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A3.1 : The Process Computer Control System of the Hydroelectric Power Stations Along the River Danube in Austria
THE PROCESS COMPUTER CONTROL SYSTEM OF THE HYDROELECTRIC POWER STATIONS ALONG THE RIVER DANUBE IN AUSTRIA W. Pillmann* and R. Stefanich** *Austrian Federal Institute of Public Health and Technical University of Vienna, Awtna **Osterreichische Brown, Boven' - Werke AG, Vienna, Awtria Abstract. In Austria the control centers of the six hydroelectric power stations along the river Danube are equipped with process computers. The control systems are designed for advanced control of the plants,including multilevel process control, interactive man-machine interfaces, advanced monitoring, security related functions and for supervising the hydroelectric plant cascade in the future. During implementation particular attentions was paid to the water level and flow rate control system in the power stations. The principles of design,the realisation of this direct digital control and the up to date experiences with the controller operation are presented in this paper. Furthermore hardware structure of the control center, the construction principles of the software package and the man machine interface are discussed briefly. Keywords. Hydro-electric power plant, power station control, digital computer applications, flow contro1,water level control, direct digital control, identification, digital simulation.
INTRODUCTION The six hydroelectric power stations along the river Danube are run by the "Osterreichische Donaukraftwerke AG", one plant is under construction and the expansion to 12 power stations is planned for 1996. Today the power plant cascade along the upper Danube in Austria from Jochenstein to Ybbs has been closed. These power plants are not independent from each other concerning energy production and discharges and should be coordinated for economical reasons. As a matter of fact the "Donaukraftwerke AG" started a project, in whose first phase the control center of each plant was equipped with process computers. The objectives of plant automation are: - the improvement of the automation system in the power plant (e.g. on-line data acquisition, data presentation, monitoring of events, printing of statistical reports) - support of control staff (monitoring and reporting of malfunctjons)
-
start-up and sequential processing of safety related functions.
In the future a central supervisory control station for the hydroelectric power plant cascade should be installed (Dang Van Mien, 1978).The scope of this center, located in Viennaincludes the
-
diversion of high water
-
smoothing of waves for navigation purposes
-
forecast of flow, discharges and water levels
coordinated operation of the power plant cascade corresponding to power consumption establishing a computer link between central supervisory control center and the Union Power Distribution Center.
The data-base needed for communication between the supervisory control center and the local control centers are already included in the power plant control system. THE RACE
The river Danube in Austria is subdiveded into reservoirs of lengths between 16 and 40 km (Fig.1). Each reservoirs is filled up by thetributaries and the upstream power plant and is drained by the down stream power plant.Therefore both plants determine simultanously the flow and the water level in the race (Fig.2).
W. Pillmann and R. Stefanich
Fig. 1. Outline map of the hydroelectric power plants in Austria
Power (m)
(130)
179
168
210
200
(187)
335
Capacity (~wyear)
(850) 1648 1143
1028
1320
1282
(1180)
1950
286
The higher the head in the power plant - that is to say the difference between upstream and downstream water level - the higher the power output.For the optimization of the energy output, one tries to regulate the discharge or the water level so that they are stationary (JANSEN 1979). As the six power plants posses a working capacity of more than 8000 GWh a year one can imagine the economic importance of the water level control and the benefit out of only some p.p.t. of an improved capacity, arising in the power plant operation. The stationary optimal plant operation with constant water levels and discharges is continuously disturbed. Reasons for this are the following:
-
a changeable supply of water of the Danube and its tributaries
-
catchment areas of different sizes
- changeable influx of surface water and
-
cumulation of retention effects along the plant cascade.
Until a short time ago, the water level control was done manually by means of continuous regulation of the discharge, based upon operation experiences and instructions. With the real-time computer system an advanced strategy can be used for water level control, including models of the race, multilevel process control and also safty related functions (Dy Liacco, 1978). THE POWER PLANT The hydraulic equipment of each power plant (see also Fig.2) consists of the - weirs to maintain the water level differences, needed for the production of hydro-power,
-
turbines coupled with generators to convert the water energy into electrical energy and
-
locks for navigation purposes.
In each power station there are from 4 to 9 turbines of two main types: Kaplan turbines with adjustable guidevane and runner blade openings and turbines with directly connected generators in a steel bulb, immersed in the water stream. The flow capacity of one turbine depends on the turbine type and ranges between 240 and 510 m3/s. During normal operation, the flow is controlled by means of turbines. If the actual flow exceeds the designed discharge quantity of turbines in the plant (Aschach 2040 m3/s1 Altenworth 2700 m3/s1the water is forced to flow over weir crests. In four power stations the weirs consists of an upper and lower weir plate which are lifted and lowered by electrical drives and chain transmissions. In the modern plants hydraulically operated weir segments are used. The operation of the locks is independent of the power station itself and fulfils the demand of shipping traffic-During the filling up and draining of the lock-chambers, the power station discharge increases up to 240 m3/s.
THE OPERATING RANGES OF WATER LEVEL CONTROL The objectives of water level control depend on river discharge between 700 to 4000 m3/s. In this particular range there are four operating modes(Osterr.Donaukraftwerke,1978) 1. In the operation range up to 1000 m3/s a continuous water flow with constant water levels in the race for navigation purposes must be guaranteed.Large control actions on turbine flow are not allowed and a maximum water level gradient must not be exceeded.
2. In the discharge range of about theaverage yearly dischange of 1400 m3/s (from 1 0 0 0 to approximately 2300 m3/s) the water level should be close to the upper limit approved by the authorities.
Process Computer Control System
tream plant r a c e
qL
discharges of locks
9~
discharges of weirs
qT
discharges of turbines
lh headwater level
qtr discharges of tributaries
It tailwater level
3. From a discharge of approximately 2300 m3/s the limit of turbine flow-capacity is attained and the level is controlled by means of adjustable weirs. The amount of control action on weirs with chain drive should be minimized in this operating range in order to prevent the driving elements from being worn out quickly.
lx
For discharge over 4000 m3/s where the turbines are completely opened and all upper weir plates are lowered, the automatic control operation is interrupted. Then the discharge is regulated manually by means of the upper and lower weir plates.
level in the race
(upper index u,d: upstream, downstream)
Discharge of u p s t r e a m plant
lx water
4. At a discharge of more than 2500 m3/s the set value of the water level near the power station must be lowered. Therefore the reference water level, measured in the race, where the level of running water enters into the banked up water level, doesn't exceed an upper limit.
pig. 2. Section of the hydroelectric power plant series.
plant
Fig. 3. Model of the race
Step- and impulse signals as well as ordinary discharge disturbances are used for testing purposes. In addition to the parameter identification a structure identification was carried out (Pillmann 1978).
CONTROL STRATEGY DESIGN In order to design the head water level control in a Danube Power Station it is justifiable from the economic point of view to carry out a process identification and a simulation for the purpose of controller disign and -testing. As experiments such a simple measurements of step responses take up several hours time, it was decided from the starttoexclude an experimental adjustment for controller design purposes. The following steps were taken for controller synthesis in each power station: Experimantal Identification of the Race By means of a least square method the multivariable system (Fig.3) was identified.
For short race segments a linearized model with the parameter ai, bi and n round the given operating point
with a low model error was computed. The parameters depend mainly on the storage capacity of the reservoir, delays and velocities of flows. Transforming Equ. 1 to frequency domain, the model structure 1
exp (-ST+_)
W. Pillmann and R. Stefanich
with a damping factor D < 1 results. (K1 proportinal band, TI integral action factor, delay time un natural frequency. Ttx This model structure was the same for all water levels in the race at some distance from the down stream plant, whereas a significant change of the model parameters in Equ. 1 could be observed for &/qd in the vicinity of down stream power station. In this case the sample transfer function (Equ.1) could be approximated through
The experimental and theoretical model made an approximate controller synthesis and filter dimensioning possible in the frequency domain. Simple checks of the control loop dynamics shows that level disturbances caused by discGarge fluctuation can be damped rigorously with a correctly adjusted disturbance variable compensation. In this case the PI-controller must perform only few changes of the actuating value for the level control operation.
Special Control Functions of the Automation System
with sufficient accuracy. K2 and Ti are model parameters. For long and winding race segments during structure identification a tendency to model structure of lower order (integral with delay time) could be observed (Schenk 1978, Neumiiller 1976)
For the automatic level control several special functions must be carried out in real-time. Some of them are summed up in the following.
-
- Discharge computation of turbines and weirs.
Theoretical Identification of the Race The Saint-Venant's partial differential equations were transformed both in time and locality to the Laplace-domain and the frequency characteristics for and 1 /qd were computed. X The theoretical frequency characteristics proved a good match with the experimental models. For the model transfer function
Figure 4 shows the cascade structure of the water level controller. The computer controller consists of a PI - level controller and a three level discharge controller for turbines and weirs. The output values of the selectors w . and wWi are directed to the T1 hydraulic regulating units. In the ccntroller a disturbance variable compensation to the discharge is realized using an on-line model of the race. During controller start-up the set values lsl and qs were put on a level with the control 'variables 1 and q. Then the value of 1,' is adjustes slowly to the level ls. Also the initial conditions of the models of the race and of the digital filters are set up.
Check of guide-vane and weir plate position after output of correcting variables to the controlling elements.
- Automatic adjustment of turbine opening limit during weir operation. - Selection of weir sequence for discharge control. - Controller start-up and change to manual operation.
-
A
-
The control of a Kaplan turbine must be well adjusted, so that the position of the blade tilts correspond automatically with the wicket gate opening so as to produce a maximum efficiency envelope curve. The connexion between guide wheel opening and blade tilt is additionally a function of the effective head, which is adjusted by the computer.So the effeciency envelope curve is optimized in the entire range of the effective head.
for x near the down stream plant lX/9 a positive gradient for the phase characteristic was found out. Therefore the modification of model structure of the race according to Equ.3 could be verified.
Controller Structure and -Synthesis
Transition from turbines to weir discharge operation.
software module is used for an energy output optimization of turbines operation. During the operation of n turbines the total flow is distributed equally. With this method a maximum of the efficiency en of the turbines can be achieved. If the efficiency en is smaller than en+l respectively en-1 a message is printed in order to turn a machine-set on or off.
- In case of a machine failure of one or several machines, the discharge of turbines must be diverted to weirs as fast as possible. In this special case the controller is out of operation and a s w r a t e safety function is activated, so that the instant opening of the weir plates can be performed. If the initial power plant discharge is obtained, the lowering of the weir plates is stopped automatically.
Process Computer Control System
h & r n
o
w
o
w
m c
.A
W. Pillmann and R. Stefanich
with the active power of generator i: p.
Water-eveL Computation
I'
the effective head of turbine i: hi and from
The control variable 1 is disturbed for a X short time (approx.up to 10 cm during 15min) in case of lock operations. The water qmntity flowing into the downstream locks takes its effect on a suction wave, an emptying of upstream locks causes a swell in the race.
h. = lh
-
lt
-
lri
-
lv
with head water level lh, tail water level It, loss of head in forebay by headwater racks 1 . and velocity head from entrance to
As the disturbances are only of short time the controller must suppress waves of this kind. The problem was solved by filtering the water level by means of a filter of 4th order making a compromise between measurement dynamics and disturbance variable suppression.
rl
scroll and in tailrace 1 From Equ.4 and 5 v' follows, that the turbine flow depends on over 20 primary measurement values. The discharge of weirs is computed from
The actual discharge q is a synthetic measurement value of weir- and turbine discharge and qT, which are computed out of dif£ere2 primary measurement values. According to the complexity of the physically coupled variables, the quantity q is proved to be very critical in regard of accuracy and stability.
qw = f (lh,lt,hcl,..hc6,hllI-.h16)
(6)
with the level of ith upper weir crest hci and the level of lower weir gate h li' In the block diagramm (Fig.5) the functional dependence of the total plant discharge q and the primary measurements is depicted.
For discharge computation the static charcteristics between head and active power is approximated for each turbine-generator set by means of a least square polynomial fit The turbine discharge is computed from
Simulation studies The race models were the basis for testing the controller software modules. Simulation studies during software implementation saved time-consuming tests in the plant and functional software errors could be reduced to a minimum. Several tests for the water level controller were carried through in a simulation model e.g.:
turbine 1
-
a
controller start-up and shut-down
-
controller dynamics in the operating ran9 from 700 to 4000 m3/s
-
water level response during transition from smoothing waves to water level control mode
- effects of race model parameters to the controller function
-
frequency of operation of the regulating units
-
control actions during heavy increasing and decreasing of discharge in the race.
I
The model tests showed, that many parameters in the softwarepackage could only be adjusted during simulation.Severa1 tests, e.g a switch over from turbine to weir operation, would not be possible in the plant itself. The obtained experiences from simulation studies proved to be extraordinarily useful during starting phase of the computer controller. Fiq. 5. Computation of the discharge control variable q.
Process Computer Control System
HARDWARE
Mostly used is assembler Macro 11, but for complex control functions (DDC, discharge computation) Fortran IV is used. The data exchange between the programm modules is carried out with FIFO (first in- first out) interfaces. In a common data segment the actual process state is stored. The data aquisition system serves as a timer for the automation system. The main duty cycles are:
A typical hardware configuration is shown in Fig. 6. Computer PDP 11/35 and recently PDP 11/34 are used with 96 k words (16 bit) core memory and a magnetic disk with a storage capacity of 2,4 Mbyte. The proces interface had be adapted especially for each power plant. For data aquisition the following peripheral moduls were used:
- Data aquisition, discharge computation, 5s discharge control
- the modular process data subsystem ED-1000
-
- a 12 bit ADC with 64 channel multiplexer
- Data exchange to supervisory control center
- INDACTIC 22 for remote data transfers.
The output of digital and analogue commands and data (opening and closing commands to turbines, start-stop impulses to weirs,plant discharge, water levels) is also realized with the ED-1000 process interface. SOFTWARE A system analysis was carried out before software implementation. The complete softwarepackage was planned hierarchically using a top-down design. The software is segmented into modules used for data aquisition, measurement processing, water level control, printing of reports and messages, data communication, man machine dialogue and others.
Water level control
-
Adjusting of efficiency envelope curve depending on the effective head
-
Statistical reports
1 hour.
The structure of software modules was implemented independently from the various plant types. Each module consists of a basis program using a plant specific data set. The module segmentation enables a relatively simple adaption to the different plant characteristics. The module structuring improves the software reliability and the software portability from plant to plant is made possible.Also software changes can be planned according to a uniform standard for all plants.
TO CENTRAL SUPERVISORY (VIENNA)
I CONTROL CENTER VIDEO DISPLAY TERMINAL FOR MAN-t1ACH I NE DIALOGUE ALAPA
ANALOG
DIGITAL
--ANALOG INPUT
(POWER, LOCAL LEVE S, HEIGHTS OF WEIRS\
REMOTE LEVELS
? iOFF
I G TAL INPUT CN STATES, DIGI TAL MEASURANDS)
COMEiAND OUTPUT
Fig. 6. Typical hardware configuration of a hydroelectric power station.
W. Pillmann and R. Stefanich MAN
-
MACHINE INTERFACE
The functional design of the man - machine interface was considered to be important. It enables the interactive control and monitoring of the plant operation using CRT and printer for documentation. The control software offers the possibilities of testing measurement values, modification of set values and controller parameters, starting the output of various reports, start-up and shut-down procedures, correction of data and reports and other functional details. The output can be directed by choice to display or printer. Significant commands, events and alarms are stored, recorded or printed. In case of process equipment failure precisely defined error messages and system diagnostics are displayed to the operators. Also an emergency process control operation with restricted control functions is provided. FIRST EXPERIENCES WITH THE CONTROL SYSTEM In the first testing phase of the water level control the control loop was closed by the control staff and computed commands were manually distributed to the turbines. This testing phase of approximately 48 hours was sufficient to adjust the controller parameters and to obtain a first insight into the action of the computer control. Problems arose with the control of the non measurable discharge. Because of nonlinear coupling of turbine discharge, machine power, effective head and water level, frequent turbine adjustement occurs. In the above mentioned method of discharge computation and by selecting turbine actuator steps of approx. 15 m3/s = 1,5 MW, the discharge control loop could be stabilized. -
power stations is presented. Continuing in detail, the water level control is discussed with its mode of action in different process operating ranges. For the controller strategy design a detailed process study including model building, parameter identification and digital simulation was accomplished. Some special automation procedures enable controller operation within the widedischarge range from 700 to 4000 m3/s. In the authors opinion, in the immediate future the closed loop water level control must be adjusted during high and low discharge quantities and feasible control methods for the hydroelectric power plant series along the river Danube must be developed on the basis of accumulated experiences. ACKNOWLEDGEMENTS The authors thank the control staff of the "Bsterreichische Donaukraftwerke AG" for their supports during measurements and Mag. Bielesz for some helpful hints. REFERENCES Dang Van Mien H., G.Davoust, J-Lecouturier, A-Tremenbert (1978). Centralized Control and Regulation of an Hydro-electric Power Plant Series. 7th IFAC World Congress Dy Liacco T.E. (1978). An Overview of Practices and Trends in Power System Control Centers. 7th IFAC World Congress Jansen P.Ph.(Ed.) et a1 (1979). Principles of River Engineering. Pitmann Pub.Lim.London Neumuller M., W-Bernhauer (1976).Stauregelung und AbfluRverhalten von Laufkraftwerken mit automatischen Verfahren. Wasserwirtschaft. Vol 66/9 p.253-256
In the second testing phase (closed loop control) a satisfying operation of water level " b s t e r r e i c h i s c h e D o n a u k r a f t w e r k e AG" and control was obtained. This is true for a large "bsterreichische Brown Boveri-Werke AG" variation of water quantity of upstream plant, (1978). Funktionsbeschreibung fur die for lock operation and disconnection of machines ProzeBrechenanlage der Donaukraftwerke. from high voltage power line. An example of the water level control during normal operaPillmann W. (1978). Strukturadaptive tion is shown in figure 7. Identifikation stetiger Regelsysteme. Diss.TU-Vienna (1978) CONCLUSION Schenk R. (1978). Digital Pegelregelung an einem FluBkraftwerk. In this paper the process computer autoRegelungstechnik und ProzeBdatenvermation system of the six Austrian hydroelectric arbeitung. 1978 H3 p.84-87.
264.40
264.30
Controlled w a t e r l e v e l Ottensheirn ( m )
-
Fig. 7. Record chart of the controlled water level and discharge of Ottensheim.
uA
INTRODUCTION The six hydroelectric power stations along the river Danube are run by the "Osterreichische Donaukraftwerke AG", one plant is under construction and the expansion to 12 power stations is planned for 1996. Today the power plant cascade along the upper Danube in Austria from Jochenstein to Ybbs has been closed. These power plants are not independent from each other concerning energy production and discharges and should be coordinated for economical reasons. As a matter of fact the "Donaukraftwerke AG" started a project, in whose first phase the control center of each plant was equipped with process computers. The objectives of plant automation are: - the improvement of the automation system in the power plant (e.g. on-line data acquisition, data presentation, monitoring of events, printing of statistical reports) - support of control staff (monitoring and reporting of malfunctjons)
-
start-up and sequential processing of safety related functions.
In the future a central supervisory control station for the hydroelectric power plant cascade should be installed (Dang Van Mien, 1978).The scope of this center, located in Viennaincludes the
-
diversion of high water
-
smoothing of waves for navigation purposes
-
forecast of flow, discharges and water levels
coordinated operation of the power plant cascade corresponding to power consumption establishing a computer link between central supervisory control center and the Union Power Distribution Center.
The data-base needed for communication between the supervisory control center and the local control centers are already included in the power plant control system. THE RACE
The river Danube in Austria is subdiveded into reservoirs of lengths between 16 and 40 km (Fig.1). Each reservoirs is filled up by thetributaries and the upstream power plant and is drained by the down stream power plant.Therefore both plants determine simultanously the flow and the water level in the race (Fig.2).
W. Pillmann and R. Stefanich
Fig. 1. Outline map of the hydroelectric power plants in Austria
Power (m)
(130)
179
168
210
200
(187)
335
Capacity (~wyear)
(850) 1648 1143
1028
1320
1282
(1180)
1950
286
The higher the head in the power plant - that is to say the difference between upstream and downstream water level - the higher the power output.For the optimization of the energy output, one tries to regulate the discharge or the water level so that they are stationary (JANSEN 1979). As the six power plants posses a working capacity of more than 8000 GWh a year one can imagine the economic importance of the water level control and the benefit out of only some p.p.t. of an improved capacity, arising in the power plant operation. The stationary optimal plant operation with constant water levels and discharges is continuously disturbed. Reasons for this are the following:
-
a changeable supply of water of the Danube and its tributaries
-
catchment areas of different sizes
- changeable influx of surface water and
-
cumulation of retention effects along the plant cascade.
Until a short time ago, the water level control was done manually by means of continuous regulation of the discharge, based upon operation experiences and instructions. With the real-time computer system an advanced strategy can be used for water level control, including models of the race, multilevel process control and also safty related functions (Dy Liacco, 1978). THE POWER PLANT The hydraulic equipment of each power plant (see also Fig.2) consists of the - weirs to maintain the water level differences, needed for the production of hydro-power,
-
turbines coupled with generators to convert the water energy into electrical energy and
-
locks for navigation purposes.
In each power station there are from 4 to 9 turbines of two main types: Kaplan turbines with adjustable guidevane and runner blade openings and turbines with directly connected generators in a steel bulb, immersed in the water stream. The flow capacity of one turbine depends on the turbine type and ranges between 240 and 510 m3/s. During normal operation, the flow is controlled by means of turbines. If the actual flow exceeds the designed discharge quantity of turbines in the plant (Aschach 2040 m3/s1 Altenworth 2700 m3/s1the water is forced to flow over weir crests. In four power stations the weirs consists of an upper and lower weir plate which are lifted and lowered by electrical drives and chain transmissions. In the modern plants hydraulically operated weir segments are used. The operation of the locks is independent of the power station itself and fulfils the demand of shipping traffic-During the filling up and draining of the lock-chambers, the power station discharge increases up to 240 m3/s.
THE OPERATING RANGES OF WATER LEVEL CONTROL The objectives of water level control depend on river discharge between 700 to 4000 m3/s. In this particular range there are four operating modes(Osterr.Donaukraftwerke,1978) 1. In the operation range up to 1000 m3/s a continuous water flow with constant water levels in the race for navigation purposes must be guaranteed.Large control actions on turbine flow are not allowed and a maximum water level gradient must not be exceeded.
2. In the discharge range of about theaverage yearly dischange of 1400 m3/s (from 1 0 0 0 to approximately 2300 m3/s) the water level should be close to the upper limit approved by the authorities.
Process Computer Control System
tream plant r a c e
qL
discharges of locks
9~
discharges of weirs
qT
discharges of turbines
lh headwater level
qtr discharges of tributaries
It tailwater level
3. From a discharge of approximately 2300 m3/s the limit of turbine flow-capacity is attained and the level is controlled by means of adjustable weirs. The amount of control action on weirs with chain drive should be minimized in this operating range in order to prevent the driving elements from being worn out quickly.
lx
For discharge over 4000 m3/s where the turbines are completely opened and all upper weir plates are lowered, the automatic control operation is interrupted. Then the discharge is regulated manually by means of the upper and lower weir plates.
level in the race
(upper index u,d: upstream, downstream)
Discharge of u p s t r e a m plant
lx water
4. At a discharge of more than 2500 m3/s the set value of the water level near the power station must be lowered. Therefore the reference water level, measured in the race, where the level of running water enters into the banked up water level, doesn't exceed an upper limit.
pig. 2. Section of the hydroelectric power plant series.
plant
Fig. 3. Model of the race
Step- and impulse signals as well as ordinary discharge disturbances are used for testing purposes. In addition to the parameter identification a structure identification was carried out (Pillmann 1978).
CONTROL STRATEGY DESIGN In order to design the head water level control in a Danube Power Station it is justifiable from the economic point of view to carry out a process identification and a simulation for the purpose of controller disign and -testing. As experiments such a simple measurements of step responses take up several hours time, it was decided from the starttoexclude an experimental adjustment for controller design purposes. The following steps were taken for controller synthesis in each power station: Experimantal Identification of the Race By means of a least square method the multivariable system (Fig.3) was identified.
For short race segments a linearized model with the parameter ai, bi and n round the given operating point
with a low model error was computed. The parameters depend mainly on the storage capacity of the reservoir, delays and velocities of flows. Transforming Equ. 1 to frequency domain, the model structure 1
exp (-ST+_)
W. Pillmann and R. Stefanich
with a damping factor D < 1 results. (K1 proportinal band, TI integral action factor, delay time un natural frequency. Ttx This model structure was the same for all water levels in the race at some distance from the down stream plant, whereas a significant change of the model parameters in Equ. 1 could be observed for &/qd in the vicinity of down stream power station. In this case the sample transfer function (Equ.1) could be approximated through
The experimental and theoretical model made an approximate controller synthesis and filter dimensioning possible in the frequency domain. Simple checks of the control loop dynamics shows that level disturbances caused by discGarge fluctuation can be damped rigorously with a correctly adjusted disturbance variable compensation. In this case the PI-controller must perform only few changes of the actuating value for the level control operation.
Special Control Functions of the Automation System
with sufficient accuracy. K2 and Ti are model parameters. For long and winding race segments during structure identification a tendency to model structure of lower order (integral with delay time) could be observed (Schenk 1978, Neumiiller 1976)
For the automatic level control several special functions must be carried out in real-time. Some of them are summed up in the following.
-
- Discharge computation of turbines and weirs.
Theoretical Identification of the Race The Saint-Venant's partial differential equations were transformed both in time and locality to the Laplace-domain and the frequency characteristics for and 1 /qd were computed. X The theoretical frequency characteristics proved a good match with the experimental models. For the model transfer function
Figure 4 shows the cascade structure of the water level controller. The computer controller consists of a PI - level controller and a three level discharge controller for turbines and weirs. The output values of the selectors w . and wWi are directed to the T1 hydraulic regulating units. In the ccntroller a disturbance variable compensation to the discharge is realized using an on-line model of the race. During controller start-up the set values lsl and qs were put on a level with the control 'variables 1 and q. Then the value of 1,' is adjustes slowly to the level ls. Also the initial conditions of the models of the race and of the digital filters are set up.
Check of guide-vane and weir plate position after output of correcting variables to the controlling elements.
- Automatic adjustment of turbine opening limit during weir operation. - Selection of weir sequence for discharge control. - Controller start-up and change to manual operation.
-
A
-
The control of a Kaplan turbine must be well adjusted, so that the position of the blade tilts correspond automatically with the wicket gate opening so as to produce a maximum efficiency envelope curve. The connexion between guide wheel opening and blade tilt is additionally a function of the effective head, which is adjusted by the computer.So the effeciency envelope curve is optimized in the entire range of the effective head.
for x near the down stream plant lX/9 a positive gradient for the phase characteristic was found out. Therefore the modification of model structure of the race according to Equ.3 could be verified.
Controller Structure and -Synthesis
Transition from turbines to weir discharge operation.
software module is used for an energy output optimization of turbines operation. During the operation of n turbines the total flow is distributed equally. With this method a maximum of the efficiency en of the turbines can be achieved. If the efficiency en is smaller than en+l respectively en-1 a message is printed in order to turn a machine-set on or off.
- In case of a machine failure of one or several machines, the discharge of turbines must be diverted to weirs as fast as possible. In this special case the controller is out of operation and a s w r a t e safety function is activated, so that the instant opening of the weir plates can be performed. If the initial power plant discharge is obtained, the lowering of the weir plates is stopped automatically.
Process Computer Control System
h & r n
o
w
o
w
m c
.A
W. Pillmann and R. Stefanich
with the active power of generator i: p.
Water-eveL Computation
I'
the effective head of turbine i: hi and from
The control variable 1 is disturbed for a X short time (approx.up to 10 cm during 15min) in case of lock operations. The water qmntity flowing into the downstream locks takes its effect on a suction wave, an emptying of upstream locks causes a swell in the race.
h. = lh
-
lt
-
lri
-
lv
with head water level lh, tail water level It, loss of head in forebay by headwater racks 1 . and velocity head from entrance to
As the disturbances are only of short time the controller must suppress waves of this kind. The problem was solved by filtering the water level by means of a filter of 4th order making a compromise between measurement dynamics and disturbance variable suppression.
rl
scroll and in tailrace 1 From Equ.4 and 5 v' follows, that the turbine flow depends on over 20 primary measurement values. The discharge of weirs is computed from
The actual discharge q is a synthetic measurement value of weir- and turbine discharge and qT, which are computed out of dif£ere2 primary measurement values. According to the complexity of the physically coupled variables, the quantity q is proved to be very critical in regard of accuracy and stability.
qw = f (lh,lt,hcl,..hc6,hllI-.h16)
(6)
with the level of ith upper weir crest hci and the level of lower weir gate h li' In the block diagramm (Fig.5) the functional dependence of the total plant discharge q and the primary measurements is depicted.
For discharge computation the static charcteristics between head and active power is approximated for each turbine-generator set by means of a least square polynomial fit The turbine discharge is computed from
Simulation studies The race models were the basis for testing the controller software modules. Simulation studies during software implementation saved time-consuming tests in the plant and functional software errors could be reduced to a minimum. Several tests for the water level controller were carried through in a simulation model e.g.:
turbine 1
-
a
controller start-up and shut-down
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controller dynamics in the operating ran9 from 700 to 4000 m3/s
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water level response during transition from smoothing waves to water level control mode
- effects of race model parameters to the controller function
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frequency of operation of the regulating units
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control actions during heavy increasing and decreasing of discharge in the race.
I
The model tests showed, that many parameters in the softwarepackage could only be adjusted during simulation.Severa1 tests, e.g a switch over from turbine to weir operation, would not be possible in the plant itself. The obtained experiences from simulation studies proved to be extraordinarily useful during starting phase of the computer controller. Fiq. 5. Computation of the discharge control variable q.
Process Computer Control System
HARDWARE
Mostly used is assembler Macro 11, but for complex control functions (DDC, discharge computation) Fortran IV is used. The data exchange between the programm modules is carried out with FIFO (first in- first out) interfaces. In a common data segment the actual process state is stored. The data aquisition system serves as a timer for the automation system. The main duty cycles are:
A typical hardware configuration is shown in Fig. 6. Computer PDP 11/35 and recently PDP 11/34 are used with 96 k words (16 bit) core memory and a magnetic disk with a storage capacity of 2,4 Mbyte. The proces interface had be adapted especially for each power plant. For data aquisition the following peripheral moduls were used:
- Data aquisition, discharge computation, 5s discharge control
- the modular process data subsystem ED-1000
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- a 12 bit ADC with 64 channel multiplexer
- Data exchange to supervisory control center
- INDACTIC 22 for remote data transfers.
The output of digital and analogue commands and data (opening and closing commands to turbines, start-stop impulses to weirs,plant discharge, water levels) is also realized with the ED-1000 process interface. SOFTWARE A system analysis was carried out before software implementation. The complete softwarepackage was planned hierarchically using a top-down design. The software is segmented into modules used for data aquisition, measurement processing, water level control, printing of reports and messages, data communication, man machine dialogue and others.
Water level control
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Adjusting of efficiency envelope curve depending on the effective head
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Statistical reports
1 hour.
The structure of software modules was implemented independently from the various plant types. Each module consists of a basis program using a plant specific data set. The module segmentation enables a relatively simple adaption to the different plant characteristics. The module structuring improves the software reliability and the software portability from plant to plant is made possible.Also software changes can be planned according to a uniform standard for all plants.
TO CENTRAL SUPERVISORY (VIENNA)
I CONTROL CENTER VIDEO DISPLAY TERMINAL FOR MAN-t1ACH I NE DIALOGUE ALAPA
ANALOG
DIGITAL
--ANALOG INPUT
(POWER, LOCAL LEVE S, HEIGHTS OF WEIRS\
REMOTE LEVELS
? iOFF
I G TAL INPUT CN STATES, DIGI TAL MEASURANDS)
COMEiAND OUTPUT
Fig. 6. Typical hardware configuration of a hydroelectric power station.
W. Pillmann and R. Stefanich MAN
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MACHINE INTERFACE
The functional design of the man - machine interface was considered to be important. It enables the interactive control and monitoring of the plant operation using CRT and printer for documentation. The control software offers the possibilities of testing measurement values, modification of set values and controller parameters, starting the output of various reports, start-up and shut-down procedures, correction of data and reports and other functional details. The output can be directed by choice to display or printer. Significant commands, events and alarms are stored, recorded or printed. In case of process equipment failure precisely defined error messages and system diagnostics are displayed to the operators. Also an emergency process control operation with restricted control functions is provided. FIRST EXPERIENCES WITH THE CONTROL SYSTEM In the first testing phase of the water level control the control loop was closed by the control staff and computed commands were manually distributed to the turbines. This testing phase of approximately 48 hours was sufficient to adjust the controller parameters and to obtain a first insight into the action of the computer control. Problems arose with the control of the non measurable discharge. Because of nonlinear coupling of turbine discharge, machine power, effective head and water level, frequent turbine adjustement occurs. In the above mentioned method of discharge computation and by selecting turbine actuator steps of approx. 15 m3/s = 1,5 MW, the discharge control loop could be stabilized. -
power stations is presented. Continuing in detail, the water level control is discussed with its mode of action in different process operating ranges. For the controller strategy design a detailed process study including model building, parameter identification and digital simulation was accomplished. Some special automation procedures enable controller operation within the widedischarge range from 700 to 4000 m3/s. In the authors opinion, in the immediate future the closed loop water level control must be adjusted during high and low discharge quantities and feasible control methods for the hydroelectric power plant series along the river Danube must be developed on the basis of accumulated experiences. ACKNOWLEDGEMENTS The authors thank the control staff of the "Bsterreichische Donaukraftwerke AG" for their supports during measurements and Mag. Bielesz for some helpful hints. REFERENCES Dang Van Mien H., G.Davoust, J-Lecouturier, A-Tremenbert (1978). Centralized Control and Regulation of an Hydro-electric Power Plant Series. 7th IFAC World Congress Dy Liacco T.E. (1978). An Overview of Practices and Trends in Power System Control Centers. 7th IFAC World Congress Jansen P.Ph.(Ed.) et a1 (1979). Principles of River Engineering. Pitmann Pub.Lim.London Neumuller M., W-Bernhauer (1976).Stauregelung und AbfluRverhalten von Laufkraftwerken mit automatischen Verfahren. Wasserwirtschaft. Vol 66/9 p.253-256
In the second testing phase (closed loop control) a satisfying operation of water level " b s t e r r e i c h i s c h e D o n a u k r a f t w e r k e AG" and control was obtained. This is true for a large "bsterreichische Brown Boveri-Werke AG" variation of water quantity of upstream plant, (1978). Funktionsbeschreibung fur die for lock operation and disconnection of machines ProzeBrechenanlage der Donaukraftwerke. from high voltage power line. An example of the water level control during normal operaPillmann W. (1978). Strukturadaptive tion is shown in figure 7. Identifikation stetiger Regelsysteme. Diss.TU-Vienna (1978) CONCLUSION Schenk R. (1978). Digital Pegelregelung an einem FluBkraftwerk. In this paper the process computer autoRegelungstechnik und ProzeBdatenvermation system of the six Austrian hydroelectric arbeitung. 1978 H3 p.84-87.
264.40
264.30
Controlled w a t e r l e v e l Ottensheirn ( m )
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Fig. 7. Record chart of the controlled water level and discharge of Ottensheim.
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