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Effect of closing the generator breaker when not yet in phase
Can anyone explain to me what is the effect of closing the generator circuit breaker of which genset is not yet in phase with the synchronizing bus?

Can anyone explain to me what is the effect of closing the generator circuit breaker of which genset is not yet in phase with the synchronizing bus? Can it create any harmful effect?

abella_rocks at yahoo. com. ph

It certainly can - effects will range from a loud bang to physical damage to the driver and/or alternator. It will depend on details of the installation, the strength of the connection to the grid, and the amount of out of synch when the breaker closes. Even if in synch and rotating a bit fast or slow there will be a very rapid acceleration/deceleration of the rotating parts which could bend shear bolts or overstress the shaft. This will be accompanied by a high transient power flow as the external supply acts to pull the incoming machine into line.

It is not a good idea.


By Rajesh Kushwaha on 14 January, 2009 - 6:59 am

I thought it will only affect the circuit breaker because of voltage difference across the contact faces of the CB.

If small amount of asynchronism is going to affect generator and drive. What will happen in case of load throw off from full load?

Can any one throw some more light on this?

In the event of a trip from load (sometimes called "load throw-off"), there is some adverse affect on breaker contacts because of the potential (voltage) difference and the amount of current flowing at the time of breaker opening. But, there are no adverse effect on rotating equipment (load couplings, bolts, shafts, etc.) as there is when synchronzing out of phase.

When an attempt is made to try to synchronize a generator out of phase with a grid or other generators, the magnetic fields of the generator are in opposition (depending on the amount of "asynchronism") to those induced by the grid. So, the generator rotor (and the shaft of the prime mover to which the generator rotor is connected via some type of load coupling) have to speed up or slow down, sometimes by as much as 180 degrees, in a very short period of time. That's where the damage comes into the picture; as fast as it has to speed up or slow down to get into synchronism with the grid field it has to suddenly stop when synchronism is achieved. And, the damage can be caused by either or both of the sudden speed change and/or the sudden speed stop.

The aftermath of such an event is not a pretty sight. And besides being very costly to remedy (if possible), it can be harmful or fatal to people in the vicinity of the damage.

That's why there are almost always "synch check" relays in place to prevent inadvertent out-of-synch attempts. I've never seen a manager move so quickly as when one of his reports grabbed a breaker close switch and flipped it when the synch scope was at six o'clock (180 degrees out of phase)! Fortunately, the synch check relay prevented the breaker closure.

Think of what happens when you try to force the south pole of one magnet to touch the south pole of another magnet. They try to repel each other. If the magnets are strong, it will take some force to make them touch, and if released quickly they will "fly" apart.

Well, that's what's happening when a generator is being synchronized out of phase, or out of synchronism. Only, the magnetic forces are much stronger (in relation to the amount of asynchronism)!

Is it the same as having a line to line fault?? Just like having a three phase system and each line is phase shifted 120 deg with others.

Someone correct my idea if I'm wrong.

If we are to synch manually, of course we have to do adjustments on the speed (considering the frequency only) to correct the phase angle. Using the the synchroscope as reference, does it have to be stable on the 12 o'clock position before closing? Also, can anyone explain to me how does the synchroscope work?

Thank you so much to anyone who will help me.

1 out of 1 members thought this post was helpful...

Blackstone, the answer is 180 deg. For Control List Technocrats, following is the proof I provided to another Forum about 6 years ago (almost to the day!):

Neglecting resistance components, then:

Ic = (Eg1*Eg2)/(Xg1+Xgt+Xg2), where, in per unit,

Ic = current flow at closure.
Eg1 = gen 1 generated (emf) voltage.
Eg2 = gen 2 generated (emf) voltage.
Xg1 = gen 1 reactance.
Xg2 = gen 2 reactance.
Xgt = system reactance of grid transformer and
OVH transmission line, if they exist.)
Xt = sum of above reactances.

Now, let theta equal the phase-angle between the two generators, so that,

Delta(E) = the vector sum of Eg1 and Eg2. Holding Eg1 at 0deg, and varying Eg2 from theta equals 0 to theta equals 180deg, yields the following relationship:

Delta(E) = Sqrt[(Eg1^2)+(Eg2^2) - 2*(Eg1)*(Eg2)*Cos(theta)]

Simplifying for the case where Eg1 and Eg2 magnitudes are equal, and using the trigonometric identity, the equation for Ic simplifies to:

Imax = 2*[|Eg1|*|Eg2|*Sin(theta/2)]/Xt, occurring when theta = 180deq.

Regards, Phil (

Synchroscope consists of stator and rotor with single winding wounded on each. the two ends of rotor winding is connected with any two phases of incoming alternator through PT as reference. also two ends of stator winding is connected with bus voltage through PT as reference.

When u use Synchroscope method, it acts as an induction motor which means rotor will rotate by induction principle. If the incoming alternator's frequency is higher than the bus frequency, rotor will rotate in clockwise. meanwhile if the incoming alternator's frequency is lower than the bus, the rotor will rotate in anticlockwise. When the roto's needle will come in 12'O clock position (neither clockwise nor anticlockwise), u need to close the breaker (switch) of incoming alternators immediately. before closing the breaker, pls ensure that u would have matched the voltages.

By Peter Nachtwey on 11 August, 2014 - 6:54 pm

I have been around a generator that was synched about 30 degrees out of phase. There was a very loud bang. The deck shook. The operators in the engine room were scared. Not good.

Fortunately the system was robust and no damage was done. There is no doubt that it is possible to really break things if the generator is synched even further out of phase.