QUENCH SIMULATION AND PROTECTION SYSTEM OF HIGH CURRENT SUPERCONDUCTING RECTIFIERS

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QUENCH SIMULATION AND PROTECTION SYSTEM OF HIGH CURRENT

SUPERCONDUCTING RECTIFIERS

H.H.J. ten Kate, A. Holtslag, L.J.M. van de Klundert

To cite this version:

H.H.J. ten Kate, A. Holtslag, L.J.M. van de Klundert. QUENCH SIMULATION AND PROTEC-

TION SYSTEM OF HIGH CURRENT SUPERCONDUCTING RECTIFIERS. Journal de Physique

Colloques, 1984, 45 (C1), pp.C1-663-C1-666. �10.1051/jphyscol:19841134�. �jpa-00223605�

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JOURNAL DE PHYSIQUE

Colloque C1, suppl6rnent au n o 1, Tome 45, janvier 1984 page Cl-663

QUENCH S I M U L A T I O N AND PROTECTION SYSTEM OF H I G H CURRENT SUPERCONDUCTING RECTIFIERS+

H.H.J. ten Kate, A.H.M. Holtslag and L.J.M. Ivan de Klundert

!&)ente University of TechnoZogy, Department of Applied Physics, P.O.B. 217, 7500 AE Enschede, The NetherZands

RGsumB - Le comportement d'un syst&me de protection est simule pendant un fonctionnement anormal d'un redresseur 21 25 k ~ . Des r6sultats typiques du comportement de quench sont pr6sentgs.

Abstract

-

The behaviour of a protection system has been simulated during fault operation of a 25 kA rectifier. Typical results of the quench behaviour are presented.

INTRODUCTION

Since several years the application of superconducting rectifiers as cryogenic power supplies for superconducting magnets is studied in our laboratories /1-6/.

This paper describes a protection system which applies a superconducting protection switch in order to force the current in the magnet to flow through a dumpresistor

(Fig. 1). In'this way the energy content of the magnet is dumped outside the helium- bath for the greater part without damaging the rectifier or the coil. The behaviour of this protection system has been simulated. Results are presented below. The reader is referred to references 1-6 for more details about superconducting rectifiers them- selves and the present state of the investigations.

REASONS TO PROTECT A S.C. RECTIFIER

A protection system for a superconducting rectifier has to prevent excessive heating of the conductors after a quench. Especially the superconductors in the rectifier switches which have a coppernickel matrix are very sensitive for this.

I sc r e c t i f ~ e r I protection 1 load magnet

- I - - 7

- u ,

+-M, k,-~,+m k2 ^,-uu.-,,

n p u y transformer , switches dump, (md surroundlngs

Fig. 1 - Full wave s.c. rectifier con- Fig. 2

-

Substitutional scheme of the nected to a s.c. magnet via a protection circuit in Fig. 1.

switch Spr and a dump resistor.

* ~ h e s e investigations in the program of the Foundation for ~undamental Research on Matter (FOM) have been supported (in part) by the Foundation for Technical Research (STW), future technical science branch/division of the Netherlands Organisation for the Advancement of pure research (zwO).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841134

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C1-664 JOURNAL DE PHYSIQUE

The critical heating is determined by the material constants of t,he superconducting wire which dictate a maximum load by current density J and time ,6f exposure.

When the current in the rectifier circuit decreases exponentially with a time constant T during the dumpprocess, the condition for the initial current density becomes:

heat and the resistivity of the s.c. wire respectively. The permissible load expressed by the constant k is for a typical superconductlng wire with copper nickel (Cu30Ni) matrix (k = 1.2 1015 ~ ~ s / m ~ if ''final = 300 K) about a factor 1500 smaller compared to superconductors with copper matrix. Combinations of permissible values for the current density and the time constant for a final temperature of 300 K are:

This describtion is correct when the heating process is truly adiabatic. If the time constant of the dumpprocess becomes several seconds or more, the heat conductivity will take part and the heat capacity of the structural materials around the conductor becomes available. It has become clear that after a quench the current density in the normal zone has to fall very fast. This is achieved through creation of an alternative p a w of current being the dump resistor connected across the load coil. In absence of a bhotection switch, the resistance % of the dump resistor has to be much smaller thhn half the value of the resistance of a rectifier switch R,. However, this cannot always be accepted. A protection switch with an off-resistance Rpr permits to choose the resistance of the dump resistor much larger. The condition for proper operation is now: Rs < % << Rs/2

+

Rpr.

SIMULATION MODEL OF THE S.C. RECTIFIER

The behaviour of the rectifier is described by a number of differential equations.

These equations are formed by use of the substitutional scheme (Fig. 2). In this scheme we recognize the transformer with its primary inductance Lp, the inductance LS1 and LS2 of both secondary parts of the transformer, the mutual inductance M1 and M2 between the primary and a secondary inductance and the mutual inductance m between both secondaryinductancesLS1 and

LS.

The switches are present by means of the off- resistances RS1 and RS2 and the off-resistance Rpr of the protection switch. Themodel permits the load coil LL to have a magnetic coupllng MSHwith its environment being a metallic coil former, a shield or the cryostat with a time constant TSp = LsH/XSH.

Mutual inductances between the load coil LL and the transformer or the skitches have

secondary circuit are included in LS1 and IS2. The five current loops are represented by five differential equations: \

(L +L ) dt(M1+M2) dt"l 0 0

G t G p

Rs l+Rs2+dtLs R +d (L +m)

sl t sl 0 0

Rsl:dt(Lsl+m) R +R +R -td L

sl pr d t sl -Rd 0

0 0

-Rd Rd+R~+dtL~ 't"s~

0 0 0

d t M s ~ where LS = Lsl+LS2+2m and dt the differential operator d/dt.

In order to solve the equations simultaneously the dependence of the resistance Ri of component i upon time t and temperature T have to be fixed:

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Fig. 3 - A run down after a quench in a 25 k A rectifier. The distribution of the currents, t h ~ dissipated energies and the temperature increase in the components of the rectifier.

R. = R. (T(t))

.

^R.(t) where R. (T) = RiO (1

+

^BliT

+

6 T ~ )

1 I. 2i

Bli,

BZi

constants.

The function Ri(t) determines at what time and in which way a quench occurs and at what time the protection switch opens. For example:

RS1 =

Rs2 = Rg2(1 + BlS2T + B ~ ~(1 ~

-

T^{exp [-(t}~ )

-

^tl)/~ll

R = R0

pr pr(l + BlprT + B ~ p r T ~ ) (1 - exp C-(t

-

^t2)/~21

,

^t2> t I'

At time tl switch S2 opens while S1 is already open. At time t2 the protection switch starts to open. The delay t2

-

tl represents the quench detection time and the response time of the protection switch. The program calculates the currents in the circuit, the dissipated energy in the resistances, the temperature of the conductors in the resistors and reviews finally the distribution of the dissipated energy over the components.

RESULTS FOR A 25 kA RECTIFIER

Next, some typical results for a 25 k~/500 W s.c. rectifier /4,5,6/ which are re- presentative for other high current S.C. rectifiers with a same protection. The rectifier energizes in this case a S.C. magnet with an energy content of 78 kJ at 25 kA. Figure 3 shows the output of a quench simulation whereby Rd = 8 mQ, R& = 100 ma and

~ g

⁼^{4 mQ.}

~ , ~

At the time tl = 10 ms a quench occurs through a creation of a normal zone in switch

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Cl-666 JOURNAL

DE

PHYSIQUE

Fig. 4

-

The distribution of the currents and the dissipated energies in the rectifier when the response time of the protection switch is 10 ms.

S2 while switch S1 is already open. Only 1 ms later, thus very fast after a quench, the protection switch opens atOt = 11 ms. After that the dump process is dominated by the time constant rd % LL/R~ = 32 ms, see Fig. 3. The increases of the temperature are also shown in Fig. 3. The dump resistor, mounted above the helium bath at a reference temperature of 50 K heats up to 180 K. The protection switch heats up from 4.2 K to 150 K. The most important result of the fast opening of the protection switch is the fast fall of the current density in the rectifier switches and the protection switch. The results become worse if the response time of the protection switch (t2 - ti) increases; for example to 10 ms in Fig. 4. The efficiency of the dump process lid is 0.885 which means that 88.5 % of the energy in the magnet is dissipated in the dump resistor.It is evident that for a maximum dump efficiency the conditions Rpr >> % and rd >> T S H have to be fulfilled. A rough indication of the dump efficiency is then given by: rid = (1

+

^%/Rpr)-1. This expression gives for the example discussed above a drmp efficiency of 0.926 while the accurate value obtained with the simulation model i.s 0.885.

FINAL REMARKS

The number of research activities in the field of superconducting rectifiers and their application in superconducting systems are very limited. One of the impedi- ments appear to be the absence of reliable protection systems for the rectifiers and their load-magnets especially at high levels of current and energy content.

With the protection system proposed in this paper a large variety of s.c. magnets may operate safe. Therefore the physics of protection switches for high currents

(10 - 25 kA) having a considerable volume because of the desired off-resistances, have to be studied to ensure that in the near future the application of S.C. rectifiers will be successful.

REFERENCES 1. Klundert, 2. Klundert,

L.J.M. van de and Kate, H.H.J. ten, Cryogenics

2

(1981)195.

L.J.M. van de and Kate, H.H.J. ten, Cryogenics

21

(1981)267.

3. Kate, H.H.J. ten, Bunk, P.B., Steffens, H.A., Klundert, L.J.M. van de, Proc.

MT7 Karlsruhe BRD, IEEE MAG-17 (1981)2067.

4. Kate, H.H.J. ten, Holtslag, A.H.M., Steffens, H.A., Knoben, J., Klundert, L.J.M.

van de, Proc. ICEC 9 KOBE 'JAPAN, Butterworth UK (1982)753.

5. Kate, H.H.J. ten, Holtslag, A.H.M., Knoben, J, Steffens, H.A., Klundert, L.J.M.

van de, Prod. ASC Knoxville USA, IEEE MAG-19 (1983)1059.

6. Kate, H.H.J, ten, Knoben, J., Steffens, H.A., Klundert, L.J.M. van de, Proc.

MT8 Grenoble France, gaper 5N1-02 of this conference.