Spin-up questions version 2nd edition 2014

[rg1]

A small addition- My diagnosis is you should improve on pu system. Read some good stuff on pu and do some questions on pu. Of course our discussion must have benefited you in that direction. I have given basic formula for Zpu in my posts. Just have a deep look into that. Try to derive those formula yourself.

 

[supra33202]

I think I got it now.

NCEES 125)

Since transformer 1 has the least capacity (1000kVA) and we don't want to overload it, then Sa=(MVAa/puZa)*Stotal/(MVAa/puZa+MVAb/puZb) So 1000=(1000/4.5)*Stotal/(1000/4.5+2000/6) Solve this for Stotal; it gives Stotal =2500kVA

The maximum total load (KVA) is 2500KVA. The NCEES answer is correct.

Just to confirm:

Transformer 1 will contribute 1000 KVA of the 2500 KVA total load.

For transformer 2:

Sb=(MVAb/puZa)*Stotal/(MVAa/puZa+MVAb/puZb) So Sb=(2000/6)*Stotal/(1000/4.5+2000/6) Solve this for Sb; it gives Sb =1500kVA

Transformer 2 will contribute 1500 KVA of the 2500 KVA total load.

And it makes sense that Sa + Sb = 1000 KVA + 1500 KVA = 2500 KVA total load.

Thanks!

 

[rg1]

I think I got it now.

NCEES 125)

Since transformer 1 has the least capacity (1000kVA) and we don't want to overload it, then Sa=(MVAa/puZa)*Stotal/(MVAa/puZa+MVAb/puZb) So 1000=(1000/4.5)*Stotal/(1000/4.5+2000/6) Solve this for Stotal; it gives Stotal =2500kVA

The maximum total load (KVA) is 2500KVA. The NCEES answer is correct.

Just to confirm: Yes very right

Transformer 1 will contribute 1000 KVA of the 2500 KVA total load. so 2500-1000=1500

For transformer 2:

Sb=(MVAb/puZa)*Stotal/(MVAa/puZa+MVAb/puZb) So Sb=(2000/6)*Stotal/(1000/4.5+2000/6) Solve this for Sb; it gives Sb =1500kVA  This is right but if you take 2000 kva supplied by T2; the Stotal will be different at 3333.33KVA implying overloading of T1.

Transformer 2 will contribute 1500 KVA of the 2500 KVA total load. yes

And it makes sense that Sa + Sb = 1000 KVA + 1500 KVA = 2500 KVA total load. yes

Thanks!

 It is my pleasure but still if I were you I will strength my pu chapter. 

 

[rg1]

Please do not always think that lower capacity source will be fully loaded, it will depend on actual Z of the source, you can have it otherwise also if Zs are like that. So be careful.