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Most utilities are yet to fully optimise their transmission infrastructure and could be investing unnecessarily in new equipment without making overhead lines work to full capacity.
Ambient conditions can significantly affect the transfer capacity of an overhead power line. However, most utilities lack detailed knowledge about these conditions and don’t have a complete picture when it comes to the true transfer capacity of a particular line. As a result, they employ unnecessarily high safety margins when transmitting power, so the inherent design capacity of overhead lines is not always optimised.
Congestion happens when more power needs to go through a line than can be accommodated, either because the true thermal capacity has been reached, or the utilities judge that the load has reached the assumed capacity. Grid operators often set these static ratings to maintain the reliability and safety of the grid. Known as ‘static line ratings’, these limits often lead to artificial congestion on power lines as utilities make false assumptions about the true capacity based on the ratings set out by operators.
Congestion forces utilities to buy more expensive wholesale electricity. Fuel shifting is another source of congestion as coal plants are replaced by gas and renewables.
Distributors can also invest in more transmission or generation capacity near to areas of high demand. However, building new power lines is costly. And newer sources of power like wind and solar are produced far away from urban areas where demand is greatest. It makes sense that optimisation technologies are first considered to ensure that the existing power lines are being used to their full potential before ones are built.
In addition to voltage and stability issues, the electricity transmitted through overhead lines is limited by the amount a cable sags between the supporting towers. The sag is determined by the ambient conditions, the current flowing, and the properties of the cable. The hotter a cable gets, the more it sags. Transmission lines must operate a minimum clearance above ground. To maintain this clearance in all weathers, utilities will only send a certain level of current though the lines for fear that if they get too hot they will sag too low.
Traditional static line ratings do not take into account the effect of ambient conditions, especially wind which cools the line even at low speeds, so utilities can’t use the true capacity of the line to its full potential and protect it from overheating.
Some utilities take a more dynamic approach to line ratings. Line ratings might be allowed to vary throughout the day or according to the time of year, depending on average ambient temperature.
While this can help increase ratings, it does not take the effect of wind or solar radiation into account. And it does not give operators the visibility to determine the true state of the transmission line and its clearance. So the true capacity of the line is not known and not optimised.
Dynamic line rating (DLR) goes one step further by using realtime data about ambient conditions, including temperature, solar radiation and wind. The realtime rating information is sent to the utilities and market operators who can then optimise the capacity of the power lines, while still maintaining the required minimum clearance.
The technology was tested by US utility Oncor. It found that DLR increased line capacity and reduced congestion, making more efficient use of infrastructure with high return on investment for relatively low cost. Oncor has already expanded its use of DLR systems in Texas.
The Electric Reliability Council of Texas estimated that congestion costs $130 million a year. Taking advantage of DLR allows utilities to reallocate their capital expenditure and reduce customers’ electricity bills.
In many situations, replacing the network isn’t feasible but something needs to be done to reduce congestion and its costs.
DLR is ideal for increasing capacity, reducing capital expenditures and saving costs for the consumer. When it comes to line rating, more knowledge really does mean more power.