Voltage Drops; ETAP vs formula, please help?

[vagojr]

Dears,

I am involved in a Load Flow analysis with ETAP; for that purpose I have included in the model the transformers, switchgear, static and rotative loads and cables.

The 3ph cable sections selected is by means of ampacity and voltage drop calculation by means of the following formula;
Vd = sqrt(3) * L/1000 * I * (R*cosPHI + X*senPHI), being cosPHI=Power Factor. The criteria is not to reach to the 2% voltage drop from switchgear to MCC/Panel; and 3% from MCC/Panel to the end-load.

The case is that after performing the Load Flow study (considering the worst case) the values of voltage drops in the ETAP are very very different spreaded by the formula. :?

So my conclusion is that the sections of long tracks could be lower but this would not comply with formula!! :happysad:
I would like to know if someone could help me to understand why this is happening. Thank you in advance

FYI: already checked are the cable length, Resistance and Reactance to be the same in ETAP and in formula (L, R and X). Results:
Trafo with tapchanger => switchgear => MCC => static & rotatory loads

• VD at MCC by Formula: 1.44%
• VD at MCC by ETAP (V=100% at switchgear) = 0,55%
• VD at MCC by ETAP (V=90% at switchgear) = 0,88%

 

[Smart $]

If results are different you have to confirm same formula and variables are being used.

The IEEE exact voltage drop formula is:

Vd = es + IRcosφ + IXsinφ - sqrt[es^2 - (IXcosφ - IRsinφ)^2]​

However, the difference between this formula and yours is barely significant if not insignificant.

 

[vagojr]

If results are different you have to confirm same formula and variables are being used.

The IEEE exact voltage drop formula is:

Vd = es + IRcosφ + IXsinφ - sqrt[es^2 - (IXcosφ - IRsinφ)^2]​

However, the difference between this formula and yours is barely significant if not insignificant.

thank you.
Anyway I am trying to investigate further and I am reaching to the conclusion of that the issue is the static loads (constant impedance), ufff... someone able to give me a shortcut?? please

 

[Phil Corso]

Vajor,..

I suggest you read the thread "Transformer Overload"! It specifically addresses your query! The solution is to use the %V-Regulation formula. It is much simpler than subtracting Er from Es!

Regards, Phil Corso

 

[vagojr]

Vajor,..

I suggest you read the thread "Transformer Overload"! It specifically addresses your query! The solution is to use the %V-Regulation formula. It is much simpler than subtracting Er from Es!

Regards, Phil Corso

thank you a lot Phil, I will investigate and let you know
regards

 

[rian0201]

Load flow used in etap is an iterative process.. This is more exact.

If you want to calculate it hand it would take a lot of time, repeating process until the answer is arrived. Plus if you will not simplify the diagram, the you have to learn ybus matrices to further simplify the process, by means of newton raphson and other methods.

Plus the effect of type of loads will also matter.

Also take note that etap will use positive tolerance in load flow analysis.


Sent from Mars

 

[Phil Corso]

... If you want to calculate it hand it would take a lot of time, repeating process until the answer is arrived. Plus if you will not simplify the diagram, the you have to learn ybus matrices to further simplify the process, by means of newton raphson and other methods. Plus the effect of type of loads will also matter. Also take note that etap will use positive tolerance in load flow analysis. Sent from Mars

Please return to Earth and read the thread, "Transformer Overload". The method recommended, "%V-Regulation" requires five inputs: three related to the Xfmr (kVA, %Z, X/R); and two related to connected Load (kVA and its PF).

Contact me if you would like to receive a copy of an Excel-solution!

Regards, Phil Corso

 

[rian0201]

No need, and im not interested sorry on that. When you said five inputs, how about constant impedance loads? Constant current? Or combination on them? How about unbalance loads?

Your method is good for small systems.

I rather use matlab or mathcad and have a complete iterative process since the first question deals with etap that uses iterative LF.


Sent from Mars

 

[vagojr]

Please return to Earth and read the thread, "Transformer Overload". The method recommended, "%V-Regulation" requires five inputs: three related to the Xfmr (kVA, %Z, X/R); and two related to connected Load (kVA and its PF).

Contact me if you would like to receive a copy of an Excel-solution!

Regards, Phil Corso

Thank you all for your kind interest and good directions.
I would like to receive this excel in order for me to understand your approach and see how I can use it as a best approximation of the formula.
Regards

 

[vagojr]

since the first question deals with etap that uses iterative LF.

Thank you very much rian0201, appreciated a lot your interest.
The case is that I though that the difference between the simplified formula and etap was not such great than finally is... lesson learnt hehe
Do you know if ETAP can calculate automatically the best cross section also by evaluating the voltage drop in addition to the ampacity?

 

[rian0201]

Yes, you need to have the cable ampacity module. You need to have the load current via load flow or user defined. Short ckt withstand, and voltage drop, there is also an option that use the transient loading and motor starting.

Etap uses the sizes that available on the database, you can place own fav vendors.

I have also an inquiry like before regarding load flow and short circuit. I tried to do a manual calculation and i have proved it two years ago. But i used mathcad to simplify the process. And i disabled the tolerance effect to check the results.


Sent from Mars

 

[steve66]

Are you sure you are using the right voltage? If I average your ETAP numbers, the ratio for ETAP to the Formula is about the same as 208 to 480V.

 

[rian0201]

Average? Can you explain further? Why average? What is the basis on the ratios? Is that sending / receiving ratio? For me doesn't make any sense. Sorry about that.


Sent from Mars

 

[vagojr]

Are you sure you are using the right voltage? If I average your ETAP numbers, the ratio for ETAP to the Formula is about the same as 208 to 480V.

I must apologize for the first post in where the VD values were not accurate; I was trying to simplify.
The real case is this;

For a 400-230V lighting system we are studying if it is better for cable sections to use Busbar or Cable from the secondary side of 1000kVA MV/LV transformer up to the LV lighting center; so we analyzing the Voltage Drop (VD) in a subpanel (fed at 380m from lighting center); the case is the following;

• Absolute VD limit from 2ry side of transformer up to the subpanel = 3%
• Absolute VD limit from 2ry side of transformer up to LC in case of cable = 0.5%
• VD up to LC in case of busbar = 0%

Using the VD formula for cable: VD = sqrt(3)*I*L*(R*cosPHI+X*senPHI)/n , being n=number of cables per phase and L, R& X characteristics of cable related with cross section.

Same scheme but only changing Busbar to Cable in Etap and analyzing every transforer tap, the ETAP results are the following;

• VD at the subpanel with BUSBAR = 4% (in real scenario is required higher cable section?!)
• VD at the subpanel with incoming CABLE = 2.4% (?!?!) (in real scenario is required less cable section?!)
• Consumptions in Cable study is higher than in Busbar study, but the losses are less.

MY CONCLUSION; this is because the loads are static loads (constant impedance): the current incresases because the voltage increases with BUSBAR (more voltage in Lighting Center). Therefore as the consumption increases, also the VD does.

I am right or am I missing something?!
Appreciated a lot your help

Thank you all for your time.
Regards

 

[Phil Corso]

... The IEEE exact voltage drop formula is: Vd = es + IRcosφ + IXsinφ - sqrt[es^2 - (IXcosφ - IRsinφ)^2]...

o It's only "Exact" if you include the effect of line-to-line, and line-to-neutral capacitance.

o You neglected to mention it is the Line-to-Neutral V-Drop.

o If the line-to-line V-Drop is required, then, Vd is multiplied by Sqrt(3).

o You neglected to mention that es is the line-to-neutral voltage of the source.

o R and X are the total resistance and reactance for the entire conductor length.

Regards, Phil Corso

 

[Smart $]

o It's only "Exact" if you include the effect of line-to-line, and line-to-neutral capacitance.

o You neglected to mention it is the Line-to-Neutral V-Drop.

o If the line-to-line V-Drop is required, then, Vd is multiplied by Sqrt(3).

o You neglected to mention that es is the line-to-neutral voltage of the source.

o R and X are the total resistance and reactance for the entire conductor length.

Regards, Phil Corso

Neglected it is.

 

[Phil Corso]

Smart A...

Don't feel bad! 3 out of 5 is 60%!

 

[Smart $]

Smart A...

Don't feel bad! 3 out of 5 is 60%!

Don't feel bad, Phil. I posted 100% more of a formula than you did. :thumbsup:

 

[vagojr]

I must apologize for the first post in where the VD values were not accurate; I was trying to simplify.
The real case is this;

For a 400-230V lighting system we are studying if it is better for cable sections to use Busbar or Cable from the secondary side of 1000kVA MV/LV transformer up to the LV lighting center; so we analyzing the Voltage Drop (VD) in a subpanel (fed at 380m from lighting center); the case is the following;

• Absolute VD limit from 2ry side of transformer up to the subpanel = 3%
• Absolute VD limit from 2ry side of transformer up to LC in case of cable = 0.5%
• VD up to LC in case of busbar = 0%

Using the VD formula for cable: VD = sqrt(3)*I*L*(R*cosPHI+X*senPHI)/n , being n=number of cables per phase and L, R& X characteristics of cable related with cross section.

Same scheme but only changing Busbar to Cable in Etap and analyzing every transforer tap, the ETAP results are the following;

• VD at the subpanel with BUSBAR = 4% (in real scenario is required higher cable section?!)
• VD at the subpanel with incoming CABLE = 2.4% (?!?!) (in real scenario is required less cable section?!)
• Consumptions in Cable study is higher than in Busbar study, but the losses are less.

MY CONCLUSION; this is because the loads are static loads (constant impedance): the current incresases because the voltage increases with BUSBAR (more voltage in Lighting Center). Therefore as the consumption increases, also the VD does.

I am right or am I missing something?!
Appreciated a lot your help

Thank you all for your time.
Regards

Could you please support me, please?
Thank you
regards

 

[rian0201]

Your inquiry is not clear. Need SLD. And again, etap uses load flow algorithm, which is an iterative process. Even if you use "exact" formula, it will never come close. Load flow is a lot more "exact" than the "exact" formula. I have been saying this and you are still insisting to prove it otherwise. Never compare it in the first place.


Sent from Mars