Preparing to Synchronize a Generator to the Grid


Thread Starter


I don't understand why if the voltage produced by the generator is not equal to the grid's voltage, synchronization won't occur although generator and grid have equal phase sequence, frequency and phase angle.


We presume you are referring to automatic synchronization.?.?.?

Does the automatic synchronization feature have a voltage matching function--that automatically makes generator voltage equal to or slightly greater than grid voltage?

When synchronizing it's customary to make the incoming generator voltage at least equal to or greater than grid voltage. This is done so that when the generator breaker closes the VAr flow will be zero (if the generator voltage is equal to the grid voltage) or slightly lagging--considered to be positive VArs in most generator applications.

If the generator voltage is less than grid voltage at the time of generator breaker closure the VAr flow will be leading, which is usually considered to be negative VArs.

Most generators are not capable of high leading VArs, and one of the consequences of high leading VArs can be pole slipping--which is extremely damaging to generators, load couplings and prime movers. There are usually generator protective relays to open the generator breaker before pole slippage occurs and to prevent unwanted heating of generator components caused by prolonged operation with high leading reactive current (leading VArs).

In the same way, when synchronizing it's customary to make the incoming generator frequency just a little bit higher than grid frequency (sometimes called speed- or frequency matching). This is done so that when the generator breaker closes and the generator and prime slow down to grid frequency the extra energy flowing into the prime mover ensures there is positive power flowing out of the generator (kW, MW).

If the incoming generator frequency was lower than grid frequency at the time of generator breaker closure then reverse power would be flowing from the grid into the generator, causing the prime mover and generator to speed up to match grid frequency. Some prime movers DO NOT like reverse power at all. Some can tolerate a little, some can tolerate much more--but it's still a load on the other generators on the grid and as such is undesirable.

So, when synchronizing (manually or automatically) it's customary (for most generator sets) to make the incoming generator frequency and voltage at least equal to or slightly greater than grid frequency and voltage to ensure positive real power flow and positive reactive current flow at the time of generator breaker closure.

Some synchronizing schemes require positive slip (generator frequency to be higher than grid frequency) and at least matched (equal) voltage or voltage slightly higher than grid voltage in order to be able to synch. Usually, this is done in the automatic synchronizer, and sometimes in the manual synch circuit as well (though not commonly, because most philosophies say that manual synchronization should rely on trained operators to understand what they're doing and the results of closing at less than optimal conditions should an emergence require doing so--so there's not usually much in the way of frequency (speed) or voltage matching permissives for manual synchronization. But, that varies on equipment and operating philosophy and site management/operational philosophy.

Phase angle is extremely important (implying frequency is within limits) during synchronization, and phase sequence is done during initial synchronization checks and doesn't usually change for most sites (unless we're talking about portable power generation connecting to a grid--and still it's a first-time verification/test; most synchronization and/or synch-check circuits don't check phase sequence because it's assumed to have been verified before initial synchronization and because those connections are usually fixed they don't ever change (but should be re-verified if high-voltage connections are every disturbed--and/or PT sensing circuits are disturbed).

So, it sounds as if there's some "permissive" based on voltage matching that is preventing synchronization, and the above should help understand why it's customary and recommended to have the generator terminal voltage at least equal to if not slightly greater than grid voltage. Who set that "permissive" and why and what it's set at--only the person(s) responsible for the operating philosophy of the generator-set you're working with can say for certain. If it's a rental generator, I would say they're trying to protect their equipment to the extent possible by ensuring that VAr flow is at least zero or slightly positive when the generator breaker closes.

If the automatic synchronization function isn't automatically matching voltages and should be--well, that's another story. Some automatic synchronization schemes (but VERY FEW) don't automatically match voltage; they only automatically match frequency (usually by making incoming generator frequency a little higher than grid frequency). Most synchronizing and synch-check relays want to see some slip (difference between incoming and running frequency) to work properly--most new-fangled digital relays actually require some slip (positive or negative) and some won't allow generator breaker closure when the synch scope is sitting stationary at 11:30- or 12:00 o'clock (this was a shock to some of us older engineers).

Hope this helps!

Most exciters (sometimes called AVRs) are set up to produce a particular voltage when the generator-set reaches rated speed just prior to synchronization. This is commonly called the Off-Line Voltage Setpoint. It is typically set to the nominal rated voltage of the system with which the gen-set will be synchronized.

Grid voltages vary throughout the day--even on the best-regulated grids. Especially when loads are very high, and in particular when air conditioning loads are very high (due to the high number of induction motors), the grid voltage can "sag" or be lower than usual. And, quite often, if the grid regulators aren't paying sufficient attention when the air conditioning load drops off in the evening when the ambient starts to cool the grid voltage can increase above nominal.

Also, depending on lots of factors like distance from nearest power generation plants, distance from nearest transmission switchyard, etc., the voltage can also be affected.

As was mentioned earlier, most automatic synchronizers are provided with the ability to "match" voltage as well as to "match" frequency. The amount of "matching" for both functions is usually adjustable in the auto synchronizers, but not always. Frequency (Speed) matching is a must--as that's how phase angle is measured, and also how some small amount of positive power flow (KW, MW) is achieved when the generator breaker is closed. Voltage matching is important, also, for a couple of reasons. First, as was mentioned, VAr flow upon synchronization is affected by the voltage level of the generator versus the grid. And, second, large differences in voltage level increase the voltage differential across the generator breaker contacts which increases the wear of the contacts. No voltage differential means there is no difference in potential between the incoming- and running contacts of the generator breaker, and that makes it easier to close as well as reduces the wear on the contacts (reduced arcing).

So, from your post it seems that for some reason one or more of the rental generators you are working with requires manual adjustment of generator terminal voltage in order to allow synchronization. That could be because the Off-Line Voltage Setpoint is not set correctly <i>for the site where the rental generators are located</i>. If the auto synchronizer module is not very full-featured, it might not be comparing the running (grid) voltage to the incoming (generator) voltage; it may only be checking to see that the generator voltage is at the Off-Line Voltage Setpoint--but something else in the synchronizing circuit may be checking to see that the two voltage are equal.

So, without knowing a lot more about the equipment and the conditions at the site it's really not possible for us to add much more to this discussion. It's certainly not unusual for synchronizing schemes to check to make sure the speed (frequency) AND voltage are both "matched" before allowing synchronization. A lot of damage can occur to the generator, the load coupling between the generator and the prime mover, and the prime mover if synchronizing parameters are not correct. And, this being rental equipment, it may be that the owner is trying to protect the equipment, OR it may be that it just wasn't fully configured for the actual site conditions for this particular usage. It could also be that the auto synchronizer isn't working correctly--IF it's supposed to match voltage prior to synchronization.

But, the thing to know about this is that voltage matching is just as important to protection of the equipment as frequency (speed) and phase angle and phase sequence. Synchronizing is serious business--and rightly so. Improper synchronization can result in damaged equipment, and even black-outs for grids. And, for humans in the wrong place at the wrong time, it can be extremely dangerous, even deadly. There are utilities and grid operating companies around the world that are more and more insisting on automatic synchronization--that is, manual synchronization of generators to their grids is not allowed. They make generating companies prove their automatic synchronization circuits won't allow breaker closure out of phase or with extremely high or low voltage, and they don't want human operators to close the breaker because of the elevated chance of serious problems.

Hope this helps!