Power cuts

Power cuts are in the news. The engineering and economics of power distribution systems are relatively simple. The politics are not.

Extensive power cuts occur when there is a spreading failure in the supply network. For this to occur there are two requirements; First there must be a triggering event, while second, the network must lack resilience. The triggering event will usually be the electric breakdown of a major component such as a transformer, but wind damage to overhead power lines is not uncommon.

Network resilience depends on two factors. One is the level of interconnectivity, which is at a maximum when every node is directly connected to every other node, an unrealisable ideal. The other major factor is spare capacity. Each generating node and each connecting branch will have a maximum amount of power that it can deliver or carry. The network will be resilient if every one of them works well  below this limit. If the requirement of network resilience is not met, there is potential for a cascade of failures that could envelope a whole nation within minutes.

The generators have to deal with a widely variable demand, which can be divided into the base load, i.e. the minimum power required over a period of time, and the variable load. Nuclear power is ideal to service the base load, while fossil fuel power (or where available hydro power) is ideal to service the variable load.

The main reason that power cuts have come into the news is that the regulatory regime has changed. In the so-called deregulated power supply system, there is a phoney free market controlled by a regulator. The justification for the word “Phoney” is the fact that people are urged by regulators to chop and change suppliers in order to keep prices down. In fact, they change nothing but their billing company and the supply still comes from the same generators over the same grid. You walk into the bank to discuss your overdraft and they immediately try to sell you gas and electricity, though they own none. Because of the consequent pressure on prices, there is less and less incentive for generators to maintain spare capacity in the form of stand-by plant. Thus the regulatory system is optimised to maximise the risk of power cuts.

Then there is the political question, mainly driven by eco-theologians and other environmental groups. The pressure has caused successive governments in places like California and Britain to duck crucial decisions in a field that requires careful planning for decades ahead. At a local level, almost every attempt to build practicable generating capacity is vigorously opposed.

The environmental buzzword is “renewable”. Apart from hydro-electricity, this is a dangerous trap. It would not be so bad if sources such as wind power were simply an irrelevance, but it is worse than that. They create a situation in which power is worthless when the wind blows and priceless when it does not, the recipe for an unstable market. They do not exist in their own economic right, but as a result of huge political subsidies (or fines for not conforming).

Another question frequently raised is the necessity for overhead power transmission. Pylons are ugly and obtrusive (like windmills, though they are at least quiet). Air, besides being omnipresent and free, has the miraculous property as an electrical insulator that it is self-healing. Underground cables require solid insulation, usually oiled paper, that is more expensive (by about an order of magnitude) to install and difficult to maintain. When solid insulation fails, it is destroyed by the arc and becomes a conductor. The damage to an underground cable is not only difficult to repair but also very difficult to locate. Local people get irate when one side of their road is dug up, then the other side, then the first side again, but that frequently happens.

It is also because of the limitations of solid insulation that power supply equipment will always have a restricted lifetime (of the order of forty years). However carefully they are built, devices such as transformers will have tiny defects, such as slivers of metal or gaseous voids. These are the locations of what are called partial discharges, minute areas of local electric breakdown that cause the gradual deterioration of the solid and liquid components of the insulation, this in addition to natural processes such as oxidation and fatigue. It is a requirement for a reliable system that there is an on-going programme of component renewal, another aspect in which the phoney market is fatally flawed.

Overhead cable systems are large and intrusive because they have to be high-voltage and therefore need a wide spacing to obviate electrical arcing. The reason for the high voltage is that the major loss of energy is through Joule heating, which is proportional to the square of current, while the power is given by the product of voltage and current; so, for a given power, the higher the voltage the lower the losses. For the same reason it is not economic to carry power over long distances, because the energy loss is proportional to length, so the generators should ideally be distributed among the users.

High voltage cables have been the subject of manufactured scares, but then what hasn’t? These are, of course, based on the usual random statistical associations and undemanding levels of significance.  Exposure to electric and magnetic fields from overhead cables is, in fact,  very low. The reason for this is the adoption of the three-phase transmission system. The voltages and (more or less) the currents in such a system sum to zero at any instant in time, because the three sine waves are relatively displaced by one third of a cycle. Thus, though the fields are very high between the cables, they decay very rapidly with distance away from them.  The other potential hazard is the phenomenon of corona. In damp weather and polluted atmospheres, an electric discharge can occur, which generates highly reactive gases such as the oxides of nitrogen and ozone. It is unlikely that the effects are serious. Not long ago, when ozone was thought to be healthy, people had ozonisers in their houses, which used Tesla coils to generate ionising discharges. There is no record of such devices doing any harm (or good for that matter).

The dominating question is whether serious extensive power cuts are inevitable. The answer has to be YES, if there is no change of policy. Since the triggering event is essentially a random phenomenon, however, their occurrence cannot be predicted, but the probability will be greatly increased during extremes of weather, just when people are most vulnerable. Everyone, but particularly the frail and elderly, should be advised not only to stock up on candles, but also to ensure that they have an alternative form of heating.

JEB
September 2003

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