Automatic water level control in steam generators is currently achieved via a three-element controller. This algorithm is based on the measurements of level, steam flow, and feedwater flow. Unfortunately, at low power the feedwater flow signal is highly unreliable, forcing the transfer to manual control. A large number of reactor trips occur under these conditions, and the nuclear industry has shown a concern for this problem. This paper proposes and validates an alternative automatic control algorithm. This new algorithm does not rely on flow signals; it uses instead the pressure measurement in the steam header. A level set point modulation is introduced that allows the algorithm to compensate for shrink and swell phenomena. The standard {Delta}-P algorithm to control feedwater pump speed also has been modified to achieve improved performance and integration. 2 refs., 12 figs

March-Leuba, C.

March-Leuba, J.

Wilson, T.L.

Wilson, T. L.

UNT Digital Library

CONF-900143—3DE90 003710A NEW APPROACH TO CONTROLLING THE WATER LEVELOF U-TUBE STEAM GENERATORSG March-LeubaJ. March-LeubaT. WilsonOak Ridge National Laboratory'Post Office Box 2008Oak Ridge, TN 37831-43010To be presented at1989 Nuclear Science SymposiumJanuary 24-26, 1990San Francisco, CaliforniaOuLip I • I^i 11 mOW CORtTBCT NO. Dt**C06-«40R2-i»00.. ftcaxorvy. C» U.S.Gownmem .'«»r>« » ncm«ia«ft».I acsni* to BUDitn or igcoOucaEKXjaaĴ J torm ni Ois ccr;i ocw» 10 ao » . 'or U.SDISCLAIMERTbis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United Slates Government nor any agency thereof, nor any of theiremployees, makec any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.'Operated by Martin Marietta Energy Systems, Inc., for the U.S. Department of Energy underContract No. DE-AC05-84OR21400.DISTRIBUTION OF THIS DOCA NEW APPROACH TO CONTROLLING THE WATER LEVELOF U-TUBE STEAM GENERATORSC March-LeubaJ. March-LeubaT. L. WilsonOak Ridge National LaboratoryPost Office Box 2008Oak Ridge, TN 37831-45010ABSTRACTAutomatic water level control in steam generators iscurrently achieved via a three-element controller. Thisalgorithm is rised on the measurements of level, steamflow, and feec«ater flow. Unfortunately, at low power thefeedwater fie* signal is highly unreliable, forcing thetransfer tc ir.a~ual control. A large number of reactortrips occur _r.der these conditions, and the nuclearindustry has ir.ov.-n a concern for this problem.This paper proposes and validates an alternativeautomatic ccr.:rol algorithm. This new algorithm doesnot rely on f.ow signals: it uses instead the pressuremeasurement :n the steam header. A level set pointmodulation _; introduced that allows the algorithm tocompensate : : : shrink and swell phenomena. Thestandard ;.-? ilprithm to control feedwater pump speedalso has beer, modified to achieve improved performanceand integra:. :r.STEAM GENERATOR DYNAMICSU-tuire ::earn generator (SGi -.-namics is veryattractive ~:z~ :ne mathematical point of view, but it isa nightmare : : : the plant operator. Its dynamic behavioris character:::-: fcy strong nonlinearities at low power andby the so-ci..ed "nonminimura phase" problem. Botheffects ne-ec :: be compensated by the control designer.Penur:i:::>ns in pressure or enthalpy can put the SGwater level s::r.il and the water inventory out of phase forlarge periodi 10 to 100 sj. This might mean that forseveral seccr.is the amount of water introduced into asystem is greyer than the amount of steam leaving it, andthe water ie-=. may be decreasing instead of increasing.This can dra-atically affect the controllability of thesysiem and - . a be vaken \ii\o accovmi.Out-of-p.-.iie perturbations can be introduced bychanges in 1- steam flow, (2) primary enthalpy, or(3) feedwate: =nthalpy. These phenomena are callednonminimur. :r,ase or shrink and swell problems."Operated by Martin Marieoa Energy Systems, Ino,for ibe US. DeoartEKStt of Energy under ContractNo. DE-AC05-iiOR21400.In addition., the strong nonlinearities present in thesystem can affect controller., design. If the system islinearized, one can get the false impression of having astable model. The system may be stable, actually, undersmall perturbations, but a large perturbation can make itunstable. This effect is more evident at low power, andit seems to be the most important source of failure infeedwater control designs.EXISTING CONTROLLERSAt present the following three control options areavailable to the nuclear industry.One-Element ControllerMinimization of the level error using a standard PED(Proportional-Integral Derivative) module leads to theone-element controller. The out-of-phase period of thisdesign can lead to instabilities.Based on this approach, some naive designs havebeen proposed for low-power operation. These designscalculate the gains for the PID level controller based ona linearized model, which may give the misleadingimpression of a stable system. Even if this were true, thePID gains would be so small that the system wouldrespond too slowly to typical power ramps.Three-Element ControllerFigure 1 shows a block diagram of a three-elementcontroller. Not only the level error is minimized, but alsothe difference between the steam and feedwater flows. Inthis way, the out-of-phase periods are compensated to acertain extent, making this design highly effective.Unfortunately, at low power trie feedwater flow signal isunreliable; therefore, the three-element controller cannotbe used.The Belgian Level ControllerBelgian nuclear power plants use an automaticfeedwater control that can perform under any operatingconditions[l]. It is based on an adaptive three-elementdesign that includes a shrink and swell compensator.When the feedwaier flow sienal becomes unreliable at lowMinimizes level error andflow differencelevelIsetPIdiff flowPI1 actuatorFig. 1. Diagram of a three-element controller.level (ft)45. -4 tripping out turbine42.240.level set point 100%level set point 20%38. - tripping out reactorFig. 2. Steam generator level scale.power, the Belgian design solves this problem by using aone-element controller with a new shrink and swellcompensator.The compensators for low power and for full powerare different. At full power the steam flow signal is used,and at low power the wide-range level signal is used. Wehave been unable to get detailed information on how thesignals actually work, but the operational experience seemsto be satisfactory.LEVEL SET POINT MODULATIONThis section presents a :the water level automaticsconditions. The algorithm iscontroller, but it uses a letechnique to compensate forThe model used to persimulation is presented, and :action .' the one-element ccshown how the gains requi:power ramps are too big iunstable at low power.modulation technique is-.ovel algorithm to controlIy under any operatingrased on the one-element• el set point modulation: ut-of-phase phenomena,•form the steam generatorhe SG dynamics under ther/.roller is described. It ised to follow the standardnd can make the systeminally, a level set pointe to compensate for theOne-Elemau Controller DynamicsUsing the standard PID modules of the MMS library,a one-element controller was added to the model. Apower ramp of 0.05%/s was simulated.The PID module was adjusted to keep the level nearthe set point during the transient. Figure 3 graphicallyrepresents the results.To keep the level error small, it was necessary tohave strong gains. The power ramp used is the onerequired by the nuclear industry for its level controlalgorithms.A strong gain keeps the level near the set point, butit also can make the system unstable. Figure 4 shows theresults of a step change in steam flow at 100% power. Ifsteam flow is reduced, steam pressure increases, inducinga contraction of the steam bubbles in the water. At thesame time, the level controller is acting ia the oppositedirection: feedwaier flow is increasing v-hsn the steam flowhas been reduced for a period of about 30 s. This out-of-phase period will produce a large overshoot.nonminimum phase phenomenon.Steam Generaxor ModelThe Modular Modeling 5%-stem (MMS) library[2] hasbeen used to develop the SG model. The modules usedrepresent the following:1. Feedwaier pump (includir.g feedwaier pump turbinedrive and turbine valvej2. Main feedwater valve and actuator3. U-tube steam generator4. Connectors and pipesThe parameters used for this model are taken from aCombustion Engineering P\VR (Arkansas Nuclear One-2).Figure 2 is a graphical representation of the level scalesin this SG.level (ti)4 3 -ptwerI**«I l « t 1JO.BI"0.6-0.5i-0.40 100 200 300 400 500 600 700 800 900 1000 1100 1200time (s)100% 10 50% pow*rFig. 3. Steam generator response to a power ramp forthe ooe-elcmeru controller.4 1 ^ _level (It) Lset = LsetQ (1)where L? is a modulated correction factor, which weobtain from4 3 t-!S70 SO WO 150 200 2M 300 350 400time (s)l l t lm now ttcm 5 92tB lo 5 »BFig. 4. Steam generator response to a steam flow stepperturbation at 100% power.level lit!42 I-0 5: -:3 '.SO 240 300 360 420 480 540 600time (s)• leim 1>cw • • : - • Ste 10 1 ;S»8Fig. 5. Steam generator response to a steam flow stepperturbation at 25% power.dLfldt = -Lrtr, - (2)At low power the results of this overshoot arecaiasiroprj:. Figure 5 presents a simulation of a stepchange in steam flow at 25% power. Although this timethe step is s-ailer in absolute value, the system becomesunstable. Moreover, the instability has a nonlinear nature.The linearization around the set point, however, wouldlead us to believe that the system is stable.Level Set Point ModulationWe have jeen that the gains necessary to follow apower ramp are too large and that the SG may becomeinsiable. This instability is mainly induced by the om-of-phase phenomenon: the level signal can be decreasingwhile the water inventory is increasing. It is necessary tocompensate the level controller in some way if a largeperturbation is introduced into the system.We have introduced a level set modulation techniqueto avoid these problems. We define Lset0 as the desiredlevel set point. In our algorithm, the actual level setpoint sent to the one-element controller will be Lset,defined as:where P is the steam header pressure signal and K^ andrp are positive gains to be adjusted by the controldesigner.Simulation ResultsThe level set modulation technique was implementedon the MMS steam generator model. A standard PIDmodule was used for the one-element controller, and thepressure signal was filtered (low-pass) before taking thenumerical derivative. The derivative was filtered againthrough a band limiter because we are interested incompensating only for large perturbations.Figure 6 graphically displays the simulation results at100% power. Figure 7 compares the dynamic behaviorof the system: Fig. 7a without level modulation, andFig. 7b with level modulation.The effect of the compensation is more dramatic atlow power. Figure 8 presents the simulation results at25% power, and Fig. 9 compares them with the onesobtained without modulation.PUMP SPEED CONTROLLERPump speed is controlled using the A-P algorithm.The idea is to operate the pump in such a way that thedifference in pressure between the steam header and thepump exit is almost constant. The problem in this designwill be that the pump speed controller does not use thelevel error information: therefore, in some specialscenarios it can act against the level controller.I n e l (It)43v/SO 100 ISO 200 250time (s)300 350 400• Kara Hoar ttoa an lo 3 OtOFig. 6. Steam generator level response to a steam flowstep perrurbauoa at 100% power with level modulation.SO 1» HO OT 260 300 380 400 0 60 120 WO 240 300 MO 42O 480 640 WOtime (a)• » flea ti*a 11*1 » < >*••tl(tl)(b)48tonl (II)•• tot feint (b) —IHM - I M J42 •I.414241SO 100 ISO 200 260 3O0 360 400 0 80 120 HO 240 300 390 420 4*0 S4O WOtime (a) time (a)i » Item tram I M c i C>* • • • • • fk» fm 1 <•* to Lib!Fie. 7. Comparison between the steam generator F l S- 9 - Comparison of steam generator respoosc: 25%response at 100% power (a) without level modulation and P°*er (a) a i a ) o u t k r v c l ™xl"la^n and (b) with level(b)'w0i kr.-el modulation. modulation on.• A valve set program is introducedis "'e that substitutes the delta-P controller•53 l O 300 250 T O <B0 £40 6&0time fs)Hecn ' ; • --ir" ' ! e * 's l i l t f iFig. 8. Steam generator level response to a steam flowsie.p ptroirtauoo: 25% power wilii te«e\ loodu'iiiotLIn our design, we have changed the control conceptfor the pumps. Two factors will contribute to thecontroller: (1) a valve set program and (2) the level error.Figure 10 is a diagram of the valve set program.Given a steam flow signal (a power signal also could beused), a function generator sets the optimal valve position.This function generator has been designed to operate off-line so as to optimize the position of the valve to ensureTABLE FILTER10. Diagram of the valve set program-safean acceptable difference of pressure and acontrollability margin.The optimal valve position signal is then filtered \ointroduce a delay (in this design, the delay was 30 s).This delay must be introduced to avoid the effects ofabrupt reactions on the controller. The filter signal isthen added to the one-element controller signal and sentto the feedwater valve. Simultaneously, the enor betweenthe actual position and the desired position of thefeedwater valve is added to the one-element controllersignal and sent to an integral box that actuates the pumpvalve.Figure 11 is a diagram of the integrated controlalgorithm: level modulator, one-element controller, andvalve set program, and Figure 12 shows the positions ofthe fiedwater and pump valves during a simulatedtransient at 100% power. The behavior of the integratedcontroller is clear: the fast proportional information issent to the feedwater valve, while the pump valve executesa slow integral movement to correct for steady state errorsand to make the feedwater valve move to the optimalposition.FUTURE WORKThe algorithm presented here is ready to beimplemented in any operating steam generator. Its designwas based on the capabilities of the Foxboro hardwareand. therefore, no new hardware development will benecessary.We believe that the capabilities of the algorithm couldbe improved by taking into account feedwater and primaryenthalpy perturbations. Both perturbations introduce anout-of-phase period into the system that ran becompensated by using the level modulation technique.Let Lp, LIp, and L« represent the modulated correctionsfor pressure, primary temperature, and feedwatertemperature. The modulated level set point signal sent tothe one-element controller will be given byLset - Lset0 + L? + L^ + L^ ,dLr'dt = -L7irf - K?(dP/dt) ,(3)(4)(5)(6)where Tp and T, represent the primary and secondarytemperature signals.On the other hand, it would also be interesting tostudy the effects induced by coupling more than one steamgenerator to the same feedwater line. In this case,perhaps the original £-P controller would perform betterdLJdt = -LJT,S - KJdT,/UT-STEAM GENERATOR DP/DT CONTROLLERFig. 11. Diagram of the integrated control system.Feedwitir Vilv* Position Pump Valva Position0.4 0.4100 150 ZOO 250Time (s)300 350 400•torn (low iron 1 M to S.OiSFig.transient.12. Steam generator valve positions during athan the proposed valve set program because, by Keepingthe pressure difference constant, it will be possible tominimize the coupling effects.Finally, we need better simulation of very low powerconditions. The MMS model used in this work failed atvery low power. It will be necessary to use the extended-range modules of the MMS library.SUMMARYIn this section, an alternative automatic controlalgorithm has been proposed. The new algorithm doesnot rely on flow signals but instead uses the pressuremeasurement in the steam header. A level set pointmodulation is introduced that allows the algorithm tocompensate for the shrink and swell phenomenon. Thestandard A-P algorithm to control feedwater pump speedhas also been modified to achieve greater performance andintegration.The new algorithm has been validated using adetailed U-tube steam generator model based on theMMS library.REFERENCES[1] R Bayens, "The Belgian PWR Steam GeneratorLevel Control System from Criticality to Full Power,"LABOREC M03-87-052/R-RB-MO, "l987.[2] Babcock & Wilcox Company, 'Modular ModelingSvstem (MMS): A Code for the Dynamic Simulationof Fossil and Nuclear Power Plants," March 1985,

A new approach to controlling the water level of U-tube steam generators

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