KVA load on parallel generators

[EEpowerOK]

Spin up has a question 3-04, with Gen A at 3 MVA and 3%, Gen B at 4 MVA at 4%, both in parallel. The answer had the KVA load at 6 MVA. Where did the 6 MVA come from?

3M/.03

Gen A= --------------------- x 6 MVA (KVA Load)

3M/.03 + 4M/.04

I've seen CI in version 1-#13 add the parallel MVAs with different Z%.

 

[Ship Wreck PE]

There is a easy way to do this.

Gen 1 times imped 2

Gen 2 times imped 1

3 x 4 = 12

4 x 3 = 12

12/12 = 1

So both generators are equal.

Generator 1 maximum is 3 MVA, generator 2 can only use 3 MVA. 3 + 3 = 6 MVA.

 

[EEpowerOK]

Thank you for the reply. I have seen that method used before, is that the only way to calculate the total load? I was trying to use the following

imped 2 new = imped 2 * (S1/S2) then S1 new= (imped 2 new / imped 1) * S2

S Total = S1 new + S2 = 8 MVA

Which is not the same result

 

[Ship Wreck PE]

Are you getting the right answer on 2-15,

4-48, and 5-61 in spinup?

 

[EEpowerOK]

Yes, on 4-48 and 5-61, I get the same correct answer using the method I explained above.

But with 3M @ 3% and 4M @ 4% I get 8M for a total not 6M.

The first time in Spin up your method was used it said by trail and error (2-15), was not sure what that meant, I got the correct answer on 2-15 too.

 

[daw4888]

Yes, on 4-48 and 5-61, I get the same correct answer using the method I explained above.

But with 3M @ 3% and 4M @ 4% I get 8M for a total not 6M.

The first time in Spin up your method was used it said by trail and error (2-15), was not sure what that meant, I got the correct answer on 2-15 too.

How do you get 8 MVA out of a 3 MVA and 4 MVA transformer?

 

[EEpowerOK]

I converted T2 imped to T1 base

imped 2 new = imped 2 * (S1/S2) then S1 new= (imped 2 new / imped 1) * S2

S Total = S1 new + S2 = 8 MVA

I think I see the difference, if I convert T1 imped to T2 base I get S total of 6MVA, convert T2 imped to T1 base get 8MVA. So how do you know which way to go? Obviously to add them is 7 MVA, I guess it can not be over the 7MVA.

 

[Flyer_PE]

This is very similar to NCEES problem 125. There's a pretty good discussion of it here.

 

[Ace1979]

I don't know if this approach is correct but the ratio of Z1 to Z2 (ie .03/.04) = .75.

Which means that for a Gen G1 fully loaded at 3 MVA, Gen G2 will only deliver 75 % of its rated power, or 4MVA*.75 = 3MVA.

The total power delivered will be 6MVA (Sg1+Sg2).