2,2222 10 0.23145scZ j jæ ö= × = Wç ÷è øGUID-A4805F4D-20EC-4B39-A39D-925513515FA5 V1 EN (Equation 7)The transformer impedance, referred to 22 kV level, is:2,2222 0.12 0.9760TZ j j= × = WGUID-CB0D94B6-5BFA-42EB-AE4A-B0EE898A0D78 V1 EN (Equation 8)Calculation of a phase-to-phase short circuit at this busbar:23 22000 / 3 91672 0.23 0.97sc phI Aj j= × =+GUID-846C7A33-F976-45DF-95E9-75124D6A24D2 V1 EN (Equation 9)The setting is chosen to 5 IBase which corresponds to 7 875 A primary current.2. Set t1 to 0.1 sThe time delay must be chosen so that the blocking signal shall be able to preventunwanted operation during feeder short circuits. 0.1 s should be sufficient.3.1.5.3 Calculating settings for step 4The first requirement is that the phase overcurrent protection shall never trip for loadcurrent during extreme high load situations. It is assumed that the transformer shall be ableto be operated up to 130% of the rated power during the limited time. The protectionresetting ratio of 0.95 shall also be considered. The minimum setting can be calculated asfollows:1 60 10001.3 21550.95 3 22puI A׳ × × =×GUID-4D6B62EC-32DC-433E-B1C8-D23E8D06D017 V1 EN (Equation 10)The next requirement is that the protection shall be able to detect all short circuits withinthe defined protected zone. In this case it is required, if possible, that the protection shalldetect phase-to-phase short circuit at the most remote point of the outgoing feeders asshown in figure 8.Section 3 1MRK 511 286-UUS AREC650 setting examples48Application manual
