Semi-Conductive Jacketed Cable Investigation
A growing number of utilities were installing semi-conductive jacketed cable on their distribution systems. This type of cable has several inherent advantages: 1) the semi-conductive jacket provides system grounding while, 2) the jacket prevents moisture ingress to the neutral conductor. On the other hand, this type of cable is subject to induced currents during ground strokes which can propagate onto the secondary neutral, into padmount transformer wingdings, and customer loads.
P2-2 was conducted at the Skeats High Power Laboratory in Philadelphia. The main objectives were to:
- Document the grounding characteristics and neutral current propagation of semi-con cable for impulses applied at the riser pole;
- Determine the magnitude of current that can be induced on the cable neutral during a ground stroke situation;
- Measure the magnitude of secondary neutral currents resulting from a ground stroke near the primary semi-con cable;
- Develop appropriate system design and field installation guidelines to improve the understanding, design, and operation of semi-con cable installations.
In order to meet these test objectives, approximately 1000 feet of semi-conductive jacketed cable was installed in trenches adjacent to the existing direct-buried insulated jacketed cable and cable-in-conduit. Counterpoise in the first cable loop used in the Phase II investigation was removed to minimize proximity effects to the semi-con cable. An overall site plan is shown below. A total of 2 loops and 2 straight cable sections were installed.
The majority of the semi-con cable testing effort addressed ground strokes. A ground stroke was simulated by suspending an electrode approximately 2 inches above ground surface. The ground stroke was moved to various compass positions from a reference CT splice on the cable neutral. The magnitude of induced current varies with the location of the ground stroke relative to the cable. Induced currents increase as the ground stroke approaches the cable trench.
Neutral-to-ground voltages and neutral current propagation were monitored at four neutral splices made below ground and extended to the surface while other neutral monitoring points were located at the beginning and end of each cable loop and straight section. Each underground lead was insulated with tape.
The secondary surge investigation demonstrated that induced currents on the primary cable can propagate into the secondary circuit of a padmount transformer. The amount of transferred current is a function of the relative resistances between the padmount transformer ground and the customer ground. Secondary surge currents increase when the padmount transformer ground increases relative to the customer ground. This is consistent with the conclusions from the earlier secondary surge investigation.
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This study was conducted in: P2-2