Patent Description:
The issue of corrosion current resulting from an electrical supply substation comprising a large copper ground bus is known and discussed in "<NPL> which proposes the solution of sacrificial anodes ("rectifiers") in reducing the corrosion of line anchors.

In accordance with the present invention, there is provided a method for protecting a plurality of electrical poles located near an electrical substation, comprising the steps of:.

The present invention recognizes that the grounding grid of an electrical substation, having a more electropositive native potential (-<NUM> mV) than the native potential of the galvanized steel poles near the substation (-<NUM> ,<NUM> mV), creates a galvanic corrosion cell which results in accelerated corrosion of the galvanized steel poles. To counter this condition, anodes are installed adjacent the grounding grid, and an impressed current is established so as to shift the effective potential (the Instant Off potential) of the grounding grid to approximately -<NUM> mV. With that impressed current being applied to the grounding grid, the metal poles no longer "see" the grounding grid as a large electropositive cathode, which eliminates the driving force for galvanic corrosion of the poles and thereby protects the poles against corrosion.

<FIG> shows a prior art electrical substation <NUM>, which includes a large, underground copper grounding grid <NUM> beneath the substation <NUM>.

In a typical prior art electrical substation, a ground wire <NUM> extends from the substation <NUM> to the nearest electrical pole <NUM> and then from one electrical pole <NUM> to the next, and each of the electrical poles <NUM> in the series is electrically connected to this ground wire <NUM> via a wire pigtail <NUM>. (It should be noted that the electrical poles <NUM> in the drawing may represent utility poles or towers, and the use of the word "pole" in this description also encompasses towers. ) The ground wire <NUM>, which may also be a neutral return or shield wire as needed for the electrical circuit or lightning protection, is electrically connected to (that is, it is in electrical continuity with) the substation <NUM>, which, in turn, is electrically connected to the copper grounding grid <NUM> via the bonding wires <NUM>. Each power pole <NUM> is also firmly planted into the ground (soil <NUM>).

The present invention includes the realization that this arrangement results in a galvanic corrosion cell that accelerates the corrosion of the poles and of any metal anchors connected to the poles, because the poles <NUM>, whether or not they are galvanized, have a much more electronegative native potential than the copper grounding grid <NUM> of the substation <NUM>. The ground wire <NUM> from the poles <NUM> to the substation <NUM> and the grounding wires <NUM> from the substation <NUM> to the grounding grid <NUM> provide an electrical pathway (electrical continuity) from each pole <NUM> to the copper grounding grid <NUM>, and the earth <NUM> itself provides an ion pathway so as to complete the electrochemical circuit. The power poles <NUM> (and any metal anchors connected to the poles <NUM>) effectively "see" the copper grounding grid <NUM> of the substation as being a cathode, having a more electropositive potential than the poles <NUM> (and anchors), and the poles <NUM> (and anchors) then become the anodes of this corrosion cell. This means that the poles <NUM> (and anchors) lose electrons and corrode. Thus, the connection of the poles <NUM> (and anchors) to the substation <NUM> and to its copper grounding grid <NUM> causes accelerated corrosion of the power poles <NUM> (and anchors) due to galvanic action.

The native ground potential of the copper grounding grid <NUM> typically is approximately -<NUM> millivolts (mV), while the native ground potential for zinc galvanized steel poles typically is from -<NUM> to -<NUM> mV, depending on the specific intermetallic layer present. When the grounding grid <NUM> and poles <NUM> are made electrically common by bonding via the pigtails <NUM>, wire <NUM>, substation <NUM>, and bonding wires <NUM>, a mixed-metal potential of about -<NUM> mV, which is calculated as the mathematical average: <MAT> results on all electrically common structures. This potential may vary depending upon soil corrosion characteristics. This large difference in potential sets up the galvanic cell, resulting in accelerated corrosion of the galvanized steel poles <NUM>, with the more electronegative metal (the galvanized poles <NUM> and anchors at - <NUM> ,<NUM> mV native potential) behaving as the anode and the more electropositive metal (the grounding grid <NUM> at - <NUM> mV native potential) behaving as the cathode.

Of course, this is an unintended consequence of grounding the poles <NUM> through the substation <NUM> to the copper grounding grid <NUM> in corrosive soils.

<FIG> and <FIG> schematically depict the solution which is the subject of this invention. As best appreciated in <FIG>, impressed current anodes <NUM> are placed around the grounding grid <NUM> to surround the grounding grid <NUM>. In this particular embodiment, the impressed current anodes <NUM> are placed on the North, South, East and West sides of the grounding grid <NUM>, at approximately the midpoint of each side of the grid <NUM>, and at a distance of about ten feet outside of the grid. In this embodiment, four anodes placed in the cardinal directions (N-S-E-W) around the grounding grid and placed at a distance of L/<NUM> (with L being the length of a given side of the grid) is appropriate. In other cases, a number, greater than four, of impressed current anodes are placed at a distance of L/<NUM> or less from the sides of the grid. Alternatively, continuous linear anodes may at times be desirable - these would be plowed in or trenched in adjacent to the grounding grid. There is no reason in theory why the anodes could not be placed inside the grounding grid, except in practicality, if the substation is located there, it would require too much disturbance of existing assets to install or repair. The impressed current anodes may be made of any suitable material. Commonly used materials for impressed current anodes include graphite, cast silicon-iron or mixed metal oxide wires. Numerous types are commercially available.

These anodes <NUM> are electrically connected to each other via an electrical wire <NUM>, which, in turn, is electrically connected via an electrical wire <NUM> to the positive (+) terminal of a direct current (DC) power source <NUM>, which in this case is a cathodic protection rectifier <NUM>. Another electrical wire <NUM> connects the negative (-) terminal of the DC power source <NUM> to the grounding grid <NUM>.

Using this arrangement, an impressed current is applied to the grounding grid <NUM> by the DC rectifier <NUM> to lower the electrochemical potential of the grid <NUM>. In this instance, an impressed current resulting in an IR free polarized potential of approximately -<NUM> to -<NUM> mV instant-off potential is applied, as measured at the grounding grid <NUM>. This instant-off potential approximates but is slightly less negative than the native potential of the galvanized steel poles <NUM>. (If a potential were applied that was more negative than the -<NUM> mV potential of the poles <NUM>, it might cause a shift in pH of the soil, which could cause accelerated corrosion of the galvanized coating on the poles <NUM>. ) This impressed current effectively reduces the potential of the grid <NUM> as "seen" by the galvanized poles <NUM> nearly back to the native potential of the poles <NUM>. This means that there is no longer a galvanic corrosion cell driving force between the poles <NUM> and the grounding grid <NUM>, so the grounding grid <NUM> no longer causes accelerated corrosion of the poles <NUM>.

The standard Instant-Off potential is measured with respect to a copper-copper sulfate reference cell. The Instant-Off measurement is captured when the Cathodic Protection current (CP current) is interrupted, and the IR drop in the soil disappears to reveal a CP potential plateau (lasting up to half a second) that best approximates the polarization between the structure and the contacting soil. In this case, the structure is the grounding grid <NUM>.

To attain the desired level of impressed current at the grounding grid <NUM>, the rectifier <NUM> is energized, and the voltage and amperage outputs are adjusted until the instant off reading at the grounding grid <NUM> is the desired reading. Instant off potential is the same as an IR free potential (where V=IR stands for Voltage= Current (I) X Resistance (R)), and the IR portion is the potential contribution which may be measured as the Cathodic Protection current flowing between the reference cell (placed atop the soil) and the structure.

It should be noted that this arrangement also provides protection to the copper grounding grid <NUM> which is susceptible to accelerated corrosion in corrosive soils due to the galvanic cell that has been created with the poles <NUM>.

While there may be variations in the protocol to establish the desired degree of protection of the grounding grid <NUM> and the poles <NUM>, a typical protocol is outlined below:.

In an actual field test, the initial current used at the rectifier at substation A was <NUM> amps. The average native potential was measured (averaging the observed native potential at a plurality of points around the grid <NUM> of substation A) as <NUM> mV, and the average "Instant Off" potential was measured (averaging the observed Instant Off potentials) as <NUM> mV.

The average polarization (AP) was then calculated: <MAT> <MAT>.

The desired shift was then calculated: <MAT> <MAT>.

Solving this equation yields <NUM> amps as the required current to use in the rectifier <NUM> for substation A, so a <NUM> amp current is used as the impressed current at this particular substation A.

<NUM> - Set rectifier <NUM> output to attain -<NUM> mV CSE (Copper-sulfate reference electrode) potential on the grounding grid <NUM> (aim for instant-off potential of about -<NUM> to -<NUM> mV).

<NUM> - Measure the cathodic protection "on" and "instant off" potentials on selected
poles to confirm that a sufficient shift in potential has been achieved. Preferably these measurements are taken at least <NUM> hours after the grounding grids <NUM> have been electrified with their corresponding rectifiers <NUM>.

<NUM> - Consider supplementing the cathodic protection at individual poles <NUM> showing a potential of less than -<NUM> mV by installing additional localized cathodic protection (such as sacrificial magnesium anodes locally at the individual poles <NUM>). It is expected that practically <NUM>% corrosion protection is obtained for poles <NUM> near substations <NUM>. However, poles <NUM> located very far from substations <NUM> may have a limited shift in potential (in the range of <NUM> to <NUM> mV shift) and therefore only partial protection is obtained. Even with low potential shifts for poles far from the substations, this can translate into a substantial addition to the life of those galvanized poles.

<FIG> is a graph showing the years of useful life for a galvanized pole or structure starting at <NUM> year useful life at zero shift in potential. It may be appreciated that a shift in potential of approximately -<NUM> mV results in an <NUM> year useful life, an increase of one order of magnitude in the useful life of the pole.

<NUM> - Wireless transmitters may be installed to monitor data from reference electrodes measuring the electrical potential at selected poles <NUM> so as to detect irregularities which may signal a change in the environmental or physical conditions surrounding the pole <NUM> which may impact its level of cathodic protection.

The electrochemical potentials are an indication of corrosion activity and as such the data can be used to monitor the corrosion activity of the poles <NUM>, the effectiveness of the cathodic protection, the level of protection, changes in soil corrosivity surrounding the poles <NUM>, and irregularities in the shield line <NUM>.

Claim 1:
A method for protecting a plurality of electrical poles (<NUM>) located near an electrical substation (<NUM>), comprising the steps of:
providing a grounding grid (<NUM>) beneath the substation (<NUM>), said grounding grid (<NUM>) being in contact with the ground;
electrically grounding the substation (<NUM>) to the grounding grid (<NUM>);
electrically grounding a plurality of electrical poles (<NUM>) to a common ground wire and electrically grounding that common ground wire to said grounding grid (<NUM>);
surrounding the grounding grid (<NUM>) with at least one impressed current anode (<NUM>) near said grounding grid (<NUM>), wherein either:
four impressed current anodes (<NUM>) are placed on the North, South, East and West sides of the grounding grid (<NUM>) at approximately the midpoint of each side of the grounding grid (<NUM>) and at distance of about L/<NUM> from the sides of the grounding grid (<NUM>), wherein L is the length of a given side of the grid;
a number greater than four, of impressed current anodes (<NUM>) are placed at a distance of L/<NUM> or less from the sides of the grounding grid (<NUM>), wherein L is the length of a given side of the grid; or
a plurality of continuous linear impressed current anodes (<NUM>) are plowed in or trenched in adjacent to the grounding grid (<NUM>);
providing a direct current power source (<NUM>) having a positive terminal and a negative terminal;
electrically connecting the negative terminal of the direct current power source (<NUM>) to the grounding grid (<NUM>) and electrically connecting the impressed current anodes (<NUM>) to each other and to the positive terminal of the direct current power source (<NUM>); and
using the direct current power source (<NUM>) to apply a direct current that reduces the effective electrical potential of the grounding grid (<NUM>).