Parallel Operation Procedure for Marine Diesel Generators in Ships

In order to put a generator onto a live bus bar or in other words, for the parallel operation generators in ships, the following conditions must be fulfilled.

  1. The incoming generator voltage is almost equal to the bus bar voltage.
  2. The incoming generator frequency is equal to or slightly higher than the bus bar frequency.
  3. With the conditions 1 and 2 met, the incoming generator circuit breaker must be closed at the exact instant when the generator voltage is in phase in opposition to the bus bar voltage (synchronism)

The following describes, as an example, how to put D/G 2 onto the main switch board (MSB) bus already served by D/G 1. Follow the procedure for proper placing of an incoming generator to the bus bar and to load shift a generator which is already powering up bus bar.

Engine Start and Voltage Establishment

Start D/G 2 engine and let D/G 2 voltage establish to the rated value.

Comparison of Voltage and Frequencies

Compare D/G 2 voltage and frequency with the bus voltage and frequency (D/G 1 voltage and frequency as measured on the bus). Read the bus voltage and frequency on the bus voltmeter and frequency meter, which are assumed to be at the rated values. To read D/G 2 voltage and frequency, set “VOLT & FREQ METER” switch on the synchro panel to D/G 2. Automatic voltage regulator will bring the D/G 2 voltage to its rated value. So no need to adjust voltages. At this step, the generator frequency is acceptable if it is sufficiently close to the bus frequency.


On confirmation that the frequency difference is small (1 Hz or less), position “SYNCHROSCOPE” switch to D/G 2 and observe the movement of the synchroscope pointer as well as the brilliance change of the synchro lamps.

Synchroscope pointer is rotating in “FAST” direction (clockwise)

The generator frequency (D/G 2 ‘s) is higher than the bus frequency (D/G 1 ‘s). The sequence of the synchro lamp brilliance change is also in clockwise direction.

Synchroscope pointer is rotating in “SLOW” direction (anticlockwise)

The incoming generator frequency is lower than the bus frequency. The sequence of the synchro lamp brilliance change is also in anticlockwise direction.

Synchroscope pointer is stopping

The generator and bus frequency are the same. The position of the synchroscope pointer (assumed to be stationary) indicates the relative phase angle. In this case, there is no synchro lamp brilliance change.

Synchroscope pointer is at 12 o’ clock position

Whether the synchroscope pointer is moving slowly or stationary this condition indicates that the generator and bus voltages are in phase (synchronism). At that moment, the two outside synchro lamps will be on at the same brightness while the middle top one will be at completely dark.

Closing the Generator ACB at Synchronism

Observing the synchroscope or synchrolamps, operate “GOVERNOR MOTOR” switch for D/G 2 to RAISE or LOWER and bring D/G 2 frequency about 0.3 Hz higher than the bus frequency (in this case, synchroscope pointer will rotate in “FAST” direction and complete one rotation in 3 or 4 seconds). Operate ACB control switch for D/G 2 to CLOSE a little prior to synchronism, anticipating a timing that D/G 2 ACB be closed at the exact instant of synchronism.

Load Division

On closure of D/G 2 ACB, operate “GOVERNOR MOTOR” switch for D/G 2 to RAISE so that D/G 2 takes on KW load and let each generator share the load in proportion to their capacity ratio. Also operate “GOVERNOR MOTOR” switch for D/G 1 if necessary to maintain the bus frequency at 60 Hz. Then position “SYNCHROSCOPE” switch to OFF.

Load Shift

To remove one of the generators running in parallel, first transfer load on that generator to other generator(s). When the KW output of this generator is reduced to almost zero, remove the generator from the bus by opening its ACB. For example, to remove D/G 1 running in parallel with D/G 2as mentioned above, operate “GOVERNOR MOTOR” switch for D/G 1 to lower; load on D/G 1 transfers to D/G 2. Also operate GOVERNOR MOTOR” switch for D/G 2 if necessary to maintain the bus frequency at 60 Hz. When D/G 1 output is reduced to about 5% of its rated output, operate “ACB CONTROL” switch for D/G 1 to open from the bus.


  1. Pay attention to the variation in bus frequency during the load division or shift control, and be sure to maintain it at 60 Hz.
  2. Pay attention to the loading condition of the generator(s) remaining on the bus during the load shift control, and do not overload it for them.
  3. If the output of a generator is reduced below 0 KW during the load shift control, the induction type reverse power relay (RP setting: 10% of the rated output, time setting: 10 sec) will operate and trip the generator ACB.

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