HARDWARE IN THE LOOP TESTING OF A STEAM TURBINE BYPASS REGULATOR USING A TI C2000 MICRO-CONTROLLER

E. Galardi, L.Pugi, N. Lucchesi+, and A Rindi

Università Di Firenze, Dipartimento di Ingegneria Industriale Via Santa Marta 3, 50139 ,Firenze,

phone: + (39) 0554796332, fax: + (39) 0554796332, email: luca.pugi@unifi.it web: www.ing.unifi.it

+Velan Abv SPA Via Coselli 13/15, Coselli, Lucca, Italy Tel. +39 0583 403587 fax 0583949920

www.abvvalves.com

ABSTRACT For a fast and safe start up and shut down of steam power plant, there is a growing interest in the optimization of turbine bypass controllers and actuators that are mainly used during transients. This work is focused on the development of a simple and fast code for real time simulation of a steam plant for the Hardware In the Loop (HIL) simulation of turbine bypass controllers and actuators. The idea is to build a Simulink library of simplified plant components such as valves, turbines, heaters and so on that could be easily assembled in order to simulate with a very simplified approach different plants and operating scenarios. The code, which is implemented for a fixed, discrete step solver should be easily compiled for a real time target such as a Texas Instrument C2000 controller (TMS320F28335 series), in order to be executed in Real Time (RT) on a low cost industrial hardware. In particular the developed code should is used for the development of a virtual environment that should be used for HIL testing of controllers and actuators.

1. INTRODUCTION

In the modern generating units [1], plant efficiency and cost of energy production are continuously improving in terms component size, working pressures and temperatures. In addition, a high flexible operation [2] (e.g., the cyclic operation) is becoming an important specification in the design even of large power plants. Different commercial and technological reasons are enforcing this trend:

• Delocalization of energy production and growing use of renewable energy sources.

• Liberalization of the energy market

A flexible exercise of the plant involves even higher reliability, availability, and duration of components which are more subjected to potentially dangerous thermo-mechanical stresses [3] especially in the transitional phases (start-up, shut-down) or load disturbances, due to rapid

changes in the steam conditions. Many companies are recently developing high efficiency Turbine Bypass Systems (TBS), which can contribute to fulfill all these specifications in order to achieve flexible plant operation, including:

Fast and repeated start-up (maximizing the components life) [4].

Quick restoration of the power supply to the network, if any disturbance occurs.

Maintaining of the main physical variables, for instance P, 푚̇ and T, in a desired working range.

Separation of boiler and steam-turbine operations in transient operating mode and load disturbances.

Bypass systems are typically used during plant transients, (e.g. start-up and shut-down operating conditions) where pressure and temperature conditions are controlled to avoid potentially dangerous working conditions such as extreme pressure and temperature or multiphase flows and to smooth thermal gradients to which are subjected many components such as heat exchangers and turbo-machines.

Typical applications are in large fossil fired steam plants and more recently in combined gas-steam turbine power plants [1]. In particular, for this second category of applications, fast and frequent start-up and shut-down transients are often required. This involves an objective difficulty to simulate the controlled plant since most of the components of the plant are working in off-design conditions during these transients.

Main components of a TBS are described in Figure 1. : an inlet steam mass flow rate goes through a DTP (acronym of Discrete Tortuous Path) lamination valve, represented in Figure 2. , whose internal pressure losses are controlled in order to produce a desired pressure drop. Since this is a control related application a smooth linear response of the valve an a noiseless vibration free behavior are high recommendable specifications, hence DTP valves are often used for this kind of applications. A DTP valve (Kwon [5])

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is composed by a set of stacked discs on which is produced a tortuous path in radial directions; by changing controlling the axial position of a piston/plug is possible to change linearly the numbers of discs through which the steam can pass through and consequently the equivalent valve orifice area. Considering the typical operating conditions of this valve, the flow in the tortuous path is chocked so the valve should be used also to proportionally control the steam mass flow rate in the plant. As stated by different works in literature, this kind of construction is robust, reliable and reduces considerably noise and vibrations associated to fluid lamination [6]-[7]. The lamination process inside this valve can be approximated as an adiabatic, isenthalpic transformation, thus in order to control the outlet steam temperature, the specific enthalpy of the flow is reduced mixing and in the main steam main flow some cold water. In order to improve the response of the turbine bypass system an efficient design of both actuators and control system is mandatory. In literature there are some examples concerning study and development of TBSs [8], [4] which are typically based on a rigid distinction between simulation and experimental results from field activities. Recently there is an increasing interest in the application of HIL techniques to large energy production facilities, including thermal plant controllers [9], electrical power management systems [10] or large Hydro-Power Plants [11]. Aim of this work is to apply the HIL approach to the testing and to the fast prototyping of both controllers and actuators for TBSs reusing a consolidated know-how and the previous experiences taken from vehicular applications as proposed by Pugi, Malvezzi and Allotta [12]-[13].

In particular, the application of HIL techniques involves the development of an efficient and modular approach to implement simulated plant models which have to be implemented in real time on an electronic control unit. In this work authors proposed to use a bond-graph approach which is often used to obtain simplified lumped models [14]-[15] able to capture the dynamical behavior of complex systems. The effectiveness of the proposed approach is also confirmed by considering previous experiences in the modeling of both Thermal Hydraulic plants [16] (complex lubrication networks of Turbo-Machines auxiliary systems) and pneumatics (railway brake plants) using customized codes [17] or commercial software [18]. As consequence the final goal of this work is to demonstrate that is possible to implement on a small cost, low performance hardware, a simplified plant model that should be used both for HIL testing of TBSs with a reasonable quality of simulated results respect to limited available computational resources. In particular to obtain this objective the plant is modeled using an hybrid approach in which simplified dynamical model of the steam boiler available in literature [27] and tabulated data from off-design simulations [20] performed offline are used at the same time.

Figure 1: Typical layout of a TBS.

Figure 2: Scheme of DTP.

2. HIL TESTING OF A TURBINE BYPASS UNIT

Aim of this work is the development of the HILTest Rig visible in Figure 3: the rig is composed by an host PC which is used for pre and post processing of data and to manage communication and data storage with the RT controller of the rig, a TI C2000 micro-controller (TMS320F28335 series). On the TI C2000 controller is implemented a real time model of a controlled Steam Plant (a two pressure level pressure power generator). According the results of the steam model plant, the turbine by-pass controller implemented on the same TI 2000 micro-controller computes a reference command (a 20kHz PWM signal, refreshed at 400 Hz) for the tested components that are respectively the DTP and the Spray Water Valves of a turbine bypass system. Valves are controlled by Positioners, these drives are also able to measure in real time the current valve opening position. These measurements are provided as feedback, through the ADC pins, to the TI C2000 controller, which computes the next simulation step of the controlled plant model considering the current state of the valve. The rig is completed by some electronic components such as PWM converters which are used to adapt the I/O of the TI C2000 controller to the tested industrial components which are typically 24V voltage feed systems controlled using current (4-20mA) analog signals.

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Figure 3: HIL testing of a Turbine Bypass Unit.

3. IMPLEMETED RT MODEL OF THE PLANT

In Figure 4 it’s shown the scheme of the benchmark plant implemented on the C2000 microcontroller: it’s a two pressure levels steam power plant whose main features and data are available in literature [8]. The plant it’s composed by a boiler which produced superheated steam through the heat exchange provided by economizer, evaporator and superheater stages. High pressure steam is expanded by an high pressure turbine stage controlled by turbine bypass system. The expanded steam is then reheated (reheater stage) and used to feed a low pressure turbine stage, also controlled by a second turbine bypass system. Finally, the steam is condensed closing the cycle (condenser stage).

For the RT implementation the plant is discretized using the

lumped approach proposed by Karnoop [14]-[15] and also adopted by authors for the development of lumped models of thermo-hydraulic circuits [16]. In capacitive lumped components (“C” in the scheme of figure 4) energy and mass balance are performed, while on resistive (‘R’ in figure 4) momentum equations are implemented.

Data concerning valve response are taken by literature [19], and the model is tuned using results of a steam plant simulation software previously developed by Carcasci and Facchini [20].

Figure 4: benchmark plant model and corresponding discretization in resistive and capacitive elements.

Referring to the scheme visible in Figure 4, for the control of the steam plant two different configurations, briefly called A and B in Table 1, have been considered:

Strategy “A”: For this configuration boiler pressure is directly controlled by the boiler internal control loop. The plant transient is modelled considering three phases:

o Start Up: the turbine bypass is gradually opened.

o Turbine Run-Up: turbines are gradually activated

o Since the Turbines are running the bypass is gradually closed and de-activated

In the first two phases the two bypass system mainly control pressure corresponding to inlet and outlet conditions of the low pressure turbine stages (DTP valves). In the third one, the DTP valves are used to control the plant flow. Spray water valves are always used to control outlet temperatures of both turbine stages.

Strategy “B”: the boiler burner is continuously controlled in order to roughly control the steam flow. Turbine bypass valve are used to control the inlet pressure levels (DTP) and the outlet temperatures of turbines (Spray Water Valve). Since the pressure of the boiler is not directly controlled these kind of control is sometime referred as Sliding Pressure Mode.

Both the control strategies are implemented considering simple PID regulators manually tuned with heuristic criteria. In order to improve numerical efficiency different part of the models are implemented with different integration frequencies as visible in Table 2. According to this approach, a complex system such as a steam plant has been successfully implemented on a low cost, but quite powerful hardware, as

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the C2000 controller, consuming about half of the available computational resources, in terms of memory and CPU performances.

Table 1: different control strategies implemented on the real time model of the steam plant.

Operating phases

Bypass Start-up

Turbine Run-up

Bypass Shut-down

HP

Leve

l

SHP A(boiler loop) B

A(boiler loop) B

A(boiler loop) B

MXHPT A/B A/B A/B

RHP A/B A/B A/B

MXHPm A

EVm B(boiler loop)

B(boiler loop) B(boiler loop)

LP

leve

l MXLPT A/B A/B A/B

MXLPm A A A

Table 2: multi-tasking implementation on TI C2000

Model

Boiler (SH,EV, ECO,RH)

Turbines (THP/ TLP)

Mixing Capacities (MXLP, MXHP)

Freq. [Hz] 0.25Hz 1Hz 10Hz

Model Control Valves Valve Controllers/

Positioners Filtering A/D and COM

Freq. [Hz] 10Hz 100Hz 500Hz

4 RESULTS

The simplified plant model and its corresponding real time implementation on TI C2000 have been verified and validated by comparing the results of the model with the corresponding data from bibliography [8]. Some results in terms of steam flows for both the proposed control strategies are shown in Figure 5. A more general comparison in terms of mean relative errors in the three simulated plant phases is shown in Table 3. It should be noticed a very good agreement between simulation results and validation data.

Table 3: comparison between results computed by the RT model implemented on TI C2000 and the corresponding data from literature [8].

Bypass Start-

up

Turbine Run-up

Bypass Shut-down

.MXHP strat Am <4% <0.1% <0.1%

.MXHP strat Bm <0.1% <0.1% <0.2%

.strat A MXLPm <0.5% <3.5% <6.5%

.strat B MXLPm <0.7% <6.5% <8.5%

.SH strat AP <0.1% <0.1% <0.1%

.SH strat BP <0.1% <0.1% <0.1%

.RH strat AP <0.1% <0.1% <4%

.RH strat BP <0.1% <0.1% <4%

.MXHP strat AT <0.4% <0.2% <2%

.MXHP strat BT <0.2% <0.3% <0.9%

.MXLP strat AT <0.2% <0.2% <3%

.MXLP strat BT <0.2% <0.4% <3%

Figure 5: comparison between results (steam flows on LP and HP turbines) computed by the RT model implemented on TI C2000 and the corresponding data from literature [8].

CONCLUSIONS AND FUTURE DEVELOPMENTS

An innovative test for HIL testing of turbine bypass system has been developed using a low cost TI C2000 micro-controller. On the controller is implemented a simplified model of a Steam Plant which has been validated comparing simulation results with data available in literature. The rig has been assembled in the ABV Velan plant of Capannori (Lucca, Italy). Future activities and publications will be focused on the optimization of turbine bypass controllers and on the identification of tested components.

REFERENCES

[1] R. Kehlhofer, F. Hannemann, F. Stirnimann, B. Rukes, Combined-Cycle Gas & Steam Turbine Power Plants, 2009, PennWell Corporation.

[2] Logar, A., Depolt, T., and Gobrecht, E. Advanced steam turbine bypass design for flexible power plants. In Proceedings of the 2002 International Joint Power Generation Conference (IJPG2002), Scottsdale,Arizona, USA, 2002, pp. 43–49, IJPGC2002-26071.

[3] F. Casella, M. Farina, F. Righetti, R. Scattolini, D. Faille, F. Davelaar, A. Tica , H. Gueguen, D. Dumur, An optimization procedure of the start-up of Combined Cycle Power Plants, Preprints of the 18th IFAC World Congress Milano (Italy) August 28 - September 2, 2011.

[4] Pasamontes, M. , Alvarez, J.D. ; Guzmán, J.L. ; Berenguel, M., Bumpless switching in control - A comparative study, IEEE Conference on Emerging Technologies and Factory Automation (ETFA), 2010 13.16September 2010 Bilbao Spain.

[5] W C Kwon, G R Kim, S C Park, J Y Yoon, Design of a tortuous path trim for a high-pressure turbine bypass valve, Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering May 1, 2010 vol. 224 no. 2 149-153.

[6] Amano, R. S. and Draxler, G. R. High-pressure steam flow in turbine bypass valve system. Part 1: valve flow. J. Propuls. Power, 2002, 18(3), 555–560.

[7] Rahmeyer,W. J., Miller, H. L., and Sherikar, S. V. Cavitation testing results for tortuous path control valve. In Cavitation and multi-phase flow, vol. 210, 1995, pp. 63–67 (ASME FED, South Carolina), ASME/JSME Fluid Engineering and Laser

258

Anemometry Conference and Exhibition, Hilton Head, South Carolina, 13–18 August 1995.

[8] Q. B. Chou, S. G. Chow, C. R. Stevens, Design and Dynamic Performance of a Steam Turbine Bypass Control System for a Large Fossil-Fired Power Generating Unit, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-98, No. 3 May/June 1979.

[9] Iacob, M. ; Andreescu, G.; Real-time hardware-in-the-loop test platform for thermal power plant control systems, IEEE 9th International Symposium on Intelligent Systems and Informatics (SISY), 2011 , Subotica 8-10 Sept. 2011.

[10] E. de Jong, R. de Graaff,P. Vaessen, P. Crolla, A. Roscoe, F.Lehfuß, G. Lauss, P. Kotsampopoulos, F. Gafaro, European White Book on Real-Time Powerhardware-in-the-Loop testing, European Distributed Energy Resources Laboratories, DERlab Report No. R- 005.0, 2012

[11] Zaev, E., Tuneski, A.; Babunski, D. ; Trajkovski, L., Nospal, A. ; Rath, G., Hydro power plant governor testing using hardware-in-the-loop simulation, Mediterranean Conference on Embedded Computing (MECO), 2012 . Bar, 19-21 June 2012.

[12] Pugi, L., Malvezzi, M., Tarasconi, A., Palazzolo, A., Cocci, G., Violani, M., HIL simulation of WSP systems on MI-6 test rig, (2006) Vehicle System Dynamics, 44 (SUPPL. 1), pp. 843-852.

[13] Allotta, B., Pugi, L., Malvezzi, M., Bartolini, F., Cangioli, F. A scaled roller test rig for high-speed vehicles (2010) Vehicle System Dynamics, 48 (SUPPL. 1), pp. 3-18.

[14] Karnopp DC and Rosenberg RC. System Dynamics, a Unified Approach. New York, NY: John Wiley & Sons Inc, 1975.

[15] Kulakowski BT, Gardner JF and Shearer JL. Dynamic Modeling and Control of Engineering Systems. 3rd ed. The Edinburgh Building, Cambridge, London: Cambridge University Press, 2007.

[16] R. Conti, G. Lo Presti, L. Pugi, E .Quartieri, A. Rindi and S. Rossin, A preliminary study of thermal hydraulic models for virtual hazard and operability analysis and model-based design of rotating machine packages,Proc IMechE Part E: J Process Mechanical Engineering 0(0) 1–17 DOI: 10.1177/0954408913499910 uk.sagepub.com/jpme

[17] Pugi L., Malvezzi M., Allotta B., Banchi L., Presciani P., A parametric library for the simulation of a Union Internationale des Chemins de Fer (UIC) pneumatic braking system (2004) Proceedings of the Institution of Mechanical Engineers, Part F: Journal of Rail and Rapid Transit, 218 (2), pp. 117-132

[18] Pugi, L., Palazzolo, A., Fioravanti, D. Simulation of railway brake plants: An application to SAADKMS freight wagons (2008) Proceedings of the Institution of Mechanical Engineers, Part F: Journal of Rail and Rapid Transit, 222 (4), pp. 321-329.

[19] Emerson Process Management, “Control Valve Handbook”, Fisher Controls International LLC, 2005.

[20] Carcasci, C., Facchini, B., 1996; "A Numerical Method for Power Plant Simulation", Transaction of the ASME Journal of Energy Resources Technology, pubblicato da the American Society of Mechanical Engineers (New York -NY), March 1996, vol.118, pp.36-43, 1996, ISSN: 01950738.

[21] Mazur, Z., Urquiza, G., Campos, R., and McMahon, B. Improvement of the turbine main stop valves with flow simulation in erosion by solid particle impact CFD. Int. J. Rotat.Mach., 2004, 10, 65–73.

[22] Miller, H. L. and Sterud, C. G. A high-pressure pump recirculation valve. In Proceedings of the Electric Power Research Institute’s Power Plant Symposium, Kansas City,MO, 1987.

[23] Miller,H. L. and Stratton, L. R. Recent advances in noise prediction for control valves, special lecture. In Proceedings of the International Symposium on Fluid control and measurement, Tokyo, 1985.

[24] W. Wagner et al., "The IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam," ASME J. Eng. Gas Turbines and Power, 122, 150-182 (2000).

[25] Malvezzi, M., Allotta, B., Pugi, L. Feasibility of degraded adhesion tests in a locomotive roller rig (2008) Proceedings of the Institution of Mechanical Engineers, Part F: Journal of Rail and Rapid Transit, 222 (1), pp. 27-43.

[26] W. Wagner et al., "The IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam," ASME J. Eng. Gas Turbines and Power, 122, 150-182 (2000).

[27] K.J. Astrom, R.D. Bell, Drum-boiler dynamics, Automatica 36 (2000) 363}378

[28] Oppenheim, Alan (2010). Discrete Time Signal Processing Third Edition. Upper Saddle River, NJ: Pearson Higher Education, Inc. p. 504. ISBN 978-0-13-198842-2.

[29] Shampine, L. F., Numerical Solution of Ordinary Differential Equations, Chapman & Hall, 1994.

[30] QI Yiwen, BAO Wen, Bumpless Switching Scheme Design for Continuous-time Controller Switched Systems, Proceedings of the 30th Chinese Control Conference , July 22-24, 2011, Yantai, China

[31] F. Casella, F. Pretolani, Fast Start-up of a Combined-Cycle Power Plant: a Simulation Study with Modelica, Modelica 2006, September.

[32] Krüger, K., Rode, M., Franke, R.: “Optimal control for fast boiler start-up based on a nonlinear model and considering the thermal stress on thick-walled components.”, Proceedings of the 2001 IEEE International Conference on Control Applications, pp. 570 – 576, 5-7 Sept. 2001.

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