Synchronizing a Breaker

Usually you want the incoming generator to be a bit faster, because it will then slow down some as it picks up its share of the load. However, I have experienced systems in which some adjustment or another was a bit off, but which would work by bringing in the incoming generator slow. This may be a bit unusual, but yes, it is possible for the synchroscope to rotate in either direction. In either case, you would monitor the load sharing and perhaps follow up with some further slight adjustments.

There is no need to have the incoming genset going at a faster speed to compensate for the loading phenomenon. The existing grid will pull the genset into speed if the synchronisation is correctly made to within a reasonable speed gap (less than 3%).

The important issue is that you must have a good load sharing and governor system for parallelling. Once the gear is correct, Synchronise with in the nearest speed (Voltage, and Sequence being OK).

The gen.set revs will drop slightly as it picks up the load, although this will be quickly compensated for by the (electronic) governor, so the revs are often set a couple of hundred rpm higher, especially when we used to do it with mechanical governed engined gen.sets (& a light bulb across the red phases as indication, it would go out when there was no voltage differential! The knife switch would then be swiftly closed) The gen.set will have to alter its speed slightly to lock into sync, an engine will always slow down / reduce revs more quickly than it can accelerate / increase revs: therefore bringing the system into synchronisation quicker. So the point of setting the speed slightly faster is it is quicker than making the engine accelerate. The rpm is unlikely to be exactly spot on, although I concede electronic governors / controllers have made this pretty accurate. As previously stated the grid / master gen.set will pull the slave unit into sync (as long as it is not too far out <3%) once electrically connected it will be held in sync but if the voltages are badly adjusted circulating current will result, this must be trimmed out by slave alternator voltage adjustment (again usually set 2 or 3 volts above bus bar / grid voltage pre-sync) to ensure slave unit is exporting power and not being 'driven ' (like an electric motor) by the grid voltage. Out of interest we used to run two alternators from the same engine via a tooth belt, we experimented one day by slipping the belt one tooth on one of the alternators (so it was 'one tooth' out of sync.) we ran the unit and on stopping found the belt had been forced back round one tooth to bring the alternator back into sync! We did this experiment several times to convince ourselves what was happening! p.s. For some reason this will not break up into paragraphs, however many times I edit it & looks ok in the box; so my apologies for 'a block of text'.

Can - yes. Should - no. Clockwise slowly - breaker in at at 5 to 12 - safest way.

If the oncoming generator does not take load (that is it must be running a tiny amount faster than the online generator), it will probably get "motored" (unless it has EXACTLY the same speed, which would mean that the lights would NOT be moving on an AC 3 Phase system!), which means that it is being "driven" by the online generator.

All systems I eve worked on had a reverse power sensing relay/circuitry that would immediately drop off the oncoming generator.......

This was true on both DC & AC systems......as you don't want a generator working as a giant motor and driving the prime mover do you?

Its a safety feature.....

Never underestimate the size of the magnetic forces when connecting out of sync, the results can put a genny and prime mover through a wall.....

Quite right, I've seen a Cummins 6ct [150kVA] rip its engine mounts when the gen.set was closed out of sync; not me!! :-) - it welded the contactor shut as well. 'the result nearly put the genny and prime mover through a wall..... '

For example.......!!!! Many thanks.

I looked for some pictures on the web, could not find any.....