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resonant grounded

resonant grounded

resonant grounded

(OP)
We are design a hydro powerplant with generator (13,8kV-85MVA) grounding by  neutral grounding transformer
with high resistance at the secondary.
Generator will be acopled in a bubble turbine under river.
We decided to limit ground fault with 15A to avoid any kind of overvoltage stress in generator winding.
Is was not possible because of high capacitive generator system(bus duct, generator, surge protector...).
We intend to use GROUND-FAULT NEUTRALIZER(GFN).
We never used it before, so we need some information about voltage, tapes for regulation, calculation.
We appreciate very much some experiene about it.
 
 

RE: resonant grounded

If you'll search for "Petersen coil grounding" on web you'll get a lot of articles as for instance:
"Protective relaying for power generation systems By Donald Reimert pp 130"
http://books.google.com/books?id=i9hXq4QUhmYC&pg=PA130&lpg=PA130&dq=GROUND-FAULT+NEUTRALIZER(GFN).&source=bl&ots=KquzTIsHK4&sig=z_6UuPKxIbMOLB57G8tlodEyK1A&hl=en&ei=Ji2cSruGB5XVjAfSuZjgBg&sa=X&oi=book_result&ct=result&;resnum=3#v=onepage&q=GROUND-FAULT%20NEUTRALIZER(GFN).&f=false
"Application of Sensitive Earth Fault Relays to Petersen Coil Earthed and Insulated Power Systems" Alstom:
http://www.areva-td.com/solutions/liblocal/docs/LegacyProducts/kceu142-242salesen.pdf
 

RE: resonant grounded

Go back the the generator vendors, much larger systems than this are high resistance grounded. Something sounds wrong. Also TVA has a lot literature on this and hydro in particular. Sometimes engineers think they know better than the vendors, this is seldom the case.  

RE: resonant grounded

Hi
Im fully agree with xxjohnh . Very big generators have a high resistor systems with 5A GF limit.

Petersen coil is good thing, but for distribution network.

best Regards.
Slava

RE: resonant grounded

(OP)
We decided to limit ground fault with 15A to avoid any kind of overvoltage stress in generator winding of 46 generators installed underwater.
Is was not possible with high resistors system grounding.
I dont know why, but in Brazil capacitive currents of equipments (busduct,generator...) are very high,
close to 30-40A.
GROUND-FAULT NEUTRALIZER(GFN) is the unique solution.
All american grounding standard Guide include this method of grounding.

  

RE: resonant grounded

In the US, GF Neutralizers are considered obsolete and almost never used.  High-resistance grounding is the preferred method for large generators.  

I would review the calculations that came up with 30 A of capacitive charging current.  This is an exceptionally high value.  Unless you have a mile of bus between the generator and the transformer, there may be an error in the calculations.   

RE: resonant grounded

(OP)
1- fews literature of reference show GFN better than high resistance grounding method.
2- I never analised calculation of hydrogenerator high resistance grounding with 5A current.
3-Last calculation of unit-connected hydrogenerator high resistance grounding were 32.4A. It was revised. They change to 29.2 A (!!??) What to do?
4-Does anyone could mention what are problems with GFN?
  

RE: resonant grounded

May I ask what your 3-phase fault current level is? You have to know this because you are going to decide whether to use reactance grounding or high resistance grounding. As far as I can remember (I have to look for my reference book on this), it is advisable to use resistors (than using reactance) if your design let-thru current is 25% of the 3-phase fault current at your generator terminals.
I will post the reference if I can find it though.

RE: resonant grounded

Utility-class generators are almost always earthed through a single phase distribution transformer with a resistive burden on the secondary. Most generator manufacturers will include the earthing package within their scope. Using an unusual earthing system will only complicate things when there are tried and tested solutions already in use in thousands of plants throughout the world. It is hard to imagine that anything in Brazil is radically different to anywhere else.
  

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

RE: resonant grounded

Quote (ScottyUK):

"Utility-class generators are almost always earthed through a single phase distribution transformer with a resistive burden on the secondary."

Very true! And that is because when using a resistor that is wire-wound of small-sized wire these are prone to damage. We use a distribution type single-phase transformer with a grid-type resistor on the secondary which is very reliable compared to wire-wound resistor!

RE: resonant grounded

(OP)
A unit Transformer consists:
- 1 step up transformer  of 300MVA-525 kV(solid grounded star) – 170MVA-13,8kV (delta)- 170MVA-13,8kV(delta)
- 4 generators of 84MVA -13,8kV – cosFi 0,9
- 2 13,8kV generation busbar(ungrounded) are connected to low voltage side of step up transformer.
Each 2 generators are connected to 1 busbar by generator circuit breaker.

Originally neutral of each  generator was grounded by  single phase distribution transformer with a resistive burden on the secondary.

It was verified very high capacive currents  to ground.
We decided to install CB in neutral of each generator, in order to keep only one grounded.

As generator wil be installed underwater was decide to reduce ground current to 15A.
It was only possible using resonant grounding.

We are verifying possibility to remove surge arrester at the generator surge cubicle ,
in order to reduce capacitive to ground current  and perhaps avoiding resonant grounding.  


burnt2x,
Beside consideration of ScottyUK (utility class), all time you request IEEE generator circuit breaker standard,
automatically you are using high impedance grounding scheme, because this standard does not admit ground to fault
more than 50A for generator CB.
 

RE: resonant grounded

(OP)
ERRATA:
...................
We are verifying possibility to remove CAPACITOR SHUNT at the generator surge cubicle ............
 

RE: resonant grounded

Quote:

burnt2x,
Beside consideration of ScottyUK (utility class), all time you request IEEE generator circuit breaker standard,
automatically you are using high impedance grounding scheme, because this standard does not admit ground to fault
more than 50A for generator CB.
Do you mean to say that if you use ". .generators. . .earthed through a single phase distribution transformer with a resistive burden on the secondary." the system is high impedance scheme? What if the secondary resistor value is such that you design allowing 600A in the neutral (or primary winding of the single-phase distribution transformer)? Do you still say it's a "high impedance grounding with 600A in the neutral during ground faults?
 

RE: resonant grounded

(OP)
I mean:
If you require adopt IEEE Std C37.013-1997- generator circuit breaker standard - in your CB generator technical specification ,  automatically you are using high impedance grounding scheme, because this standard does not admit grount to fault more than 50A for generator CB.

I really dont known explain why this.



  

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