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PARTIAL LOADING OPTIMAL CONTROL AS STEAM TURBINE EFFICIENCY IMPROVEMENT
Z. Domachowski
Technical University of Gdansk Department of Automatics and Turbine Propulsion
Narutowicza 11/12, 80-952 Gdansk, Poland phone: (48 58) 3472662, fax: (48 58) 3414712,
e-mail: [email protected]
Proceedings of 2000 International Joint Power Generation Conference
Miami Beach, Florida, July 23-26, 2000
IJPGC2000-15014
ABSTRACT A steam power plant partial loading optimal operation is discussed. It is reasonable in the case of all turbosets different from base load operation mode ones. It requires an appropriate algorithm automatically performed of steam turbine control valve sequence. A quantity governing of steam turbine has been assumed as well as individually driven turbine control valves. Under any steam turboset partial loading there is possible to choose an unique steam turbine control valve sequence which would be optimal from the specific heat consumption point of view. The steam turbine efficiency maximum is then the turboset partial loading optimal operation criterion. An algorithm aiming to analyse the steam turbine efficiency as a function of turbine control valve sequence has been performed. Simulation research has been carried out aiming to investigate the influence of some variants of turboset governor architecture and parameters on several turboset performance characteristics. An algorithm and computer program of turboset partial loading optimal
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operation have been observed as operating correctly. A turboset partial loading optimal control system has cooperated satisfactorily with a boiler control system. INTRODUCTION An optimum efficiency operation characterizing an optimal energy conversion is among others the aim of any steam turbine design. As usual the specific heat consumption is minimal at nominal rating of turboset. However only a part of turbosets are operated at nominal rating. These ones are the so called base load generation units. An efficiency of steam turboset operated under partial loading is a function of turboset power output. Such a function depends among others on the steam turbine control type. From three types of steam turbine control, i.e. slide pressure control, throttle control, and nozzle governing, the last one is the most efficient to be applied to partial loading steam turbines. A steam turbine nozzle governing is in other words a quantity governing of steam
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Fig.1. Block diagram of turboset partial loading optimal control system
f - electric power grid frequency, ω - turboset rotor angular velocity, PT - turboset output, DD - computer device, M - turboset rotor angular velocity meter, PJ - proportional-differential controller, SR - servomotor, T - turboset, VS - turboset partial loading optimal control algorithm
M
VSDDPJ SR T
ωωωω
f
PTPR
+
-
turbine power output. The quantity of steam is adapted to the required steam turbine power output. For such a purpose there are some steam turbine control valves opening sequentially. The control valve opening sequence is either predetermined, when all control valves are commonly driven by one servomotor, or adjustable, when control valves are driven individually. Any sequence of the control valve opening is characterised by one curve representing the steam turbine efficiency versus steam turbine power output, see Fig.2. Under partial loading steam turboset operation, in the case of quantity governing with individually driven steam turbine control valves, there is possible to choose a unique control valve sequence which would be optimal under the specific heat consumption criterion. Then the steam turboset rated power can be divided into several ranges in which there is a unique control valve sequence optimal from the turboset efficiency point of view, see Fig.2. This is the principle of the steam turboset partial loading optimal control. A high part load efficiency of steam turbine is required e.g. for all turbosets participating at the secondary control of electric grid frequency.
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Decreasing the specific heat consumption of a turboset has an advantage as well from economic as from ecological point of view. On one hand it involves the reduction of electric power cost. On the other hand it results in the diminution of the primary energy consumption and by the same of the environment pollution. STEAM TURBOSET PARTIAL LOADING OPTIMAL OPERATION CONCEPT Any power plant participating at the secondary frequency control of an electric power system is changing its power output of a required value accordingly to the command of an electric power system frequency controller. First, the required new turboset power output has to be achieved in a minimal time, as well from technical as from economic point of view. Second, the new turboset loading has to satisfy a maximum efficiency criterion. In such a way a problem of the turboset partial loading optimal control arises. It concerns an appropriate turboset control to minimise the specific heat consumption of turboset.
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The choice of an appropriate sequence of turbine control valve opening satisfying the requirement of turboset efficiency optimal control makes the principle of an algorithm of turboset partial loading optimal control. The task of control system is to adapt a turboset governor in a way to achieve an optimal point of the steam turbine internal efficiency. That adaptation consist in the choice of an adequate steam turbine control valve sequence (Domachowski, 1994). A simplified block diagram of such a control system is represented in Fig.1. On line computation, in a close-loop computer turboset control system, of steam turbine internal efficiency is a basis of any concept of turboset partial loading optimal control. Therefore in a considered concept of steam turboset partial loading governor, see Fig.1, there are among others the following blocks of computer nature: DD - representing quantization and sampling of control signal, VS - representing calculation of turboset partial loading optimal control algorithm. The other blocks of steam turboset partial loading governor, see Fig.1, represent PJ - the assumed proportional integrating controller, SR - the steam turbine control valve servomotor, M - the turboset rotor angular velocity meter. The above mentioned steam turboset partial loading control system has some particularities. Namely it is of computer nature containing a specific turboset partial loading optimal control software. This software enables an optimal control procedure consisting in choosing a steam turbine control valve sequence to achieve a locally possible steam turbine internal efficiency optimum.
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STEAM TURBINE MAXIMUM INTERNAL EFFICIENCY ALGORITHM Basing on the knowledge of the • amount of steam turbine control valves, • cross section of particular turbine nozzle
boxes, • rated values of steam parameters inside the
turbine, • steam pressure drop in the turbine control
valves it is possible to analyse the steam turbine efficiency, under steam turbine partial loading, as function of steam turbine control valve sequence. As an example let us assume a steam turbine equipped with four control valves denoted A, B, C, D. Suppose that e.g. in the following sequence of steam turbine control valve opening A → A + B → A + B + C → A + B + C + D there is a relationship between full opening of control valve and turboset power output percentage represented in Table 1. Table 1. Turboset power output versus control valve opening steam turbine control valve full opening
turboset power output, PT / PTo , %
A 50 A + B 80
A + B + C 95 A + B + C + D 100
Simplifying the problem additionally assume the turboset partial loading, PT , is not less than 50% of turboset rated power, PTo . In such a case the following six sequences of steam turbine control valve opening are possible:
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I: A, B, C, D (A → A + B → A + B + C → A + + B + C + D), II: A, C, B, D (A → A + C → A + C + B → A + + C + B + D), III: A, D, B, C (A → A + D → A + D + B → A + +D + B + C), IV: A, B, D, C (A → A + B → A + B + D → A + B + D + C), V: A, C, D, B (A → A + C → A + C + D → A + + C + D + B), VI: A, D, C, B (A → A + D → A + D + C → A + D + C + B). Each of the above steam turbine control valve sequences is characterised by a specific turboset efficiency as function of turboset power output. All of them are represented in Fig.2. On the axis of abscissa there are denoted turboset power output points corresponding to several variants of full opened control valves. E.g. the point denoted A means there is the valve A full opened, the point denoted A + D means both the valve A and D are full opened, and so on.
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The upper envelope of a set of curves, see Fig.2, represents the maximum possible turboset efficiency under turboset partial loading. To satisfy the required turboset efficiency optimal control it is necessary to choose an appropriate sequence of steam turbine control valve opening. This is the principle of an algorithm of turboset partial loading optimal control. TURBOSET PARTIAL LOADING OPTIMAL CONTROL SIMULATION ANALYSIS Some simulation investigations have been carried out on the presented above concept of turboset partial loading optimal control. With the aid of computer simulation several control characteristics have been verified. They have to be analysed for at least two reasons. First of all, differing from a conventional turboset control system, the operation of turboset partial loading optimal control system consists, among others, in adapting a steam turbine control valve sequence
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to achieve a locally possible steam turbine internal efficiency optimum. Besides, as containing a specific turboset partial loading
a )
b)
Fig.3. Unit step response of the turboset power output (a), and servomotor output (b) to the turboset power output reference (in a linear turboset control system it has been assumed GPJ =3,0(1+1/0,5s), GSR = 1/(1 + 0,2s) - see Fig.1)
95 100 105 110 115 120 125 130 135
.800
.920
.960
1.00
1.04
1.08
t,s
PT
95 100 105 110 115 120 125 130 135
.800
1.00
1.20
1.40
t,s
CV
1.60
1.80
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optimal control software, that turboset control system is of computer nature (Frater, 1986; Horn, 1988). The turboset partial loading optimal control simulation analysis has included the following aspects: • verification of the algorithm of steam turbine
control valve sequence changing, • investigation of an influence of turboset
control architecture and parameters on turboset control transients,
• analysis of an interaction between a boiler control system and a turboset one.
Standard unit step inputs have been applied of turboset power output reference signal, PR , on one hand, and of electric power grid frequency signal, f, on the other hand. An integral of turboset power output squared error has been served as a turboset control transient optimum criterion. Summarising results of above mentioned simulations it may be stated as it follows: • as well an algorithm as a computer program of
turboset partial loading optimal control have been operating correctly,
• abrupt automatic changes of steam turbine control valve sequence, occurring as result of turboset partial loading optimal algorithm operation, in which some control valves are being abruptly closed, and some others are being abruptly opened, do not provoke significant disturbances in turboset control transients, see e.g. Fig.3 - a steam turbine control valve sequence has been changed at PT = 0,93,
• in a considered turboset computer control
system both parameters characterising the computer device, DD - see Fig.1, i.e. the sampling time value, Ti , and the signal quantization value, Qi , influence turboset control transients; for to achieve a required
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rapidity and accuracy of turboset control system it would be enough to apply as appropriate values Ti < 0,05, and Qi < 0,005,
• under three speed change levels of the turboset power output reference signal, dPR /dt = 0.05; 0,1; 0,5 min -1 , the consecutive turboset control transients have been observed as fully satisfactory.
FINAL REMARKS Stability, accuracy, reliability, and safety are as usual associated to the notion of turboset automatic control. It is very rare to mention the turboset efficiency in a context of turboset control. However an appropriate turboset control is able to improve the specific heat consumption of turboset. This is the case, among others, of turboset partial loading optimal control. On one hand, decreasing energy consumption results in economic profit as a decrease of energy consumption per unit. On the other hand it brings about ecological advantages as decreasing consumption of energy resources as well as limiting air and biosphere pollution. Assume an improvement of steam turbine efficiency, resulting from an application of turboset partial loading optimal control, as being equal to about 0,5% - see the example of Fig.2. It may seem to be not so much. Nevertheless even a small increase of the electric power production efficiency results in considerable profits as well of economy as of ecology nature. This is because a large amount of electric energy is produced in steam power plants supplied with fossil fuels. As an example there is the Polish electric power sector as well as those ones of some other Central and Eastern Europe countries. In Poland over 90% of the electric power production comes from steam power stations fuelled with coal and lignite. Moreover the quality of the fuel containing much sulphur and fly-ash is low. Therefore any improvement of the
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electric power production efficiency has as a result, among others, a diminution of the amount of pollutants, i.e. of sulphur dioxide, nitrogen monoxide and dioxide, fly ash. The above discussed steam turboset partial loading optimal control may be recommended for steam power plants. It brings about as technical as economic and ecological advantages. Its application is reasonable in the case of turboset participation at the electric power system frequency secondary control. It requires a computer architecture of steam turboset control system as well as an appropriate software. Anyway it could be achieved within a short time, and without particular funds. REFERENCES Domachowski Z.,1994, �Economic and Ecological Aspects of Turboset Partial Loading Optimal Control�, Proc. The Tenth Conference on Steam and Gas Turbines for Power and Cogeneration Plants, Karlovy Vary, Czech Republic, October 19-20 Frater R.L., 1986, �Microprocessor Turbine Control Operating Experience�, Proc. The American Power Conference, Chicago, USA Horn P.A.L., 1988, �The Application of Digital Control to Turbine- Generators�, Proc. The IEEE International Conference on Refurbishment of Power Station Electrical Plant, London
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