40 ka superconducting dc transformer for the fresca test...

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: [email protected]

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


Module Channel Signal Comments 2 B 1VT19

- 2VT19

total sample


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



- 1VT31

transformer primary 1st quarter

5 B 1VT31 -

2VT30

transformer primary 2nd quarter


- 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).

- 42 -

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


1 B 1VT19 2VT19 2VT25



- 2VT19

total sample


2VT20 2VT01

total sample differential 3 wires

- 44 -


2VT34 1VT20


4 B 1VT32 2VT32 1VT30



- 1VT31

transformer primary 1st quarter

5 B 1VT31 -

2VT30

transformer primary 2nd quarter


- 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%.

40 ka superconducting dc transformer for the fresca test...

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