What are the condition for parallel operation of the generators?

Answer 1

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.

Answer 2

parallel operation of generators is nothing but giving the same source of energy to the two or more units.

Answer 3

Voltage , phase sequence and frequency should be same.

Answer 4

voltage of both generators must be equal.