3 Phase Power Balanced

[ElectroEnthusiast]

Why does 3 phase needs to be balanced? What is the requirement?
Also, i heard that unbalanced 3 phase is expensive. Why/How so?
How do they balance 3 phase in distribution lines?

 

[chuckey]

If the three phase is not balanced then you get neutral current with a balanced load. The neutral point on a transformer should be at zero volts. In a factory I worked in we had a 6V bulb connected between neutral and true earth and it glower most of the time and occasionally burnt out. In broadcasting where you have 250 KW DC supplies , unbalance in the phases causes causes 50HZ ripple and humbars on the TV pictures. So we had independent voltage stabilisers on each phase. The distribution phase balance is done by automatic tap changers on transformers.
Frank

 

[ElectroEnthusiast]

Practically, i've seen that it is not possible to distribute the load equally. How does the current on neutral happen? Neutral is to be grounded, it will be on zero voltage as long as it is grounded. Do they use DC supply for broadcasting? I'm confused, if they do so then how would 50HZ ripple occur.

Please elaborate both in terms of Wye-network and triangle network.

 

[chuckey]

" Do they use DC supply for broadcasting? I'm confused, if they do so then how would 50HZ ripple occur." If the three phase is balanced the ripple is 300 HZ, the unbalance (extra voltage on one phase!!) results in a 50 HZ component.
"Practically, i've seen that it is not possible to distribute the load equally", its easy as a consumer, the problem is that the mains distribution agencies, would feed a street with three phase with blocks of houses on each phase, in general the currents balance but not exactly, i.e. a supply current of 500 A and a neutral current of 50 A. Whilst our load on the three phase would be balanced to 5 A in a current of 500 A.
Frank

 

[ElectroEnthusiast]

Thanks chuckey, i was able to understand a few things. Any simulations for this?

a supply current of 500 A and a neutral current of 50 A. Whilst our load on the three phase would be balanced to 5 A in a current of 500 A.

Case 1) Block A uses 500A, Block B uses 500A & Block C uses 500A - So balanced.
Case 1) Block A uses 500A, Block B uses 50A & Block C uses 10A - So Unbalanced. What will be the neutral current?

 

[chuckey]

if you draw a vector diagram with 500 vertical, 10 @120 degrees and 50 @ 240 degrees. So you have to resolve the 50 and 10 component into 0 and 90 degree components. So 10 @120 degrees = -5 @0 degrees and 8.66 @ 90. 50 @ 240 = -25 @ 0 degrees and 43.3 @ -90 degrees. so 0 degree components = 500 -5 -25 = 470, 90 degree , 0 (no 90 degree component on the 500 vector) +8.66 - 43.3 = -34.64. So the neutral current is the vector sum of 470 and 34.64 = 471.27 Amps.
Frank

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[FvM]

Practically, i've seen that it is not possible to distribute the load equally.

Yes. For this reason, power distribution transformers use circuits that tolerate unbalanced single phase load up to the rated current. This are particularly delta-wye or wye-zickzack transformers or (with lower unbalanced load capability) wye-wye with compensation windings.

 

[ElectroEnthusiast]

This are particularly delta-wye or wye-zickzack transformers or (with lower unbalanced load capability) wye-wye with compensation windings.

Any diagram/link for this.
Why exactly should the 3 phase be balanced. I know, it is to control neutral current. What happens if unbalanced? I also heard that i will be charged more if i draw current from an unbalanced system. Is it true?