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Quench detection and data acquisition system for SST-1 superconducting magnets
Fusion Engineering and Design 74 (2005) 819–823
Quench detection and data acquisition system for SST-1 superconducting magnets A.N. Sharma ∗ , C.J. Hansalia, Y. Yeole, G. Bansal, S. Pradhan, A. Moitra, Y.C. Saxena Institute for Plasma Research, Bhat, Gandhinagar 382428, Gujarat, India Available online 9 August 2005
Abstract Superconducting magnet system of steady state superconducting Tokamak (SST-1) shall be operating in a very noisy environment. Presences of high inductive voltages in the magnets during off-normal events like VDE, plasma disruption and PF magnet ramp ups, etc. has made quench detection and data acquisition a challenging task. A hybrid of analog electronic circuits and software-controlled data acquisition system has been developed and tested to safeguard the magnets. This paper will describe the electronic hardware circuits developed for signal conditioning, high voltage suppression, fail-proof quench detection and for noise elimination algorithms and their testing. The SST-1 superconducting magnets will have large number of sensors like voltage taps, Venturi flow meter, strain gages, hall probes, pressure sensors, temperature sensors and displacement transducers. A real time data acquisition system has been designed using VMEbus for monitoring and storing signals from all these sensors and initiating control action in case of offnormal events. The paper will also describe the configuration of the data acquisition system with emphasis on hardware used and the software developed for it. © 2005 Elsevier B.V. All rights reserved. Keywords: Quench detection; Superconducting magnets; Sensors; Signal conditioning; VME; Data acquisition
1. Introduction The steady state Tokamak (SST-1) is in commissioning phase at Institute for Plasma Research in India. The SST-1 is a medium size tokamak to study the plasma behavior in steady state operation of 1000 s. In SST∗ Corresponding author. Tel.: +91 79 2396 9001x327/404; fax: +91 79 2396 9017. E-mail address: [email protected] (A.N. Sharma).
0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.06.288
1, all the 16 toroidal field (TF) magnets and 9 out of 11 poloidal field (PF) magnets are superconducting [1,2]. The base conductor for SST-1 superconducting magnets is a NbTi/Cu-based cable-in-conduit conductor. The superconducting magnets in SST-1 will be cooled by supercritical helium at 4.0 atm and 4.5 K. The energy stored in TF and PF coils system is 56 and 12 MJ, respectively. So, an efficient and fail-proof quench detection system was required to avoid damages to the magnets. A lot of coil diagnostics is also
820
A.N. Sharma et al. / Fusion Engineering and Design 74 (2005) 819–823
planned to have detailed information about the magnet behavior. A rugged, real time data acquisition is also designed for communication with other subsystems and magnet data storage.
2. Monitoring of SST-1 superconducting magnets SST-1 superconducting magnets will be monitored during all operational scenarios like cool down, ramp up, flat top, plasma breakdown, dumping/ramp down and warm up. About 500 sensors, like voltage taps, Venturi flow meter, hall probes, pressure sensors, temperature sensors and displacement transducers will be mounted on the magnets. These sensors are selected keeping in mind the magnetic field present in the tokamak environment as well as their cryogenic compatibility. These sensors produce very low level output signals and so they impose very strict requirements on the signal conditioning and data acquisition system. The background of the tokamak hall is very harsh because of presence of varying high electro-magnetic fields, low temperatures, high common-mode voltages, etc. So, for accurate measurement, a lot of attention was paid in grounding, shielding, signal driving up to data acquisition system. The signal conditioning schemes can be broadly divided into two categories, one for voltage taps and the other for miscellaneous sensors.
3. Signal conditioning of voltage taps Signal conditioning of voltage taps is crucial from the view that it will have very large common-mode voltages (of the order of 5 kV) in off-normal events plasma disruption, VDE etc. Also, as all TF and PF coils are inductively coupled with each other, voltage taps will also have a lot of inductive voltage induced on them, which makes quench detection very difficult. In SST1, multi level protection from high voltages is planned. First stage is a RC filter combination to bypass high frequency spikes. The R used here is high voltage and high wattage resistor. This resistor will also act as fault current limiter to prevent the voltage tap burn-out. The next block for high voltage spike suppression is combination of metal oxide varistors and back-to-back zener diodes. After this isolation, amplifier isolates the voltage taps
and low pass filters do noise filtering. Cancellation of noise levels are also planned through the subtraction scheme by balancing adjacent layers or pancakes of the same coil or by balancing voltages between same layers in symmetrically placed coils wherever possible. In general, this method is less sensitive compared to balancing in a bridge circuit. The subtraction scheme has been successfully used in a number of large volume superconducting magnets [3,4]. TF magnet has six double pancakes. Its quench detection schematic is shown in Fig. 1. As shown, each half of the TF magnet has its own subtraction circuit. The algorithm adopted is, Vo = 0.5 (V1 + V3 ) − V2 where V1 , V2 and V3 are the voltages across first, second and third double pancake and Vo is difference voltage of the above equation. The same algorithm is applied on the last three double pancakes of the coil. For redundancy, total voltage drop across first three double pancakes is then subtracted from the last three. All three difference signals are then compared with threshold voltage and threshold time. These signals are evaluated by OR gates and sent to latch and relay circuit. From there, this signal is sent to power supply group to inform about magnet quench and dumping initiation. The quench detection circuits also drive the voltage signals across each magnet section to VME data acquisition system for post analysis. Similar circuits are implemented for all PF coils, current leads, etc. All these cards were tested by simulating TF and PF magnets by using different chains of resistances and a constant current source and a function generator. Different scenarios of SST-1 operation were simulated and tested for noise elimination, false triggering and fail-safe operation. The cards were also subjected to high voltages using high voltage power supplies and capacitive discharges.
4. Signal conditioning of sensors Multi channel cards were developed for all the sensors. As shown in Fig. 2, each channel has individual amplifier, low pass filter for noise filtering and optical isolation to avoid grounding problem. Much iterations were done to achieve best decoupling and ground, power line layouts on the PCB. Separate grounds for
A.N. Sharma et al. / Fusion Engineering and Design 74 (2005) 819–823
821
Fig. 1. TF quench detection card scheme.
digital section ground and analog section were provided. These signals are then sent to VME data acquisition system for storage and display in engineering units. The power supplies for sensors were also developed, wherever they were not commercially available. All these sensors are thermal anchored at 77 and 4.2 K to avoid heat load on the system. Temperature sensor leads go up to feedthroughs on the cryostat plate using phos-
phor bronze wire as they have low thermal conductivity. This reduces error in temperature measurement. All signal conditioning cards and quench detection cards are kept in an EMI shielded enclosure and all signal cables are shielded and grounded. Power supplies for all these cards are isolated from main grounding using isolation transformers.
5. Data acquisition system for SST-1 magnets
Fig. 2. Interface of sensors to VME data acquisition.
In a tokamak-like application, data acquisition system is required to acquire, digitize, apply algorithm, make decision, initiate control sequence and communicate with other subsystem all in real time. To satisfy all these requirements, a complete VMEbus-based real time data acquisition system is acquired from different vendors. It includes VG4 processor from SBS, analog input modules (AVME 9660+ IP330) and analog output board (IP 220) from ACROMAG, digital I/O modules (VMICVME-2528) from VMIC. The application development is done on VxWorks OS on Tornado 2.0.2 Platform where C language is used as back end tool, LabView and Matlab as front end tool and
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Fig. 3. VME data acquisition system layout of SST-1 magnets.
TCP/IP socket programming is used for communication with other subsystems. Fig. 3 shows the data flow diagram of the VME data acquisition system of SST-1 magnets. For the safety of SST-1 magnets, in the case of the voltage taps failure, redundant method of detecting quench is also planned. In-house fabricated Venturi flow meters shall be placed at the inlet of each of the TF magnets and at the inlet of the PF1 and PF3, which are more prone to quench. Initial quench zone will be quickly helium depleted as a result of normal zone growth and a backpressure shall be felt on the incoming flow leading to decrease of flow at the inlet. The flow rate data is monitored and acquired in VME data acquisition (daq) system. When the flow will decrease to a predetermined fraction of the nominal flow, the event shall be considered as a quench and VME daq system will initiate control for dumping of the magnets. There are absolute pressure transducers placed at the inlet header and outlet header. The increased pressure at the outlet of the magnets will also be used as an indication of the quench as a result of adiabatic compression of helium following a transient disturbance. However, these sensors are for redundancy only, as their response to quench will be very slow compared to voltage tap sensors.
VME daq system will also send minimum, maximum and average of all temperature sensor values to cryogenic group through the analog output cards. These values will be used to control the cool down process. The VME daq will use a ring buffer arrangement to store pre trigger and post trigger data. It will also be storing all the relevant data coming from all the sensors for all the SST-1 operating scenarios.
6. Results A fail-safe quench detection system has been developed and tested. In the due course of testing, the cards were validated both for ‘no-quench’ and ‘quench scenarios with superimposed high frequency noise’. In both these scenarios, the circuits were tested for threshold voltages set at 100 mV and threshold time set at 100 ms of response. The circuits performed in the expected lines. In order to validate the performance of the circuit during the plasma break down and current start-up scenarios, where an induced voltage as high as 3.5 kV may get induced, the circuit was validated against a 3.5 kV impulse source. The front end signal conditioning cards were also
A.N. Sharma et al. / Fusion Engineering and Design 74 (2005) 819–823
developed for the sensors used in SST-1 magnets in steady state operating scenarios. A real time, stand alone data acquisition system incorporating 512 analog input channels, 16 analog output channels and 128 digital I/O channels has been successfully developed. The data acquisition system thus takes care of the entire magnet operational scenarios including cool down, warm up, cool down following a quench, the off-normal events like vertical displacement events, etc. and the nominal operational scenarios. This data acquisition system is presently getting commissioned for the SST-1 Tokamak and is expected to get finally integrated by early November 2004.
823
References [1] The SST Team, Conceptual design of SST-1 Tokamak, in: 16th IEEE/NPSS Symposium on Fusion Engineering, vol. 1, Urbana Champaign, USA, 1995, p. 481. [2] S. Pradhan, Superconducting cable-in-conduit conductor for SST-1 superconducting magnets, in: Second IAEA meeting on Steady State Tokamaks, October 25–29, Kyushu University, Japan, 1999. [3] J.L. duchateau, D. bessete, D. Ciazynski, J. Pierre, E. Rouanet, P. Riband, et al., Monitoring and controlling toresupra toroidal field system status after a year of operation at nominal current, IEEE trans. on Magnetics, vol. 27, No. 2, March 1991, pp. 2053–2056. [4] R.G. Benson, R.B. Goldfrab, E.S. Pittman, Quench circuit for electronic instruments used with superconducting magnets, Cryogenics 26 (1986) 482–483.
Quench detection and data acquisition system for SST-1 superconducting magnets A.N. Sharma ∗ , C.J. Hansalia, Y. Yeole, G. Bansal, S. Pradhan, A. Moitra, Y.C. Saxena Institute for Plasma Research, Bhat, Gandhinagar 382428, Gujarat, India Available online 9 August 2005
Abstract Superconducting magnet system of steady state superconducting Tokamak (SST-1) shall be operating in a very noisy environment. Presences of high inductive voltages in the magnets during off-normal events like VDE, plasma disruption and PF magnet ramp ups, etc. has made quench detection and data acquisition a challenging task. A hybrid of analog electronic circuits and software-controlled data acquisition system has been developed and tested to safeguard the magnets. This paper will describe the electronic hardware circuits developed for signal conditioning, high voltage suppression, fail-proof quench detection and for noise elimination algorithms and their testing. The SST-1 superconducting magnets will have large number of sensors like voltage taps, Venturi flow meter, strain gages, hall probes, pressure sensors, temperature sensors and displacement transducers. A real time data acquisition system has been designed using VMEbus for monitoring and storing signals from all these sensors and initiating control action in case of offnormal events. The paper will also describe the configuration of the data acquisition system with emphasis on hardware used and the software developed for it. © 2005 Elsevier B.V. All rights reserved. Keywords: Quench detection; Superconducting magnets; Sensors; Signal conditioning; VME; Data acquisition
1. Introduction The steady state Tokamak (SST-1) is in commissioning phase at Institute for Plasma Research in India. The SST-1 is a medium size tokamak to study the plasma behavior in steady state operation of 1000 s. In SST∗ Corresponding author. Tel.: +91 79 2396 9001x327/404; fax: +91 79 2396 9017. E-mail address: [email protected] (A.N. Sharma).
0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.06.288
1, all the 16 toroidal field (TF) magnets and 9 out of 11 poloidal field (PF) magnets are superconducting [1,2]. The base conductor for SST-1 superconducting magnets is a NbTi/Cu-based cable-in-conduit conductor. The superconducting magnets in SST-1 will be cooled by supercritical helium at 4.0 atm and 4.5 K. The energy stored in TF and PF coils system is 56 and 12 MJ, respectively. So, an efficient and fail-proof quench detection system was required to avoid damages to the magnets. A lot of coil diagnostics is also
820
A.N. Sharma et al. / Fusion Engineering and Design 74 (2005) 819–823
planned to have detailed information about the magnet behavior. A rugged, real time data acquisition is also designed for communication with other subsystems and magnet data storage.
2. Monitoring of SST-1 superconducting magnets SST-1 superconducting magnets will be monitored during all operational scenarios like cool down, ramp up, flat top, plasma breakdown, dumping/ramp down and warm up. About 500 sensors, like voltage taps, Venturi flow meter, hall probes, pressure sensors, temperature sensors and displacement transducers will be mounted on the magnets. These sensors are selected keeping in mind the magnetic field present in the tokamak environment as well as their cryogenic compatibility. These sensors produce very low level output signals and so they impose very strict requirements on the signal conditioning and data acquisition system. The background of the tokamak hall is very harsh because of presence of varying high electro-magnetic fields, low temperatures, high common-mode voltages, etc. So, for accurate measurement, a lot of attention was paid in grounding, shielding, signal driving up to data acquisition system. The signal conditioning schemes can be broadly divided into two categories, one for voltage taps and the other for miscellaneous sensors.
3. Signal conditioning of voltage taps Signal conditioning of voltage taps is crucial from the view that it will have very large common-mode voltages (of the order of 5 kV) in off-normal events plasma disruption, VDE etc. Also, as all TF and PF coils are inductively coupled with each other, voltage taps will also have a lot of inductive voltage induced on them, which makes quench detection very difficult. In SST1, multi level protection from high voltages is planned. First stage is a RC filter combination to bypass high frequency spikes. The R used here is high voltage and high wattage resistor. This resistor will also act as fault current limiter to prevent the voltage tap burn-out. The next block for high voltage spike suppression is combination of metal oxide varistors and back-to-back zener diodes. After this isolation, amplifier isolates the voltage taps
and low pass filters do noise filtering. Cancellation of noise levels are also planned through the subtraction scheme by balancing adjacent layers or pancakes of the same coil or by balancing voltages between same layers in symmetrically placed coils wherever possible. In general, this method is less sensitive compared to balancing in a bridge circuit. The subtraction scheme has been successfully used in a number of large volume superconducting magnets [3,4]. TF magnet has six double pancakes. Its quench detection schematic is shown in Fig. 1. As shown, each half of the TF magnet has its own subtraction circuit. The algorithm adopted is, Vo = 0.5 (V1 + V3 ) − V2 where V1 , V2 and V3 are the voltages across first, second and third double pancake and Vo is difference voltage of the above equation. The same algorithm is applied on the last three double pancakes of the coil. For redundancy, total voltage drop across first three double pancakes is then subtracted from the last three. All three difference signals are then compared with threshold voltage and threshold time. These signals are evaluated by OR gates and sent to latch and relay circuit. From there, this signal is sent to power supply group to inform about magnet quench and dumping initiation. The quench detection circuits also drive the voltage signals across each magnet section to VME data acquisition system for post analysis. Similar circuits are implemented for all PF coils, current leads, etc. All these cards were tested by simulating TF and PF magnets by using different chains of resistances and a constant current source and a function generator. Different scenarios of SST-1 operation were simulated and tested for noise elimination, false triggering and fail-safe operation. The cards were also subjected to high voltages using high voltage power supplies and capacitive discharges.
4. Signal conditioning of sensors Multi channel cards were developed for all the sensors. As shown in Fig. 2, each channel has individual amplifier, low pass filter for noise filtering and optical isolation to avoid grounding problem. Much iterations were done to achieve best decoupling and ground, power line layouts on the PCB. Separate grounds for
A.N. Sharma et al. / Fusion Engineering and Design 74 (2005) 819–823
821
Fig. 1. TF quench detection card scheme.
digital section ground and analog section were provided. These signals are then sent to VME data acquisition system for storage and display in engineering units. The power supplies for sensors were also developed, wherever they were not commercially available. All these sensors are thermal anchored at 77 and 4.2 K to avoid heat load on the system. Temperature sensor leads go up to feedthroughs on the cryostat plate using phos-
phor bronze wire as they have low thermal conductivity. This reduces error in temperature measurement. All signal conditioning cards and quench detection cards are kept in an EMI shielded enclosure and all signal cables are shielded and grounded. Power supplies for all these cards are isolated from main grounding using isolation transformers.
5. Data acquisition system for SST-1 magnets
Fig. 2. Interface of sensors to VME data acquisition.
In a tokamak-like application, data acquisition system is required to acquire, digitize, apply algorithm, make decision, initiate control sequence and communicate with other subsystem all in real time. To satisfy all these requirements, a complete VMEbus-based real time data acquisition system is acquired from different vendors. It includes VG4 processor from SBS, analog input modules (AVME 9660+ IP330) and analog output board (IP 220) from ACROMAG, digital I/O modules (VMICVME-2528) from VMIC. The application development is done on VxWorks OS on Tornado 2.0.2 Platform where C language is used as back end tool, LabView and Matlab as front end tool and
822
A.N. Sharma et al. / Fusion Engineering and Design 74 (2005) 819–823
Fig. 3. VME data acquisition system layout of SST-1 magnets.
TCP/IP socket programming is used for communication with other subsystems. Fig. 3 shows the data flow diagram of the VME data acquisition system of SST-1 magnets. For the safety of SST-1 magnets, in the case of the voltage taps failure, redundant method of detecting quench is also planned. In-house fabricated Venturi flow meters shall be placed at the inlet of each of the TF magnets and at the inlet of the PF1 and PF3, which are more prone to quench. Initial quench zone will be quickly helium depleted as a result of normal zone growth and a backpressure shall be felt on the incoming flow leading to decrease of flow at the inlet. The flow rate data is monitored and acquired in VME data acquisition (daq) system. When the flow will decrease to a predetermined fraction of the nominal flow, the event shall be considered as a quench and VME daq system will initiate control for dumping of the magnets. There are absolute pressure transducers placed at the inlet header and outlet header. The increased pressure at the outlet of the magnets will also be used as an indication of the quench as a result of adiabatic compression of helium following a transient disturbance. However, these sensors are for redundancy only, as their response to quench will be very slow compared to voltage tap sensors.
VME daq system will also send minimum, maximum and average of all temperature sensor values to cryogenic group through the analog output cards. These values will be used to control the cool down process. The VME daq will use a ring buffer arrangement to store pre trigger and post trigger data. It will also be storing all the relevant data coming from all the sensors for all the SST-1 operating scenarios.
6. Results A fail-safe quench detection system has been developed and tested. In the due course of testing, the cards were validated both for ‘no-quench’ and ‘quench scenarios with superimposed high frequency noise’. In both these scenarios, the circuits were tested for threshold voltages set at 100 mV and threshold time set at 100 ms of response. The circuits performed in the expected lines. In order to validate the performance of the circuit during the plasma break down and current start-up scenarios, where an induced voltage as high as 3.5 kV may get induced, the circuit was validated against a 3.5 kV impulse source. The front end signal conditioning cards were also
A.N. Sharma et al. / Fusion Engineering and Design 74 (2005) 819–823
developed for the sensors used in SST-1 magnets in steady state operating scenarios. A real time, stand alone data acquisition system incorporating 512 analog input channels, 16 analog output channels and 128 digital I/O channels has been successfully developed. The data acquisition system thus takes care of the entire magnet operational scenarios including cool down, warm up, cool down following a quench, the off-normal events like vertical displacement events, etc. and the nominal operational scenarios. This data acquisition system is presently getting commissioned for the SST-1 Tokamak and is expected to get finally integrated by early November 2004.
823
References [1] The SST Team, Conceptual design of SST-1 Tokamak, in: 16th IEEE/NPSS Symposium on Fusion Engineering, vol. 1, Urbana Champaign, USA, 1995, p. 481. [2] S. Pradhan, Superconducting cable-in-conduit conductor for SST-1 superconducting magnets, in: Second IAEA meeting on Steady State Tokamaks, October 25–29, Kyushu University, Japan, 1999. [3] J.L. duchateau, D. bessete, D. Ciazynski, J. Pierre, E. Rouanet, P. Riband, et al., Monitoring and controlling toresupra toroidal field system status after a year of operation at nominal current, IEEE trans. on Magnetics, vol. 27, No. 2, March 1991, pp. 2053–2056. [4] R.G. Benson, R.B. Goldfrab, E.S. Pittman, Quench circuit for electronic instruments used with superconducting magnets, Cryogenics 26 (1986) 482–483.