Current Unbalanced

Figure 12.5 Equivalent stator currents during unbalanced voltage...

The unbalanced voltage will produce an additional rotor current at nearly twice the supply frequency. For example, for a 2% slip, i.e. a slip of 1 Hz, the negative sequence stator current, due to an unbalanced supply voltage, will induce a rotor current at a frequency of (2/- 1) = 99 Hz for a 50 Hz system. These high-frequency currents will produce significant skin effects in the rotor bars and cause high eddy current and hysteresis losses (Section 1.6.2(A-iv)). Total rotor heat may be represented by... 

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. 

Figure 20.19 In a healthy system the unbalanced current (other than a ground fault or phase to phase and ground fault) will flow through the neutral and not the ground...

This CBCT will detect the unbalanced current... 

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. 

Consider a bus duct having a rated current of 4000 A and an unbalanced current in the middle phase of 4400 A. Determine the size of the reactor to achieve a balanced voltage system. 

Designing a reactor for the middle phase to balance a large unbalanced current-carrying system... 

---