power station synchronization requirement

[ldcktm]

Hi everybody
very good morning

I have a question regarding the synchronization of two power station to grid
we say voltage should be equal but i would like to know what is the % practical voltage difference (tolerance) that can exist between the two system just before the synchronization .

I know other requirements of phase sequence and frequency should also be met. I just want to share the practical experiences of other engineers they have observed.

 

[waross]

That question is not as simple as it seems.
Why is there a voltage difference?
If is a phase angle difference, the voltage may be off by 30% or more. Not a good idea but it will quickly correct itself. As long as it doesn't correct so fast that it trips the main breaker, blows a coupling or shears a drive key. I have seen small (350 KVA) machines synchronized with a calculated phase angle voltage difference of 50%. Possible but not recomended.
If the voltage difference is a difference in RMS voltages, there will be a reactive current driven by the voltage difference.
Up to a megawatt or so, if your voltmeters appear to show the same voltage you are good to go. Your voltage error will probably be less than 5%. Close the breaker and then trim the power factor. (By adjusting the voltage setting.)
Frequency. You want the synchroscope needle to be moving slower than the second hand on a clock. The second hand moves at 1 RPM. 1/4 to 1/2 RPM is better for synchronizing.
On 60 Hz that's a frequency error of 0.4% to 0.8%.
Phase sequence must be the same.
Phase angle difference should be within plus or minus 12 degrees. That's between 2 minutes before the hour to 2 minutes after the hour on a clock face. If you close when the synchroscope is at 5 minutes to the hour, you have a 30 degree phase angle and a voltage difference of 50%.
We have a regular who has extensive experience with much larger sets. He will recommend different tolerances. The big sets are less tolerant of errors than the small sets.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

Idcktm,
10% voltage difference is not uncommon.
It is the frequency difference / phase angle that you want to closely monitor when it comes to generator synchronising with the grid.
In any case, it is important to see that the generator voltage is more than the grid voltage to ensure there won't be pole slipping protection operation the moment you close the circuit breaker (because the generator field is weak)

 

[ScottyUK]

Below are a couple of comments lifted from a previous discussion on this topic. My settings are from a 15 year old plant; Slava's information is from a new plant. Both are applied to big machines.

Most machines are braced for a three phase fault across the terminals. An out of sync closure under worst case conditions is likely to be about double that in terms of the forces on the machine. Something will likely break in the engine.

For ballpark figures for a synchroniser I'd look at something like 10% voltage, 20° phase, 0.1Hz ? freq as the settings for the backup check sync relay and expect the synchroniser to be achieving more like 2%, 5°, 0.05 Hz. Those figures are from a much bigger machine, and Bill is going to tell me I am far too conservative. 'Conservative' was one of those things which prevented bits of turbogenerator being scattered across Teesside.

Scotty, first of all, I wish to you a good luck in your new job. Im very happy for you. ( btw, thanks to RMW for this information)
Second, xmmm, Im more coservative:
1. for autosynchronazing 1-2deg, 2% delta voltage and 0.02Hz delta.
2. For back up synchrocheck relay 7-8%deg, 5% delta voltage
and 0.05Hz delta.
This is a our setting in last "first synchronazing" for the 150MW block.

If you want to read the whole thread it is here: thread238-233765.


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If we learn from our mistakes I'm getting a great education!

 

[BigIanH]

Depending on the size of the generator, I only have experience up to 2.5mW.
Generally I set the phase angle to 3 degrees for a nice stable deisel machine and up to 8 degrees for a gas machine (I'm talking reciprocating machines, not turbines).
Voltage difference, normally I would use a 5% window maximum.
Most modern voltage matching equipment will match within a volt or so (if given enough time and the engine is reasonably stable).

What is just as important as voltage and phase angle is the dwell time.
This is how long the generator output is within the "phase window" before breaker close. Set this too short and a possible crash synch can happen if the engine is unstable.
Stable deisels, can be set anything up to 0.5 seconds, Gas machines usually around 0.3 seconds.
I have used these parameters on dozens of machines with no problems.

 

[ldcktm]

Thanks a lot for valuable suggestion and its a great forum and I really appreciate you guy's help

 

[Downstream]

I would set the voltage difference at least below the following: That when the delta volts rms, between the incoming bus and the running bus divided by the subtransient reactance of the generator, does not exceed the generator nameplate rating in amps.