Dealing with transients

When a new demand or load is added or subtracted from the system it affects the supply: continuing our occasional water analogy, it causes ripples in the supply. As with water, the size of the ripples, how far they extend and their effect depend on the size of the pebble and the size of the pool.

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Other disturbances can also produce transient or short-lived effects on the transmission or distribution grid. A short circuit is one example caused by an impact on the cable or in the switchyard. This is unlikely in the case of transmission cables, since they are higher, but relatively common on the distribution network. There are regular examples of agricultural or industrial equipment striking overhead lines if their drivers misjudge their relative heights. Animals are also a frequent cause, espe- cially small climbing creatures such as squirrels, or trees and other vegetation can be blown against the line. Lightning strikes are also frequently to blame – one effect that is important for the transmission network. In many cases now, the network is equipped with automatic circuit breakers that switch in the event of a short circuit and automatically reclose a few seconds later to bring the line back into operation. In this case there is no loss of power (or a loss of only a few seconds) but the effect is to send more ripples across the network. In some cases it can cause a voltage or frequency ‘collapse’.

Such transients have an effect. As computer operators know, power electron- ics systems are very vulnerable to them, so most computer shops sell socket sets that include surge arrestors, which counter transient changes in the supply. Which transients have to be managed depends on how sensitive your computer is.

Similarly, equipment connected to the distribution or transmission networks has to be protected against transients or faults in the grid supply, and the characteristics of each type of generation determine how much protection is required and when, in certain circumstances, generators can help even out faults and add stability to the grid.

Large disturbances in the grid can affect any generating plant and, beyond set limits, most will automatically disconnect from the grid to limit the damage to the plant. But, as we have seen, the sudden removal of supply from the system can create its own fault: the result can be a domino effect that propagates faults far beyond the original area and jeopardizes the running of the system.

Two such events happened within a few months of each other a few years ago. In the USA in August 2003 the loss of a transmission line in the north required power to be switched to transport across another part of the network. The unanticipated extra load caused several power stations to ‘trip’, and faults quickly cascaded throughout the interconnected system in the north-east USA, eventually causing blackouts that lasted several hours in New York and many of the surrounding states. Power cuts extended up to Canada. A few months later, similar effects caused blackouts in Italy. In both cases, the original event was a short circuit on one power line caused by a tree striking the line.

For electricity-system operators, containing such an incident is easier if there are more connections and an extensive grid: having a lot of transmission and distribution lines gives the operators different options for switching power around so that no one part of the system becomes overloaded. Generators (and loads) also ideally have some ability to ‘ride through’ faults, so they are less likely to disconnect and cause the problem to cascade. Some forms of conventional thermal and nuclear generation are valuable in this respect. Because they include heavy rotating machinery, the plant has huge mechanical momentum. A brief fault is not enough to interrupt the turning

Interacting with the electricity grid 117 generator and it will take several seconds before a fault develops that will cause it to separate from the network.

Some other forms do not have this effect. Wind turbines, for example, have in the past been designed not to ride through faults but to disconnect immediately, because the turbine characteristics are such that the turbine risked being damaged by the connection. This approach was taken because turbines were relatively small and separated generators even in networks where they were widely used, and so the effect of disconnecting one small turbine from the network was very small. The situation changed somewhat when wind started to be installed in much bigger quantities and in wind farms with a single connection that represented an important input to the grid, frequently in areas where the grid itself was spread thin and had little other capacity around to share the burden. At this point, fault ride-through became an important issue for wind. When a cyclist wobbles as he hits a pothole, it is not too important (except to the cyclist) whether he rides through it or falls off. But, if it happens in a group of cyclists, the resulting pile-up could bring traffic to a halt for miles.

In practice, for large wind farms, electronic management systems can be incor- porated that allow the wind farm to mimic the ride-through ability of a generator with large rotating machinery, and this is likely to be cost-effective in a system where faults and unavailability are penalized.