422 IEEE TRANSACTIONS ON MAGNETICS, VOL. 28, NO.l, JANUARY 1992

CRYOTRON CONVERTERS FOR THE SUPPLY OF SUPERCONDUCTING MAGNET WINDINGS

Yu. V . Skobar ikh in, Yu .A. Bashk irov, V.M.Ivanov and Yu.P.Chernavsky

Krzh izhanovsky Power Engineering Institute, Leninsky prospect, 19, 117927 Moscow, USSR

Abstract

Examined are cryotron converters for the supply of superconducting d.c. magnet windings for use in the power industry, transport, medicine, magnetic separation, electrophysical systems, etc. They were made and studied for currents up to 1 kA, while one for 10 kA is being designed. Electronic control systems ensure the operation of the cryotron con- verters in fully automatic modes of inductive commutation at an efficiency of 96-98% in the "cold" zone.

I. INTRODUCTION

Cryotron converters feature advantages such as low losses in the "cold" zone, the absence of high- -current leads (the overall cross-section of the current leads into the "cold" zone is under 1 sq mm) and movable elements, low power consumption, manufacturability, computer compatibility, and others. Superconducting switches - cryotrons whose rectifying effect is based on the superconducting to normal pha- se transition under the action of the magnetic or heat pulse serve as rectifying elements.

The cryotron converters ensure the lead-in, lead- out and stabilisation of current in the superconduc- ting winding as well as the reversal of the magnetic field polarity. With the disconnection of the conver- ter the magnetic system automatically switches to "frozen" flux conditions.

Functionally, these converters comprise the "cold" and "warm"sections connected by low-current

conductors (Fig.1). The "cold" section consists of a superconducting transformer connected to the cryo-

Fig.1. The converter

A.V. Novosel tsev, S . V . Nepogodyev Institute of Energy Conservation, the Ukrainian Academy o f Sciences,

U1 .zei inskogo, 11, 252070 Kiev, USSR

tron rectifier of a two-semiperiod single-phase cir- cuit as well as instrument pickups and protection elements. The "warm" section is made up of electronic control systems whose circuit engineering depends on the converter's functional purposes.

11. ELEMENTS OF THE "COLD" SECTION

A . The cryotrons

Magnetically controlled cryotrons with switching elements of Pb-2.27% Sb foil were used in the first odels designed at the Krzhizhanovsky Power Engine- ering Institute. Its resistance in normal state at 4.2 K was 1.26 mob. The liquid helium-cooled switch is placed inside the control coil made of a 0.33 nun-diameter Nb-Ti wire of 0.02 H inductance. The 1000 A cryotron of 46mm x 60mm x 140mm weighs 0.35 kg

The models with magnetically controlled cryotrons exhibited though some essential drawbacks complica- ting their practical use: poor mass and dimension characteristics, low resistance in the normal state (the cause of relatively low efficiency) and weak protection against leakage fields. The above draw- backs are due to the necessity to use type I super- conductors of low critical magnetic fields in such cryotrons.

Thermally controlled cryotrons with type I1 su- perconductors allow to eliminate these drawbacks, while the cryotron switching into normal state by an initiating heat pulse (under 1 ms) and its further retention in that state by the heat released by the . back current flowing across the gate allows to mini- mize the control-associated heat losses in the "cold" zone to the milliwatt level.

We have designed thermally controlled cryotrons for 100, 1000 and 10000 A transport currents. Nb-Ti wires of close thermal contact with the heaters were used as switching elements. These components were heat-insulated from the coolant. The geometry and thermophysics of the heat insulator have a tangible effect upon the cryotron characteristics such as time for switching into superconducting state tc; energy E m , required for heating the gate (initiation ener- gy) and back current 1st through the gate retaining the latter in resistive state. The cryotron parame- ters can be obtained by means of design relations combining the constructional and physical characteri- stics as well as experimentally by means of instru- ment devices developed at the Krzhizhanovsky Power Engineering Institute. Table 1 offers the main cha- racteristics of thermally controlled cryotrons with .'b-Ti conductors.

[I].

B . SuperconductinE transformers

Transformers without ferromagnetic cores and with two superconducting windings were used in these mo- dels: a multiturn primary one connected to the cont- rol units by low-current wires and a multi-ampere se-

0018-9464/92$03,00 @ 1992 IEEE

423 TABLE 1

CHARACTERISTICS OF THERMALLY CONTROLLED CRYOTRONS

Parameter Rated current

100 A 1000 A 10000 A

Reverse voltage in the gate UR, mV 100 100 60

Gate resistance R, mOhm 230 40 2.0

Back current of the gate Ist, A 0.42 2.4 30

Initiation energy Ein, mJ 0.45 3.5 35

perconducting state tc, ms 6.0 7.0 10.0 Time for switching into su-

Time for switching into normal state tin, ms 0.3 0.3 0.5

Length/diameter, mm 19/0.95 25/1.35 16/12

condary one with a middle tap and two half-windings. cryo-

trons [l] the secondary winding was made of copper- stabilised Nb Sn foil which consisted of two 3-turn

half-windings of 0.187 H. The primary winding was made of a 0.33 m-diameter Nb-Ti wire. It had 3120 turns and 0.089 H inductance.

In converters with thermally controlled cryotrons the secondary windings were made of Nb-Ti wires and buses. Two types of transformers were designed: a coil one with comparable axial and radial dimensions and a ring one whose diameter, with small axial height, exceeds that of the magnet winding. The lat- ter improves the converter layout characteristics. The transformer characteristics of converters with thermally controlled cryotrons are given in Table 2.

In the models with magnetically controlled

3

C. Instrument pickups and protection elements

Voltage pick-ups A, B, C and D (Fig.1) were in stalled in the secondary winding circuit in order to monitor and adjust the converter in the "cold"

TABLE 2

CHARACTERISTICS SUPERCONDUCTING TRANSFORMER

-~ ~

Parameter Current in secondary winding

100 A 1000 A 10000 A

Primary winding

Diameter of Nb-Ti wi- re in Cu matrix, mm 0.33 0.33 0.5 number of turns 3840 156 2240 inductance, H 0.136 0.115 0.481

Secondary winding

Dimensions of Nb-Ti conductor in Cu matrix 8 1 . 2 7 mm 7.0 mmz 7.0 mmz number of turns 19x2 2x2 1x2 number of parallel buses 1 1 10 half-winding induc- tance, J./ H 8.9 0.97 0.108

Dimensions with framework

axial height, m 62.5 22 95

inner diameter, mm 8 112 90 outer diameter, mm 50 138 137

zone. In the absence of direct leads there is no way of conventional measuring the magnet current, and a need arose for indirect current measurement by means of Hall probes. Two Hall probes were mounted on the magnet: one for magnetic field measurement and the other was connected to the control system.

DSch 122-25 type were used to protect the cryotron converter from damage in case of an emergency. These diods are free of the anomalities of V-A characteris- tics at cryogenic temperatures and parallel connec- tion of unidirectional diods is therefore possible to protect high-current magnets (above 150 A).

The Schottky gallium arsenide diods of the

111. THE CONTROL SYSTEM

The system supplies heteropolar rectangular pul- ses to the primary winding and control elements of the cryotrons, forms the given algorithm and controls the pulses ensuring thus operation in the economical mode of inductive commutation [2].

The functional diagram of the control system is given in Fig.1. The system consists of a digital pro- cessor forming the required consecutiveness of pulses (algorithm) and responding to signals from the cont- rol line. Since the above consecutiveness of pulses functionally involves the magnet current recorded by the Hall pickups, the unit of switching pulse forma- tion is used which generates the control signal and is functionally dependent on the magnet current.

driver supplying the primary winding can be made in two versions:(a) with a d.c.amplifier for low-capa- city ones and (b) with voltage invertor for high-ca- pacity ones. In both versions it is required to redu- ce the output resistance of the driver and the cable line as a whole. For version (a) it means negative feedback covering the cable line and for (b) it imp- lies special designs lowering the voltage drop in open invertor switches. The power unit furnished with a capacitive storage compensated the pulsation of current consumed by the power driver.

Depending on the converter capacity the power

IV. THE MAIN DESIGN RELATIONS

Pumping and commutational processes take place during the operation of cryotron converters. In the pumping interval (tu) one of the cryotrons is in nor- mal state, a rectangular supply voltage pulse of amp- litude U is applied to the primary winding of the transformer and rectified pulse Um to the magnet's winding. In the commutational interval (tk) voltage pulse U is applied to the transformer's primary win- ding, with both cryotrons being in a superconducting state and the magnet's winding having a zero voltage. During the pauses (tpauz) the cryotron switching is carried out, and the transformer and the magnet are without voltage. The duration of the semiperiod (cyc- le) t c is determined as a total of: tc = tu + tpauz + tk, and the frequency of the con- verter's operation as fc = 1/(2tc).

In converters with thermally controlled cryo- trons tu and tpuz did not change during the opera- tion, while the duration of the commutational inter- val did according to the linear functional relation to the magnet's current (Im): tk = kt-Im, where kt is coefficient of the control function.

During the converter's operation the current in the magnet's winding changes discretely exclusively in pumping intervals. In the case of tu = const the dependence of current ( I m ) on the number of cycles (n) is linear. With the implementation of algorithm tu = const, tpauz - const, and tk - var the time de- pendence Im(t) will be nonlinear and can be described by arithmetical series.

Analysing the converter's operational effective-

424 ness in inductive commutation mode we can use the relation for efficienc in the "cold" zone: ? = [l + 4 1st / Im ] , whlch takes into account the losses due to back currents in the cryotron gates which amount to about 80 X of the overall losses occurring in the "cold" zone.

-7 .

V. THE PROTOTYPES AND EXPERIMENTAL MODELS

Various models of cryotron converters were de- signed and studied in the past decade at the Krzhizhanovsky Power Engineering Institute. They include one with magnetically controlled cryotrons and a control system ensuring its operation under inductive and resistive commutation. The poor perfor- mance of this model denied its practical employment. The use of thermally controlled cryotrons allowed to build commercial prototypes of converters for 100 A. Another model of this kind has been designed for 1 kA, and a commercial prototype with improved layout characteristics is now being made on its basis. The elements of a converter for 10 kA have been designed whose manufacture is to begin in the near future. The characteristics of converters are given in Table 3.

Work is under way on the design of a converter demonstration prototype with elements of high-Tc ma- terials. The high-current part of this converter is made in the form of a solid construction which allows to exclude ohmic resistance between the elements [ 3 ] .

VI. EXPERIMENTAL RESEARCH

The current input, output and stabilisation in the magnet were investigated, fault conditions were simulated which confirmed the serviceability of the protection unit, and inductive and resistive commu- tation modes were studied. The causes of instability in the operation of converter elements were identi- fied on whose basis optimal algorithms were developed and the unit of the control system were optimized.

Fig.2 shows the experimental dependences of the magnet current on time Im(t) obtained in the model with magnetically controlled cryotrons and demon- strating the converter's ability to reverse the mag- net current. During the tests a current of 1020 A was attained with about 800 J power input into the magnet [l].

Shown in Fig.3 are the Im(t) characteristics for the input and output obtained while investigating a 100-ampere converter connected to a 1.6 H magnet. Stored in the magnet was energy of about 10 kJ.

TABLE 3

CHARACTERISTICS OF CONVERTERS

Parameter Magnet's current

100 A 1000 A 1000 A 10000 A

cryotron control thermal magnetic thermal thermal

magnet induc-

voltage in magnet winding Um, mV 50 20 50 30

frequency fc, Hz 4.3-5 10-13 4.1-4.6 3.0-3.6

kt ,y s/A 160 6.07 21.4 4.96

current and 3 A 5 A 8.3 A 17 A voltage supply to 6.5 V 25 V 18 V 85 V the transformer efficiency 0.97 0.85 0 . 9 8 0.98

tance, H 1.6 0.0015 0,0015 -

800 d - 600

; 400 ~ 200

+I

Z o :: -200 -400

; -600 VI

0 6 -800 2 -1000

0 20 40 60 80 100 120

Time (sec)

Fig.2. The experimental curves of the reversal of magnet's current

20 40 60 80 100 120 140 Time (min)

Fig.3. The experimental dependences of h(t) for different modes

Fig.4 offers the oscillograms of current ip in the transformer's primary winding and voltage Um in the magnet's winding to the following scale: ip - 2 A/mark and Um - 50 mV/mark. Different forms of cur- rent ip and polarity of voltage pulses Um are charac- teristic for the input and output conditions. A chan- ge in the relative voltage pulse duration is observed with the growth of the magnet's current Im. The insi- gnificant switching excursions on the voltage oscil- lograms are the result of imperfect adjustment of the inductive commutation modes (the deviation from kt is about 1-2 X ) .

VII. CONCLUSION

The work on cryotron converters has advanced by now beyond the purely research stage, and their full- -scale application will be soon possible in systems based on superconducting magnets.

The possibility of controlling the discrete quan- tity of current input into the magnet's winding will allow to build precision devices on the basis of such

425

C ) d)

Fig.4. The oscillograms o f ip and Um for the lead-in and lead-out modes at different currents in the magnet: a) lead-in at 1 ~ 5 0 A : b) lead-in at Im=100A; c) lead-out at Iw100A; d ) lead-out at Im=SOA.

magnets within an accuracy of current setting several orders higher than by using the conventional power sources.

REFERENCES

[l] V.Ye.Ignatov, A.V.Kuzmin, Yu.V.Skobarikhin. Ex- perimental model for a 1000 A superconducting converter// Elektrichestvo. 1983. N 4 , p.56-57

[ Z ] Introduction into the Theory of static super- conducting converters/ V.Ye.Tonka1. A.V.Novo- seltsev, Yu.V.Skobarikhin, S.V.Nepogodyev. Kiev, Naukova Dumka, 1990.

[ 3 ] The design of high-Tc converters./ S.V.Nepogo- dyev, A.V.Novoseltsev, Yu.M.Klepov, Yu.V.Skoba. rikhin, D.P.Moiseyev// In: Vysokotemperaturnaya sverhprovodimost. 1990, 2nd ed., p.91-98.