Presented By:
Ronald Proffitt
Technical Consultant
North American Substation Services
TechCon 2022
Arguably, the two most important substation/plant components related to Safety and Reliability are the station battery and ground grid. If they do not function correctly, personnel Safety and equipment Reliability are at risk. In recent years with the focus on FERC PRC guidelines, the battery gets plenty of attention. Unfortunately, the ground grid mostly remains “out of sight, out of mind.”
Utilities and Plants use a variety of different grounding strategies and designs. Regardless of the design, there are fundamental tests that can be performed to assure the basic objectives of the substation ground grid are met. Those objectives are 1) to provide low resistance paths for current to flow while 2) maintaining personnel step and touch potentials and equipment bonding are at Safe levels. This paper reviews basic ground grid components, common problems, fundamental maintenance, and performance test practice, and a few examples of problems identified.
Introduction
Substation ground grid design can vary greatly. Regardless of the specific design, basic electrical components are utilized. These components usually include conductive cable runs, grounding rods, and connectors. The components are connected according to the specific design such that all substation power equipment and ancillary components are bonded together. This will allow for any stray, unbalanced, or fault current to flow through the ground conductors while maintaining step and touch potentials at Safe levels and protect sensitive digital instrumentation and controls from failure. If the ground grid remains intact and equipment bonded, generally the ground mat will perform as designed.
Problem Statement:
The biggest problem with a ground grid is that most of the components are buried. So, “out of sight, out of mind.” As a utility or plant owner, we understand the importance that routine testing plays in assuring ground mat performance. However, let’s face it, testing is rarely performed or if done, the data is either unreliable, lost, or unreviewed. Perhaps repair orders get created but are relegated to “low priority” in the budgeting process? The compliance box gets checked but actual repair work is difficult to materialize.
Typical substation grid:
A typical ground mat is made up of parallel runs of 4/0 copper, copper-clad steel (CCS), or equivalent low resistance electrical conductors, bonded together underground with either thermal or mechanical connectors in an X and Y grid layout. The grid usually undercovers the entire station and extends several feet outside of the fence line and gate(s). If a substation or switchyard is adjacent to an industrial plant or power station, multiple conductor runs may be used to bond the respective yards together. All power equipment, control houses, and fences are electrically bonded to the ground mat. An insulated layer of gravel may also be an integral component of the substation grounding design.
Again, most of the grid components are buried and not visually accessible for inspection. It is difficult to know the actual condition of your ground grid system. Was it installed and connected as shown on the station prints? Has it remained intact and functional over time and through years of revisions/construction projects, etc.?
Step potential is the voltage difference between the feet of a person standing or walking through a substation or near an object. If a fault should occur, the ground mat has the be effective enough to keep the step voltage at Safe levels for our employees.
Touch voltage is the voltage difference measured when a person contacts a metallic conductive object in reference to the ground mat. The mesh voltage is when a larger portion of a person’s body is in contact with the metallic object referenced to the ground mat. Again, if a fault should occur, the ground mat must be effective enough to flow current while maintaining voltage differences at a Safe level.
What can cause problems?
We’ve just reviewed that the components that comprise a ground grid system are rather simple. Primarily the grid is made up of conductors and connectors. There are generally two reasons why ground mats deteriorate or fail to perform. 1) Current cannot flow adequately if conductors either open-circuit or connectors deteriorate causing open or high resistance connections or 2) the design or installation thereof, is not adequate to safely mitigate available fault current.
Conductors and connections get compromised for a variety of reasons:
- Just because ground conductors are shown on prints, doesn’t mean they were installed accordingly. They may not even exist.
- Often grading or construction in and around the substation can damage the ground grid.
- Grounds get cut or damaged when failed equipment is replaced. • When equipment failures occur, fault currents can be a high enough magnitude to damage ground connections and/or melt conductors. The actual condition is unknown unless verified.
- Copper theft. Thieves steal copper ground risers. Fig. 3.
- Connectors deteriorate with age. Older style mechanical connectors are especially prone to deterioration. Even thermal connectors can deteriorate over time if not applied correctly.
To identify open conductors and poor connections, the testing process injects current and measures voltage drop to specific reference points.
What does testing look like?
Testing can vary widely. Some utility operators use handheld impedance meters to spot-check the ground risers as a simple health check.
NASS’s process is a little more elaborate but begins with the basics.
Once a job is requested, NASS works with the Customer to obtain and review prints to understand the specific ground mat installation. Often through the life of a substation, the grid has been modified/added to or “overlayed” to meet present-day requirements. To develop a test plan that satisfies requirements, it is very important to understand what’s supposed to be in the ground and how it’s connected.
Once a test plan is developed, the next important part of the process is to tag and label all test points. Ground risers and reference point are labeled with durable brass tags or equivalent as shown.
Maintenance Testing – Grid Integrity
Basic testing of the ground mat employs the use of a high current DC power supply (300A continuous rating), calibrated digital voltmeter, and a DC clamp-on ammeter to perform Integrity (Point-to-Point) measurements. Testing can be performed while the substation is energized. Since measured voltage drop is very sensitive, clean connections are imperative. Every ground riser is tested. Voltage drop is recorded from the riser test point to a selected reference point. The magnitude of current flow is recorded in both directions. That is, the magnitude of current flow supplied by the 300A source is recorded UP the riser into the steel and DOWN the riser into the ground mat. Depending on adequate grounding of the equipment, the SPLIT of current should make sense. Also, the voltage drop has to meet certain guidelines. The voltage drop guidelines will vary depending on the size and type grounding conductors used.
All riser current and voltage values are entered into a spreadsheet for evaluation. Localized mat issues are identified. Simple repairs can be made on-site if the Customer so desires. However, many times, larger more systemic problems also become obvious that require budgeting for larger repairs or perhaps even an entire station “overlay” or redesign project.
Performance Testing – Soil Resistivity, Ground Mat Impedance, and Ground “System” Impedance
While Maintenance Integrity testing is adequate to identify that the existing ground mat is installed and connected as designed, often utility customers request more elaborate substation grounding studies.
Trained and qualified test teams can perform more elaborate ground grid validation and studies. Using the Smart Ground Multimeter (SGM), accurate measurements of soil resistivity, ground mat impedance to remote earth, voltage (step, touch, and transfer) studies, and System modeling can be accomplished.
Measurement of Soil Resistivity:
Soil resistivity from region to region across the continent can vary widely. Knowing accurate soil resistivity values at a specific substation location is an important component in determining Safe step and touch potentials are met and grid design is optimal.
Using the SGM, simultaneous measurements are made on nine probes located along a line on the soil surface. The measurement arrangement is illustrated in Figure 7. The measurements obtained from the nine pins (several tests are run at varying pin spacings) are processed by error correction and estimation algorithms to construct a two-layer or three-layer soil model.
Ground Mat and System Impedance Test Method:
The overall substation ground mat impedance is measured to “remote earth.” A source current is injected into the ground mat referenced to the return electrode some distance away (maybe as far as a mile away) while the voltage drop is measured at prescribed probe distances. A typical measurement arrangement is shown in Figure 9 below.
When performing a system study, obviously obtaining an accurate ground mat impedance value is an important input component for the SGM intelligence to model step and touch potentials.
By switching modes on the SGM, using the same test connections, a “System” impendence can also be measured. The ground impedance measurement function can be applied to any existing grounding system of an energized or de-energized facility. From these data, the impedance of the grounding system under test is extracted using SGM program intelligence. The measured impedance of the grounding system is obtained as a function of frequency. The estimated impedance is reported in a plot of impedance magnitude and phase versus frequency. Note that the measured ground impedance is the combination of the impedance of the grounding system under test, in parallel with the impedance to ground of all shield wires, neutral wires, and other grounded metallic structures connected to the grounding system under test. An estimation algorithm “fits” the measurements to the model of the grounding system, shield wires, neutrals, etc. No knowledge of the type and number of shield wires, neutrals, etc. is required.
Once soil resistivity, substation ground mat impedance, and “System” impedance is known, those values can be inputted into the program model along with other factors provided by the utility such as maximum available fault current and IEEE Std.80 reduction factors and Safety parameters. Ground Potential Rise (GPR) and Equipotential Plots can then be projected and displayed. Hence, determination can be made if the existing infrastructure support Step and Touch potentials that are within Safe levels or if modification are necessary.
Conclusion:
Ground grid integrity is one of the most important factors in the Safe and Reliable operation of the substation. However, it is easily forgotten since most of its components are buried. Unfortunately, the station ground grid only gets attention either when sensitive digital controls or equipment continuously fail during switching events and storms, or even worse, employees get shocked. As explained in this paper, the grid components are rather basic and integrity testing can be performed with a high current source and handheld instruments. However, it is difficult for untrained personnel to make measurements, collect data in an organized format and have that data reviewed so that informed decisions can be made. Let’s face it, it’s not “rainy day work” for an in-house maintenance crew to perform and expect reliable results.
Using specifically trained and experienced crews that routinely perform ground grid testing is prudent practice. Also, using the SGM or equivalent to conduct the more elaborate soil resistivity, ground mat, and System impedance testing and modeling to determine adequate design and substation grid performance is best accomplished with trained personnel. Obtaining a detailed final report helps the Utility Client with justifying cost-effective, repair options to maintain their ground grids Safe and Reliable.
Case Study Example 1:
- Substation expansion had recently been completed.
- Capacitor bank and additional cable troughs had been added.
- Ground grid testing had been performed prior to construction.
- Components started failing during switching of the new capacitor bank. • Upon further investigation post-work ground grid testing was performed. • Testing discovered that during construction, digging inside the station had created an “islanding” issue with the new capacitor bank.
- Switching created transients that were introduced into the control house via the messenger ground and control cables.
Case Study Example 2:
- New 1500 MW Generating Station
- Collector Station and power equipment had been installed
- Control house was complete
- Generation plant construction had been completed
- Start-up operations had begun
- Unusual line events continued to trip generators off-line with no apparent explanation
- Discovered that during final construction activities installing hard surface road the preparation required a final grade
- During the process of final grading, sub-surface grounds were cut between plant and collector station
- No indication or evidence of the damage was detected until performing the ground grid testing
- Site had been properly tested prior to final grading and start-up
References:
- A. P. Sakis Meliopoulos, February 2020, “Smart Ground Multimeter (SGM) Operating Manual”
- IEEE Standard 80 (2013 Edition), “IEEE Guide for Safety in AC Substation Grounding”
