Lesson 2
DC GENERATORS
Module 2J
Parallel Operation
of DC Generators
Engr. GABRIEL M. SALISTRE, JR.
Preliminaries
 In generating station or power
plants, generators generally are
connected and running in parallel.
 Isolated one large generator is
outdated
 Several smaller generators are
running in parallel rather than
large single unit’s capable of
supplying the maximum peak load.
 Smaller generators can be run
single or in various combination to
suit the actual load demand.
Advantages of Parallel Operations
 Continuity of Service (Smooth Supply of Power)
 In case of breakdown of generator, the supply of power will not get
disrupted.
 If something goes wrong in one generator, the continuity of power
can be continued by other healthy units
 Uninterrupted power supply is the main criteria of the electricity
generating station
Advantages of Parallel Operations
 Efficiency (Cost Effectiveness)
 Generator is maximum when it runs with its full load capacity
 Electrical demand fluctuates during the entire 24 hours and also during the
various seasons of the year.
 Each generator connected in parallel can be operated with its full load
capacity by connecting and disconnecting a required number of generators
in the system as per the load demand.
 Economical to use single small unit when load is light. When load
increases, larger unit can substitute a smaller one or another smaller unit
can be connected run in parallel with those in in operation.
Advantages of Parallel Operations
 Repair and Maintenance (Easy to maintain)
 Routine or periodic maintenance of generator is required in
generating station
 One unit can be shutdown for a routine check without interrupting
the service
 Other generators in the system take care of the load demand during
the routine check up or breakdown maintenance of any generator
Advantages of Parallel Operations
 Easy to Increase Plant Load Capacity
 The demand of electricity is increasing day by day.
 Instead of running a single large generator, a
moderated size generator can be connected in parallel
to the system according to the increased load demand.
Ideal Conditions for Parallel Operation of DC Generator
 The incoming generators voltage should be same as bus-
bar voltage.
 The polarity of all generators running in parallel must be
same.
 The + and – voltage terminals of generators must be connected to
+ and – voltage terminal of bus-bars (otherwise a serious shortcircuit will occur).
 Equalizer bar should be used for
compound and series generators.
 Induced emfs of generators should
be preferably same (otherwise
circulating currents result ).
Shunt DC Generators in Parallel
 Shunt generators are most suited for stable parallel
operation due to their slightly drooping characteristics.
 If there is any tendency for a generator to supply more
or less than its proper share of load, it changes system
voltage which certainly opposes this tendency. This
restores the original division of load. Thus the shunt
generators automatically remains in parallel, once they
are paralleled.
Load Sharing Characteristics of Shunt Generators in
Parallel
With External Characteristics Given (Power Ratings, Voltage
Ratings, Voltage Regulations, etc)
Load Sharing Characteristics of Shunt Generators in
Parallel
Load power and currents before
changes on bus load
where:
Load Sharing Characteristics of Shunt Generators in
Parallel
Load currents after changes on
bus load
where:
Load Sharing Characteristics of Shunt Generators in
Parallel
Terminal voltage after changes on
bus load
where:
Load Sharing Characteristics of Shunt Generators in
Parallel
Load power after changes on
bus load
where:
Load Sharing Characteristics of Shunt Generators in
Parallel
With Internal Characteristics Given (Generated Voltage
Resistances of Windings, etc)
Characteristics of Shunt Generators in Parallel
 Two parallel shunt generators having equal no – load
voltages share the load in such a ratio the load current of
each machine produces the same drop in each generator.
 In the case of two parallel generators having an unequal no
– load voltages, the load currents produces sufficient
voltage drops in each so as to keep their terminal voltage
the same.
 The generator with the least drop assumes greater share on
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.
Compound DC Generator in Parallel
Series DC Generator in Parallel
The Equalizer Bus Bar
 In order to make the parallel operation
of compound and series DC generator
stable and smooth, a conductor of low
resistance known as " Equalizer Bar " is
connected between the
two armature ends of two generators.
 The operation of the equalizing bar is to
make an equal flow of field currents in
two generators series field coils.
 When the current through the winding of
one generator increases due to the
sharing of load, the current passes
through the equalizing bar.
Voltage Regulation
 The regulation of a generator refers to the voltage
change that takes place when the load changes.
 It is usually expressed as the change in voltage from a
no-load condition to a full-load condition, and is
expressed as a percentage of full-load.
Voltage Regulation
where:
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Plotting the External
Characteristics
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Plotting the External
Characteristics
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