Mitigation Technologies

Mitigation techniques for stray voltage and elevated NEV leading to nuisance shocking range from isolation devices to impedance-reduction techniques. Each method has its proven success stories as well as its shortcomings. Traditional mitigation techniques include:

• Load balancing
• Re-sizing neutral conductors
• Isolation
• Improved grounding techniques
• Equipotential planes

Load Balancing

On three-phase, grounded-wye distribution systems with 60-Hz, equally balanced phase currents, the net neutral current should be zero, that is the neutral current from the three phases effectively cancels out. Unfortunately, in the real world perfect balancing can be upset by many factors such as phase shift, load unbalance, and harmonic currents. These phenomena can cause current to flow in the neutral conductor and into the ground rod at the neutral-to-ground bonding point, which creates a proportional NEV. Balancing the phase currents can reduce the 60-Hz-caused NEV across the entire distribution system.

Regarding load balancing on the customer side of a facility's main step-down transformer, the results will be very similar in terms of NEV levels produced and in terms of cancellation (the exception being that the NEV level due to customer loads affects only those customers connected at that transformer and does not directly affect the other customers on the distribution system).

Re-sizing Neutral Conductors

Currents returning on grounded wye-connected power systems cause a voltage drop across the impedance of the neutral conductor. Because the neutral conductor is grounded, the impedance of the earth return path in parallel with the impedance of the neutral return path dictates the percentage of earth current and the corresponding NEV at that neutral-grounding point. A very simplified way to look at this is to examine a circuit with a current source (neutral return current) and two current paths (neutral path and earth path). It is well known that, all else being equal, electricity will follow the conducting path with the lowest impedance. If each path of a two-path system has an identical impedance, then half of the current will flow through each path. If we now lower the impedance of the neutral path (relative to the earth path), more current will now flow through the lower-impedance neutral. Therefore, reducing the impedance of the neutral effectively l reduces the amount of current flowing through the earth path and lowers corresponding NEV at that neutral-to-ground bonding point.

Isolation

A simple and effective means of keeping stray voltage from getting to animal and human contact points would be to not bond the primary and secondary neutral conductors at the service transformer. If there is no metallic connection, there can be no voltage and no opportunity for current flow from the primary neutral.

Isolation can be accomplished by a physical separation (i.e. no connection whatsoever) or through the use of a separate isolation transformer. The key considerations in such an approach, however, are the adverse effect on protection of the load-side of the transformer in the event of a lightning strike, a system fault, or a wiring error. Therefore, the benefits of isolating the secondary neutral must be weighed against the chances of these abnormal events occurring and the undesirable consequences that would result. While isolation has some drawbacks, the states of Michigan and Wisconsin allow primary/secondary neutral isolation if the end user requests it.

Improved Grounding Techniques

In terms of NEV, stray voltages, and even MOEV, it is generally accepted that reducing the impedance of all return paths - neutral or to earth - can either reduce voltage levels at remote earth or allow breakers to operate to clear MOEV-related faults when they occur. From the standpoint of reducing the voltage at remote earth, any impedance reduction will provide a corresponding reduction in NEV due to a reduced voltage drop, given the same currents.

To reduce the NEV levels of utility systems, two improved grounding options are effective:

• Install a substation ground grid to reduce the impedance of the remote earth
• Improve the grounding impedance of power poles

Obviously, the cost of improving pole ground impedance on a large scale could be cost prohibitive because there can be literally thousands of pole grounds versus only dozens of ground rods for a substation ground grid. Even so, improved pole grounding can be a viable option for improving NEV levels at those locations with particularly intransigent nuisance shocking problems. In either case, the objectives are to reduce the ground impedance and the subsequent voltage at remote earth.

Preliminary modeling of ground impedance suggests that NEV levels can be reduced 10 to 20% by reducing ground-grid impedance by 50 percent. Improvements are most prominent at the substation, but drop off rapidly one-half circuit mile away from the substation. On the other hand, reducing customer and pole ground impedances by only 25% results in comparable reductions of NEV - 10 to 20% with consistent results across the entire circuit. So, the higher cost of pole-by-pole improvement in grounding yield the benefit of improvements over a much wider area than improvements only at the substation.

Equipotential Planes

Similar to the ground-reference structures used for computer rooms and the ground mats that minimize step potentials at utility substations, the equipotential plane is a useful means of minimizing nuisance shocking at animal contact points. The application is actually simpler than reducing the impedance of electrical return paths in that high-frequency bonding and grounding considerations are not an issue.

An equipotential plane typically consists of a conductive wire mesh installed under the area where nuisance shocking has been reported, and attaching most (if not all) conductive materials in the area directly to the mesh. The technique, while not necessarily eliminating the original fundamental causes of the nuisance shocking problem (stray voltage, NEV, etc.), essentially "move the problem" away from areas where people, animals, or equipment are likely to insert themselves into the conducting path. If done properly, the typical results at animal contact points are voltage levels measured in tenths of a volt. However, retrofitting such a grid can be expensive particularly if it requires excavation or even cutting grooves in an existing concrete floor to insert the metallic mesh. Also, care must be taken to ensure that the edge of the mesh doesn't simply move the nuisance shocking to another location. For example, voltage-gradient ramps may be required to entice livestock to step onto the plane because it will likely be at a voltage potential different from the surrounding earth.