Page 213 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

• Under these conditions, the generator 2 will not take any load as its induced e.m.f. is same as bus bar voltage
and there will not be any flow of current between two points at same potential.
• This generator is thus said to be floating on bus bars.
• In order that the generator 2 should supply current, it is necessary that its induced e.m.f. should be more than
the bus bar voltage.
• field of gen2 is strengthened till it takes proper share of load.
• field of generator 1 is weakened to maintain the bus bar voltage constant.
Load sharing of dc generators

• To take a generator out of service, its field is weakened until the ammeter of the generator to be cleared reads
zero.
• This method helps in avoiding any shock or sudden disruption to the prime-mover or to the system itself.

• Shunt Generator has slightly drooping voltage characteristics,
• most suited for stable parallel operation.
• Therefore, once paralleled, they’re automatically command in parallel.
• Two parallel shunt generators having equal no-load voltages share the load in such a ratio that the load current
of each machine produces the same drop in each generator.
• In the case of two parallel generators having unequal no-load voltages, the load currents produce sufficient
voltage drops in each so as to keep their terminal voltage the same.

• The generator with the least drop assumes greater share of the change in bus load.
• Paralleled generators with different power ratings but the same voltage regulation will divide any oncoming bus
load in direct proportion to their respective power ratings.

Parallel operation of dc compound generators
• Rising characteristics of the usual compounded generators
• the parallel operation of such generators is unstable

• Let each generator is taking its proper share of load.
• Let us now assume that for some reason, generator No.1 takes a slightly increased load.
• In that case, the current passing through its series winding increases;
• further strengthens its field and so raises its generated e.m.f. thus causing it to take still more load.

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CITS : Power - Electrician & Wireman - Lesson 30-37