CHAPTER 5: SETTINGS SYSTEM SETUPG30 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL 5-1375there is a grounding bank on the Delta winding of the power transformer within the zone of protection, a ground faultresults in differential (zero sequence) current and false trips. In such a case, it is necessary to insert a zero sequence currenttrap with the Wye connected CTs on the Delta winding of the transformer.In general, zero sequence removal is necessary if zero sequence can flow into and out of one transformer winding but notthe other winding. Transformer windings that are grounded inside the zone of protection allow zero sequence current flowin that winding, and therefore it is from these windings that zero sequence removal is necessary.The G30 performs this phase angle compensation and zero sequence removal automatically, based on the settingsentered for the transformer. All CTs are connected Wye (polarity markings pointing away from the transformer). All currentsare phase and zero sequence compensated internally before the calculation of differential and restraint quantities.The phase reference winding (w f ) is the winding to have a phase shift of 0° applied to it. The phase reference winding ischosen to be the delta or zigzag (non-wye) winding with the lowest winding index, if one exists. For a transformer that hasno delta or zigzag windings, the first winding is chosen.The phase compensation angle (Φcomp ), the angle by which a winding current is shifted to refer it to the phase referencewinding, is calculated by the G30 for each winding as follows:Φcomp [w] = | Φ[w f ] – Φ[w] | where Rotation = “ABC” Eq. 5-14Φcomp [w] = | Φ[w] – Φ[w f ] | where Rotation = “ACB” Eq. 5-15In our example, the phase reference winding is winding 1, the first delta winding (that is, wf = 1). The phase compensationangle for each winding is then calculated as follows (assuming Rotation = “ABC”):Φcomp [1] = 0° – 0° = 0° Eq. 5-16Φcomp [2] = 0° – (–30°) = 30° = 330° lag Eq. 5-17The following table shows the linear combination of phases of a transformer winding that achieves the phase shift andzero sequence removal for typical values of ΦcompwhereIA [w] = uncompensated winding ‘w’ phase A currentIAp [w] = phase and zero sequence compensated winding ‘w’ phase A current
