Dynamic System Rating for Transmission Corridors

The integration of high levels of Renewable Energy Capacity within existing electrical grids, which were designed to transmit power from conventional generation sources to major load centers is rapidly changing, where network congestion is one example of the problems faced by many operators in the acceleration of decarbonized system. This includes existing congested corridors coupled with new areas of congestion within sections of the network that would not traditionally have been associated with generation. Not only does this lead to limited ability to rapidly deploy and connect new renewable sources but it can also lead to curtailment of the existing fleet at significant constraint costs and could potentially provoke voltage quality and stability issues. Whilst the transmission capacities of the network can be improved through network reinforcements, this may in many cases prove prohibitively expensive and time-consuming (multi-year) thus delaying the renewable deployment. One of the key constraining factors on network capacity is the thermal limit of the conductor and therefore there has been widespread testing and implementation of both sensor-based and sensor-less Dynamic Line Rating technology which monitors the dynamic thermal rating of the line bases based on measurements of current, temperature or sag.The thermal limit is not the only limiting factor to network capacity, and dynamically increasing the rated capacities within the network could result in voltage quality and stability risk, in some applications. Dynamic Line Rating technology alone is insufficient as a basis for autonomous control actions to adapt and optimize network power flows. This paper proposes monitoring of not only the thermal limits but also potential quality and stability issues that may arise, by the online calculation of Dynamic Power Ratings for a local network zone.To utilize the additional power transfer capacity available and mitigate existing network congestion, control actions can be taken which will be highly dependent upon the local resources. For these control actions to be effective, they need to be realized in very small latencies approaching real-time; the paper proposes a local approach to the required control actions, rather than rely on the centralized controls typically managing grid power flows. This fast acting, intelligent, edge-based control response is called Zonal Autonomous Control (ZAC).Such is the scale of network congestion and consequently on renewable energy curtailment, that curtailment penalties in the United Kingdom, for example, have reached nearly £1Bn per annum [1]. This is because more than 3.8 million MWh would otherwise have been generated and transmitted to the grid if it were not for network congestion: put simply the network was not built with these new generation sites in mind, and power flows are limited due to thermal, voltage quality or stability constraints. In some areas of the network where angular stability is a key concern, protection schemes can be installed to mitigate this risk, thus realizing further capacity in the corridor until the next limit is reached, such as thermal. Indeed, the same can be said if the thermal limit is increased as the threshold for voltage and/or angular stability may be reached before the full thermal capacity can be used when monitoring the line ratings. Therefore, a holistic approach which not only monitors the thermal line ratings but also the impact of increasing the capacity on other stability margins must be assessed.

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