Current Negative phase sequence

Modem relays include a function for detecting the negative phase sequence currents, with settings typically in the range of 10% to 50% of the nominal relay positive sequence current. High power rating motors may need a lower limit than 10%. 

The shape of the curve for negative phase sequence current operations varies with the manufacturer. Some prefer an whilst others an inverse time characteristic. Time settings are typically in the range of 10 to 120 seconds. 

T) Maximum hot - phase heating (positive and negative sequence currents in phase). 

Figure 4.7c shows the fifth harmonic phasors. Note that the phase sequence of the fifth harmonic currents is clockwise and opposite to that of the fundamental. The fifth harmonics are negative sequence harmonics. 

This occurs when the positive and negative sequence components fall in phase in which case the equivalent stator current will become... 

In the event of single phasing, in star-connected windings, one of the phases of each positive and negative sequence components will counter-balance each other, to produce zero current in the open phase (Figure 12.8(a)). The magnitude of these components in the two healthy phases are equal, i.e. I, = 1, and the maximum heat of the stator as in equation (12.4)... 

Protection against current imbalance. Measurement of phase negative sequence current Time-overcurrent relay... 

In this countermeasure, the faulted phase is opened instandy and healthy phases are opened about 10 s later when the DC component has decayed enough. When this countermeasure is applied near a generator (especially when the cable line offers a radial path to the generator), it is necessary to evaluate the negative sequence current capability of the generator as Countermeasure (3) causes an unbalanced operation for a pro-longed duration. 

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