Single-wire Earth Return - Description

Description

SWER is a good choice for a distribution system when conventional return current wiring would cost more than SWER’s isolation transformers and small power losses. Power engineers experienced with both SWER and conventional power lines rate SWER as equally safe, more reliable, less costly, but with slightly lower efficiency than conventional lines. (except when bushfire is a risk ).

Power is supplied to the SWER line by an isolating transformer of up to 300kVA. This transformer isolates the grid from ground or earth, and changes the grid voltage (typically 22 or 33 kilovolts line to line) to the SWER voltage (typically 12.7 or 19.1 kilovolts line to earth).

The SWER line is a single conductor that may stretch for tens or even hundreds of kilometres, with a number of distribution transformers along its length. At each transformer, such as a customer's premises, current flows from the line, through the primary coil of a step-down isolation transformer, to earth through an earth stake. From the earth stake, the current eventually finds its way back to the main step-down transformer at the head of the line, completing the circuit. SWER is therefore a practical example of a phantom loop.

In areas with high-resistance soil, the resistance of the soil wastes energy. Another issue is that the resistance may be high enough that insufficient current flows into the earth neutral, causing the grounding rod to float to higher voltages. Self-resetting circuit breakers usually reset because of a difference in voltage between line and neutral. Therefore, with dry, high-resistance soils, the reduced difference in voltage between line and neutral may prevent breakers from resetting. In Australia, locations with very dry soils need the grounding rods to be extra deep. Experience in Alaska shows that SWER needs to be grounded below permafrost, which is high-resistance.

The secondary winding of the local transformer will supply the customer with either single ended single phase (N-0) or split phase (N-0-N) power in the region’s standard appliance voltages, with the 0 volt line connected to a safety earth that does not normally carry an operating current.

A large SWER line may feed as many as 80 distribution transformers. The transformers are usually rated at 5 kVA, 10 kVA and 25 kVA. The load densities are usually below 0.5 kVA per kilometer (0.8 kVA per mile) of line. Any single customer’s maximum demand will typically be less than 3.5 kVA, but larger loads up to the capacity of the distribution transformer can also be supplied.

Some SWER systems in the USA are conventional distribution feeders that were built without a continuous neutral (some of which were obsoleted transmission lines that were refitted for rural distribution service). The substation feeding such lines has a grounding rod on each pole within the substation; then on each branch from the line, the span between the pole next to and the pole carrying the transformer would have a grounded conductor (giving each transformer two grounding points for safety reasons).