This Paper Presents the Transients and Steady State Performance of a Two Area Interconnected Thermal Power System Considering Static Synchronous Series Compensator

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DESIGN OF LFC FOR TWO AREA INTERCONNECTED POWER SYSTEM

CONSIDERING SSSC IN SERIES WITH THE TIE-LINE

ABSTRACT

This paper presents the transients and steady

state performance of a two area interconnected

thermal power system considering Static

Synchronous Series Compensator (SSSC) in series

with the Tie-line. It is possible to stabilize the system

frequency and Tie-line power oscillations by

adjusting the output voltage of SSSC which is

expected to provide new ancillary service for the

future power system. A control strategy SSSC is

proposed to provide active control of system

frequency gain setting of the integral controller

without and with SSSC are optimized using Integral

Squared Error (ISE) Technique .The conventional

controllers are designed and implemented in two area

interconnected power system with SSSC which is

quite capable of suppressing the frequency and Tie-

line power oscillations effectively under the

occurrence of sudden load changes in area-1 as

compared to the obtain without SSSC.

Key Words: LFC-Load Frequency Control,

SSSC- Static Synchronous Series Compensator.

INTRODUCTION

In any interconnected power system, the

generation normally comprises of a mix of thermal,

hydro, nuclear and gas power generation. However

owing to their high efficiency to the nuclear plants

are usually kept at base load close to their maximum

output. Gas power generation is ideal for meeting the

varying load demand at such a plant for a very small

percentage of total system generation. Thus the

natural choice of LFC falls and either thermal or

hydro unit. The LFC of an interconnected power

systems has two principle aspects maintenance a

frequency and power exchange over inter area tie

line on the respective scheduled values. For

successful operation of the system the following basicrequirements are to fulfilled Generation must be

adequate to meet all the demand The system

frequency must be maintained within the narrow and

rigid limits

In the case of interconnected operation the tie

line power flow must be maintain at the specified

value. Under the steady state condition there is a

balance between real power generation and Demand.

Any sudden change in the demand is immediately

indicated by changing in speed or frequency. There is

an oscillation in the frequency till the steady state

condition is achieved. To damp-out these oscillations

energy storage device orstatic synchronous series

compensator can be included [1] in the system and

the same can meet the sudden change in load. The

stabilization of frequency oscillations is an

interconnected power system became challenging

when implemented in future competitive

environment.

The analysis of an interconnected power

system some area are considered as the channels of

disturbance in this situation. The conventional

frequency control (i.e.) the governor may no longer

be able to attenuate the large frequency oscillations

due to slow response.

The tie-line power flow, which is

controlled by static synchronous series compensator

(SSSC) installed in the series with tie line in between

two area of an interconnected power system has thepossibility to control the system frequency also in a

positive manner. The proposed control strategy will

be a new ancillary service for the stabilization of

frequency oscillations of an interconnected power

system.

MODELING OF A TWO AREA

REHEAT THERMAL POWER SYSTEM

In an uncontrolled power systems, large

deviation (maximum permissible 0.5 Hz) of frequency cannot be tolerated and, therefore, some



suitable control strategy have to be develop to

achieve much better frequency consistency. Even if

the frequency of a system is kept with-in rather

narrow tolerances, in itself does not necessarily

provide for the accuracy of synchronous clocks since

such clocks measure the integral of the frequency

may show up cumulatively in synchronous time.

The intolerable dynamic frequency changes

in load can be controlled and also the synchronous

clocks run on time, but not without error during

transient period. To achieve better frequency

constancy the integral controller is added to the

uncontrolled to area non-reheat thermal power

system.

INTEGRAL CONTROLLER

Integral controller operation is bestcontinuous change in load because it produces the

control signal according to the integral part of the

error signal. The developed controller design

relations based on a performance index that considers

the entire closed loop response. The performance

indexes calculations can be obtain in any one of the

four criterions Integral of the square error (ISE) Time

multiplied integral square error criterions. Integral of

the absolute value of the error (IAE) Where: Error

signal e(t) is the difference between the set point andthe measurement.

Integral part of the time – weighted absolute

error (ITAE) The integral of the square error (ISE)

gives the good performance than the other criterions

as if requires less computation and more accuracy.

TRANSFER FUNCTION MODEL

Transfer function model of the steam

governors and turbines are done based on the IEEE

committee report on dynamic models for steam

turbine in power system studies. Fig. 1.1 shows block

diagram of the two area reheat thermal power system

using integral controller.

STATE SPACE ANALYSIS

The power system model considered being a

linear continuous time dynamic system, and the

power system model can be represented by the

standard state space model as

d Bu Ax x

Where x -- State variable vector

U --Control vectorP --Disturbance vector

A--System Matrix

B--Input matrix

-- Disturbance matrix

The above matrices are constant and

of compatible dimensions associated with them which

in turn depend on the system parameters and the

operating point for the system considered.

222,2121111 ,,,,,,, GT tieGT PP XeF PPP xeF x

T C C PPu 21

T d d PPd 21

Modeling of a Two-Area interconnected

Thermal Power System with Reheat Turbines

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2 Mathematical model of the SSSC

In this study, the mathematical model of the

SSSC for stabilization of frequency oscillation is

derived from the characteristics of power flow control

by SSSC [5] .by adjusting the output voltage of SSSC

(SSSC ),the tie-line power flow (P12+j12),can be

directly controlled as shown in fig 1. Since the SSSC

fundamentally controls only the reactive power, then

the phasor SSSC is perpendicular to the phasor of

line current , which can be expressed as

Vsssc = j VSSSC ( ̅)

Where Vsssc and I are magnitudes of

VSSSC and respectively

Where /I is an unit vector of line current.

In fig 1, the line current = [1- 2- j VSSSC ( / I) ]

Jxl

Where xl is the reactance of tie – line , 1 and 2

are the bus voltage.

The complex power from fig 3.1

T

T

T

T



P12 + jQ12 = I V

*

1

Where I *

is conjugate of substituting (2) in (3) we

get

P12+ jQ12 = * + sin ( ) – VSSSC (1 I

*

/Xl I)

+ j ((

)-((

)cos ( )))

Where 1 = Viejδ1 and 2 = V2ejδ2 in (4) ,the

second term of right hand side

1 is P12+ jQ12 . as a result, the relation in the real

part of (4) gives.

P12=( ) sin ( ) – ( P12 / XlI) Vsssc

The second term of right hand side is active

power controlled by SSSC. Here, it is assumed that

V1 and V2 are constant, and the initial value of

VSSSC is zero. i.e ,VSSSC =0. By linearizing (5)

about an operating point.

SSSC ollV I X

P

X

V V P

1202120102112 )(

)cos(

....(3.6)

where subscript „0‟ denotes the value at operating

point .

by varying the SSSC output voltage VSSSC ,the

power output of SSSC can be controlled as.

Psssc = (p120 / X l I0 ) Vsssc

In equation (6) implies that the SSSC capable of controlling the active power independently. In this study

the SSSC is represented by the power flow controller

where the control effect of active power by SSSC is

expressed by PSSSC instead of (P120 / XlIO)

VSSSC, as a results (6) can be expressed as

P12 = P T12 - PSSSC

Where P = [ ⁄ ] cos ( ) )

=T12 )

Where T12 is a synchronizing power co-efficient

SIMULATION RESULT

The system state space equations are developed from the

transfer function model of two area reheat thermal power

system. The integral controller is design for to area reheatthermal power system using ISE criterion (fig 1.1). The

system frequency deviation response and tie line power

deviation response were obtained, using Matlab version

7.01

Integral controllers using output feedback

designed on the basis of intergral square Error criterion

are implemented in the interconnected two – area thermal

reheat power system without and with SSSC units the

performance of these controllers is as shown in fig. 3.5 to

3.6. the proposed controller is Implemented in a two – area interconnected power system without and with SSSC

units for a step load disturbance of 0.01 p.u. Mw in area 1

and the responses of the frequency deviations ΔF, tie-line



power deviating ΔPtie and control input deviations ΔPc

are obtained.

The gain values and the cost function values of the

various decentralized integral controllers for the two –

area thermal reheat power system without and with SSSC

are given in table 1.

From figure. it is evident that the dynamic responses

have improved significantly with the use of SSSC units.

From the tabulations, it can be found that the

controller designed for two – area thermal reheat power

system with SSSC units have not only reduces the cost

function but also ensure better stability, more over

possesses less over / undershoot and faster settling time

when compared with the controller designed for two –

area thermal reheat power system without SSSC units.

Cost curve of two area interconnected thermal (Reheat)

power system with integral controller considering

SSSC in the tie-line

CONCLUSION

In this paper, modeling and analysis of two area

interconnected reheat thermal power system without and

with Static Synchronous Series Compensator (SSSC) arecarried out. The optimum value of Integral controller for

a two-area reheat interconnected thermal power system is

designed using ISE criterion method by minimizing its

performance index. Then the output response of the LFC

is investigated by using Integral controller without SSSC.

The same procedure is repeated for the system with

SSSC. Small rating SSSC units are connected in series

with tie-line of two – area interconnected power system

and responses show that they are capable of consuming

the oscillations in area frequency deviations and tie-line

power deviations of the power system.

Further SSSC units reduce to over / under shoot and

settling time of the output responses. Hence it may be

concluded that SSSC units are efficient and effective for

improving the dynamic performance of Load frequency

control of Inter connected power system that of the

system without SSSC Units.

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0

0.0002

0.0004

0.0006

0.0008

0.001

0 0.5 1 1.5 2

P e r

f o r m a n c e I n d e x [ J ]

Gain Ki



Frequency Deviation in Area 1 Of A Two Area

Interconnected Thermal (Reheat) power system For0.01 pu Mw Step Load change in Area 1

Documents

This Paper Presents the Transients and Steady State Performance of a Two Area Interconnected Thermal Power System Considering Static Synchronous Series Compensator