Spin-up questions version 2nd edition 2014

supra33202 said:

I hope you can help me with the following questions.

1) Q1-64, can you show me the reference regarding equivalent circuit fault?

2) Q2-11, can you show me the reference regarding equivalent circuit fault?

3) Q3-04, how do you get 6 MVA for the load? Should that information (load MVA) given in the question?

4) Q3-47, can you show me how to get reactance?

5) Q3-68, can you show me the reference regarding equivalent circuit fault?

6) Q3-78, how do you get 12 MVA for the load? Should that information (load MVA) given in the question?

7) Q4-48, how do you get 6MVA for the load? Should that information (load MVA) given in the question?

8) Q5-61, how do you get 6MVA for the load? Should that information (load MVA) given in the question?

Thanks!

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supra33202 said:

1) Q1-64:Explained in Glover, very nicely.  

3) Q3-04: Good question. To be same as parallel operation of Xmers.  Start from basics. Za=Zpu*KV**2/MVA ; I1*Z1=I2*Z2 This gives I1*.03/3=I2*0.04/4==== I1=I2; Voltage is equal because of parallel operations so MVA1=MVA2; total 6MVA

4) Q3-47:V= 170/sqrt2@30 and I= 5/[email protected] =====Z=V/[email protected]====13.3+31i So answer is 31 Ohms                

Thanks!

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rg1 said:

Hope is has made sense. Thanks

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supra33202 said:

1) I understand Q3-47 now. Thanks!

2) I still need help with Q3-04. Here is the solution.

I still don't know how to get 6 MVA for the load.

Now, I am thinking maybe because generator A has max capacity of 3MVA and to match generator A, generator B should use 3 MVA out of 4 MVA capacity only. So the total MVA load should max out at 6MVA? Match means Gen B if hooked in parallel to A , it has to supply 3MVA to satisfy the condition I1*Z1=I2*Z2.

Leave Generators for time being- think of two similar resistances placed in parallel at Voltage V. Will they not carry same current? Same analogy applies here.

Actually, I still don't understand the principle. Generator B can contribute more than generator A. Why limit generator B's capacity?- Because they are in parallel. If Gen B supplies more current than this the condition I1Z1=I1Z2 will be violated, It is not the capacity alone, you have to satisfy other conditions too. This is same as driving a car which can go up to 100 mph on a alley having speed limit of 20 mph or a three legged race where both parterres have to run at same speed be it Usain Bolt on one side.

Please advise.

Thanks!

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rg1 said:

To supply power while operation in parallel by any two sources, the terminal Voltage has to be equal because they are connected to same terminals. My advise will be instead of using the formula, understand the problem, go from basics of sharing load by two resistances in parallel.The condition of I1*Z1=I1Z2 has to be met while operation in parallel. where Z is actual Z, not per unit. Because Actual Zs of both generators are same in our case; when put in parallel they share same current and hence same MVA. I hope it is okay now. If you still have a doubt we can discuss a little more.

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supra33202 said:

I got it now.

Thanks!

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rg1 said:

It is my pleasure.

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Stephen2awesome said:

So, MVAload would just be the lowest MVA rated xfmr *2? 

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rg1 said:

If I have understood you correctly and if the question is pointing to my posts regarding the question in question, you are absolutely right. In this particular question the answer should be 6MVA (3*2 or 3+3). 

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supra33202 said:

NCEES practice exam question 125 has a similar problem as spin-up Q3-04. However, they use different method to solve it. It changes the impedance base.

I used Spin-up method, and I got 2000 KVA.

NCESS answer is 2500 KVA.

Could you talk about it?

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rg1 said:

Both methods give same results of 2500KVA. Can you share your calculations to locate the error. 

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supra33202 said:

Problem:

Transformer#1: 1000 KVA, Z1= 4.5%

Transformer#2 : 2000 KVA, Z2= 6.0%

What is the max total load (KVA) without overloading?

Solution:

Since the max capacity that transformer#1 can contribute without overloading is 1000 KVA, You are right upto this point.  the load should be 1000 * 2 = 2000 KVA.\ How you concluded this  that transformer 2 will share the same load as T1. That is only possible when both Transformers have same impedance. Remember I1Z1=I2Z2. You have derailed here. ( In the earlier question both generators had equal Zs ( Not pu Zs), which resulted in equal sharing of loads, that may not be the case always, so better use I1Z1=I2Z2) 

You take load of transformer one and put in the formula to find Total load . You will get 2500 as answer. So by giving 1500 to T2  it is also not Overloaded and we get the answer.

Transformer#1: (1000KVA/4.5) / [ (1000KVA/4.5)+(2000KVA/6) ]   *   2000KVA = 800 KVA

Transformer#1: (2000KVA/6) / [ (1000KVA/4.5)+(2000KVA/6) ]   *   2000KVA = 1200 KVA

Max total load = 800 + 1200 = 2000 KVA

Answer shows 2500 KVA. Please advise.

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rg1 said:

I have explained in red. Hope it will make sense. I have not fully solved it for you to make you understand the concept and use of formula. If it does not make sense, we can discuss further. Think about using the basic formula of equalizing I1Z1 and I2Z2 where Z are actual Zs not the per unit and do the maths after that.

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supra33202 said:

1) Q3-04 has the percent impedance not per unit impedance?

2) NCEES 125 has per unit impedance not percent impedance?

How do we know that?

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rg1 said:

Percent and per unit are same. 5% means 5/100=0.05 per unit. Now their relation with actual quantity in case of Z (Impedance) is Zpu=Zactual/Zbase where Z base is KV**2/MVA so Zpu=Za*MVA/KV**2 or Za=Zpu*KV**2/MVA.

This per unit or percent was not making any difference in the answer here because they were appearing in both Numerator as well as denominator.

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supra33202 said:

I am still confused. Okay let us do it.

Spin Up Formula 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 Is it okay now. Now you can ask why not use the same formula for T2 taking 2000KVA. You will get a different result for Stotal and  you will find that T1 will be overloaded in that case. Just do it and see for yourself for better understanding of the case.

Why can't I use the same method (spin-up Q3-04) to solve NCEES 125 problem?

Based on my understanding, they are asking the same thing - maximum total load (KVA) without overloading either transformers. Yes they are asking the same thing and you have same answer by many methods including the one of spin up.

" ( In the earlier question both generators had equal Zs ( Not pu Zs), which resulted in equal sharing of loads, that may not be the case always, so better use I1Z1=I2Z2)  "

I know both problem's transformers have different per unit impedance.

Q3-04 has equal actual impedance? How do you know that? Zactual=Zpu*KV**2/MVA. Because voltage is same for both sources assume say 1kV and then try to do it yourself. If you do not get, I will do it for you. 

How do we know NCEES doesn't have equal actual impedance? Zactual=Zpu*KV**2/MVA. Because voltage is same for both sources assume say 1kV and then try to do it yourself. If you do not get, I will do it for you. 

"Percent and per unit are same. 5% means 5/100=0.05 per unit. Now their relation with actual quantity in case of Z (Impedance) is Zpu=Zactual/Zbase where Z base is KV**2/MVA so Zpu=Za*MVA/KV**2 or Za=Zpu*KV**2/MVA."

I agree.

Do you think you can spend some time to solve NCEES 125 problem? We will discuss it till you get it!!! 

Thanks!

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