OMICRON Magazine

Magazine | Issue 1 2019 Distance protection and impedance-based fault location on overhead lines and power cables is being used at utili- ties all over the world. The Z 1 positive-sequence imped- ance and Z 0 zero-sequence impedance are significant char- acteristics of the protected line which must be understood precisely in order to make the relay decide whether to trip or not to trip with reliability. Sending out the line crew requires accurate knowledge of the fault location in order to keep the outage short. Therefore, fault locators that also depend on accurate line parameters are being used. Measurements or calculations? Line parameters can either be measured or calculated, based on the geometry of the conductors and soil proper- ties. When calculating line impedances, multiple simpli- fication steps are taken which do not apply in the real world. Therefore, the results are often not precise enough. In particular, line-to-ground impedances, and therefore Z 0 , are influenced by the physical characteristics that are present, such as metal pipes or neighbouring cables in the ground. This is why they are not sufficiently accurate when calculated. Furthermore, it is assumed that the soil has uniform resistivity which is usually not the case due to different soil properties in different layers of soil. However, a measurement reflects all factors and obtains true values. Therefore, Anchorage ML&P, a utility in Anchorage, USA, has started to measure line impedances to verify and optimize distance relay parameterization. Although Anchorage ML&P uses line differential protection for its primary form of transmission line protection throughout its system, its backup protection is distance protection. In May 2018 a 138-kV overhead line with a length of 4.4 mi / 7.1 km was measured. Overhead lines in Alaska are usually not equipped with a ground wire, as lightning is not a common occurrence. This means, that the entire fault current in the event of a single-line-to- ground fault (SLG fault) returns via soil and therefore via a path with unknown properties. Multi-functional test system For the measurement, Anchorage ML&P used the CPC 100 as a multifunctional main unit which generates frequency variable test signals and measures current and voltage by applying digital filtering for effective noise suppression. This main unit was combined with the CP CU1 accessory, a coupling unit which ensures galvanic isolation between the line under test and the main unit. Additionally, the CP GB1 Grounding Box was used which comes with surge arrestors capable of diverting high fault currents in case of unexpected over-voltages from the line side. Although, Anchorage ML&P originally obtained the CPC 100 + CP CU1 test system in order to verify the effectiveness of its ground grids by means of measur- ing the Ground Potential Rise, Step & Touch Potentials and Point-to-Point resistances, it saw the verification of its transmission line impedances as a benefit to the test system. precise measurements Seven loop impedances are being measured in order to determine the Z 1 positive-sequence impedance and reLIABLe dIStAnCe proteCtIon Get precise line parameters with the CPC 100 + CP CU1 35

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