Parallel Operation of DC Generators: In a dc power plant, power is
usually supplied from several generators of small ratings connected in
parallel instead of from one large generator. This is due to the following
reasons:
(i) Continuity of service: If a single large generator is used in the power
plant, then in case of its breakdown, the whole plant will be shut down.
However, if power is supplied from a number of small units operating in
parallel, then in case of failure of one unit, the continuity of supply can be
maintained by other healthy units.
(ii) Efficiency: Generators run most efficiently when loaded to their rated
capacity. Therefore, when load demand on power plant decreases, one or
more generators can be shut down and the remaining units can be
efficiently loaded.
(iii) Maintenance and repair: Generators generally require routinemaintenance
and repair. Therefore, if generators are operated in parallel,
the routine or emergency operations can be performed by isolating the
affected generator while load is being supplied by other units. This leads
to both safety and economy.
(iv) Increasing plant capacity: In the modern world of increasing
population, the use of electricity is continuously increasing. When added
capacity is required, the new unit can be simply paralleled with the old
units.
(v) Non-availability of single large unit: In many situations, a single unit
of desired large capacity may not be available. In that case a number of
smaller units can be operated in parallel to meet the load requirement.
Generally a single large unit is more expensive.
Connecting Shunt Generators in Parallel: The generators in a power
plant are connected in parallel through bus-bars. The bus-bars are heavy
thick copper bars and they act as +ve and -ve terminals. The positive
terminals of the generators are .connected to the +ve side of bus-bars and
negative terminals to the negative side of bus-bars. Fig. (1) shows shunt
generator 1 connected to the bus-bars and supplying load. When the load
on the power plant increases beyond the capacity of this generator, the
second shunt generator 2 is connected in parallel with the first to meet the
increased load demand. The procedure for paralleling generator 2 with
generator 1 is as under:
(i) The prime mover of generator 2 is brought up to the rated speed. Now
switch S4 in the field circuit of the generator 2 is closed.
(ii) Next circuit breaker CB-2 is closed and the excitation of generator 2
is adjusted till it generates voltage equal to the bus-bars voltage. This is
indicated by voltmeter V2.
(iii) Now the generator 2 is ready to be paralleled with generator 1. The
main switch S3 is closed, thus putting generator 2 in parallel with
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generator 1. Note that generator 2 is not supplying any load because its
generated emf is equal to bus-bars voltage. The generator is said to be
"floating" (i.e. not supplying any load) on the bus-bars.
Figure(1)
(iv) If generator 2 is to deliver any current, then its generated voltage E
should be greater than the bus-bars voltage V. In that case, current
supplied by it is I = (E - V)/Ra where Ra is the resistance of the armature
circuit. By increasing the field current (and hence induced emf E), the
generator 2 can be made to supply proper amount of load.
(v) The load may be shifted from one shunt generator to another merely
by adjusting the field excitation. Thus if generator 1 is to be shut down,
the whole load can be shifted onto generator 2 provided it has the
capacity to supply that load. In that case, reduce the current supplied by
generator 1 to zero (This will be indicated by ammeter A1) open C.B.-1
and then open the main switch S1.
Load Sharing: The load sharing between shunt generators in parallel can
be easily regulated because of their drooping characteristics. The load
may be shifted from one generator to another merely by adjusting the
field excitation. Let us discuss the load sharing of two generators which
have unequal no-load voltages. Let E1, E2 = no-load voltages of the two
generators R1, R2 = their armature resistances
V = common terminal voltage (Bus-bars voltage). Then
1
1
1 R
I E V
and
2
2
2 R
I E V
Thus current output of the generators depends upon the values of E1 and
E2. These values may be changed by field rheostats. The common
terminal voltage (or bus-bars voltage) will depend upon (i) the emfs of
individual generators and (ii) the total load current supplied. It is
generally desired to keep the busbars voltage constant. This can be
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achieved by adjusting the field excitations of the generators operating in
parallel.
Compound Generators in Parallel: Under-compounded generators also
operate satisfactorily in parallel but over compounded generators will not
operate satisfactorily unless their series fields are paralleled. This is
achieved by connecting two negative brushes together as shown in Fig.
(2) (i). The conductor used to connect these brushes is generally called
equalizer bar. Suppose that an attempt is made to operate the two
generators in parallel without an equalizer bar. If, for any reason, the
current supplied by generator 1 increases slightly, the current in its series
field will increase and raise the generated voltage. This will cause
generator 1 to take more load. Since total load supplied to the system is
constant, the current in generator 2 must decrease and as a result its series
field is weakened. Since this effect is cumulative, the generator 1 will
take the entire load and drive generator 2 as a motor. After machine 2
changes from a generator to a motor, the current in the shunt field will
remain in the same direction, but the current in the armature and series
field will reverse. Thus the magnetizing action, of the series field opposes
that of the shunt field. As the current taken by the machine 2 increases,
the demagnetizing action of series field becomes greater and the resultant
field becomes weaker. The resultant field will finally become zero and at
that time machine 2 will be short circuited machine 1, opening the
breaker of either or both machines.
Figure (2)
When the equalizer bar is used, a stabilizing action exists and neither
machine tends to take all the load. To consider this, suppose that current
delivered by generator 1 increases. The increased current will not only
pass through the series field of generator 1 but also through the equalizer
bar and series field of generator 2. Therefore, the voltage of both the
machines increases and the generator 2 will take a part of the load.
parallel operation of generators is nothing but giving the same source of energy to the two or more units.
Voltage , phase sequence and frequency should be same.
voltage of both generators must be equal.
To avoid possible current loop through multiple neutral points
CONDITIONS REQUIRED FOR TRANSFORMER PARALLEL operation are.. 1. Voltage turn ratio. 2. phase sequence 3. Impedence should be same 4. polarity Hitesh
Normal shaft generators do not have droop-control for frequency and voltage. Semiconductor (drive) based variable speed shaft generators may have those today.
Generally speaking, the outputs of generators can be coupled by connecting the generators in parallel. In generators that produce DC (direct current), the voltages need to be equalized and the polarities of the connections needs to be observed. Both negatives are connected together and both positives connected together to create one parallel output. In an AC setup, the generators need to be running about the same voltage, and must be connected in phase for the setup to be successful. In practice, the generator that is being brought online is run up, and it frequency is allow to be a little bit faster than the unit that is online. When the phase difference is being observed, and the difference is approaching zero, at a "couple of degrees before" the zero, the generator that is being brought online is then connected to the grid and it "locks in" so that all the generators are running in phase. Connecting DC generators positive to negative is an invitation to disaster, and bringing an AC generator online 180 degrees out of phase with an running generator is the same kind of disaster. Shutting a breaker to parallel generators without insuring polarity concordance (in the case of the DC ones) or phase concordance (in the case of the AC ones) will pit one generator against the other, and severe damage will result.
No it is not normal. Ideally both the generators share the load equally provided both generator ratings and other parameters are same.
there will be circulating current between the generators sufficiently large that might damage the machines.
To avoid possible current loop through multiple neutral points
CONDITIONS REQUIRED FOR TRANSFORMER PARALLEL operation are.. 1. Voltage turn ratio. 2. phase sequence 3. Impedence should be same 4. polarity Hitesh
The generators also have resistance which causes power loss and voltage drops. Connecting them in parallel reduces the total resistance that reducing the voltage drop as well as increasing the total current.
List out the conditions to be satisfied for running two or more DC shunt generators in parallel?
the operation of connecting an alternator with other alternator or with common bus-bars is known as synchronizing.Generally alternator used in power system where they are connected in parallel with other alternator.condition for parallel operation of an alternator1. The terminal voltage of incoming alternator must be same as the bus-bars voltage.2. The speed of incoming alternator must be such that its frequency f =pn/120 as the same frequency of the bus-bars.3. The voltage of incoming alternator and the bus-bars voltage are in same phage.
Restate your question. Generators don't run in series. Generators always run on parallel synchronizing so your question is wrong.
load demand
Transformers can be connected in parallel, if they have the same primary and secondary voltages and equal operating frequency.
They were able to operate the entire day (24 hours).
yes,but the two dismaler generators should have the following similarity; 1]same voltage rating 2]same frequency 3]same rpm.
Normal shaft generators do not have droop-control for frequency and voltage. Semiconductor (drive) based variable speed shaft generators may have those today.