8/6/2019 Dynamics Problems of Frequency and Active

1/6

Abstract -- One of the main problems in the area of frequencycontrol is providing proper dynamics of frequency and activepower deviation. The paper deals with analysis of the frequencyand active power deviation dynamics during turbine governoraction. For providing effective frequency control dynamiccharacteristics of governors should be very similar. The authorssuggest a new method for control of the output power of generators using a necessary dynamic law. It allows ensuringfrequency and active power control in a mode, adaptive to theoperational conditions. Developed dynamic model was used foranalysis of frequency and active power behavior duringdisturbances in the power system. Results of mathematicalmodeling were compared with the field tests carried out at thehydro power plant. Field tests proved effectiveness of suggestednew control method.

Index Terms -- dynamics, frequency control, governor,mathematical modeling, testing.

I. I NTRODUCTION

ESTRUCTURING of the electrical industry andtransnational networks causes new technical and political

problems. Unification of Western and Eastern electricalutilities forces to revise some technical aspects of the power system control. Among other aspects there is the problem of frequency and active power control. Principles of frequencycontrol in the Eastern European utilities and UCTE power system were different. There is a trend to develop commonapproach in that area. One of the main problems in the area of frequency control is providing proper dynamics of frequencyand active power deviation. Improper selection of frequencycontrol characteristics for generators, participating infrequency control, will lead to the swings of power among thegenerators. UCTE requirements determine dynamics of the

primary control and, hence, power system frequency deviationdynamics [1], [2]. The behavior of governor for each generator

is not specified. The paper deals with an analysis of thefrequency and active power deviation dynamics during turbinegovernor action. The authors suggest a new control method,which allows providing necessary deviation dynamics duringthe frequency and active power control. Results of amathematical modeling of the active power control as well asfield test results of generator output power are presented in the

paper.

II. P ROBLEMS OF UNIFICATION OF THE WESTWRN AND EASTERNELECTRIC POWER SYSTEMS

After the breakdown of the former Soviet Union newindependent States were created with independent electricutilities. Eastern European countries created joint power system CENTREL that later was connected to the UCTEsystem. The Baltic States utilities work in parallel with theRussian and some other new independent utilities. There is atrend to investigate possibility of unification of the Russian

power system with UCTE/CENTREL system.The basic objectives of the UCTE or any other large

integrated power system is to obtain the best possibleutilization of power generating facilities mainly through theexchange of electrical power and energy [3], [4]. Unificationmeans fulfillment of common technical requirements by all

power utilities of a joint power system. Frequency control problem is one of the most important items to be discussed before unification.

At a present moment UCTE frequency control approachdiffers very much from frequency control principles of theformer Soviet Unions Joint power system. UCTE power

systems developed common approach to the frequency control.The differences are in parameters of electric power equipment,main features of transmission network, quality of control andstandards. Besides, Western systems are more concentrated,while Eastern systems have weak ties with some regions. Thefirst ties between Western and Eastern regions can be weak atthe beginning.

Common requirements were developed for the primary andsecondary frequency control of the joint power system [2], [4].It is possible to accept common standards for quality of control, but difference among power systems will exist. It willinfluence problem of stability and quality of control.

The primary control of frequency in the Russian joint power system has uncoordinated and random character [5]. Quality of frequency control (frequency deviations and dampingfrequency) in Western and Eastern power systems are differenttoo [6]. Droop of governors characteristics change in a rangeof 7% to 30% for joint power system depending on the number and structure of parallel operating generators. The secondarycontrol of frequency is used for provision of operational loadcurves, keeping exchange of electrical power and energy andautomatic frequency control. Especially dedicated hydro

power plants are used for the fast frequency control.

Dynamics Problems of Frequency and ActivePower Control in Electric Power System

V. Chuvychin, Senior Member, IEEE, N. Gurov, Nonmember, A. Skutelis, Ph.D. Student, Nonmember, V. Strelkovs, Graduate Student, Nonmember

R

0-7803-7967-5/03/$17.00 2003 IEEE

Paper accepted for presentation at 2003 IEEE Bologna Power Tech Conference, June 23th-26th, Bologna, Italy

8/6/2019 Dynamics Problems of Frequency and Active

2/6

New requirements to the frequency control in joint power system of Russia are now under consideration [5], [6].

III. D YNAMIC REQUIREMENTS TO ACTIVE POWER CONTROL

One of the main requirements to the primary control of frequency and active power is uniform distribution of generators involved into control process [2]. The moregenerators will be involved in the frequency control process,the more smooth frequency control process takes place. Whenonly selected number of generators participates in frequencycontrol, their dynamic characteristics should be very similar.

Generating units, participating in the primary and secondarycontrol, should fulfill dynamic requirements concerning thetime of active power deviation [2], [3]. Not identical dynamiccharacteristics of control units can cause unnecessary power swings among generating units or power plants.

The main dynamic requirement determines primaryfrequency control activation time. The deployment time of the

primary control reserves of the various controlled areas should be as similar as possible, in order to minimize dynamic

interaction of controlled areas. The primary control reservesmust be fully activated within 30 seconds. There was arequirement to activate 50% of reserves in 5 seconds. Later characteristic of activation reserves with respect to time wasaccepted as linear law [2].

Coordination of dynamic characteristics of Western andEastern power systems can be provided not only by similar dynamic characteristics of all generating units, but also by

providing required characteristics at the borders of controlledareas independently on characteristics of generators inside thearea. Results of most theoretical investigations presented in

paper are associated with behavior of power and frequency at

the borders of controlled areas.

IV. N EW METHOD TO CONTROL THE OUTPUT POWER OF AGENERATOR

The cause of research, carried out by the authors, wasdesire of local electric power utility Latvenergo to investigatedynamic features for existing turbine governors of the hydro

power plants of Latvenergo. The authors performed analysis of operational conditions for Latvenergo electric power system.The criterion for quality of control for analyzed equipment isits ability to satisfy frequency and active power controlrequirements.

The aim of investigation was to find a method for increasing stability of frequency and active power control. Theauthors suggest a new method for control of the generatorsoutput power of using a necessary dynamic law. Advantage of the new method is illustrated by comparison of the new andconventional control methods.

Let us compare differences of two methods using as anexample active powers variation process during the start of a generator. Fig. 1 illustrates behavior of generators output

power during the start of generator at the Riga Hydro Power Plant for the case when existent governor was in operation.

Test requirement was to adjust 50 MW of output power.During the transient overshooting of output power up to 75MW takes place. The reason was that normally parameters of governor are adjusted for some average operational conditionand for extreme conditions parameters will not be optimal.

Fig. 1. Increase of active power during the start of generator

Active power variation character depends mainly on parameters of the turbine governor. Conventional turbinegovernors in general are control arrangements with a feedback determined by position of a servomotor. Quality of the control

process depends on the number of parameters: value of thewater head, active power setting and governors feedback

parameters. Thus, for the same valve opening of a turbine anoutput power of generator will be different for different levelsof the water head. The water head level is a variable quantityand its measuring accuracy depends very much on the number of neighbor generators in operation. Hence, output power variation character is not stable and for some conditionsfluctuation during the change of output power are inevitable.

This is the main drawback of active power control withconventional governor systems. There is a need in new controlmethods, which are adaptive to a change of operationconditions.

Control system will be adaptive to the change of operational conditions in case when dynamic law is used for control. The chapter describes suggested approach.

It is convenient to use phase plane for providing necessarycontrol dynamics [9]. For considered case it will be phase

plane dP/dt (rate of change of the output active power) as afunction of the active power P.

Fig.2 illustrates an example of a possible controlcharacteristic for control of output active power during thestart of a generator or frequency control. Suggested methodallows to control not only generators power, but also a rate of

power change.During the start of the generator or frequency control

process a point at the phase plane can describe its operationalstate with coordinates dP/dt and P. When change of the power does not coincide with the control characteristic,microprocessor-based turbine governor with the given controlcharacteristic should adjust output parameters. In case the

point, located below the control characteristic, describes

8/6/2019 Dynamics Problems of Frequency and Active

3/6

operational state, governor creates a signal to increase theopening of the valve. It results in the water flow increase andhence, increase of dP/dt value. When initial rate of change of the output active power is larger than its setting (operational

point is above the operational characteristic), governor createssignal to decrease water flow through a turbine.

Fig. 2. Possible control characteristic

The acceleration of the generator starts at point 0 (Fig.2.)with no-load operational condition. Increase of the water flow

through the turbine results in increasing of the output power until rate of change of power reaches the given settingdP/dt SET . (Point 1). Then generators output power increaseswith a constant speed (section 1..2). The closer is value P N tothe power setting, the slower is increase of the power (section2..3). Adjustable section 3..4 characterizes dead zone P of the power control.

When active power exceeds setting value the state pointwill be in the range of operational characteristic, described bysection 4...6. Here the rate of change of power is negative and,hence, the control system will minimize the output power of generator.

Fig. 3 represents an example of generators active power behavior during the start of a hydro unit at Riga Hydro Power Plant. Here initial setting of the rate of change of power wasassumed equal to dP/dt=1.5 MW/s and dead zone to P=0.

Fig. 3. Active power behavior during the start of generator

Time diagram illustrates correspondence of the process tothe preset control characteristic. Approximately during 30seconds the output power increases with a constant presetvalue of dP/dt. Then the rate of change of power decreases and

the transient process becomes slower. The total time of the process is about 35 seconds.

Suggested method ensures smooth variation of generators power for any water head level and other operationalconditions. Some restraining should be taken intoconsiderations for parameters of control characteristic. It refersto the maximal value of dP/dt SET setting and location of point 2at the control characteristic. The maximal value of dP/dt SET is

limited by dynamic property of a water flow through theturbine and by maximal allowed speed of a servomotorsmovement for actuating mechanism of a turbine. To avoidwater hammer phenomenon a turbine valve should not beopened too fast.

Slope 2-5 of control characteristic (Fig. 2) should not havesteep front. Due to inertial character of a turbine governor result of input action appears with a time delay. Oscillations of output power can take place in case slope 2-5 has a steep front.Fig. 4 presents example of control characteristic with a steepfront of slope 2-5 and Fig. 5 illustrates an example of generators active power behavior. Process is not stable and

harmonic oscillations of the output power of generator take place.

Fig. 4. Control characteristic with a steep front

Fig. 5. Oscillations of active power during the start of generator

Suggested new control method can be used for control of any type of generating unit for the hydro and steam operated

power plants.

V. M ATHEMATICAL MODELING OF CONTROL PROCESS

Mathematical modeling of active power behavior wascarried out The new control method can find interesting

8/6/2019 Dynamics Problems of Frequency and Active

4/6

application for the primary and secondary frequency control inthe power system. Behavior of power and frequency at the

borders of controlled areas is considered as an example. Theworst case is frequency control of two power systems,connected through the weak tie line.

Fig. 6 shows two power systems I and II connected bytransmission L with power flow P L. Each power system haslocal load P I and P II. The modeling of frequency and active

power behavior were carried out for such system in case of sudden disturbance. Frequency deviation takes place in both

power systems during the change of a system load. Thecharacter of frequency deviation depends on the system

parameters and parameters of the turbine governors. When parameters of both systems are different, instant values of frequency during transients differ in system I and II. Hence,control actions of governors in both systems are different too.It causes unnecessary equalizing power flows through tietransmission and, hence, deterioration or control quality andsystem stability.

Fig. 6. Power systems I and II connected via transmission line

The authors developed mathematical model for consideredcase (Fig. 6). Each system was represented by an equivalentgenerator and governor system for the hydroelectric unit.Hydraulic parameters of the waterways are also taken intoaccount. During the analysis transient processes weredescribed by differential equations. Equations determine

deviation of control value during the disturbance. Results of comparison of conventional control system and system withnew dynamic control is presented in this chapter.

Reaction of the system to a sudden change of a system loadwas analyzed. The change of control reactions of the systemgovernors can be expressed as

P NP k f ; K (1)f S N

= =

where P is a change of control action;f =f -f N is a frequency deviation from the rated value;K is proportionality factor;S is droop of governor.

Transient process was investigated for the case, when both power systems I and II have equal generating capacity and thesame system and governor parameters, same droop and load-damping factor. In this case systems are more sensitive todisturbance and there is more visual comparison of differentcontrol modes. Due to the different load at each system there isequalizing power flow through the transmission tie

E EI IIP sin (2)L X

=

where E I and E II are the equivalent EMF of two power systems;

X is mutual impedance of two power systems; is phase angle between the equivalent EMF.

The authors conducted modeling of dynamics of active power behavior when generators were used for a primaryfrequency control for different values of disturbances and

power system parameters. Output powers of each equivalentgenerator, power flow through the tie line, instant values of frequency in each power system and angle d were calculated.Below results of analysis for few calculation cases are

presented.

Fig. 7. Transient process during the P I = 0,27 disturbance of the system

1. Both power systems have generated capacity equal to 1 inrelative values. Load-damping factor for both system D=1.5.Governors droop value is assumed 0.05. System loads are P I =0.95 and P II = 1.05. Hence, there is a power flow P L = 0.05from system I to system II. Maximal tie capacity was assumedequal to 0.1 in relative values. Parameters of both power systems and governors are equal. The maximal possible

8/6/2019 Dynamics Problems of Frequency and Active

5/6

disturbance was considered increase of the load P I = 0.27.Fig. 7 illustrates time response of P L, and frequency values

for conventional control system and mentioned disturbance.The main conclusions from analysis of considered case are: After disturbance during the transient process instantvalues of frequencies in the power systems I and II aredifferent. Control response of both systems is absent during the first1-2 seconds due to the water hammer phenomena. The differences of frequencies are the reason of differentcontrol responses for both power systems. It causes dampedoscillation of tie power P L and angle . In considered case

peak angle varies from 30 to - 120 and tie line power P L does from 0.05 to 0.1, which is equal to peak power capacityof the tie line. Oscillation intensity is growing with reductionof governor droop value.

Fig. 8. Transient process during the P II = 0,05 disturbance of the system

2. Possibility of violation of the system stability depends notonly on the value of disturbance but also on its location. Fig.8 presents response of power P L, and frequency values for increase of the load P II=0.05. In spite of the small value of disturbance, compared with the first calculated case,oscillation of the power is close to the peak value.

When two power systems have different generating unit andgovernor parameters the transient process is more complicated.

3. Application of the new method for control of frequencyand active power in two power systems connected by weak tieline was considered. Each generating unit, participating infrequency control, should be equipped with suggested dynamiccontrol system (Fig. 2). Speed droop characteristic of agovernor determines necessary output power deviation P for

the given deviation of a system frequency f. This power deviation is a control task for dynamic control system. Thenew value of power is P N=P N+P. Analysis of transient

processes was made for the same disturbances as for conventional control systems. Calculation results showed thatfor new control method transient process was much shorter,with much smaller oscillation amplitude. Fig. 9 presentscomparison of angle behavior for conventional and newcontrol methods. System parameters are assumed similar as for analysis of case 1. Increase of P II=0.085 was considered assystem disturbance. For the new control system transient

process of angle came to the end in 5 seconds. For

conventional control system transient process lasts almost 30seconds.

Fig. 9. Comparison of transient processes character for conventional and newcontrol methods

VI. R ESULTS OF A FIELD TEST OF THE RIGA HYDRO POWER PLANT S GENERATORS

Riga Hydro Power Plant is equipped with newmicroprocessor-base governors, developed and implemented

by the authors. The field tests were conducted for investigationof dynamic properties of the real generating unit. Besides, thenew control law and influence of governors feedback

parameters were investigated. The chapter describes results of the tests.

The main aim of the field test was to make sure that newcontrol method is efficient and suitable for practicalimplementation. Fig. 1 represents active power variation

process for conventional governor system. Fig. 10 illustratesstart of Riga Hydro Power Plant generator for similar operational condition with new dynamic control method.Initial rate of change of power is equal to 2.5 MW/s (section 1-2 of Fig. 2). Point 2 of characteristic corresponds to 0.8 of P N,and final setting is equal to P N=50MW. The fast and smooth

8/6/2019 Dynamics Problems of Frequency and Active

6/6

acceleration of power take place. Output power is steady andequal to 50MW in 25 seconds.

Fig. 10. Increase of active power during the start of generator with newgovernor

Fig. 11 shows output power response for the case whencontrol requirement is an increasing generators output from

50MW to 67MW and then restoring previous value. This test proved ability of suggested dynamic control method to provide power system frequency control

Fig. 11. The illustration of output power control

VII. C ONCLUSIONS

1. Unification of electrical power systems causes necessityin co-ordination of frequency and active power controlmethods.

2. In two electric power systems, connected by weak tieline, during the system disturbance:

dynamics of transient processes is different; responses of governors are different; unnecessary fluctuations of power in tie line take place,which reduce stability margin and quality of the transient

process.3. There is a need in providing identical dynamic

characteristics for all units, participating in the frequencycontrol.

4. New method to control the output power of a generator issuggested, that allows providing a necessary dynamiccharacteristic of control.

5. Mathematical modeling and field tests of the active power behavior showed efficiency of suggested method.

VIII. A CKNOWLEDGEMENT

The authors would like to thank the head of electricaldepartment at Riga Hydroelectric Power Plant Albert Nelsonand engineer Jury Kozlovski for the help during experimentalinvestigation.

IX. R EFERENCES [1] UCTE: Summary of the current operating principles of the UCTE. 1998.[2] UCTE: Ground Rules concerning primary and secondary control

of.frequency and active power within UCTE. 1998.[3] I. Biernacka, I. Radzio, Supervision of the Application of

Rules.Concerning Primary and Secondary Control of Frequency in theInterconnected Power Systems UCTE/CENTREL, APE , Jurata, 2001.

[4] Problems of frequency control, Scientific Seminar , DC Baltia, Riga, 24 25 of May, 2001.

[5] J. N. Kucherov, A. F. Bondarenko, F. L. Kogan, L. N. Kasjanov, J. R.Itelman, A. N. Komarov and G. S. Kiselev, Technical aspects of

preparation for parallel operation of the Joint Power System of Russiaand Western power systems, Elektrichestvo, Nr.1, pp. 19-29, 2000. (inRussian).

[6] E. A. Marchenko, Analysis of frequency fluctuations in Joint Power System of Russia and Western power systems, Elektrichestvo, Nr.2, pp.2-7, 2001. (in Russian)

[7] Bujko J., Zielinski Z., Lipko A., Adaptation of the National Power System for Parallel Operation with the UCPTE Systems, Energetyka,

Nr.3, March 1996, pp. 123-128, Poland (in Polish).[8] Obolenski W., Wrona J., The Secondary Control of Power and

Frequency in New Conditions of NPS Operation, Energetyka , Nr.3,March 1996, pp. 133-136, Poland (in Polish).

[9] Gurov N., Chuvychin V., Application of the phase plane method for control of normal and emergency conditions in the power system,Problems of present day electrotechnics 2000, V InternationalConference , 6-8 June, 2000, pp. 120-124, Kiev, Ukraine (in Russian).

X. B IOGRAPHIES

Vladimir Chuvychin (M1979, SM1990) was born in Moscow region,Soviet Union, on January 17, 1941. He received diploma engineer degree in1965, Candidate of Technical Science degree (Ph.D) in 1975 and Dr.habil.sc.degree in 1997 from Riga Technical University, Latvia.

Since 1965 he is with Riga Technical University, Faculty of Electricaland Power Engineering where is currently Professor and Vice Dean inresearch affairs. He was a visiting Fulbright Scholar at the University of Texas, Arlington, in 1990 and at the University of Washington, Seattle in1994. His research interests include protective relaying and power systemautomation and control. He holds many patents in this area.

Nikolaj Gurov (nonmember) was born in Russia on April 2, 1932. Hereceived diploma engineer degree in 1964, Candidate of Technical Sciencedegree (Ph.D) in 1974 and Doctor of Science in engineering degree in 1992from Riga Technical University, Riga, Latvia. He is presently PrincipalResearcher in the Faculty of Electrical and Power Engineering, RigaTechnical University, Riga, Latvia. His research interests include power

system emergency control and automation. He also holds many patents in thisarea.Andris Skutelis was born on December 18, 1976. He received his B.Sc.,

M.Sc. in Power Engineering from Riga technical university, Riga, Latvia in1998 and 2001, respectively. Now he is Ph.D. student at the Faculty of Electrical and Power Engineering. His research interest include control of hydro power plants.

Vadims Strelkovs was born in Riga, Latvia, on May 28th, 1979. Hereceived bachelors degree in 2002 from Riga Technical University, Riga,Latvia. He is currently graduated student, Faculty of Electrical and Power Engineering and also a part time lecturer there. His research interests include

power system emergency control and automation as well as their mathematical modeling.