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Home / Technical Articles / Preparing to synchronize a generator to the grid
Preparing to synchronize a generator to the grid
Preparing to synchronize a generator to the grid (on photo: General Electric steam turbine generator)

Conditions

In order to synchronize a generator to the grid, four conditions must be met:

  1. Phase Sequence
  2. Voltage Magnitude
  3. Frequency
  4. Phase Angle
Synchronizing a Generator to the Grid
Figure 1 – Synchronizing a Generator to the Grid

1. Phase Sequence

The phase sequence (or phase rotation) of the three phases of the generator must be the same as the phase sequence of the three phases of the electrical system (Grid).

The only time that the phase sequence could be wrong is at initial installation or after maintenance. There are two possible problem sources.

The generator or transformer power leads could actually be interchanged during maintenance orthe potential transformer leads could be interchanged during maintenance.


2. Voltage Magnitude

The magnitude of the sinusoidal voltage produced by the generator must be equal to the magnitude of the sinusoidal voltage of the grid.

If all other conditions are met but the two voltages are not the same, that is there is a voltage differential, closing of the AC generator output breaker will cause a potentially large MVAR flow.

Recall that before a generator is synchronized to the grid, there is no current flow, no armature reaction and therefore the internal voltage of the generator is the same as the terminal voltage of the generator.

If the generator voltage is higher than the grid voltage, this means that the internal voltage of the generator is higher than the grid voltage. When it is connected to the grid the generator will be overexcited and it will put out MVAR.

If the generator voltage is less than the grid voltage, this means that the internal voltage of the generator is lower than the grid voltage. When it is connected to the grid the generator will be under-excited and it will absorb MVAR.


3. Frequency

The frequency of the sinusoidal voltage produced by the generator must be equal to the frequency of the sinusoidal voltage produced by the grid.

Generator Slower than Grid
Figure 2 – Generator Slower than Grid

In Figure 2 above the generator is slower than the grid.

The synchroscope would be rotating rapidly counter clockwise. If the generator breaker were to be accidentally closed, the generator would be out of step with the external electrical system. It would behave like motor and the grid would try to bring it up to speed.

In doing so, the rotor and stator would be slipping poles and damage (possibly destroy) the generator as described previously. The same problem would occur if the generator were faster than the grid.

The grid would try to slow it down, again resulting in slipping of poles.

Generator at Same Speed asGrid but not in Phase
Figure 3 – Generator at Same Speed asGrid but not in Phase

Figure 3 shows the condition where the generator and grid have matching speed. The high points and zero crossings of the sinusoidal voltages occur at the same rate of speed.

However, if you notice in 2 with the grid and a phase angle exists between them. This would appear as a non-rotating synchroscope (both generator and grid at same frequency), where the pointer would appear stuck at about 9:00 o’clock (generator lagging grid).

If the generator breaker were to be closed at this time, the grid would pull the generator into step.

However, this again would cause a large current in-rush to the generator and high stresses on the rotor/stator with subsequent damage to the generator. If the generator were leading the grid, it would try to immediately push power into the grid with the same destructive forces as mentioned.

Hence the generator must be brought to a point where the grid voltage waveform exactly matches what it is producing.

4. Phase Angle

As previously mentioned, the phase angle between the voltage produced by the generator and the voltage produced by the grid must be zero.

The phase angle (0 to 360°) can be readily observed by comparing the simultaneous occurrence of the peaks or zero crossings of the sinusoidal waveforms.

If the generator breaker is closed when they match exactly, the connection will appear smooth and seamless.

At that instance (Figure 4 below), the pointer on the synchroscope would indicate 12:00 oíclock.

The worst case occurs if the generator is exactly out-of phase, with a phase angle of 180° and the synchroscope pointing at 6:00 o’clock.

Generator in Phase with Grid
Figure 4 – Generator in Phase with Grid

Synchronisation of Generators to a Busbar (VIDEO)

Cant see this video? Click here to watch it on Youtube.

Resource: Science and Reactor Fundamentals – Electrical CNSC Technical Training Group

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

50 Comments


  1. James Senior
    Oct 26, 2020

    hello i am currently doing a university course in electrical engineering i was wondering if you anyone could give me information on why the frequency needs to be synchronised first and then followed by the voltage probably very basic answer just cant find it. thanks for any help


    • Thomas
      Nov 06, 2020

      The angular velocity will accelerate/decelerate if frequency is not synchronised first, thus preventing, voltage synchronism.


    • Abdul Salam Kousha
      Nov 10, 2020

      first we adjust the frequency by increasing/decreasing engine speed.
      . by adjusting the frequency also the voltage will be affected
      E or V (generator)=N(stator)* Magnatic Field of the Rotot * w
      as w=2*pi * f
      so now f is adjusted already and it is fixed after that we will adjust V(generator)
      and that will be done by AVR or manually by VR
      adjusting V(generator) is actually done by adjusting the current of the (stator excitor) which would change the magnatic field of the main rotor and eventually the V(generator) would be changed(adjusted).
      if you adjust the voltage first then adjusting the frequency in this case will lead the V(generator) to be changed as per the equation mentioned before


  2. James Senior
    Oct 26, 2020

    hi there i am currently doing a university course in electrical engineering i was wondering if you anyone could give me information on why the frequency needs to be synchronised first and then followed by the voltage probably very basic answer just cant find it. thanks for any help


  3. vafo
    Sep 19, 2020

    How to synchronize 33KV star with 33kv delta


  4. Ahmed
    Jun 22, 2020

    When I question the synchronization of generator 1 with generator 2, generator 2 is disconnected at 500 kW load When I question the synchronization of generator 1 with generator 2, generator 2 is disconnected at 500 kW load with trip cutter separated


  5. Bhumika Sarode
    Sep 23, 2019

    very helpful


  6. Daramola Adebola
    Sep 06, 2019

    Pls. how do i solve this problem. i run a drilling rig with 3 Generators, whenever i tried to synchronized 2 gen the power percentage increased to 65% without load, please what would do resolve this problem.

    thanks


  7. Milan darji
    Mar 19, 2019

    Respected sir
    My question is related to inter connected power system please tell me how to decide after synchronizing of generator with grid how much power will flow from generator.

    Assume that gride voltage is 1P.U at ange 0 degree.

    And my second question is how to flow power from generator as our requirement means if i want to flow power from my synchronized generator only 5MW so which setting is to be changed

    Assume that gride voltage is 1P.U at ange 0 degree also i have one master generator unit.


  8. RITESH KAKKAR
    Aug 13, 2018

    Hello
    How this led meter is working with this generator synchronization please tell


  9. RITESH KAKKAR
    Aug 13, 2018

    HELLO
    what is use of this LED rotating in this panel


    • Saurabh Sinkar
      Dec 02, 2018

      That LED will indicate the phase angle difference between the voltage generated by DG and grid voltage. When speed of governor is increase the frequency of generated voltage also increases in same proportion. Synchroscope is LED is showing how much difference is their in the frequency of dg and grid voltage. If LED is in too slow region then we need to raise governor speed and vice versa.
      The guy first raises speed manually and then put in auto mode which makes DG frequency same as grid frequency by slowing down governor.
      I hope you are cleared.


  10. Jesus
    Aug 11, 2018

    Please the generator voltage,the frequency at the speed of 100 percent are okey before synchronization. the problem is upon turning on the synchro switch it will grab the load from the grid


  11. Papoo Sankaran
    Aug 08, 2018

    Synchronization with Grid explained in most simple manner with clarity. Thanks for sharing

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