4.2.4.6 Example how to connect the open delta VT to the IED for lowimpedance earthed or solidly earthed power systems SEMOD55055-199 v6Figure 22 gives an example about the connection of an open delta VT to the IEDfor low impedance earthed or solidly earthed power systems. It shall be noted thatthis type of VT connection presents secondary voltage proportional to 3U0 to theIED.In case of a solid earth fault close to the VT location the primary value of 3Uo willbe equal to:3 3Ph PhPh EUUo U--= =EQUATION1926 V1 EN-US (Equation 7)The primary rated voltage of such VT is always equal to UPh-E Therefore, threeseries connected VT secondary windings will give the secondary voltage equalonly to one individual VT secondary winding rating. Thus the secondary windingsof such open delta VTs quite often has a secondary rated voltage close to ratedphase-to-phase VT secondary voltage, that is, 115V or 115/√3V as in this particularexample. Figure 22 gives an overview of the actions which are needed to make thismeasurement available to the built-in protection and control functions within theIED.Section 4 1MRK 504 169-UEN AAnalog inputs62 Transformer protection RET650 2.2 IECApplication manual
