Voltage Line or Phase Voltage

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BatmanisWatching1987

Senior Member
Location
NJ
Occupation
Jr. EE
How do you determine if the voltage in a given problem is Line or Phase Voltage given unless otherwise stated.

Here is an example

"A three phase, 230 kV transmission line has a reactance of 50 ohms per phase and aresistance of 10 ohms per phase. The current per phase is 100 amps. What are the total active
power losses in the transmission system?"
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Disclaimer - I don't do transmission lines (except for the one on the PE test) Translation - I may be all wet.
How do you determine if the voltage in a given problem is Line or Phase Voltage given unless otherwise stated.

Here is an example

"A three phase, 230 kV transmission line has a reactance of 50 ohms per phase and aresistance of 10 ohms per phase. The current per phase is 100 amps. What are the total active
power losses in the transmission system
?"

Assumption: This is a PE practice problem. Because if it were life, you would know.

Unless specified otherwise, the voltage will be line to line.

Curiosity Q: For this problem why would it matter?
 

BatmanisWatching1987

Senior Member
Location
NJ
Occupation
Jr. EE
I also thought it was VL but it was actually VP

Since I was trying to first find the Vdrop, then use S3P = sqrt(3)*Vdrop*I to find he value for P+jQ

But I needed to use the formula S3p=3*Vdrop*Ip to get the correct answer
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
According to my EE friend, a transmission line voltage always refers to the line-to-line voltage.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
I also thought it was VL but it was actually VP

Since I was trying to first find the Vdrop, then use S3P = sqrt(3)*Vdrop*I to find he value for P+jQ

But I needed to use the formula S3p=3*Vdrop*Ip to get the correct answer

Adding to gar's post: Yes, you do need to work the power per phase. Cause there is real power lost in each phase. But the V(L-L) or V(P-N) don't have any thing to do with it.

Or you could use P = Real(3*(I^2)*Z) The vars don't count.

If this is a PE test practice, I highly suggest you purchase the solutions manual.

I also thought it was VL but it was actually VP
Okay - So why did you ask?

the worm
 

BatmanisWatching1987

Senior Member
Location
NJ
Occupation
Jr. EE
Adding to gar's post: Yes, you do need to work the power per phase. Cause there is real power lost in each phase. But the V(L-L) or V(P-N) don't have any thing to do with it.

Or you could use P = Real(3*(I^2)*Z) The vars don't count.

If this is a PE test practice, I highly suggest you purchase the solutions manual.


Okay - So why did you ask?

the worm

I thought it was VL but it was actually VP. Hence why, I was wondering if I missed read the problem knowing it was actually VP given.

I initially used P=3*I2*Z but then I wanted to check my answer but I got 2 different answer when I used S3P = sqrt(3)*Vdrop*I
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
190313-2434 EST

Batman:

From your last post I am not sure that you clearly, intuitively, understand the question, assumptions, and the solution.

.
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
How do you determine if the voltage in a given problem is Line or Phase Voltage given unless otherwise stated.

Here is an example

"A three phase, 230 kV transmission line has a reactance of 50 ohms per phase and aresistance of 10 ohms per phase. The current per phase is 100 amps. What are the total active
power losses in the transmission system?"

Voltages are always expressed as line values unless specified otherwise. The nature/context of the problem will usually be obvious if they referring to line-to-neutral or line-to-ground values.
 

LMAO

Senior Member
Location
Texas
How do you determine if the voltage in a given problem is Line or Phase Voltage given unless otherwise stated.

Here is an example

"A three phase, 230 kV transmission line has a reactance of 50 ohms per phase and aresistance of 10 ohms per phase. The current per phase is 100 amps. What are the total active
power losses in the transmission system?"

This is a poorly written problem. It should either give you the line current or specify whether the load is Y or Δ. Line voltage is irrelevant here.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Disclaimer still in effect

I thought it was VL but it was actually VP. Hence why, I was wondering if I missed read the problem knowing it was actually VP given.

I initially used P=3*I2*Z but then I wanted to check my answer but I got 2 different answer when I used S3P = sqrt(3)*Vdrop*I
It is line loss, VL, VP doesn't matter. As you note, Vdrop does.
Consider:
Power dissipation in each phase conductor = Vdrop*I​
(sqrt(3) is not part of this) The only things that matter are the phase conductor current and the Voltage drop on that conductor.​

Vdrop = I*Z

Substituting

Complex Power(each phase conductor) = (I*Z)*I = I^2*Z

But you only want the real part
P = Real(I^2*Z)
Total power dissipated = 3*P​


Still curious:
Q1: How is it you know the "230KV" was VP, and not VL?
Q2: Why Would it matter?


The Worm (turneth and begins to dig)
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
Q1: How is it you know the "230KV" was VP, and not VL?
Q2: Why Would it matter?

Agreed. What is important is the current, the impedance, and the voltage drop. The voltage itself isn't relevant. Just because information is given in a problem setup, that doesn't mean it is needed to solve the problem.
 

Sahib

Senior Member
Location
India
Agreed. What is important is the current, the impedance, and the voltage drop. The voltage itself isn't relevant. Just because information is given in a problem setup, that doesn't mean it is needed to solve the problem.
Perhaps he wanted to crosscheck?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
190314-1254 EDT

In looking at this question --- it was very well designed and written.

It provided sufficient information for one to solve the problem without having to make assumptions, and it provided enough additional information to confuse one that did not really understand the subject and problem.

.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
Perhaps he wanted to crosscheck?

Crosscheck what? The voltage doesn't enter into any of the equations. The differential voltage does, but that's just voltage drop.

When there is information in a test problem that is ambiguous, e.g., the voltage in a transmission line is given but it isn't clear whether it's phase to neutral or phase to phase, that's a pretty strong clue that the information isn't needed to solve the problem.
 
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