The Price of Moving Power and Getting More Bang for the Dollar
The whole objective of an electricity power grid is to move power from a remote source hundreds if not thousands of kilometres away to the consumer as efficiently as possible. The operative word in this opening statement is "efficiently". The word efficiency is a broad term that and may be considered from many different perspectives. Engineers generally quantifies efficiency as power in versus power out of a system while the environmentalist will measure it by the
net effect on the environment including the visual impact. The economist will look at it from the perspective of net present value offset by the rate of return on the investment. There is no right or wrong answer when examining the
efficiency of a piece of infrastructure. It is however a point of debate when formulating the business case for capital investment into infrastructure.
More often than not the business case for committing capital into infrastructure projects is driven by demand and necessity. We have a tendency to take a reactive approach to such matters as it is often politically unpopular to spend "tax payer’s money" which is a finite resource on infrastructure without a clear and present need. For example, the
Sydney Harbour Bridge and the Snowy-hydro projects would be difficult to justify in the present political environment. As such it is imperative to deliver these key projects without over expending capital. With this in mind it’s up to the engineers to find the most economical solution to move power from one regional area to another. Cable
selection plays an important role in this relationship and can make the difference. For example, an 11kV 240mm2 copper underground cable and an 11kV 400mm2 aluminium underground cable are both capable of delivering 6MVA of apparent power. However, the difference between the two cables on a price per metre basis is substantial. The 11kV 400mm2 aluminium cable cost is approximately 70% of the 11kV 240mm2 copper cable on a per linear metre basis. Therefore on a purchase cost per MVA basis the aluminium option is approximately 30% more cost effective then the copper cable. Further to this, additional savings can be gained if overhead construction is adopted as opposed to an underground construction. As a general rule of thumb the cost of underground is approximately 3 to 5 times more than overhead construction for the same amount of power transported on a per kilometre basis. For example an 180MVA 132kV underground feeder generally costs $1500/m (AUD) whereas the same power can be transported in a 132kV overhead feeder from approximately $500/m (AUD).
These general guidelines can be used to determine the most cost effective method of transmitting and distributing power depending on the site environmental constraints. Using these rules the power engineer can make informed decisions about the construction methodology to ensure the optimal “Bang for the Dollar” is achieved and key stakeholders such as the tax-payer is getting the best return on their investment.