Generators Paralelling - Utility Closed Transition

[living2learn]

System: (2) 12kV, 2500MW Generators that parallel to a common bus and connect to the main switchgear and power some large loads. The system is a closed transition with the Utility. During normal operation only one utility is present.

I have attached a diagram of the electrical system.

The Problem: When utility is present all the PT’s in the electrical system read the same voltage - life is great. Even though they are configured differently the relay compensates. Now when utility is disconnected and the generators are online the PT voltage at the generators and generator switchgear read 122.7V while the main switchgear (1500’ away) reads 128.6V. How did the voltage rise?? Clearly not enough capacitance in the system, but the system was unloaded during this measurement. The waveform is slightly distorted at 1-2% Vthd.

thoughts?

 

[collies99]

Wow, at first you had the largest generstirs in the world, but your sketch cleared that up.

what type of generator grounding system di you have ?

is there any xformer delts connecting winding on the 12 kv bus?

 

[waross]

If everything is open and the only load is the PTs there may be enough cable capacity to drive up the voltage.

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

 

[collies99]

Waross suposition could be correct, but I am wondering where the harmonics is generated ?

 

[waross]

The magnetizing current is non linear and so may generate harmonics.

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

 

[living2learn]

Thanks for the replies.

collies99

The generators are solidly grounded
I believe there are no transformers connected to the system.

Everything is open but all the feeder breakers are closed that each feed a mile worth of 500kcmil conductors.

So when the gens were running the only load on the system where a bunch of cables.

How much capacitance would it take?

Quick way to calc voltage rise...assume all capacitance is at end of line, which is worst case. Vrise = I*sin(theta) whier I is the per phase current into the capacitance in amps, X is the reactance up to the point of the capacitors in ohms and sin(theta)= one since a capacitor is 90 degrees.

assuming the gens will maintain a pu voltage of 1, then the only x is teh line reatance which I got to be .11 ohms. I calculated the capacitance chargeing current to be...around 1 Amp
so the rise would be .11*1 = .11volts?

another way i thought would be I am 5% out of whack 12.2kV vs. 12.8kV so I need 600V
600V=12kV/1.73 * (Xc)^2
600/7200=(Xc)^2
.0833=(Xc)^2
.288=Xc
from a cable table with single conductors in conduit-non magnetic increase 20% random lay = .0384 ohm/1000 ft
Xc=.288 needed and .0384 ohms per 1000'
.88/.0384 = 7400ft which is 1.4 miles of cable - which we would have.

my thougts are a little all over the place and will upload my other math I did at home which was clearer.

 

[collies99]

Even if one knows the capacitance per fiit, the voltage rise is nit linear as per the ferranti effect, that would be exponential.

 

[living2learn]

Awesome. The more I learn the more I realize I dont' know S*$#

Can this be hand calc'd or just way to complicated and software must be utilized? I have a difficult time when I do not grasp something and it slowly drives me insane. I will digest this over the next day or so and come back with my results. If anybody has any words of wisdom before I partake on this journey, let me know.

good arcitles, examples etc...

This has turned out to be quite an issue and have been tasked with "well if you think this is the problem then prove it, then we will test your theory"

 

[collies99]

http://www.powerqualityworld.com/2011/09/ferranti-effect.html?m=1

the math is beyond me, but if you can get the equivalent bus capacitance per unit length to you desired destination, where the VTs are, it,may come close to your measured values.

the links requires a pi model versus t.

 

[living2learn]

Would adding a transformer to the circuit be enough inductance to lower the voltage? In anybodys opinion how much load would need to be added to lower the voltage??

10%-20%....

Thoughts?

 

[living2learn]

Would adding a transformer with nothing connected to it cancel out the capacitance in the cables??

How much load do you think we need to cancel out the capacitance?

10%...

thoughts?