Twenty years ago I was standing in a 15-kv substation when a distribution buss CT failed in a fault and exploded. A piece of debris penetrated a nearby OCB and started an oil leak. The investigation turned up a poor ground connection that could not protect the CT from current imposed in the buss fault. On another occasion I participated in the investigation of a failed million pothead on a distribution riser leaving the sub. The investigation turned up an improperly installed powder-actuated wedge connector on the cable terminal’s ground. On yet another occasion a “buzzing” gate post turned out to be minus the post-to-post ground buss outside the gate that had been pulled from the ground by a copper thief. That station gate was 3 feet from a public sidewalk. Imagine that!
How Substation Grounds are Supposed to Work
The methodology and construction specs vary, but the design criteria is the same. The ground system has three purposes, all of equal importance. Conduct faults to earth; limit voltage rise on the station mat; and eliminate step and touch potentials.
There are two components to the station’s grounding system made up of the main grounding buss and the grounded mat. The main buss is larger copper conductor, usually 4/0, that rings and crosses the station. The buss is usually trenched in with leads brought up in critical locations. Rods are installed along the buss anywhere from 20 feet apart to 100 feet apart according to the calculated resistivity of the earth. Woven between the buss is the mat. Mats are often constructed of smaller copper anywhere from #4 to #10, but can be larger. They are cross connected to form 12-inch to 3-foot squares, all depending on the design criteria. Mats and buss are usually exothermically welded. Years ago we used to braze them, but learned that heat in a prolonged fault can melt the connections.
The buss is principally designed to conduct anticipated fault currents to earth, while the mat is principally designed to provide against step potential and as a shield from currents and voltage rise injected into the earth during a fault. The mat must be carefully laid and effectively cross connected. In some installations the mat is trenched in similar to, but not as deep as the main grounding buss. In other installations the mat is laid over graded and compacted earth and then covered with good clay soil, packed and tamped. After the cover soil is tamped, a layer of crushed stone is laid. The stone layer can be anywhere from 1 to 3 feet deep.
The stone is not there for architectural reasons. The crushed stone bed provides an insulating buffer between workers and the mat. The stone layer’s resistance is sometimes 3,000 ohms/meter or more between the worker and the mat.
A common question about equi-potential in substations comes up often: “Why add resistance to the mat if you want equi-potential?” The answer is in the basic electrical principal that makes equi-potential protection work. Keep in mind the phrase “equal potential.” If you are walking across a substation when a 50-kv 20,000 amp fault hits the ground grid, you are protected against step potential at your feet by the grid. Your feet are at “equal potential.”
EPIC Protection: “Equal Potential – In-equal Current”
If you are holding a switch handle, another property of the equi-potential system is engaged. At the handle, you are a parallel resistor with the grounding system that is conducting fault into the ground grid that you are standing on. In the law of parallel resistances, the voltage across all paths in the circuit is the same. At the handle, you are still at equal-potential (voltage) with the fault current flowing in the ground system. Voltage without current, even at high levels, is not usually deadly.
On the other hand, now that you are in the path of the fault, you also have to worry about current. The equi-potential principal, designed into the station’s grounding system, protects you here too. The current across all parallel resistances in the current path is proportional to the resistance in each of the parallel paths. The station’s ground grid is practically of zero resistance while you are more than 4,000 ohms across your body and the stone layer to the mat. Just like overhead EPZ, the mat and resistance of your body took the voltage but minimized the current to a survivable level. The next question is usually “Should I wear rubber gloves when I switch in a station?” My answer is “I do!”
Back to the Mat
You can see how important the mat is to the survival of the station and to workers in and people around the station. If the mat is disturbed or damaged, it is critical that the mat be excavated back to undamaged areas and reconstructed to its original design integrity. When the mat is installed, leads are brought up at every device, column, fence post and transformer location. Foundations are tied in and a grid extension is laid outside the boundary of the station, often several feet outside the fence line. The integrity of each of these connections to the grid is important and must not be compromised.
Things to Look For
Ground buss and mat resistance is usually monitored by substation departments. Test wells are often installed so that fall of potential tests can be compared to the original installation test data to monitor for degrading conditions. These tests don’t always turn up damage that might affect you in the station. It’s not an exhaustive list, but here are some things to watch for.
Deep wheel tracks: Station stone is sometimes deep and, if constructed of larger rock, hard to walk in. If deep wheel furrows are noted in the station, avoid walking in those areas until someone verifies that the mat below the wheel marks is not damaged.
Dead grass: Dead grass along a boundary fence is sometimes vegetation control, but it can be a hot fence. Look at fence sections to examine for ground grid connections. By the way, if you have to remove a fence section to shuttle in equipment, the fence opening should be jumpered just like an open neutral.
Rust: Rust indicates some level of cathodic activity. Cathodic activity can cause connections to degrade and increase resistance to current flow in a fault. Rust at the bottom of a structural member may also indicate cathodic activity that may affect the grounding system. Report rusting steel or rusting ground connections to your substation department.
Burn Marks: Station class arrestors have caps in their bases that blow out in an over- voltage. They often leave scorch marks at their bases. Failed arrestors limit the station protective system’s ability to conduct faults safely to ground. Burns at other ground connections indicate trouble too.
Missing Ground Straps: Be vigilant when approaching fences, equipment and steel structures. Look for open diameter holes that were punched in for bolt ground connections. Look for paint discolorations where connectors used to be. Ground connections bleed off objectionable, if not deadly, induced voltages on equipment. If you can’t observe ground connections, handle surfaces with rubber gloves.
Cracked Equipment or Column Foundations: Today substation concrete foundations are often constructed with the rebar and mounting bolts bonded together and connected to the grid in a Ufer connection to establish equi-potential with the station’s grounding system. If fault currents are incidentally transferred into concrete that is not well bonded, moisture in the concrete heats up to steam. Steam at 1,800 times the moisture volume can crack concrete. Cracked concrete may not be incidental. It may be evidence of a greater hazardous potential condition.
Substations can be great places to work. Sometimes you just have to keep your eyes open, head down and hands in your pockets. Be safe.