University of Strathclyde Small Scale Hydro: Novel Approaches to Generation & Transmission

Hillside

Medium Voltage Background Information

Overhead/Underground:

Whenever power is supplied along a cable, a voltage drop occurs across cables. The National Grid stipulates that there should be no more than a +/- 6% difference in supply voltage.

For an 11kV connection, this would give a permissible difference of +/-660V and for a 33kV transmission connection, +/-1980V difference. Using the power, current, resistance relationship P=I2R, we can make a comparison of power transmission losses between 11kV and 33kV cables. Looking at a per km basis for an estimated 2MW of power generated (as decided at the start of this section), to be transmitted to the rest of the grid, the 33kV voltages offers just over 450W/km of losses where as transmitting the same amount of power at the lower voltage of 11kV sees losses that are up by almost a factor of 10 at around 4050W/km. Taking this into consideration, running the transmission lines at higher voltages lowers the losses. It does however increase the cost of materials used in construction of the cable and switchgear required. 

Underground cables have less of a visual impact than overhead cables strung up across poles. In areas where visual impact may be of concern, e.g. long distance paths, scenic views, sites of historic importance, and outdoor recreation, this consideration will need to be made. It should also be noted that any underground cable used would be subject to the same losses as any overhead cables.


Under water Cable Specifications

To run power submerged in water over any substantial distance, marine grade XLPE (crossed linked polyethylene) cable is the most suitable product. The cost of this sub-sea power cable is in the same region as cable used for under-grounding of distribution lines. While Marine XLPE is usually made to order in specific lengths for individual projects it is important to note the length is constrained by transportation limitations.

The specifications for a sub-sea cable to satisfactorily carry the power requirements of 2MW at 33kv are as follows: The cross sectional area of each conductor is 185mm2. This gives the cable an overall diameter of 12cm. Its weight is around 12kg/m, giving a resultant weight of 12000kg/km or 60000kg for the 5km of cable to run through the supply pipe for the supply tunnel. A cut away view of a typical submarine power cable is shown on the right to give an idea of the construction of a sub-sea cable. 

Image courtesy of ABB Subsea Cable Systems

Using these specifications, modelling of a potential tunnel anchor block was carried out. CFD analysis of the flow of water through the supply tunnel and over the cable was produced to try to understand any changes in pressures around the cable and velocity variations. Velocity is important as above 3ms-1 there is a risk of erosion by particles in the water. Below this velocity there is risk of corrosion however this more common in salt water environments. In the freshwater situation we are concerned with, there is little or no salt, hence less chlorides and less corrosion potential.

To read more about the CFD analysis on placing the cable in the tunnel, please click here.