By Will Knapek
Between 80-90 percent 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 its protection zone and other undesired consequences, such as under-tripping or unintended automatic reclosing initiation.
The problem escalates because of recent 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 the subject of heavy scrutiny. Validation of accurate distance relay settings is now a topic of discussion by electric power utilities as well as professional technical committees, such as the Institute of Electrical and Electronics Engineers (IEEE) Power Systems Relaying Committee. It quickly becomes apparent 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;
• Polarizing current (3I0, I2);
• Supervising element (3I0);
• Z0/Z1 (zero-sequence compensation); and
• 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 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 the ground fault loop calculation can be simplified and treated similarly to the phase-to-phase fault loop calculations performed in the protective device. If the k factor is not accurate, therefore, fault reach (distance) will be incorrectly calculated. Transmission line impedances, used for k factor, are often calculated by line constants programs. Because of the many variables required, line parameter calculations are prone to error, particularly in the zero-sequence impedance value of the line. Utilities, for example, often assume fixed soil resistivity values (10Ωm, 100Ωm, etc.) applied across their system models, even in cases where the transmission line might span over soil types different from those assumed in the line constants program. Because of the uncertainties related to soil resistivity and 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 just described.
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 under or overreach-resulting in a misoperation. In order words, relay sensitivity to detect ground faults will be affected.
In addition, 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 by utility crews to promptly locate and remove foreign objects from the primary system-and quickly repair damaged lines. Short circuit and coordination studies also depend on accurate modeling data to enable the protection engineer to correctly set relays.
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 described uncertainties. In the past, line parameter measurement was considered prohibitive and costly because it required large, high-power equipment to overcome nominal frequency interferences and off-nominal frequency injection was not possible. With modern digital technology and creative design, OMICRON has overcome these challenges with the CP CU1 coupling unit, an extension to the CPC 100.
OMICRON's CP CU1 provides a solution to take actual impedance measurements. The CPC 100, a multifunctional primary test set system, and the CP CU1 combine to provide safe and high accuracy measurements of the line's sequence impedances-up to 200 miles-and k factors; mutual coupling between parallel transmission lines; and between power and communication lines. In addition, substation ground impedances and step and touch voltages can be measured. The system safely couples the test equipment with the transmission line, including internal surge arrestors and coupling transformers.
The CPC 100 + CP CU1 system also includes the CP GB1 grounding box to connect the CP CU1 to the line and provide an auxiliary injection point a safe distance from the operator. Off-frequency injection allows the application of lower level test signals and overcomes the problem of nominal frequency interference. System measurement accuracy varies from 0.3 percent to 1.0 percent, depending on the load impedance.
About the author: Will Knapek is an application engineer for OMICRON Electronics Corp., USA. He has a BS from East Carolina University and an AS from Western Kentucky University, both in Industrial Technology. Knapek has been active in testing since 1995 and is certified as a Senior National Institute for Certification in Engineering Technologies (NICET) technician and a former International Electrical Testing Association (NETA) Level IV technician. Knapek is also a member of IEEE.