Grounding Considerations for Line...
Transcript of Grounding Considerations for Line...
![Page 1: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/1.jpg)
Grounding Considerations for
Line Protection
Joe Mooney, Jared Mraz, David Lewis, Aaron Findley, Molli DooleyPOWER Engineers, Inc.
Presented by: Molli Dooley
![Page 2: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/2.jpg)
i. Introduction
ii. Resistive Reach in Line Protection
iii. System Grounding Principals
iv. Analysis of Transmission Line Grounding
v. Relay Response
vi. Conclusions
vii. Questions
Presentation Contents
![Page 3: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/3.jpg)
Introduction
• Faults to ground result in current flowing through the power system to/from the earth.
• Ground fault current is limited by:• Source impedance• Line impedance• Arc resistance• Tower footing resistance
![Page 4: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/4.jpg)
Introduction Line and Source impedance are typically part of a base model
Arc, footing, and any other resistances are added as a fault resistance on a case by case basis.
![Page 5: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/5.jpg)
Accommodating Fault Resistance
• Common methods for detecting ground faults include-
– Directional Ground Overcurrent– Directional Negative-Sequence– Line Current Differential– Mho Ground Distance– Quadrilateral Ground Distance
![Page 6: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/6.jpg)
Accommodating Fault Resistance
• Quadrilateral Elements, wide resistive reach.• Mho Expansion (temporary)• Directional Ground O.C.
![Page 7: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/7.jpg)
Sources of Fault Resistance
• How much resistive coverage is needed?
![Page 8: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/8.jpg)
Sources of Fault Resistance - Arc
• Empirically derived Warrington equation
• Empirically derived Westinghouse equation
![Page 9: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/9.jpg)
9
Sources of Fault Resistances –Tower Grounding
• Typically ground rod(s), grid, or buried counterpoise
• Imperfect Ground Connection
• Resistance depends on soil resistivity, and installed conductor
![Page 10: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/10.jpg)
Impact of soil resistivity
Zero Sequence Self Impedance of a conductor
• A function of resistivity (ρ), but relatively insensitive to it compared to footing resistance
![Page 11: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/11.jpg)
Tower Grounding
• Footing resistance can vary from tower to tower due to changes in soil resistivity
• Most utilities set a maximum allowable footing resistance determined by insulation coordination studies
• RUS Design Manual of High Voltage Transmission Lines recommends a footing resistance of 25 ohms or less in lightning prone areas
![Page 12: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/12.jpg)
Fault Current Distribution
![Page 13: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/13.jpg)
Simulation of fault current split
Current to Ground vs distance from the faulted tower. Each bar represents the current through a tower ground.
![Page 14: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/14.jpg)
Estimating Equivalent Impedance
• Ideally we want to estimate the apparent impedance the relay would see– With continuous shield wires, the fault current distributes
through multiple towers reducing the apparent resistance.
– Without shield wires, the resistance for ground faults would be based on a single tower footing resistance
![Page 15: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/15.jpg)
Infinite Half Line
Simple representation of transmission line grounding system, assuming equal footing resistance and span lengths
![Page 16: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/16.jpg)
Approximation for Midline Faults
Midline faults offer two paths for current distribution and the effective apparent impedance of the overhead shield system is approximately half of the infinite half line
![Page 17: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/17.jpg)
Apparent Impedance
The apparent impedance at the fault location is the parallel combination of the equivalent overhead system, and the resistance of the faulted tower.
![Page 18: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/18.jpg)
Equivalent Shield impedance Profile
As noted there is a minimum length for the approximation to be valid. If we are closer to a substation than Lmin, the actual impedance will be less, approaching the substation grid resistance.
![Page 19: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/19.jpg)
Equivalent Shield impedance Profile
On radial lines with no remote station, the apparent impedance will rise as the available paths for distribution fall down to a single direction.
![Page 20: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/20.jpg)
Impedance seen at the relay
• The apparent impedance seen by the relay with grounded shield wires
• 5, 25, 50, 200 ohm footing resistances
![Page 21: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/21.jpg)
Impedance seen with in-feed
• The apparent impedance seen by the relay with
• A Radial line• Equal contribution from
both ends of the line• Remote source
contributes twice the fault current as the local source.
![Page 22: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/22.jpg)
Summary• For unshielded or segmented lines,
tower footing resistance is a significant factor, 15-200 ohm.
• Grounded shield wires reduce apparent footing resistance– Down to the same order of
magnitude as arc resistance– 1-5 ohms typical
• Unlike arc resistance, apparent footing resistance is amplified by in-feed from other terminals
![Page 23: Grounding Considerations for Line Protectionprorelay.tamu.edu/wp-content/uploads/sites/3/2019/... · Grounding Considerations for Line Protection Joe Mooney, Jared Mraz, David Lewis,](https://reader030.fdocuments.us/reader030/viewer/2022040906/5e7b0b7a6393333ba529eeac/html5/thumbnails/23.jpg)
23