DEB-oriented Modelling and Control of Coal-Fired Power Plant

Transcription

1 Prerints of the 9th World Congress he International Federation of Automati Control Cae own, South Afria. August 4-9, 4 DEB-oriented Modelling and Control of Coal-Fired Power Plant Li Sun*. Junyi Dong*. Donghai Li*. Xi Zhang** Yali Xue*. *State Key Lab of Power System, Deartment of hermal Engineering, singhua University, Beijing, China ( lidongh@mail.tsinghua.edu.n) Corresonding author. ** Guangdong Power est and Researh Institute, Guangzhou, China, ( zhangxi_sjtu@63.om) Abstrat: Diret-energy-balane (DEB) based oordinated ontrol strategy is ritial in ahieving good load following and steam ressure stability in modern ower lant. Dealing with the adverse imat of erturbation in oal uality, however, is still a hallenge to be resolved. In this aer, a 3MW drum-boiler unit model is first established and verified by exerimental data, for the urose of DEB and alorifi value flexibility study. Unlike onventional measurement orretion methods, the heat value variation is onsidered here as a art of the internal disturbane, whih is estimated and omensated in an imroved ative disturbane rejetion ontrol (ADRC) struture. he roosed ontrol strategy brings a novel design onet for a ritial roblem of heat variation in any oal-fired ower lant. he simulation results demonstrate that the dynami erformane of main steam ressure an be signifiantly imroved.. INRODUCION he oal-fired ower lant has ahieved a raid develoment in reent years and will undoubtedly be the main ower generation unit for a long time. Inreasingly strit demand on load and freueny regulations (Edlund et al., 8), however, is ut forward to ensure the safety of ower grid. Generally, the main reuirements for load ontrol system are listed as follows. Eletri ower outut an be adjusted raidly as demand by automati disath system (ADS). he regulation rate in China is usually.5%-% full load er minute. Main steam ressure an be maintained in a limited range (usually ±.4MPa)desite variations of the load. he outut ower and main steam ressure must be maintained well while the heat value of the oal varies. In modern ower lant, diret energy balane (DEB) based oordinated ontrol strategy an fulfil the first two goals well. he boiler follows turbine (BF) mode is usually adoted in DEB, in whih the governor valve is resonsible for traking the grid demand raidly. At the same time, some existing feedforward strategy in DEB an guarantee the throttle ressure within the bounds. So the urgent obstale faing field engineers is the third reuirement. o reah this goal, we should first develo a simle and suitable model for DEB ontrol researh. he lassial model roosed by (Bell and Astrom, 987) whih reresenting a 6 MW oil fired ower lant was widely aeted as a real lant and a number of ontrol strategies were researhed based on this model ((Lee et al., 8), (Lu et al., ) and (Wu et al., 3)). However, this model differs from modern ower lant in () the time delay negleted due to the fast oil uming roess while the oal onveying roess needs to ost uite a long time; () the heat value of oil stayed relatively steady omared with raw oal; (3) the ontrolled ressure in oil fired lant is drum ressure while main steam ressure is more referred in oal-fired lant. In addition, some arameters needed by DEB annot be rovided by Bell- Astrom model. In onventional DEB struture, the solution to the variation of heat value is usually deendent on the measurement methods. he traditional way was to adot the alorifi value orreting method. he results, however, were not reliable due to the oor alulation auray. he modern on-line measurement is under researh but the assoiated high ost limited its aliations. Ative disturbane rejetion ontrol (ADRC) strategy (Han, 9) reeived more attention reently (Gao 3), whih was alied suessfully in the Parker Parflex hose extrusion faility (Zheng and Gao, ) and ALSOM gasifier (Huang et al., 3). hese aliations demonstrate its enormous otential for roess ontrol. In this study, the erturbation of heat value and other unknown dynamis were regarded as the disturbane whih is then estimated using the imroved extended state observer (ESO) and omensated in real time. Simulation results demonstrate the sueriority of the roosed sheme over the onventional method. Coyright 4 IFAC 43

2 Cae own, South Afria. August 4-9, 4. A SIMPLIFIED PLAN MODEL FOR DEB DESIGN. Plant Desrition he unit of a drum-boiler 3MW ower lant in Guangdong Provine, China, is seleted for the modelling study. In this setion, we build a nonlinear model based on the first law of thermodynamis. A shemati view of a oal-fired ower lant is shown in Fig.. he number of arameters was redued signifiantly through some assumtions and simlifiations so that the arameters an be alulated simly. P HP µ LP urbine Generator LP N E In rinial, the ure delay resulted from the onveyor belt, τ, and rimary air ie, τ, whih an be exressed as: ue τ i s = B (3) o s f = e τ (4) where u B is the boiler demand, f is the mass flow of oal blowing into the burner. o write energy balane euations, let V f denotes the volume of the furnae, Q denotes the radiant energy released, Q r the lower heating value (LHV) and η the ombustion effiieny. Furthermore, let subsrits a, s, g, f and w refer to the inlet air, steam, flue gas, furnae and water wall, resetively. D,, m, and is the mass flow, seifi heat aaity, mass and Kelvin temerature. ad net Coal µ B Boiler he energy balane for the furnae is Air dg ad V f g = f Qnetη D a a a D g g g Qr (5) Fig.. A shemati view of a oal-fired ower lant he system dynamis an be roughly searated into two different time sales. he dynamis governing the oal and steam flows are relatively fast, whereas dynamis from heat transfers are muh slower. In this aer, we ignored the slow roesses in steam and water iruit as no temerature or enthaly arameters are needed in the DEB ontrol system. However, the slow harateristis in the ombustion roess still remained to reflet the fundamental feature of oal-steam roess, whih is also the original diffiulty for DEB ontrol and oal uality flexibility.. Coal-Steam ransformation Channel he erformane of oal-steam hannel has a signifiant imat on the overall dynami resonse of oal-fired ower lants. hus, the hannel model should reflet the seifi features of large inertia and time-delay in the oal ulverizing, ombustion and heat release roesses. he mass balane for the ulverizer is i where o dm = () i and o reresent the mass flow of oal moving into and out of the ulverizer, resetively. M is the oal storage mass. With oerating exeriene, the mass flow of ulverized oal an be onsidered roortional to the mill load. o = M where is a time onstant whih an be identified. () he energy balane euation for water-wall is dw m w w = Qr Q (6) where Q is the heat transferred from water-wall to the flowing steam inside. In addition, two heat transfer euations should be inluded to ensure the losure of the euations. Q K (7) 4 4 r= ( g w) Q= KA( ) (8) w w s where K and K is the radiation and onvetion heat transfer oeffiient, resetively. Considering the water and steam in the drum and risers are both in the saturation ondition, the heat-mass transformation euation lists below. Q Dr s s where r s is the latent heat of vaorization of water. Finally, ombining the E. ()-(9), we an alulate the real-time steam rodution rate, D s, aording to boiler demand, B..3 Steam-Pressure ransformation Channel In the roosed model, the boiler steam system was divided into two arts as deited in Fig., whih ould be onsidered as two single-hase ies. he onservation law of mass is listed below. D ρ A = t x (9) () 44

3 Cae own, South Afria. August 4-9, 4 Here A is the ross-setional area, and both of the mass flow in the ie, D, and steam density, ρ, are the funtion of x-axis osition and time. In addition, the resistane formula for the ie is ρ = d x he frition loss er unit length on Dary-Weisbah euation. d () d an be alulated based D = ξd () ρ So far, the losure of the euations in the steam-ressure hannel was also ahieved..4 Pressure-Power ransformation Channel he ressure of the governing stage,, an be exressed as: = k µ (3) where is the main steam ressure, µ is the osition of turbine valve atuators, and k is the roortional oeffiient. Let suersrit ~ denote rated oerating ondition, th refer to the steam temerature in the throttle. Assuming the exhaust ressure, n, is muh less than, Flugel-based model of inlet steam mass flow, D, an be simlified as n th D = D ( ) = k k th n n (4) where, k is a stati gain arameter varying with oerating oint. Let subsrit r and e denote reheater and eonomizer, h refer to the enthaly, ϕ turbine effiieny, H denote the heat released in the ondenser. Based on energy onservation, the outut ower an be exressed as Ne = ϕ( D ( h h ) H D ( h h )) (5) o i e r r r Atually, the heat transfer amount in the reheater and ondenser is usually roortional to the urrent load ower aording to engineering exerienes. So, a good aroximation of Ne is Ne = ϕ( D ( h h )) βne (6) e Simlify the exression by ombining like terms: For simliity, the dynami harateristis of turbine an be desribed by three euivalent first order links, onsidering the storage aaity or inertia of nozzle hamber and reheater (de Mello (99)). hus, the E. (5), (6) an be resetively rewritten as: = kµ (8) D D = k (9) With identified time-varying arameters k i, the ritial oeration arameters of turbine an be easily alulated from the revious hannel and inut variables. 3. HE SAE SPACE MODEL 3. Further simlifiations he model above an ature the gross behaviour of the lant well. he model does, however, have two serious defiienies, whih make it not ratial for system synthesis. he first is there are too many intermediate variables existing in the euations, whih will inrease the aumulative errors; the other is the large omutational ost due to strong nonlinearity and artial differential euations. Additional simlifiations based on linearization and lumed arameter method (LPM) an be made if we are only interested in the oordinated ontrol level suh as DEB. Many unimortant intermediate variables are summarized as a time-varying item whih an be identified through exerimental data. Due to the age limit, this art is omitted, whih is onsidered to be resented in an extended version. 3. he affine nonlinear state-sae model here are many alternative hoies of state variables to obtain a ontrol-oriented model. In this setion, we will build a model with six states, x = f Ds b D, whih gives insight into the key hysial mehanisms that affet the dynami behaviour of the ower lant remarkably. By u = u ( t τ ) µ and system defining ontrol inut [ B ] outut [ ] y = Ne, the affine nonlinear state-sae model an be exressed as x = f( x ) g( xu ) () y= hx ( ) () where Ne = k D (7) 3 where, k= ϕ ( h he) 3 β, whih is also a stati arameter deending on oerating oint. 45

4 Cae own, South Afria. August 4-9, 4 x.37.6 x3 ( x x) 5 (.3 x 9.4 x4 x3 x4 ) 6 f( x) = gx ( ) = (9.4.3 x4 x3 x4 x6 ).84 x 4 7 x 5 (74.86 x5 x6 ) [ ] 4 6 hx ( ) = x.3x 3.3 Parameter identifiation and model validation In the state-sae model roosed above, there are six inertia onstants,,, 5, 6, 7 and time-delay τ to be identified. In this setion, geneti algorithm (GA) was adoted to otimize these arameters by minimizing the error between the model outut and exerimental data. Another grou of test data was alied to validate the high reision of the identified model, as shown in Fig.. MW t/h % Main Steam Flux Valve Position 8 Outut Power 6 5 Fig.. Model validation ( MPa Ma t/h Main Steam Pressure Drum Pressure Feed Coal Flux 8 5 est data; ---- Model outut) 4. A MODIFIED DEB SAEGY BASED ON ADRC 4. Analysis of the onventional DEB struture Enhaned stability reuirements for oal-fired ower lant in the ast thirty years have led to a raid rogress in oordinated ontrol systems, wherein the diret energy balane ontrol strategy was most widely used. It balanes db the heat released from the boiler, ( Qm = Cb ), with r the energy demanded by the turbine-generator, ( Qr = ). Here, r is the referene of main steam ressure, and C b is the thermal storage oeffiient. Under this struture, the dynami regulation of main steam ressure mainly relies on the feedforward ation while PI ontroller is resonsible for eliminating stati error. As long time-delay exists in the ressure loo, a weak PI ontroller is referred to avoid freuent adjustment whih is harmful for oal mill and onsumes extra energy. However, the weak PI may fail in disturbane rejetion sine the unknown disturbanes an only be rejeted through feedbak loo. So, the rimary drawbak of the traditional DEB strategy is the slow harateristis in disturbane rejetion. 4. Ative disturbane rejetion ontrol 4.. Basi riniles Ative disturbane rejetion ontroller (ADRC) was originally roosed by (Han, 999), aiming to design a novel ontrol strategy whih was indeendent on aurate lant model. he entral idea of ADRC was to treat the nonlinear, ouling and disturbanes in the lant as an extended state, whih would be atively estimated and omensated for in real time. Assume the roess order lant: (,,,, ω) G an be modelled as a general first y = g t y y bu () where b is the lant arameter and g reresents the high order, nonlinear, ouling, disturbanes, et. in the lant. Define f = g ( b b ) u and then we an get y = f bu (3) Define x = y as a state variable and x = f as an extended state variable. On the remise that x is measurable and f is differentiable, (3) an be written in the anonial state sae form as: x = x bu, x = h. (4) y = x. o estimate f, design an extended state observer (ESO) for lant exressed in (4) as follows: z = z β ( x z) bu z = β ( x z ) (5) where β, β,b are the observer arameters. When ESO is aurately tuned, z, z will trak y, f, resetively. With the estimated extended state z, the ontrol law is onstruted as: 46

5 Cae own, South Afria. August 4-9, 4 u ( u z ) = (6) b Combining (3) with (6) to get a simlified lant as follows: u z y = f bu z b = u (7) b ill now, the extended state f named generalized disturbane is omensated after being estimated by ESO. Design a roortional ontroller for the redued lant in (7) as follows: u = k ( r y) (8) where r is the referene inut. Combine (7)-(8) to obtain the losed-loo dynami euation: y ky= kr (9) Conduting Lalae ransformation, we an get hus simly tuning of ω, ω, b is reuired to obtain a o satisfatory set of ADRC arameters. 4.3 Modified framework of DEB based on ADRC In this setion, a modified framework is roosed, whih regards oal uality erturbation as internal disturbane. Based on an imroved ESO with inut-delay roosed by (Zhao and Gao, 3), the internal disturbane ourring in the time-delay roess an be atured synhronously. However, the estimated total disturbane ˆd, inludes not only the internal disturbane, but also external disturbane and slight flutuation, whih would result in invalid ontrol effort. As is known, the external disturbane is mainly from valve ation, whose dynami harateristi is muh faster than boiler. Based on the different freueny band of internal and external disturbanes, a strong filter after ESO is introdued here to searate the external disturbanes and unavoidable flutuation from the estimated total disturbane. he shemati of the framework is illustrated below. k G () s = l s k (3) Finally, the derivation roess above an be illustrated as r k u /b u G y Pressure setting Load Demand Q r ADRC Q m k Filter d Saturation /b ESO Inut Delay Feed Forward µ u B Saturation Heat Value erturbatin t Boiler urbine Units b Ne du Cb z ESO Fig.3. Diagram of ADRC 4.. uning rules for ADRC arameters o simlify the arameter tuning roedure, (Gao. 3) roosed a ratial method based on bandwiharameterization. In this aroah, k, β, β are ertain funtions of the ontroller or observer bandwih as follows: k = ω, β = ω, β = ω (3) o o hus the ADRC arameters dereased to 3, whih are ω ω, b., o In order to ahieve the desired ontrol erformane, the following exerienes would be helful in further tuning. he larger the ω o is, the stronger traking ability the ESO has. But a large ω results in the sensitivity to o noises. A large ω or a small b auses the strong ontrol ation, whih leads to fast resonse but also overshoot and flutuation at the same time. Fig.4. he shemati of modified DEB struture A Butterworth filter was adoted in the struture, whih was often referred as a maximally flat magnitude filter. 4.4 Simulation results he inut delay was assigned eual to the identified time delay in setion 3.3. he ontrollers arameters were set below. able (a) Controller arameters Control Loo Parameters of onventional DEB Feedbak Loo P =, I =. Feedbak Loo P =.4, I =. Feed forward Loo K =.43, Kd =, =.855 f f able (b) Controller arameters Control Loo Parameters of modified DEB Feedbak Loo ω = 3., ω =.4, b =., o =.5 f Feedbak Loo ω =., ω =., b =. o Feed forward Loo K =.6, Kd = 8, =.855 f f Cut-off freueny f =.5 47

6 Cae own, South Afria. August 4-9, 4 During the simulation, the load demand is redued by 4MW at t = s with a seed of -6MW/min; After the outut ower and the main steam ressure reahed stable, the heat value (k 4 ) redued at t =7s. he simulation results were shown in Fig. 5. MW % Outut Power 6 Valve Position MPa t/h Main Steam Pressure 6 Feed Coal Flux 3 Bound 9 6 Fig. 5. he omarison of both strutures ( Conventional; ---- Modified; ---- Referene) he simulation results show the refet the erformane of both strutures in load traking. Less ontrol effort, however, shows the advanement of ADRC ontroller. Main steam ressure during the load down roess ould be dragged bak more uikly under the modified framework. 5. CONCLUSION A state sae model was built under the DEB framework for the urose of ontrol design. he omarison with exerimental data demonstrates a high auray of this model. More imortantly, the model adots a novel onetion of the roblem where the heat value variation, a ritial roblem in oal-fired ower lant, is treated as a art of disturbane, to be estimated and rejeted. his leads to a more effetive ontrol strategy based on ADRC where the weakness of the existing measurement methods is overome. In artiular it is shown that the method roosed an be feasibly imlemented with a small but key revision under the existing framework. Demello, F. P. (99). Dynami-Models for Fossil Fueled Steam Units in Power-System Studies. IEEE ransations on Power Systems, 6(), Edlund, K., Bensen, J. D., Børresen, S., & Mølbak,. (8). Introduing model reditive ontrol for imroving ower lant ortfolio erformane. In: Proeedings of the 7th IFAC World Congress. Gao, Z. (3). Saling and bandwih-arameterization based ontroller tuning. In: Proeedings of the Amerian Control Conferene. Gao, Z. (3). On the entrality of disturbane rejetion in automati ontrol. ISA transations. (in ress) Han, J. (999). Nonlinear design methods for ontrol systems. In Pro. of the 4th IFAC World Congress. Han, J. (9). From PID to ative disturbane rejetion ontrol. Industrial Eletronis, IEEE transations on, 56(3), Huang, C. E., Li, D., and Xue, Y. (). Ative disturbane rejetion ontrol for the ALSOM gasifier benhmark roblem. Control Engineering Pratie,, Lee, J. D., Moon, U. C., Lee, S. C. & Lee, K. Y. (8). An Adative Dynami Matrix Control of a Boiler-urbine System. In: 7 th IFAC World Congress. Lu, Z., Lin, W., Feng, G., & Wan, F. (). A Study of Nonlinear Control Shemes for a Boiler-urbine Unit. In IFAC symosium in Nonlinear Control Systems ( ). Wu, X., Shen, J., Li, Y. & Lee, K. Y. (3). Data-Driven Modeling and Preditive Control for Boiler urbine Unit. Energy Conversion, IEEE ransations on, 8(3), Zhao, S. & Gao, Z. (3). Modified ative disturbane rejetion ontrol for time-delay systems. ISA transations. (in ress) Zheng, Q. & Gao, Z. (). An energy saving, fatoryvalidated disturbane deouling ontrol design for extrusion roesses. In: Proeedings of the th World Congress on Intelligent Control and Automation. ACKNOWLEDGEMEN his work has been suorted in art by the National Natural Siene Foundation of China (No ) & Prodution- Study-Researh Cooeration Projet of singhua University and Guangdong Power est and Researh Institute. REFERENCES BELL, R. D. and ASROM, K. J. (987). Dynami models for boiler-turbine-alternator units: data logs and arameter estimation for a 6 MW unit. In: Reort, Lund Institute of ehnology, Deartment of Automati Control. 48