Automatic generation control of an isolated small-hydro power plant

Electric Power Systems Research 76 (2006) 889–896

Automatic generation control of an isolated small-hydro power plant

S. Doolla ∗, T.S. BhattiCentre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

Received 11 July 2005; received in revised form 3 October 2005; accepted 19 November 2005Available online 4 January 2006

Abstract

The paper presents a new technique of automatic generation control of an isolated small-hydro power plant. Presently surplus generation due todecrease in load is dissipated in a dump load. But surplus water can be used for irrigation, which is the primary requirement of the local communityfor their survival at large number of locations. It is being wasted in the dump load and the new scheme proposed in this paper eliminates the dumpload, while the input power of the hydro plant is controlled by an on/off and a servo controlled valves. The on/off control linearly raises or lowersthe generation by 50%, while the servo motor controls the generation to the required extent. It is shown that the rate of opening or closing of theon/off control valve has considerable effect on the transient response of the system. Finally, the dynamic responses due to step disturbance fordifferent nominal loadings of the system, gain settings, rate of closing/opening of the on/off control valve are also presented.©

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eywords: Automatic generation control; Small-hydro; Dump load; On/off control; Servo motor

. Introduction

In today’s world electrical energy is very essential and itsvailability plays a significant role in the upliftment of remote,ackward or grid supply starved society. The gap betweenemand and supply is increasing day by day and often it is notossible to meet the demand of big consumers. Therefore, a largeumber of villages either have no electrical connection or get-ing erratic electric power only for few hours in a day from grid.he dependence on fuel (diesel systems) and also on grid sup-ly can be reduced by having stand-alone generations to meethe local requirements in these areas/locations. Some of theseeographical areas have large number of small-hydro streams,hich can be used for stand-alone power generation.The energy in flowing water of small streams can be tapped

y small-hydro power plants. This clean source of power, there-ore, plays a vital role in rural electrification in the case ofeveloping countries [1–3]. Moreover, small-hydro power hashuge, as yet untapped potential in most areas of the world

nd can make a significant contribution to future energy needs4]. Small-hydro power generation is already an effective and

research and development of controls for this technology. Mostof the small-hydro plants usually require a weir, and they do notrequire a dam or large water reservoir. Different technologiesof small-hydro power, innovations being developed and barriersfor further development were discussed in detail in ref. [5].

The maintenance of system parameters like frequency, volt-age, etc., within certain limits is essential for proper operationand efficient use of power produced. The system frequency canbe maintained constant by eliminating the mismatch betweengeneration and load. A conventional speed governor with supple-mentary integral control can be used to maintain the frequencyconstant both for grid connected and isolated mode operation. Ingeneral the generation control mechanism is not used due to pro-hibitively high cost; therefore, frequency is maintained by loadmanagement. A complete mathematical model for an improvedload controller when applied for a micro/mini hydro, wind, dieselelectric system are discussed in detail in refs. [6–12]. In a stand-alone small-hydro generation system due to non-availability ofstorage facility, the total input has to be converted into electricalenergy. Any variation in power demand is controlled by a resis-tive load called dump load. Since the input to the generator is

fficient proven technology, but there is considerable scope for

∗ Corresponding author. Tel.: +91 11 2659 6315; fax: +91 11 2658 1121.

essentially constant, the excess power due to decrement in loadis dumped into the dump load [13,14]. One of the main reasonsfor non-exploiting isolated small-hydro power systems in thehigher capacity range is due to the limitation on the size of theavailable dump loads [15,16]. It is also one of the reasons on
E-mail address: [email protected] (S. Doolla).
378-7796/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.epsr.2005.11.002

890 S. Doolla, T.S. Bhatti / Electric Power Systems Research 76 (2006) 889–896

Fig. 1. Proposed scheme of two pipes equipped with on/off control and servo motor control valves.

the existing systems that the resources are not used to the fullestavailable capacity.

In most of the sites of small-hydro plants it has been observedthat the primary requirement of the local community is water forirrigation of agricultural land as their survival depends upon it.But if electricity is available, it will enhance the living standardsby helping in better education, hospital, communication, facili-ties, etc. Once water enters the stream from tailrace, it requirespower for pumping to the fields. Therefore, if surplus water isavailable before the entry to the penstock it can easily be divertedto the fields. In general, the load factor of small-hydro plants isless than 50%. Therefore, more than 50% water can be availableas surplus if proper generation control strategies are employedinstead of using dump load. A new control scheme is proposed inthis paper by which the dump load is eliminated and frequencyis maintained at the desired level.

In the proposed control scheme, the penstock flow is regu-lated through two longitudinal small sections of pipes as shownin Fig. 1. One pipe is fitted with on/off control valve with 50%of flow rate under maximum rated load conditions in on state.The second pipe is fitted with a valve, which is controlled bya servo motor. The flow rate in the second pipe is continuouslycontrolled by controlling the input signal to the servo motor.The first control valve is either fully open or closed depend-ing upon the loading condition. When the load is less than 50%of the rated maximum load, on/off control valve will remaincd

Fig. 2. (a) Principal schematic of servo control valve. (b) Closed loop transferfunction for servo motor valve control.

disturbance in load, the servo motor controlled valve changesthe flow rate so as to maintain the system frequency constant.The water head is maintained constant by overflow of excesswater through spillway and diverting to the fields through achannel for irrigation. This method therefore eliminates the con-ventional frequency control by additional load management, i.e.dump load. The decision of the time for on/off the control valve,when the load increases or decreases plays a vital role in thesystem dynamics. To study the dynamics, a transfer functionmodel is developed for the system along with the servo motorand on/off controller. Transient responses are shown for dif-ferent loading conditions and for low, medium and high headinstallations.

o pow

losed and the servo motor controlled valve will take care of theeviation in frequency due to load variation. Whenever there is

Fig. 3. Transfer function block diagram of small-hydr
er system with on/off and servo motor valve controls.

S. Doolla, T.S. Bhatti / Electric Power Systems Research 76 (2006) 889–896 891

Fig. 4. (a) Decision making system for on/off control valve. (b) Integrator (dual) for on/off control valve.

FAi

ig. 5. (a) Transient responses of system Case A for step changes in load, �PL showifor step changes in load, �PL showing deviations in valve position, �XS. (c) Trans

n power generation, �PG.

ng deviations in system frequency, �F. (b) Transient responses of system Caseient responses of system Case A for step changes in load, �PL showing change


2. Servo motor valve control

Servo motor control systems are finding widespread applica-tions, mostly in automatic production. This includes nearly alltypes of packaging machinery, material handling, assembly andother applications including robotics [17,18]. The programma-bility feature of servo systems increases its adaptability to dif-ferent control applications. A very common method of positioncontrol using servo motor is by controlling armature voltage andkeeping field constant [19–21]. A servo motor has the advantagesof linear speed–torque characteristics, quick response, availableat all power ranges [22]. A dc servo motor with armature controlfor controlling the water flow rate in the penstock and its closedloop transfer function are shown in Fig. 2. The flow of water iscontrolled by controlling the position of the valve. The transferfunction of dc servo motor is given as [22]:

θ(s)

Ea(s)= K

s[LaJs2 + (Lab + RaJ)s + Rab + KKb](1)

where Ra is the armature resistance (ohm); La is the armatureinductance (henry); θ is the angular displacement of the motorshaft, radian; J is the equivalent moment of inertia of the motorand load (kg m2); b is the equivalent viscous-friction coefficientof the motor and load (N m/rad/s); K is the angular velocity andvoltage constant in rpm/voltage; Kb is the constant.

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Fig. 6. Transient responses of system Case B for step changes in load, �PL

showing deviations in system frequency, �F.

Fig. 7. (a) Transient responses of system Case B for a step change in load, �PL

(21 kW) for different integral gain values KIS, showing deviations in systemfrequency, �F. (b) Transient frequency deviation responses of system Case Bfor a step change in load, �PL (21 kW) for different values of the proportionalgain KPS, showing deviations in system frequency, �F.

The inductance La in the armature circuit is usually small anday be neglected. If La is neglected, then the transfer function

f (1) is given by:

θ(s)

Ea(s)= Kv

s(Tvs + 1)(2)

here

v = K

Rab + KKb(motor gain constant) (3)

v = RaJ

Rab + KKb(motor time constant). (4)

. System description

Fig. 3 shows the transfer function block diagram of an isolatedmall-hydro power system with servo motor and an on/off valveontrol. The model is based on small signal analysis and the ratef increase or decrease of generation by the on/off control valves therefore taken linear. The first-order transfer function is dueo delay in measurement or monitoring of the system frequencyrecedes the servo motor transfer function. The integral gainIS eliminates the frequency deviations by varying the servootor valve within limits of minimum and maximum value of

imiter 2. As the load increases, frequency will decrease and theervo motor will increase the flow rate to maintain the frequencyonstant (�f = 0). If �XS reaches the upper limit but frequencyeviation is negative, i.e. the frequency is lower than the nominalalue, the on/off valve is activated to ‘on’ position increasing theeneration, �PG. It is vice versa when the load decreases. Theystem damping (load frequency characteristics) [23] is given


by:

D = PoL

f oPR(5)

The system gain constant and time constant are given by

KP = 1

D(6)

TP = 2H

f oD(7)

KP and TP will, therefore have different values depending uponloading. PR is the power capacity of the small-hydro power plant.

The time taken by water to travel the penstock under idealcondition is given by

tP = l

v= l√

2gh(8)

where ‘l’ is the length of the penstock, ‘h’ the available head ofthe water and v is the velocity of water in the penstock. The time

constant TW in the transfer function represent the delay of waterin the penstock and is proportional to tP, therefore

TW = ktP (9)

The value of TW indicates the low, medium or high head instal-lation. The numerator term (zero) in the penstock turbine timeconstant indicates that the increase or decrease in generationis momentarily opposite when the control valve is opened orclosed, respectively, and this effect is more as head increases.

The detail of the control logic is shown in Fig. 4(a and b).The settling time is used as threshold value for all the switches.Implementation of such control logic is quite possible with lowcost analog/digital circuits.

4. Simulation, results and discussion

A typical example of an isolated small-hydro power systemis considered for simulation. The details of the system alongwith data are given in Appendix B. If the load �PL varies such

FiKTg

ig. 8. (a) Transient responses of system Case B for a step change in load, �PL (21 kn system frequency, �F. (b) Transient response of the on/off valve position, �Xon/o

G. (c) Transient response of the servo motor controlled valve position, �XS, of theransient responses of system Case B for a step change in load, �PL (21 kW) for difeneration, �PG.

W) for different rate of closing of on/off control valve, KG, showing deviations

ff, of the system Case B for a step change in load, �PL (21 kW) for differentsystem Case B for a step change in load, �PL (21 kW) for different KG. (d)

ferent rate of closing of on/off control valve, KG, showing deviations in power


that 0 < PoL + �PL ≤ 0.5PL,Max or 0.5PL,Max < Po

L + �PL ≤PL,Max only the servo motor controlled valve vary between min-imum and maximum value so as to maintain the frequencyconstant and there will be no action of the on/off control valve.This is depicted by transient responses Case A (Appendix B)for a step disturbance of ±21 kW as shown in Fig. 5. It isobserved that the frequency deviations, �F vanishes in about200 s, Fig. 5(a). The servo controlled valve attains its new posi-tion, �XS as shown in Fig. 5(b). Initially when the valve opens toincrease the power generation, �PG, it decreases first and thenincreases and vice versa as shown in Fig. 5(c).

For the Case B (Appendix B), where the nominal loadis Po

L > 0.5PL,Max and the load disturbance occurs such that0 ≤ Po

L + �PL ≤ 0.5PL,Max, the on/off control valve of Fig. 1closes to reduce the generation by 0.5PL,Max. The transient fre-quency responses of the system for different step change inload are shown in Fig. 6. It is observed that a positive steadystate error exists in frequency if no corrective action is taken.After 125 s occurrence of the disturbance, valve 1 of the on/offcontrol begins to close to reduce the generation. The time tostart corrective action is taken as 125 s in the simulation, butin practice it may be less and can be considered to start whenthe steady state error persists within certain range. A suddendecrease in frequency is observed and the servo motor startsincreasing the generation. The value of the negative frequencydeviation depends on the size of the load disturbance. After thata steady state error exists in frequency as long as the on/off con-trol valve is closing and the servo motor control valve is chasingto open. Finally, the servo motor controlled valve opens to thedesired level and eliminates the frequency deviation, �F = 0,when on/off control valve completely closes. The effect of theintegrator and proportional gains of the servo motor controller onthe transient system frequency responses are shown in Fig. 7(aand b), respectively. It is observed that the increase in the valueof integrator gain, KIS reduces the negative steady state error infrequency but increase the negative peak value and vice versa,but the settling time almost remains the same. It is also observedthat the increase in the value of proportional gain KPS makes thesystem more oscillatory but reduces the negative peak of fre-quency response and vice versa. The closing time of the on/offvalve has considerable effect on the transient responses of thesystem as shown in Fig. 8 for step load,�PL = −21 kW. The tran-sient responses of system frequency, �FS, on/off control valveposition, �Xon/off, servo motor control valve position �XS andturbine power generation �PG are shown in Fig. 8(a–d), respec-tively. It is concluded that rapid closing of the on/off controlvalve results in large deviation in frequency and power gener-ation but reduces the settling time of the oscillations. On theother hand, the slow closing of the on/off control valve reducesfluctuations in the system frequency and power generation butincreases the settling time of the responses.

The closing rate of the on/off control valve has consider-able influence on the transient response of the system. Transientresponses for initial high rate of closing and subsequent slow rateof closing of the on/off control valve are shown in Fig. 9(a andb). Comparing the transient responses of the system frequency itis observed from Figs. 6 and 9, that the performance deteriorates

Fig. 9. (a) Transient responses of system Case B for a step change in load,�PL (21 kW) for different rate of closing of on/off control valve, KG, showingdeviations in frequency, �F. (b) Transient response of the on/off control valvein position XS, of the system Case B for a step change in load, �PL (21 kW) fordifferent KG.

if the initial rate of closing of the on/off control valve is high.Transient responses for initial slow rate of closing and subse-quent high rate of closing of the on/off control valve are shownin Fig. 10(a and b). Comparing Figs. 6 and 10, it is concludedthat the transient responses improve by selecting initial slow rateof closing and later on high rate of closing of the on/off controlvalve.

For the case C (Appendix B), where the nominal load isPo

L < 0.5PL,Max and the load disturbance occurs such thatPo

L + �PL > 0.5PL,Max, the on/off control valve 1 of Fig. 1opens to increase the generation by 0.5PL,Max. The transientfrequency responses of the system for different step change inload are shown in Fig. 11.

The transient responses shown in Figs. 5–11 are for high headinstallations, i.e. TW = 4.0 s. The influence of TW (head) on thetransient performance of the system (Case B) has been investi-gated. The settling time of the responses remaining same for stepdisturbance of +21 kW and for different TW, as shown in Fig. 12.For low head installations, the peak value of the frequency


Fig. 10. (a) Transient responses of system Case B for a step change in load,�PL (21 kW) for different rate of closing of on/off control valve, KG, showingdeviations in frequency, �F. (b) Transient response of the on/off control valvein position XS, of the system Case B for a step change in load, �PL (21 kW) fordifferent KG.

Fig. 11. Transient responses of system Case C for step changes in load, �PL,showing deviations in frequency, �F.

Fig. 12. Transient responses of system Case B for step change in load, �PL

(+21 kW) for various changes in head (TW), showing deviations in frequency,�F.

deviations reduces marginally. Also the momentary oppositefrequency deviations due to opening of the valve reduces withreduction in TW. Therefore, the hydro plants of low head instal-lations are more stable than the high head installations. The stepdeviation in frequency of ±1 Hz occurs during switching onor off of the control valve and the peak deviation in frequencydepends upon the valve of the load disturbance.

5. Conclusions

A new method of automatic generation control of an isolatedsmall-hydro power plant is proposed to eliminate the conven-tional dump load controller. It is very important as the savedwater can be used for irrigation, which is the primary need ofthe local community for their survival. It is observed that thenew technique effectively eliminate the frequency deviationsdue to load disturbances for different nominal loadings of thesystem. From simulation results it is clear that the system ismore dynamically stable if the on/off control valve has rate ofclosing or opening initially low and later on high. Some of thesystem transient responses due to step disturbances are shownfor different nominal loadings and controller gains.

Appendix A. List of symbols

�

HKKKKKKp�

�

F frequency deviation (Hz)inertia constant of the generation system (s)

G integral gain constant for on/off control valveIS integral gain constant for servo systemM error and measuring circuit gain constantp power system gain constant (Hz/p.u.)PS proportional gain controller constant for servo systemv gain constant for servo system.u per unit of powerPG change in generation (p.u)PL change in load (p.u)


TM time taken for measuring data (s)TP power system time constant (s)Tv Servo motor time constant (s)TW nominal starting time of water in penstock (s)�XS change in servo motor controlled valve position (p.u)�Xon/off change in on/off control valve position (p.u)

Appendix B

Ratings and the data of the typical example of isolated powersystem studied.

Capacity if the small-hydro power plant, PR = 1200 kW.Maximum nominal load on the system, PL,Max = 1000 kW.System nominal frequency, fo = 50 Hz.Inertia constant of the generator, H = 5 s.Time constants:

TM = 0.02 s, Tv = 0.1 s;TW = 1.0 s (low head), 2.2 s (medium head) and 4.0 s (highhead).

Gain constants:KM = 0.004, Kv = 2.5, KIS = 0.4 and KPS = 8.52;

Case A. Nominal load = 900 kW

D(Po + P0 )/PR 900

K

T

C

C

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[

[

[

[

[

[

[

[

[

[

[

[

= L D

f o =1200 × 50

= 0.015 p.u/Hz

P = 1

D= 66.667 Hz/p.u

P = 2H

f oD= 2 × 5

50 × 0.015= 13.333 s

Initial on/off control valve state (open) = 500 kW = 0.4167 p.u.Servo motor initial state = 400 kW = 0.3333 p.u.On/off control limiter = −0.41667 to 0 p.u.Servo motor control valve limiter = 0.0833 to −0.333 p.u.

ase B. Nominal load = 520 kW

D = 8.667 × 10−3 p.u/Hz,KP = 115.3846 Hz/p.u, TP = 23.0769 s.Initial on/off control valve state is open = 500 kW = 0.4167 p.u.Servo motor initial state = 20 kW = 0.01667 p.u.On/off control limiter = −0.41667 to 0 p.u.Servo motor control valve limiter = −0.0167 to 0.4 p.u.

ase C. Nominal load = 480 kW

D = 8.0 × 10−3 p.u/Hz, KP = 125 Hz/p.u, TP = 25 s.Initial on/off control valve state is closed = 0 p.u.Servo motor initial state = 480 kW = 0.4 p.u.On/off control limiter = 0 p.u to 0.4167 p.u.Servo motor control valve limiter = −0.4 to 0.01667 p.u.

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Automatic generation control of an isolated small-hydro power plant