SOLVINGillGH-VOLTAGEPROBLEMSIN WIRELESS/UTILITY COLLOCATIONS

Clayton HallmarkGrounding Systems LLCChagrin Falls, OH 44023

Ernest M Duckworth, Jr., P.E.Positron Industries, Inc.

Sed alia, CO 80135

ABSTRACT

Wireless communications providers are using electric-utility transmission towers in high-voltagecorridors throughout the world as sites for their equipment and antenna locations. This collocation withhigh-power transmission lines offers challenging engineering problems because of the effects of groundpotential rise (GPR). In the absence of actual test results, calculated GPR levels must be used indetermining the engineering design necessary to properly isolate wire-line communications fromdamaging GPR effects. These calculated GPR levels can be very large; and if it were not for the abilityto reduce these levels by improving the grounding system, there would be limitations on use of wire-linecommunications serving some of these locations. Limiting the use of wire-line communications servingcell sites in high-voltage corridors would limit cost-effective engineering design. New methodologies forgreatly improving small cell-site grounding systems are key to reducing GPR in high-voltage corridors tolevels that can be safely handled by isolation equipment.

I. INTRODUCTION

Isolation equipment is readily available that will protect wire-line communications facilities enteringPCS locations within high-voltage corridors from a GPR as high as 50 kV rms and 90 kV surge. Properlyinstalled, this isolation equipment will offer many years of maintenance-free, reliable protection from theeffects of GPR

Those PCS locations within high-voltage corridors that have overhead ground conductors (OGC) withno neutral will experience theoretical GPR levels under 45 kV peak, provided that the PCS groundingsystem resistance is less than 5 ohms. If a neutral is also present in the overhead, the theoretical GPRlevels will be less than 20 kV peak. This represents the vast majority of the type of high-voltagecorridors in use today, and these magnitudes can easily be isolated with equipment available on themarket.

The PCS locations within high-voltage corridors that have no OGC and no neutral will experiencemuch higher theoretical GPR levels, even with a 5-ohm PCS grounding system at the tower base. This isbecause all of the fault energy will pass down through the single tower into the ground. Worst case

theoretical GPR levels under these conditions could reach a maximum of85 kV. Note: Actual real-lifeGPR levels much over 30 kV peak asymmetrical may not occur, because earth ionization increases theearth conductivity if the current density becomes high enough.

Obtaining less than 5 ohms for a PCS grounding system in poor resistivity soils may be very difficultat a cell site with a small grounding system. However, significant grounding improvement to these smallgrounding systems can be obtained without expensive or elaborate grounding systems, as one of theauthors has shown.

II. GROUND POTENTIAL RISE (GPR)\ffi

l".'1'"

Electrical damage from ground potential rise (GPR) throughout the wireless industry has an estimatedcost in the many millions of dollars each year, but few engineers in the industry are even aware of thephenomenon.

Most times, the first sign that something is wrong comes right after a thunderstorm or after a fault onthe power line. Suddenly, the wire-line service coming into your cell site has failed, and the delicatecircuitry of your communications equipment is damaged. This is often misdiagnosed as an unavoidablemaintenance problem, and much money is spent on repairing equipment and replacing protective fusesand gas tubes -to say nothing of potential lost revenue. In the worst case, the safety of personnelworking at the site may be seriously compromised.

ill. SOLVING THE MYSTERY

In reality, this type of damage very well could be due to a phenomenon called ground potential rise(GPR).

When a ground fault occurs at a power substation, some of the fault current will return to its source,namely the substation transformer, via the earth, through the substation's ground grid (Figure 1). Thisground grid has its own characteristic impedance. Following Ohm's law, a current passing through animpedance will result in a voltage. This increase in the potential of the grounding system, referenced toremote earth, is called ground potential rise (GPR).

Figure 1. Development ofGPR from power system fault.

As Figure 2 shows, if your telecommunications lines coming into a cell site are copper, and if theselines are not properly isolated, they provide a path for the voltage impulse coming up from the groundingsystem, whether from lightning or a power fault as discussed earlier. Nonnally, communicationsengineers look upward for threats in the electrical environment; but this one comes from below, from thevery grounding system that is part of the electrical protection scheme. This threat is real and cancompromise personnel safety and damage equipment.

Figure 2. Communications location without isolation protection.

These GPR surge currents develop on the grounding system and are sent out onto your conductivecopper communications lines back to a remote ground, which in this case is the serving central office(CO). This is why ordinary surge protection devices such as gas tubes are ineffective in protectingagainst GPR.

However, special high-voltage protection (HVP) isolation devices -including isolationtransfonners, optical couplers, and fiber optics -interrupt the conductive paths that carry the GPRcurrents (Figure 3). These devices provide an isolation gap rated at 50kV fillS and 90 kv for surges. The;highest service reliability may actually be from wire-line facilities using passive isolation equipment, i.e.,isolation transfonners. Active isolation equipment using optical isolators requiring power will lower thereliability of a TI carrier or HDSL service and needlessly expose maintenance personnel more frequentlyto possible hanD.

Figure 3. Communications location with isolation protection.

IV. GET TO KNOW THE STANDARDS

Follow existing national codes and IEEE standard installation procedures while using HVP devices.The most important standards include:

.ANSI/IEEE Standard 487-1992 -Guide for the protection of wire-line communication facilitiesserving electric power stations.

.ANSI/IEEE Standard 367-1996 -Recommended practice for detennining the electric power stationground potential and induced voltage from a power fault.

.ANSI/IEEE Standard 80-1997 -Guide for safety in AC substation grounding.

.NFP A 70-1999 -National Electrical Code.

Communications protection engineers should not turn a blind eye to GPR damage because theybelieve special HVP devices are more expensive than gas tubes. Consider ongoing costs for continuallyreplacing damaged equipment year after year. Also consider that the costs of labor for repairs and thelost revenue from downed communications lines can easily surpass the cost of GPR protection. Anddon't forget personnel safety and liability issues: employees working in, on, or around equipmentconnected to a remote ground potential are at a safety risk if standards and codes are not followed.

Properly protected GPR locations, designed and maintained by trained employees, will reduce overallcosts, improve productivity, and increase circuit reliability over any time period.

V. MODERN GROUNDING TECHNOLOGIES

As can be seen in Figure 2 above, the GPR voltage can be reduced by bringing the resistance at thecommunications location (cell site) to a low level with respect to the remote ground location, effectivelyshorting out the GPR. The trend among wireless service providers is to specify a resistance to remoteearth of 5 ohms or less. A low ground resistance produces the following benefits:

.Reduces touch and step potentials, which are dangerous to personnel

.Reduces voltages across insulators that can cause current flashover across the insulators

.Reduces the likelihood of sideflashing, or arcing through air, between exposed and groundedstructures and components

.Diverts lightning current around concrete tower foundations, which can be exploded by the current

.Facilitates the discharge to ground of currents intercepted by protectors and arresters

.Keeps GPR within the specifications of HVP isolators.

Modem lightning research has led to improved understanding of the lightning threat. It has shown thewaveshape and magnitude of lightning strokes. This has shown the importance of low-impedance as wellas low-resistance grounds, since the destructive voltages developed by fast transients such as lightningdepend more on the inductive-reactance component of the impedance than on the resistance. However, itis usually easier to calculate, predict, and measure the resistance of an electrode than the inductive andcapacitive reactances. Fortunately, if we design and install an electrode for a low resistance such as 5ohms, it also tends to have low reactances.

VI. THINK LATERAL

When a really low resistance is required, the best advice for the grounding designer is to "thinklateral," especially when the soil is highly resistive or too thin to allow driving rods. A flat electrode ofsignificant lateral extent, at a shallow depth of only 30 in., may be the best or only option. It resemblesa buried plate, which provides a highly capacitive electrode. The resistance of an electrode is inverselyproportional to the capacitance. In fact, the fonnula for resistance of any earth electrode is based on thecapacitance between the buried electrode and its hypothetical image above the earth.

Conductive cement such as EarthLink from Grounding Systems provides an easy, economical way todesign and install extensive electrodes. The cement is employed as a backfill material around commonlyused metallic electrodes such as driven rods and buried wires (counterpoise) and rings, increasing theircross-sectional area by a factor of 100 or 200 (for a 4/0 wire) or even more. In many adverse groundingsituations, the conductive electrode may be the only economic and practical method of obtaining 5 ohms.

Cement can be used to augment almost any kind of electrode, and the results are easy to calculate andpredict and are permanent. Cement is well known to contractors to protect buried metal from corrosion.Not just any kind will do for grounding, however. Conductive cements have over 200 times lowerresistivity than ordinary cement -low enough that standard formulas can be used for calculating theresistance of electrodes made with them, just as if the electrodes were made of metal.

Figure 4. Conductive cement effectively enlarges the wire, creating a conductive plate.

Figure 4 shows a horizontal-strip configuration, or groundbed, and the formula for calculating itsresistance. The most common installation procedure follows:

1. Dig a trench, 30 in. deep, 20 in. wide, and as long as required to obtain the desired resistance. (Thelength is a design calculation, discussed later.) Center a 4/0 stranded wire in the bottom of the trench.

2. Pour in the cement as a dry powder (it will later absorb moisture and harden) by dragging an open bagof it down the trench. Use one 50-lb bag every 10ft. Heap the cement up as shown.

3.'Lift the wire slightly so it is completely covered by the cement for corrosion protection. Tamp thecement with feet or a shovel toward the tapered edges.

4. Carefully shovel in a 4-in. layer of soil and tamp it down.

5. Push in the rest of the removed soil using construction equipment.

VB. DESIGNING A HORIZONTAL ELECTRODE

The design procedure is as follows:

1. Decide upon the desired resistance of the electrode.

2. Measure the soil resistivity with an earth tester.

3. Detennine the required length from the table, based on the desired resistance (5 or 10 ohms) and thesoil resistivity.

Length for5-0hmGround

Length for10-0hmGround

SoilResistivity

5000 Q-cm7000 Q-cm

10,000 Q-cm15,000 Q-cm20,000 Q-cm30,000 Q-cm50,000 Q-cm

100,000 Q-cm

10m (33 ft)16 m (52.5 ft)26 m (85 ft)44 m (144 ft)63 m (207 ft)105 m (344 ft)194 m (636 ft)440 m (1444 ft)

3 m (9.8 ft)6 m (20 ft)10 m (33 ft)18 m (59 ft26 m (85 ft)44 m (144 ft)84 m (276 ft)194 m (636 ft)

Table 1. Table of lengths for 5- and IO-ohm grounds. Use the formula for intermediate values.

vIn. GROUND RING

A typical pad-mounted wireless site has a buried ground ring around the pad, about 2 ft out from thepad, and another ring around the antenna. The formula given in Figure 4 applies; however, theresistance thus obtained must be multiplied by 1.12 to account for the reduced grounding efficiency of asquare ring compared to a straight strip. For example, if the two rings require 145 running feet (44 m)just to surround the pad and antenna, the table shows this would give about 5 ohms in 15,000 ohm-cmsoil (about 1.5 times the average U.S. soil resistivity). Multiplying by 1.12, the resistance would beabout 5.6 ohms. A still lower resistance could be achieved by extending radials from the four outercorners of the configuration.

IX. GIRD THE GRID

Meanwhile, back at the substation, the source of the GPR from power faults, the GPR can be reduced bylowering the resistance of the grounding grid. If conductive cement is used to surround grid wires on a10-by-10-ft spacing, the grid area can be reduced by 10 or 20 percent, with a concomitant money savingand reduction in the extent of the critical 300-V GPR contour. Use IEEE Std. 80-1997 data or EPRISubstation Grounding Workstation software and assume strip conductors of2-in.-by-18-in. cross section.For further information, refer to manufacturers' application notes.

Existing ground grids also can be improved by extending the grid area by 10 or 15 percent and usingconductive cement. In one application in high-resistivity soil, grid resistance was reduced from 10 ohmsto 2 ohms. In another, resistance was reduced from 0.96 to 0.2 ohm. Consolidated Edison and BostonEdison have used conductive cement to ground transmission towers and substations.

X. EMBEDDED GROUND ROD

About 50% of the resistance between a ground rod and remote earth is in a shell within the first 6 in.from the rod. If this shell is shorted out by encasing the rod in 6 in. of conductive cement, as shown inFigure 5, the resistance is halved. This is a good example of how the resistance of any electrode can bedecreased without making the electrode longer. This is important wherever bedrock limits the length ofground rods or when property lines limit the length of a horizontal electrode.

CONNECTINGWIRE

R=.E!-[ In ',-In '.] +..!.. [ In 4L -1 -In r.2nL 2nL 7 FT, IN 12-IN. DIA

AUGERED HOLE(6 -IN. RADIUS)

POURED CYLINDER OFCONDUCTIVE CEMENT

NOTE:Po (CEMENT)=20 C1-cmp,=SOIL RESISTIVITY INr.= RADIUS OF ROD = O.79cmr,= RADIUS OF CEMENT = 15.23cmL = LENGTH = 244cm

~

1ONE FOOT OF

8-FT. RODDRIVEN INTO SOil

NOT DRAWN TO SCALE

~

d = 1.6 cm for 5/8 in. rod

Figure 5. This embedded ground rod takes advantage of the fact that 50 percent of the earth resistance iswithin 6 inches of the rod.

REFERENCES

Positron Industries, Inc., Teleline Isolator Product Guide, Montreal, Quebec, Canada, 1999.Grounding Systems Co., Application Note TD-l, Ground Grid Improvements and Extensions, ChagrinFalls, OR, 1999.C. L. Hallmark, Horizontal Strip Electrodes for Lowering Impedance to Ground, INTELEC 97Proceedings, Sec. 17-2, pages 368-375.Gilbert Sharick, Grounding and Bonding, Vol. 13 of abc TeleTraining Basic Series, abc TeleTraining,Geneva, IL, 1999.