Loading Single phase, unbalanced

Voltage unbalance (negative phase sequenceJ This causes negative sequence components and results in excessive heating of the motor windings. An unbalanced voltage may occur due to unevenly distributed single-phase loads. 

When a three-phase four-wire system feeds non-linear or single-phase loads this balance is upset and the unbalanced current flows through the neutral. The same relationship can be expressed as... 

Loads of welding sets, which have intermittent duty cycles, are mostly single phase, cause low p.f. and stress the generator windings intermittently. Unbalanced loads, such as single phase loads, distributed unevenly cause current unbalance and low p.f. 

In a G/F the three CTs will also measure the unbalanced load current, if any, in addition to G/F current. For an appropri.ite setting of the relay, therefore, it will be essential that the likely system unbalanced current be measured and the relay set in excess of this to detect a G/F, For systems feeding single-phase or unbalanced loads, prone to carrying high and widely fluctuating unbalanced neutral currents, it may be difficult to determine ihe likely amount of unbalance and provide a suitable setting for the G/F relay. In such cases use of four CTs or core-balanced CTs (if it is a four w ire system) would be more appropriate. 

In LT systems, where the neutral is groiintled, the neutral as well as the ground conductor tnay have to carry unbalanced currents up to hall the rating of the line currents due to single-phase loads. A ground conductor should also be rated for the same size as the neutral, irrespective of the setting of the relay. 

Star windings are used when a neutral connection is required for earthing or for un-balanced loads (these are usually groups of single-phase loads placed between separate phases of the supply and its neutral. Each group may not be identical and hence the system will be unbalanced). 

Three-phase versions consisting of single-phase units connected in delta-delta, delta-wye, or wye-wye can be unstable when working into unbalanced loads. Increased stability for three-phase operation can be achieved with zigzag and other special winding conhgurations. 

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