1.
Why almost all large size Synchronous
machines are constructed with rotating field system type?
The following are the principal
advantages of the rotating field system type construction of Synchronous
machines:
·
The relatively small amount of power,
about 2%, required for field system via slip-rings and brushes.
·
For the same air gap dimensions, which
is normally decided by the kVA rating, more space is available in the stator
part of the machine for providing more insulation to the system of conductors,
especially for machines rated for 11kV or above.
·
Insulation to stationary system of conductors is not subjected to
mechanical stresses due to centrifugal action.
·
Stationary system of conductors can
easily be braced to prevent deformation.
·
It is easy to provide cooling
arrangement for a stationary system of conductors.
·
Firm stationary connection between
external circuit and system of conductors enable he machine to handle large
amount of volt-ampere as high as 500MVA.
2.
Write down the equation for frequency of emf induced in an
Altenator.
Frequency
of emf induced in an Alternator,f ,expressed in cycles per second or Hz, is
given by the following equation
F = (PN)/120 Hz,
Where
P- Number of poles N-Speed in rpm
3. How are alternators classified?
According to type of
field system
·
Stationary field system type
·
Rotating field system type
According to shape of
field system
·
Salient pole type
·
Smooth cylindrical type
4. Name the types of Alternator based on their rotor construction.
Alternators
can be classified into the following two types according to its rotor
construction
·
Smooth cylindrical type alternator
·
Salient pole alternator
Steam turbines are found to
operate at fairly good efficiency only at high speeds. The high speed operation
of rotors tends to increase mechanical losses and so the rotors should have a
smooth external surface. Hence, smooth cylindrical type rotors with less
diameter and large axial length are used for Synchronous generators driven by
steam turbines with either 2 or 4 poles.
6. Which type of
Synchronous generators are used in Hydro-electric plants and why?
As the speed of operation is
low for hydro turbines use din Hydro-electric plants, salient pole type
Synchronous generators are used. These allow better ventilation and also have
other advantages over smooth cylindrical type rotor.
7. What are the advantages of salient pole type construction used for
Synchronous machines?
Advantages of salient-pole type
construction are :
·
They allow better ventilation
·
The pole faces are so shaped that the
radial air gap length increases from the pole center to the pole tips so that
the flux distribution in the air-gap is sinusoidal in shape which will help the
machine to generate sinusoidal emf
·
Due to the variable reluctance the
machine develops additional reluctance power which is independent of excitation
8.
Why is the stator core of Alternator laminated?
The stator core of Alternator
is laminated to reduce eddy current loss.
9.
How does electrical degree differ from mechanical degree?
Mechanical
degree is the unit for accounting the angle between two points based on their
mechanical or physical placement.
Electrical
degree is used to account the angle between two points in rotating electrical
machines. Since all electrical machines operate with the help of magnetic
fields, the electrical degree is accounted with reference to the magnetic
field. 180 electrical degree is accounted as the angle between adjacent North
and South poles.
10.Define winding factor.
The winding factor Kd is defined as the ratio of phasor addition of emf induced in all
the coils belonging to each phase winding to their arithmetic addition.
11. Why are Alternators rated in kVA and not in kW?
The continuous power rating of any machine is generally defined as the power the machine or apparatus can deliver for a continuous period so that the losses incurred in the machine gives rise to a steady temperature rise not exceeding the limit prescribed by the insulation class.
Apart
from the constant loss incurred in Alternators is the copper loss, occurring in
the 3 –phase winding which depends on I2
R, the square of the current delivered by the generator. As the current is
directly related to apparent – power delivered by the generator , the
Alternators have only their apparent power in VA/kVA/MVA as their power rating.
12. What are the
causes of changes in voltage in Alternators when loaded?
Variations in terminal voltage in Alternators on
load condition are due to the following three causes:
·
Voltage variation due to the
resistance of the winding, R
·
Voltage variation due to the armature reaction effect, Xa
13.
An Alternator is found to have its terminal voltage on load
condition more than that on no load. What is the nature of the load connected?
The nature of
the load is of leading power factor , load consisting of resistance and
capacitive reactance.
14. Define the term voltage regulation
of Alternator.
The
voltage regulation of an Alternator is defined as the change in terminal
voltage from no-load to load condition expressed as a fraction or percentage of
terminal voltage at load condition ; the speed and excitation conditions
remaining same.
15. What is the necessity for
predetermination of voltage regulation?
Most
of the Alternators are manufactured with large power rating , hundreds of kW or
MW, and also with large voltage rating upto 33kV. For Alternators of such power
and voltage ratings conducting load test is not possible. Hence other indirect
methods of testing are used and the performance like voltage regulation then
can be predetermined at any desired load currents and power factors.
16.
Name the various methods for predetermining the voltage regulation
of 3-phase Alternator.
The
following are the three methods which are used to predetermine the voltage
regulation of smooth cylindrical type Alternators
·
Synchronous impedance / EMF method
·
Ampere-turn / MMF method
·
Potier / ZPF method
17. How
synchronous impedance is calculated from OCC and SCC?
Synchronous impedance is
calculated from OCC and SCC as
|Zs|
= E0/Isc(for same If)
18.
What are the advantages and
disadvantages of estimating the voltage regulation of an Alternator by EMF
method?
Advantages:
·
Simple no load tests (for obtaining
OCC and SCC) are to be conducted
·
Calculation procedure is much simpler
Disadvantages:
·
The value of voltage regulation
obtained by this method is always higher than the actual value
19.
Why is the synchronous impedance method of estimating voltage
regulation considered as pessimistic method?
Compared
to other methods, the value of voltage regulation obtained by the synchronous
impedance method is always higher than the actual value and therefore this
method is called the pessimistic method.
20.
In what way does the ampere-turn method differ from synchronous
impedance method?
The ampere-turn /MMF method is the converse of the EMF method in
the sense that instead of having the phasor addition of various voltage
drops/EMFs, here the phasor addition of MMF required for the voltage drops are
carried out. Further the effect of saturation is also taken care of.
21.
What are the test data required for predetermining the voltage
regulation of an Alternator by MMF method?
Data required for MMF
method are :
·
Effective resistance per phase of the 3-phase winding R
·
Open circuit characteristic (OCC) at
rated speed/frequency
·
Short circuit characteristic (SCC) at
rated speed/frequency
22.
Why is the MMF method of estimating the voltage regulation
considered as the optimistic method?
Compared
to the EMF method, MMF method, involves more number of complex calculation
steps. Further the OCC is referred twice and SCC is referred once while
predetermining the voltage regulation for each load condition. Reference of OCC
takes care of saturation effect. As this method require more effort, the final
result is very close to the actual value. Hence this method is called
optimistic method.
23. State the condition to be
satisfied before connecting two alternators in parallel
The
following are the three conditions to be satisfied by synchronizing the
additional Alternator with the existing one or the common bus-bars.
·
The terminal voltage magnitude of the
incoming Alternator must be made equal to the existing Alternator or the
bus-bar voltage magnitude.
·
The phase sequence of the incoming
Alternator voltage must be similar to the bus-bar voltage.
·
The frequency of the incoming
Alternator voltage must be the same as the bus-bar voltage.
24.
How do the synchronizing lamps indicate the correctness of phase
sequence between existing and incoming Alternators?
The
correctness of the phase sequence can be checked by looking at the three sets
of lamps connected across the 3-pole of the synchronizing switch. If the lamps
grow bright and dark in unison it is an indication of the correctness of the
phase sequence. If on the other hand, they become bright and dark one after the
other, connections to any two machine terminals have to be interchanged after
shutting down the machine.
25.
What are the advantages and disadvantages of three dark lamps
method of synchronizing?
Advantages:
·
The synchronous switch using lamps is
inexpensive
·
Checking for correctness of the phase sequence can be obtained in
a simple manner which is essential especially
when
the Alternator is connected for the first time or for fresh operation after
disconnection
Disadvantages:
·
The rate of flickering of the lamps only indicates the frequency
difference between the bus-bar and the incoming Alternator. The frequency of
the incoming Alternator in relation to the bus-bar frequency is not available.
26. How
synchronoscope is used for synchronizing Alternators?
Synchronoscope
can be used for permanently connected Alternators where the correctness of
phase sequence is already checked by other means. Synchronoscope is capable of
rotating in both directions. The rate of rotation of the pointer indicates the
amount of frequency difference between the Alternators. The direction of
rotation indicates whether incoming Alternator frequency is higher or lower
than the existing Alternator. The TPST switch is closed to synchronise the
incoming Alternator when the pointer faces the top thick line marking.
27.
Why synchronous generators are to be constructed with more
synchronous reactance and negligible resistance?
The presence of more resistance
in the Synchronous generators will resist or oppose their synchronous
operation. More reactance in the generators can cause good reaction between the
two and help the generators to remain in synchronism in spite of any
disturbance occurring in any one of the generators.
28. List the factors that affect the
load sharing in parallel operating generators?
The
total active and reactive power delivered to the load, connected across the
common bus-bars, are shared among Synchronous generators, operating in
parallel, based on the following three factors
·
Prime-mover characteristic/input
·
Excitation level and
·
Percentage synchronous impedance and
its R/X ratio
29. How does the
change in prime mover input affect the load sharing?
An
increase in prime-mover input to a particular generator causes the active-power
shared by it to increase and a corresponding decrease in active-power shared by
other generators. The change in reactive power sharing is less appreciable. The
frequency of the bus-bar voltage will also subjected to slight increase in
value.
30. How does change in excitation
affects the load sharing?
The decrease in excitation in
one generator causes the reactive power shared by it to decrease and a
corresponding increase in reactive-power shared by
other
generators. The change in active-power sharing is less appreciable. There will
be a slight decrease in terminal voltage magnitude also.
31.
What steps are to be taken before disconnecting one Alternator
from parallel operation?
The
following steps are to be taken before disconnecting one Alternator from
parallel operation
·
The prime-mover input of the outgoing generator has to be
decreased and that of other generators has to be increased and by this the
entire active-power delivered by the outgoing generator is transferred to other
generators.
·
The excitation of the outgoing
generator has to be decreased and that of other generators have to be increased
and by this the entire reactive-power delivered by the outgoing generator is
transferred to other generators.
·
After ensuring the current delivered
by the outgoing generator is zero, it has to be disconnected from parallel
operation.
32. What is meant
by infinite bus-bars?
The source or supply lines with
non-variable voltage and frequency are called infinite bus-bars. The source
lines are said to have zero source impedance and infinite rotational inertia.
33.
How does increase in excitation of the Alternator connected to infinite
bus-bars affect this operation?
Increase
in excitation level of the synchronous generator will effectively increase the
reactive component of the current supplied by the generator and hence the
active power delivered.
No comments:
Post a Comment