Paralleling trfrs from different sources
Paralleling trfrs from different sources
(OP)
I have a YNd1 trfr fed by a generator and a YNd11 trfr fed from the utility network. Sync check is done on the LV side of both. It is required to run these in parallel. Can this be done or must the vector groups be the same?
I tend to think not - so long as the respective currents are in phase and same phase rotation.
Thanks.
I tend to think not - so long as the respective currents are in phase and same phase rotation.
Thanks.






RE: Paralleling trfrs from different sources
Bill
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"Why not the best?"
Jimmy Carter
RE: Paralleling trfrs from different sources
RE: Paralleling trfrs from different sources
RE: Paralleling trfrs from different sources
The simplest governor control is droop. When the generator is in parallel with a much larger system you adjust the load by adjusting the frequency set point. With 3% droop, and the frequency set at 60 Hz (or 50 Hz) the generator will not output real power. (I have seen generators operated at 0.0% real load in order to adjust voltage and or to correct power factor.)
As you increase the frequency set point the generator contribution to the load increases. At a set point of + 1.5% or 60.9 Hz the generator should output 50% of its rated kW. At a set point of 3% or 61.8 Hz the generator should output 100% of its rated kW. Note that the frequency is set at 60 Hz by the much larger utility system. You are not changing the frequency, you are changing the frequency set point so as to let the proportional band of the governor know how much power you want out.
Close. You have the right idea. The kW and kVars produced combine to produce KVA that is at the same angle as the system impedance with an islanded set.
Things change quite a bit when you go from islanded to parallel with a large system.
Frequency setting: Islanded; Controls the frequency (subject to 3% droop).__ In Parallel with a large system; Controls the power generated
Voltage setting: Islanded; Controls the voltage.___ In Parallel with a large system; Controls the production of VARs
If you want to match the generator output with the local load you will need a load control panel capable of measuring the local load and setting the governor accordingly. Alternately you may measure the load on the utility connection and set to control for 0.0% kW and KVARs.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Paralleling trfrs from different sources
RE: Paralleling trfrs from different sources
RE: Paralleling trfrs from different sources
In this application how does the generator know about phase displacement? If it does know, does it care?
RE: Paralleling trfrs from different sources
RE: Paralleling trfrs from different sources
RE: Paralleling trfrs from different sources
pwrtran - there can only be a 60degree phase shift if the generator and the utility both are coupled to the same source (i.e. same reference). This is clearly not the case here so the way I see it the angle between them could be anything before synch.
David - as Scotty says how does the gen know about phase displacement? The way I see it, as I said above, the angle can be anything as the gen and the utility do not have a common reference. This leads to my next question though, once the islanded generator has been fired up, how does one change it's voltage angle relative to that of the utility voltage?
Thanks.
RE: Paralleling trfrs from different sources
Prior the synchronizing the generator prime mover throttle controls the speed and unless the electrical frequencies are exactly matched, the phase angle between the utility and the generator will be continuously varying.
RE: Paralleling trfrs from different sources
Thanks David.
RE: Paralleling trfrs from different sources
First let's consider proportional band as in a PID controller and assume for now an islanded generator.
If the Proportional setting of a PID controller is set at 3% and the set point is 60 Hz. the controller will open the throttle as the frequency drops below 60 Hz. When the frequency has dropped 3% to 58.2 Hz the generator will be at full output.
But most loads do better with a slightly higher frequency than with a slightly lower frequency.
So we offset the set point up 3% to 61.8 Hz and let the machine drop down to 60 Hz at full load.
Now we could set the set point at 61.8 Hz and call the proportional band 0.02912621359223300970873786407767
It's easier to call it the droop.
I guess we may be able to call it 3% proportional band with a 1.8 Hz offset.
(For 50 Hz, use 1.5 Hz, still 3%)
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Paralleling trfrs from different sources
RE: Paralleling trfrs from different sources
The angles that you are worried about must be resolved before synchronism will occur.
When a set is being brought on-line, the frequency is generally slightly higher than the grid frequency. When the synchroscope or the sync check relay indicates synchronism the breaker is closed, manually or automatically. The generator has been running slightly fast so that it slows slightly when connected to the grid. The governor sees this drop in frequency as an error and opens the throttle slightly (proportional action) with the result that the generator picks up a little load, enough load to assure stability.
I have seen the effect that you are concerned with. It was the result of a wiring error that introduced a 60 deg. error between the generator reference voltage and the bus reference voltage.
The breaker often tripped immediately it was closed. Once or twice a year a key would be sheared off in a coupling.
Upon a successful closing the syncro-scope would jump to 2 O-clock as the generator and engine were rather violently shifted 60 electrical degrees.
I never did find out exactly how many years this happened before it was rectified, but more than a couple of years.
Bill
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"Why not the best?"
Jimmy Carter
RE: Paralleling trfrs from different sources
I have seen something like that once, maybe not quite 60° out of phase but well away from where it should have been. A defective auto-synchroniser, a badly set backup check-sync relay, a manual sync by someone who wasn't paying attention, and a 400MVA set makes for a real exciting moment. It was bad enough in the HV substation where we heard the whole place vibrate, but apparently not anywhere near as scary as on the turbine deck where I'm told the whole structure felt like a ship in rough seas. While I doubt it moved much at all, the guys were pretty shaken by it.
RE: Paralleling trfrs from different sources
Gunnar shared a similar story a few years back. I think that was a hydro installation and a very slow closing breaker.
I remember another event in Western Canada back in the 70s. The load dispatcher called a hydro plant with instructions to put another set on-line. The operators replied that that wasn't possible as the synchroscope was not working. After some discussion and following direct orders the operators closed the switch. If memory serves, the unit transformer was badly damaged.
I believe that 90 electrical degrees out is the worst case mechanically and 180 electrical degrees out is the worst case electrically.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Paralleling trfrs from different sources