Wye connected unbalanced 3 phase grounded neutral

Status
Not open for further replies.

gvajay

Member
In case this is a stupid question then I apologize in advance...

Assuming an unbalanced 3 phase installation results in (lets say) 5A of current on the neutral conductor (grounded), then is that wasted, in that it went to ground directly rather than through an electrical appliance first?

The idea is that incoming power is metered (and paid for) based on what comes into the facility via the 3 hot phases, so that anything leaving the facility (via neutral / ground) was paid for but was wasted via phase imbalance -> neutral -> ground.

I am hoping this is in fact a dumb question resulting in the answer that of course unbalanced 3 phase is both wasteful and expensive for "paying customers" (as opposed to for utilities).

Most of the data centers we come across have 3 phase power incoming, yet (often) use 1 phase 220V circuits to power equipment, and over time as circuits are added moved and equipment is refreshed, what started out balanced, can become badly unbalanced.

Does this cost the datacenter manager real money?


Thanks!

Gabor
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Assuming an unbalanced 3 phase installation results in (lets say) 5A of current on the neutral conductor (grounded), then is that wasted, in that it went to ground directly rather than through an electrical appliance first?

That description is inaccurate.

1) Electricity does not try to go to ground, it wants to go back to its source.

2) The unbalanced current must go through a load (electrical appliance) that is connected Line-Neutral, unless you have a short circuit in your system.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
There are no stupid questions. But there are questions that lead the listener to know that the speaker has a misconception (or put a better way, has an opportunity to learn something new). This is one of those. Let me try to explain.

A three-phase system, such as the one you are describing, has a total of five wires. The ground wire (i.e., the ?equipment grounding conductor?) has no purpose except in the rare event of an equipment fault. So let?s not talk about that one, for now.

Three phase wires and one neutral wire carry current to the loads, and the same four wires carry current back from the load to the source. Current that leaves the source via Phase A might return to the source via Phase B, or via Phase C, or via the neutral wire. I could make similar statements about current that leaves the source via Phase B, and about current that leaves the source via Phase C. If the loads are balanced, then all the current returns to the source via Phase A, or Phase B, or Phase C, and the neutral current is zero. If there is any imbalance, the some current will return to the source via the neutral.

Most of the time, the source is a transformer that is connected in a WYE configuration. That means that the Phase A, Phase B, and Phase C wires are all connected to each other at the center point of the WYE, and the neutral wire is connected to this same point. That is the point from which all currents originate, as they leave the source and head towards the loads. That is also the point to which all currents return. Planet Earth is not a participant in this current flow.
 

gvajay

Member
3 phase power "loss" Opportunity to learn something new is much appreciated - Thanks!

3 phase power "loss" Opportunity to learn something new is much appreciated - Thanks!

Thanks so much for the patient replies (Charlie, Brian, Jim) to my question (as I work to understand this better).

Unfortunately, I think I'm starting to -

To recap - I am trying to create additional value to customers using our power, heat, humidity monitoring solution (in pilot now) which measures circuit level power consumption, (as well as facility temperature (and humidity) in datacenters). Knowing what is consumed is the first step in knowing how to manage it better (virtualization, consolidation projects, etc.). Before / after measurements of consumption yield clear proof of savings, and can can drive future initiatives.

The current (sorry) issue of savings accruing from monitoring and controlling phase imbalances in datacenters where 3 phase is supplied, but 208V single phase is consumed by computing equipment, was thought to be a potentially valuable additional capability that is a byproduct of the available measurements. We know V*A incoming on the 3 phases (measured), as well as consumption on all the circuits (also measured).

Mike Holt's forum has already been the best source of info specific to understanding 3 phase imbalances, and I look forward to digging into (i.e. reading) all the topics as time permits.

So, is there any potentially significant value for an electricity consumer (as above) to balance power consumption between phases? As I understood Charlie's answer, the neutral wire in a wye connection already does that, and that savings would be minimal when only looking at imbalance.

Finally for now, does this phase imbalance affect the facility's power factor? This would be a much smaller effect than the (misunderstood wishful thinking approach) which used supply voltage*imbalance current -> planet earth (as wasted (but paid for) power)?

(BTW, I didn't make that up - an electrician colleague told us he routinely saves factory floor installations up to 50% by fixing phase imbalances that grew over time (additions and changes) - which got me going on adding this to our project)? (In residential power (120V single phase), neutral is connected to ground but incoming = outgoing so the ground wilre current is zero (as I understand how a GFI works), but I thought (and hoped for this project :) ) that a 3 phase imbalance current in neutral might find its way to ground, and therefore create a real savings opportunity. I do see that there is current on the neutral, but no voltage and therefore no power and apparently none of that current ends up going into ground unless something is defective. Still looking for value in presenting phase imbalance information to the datacenter/facility manager.

The pictures and docs refernced below show that the neutral in a 3 phase installation is usually grounded -

From
http://www.teal.com/newsletter/AppsNote15.pdf,

as well as from
http://books.google.ca/books?id=8Zg...=X&oi=book_result&resnum=7&ct=result#PPA74,M1

"Grounding of Delta and Wye Systems -
It is common practice to ground the neutral, or center
point of a Wye connected source. In most cases, this
grounding is required by the United States' National
Electric Code (NFPA-70). By grounding a Wye system,
the voltages to ground are stabilized and controlled."

Thanks and regards,
Gabor
 
Status
Not open for further replies.
Top