Unbalanced loads on delta primary system?

[Hahlbeck]

How do the primary phases balance out on delta primary distribution systems, given the unbalanced load on the three phases and the lack of a neutral to carry the difference of the unbalanced current? I've been told the equipment (transformers, etc) being attached line-to-line allows the primary to balance itself somewhat, and also that some stray current will make it's way to ground. Anyone have a more detailed explanation, or is anyone aware of any resources offering a good explanation of the relevant concepts/theories?

It seems that they're rare enough in the US these days that not a lot of folks are very familiar with the theory. I'm five years into doing overhead design work and this is my first time encountering delta circuits. The work I'm doing doesn't really require perfect understanding but I'd like to understand a little more about how it works.

Thanks for reading and for any insight you've got!

- Hahlbeck

 

[Ingenieur]

the phase currents will be unbalanced as will their sum, the line currents
no ground current

http://www.electrical-engineering-assignment.com/12-unbalanced-delta-connected-load

 

[electrofelon]

How do the primary phases balance out on delta primary distribution systems, given the unbalanced load on the three phases and the lack of a neutral to carry the difference of the unbalanced current? I've been told the equipment (transformers, etc) being attached line-to-line allows the primary to balance itself somewhat, and also that some stray current will make it's way to ground. Anyone have a more detailed explanation, or is anyone aware of any resources offering a good explanation of the relevant concepts/theories?

It seems that they're rare enough in the US these days that not a lot of folks are very familiar with the theory. I'm five years into doing overhead design work and this is my first time encountering delta circuits. The work I'm doing doesn't really require perfect understanding but I'd like to understand a little more about how it works.

Thanks for reading and for any insight you've got!

- Hahlbeck

Here in central NY, I would say a majority of the distribution is 4800V delta. Super prevalent. Some people who grew up around MGN's think thats the way it always is - not so.

Think about it: How is a delta any different from a wye in terms of balancing? You can connect more stuff to two phases of a wye just like you can a delta.

 

[Ingenieur]

I think he thinks since in a wye the n carries the imbalance
he thinks the delta has a similar thing, possibly the earth

the sum of currents to a delta = 0, balanced or unbalanced
just like a wye without a neut
the wye is a bit easier to comprehend due to the centerpoint
kcl all i summed at a node = 0 or entering = leaving

 

[Hahlbeck]

I think he thinks since in a wye the n carries the imbalance
he thinks the delta has a similar thing, possibly the earth

the sum of currents to a delta = 0, balanced or unbalanced
just like a wye without a neut
the wye is a bit easier to comprehend due to the centerpoint
kcl all i summed at a node = 0 or entering = leaving

Yeah, you're right that I was thinking the delta has a similar thing to the next on a wye circuit... I used to work with transmission line design where there's no neutral, because the 3 phases are assumed to always be balanced. Then I started working on wye distribution circuits and the neu made sense as carrying the phase imbalance. Now delta and it just doesn't make intuitive sense to me... I'm not quite following the explanation of the sum of currents being zero but thanks for that. Gives me a more specific direction to look into.

 

[Hahlbeck]

Here in central NY, I would say a majority of the distribution is 4800V delta. Super prevalent. Some people who grew up around MGN's think thats the way it always is - not so.

Think about it: How is a delta any different from a wye in terms of balancing? You can connect more stuff to two phases of a wye just like you can a delta.

Thanks for the reply - interesting that it's very prevalent in central NY. Any chance you know of any books or manuals that folks there use when they're just starting to learn about it?

 

[Ingenieur]

look at it like this, you only have 3 wires, and all current sent, can only return on those wires regardless of load configuration


assume unbalanced ph loads of:
ph a = 100/0 deg = 100 + 0j
ph b = 50/120 deg = -25 + 43.3j
ph c = 0/-120 deg = 0 - 0j

so summing line currents: (signs matter: all line i's leaving, pha and phb into ia, phc into ib)
pha + phb = line ia = 100 + 0j +(-25 + 43.3j) = 75 + 43.3j = 86.6 / 30 deg
+(-phb) + phc = ib = -(-25 + 43.3j) + 0 - 0j = 25 - 43.3j = 50/-60
+(-phc) +(- pha) = ic = -0 + 0j -100 - 0j = -100 = 100/180

I total = ia + ib +ic = 75 + 43.3j + 25 -43.j -100 = 0 + 0j = 0/0 deg

 

[kwired]

Same thing happens in majority of the dry type transformers installed (3 phase of course). Primary winding is delta connected in most cases. Doesn't matter if secondary is wye or delta - any unbalancing of load will result in unbalanced current on the primary, but no matter how unbalanced it is, all current is on the three supply conductors.

 

[GoldDigger]

If you are having trouble convincing yourself, just consider the case of a single line to line load on a delta.
The currents on the two line wires are equal and opposite, while the third line wire carries no current.
Extending that, you can put any load you want on each line to line phase and then just add up (vectorially) their line current values to get the total current.
What I find interesting about a single phase load is the from the distribution transformer's perspective that load current is supplied by the parallel circuit of the directly connected coil and the series combination of the other two coils.



Sent from my XT1585 using Tapatalk

 

[kwired]

What I find interesting about a single phase load is the from the distribution transformer's perspective that load current is supplied by the parallel circuit of the directly connected coil and the series combination of the other two coils.



Sent from my XT1585 using Tapatalk

And if you lose one of the incoming supply lines you still have output on the secondary, primary current just gets distributed to the two working lines.

 

[GoldDigger]

And if you lose one of the incoming supply lines you still have output on the secondary, primary current just gets distributed to the two working lines.

Yes, a closed delta becomes instead an open delta with all of the performance limits that entails. But you still have three phase voltage available. In a wye source, however, losing one secondary winding leaves you with a totally missing phase.

Sent from my XT1585 using Tapatalk