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FEATURE | ELECTRICAL SUBSTATIONS

BY ALEX MOGILEVSKY, CEATI International & SHAWN OTAL, BABAK JAMALI, BYRON MA, METSCO Energy Solutions

Grounding systems are a crucially important component of any power supply system, because they directly impact public and employee safety, supply system

reliability, power quality and life expectancy of power equipment. In spite of their crucial role in safe, reliable, and economic operation of power supply systems, grounding systems do not receive the same level of preventative maintenance as other infrastructure assets.

While utilities have generally adopted state-of-the-art risk-based main tenance practices for other important assets, very little work has been un dertaken in improving and modernizing the maintenance practices for grounding systems. A newly published “Grounding System Maintenance Guide” commissioned by CEATI International’s Grounding and Lightning Interest Group, provides information that can help utilities modernize the maintenance practices for their grounding systems. Refer to the “Guide Objectives” sidebar for further information.

Standards addressing design and measurement procedures

1

3

2

4

Identify and document the common degra-dation and deterioration processes associated

with various components of grounding systems

Identify and document the common degra-dation and deterioration processes associated

with various components of grounding systems

Develop an objective yardstick for measure-ment and benchmark the functional state of

grounding systems through the use of appropriate health index algorithms

Specify appropriate inspection techniques and test methods to assess and benchmark

the health and condition of grounding systems in a cost- effective manner

GUIDE OBJECTIVESCEATI International’s “Grounding System Maintenance Guide”, which was developed with the support of Grounding and Lightning program members from 19 North American electrical utilities, has been produced to achieve a number of objectives.

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Currently available grounding standards address grounding system de sign and measurement procedures but they do not address the scope and frequency of maintenance activities needed to be performed on different types of grounding systems as outlined in this feature.

PREVENTATIVE MAINTENANCEPreventative maintenance of grounding systems involves inspections and testing to correctly and precisely assess the functional state of grounding system components, allowing timely replacement of the degraded com ponents when their performance becomes unacceptable. The best strategy in assuring the readiness of ground grid components for reliably carrying out their intended functions at all times consists of a four-step plan.

Step 1Collect, monitor and manage the relevant information about ground grid components, that is, maintaining data bases showing past, scheduled and planned maintenance activities and detailed results of prior inspections and testing that could be used as benchmarks for interpretation of in spection and test results.

Step 2Evaluate the consequences of risk associated with failure of the ground grids in carrying their intended function. Use this risk profile to determine the optimal scope and schedule of preventative maintenance activities in the form of periodic testing to assess the condition of various components.

Step 3Employ the information of various ground grid health parameters col lected through visual inspection and testing, to determine the overall health and condition of ground grid in form of health index.

Step 4Prioritize the mitigation work to rectify defects based on the health indices and implement the optimal intervention activities.

SYSTEM COMPONENTS & DEGRADATION MODESA simple grounding system for a pole may consist of a single rod driven into earth and bonded to the case of a distribution transformer and sys tem neutral through a riser conductor and appropriate connectors. A grounding system for pad-mounted distribution equipment may consist of two to four ground rods installed at the corners of the rectangular pad and connected in parallel through a bare conductor buried horizontally in form of a ring or counterpoise. The grounding system for a substation consists of a large number of components that include ground rods, bur ied interconnecting mesh, ground leads and bonding conductors with adequate redundancy, grounding connectors, voltage gradient mats, sur face stone, and fence grounding.

Each of these grounding components performs a different task in a grounding system. For example, ground rods facilitate the conduction of fault current from the upper layer of the soil (which is typically frozen during winter conditions) to the bottom layers, and surface stone provides an upper layer of high resistivity material to increase the safe ground po tential limits during a fault.

Each aforementioned component experiences different forms of degradation in health with time. For example, steel

ground rods provide acceptable electrical and mechanical characteristics upon installation, but with passage of time, these rods are subjected to corrosion, which results in a reduction in the rod diameter. Similarly, the surface stone may break into smaller particles (fines) with passage of time, allowing vegetation to grow, which reduces the resistivity of the surface stone, and lowers the safe limits of touch and step potentials. Touch potential is the voltage between energized equipment and the feet of a nearby subject who is in direct contact. Step potential is the voltage between the feet of a nearby subject who is standing within proximity of energized electrical equipment.

Dissimilar to the conventional fixed assets employed on power systems, that is, transformers, circuit breakers, protection and control (P&C) components, among others, grounding systems do not generally experience a sudden and total failure in service, but their performance degrades gradually over life.

The end-of-service life for a grounding system is deemed to be when the performance has degraded to a level at which it can no longer meet the performance level of its intended functions.

The grounding system degradation rates with time depend on a number of external factors that include short circuit level, how much of the unbalanced load current flows through ground grid, acidity of surrounding soils causing corrosion of below grade components, construction and maintenance activity in the area and theft of copper conductors.

CONDITION ASSESSMENTThe condition assessment techniques for grounding systems can be grouped into three general categories: visual inspections, in-situ electrical measurements, and lab testing on excavated components.

Visual inspections are extremely cost effective in assessing the condition of above-grade components. Through visual inspections, maintenance personnel can confirm conductor size and conductor redundancy for ground leads and bonds and reveal any mechanical damage to conductor strands

Circuit network diagram of a ground grid

Ground

Node a Node b

Node 1R1R2

R3Node 2

Node 3

Branch 1

Branch 2

R ab

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including damage to the grid caused by copper theft.It is not practical to assess the condition of below-grade

components through visual inspections. Therefore, condition of ground grid components, installed below grade, can be determined through in-situ electrical measurements, including integrity tests, fall-of-potential measurements, and step-and- touch potential surveys.

These tests provide fairly accurate results of the overall health and condition of the ground grid. However, in assessing its effectiveness in meeting the intended safety objectives, these tests may not reveal the underlying cause of its unsatisfactory performance. When a grounding system is in poor condition, testers can identify the root cause of its less-than-stellar performance by excavating a

representative sample of the ground grid components and performing inspections and laboratory tests on affected components to determine the degree of degradation. Inspections and testing of a representative sample of excavated ground rods and below-grade conductors and connectors provide reliable indication of the overall health of the grounding system.

Through excavations, testers can verify and assess the conductor sizes and any loss in diameter of ground rods due to corrosion. Mechanically stressed connectors can be detected and their resistance can be measured. Ground rods may be pulled and tested in a lab or tested non-destructively at the work site to measure metal loss due to corrosion.

HEALTH INDEXINGIn order to quantify the condition of a grounding system relative to critical long-term degradation factors that cumulatively lead to its end-of-life, utilities can develop the Health Index of the grounding system. Table 1 shows grounding system health index formulation.

As described in Table 1, computing the Health Indices for grounding systems requires assignment of condition ratings to various health and condition indicators. The CEATI guide provides guidelines to maintain

Life expectancy of ground grid components

Surface stone resistivity testing

Com

pone

nt L

ife E

xpec

tanc

y (y

ears

)Component Life Expectancy

100

Ground rods Buried interconnecting mesh

Ground leads connecting ground electrode with supply system neutrals

Bonding conductors connecting ground electrode with equipment frames

Buried ground grid connectors

Above-grade grounding and bonding connectors

Voltage gradient mats

Surface Stone

Mean

Maximum

Minimum

908070

6050

53 5056

51 53 47 4439

40

3020

10

0

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objectivity, consistency, and uniformity in assignment of the condition ratings—particularly when a large team is involved in condition assessment. The guidelines can be reviewed and modified by a utility, if necessary, to meet its specific plans and objectives. As an example, Table 2 provides a guideline to calculate condition rating of risers/bonds to buildings/fences/gates.

After the health index of the grounding system has been determined based on the criteria provided in Table 1, overall condition of the grounding system can be expressed and interpreted through the scale provided below in Table 3.

Health indices, determined in this manner, can be used to benchmark the degree of degradation in performance of a grounding system in providing its intended functions. Additionally, health indices can be used to correctly direct investments of the correct size and scope and at the

appropriate time into repairs, refurbishment, and replacement of grounding systems, ensuring economic efficiency of the investments. ET

Shawn Otal (is the chief executive officer (CEO) of METSCO Energy Solutions Inc. and a senior electrical engineer with over 35 years of experience. Babak Jamali is a professional engineer with more than 15 years of experience in power systems. Byron Ma is an apprentice engineer specializing in system planning and ground grid investigations. Alex Mogilevsky is a senior program manager, transmission and distribution programs at CEATI International with over 25 years of experience in program/project management.

CONTACT: alex.mogilevsky@ceati.com

# Condition Criteria Weight Condition Rating Factors Maximum Score

1 Quality of Original Grounding System Installation 4 A,B,C,D,E 4,3,2,1,0 16

2 Condition of Risers/Bonds to Equipment/Structures/Neutral/ Overhead Ground wires/Attachment Points

4 A,B,C,D,E 4,3,2,1,0 16

3 Condition of Bonds to Buildings, Fences, Gates 3 A,B,C,D,E 4,3,2,1,0 12

4 Condition of Voltage Gradient Mats 2 A,B,C,D,E 4,3,2,1,0 8

5 Condition of Surface Stone 2 A,B,C,D,E 4,3,2,1,0 8

6 Surface Stone Resistivity 2 A,E 4,0 8

7 Grid and Bond Integrity 4 A,B,C,D,E 4,3,2,1,0 16

8 Current Injection Test 4 A,B,C,D,E 4,3,2,1,0 16

Max Score 100

Health Index Condition Description Requirements

85-100 Very Good No noticeable aging or deterioration of grounding system components

Normal maintenance

70-85 Good Minor deterioration of some grounding system components

Normal maintenance

50-70 Fair Significant deterioration of some grounding system components

Increase diagnostic testing, possible replacement of degraded components depending on criticality

30-50 Poor Widespread deterioration of most grounding system components

Start planning process to replace or rebuild, considering risk and consequences of failure

0-30 Very Poor Extensive serious deterioration of most grounding system components

At end-of-life, immediately assess risk; replace or rebuild based on assessment

Condition Rating Description

A Adequate number of riser conductors thread through fence fabric and are bonded to barbed wire, gates bonded side-to-side, with gradient control grids under swing area and buildings

B Normal signs of wear with respect to the above characteristics

C A few (< 5 percent) of the risers/bonds have some deficiencies

D A significantly large fraction (> 5 percent and <50 percent) of the risers/bonds have serious deficiencies

E A majority (> 50 percent) of the risers/bonds have serious deficiencies

Table 1: Health Index Formulation

Table 3: Grounding system health index scale

Table 2: Condition of risers/bonds to buildings/fences/gates