Unbalance three phase loads 2

[RonH]

I need to know how much current unbalance is acceptable for a three phase delta load. One phase will draw 37.5A, the second one 33A, and the third one 30A. I calculated the unbalance of current to be about 12%. A potential problem is that one of the legs will be switched off, thus dropping the current to zero in that leg. Is this acceptable? Or do I need to balance the loads. The voltage unbalance is less that 3%.

 

[jbartos]

Please, would you clarify nature of loads involved. Motors are very vulnerable to the voltage unbalance (e.g. over 3%). Resistive heaters could potentially remain unharm. However, they may produce less heat.

 

[RonH]

The loads for the above questions are: phase 1 and 2 are fluorescent lamps, and phase 3 are HID type lamps. Both types of lamps use ballast. The system has been power factor corrected to 0.9 to 1.0. The HID lamps would be turned off at various times.

RonH

 

[jbartos]

The lighting systems are known to have the need for great diversity of connections to branch circuits and examination of branch circuits in lighting panels to have them balanced. The three-phase lighting panels with many branch lighting circuits tend to statistically balance the three-phase power distribution system for lights and load power supply (transformers) more symmetrically. If one leg in the closed delta three-phase transformer is lightly loaded, then the situation resembles open delta power supply. This open-delta power supplies are widespread in mixed residential-commercial areas. There are two overhead transformers installed on poles, which may be frequently seen. The three phase open-delta power supply power quality is considered reasonably good by the Utilities in spite of occasional noticeable three-phase voltage unsymmetries.

 

[ojanja]

A 2 to 4% unbalnce is acceptable provided the loads are not motor type. Nvertless you should try to avoide current unbalance. Unbalnce can cause neutral conductors to over heat and hence causes nuisance tripping.

 

[jbartos]

Suggestion to "ojanja Jan 3, 2001 posting": The 3-phase delta connection does not have neutral conductors; therefore, the overheat and nuisance tripping are inapplicable.

 

[roliver]

I think the question here should be how much voltage unbalance between phases in a delta load can be tolerated before excessive circulating current develops in the windings of the transformer. A delta-connected load served from a close delta connected secondary winding of a transformer that is wye(ungrounded) on high side, tends to balance the phase voltages by generating the circulating current in the winding. I am not sure based on original question what scenario we have here. Except the very first response asking more clarification and reference to voltage unbalance of 3%, would be on the right track.

 

What about the affects of an unbalanced load on a generator.

I have a similar scenario. My power factor is anticipated to be between 0.8 and 0.9, but my imbalance looks to be to much. The current between the phases varies with one phase at 35 amps and another at 13 amps the third phase would be also around 13 amps. All the loads are single phase (lighting, power supplies battery chargers, inverters). The three phases are "Y" connected to a distribution panel and the three phases are basically used independantly. The whole system could have been designed with a single phase.

Other than high neutral currents what kind of problems would such an imbalance create.

 

[Stoshly]

The system is 4 wire. All devices used are 220 volts.