Jump to content
Engineer Boards

​ ​ ​
 photo CHPE_AnimatedWebBanner_650x1202_zps5704d467.gif
spinup

Question for the Week (Power PE)

Recommended Posts

The next exam is on April 5th, 2019. There are 15 weeks left. Each week until the exam a question will be posted on www.spinupexams.com/

Previous weeks will also be contained on the website.

Week 1 Question For The Week can be found at:

www.spinupexams.com/

Under the "Question For The Week" tab.

  • Thanks 1

Share this post


Link to post
Share on other sites

View Question

diagram4004


The above 3 phase system has a transmission line with impedance and distance stated below (Z and D). The voltage source is 480V ∠0°, and the load is drawing 75A with a 0.85 power factor lagging. What is the voltage at the load end?
Z = 0.1 + j0.02 per 1,000 ft.
D = 1750 ft

 

CA. 457V
B. 264V
C. 269V
XD. 467V


 

Solution

Vload = vsource - (Iload)(Zload/line)
= ( (480∠-30° / √3) - (75∠-cos-10.85)(0.1 + j0.02)(1750 / 1000) )
√3
=( (277∠-30°) - (75∠-31.8°)(0.1∠11.3°)(1750/1000) )√3
=( 277∠-30° - 13.4∠-20.5° )√3
=( 264∠-30.5° )√3
=457V

Can someone tell me why you would divide by √3 for Zload /line?  I thought impedance is usually given per phase and there is no need to multiply/divide 3 phase impedance  by √3 like we do to 3 phase delta/wye current and voltages. 

 

Edited by roy167

Share this post


Link to post
Share on other sites

Week_1_QFTW_Question_Solution.png

Why isn't VAN not equal 277 angle -30? The magnitude of answer would be same but angle would be different.  In case of Wye, Phase voltage lags line voltages by 30 degrees. 

The funny thing is, if you take 277<0  and multiply that with answer provided in B, ( S= VI *) then you do get 25 KVA,0.75  lagging. 24.9K<41.4 😡

Only inference I draw is, in practice this situation won't exist. Something has to change, either source has to be 277<-30,  or load information is not correct. 

 

Edited by roy167

Share this post


Link to post
Share on other sites
18 hours ago, roy167 said:

View Question

diagram4004


The above 3 phase system has a transmission line with impedance and distance stated below (Z and D). The voltage source is 480V ∠0°, and the load is drawing 75A with a 0.85 power factor lagging. What is the voltage at the load end?
Z = 0.1 + j0.02 per 1,000 ft.
D = 1750 ft

 

CA. 457V
B. 264V
C. 269V
XD. 467V


 

Solution

Vload = vsource - (Iload)(Zload/line)
= ( (480∠-30° / √3) - (75∠-cos-10.85)(0.1 + j0.02)(1750 / 1000) )
√3
=( (277∠-30°) - (75∠-31.8°)(0.1∠11.3°)(1750/1000) )√3
=( 277∠-30° - 13.4∠-20.5° )√3
=( 264∠-30.5° )√3
=457V

Can someone tell me why you would divide by √3 for Zload /line?  I thought impedance is usually given per phase and there is no need to multiply/divide 3 phase impedance  by √3 like we do to 3 phase delta/wye current and voltages. 

 

Vload = vsource - (Iload)(Zload/line)
= ( (480∠-30° / √3) - (75∠-cos-10.85)(0.1 + j0.02)(1750 / 1000) )
) ---> (L-N)
= 263.8 <-30.48 ---> (L-N)
= √3 x 263.8 <(-30.48+30)---> (L-L)

=457<-0.48V---> (L-L)

 √3 there in the solution is just conversion from L-N to L-L.

Edited by Sdhabik

Share this post


Link to post
Share on other sites
17 hours ago, roy167 said:

Week_1_QFTW_Question_Solution.png

Why isn't VAN not equal 277 angle -30? The magnitude of answer would be same but angle would be different.  In case of Wye, Phase voltage lags line voltages by 30 degrees. 

The funny thing is, if you take 277<0  and multiply that with answer provided in B, ( S= VI *) then you do get 25 KVA,0.75  lagging. 24.9K<41.4 😡

Only inference I draw is, in practice this situation won't exist. Something has to change, either source has to be 277<-30,  or load information is not correct. 

 

I agree with you. VAN shall be 277<-30. These kind of things we have to be careful during calculation. 

Share this post


Link to post
Share on other sites

=457<-0.48V---> (L-L)

Yes that solution is correct. I was confused because they hadn't given whether source/load is wye/delta. Since you are first converting everything in per phase and then back to 3 phase. You still have to use -30, +30 phase shift. 

Edited by roy167

Share this post


Link to post
Share on other sites

Two months until the exam.

Week 7 Question For The Week posted at:

www.spinupexams.com/

Under the "Question For The Week" tab.

Previous weeks are also contained on the website. 

  • Like 1

Share this post


Link to post
Share on other sites

Week 8 Question For The Week posted at:

www.spinupexams.com/

Under the "Question For The Week" tab.

Previous weeks are also contained on the website. 

Question below:

image.png.02629c5ae20bbbc529e74d754293a242.png

  • Like 1

Share this post


Link to post
Share on other sites

Week 12 Question For The Week posted at:

www.spinupexams.com/

Under the "Question For The Week" tab.

Previous weeks are also contained on the website. 

  • Like 1

Share this post


Link to post
Share on other sites
9 minutes ago, spinup said:

Week 12 Question For The Week posted at:

www.spinupexams.com/

Under the "Question For The Week" tab.

Previous weeks are also contained on the website. 

Good little problem, I calculated it wrong (35.15% )by using angle. That goes to show there are so many ways one can be tipped even with simple problems! I have never come across a problem which gave an angle and asked to find voltage regulation.  

Share this post


Link to post
Share on other sites

Wouldn't it be wonderful if all the questions were this easy. More protection questions is what I need.

Share this post


Link to post
Share on other sites
On 3/10/2019 at 8:45 AM, roy167 said:

Good little problem, I calculated it wrong (35.15% )by using angle. That goes to show there are so many ways one can be tipped even with simple problems! I have never come across a problem which gave an angle and asked to find voltage regulation.  

I saw similar question in one of the Complex Imag test. I did the same thing of including the angle & got the wrong answer.

Share this post


Link to post
Share on other sites
9 hours ago, Nashi said:

Wouldn't it be wonderful if all the questions were this easy. More protection questions is what I need.

I agree. Not sure what % of questions on the actual exam are of this level.

Share this post


Link to post
Share on other sites
17 hours ago, Messi said:

I saw similar question in one of the Complex Imag test. I did the same thing of including the angle & got the wrong answer.

 

On 3/10/2019 at 9:45 AM, roy167 said:

Good little problem, I calculated it wrong (35.15% )by using angle. That goes to show there are so many ways one can be tipped even with simple problems! I have never come across a problem which gave an angle and asked to find voltage regulation.  

For Voltage Regulation just remember it's the magnitude of the full load and no load voltage so you would not use the angle.

\text{Percent } VR = \frac{|V_{nl}| - |V_{fl}|}{|V_{fl}|} \times 100

Share this post


Link to post
Share on other sites
20 minutes ago, daydreambeliever said:

 

For Voltage Regulation just remember it's the magnitude of the full load and no load voltage so you would not use the angle.

\text{Percent } VR = \frac{|V_{nl}| - |V_{fl}|}{|V_{fl}|} \times 100

Now I know, I did not even look up the formula and did it from my head not knowing there is magnitude sign in the formula. Attention to every little detail is so key. You may not even need to look up the formula and think I got this right, you may know the steps, process everything and still could end up picking up the wrong answer, unless you have solved a similar problem before hand. 

 

When you use angle you get 35% regulation which seems odd, but you can't go by that because, it is a problem and not real word situation. Otherwise such a high regulation should make you rethink.  

Edited by roy167

Share this post


Link to post
Share on other sites
6 minutes ago, roy167 said:

Attention to every little detail is so key.

Yes! I've messed up several problems lately because of this. Formulas that I thought I had memorized correctly but made small errors. I've also made several errors due to entering information into my calculator too quickly and fat-fingered the wrong button. When you solve a problem and your answer is NONE of the choices you know you've REALLY screwed up! :laugh:

Share this post


Link to post
Share on other sites
1 hour ago, daydreambeliever said:

 When you solve a problem and your answer is NONE of the choices you know you've REALLY screwed up! :laugh:

I would take that any day over, you solve the problem incorrectly and answer is one of the choice! If the answer doesn't match, at least that forces you to rethink!   

Share this post


Link to post
Share on other sites
13 minutes ago, roy167 said:

If the answer doesn't match, at least that forces you to rethink!

HAHA...very true!

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

×
×
  • Create New...