52407115 voltage stability[1]

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A

SEMINAR REPORT

ON

AVOIDING RISK OF VOLTAGE INSTABILITY IN A

POWER SYSTEM THROUGH REACTIVE POWER

RESCHEDULING AND LOAD SHEDDING

(SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE

AWARD OF THE DEGREE OF)

BACHELOR OF TECHNOLOGY

2010 2011

JAIPUR NATIONAL UNIVERSITY, JAIPUR

(A Venture of seedling group of institutions)

BY

MANISH KUMAR SHARMA


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AB T ACT

As th use of renewable energy sources (R s) increases worl wi e, there is a rising interest

on their im acts on power system operation and control. The important impacts of a large

penetration of variable generations in area of operation and control can be summarized in the

directions of regional overloading of transmission lines in normal operation as well as in

emergency conditions, reduction of available tie-line capacities due to large load flows,

frequency performance, grid congestions, increasing need for balance power and reserve

capacity, increasing power system losses, increasing reactive power compensation, and

impact on system security and economic issues. The distributed power fluctuation (due to

using of variable generations) negatively contributes to the power imbalance, frequency and

voltage deviations. Significant disturbance can cause under/over frequency/voltage relaying

and disconnect some lines, loads and generations. Under unfavourable conditions, this may

resultin a cascading failure and system collapse. Here we describe a procedure forimproving

voltage stability condition of a power system by reactive power rescheduling or load

shedding. For this purpose, a voltage stability index and its threshold value is used as the

basis. Sensitivity factors are derived to relate change in voltage stability index for changes in

reactive power at generation buses and changes in load atload buses. Using these sensitivity

factors, a procedure is proposed for avoiding risk of voltage instability in a power system by

applying reactive power rescheduling orload shedding.


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ACKNOWLE E ENT

I would like to convey my sincere thanks to Prof.DEVENDRA DODA (HOD, EE) for

giving us such a wonderful opportunity to enhance our skills throughthese seminars.

I would like to express my deep sense of gratitude, indebtedness to Mr. DUR

ESH

NANDAN PATHAK(lecturer, EE, JNU, Jai pur), and all the faculty mem bers fortheir

guidance, ever inspiring help, affectionate encouragement & motivation .They have been a

great source ofinspiration forme.Ive been receiving valuable suggestions fromthem.

I am also thankfulto Mrs. DEEPIKA CHAUHAN who helped me in my seminar withhis

fullinterest. The uphilltask for completing this seminar report would have been impossible

without support of all staffmember.

No words are sufficientto express my gratitude to colleagues fortheir

exemplary patience, understanding, co-operation & for creating congenial environment to

carry outthis work.

NAVEEN KUMARMEENA


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FIGURE INDEX

TOPICS PAGENO.

Figure1. Flow chart for reactive power rescheduling or/and load Shedding to improve

voltage stability condition of a power system.................... ....................................................19 Figure1 continued..................................................................................................................20

Figure1 continued.................................................. ................................................................22

Figure1 continued..................................................................................................................23 Figure2. IEEE30Bus system................................................................................................24


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INTRODUCTION

The problem related to voltage instability in a power systemis a major concern for power

system operation and planning. A major factor contributing to voltage instability is thevoltage drop that occurs when active and reactive power ow throughinductive reactance of

the transmission network; this limits the capability ofthe transmission network for power

transfer and voltage support. The powertransfer and voltage support are furtherlimited when

some ofthe generators hittheir eld or armature currenttime-over load capability limits.

Voltage stability is threatened when a disturbance increases the reactive power demand

beyond the sustainable capacity ofthe available reactive power resources. Voltage collapse is

characterized by a slow variation in system operating point, due to increase in the loads, in

such a way thatthe voltage magnitude gradually decreases until a sharp accelerated change

occurs.Ithas been observed that voltage magnitudes, in general, do not give a good

indication of proximity to voltage stability limit.In recentliterature, many voltage stabilityand voltage collapse prediction methods have been presented. Some ofthe important ones

are:

(1) Voltage collapse index based on a normalload ow solution (L-index);

(2) Voltage collapse index based on closely located power ow solution pairs;

(3) Voltage collapse index based on sensitivity analysis; and

(4) Minimum singular value of Newton-Raphson power ow Jacobian matrix.

These methods assess the closeness to the critical loading by looking in to the voltage

stability sensitivity indices orthe smallest Eigen value or singular value ofload ow Jacobian

matrix. Index presented in gives a scalar number to each load bus, called the L-index, to

indicate the proximity of voltage collapse for a power system. The index value ranges from0

to 1. The bus with largest value of L is the most vulnerable bus in the system. In the

procedure for calculation of L has been simplied using some acceptable approximations;

this reduces the computational burden considerably. A reliable voltage stability index Iihas

been proposed, whose threshold value is found to be varying marginally between 0.43to 0.52

as againstthe theoreticalthreshold value of0.5. The decrements ofthis index are rapid with

respectto load variation as it approaches the proximity of voltage collapse. Therefore, index

nearto 0.7is an alarming situation so far as voltage stability of a power systemis concerned.

For a voltage stability index to be effective and useful, it should possess the following

qualities: (1) Indicator/index should be related to the controllable parameters of a power

system through a simple function; and (2) some corrective measures could be derived from

the indices. The proposed procedure aims atinvolving only sensitive generation buses and/or

load buses for reactive power rescheduling and/or load shedding, respectively, to improve

voltage stability condition of a power system. Load shedding option is undertaken whenreactive power rescheduling of generation buses cannot improve voltage stability index of a

bus to a desired value.


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2.CLASSIFICATIONOF INSTABILIT MECHANISMS

The objective ofthis section is to relate the above concepts oflarge disturbance and time

scale decomposition to power system phenomena and models. We provide a classication of

loss of stability mechanisms relevantto voltage phenomena. State approximation to be used

and the overall systeminstability to be decomposed in several well dened categories. Let us

assume a large disturbance and considerthe possible unstable system responses thatmight

result.

2.1 Transient Period

In the transient period immediately following the disturbance the slow variables do not

respond yet and may be considered constant. The three majorinstability mechanisms are

T1: loss of equilibrium ofthe fast dynamics.

T2: lack of attraction towards the stable post-disturbance equilibrium ofthe fast dynamics.

T3: post-disturbance equilibrium oscillatory unstable. The transient period is the usualtime

frame of angular stability studies. Forinstance, the loss of synchronism following too slow a

fault clearing is a typical T2mechanism. This is also the time frame oftransient voltage

stability, which results fromloads trying to restore their powerin the transienttime frame.

Typical examples are induction motorloads and HVDC components.An example of T1voltage instability is the stalling of an induction motor fed through a long transmission line,

after some circuittripping makes the transmission impedance too large.Motor stalling causes

the voltage to collapse. The motormechanical and electricaltorque curves do notintersect

any longer, leaving the system without a post-disturbance equilibrium. An example of T2

voltage instability is the stalling ofinduction motors after a short-circuit.In heavily loaded

motor and/or slowly cleared fault conditions, the motor cannot reaccelerate afterthe fault.The mechanical and electricaltorque curves intersect but at fault clearing, the motor slip is

largerthan the unstable equilibrium value.

2.2 Power System Stability

Power system stability is the ability of an electric power system, for a given initial operating

condition, to regain a state of operating equilibrium after being subjected to a physical

disturbance, withmost system variables bounded so that practically the entire system remains

intact.

2.3 Voltage Stability

Voltage stability refers to the ability of a power systemto maintain steady voltages at all

buses in the system after being subjected to a disturbance from a given initial operating

condition. Characterized by loss of a stable operating point as well as by the deterioration of

voltage levels in and around the electrical centre ofthe region undergoing voltage collapse


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T e c ange in bus voltage vector [ V ] wit respect to c ange in reactive power

injection vector [Q] can be expressed as:

Substituting [V ] in Eq ( ) fro Eq ( ), we ave

T e sensitivity factors [] relate c ange in t e index value for kt

bus wit respect to c ange

in reactive power injections at t e buses, but reactive power resc eduling can becarried outonly in t e generation buses, as suc , representing Eq ( ) only for generationbuses we ave


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T e Jacobian atrix of NR load ow analysis relates c ange in real and reactive power

injections wit respect to c ange in bus voltage angles and bus voltage agnitudes asfollows:-

Substituting [ V ] in Eq ( ) fro Eq ( ) we ave


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T e above equation can be expressed as:

w ere [ ] and [] are t e sensitivity factors (SFs) relating [P] and [Q] to t e c ange in t e

index value of kt bus Assu ing load power factors do not c ange wit c ange in load

values, Eq ( 5) can be written as:


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3.Determination ofSuitable Value of MG, ML, and Solution Procedure:-

The proposed procedure aims at involving sensitive generation buses and/or load buses for

reactive power rescheduling and/or load shedding, respectively, to improve the voltage

stability condition of a power system. Load shedding option is undertaken when reactive

power rescheduling of generation buses cannot improve the voltage stability index of avulnerable load bus to its desired value. To ensure participation of sensitive generation buses

and/orload buses, buses having SF values more than cut off values cut and cut are selected

for reactive power rescheduling and/or load shedding, respectively. A factor (KM > 1) is

included for the purpose of ensuring that sufficient num ber of buses are included during

improvement of the value of Ik and all the rescheduled (reactive power) generators or load

shedding buses do no get xed at their limits. A value of KM 1.1 1.2 has been found to

work well formost ofthe systems. The procedure adopted for the selection of participating

buses is as follows:

1. Store the bus number in arrays BRj and BLj for j = 1. . .N according to descendingorder of the sensitivity factor associated with each bus for reactive power rescheduling and

load shedding, respectively (buses having highest sensitivity factor are ranked as one, bus

with nexthighest sensitivity ranked two, and so on) and setM = 0.

2. Set j = 1 and Iach = 0.3. i = BRj.4. Ifith bus is not a generation bus; go to step 6.5. If Iach > KMIk go to 15

M = M + 1

Iach = Iach + iQilimit

6. j = j + 1.

7. If j N; go to step 3.

8. Set j = 1 and ML = 0.

9.i = BLj.

10.Ifith bus is not a load bus; go to 13.

11.If j < 0 go to step 13.

12.If Iach > KMIk go to 15.

ML = ML + 1Iach = Iach + i PDi

limit

13. j = j + 1.

14.If j N; go to step 9.

15. Stop.


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5.Simulations Results and Discussion

To verify the applicability of the proposed procedure, simulations were carried out on

IEEE30 bus system shown in Figure 2. To demonstrate the effectiveness of the procedure,

both reactive power rescheduling and load shedding were carried out according to the

requirements to improve the value ofthe indexIk ofmost vulnerable bus to its desired value

Idesk . The system loading is adjusted in such a way that voltage stability indices of a few

buses ofthe system falls below 0.75. Table 1 shows the generations, loads, and voltage

conditions at these generation buses. Table 2 presents the loads, voltage condition, and

voltage stability index of load buses of the system. It shows the minimum value of voltage

stability index is appearing at load bus 19, the value is I19 = 0.718003. From voltage

instability point of view, this bus is the most vulnerable bus. Simulations were carried outto

improve this index to different desired value with different reactive power limits on the

generation buses.Minimum value ofload, which cannot be shed atload bus, is taken as 20%

ofthe initialload atthe bus. Table 3 presents the two different reactive powerlimits applied

to the generation buses for simulation purpose. Power limits on the generation buses.

Minimum value ofload, which cannot be shed atload bus, is taken as 20% ofthe initialload

atthe bus. Table 3 presents the two different reactive powerlimits applied to the generation

buses for simulation purpose.

5.1.Desired Value of I19des

= 0.75

Case-I reactive power limits are applied to the generation buses for improving value ofthe

voltage stability index at bus 19. The results obtained from simulation are presented in Tables

4 and 5.

The simulation results show that the voltage stability index is improved to 0.744932 from

0.718003through reactive power rescheduling.In this case, load shedding is not required for

improvement ofthe voltage stability index.


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5.2. Desired Value of I19des

= 0.8

At rst, Case-I reactive power li its are applied to t e generation buses for i proving value

of t e voltage stability index at bus T e results obtained fro si ulation are presented in

Tables and


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6.CONCLUSIONThe proposed procedure for reactive power rescheduling or load shedding aims atimproving

voltage stability condition of a power system from proximity of voltage collapse.Index Ik, the

voltage stability index ofthe most vulnerable bus of a power system, is used as the basis for

the improvement of voltage stability condition of the system. Sensitivity factors are derivedto relate change in voltage stability index for change in reactive power at generation buses

and change in load at load buses. The sensitivity factors are used to determine the required

reactive power rescheduling and/orload shedding to im prove voltage stability condition of a

power system. The proposed procedure aims at involving only sensitive generation buses

and/or load buses for reactive power rescheduling and/or load shedding, respectively, to

im prove voltage stability condition of a power system. Load shedding option is undertaken

when reactive power rescheduling of generation buses cannotimprove voltage stability index

of a bus to its desired value. The proposed procedure for improving voltage stability

condition by reactive power rescheduling or load shedding can provide usefulinformation to

power system planner/operator to undertake corrective action to avoid risk of voltage

instability in a power system.


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7. APPENDICES

N Total number of buses in the system

NG Total number of generation buses in the system

NL Number ofload buses in the system

Pi Injected active power atith bus

Qi Injected reactive power atith bus

QGi(max) Maximumlimit of reactive power generation atith bus

QGi(min) Minimumlimit of reactive power generation atith bus

PDi(min) Limit on load shedding ofthe ithload bus

Si Pi + jQi

Vi Magnitude of voltage atith bus

i Angle ofthe bus voltage atith bus

Gi j + jBi j Element of Y -BUS matrix atith row and j th column


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8. REFERENCES

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2. Kessel, P., and Glavitsch, H., Estimating the voltage stability of a power system, IEEE

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3. Clark, H. K., New challenges: Voltage stability, IEEE Power Eng. Rev., pp. 33 37,

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5. Crisan, O., and Liu, M., Voltage collapse prediction using an im proved sensitivity

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6. Lof, P. A., Anderson, G., and Hill, D. J., Voltage stability indices of stressed power

system, IEEE Trans. PWRS, Vol.8, No.1, pp.326335, 1993.

7. Tiranuchit, A., and Thomas, R. J., A posturing strategy against voltage instability in

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52407115 voltage stability[1]

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