1600

Modern Protection Systems D.A.Judd, L.J.Kettle, H.Menown, B.P.Newton, N.S.Nicholls and R.Sheldrake. EEV, Chelmsford, Essex, CM1 ZQU, UK. Abstract

In this paper two thyratroo crowbar systems are described. The first, contained in an oil filled tank, is a completesystem to protect a poweredfrom a lOOKV, 2OA d.c. supply. In the secondsysten a CX1722glass thyratron, operating in air, has beenretro-fitted to a test eqnipmt, in place of a spark gap, to protect an (IOT]. Thesupply here is 32KV, 4A d-c. Thethyratron will operateon fault downto 5KV E.T. whereasthe spark-gapwould not trigger below25KV.

Au important paraleter in the choice of the crowbar thyratron is the total conlolbs to be switched. It is often not appreciatedthat mostof the conlolbs conductedby the thyratron coma fror the power-supply follow-on current rather than the initial storage discharge. Keasuremts of these follow-on currents are presented. 1 Introduction

Traditional television andother sirilar low power d.c. and pulse used in Linear Acceleratorsdo not usually needcrowbars. Thestored energy is small and the circuit breakersare usually fast enough to protect the klystron in the event of an arc. This is not the case, however, with high and super power klystrons’, nor with the Klystrode2or the recently developedIOT3. FIG. 2. 100KV CROWBAR

The electrical circuit involving a crowbar is Theheart of the systemis the crowbarthyratron, which well known]and is shownin fig.1. is a five gap double cathodetube, type noaberCX1194B. For very long term, reliable, voltage hold-off in a crowbar application, a rating of only 2OKVper high voltage gap is appropriate. This thyratron has two anxiliary grids, grid 1 and grid 2, at eachend. Grid 1 is driven with a d.c. current of 100x$ and the aonitoring of this current together with the voltage drop across the grid 1 allows a * CROWBAR LOAIJ thyratron ‘ready to fire’ interlock to be incorporated. Like the external trigger signal, this interlock signal is POWER SUPPLY transaitted optically betweenthe floating negative high F1G.f. CROWBAR IN A PRACTICAL CIRCUIT voltage deck and the low level electronics. This is the essential, necessary feature for long terr reliable 2 1OOKV Crowbar System operation of the crowbarsystem.

Thecrowbar systen has beendesigned so that the only The systemwas tested using the well known shorting essential connectionsrequired are high voltage in, high techniquewith 4uFof storage capacitanceand two I voltage out, pulse groundand lains inpnt(l.ZKVA). Direct 109 series . The stray inductancesLI and LZ are triggering fror the fault current, via a Rogowskicoil, is estimted to be approzilately equal and each about 4plI. incorporated and provision is also rade to trigger the These tests show that a short circuiting wire down to crowbarvia an optical link for test purposes. Fig.2 shows 0.0156~~ cross-sectional area (38 SWG)is protected. a photograph of a crowbartank with the thyratroo sub- Oscillograls in fig 10 showthe faolt and the crowbar asseably. currents and showthat the delay betweenapplication of the 1601 and the instant of crowbar conduction is The follow-on current fro1 this particular 0.36~~. Whenthe crowbaris fired externally via the fibre controlled powersupply’ is exceptionally low, especially optic the delay betweenthe low level electronics signal coapared to the results fror the IOT power supply (see and crowbarconduction was 1.2~~. Theminim E.T. at which below), and illustrates the excellent control that can be the crowbarwill fire reliably on an applied short circuit achievedby a thyristor regulated supply. is 15KV, and the minim H.T. at which the crowbar will fire reliably with an external trigger signal is 1OKV. 3 1.O.T Crowbar System The syster has beenshown to hold off 1OOKVreliably for long periods. The syster operated for three weeks Fig.6 is a circuit diagrar of the IOT power supply, continuously, without a thyratron prefire. and fig.7 is a photographof the crowbarsystem, which is The crowbarhas beenoperating satisfactorily since built arounda single gap CX1722glass thyratron. August 1991, and np to larch 1992has clocked up 1010 crowbars so far. The surge current fro1 the storage capacitor whenthe crowbarfires is shownin fig.3. Fig.4 showsonly the follow-on current fro1 the powersupply on- load (i.e. without the capacitor surge). The total conlolbs the crowbarswitches is 0.7CMI, lade up of 0.4C fror the storage capacitor and 0.3C fro1 the powersupply follow-on. CROWBAR LOAD Pig.5 showsthat the ‘let-through’ to the fault is 85. (IOU FIG.6. IOT POWERSUPPLY.

vert: Since the E.T. is 32KVm.ax a single gap thyratron is lKA/div used. Other essential features fror the 1OOKVsysten, such as autonatic triggering fron the fault current and grid 1 current interlock, are also incorporated. A reliable, air horiz: spaced isolation transformeris usedto provide power for lOps/div the thyratron heaters and trigger electronics. The systen could be nademe corpact by using a potted isolation transfoner.

1OOKV 4uF

FIG.3. CURRENTSURGE ON FAULT.

vert: 13.6A/div

horiz: 2.5rs/div

FIG.4. POWERSUPPLY FOLLOW-ON.

! I upper: 2KAldiv

lower: lKA/div

horiz: Sps/div \ 1 1 1 1 1 11OOKV

FIG.! 5.r_LLLI”’ CROWBAR& "LET-THROUGH" FIG. 7. THE IOT CROWBAR. The results aboveillustrate the severe conditions which a crowbarswitch lost often handle. Fig. 8 shows a high forwardpeak current and subsequentcurrent reversal Pert: through the crowbar caused by the underdawped O.lpP 2KAldiv capacitor. The high frequencyoscillations soperilposedon the current pulse are due to the cable between the power horiz: supply and the test equipwent. Zps/div Thepower supply follow-on current is shownin fig. 9. Heasurewentsof the follow-on coulowbsin thyratroo test modulatorsat EKVindicate that 2Cto 3Cis cowwon,so the 30KV level of 5.7C found in this IOT test equipwent is not 16pF exceptionally high.

FIG. 8. CURRENTSURGE ON FAULT. Undercapacitor dischargeconditions, with discharge currents of several kilo-alps, the CX1722is rated at only ~ I 0.6C. Undercrowbar conditions, however,where lost of the I I vert: conductedcharge coaes from the follow-on current, which is 30A/div of the order of tens of alps, then the results presented here indicate that the coulowbrating can be increasedby a horiz: factor of at least 10 for such crowbarservice. lOOws/div 4 Conclusions

FIG. 9. POWERSUPPLY FOLLOW-ON. Two thyratron crowbarsystems, operating under very different conditions, havebeen designed and installed in The autoaatic triggering incorporated into the production test equiprent. Both operateover a wide dynaric thyratron systemgives a delay betweenfault and start of range, and their ability to provide a ‘ready to fire’ thyratron conduction of 0.3~s. Whenthe thyratron is interlock gives a reliability that other devices cannot triggered via the fibre optic, the delay between the provide. initiation of the low level signal and thyratron conduction is 1.2~s. The thyratron will operateon a fault downto 1KV In one of these applications the (31722 glass andwith external triggering downto 5OOVd.c. thyratron has beenshown to satisfactorily handle a high peakforward current, subsequentcurrent reversal, and a Figs.8 and 9 showthe fault current through the high powersnpply follow-on current. The total conducted crowbar thyratron in the equipwent. The total charge chargeof 6.2Cis over ten tines the coulombrating of this switchedis 6.2C, with 0.5Cfror the storage capacitor and thyratron for capacitor discharge conditions. Prow this the reaaining 5.7Cfrom the powersupply follow-on current. result it is concludedthat a realistic coulowbrating for Fig.10 showsthe the ‘let-through’ to the fault in the test the larger CX1194B,under crowbarconditions, will be in circuit to be 1.2wC. excessof 2OC.

The crowbarhas beenoperating satisfactorily since installation in November1991, and up to l’st Harch1992 it 5 References had clockedup 750 firings. 111N.S.Nicholls, E.Henown and R.J.Wheldon. ‘Double EndedHydrogen for CrowbarProtection of upper: High Power TWTSystems’ I 13’th Power Modulator ZOOA/div Symposiur,Buffalo, N.Y., June 1978.

lower: [II D.E.Preist and H.B.Schrader. ‘The Klystrode - An I +-++=&---I ’ 2OOA/div Unusual Transritting Tube with Potential for I I VHF-TV’. Proc. IEEE,Vo1.70, No.11, Nor 1982. horiz: Sps/div [31 G.T.Clayworth, E.P.Bohlen and R.Eeppinstall. ‘Sole Exciting Adventures in The IOT Business’. 3OKV International BroadcastEngineer, Kay 1992. t t t 16pP FIG.lO. CROWBAR& "LET-THROUGH"CURRENTS [41 N.S.Nicholls et al. To be published.