Substation and Equipment Grounding
Summarize the current measurement equipment and techniques for evaluating the condition of energized substation grounding systems; discuss
the advantages, disadvantages, and limitations of each technique. Test substations with available equipment to determine effectiveness and
validate a method for pinpointing the substation grounding issues. Develop recommendations for PPL on the best practice for substation
grounding system testing.
Substation Importance
The Problem
• At the heart of the transmission and distribution system
• Steps up voltage to carry the electricity across long distances
• Steps down voltage to bring the electricity to residential and
commercial customers
• Many substations were constructed over 50 years ago and electric companies
do not know what condition the grounding system is currently in
• The effects of corrosion through water content, pH value, soluble salt content,
and soil resistivity of the soil at the different sites is unknown
• With a degraded grounding systems, the likelihood of transfer, step, or touch
potential rises. This is a dangerous temporary voltage that goes through
equipment or personnel
• Traditional testing methods currently used to test grounding systems requires
a de-energized substation, which is only practical before it comes online.
• When the substation is energized it is tied in with all equipment, neutral
wires, transmission lines, and OHGW, it is impossible to isolate the test
procedures from the background noise
• No standards exist for continued safety checks on the grounding system after
any substation is in-service
Grounding
• The ground system is a critical part of a substation
• It is a network of copper wires below the ground that is
connected to each piece of equipment
• Offers a safe, low resistance path to earth during normal or
fault conditions
• Ensures operational performance of the equipment and
safety of personnel on site
• Provides reliable and continuous flow of electricity by
preventing damage to equipment
Current Test Procedures
• Fall of Potential: most common procedure used for
disconnected substations to measure ground
resistance
• Three-point method
• Lazy Spike
• Slope Method
• Clamp-On
• Wenner Four Point Method: for soil resistivity
Current Equipment
• AEMC Instruments: Ground Resistance Tester Model
6472
• Fluke: GEO Earth Ground Testers 1623/1625
• Megger: digital Earth Tester DET2/2
• EPRI Smart Ground Multimeter
Current Industry Studies
Conclusions
• Big gap in market for technologies that check the integrity of
energized substations ground grids
• The current test protocols and equipment have limited
applications for in-service substations
• There has been limited and inconsistent research in this area
until recently
• The research currently available points towards a unique (not
60Hz) current injection into the ground as best method, this
allows for the differentiation of the background noise.
• Coupling results of current injection with simulation
software may lead to the ability to be able to pinpoint break
points in ground system
• Testing and Evaluating Grounding Systems of High Voltage Energized Substations:
Alternate Approaches (1999)
• Conventional Method
• Transition Method
• 70Hz Method
• Practical Testing of Grounding Systems by Current Injection (2005)
• The test injects a unique frequency into the ground to differentiate the
background noise from the test signals
• It couples the results with modeling software to understand faults in the ground
grid
• EPRI Condition Assessment of Substation Ground Grids: Phase 2 –Concept Testing
(2008)
• This study goal is to develop a method and instrumentation to determine integrity
of the grounding grid that is inexpensive, reliable and easy to use
• EPRI Lenox Test Grid
• Injected currents into subsurface grid can be detected using magnetic pickup coils
• The more information known about the grid, the better the test results
Author:
Megan McLoughlin
Lehigh University 2011
M.Eng Energy Systems
Engineering
Sponsor:
PPL Electric
Mentors:
Wayne Wittman (PSEG)
David Soyster (PPL)
Bob Lally (PPL)
Special Thanks to:
Dr. John Coulter
Shaku Jain-Cocks