Why Does Connecting Unsynchronized Generator and Utility via Transfer Switch Cause Damage?

Hello all,

Suppose we have two power sources like a generator / UPS or any utility power source. Both are not synchronized and are operating through transfer switch. Because of technical problems (i.e. when both power supplies combine), then there will be a blast. I just want to ask why.

Thanks!

phase angle / synchronization (http://wiki.answers.com/Q/Procedures_to_synchronize_generators_in_parallel)

The process of paralleling generators is called synchronization. What this means is that both generators have to be in phase with each other and running at the same speed. The generator that is running is called the lead generator and the one that you are trying to parallel is called the lag generator. There are two types of instrumentation used in synchronising, one is a light bank and the other a bit more sophisticated is a synchroscope meter. The synchroscope indicates whether the lag generator is faster, slower or in phase with the bus. The lag generator is brought up to speed just under the RPM of the lead generator. The synchroscope will be revolving very slowly in a clockwise rotation at this point. When the hand on the synchroscope reaches the 11 o'clock position the switch is thrown to bring the generator on to the bus tying the two generators together.

The light system of paralleling generators has lamps hooked in series and connected between the hot lead of the lead generator and the hot lead of the lag generator. When one generator is running faster than the other one, the lights will blink off and on as the generators come in and out of phase with each other. Adjusting the throttle of the lag generator and bringing it closer to synchronise speed will cause the lights will start to pulsate slower and slower until they go off for a period of seconds and then on for a period of seconds. At this point the two generators are electrically paralleled. Count the time between on off on of the lights. At half the off time (zero voltage) throw the switch and the generators will lock together, the two generators will be running in parallel with each other. Manually to keep them paralleled requires a bit of work, as the faster generator will try and take the load and motor the other one. This is where a governor is a great asset. If the generators are tied together out of phase two things could happen. The least problematic would be the generators breaker will trip. The second and most dangerous is if the lag generator is just a little bit out of phase when the switch is thrown, then the generator will be jerked into phase lock as momentary heavy current flows between the two generators. The sudden loading of the generator has been known to break crankshafts of the engine driving it.

*my dear sir steve my question is still same why there will be an electrical accident when two power supplies which are not synchronized and there is no parallel operation. one we have utility power supply and second one is a back up power supply.... when both combines by mistake then there will be a blast/ accident but what happened actually which cause this accident??? *

What happened is you "paralleled" the two generators (grid and engine gen?), and they were out of phase. The smaller unit tried to sync up, and broke. I don't know how to explain it any simpler.

First question: why did nobody answer ?

Answer: (At a guess) Because they were busy or asleep or doing something else.
Not everybody lives in front of their PC.
Some of us do other things for greater or lesser parts of the day.
You may be in a different time zone in a different part of the world to the rest of us.
__________

Second question:Why do AC supplies blow each other up if they are not synchronised ?

Answer: Think of the two AC supplies as being like two high voltage, for example 250V, batteries, which are having their polarity repeatedly switched over.

If they are in phase, they remain like the two 250V batteries in parallel supplying DC. As long as they have the same polarity, they work together to supply the load. No problem.

If the two 250V batteries both repeatedly have their polarity switched over simultaneously, they remain perfectly in parallel, supplying the load. No problem.

But if one 250V battery is switched out of synchronisation with the other, they will become two 250V batteries circularly connected in series, shorting circuiting each other.
Result: 500 volts driving current through only the power sources' low internal resistance.

That is what is happening in the two outof-sync generators. So the two generated voltages are added together, 500 Volts, driving current through each other's very low internal impedance coils.

Result: MASSIVE OVERLOAD CURRENT through both generators' coils !

ALL OF THE POWER GENERATED immediatly produces MASSIVE HEAT in BOTH of the generator coils' resistance !

The copper coils IMMEDIATELY MELT or become WHITE HOT PLASMA ! (OK, I maybe guessing a bit there.)

All of the electrical cables in the circuit are subjected to the MASSIVE OVERCURRENT, so they BLOW UP like fuses !

NOW CAN YOU IMAGINE why that produces ONE MASSIVE BLAST ! ! ! ! ! !

(Pardon me shouting, but I think it produces an appropriately dramatic effect.)

Your answer is simply superb Frank Bushnell.
Wow! what an example you choose...

Thank you for that compliment !
I like to think I have a talent for simplifying things to make them comprehensible to everyone.

Sorry for late reply i was onboard a vessel with out internet.
You have to see the freq. + voltage + timing at the same time to be good.
If you draw a sinus on paper and then a second with 90 degree delay and then think that that is phase one from gen. 1 and gen.2 set on that way up for a three phase.
even a 10 degree synchronization mistake gives a big short cut.
And it can also great mechanical damage to the engine.