The topic of this paper is the studies that were made in the framework of the renovation project of the CERN SPS accelerator's power converters. This project was initiated because these converters were designed 20 years ago and they will need to work another 20 years, due to the use of the SPS machine as the LHC injector

Ozturk, E

Royer, M

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN - SL DIVISIONCERN-SL-98-076   POThe Novel Control Loops of the PulsedPower Converters for the CERN SPS MachineE. Ozturk*, M. RoyerAbstractThe topic of this paper is the studies that were made in the framework of therenovation project of the CERN SPS accelerator’s power converters. Thisproject was initiated because these converters were designed 20 years ago andthey will need to work another 20 years, due to the use of the SPS machine asthe LHC injector.EP2 Forum, ESRF - Grenoble, France, 21-22 October 1998*) SERCO EUROPE LTD, Teddington, EnglandGeneva, Switzerland01.12.98SL/Div RepsThe Novel Control Loops of the PulsedPower Converters for the CERN SPS MachineE. Ozturk, M. RoyerCERN, Geneva, SwitzerlandIntroductionAlthough the project covers the complete renovation of the converters, thispaper treats only the design of the passive filter and the control loops.During the design, great care has been taken to minimise the dependency ofthe control loops on the load, the power converter’s characteristics (Vmax,Imax) and the passive filter, in order to lessen the down-time of the SPSmachine, during power converter repairs.  Therefore, adjustments on the controlloop cards have been eliminated.Figure 1 depicts control loops of a typical thyristor phase-controlled powerconverter. We will study each control loop in detail.  State feedback structureshave been chosen to facilitate the parameterisation of the control loops.  Theuse of a passive card called “the configurator” containing such parametersallows both modularity and independence.  The use of precision components(op. amps and high-precision resistors) also makes high precision andperformance possible with no adjustments.  Practical results will be given toillustrate the design.IRefControlled RectifierPassive FilterGatecontrolcircuitMainsDampingLOOPVoltageLOOPCurrentLOOP LOADStateObserverFigure 1 : The power converter and its control loop schemeElectronic Damping LoopThe former passive filter was damped by a resistor, which was calculated togive a damping factor of 1.  The passive filter has been modified to enhance the                                           *  Ozturk - SERCO LTD.rejection ratio at 50 Hz and its harmonics.  We achieved this by adopting alightly damped passive filter.  The frequency responses of these two variantsfilters are shown in Fig. 2.-60-50-40-30-20-1001020301 10 100 1000F  (Hz.)dBLightly Damped Passive Filter  Former Passive FilterFigure 2 : Passive filters frequency response comparisonAs a consequence, the passive filter can resonate. To avoid this, the filter isplaced within a control loop.  We have decided to use modern controltechniques, e.g. state feedback.  This technique requires the accessibility of thestate variables, in our case the current through the filter inductance and thecapacitor voltage, the latter being the only available information.  Thus we needto build a state ‘observer’ in order to reconstitute the missing state variable.The scheme of a control loop using a state observer is shown in Figure 3.xk +-vuu cK³BACMy+-++CAx xBP r o c e s sS t a t e  O b s e r v e rS t a t ef e e d -b a c k³Figure 3 : Typical state feedback controller using a state observerModelling the Passive FilterElectrically the passive filter is illustrated in the figure 4.  Using the conventionof this figure, the matrices for the state equations for the passive filter are : 010C BVu   ¸¸¸¸¸¸¹·¨¨¨¨¨¨©§ FCRdFCFCFCRdFCRdFCRdFCLFA121110212121010¸¸¸¸¸¹·¨¨¨¨¨©§ 001LFB0 D¸¸¸¹·¨¨¨©§ 321xxxxFVy  C1FLFRdC2FV B VFx3x2iC1F iC2Fx1Figure 4 : Electrical equivalent circuit of the passive filterKeeping in mind the fact that the passive filter is lightly damped, we can use asecond order approximation.  In this case the model is given in the controllablecanonical form by :»¼º«¬ª baA10, »¼º«¬ª 10B , > @baC  , D = 0.With : )F2CF1C(LF1a , )F2CF1C(LFRdF1Cb .Determination of the ControllerFeed-back Matrix KThe feedback gains of the controller are obtained easily because the model isdescribed in the controllable canonical form.  The feedback matrix K in our case(SISO) is a simple vector of 2 rows : »¼º«¬ª 10kkK .  Let us assume that vZ , v[  arethe natural angular frequency and the damping factors of the closed looprespectively.  In this case the gains are given by :b2kakvv12v0Z[ Z This means that the closed loop bandwidth and damping factor are independentand can be assigned at will.Forward Gain kGenerally state feedback controllers modify the DC gain of the process.  Theforward gain k is used to bring the DC gain of the closed loop system to 0 dB.In our case this gain is given by: k ka 0 1Determination of State ObserverIn order to represent the true state variables, the dynamics of the state observermust be faster than the original process, i.e. the poles of the state observermust be situated more to the left in the s plane than the passive filter poles. Tolimit the noise sensitivity of the state observer we have chosen a factor of fourand an optimum damping factor (0.7) for the fastest settling time.  This gives usthe observer feedback vector »¼º«¬ª 10mmM defined by :1amaba2m2o122ooo0 Z Z[Z The observer natural angular frequency is oZ  and the damping factor is o[ .Numerical ApplicationFor a passive filter at 25 Hz, closed loop bandwidth at 80 Hz, damping factor of0.7 and the bandwidth of the state observer at 100Hz. The parameters are :k0 = 228000,   k1 = 698,   k = 10.24,   m0 = 0.0275,   m1 = 15.Voltage LoopAs we have seen in the previous paragraph, the electronic damping loop has nointegral term in the control law, therefore slow mains disturbances are notrejected by the electronic damping loop.We have added a new controller on top of the electronic damping loop. This iscalled the voltage loop, see figure 5.  The controller herein is made of a simpleintegral term : C(s) = 1/Wi.s  and it is sufficient to fulfil the job of rejecting slowmains disturbances. The integration time is calculated to give a closed-loopbandwidth of 50 Hz.Us+-Controller E. Damping loop +Passive FilterUc Load (R,L)URefFigure 5 : The voltage loopCurrent LoopThe current reference of the power converters is a series of pulses (trapezoids)which implies additional constraint on the current loop; the step response mustbe absolutely without overshoot, while being fast enough to have an acceptableramp-error.  This is achieved by employing a state feedback controller withfeed-forward.  The presence of the feed-forward term in the control law givesanticipation, speed of response and also introduces an adjustable closed-loopzero allowing pole-zero cancellation.  Thus, the closed-loop transfer function isreduced one order and simplified.  The block diagram model of the current loopis shown in figure 6.Taking into account that the closed-loop step response must be withoutovershoot, in order to stay on a pre-characterised hysteresis curve of themagnet, the controller parameters are dimensioned in order to have negativereal-axis poles.  Furthermore, those poles are triple i.e. placed on the samelocation for rapidity.  As stated before, one of the poles is cancelled by the zero.Therefore, the complete closed-loop system can be seen as a second orderplant with double poles.  So the step response is without overshoot.ucmu’cmIref--kwkRk11sTiIxRV maxLimiterUsURéf sVpW115max10 11Iksmmmax WI dV/dt maxLimiterCOTROLLERDCCT + LOAD MODELPOWER CONVERTER+ V. LOOP MODELFigure 6: The current loopLimitersActive limiters are used to respect physical limits of the power converter.Without this respect, the controller will fall into saturation, thus loosing control ofthe power converter and can lead to converter instability.Vmax LimiterThis sub-circuit limits the reference signal of the voltage loop within the powerconverter voltage capability.  In case of limitation, a signal is fed to theintegrator to limit its output.  When the limitation condition disappears, thissignal returns to 0.dV/dt LimiterBecause the rectifying bridge cannot change instantaneously from rectifiermode to inverter mode, it is necessary to limit the rate of change of theelectronic damping loop input.  This is equivalent to limiting the power converteroutput’s rate of change.  This limiter is also an active circuit, i.e. when limitationoccurs a signal is fed to the integrator, limiting its output and thus avoiding lossof control.The Loops CardThe analogue behavioural model, without limiting circuit, is given in figure 7.This system can be realised using op. amps and analogue multipliers.  In asystem where a parameter depends on the power converter characteristics oron the load, an analogue multiplier is used and a potentiometer is placed on apassive card called the “configurator” for adjustment purposes.k F(s)++-vu Usuc110sm a+-kk a0 1 0 00 1 m b km ak1111sb m( )akbk10m kb m k1 11 1( )++cx2cx1kb m k11 1( )1Wi s+-uRef1T si kRkd1+-+-kW+IRefsksIVmmW 112maxmaxIFigure 7 : Analogue behavioural model of the whole systemResultsTime Domain Response0123450 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5Time (s)VInput OutputFigure 8 : Step response of the current loopFrequency Domain Responses-80-70-60-50-40-30-20-100100.1 1 10 100 F in HzdB-400-350-300-250-200-150-100-500DegreesAmplitude PhaseFigure 9 : Current loop response-80-60-40-20020401 10 100 1000F in HzdBElectronic D. Loop Gain Passive Filter GainFigure 10 : Passive filter and electronic damping loop responses-80-70-60-50-40-30-20-100101 10 100 1000F in HzdB-400-350-300-250-200-150-100-500DegreesAmplitude Phase Figure 11 : Electronic damping loop response-80-70-60-50-40-30-20-100101 10 100 1000 F in HzdB-400-350-300-250-200-150-100-500DegreesAmplitude PhaseFigure 12 : Voltage loop responseConclusionAs the loops card has all the adjustments placed on the configurator card, it isindependent of the power converter and is thus interchangeable.  Therefore, themaintenance is easy and could be done by non-specialist personnel.  Acompletely parameterised control loop allows the change of the loadparameters against the small penalty of voltage level adjustments.The lightly damped passive filter has greatly reduced the output ripple.  Themodelling of the whole power converter would enable a rapid porting of thecontrol loops into the digital domain.Practical implementation of the control loop has pointed out the essential role ofthe active limiters, which keep the system under control (avoiding controllerwindup) when transients occur.ReferencesB.C. Kuo Automatic control systems.Edition : Prentice Hall Collection : International Editions.  SeventhEdition 1995.H. Bühler Conception de systèmes automatiques.Edition Presses Polytechniques et Universitaires Romandes.Collection électricité.E. Ozturk Commande de convertisseurs de puissance pulsés.Mémoire CNAM 1998.

The Novel Control Loops of the Pulsed Power Converters for the CERN SPS Machine

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