OMICRON Magazine

the Z 0 zero-sequence impedance. This includes all three phase-to-phase loops, all three phase-to-ground loops and one loop where all three phases are shortened and measured to ground. Overall, the test took less than one hour. After the test, the dedicated Primary Test Manager software created a test report which comprised all of the relevant data: › Z 1 positive-sequence impedance › Z 0 zero-sequence impedance › k 0 zero-sequence compensation factor › Deviation between measured and calculated values for Z 1 and Z 0 if calculated values are available › Actual zone reach based on present relay parameters › Accuracy of the impedance-based fault locator based on present relay parameters Table 1 compares the measured values for Z 1 and Z 0 which were derived from the measurement of the seven loop impedances with the calculated values. As expect- ed, the deviation of the positive-sequence impedance is negligible as the positive-sequence impedance can be accurately calculated from the geometry of the conduc- tor arrangement. However, quite high deviations have been observed for the zero-sequence impedance. The X value of Z 0 , which is the more crucial component aside from the R value, was previously determined to be almost 70% too high from the calculation based on the geometry and soil properties. error calculation Z 1 Z 0 r in Ω X in Ω r in Ω X in Ω Measured value 0.722 1.938 1.095 5.067 Calculated value 0.740 1.940 1.450 8.500 error (ref. to measured value) 2.54% 0.09% 32.42% 67.75% Table 1: Comparison of measured and calculated values In order to visualize the actual zone reaches and the accuracy of the impedance-based fault locator, the Primary Test Manager software allows you to enter relay parameters in order to produce the corresponding charts. The primary X-value for the zone is entered in order to evaluate the zone reaches. In this particular case, zone 1 was subject to evaluation with a desired grading factor of 90%. This means that the X value is 90% of the X value of the calculated positive-sequence impedance. Furthermore, the k 0 zero-sequence compensation factor (magnitude k 0M and angle k 0A ) is entered per its defini- tion in the SEL 311 manual: k 0 = Z 0 – Z 1 3 × Z 1 distance protection parameters X 1 prim zone 1.746Ω k 0M 1.068 k 0M in ° 14.15 For phase-to-phase faults the reach is in the 90% range which is expected because there is almost no deviation between the calculated and measured Z 1 . As expected, the zone reaches for phase-to-ground faults are far away from 90% which is caused by the zero-sequence compensation factor being derived from the calculated Z 0 . In this case the relay would tend to overreach, which means that in the event of a SLG fault on the subsequent line (up to more than 120% of the measured line) the relay of the measured line would also trip, in addition to the relay which protects the subsequent line. 150 0 90 A–B B–C A–C A–G B–G C–G 92.82 91.77 85.97 121.56 124.83 129.01 Zone reach in % of line length 36