Operation modes of Alternator or generator – Grid mode & Island mode.

An alternator or generator generates power might be used to supply either two purpose. The first is to supply power directly to the load or to consumer. The second one is to supply power to the grid. As example in case of power plant or power station several alternator supplies power to the grid, there is no direct consumer.

Operation modes of Alternator

When the alternator supplies power to the load or consumer directly, the criteria of the load affects the alternator. We can not control the load(KW or MW) and power factor but the terminal voltage and frequency can be controlled. This is called ISLAND mode.

When the alternator is connected to the Grid, in fact synchronized with the grid- alternator swings with the grid. The frequency, voltage we can not control but the load in KW or MW supply we can control.This is called GRID mode.

So The operation mode of alternators primarily divided in to two general group. One is Island mode and the second is grid mode.

Island mode of Alternator:

In Island mode there is no grid connected, only a certain isolated area is covered. This happens in case of marine or ship and factories with own supply & own consumption. The amount of load supplied we can not control as it totally depends on load demand in the covered area, but the voltage and frequency we can control.

In case of Island mode-

  • Active and Reactive power supplied to the bus bar depends on load /
  • Frequency and voltage of the bus bar we are able to control.

Grid mode of power generation:

In grid mode the power generation station or power plant is in parallel with the grid. As the total plant is connected to grid- the frequency & voltage is uncontrollable, depends on the grid voltage & frequency. But the load is controllable, so is power factor, as we can set the load of an unit to 5MW or 8MW and set the power factor to .80 and alternator will generate power following the commands.

In grid mode-

  • Frequency and voltage depends on grid
  • Active and reactive power supplied to the grid we are able to control
alternator in power plant
Turbine Generator deck for the Number One Unit at Diablo Canyon Nuclear Power Plant

One thought on “Operation modes of Alternator or generator – Grid mode & Island mode.”

  1. Assalamu’alaikum,

    May I have know your country? If Indonesian, I would write in Bahasa.
    Your articles are very useful.


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