Grounding Design Calculations – Part Sixteen

In Article Grounding Design Calculations – Part One ", I indicated the following:

 Grounding System Design Calculations according to type of the building The procedures for performing the Grounding System Design Calculations can differ slightly according to the type of the building as follows: Domestic, commercial and industrial premises, High and medium voltage electricity substations.

 First: Domestic, commercial and industrial premises We mean by domestic, commercial and industrial premises, all installations up to 1,000 V ac and 1,500 V dc - between phases, with some minor exceptions.

And I explained Methods of Grounding Design Calculations of Domestic, commercial and industrial premises in the following Articles:

 Second: High And Medium Voltage Electricity AC Substations

I began explaining Grounding Design Calculations for second type of buildings: AC Substations in Article
Grounding Design Calculations – Part Ten "  where I explained the following:

• Design Procedures for grounding system design as per IEEE 80: Guide for safety in AC substation grounding,
• Step#1: Field Data Collection,
• Step#2: Earthing Grid Conductor Sizing.

Also in " Grounding Design Calculations – Part Eleven ", I explained Step#3: Calculation Of Tolerable Touch And Step Voltages.

And in Article " Grounding Design Calculations – Part Twelve ", I explained Step#4: Preliminary Design of Grounding System for AC Substations.

And In Article " Grounding Design Calculations – Part Thirteen ", I explained Step#5: Calculation of the Preliminary Grid Resistance, Rg, Of the Grounding System in Uniform Soil

And In Article " Grounding Design Calculations – Part Fourteen ", I explained Step#6: Determination of Maximum Grid Current, IG.

Also, in Article " Grounding Design Calculations – Part Fifteen ", I explained the following steps:

• Step#7: Calculation of Maximum Grid Potential Rise and Comparing With the Tolerable Touch Voltage from Step#3
• Step#8: Calculation of Mesh and Step Voltages

Today, I will continue explaining other steps from the design procedures of grounding system for AC Substation.

 Design Procedures of Grounding System for AC Substations - Continued

Design Procedures

The design process of a substation grounding system requires many steps. The following steps were established by the IEEE Standard 80-2000 for the design of the ground grid:

• Step#1: Field Data Collection,
• Step#2: Earthing Grid Conductor Sizing,
• Step#3: Calculation of tolerable touch and step voltages,
• Step#4: Preliminary design of grounding system,
• Step#5: Calculation of of the preliminary Grid Resistance, RG, of the grounding system in uniform soil.
• Step#6: Determination of Grid current, IG.
• Step#7: Calculation of maximum grid potential rise and comparing with the tolerable touch voltage from step#3. If the GPR of the preliminary design is below the tolerable touch voltage, move to step#12 (no further analysis is necessary). If not, continue to step#8.
• Step#8: Calculation of mesh and step voltages.
• Step#9: Comparing the computed mesh voltage from step#8 with the tolerable touch voltage from step#3. If the computed mesh voltage is below the tolerable touch voltage, continue to step#10. If not, move to step#11 for revising the preliminary design.
• Step#10: Comparing the computed step voltage from step#8 with the tolerable step voltage from step#3.If the computed step voltages are below the tolerable step voltage, move to step#12. If not, move to step#11 for revising the preliminary design.
• Step#11: Preliminary Design modification; If either the step or touch tolerable limits from step#3 are exceeded, revision of the grid design is required.
• Step#12: Detailed final design. After satisfying the step and touch voltage requirements, additional grid and ground rods /conductors may be required. The final design should also be reviewed to eliminate hazards due to transferred potential and hazards associated with special areas of concern.

The block diagram in Fig (1) illustrates the Design procedures.

 Fig (1)

Step#9: Comparing The Computed Mesh Voltage From Step#8 With The Tolerable Touch Voltage From Step#3

 If Action Em < Etouch Continue to Step#10: Comparing the computed step voltage from step#8 with the tolerable step voltage from step#3 Em > Etouch Move to Step#11: Preliminary Design modification

Step#10: Comparing The Computed Step Voltage From Step#8 With The Tolerable Step Voltage From Step#3

 If Action Es < Estep Move to Step#12: Detailed final design Es > Estep Continue to Step#11: Preliminary Design modification

Example#1:

Using the same data of Example#2 in previous Article Grounding Design Calculations – Part Fifteen verify that the earthing grid design is safe or not.

Solution:

From example#2 referred above, we get the following results:

• Etouch (for 70 Kg Person) = (1000 + 1.5 Cs ρs) x 0.157 / √ts = (1000 + 1.5 X 0.7207 X 3000) 0.157 / √0.15 = 1720.04 V
• Estep (for 70 Kg Person) = (RB + 2Rf) x IB = (1000 + 6 Cs ρs) x 0.157 / √ts = (1000 + 6 X 0.7207 X 3000) 0.157 / √0.15 = 5664.03 V
• Em = 1661 V
• Es = 728 V

Step#1: Comparing the Computed Mesh Voltage Em with the Tolerable Touch Voltage Etouch

We find that: Em < Etouch

So, the earthing system passes the touch potential criteria and we will Continue to Step#10: Comparing the computed step voltage from step#8 with the tolerable step voltage from step#3

Step#2: Comparing The Computed Step Voltage Es With The Tolerable Step Voltage Estep
We find that: Es < Estep

So, the earthing system passes the touch potential criteria and we will  Move to Step#12: Detailed final design.

Having passed both touch and step potential criteria, we can conclude that the earthing system design is safe.

 Step#11: Preliminary Design Modification If either the step or touch tolerable limits from step#3 are exceeded, revision of the grid design is required. These revisions may include modifying the following design parameters: D Spacing between parallel conductors, m n Geometric factor composed of factors na, nb, nc, and nd LC Total length of grid conductor, m LT Total effective length of grounding system conductor, including grid and ground rods, m

 Methods for Preliminary Design Modification If the calculated grid mesh and step voltages are greater than the tolerable touch and step voltages, then the preliminary design needs to be modified. The following are possible remedies: 1- Decrease Total Grid Resistance: If the total grid resistance is decreased, the maximum GPR is decreased; hence the maximum transferred voltage is decreased. effective ways to decrease the grid resistance are: More grid conductors to increase the area occupied by the grid, More earthing electrodes, Increasing cross-sectional area of conductors, Deep driven rods or wells can be used also if area is limited and the rods penetrate lower resistivity layers. 2- Decrease Grid Spacings: By increasing the number of parallel conductors in each direction, the condition of the continuous plate can be approached more closely. Dangerous potentials within the substation can be eliminated. For the perimeter, the effective ways to control perimeter gradients and step potentials are: A ground conductor can be buried outside the fence, Increase the density of ground rods at the perimeter, Bury two or more parallel conductors around the perimeter at successively greater depth as distance from the substation is increased, Vary the grid conductor spacing with closer conductors near the perimeter of the grid. 3- Increase the thickness of the surface layer: a practical limit may be 6 inches. 4- Limit total fault current: If feasible, limiting the total fault current will decrease the GPR and gradients in proportion. 5- Diverting greater part of the fault current to other paths by one of the following methods: Connecting overhead ground wires of transmission lines, Decreasing the tower footing resistances in the vicinity of the substation. 6- Barring access to limited areas: if practical, can reduce the probability of hazards to personnel. 7- Consider soil treatments to lower the resistivity of the soil. 8- Greater use of high resistivity surface layer materials.

 Step#12: Detailed Final Design After satisfying the step and touch voltage Criteria, we come to finalize our grounding grid design which should be reviewed to: Investigate possible special areas of concern in the substation grounding network,This includes an investigation of grounding techniques for substation fence, switch operating shafts, rails, pipelines, and cable sheaths, Eliminate hazards due to transferred potential and hazards associated with special areas of concern. The result of this investigation may require additional grid and ground rods /conductors especially if: The grid design does not include conductors near equipment to be grounded, The grid design does not include Additional ground rods at the base of surge arresters, transformer neutrals, etc. The most important areas of concern are: Service areas, Grounding of substation fence, Switch shaft and operating handle grounding, Control cable sheath grounding,  GIS bus extensions, Surge arrester grounding, Separate grounds, Transferred potentials by Communication circuits, Rails, Piping, etc.

Important Areas Of Concern

1- Service Areas:

They include the areas within the substation fence that used for storage, staging, or general service areas. Step and touch potentials should be checked to determine if additional grounds are needed in these areas.

2- Switch Shaft And Operating Handle Grounding:

Operating handles of switches represent a significant concern of electrical shock if the handles are not adequately grounded. So, touch and step voltages near the operating handle should be within safe limits. Additional means are taken to provide a greater safety factor for the operator as follows:

1. The switch operating shaft can be connected to a ground mat on which the operator stands when operating the switch. The ground mat is connected directly to the ground grid and the switch operating shaft (see fig.2).
2. Providing a direct jumper between the switch shaft and the ground mat provided a jumper from the switch shaft to the adjacent grounded structural steel. This jumper may be a braid or a braidless grounding device (see fig.3).

 Fig.2: Ground Mat

 Fig.3: Switch shaft and operating handle grounding

3- Grounding Of Substation Fence:

The substation grounding design should be investigated that the touch potential on the fence is within the calculated tolerable limit of touch potential. Step potential is usually not a concern at the fence perimeter, but should be checked to verify that a problem does not exist.

The metal fence grounding requirement may be accomplished by:

1. Bonding the fence to the substation grounding grid, or,
2. Bonding the fence to a separate underground conductor below or near the fence line.

As per The National Electrical Safety Code (NESC), the various fence grounding practices are:

1. Fence is within the ground grid area and is connected to the substation ground grid.
2. Fence is outside of ground grid area and is connected to the substation ground grid.
3. Fence is outside of ground grid area, but is not connected to the substation ground grid. The fence is connected to a separate grounding conductor.
4. Fence is outside of ground grid area, but is not connected to the substation ground grid. The fence is not connected to a separate grounding conductor. The contact of the fence post through the fence post concrete to earth is relied on for an effective ground.

Several different practices are followed for fence grounding which are (see fig.4):

1. Ground only the fence posts, using various types of connectors
2. Ground the fence posts, fabric and barbed wire.

 Fig.4: Fence grounding method
However the following notes must be considered when grounding a substation fence:

1. The grounding grid should extend to cover the swing of all substation gates (see fig.5),
2. The gate posts should be securely bonded to the adjacent fence post utilizing a flexible connection.

 Fig.5: Gate grounding method

4- Control Cable Sheath Grounding:

Metallic cable sheaths, unless effectively grounded, may attain dangerous voltages with respect to ground. These voltages may result from:

1. Insulation failure,
2. Charges due to electrostatic induction, and flow of currents in the sheath, or
3. The voltage rise during faults discharging to the substation ground system to which the sheaths are connected.

Metallic cable sheaths are grounded by connecting wire or strap to the permanent ground, the used wire or strap must be sized to carry the available fault current.

However the following notes must be considered when grounding Control cables:

1. Sheath currents on single-conductor cables can be reduced by grounding one end of the sheaths only, when The cable length is not excessive. For long cables, the sheath should be grounded at both ends and at each splice,
2. The sheaths of shielded control cables should be grounded at both ends to eliminate induced potentials,
3. Another solution for grounding the sheaths of shielded control cables is to run a separate conductor in parallel with the control cable connected to the two sheath ground points,
4. The induced voltages on Nonshielded cables can be reduced by as much as 60% by grounding both ends of an unused wire.

5- GIS Bus Extensions.

6- Surge Arrester Grounding:

Surge arresters should always be provided with a reliable low-resistance ground connection by one of the following methods (see fig.6):

1. Providing separate ground leads from arresters mounted on metal structures,
2. Using the arrester mounting structures as the surge arrester ground path because the large cross section of the steel members provides a lower resistance path than a copper cable of the usual size.

 Fig.6: Surge Arrester Grounding

However the following notes must be considered when grounding Surge arrester:

1. Arresters should be connected as close as possible to the terminals of the apparatus to be protected and have as short and direct a path to the grounding system as practical,
2. Adequate electric connections from the structure to both arrester ground lead and ground grid must be ensured,
3. Ensure that the steel cross sectional area is adequate for conductivity, and that no high resistance is introduced into joints from paint films, rust, etc.

7-  Separate Grounds:

The practice of having separate grounds within a substation area is rarely used for the following reasons:

1. Higher resistances for separate safety and system grounds are produced than would be the case for a single uniform ground system.
2. In the event of insulation failures in the substation, high currents could still flow in the safety ground.
3. Because of a high degree of coupling between separate electrodes in the same area, the safety objective of keeping the GPR of the safety grounds low for line faults would not be accomplished.
4. Often dangerous potentials would be possible between nearby grounded points because decoupling of the separate grounds is possible, at least to some extent.

8-  Transferred Potentials:

A serious hazard may result during a ground fault from the transfer of potential between the substation ground grid area and outside locations (see fig.7). This transferred potential may be transmitted by communication circuits, conduit, pipes, metallic fences, low-voltage neutral wires, etc. The danger is usually from contact of the touch type. A transferred potential problem generally occurs when a person standing at a remote location away from the substation area touches a conductor connected to the substation grounding grid. The importance of the problem results from the very high magnitude of potential difference, which is often possible. This potential difference may equal or exceed (due to induced voltage on unshielded communication circuits, pipes, etc.) the GPR of the substation during a fault condition.

 Fig.7:Transferred Potentials

Various means can be taken to protect against the danger of transferred potentials for :

1. Communication circuits,
2. Rails,
3. Low-voltage neutral wires,
4. Portable equipment and tools supplied from substation,
5. Piping,
6. Auxiliary buildings,
7. Fences.

 What is done after finishing the detailed final design of the substation grounding system as a paper work? After the detailed final design of the substation grounding system is finished , the following steps are usually followed: Construction of grounding system, Field measurement of resistance of Grounding system as constructed, Review of design steps from step #5, to step#11 based on actual Measurements, Investigation of transferred potentials and Special danger points.

In the next Article, I will explain and introduce the best ever excel spreadsheet for Grounding System Design for AC Substation. Please, keep following.