A dc-dc converter has been developed for retrofitting inside the vacuum space of the HTS rotor of a synchronous generator. The heavy copper sections of the current leads used for energising the HTS field winding were replaced by cryogenic power electronics; consisting of the converter and a rotor control unit. The converter board was designed using an H-bridge configuration with two 5A rated wires connecting the cryogenic boards to the stator control board located on the outside of the generator and drawing power from a (5A, 50V) dc power source. The robustness of converter board was well demonstrated when it was powered up from a cold start at 82K. When charging the field winding with moderate currents (30A), the heat in-leak to the ‘cold’ rotor core was only 2W. It continued to function down to 74K, surviving several quenches. However, the quench protection function failed when injecting 75A into the field winding, resulting in the burn out of one of the DC-link capacitors. The magnitudes of the critical currents measured with the original current leads were compared to the quench currents, which was defined as the current which triggered quench protection protocol. The difference between the two currents was rather large, (~20A). However, additional measurements using a single HTS coil in liquid nitrogen found that this reduction should not be so dramatic and in the region of 4A. Our conclusions identified the converter’s switching voltage and its operating frequency as two parameters, which could have contributed to lowering the quench current. Magnetic fields and eddy currents are expected to be more prominent the field winding and its impact on the converter also need further investigation

Bailey, W

Forsyth, A

Jia, C

Wen, Hauming

Yang, Y

English

AbstractA dc-dc converter has been developed for retrofitting inside the vacuum space of the HTS rotor of a synchronous generator. The heavy copper sections of the current leads used for energising the HTS field winding were replaced by cryogenic power electronics; consisting of the converter and a rotor control unit. The converter board was designed using an H-bridge configuration with two 5A rated wires connecting the cryogenic boards to the stator control board located on the outside of the generator and drawing power from a (5A, 50V) dc power source. The robustness of converter board was well demonstrated when it was powered up from a cold start at 82K. When charging the field winding with moderate currents (30A), the heat in-leak to the ‘cold’ rotor core was only 2W. It continued to function down to 74K, surviving several quenches. However, the quench protection function failed when injecting 75A into the field winding, resulting in the burn out of one of the DC-link capacitors. The magnitudes of the critical currents measured with the original current leads were compared to the quench currents, which was defined as the current which triggered quench protection protocol. The difference between the two currents was rather large, (∼20A). However, additional measurements using a single HTS coil in liquid nitrogen found that this reduction should not be so dramatic and in the region of 4A. Our conclusions identified the converter's switching voltage and its operating frequency as two parameters, which could have contributed to lowering the quench current. Magnetic fields and eddy currents are expected to be more prominent the field winding and its impact on the converter also need further investigation

Bailey, Wendell

Yang, Yifeng

Forsyth, Andrew

Jia, Chungjiang

Elsevier - Publisher Connector 

A Cryogenic Dc-Dc Power Converter for a 100kW Synchronous HTS Generator at Liquid Nitrogen Temperatures 

 Physics Procedia  36 ( 2012 )  1002 – 1007 
1875-3892  © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors. 
doi: 10.1016/j.phpro.2012.06.096 
Superconductivity Centennial Conference 
A cryogenic dc-dc power converter for a 100kW synchronous 
HTS generator at liquid nitrogen temperatures 
Wendell Baileya *, Hauming Wena, Yifeng Yanga, Andrew Forsythb, Chungjiang 
Jiab 
aCryogenics, Faculty of Engineering and the Environment, University of Southampton, Southampton, SO171BJ, United Kingdom 
bSchool of Electrical and Electronic Engineering, University of Manchester, Manchester, MN63PL, United Kingdom 
 
Abstract 
A dc-dc converter has been developed for retrofitting inside the vacuum space of the HTS rotor of a synchronous 
generator. The heavy copper sections of the current leads used for energising the HTS field winding were replaced by 
cryogenic power electronics; consisting of the converter and a rotor control unit. The converter board was designed 
using an H-bridge configuration with two 5A rated wires connecting the cryogenic boards to the stator control board 
located on the outside of the generator and drawing power from a (5A, 50V) dc power source. The robustness of 
converter board was well demonstrated when it was powered up from a cold start at 82K. When charging the field 
winding with moderate currents (30A), the heat in-leak to the ‘cold’ rotor core was only 2W. It continued to function 
down to 74K, surviving several quenches. However, the quench protection function failed when injecting 75A into 
the field winding, resulting in the burn out of one of the DC-link capacitors. The magnitudes of the critical currents 
measured with the original current leads were compared to the quench currents, which was defined as the current 
which triggered quench protection protocol. The difference between the two currents was rather large, (~20A). 
However, additional measurements using a single HTS coil in liquid nitrogen found that this reduction should not be 
so dramatic and in the region of 4A. Our conclusions identified the converter’s switching voltage and its operating 
frequency as two parameters, which could have contributed to lowering the quench current. Magnetic fields and eddy 
currents are expected to be more prominent the field winding and its impact on the converter also need further 
investigation. 
 
© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Horst Rogalla and 
Peter Kes. 
Keyword: HTS generators; power converter; synchronous machines; cryogenic devices.  
 
* Corresponding author. 
E-mail address.wosb@soton.ac.uk 
Available online at www.sciencedirect.com
© 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors.
Open access under CC BY-NC-ND license.
Open access under CC BY-NC-ND license.
 Wendell Bailey et al. /  Physics Procedia  36 ( 2012 )  1002 – 1007 1003
1. Introduction 
Following the construction and testing of superconducting generators, which operate in the temperature 
range of 60K to 80K, [1-2]. A project has been carried out by a group of Universities in partnership with 
Rolls Royse to investigate the feasibility of building a cryogenic power converter, for energising the 
superconducting field winding for these types of machines. The insertion of a cryogenic dc power source 
onboard the cold rotor, with a high-voltage/low-current ac input and a low-voltage/high-current dc output, 
could reduce the heat in-leak incurred by the current leads, [3].  
Cryogenic power electronics provide other promising benefits over their room temperature counterparts 
in terms of a reduced size, weight (increased power density), improved efficiency, improved switching 
speed and improved reliability. Passive devises such as inductors, capacitor and interconnects will also 
improve by the lowered resistance of their constituents metal conductors or use of superconductors, [4].  
With the help of cryogenic power electronics; the manufacture of an HTS generator unit, fully integrated 
with the converter, cooling and coupling components could become a real possibility. Designers of light 
aircrafts, submersible sea crafts and wind farm would welcome such a generator. 
2. Details of the HTS generator and the dc-dc power converter design 
The generator selected to retro-fit the cryogenic power converter to was a 100kW synchronous generator, 
which used a conventional 3-phase (198A, 415V). The rotor was designed to withstand the torque 
generated at 3000 R.P.M and consisted of a magnetic iron core and a field winding built from a stack of 
ten identical HTS pancake coils, each separated by flux diverting ring. The winding had a critical current 
of 74.5A at 77K and had undergone an intensive test program at low temperatures, described in more 
detail in [5]. The calculated heat in-leak of the original current leads was 7W at 100A. 
The field winding controller, shown in Figure 1(a) consisted of two parts i) the H-bridge dc-dc converter 
board and, ii) the control board. The function of the converter board was to charge, maintain and 
discharge current to the field winding. It was built using a thermally conductive PCB board and used two 
parallel banks of MOSFETS to help minimise conduction losses. All board components underwent 
cryogenic testing, to verify their suitability to operate at low temperatures. The circuit diagram in Figure 
1(b) identifies the position of the converter board within the system circuit. Figures 1(c), 1(d) and 1(e) 
show which transistors S1, S2, S3 and S4, were active while a particular function was in operation. 
During the charge sequence (1a), transistors S1 and S4 were active and current was drawn from a small 
(5A, 50V) dc power source positioned outside the generator. The converter delivered current to the field 
winding in ‘packets’ imitating the operation of stepping power source. Once charged, S1 was opened and 
the current freewheeled in the closed loop (1b), formed by S2, S4 and the winding. During the discharge 
sequence (1c), a dump resistor was activated and the energy in the coil was dissipated through S2 and S3.  
Measurement of the field winding inductance, (630 mH at low frequency and 6.8 mH at 10 kHz) and the 
winding resistance (5Ω at 10 Hz and 200Ω at 10 kHz), highlighted the importance to try to minimise the 
operating frequency of any dc converter designed for this type of application. The maximum voltage limit 
for the charging circuit was set to 40V; to enable the use of cryogenically tested low-loss MOSFETS, 
without affecting the converters ability to charge the field winding rapidly under transient conditions. The 
maximum charging rate was ~2.4s to reach the maximum design current limit set at 150A. The control 
board was split into two parts; i) the rotor control board and, ii) the stator control board. The rotor control 
board interfaced with the dc converter and together they were fixed to the cold rotor. It was fabricated 
1004   Wendell Bailey et al. /  Physics Procedia  36 ( 2012 )  1002 – 1007 
upon a normal FR-4 PCB substrate and had three primary functions; i) control and sense the field current, 
ii) feedback the current signal to the stator board and execute commands sent by the stator board, iii) 
activate the quench protection protocol.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.1. (a) DC converter board assembly; (b) System circuit diagram; (c) Charging circuit; (d) Freewheeling circuit; (e) Discharging 
circuit. 
 
The stator control board shown in Figure 2(a), remained on the outside the machine and communicated to 
the ‘cold’ rotor board through a one wire link, connected via the generators 3rd slip ring. This section of 
the control circuit monitored the field winding current feedback signal and compared this signal to the 
reference current. The current feedback signal was updated at 1ms intervals. The reference current was set 
manually, and a PI controller generated the reference current to activate the field winding charger; a buck 
converter operating in peak current control mode. 
3. Mounting of the dc-dc converter to the superconducting rotor. 
The dc converter and the rotor control boards were assembled next to each other on a single aluminium 
support plate, which was then bolted to the side of the iron core, as shown in Figure 2(b). The final 
dimension of the dual board assembly was 390 mm long x 68 mm wide x 37 mm in height. The total 
weight of the assembly was 0.4 kg at a distance 150 mm from the centre of the machine, giving a 
centrifugal force of ~1500N if rotated at the balanced speed of 1500 R.P.M. In order to avoid re-
balancing the rotor, only stationary tests were carried out during this project. 
 
The heat generated by the power board when charging the field winding to ~150A, was estimated to be 
between 5W to 7W. Semi-flexible copper strips, soldered to the substrate of the converter board incepted 
the heating and compensated for the contraction of the board assembly. The other end of each strip was 
bolted to the ‘cold’ copper radiation shield. An HTS bus bar was soldered to each end of the field winding 
terminations. The other end of the bars ran parallel with the sides of the converter. Semi-flexible copper 
strips were used as ‘jumpers’ between the bus bar and the current tracks on the converter board.  
 
H-bridge 
control 
Current 
feedback 
H-bridge DC 
converter board 
Bracket to fix board 
assembly to rotor 
Slip rings 
S3 
Current tracks for field 
winding connection 
MOSFETS 
Rotor control 
board 
VCCS 
Cold environment 
Room temperature 
Charge 
Discharge 
S4 
Winding 
Winding 
DC-link capacitor 
S4 
S4 
S3 
S2 
S2 
S1 
S1 
S3 
Winding 
H-bridge 
converter 
1(a) 
1(b) 
1(c) 
1(d) 
1(e) 
 Wendell Bailey et al. /  Physics Procedia  36 ( 2012 )  1002 – 1007 1005
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.2. (a) Stator control board; (b) Cross-sectional view of the rotor assembly, the power board assembly and the vacuum outer wall. 
4. Testing of the dc-dc power converter 
The first test for the dc converter assembly was to verify whether the board could switch on from a cold 
start. The rotor was cooled to 82K using liquid nitrogen delivered directly from a Dewar in an open-cycle. 
Figure 3(a), shows the switching waveform captured as the power electronics responded from the cold 
start. The square-wave (Ch 4) shows the communication signal relayed between the micro-processors 
onboard the rotor and stator control boards. The temperatures of the iron rotor core, a single coil in the 
field winding and the converter board were all monitored using wireless telemetry. 
Using the closed-cycle cooling system described in [5] primed with liquid nitrogen, the rotor was cooled 
down to 77K. A series of board control tests were carried out up to 40A on the way down to 77K. Figure 
3(b), shows a set of temperature profiles for a 40A ‘charge-freewheel-discharge cycle’. The temperature 
of the rotor core remained stable at ~79K throughout the cycle. The temperature of the coil reacted in 
conjunction with the injection of current. The temperature of the converter board also increased but 
lagged slightly behind that of the coil, which gave some indication about the effectiveness of the thermal 
links. The temperature of the coil recovered in about 5 minutes once the current was discharged.  
The voltage waveforms in figure 3(c), for a 30A charge cycle were used to assess the amount of power 
dissipated by the converter assembly. The average power injected into the DC-link capacitor was 0.9W. 
The power injected through the one-wire communication line was about 1W. Therefore the additional 
heat load on the cryogenic cooling system due to locating the power electronics assembly in the cold 
environment was small (~2W at 30A). A discharge voltage waveform is presented in figure 3(c). During 
the transient, the current sensing power MOSFET turned on and sensed the current which was modulated 
and sent out by the one-wire communication system. A 47Ω dump resistor was switched on across the 
ends of the DC-link capacitor. It took about 120 seconds to fully discharge the 40A freewheeling in the 
field winding. The trace captured the negative feedthrough current with a peak value of 0.8A 
9% Ni-steel 
magnetic core 
Vacuum outer wall 
HTS coils and 
flux diverting 
ring 
Reference current 
set dial 
PI controller 
Buck converter 
(charger) 
Converter board 
assembly 
Dump resistor 
2(a) 
Communication 
controller 
2(b) 
1006   Wendell Bailey et al. /  Physics Procedia  36 ( 2012 )  1002 – 1007 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.3. (a) Voltage waveform showing the operation of the one-wire communication link (100μs/div), Ch4 = 20V/div; (b) Charge-
freewheel-discharge cycle at 40A; (c) Voltage waveform for a 30A charge cycle, (200ms/div), Ch2 = 20V/div, Ch4 = 0.5A/div, Ma 
= 1W/div; (d) Voltage waveform for a 40A discharge cycle, (100ms/div), Ch1 = 1V/div, Ch2 = 20V/div, Ch4 = 0.5A/div; (e) 
Testing of quench protection; (f) Hysteresis control (5s/div), Ch1 = 0.5V/div equivalent to 25A/div, Ch2 = 20V/div, Ch3 = 0.5A/div 
A quench protection algorithm was pre-programmed into the rotor controller. Its sensitivity was set to 
detect an increase in the field windings resistance of 50mΩ occurring in 100ms. Within 0.5s, a  
freewheeling pathway was opened by switching on all transistors in the H-bridge circuit to limit the onset 
of the quench. Figure 3(e) presents some temperature responses obtained while initiating a series of 
quenches in the field winding to test this function. The winding was charged to 40A at 1.2A/s, as the coil 
temperature rose from 86K to 88K. A current induced quench, highlighted by the sudden runaway in the 
coil temperature was averted as the winding protection protocol was automatically triggered. The current 
was safely discharged and the coil temperature fully recovered in about 2.5 minutes. 
A second group of tests were carried out delivering liquid air from the cooling system to cool the rotor 
down to 62K if required. The functionality of the 5A hysteresis band written into the programming of the 
stator controller was confirmed in figure 3(f). This band prevented unnecessary switching between the 
charge and discharge modes during normal operation. In the waveform, the feedthrough current became 
negative as the circuit was switched into the discharge mode. Once the coil current decreased to 35A, the 
circuit automatically switched back to the charge mode and the coil current began to increase to 40A. Our 
final test was to try to inject as much current into the field winding using the dc converter system, at 
temperatures of 70K and below. Figure 4(a) shows how the current was ramped to 75A at 5A/s, and the 
coil temperature increased to 75K. Unfortunately, during an unexpected quench; the protection protocol 
failed and the converter board temperature suddenly increased at a much faster rate than in the coil. The 
result was a burn-out of one of the dc-link capacitors. Figure 4(b) presents the voltage waveform captured 
during this event. The feedthrough current shows a sudden spike as the energy is discharged through a 
weak point on the converter board and not through the dump resistor. The currents, at which the field 
Feedthrough current (Ch4) 
Comm. voltage (Ch4) 
Polarity check, quench and charge bit DC-link voltage (Ch2) 
Injected power (Ma) 
3(a) 
3(b) 
3(c) 
3(d) 
3(f) 
3(e) 
8 bit current data  
Feedthrough current (Ch 1) 
Feedback current signal (Ch 4) 
DC-link voltage (Ch2) 
Feedback current signal (Ch 1) 
DC-link voltage (Ch2) 
Feedthrough current (Ch3) 
 Wendell Bailey et al. /  Physics Procedia  36 ( 2012 )  1002 – 1007 1007
winding quenched when charging with the dc converter, are compared against the critical currents 
measured using the original copper current leads in figure 4(c). During inspection, the quench protection 
protocol seemed to discharge the current in the field winding prematurely; with a shortfall in current of 
~20A seen over the entire temperature range. However, some bench-top board tests using the same 
quench protection criteria to protect a single HTS coil in liquid nitrogen, gave a much better agreement, 
(within 4A). The single coil tests were repeated several times and confirmed that the switching voltage 
waveform of the dc converter resulted in a slightly lower quench current. The effects of eddy currents and 
magnetic fields generated in the interaction between the field winding with the dc converter are unknown. 
Further investigation is required to see if these effects also cause reductions in the quench current.  
 
 
 
 
 
Fig.4. (a) DC converter board failure due to quench; (b) Voltage waveform during quench, Ch1= 0.5V/div equivalent to 25A/div, 
Ch2 = 20V/div, Ch4 = 0.5V/div; (b) Plot of the quench currents measured for the HTS winding when charging with the dc 
converter, compared to the critical currents obtained for the same field winding measured when using the original current leads. 
5. Conclusions 
The design of the dc-dc power converter was successful at reducing the size and weight of the power 
equipment needed to electrically drive an HTS generator. Potential reliability issues and heat dissipation 
levels were realised for such a device. The power board losses rise with the square of the winding current, 
although these can be further reduced by placing more MOSFETS in parallel. The current injected to the 
field winding before the quench protection protocol intervened was ~20A less than the measured critical 
current of the winding. Further work is needed to understand how the operating frequency and the 
magnitude of the switching voltage affect the quench current in order to optimise this type of power 
converter.  
References 
[1] Al-Mosawi MK, Goddard K, Yang Y, Beduz C. Construction of a 100 kVA high temperature superconducting synchronous 
generator. IEEE Trans Appl Supercond 2005;15:2182-85. 
[2] Bailey W,Goddard K,Al-Mosawi, MK, Yang.Y, The design of a lightweight HTS synchronous generator cooled by 
subcooled liquid nitrogen”, IEEE Trans Appl Supercond 2009;19:1674-77. 
[3] Kondo Y, Fukano S, Ninomiya A, Ishigohka T. Cryogenic low-voltage/high current dc power source using multi-parallel 
connected MOSFETS. IEEE Trans Appl. Supercond 2009;19:2337-40. 
[4] Haldar P et al. Improving perfromance of cryogenic power electronics. IEEE Trans. Appl.Supercond 2004;15:2370-75. 
[5] Wen H, Bailey W, Goddard K, Further testing of an’iron-cored’ HTS synchromous generator cooled by liquid air. IEEE 
Trans Appl Supercond 2011;21:1163-66. 
 
4(a) 4(b) 4(c) 
DC-link voltage (Ch2) 
Feedback current 
signal (Ch1) 
Feedthrough current (Ch4) Quench 


Bailey, W.O.S.

Yang, Y.

Forsyth, A.

Jia, C.

Southampton (e-Prints Soton)

  Physics Procedia           Physics Procedia  00 (2011) 000–000 www.elsevier.com/locate/procedia  Superconductivity Centennial Conference A cryogenic dc-dc power converter for a 100kW synchronous HTS generator at liquid nitrogen temperatures Wendell Baileya *, Hauming Wena, Yifeng Yanga, Andrew Forsythb, Chungjiang Jiab aCryogenics, Faculty of Engineering and the Environment, University of Southampton, Southampton, SO171BJ, United Kingdom bSchool of Electrical and Electronic Engineering, University of Manchester, Manchester, MN63PL, United Kingdom  Abstract A dc-dc converter has been developed for retrofitting inside the vacuum space of the HTS rotor of a synchronous generator. The heavy copper sections of the current leads used for energising the HTS field winding were replaced by cryogenic power electronics; consisting of the converter and a rotor control unit. The converter board was designed using an H-bridge configuration with two 5A rated wires connecting the cryogenic boards to the stator control board located on the outside of the generator and drawing power from a (5A, 50V) dc power source. The robustness of converter board was well demonstrated when it was powered up from a cold start at 82K. When charging the field winding with moderate currents (30A), the heat in-leak to the ‘cold’ rotor core was only 2W. It continued to function down to 74K, surviving several quenches. However, the quench protection function failed when injecting 75A into the field winding, resulting in the burn out of one of the DC-link capacitors. The magnitudes of the critical currents measured with the original current leads were compared to the quench currents, which was defined as the current which  triggered  quench  protection  protocol.  The  difference  between  the  two  currents  was  rather  large,  (~20A). However, additional measurements using a single HTS coil in liquid nitrogen found that this reduction should not be so dramatic and in the region of 4A. Our conclusions identified the converter’s switching voltage and its operating frequency as two parameters, which could have contributed to lowering the quench current. Magnetic fields and eddy currents are expected to be more prominent the field winding and its impact on the converter also need further investigation.  © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Horst Rogalla and Peter Kes. Keyword: HTS generators; power converter; synchronous machines; cryogenic devices.   * Corresponding author. E-mail address.wosb@soton.ac.uk 2  Wendell Bailey/ Physics Procedia 00 (2011) 000–000 1. Introduction Following the construction and testing of superconducting generators, which operate in the temperature range of 60K to 80K, [1-2]. A project has been carried out by a group of Universities in partnership with Rolls Royce to investigate the feasibility of building a cryogenic power converter, for energising the superconducting field winding for these types of machines. The insertion of a cryogenic dc power source onboard the cold rotor, with a high-voltage/low-current ac input and a low-voltage/high-current dc output, could reduce the heat in-leak incurred by the current leads, [3].  Cryogenic power electronics provide other promising benefits over their room temperature counterparts in terms of a reduced size, weight (increased power density), improved efficiency, improved switching speed and improved reliability. Passive devises such as inductors, capacitor and interconnects will also improve by the lowered resistance of their constituents metal conductors or use of superconductors, [4].  With the help of cryogenic power electronics; the manufacture of an HTS generator unit, fully integrated with the converter, cooling and coupling components could become a real possibility. Designers of light aircrafts, submersible sea crafts and wind farm would welcome such a generator. 2. Details of the HTS generator and the dc-dc power converter design The generator selected to retro-fit the cryogenic power converter to was a 100kW synchronous generator, which  used  a  conventional  3-phase  (198A,  415V).  The  rotor  was  designed  to  withstand  the  torque generated at 3000 R.P.M and consisted of a magnetic iron core and a field winding built from a stack of ten identical HTS pancake coils, each separated by flux diverting ring. The winding had a critical current of 74.5A at 77K and had undergone an intensive test program at low temperatures, described in more detail in [5]. The calculated heat in-leak of the original current leads was 7W at 100A. The field winding controller, shown in Figure 1(a) consisted of two parts i) the H-bridge dc-dc converter board  and,  ii)  the  control  board.  The  function  of  the  converter  board  was  to  charge,  maintain  and discharge current to the field winding. It was built using a thermally conductive PCB board and used two parallel  banks  of  MOSFETS  to  help  minimise  conduction  losses.  All  board  components  underwent cryogenic testing, to verify their suitability to operate at low temperatures. The circuit diagram in Figure 1(b) identifies the position of the converter board within the system circuit. Figures 1(c), 1(d) and 1(e) show which transistors S1, S2, S3  and S4, were active while a particular function  was in operation. During the charge sequence (1a), transistors S1 and S4 were active and current was drawn from a small (5A, 50V) dc power source positioned outside the generator. The converter delivered current to the field winding in ‘packets’ imitating the operation of stepping power source. Once charged, S1 was opened and the current freewheeled in the closed loop (1b), formed by S2, S4 and the winding. During the discharge sequence (1c), a dump resistor was activated and the energy in the coil was dissipated through S2 and S3.  Measurement of the field winding inductance, (630 mH at low frequency and 6.8 mH at 10 kHz) and the winding resistance (5Ω at 10 Hz and 200Ω at 10 kHz), highlighted the importance to try to minimise the operating frequency of any dc converter designed for this type of application. The maximum voltage limit for the charging circuit was set to 40V; to enable the use of cryogenically tested low-loss MOSFETS, without affecting the converters ability to charge the field winding rapidly under transient conditions. The maximum charging rate was ~2.4s to reach the maximum design current limit set at 150A. The control board was split into two parts; i) the rotor control board and, ii) the stator control board. The rotor control board interfaced with the dc converter and together they were fixed to the cold rotor. It was fabricated   Wendell Bailey, Physics Procedia 00 (2011) 000–000  3 upon a normal FR-4 PCB substrate and had three primary functions; i) control and sense the field current, ii) feedback the current signal to the stator board and execute commands sent by the stator board, iii) activate the quench protection protocol.                   Fig.1. (a) DC converter board assembly; (b) System circuit diagram; (c) Charging circuit; (d) Freewheeling circuit; (e) Discharging circuit.  The stator control board shown in Figure 2(a), remained on the outside the machine and communicated to the ‘cold’ rotor board through a one wire link, connected via the generators 3rd slip ring. This section of the control circuit monitored the field winding current feedback signal and compared this signal to the reference current. The current feedback signal was updated at 1ms intervals. The reference current was set manually, and a PI controller generated the reference current to activate the field winding charger; a buck converter operating in peak current control mode. 3. Mounting of the dc-dc converter to the superconducting rotor. The dc converter and the rotor control boards were assembled next to each other on a single aluminium support plate, which was then bolted to the side of the iron core, as shown in Figure 2(b). The final dimension of the dual board assembly was 390 mm long x 68 mm wide x 37 mm in height. The total weight of the assembly  was  0.4 kg at a distance 150  mm  from the centre of  the  machine,  giving a centrifugal  force  of  ~1500N  if  rotated  at  the  balanced  speed  of  1500  R.P.M.  In  order  to  avoid  re-balancing the rotor, only stationary tests were carried out during this project.  The heat generated by the power board when charging the field winding to ~150A, was estimated to be between 5W to 7W. Semi-flexible copper strips, soldered to the substrate of the converter board incepted the heating and compensated for the contraction of the board assembly. The other end of each strip was bolted to the ‘cold’ copper radiation shield. An HTS bus bar was soldered to each end of the field winding terminations. The other end of the bars ran parallel with the sides of the converter. Semi-flexible copper strips were used as ‘jumpers’ between the bus bar and the current tracks on the converter board.   H-bridge control Current feedback H-bridge DC converter board Bracket to fix board assembly to rotor Slip rings S3 Current tracks for field winding connection MOSFETS Rotor control board VCCS Cold environment Room temperature Charge Discharge S4 Winding Winding DC-link capacitor S4 S4 S3 S2 S2 S1 S1 S3 Winding H-bridge converter 1(a) 1(b) 1(c) 1(d) 1(e) 4  Wendell Bailey/ Physics Procedia 00 (2011) 000–000                   Fig.2. (a) Stator control board; (b) Cross-sectional view of the rotor assembly, the power board assembly and the vacuum outer wall. 4. Testing of the dc-dc power converter The first test for the dc converter assembly was to verify whether the board could switch on from a cold start. The rotor was cooled to 82K using liquid nitrogen delivered directly from a Dewar in an open-cycle. Figure 3(a), shows the switching waveform captured as the power electronics responded from the cold start. The square-wave (Ch 4) shows the communication signal relayed between the micro-processors onboard the rotor and stator control boards. The temperatures of the iron rotor core, a single coil in the field winding and the converter board were all monitored using wireless telemetry. Using the closed-cycle cooling system described in [5] primed with liquid nitrogen, the rotor was cooled down to 77K. A series of board control tests were carried out up to 40A on the way down to 77K. Figure 3(b), shows a set of temperature profiles for a 40A ‘charge-freewheel-discharge cycle’. The temperature of the rotor core remained stable at ~79K throughout the cycle. The temperature of the coil reacted in conjunction  with  the  injection  of  current.  The  temperature  of  the  converter  board  also  increased  but lagged slightly behind that of the coil, which gave some indication about the effectiveness of the thermal links. The temperature of the coil recovered in about 5 minutes once the current was discharged.  The voltage waveforms in figure 3(c), for a 30A charge cycle were used to assess the amount of power dissipated by the converter assembly. The average power injected into the DC-link capacitor was 0.9W. The power injected through the one-wire communication line was about 1W. Therefore the additional heat load on the cryogenic cooling system due to locating the power electronics assembly in the cold environment was small (~2W at 30A). A discharge voltage waveform is presented in figure 3(c). During the transient, the current sensing power MOSFET turned on and sensed the current which was modulated and sent out by the one-wire communication system. A 47Ω dump resistor was switched on across the ends of the DC-link capacitor. It took about 120 seconds to fully discharge the 40A freewheeling in the field winding. The trace captured the negative feedthrough current with a peak value of 0.8A   9% Ni-steel magnetic core Vacuum outer wall HTS coils and flux diverting ring Reference current set dial PI controller Buck converter (charger) Converter board assembly Dump resistor 2(a) Communication controller 2(b)   Wendell Bailey, Physics Procedia 00 (2011) 000–000  5               Fig.3. (a) Voltage waveform showing the operation of the one-wire communication link (100μs/div), Ch4 = 20V/div; (b) Charge-freewheel-discharge cycle at 40A; (c) Voltage waveform for a 30A charge cycle, (200ms/div), Ch2 = 20V/div, Ch4 = 0.5A/div, Ma = 1W/div; (d) Voltage waveform for a 40A discharge cycle, (100ms/div), Ch1 = 1V/div, Ch2 = 20V/div, Ch4 = 0.5A/div; (e) Testing of quench protection; (f) Hysteresis control (5s/div), Ch1 = 0.5V/div equivalent to 25A/div, Ch2 = 20V/div, Ch3 = 0.5A/div A quench protection algorithm was pre-programmed into the rotor controller. Its sensitivity was set to detect  an  increase  in  the  field  windings  resistance  of  50mΩ  occurring  in  100ms.  Within  0.5s,  a freewheeling pathway was opened by switching on all transistors in the H-bridge circuit to limit the onset of the quench. Figure 3(e) presents  some  temperature responses obtained  while initiating a series of quenches in the field winding to test this function. The winding was charged to 40A at 1.2A/s, as the coil temperature rose from 86K to 88K. A current induced quench, highlighted by the sudden runaway in the coil temperature was averted as the winding protection protocol was automatically triggered. The current was safely discharged and the coil temperature fully recovered in about 2.5 minutes. A second group of tests were carried out delivering liquid air from the cooling system to cool the rotor down to 62K if required. The functionality of the 5A hysteresis band written into the programming of the stator controller was confirmed in figure 3(f). This band prevented unnecessary switching between the charge and discharge modes during normal operation. In the waveform, the feedthrough current became negative as the circuit was switched into the discharge mode. Once the coil current decreased to 35A, the circuit automatically switched back to the charge mode and the coil current began to increase to 40A. Our final test was to try to inject as much current into the field winding using the dc converter system, at temperatures of 70K and below. Figure 4(a) shows how the current was ramped to 75A at 5A/s, and the coil temperature increased to 75K. Unfortunately, during an unexpected quench; the protection protocol failed and the converter board temperature suddenly increased at a much faster rate than in the coil. The result was a burn-out of one of the dc-link capacitors. Figure 4(b) presents the voltage waveform captured during this event. The feedthrough current shows a sudden spike as the energy is discharged through a weak point on the converter board and not through the dump resistor. The currents, at which the field  Feedthrough current (Ch4) Comm. voltage (Ch4) Polarity check, quench and charge bit  DC-link voltage (Ch2) Injected power (Ma) 3(a) 3(b) 3(c) 3(d) 3(f) 3(e) 8 bit current data  Feedthrough current (Ch 1) Feedback current signal (Ch 4) DC-link voltage (Ch2) Feedback current signal (Ch 1) DC-link voltage (Ch2) Feedthrough current (Ch3) 6  Wendell Bailey/ Physics Procedia 00 (2011) 000–000 winding  quenched  when  charging  with  the  dc  converter,  are  compared  against  the  critical  currents measured using the original copper current leads in figure 4(c). During inspection, the quench protection protocol seemed to discharge the current in the field winding prematurely; with a shortfall in current of ~20A  seen  over  the  entire  temperature  range.  However,  some  bench-top  board  tests  using  the  same quench protection criteria to protect a single HTS coil in liquid nitrogen, gave a much better agreement, (within 4A). The single coil tests were repeated several times and confirmed that the switching voltage waveform of the dc converter resulted in a slightly lower quench current. The effects of eddy currents and magnetic fields generated in the interaction between the field winding with the dc converter are unknown. Further investigation is required to see if these effects also cause reductions in the quench current.       Fig.4. (a) DC converter board failure due to quench; (b) Voltage waveform during quench, Ch1= 0.5V/div equivalent to 25A/div, Ch2 = 20V/div, Ch4 = 0.5V/div; (b) Plot of the quench currents measured for the HTS winding when  charging with the dc converter, compared to the critical currents obtained for the same field winding measured when using the original current leads. 5. Conclusions The design of the dc-dc power converter was successful at reducing the size and weight of the power equipment needed to electrically drive an HTS generator. Potential reliability issues and heat dissipation levels were realised for such a device. The power board losses rise with the square of the winding current, although these can be further reduced by placing more MOSFETS in parallel. The current injected to the field winding before the quench protection protocol intervened was ~20A less than the measured critical current  of  the  winding.  Further  work  is  needed  to  understand  how  the  operating  frequency  and  the magnitude of the switching voltage affect the quench current in order to optimise this type of power converter.  References [1] Al-Mosawi MK, Goddard K, Yang Y, Beduz C. Construction of a 100 kVA high temperature superconducting synchronous generator. IEEE Trans Appl Supercond 2005;15:2182-85. [2]  Bailey  W,Goddard  K,Al-Mosawi,  MK,  Yang.Y,  The  design  of  a  lightweight  HTS  synchronous  generator  cooled  by subcooled liquid nitrogen”, IEEE Trans Appl Supercond 2009;19:1674-77. [3] Kondo Y, Fukano S, Ninomiya A, Ishigohka T. Cryogenic low-voltage/high current dc power source using multi-parallel connected MOSFETS. IEEE Trans Appl. Supercond 2009;19:2337-40. [4] Haldar P et al. Improving perfromance of cryogenic power electronics. IEEE Trans. Appl.Supercond 2004;15:2370-75. [5] Wen H, Bailey W, Goddard K, Further testing of an’iron-cored’ HTS synchromous generator cooled by liquid air. IEEE Trans Appl Supercond 2011;21:1163-66.  4(a)  4(b)  4(c) DC-link voltage (Ch2) Feedback current signal (Ch1) Feedthrough current (Ch4)  Quench 

Construction of a 100 kVA high temperature superconducting synchronous generator.

Cryogenic low-voltage/high current dc power source using multi-parallel connected MOSFETS.

Further testing of an’iron-cored’ HTS synchromous generator cooled by liquid air.

Improving perfromance of cryogenic power electronics.

The design of a lightweight HTS synchronous generator cooled by subcooled liquid nitrogen”,

A cryogenic dc-dc power converter for a 100kW synchronous HTS generator at liquid nitrogen temperatures

Wendell Bailey

Hauming Wen

Yifeng Yang

Andrew Forsyth

Chungjiang Jia

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A Cryogenic Dc-Dc Power Converter for a 100kW Synchronous HTS Generator at Liquid Nitrogen Temperatures

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A Cryogenic dc-dc power converter for a 100 kW synchronous HTS generator at liquid nitrogen temperatures

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A cryogenic dc-dc power converter for a 100kW synchronous HTS generator at liquid nitrogen temperatures

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The University of Manchester - Institutional Repository