Basic 3 phase power balance theory 1

[dav08a]

Dear all,

i have been in Power industry for quite a few years, yet i still dont understand this simple basic theory. hope the expert here could enlighten me.

The power system is 3 phase 120deg balance. however, when it come to consumer side, it will be step down to 415Vac and distribute single phase (230Vac P-N) to household.

a) how the utility do to make sure the consumer use the power in balance way? i know the utility will distribute the ABC, and by probability to balance it. but still some household use more power than other, and there is possibility 3 phase cannot get balance.
b) if the 3 phase are not balance, this mean they will be current flowing in neutral wire. then, isn't this will trip the ELCB?

please advise.

 

[waross]

The household current over a small sample will probably not balance.
Current will be flowing in the neutral wire.
The residual current measuring device will measure the neutral current in addition to the three phases. Including the neutral current assures that the currents will sum to zero. In the event that the three phase currents plus the neutral current do not sum to zero, the device infers that the unbalance current is flowing trough the ground path and initiates a trip.

Bill
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"Why not the best?"
Jimmy Carter

 

[PHovnanian]

if the 3 phase are not balance, this mean they will be current flowing in neutral wire. then, isn't this will trip the ELCB?

Probably not. It depends on the neutral and ground impedances. Typically, neutral impedances are quite low compared to ground impedance, so an imbalance returning in the neutral will not cause an ELCB (the voltage sensing kind) to trip. Current sensing RCDs only look at the phase and neutral currents and trip only when these don't sum to zero. So they are insensitive to load current imbalances.

 

[ScottyUK]

Are the voltage sensing type still made? They were banned by the UK wiring regulations years ago. I only ever saw a handful and I haven't seen one for maybe twenty years. I suppose if I worked on domestic or light commercial installations I might have seen a few more, but they certainly aren't common these days.


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If we learn from our mistakes I'm getting a great education!

 

[pwrengrds]

Utilities do balance the circuits by adding load to each phase to make up a balanced 3 phase circuit. Each house or single phase line is a very small load, and the loads are all about equal, all they have to do is put every third one on a phase and if the unbalance is too, large swap some houses to a different phase. Adding residential housing is a slow process and they have a lot of time to balance it.

If they did put all of the houses on one phase it would trip that phase on overload and on ground current, but they are smart enough to avoid that.

It's the same in any three phase panel that feeds single phase loads, they have to be balanced or it will cause problems in cases of large imbalances.

 

[dav08a]

How about some household that actually direct take in 3 phase power from the utility? they have the 3 phase wiring in the house, which by theory they should split the 3 phase balance to household electrical appliance. however in real life, there must be some unbalance, in this case, all the current flow in neutral line?

 

[PHovnanian]

Three phase customers should balance their single phase loads across each phase to the greatest extent possible. That will depend on the nature of the three phase supply and the loads.

For example, here in the USA there are Y-D transformer configurations that supply one center tapped 240V phase (to supply two 120V groups of loads) plus a 'wild leg' that completes the delta. Three phase 240V loads are connected to all three lines and are for all practical purposes balanced. It is also possible to connect 2 wire, 240V single phase loads across any two lines. Given enough of these, it is possible to balance them as well. But 120V loads can only be connected between the one center tapped delta winding. Because of this, achieving a perfect balance is not likely.

This should not be a problem on a 230/415V Wye system, as it is symmetrical and there are no such restrictions on single phase load connections.

In practice, there will be some imbalance. And that will result in neutral currents. Neutral currents are not a major problem as long as neutral conductors and transformers are sized to accommodate them. They also should not be a problem for modern RCCB/Ground Fault protection systems as these look at the vector sum of line and neutral currents and trip only when these don't sum to zero. Imbalance can be a problem for utility distribution protection, as most utilities combine the neutral and protective ground functions in one conductor. Therefor a large load imbalance cannot be differentiated from a ground fault and, if the ground fault trip levels are set low, an imbalance can result in nuisance GF tripping. A common scenario is one where a three phase distribution system feeds numerous single phase taps. During a windstorm and given a certain amount of bad luck, several of these laterals will be knocked off line (when their fused cutouts blow) all tapped off the same phase. This can result in the substation load becoming unbalanced and tripping the entire circuit off line. In my area, it is also common practice for overhead lines to be phased A-B-C, with the B phase always in the center (pole top) position). Even the fused three phase laterals tend to get their A and C phase fuses blown more often than B phase (that's where the tree branches hit). That too can lead to a circuit imbalance.

 

[waross]

Three phase residential and small agricultural applications are for the most part for a single large motor or group. Residential, air conditioning and agricultural for a large irrigation pump or a group of grain drier fans.
There will be a 120:240 Volt service similar to the common North American residential service.
One or two transformers are added to supply the three phase.
With a wye:delta transformer bank there is a choice of connecting the wye point to the system neutral or leaving it floating. Both methods have issues. Pick your issue and live with it.
I prefer the two transformer "Open delta" connection. I believe that this may be called a "V" connection in IEC land.
The open delta configuration may have phase angle error and/or voltage unbalance issues on a long rural distribution line with a voltage drop on the neutral. A simple solution is to oversize the motor 10%, 15% or 20% and the motor will have the extra capacity to do a good job of bringing the service close to a balanced condition.

Bill
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"Why not the best?"
Jimmy Carter