Operation generators in parallel

In order to parallel AC Generators satisfactorily, certain basic conditions have to be met. These are as follows:

(1) All systems must have the same voltage.

(2) All systems must have the same phase rotation.

(3) All systems must have the same frequency.

(4) All systems must have the same angular phase relationship.

(5) Systems must share the load with respect to their ratings.

Main generator units (gas turbine, steam turbine or diesel drives) have to be run in parallel to share а total load if it exceeds the capacity of а single machine. Changeover of main and standby generator units requires а brief parallel running period to achieve а smooth transition without blackout. For simplicity and security, it is not normally possible to run а main generator in parallel with either the emergency generator or а shore supply (except in cases where the ship’s power plant is constructed to allow this type of operation). Circuit breaker interlocks are used to prevent such an arrangement.

Parallel running is achieved in the two stages of synchronizing and load sharing. Both operations are usually carried out automatically, but manual control is still in common use and is generally provided as а backup to the auto control.

The generator already on the bars is called the running machine and the generator to be brought into service is the incoming machine.

To smoothly parallel the incoming generator, it must be synchronized with the live busbars [3].

Figure 73. Two generators to be synchronized

 

To achieve smooth manual synchronizing, the incomer must be brought up to speed to obtain approximately the same frequency that is shown on the busbar frequency meter, eg 60 Hz.

The incoming generator voltage is set by its AVR to be equal to the busbar voltage. Fine tuning of the speed can now be observed on the synchroscope or synchronizing lamps. The incomer is adjusted so that the synchroscope indicator rotates slowly clockwise (fast direction) at about 4 seconds per indicator revolution.

The circuit breaker should be closed as the indicator approaches the 12 o’clock (in-phase) position. The breaker closing between 5-to and 5-past the 12 o’clock synchroscope position is satisfactory as long as the pointer rotation is fairly slow.

А traditional pointer type synchroscope is usually short time rated (eg up to 20 minutes) to avoid overheating. Do not forget to switch it off after а paralleling procedure. Modern synchroscope indicators use а circular set of LEDs that sequentially light up to show the phase difference between the generator voltages [3].

As а backup, or alternative, to the synchroscope, а set of lamps may be used. The correct synchronized position may be shown by either of the following methods:

- Lamps dark method (2 lamps)

- Lamps bright method (2 lamps)

- Sequence method (3 lamps).

Figure 74. Synchronizing instruments

 

ln each case, the lamps are connected between the incoming generator and the busbars. The sequence method, as shown in Figure “Synchronizing with three lamps”, is preferred as it displays а rotation of lamp brightness that indicates whether the incoming machine is running fast (clockwise) or slow (anticlockwise). As with the synchroscope, the lamp sequence must appear to rotate slowly clockwise. Correct synchronization occurs when the top or key lamp is dark and the two bottom lamps are equally bright [3].

 

 

Figure 75. Synchronizing with three lamps

 

Adjust the generator speed until the voltmeter very slowly fluctuates from zero to maximum. Close the breaker when the voltmeter indication passes through zero.

А check synchronizing unit, as shown in Figure “Synchronizing unit”, has an electronic circuit to monitor the voltage, phase angle and frequency of the incoming generator with respect to the busbars. Circuit breaker operation is initiated by the watchkeeper, but the check synchronizing monitor only allows а permit-to-close signal when all the synchronizing conditions are within acceptable limits. This method provides а useful safeguard against operator error but retains overall watchkeeper control for adjusting the voltage and frequency [3].

 

Figure 76. Synchronization unit

 

Auto synchronizing of an incoming generator does everything an operator would do. It senses and controls the voltage and frequency then initiates а circuit breaker close signal at the correct instant. The auto synchronizing equipment uses electronic circuits to monitor the size of voltage, frequency, and phase angle difference, then acts to regulate them until they are equal to the existing busbar conditions.

Usually, the check or auto synchronizer units are switched between а set of generators as and when required.

When an incoming generator has been successfully synchronized, the synchronizing equipment should be switched off.

The total busbar load can now be shared between generators or totally transferred to the new machine. In parallel operation, а generator governor directly controls power (kW), while its AVR trimmer controls reactive power (kVAr) or “power factor”.

Manual kW load sharing is achieved by raising the governor setting of the incoming machine while lowering the setting on the running machine. The balance of power sharing is dictated by the governor (speed) droop of each generator prime mover. Current (or kVAr) sharing is set by the voltage droop of each generator AVR.

For equal load sharing of kW and kVAr, each machine must have similar droop characteristics, which are typically 2-4% between no-load and full-load values. An overall balance of load sharing for kW and kVAr can be seen by comparing the power factor () meters of each generator [3].

Autoload sharing equipment compares the kW loading of each generator (via CTs and VTs) and any difference is used to provide an error signal to raise/lower the governor setting of each prime mover as necessary. The equipment is usually trouble free, requiring little maintenance other than an occasional visual inspection, cleaning and checking the tightness of the connections [3].

Manual load sharing is the normal fallback if the auto control equipment fails.