Demonstration project 6 Improving the flexibility of the grid (FLEXGRID)

Main findings

  • Real-time monitoring of the temperature of the power cables allows over 10% more wind-generated power to be brought onto the transmission grid.
  • Excess energy can be re-directed to lines with spare capacity in order to use the transmission capacity more efficiently, increasing the controllability and security of the grid and reducing the need for curtailments.

Project description

The integration of renewable energy has had a significant impact on the flow of energy through the power grid, making it necessary to increase network development while maximising the utilisation of the existing facilities. Even when the grid has been adapted to the changes, not all wind power can always be integrated, leading to curtailments. These curtailments sometimes happen when the nominal static ratings (the maximum power that can be transmitted) of the lines are exceeded, making the operation of the line unsafe. However FLEXGRID demonstrate altives to help optimise infrastructure and avoid curtailment of renewable generation.

Firstly the power line conditions were monitored in order to assess their maximum capacity in real time based on the real conditions rather than using static seasonal ratings: so-called 'dynamic line rating'.

The use of dynamic line rating is closely related to wind power integration because when the wind blows harder, wind power production increases and the load factor is higher. More transmission capacity is needed in these situations. However, there is more transmission capacity because of the higher cooling effect of the wind over the lines allowing for more power to flow through them.

The temperature along the line is recorded in real time, allowing operators to obtain the transmission capacity rating. In order to assess the temperature, a special kind of conductor called an 'Optical Phase Conductor' (OPPC) is used. OPPC is a conventional conductor in which one of the wires has been replaced by a stainless steel tube in which a beam of fibre optics is integrated. Through a DTS (Distributed Temperature Sensor) system located in the substation, a laser pulse is sent through the fibre optics. By processing this signal it is possible to monitor the temperature to a 2ºC accuracy along the entire line. Alongside this technology, two hour and four hour forecast algorithms will be developed which can be integrated into any other operation tool.

Secondly, grid operators should be able to dynamically control the power that flows through the line to avoid surpassing the limit by implementing power flow control devices. These devices allow excess energy to be re-directed from one line to another with spare capacity in order to use the transmission capacity more ef-ficiently, thus increasing the controllability and security of the grid. The use of power flow control devices is closely related to wind power integration because it is an energy source with higher variability requiring an instantaneous power flow control. The so-called 'Overload Line Controller' is characterised by the integration of traditional power solutions (reactors and switches) and an advanced control system typically implemented in Flexible AC Transmission System (FACTS) solutions based on power electronics.

This solution offers an attractive alternative in terms of functionality and cost, in comparison with other solutions. It is composed of nine reactors mechanically arranged in series, driven via individual switches and in parallel with each of the reactors. Through the optimum and controlled action of these reactors, it is possible to achieve staggered limitation of the desired power flux, allowing the overload control of the affected line in a steady and a contingency state.

Results in detail

The results of the demonstration are striking. Dynamic Line rating:

  • Acquisition of the maximum transmission capacity in real time. Most days the dynamic ratio, throughout the day, is more than 10% higher than the seasonal rating. In days with more wind, it could be more than 25% higher over the day.
  • Monitoring along the entire line without having to make estimations or extrapolations. The demo records temperature every 2 metres and every 10 minutes. This provides the operator with complete certainty.
  • 'Hot Spots' – areas which need maintenance - can easily be detected with this technology.
  • Possibility of studying the correlation between wind production and the increase of transmission capacity in lines affected by local weather conditions.

Overload line controller:

  • Control of power flows in steady state and contingency state. Control strategies based on real-time measurements and set points from dispatching centre.
  • Lower cost device and maintenance compared with other power electronics solutions.
  • Easily transferable to another location if necessary. Easily scalable solution in terms of size and the number of steps.

Both developments have been validated by simulations, lab testing and finally infeld demonstrations. A sensible application of these new technologies would provide extra capacity for the integration of renewa-bles, as well as an increase in operational safety. With the same level of safety, these developments will allow a more efficient management of the electrical grid:

  • Increasing the availability of infrastructure.
  • Optimising the management of the networks.
  • Balancing the loads on the lines.
  • Setting out immediate corrective actions.