×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# Three phase unbalanced load flow2

## Three phase unbalanced load flow

(OP)
A balanced 240V a.c. phase-to-phase three phase ( A-B-C ) star connected source of equal negligible positive and negative sequence impedance and no connection to earth (ground) supplies single phase 240V a.c. incandescent lamps.
Each lamp is connected between 240V phase-to-phase at 5 m intervals along a three core ( A-B-C ) cable with the objective of achieving three balanced line cable currents ( A-B-C ) . Eg. lamp connections A-B, B-C, C-A .....
Can anyone suggest how the load flow currents and can be calculated

### RE: Three phase unbalanced load flow

The incandescent lamps in the said case are connected in Delta fashion and are said to be presenting balanced load among all three phases.
Hence, the current in A-B-C lines will be that in any of the three circuits (same current as the loads are balanced) multiplied by sqrt3.

### RE: Three phase unbalanced load flow

Dear Mr. EddyWirbelstrom (Electrical)(OP)19 Sep 22 11:18
" ... A balanced 240V a.c. phase-to-phase three phase ( A-B-C ) star connected source.... no connection to earth .... supplies single phase 240V a.c. incandescent lamps. Each lamp is connected between 240V phase-to-phase....( A-B-C ) cable ...achieving three balanced line cable currents ( A-B-C ) . Eg. lamp connections A-B, B-C, C-A .....Can anyone suggest how the load flow currents and can be calculated "
1. Your source ABC is in Y formation. Neutral is non-earthed. VAB =VBC =VCA =240 V .
1.1 Attention: Take note that VAB , VBC, VCA are 120deg apart.
2. Your 240V lamps are connected in D formation, across AB, BC, CA.
3. You NEED to know the wattage of the lamps in order to calculate the current through each lamp.
3.1. The Current through each lamp will be the same=balance = i =W/V, where V=240V , W=wattage W.
4.1. The current from the source will the same=balance IA=IB=IC ... A.
5. The source current IA=IB=IC = 1.732 x current through each lamp, see above 3.1.
Che Kuan Yau (Singapore)

### RE: Three phase unbalanced load flow

If the phases are not balanced, you can calculate the current from B-A, A-C, and C-B vectorially, knowing the Ø-Ø voltage and the resistance of the lamps (V²/P where V is rated voltage and P is rated watts). IBA = VBA/R, IAC = VAC/R, ICB = VCB/R. Then calculate the line current vectorially IA = IAC - IBA, IB = IBA - ICB, IC = ICB - IAC.

### RE: Three phase unbalanced load flow

If we neglect the supply cable impedance and consider sum of all lighting lamps per each phase, we get a triangle of unbalance resistances. If we transform this triangle into a star, we get the currents per phase. The voltages are equals 240/sqrt(3).

### RE: Three phase unbalanced load flow

2
Zanoter4,

OK except that there is no neutral, so the phase voltages are not defined. If you assume balanced 3Ø voltages, then if the loads are unbalanced, the phase currents will not sum to zero. They have to aum to zero because there is no neutral. See the attached Excel workbook.

### RE: Three phase unbalanced load flow

Thank you very much, jghrist. Because the neutral is not in the centre VAN,VBN and VCN are not equal to the phase voltages and then the sum of the currents is not equal to zero. It is necessary to calculate the potential of the neutral point first.

### RE: Three phase unbalanced load flow

The phase voltages of an ungrounded Y are defined with respect to each other, but not ground. For this problem all of the loads are between phases so the relationship of ground to any phase does not matter.

If it is convenient to choose to pick a point in the center of the Y for the purpose of calculation, the fact that it is not defined with respect to ground does not matter either.

Caution - sometimes the ground reference matters, it depends on the question.

### RE: Three phase unbalanced load flow

Thank you, FacEngrPE . In the open post it is written "star connected source”. New VN (load neutral point potential) it is the voltage referred to the source star connection point -grounded or not.

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

#### Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a partâ€™s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!