- Email: [email protected]
Generic power supply feedback controller for control of plasma parameters in SST-1
Fusion Engineering and Design 137 (2018) 331–337
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Generic power supply feedback controller for control of plasma parameters in SST-1
T
⁎
Dinesh Kumar Sharma , Akhilesh Kumar Singh, Subrata Pradhan Institute for Plasma Research(HBNI), Bhat, Gandhinagar 382 428, Gujarat, India
A R T I C LE I N FO
A B S T R A C T
Keywords: IGBT H bridge inverter VME RCC VxWorks and hysteresis band control
Steady State Superconducting Tokamak (SST-1) uses Radial Control Coil (RCC) for radial control of the plasma in initial experiments. RCC is a resistive single turn coil. There are two radial coils placed symmetrically up and down. These coils are connected either in saddle or parallel configurations. The requirement of rate of rise of current in RCC is 1 MA/s with peak current as ± 10 kA. The power supply consists of IGBT based 72 H bridge inverter modules connected in parallel, L-C filter, 12 pulse SCR rectifier circuit, water cooling system and transformer. A versa module euro card (VME) bus based Generic Controller (GC) has been developed to control current in RCC in feedback loop. GC controls gate pulse firing of 72 IGBT based H bridge inverter module using hysteresis band control. GC provides 2.5 kHz to 10 kHz switching with fiber optic gate pulse interface to RCC power supply. This paper describes existing controller along with associated issues, development of VME bus based GC, its hardware and software with flow chart, RTOS VxWorks programming, integration of GC with H bridge inverter, MATLAB simulation, testing results of random and step perturbation with power supply operations.
1. Introduction Steady State Superconducting Tokamak (SST-1) [3] is a new generation of tokamak with major objectives of steady state operations in advanced confinement configurations of tokamak plasmas. Loss of equilibrium and force balance conditions of the plasma column inside SST-1 leads to its eventual disruptions on the vessel wall. Longer the plasma column is kept in a forced balanced condition [3] with its position being maintained, longer is the plasma duration. For these things to happen, one of the prescriptions is to maintain the position of the formed plasma column by diagnosing the influencing external field parameters and controlling them. Thus, fast plasma position control in SST-1 is a necessity to obtain longer plasma duration [4]. Elongated SST-1 plasma columns in future will have more demanding control against the vertical moments of the plasma column as compare to radial control [4]. Envisaging all these requirements, an IGBT inverter power supply has been installed with a primary aim of fast vertical plasma position control of SST-1. IGBT inverter power supply is designed to respond to two types of perturbations [8]. In random perturbation scenario, a 100 Hz sinusoidal signal is required to be generated by the power supply. This requirement leads to peak reversal of current in IGBT inverter power supply from +10 kA to -10 kA. For step perturbations typical of plasma beta crashes, 1 MA/s ramp rate was fixed for ⁎
IGBT inverter power supply. On the basis of physics requirements of SST-1 plasma operations, the electrical parameters of the power supply have been optimized and implemented in the design. Since the bus bar inductance and resistances are higher in comparison to the feedback coil L and R parameters; the power supply was located near to SST-1 machine in magnet protection hall (MPH). An IGBT inverter power supply is currently being used for radial position control for initial experimental phase of SST-1 operation. Power supply is a multi-modular high current and fast responding H bridge IGBT based inverter. Central control system of SST-1 provides trigger to initiate and VME master control card, subsequently, the current profile in RCC [7] evolves in real time to generate the required radial magnetic field appropriate for fast radial position control. Power supply specifications are listed in Table 1. The output voltage of the inverter is ± 133 V and output current as ± 10 kA. The maximum slew rate is 1 MA/s. This had been tested on a water-cooled dummy load. Large power IGBT cannot switch very fast. Thus, cost effective and smaller IGBT of rating 300 A each has been chosen. The maximum switching frequency is 10 kHz. A large capacitance of 4.32 F [2] has been equally distributed amongst all the 72inverter modules. This high value of capacitance was necessary towards limiting ripple to a lower value in inverter input dc bus as well as to limit over voltage resulting from the inductance of the coil. For fast
Corresponding author. E-mail address: [email protected] (D.K. Sharma).
https://doi.org/10.1016/j.fusengdes.2018.10.014 Received 5 July 2018; Received in revised form 22 September 2018; Accepted 12 October 2018 Available online 18 October 2018 0920-3796/ © 2018 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 137 (2018) 331–337
D.K. Sharma et al.
Table 1 RCC power supply specifications. DC Voltage Ripple Current Controller response time Random perturbation Ramp rate L and R with bus bar
133 V 1% ± 10 kA 0.1 ms 100 Hz 1 MA/s 64 μH and 1.5 mΩ
Fig. 3. H bridge inverter circuit.
simplifying design. In normal inverters, freewheeling diode does not dissipate much energy. In this case however, IGBT Q1 and Q4 conduct in the On state. As Q1 and Q4 are switched Off, freewheeling diode D2 and D3 conduct for almost equal time. The existing controller used for RCC power supply was a hybrid one. However, there were issues associated with its operations. Foremost issue for this hybrid controller was that, it was housed in a customized rack. Intermittently grounding issues arose because of poor grounding connections. During the operation, it was often observed that hybrid controller does not operate properly and On/Off switch does not work. For reference connections, VME backplane P2 connector’s pins were wire mapped for 16 bits with a clock signal. That was infact the reason for Async module to be housed inside customized rack. During SST-1 campaigns, it was observed many times that hybrid controller did not operate because of the reasons stated above. Hence, it was finally decided to replace it with VME bus based rugged controller.
Fig. 1. IGBT H bridge inverter schematics with GC.
response, VME based controller and FO gate pulsing have been implemented. These large numbers of modules together spans about 6 m in real space. A tree structure for bus bar has been devised to equalize the bus bar impedance. All the above features together provided the solutions for better performance of the power supply with prescribed load requirements. Fig. 1 shows the H bridge inverter of RCC power supply along with GC. This power supply consists of transformer, 12 pulse SCR rectifier, LC filter and IGBT inverter, VME control rack with FO (fiber optic) gate pulse firing interface, interface to central control on reflective memory link (RFM). There are two rectifiers of 2.5 kA each for star and delta winding (Fig. 2), which, together produces 133 VDC and 5 kA current [2]. Rectifier is a twelve pulse SCR circuits with inductor of 150 μH in positive and negative line. Rectifier current and voltage can be controlled in local and remote mode. In local mode Iset and Vset can be adjusted manually through a pot. For faster ramp rate (1 MA/s) Vset is kept as 80% and Iset is kept as 100%. This current and voltage setting can also be dynamically adjusted via VME AO channel or on Ethernet link through graphical user interface. Fig. 3 shows the H bridge inverter configuration of RCC power supply. Each inverter consists of 4 IGBT modules forming H bridge [6] configuration and are mounted on water-cooled heat sink for
2. Generic controller A VME bus based controller was developed for switching of IGBT and feedback control of current in RCC. VME bus is an open architecture. The VME modules are non-proprietary type. Further, they are easily available and programmable. After analyzing the switching frequency requirements, interface requirements, feedback processing of current requirements, step and random perturbation references; it was decided to go for open architecture like VME bus system. Further, VME bus based system is being extensively used in Institute for Plasma Research (IPR) within different groups and hence platform compatibility amongst various sub-systems becomes better. As the name is generic, this controller’s use can be extended to different requirements within the timing constraints of hardware and software without needing any alterations. For RCC power supply; Motorola MVME3604 processor card, MVME 2528 DIO module, IP480 timer modules, fast and simultaneous sampling VGD4 analog in module for fast processing of analog signal, IP330 AI for feedback current from DCCT (direct current current transducer), Async module with clock have been used. VME bus system hardware system of GC is shown in the Fig. 4. RTOS VxWorks was used for the application development. Application was developed in Tornado 2.0.2. In this application, binary semaphore and task priority have been used. RTOS VxWorks has minimum interrupt latency among its peers. Interrupts can be directly configured and linked to the interrupt handler routine. Through task synchronization via interrupt a fast response application was developed. For switching of IGBT inverter
Fig. 2. Rectifier circuit of RCC power supply. 332
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2.2. Generic controller software RTOS VxWorks and Tornado 2.0.2 was used for the software application developments. Project utility of Tornado was used in the application development. Each VME module has two parts for software development. There is a configuration part and a processing part. Configuration program configures the module according to its hardware. Once configured, channels can be acquired on interrupt or external clocks. Application program uses binary semaphore and task creation technique of VxWorks. VxWorks semaphore is highly optimized and provides the fastest inter task communication mechanism. Semaphores are primary means for addressing the requirement of mutual exclusion and task synchronization. For mutual exclusion, semaphore accesses to shared resources and for synchronization, semaphores coordinate a task’s execution with external event. Program is written in C/C++ language. There are two tasks running in applications named as APC_On_Off and APC_ref. APC_On_Off has higher priority in comparison to APC_ref. APC_On_Off checks power supply On status. If it is On, it provides semaphore to APC_ref routine. APC_ref checks RCC power supply On status and current reference (Iref) provided by central machine control on RFM. Iref is the reference current required in RCC. It is generally a trapezoidal or triangular waveform with varying ramp rate (100 kA/s to 1 MA/s) and flat top time (100 ms to 300 ms). If Iref of RCC power supply is positive, then it calls apc_p routine for forward FO gate pulse firing of IGBT inverter using hysteresis band control techniques. If Iref of RCC power supply is negative (below 0 × 8000), then APC_ref calls apc_n routine for reverse FO gate pulse firing of IGBT inverter using hysteresis band control technique. The apc_p and apc_n both programs checks status of RCC power supply and reads feedback current (Ifback). Ifback is the actual current measured by DCCT (bipolar 20 kA/10 V). The apc_p and apc_n both programs provides switching to IGBT pair (1,4) (forward current) and (2,3) (reverse current). If there is a change in the status of power supply and changes in Iref (from positive to negative or vice versa, APC_ref and APC_On_Off takes over the precedence and takes semaphore. The software architecture is shown in the Fig. 6.
Fig. 4. VME hardware system.
power supply, its core routine uses binary semaphore and task priority creation. Binary semaphore can be directly linked to resource sharing on a logical event.
2.1. Generic controller hardware Fig. 5 shows the hardware architecture and its functions and interfaces of VME modules used in GC. MVME 3604 Motorola is the processor card. IP330 acquires feedback DCCT current for processing in hysteresis band control. DI card reads status (On/Off) of RCC power supply. DO card is used for providing FO forward and reverse IGBT gate pulse firing. Async and RFM (reflective memory) [7] is FO interface to central control system. Timer module provides 10 kHz clock required for IP330 AI module and senses external events. VGD4 is a fast 85-kilo sample per second AI module for plasma current processing on external events.
2.3. Hysteresis band controller Hysteresis band controller controls the switching of IGBT (On/Off) based on hysteresis curve. There is positive and negative band. The band can be adjusted in the software algorithm. Here for RCC power supply, 100 A band has been fixed. As acquisition rate is at 10 kHz and
Fig. 5. GC Hardware Architecture.
Fig. 6. GC Software Architecture. 333
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Gate pulse signals are passed through M57962, which is a hybrid integrated circuit for driving IGBT. This device operates as an isolation amplifier for these modules and provides required isolation between the input and output with an optocoupler. Built-in desaturation detector provides short circuit protection. A fault signal is provided if the short circuit protection is activated. Output of M57962 is used to provide gate pulse trigger to pair (1,4 and 2,3) of IGBT
Fig. 7. Hysteresis band control schematics.
4. Results
ramp rate is of 1 MA/s. 10 kHz corresponds to 100 μs and that corresponds to 100 A with 1 MA/s ramp rate. Software algorithm is designed on the basis of error signal. An application program acquires Iref and Ifback at 10 kHz acquisition rate. The program calculates error signal (Ifback- Iref) in real time. If the error signal crosses by +100 A then IGBT is switched Off. And again error crosses –100 A, then IGBT is switched On. So for both positive and negative going pulses IGBT are switched Off and On. Hence output DC current will remain within the band. This technique is known as hysteresis band control. Hysteresis band control logic is shown in Fig. 7.
After successful lab testing, GC was integrated with IGBT based inverter power supply of RCC in magnet protection hall (MPH) where, IGBT inverter power supply is located. Power supply of RCC can be operated in local and remote mode. For initial testing, local mode of operation was preferred. A fixed Iref of 400 A was provided to GC. Application program is downloaded in VME target MVME3604. APC_On_Off and APC_ref tasks are spawned through shell window of Tornado. RCC power supply On is provided through a manual switch. Then current in dummy load coil rises and stays at flat top. At flat top and rise and fall of current is being controlled through hysteresis band control techniques. Generally a trapezoidal reference is given to RCC. Iref ramps up at ramp rate of 100 kA/s and stays at flat top for 100 ms and then ramps down with same rate. IGBT switching requirement is from 2.5 kHz to 10 kHz. The maximum switching frequency is fixed from heat sink design and cooling considerations. As switching frequency is increased, switching losses increase. Testing on GC for RCC power supply has to be performed in three different ways. For random perturbation, a 100 Hz sinusoidal signal is to be generated. For step perturbation, 1 MA/s ramp rate is needed in Ifback. At last, a switching frequency is to be generated varying from 2.5 kHz to 10 kHz. Current sharing of individual IGBT inverter module is being controlled through passive techniques. Current sharing is well within 10% variations.
3. Interface circuit Hardware interface of FO IGBT gate pulse firing to VME is described in the Figs. 8 and 9. Two channels of VME DO card are used for gate pulse firing. Channel 1 is reserved for forward gate pulse firing (1, 4) and channel 2 is fixed for reverse gate pulse firing (2, 3). TTL high signal at DO corresponds to IGBT On and TTL low at digital out channel corresponds to IGBT Off. On and Off duration time is decided by Iref and Ifback. Two TTL channels from VME DO module are connected to NAND driver. NAND gate output is connected to base of transistor drive (open collector) circuit as shown in Fig. 9. Single DO channel provides control trigger to FO forward or reverse module. These modules are housed in customized rack as shown in Fig. 8. There are 9 FO modules. Each module provides FO triggers for 8 IGBT inverter modules. The gate pulse to IGBT is on FO link to avoid interference in gate triggering. HFBR 1521 is FO transmitter. HFBR2521 is FO receiver. On corresponds to low signal and high pulse corresponds to Off signal. This signal along with ground goes to OR gate and then it goes to Schmitt trigger input CD40106B. The trigger switches at different points for positive and negative going signals. The difference between positive going voltage and negative going voltage is defined as hysteresis voltage. Interlock is also incorporated between two Schmitt inverter trigger so as to block the On of all IGBT simultaneously or to avoid dead short circuit. There is 4 μs dead bands to avoid short circuit.
4.1. IGBT switching frequency First testing of GC interface to RCC power supply was to operate power supply on dummy load coil with switching frequency varying from 2.5 kHz to 10 kHz. Previous hybrid controller used to operate RCC power supply at 3 kHz switching frequency. Then switching frequency is varied to higher side up to 8.37 kHz as shown in Fig. 10. Thus, varying switching frequency from 3 kHz to 8.37 kHz successfully completes the first benchmark for GC.
Fig. 8. Gate trigger distribution rack. 334
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Fig. 9. Interface circuit schematics.
4.2. Random perturbation
4.4. MATLAB simulation
Another benchmark for the controller is to provide sinusoidal tracking of signal for random perturbation as desired in the initial physics requirements. A signal of 100 Hz sinusoidal was given to the GC as Iref. DCCT provides feedback current where ± 20 kA corresponds to ± 10 V. Peak current of 3000 A (1.5 V peak voltage) was achieved. Signal A is Iref and signal B is Ifback measurement (Fig. 11).
A MATALAB model was built for H bridge inverter as shown in the Fig. 13. FO forward (positive current) and FO reverse (negative current) circuit simulation was performed for H bridge circuit of RCC power supply. Load parameters was chosen as R = 7.6 mΩ and L = 50 μH. Switching frequency is being controlled by current reference and feedback current. Voltage was chosen as 133 V. RCC power has DC choke of 150 μH in each arm. Capacitor across the circuit was at 4.32 F A step current reference of +5 kA to -5 kA was provided to H bridge IGBT inverter. Simulated output voltage waveform along with load current is shown in the Fig. 14. The fall time from +5 kA to -5 kA was calculated as shown in the Fig. 15. This fall time is 5 ms which corresponds to 2 MA/s ramp rate in RCC dummy load coil. This ramp rate matches with actual load current profile as shown in the Fig. 12.
4.3. Step perturbation Third and the last bench mark for the controller is to provide current changes at ramp rate of 1 MA/s. Flat top current of 5 kA was set in dummy load coil. Now, reference is changed to −5 kA and oscillogram is captured. Current of 10 kA changes in 5 ms. GC provides 2 MA/s ramp rate in dummy load coil. Hence the GC fulfills step perturbation requirement.
Fig. 10. Switching frequency of output DC voltage at 8.37 kHz from GC. 335
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Fig. 11. Random Perturbation (100 Hz sinusoidal) in dummy load coil.
Fig. 12. Step perturbation (2 MA/s ramp rate) in dummy load coil.
Fig. 14. Output load current and voltage switching profile (simulated).
Fig. 15. Fall time from +5 kA to -5 kA (simulated).
5. Conclusion and future works Fig. 13. MATLAB model of H bridge inverter.
Development of VME bus based generic controller for RCC power supply has been presented in this report. GC has been successfully developed, programmed and integrated with RCC power supply. GC has
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References
been tested on load in a water-cooled dummy load coil for switching frequency, random and step perturbations. Simulation results are matching with actual load current testing. With GC enhanced feedback processing is possible. Rectifier Vset and Iset can be dynamically controlled in real time. Interface to other subgroups is easier because of same platform. During this development of GC, a novel technique for IGBT switching and current control has been developed. This technique will be further explored for 12-pulse SCR phase control in PF (poloidal field) power supply. This technique can be used directly for switching frequency and current control of any IGBT based power supply.
[2] Dinesh Kumar Sharma, et al., Multi modular high current fast response IGBT inverter power supply of SST-1 tokamak, Power Electronics Applications, EPE’15 ECCEEUROPE (2015). [3] Subrata Pradhan, et al., “SST-1 status and plans” IEEE transactions on plasma science, IEEE Trans. Plasma Sci. 40 (March (3)) (2012) 614–621. [4] Subrata Pradhan, et al., First experiments on SST-1, Nucl. Fusion 55 (2016) 104009. [6] Haihong Huang, et al., Paralleling inverter analysis of EAST fast control power supply, Fusion Energy (2014) 456–462. [7] Dinesh Kumar Sharma, et al., Adaptation of fast responding power supply for radial position control in SST-1, DAE-BRNS Indian Particle Accelerator Conference, INPAC 45 (2013) INIS Issue 38(1). [8] B. Indranil, et al., Plasma position control in SST-1 tokamak, Pramana J. Phys. Indian Acad. Sci. 55 (December) (2000) 741–750.
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Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Generic power supply feedback controller for control of plasma parameters in SST-1
T
⁎
Dinesh Kumar Sharma , Akhilesh Kumar Singh, Subrata Pradhan Institute for Plasma Research(HBNI), Bhat, Gandhinagar 382 428, Gujarat, India
A R T I C LE I N FO
A B S T R A C T
Keywords: IGBT H bridge inverter VME RCC VxWorks and hysteresis band control
Steady State Superconducting Tokamak (SST-1) uses Radial Control Coil (RCC) for radial control of the plasma in initial experiments. RCC is a resistive single turn coil. There are two radial coils placed symmetrically up and down. These coils are connected either in saddle or parallel configurations. The requirement of rate of rise of current in RCC is 1 MA/s with peak current as ± 10 kA. The power supply consists of IGBT based 72 H bridge inverter modules connected in parallel, L-C filter, 12 pulse SCR rectifier circuit, water cooling system and transformer. A versa module euro card (VME) bus based Generic Controller (GC) has been developed to control current in RCC in feedback loop. GC controls gate pulse firing of 72 IGBT based H bridge inverter module using hysteresis band control. GC provides 2.5 kHz to 10 kHz switching with fiber optic gate pulse interface to RCC power supply. This paper describes existing controller along with associated issues, development of VME bus based GC, its hardware and software with flow chart, RTOS VxWorks programming, integration of GC with H bridge inverter, MATLAB simulation, testing results of random and step perturbation with power supply operations.
1. Introduction Steady State Superconducting Tokamak (SST-1) [3] is a new generation of tokamak with major objectives of steady state operations in advanced confinement configurations of tokamak plasmas. Loss of equilibrium and force balance conditions of the plasma column inside SST-1 leads to its eventual disruptions on the vessel wall. Longer the plasma column is kept in a forced balanced condition [3] with its position being maintained, longer is the plasma duration. For these things to happen, one of the prescriptions is to maintain the position of the formed plasma column by diagnosing the influencing external field parameters and controlling them. Thus, fast plasma position control in SST-1 is a necessity to obtain longer plasma duration [4]. Elongated SST-1 plasma columns in future will have more demanding control against the vertical moments of the plasma column as compare to radial control [4]. Envisaging all these requirements, an IGBT inverter power supply has been installed with a primary aim of fast vertical plasma position control of SST-1. IGBT inverter power supply is designed to respond to two types of perturbations [8]. In random perturbation scenario, a 100 Hz sinusoidal signal is required to be generated by the power supply. This requirement leads to peak reversal of current in IGBT inverter power supply from +10 kA to -10 kA. For step perturbations typical of plasma beta crashes, 1 MA/s ramp rate was fixed for ⁎
IGBT inverter power supply. On the basis of physics requirements of SST-1 plasma operations, the electrical parameters of the power supply have been optimized and implemented in the design. Since the bus bar inductance and resistances are higher in comparison to the feedback coil L and R parameters; the power supply was located near to SST-1 machine in magnet protection hall (MPH). An IGBT inverter power supply is currently being used for radial position control for initial experimental phase of SST-1 operation. Power supply is a multi-modular high current and fast responding H bridge IGBT based inverter. Central control system of SST-1 provides trigger to initiate and VME master control card, subsequently, the current profile in RCC [7] evolves in real time to generate the required radial magnetic field appropriate for fast radial position control. Power supply specifications are listed in Table 1. The output voltage of the inverter is ± 133 V and output current as ± 10 kA. The maximum slew rate is 1 MA/s. This had been tested on a water-cooled dummy load. Large power IGBT cannot switch very fast. Thus, cost effective and smaller IGBT of rating 300 A each has been chosen. The maximum switching frequency is 10 kHz. A large capacitance of 4.32 F [2] has been equally distributed amongst all the 72inverter modules. This high value of capacitance was necessary towards limiting ripple to a lower value in inverter input dc bus as well as to limit over voltage resulting from the inductance of the coil. For fast
Corresponding author. E-mail address: [email protected] (D.K. Sharma).
https://doi.org/10.1016/j.fusengdes.2018.10.014 Received 5 July 2018; Received in revised form 22 September 2018; Accepted 12 October 2018 Available online 18 October 2018 0920-3796/ © 2018 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 137 (2018) 331–337
D.K. Sharma et al.
Table 1 RCC power supply specifications. DC Voltage Ripple Current Controller response time Random perturbation Ramp rate L and R with bus bar
133 V 1% ± 10 kA 0.1 ms 100 Hz 1 MA/s 64 μH and 1.5 mΩ
Fig. 3. H bridge inverter circuit.
simplifying design. In normal inverters, freewheeling diode does not dissipate much energy. In this case however, IGBT Q1 and Q4 conduct in the On state. As Q1 and Q4 are switched Off, freewheeling diode D2 and D3 conduct for almost equal time. The existing controller used for RCC power supply was a hybrid one. However, there were issues associated with its operations. Foremost issue for this hybrid controller was that, it was housed in a customized rack. Intermittently grounding issues arose because of poor grounding connections. During the operation, it was often observed that hybrid controller does not operate properly and On/Off switch does not work. For reference connections, VME backplane P2 connector’s pins were wire mapped for 16 bits with a clock signal. That was infact the reason for Async module to be housed inside customized rack. During SST-1 campaigns, it was observed many times that hybrid controller did not operate because of the reasons stated above. Hence, it was finally decided to replace it with VME bus based rugged controller.
Fig. 1. IGBT H bridge inverter schematics with GC.
response, VME based controller and FO gate pulsing have been implemented. These large numbers of modules together spans about 6 m in real space. A tree structure for bus bar has been devised to equalize the bus bar impedance. All the above features together provided the solutions for better performance of the power supply with prescribed load requirements. Fig. 1 shows the H bridge inverter of RCC power supply along with GC. This power supply consists of transformer, 12 pulse SCR rectifier, LC filter and IGBT inverter, VME control rack with FO (fiber optic) gate pulse firing interface, interface to central control on reflective memory link (RFM). There are two rectifiers of 2.5 kA each for star and delta winding (Fig. 2), which, together produces 133 VDC and 5 kA current [2]. Rectifier is a twelve pulse SCR circuits with inductor of 150 μH in positive and negative line. Rectifier current and voltage can be controlled in local and remote mode. In local mode Iset and Vset can be adjusted manually through a pot. For faster ramp rate (1 MA/s) Vset is kept as 80% and Iset is kept as 100%. This current and voltage setting can also be dynamically adjusted via VME AO channel or on Ethernet link through graphical user interface. Fig. 3 shows the H bridge inverter configuration of RCC power supply. Each inverter consists of 4 IGBT modules forming H bridge [6] configuration and are mounted on water-cooled heat sink for
2. Generic controller A VME bus based controller was developed for switching of IGBT and feedback control of current in RCC. VME bus is an open architecture. The VME modules are non-proprietary type. Further, they are easily available and programmable. After analyzing the switching frequency requirements, interface requirements, feedback processing of current requirements, step and random perturbation references; it was decided to go for open architecture like VME bus system. Further, VME bus based system is being extensively used in Institute for Plasma Research (IPR) within different groups and hence platform compatibility amongst various sub-systems becomes better. As the name is generic, this controller’s use can be extended to different requirements within the timing constraints of hardware and software without needing any alterations. For RCC power supply; Motorola MVME3604 processor card, MVME 2528 DIO module, IP480 timer modules, fast and simultaneous sampling VGD4 analog in module for fast processing of analog signal, IP330 AI for feedback current from DCCT (direct current current transducer), Async module with clock have been used. VME bus system hardware system of GC is shown in the Fig. 4. RTOS VxWorks was used for the application development. Application was developed in Tornado 2.0.2. In this application, binary semaphore and task priority have been used. RTOS VxWorks has minimum interrupt latency among its peers. Interrupts can be directly configured and linked to the interrupt handler routine. Through task synchronization via interrupt a fast response application was developed. For switching of IGBT inverter
Fig. 2. Rectifier circuit of RCC power supply. 332
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2.2. Generic controller software RTOS VxWorks and Tornado 2.0.2 was used for the software application developments. Project utility of Tornado was used in the application development. Each VME module has two parts for software development. There is a configuration part and a processing part. Configuration program configures the module according to its hardware. Once configured, channels can be acquired on interrupt or external clocks. Application program uses binary semaphore and task creation technique of VxWorks. VxWorks semaphore is highly optimized and provides the fastest inter task communication mechanism. Semaphores are primary means for addressing the requirement of mutual exclusion and task synchronization. For mutual exclusion, semaphore accesses to shared resources and for synchronization, semaphores coordinate a task’s execution with external event. Program is written in C/C++ language. There are two tasks running in applications named as APC_On_Off and APC_ref. APC_On_Off has higher priority in comparison to APC_ref. APC_On_Off checks power supply On status. If it is On, it provides semaphore to APC_ref routine. APC_ref checks RCC power supply On status and current reference (Iref) provided by central machine control on RFM. Iref is the reference current required in RCC. It is generally a trapezoidal or triangular waveform with varying ramp rate (100 kA/s to 1 MA/s) and flat top time (100 ms to 300 ms). If Iref of RCC power supply is positive, then it calls apc_p routine for forward FO gate pulse firing of IGBT inverter using hysteresis band control techniques. If Iref of RCC power supply is negative (below 0 × 8000), then APC_ref calls apc_n routine for reverse FO gate pulse firing of IGBT inverter using hysteresis band control technique. The apc_p and apc_n both programs checks status of RCC power supply and reads feedback current (Ifback). Ifback is the actual current measured by DCCT (bipolar 20 kA/10 V). The apc_p and apc_n both programs provides switching to IGBT pair (1,4) (forward current) and (2,3) (reverse current). If there is a change in the status of power supply and changes in Iref (from positive to negative or vice versa, APC_ref and APC_On_Off takes over the precedence and takes semaphore. The software architecture is shown in the Fig. 6.
Fig. 4. VME hardware system.
power supply, its core routine uses binary semaphore and task priority creation. Binary semaphore can be directly linked to resource sharing on a logical event.
2.1. Generic controller hardware Fig. 5 shows the hardware architecture and its functions and interfaces of VME modules used in GC. MVME 3604 Motorola is the processor card. IP330 acquires feedback DCCT current for processing in hysteresis band control. DI card reads status (On/Off) of RCC power supply. DO card is used for providing FO forward and reverse IGBT gate pulse firing. Async and RFM (reflective memory) [7] is FO interface to central control system. Timer module provides 10 kHz clock required for IP330 AI module and senses external events. VGD4 is a fast 85-kilo sample per second AI module for plasma current processing on external events.
2.3. Hysteresis band controller Hysteresis band controller controls the switching of IGBT (On/Off) based on hysteresis curve. There is positive and negative band. The band can be adjusted in the software algorithm. Here for RCC power supply, 100 A band has been fixed. As acquisition rate is at 10 kHz and
Fig. 5. GC Hardware Architecture.
Fig. 6. GC Software Architecture. 333
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Gate pulse signals are passed through M57962, which is a hybrid integrated circuit for driving IGBT. This device operates as an isolation amplifier for these modules and provides required isolation between the input and output with an optocoupler. Built-in desaturation detector provides short circuit protection. A fault signal is provided if the short circuit protection is activated. Output of M57962 is used to provide gate pulse trigger to pair (1,4 and 2,3) of IGBT
Fig. 7. Hysteresis band control schematics.
4. Results
ramp rate is of 1 MA/s. 10 kHz corresponds to 100 μs and that corresponds to 100 A with 1 MA/s ramp rate. Software algorithm is designed on the basis of error signal. An application program acquires Iref and Ifback at 10 kHz acquisition rate. The program calculates error signal (Ifback- Iref) in real time. If the error signal crosses by +100 A then IGBT is switched Off. And again error crosses –100 A, then IGBT is switched On. So for both positive and negative going pulses IGBT are switched Off and On. Hence output DC current will remain within the band. This technique is known as hysteresis band control. Hysteresis band control logic is shown in Fig. 7.
After successful lab testing, GC was integrated with IGBT based inverter power supply of RCC in magnet protection hall (MPH) where, IGBT inverter power supply is located. Power supply of RCC can be operated in local and remote mode. For initial testing, local mode of operation was preferred. A fixed Iref of 400 A was provided to GC. Application program is downloaded in VME target MVME3604. APC_On_Off and APC_ref tasks are spawned through shell window of Tornado. RCC power supply On is provided through a manual switch. Then current in dummy load coil rises and stays at flat top. At flat top and rise and fall of current is being controlled through hysteresis band control techniques. Generally a trapezoidal reference is given to RCC. Iref ramps up at ramp rate of 100 kA/s and stays at flat top for 100 ms and then ramps down with same rate. IGBT switching requirement is from 2.5 kHz to 10 kHz. The maximum switching frequency is fixed from heat sink design and cooling considerations. As switching frequency is increased, switching losses increase. Testing on GC for RCC power supply has to be performed in three different ways. For random perturbation, a 100 Hz sinusoidal signal is to be generated. For step perturbation, 1 MA/s ramp rate is needed in Ifback. At last, a switching frequency is to be generated varying from 2.5 kHz to 10 kHz. Current sharing of individual IGBT inverter module is being controlled through passive techniques. Current sharing is well within 10% variations.
3. Interface circuit Hardware interface of FO IGBT gate pulse firing to VME is described in the Figs. 8 and 9. Two channels of VME DO card are used for gate pulse firing. Channel 1 is reserved for forward gate pulse firing (1, 4) and channel 2 is fixed for reverse gate pulse firing (2, 3). TTL high signal at DO corresponds to IGBT On and TTL low at digital out channel corresponds to IGBT Off. On and Off duration time is decided by Iref and Ifback. Two TTL channels from VME DO module are connected to NAND driver. NAND gate output is connected to base of transistor drive (open collector) circuit as shown in Fig. 9. Single DO channel provides control trigger to FO forward or reverse module. These modules are housed in customized rack as shown in Fig. 8. There are 9 FO modules. Each module provides FO triggers for 8 IGBT inverter modules. The gate pulse to IGBT is on FO link to avoid interference in gate triggering. HFBR 1521 is FO transmitter. HFBR2521 is FO receiver. On corresponds to low signal and high pulse corresponds to Off signal. This signal along with ground goes to OR gate and then it goes to Schmitt trigger input CD40106B. The trigger switches at different points for positive and negative going signals. The difference between positive going voltage and negative going voltage is defined as hysteresis voltage. Interlock is also incorporated between two Schmitt inverter trigger so as to block the On of all IGBT simultaneously or to avoid dead short circuit. There is 4 μs dead bands to avoid short circuit.
4.1. IGBT switching frequency First testing of GC interface to RCC power supply was to operate power supply on dummy load coil with switching frequency varying from 2.5 kHz to 10 kHz. Previous hybrid controller used to operate RCC power supply at 3 kHz switching frequency. Then switching frequency is varied to higher side up to 8.37 kHz as shown in Fig. 10. Thus, varying switching frequency from 3 kHz to 8.37 kHz successfully completes the first benchmark for GC.
Fig. 8. Gate trigger distribution rack. 334
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Fig. 9. Interface circuit schematics.
4.2. Random perturbation
4.4. MATLAB simulation
Another benchmark for the controller is to provide sinusoidal tracking of signal for random perturbation as desired in the initial physics requirements. A signal of 100 Hz sinusoidal was given to the GC as Iref. DCCT provides feedback current where ± 20 kA corresponds to ± 10 V. Peak current of 3000 A (1.5 V peak voltage) was achieved. Signal A is Iref and signal B is Ifback measurement (Fig. 11).
A MATALAB model was built for H bridge inverter as shown in the Fig. 13. FO forward (positive current) and FO reverse (negative current) circuit simulation was performed for H bridge circuit of RCC power supply. Load parameters was chosen as R = 7.6 mΩ and L = 50 μH. Switching frequency is being controlled by current reference and feedback current. Voltage was chosen as 133 V. RCC power has DC choke of 150 μH in each arm. Capacitor across the circuit was at 4.32 F A step current reference of +5 kA to -5 kA was provided to H bridge IGBT inverter. Simulated output voltage waveform along with load current is shown in the Fig. 14. The fall time from +5 kA to -5 kA was calculated as shown in the Fig. 15. This fall time is 5 ms which corresponds to 2 MA/s ramp rate in RCC dummy load coil. This ramp rate matches with actual load current profile as shown in the Fig. 12.
4.3. Step perturbation Third and the last bench mark for the controller is to provide current changes at ramp rate of 1 MA/s. Flat top current of 5 kA was set in dummy load coil. Now, reference is changed to −5 kA and oscillogram is captured. Current of 10 kA changes in 5 ms. GC provides 2 MA/s ramp rate in dummy load coil. Hence the GC fulfills step perturbation requirement.
Fig. 10. Switching frequency of output DC voltage at 8.37 kHz from GC. 335
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Fig. 11. Random Perturbation (100 Hz sinusoidal) in dummy load coil.
Fig. 12. Step perturbation (2 MA/s ramp rate) in dummy load coil.
Fig. 14. Output load current and voltage switching profile (simulated).
Fig. 15. Fall time from +5 kA to -5 kA (simulated).
5. Conclusion and future works Fig. 13. MATLAB model of H bridge inverter.
Development of VME bus based generic controller for RCC power supply has been presented in this report. GC has been successfully developed, programmed and integrated with RCC power supply. GC has
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References
been tested on load in a water-cooled dummy load coil for switching frequency, random and step perturbations. Simulation results are matching with actual load current testing. With GC enhanced feedback processing is possible. Rectifier Vset and Iset can be dynamically controlled in real time. Interface to other subgroups is easier because of same platform. During this development of GC, a novel technique for IGBT switching and current control has been developed. This technique will be further explored for 12-pulse SCR phase control in PF (poloidal field) power supply. This technique can be used directly for switching frequency and current control of any IGBT based power supply.
[2] Dinesh Kumar Sharma, et al., Multi modular high current fast response IGBT inverter power supply of SST-1 tokamak, Power Electronics Applications, EPE’15 ECCEEUROPE (2015). [3] Subrata Pradhan, et al., “SST-1 status and plans” IEEE transactions on plasma science, IEEE Trans. Plasma Sci. 40 (March (3)) (2012) 614–621. [4] Subrata Pradhan, et al., First experiments on SST-1, Nucl. Fusion 55 (2016) 104009. [6] Haihong Huang, et al., Paralleling inverter analysis of EAST fast control power supply, Fusion Energy (2014) 456–462. [7] Dinesh Kumar Sharma, et al., Adaptation of fast responding power supply for radial position control in SST-1, DAE-BRNS Indian Particle Accelerator Conference, INPAC 45 (2013) INIS Issue 38(1). [8] B. Indranil, et al., Plasma position control in SST-1 tokamak, Pramana J. Phys. Indian Acad. Sci. 55 (December) (2000) 741–750.
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