Determining the Probability of Nuisance Operations of Distribution Fuse Links Due to Lightning-Induced Current Surges.

The probability of nuisance operations of distribution fuse links due to lightning-induced current surges is the subject of much discussion today. This discussion is driven, in part, by the increasing practice of relocating the surge arrester to the transformer tank. Historically, the arrester was installed on the cross-arm on the source-side of the fuse cutout. This location protected the fuse link from lightning surges, thereby allowing the selection of the smallest practical fuse-link ampere rating to provide optimal overcurrent protection for the transformer.

It has recently been suggested that overvoltage protection of the transformer, thought to be adequate by historical standards, could be improved by moving the surge arrester to the transformer tank. This eliminates the L×di/dt voltage associated with the arrester ground wire which, in many instances, can overstress the transformer’s winding-to-ground insulation and lead to dielectric failure of the transformer.

One consequence of the practice of relocating the surge arrester to the transformer tank is that the probability of nuisance operation of the fuse link protecting the transformer will increase significantly — from essentially zero when the arrester is mounted on the cross-arm on the source-side of the fuse link, to as high as 5–6% when the arrester is relocated to the transformer tank. Using larger fuse-link ampere ratings having a conventional speed characteristic (i.e., “K” Speed or “T” Speed) will result in a lower probability of nuisance operations. However, these larger fuse-link ratings will provide a lesser degree of overcurrent protection for the transformer.

It is possible to achieve both excellent overvoltage protection for the transformer (by relocating the arrester) and excellent overcurrent protection for the transformer by using either high-surge-capacity S&C “DR” Speed Positrol® Fuse Links or S&C Fault Tamer Fuse Limiters. “DR” Speed Positrol Fuse Links and Fault Tamer Fuse Limiter Fuse Cartridges have a high adiabatic minimum-melting I2t value — roughly equivalent to that of a 20K-ampere fuse link — which has a worst case probability of nuisance operation due to lightning-induced current surges of 1.5% or less per year (rural system, severe isokeraunic level).

The probability of nuisance fuse-link operations due to lightning-induced current surges has been determined using information published in EPRI Report EL-1140, “Study of Lightning Current Magnitude Through Distribution Arresters.” This report contains data on the probability of occurrence of lightning strokes having specific I2t values, for a number of different conditions such as urban systems versus rural systems, and for different isokeraunic levels; low-to-moderate (14.8 year mean time between strokes) and severe (9.2 year mean-time between strokes).

The probability of nuisance operations of “K” Speed and “T” Speed Fuse Links applied on urban systems, for both severe and low-to-moderate isokeraunic levels, is shown in Figure 1. The probability of nuisance operations of “K” Speed and “T” Speed Fuse Links applied on rural systems, for both severe and low-to-moderate isokeraunic levels, is shown in Figure 2. Rural distribution systems are slightly less well shielded from lightning strokes, and thus have a higher probabilities of nuisance fuse link operations compared to urban distribution systems, all other factors being the same.

The surge arrester is located on the transformer tank in both figures. Also, it is assumed that the fusing ratio (ratio of fuse-link ampere rating divided by the full-load current of the transformer) is 1.5, and the transformer is loaded to not more than 150% of nameplate. A larger fusing ratio (e.g., 2.0 instead of 1.5) will result in a slight reduction of the probability of nuisance operation of the fuse link. Higher transformer loading levels (e.g., 200% of nameplate instead of 150%) will result in a slight increase in the probability of nuisance operation of the fuse link. The preloaded high-frequency average-melting I2t of the fuse link was used in this analysis.

Figure 1. Probability of nuisance fuse-link operations due to lightning-induced current surges.
Fuse-link is on source side of surge arrester, urban system.

Figure 2. Probability of nuisance fuse-link operations due to lightning-induced current surges.
Fuse-link is on source side of surge arrester, rural system.