RGPV EX 503 Unit IV

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Prof MD Dutt HOD Ex Department SRCT Bhopal

READING MATERIAL FOR B.E STUDENTS

OF RGPV AFFILIATED ENGINEERING COLLEGES

SUBJECT ELECTRICAL MACHINES II

Professor MD Dutt

Addl General Manager (Retd)

BHARAT HEAVY ELECTRICALS LIMITED

Professor(Ex) of EX Department

Bansal Institute of Science and Technology

KOKTA ANANAD NAGAR BHOPAL

Presently Head of The Department ( EX)

Shri Ram College Of Technology

Thuakheda BHOPAL

Sub Code EX 503 Subject Electrical Machines II

UNIT IV Synchronous Machines II

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RGPV Syllabus

EX503 ELECTRICAL MACHINES II

UNIT IV SYNCHRONOUS MACHINES II

Salient pole machine, two reaction theory equivalent circuit model and phasor

diagram. Determination of Xd and Xq by slip test, SCR and its significance,

Regulation of salient pole alternator, power angle equation and characteristics,

synchronizing of alternator with infinite bus bar, parallel operation and load

sharing, synchronizing current, synchronizing power and synchronizing coefficient

, synchroscope and phase sequence indicator, effect of varying excitation and

mechanical torque

INDEX

S No Topic Page

1 Salient pole machine, two reaction theory equivalent circuit

model and phasor diagram

3,4,5

2 Determination of Xd and Xq by slip test, SCR and its

significance

6,7,8

3 Regulation of salient pole alternator 9,10

4 Power angle equation and characteristics 11,12

5 Synchronizing of alternator with infinite bus bar 13,14,15

6 Parallel operation and load sharing 15

7 synchronizing current, synchronizing power and

synchronizing coefficient

16,17

8 Synchroscope and phase sequence indicator 17,18

9 Effect of varying excitation and mechanical torque 18,19,20,26

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SALIENT POLE MACHINE, TWO REACTION THEORY, EQUIVALENT

CIRCUIT MODEL AND PHASOR DIAGRAM

Salient pole machines have the non uniform air gap. In case of cylindrical

rotor machines the air gap is uniform. The protruding or projected pole

structure of the rotor of salient pole machine makes the air gap highly non

uniform. The axis of symmetry of north magnetic poles is negative d axis.

The axis of symmetry half way between adjacent north and south poles is

quadrature axis or q axis.

TWO REACTION THEORY:- The theory proposes to resolve the armature m.m.f

into two components, with one located along the axis of rotor salient pole, It

is known as direct axis or d axis component. The other component is located

perpendicular to the axis of the rotor salient pole, It is known as the

quadrature axis or q axis. The d axis component of armature m.m.f Fa is

denoted Fd and the q axis component by Fq. The component Fq results in a

cross magnetizing effect. If Ψ is the angle between armature current Ia and

excitation voltage Ef, the amplitude of Fa of armature m.m.f is than

Fd = Fa sinΨ , Fq = Fa cos Ψ

The armature current produces stator magnetic motive force Fs the m.m.f

lags behind Ia by 90º. The m.m.f Fs produces magnetic field Bs along the

direction of Fa. The Fs stator m.m.f is resolved into two components namely

Fd and Fq.

If Φd = direct axis flux and Φq = quadrature axis flux

Rd = direct axis reluctance path

x path, Rq = q axis reluctance path

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Than Φd = Fd/ Rq Φq = Fq/Rq

Since Rd < Rq The direct axis component of mmf Fd produces more flux

than the quadrature axis component of m.m.f Fq. The direct and quadrature

axis stator fluxes produces voltages in stator windings by armature reaction

Ead = direct axis component of armature reaction voltage

Eaq = quadrature axis component of armature reaction voltage

Since each armature reaction voltage is directly proportional to its stator

current and lags behind by 90º. Therefore the armature reaction voltages can

be written as

Ead = - JXad Id

Eaq = -j Xaq Iq

The value of Xaq is always less than Xad since the e.m.f induced by a given

m.m.f acting on direct axis due to its higher reluctance. The total voltage

E’ = Ef +Ead + Eaq

E’ = Ef –jXad Id– jXaq Iq

Xad = armature reaction reactance in the direct axis per phase

Xaq = armature reaction reactance in the quadrature axis per phase

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Since the emf induced by a given m.m.f acting on direct axis is smaller than

for the quadrature axis due to its higher reluctance The total voltage induced

in the stator is sum of e.m.f’s induced by the field excitation.

E’ = Ef +Ead + Eaq

E’ = Ef –jXad Id– jXaq Iq

The voltage E’ is equal to terminal voltage V plus the voltage drop in

resistance and leakage reactance of the armature.

E’ = V +Ia Ra + jXlIa

Ia is split into two components

Ia = Id +Iq

Ef = V +IaRa +jXadId +jXaqIq +jXlIa

Ef = V Ra ( Id + Iq) +jXad Id +jXaqIq +jXl( Id +Iq)

Ef = V + Ra (Id+Iq) +j(Xl+Xad) Id +j (Xl+Xaq) Iq

Xd = Xl+Xad

Xq = Xl+Xaq

We get Ef = V +IaRa + JXd Id + jXqIq

Phasor Diagram:- The complete Phasor diagram of a salient pole generator based

on the two axis theory is drawn here below with lagging power factor.

PHASOR DIAGRAM

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Simplified Phasor diagram of salient pole synchronous generator at lagging power

factor. In the above diagram angle Ψ =Φ+δ is not known for given values of

V. Ia and Φ, The components Id and Iq of the armature current are usually

not given. These component currents depend upon δ which is to be

determined.

Iq = Ia _Id

Ef = V +RaIa +jXdId +JXq(Ia_Id)

Ef = V +RaIa +jXqIa +j(Xd-Xq)Id

We have

Id = Iasin Ψ and Iq = Ia cosΨ

In the following Phasor diagram drawn below BC is drawn at 90º to Ia and CD is

drawn perpendicular to Ef In Triangle BCD ∟BCD =Ψ

cosΨ =CD/BC = XqIq/BC , BC = XqIq/cosΨ = Xq ( Iacos Ψ) = XqIa

cos Ψ

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Thus the line BC represents the Phasor JXqIa and its end point M such that the

direction of Ef. Now BC is extended to point M such that the distance BM =

XdIa or CM =(Xd-Xq)Ia. The line MN is drawn perpendicular to Ef making

the angle Ψ at point M, which makes CN = (Xd-Xq)Ia sin Ψ = (Xd_Xq)Id.

The point N is the end point of Ef, from triangle OCK

Tan Ψ +CK/OK = KB+BC = VsinΦ +XqIq

OL +LK VcosΦ +RaIa

Ψ = Tan ¯¹ VsinΦ +XqIq

VcosΦ +RaIa

Ψ = δ +Φ or δ = Ψ – Φ

Once δ is known , Id and Iq can be easily found. The magnitude of excitation

voltage Ef can be determined either from the Phasor diagram or from the

following equation

IEfI =Vcosδ + RaIq + Ra Id

Determination of δ If the armature resistance is neglected the equation becomes

Tanδ = XqIa cosΦ

V + XqIa sinΦ

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DETERMINATION OF Xd and Xq BY SLIP TEST,

In a simple no load test or slip test, a small voltage at rated frequency of rated

value is applied to the stator winding. The field winding is kept open circuited. The

rotor is driven by an auxiliary motor at a speed slightly less or more than

synchronous speed. Note the direction of rotation it should be same as that the

rotating field of stator . a small voltage reading V2 is indicated by the voltmeter

across the open field winding terminals shows that the direction of rotation of rotor

is proper. Since the rotor is rotating at speed ήr close to ήs there will be small slip

between the rotating magnetic field produced by the armature mmf and the field

poles is equal to slip speed ήs – ήr . Since stator m.m.f moves slowly past the

actual field poles, There will be an instant when the peak of armature mmf wave is

in line with axis of actual salient field poles. This axis is along the d axis. In this

position, the armature flux linkage with field winding is maximum and the rate of

change of this flux linkage is zero. Therefore the induced voltage across the field

winding is zero. The d axis can therefore be located on the oscillogram. From the

figure Xd = ab/cd , also the ratio of armature terminal voltage per phase to the

corresponding current per phase to the armature current gives Xd

Also, the ratio of armature terminal voltage per phase to the corresponding current

per phase gives Xd.

After one quarter of slip cycle the peak armature wave is in line with q axis. In this

position, the reluctance offered by long air gap is maximum as shown in figure b .

a large magnetizing current is needed to establish the same air gap flux. This

maximum current Imax is measured from the line ammeter A. Also in this position,

the armature flux linking the field winding is zero, and the arte of change of this

flux linkage is maximum. Consequently the field winding is zero, and the rate of

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change of flux linkage is maximum. Consequently, the induced voltage across the

field winding is maximum. Thus q axis can also be located on the oscillogram.

From this

Xq = a’b’/c’d’.

Also the ratio of armature terminal voltage per phase to the corresponding

armature current per phase given Xq.

The slip test is generally used to find out ratio Xd/Xq . the direct axis synchronous

reactance Xd is determined from open and short circuit test as in the case of

cylindrical rotor machine. Knowing Xd from O.C and S.C test and ratio Xd/ Xq

from slip test we can find out Xq.

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SCR AND ITS SIGNIFICANCE

The short circuit ratio (SCR) of synchronous machine is defined as the ratio of the

field current required to generate rated voltage on open circuit to the field current

required to circulate rated armature current on short circuit.

The short circuit ratio SCR can be calculated from the open circuit characteristics

(O.C.C) at rated speed and short circuit characteristics of three phase synchronous

machine.

SCR = If for rated O.C Voltage = Oa

If for rated S,C Current Od

Since triangle Oab and Odc are similar

SCR = Oa = ab

Od de

The direct axis synchronous reactance Xd is defined as the ratio of open circuit

voltage for given current to the armature short circuit current for the same field

current

XdΩ = ac

ab

The per unit value of Xd is given by

Xdpu = XdΩ

Base impedance

But base impedance = Per phase rated voltage

Per phase rated armature current

= Vrated = ac Ω

Ia rated de

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Therefore Xd pu = ac · de = de

ab ac ab

SCR = ab = 1 = 1

de ( de/ab) Xd pu

The SCR is equal to the reciprocal of per unit value of the direct axis synchronous

reactance.

SIGNEFICANCE OF SCR

The SCR is an important factor for the synchronous machine. It affects the

operating characteristics, Physical size and cost of the machine. With a low value

of S.C.R a synchronous generator has a large variation in terminal voltage with a

change in load. That is the machine is very sensitive to load variations. In order to

keep the terminal voltage constant, field current is to be varied over a wide range.

The synchronizing power is small if the SCR small. Since the synchronizing power

keeps the machine in synchronism, a low SCR is less stable when operating in

parallel with other generators. But the armature current under shrt circuit

conditions is small for a low SCR.

A synchronous machine with high value of SCR has better voltage regulation and

improved steady state stability limit but the short circuit fault current in the

armature is high.

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REGULATION OF ALTERNATOR

Draw the Phasor diagram OB = V to represent 100% voltage at full load. The

current Phasor I = OA is drawn at angle Cos¯¹θ, the lagging P.F behind V, The

resistance voltage drop IR is drawn parallel to I. The leakage reactance drop Ix is

drawn oerpendicular to I. The Phasor sum of

V,Ir = IXs ( OD=E)

Therefore OD = V volts

From OCC the field current required to get V volts is say Io, draw OM

perpendicular to Phasor E to represent excitation required to induce emf E. The

field current equivalent to full load armature reaction on short circuit is MN and is

parallel to current Phasor I. Take a point S on MN such that MS= KMN, where K

is the ratio of cross reaction to the direct axis reaction per ampere turn.

Suppose K = 0.5 in that case it will be at mid point of MN. Join OS and draw a

perpendicular NG on OS produced. The OG is required excitation, OY is the

Phasor sum of OM & MH. Measure OG in amps, from OCC find out the voltage

fro OG amps The EMF induce is Ex

Therefore the % regulation = Ex-Vo X100

Vo

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POWER ANGLE EQUATION AND CHARACTERISTICS,

The resistance Ra of the armature has negligible effect on the relationship between

the power out put of a synchronous machine and its torque angle δ. It may

therefore may be neglected. The power diagram of lagging P.F for salient pole

alternator neglecting Ra is drawn

Complex power output per phase SiΦ = V Ia۰

Taking Ef as the reference Phasor

V = V∟-δ = Vcosδ – jVsinδ

Iq +jId =Ia۰

erThrehT SiΦ = V Ia ۰

= ( V cosδ -jsinδ) ( Iq+jId) Equation I

XqIq = CD = AM = Vsinδ

Therefore Iq = Vsinδ /Xq

XdId = AC= MD=OD-OM = Ef –Vcosδ

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Id = (Ef- V cosδ)/Xd

SiΦ = (Vcosδ – jVsinδ) (Vsinδ /Xq) + j(Ef- V cosδ)/Xd

Substituting the values of Iq and Id in the equation I we get

SiΦ = (Vcosδ – jVsinδ) [(Vsinδ /Xq) + j(Ef- V cosδ)/Xd]

= [V²cosδsinδ/Xq + VEfsinδ/Xd -V²cosδsinδ/Xd] + j [VEfcosδ/Xd -V²cos²δ/Xd

- V²sin²δ/Xq]

= [VEfsinδ/Xd +(V²/2)[ (1/Xq-1/Xd)sin2δ] +j [ (VEf cos /Xd) –

(V²/2XdXq)(Xd+Xq) – (Xd-Xq) Cos2δ]

Also S1Φ = P1Φ + j Q1Φ

P1Φ = [VEfsinδ/Xd +(V²/2)[ (1/Xq-1/Xd)sin2δ] real power in watts for single

phase

P3Φ = 3 P1Φ =3 [VEfsinδ/Xd +(V²/2)[ (1/Xq-1/Xd)sin2δ]

The reactive power

Q1Φ =[ (VEf cos /Xd) – (V²/2XdXq)(Xd+Xq) – (Xd-Xq) Cos2δ]

Q3 Φ = 3 Q1Φ = 3[ (VEf cos /Xd) – (V²/2XdXq)(Xd+Xq) – (Xd-Xq) Cos2δ]

The real power is the same as that obtained in the case of cylindrical rotor

machine. The reactive power depends upon the saliency defined by the quantity [

1/Xq-1/Xd]. The saliency disappears when Xd= Xq That is for cylindrical

rotor)Also this term disappears even when there is no field current ( Ef=0). The

equation for active power and reactive power is applicable for the generator and for

the synchronous motor. The torque angle δ is positive for generator and negative

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for motor. P- Q curve i.e verses δ curves for salient pole machine is as shown

below.

The electromagnetic torque or torque developed for a 3 phase synchronous

machines is given by

Tem = 3P1Φ/ώm = 3 [ Efsinδ + (Xd – Xq)sin2δ ]

2Пήs Xd 2Xd Xq

The resulting torque so developed has two components , The first term represents

Texc due to field excitation

Texc = 3 Efsinδ

2Пήs Xd

The second term is the reluctance torque Trel = 3 [(Xd – Xq) ]sin2δ

2Пήs 2Xd Xq

The reluctance torque is independent of excitation and exists only if the machine is

connected to a system receiving reactive power from the synchronous machine

operating in parallel with terminal voltage V.

The reluctance torque is due to the saliency of field poles which tend to align the

direct axis with that of the armature mmf. It is to be noted that if there is no field

current Ef=0, the first term

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VEf sinδ/ Xd becomes zero, and the machine still has some power generation.

However it is impractical to operate the synchronous machine without field

excitation on a power system, because it would supply only about 25% or less of

its real power rating. Also it will absorb an excessive amount of reactive power.

The maximum reluctance torque occurs at δ =45º.

The power angle ( P –δ) curve for a salient pole machine is shown . It is to be

noted that peak power or steady state limit occurs at a value of δ less than 90º,

The value δmax depends on the relative magnitude of V, Ef and saliency.

SYNCHRONISING OF ALTERNATOR WITH INFINITE BUS BAR.

In a power system more than one alternator operate in parallel. The machines may

be located at different places. The machines, connected to the same bus but

separated by transmission lines of low reactance’s. The capacity of system is so

large that its voltage and frequency may be taken as constant. The connection or

disconnection of single machine or a small load on such system would not effect

the voltage and frequency. The system behaves like a large generator having

virtually zero internal impedance and infinite rotation inertia. Such a system of

constant voltage and constant frequency regardless of load is called infinite bus bar

system.

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The characteristics of infinite bus bars are as follows:-

a) The terminal voltage remains constant, because incoming machine is too

small to increase or decrease it.

b) The frequency remains constant, because the rotational inertia is too large to

enable the incoming machine to alter the speed of the system.

c) The synchronous impedance is very small since the system has large number

of alternators in parallel.

In an isolated operation, the change of excitation changes the terminal voltage,

the power factor depends on the load only. When an alternator is working in

parallel with an infinite bus and its excitation is change, The P.F of the machine

changes. However change in excitation does not change the terminal voltage.

Which is held constant by the system.

AN ALTERNATOR CONNECTED TO AN INFINITE BUS HAS THE

FOLLOWING OPERATING CHARACTERISTIC

1) The terminal voltage and frequency of generator are controlled by system to

whom it is connected.

2) The governor set point of the alternator control the real power supplied to

infinite bus

3) The field current in the alternator controls the reactive power supplied by the

alternator to the infinite bus. Increasing field current in the alternator

operating in parallel with an infinite bus increases the reactive power output

of the alternator.

OBTAINING INFINITE BUS

a) Proof of voltage remaining constant

V= terminal voltage, E = induced EMF in each generator, Zs =

Synchronous impedance, n=no off generators in parallel.

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V=E-IZseq, Zseq= Zs/n

When n is very large Zseq 0, Izseq = 0

Therefore V = E constant.

b) Proof of frequency remaining constant

Let J = Moment Inertia of each alternator

Total moment of inertia = J1 +j2+j3 +……….. Jnd = nf

Acceleration of alternator = accelerating torque / moment of inertia

τa = τa

∑j nj

If n is very large so n j will also be large

Therefore acceleration = 0 and thus the speed is constant.

SYCHRONISING POWER, PARALLEL OPERATION AND LOAD

SHARING OPERATION ON INFINITE BUS

SYCHRONISING POWER:- When a synchronous machine is

synchronized to infinite bus bars has a inherent tendency to remain in

synchronism. Consider a synchronous machine is transferring steady

state power P o at a steady load angle δ o, Suppose due to transient

disturbances, the rotor of generator accelerates, so that the load angle

increases to dδ. The operating point of the machine shifts to a new

constant power line and the load on the machine increase to P o +dδ.

Since Te steady power input remains unchanged this additional load

decreases the speed of the machine and brings it back to synchronism.

Similarly, if due to transient disturbances the rotor of the machine

retards, so that the load angle decreases, the operating point of the

machine shifts to a new constant power line and the load ion the machine

decreases to P o –dδ. Since the steady power remains same, the reduction

in load accelerates the rotor, consequently the machine again comes in

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synchronism. It is seen that the effectiveness of this correcting action

depends on the change in power transfer for a given change of load angle.

A measure of effectiveness is given by Synchronizing power coefficient.

It is defined as rate at which the synchronous power P varies with load

angle. It is also called stiffness factor, rigidity factor or stability factor

and is denoted by Psyn

Psyn = ∆ dP/dδ

In a salient pole machine

P= VEfsinδ/Xs +(V²/2)[ (1/Xq-1/Xd)]sin2δ

Psyn = VEfcosδ/Xs +V²[ (1/Xq-1/Xd)]cos2δ

PARALLEL OPERATION

Electric power system are interconnected for economy and reliable operations.

Interconnection of ac power system requires synchronous generator’s to operate in

parallel with each other. In a generating stations two or more generators are

connected in parallel. In an interconnected system forming a grid the alternators

are located at different places. They are connected in parallel by means of

transformers and transmission lines.

An arrangement of generators for parallel operation enable a plant engineer to

adjust the machines for optimum efficiency and greater reliability. As the load

increases beyond the generating capacity of the connected units, Additional

generators’ are paralleled to carry the load. Similarly as the load demand falls off

one or more of the machines are generally taken off the line to allow the units to

operate at a higher efficiency.

ADVANTAGES

1. Several alternators can supply a bigger load than single alternator.

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2. During light load, one or more alternators may be shut down, and

those remaining operates at or near full load, and thus more

efficiency.

3. When one machine is taken out of service for maintenance and

inspection, the remaining machines maintain the continuity of supply.

4. If there is a breakdown of a generator, there is no interruption of

power supply.

5. In order to meet the increasing future demand of load more machines

can be added without disturbing the original machines.

6. The operating cost and cost of energy generated are reduced when

several generator’s operate in Parallel.

CONDITION FOR PARALLEL OPERATION

Most of the synchronous machines will operate in parallel with other synchronous

machine and process of connecting one machine in parallel with another machine

or infinite bus bar system is known as synchronizing. Those machines already

carrying load are known as running machines, while alternators which is to be

connected in parallel with the system is known as incoming machine. Before the

incoming machine is to be connected to the system , the following conditions are to

be satisfied.

i) The phase sequence of the bus bar voltage and the incoming

machine voltage must be same.

ii) The bus bar voltages and the incoming machines voltage must

be in phase.

iii) The terminal voltage of incoming machine should be equal to

that of the alternator with which it is to be run parallel or with

bus bar voltage

iv) The frequency of the generated voltage of the incoming

machine must be equal to the frequency of voltage of bus bar.

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SYNCHRNOUS MACHINES LOAD SHARING OPERATION ON INFINITE

BUS BAR

Consider two alternators are running in parallel . The frequency load

characteristics of two machines is shown below.

W1 = full load rating of machine 1

W2 = full load rating of machine 2

P1 = Power shared by machine 1

P2 = Power shared by machine 2

P = Total power supplied by two machines

fo1 = No load frequency of Machine 1

fo2 =No load frequency of Machine 2

fl1 = Full load frequency of machine 1

fl2 = Full load frequency of machine 2

f = common operating frequency when both the machines are

running in parallel.

MACHINE 1

Drop in frequency from no load to full load = fo1 -- fl1

Drop in frequency per unit rating = fo1 -- fl1

W1

Drop in frequency for a load P1

fl =P1 fo1 -- fl1

W1

Operating frequency = No load frequency – drop in frequency

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f = fo1 -- P1 fo1 -- fl1

W1

MACHINE 2

Similarly for alternator 2 , the operating frequency is

f = fo2 -- P2 fo2 -- fl2

W2

fo1 -- P1 fo1 -- fl1 = fo2 -- P2 fo2 -- fl2

W1 W2

We also know that P = P1+ P2

The above equation is used to determine P1 and P2 and f.

EFFECT OF SYNCHRONIZING CURRENT , HUNTING OF ALTERNATOR

Effect of synchronizing current

Consider a synchronous machine with terminal voltage Vt. Operating at rotor angle

δ, drawing armature current Ia and having excitation emf Ef. A sudden

disturbances causes its rotor angle to increase (δ +∆δ). As we have seen above it

develops synchronizing power Ps which counters the change. The synchronizing

power arise from synchronizing emf Es and synchronizing current Is. As the

terminal voltage is constant the armature equation before after change are

׀ ׀ ( ) = ( )

E s = E f ( ) - E f∟-δ = change in Ef

I s = I a , -I a Change in I a

= E f ( ) - E f∟-δ = - j (I a -I a)Xs

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E s = - jI s Xs where E s is synchronizing emf, and I s = synchronizing current.

I s = E s/- jI s Xs

HUNTING OF ALTERNATOR

A steady state operation of alternator is a condition of equilibrium in running

conditions. With the sudden increase in the load the speed drops and similarly with

decrease of load the speed increases. This type of changes causes hunting of

alternators. The main reasons for hunting are as follows;-

a) Sudden change of load

b) Faults occurring in the system which the generator supplies.

c) Sudden change in field current.

d) Cyclic variation of load.

EFFECTS OF HUNTING

1) It can lead to loss of synchronism

2) It can cause variations of the supply voltages

3) It increases the possibility of resonance

4) Large mechanical stresses may develop in the rotor shaft.

5) The machine losses increases and the temperature of the machine rises.

Out of these effects , the first one is the most important and it is to be

avoided.

The hunting of alternator causes the circulating current also. It always

preferred that the alternator should not operate on hunting condition.

If the hunting prevails in the machine the electrical power fed to the mains

and shaft oscillates, which in turn causes shaft fatigue.

REDUCTION OF HUNTING

Following are some of the technique used to reduce hunting

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a) Use of Damper Winding

b) Use of Fly Wheel

c) By designing synchronous machine.

DAMPER WINDING :- Additional damping is provided in the salient pole

synchronous machine by means of damper bars located in the main pole of the

machine and short circuited through round rings at both ends. These bars acts like a

squirrel cage induction motor. These damper windings are used in synchronous

motors for the starting purpose.

USE OF FLY WHEEL:- The prime mover is provided with a large and heavy fly

wheel. This increases the inertia of the prime mover and helps in maintaining the

rotor speed. The Large Hydro generators having big diameter compare to core

length also works as fly wheel.

DESIGN:- By designing synchronous machines with suitable synchronizing power

coefficients.

SYNCHROSCOPES AND PHASE SEQUENCE INDICATORS

The phase sequence of the generator is checked carefully at the time of installation.

By means of synchroscope the voltage from one phase of incoming machine with

that of the corresponding phase of the three phase system is compared. The

position of the pointer of synchroscope indicates the phase difference between the

voltages of incoming machine and the infinite bus. When the frequencies are equal,

the pointer is stationary. When the frequencies differ, the pointer rotates in one

direction or the other direction. The direction of motion of the pointer shows

whether the incoming machine is running too fast or too slow, that is whether the

frequency of incoming machine is higher or lower than that of infinite bus bar. The

speed of rotation of the pointer is equal to the difference between the frequencies

of incoming machine and infinite bus. The frequency and phase position are

controlled by adjusting the prime mover input to the incoming machine. When the

indicator moves very slowly ( that is the condition when frequencies are almost

same) and passes through the zero phase point ( vertical u position), the circuit

breakers is closed and the incoming alternator is connected to the bus. It is to be

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noted that synchroscope checks relationship only with one phase. It gives no

information for the PH sequence.

The procedure is used for synchronizing synchronous motors with bus bars. When

the motor reaches synchronous speed, the d.c field is excited and if the load is not

excessive, The motor pulls in synchronism with the bus.

PHASE SEQUENCE INDICATORS

The phase sequence indicator is a device used to compare the phase sequence of

three phase generators or motors. One type of phase sequence indicator is tiny

three phase induction motor having and aluminum disc mounted for free rotation

on a cushioned glass hard bearing and having field winding placed at 120º

geometrical degree intervals about the rotor axis. One terminal of each winding is

extended beyond the case to a distinctly colored conductors and to altered test clip

for attachment to one conductor of the system or machine.

The flexible cable withstands severe handling and is meant for suspending the

phase sequence meter from the live wires or terminals with test clips. The cables

are anchored in the sealed housing. It can be used to correlate the motor or

generator conductor or terminal markings with the phase sequence of applied or

generated voltage. This type phase sequence meter avoids reversal of a generator

phase rotation when being paralleled, which would avoid short circuits. The three

leads of the Phase sequence indicators are colored RED, YELLOW and BLUE.

The RED colored wire is for phase A and Yellow for B and blue for C Phases. The

rotor in the instrument can be observed through the three ports at which it turns so

that we can note the direction in which it is rotating. The rotor can be started by

means of a momentary switch, it stops when we release the switch. It is necessary

to determine the phase sequence of line and alternator’s before synchronizing. This

can be easily checked by using Phase Sequence Indicators./

EFFECT OF VARYING EXCITATION AND MECHANICAL TORQUE

From the power curves for salient pole machines drawn in relation with δ. The

torque angle is positive for generator and negative for motor. The electromagnetic

torque or torque developed for a 3 phase synchronous machine is given by

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τem = 3P1Φ/ώm = 3 [ V Efsinδ + (Xd – Xq)sin2δ ]

2Пήs Xd 2Xd Xq

It is be noted that the resulting torque has two components, the first term represents

the torque τexc due to field excitation

τexc = 3 V Efsinδ

2ПήsXd

The second term is known as reluctance torque

τrel = 3 [(Xd – Xq) ]sin2δ

2Пήs 2Xd Xq

The reluctance torque is independent of excitation and exists only if the machine is

connected to a system receiving reactive power from other synchronous machines

operating in parallel with the terminal voltage V.

The reluctance torque is due to the saliency of the field poles which tend to align

the direct axis with that of the armature m.m.f.

It is to be noted that if there is no field current or no field excitation

Ef = 0

The term V Efsinδ becomes zero, and the machine still has some power

Xd generation capability

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However, it is impractical to operate a synchronous generator without field

excitation on a power system., because it would supply only 25% or less of its real

power rating. Also it would absorb an excessive amount of reactive power.

If an attempt is made to cause the machine to act as a generator or motor no field

current ( V supplied by the bus to which the machine is connected) , the poles shift

relative to the stator field, thus increasing the reluctance of the flux path as the

torque increase, For this reason the torque given by the following equation is called

the reluctance torque.

τrel = 3 [(Xd – Xq) ]sin2δ

2Пήs 2Xd Xq

The maximum value of this reluctance torque occurs at δ =45º It is to be noted that

peak power or steady state limit occurs at a value of δ less than 90º. The value of

δmax depends on the relative magnitude of V,Ef and saliency.

In general the value of δ decides whether the machine is overexcite generator or

motor supplies reactive power to the bus bars, and an under excited generator or

motor consumes or absorbs reactive power from bus bars.

The power angle ( P -- δ) curve for a salient pole machine drawn is as follows

RGPV EX 503 Unit IV

Engineering

Transcript of RGPV EX 503 Unit IV