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synchronization of diesel-generator group

synchronization of diesel-generator group

synchronization of diesel-generator group

(OP)
It is thought to operate in parallel two diesel-generator groups apart from each other by 3.5 km.

generators: 1 MVA-60Hz-13.8kV
3 single cable of 50mm2, Xc=8.993 ohm.km , length=3.5 km

 1-Is it possible to synchronize within tolerable limits overvoltage?
2-Is there any calculation expedient to make this assessment?
 

RE: synchronization of diesel-generator group

(OP)
more information:
generator: 1MVA-440V-60Hz-1800rpm- cosfi 0.8
stepup transformer: 1MVA-0.44/13.8kV

RE: synchronization of diesel-generator group

If you can run in droop there should be no problem. If you have a group of generators controlled by a load control panel and the load control panell may be set to respond to load changes with a frequency droop no problem. You may run one group in isochronous and run the second in base mode, but if the load drops below the base load setting the frequency may rise and the isochronous group may suffer light load issues such as wet stacking.
Cheap and easy, go droop.
For tight frequency control and good load sharing, you will probably need communications between the sites.
 

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

RE: synchronization of diesel-generator group

(OP)
waross,
I am talking about synchronization , not generators in parallel.
I am worried about long distance of generators could carrying voltage above limits standard for synchronization.
 

RE: synchronization of diesel-generator group

odlanor

To restate your question (so you can see if I understand what you are asking): Will the voltage rise caused by the cables' capacitive reactance through the stepup transformer reactance result in the voltage at the paralleling breaker to be beyond limits for synchronization?

What is the stepup transformer %X (%Z and X/R ratio)? What is the sync check voltage limit, or phase angle limit you are using?

Knowing the transformer %X, it is easy to calculate the voltage rise due to cable capacitance. This can be translated into an equivalent phase angle difference.

RE: synchronization of diesel-generator group

And are you certain that Xc=8.993 ohm.km cable capacitive reactance is correct?

RE: synchronization of diesel-generator group

(OP)
PHovnanian,
1-...Will the voltage rise caused by the cables capacitive reactance through the stepup transformer reactance.....
Yes.
2-What is the stepup transformer ......
It is not important now. Firstly I have to calculate answer 1.
Generators has limit voltage +-5% and stepup transf. +-10%.
If you energize generator with 0.95 pu voltage and terminal remote of line exceed 1.10 or 1.15 pu voltage ,
your cable exceed standards of operation.
3-And are you certain that Xc=8.993 ohm.km cable capacitive reactance is correct........
I dont know. I guess this refers to a cable of 50mm2 of 3.5 km.

PHovnanian,
I do not work with diesel generation. I usually work with hidraulic generators(50 MVA and above) connected to overhead 500kV transmission lines. This problem sometimes appears in that configuration.
I would like to find some literature about cable transmission line  with this problem.  

RE: synchronization of diesel-generator group

(OP)

PHovnanian,
The cable connecting C1 and C2 has the following characteristics.
 • Section: 50mm2 (one cable per phase)
 • Material: Electrolytic annealed copper conductors, stringing class 2
 • Nominal voltage: 8,7 / 15 kV
 • Insulation: EPR
 • Coverage: PVC
 • Type: Compact Eprotenax Prysmian (adopted as the reference)
 Prysmian's catalog, the value of Xc of this cable is
7991 Ohm * km

RE: synchronization of diesel-generator group

Quote:


2-What is the stepup transformer ......
It is not important now. Firstly I have to calculate answer 1.

The stepup transformer impedance certainly is important as it will contribute the largest part of the voltage rise due to the line capacitive reactance.

However, I did a little back of the envelope calculation assuming a transformer reactance of 10%. The resulting voltage rise given a more reasonable Xc of 7991 Ohms/km is about 1.8 volts. This should have an insignificant effect on paralleling the systems.
 

RE: synchronization of diesel-generator group

(OP)
Thank you. I wonder how you did the calculation?

RE: synchronization of diesel-generator group

Quote:

I wonder how you did the calculation?

Base Z of transformer is 5.24 Ohms on 13.8 kV side, so assumed Xt of 10% is 0.524 Ohms.

Xc per phase of 3.5 km of cable: 7991/3.5 = 2280 Ohms

Charging current (Ic): 13800 / ( 1.732 * 2280 ) = 3.49 Amps

Voltage rise across transformer X: Xt * Ic = 1.83 Volts

RE: synchronization of diesel-generator group

(OP)
PHovnanian
I found an old example of this calculation, for overhead transmission and made an adjustment for cable lines.
 It refers to to calculations of the ABCD parameters of a transmission line.
I guess this is a better aproach.

Vn    13800    volt        
Rac    0,535    ohm/km    S=20cm    prysmian
Xl    0,33    ohm/km        
len    3,5    km        
Xc    7991    ohm.km        
                
symmetrical T-network            
Vs=  (1+ ZY/2)*Vr + Z (1 +ZY/4) * Ir        
Ir=0                
Vr= Vs/(1+ ZY/2)            
                
Vs    7967,67    - send voltage        
R    1,87            
X    1,16            
Y    0,00            
Z    2,20            
ZY2+1    1,00            
Vr    7967,35    volt    receiving voltage    

 

RE: synchronization of diesel-generator group

(OP)
PHovnanian
I found an old example of this calculation, for overhead transmission and made an adjustment for cable lines. It refers to to calculations of the ABCD parameters of a transmission line.I guess this is a better aproach.

Vn =   13800; %    volt  
Rac =   0.535 ;%   ohm/km
% S=20cm    prysmian
Xl  =  0.33  ;%  ohm/km
len  =  3.5 ;%   km    
Xc = 7991;%    ohm.km  
% symmetrical T-network      
% Vs=  (1+ ZY/2)*Vr + Z (1 +ZY/4) * Ir
% Ir=0                Vr= Vs/(1+ ZY/2)   
Vs =   Vn/sqrt(3);%   - send voltage   
Z=Rac + j*Xl;
Xc= -j* Xc/len;
Y  =  1/Xc;          
Vr = Vs / ( 1 + Z * Y / 2)  
Vr =  abs(Vr);   %  receiving voltage
format bank
display(Vs);  %  7967.43 V
 display(Vr); %  7967.60 V
  

RE: synchronization of diesel-generator group

That gives you the voltage rise due to the charging current and the impedance of the line itself. But there is no allowance for the transformer impedance at the sending end.

For a relatively short (3.5 km) line, the transformer impedance and associated voltage rise is much greater than that due to the line impedance.

Either way, your figure shows a rise of 0.23 Volts. I figured 1.83 volts rise due to the transformer itself (with a quick and dirty formula). Even adding the two together, I doubt you'd see any adverse effects on the ability to parallel these systems.

RE: synchronization of diesel-generator group

(OP)
PHovnanian (Electrical)
Yes. I agree with you. Thank you!

RE: synchronization of diesel-generator group

(OP)

PHovnanian (Electrical)
I am sorry to have to reconsider my point of view. I do this because I believe there is an error of interpretation  in your calculation:
 
Voltage rise across transformer X: Xt * Ic = 1.83 Volts

Ic is a shunt current in the circuit  of cable line.

Expression Xt * Ic  represents a drop voltage into transformer.
Ic is a serie current in the circuit  of transformer.
Both parameters current  are  differents.
 

RE: synchronization of diesel-generator group

Quote:

Both parameters current  are  differents.
Why do you say that?

The line charging current is indeed due to the shunt admittance of the line's capacitance. But to reach the line from the generator, it must pass through the transformer's series impedance.

Draw a one line diagram to visualize this.
  

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