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Manual Synchronization
Manual synchronization of generator onto the grid.

How is generator manually synchronized onto a live bus.

edmund.adu@vra.com

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

yawadu,

>How is generator manually synchronized onto a live bus.

Nothing like a simple question.

Prior to synchronization, the generator needs to be at or very near rated speed, because speed and frequency of the generator output are directly related.

Prior to synchronization, the generator terminal voltage needs to be at or very near rated voltage (which should be approximately equal to nominal grid voltage).

The generator breaker needs to be in the racked-in position with the primary and secondary disconnects made up (connected) and ready to synchronize.

All lock-out relays must be in the reset position.

There should be a switch or handle on the generator control board/panel to select MANUAL or AUTOMATIC synchronization, and MANUAL synchronization needs to be selected.

The operator then needs to manually adjust the prime mover speed (which adjusts the generator speed and frequency) to make the generator frequency just slightly greater than the frequency of the grid the generator is being synchronized to. Usually, there is a a scope with a moving needle called a synchroscope which is used during synchronization. When the needle is rotating in the clockwise direction, the frequency of the generator is faster than the frequency of the grid the generator is being synchronized to; the faster the needle is rotating the higher the generator frequency is with respect to grid frequency. When the needle is rotating in the anti-clockwise direction, the frequency of the generator is slower than the frequency of the grid the generator is being synchronized to; the faster the needle is rotating in the anti-clockwise direction the slower the generator frequency is with respect to the grid frequency. The general rule is that immediately prior to synchronization the synchroscope needle should be rotating in the clockwise direction at the rate of one rotation every five or six seconds. It could be a little slower, or it could be a little faster, but it should be rotating "slowly" in the clockwise direction during manual (or even automatic synchronization).

Then the operator uses the AVR (Automatic Voltage Regulator) adjust switch to make the generator terminal voltage equal to or just slightly higher than the voltage of the grid the generator is being synchronized to. It could actually be a little lower than grid voltage, but that's not the general rule for synchronizing--it should be equal to or slightly greater than grid voltage.

Then, the operator puts his (or her) hand on the generator breaker close switch handle. Watching the synchroscope needle rotate (slowly in the clockwise direction) the operator waits for the needle to approach the 11 o'clock position (just slightly before 12 o'clock/straight up) and then rotates the generator breaker close switch handle in the CLOSE direction and holds it in the CLOSE position for a couple of seconds and releases the handle (it should return to mid-position by spring action). Once the generator breaker is successfully closed, meaning the generator is synchronized to the grid, the synchronization selector switch should be placed back in the OFF position.

In the "background" there should be a synchronizing check relay which is monitoring the frequency of the generator and the frequency of the grid the generator is being synchronized to--and newer, more modern synch check relays monitor BOTH frequency and voltage of both the generator and the grid. The synchronizing check relay WILL NOT allow the operator to close the generator breaker with the generator breaker close switch handle if the frequency and phase angle of the generator and grid are not within allowable limits. This is to prevent damage to the generator breaker, the generator, the coupling between the generator and the prime mover, and the grid by closing the generator breaker out of phase with the grid voltage sine waves.

Now, why do we do all these things? First, it is CRITICAL that the generator voltage sine waves be properly matched (in phase) with the grid frequency voltage sine waves (most synchronous generators are three-phase generators). If the sine waves of the phases of the generator are NOT "aligned" or very nearly aligned with the sine waves of the phases of the grid, then VERY GREAT mechanical forces--and electrical forces, too--can result which can cause mechanical and electrical damage to BOTH the generator and it's prime mover, and the grid components also. When the synchroscope needle is pointing at 12 o-clock (straight up--vertical) this means the phases are perfectly in alignment. "So," you ask, "you said to close the breaker BEFORE the synch scope needle reached 12 o'clock/straight up--which means the sine waves are NOT exactly in phase (aligned)?!?! WHY??!!?!" Because, the breaker has some mechanical linkages in the operating mechanism which take some time to actually operate and close the generator breaker contacts. So, we send the command to close the breaker slightly ahead of the 12 o'clock/straight up needle position so that when the breaker's main contact do close they will be as close as possible to in phase and aligned.

Why does the operator adjust the generator terminal voltage to be equal to or slightly greater than the grid voltage? Because, if the generator terminal voltage is equal to the grid voltage when the generator breaker closes there will be zero VAr (Volt-Ampere Reactive) current flow across the generator breaker between the generator and the grid. When the generator terminal voltage is slightly greater than the grid voltage when the generator breaker closes there will be a slight amount of VAr current flowing out of the generator breaker and to the grid--which is okay. Most generators are designed to produce reactive current and supply reactive current to the grid. And, a small amount of "positive" reactive current during synchronization and initial loading is desirable. As opposed to "negative" reactive current (VArs) flowing into the generator from the grid if the generator terminal voltage is less than the grid voltage when the generator breaker is closed. There are protective relays monitoring the generator which may open the generator breaker if the magnitude of VAr current flowing into the generator is "excessive" at the time the generator breaker is closed during synchronization. So, it's desirable to have a small amount of VAr (reactive) current flowing out of the generator at the time of synchronization, to prevent a possible breaker trip.

Finally, by having the synchroscope rotate in the clockwise direction, the operator is making the generator frequency be just slightly higher than the grid frequency. This means that the prime mover is producing slightly more torque than is required to keep the generator spinning at the exact speed (frequency) of the grid. When the generator breaker closes during synchronization the generator rotor is "captured" by the grid and slows down immediately to match grid frequency. BUT, the prime mover governor still maintains that slight extra energy flow-rate into the prime mover. This cause the Watts (or kW or MW) being produced by the generator to increase to some positive level flowing out of the generator at the time of synchronization.

If the synchroscope was rotating slowly in the anti-clockwise direction when the generator breaker was closed, that means the generator frequency is less than grid frequency and the prime mover is not producing enough torque to keep the generator spinning at the same frequency as the grid. When the generator breaker is closed with the prime mover not producing the required power to keep the generator spinning at grid frequency the grid will still "capture" the generator rotor and will speed up the generator rotor to make it match grid frequency--and to do that will require that power from the grid (Watts, kW, MW) will be flowing from the grid into the generator to keep the generator rotor (and the prime mover!) spinning at grid frequency/speed. This is called "reverse power", and most generators can tolerate reverse power--but some prime movers (like steam turbines and reciprocating engines) can NOT tolerate reverse power. So, there is at least one reverse power protective relay monitoring the direction of real power (Watts, kW, MW) into and out of the generator to protect the prime mover and generator against the effects of reverse power. By having the generator rotor spinning a little faster than grid frequency when the generator breaker is close during synchronization that ensures that no reverse power will flow into the generator from the grid, possibly opening the generator breaker.

Most, but not all, automatic synchronization relays/schemes do exactly the same as above--adjust generator frequency and phase angle and voltage (sometimes called "speed matching" and "voltage matching") when synchronizing; some only match frequency and phase angle. Prior to the first synchronization of any generator, it is critical to verify the three phases of the generator output are matched to the grid phases, and that all of the metering signals (from PTs (Potential Transformers) and CTs (Current Transformers)) are all providing the proper signals in the proper directions. And, any time those metering circuits are disturbed during any kind of maintenance or upgrade they should be re-checked as they are critical to proper synchronization and unit (and personnel!) protection.

Hope this helps! Synchronization is VERY CRITICAL to the proper operation of a power plant. Done improperly, it can have disastrous and deadly effects. That's why, as was noted above, there is almost always a synchronizing check relay in the manual synchronization generator breaker close circuit to prevent an operator from unintentionally closing the generator breaker when the phases are not aligned--which can cause the most damage to equipment (and personnel if they are too close!).