A properly designed and installed grounding system ensures reliable performance of electrical substations.
Just how important is substation reliability? Fast clearing of faults, made possible by good grounding, improves the overall safety and reliability of an electrical system. Therefore, substation reliability must be as "built-in" as possible because of the high available fault current levels present and unlikely occurrence of follow-up grounding inspections.
The following 11 basic tips, if put into practice, will enhance your substation grounding system.
Tip 1: Size conductors for anticipated faults
Conductors must be large enough to handle any anticipated faults without fusing (melting). Table 1, which is derived from IEEE 80-1986, IEEE Guide for Safety in AC Substation Grounding, lists the maximum allowable fault current (in kA) for various conductor sizes and fault durations. Fig. 1 shows maximum-allowable-fault-current-versus-time performance curves for the same conductor sizes listed in Table 1.
Failure to use proper fault time in design calculations creates a high risk of melted conductors. For example, a 4/0 AWG conductor can withstand 42,700A for 0.5 sec before fusing. However, this same conductor can withstand only 13,500A for 5 sec. IEEE 80 suggests using 3.0 sec for small substations. This time is equal to the short-time rating of most switchgear.
Tip 2: Select the right connector
The connections between conductors and the main grid, and between the grid and ground rods, are as important as the conductors themselves in maintaining a permanent low-resistance path to ground. You must consider the type of bond the connection creates with the conductor or ground rod and temperature limits.
The most frequently used grounding connections are mechanical pressure-type (bolted, compression, and wedge) and exothermically welded. Pressure-type connections produce a mechanical bond between conductor and connector. This connection either holds the conductors in place or squeezes them together, providing surface-to-surface contact with the exposed strands. The exothermic process fuses the conductor ends together to form a molecular bond between all strands of the conductor.
Temperature limits are important considerations. How effectively a connection carries current indicates how well it will maintain low resistance. IEEE 80 rates the maximum allowable temperature limits for both pressure-type and welded connections. IEEE 837 gives additional information.
Tip 3: Pay attention to ground rod length, number, placement, and spacing
The length, number, and placement of ground rods affect the resistivity of the path to earth ground. Each doubling of ground rod length reduces resistivity 45%, if you're working with uniform soil conditions. Usually, soil conditions are not uniform, so it's vital to obtain accurate resistivity data by measuring ground rod resistivity with appropriate instruments.
For maximum efficiency, rods should be placed no closer together than the length of the rod. Normally, this is 10 ft. Each rod forms an electromagnetic shell around it, and when the rods are too close, the shells actually interfere with each other.
For economic reasons, there's a limit to the maximum distance between rods. Normally, this is 20 ft. At more than 20 ft, the cost of real estate and additional conductor needed to connect the rods is not economically attractive. Four interconnected rods on 100-ft centers will reduce resistivity 94% over one rod but require at least 400 ft of conductor. On the other hand, four rods placed 20 ft apart will reduce resistivity 81% over one rod and use only 80 ft of conductor. Additionally, the 20-ft spacing uses only 4% of the real estate consumed by the 100-ft spacing.
Tip 4: Prepare the soil
Soil conductivity is an important consideration in substation design. The lower the resistivity, the easier it is to get a good ground. In areas where soil conductivity is low or where dry weather can change soil conductivity, consider using a ground-enhancement material. Another option, especially in areas where deep frost occurs, is to use deep-driven rods.
Tip 5: Eliminate step and touch potential
Limiting step and touch potential to safe values in your substation is vital to employee safety. Step potential is the voltage difference between a person's feet and is caused by the dissipation gradient of a fault entering the earth. Just 30 in. away from the entry point, voltage usually will have been reduced by 50%. For example, a 1000A fault in a 5-ohm grounding system will enter the earth at 5000V. So, 30 in. away, less than the distance of a normal step, a fatal potential of 2500V will exist. This is shown in Fig. 2.
Touch potential represents the same basic hazard, except the potential exists between the person's hand and his or her feet. However, since the likely current path runs through the arm and heart region instead of through the lower extremities, the danger of injury or death is even greater.
In both situations, the potential essentially can be eliminated by an equipotential wire mesh safety mat installed just below ground level, as shown in Fig. 3. Connected to the main ground grid and any switches or equipment a worker might touch, an equipotential mesh will equalize the voltage along the worker's path and between the equipment and his or her feet. With the voltage difference (potential) thus essentially eliminated, the hazard to personnel is virtually eliminated as well.
An equipotential wire mesh safety mat is usually fabricated from #6 or #8 AWG copper or copper-clad wire to form a 24 x 24-in. or 24 x 48-in. mesh. Many other mesh sizes are available. To ensure continuity across the mesh, all wire crossings are brazed with a 35% silver alloy. Interconnections between sections of mesh, and between the mesh and the main grounding grid, should be made so as to provide a permanent low-resistance high-integrity connection.
Tip 6: Ground the foundation
Because it's nearly impossible to isolate a metal structure from its foundation, the use of "Ufer" grounds has significantly increased in recent years. Ufer grounds utilize the concrete foundation of a structure plus building steel as a grounding electrode. Even if the anchor bolts are not directly connected to the reinforcing bars (rebar), their close proximity and the semi-conductive nature of concrete will provide an electrical path. Two additional facts need to be considered in Ufer grounding.
A high fault current (lightning surge or heavy ground fault) can turn moisture in the concrete to steam. This steam, attempting to expand to 1800 times its original volume, produces forces that may crack or otherwise damage the concrete. In an actual installation, a major utility used only the footers for grounding electrodes on a 765kV line. Later inspection found 90 foundations with fractures, some severe.
The presence of even a small amount of DC current will ca use corrosion of the re bar. Because corroded steel expands to more than two times its original volume, this expansion creates extremely large forces on the concrete. Although AC leakage will not cause corrosion, the earth will rectify a small percentage of the AC to DC. In situations where the anchor bolts are not bonded to the rebar, concrete can disintegrate in the current path.
To reduce concrete damage, you can limit the short duration current or provide a metallic path from the rebar through the concrete to an external electrode. That external electrode must be sized and connected to protect the concrete's integrity.
Proper design of Ufer grounds, as shown in Fig. 4, provides for connections between all steel members in the foundation and one or more metallic paths to an external ground rod or main ground grid. This gives faults a low-resistance path through the concrete to the earth.
Tip 7: Ground the fence
Utilities vary in their fence-grounding specifications, with most specifying that each gate post and corner post, plus every second or third line post, be grounded. All gates should be bonded to the gate posts using flexible jumpers. All gate posts should be interconnected. In the gate swing area, an equipotential wire mesh safety mat can further reduce hazards from step and touch potentials when opening or closing the gate.
Some substation designs require fence grounding to be isolated from the main ground grid; others require it to be tied into the grid. Tying the fence ground into the main ground grid, as shown in Fig. 5, will reduce both grid resistance and grid voltage rise. A word of caution here: Internal and perimeter gradients must be kept within safe limits because the fence is also at full potential rise. This can be accomplished by extending the mesh with a buried perimeter conductor that is 3 ft or 4 ft outside the fence and bonding the fence and the conductor together at close intervals.
Tip 8: Ground all disconnect switch handles
To protect the switch operator in case of a fault, place a safety mat on or under the earth's surface at all switch handles. There are four types of safety mats.
* A steel grate or plate on supporting insulators. This works only if the operator can be kept completely isolated on the grate. Therefore, insulators must be kept clean. Any vegetation in the vicinity should be cut or eliminated completely.
* A steel grate on the surface, permanently attached to the grounded structure. This arrangement has the operator standing directly on the grate.
* Bare conductor buried (in a coil or zigzag pattern) under the handle area and bonded to the grounded structure.
* Prefabricated equipotential wire mesh safety mat buried under the handle area and bonded to the grounded structure, as in Fig 6 (on page 46). This is likely to be the least expensive choice.
In all but the first arrangement, both the switch operating handle and the personnel safety grate (or mat) should be exothermically welded to structural steel, thus ensuring nearly zero voltage drop.
Tip 9: Ground all surge arrestors
Surge arrestors pass surge energy ("spikes") to ground. To transfer current at minimum voltage drop (which provides maximum protection), each surge arrestor groundlead should have a short direct path to earth and should be free of sharp bends.
To use transformer tanks or structures as the grounding path, you must ensure that multiple paths to ground are both available and secure (this includes making effective connections). Whenever there is any question about the adequacy of these paths, use a separate copper conductor between the arrestor and the ground terminal (or main grounding grid). Because steel structures (due to their mass) have a lower impedance than a separate copper conductor, connect any separate conductor to the structure near the arrestor.
Tip 10: Bond and ground all cable trays
The NEC in Art. 318 details the requirements for cable trays, which cannot be treated the same as conduit. All metallic tray sections must be bonded together because mechanical splice plates do not provide an adequate path for fault currents. Therefore, the bonding jumpers (either the welded type used on steel trays or the lug type) must be placed across each spliced joint. If a metallic tray comes with a continuous grounding conductor, the conductor can be bonded inside or outside the tray. When cable tray covers are used, they should be bonded to the tray with a flexible conductor. The trays themselves should be bonded to the building steel (usually at every other column) and to all conduits containing conductors common to the cable tray system.
Tip 11: Pay attention to temporary grounding
When personnel work on high-voltage electric structures or equipment, any conductive bodies should be grounded. The usual grounding method is to attach a flexible insulated copper cable with a ground clamp or lug on each end, as shown in Fig. 7. These flexible jumpers require continual inspection and maintenance.
For cable connections to clamps, welded terminations (either a welded plain stud or a threaded silicon bronze stud welded to the conductor end) will provide a secure, permanent connection. The electrician solidly ties the clamp or lug to ground, then attaches the other clamp to the cable being grounded. The selection of the right ground for the first step is critical, as the following two incidents illustrate.
Case History No. 1. A utility company ran a series of tests to determine the effectiveness of grounding jumpers. Using a 20,000A short circuit, it discovered clamping directly to the tower structure was ineffective in that the high contact resistance between the clamp and the structure surface caused the clamp to be blown off.
Case History No. 2. A firm tested a stainless steel stud that projected through a hole in a structure; it was secured with a washer and nut on each side. The testers attached a clamp the stud, then subjected it to a 20,000V test. The short circuit caused the stud to burn off at the structure.
A solution to the high-resistance structure attachment involves welding a copper stud to the structure. Attaching a ground clamp to this copper stud provides a low-resistance contact. A special note: The stud must have an enlarged end located away from the structure, because a fault would tend to move the jumper toward this end of the stud. Fig. 8 shows some common studs, which should be welded or bolted to steel surfaces and fabricated to fit specifications.
Table 1. Maximum allowable current, in kA, for copper conductors using exothermic connections.
For copper conductors, the exact formula from IEEE Std. 80 can be simplified to:
I = A / K [square foot of S]
Where:
I = RMS current in amperes
A = Conductor size in circular mils
S = Current duration in seconds
K = Constant for maximum allowable temperature