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


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

About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on


  1. Chibuike
    Dec 03, 2016

    I have 40kva And 60kva Hw can I sychoronize The both of Generator

  2. Manoj kumar sinha
    Apr 17, 2016

    We have two generator of 15MW having 11KV voltage which runs in parallel.when load requirment exceeds 30MW we have to syncronised generators with grid which is 132/11KV.When grid runs in parallel with generator the average powerfactor of grid is poor due to which we have to pay baby penalty to EB.But when load puts on grid indipentantly running as well as average powerfactor is good.Please guide me why such type of condition occur and also how to avoid poor powerfactor during synchronised condition.

  3. Muhammed
    Mar 05, 2016

    Dear Edvard,

    Please tell the advantanges and disadvantages of having GCB at STG, in order to avoid direct synchronizing with HV breaker.

  4. mohammad
    Jan 14, 2016

    thank you for this page …. but i not understand 4. Phase Angle part

    Dec 23, 2015

    I would like to synchronise 440/220 volts 3 phase 4 wire generator with grid transformer of 11000/433V delta wye. How do I do that?

  6. Capt. Waris Shaheen
    Sep 06, 2015

    thanks for providing a real good and easy concept of synchronization. Well done, Keep it up. A good service to the electrical engineering community.

  7. joseph
    Sep 05, 2015

    My problem is that have one generator run in meanwhile the two are connected to synchronizing panel (both Gen has 1000kw rating each) so when on the second Gen and synchronize the the total load move the new gen just on please what is the Problem

  8. Ben
    Aug 21, 2015

    Wow! this is great!This is good write up,its very helpful.God bless….

  9. Bryan Marome
    Aug 16, 2015

    Thank you for shearing your wonderful knowleadge.

  10. Neil Isaac
    May 15, 2015

    For emergency generation purposes where your site is normally powered by the Grid the benefit of running in this fashion is that you ensure that when running in parallel, if the grid supply is lost then you ensure 100% of your site load can be comfortably transfered to the emergency generator. I have direct experience of manually closing generator circuit breakers to the grid supply, you soon learn to have all the parameters as close to the grid as possible. The main problem is getting the generator speed (frequency) controlled so that the frequency is synchronised as close to the grid as possible.

  11. senthil kumar
    Apr 27, 2015

    i need to synchronize my unit with grid. but last time we done it but it make fluctuation in voltage and some cards are buringi…i dont know why… any one can aswer for my querry…

  12. Sunil Khandelwal
    Mar 31, 2015

    I want to synchronize the Gas Turibine Generator with the Grid at 11kV Level. The Generator is generating Voltage at 11kV with Dyn11, 11/ 11kV Generator Isolation Transformer. We are receiving Grid Power through Yy0, 132/11kV Transformer. Can they be made parallel? Can Voltages be at the same Phase Angle.

  13. victor castillo
    Jan 31, 2015

    I need how to connect and synchronize the mains with a generator
    I need to couple with synchroscope

  14. Max
    Nov 20, 2014

    Sir my question is practically we keep the generator voltage and frequency slightly higher than the grid voltage. Why is this done?

  15. Joe
    Sep 05, 2014

    I work at a powerhouse and we have 6 30MW hydro generators. We have 3 to a bus. last week the CO2 was set off by our 87G Relay on unit 4. Everything tested great. Yesterday the same thing happend to a different unit on the same bus. the operator on duty said as sood as he put the sycronizor in auto it closed the breaker. The sycn. was at 3 o’clock and it tripped the 87G. these 2 units are on the same bus. Since the first trip on unit #4 we have run unit #5 several times. Any thoughts on whats going on here.

    • Satya
      Sep 13, 2014

      Did you check with the synchronizer? is it OK? do check for abnormalities in the bus during switching. you might come to a conclusion.

  16. Awais Khan
    Aug 23, 2014

    How we can connect two generator one is150kva and other 180kva gen .when load increase on 150kva then 180kva will start .180kva is stand by.

  17. Adebayo Johnson
    Aug 21, 2014

    Thanks for this article. However, there is no discussion on how to achieve same frequency, voltage and phase. The discussion on this will be helpful

    • Sean
      Mar 21, 2016

      My experience is limited to marine installations but the operating principles are the same.

      The frequency of the incoming generator is regulated by varying the speed of the prime mover. The governor should handle that.

      For the voltage, that is regulated by the AVR and it does this by varying the rotor field excitation on the generator.

      Just as with the frequencies, for the voltage phase angles, a synchroscope or synchronising lamps will be the indicator as to when the voltages are in phase.

      For the phase sequences, both the generator and grid busbars are checked using the same phase sequence meter. As mentioned by Edvard, the terminals can be interchanged if the phase sequence isn’t the same.

      My explanation isn’t that detailed by I hope it helps.

  18. A. Murshed
    Jul 07, 2014

    Phase angel is the angel difference between the generator voltage and the grid voltage. While phase sequence means that the three phases of the generator must match with the same phases on the grid.
    For example: A, B, and C are the three phases of the generator. Then to have the same phase sequence with the grid connect them to their match on the grid A to A etc…
    Note down that u dont need to confirm this condition everytime u synchronyze the gen. with the grid. Because the gen. Is connected permenantly.

  19. Harshal
    May 12, 2014

    When paralleling the Generator to Grid, What % of variation in both the voltages are allowed & what is the criteria for it?

  20. Bilal Hussain
    Feb 20, 2014

    there is something that’s i want to ask you phase angle and phase sequence is it same function .. thats we done during synchronizing ..

  21. Yasin Kasirga
    Dec 13, 2013

    Very neat, very helpful.
    I have not come across any source e explaining the Synchronization concept this simple and they say “simplicity is the ultimate sophistication”
    Great work!

  22. komichi
    Jul 24, 2013

    many thanks you are a real kind man; you help many many people around world

    many thanks

    Mar 31, 2013

    very helpful, i must say, never seen such a useful and efficient portal

  24. croger2012
    Mar 27, 2013

    Good article. It would be desirable to improve the PDF generator in Spanish to save it and print it, is failing

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