An Engineer must be familiar with the following topics in order to design substation/plant ground grids:
_ Ground Grid Concepts _ Surface Soil Resistivity _ Ground Potential Rise _ None of Influence _ Transfer Potential
_ Step-and-Touch Potential
_ Grid Depth and Number of Ground Rods _ Wire Sizing
_ Fault Times
Ground Grid Concepts
As explained in SAES-P-111, grounding systems perform the following main functions: _ To safeguard a person from electric shock by ensuring that, under fault
conditions, all surfaces with which a person is in simultaneous contact, including those of metallic equipment and the ground, remain at safe relative potentials.
_ To safeguard electrical equipment by grounding power systems to ensure that, under fault conditions, both voltages and currents are within predictable limits and that the protective devices will operate reliably and with appropriate discrimination.
_ To provide a path to ground from lightning arrestors that might operate due to direct lightning strikes, to lightning induced surges, or to switching surges.
_ To reduce the possibility of static electricity discharge that would present a fire risk in hazardous areas.
A grounding system consists of the grounding conductors that connect all items to be grounded and of a grounding electrode or grounding electrodes. The use of multiple grounding electrodes is known as a ground grid, and the ground grid forms the medium of
Ground Grid Concepts (Cont'd)
The purposes of the ground grid are to provide a low resistance path to earth for fault currents and to limit the rise of ground potentials that could generate surface gradients that are unsafe for human contact.
The following factors influence the design of a grounding system:
_ The maximum prospective ground-fault current that can pass between the fault location and the system neutral point or points and the duration of the ground-fault current flow. The size of the ground fault current governs the grounding conductor size.
_ The proportion of the ground fault current that will pass between the grounding system and the body of earth and the duration of the current flow. This factors govern the electrode design.
_ Site soil resistivity.
_ The degree of exposure to mechanical damage and corrosion. This factor will influence the choice of materials and the manner of installation.
Surface Soil Resistivity
The grid resistance and the voltage gradient within a substation are directly dependent on the soil resistivity. The surface soil resistivity is the resistivity of the upper layer of the soil. This resistivity is important because it helps limit the body current through addition of resistance to the equivalent body resistance. That is, if the upper layer of the soil is high in resistivity, the amount of current through the body of a person in contact with an energized component is reduced. A thin layer of crushed rock on the surface can raise the surface soil resistivity and can result in higher permissible step and touch voltages.
Because the effective length of the grid conductor is inversely proportional to the permissible body contact voltages, an increase in surface soil resistance allows a greater body contact voltage; consequently, a shorter grid length can be used for the same area. The result is greater grid spacing.
Ground Potential Rise (GPR)
Ground potential rise (GPR) is an AC potential difference between remote earth (reference of zero potential) and local ground. The magnitude of the maximum expected GPR determines the type of protection that is required for communication equipment as follows:
_ Locations at which the maximum expected GPR (or voltage stress magnitude) is less than 300V are classified as low risk sites. The amount of protection that is required for circuits at these sites is minimal and depends upon the reliability needs.
_ Locations at which the maximum expected GPR (or voltage stress magnitude) is between 300V and 1500V are classified as moderate hazards. Protection must be applied to all circuits. The acceptable protection methods should be determined by SAES-T-887.
_ Locations at which the maximum expected GPR is above 1500V are considered severe hazard sites. Protection methods will include isolating or neutralizing transformers as determined by SAES-T-887. Ground potential rise (GPR) is essentially the product of the following:
_ The total ground grid resistance to a remote earthing point (outside the zone of influence of the GPR at the power system fundamental frequency).
_ The total net fault current that flows through the ground grid. GPR can be expressed by the following equation:
GPR = IG x RG
where: IG = portion of the total fault current that flows through the
grid to remote earth.
Ground Potential Rise (GPR) (Cont'd)
The total prospective ground-fault current at a location should be determined through a system fault current study. The sub-transient symmetrical R.M.S. value of the ground-fault current "Ig" should be used. Frequently, a fault level analysis or relaying study will have been performed for the location and will give the required information. For all power plants, it should be assumed that the maximum ground-fault current is equal to the breaker's symmetrical interrupting capability.
The Electrical Engineer should determine the proportion of this current that is passing between the electrode and earth; he should then apply a factor for future system growth. This factor is a matter of judgement and is based on the following indicators:
_ Nearness of the calculated three-phase symmetrical fault level for the location to the circuit breaker interrupting capacity. If these values are close, the location is at a point of high fault level and the chance of future increases are limited.
_ The probability of development in the area (especially power generation). A remote location on the fringe of an established oil field can increase little in fault level. A location in an area of future development can increase greatly.
The computer program MALT, developed for Saudi Aramco installations, can analyze the effects of buried grounding electrodes. MALT should be utilized to review the grounding electrode design for all major industrial facilities (e.g., desalting facilities, seawater injection plants, gas plants, etc.). MALT can be used to determine the following:
_ The resistance to remote earth of grounding electrodes (grid) in a one layer soil model (regardless of shape, depth of burial, and size of conductors).
_ The resistance to remote earth of electrodes (grid) in a two-layer soil model (regardless of shape, depth of burial and size of conductors). _ The GPR of a given site.
Zone of Influence
The elevated potential of the industrial site grounding electrode during a ground fault results in a rise in potential of the earth inside and outside of the plant boundaries. The potential
Zone of Influence (Cont'd)
A potential contour survey can locate the hazardous potential gradients near grounded electrical structures for each fault type and location. The voltage drop to points surrounding the structure are measured from a known reference point and is plotted on a map of the location. A potential contour map then can be drawn through connection of the points of equal potential with continuous lines. If the contour lines have equal voltage differences between them, greater hazards are indicated by closer lines. Actual gradients that are due to ground fault currents are obtained through multiplication of test current gradients by the ratio of the fault current to the test current. A typical contour map of a substation grid is shown in Figure 7. The area that is contained by the perimeter (B) in Figure 7 is termed the zone of influence of the GPR. The permissible magnitude of the voltage that is along the perimeter (B) is by choice or design and is often limited by an agreement among the authorities concerned to a maximum of 300V.
The most accurate measurements of potential gradients are made through use of the volt- ammeter or current injection method. A known current, usually between 1 and 100 A and between 55 and 70 Hz, is injected into a remote ground test electrode through use of an insulated conductor. A current that is greater than 50 A (personnel and equipment safety considerations are to be observed) is preferred where the ground impedance is less than 1_. Where electronic measuring instruments are used (for example, a digital frequency selective voltmeter), a test current much less than 50 A is satisfactory.
This procedure would not apply where insulated overhead ground wires are employed and where calculations would be required. A remotely located ground test electrode is necessary to prevent gradient distortion from the mutual impedance of inadequately spaced ground electrodes. The distance between the ground under test and the remote current electrode can vary from less than 300 meters for a small ground grid or an isolated station to a kilometer or more for larger installations and for installations in densely populated areas. Measurements of the potential should be made with a very high impedance meter that is connected between the ground grid and a test probe, which is driven into the earth along the profile lines radial to the power station. Unless suitable means are employed to mask out the residual ground current and the other interference, the test current must be of sufficient magnitude to do the masking. External power frequency and harmonic components are removed through use of filtering. At the same time, in order to avoid variations in voltage gradients during a series of measurements, care must be taken to prevent heating and drying of the soil that is in contact with the ground grid or the test electrode. Low-current test methods will produce approximate results.
Economics and the necessary or the desired accuracy that is required will dictate the use of these methods or other methods and the number of measurements to be made.
Zone of Influence (Cont'd)
When more than one overhead or underground cable is connected to a substation, potential gradients in and around the substation can be quite different for faults that are on different lines or cables. Similarly, faults at different locations in large substations also can result in differences in potential gradients in and around the power station. Potential gradients in and around a large substation should be determined for two or more fault conditions.
Underground metallic structures, metallic structures on the surface of the earth, metallic fences, and overhead ground wires that are near a substation, whether connected to the ground grid or not, will usually have a significant effect on the potential gradients and should be considered in potential gradient measurements. These structures include neutral conductors, metallic cable sheaths, metallic water and gas lines, and railroad rails.
When a potential gradient study cannot be economically justified, potential gradients can be calculated from ground resistance and soil resistivity measurements. The accuracy of such calculations will depend on the accuracy of the measurements and on the unknown abnormalities of the earth around and below the ground grid. The adequacy of such calculations then can be verified with relatively few potential gradient measurements.
Depending upon the magnitude of a GPR, the following effects can arise outside a substation or adjacent to a power line grounding electrode or transmission line tower (within the zone of influence of the GPR):
_ The potential can be transferred through a metal part, bonded with or coupled resistively to the plant grounding electrode(s), to remote locations.
_ The touch voltage between a part that is grounded to the plant grounding electrode and a local ground (for example, a high-voltage interface ground) can be excessive.
_ Reversed touch voltage (or voltage stress) between the local ground and a part having a lower or even zero potential (for example, a telephone cable protection interface) can become excessive.
Zone of Influence (Cont'd)
Paragraph 4.1.6 in SAES-P-111 describes a situation in which a new grounding system is connected to an existing grounding system. The resultant zone of influence is that of the composite system. SAES-P-111 paragraph 4.1.6 reads as follows:
"Where a new grounding system is connected to or located within the zone of influence of an existing grounding system, the two grounding systems shall be interconnected by a minimum of two conductors per grid. The designer of the new grounding system shall be responsible to review the overall grounding system. Recommendations for upgrading the existing system(s), if required, shall be made to the Operations Department of the respective area."
Transfer Potential
Transfer is defined as the relocation of a hazardous potential from a ground-grid area to points that are outside of the ground-grid area. A serious hazard may result during a fault from the transfer of potentials between the ground-grid areas and outside points. This transfer of potentials is done by conductors (such as communication and signal circuits, low-voltage neutral wires, conduit, pipes, rails, and metallic fences). The danger usually is from contacts of the touch type. The importance of the problem results from the very high magnitude of potential difference that is often possible. Induced voltages on unshielded communication circuits, static wires, and pipes, can result in transferred potentials exceeding the sum of the GPR's of both the faulted substation and the source substation. Rails entering the station, when connected either intentionally or otherwise to the ground grid, can theoretically create a hazard at a remote point by transferring the grid potential rise during a fault. Similarly, if the rails are grounded remotely, a hazard can be introduced into the station area.
Hazards are possible where the neutrals of low-voltage feeders or secondary circuits that serve points that are outside of the station area are connected to the station ground. When the potential of the station ground grid rises as the result of ground-fault current flow, all or a large part of this potential rise can appear at remote points as a dangerous voltage between this grounded neutral wire and the adjacent earth. Where other connections to earth are also provided, the flow of fault current through these connections can, under unfavorable conditions, create gradient hazards at points that are remote from the station. Each installation should be reviewed for transfer potential hazards, and corrective action should be taken as required.
Transfer Potential (Cont'd)
For communication circuits, schemes have been developed that involve protective devices and insulating and neutralizing transformers to safeguard personnel and terminal communication equipment. The introduction of fiber optics to isolate the substation communications terminal from the remote terminal can eliminate the transfer of high potentials. Fiber optics should be considered when potentials cannot be easily controlled by more conventional means.
Rail hazards can be removed by removable track sections where the rails leave the ground- grid area, or through installation of several insulating joints in the rails that are leaving the grid area. A second set of insulating joints that are beyond the first set would protect against the shunting of a single set by a metal car or the soil itself and would also reduce the remote hazard of potential differences across a joint itself. The insulating joints must be capable of withstanding the potential difference between remote earth and the potential transferred to the joint. Adequate creepage distance should be ensured to offset any pollution or contamination problems.
Step and Touch Potential
Step potential (voltage) is the difference in surface potential that is experienced by a person that is bridging a distance of one meter with his feet, without contacting any other grounded object.
Touch potential (voltage) is the potential difference between the ground potential rise and the surface potential at the point where a person is standing with his hands in contact with a grounded structure. The maximum touch voltage is to be found within a mesh of a ground grid.
Death can occur from step and touch potentials, depending on the magnitude and the duration of the fault. Body conditions that can reduce resistance, such as wet hands or shoes, also can increase the probability of death or injury.
Step and Touch Potential (Cont'd)
As given by IEEE Std. 80, the following formulas are used to determine the allowable step- and-touch potentials:
_ The maximum driving voltage of any accidental circuit should not exceed the following limits:
- For step voltage the limit is for a man weighing 50 kilograms is: Similarly, the touch voltage limit is:
where:Cs (hs1 K) = 1 for no protective surface layer.
_s = the resistivity of the surface material in ohm - M = 150 (assumed measured value) or 3000 ohm-m for crushed rock.
ts = duration of shock current in seconds - (1 second
for Saudi Aramco installations).
Grid Spacing
If either the step voltage limit or the touch voltage limit are exceeded, a revision of the grid design is required. The revision may include smaller conductor spacings, and adding additional ground rods. Details of the potential revision can be found in IEEE Std. 80, Section 14.7.
Even if the step voltage limit or touch voltage limits are met, additional grid conductors and ground rods can be required if the grid design does not include conductors that are near the equipment to be grounded (such as surge arrestors and transformers).
Step and Touch Potential (Cont'd)
Another method to improve step and touch potentials is addition of crushed rock on the surface of the soil. Crushed rock will change the K factor in the CS (hS1K) portion of the step voltage limit and touch voltage limit formula as follows:
where: _s = crushed rock resistivity in ohm-m
_ = earth resistivity in ohm-m
The reduction factor Cs is also changed as a function of the change in K and the change in the thickness of the layer of crushed rock (hs), as shown in Figure 8.
Reduction Factor Cs as a Function of Reflection Factor K and Crushed Rock Layer Thickness hs
Step and Touch Potential (Cont'd)
Example: Find the ETouch 50 for the following conditions: _s = 2000 (using crushed rock)
ts = 1
hs = 0.1
_ = 150
for this example, K is determined as follows: From Figure 8, Cs = 0.58
Therefore:
= 318V
Grid Depth and Number of Ground Rods
Saudi Aramco Engineering Standard SAES-P-111 states that ground grids are to be buried to a minimum depth of 460 mm (18 in.). This method effectively reduces step and touch voltages on the earth's surface.
The total length of the grid reduces the step and touch voltages and the grid resistance. The grid length includes both conductor length and ground rod length.
The physical conditions at a substation dictate the number and the length of the ground rods vs. the conductor grid length. The ground rods are normally installed at the perimeter of the grid to moderate the increase of the surface gradient that is near the peripheral meshes. Ground rods should also be installed at major equipment and especially at lightning arresters. Rods that penetrate the lower resistivity soil are far more effective in dissipating fault currents when a two-or-multilayer soil is encountered and when the upper soil layer has higher resistivity than the lower soil layers. Ground rods that are in proximity are far less effective at dissipating fault currents than individual ground rods that are well spaced.
Wire Sizing
In AWG, the numbers are regressive: that is, a larger number denotes a smaller wire. Each wire size (in AWG) often is represented in circular mils. One circular mil (cm) is the area of a wire with a diameter of 0.001 inches. The cm measure is simply the diameter in mils squared.