11 Synchronous Generator and Motor Operation

8/10/2019 11 Synchronous Generator and Motor Operation

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Synchronous GeneratorOperation


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Effect of Generator Loads Lagging Power Factor

If lagging loads (+Q or inductive reactive power loads) are added to a generator,the phase voltage V f and the terminal voltage V T decrease.

Keep field excitation the same'

A A E E

A s A jX f E V I


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Effect of Generator Loads Unity Power Factor

A s A jX f E V I

If unity-power-factor loads (no reactive power) are added to a generator,the phase voltage V f and the terminal voltage V T slightly decrease.


A A E E


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Effect of Generator Loads Leading Power Factor

A s A jX f E V I

If leading loads (-Q or capacitive reactive power loads) are added to a generator,the phase voltage V f and the terminal voltage V T may increase.


A A E E


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Generator Voltage Regulation

100%nl fl

fl

V V VRV

Lagging Load -> large positive voltage regulationUnit Power Factor Load -> small positive voltage regulationLeading load -> may be negative voltage regulation


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Generator V Curves

The shape is like the letter V For each fixed real power, plot

armature current vs. fieldcurrent.


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Example 1 (1)

A 480 V, 60 Hz, D connected, four pole synchronous generator has the OCC curve shown in the figure.

This generator has a synchronous reactance of 0.1 W and an armature resistance of 0.015 W .At full load, the machine supplies 1200 A at 0.8 PF lagging. Under full load conditions, the frictionand windage losses are 40 kW, and the core losses are 30 kW. Ignore any field circuit losses.

(a) What is the speed of rotation of this generator?(b) How much field current must be supplied to the generator to make the terminal voltage 480 V at

no load?(c) If the generator is now connected to a load and the load draws 1200 A at 0.8 PF lagging, how much

field current is required to keep the terminal voltage equal to 480 V?(d) How much power is the generator now supplying? How much power is supplied to the generator

by the prime mover? What is the machines overall efficiency? (e) If the generators load were suddenly disconnected from the line, what would happen to its

terminal voltage?(f) Finally, suppose that the generator is connected to a load drawing 1200 A at 0.8 PF leading , howmuch field current would be required to keep V T at 480 V?


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Example 1 (2)

sg1.m


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Example 2 (1)

After the MatLab program can work, please change the load current to be 60 Aat 1.0 PF, and 60 A at 0.8 PF leading and redo the above.

A 480V, 60 Hz, Y connected, six pole synchronous generator has a synchronous reactance of 1 W and

an armature resistance of 0.1W

.

At full load, the machine supplies 60 A at 0.8 PF lagging. Under full load conditions, the frictionand windage losses are 1.5 kW, and the core losses are 1.0 kW. Ignore any field circuit losses.

(a) What is the speed of rotation of this generator?(b) What is the terminal voltage of this generator at full load assuming the field excitation current

keeps the same as no load?(c) What is the efficiency of this generator at full load?(d) How much shaft torque must be supplied by the prime mover at full load? How large is the

induced counter torque?(e) What is the voltage regulation of this generator ?

sg2.m


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Example 2 (2)

s A s A A R jX f V I I Edq

0 sin cos sin A A s A s A A

R I X I E q q d I I

Imaginary part of

Let the angle of : =0.f f

q VV

becomes:

cos sinsin A A s A s A

A

X I R I

E

q q d

I I

Note: is negative when current is lagging.

cos cos sin A A A s A s A

V E R I X I f d q q I I

Aq

I

Af q q q V I


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Synchronous GeneratorCapability Curve


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Generator Capability Curves (1)

(1) Stator Copper Loss (stator heating):

(2) Rotor Copper Loss (rotor heating):

(3) Prime- movers Power Limit.

23SCL A s

P I R

2

RCL F F P I R

The maximum allowable heating of the rotor sets a maximumfield current I F for the machine. Its equivalent to set a maximum

E A for the machine.

The maximum allowable heating of the stator sets a maximum phase current I A for the machine. Its equivalent to set a maximum

apparent power for the machine. (power factor is irrelevant)


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rotor field current sets the rated power factor

(rotor heating)

(stator heating)

A s A jX f E V I


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A s A jX f E V I

Assume V f keeps rated value. Multiply the above figure by3

S

V

X f

P

Q


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P

Q

capability curve

Q

P

flip


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Q

P

Add prime movers power limit (real power)


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Example 3

sg3.m

A 480V, 50 Hz, Y connected, six pole synchronous generator is rated at 50 kVA at 0.8 PF lagging. Ithas a synchronous reactance of 1.0 W per phase. Assume that this generator is connected to a steamturbine capable of supplying up to 45 kW. The friction and windage losses are 1.5 kW, and the corelosses are 1.0 kW.

(a) Sketch the capability curve for this generator, including the prime-mover power limit.(b) Can this generator supply a line current of 56 A at 0.7 PF lagging? Why or why not?(c) What is the maximum amount of reactive power this generator can produce?(d) If the generator supplies 30 kW of real power, what is the maximum amount of reactive power that

can be simultaneously supplied?


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Synchronous MotorOperation


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UCF Synchronous Motor and Generator (1)

motor generator

net Rind k BBT

d sinnet Rind BkBT


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UCF Synchronous Motor and Generator (2)

GeneratorMotor

s A s A A R jX f V I I E s A s A A R jX f V I I E


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UCF Torque of Synchronous Motor

d sinnet Rind BkBT

A s A s A R jX f V E I I

)(forsin3

cos3

s A s

A

A

X R X

E V

I V P

d

q

f

f

sin3 d f

sm

A

mind X

E V P T

Pull-out torque: when sin d =1, the maximum torque the machine can get.net R BkBT max 3max

sm

A

X

E V T

f

omaxTypically take 3 (or sin 1 / 3, 19.47 )

in the design (leave margin).

full load T T d d


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UCF Effect of Load Change

magnets) permanentusingorfixed(whenfixed:

course)electrical(fromfixed: sin3cos3

F A

s

A

A

I E

V

X

E V

I V P

f

f

f d q

2(load increases) sin more heat (3 )

(leading lagging) A A s P I I Rd

q

q

At full load, typically pick up cos 1 in the design.

sin , cos A

s A

A

V E

X E I

f

q

d d


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UCF Example 1

Details in sm1.m


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UCF Effects of Field Current Change

A s A s A

A s A

R jX jX

f

V E I I

E I

q


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UCF Underexcited Synchronous Motor


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UCF Overexcited Synchronous Motor

Behaves like a capacitor: can be used for power factor correction.Called synchronous capacitor or synchronous condenser .


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UCF Synchronous Motor V Curves


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UCF Example 2

sm2.m


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UCF

Power factor Correction Using OverexcitedSynchronous Motor - Example 3

sm3.m

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11 Synchronous Generator and Motor Operation

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