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Paralleling trfrs from different sources
2

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.

RE: Paralleling trfrs from different sources

Phase rotation, voltage and synchronization are all that is required. Vector group is not important but remember that the throttle position of the prime mover determines how much power as KiloWatts is delivered and the voltage setting determines how much reactive power in Kilo-Volt-Amps-Reactive is delivered or absorbed and in what direction.

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

RE: Paralleling trfrs from different sources

(OP)
Thanks waross. Let's say the generator via the step-up transformer feeds load. Load and system impedances (as seen by the generator) will result in a certain Z at a given angle. I take that the gen governor and AVR then adjust themselves so that the kW and kVars produced are more or less at the same angle as the system impedance?

RE: Paralleling trfrs from different sources

Depends on your control strategy. You could run the generator at any loading up to its maximum capacity and at any power factor within its capability curve.

RE: Paralleling trfrs from different sources

2
What David said.
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.

Quote:

kW and kVars produced are more or less at the same angle as the system impedance?
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

The throttle (power) can also be controlled independently of what's happening on the power system. It can be set for base load operation, where it is set to some percentage of unit rating and left there for extended periods. It can also be operated in a fuel following mode, like digester gas fueled units that are run to consume all the fuel produced.

RE: Paralleling trfrs from different sources

You will have 60 degree out off phase if you parallel two transformers without doing some efforts. You need to swap any two phases on one transformer on both primary and secondary side.

RE: Paralleling trfrs from different sources

pwrtran,

In this application how does the generator know about phase displacement? If it does know, does it care?

RE: Paralleling trfrs from different sources

The generator and the utility will be 60 degrees out of phase, but won't know or care.

RE: Paralleling trfrs from different sources

Awww David, you've gone and spoiled the fun... tongue

RE: Paralleling trfrs from different sources

(OP)
waross - thanks for the very informative post. I gained much from it. I understand the concept of real power output being coupled to the frequency setting but what is droop?

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

Scotty and I were typing at the same time, he's just faster. Once they're connected the phase angle will remain close to 60 degrees; off by the power angle. The generator will have a very good idea what phase angle the utility is when seen from the generator side of the generator transformer, but other angles are of no consequence.

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

(OP)
Got it! Once they're in synch the gen and utility are 60 degrees out of phase of course since when starting from the point of synch and travellling through the trfrs the one goes +30 degrees whilst the other goes -30 degress. So in this instance the reference is the side of the trfrs where they are synched.

Thanks David.

RE: Paralleling trfrs from different sources

Droop.
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

I am kind of with you guys but once they are synced via two different transformers the voltage vectors from the generator transformer will be bonded to the utility transformer, and the generator speed will be either elevated or dragged down before stabilization. After that only the power angle or the torque angle matters and there will be no angular displacement between two transformers.

RE: Paralleling trfrs from different sources

When they are in sync they are in sync.
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
--------------------
"Why not the best?"
Jimmy Carter

RE: Paralleling trfrs from different sources

Bill,

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

Ours were relatively tiny diesels. 600 KWs and 350 KWs. Our system served about 5000 homes. The voltage reference from the generators was taken phase to phase and the bus reference was taken phase to ground. Usually the breaker tripped. Sometimes a 350 KW set would shear a key. I am still amazed at how little damage was done over the years.
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

(OP)
waross - thanks for the droop explanation. I'm still in school every day.

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