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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH European Laboratory for Particle Physics
40 kA Superconducting DC Transformer for the FRESCA test station
O. Vincent-Viry / AT-MAS
Keywords: superconducting, transformer, Fresca
Abstract This note presents the superconducting transformer that is now operated on the FRESCA test station. It is divided in two parts: part 1 from page 1 to 30 is the note itself, part 2 from page 31 to the end is annexes. After an introduction of the objectives and functioning of the superconducting transformer, the note describes the sample insert that holds the transformer, its current regulation system and its protection system. The conclusion finally gives the performances of the transformer (cryogenic aspects as well as measurement accuracy). This is an internal CERN publication and does not necessarily reflect the views of the LHC project management.
LHC Project Note 3552004-09-24
Author: Olivier.vincent-viry@cern.ch
Contents: 1.Introduction 1.1.Objectives 1.2.Functioning of a superconducting transformer 1.3.Energy stored in the transformer
1.4.Scope of the report 2.Transformer sample insert 2.1.Sample insert 2.2.Superconducting transformer 2.3.Rogowski coil 2.4.Secondary heaters 2.5.Sample connexion 3.Current regulation of the transformer
3.1.Measurement of secondary current 3.2.Regulation scheme 3.3.Test of the current regulation system
3.3.1.Open loop tests 3.3.2.Closed loop tests
3.3.2.1.Current oscillations during a ramp 3.3.2.2.Oscillation of the measured voltage during a ramp 3.3.2.3.Current cycle
3.4.Conclusion 4.Protection of the transformer
4.1.Introduction 4.2.Protection strategy 4.3.Protection system 4.4.Test of the protection system 4.4.1.Case 1 4.4.2.Case 3 4.4.3.Hot spot estimation 4.5.Conclusion
5.Conclusion References Annex 1: current regulation system Annex 2: analytical modelisation of the regulation Annex 3: SI-selection box Annex 4: hot spot calculation Annex 5: modifications in Labview flowchart Annex 6: possible improvements of the system.
- 1 -
1.Introduction 1.1.Objectives
The FRESCA test station is the CERN facility for the acceptance tests of the
superconducting cables for the LHC [1]. Its main features are: independently cooled 88mm aperture - 10T background magnet, test currents up to 32kA, temperature between 1.8 and 4.3K, measurement length of 60cm, field perpendicular or parallel to the cable broad face. The samples tested in FRESCA are fed by an external 32kA power supply through copper current leads self-cooled by the vapour from the He bath. Typical He consumption due to these leads is about 550 litres of He for a standard day of tests (10 h). In order to decrease the He consumption of the station and to increase the current available in the sample (for special cables), it has been decided to build a sample insert in which a superconducting DC transformer replaces the function of the external 32kA power supply. The large copper leads are therefore replaced by small 100A leads, feeding the primary of the transformer. The transformer should also allow higher currents in the samples while keeping a precision of 0.5 % for the measurement of the sample current. As voltages in the range of a few µV are measured on the samples, the system also needs a current regulation to avoid inductive voltage disturbances during measurements. The size of the transformer is determined by the space available in the sample insert between the λ-plate and the radiation shields. For the measurement of the secondary current, which is the crucial point when operating a superconducting transformer, the system uses 2 Rogowski coils associated with a digital integrator. Hall probes could also have been used, but apart from the fact that this method is less precise, Hall probes need re-calibration at every thermal cycle thus leading to a less convenient system. 1.2.Functioning of a superconducting transformer A DC superconducting transformer acts just like a classical transformer with copper winding. But with a copper transformer, when the primary current is suddenly ramped up and then kept constant, the secondary current decreases from its maximum value to 0 with a time constant in the µs range, whereas, if the secondary is superconductive, the secondary current will keep flowing at its maximum value (no resistance). In practical cases, the secondary is never totally superconducting (connection between the secondary of the transformer and the sample) thus giving a time constant in the 1000s range or more.
- 2 -
The transformer is modelled according to the following sketch (Fig. 1):
Rp Rs
Lp Ls
M
U Lsam
Ip Is
Fig. 1 Modelling of the transformer
The equations describing the functioning of the transformer, and corresponding to the previous sketch, are:
⎢⎢⎢⎢
⎣
⎡
+−=
+=
dtdIpLpUIpRp
dtdIsM
dtdIsLtotIsRs
dtdIpM
.
. (1)
with Ip, Is: primary, secondary current Rp, Rs: primary, secondary resistance Lp, Ltot: primary, secondary total inductance M: mutual inductance between transformer primary and secondary U: voltage of the power supply feeding the primary of the transformer The secondary resistance is due to the connection between the sample and the secondary of the transformer. The total inductance of the secondary is the inductance of the whole secondary loop, that is transformer secondary + sample: Ltot = Ls + Lsample. Assuming that the self-inductances and the resistance Rs are independent of the current, and also assuming a linear increase of the primary current Ip(t) = k.t, the current ratio Is/Ip is then:
⎟⎠⎞
⎜⎝⎛ −= t
LtotRs
LtotM
IpIs
211 with s
RsLtot 1000> (2)
From (2) it is clear that the larger is the mutual inductance M between primary and secondary, the larger will be the current ratio, thus allowing larger current in the secondary.
- 3 -
Equations (1), applied for a linear increase of Is, yield the following result (Fig. 2):
Fig. 2: Simulation of a superconducting transformer using (1) The above simulation shows that the primary current needs to be increased to keep the secondary current linearly ramped up. In the same way, in order to keep the secondary current constant, the primary current must be slowly increased. The increase of primary current compensates the dissipation of energy in the resistance of the secondary, and is linked to the time constant Ltot/Rs. This particular example, as well as equation (2), clearly shows the need for a current regulation of the transformer. 1.3.Energy stored in the transformer The equations corresponding to the previous simulation (Fig. 2) are:
10 tt ≤≤ ttI
tIs s
1
1)( = with 11 )( pItIp = and 11 )( sItIs = (3)
21 ttt ≤≤ 1)( sItIs = 1)( pItIp = (4) The power P and the energy E stored in the transformer are given by:
dtdIsIpM
dtdIpIpLpP .. −= ∫=
t
dttPE )( (5)
0
10000
20000
30000
40000
50000
60000
0 200 400 600 800 1000 1200 1400time [s]
I_se
c [A
]
0
10
20
30
40
50
60
70
I_pr
im [A
]
I_secI_prim
- 4 -
For 10 tt ≤≤ , we have:
⎟⎠⎞
⎜⎝⎛ −+⎟
⎠⎞
⎜⎝⎛ −+= 11
213
6.
8..)( 2
21
21
21
21
21
21 kI
Ltotk
tIRstI
MRsLptE sss (6)
with LtotLp
Mk.
=
The energy stored in the transformer at t = t1 increases if Rs, Is1 or t1 increases and decreases if k (or M) increases. The larger M is, the smaller is the amount of magnetic energy stored in the transformer. In case of a quench, the hot spot in the winding will then be also lowered. For 21 ttt ≤≤ , we have:
( )1211
21
222
12
2
12 ..22
.)()( ttIIM
RsLpttI
MRsLptEtE sps −+⎟⎟
⎠
⎞⎜⎜⎝
⎛−+= (7)
The energy stored in the transformer at t = t2 increases if Rs or t2 increases. This stresses the importance of keeping Rs as small as possible in order to minimise the amount of energy stored in the transformer during a plateau of the secondary current. 1.3.Scope of the report Section 2 is the description of the components of the sample insert holding the transformer: sample insert, transformer, Rogowski coils, secondary heaters and samples.
Section 3 deals with the current regulation system of the transformer: how secondary current is measured, scheme of the regulation system, tests and performances. Section 4 deals with the protection system of the superconducting transformer: protection strategy, description of protection system and tests. Section 5 concludes this report with the performances of the superconducting transformer and also gives some possible improvements of the system.
- 5 -
2.Transformer sample insert: This section presents the sample insert on which is mounted the superconducting transformer. The various sensors placed in the insert are described in detail and the characteristics of the transformer as well as the current measuring system are given. 2.1.Sample insert: The insert, which holds the transformer (so called SI 3)(Fig. 3), is built on the same basis as the two sample inserts using copper leads (so called SI 1, SI 2).
transformer
λ plate
connectors
copper plait
top plateconnectors
radiation shield
Rogowski coil
connexion with sample
Fig. 3: Sample insert 3(SI 3) (See Fig. 4 for detail of the transformer) There are only 4 radiation shields (as compared to 5 in SI 1 and 2). The current leads from the top plate to the primary of the transformer (inside the cryostat), consist of two flat laminated copper plaits (16 mm2 copper section). These two plaits are overlapped with nylon spiral wrap to avoid short-circuit while allowing He gas to enter and cool the leads. On the top plate, the connections between the copper plaits and the currents leads to the power supply are made through S10AR-N plugs from Multicontact. For cryogenic control (ABB system), 3 temperature sensors are mounted on the sample insert:
- one Platinum 100 (TT839), located below the λ-plate - two Platinum 100 (1TT843 and 2TT843), located on the transformer body
- 6 -
2.2.Superconducting transformer: The primary winding of the transformer is a solenoidal coil of 10850 turns of NbTi wire from Alstom:
Wire: Filament diameter 45 [µm] Cu/SC 1.35 RRR 82 wire diameter 0.5 [mm] wire diam.(insulated) 0.542 [mm] wire cross section 0.1963 [mm2] wire cr. sect. (insulated) 0.2307 [mm2] wire resistance 300K 0.158 [Ω/m]
Winding: inner, outer radius of the primary 70, 88 [mm] height primary 160 [mm] thickness primary ~18 [mm] turns primary 10850 compaction factor (primary) 0.870 number of layers 33 self inductance 11.75 [H]
The primary winding is impregnated with epoxy resin. The connection between the primary superconducting wire and the copper plait is made through a copper piece on which is soldered the wire (SnAg soldering) and also clamped the plait. The secondary winding consists of 7 turns of LHC 01E00113 cable, on which has been soldered (all along), using SnAg, a copper strip of 1 mm thickness for mechanical and electro-dynamical stability.
Cable reference 01E00113A Number of strands 28 strand diameter 1.065 [mm] cable width 15.1 [mm] cable mid-thickness 1.900 [mm]
For transformer protection, 5 pairs of voltage taps are soldered on the primary winding:
- 2 located on the soldering between the copper plaits and the primary wire - 3 located inside the winding.
Also 1 pair and 4 voltage taps are soldered on the secondary winding: - 1 pair located at the middle of the secondary winding - 1 voltage tap on each side of the secondary heater, for both heaters
(See section Protection for more details)
- 7 -
The secondary winding is maintained against the primary winding by means of G10 (glass fibre) bars and pushing screws (flat head), see Fig. 4.
secondary winding
7 turns of LHC 01E00113 inner cable with 1mm copper soldered all along
connection primary wire - copper plait
secondary heater location
G10 bar maintaining secondary winding (with 8 M5 screws)
primary winding (inside): 10850 turns of NbTi wire
M5 screw
Fig. 4: Superconducting transformer
The mutual inductance between primary and secondary winding is 8.77 mH. 2.3.Rogowski coils: The current flowing in the secondary of the transformer is measured by means of two Rogowski coils placed at the two extremities of the transformer (see Fig. 3). They consist in toroidal coils with rectangular cross section, through which passes the secondary cable. Each Rogowski coil delivers a pick-up voltage proportional to the time derivative of the current flowing through its aperture and is theoretically insensitive to external surrounding magnetic field and to the exact position of the current within the aperture.
Rogowski coil coil height 70 [mm] coil inner radius 22 [mm] coil outer radius 60 [mm] material of the core (G10) material of the wire copper wire diameter 0.1 [mm] number of turns 5528 number of layers 4 glue* Stycast insulation Mayla external layer fiber glass
- 8 -
*Stycast glue in addition with fiber-glass is used to fix wires and solidify the coil. 2.4.Secondary heaters: Two heaters are mounted on the secondary winding for transformer operation and protection (Fig. 5). (See section Current regulation and Protection for details) The heaters are Minco thermo-foils heaters with Kapton insulation of dimensions: 19.1 * 76.2 mm and a resistance of 21.1Ω each. They are located just above the Rogowski coils (see Fig. 3 and 4). An Allen-Bradley temperature sensor is fixed onto each heater.
Minco Heater (kapton isolated)
temperature probe (Allen Bradley)
copper strip (1mm thick)
secondary cableindium strip
thermal conducting paste
G10 piece (screwed)
maintaining bar
Fig. 5 Details of the heater mounting
2.5.Sample connection: The cable samples to be measured consist of two pieces of 2.3m long cables, soldered together at the bottom (SnAg soldering over about 25cm). The two samples are then mounted in their sample holder [1], and their free extremity is connected to the current leads (here the transformer secondary) by clamped connectors (see Fig. 3) located below the λ-plate (with a typical resistance of a few nΩ)(see also Fig. 6).
- 9 -
3.Current regulation of the transformer: Testing superconducting cables requires measuring very small voltages in the range of a few µV. The current flowing in the samples has then to be very linear (linear ramp or plateau) to avoid that additional inductive voltages due to current changes disturb the measurement. The secondary of the transformer connected with the samples forms a loop which is not totally superconducting due to connection resistances (bottom connection between the 2 samples + connections between each sample and the secondary of the transformer, for a total of 1 –10 nΩ)(Fig. 6).
resistive partsoldering SnAg
resistive partclamped contact with
indium strip in between
sample 1
primary winding10850 turns of NbTi wire
secondary winding
copper plait
λ plate
sample 2
Fig. 6 Transformer and samples
The current feeding the primary has then to be regulated in order to counter-balance the resistive losses in the secondary loop (secondary + samples) and so to insure a very linear secondary current. Moreover the current regulation will allow other types of secondary current waveforms that would be needed for others tests on the samples. 3.1.Measurement of secondary current: The current flowing in the secondary is measured by means of two Rogowski coils associated with a digital integrator. A Rogowski coil generates a voltage proportional to the time derivative of the current flowing through its aperture. An integration of the pick-up signal is then necessary to reconstruct the current waveform.
- 10 -
A perfectly wound Rogowski coil is theoretically insensitive to an external magnetic field change. However, due to winding imprecision, each practical Rogowski coil is somewhat sensitive. In order to minimise the perturbations (caused by a change in the magnetic field generated by the transformer, or a change in the background field), the two coils are connected in anti-series. The calibration measurements for the Rogowski coils give:
coil 1 coil 2 resistance (300K) 2577 2597 [Ω] self-inductance 445 442.7 [mH]
mutual inductance with transformer primary (*) 0.45 0.26 [mH]
(*) The mutual inductance measurement has been done with coils mounted at their definitive location on the sample insert 3. The difference between the measurements for the two coils is certainly due to a difference in the precision of the coils winding. Connected together in anti-series, the Rogowski coils give a voltage of V = 0.1638 mV for a current through the aperture of the coils ramped at a rate of 1A/s. The pick-up voltage given by the Rogowski coils, connected in anti-series, can be approximated with:
( )dtdIs
dtdIpMMKV RPRPRog
321 10.1638.0 −
−− +−= (8)
with K: coupling coefficient of the transformer
MP-R1, MP-R2: mutual inductances between primary and Rogowski coils
Assuming that dt
dIpLtotM
dtdIs
= (at the beginning of a run), and with a secondary
inductance of 9 µH (see section 3.3.1.), the error on the current measurement due to the stray field of the transformer is then only 0.02 %. The digital integrator [2] has a time constant of 1s. A 12 bit +/-10V analogue converter provides an output voltage proportional to the integrator value (gain selected: 1 V.s/V), which is then used by the regulation system. The maximum resolution for the measured current is then 30A. As the current measurement with the Rogowski coils is a relative measurement, one has to know the initial value of the current flowing through the coil when the integration is launched in the digital integrator. Heaters are mounted on the transformer secondary to warm up the cable into its normal state. This assures that the secondary current is 0 when the digital integrator starts to integrate the Rogowski signal (of course the primary current is 0 during the heating up).
- 11 -
The operator has to set and minimise the drift of the digital integrator before using the transformer, but still the drift will limit the maximum duration of a run, in order to keep the error on the current measurement below 0.5% (Fig. 7).
Fig. 7 Typical drift of the digital integrator For example, the maximum duration of a plateau at 15 kA is around 15 mn if one wants to keep the drift of the digital integrator below 0.5% of the plateau current. 3.2.Regulation scheme: The feedback loop is only an adaptation of the output signal, the feedback signal is then compared to a set signal and the difference goes into the system input via a proportional integral corrector (Fig. 8).
Yokogawa programmable
DC sourcecorrector
proportional - integralLakeshore
power supply
Transformer
Rogowski coils
digital integratoractive filter
attenuateur
primary current
secondary current
inverter
+-
Fig. 8 Transformer current regulation scheme
0
50
100
150
200
250
300
350
0 600 1200 1800 2400 3000 3600
t [s]
I [A
]digital integrator drift
- 12 -
The regulation system consists of: - Yokogawa programmable DC source
used as the generator of control voltage (such as voltage ramps) and controlled by Labview (GPIB).
- Lakeshore power supply 4-quadrant bipolar +/-125A / 1kVA power supply, used as an amplifier (G = 100A / V) to feed the transformer primary.
- Active filter 2nd order Butterworth type filter used both to filter the signal and to suppress the voltage drops at the analogue output of the digital integrator. The inverter and the attenuator are used to adjust the feedback signal to the set signal before entering the comparator. The corrector [3] and the attenuator have adjustable gains. Characteristics and settings of the regulation system components are detailed in Annex 1. 3.3.Test of the current regulation system: 3.3.1.Open loop tests: This type of test is used to adjust the magnitude of the feedback signal to the one of the set signal (from the Yokogawa programmable DC source) and also to measure the time constant Ltot/Rs of the total secondary loop (transformer secondary + samples). The set signal is directly send to the Lakeshore power supply, according to the following figure:
Lakeshore power supply
transformer
Rogowski coils
digital integratorattenuator active filter
Yokogawa wave generator
V_yokogawa I_lakeshore
I_sec
V_IN-
OPEN LOOP
Fig. 9 Open loop configuration
With the attenuator gain set to –0.0693, the set and feedback voltage signals coincide perfectly at the end of the ramp (Fig. 10)(CD means Cool-Down number).
- 13 -
Fig. 10 Ramp up of primary current in open loop (CD143 and CD155) For previous examples, assumption is made that the duration of the ramp is sufficiently low to neglect the de-energization of the transformer. There is a constant delay between feedback and set signals, which seems inversely proportional to the ramp rate of the primary current (or set voltage)(Fig. 10):
Vset ramp rate delay [V/s] [ms]
CD 143 0.01 400 CD 155 0.005 200
Since the delay is constant, it will not cause a variation in dIs/dt and hence will have no effect on the U-I curve measured on the samples. (A theoretical explanation for this constant delay can be found in Annex 2). If, after the ramp, the set voltage (and so the primary current) is kept constant, the slow decrease of the secondary current gives the time constant Ltot/Rs of the total secondary loop (transformer secondary + samples). Once Ltot/Rs is known, the following run is performed: ramp up of the secondary current to a value (15 kA for example) and then set of the control voltage (from the Yokogawa) to 0V. The secondary current then decreases to 0A under the compliance voltage of the Lakeshore power supply (+/- 5V), this gives very stable waveforms for primary and secondary current, allowing to determine Rs and Ltot from equation (1) and the value of Ltot/Rs.
0
0.02
0.04
0.06
0.08
0.1
0.12
10 15 20 25t (CD 143) [s]
V (C
D 1
43) [
V]
0
0.04
0.08
0.12
0.1610 15 20 25 30 35 40 45
t (CD 155) [s]
V (C
D 1
55) [
V]
V_set (CD 143) V_IN- (CD 143) V_set (CD 155) V_IN- (CD 155)
- 14 -
Ltot/Rs Rs Ltot [s] [nΩ] [µH]
CD143 1924 5.25 10.1 CD155 2580 3.85 9.9
The magnitude of the measured Rs are in accordance with the ones usually measured in the samples. From the numerous measurements done on the FRESCA test station with SI-1 or SI-2, the self-inductance of the sample is known to be around 1 µH. This gives a self-inductance for the secondary of the transformer (alone) of about 9 µH. 3.3.2.Closed loop tests: The purpose here is to optimise the corrector settings in order to minimise the oscillations of the secondary current. The system is now in normal operating configuration (Fig 11). The corrector settings are: Tc = 0.12s (RC1) Kc = -0.17 (Attenuator pos. 1) (Tc is the time constant of the corrector integrator and Kc is the gain of the proportional amplifier).
Lakeshore power supply transformer
Rogowski coils
digital integrator
attenuator active filter
Yokogawa programmable
DC source
V_set I_lakeshore
I_sec
gain = -0.0693
V_IN-
corrector
V_out_corrector
V_out_regul
CLOSED LOOP
+
-
Fig. 11 Closed loop configuration
With the previous parameters the regulation works properly and is stable. (The secondary current ramp rates considered where in the range of 0 to 1000 A/s and several types of runs have been investigated: ramps, plateaus, cycles).
- 15 -
3.3.2.1.Current oscillations during a ramp: When programming a linear ramp with dIs/dt = 200 A/s, the following waveform is typically measured:
Fig. 12 Secondary current and deviation from linear fit CD 155, B = 4.8T, direction a, 4.3K
The current flowing in the secondary (and so in the sample) is very linear; the maximum deviation between the current and a linear fit is 30A (corresponding to the resolution of the digital integrator)(Fig. 12). This maximum deviation does not depend nor on the ramp rate neither on the secondary current. 3.3.2.2. Oscillation of the measured voltage during a ramp: The voltage over the cable sample is measured over 540, 590 or 610 mm (depending on the selected voltage tap) of the sample, in the field generated by the background magnet. The following typical waveforms (UI curves) are measured during a linear ramp until the sample quenches: Fig. 13 voltage measured over 540mm (voltage tap GL), CD 155, sample 1, secondary current ramp rate = 400A/s, B = 6T, T = 4.3K
y = 212.91x - 6116.9R2 = 1
0
5000
10000
15000
20000
25000
20 40 60 80 100 120 140 160
t [s]
Is [A
]
-30
-20
-10
0
10
20
30
delta
I [A
]
Is Is - linear fit Linear (Is)
-10-8-6-4-202468
10
0 5000 10000 15000 20000
I [A]
U [u
V]
U_raw U_corrected
- 16 -
After correction for offset and slope in the flat part of the curve [4], the voltages between 0 and 16kA are plotted in Fig. 14:
Fig. 14 measured voltage oscillations during ramping up of the current (400 A/s) The oscillations of the voltage measured over the sample are in the range of +/- 0.5 µV. The oscillation magnitude is linked to the ramp rate of the secondary current increase: for a ramp rate of 40A/s the oscillations are in the range of +/- 0.17 µV. The oscillations do not depend on the secondary current. The level of oscillations obtained is acceptable (in the same range as the noise from the 32kA power supply). The measurements are therefore perfectly usable for critical current determination, or any other analysis. 3.3.2.3.Current plateau after ramp: As for current ramps, the current oscillations during a plateau are minimised. With a ramp rate of 870 A/s, the current overshoot at the beginning of the plateau is low: 1.5 % (Fig. 15).
Fig. 15 Waveforms for a ramp at 870 A/s + plateau at 8700 A, CD 143
-0.5-0.4-0.3-0.2-0.1
00.10.20.30.40.5
0 2000 4000 6000 8000 10000 12000 14000 16000
I [A]
U [u
V]
U_corrected
-0.01
0.01
0.03
0.05
0.07
0.09
0.11
40 45 50 55 60 65 70 75t [s]
V [V
]
-1
1
3
5
7
9
11
13
I [A
] or [
kA]
V_set [V] V_IN- [V] V_out regul [V] V_out_corrector [V] I_lakeshore [A] I_sec [kA]
- 17 -
(See Fig. 11 for the naming of the above plots) Plateau secondary current: 8700 A Current overshoot after ramp: 130 A (+1.5%) Current oscillations during plateau: 30 A (resolution of the digital integrator) Current oscillations during ramp: < 30A after 3s The current overshoot decreases with the ramp rate. 3.4.Conclusion: The current regulation of the superconducting transformer is stable and works properly. The measurement of the secondary current, using Rogowski coils and a digital integrator, has a maximum resolution of 30A. The error on the measurement of the secondary current due to the stray field of the transformer is negligible. The drift of the digital integrator, even if it is minimised, limits the maximum duration of a run (to keep the error below 0.5%). The adjustments of the regulation parameters (corrector and feedback attenuator gains) allow to reach very low oscillations of the current in the samples (in the range of 30A) and very low overshoot in case of a step in ramp rate. The resulting noise on the voltage measured over the samples is in the range of +/-0.5 µV for high ramp rates (around 500A/s) and +/- 0.2 µV for low ramp rates (around 50A/s), which is sufficiently low to perform waveforms analysis.
- 18 -
4. Protection of the transformer: 4.1.Introduction: This section presents the protection system of the superconducting DC transformer implemented on the FRESCA test station. The transformer consists of a primary winding of about 11000 turns of superconducting NbTi wire on which has been wound a secondary made of 7 turns of LHC inner cable. It is designed to provide a current up to 40 kA to the samples and is powered by a 4-quadrant bipolar 125A/1kVA Lakeshore power supply. In case of a quench of the sample being tested or a quench of the primary winding, the transformer, which is self protected, has to be quickly de-energized to avoid overheating the primary winding. 4.2.Protection strategy: According to the measured Ltot, Lp, M of the transformer and for a standard Ic measurements run, the maximum energy stored in the transformer is in the range of 10 kJ (calculated from equations (1), (6) and (7). Giving the characteristics of the primary winding, a protection system in which the primary is self-protected is feasible. Of course, as stressed in equation (7), a very long plateau even at low secondary current could still lead to higher energy stored in the transformer and possibly damage the primary winding in case of quench. Giving the high inductance of the transformer primary (11.75 H), using an external dump resistor to extract energy from the transformer, within about 100ms, could lead to very high voltage across the dump resistor. Moreover this solution requires some more components (100A quick switch, resistor) leading to a more complicated system. So the self-protected transformer has been chosen. When operating the transformer, several types of quench can occur:
1- Quench of the samples (which are part of the secondary winding) without quench of the primary.
This happens in normal operating mode (when measuring the sample critical current for example) for secondary current < 25 kA.
2- Quench of the primary winding, without quench of the secondary. This can happen for example during a very long plateau. Indeed, to keep the secondary current constant, the primary current has to be slowly increased to compensate for the resistive losses in the resistive parts of the secondary (soldering between the two samples, connections sample – secondary winding). A sufficiently long plateau, even at low secondary current, can lead the primary to quench.
- 19 -
3- Quench of both primary and secondary windings. This happens when the sample quenches at high current (>25 kA). The quench of the secondary causes the primary to quench as well (quench back due to the strong coupling between primary and secondary windings).
4- Externally triggered quench. The operator can trigger a quench (emergency stop located in the FRESCA control room), or there can be erratic detection of a quench, or cryogenic problems, …
In each of the previous cases, the transformer has to be protected. This is achieved by both ordering the primary current to 0 and firing heaters located on the secondary winding, as soon as a quench is detected. The secondary heaters should be powered during at least 10s (maximum time for the primary current needs to go down to 0A). For the moment, the selected duration for the secondary heaters is 16s (adjustable timing relay in the slow security box, see Fig. 17). Ordering the primary current to 0 on the Lakeshore power supply is made by shortening the control voltage from the Yokogawa programmable DC source. In this case the current is ramped down under the maximum voltage acceptable by the Lakeshore (compliance voltage, set to +/- 5V). Firing the secondary heaters pushes a part of the secondary winding into its normal state, hence increasing the total resistance of the secondary. Due to coupling between primary and secondary, a larger part of the energy stored in the transformer is thus dissipated in the secondary. The protection system is totally hardwired. 4.3.Protection system: Several voltage taps and temperature probes are implemented on the transformer in order to monitor its state and detect a possible quench (Fig. 16).
copper plait
primary winding
secondary winding
NbTi wire
LHC cable 01E00113 + 1mm copper strip soldered
cable 01E00113 (alone)
samples
1VT33/1VT34
1VT31/1VT32
1TT43
1TT60
2TT60
1VT20
1VT19
2TT43
heater
1VT30/2VT30
2VT33/2VT34
2VT31/2VT32
1VT25/2VT25
2VT20
2VT19
heater
λ plate
(current lead)
Fig. 16 Localisation of sensors implemented on the transformer
- 20 -
5 pairs of voltage taps are soldered on the primary: - 1VT33/1VT34 and 2VT33/2VT34 are located on the soldering between the
copper plait feeding the transformer and the primary winding (NbTi wire). - 1VT31/1VT32, 1VT30/2VT30 and 2VT31/2VT32 are placed inside the winding
(two wires for each pair for redundancy, soldered together at the same place on the winding).
1 pair and 4 voltage taps are soldered on the secondary: - 1VT25/2VT25 are located at the middle of the secondary winding (two wires
soldered at the same place, directly on the copper strip). - 1VT20, 1VT19 and 2VT20, 2VT19 are placed on each side of the heaters.
The voltage taps are then connected to quench detection modules [5] located in the FRESCA protection rack.
The FRESCA test station uses two types of sample insert to provide current to the sample: with copper leads (SI-1 and SI-2) or with the superconducting transformer (SI-3). The protection system has then to handle these two types of insert and to be easily switchable from one to another. The voltage taps signals coming from the cryostat (transformer, copper leads, sample) are connected to a so-called “S.I. selection box” that distributes the signals to the quench detection modules according to the type of sample insert that is selected. (See Annex 3 for details about the SI selection box) As soon as the modules detect a quench, they deliver a so-called “interout” signal to a slow security box compiling also other types of alarm signal. Then, actions are taken to de-energize safely the transformer (set voltage to 0 and fire heaters)(Fig. 17).
24V
interout
E. S. control roomnot used
A2
A3 B3
15 18
slow security box
16’
MT2
MT2
A1A2
142434
112131
122232
output 1output 2
TTI power supply
to heaters on transformer secondary
from regulation (control voltage)
Lakeshore control input Ip m
mIp
transformer rack
auto maintaining reset (labview) measurement rack
MT323024Siemens 3RP15-05-1BP30 measurement
rack
Fig. 17 Transformer protection scheme
- 21 -
The secondary heaters are connected to a 32V, 3A TTI power supply, either to output 1 or to output 2. Output 1 is used for normal operation (secondary current measurement, see 3.1.). Output 2 is always set to a voltage (selected by the operator) and, in case of a quench detected, is connected to the heaters during 16s (time also adjustable). The maximum power dissipated in the heaters (connected in parallel) is 98W. One Allen-Bradley temperature sensor is mounted near each heater to detect if the heater works properly and block energisation of the transformer if the secondary winding is not in its superconducting state. 4.4.Test of the protection system: First, the protection system is triggered manually, in order to check that it works properly (control voltage set to 0V and firing of the heaters). 4.4.1.Case 1: The current is linearly ramped up in the secondary (and so in the sample) until the sample quenches. This test is performed with a “high” magnetic field applied on the sample (7T) in order to keep the quench current below 20kA. The sample consists of two cables 01E00127DR-5 (S1) and 01E00128HR-5 (S2). The following waveforms are measured (Fig. 18):
Fig. 18 test of the transformer protection (case 1), CD143, T = 4.2K, B =7T, ramp rate of secondary current = 440A/s
-15
-10
-5
0
5
10
15
20
0 5 10 15 20 25 30 35 40 45 50 55 60 65
t [s]
I [A
] or [
kA] o
r V [u
V]
0.00
0.04
0.08
0.12
0.16
0.20V
[V]
V_lakeshore [V] I_lakeshore I_sec [kA]- V_EN_S1 [uV] V_EN_S2 [uV] V_set [V]
- 22 -
V_EN_S1: voltage measured over 610mm on the cable 1 of the sample. V_EN_S2: voltage measured over 610mm on the cable 2 of the sample. These two signals show the resistive transition of the sample that triggers the protection system. The secondary current at quench is 14120A. When the protection is activated, the control voltage is disconnected from the Yokogawa DC source (V_set) and set to 0V. The transformer is then de-energized, accelerated by the growing resistance in the secondary loop (quench of the sample). When the quench occurs, the secondary current decreases very quickly, so quickly that the digital integrator can’t follow. The measurement of the secondary current is therefore not valid after the quench, which is not a problem since the protection disconnects the regulation system from the Lakeshore power supply. Initially the current in the primary winding (I_lakeshore) decreases very quickly. As soon as there is no current flowing in the secondary, it decreases linearly to 0A under the compliance voltage of the Lakeshore power supply (-5V) untill the complete de-energization of the transformer. The fact that the decrease of the primary current is linear shows that the primary winding does not quench, indeed according to (1) we have:
dtdIpLpUIpRp
dtdIsM +−= . (1)
The decrease of the primary current (Fig. 18), described by the equation:dt
dIpLp≈− 5
shows that Rp is negligible and so that the primary winding remains superconducting. The voltages in the primary winding at quench are also recorded (Fig. 19):
Fig. 19 Voltages in the primary winding, at quench (case 1) CD143, T = 4.2K, B =7T, ramp rate of secondary current = 440A/s
-40-30-20-10
0102030405060
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55
t [s]
V [V
]
quarter 1 quarter 2 quarter 3 quarter 4
- 23 -
quarter 1: voltage measured between 1VT33 and 1VT31 (see Fig. 16) quarter 2: voltage measured between 1VT31 and 2VT30 quarter 3: voltage measured between 2VT30 and 2VT31 quarter 4: voltage measured between 2VT31 and 2VT33 The peak voltage in the primary winding is around 50V and is distributed over 9 layers of winding (quarter 1). The voltage named quarter 1 in Fig. 18 is the voltage over the 9 most external layers of the primary winding, that is the ones that are closest to the secondary winding (and so that have the larger mutual inductance with the secondary). These different mutual inductances between quarters 1 to 4 and the secondary explain the difference in the waveforms in Fig. 19. 4.4.2.Case 3: The magnetic field applied to the sample is now decreased to 4.3T. The critical current (and hence quench current) of the sample will then be higher. The following waveforms are then recorded (Fig. 20):
Fig. 20 test of the transformer protection (case 3), CD143, T = 4.2K, B =4.3T, ramp rate of secondary current = 440A/s
The secondary current at quench is 25230A. Once the secondary current is 0A, the primary current decreases with a typical exponential curve indicating that the resistance of the primary is no longer negligible. This means that a part of the primary winding has become resistive.
-15
-10
-5
0
5
10
15
20
25
30
35
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
t [s]
I [A
] or [
kA] o
r V [u
V]
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
V [V
]
V_lakeshore [V] I_lakeshore I_sec [kA]- V_EN_S1 [uV] V_EN_S2 [uV] V_set [V]
- 24 -
This is also observed on the voltages measured over the primary winding (Fig. 21):
Fig. 21 Voltages in the primary winding, at quench (case 3) CD143, T = 4.2K, B =4.3T, ramp rate of secondary current = 440A/s
The peak voltage in the winding (83V) is again over the part of the winding that has the larger mutual inductance with the secondary (quarter1). 4.4.3.Hot spot estimation: As the transformer is self-protected, particular attention must be paid to the increase of temperature in the primary winding when it quenches, in order to avoid over-heating. A conservative estimation of the temperature in the winding can be made by considering a unit cell of the winding, where the quench starts:
dTCdtJ pCu =2ρ (9) ρCu: resistivity of the copper in the wire Cp: specific heat of the wire T: temperature of the unit cell J: current density in the copper. (See Annex 4 for more details) We assume no cooling and no propagation of heat along the wire.
-80
-60
-40
-20
0
20
40
60
80
100
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55
t [s]
V [V
]
quarter 1 quarter 2 quarter 3 quarter 4
- 25 -
Estimations of the hot spot temperature in the primary winding, for current ramp till sample quench (at 1.9K), are reported below (Fig. 22).
Fig. 22 Hot spot estimation and maximum voltage in the primary winding CD176, 1.9K, current ramp rate around 200 A/s in the secondary
According to the previous figure, the estimated hot spot in the primary for a quench current in the secondary around 40kA should be well below 50K, which will not cause any over-heating of the winding. The maximum voltage should not exceed 180V (over 9 layers of winding), so there is no risk of damaging the insulation. 4.5.Conclusion: The transformer protection system is now integrated in the FRESCA test station. It works properly and has proved its reliability. A sample insert selection box allows easily switching from the protection scheme designed for inserts with copper current leads to the one designed for the insert with transformer. If the protection is activated (quench detected in the transformer or sample, or emergency stop), the control voltage of the Lakeshore power supply is set to 0, forcing the primary current to ramp down under a maximum voltage of 5V. In the same time two heaters mounted on the transformer secondary are switched ON, forcing a part of the secondary to normal state. The system is safely protected for any type of quench (in the secondary winding only, in both primary and secondary, for an externally triggered quench). The hot spot estimation, as well as the measured voltage across the winding at quench, shows that the transformer can be used safely for current ramps in the sample up to 40kA. The only risk of damaging the primary winding is for very long plateaus of the secondary current, as the energy stored in the transformer is directly related to the duration of the plateau.
10.0
15.0
20.0
25.0
30.0
35.0
40.0
20000 25000 30000 35000 40000
I_sec quench [A]
Hot
spo
t [K
]
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
V_m
ax [V
]
hot_spot V_max
- 26 -
5.Conclusions The superconducting transformer is now implemented in the FRESCA test station and fully operational. The protection system works properly and has shown its reliability. From the cryogenic point a view, an estimation during a typical day of testing on FRESCA gives a reduction of about 45% of the He consumption of the inner cryostat when the insert with the transformer is used. This estimation is based on the measured total gas flow of the station. Fig. 23 shows the typical decrease of the He level in the inner cryostat, only due to thermal losses (so-called week-end mode, with He input blocked):
Fig. 23 decrease of He level in inner cryostat (week-end mode) The reduction of the He consumption in week-end mode (used at night for example) is of 60 %. The save of He is higher during week-end mode than during tests because, for Ic runs at high currents, the primary quenches and releases the energy stored in the transformer directly in the He bath. Concerning the secondary current measurement, a precision of 0.5% can be reached, but the drift of the digital integrator has to be taken into account: it limits the maximum duration of the run. Yet for typical Ic measurement runs (duration of a few minutes) the digital integrator drift is negligible, the accuracy of the secondary measurement being 30 A.
050
100150200250300350400450
0 50 100 150 200
t [mn]
He
leve
l [m
m]
LT830 SI2 LT830 SI3
- 27 -
The following table shows the concordance of critical measurements made on two samples using sample insert with the transformer (SI 3) and sample insert with copper leads (SI 2): sample 1: 01B10543AR-5 sample 2: 01B10544AR-5 4.3 K 4.3 K error on error on error on error on Ic at 6T Ic at 7T Ic at 6T Ic at 7T [%] [%] [%] [%]
dir a 1.34 0.93 0.78 0.53 dir b 0.28 0.44 0.14 0.22 dir d -0.42 -0.42 -1.07 -0.75
A 1% accordance between the two measurements is reached, which is the reproducibility of the critical current measurements performed on the FRESCA test station. Further investigations of the concordance between measurements using or not the transformer should of course be made. The maximum critical current measured using the superconducting current is of 36 kA. This maximum current is for the moment limited only by the mechanics of the transformer secondary: when the secondary current reaches 36 kA, some movements of the secondary cable occur and lead to premature quenching. A re-enforcement of the transformer secondary will allow currents higher than 40 kA, since the hot spot estimations and the voltages measured across the winding at quench have shown there is sufficient safety margin. The use of a superconducting transformer to perform measurement on superconducting cables is relevant and has shown its reliability: it allows to save some He and to reach high currents in the sample with sufficient precision. With the development of other superconducting cables such as Nb3Sn cables, the need for high currents will increase, using superconducting transformer is a very simple and reliable solution for that. Annex 6 described some possible improvements of sample-inserts using a superconducting transformer. Acknowledgements: Many thanks to J.L. Servais for the winding of the transformer primary, to the FRESCA team: A. Verweij, S. Geminian, R. Rota and C.H. Denarie for their help building the transformer and the many fruitful discussions.
- 28 -
References: [1] A.P. Verweij, et al “1.9K Test Facility for the reception of the Superconducting Cables for the LHC” [2] P. Galbraith “Documentation: Portable Digital Integrator” AT-MAS/PG/fm, Technical note 93-50, January 1993 [3] L. Poujol “Documentation: regulation of a superconducting transformer” AT-MAS/FRESCA doc., June 2001 [4] A.P. Verweij “Labview analysis of an UI measurement in FRESCA” AT-MAS/FRESCA doc., March 1999 [5] P. Legrand “Documentation: Module Potentiel Aimant” AT-MA/PL, AT680-2029-050, Technical note 91-21, December 1991 “Module Potentiel Aimant” AT680-2029-070, Version 3, February 1997 [6] C. Berriaud, A. Donati “A device for measuring high current at cryogenic temperatures”
CEA Saclay / DAPNIA / SACM
- 29 -
Annex 1 Here is detailed the current regulation system of the transformer. All components of the regulation system are shown, as well as the auto-maintained relays controlled by Labview used for the transformer operation and protection. Contents: 1.regulation loop 1.1.Corrector electronic board 1.2.Feedback electronic board 1.3.Definitive settings of adjustable components 2.Automaintaining resets (controlled by Labview) 2.1.Reset “control voltage = 0” 2.2.Corrector integrator reset 2.3.Quench indicator reset 3.Emergency Stop buttons 4.SI selection box
- 30 -
1.Regulation loop:
+
-
+
-+
-
+
-
+-
+-
+-
+-
100kΩ
100kΩ
100kΩ
200kΩ 100kΩ
100kΩ100kΩ
100k
Ω
100k
Ω10
kΩ
10kΩ
10kΩ
10kΩ
100kΩ
1µF
1µF
1µF
100k
Ω
adjustable
adjustable
Yokogawa
attenuator
Lakeshore Power Supply
G = 100 A/V
SuperconductingTransformer
Rogowski coilsdigital
integrator
inverter follower
corrector electronic board
feed
back
ele
ctro
nic
boar
d
inne
rcry
osta
t
follower
attenuator
active filter
comparator
integrator
1.1.Corrector electronic board: Supply requirements: +/- 15V Electronic functions:
- Attenuator with follower Two positions available: pos.1 and pos. 2 (selected by a switch, with luminescent diode indicator) Gains adjustable with a setscrew for each position High impedance output
- Integrator for integral regulation Integration/ drift adjustment mode (with a setscrew) Two positions available (two time constant) RC1 and RC2 (selected by a switch, with luminescent diode indicator) Reset function
- 31 -
1.2.Feedback electronic board: Supply requirements: +/- 15V Electronic functions:
- Active filter - Follower - Inverter - Attenuator #1 (adjustable gain) - Attenuator #2 (adjustable gain) (*) - Amplifier (adjustable gain from –1000 to –1) (*) - Comparator with gain of 1 (and adjustable offset)
(*): not used 1.3.Definitive settings of adjustable components: Corrector board: Proportional amplifier Attenuator Pos. 1 G = -0.17 Integrator RC1 Note that the drift of the corrector integrator has to be checked every day. Feedback loop: Proportional amplifier Attenuator 1 G = -0.0693
- 32 -
2.Automaintaining resets (controlled by Labview): 2.1.Reset “control voltage = 0”:
15 V
Reset Auto-maintien (back regulation box)
mIp (from corrector output )
Ipm
1 2 3 4 Burndy 4 pins (back regulation box)
from PR (1,2) via box
Rel
ay M
T2
to Lakeshore back
from box (11, 14 on relay MT323024)
RI+RI-
DO channel 5, open = reset(default = close)
not used
location: transformer rack
(Colours used above are the ones of the wires, located in the transformer rack) This auto-maintained relay is used to force to 0V the control voltage sent from the regulation system to the Lakeshore power supply. The control voltage is forced to 0 until reset from the Labview control software (DO 5).
- 33 -
2.2.Corrector reset:
D1C3D3
CNCNO
scanner (DO channel 6):open
close
C = NC
C = NO
: no reset
: reset
15 V
Rel
ay M
T2
on corrector board electronic card
DO channel 6, close = reset(default = close)
location: transformer rack
The corrector reset is used to force to 0 the output of the corrector integrator, thus sending a control voltage equal to 0 to the Lakeshore power supply. The reset is released by Labview (DO 6) just before a run (ramp to quench, cycle…) when the system is ready to go and is re-activated just after. 2.3.Quench indicator reset:
Slow security box rear panel
Matrice Securité rear panel
Outputs
card1
Interout
inputs
to DI7 (MR)
Active modules for Card 1: 1-6Active modules for Card 2: 1-6, 16Active modules for Card 3: 1-6, 16
card2card3
14 15 16
DO channel 7, close = reset(default = open)
(from MR)
cards outputsMatrice sécurité inputs
Card 16 output: red : output = 5Vgreen : output = 0V
0V
15V
(Matrice sécurité inputs 1 to 10 come from quench detection modules)
(Quench indicator)
…
location: protection rack
(Colours used above are the ones of the wires, located in the protection rack)
- 34 -
Three electronic cards are connected to the quench detection modules: Card 1 triggers the protection system Card 2 is used as a quench indicator for the Labview control system (DI 7) Card 3 is used for Card 2 auto-maintain. An auto-maintaining system is needed because the pulse given by Card 2 in case of a quench detected is too fast to be read by Labview. The Labview control software resets the quench indicator at the beginning of each run (DO 7). 3.Emergency Stop buttons:
The emergency stop button located in the control room is connected to the protection rack. Pushing this alarm button will de-energize the transformer (control voltage set to 0V and firing secondary heaters) Note that the heaters will remain powered as long as the emergency stop button is pushed (+16 s). Do not leave the emergency stop button pushed especially during cryostat warming up!
The emergency stop button located on the transformer rack sets the control voltage of the Lakeshore power supply to 0V when pushed. It has no action on the heaters. Use this button for a safe starting at beginning of a test or for any operation when you need to be sure the transformer is and remains de-energized. 4.SI selection box: Two types of sample inserts are used on the FRESCA test station: SI 1, 2 (copper current leads) and SI 3 (transformer). The protection system has then to handle these two types of insert and to be easily switchable from one protection scheme to another: Module # 1 is used for S.I. 1, 2 and S.I.3 but with different cabling Module # 2A is used only for S.I. 1, 2. It must be shortened if S.I. 3 is used. Module # 2B is used for S.I. 1, 2 and S.I.3 with same cabling Module # 3A is used only for S.I. 1, 2. It must be shortened if S.I. 3 is used. Module # 3B is used for S.I. 1, 2 and S.I.3 with same cabling Modules # 4, 5, 6 are used only for S.I.3. They must be shortened if S.I. 1 or 2 is used. Modules # 7, 8, 9, 10 are used only for quench localisation in the samples and not for protection. The voltage taps signals coming from the cryostat (transformer, copper leads, sample) are connected to a so-called “S.I. selection box” that distributes the signals to the quench
- 35 -
detection modules according what type of sample insert is selected (button on SI selection box front panel, PR).
2VT202
1 1VT241VT20
1VT192VT214
3 1VT211VT25
2VT252VT246
5 2VT19
7 1VT23
1VT2413
9 2VT23
2VT0115
14 2VT24
b20
19 a
d22
21 c
f24
23 e
25
28
27
30
31
34
33
36
24 V
ON SI 3OFF SI 1 & 2
switch:
5,6 kΩ
lower card slow security box (SI 1&2)upper card slow security box (SI 1&2)
led on: SI 3 selectedled on: SI 1&2 selected
10
1VT19
11 12
2VT19
16
1VT20
17
2VT20
18
2VT01
module “potentiel aimant” input (2*3 input/ module; 10 modules)
1A1B
2A3 A
4A4 B
2B 3B
24 V
MT2 relay
g
hi
j
k
lm
n (24V for leds)
5A
5B
6A
6B
Location: protection rack
Sample Insert selection box The MT2 relays used in the S.I. selection box withstand only 750 V pin to pin, now in case of a quench of the primary winding, the tensions can be higher. In order to protect the MT2 relays, some high voltage resistors are used to decrease the voltage level.
- 36 -
10MΩ
10MΩ
10MΩ
10MΩ
1MΩ
1MΩ
1MΩ
1MΩ
1VT33
1VT31
2VT30
2VT31
2VT33
g
h
i
j
k
l
m
n
10MΩ
10MΩ
1MΩ
1MΩ
1MΩ
10MΩ
1MΩ
10MΩ
1VT34
a
1VT34
b
c
1VT32
d
1VT30
2VT32
e
f
4A4B
5A
5B
6A
6B
Resistor bridges for protection of MT2 relays The quench detection modules used for the protection of the sample insert with transformer (S.I. 3) are connected as summarized below: Module Channel Signal Comments 1 A 1VT20
2VT20 1VT25
transformer secondary 1differential 3 wires
1 B 1VT19 2VT19 2VT25
transformer secondary 2differential 3 wires
Module Channel Signal Comments 2 B 1VT19
- 2VT19
total sample
Module Channel Signal Comments 3 B 1VT20
2VT20 2VT01
total sample differential 3 wires
- 37 -
Module Channel Signal Comments 4 A 1VT34
2VT34 1VT20
transformer primary 1differential 3 wires
4 B 1VT32 2VT32 1VT30
transformer primary 2differential 3 wires
Module Channel Signal Comments 5 A 1VT33
- 1VT31
transformer primary 1st quarter
5 B 1VT31 -
2VT30
transformer primary 2nd quarter
Module Channel Signal Comments 6 A 2VT30
- 2VT31
transformer primary 3rd quarter
6 B 2VT31 -
2VT33
transformer primary 4th quarter
If any of a modules “potentiels aimant” triggers off, a so-called “interout” signal is sent to the slow security box where actions are taken to protect the transformer.
- 38 -
Annex 2 Here is shown an analytical modelisation of the regulation system of the transformer. The equations describing the functioning of the transformer are:
⎢⎢⎢⎢
⎣
⎡
+−=
+=
dtdI
LUIRdt
dIM
dtdI
LIRdt
dIM
primprimprimprim
totprim
.
.
sec
secsecsec
(1)
with Iprim, Isec: primary, secondary current Rprim, Rsec: primary, secondary resistance Lprim, Ltot: primary, secondary total inductance M: mutual inductance between transformer primary and secondary U: voltage of the power supply feeding the primary of the transformer The secondary resistance is due to the connection between the sample and the secondary of the transformer. The total inductance of the secondary is the inductance of the whole secondary loop, that is transformer secondary + sample. The first equation of (1) gives the Laplace transfer function of the transformer:
sec.)(
RpLMppH
tot += (2)
-KcTc.p-1 Kl
Ltot.p + Rsec
M.p
Lrog.p
p1
1 + m.Ta.p + Ta2.p2
1-1-Kr
active filterinverter
attenuator digital integrator
Rogowski coils
transformer
LakeshoreintegratorattenuatorYokogawa
control voltage
feedback
correctorIprim
Isec
Analytical modelisation of the regulation loop
- 39 -
Numerical values: Kc = 0.17 corrector (gain) Tc = 0.1s corrector (integrator) Kl = 100 A/V Lakeshore power supply (acts like a A/V amplifier) M = 8.77 mH mutual inductance of the transformer Ltot = 10 µH self-inductance of the transformer secondary Rsec = 5 nΩ resistance of the transformer secondary Lrog = 0.1638 mV (at dIsec/dt = 1A/s) Pick-up voltage of the Rogowski coils Ta = 0.1 s Active filter m = 1.9 Kr = 0.0693 Feedback loop (attenuator) The transfer functions in open and closed loop (FTBO and FTBF) of the whole system read:
)1)(1( 22 pTpmTpTKKFTBO
aaA
BA
+++= (3)
with cs
lcA TR
MKKK = , rogrB LKK = and
s
totA R
LT =
3222
22
][][][]1[)1(
pTTpTmTTpTmTKKpTpmTK
FTBFAaAaaAaBA
aaA
++++++++
= (4)
The poles of the previous transfer functions are: FTBO: p1 = -0.0005 p2 = -9.5 + 3.1225i p3 = -9.5 – 3.1225i FTBF: p1 = -11.6778 p2 = -3.66135 + 1.04452i p3 = -3.66135 - 1.04452i The poles of both transfer functions have real parts that are negative; the system is then stable in both open and closed loop.
- 40 -
The system is still stable for 1 nΩ < Rsec < 50 nΩ which are minimum and maximum resistance of the secondary loop (due to connection between samples and transformer secondary). The system is also stable for 1µH < Ltot < 20µH. The self-inductance of the total secondary loop is extremely stable around 10µH since the sample is fixed inside the sample holder and since the secondary winding is also fixed on the transformer body. This is a first indication that the regulation system will be stable. Looking more precisely the FTBO transfer function we see two time constant TA = 2000s and Ta = 0.1s. The transformer is used in a DC mode, this means that the “pulsation” w (p = iw, although here we cannot speak of a real pulsation) is very low.
The system will more or less behave like a first order system pT
KKpHA
BA
+=
1)( .
We will then observe a constant delay between the signal from the Yokogawa programmable DC source (set voltage) and the signal sent to the Laskeshore power supply, which is characteristic for a first order system.
- 41 -
Annex 3 Here is detailed the so-called SI-selection box. Two types of sample inserts are used on the FRESCA test station: SI 1 and 2 (with copper current leads) and SI 3 (with the transformer). The protection system has then to handle these two types of insert and to be easily switchable from one protection scheme to another: The voltage taps signals coming from the cryostat (transformer, copper leads, sample) are connected to a so-called “S.I. selection box” that distributes the signals to the quench detection modules according to the type of sample insert that is selected (button on SI selection box front panel, PR). . Ten detection modules [5] are used in the protection system: Module # 1 is used for S.I. 1, 2 and S.I.3 but with different cabling Module # 2A is used only for S.I. 1, 2. It must be shortened if S.I. 3 is used. Module # 2B is used for S.I. 1, 2 and S.I.3 with same cabling Module # 3A is used only for S.I. 1, 2. It must be shortened if S.I. 3 is used. Module # 3B is used for S.I. 1, 2 and S.I.3 with same cabling Modules # 4, 5, 6 are used only for S.I.3. They must be shortened if S.I. 1 or 2 is used. Modules # 7, 8, 9, 10 are used only for quench localisation in the samples and not for protection. Signals from the cryostat go to the rear panel of the SI-selection box through 3 Burndy plugs:
- V_LEADS: voltages over the copper current leads (only with SI1 and 2) - TCS2: voltages over the sample and over the connections between the two
samples and the sample insert - TCS9: voltages over the transformer winding (primary and secondary)
(only with SI3) The detailed wiring of V_LEADS, TCS2, TCS9 can be found in the FRESCA documentation (/Mms_seca/Fresca/Wiring).
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2VT202
1 1VT241VT20
1VT192VT214
3 1VT211VT25
2VT252VT246
5 2VT19
7 1VT23
1VT2413
9 2VT23
2VT0115
14 2VT24
b20
19 a
d22
21 c
f24
23 e
25
28
27
30
31
34
33
36
24 V
ON SI 3OFF SI 1 & 2
switch:
5,6 kΩ
lower card slow security box (SI 1&2)upper card slow security box (SI 1&2)
led on: SI 3 selectedled on: SI 1&2 selected
10
1VT19
11 12
2VT19
16
1VT20
17
2VT20
18
2VT01
module “potentiel aimant” input (2*3 input/ module; 10 modules)
1A1B
2A3 A
4A4 B
2B 3B
24 V
MT2 relay
g
hi
j
k
lm
n (24V for leds)
5A
5B
6A
6B
Location: protection rack
Sample Insert selection box The MT2 relays used in the S.I. selection box withstand only 750 V pin to pin, now in case of a quench of the primary winding, the tensions can be higher. In order to protect the MT2 relays, some high voltage resistors are used to decrease the voltage level.
- 43 -
10MΩ
10MΩ
10MΩ
10MΩ
1MΩ
1MΩ
1MΩ
1MΩ
1VT33
1VT31
2VT30
2VT31
2VT33
g
h
i
j
k
l
m
n
10MΩ
10MΩ
1MΩ
1MΩ
1MΩ
10MΩ
1MΩ
10MΩ
1VT34
a
1VT34
b
c
1VT32
d
1VT30
2VT32
e
f
4A4B
5A
5B
6A
6B
Resistor bridges for protection of MT2 relays The quench detection modules used for the protection of the sample insert with transformer (S.I. 3) are connected as summarized below: Module Channel Signal Comments 1 A 1VT20
2VT20 1VT25
transformer secondary 1differential 3 wires
1 B 1VT19 2VT19 2VT25
transformer secondary 2differential 3 wires
Module Channel Signal Comments 2 B 1VT19
- 2VT19
total sample
Module Channel Signal Comments 3 B 1VT20
2VT20 2VT01
total sample differential 3 wires
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Module Channel Signal Comments 4 A 1VT34
2VT34 1VT20
transformer primary 1differential 3 wires
4 B 1VT32 2VT32 1VT30
transformer primary 2differential 3 wires
Module Channel Signal Comments 5 A 1VT33
- 1VT31
transformer primary 1st quarter
5 B 1VT31 -
2VT30
transformer primary 2nd quarter
Module Channel Signal Comments 6 A 2VT30
- 2VT31
transformer primary 3rd quarter
6 B 2VT31 -
2VT33
transformer primary 4th quarter
If any of a modules “potentiels aimant” triggers off, a so-called “interout” signal is sent to the slow security box where actions are taken to protect the transformer.
- 45 -
Annex 4 Here are shown the equations used for the hot-spot estimation in the primary winding in case of a quench. A conservative estimation of the temperature in the winding can be made by considering a unit cell of the winding, where the quench starts:
dTCdtJ pCu =2ρ (1) T: temperature of the unit cell ρCu: resistivity of the copper in the wire: The RRR value of the copper is 82; this gives:
( ) ⎟⎠⎞
⎜⎝⎛ ++= −−
21.10.55.510.064.2 41710 BTCuρ for T < 60K (2)
( ) ⎟⎠⎞
⎜⎝⎛ +−+−= −−−
21.10.477.7.10.074.710.292.3 215119 BTTCuρ for T > 60K (3)
with B: magnetic field applied on the wire.
Cp: specific heat of the wire:
NbTipCupp CCC −− ++
+=
ααα
11
1 (4)
with α: copper to non copper ratio of the wire
TTC Cup .4.97.75.6 3 +=− (5)
BTTC NbTip ..8.69.55.50 3 +=− (6) J: current density in the copper:
CuSIJ = (7)
with I: current in the wire SCu: surface of copper in the wire section.
- 46 -
Annex 5 Here are described the modifications to the Labview flow chart for the operation of the sample insert with the transformer (SI 3). When running the Labview software fresca operating the FRESCA test station, the first step is to load the so-called agenda and traveller files:
- The agenda file holds all parameters defining the runs to be performed (current ramp, current cycle).
- The traveller file holds names of the samples to be tested, the sample insert used (SI 3: transformer), the sample holder used …
At the beginning of an agenda: 1.If SI 3 is used, first a window is shown and asking to confirm that:
- The sample insert used is the one with the transformer - The parameters for SI 3 have been loaded in the cryogenic control software (ABB
system) - The temperature probes located next to the heaters on the transformer secondary
are powered (1TT860 and 2TT860). - The analog output of the digital integrator is connected to the acquisition system
(so that Labview can read the secondary current). 2.Then a second window is displayed asking the voltage to be set on the output 2 of the TTI power supply powering the heaters located on the transformer secondary. The output 1 is automatically set to 32V and output 2 OFF (this output is used to check that the transformer is de-energized). The output 2, connected to the heaters when the protection system is triggered, should be set to 32V. 3.
- Digital output 5 is set to “close” (DO5 is the “control voltage = 0” reset for the protection, see Annex 1).
- Digital output 6 is set to “open” (DO6 is the reset for the integrator of the corrector in the regulation loop, see Annex 1).
4.Set up of the digital integrator:
- Input range = 5V - Output range = 1V.s/V
- 47 -
At the beginning of each run: 1.Check that voltages from 1TT860 and 2TT860 < 1.08mV (T< 5K). 2.Set up of the Lakeshore power supply: Vmax = +/- 5V (compliance voltage, see section protection). 3.Set up of the Yokogawa programmable DC source: current ramp or current cycle parameters are loaded. 4.DO5 = “open” then “close” (reset of “control voltage = 0”). DO6 = “close” then “open” (release the integrator of the corrector). DO7 = “close” then “open” (reset of the “quench indicator”, see Annex 1). 5.Start of the digital integrator. Start of the Yokogawa programmable DC source. At the end of a run: 1.If the sample has not quenched at the end of the current ramp (Digital input 7 not activated, see Annex 1) a ramp down to 0V is ordered on the Yokogawa programmable DC source.
2.If the sample has quenched, the Yokogawa programmable DC source is set to 0V (after it has finished to ramp up). 3.Stop digital integrator 4.The de-energization of the transformer is checked with the following procedure:
- Set primary current to 0A on the Lakeshore power supply. - Reset and start digital integrator for the measurement of secondary – current - Fire secondary heaters during 10s, if ∆Ιsec > 100Α, then the procedure is repeated.
Summary of connections with Labview:
Yokogawa programmable DC source: GPIB 21 Lakeshore power supply: GPIB 17 TTI power supply: GPIB 12 Digital integrator: GPIB 3
Corrector board (reset): DO6 Reset “control voltage = 0”: DO5 Reset “quench indicator”: DO7 Quench indicator: DI7
- 48 -
Annex 6 Here are given some possible improvements of systems using superconducting transformer to provide high current to superconducting cables. First special attention should be paid to the impregnation of the primary winding. The external thickness of the resin, above the primary wire, has to be minimised in order to increase the mutual inductance between secondary and primary. The external surface of the resin in which is embedded the winding has also to be as smooth as possible. This allows to have a good and tight positioning of the secondary cable against the primary. The mechanics of the transformer secondary has to be carefully dealt with, else the magnetic forces at high secondary current will lead to movements of the secondary cable and premature quenching. The bars keeping the secondary cable in place could for example be made-to-measure after the secondary has been winded. The current measuring system (Rogowski coils + digital integrator) could be replaced by a DCCT modified for a functioning in cryogenic conditions [6]. The secondary current is so directly measured; this allows to get rid of the drift problems limiting the run duration, since an integrator is no longer needed. And the precision of the current measurement is increased to 10-4%.
Recommended