COMMERCIAL FEATURE Solving relay misoperation with line parameter measurements B etween 80-90% of all power system faults involve ground. Many protective relaying schemes depend on ground distance protection to accurately sense and locate ground faults on multi-terminal sub-transmission and transmission lines. In addition to the need for dependable ground fault detection, protective relaying must provide adequate selectivity to avoid over tripping for faults outside of its zone of protection and other undesired consequences, such as under tripping or unintended automatic reclosing initiation. The problem escalates due to recent major power system disturbances in North America, such as the northeast blackout of 2003. Correct application and settings of protective devices, particularly distance relays, have become subject of heavy scrutiny lately. Validation of accurate distance relay settings is now a major topic of discussion by electric power utilities as well as professional technical committees such as the IEEE power systems relaying committee. It becomes apparent very quickly that the accuracy of line parameter values may affect many people. Although ground distance relay design, characteristics and implementations vary, some of the typical parameters required to set a ground distance relay include the following: • Zone impedance reach and characteristic angle • Blinder positions, resistive reaches and angles • Directional supervision limiting angle • Polarising current (3I0, I2) • Supervising element (3I0) • Z0/Z1 (zero sequence compensation) • Z0M/Z1 (zero sequence mutual coupling compensation) Relay manufacturers have different methods of calculating zero sequence compensation, also known as the ‘K-factor’, but generally it is defined as the ratio ESI AFRICA ISSUE 3 2014 between the zero sequence impedance Z0 and the positive sequence impedance Z1 of a given transmission line. The K-factor is used to ‘correct’ the ground impedance calculation so that the ground fault loop calculation can be simplified and treated similarly to the phase-to-phase fault loop calculations performed in the protective device. Therefore, if the K-factor is not accurate, fault reach (distance) will be calculated incorrectly. Transmission line impedance (used for K-factor) is often calculated by line constants programmes. Due to the large number of variables required, line parameter calculations are prone to error, particularly in the zero sequence impedance value of the line. For example, utilities often assume fixed soil resistivity values (10Ωm, 100Ωm, etc.) applied across their system models, even in cases where the transmission line may span types of soils different from those assumed in the line constants programme. Due to uncertainties related to soil resistivity and actual transmission tower grounding, the calculation of Z0 of a given line is more susceptible to error than its Z1. This is because the calculation of Z1 is independent of the ground path impedance. For parallel transmission lines, the accurate calculation of zero sequence mutual impedance Z0M is also prone to the errors described above. Such errors in the estimation and calculation of line parameters will affect accuracy of settings used in transmission line protective devices, particularly in distance and overcurrent relays, causing them to either under or overreach, resulting in a misoperation. In other words, relay sensitivity to detect ground faults will be affected. Additionally, Z0 and Z1 are used as inputs by many digital relays to calculate the location from the line terminal to the fault. Accurate fault location data is needed for utility crews to promptly locate and remove foreign objects from the primary system, and repair damaged lines as quickly as possible. Moreover, short circuit and coordination studies also depend on accurate modelling data to enable the protection engineer to set relays correctly. The alternative to line parameter calculation is taking actual measurements on a given transmission line to accurately determine its impedances and K-factor. Measuring the line impedance using the proper techniques, equipment and safety precautions provides the opportunity to eliminate the uncertainties described above. In the past, line parameter measurement was considered prohibitive and costly, since it required large high- power equipment to overcome nominal frequency interferences, since off-nominal frequency injection was not possible. With modern digital technology and ingenious design, OMICRON has overcome these challenges with the CP CU1 coupling unit, an extension to the CPC 100. ESI ABOUT THE Author: Will Knapek is an application engineer for Omicron Electronics Corp, USA. He holds a BS from East Carolina University and an AS from Western Kentucky University, both in industrial technology. He retired from the US army as a chief warrant officer after 20 years of service, of which 15 were in the power field. He has been active in the testing field since 1995 and is certified as a senior NICET technician and a former NETA level IV technician. Will is also a member of IEEE. www.omicron.at 71