was done on the interrupter, with more than 1600 interruptions at 66 kA at 24 kV; only very few re-strikes were recorded, in agreement with the expected statistic. One of the most important findings was a stringent limitation of the allowable pre-arcing I2 t before opening. This was a new effect which had not been evident in previous experiments and is unknown from AC applications. A successful interruption was found to be strongly dependant upon a limitation of the prearcing I2 t at less than 2.3 × 109 A2 s. When the I2 t went above this threshold, the number of failed interruptions increased beyond an acceptable limit possibly due to excessive vaporized material from the contacts. The limit I2 t corresponds to a time of 470 ms at I = 70 kA and is adequate taking into account the commutation times from the BPS to the VCB. It was originally advised to reverse the direction of the current by rotating the tubes every few hundreds shots in order to equalize the wearing of the contacts (the anode is normally more eroded than the cathode). However, it was experienced that after the inversion, the interruption performances could be seriously degraded, possibly due to some mechanical realignment of the contacts. The “inversion” procedure was therefore, soon dismissed. The tests were performed on six nominally identical interrupters identified as tubes 5–10. Besides the standard conditions, further tests were performed also: - in the presence of an external magnetic field; - in the absence of the series saturable reactor; - at low current (e.g., 5 kA) but at full counter-pulse capacitor charging voltage. During the second test campaign, life tests were carried out over 250 pulses and the combined operation of the BPS and VCB was normally used fully reproducing the ITER operating conditions of the CCU (Fig. 5). 4.1. Tests with an external B field The AMF produced by the VCB (250–300 mT at 70 kA) plays an essential role for the creation of a diffuse arc and for the breaking capability of the interrupter. In ITER, an external B field of 25 mT, essentially axial, is expected in the location of installation of the CCUs.
Fig. 5. Combined operation of BPS and VCB. (a) BPS and VCB currents (in kA). (b) Total recovery voltage across VCB and on each interrupters (in kV).
External fields may distort or decrease the AMF. However, in the VCB configuration adopted for ITER, a vertical field would produce a reduction of the AMF in one tube but an enhancement of the AMF of the other tube. There was very little experience about the behaviour at interruption of VCBs with substantial external B fields (>10 mT). The shielding necessary to decrease the field to a safe level (e.g., 2.5 mT) would be quite massive and the presence of ferromagnetic material could affect the interruption. An extensive campaign was therefore performed to assess the interruption capabilities with B field of approximately 40 mT generated by a current circulating in two turns placed around the tubes. Tests at different current levels, from 10 to 70 kA, were successfully executed. In addition, a full life test (250 shots on interrupters 7 and 8) was also carried out with only one single tube re-strike while the interruption was always successful. The mechanical operability of the VCB, including actuator, was also tested in the presence of external magnetic fields. Correct operation was possible for B = 25 mT and in general no malfunction was recorded up to ca. 50 mT.
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4.2. Tests without saturable reactor and at low current and full charged counter-pulse capacitor The same interrupters were also subjected to 40 tests at full current and in the presence of magnetic external field (ca. 40 mT) after excluding the series saturable reactor from the circuit. The current derivative at interruption increased to ca. 200 A/s from ca. 100 A/s in presence of the saturable reactor. Also this test campaign was successful and no re-strike was recorded. These results prove therefore that the saturable reactor is not essential for the interrupting capabilities of the VCB [10]. The removal of the reactors from the circuit could yield some cost and space saving advantages. A CCU may be required to intervene and discharge the magnet at any moment. This means that the current to be interrupted cannot be anticipated. The counter-pulse capacitor will always be charged at the voltage suited to the maximum current and therefore, not optimised for interrupting lower currents (larger negative recovery voltage). More than 40 pulses in this operating condition (down to 5 kA) were successfully performed without any abnormal result. 4.3. A pending issue When the two last tubes available for testing (numbers 9 and 10) were tested in ‘standard conditions’, after a few successful pulses (seven interruptions at increasing currents from 8 to 50 kA), the first attempt to interrupt 70 kA failed. While single ‘restrikes’ can be occasionally experienced, it has never happened, over thousands of pulses, that two interrupters operated within their nominal ratings, failed to interrupt simultaneously. The occurrence of the failure at the first attempt to interrupt 70 kA rules out the possibility of a ‘statistical blip’. The detailed investigation of the interrupters 9 and 10 did not reveal anything abnormal either mechanical or electrical and the metallographic inspection of the contacts showed that the copper/chromium matrix was identical to the one of all the other successful interrupters. The only identified difference was the sequence of pulses to which the interrupters had been subjected. In fact, the successful interrupters had per-
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formed several tens of operations at lower currents before operating at full performances. Interrupters 9 and 10 instead executed only seven pulses before the first 70 kA interruption was unsuccessfully attempted. It could be concluded therefore that the contacts may need some training, or conditioning, before reaching their full interrupting capabilities. Further investigations are being carried out to verify this thesis.
5. Life assessment of the CCU The VCB performed well in excess of the ITER requirements for the CCUs. Interrupters 7 and 8 performed about 600 successful pulses, of which more than 500 at 70 kA, in different operating conditions (“standard”, with external magnetic field, without saturable reactor) with only one single tube re-strike. The final visual inspection showed that the contacts were in good conditions and had some further operation life. By the end of the tests, the BPS70 had operated without maintenance (the arc contacts were replaced only once) for about 1300 pulses. No malfunctioning was ever recorded and the main contacts were in almost pristine conditions. The arc contacts were still in good operating conditions.
6. Conclusions An extensive and exhaustive R&D programme has been completed by the EU HT and the development of a full scale, full rating switching unit for ITER was achieved. Both the BPS and the VCB have been successfully developed and thoroughly tested in conditions closely reproducing the ITER ones, fulfilling the ITER requirements. Some statistics: - in total, about 3000 interruptions have been carried out, 80% of them at currents of 66 kA and 70 kA; - the BPS60 and BPS70 were operated more than 6000 times (4000 operations purely mechanical); - about 1500 pulses, 800 of which at full current, were performed with the combined operation of BPS and VCB.
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A few important constraints in the operation conditions have been identified, in particular:
References
- the limitation of the pre-arcing I2 t at 2.3 × 109 A2 s for the VCB; - the importance to define clearly the cooling conditions of the external connections to the BPS terminals.
[1] ITER Design Description Document, DDD 4.1, Pulsed Power Supplies, 2001. [2] ITER Design Requirements and Guidelines Level 2, DRG2, Part 2: Magnet System. [3] ITER Design Description Document, DDD 1.1, Magnet Engineering, 2001. [4] P.L. Mondino, T. Bonicelli, V. Kuchinskiy, A. Roshal, Coil power supply components, Fusion Eng. Des. 55 (2001) 325–330. [5] F. Bellina, Design, construction and operation of the 66 kA life test facility for the ITER magnet protection vacuum circuit breakers, in: Proceedings of the 17th IEEE/NPSS Symposium on Fusion Engineering, San Diego, CA, 1997, pp. 1137– 1140. [6] T. Bonicelli, The development and testing of a 66 kA by-pass switch with arc commutation capability for the ITER coils power supply system, in: Proceedings of the 17th IEEE/NPSS Symposium on Fusion Engineering, San Diego, CA, 1997, pp. 1129–1132. [7] S.R. Shaw, T. Bonicelli, M. Huart, C. Lescure, The JET DC high power test facility and life testing of a high current ITER switch, in: Proceedings of the 21st Symposium on Fusion Technology, Madrid, Spain, 2000, pp. 99–103. [8] IEC 60694, Common Specifications for High-Voltage Switchgear and Controlgear Standards, 2002. [9] I. Benfatto, Life tests on vacuum switches breaking 50 kA unidirectional current, IEEE Trans. Power Deliv. 6 (2) (1991) 824–832. [10] P. Bettini, A. De Lorenzi, A. Maschio, Parametric analysis of current extinction in counter-pulse system, Plasma Dev. Operat. 5 (4) (1998).
A successful test campaign proved that the VCB can safely interrupt ‘low’ currents also when the counterpulse capacitor is fully charged. Several pulses, all successful, were carried out without series saturable reactor, which could yield advantages under the cost and layout viewpoints. The capability of the VCB to operate in presence of an external magnetic field without any shielding was proved up to ca. 40 mT, in excess of the ITER requirements.
Acknowledgments The authors would like to thank J. Neuser (Ritter), R. Renz (Siemens AG), E. Gaio, V. Toigo, F. Milani, L. Zanotto (Consorzio RFX), C. Lescure (UKAEA) and B. Bareyt (ITER IT) for their contributions. The continuous support and help from P.L. Mondino (previously with the ITER IT now with EFDA-CSU Garching) is gratefully acknowledged.