Drift in Synchro

[AusLee]

Hello,

I have 2 generators running in parallel. I was doing an off load test+synchro. The gensets entered in parallel on the busbar with no problem.

However, after a few minutes, i started reading the power on the circuit breaker digital trip unit: one generator was producing positive power while the other was receiving the same amount but in negative.

There is a return power protection realy set at 10% of full rated kW.

With time, the active power produced by one of the generator kept increasing while the other was decreasing at the same rate. I expected the automatic synchro system to do something about this, but instead it let the system drift until after 19 minutes the system triped because of excess return.

My question is what could be the problem in this case? is it the controller in the synchro panel? or just a bad setting somewhere?

 

[ScottyUK]

Do you have governor droop control enabled on at least one unit? Droop modifies the speed controller reference to allow the frequency to drop slightly as the load increases, and rise slightly at low load. Droop is usually quoted in % speed at full rating - if you want both units to share load equally, the droop setting should be the same on both units. If constant frequency is important, put one set - the bigger one if they are different sizes - into isochronous mode as the 'master'.

http://www.tpub.com/content/construction/14027/css/14027_58.htm

----------------------------------

I don't suffer from insanity. I enjoy it...

 

[davidbeach]

"Off-load test" - does that mean you were running the two units in parallel for an extended time with no load? A well designed, well implemented control system should handle that without difficulty, but it is a trick that many systems don't manage, or don't manage well. It probably takes active load management to get there and hold it. Adding some load for the generators to supply would likely help considerably.

 

[waross]

With any very small difference in speed setting on the governors this may happen.
What type of governors and load control do you have?
respectfully

 

[Alex68]

Moreover if the two units at the same time are still in isochronous mode (required before the parallelling) after the closing of the paralleling CB, the system becomes unstable.
Actually the isochronous speed control requires the activation of a PI regulator. So two PI regulators working indipendently on the same system bring it exactly to the unstable condition you described.
The off-load test makes the system much more unstable because no damping element (=resistive load) is connected.