Voltage Line or Phase Voltage

[BatmanisWatching1987]

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]

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]

I also thought it was V_L but it was actually V_P

Since I was trying to first find the Vdrop, then use S_3P = sqrt(3)*Vdrop*I to find he value for P+jQ

But I needed to use the formula S_3p=3*Vdrop*Ip to get the correct answer

 

[gar]

190313-2031 EST

Batman:

You need to clearly read and analyze the question. If you are given R and I what is the dissipation in R?

.

 

[LarryFine]

According to my EE friend, a transmission line voltage always refers to the line-to-line voltage.

 

[iceworm]

I also thought it was V_L but it was actually V_P

Since I was trying to first find the Vdrop, then use S_3P = sqrt(3)*Vdrop*I to find he value for P+jQ

But I needed to use the formula S_3p=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 V_L but it was actually V_P

Okay - So why did you ask?

the worm

 

[BatmanisWatching1987]

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 V_L but it was actually V_P. Hence why, I was wondering if I missed read the problem knowing it was actually V_P given.

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

 

[gar]

190313-2434 EST

Batman:

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

.

 

[xptpcrewx]

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.

 

[Rho_HV]

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]

Disclaimer still in effect

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

I initially used P=3*I^2*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]

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]

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]

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]

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.