Combined-Cycle Plant Simulation Toolbox for Power Plant Simulator.

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1 Combined-Cycle Plant Simulation oolbox for Power Plant Simulator. A. Salehi, M.Eng.Sc., A.R. Seifi, Ph.D. 2*, and A.A. Safavi, Ph.D. 3 Department of Electrical Engineering, School of Engineering Shiraz University, Shiraz, Iran. a_hamid_salehi@hotmail.com 2* seifi@shirazu.ac.ir 3 safavi@shirazu.ac.ir ABSRAC ith the availability of powerful and high performance processors, advanced numerical methods, and flexible and capable software, there is a great opportunity to develop high performance simulators for analysis of and training on complex systems. Power plants are one group of complex systems with serious impacts on the economy and operations of industries. his paper addresses the development of a set of system component simulation modules combined with a control structure in a common software framework for a typical combined-cycle power plant. he simulation toolbox was designed for educational purposes using SIMULINK based on Object Oriented Programming (OOP) and the C programming language. he simulation toolbox will be utilized to assess the long-time behavior of control systems and the overall plant performance following system disturbances. he developed simulation tool is able to use all MALAB toolboxes for research and education studies. (Keywords: gas power plant, combined-cycle power plant, modeling and simulation, MALAB, SIMULINK ) INRODUCION he generalized trend in power generation all over the world is towards increasing the use of combined-cycle power plants. he need for modeling and simulation of these types of plants and their controllers is crucial to the understanding of their dynamic characteristics and impacts on power systems. It becomes important to assess the behavior of control systems and the overall plant performance following system disturbances. here are several different combined-cycle configurations and control variation available [- 6]. his paper discusses the development of a physical simulation toolbox for a typical combined-cycle power plant with standard configuration. he heart of a plant simulation is its modeling block, which for a power plant is comprised of highly nonlinear and complex algebraic, and differential equations [6-0]. Various approaches such as modular technique and Object Oriented Programming (OOP) [-3] could be utilized for this purpose. he power plant will be modeled in the SIMULINK environment based on OOP and the C-programming language, to create a new toolbox for constructing plant simulation. An important feature of this environment is building the Dynamic Link Library (DLL) of m-files and c- files of the block diagrams of this toolbox using a Visual C++ program linked with the MALAB. he developed simulation tool is able to use all MALAB toolboxes for research studies. One of the major objectives in combined-cycle power plants is to maintain a good overall system dynamic performance and keep the efficiency high. his requires the inclusion of a control strategy for different subsystems in the combinedcycle power model. he developed simulation toolbox could be utilized for long-term stability analysis of the typical power systems. he structure of the paper is as follows. After the above general introduction, design of a tool to construct the combined cycle plant simulation toolbox will be discussed and the simulation toolbox will be described. Different tests were undertaken to evaluate the performance of the typical plant simulation in a long-term stability analysis in the presence of disturbances. he he Pacific Journal of Science and echnology 97 Volume 9. Number. May-June 2008 (Spring)

2 authors offer their conclusions in the paper s final section. DESIGN OF OOL O CONSRUC HE COMBINED CYCLE PLAN SIMULAION OOLBOX o design a simulator, the programming language and the required software should first be selected. Modeling and programming can be run using any of the major programming languages. his method requires advanced techniques in programming or the simulator will not be economic or flexible. An alternate idea is to use simulator software to simulate and analyze the necessary calculations; this would be a better method for training and other applications. One of the best software packages for this application is MALAB and its special simulator toolbox called SIMULINK. his software can be developed as a simple OOP program. However, there are no block sets for power plant components in the default libraries. herefore, it is possible to prepare the C-codes of system equations and add a new library in SIMULINK. he equations of each part of the plant (such as furnace, turbine, etc.) are functions defined by C- programming, which are fast enough in MALAB environment when they are converted to DLLfiles. Using these files, all of the components of the power plant will be constructed as block sets, which have SIMULINK properties and are added to a library. Figure shows this for the furnace block. Connecting these components together can simulate a complete power plant. o generate an environment for a simulator which acts in real time, a block set called real time clock is added to reduce the calculation time to actual time of the system. So a simulator based on object-oriented programming, C-programming, and SIMULINK toolbox of MALAB is constructed which contains several programming languages and software. All of the components of the power plants have their special differential equations and state variables. Since the numbers of state variable of the power plant are so large, SIMULINK cannot calculate the initial conditions of the system. So, the initial conditions of the plant have been calculated by the method in [4]. According to the equations of the system and measurable data in a power plant, initial conditions of each block can be computed. Figure : he Matching of Constructed Block with Default MALAB Library for Furnace. he Pacific Journal of Science and echnology 98 Volume 9. Number. May-June 2008 (Spring)

3 MODELING REUIREMENS he governing equation of combined-cycle power plant behaviors are highly nonlinear and complex and need to be accurately determined of acceptable responses are to be achieved. he components of the combined-cycle power plant are [6]: Gas urbine A typical gas turbine is divided into five interconnected subsystems: -Fuel system 2-Compressor 3-Combustion chamber 4-urbine 5-Generator he governing equations of gas turbine behaviors are highly nonlinear and complex and need to be accurately determined if acceptable responses are to be achieved. All of the subsystems have been modeled by both algebraic and differential equations [6]. Figure 2 depicts the gas turbine internal structure. Steam Power Plant A typical steam power plant contains six main parts: -Boiler 2-urbine 3-Condenser 4-Feed water system 5-Generator 6-Miscellaneous components hese parts will be described briefly, in the following text: Boiler he boiler contains the following components: Furnace Drum and Riser Superheater Reheater he order of the dynamic mechanistic equations of the open loop boiler, without PID controllers and actuators, is 4 with 22 outputs and 4 input variable including 42 algebraic equations as is shown in Figure 3. Figure 2: Gas urbine Internal Structure. he Pacific Journal of Science and echnology 99 Volume 9. Number. May-June 2008 (Spring)

4 Figure 3: Boiler Internal Structure. As mentioned, the number of governing equation for each element is quite high and complex. For instance, the governing equations of "furnace" are shown in Appendix A. there are ten inputs and twelve outputs. urbine he components of turbine of the typical plant are: High Pressure (HP) Intermediate Pressure (IP) Low Pressure (LP) he order of the dynamic equations of the open loop turbine is 0 with outputs and inputs and 32 algebraic equations [6]. Condenser he condenser is constructed from the integration of the following components: Shell material Steam ube Material Liquid he model includes 6 differential equations with 20 algebraic equations. It consists of 8 outputs and 4 inputs (without PID controllers and actuators) [6]. Feedwater System he feedwater system is constructed of five components: Deaerator Pump Economiser wo Valve he model includes 5 differential equations and 9 algebraic equations with 3 outputs and 0 inputs (without PID controllers and actuators) [6]. Generator he generator model includes the simple equation (swing equation), plus a stochastic noise generator to represented the real power electrical load fluctuations. Miscellaneous Components Apart from the components described above, a power plant includes other elements. Among them, the most important from control strategy viewpoints are valves (liquid and gas), actuators and elements with simple dynamics, which are represented via simple algebraic equations. he Pacific Journal of Science and echnology 00 Volume 9. Number. May-June 2008 (Spring)

5 Figure 4: Gas urbine Controller. Figure 5: Overall Structure of the Gas urbine. CONROLLER MODELING In order to properly assess the performance of a supervisory plant controller, it is important that the lower level component controllers be adequately represented in the power plant simulation. PID controllers were used in each control loop. In the following, the low-level control loops for each component are described. Gas urbine Control Configuration In a gas turbine, the main control loop adjusts the fuel flow to ensure the correct output power and frequency and also adjusts the air flow that control the exhaust gas temperature [, 3, 6, 4, 5]. he block diagram of the control system is presented in Figure 4. Steam Power Plant Control Configuration Steam power plant control configuration will be described briefly, in the following text: Boiler Control Configuration he assignment of input and output variables for the boiler control is as follows [6]: Superheated steam pressure is regulated by adjusting the fuel flow Drum water level is regulated by adjusting the feedwater flow Superheated steam temperature is regulated by attemporation flow Furnace air pressure is regulated by the air flow to the furnace Reheated steam temperature is regulated by title angle he Pacific Journal of Science and echnology 0 Volume 9. Number. May-June 2008 (Spring)

6 Steam urbine Control Configuration he inputs to the steam turbine controller, which are outputs from the steam turbine, the generator and the admission valves, are [6]: Delivered mechanical power Electrical frequency (rotation speed of the generator) Steam pressure at the inlet to the turbine he first and second are controlled by throttle valve position (CV and IV) and the last is controlled by bypass valve position (BV). Condenser Control Configuration he condenser controller consists of only one loop regulating the temperature of the condensate by adjusting the cooling water flow [6]. Feedwater System Control Configuration wo primary control objectives exist for the feedwater system; to maintain the vessel pressure and liquid level in the deaerator at desired values. Steam pressure is regulated by adjusting the flow of extraction steam supplied to the deaerator. Similarly liquid level is regulated via manipulation of inlet makeup flow [6]. he overall structure of the gas turbine and steam power plant are depicted in Figure 5 and Figure 6 respectively. COMBINED-CYCLE POER PLAN SIMULAION he distinguishing feature of combined-cycle power (CC) plant is the joint production of electricity from a gas turbine and steam turbine, where the high heat content of the gas turbine exhaust flow is utilized to generate additional electricity by passing it through a waste heat boiler that raises steam for admission to the steam turbine. Figure 7 depicts the overall structure of the typical combined-cycle power plant. he base load operating characteristics of the selected combined-cycle power plant is presented in the able. he simulation toolbox for this typical combinedcycle power plant is developed in two steps; first gas turbine and steam power plants are simulated separately and then these two simulators are combined. Simulation of the steam power plant was fully addressed by the authors in [6, 7]. Performances of the gas turbine simulator and the overall combined-cycle power plant simulator are evaluated via simulation. o evaluate the developed gas turbine simulator performance, two datasets related to the following tests are presented. est : a +0% ramp in demanded power output of duration 200 seconds. est 2: a +0% ramp in demanded power output of duration 200 seconds and a +0% ramp in demanded exhaust temperature of duration 200 seconds. hese tests were performed in the normal conditions. Increasing electrical power causes a decrease in mechanical speed. As shown in Figure 8 and 9, the control system compensates it and returns the speed to its normal value. In addition, the temperature converges to its desired value. he satisfactory performance of the controllers is obvious from Figure 8 and 9. he following tests were done to evaluate the performance of the combined-cycle power plant control system. est : decrease the electrical power of the steam power plant from to 0 M and decrease the furnace air pressure by 0% of duration 200 seconds. hen the electrical power decreases in a real power plant, the fuel rate and the output steam rate from control valves are expected to decrease. Variations of the superheated steam pressure, air furnace pressure, and level of the drum water, superheated steam temperature, steam turbine power output, steam turbine speed, and condensate water temperature, level of deaerator water, gas turbine power output and gas turbine speed are depicted in Figure 0. It is obvious from that a 0% decrease in the generated electrical power is approximately equivalent to a 0% decrease in fuel rate, which seems to be reasonable. est 2: failing of the cooling water control valve of the superheater steam. Failing of a control valve, is one of the usual disturbances occurring in power plants. he developed simulation toolbox is able to simulate such disturbances under normal conditions. For example, the failing of the cooling water control he Pacific Journal of Science and echnology 02 Volume 9. Number. May-June 2008 (Spring)

7 valve of the superheater steam for previous disturbance was investigated. Figure 6: Overall Structure of the Steam Power Plant. Figure 7: Overall Structure of the ypical Combined-Cycle Power Plant. he Pacific Journal of Science and echnology 03 Volume 9. Number. May-June 2008 (Spring)

8 Exhaust Gas emperature[k] Air Flow to Compressor[kg/sec x Gas urbine Power urbine Speed[PU] Fuel Flow to Combustor[kg/sec] Figure 8: Closed-loop response of gas turbine for a +0% ramp in demanded power output of duration 200 seconds. Exhaust Gas emperature[k] Air Flow to Compressor[kg/s x Mechanical urbine Speed[PU] Fuel Flow to Combustor[kg/se Figure 9: Closed-loop response of gas turbine for: a +0% ramp in demanded power output of duration 200 seconds and a +0% ramp in demanded exhaust temperature of duration 200 seconds. he Pacific Journal of Science and echnology 04 Volume 9. Number. May-June 2008 (Spring)

9 Superheated Steam Pressure [pa] 4.8 x Air Furnace Pressure [pa].3 x Level of Drum ater [m] Steam urbine Power Output [] Condensate ater emperature [K] x x 07 Superheated Steam emperature [K] Steam urbine Speed [P.U] Level of Deaerator ater[m] Gas urbine Power Output [] Gas urbine Speed [PU] Figure 0: Closed-loop response of typical combined-cycle power plant for: decrease the electrical power of the steam power plant from to 0 M of duration 200 seconds and decrease the furnace air pressure by 0% of duration 200 seconds. he Pacific Journal of Science and echnology 05 Volume 9. Number. May-June 2008 (Spring)

10 Superheated Steam Pressure [pa] 4.8 x Air Furnace Pressure [pa].3 x Level of Drum ater [m] Superheated Steam emperature [K] x 07 Steam urbine Power Output [] Condensate ater emperature [K] Steam urbine Speed [P.U] Level of Deaerator ater[m] Gas urbine Power Output [] 3.8 x Gas urbine Speed [PU] Figure : Closed-Loop Response of a ypical Combined-Cycle Power Plant for Failing of the Cooling ater Control Valve of the Super-heater Steam. he Pacific Journal of Science and echnology 06 Volume 9. Number. May-June 2008 (Spring)

11 able : ypical Operating Characteristics of the Selected ypical Combined-Cycle Power Plant at Base Load. to some extent due to the action of the controller of the reheater steam temperature. POER Gas urbine Steam Power Plant otal Net Power Gas urbine Output power Exhaust gas flow Exhaust gas temperature Compression ratio : Steam Power Plant Boiler Superheated steam pressure 45 Superheated steam temperature 77 Superheated steam flow 2 Reheated steam pressure 3 Reheated steam temperature 727 Furnace fuel flow 4 M M M M kg/s bar kg/s bar kg/s CONCLUSIONS he simulation toolbox of a combined-cycle power plant was developed within MALAB and SIMULINK. his approach was employed so that the resulting power plant toolbox was fast, in terms of computational speed, robust, in terms of convergence problem, efficient and flexible, in terms of modeling capabilities and yet can use various capabilities of MALAB and SIMULINK. PID controllers were designed for different control loops of the power plant, using the NCD Block set of MALAB, to improve the overall dynamic performance of the system. he developed simulation toolbox was successfully used to analyze and simulation of the combined-cycle power plant and its controllers at the presence of different disturbances. Steam urbine otal output power Extraction steam flow HP section outlet pressure HP section outlet temperature HP section output power IP section outlet pressure IP section outlet temperature IP section output power LP section outlet pressure LP section outlet temperature LP section output power Condenser Operating pressure Condensate flow Condensate temperature Feedwater System Deaerator operating pressure Economiser outlet water flow Economiser outlet water temperature M Kg/s bar M bar M bar M mbar kg/s mbar kg/s REFERENCES. orking Group on Prime Mover and Energy Supply Models for System Dynamic Performance Studies Dynamic Models for Combined Cycle Plants in Power System Studies. IEEE rans. On Power Systems. 9(3): Bagnasco, A., B. Delfino, G.B Denegri and S. Massucco Management and Dynamic Performances of Combined Cycle Power Plants During Parallel and Islanding Operation. IEEE rans. On Energy Conversion. 3(2): Zhang,. and P.L. So Dynamic Modeling of a Combined Cycle Plant for Power System Stability Studies. IEEE rans. On Power Systems Lu, S. and B.. Hogg Power Plant Analyzer A Computer Code For Power Plant Operation Studies. IEEE ransaction on Energy Conversion. (4): Figure shows the variations of the superheated steam pressure, air furnace pressure, level of the drum water, superheated steam temperature, steam turbine power output, steam turbine speed, condensate water temperature, level of deaerator water, gas turbine power output and gas turbine speed. Although the temperature of the superheater steam has increased, this increase has been compensated 5. Lu, S Dynamic Modelling & Simulation of Power Plant Systems. Journal of Power & Energy, Proceeding Part A. 23: Ordys, A., R. Katebi, M. Johnson, and M. Grimble Modelling & Simulation Of Power Generation Plants. Springer-Verlag. 7. Knowles, J.B Simulation & Control of Electrical Power Stations. Research Studies Press LD. he Pacific Journal of Science and echnology 07 Volume 9. Number. May-June 2008 (Spring)

12 8. Yang, P. and B.. Hogg Continuous-ime Generalised Predictive Control Of A Boiler Model. IFAC-Symposium on Control of Power Plant & Power Systems. Munich, Germany elfonder, E Constrained Control Concepts In Power Plant & Power Systems For Avoiding Emergency Conditions. IFAC Symposium On Control Of Power Plant & Power Systems. Munich, Germany [Maffezzoni, C Issues in Modelling & Simulation of Power Plants. IFAC-Symposium On Control Of Power Plant & Power Systems. Munich, Germany Eborn, J. and B. Nilsson Simulation of a Power Plant Using an Object-Oriented Model Database. IFAC 3th riennial orld Congress. San Francisco, CA Breckling, B. and C. Eschenbach Object Oriented Simulation of Plant-Environment Interaction. ASU Newsletter. 24(2): Lu, S., E. Swidenbank, and B.. Hogg An Object-Oriented Power Plant Adaptive Control System Design ool. IEEE ransactions On Energy Conversion. 0(3): Rowen,.I Speedtronic Mark IV Control System. Gas urbine Refrence Library (AGR880). 5. Yacobucci, R.B. 99. A Control System Retrofit for a GE frame 5 urbine/generator Unit, IEEE rans. on Energy Conversion. 6(2): Seifi, A.R A Research Oriented Fossil Fuel Steam Power Plant Simulator. Ph.D. hesis, Dept. of Electrical Engineering, arbiat Modarres University of ehran, Iran. 7. Seifi, H. and A.R. Seifi, An Intelligent utoring System for a Power Plant Simulator. Electric Power Systems Research. 62:6-7. APPENDIX A his appendix represents furnace differential and algebraic equations with the equations representing steady state values. Inputs F : Fuel flow to furnace (kg/s) A : Air flow to furnace (kg/s) G : Gas flow to furnace (kg/s) h G : Inlet gas enthalpy (J/kg) h A : Inlet air enthalpy (J/kg) θ : ilt angle coefficient (rad) st : emperature of superheater metal tubes () rh : emperature of reheater metal tubes () : emperature of economizer metal tubes () et Parameters C F : Fuel calorific (J/kg) : Specific heat of exhaust heat at constant C pg pressure (J/kg K) C gs : Combustion gas specific heat capacity (J/kg K) k : Attenuation coefficient k es : An experimental coefficient (J/kg K) k : Friction coefficient (m.s) f k gs : An experimental coefficient (J/kg K) k rs : An experimental coefficient (J/kg K) V F : Combustion chamber volume (m 3 ) R : Stoichiometric air/fuel volume ratio h S ref : Reference exhaust gases enthalpy condition (J/kg) ref : Reference exhaust gases temperature condition () σ : Stefan-Boltzman constant : Ideal gas constant for exhaust gases R States X F : ( ρ h ) (J/m3 ) ρ : Density of exhaust gas from the boiler (kg/m 3 ) Outputs ir : Heat transfer to the riser gs : otal heat transfer to the superheater (J/s) rs : Heat transfer to the reheater (J/s) es : Heat transfer to the economizer (J/s) P G : Furnace air pressure (pa) : Heat transfer by radiation to the superheated (J/s) is h : Mass flow of exhaust gas from the boiler (kg/s) : Enthalpy of exhaust gas from the boiler (kg/s) he Pacific Journal of Science and echnology 08 Volume 9. Number. May-June 2008 (Spring)

13 g : Gas temperature at the superheater () gr ge : Gas temperature at the reheater () : Gas temperature at the economizer () gl : Boiler exhaust gas temperature () Algebraic Equation X F h = ρ h href g = + C pg PG = R ρ g = k P ir is gs gr rs ge f F G 4 g = θkv σ ρ F 4 g = ( θ ) kv σ = is + k gs = g + C = k = rs gr 0.6 gs 0.6 ( gr C ref ρ ( g ( gs 0.6 ( ge is rh rs ) st es = kes et ) gl = ge es C gs ( A + γg F Rs ) y = 00 F Rs Differential Equation d dt X d ρ dt = ( CF V F F A A G G ir is F = ( V F ) gs ) y + h + h Rs ( + ) h ) 00 F + A + G ) interests are in the fields of combined cycle power plant, neural networks. and fuzzy systems. Ali Reza Seifi, was born in Shiraz, Iran, on August 9, 968. He received his B.S. degree in Electrical Engineering from Shiraz University, Shiraz, Iran, in 99, his M.S. degree in Electrical Engineering from he University of abriz, abriz, Iran, in 993 and his Ph.D. degree in Electrical Engineering from arbiat Modarres University (.M.U), ehran, Iran, in 200. He is currently an Assistant Professor in the Department of Electrical Engineering, School of Engineering, Shiraz University. His research areas are power plant simulation, power systems, electrical machine simulation, power electronics, and fuzzy optimization. Ali Akbar Safavi, received his B.S. degree in Electrical Engineering from Shahid Chamran University, Iran, in 987, his M.S. degree in Control Engineering from the University of NS, Australia, in 992, and his Ph.D. in Process Systems Engineering was completed at Sydney University in 995. In 996, he was a Postdoctoral Fellow at Sydney University. He is currently an Associate Professor in the Department of Electrical Engineering, School of Engineering, Shiraz University. His research interests are model predictive control, wavelets, neural networks, system identification, networked based control, and information technology. SUGGESED CIAION Salehi, A., A.R. Seifi, and A.A. Safavi Combined-Cycle Plant Simulation oolbox for Power Plant Simulator. Pacific Journal of Science and echnology. 9(): Pacific Journal of Science and echnology ABOU HE AUHORS Abdolhamid Salehi, was born in Shiraz, Iran, in, 972. He received his B.S., and M.S., degrees in Electrical Engineering from Shiraz University, Shiraz, Iran, in 999 and 2003, respectively. He has been employed with Gas Company since Presently he serves in the telecommunications department. His research he Pacific Journal of Science and echnology 09 Volume 9. Number. May-June 2008 (Spring)