A Review of Engineering Recommendations P15, P17 and P27 (Transformers, Cables and Overhead Lines)
Overview
The findings from the Customer-Led Network Revolution project have provided us with the opportunity to review the contents of a number of Engineering Recommendations. In this report the analysis from Real Time Thermal Rating (RTTR) equipment for Transformers, Overhead Lines and Underground Cables is compared to the contents of Engineering Recommendations P15 (Transformers), P17 (Underground Cables) and P27 (Overhead Lines).
A key finding is that the approaches and rating values in P15, P17 and P27 are not conservative. Particularly for overhead lines, often RTTR equipment produced values below those specified in P27. Through further analysis of the load curve at the CLNR trial sites, we have shown that the probability of exceedance using the generic ratings given in the Engineering Recommendations is low, so long as the correct site specific parameters are selected. We have therefore identified no case to change the generic ratings in these Engineering Recommendations.
In P15, P17 and P27 there is currently no explicit coverage for the use of localised site specific data (e.g. wind and load profiles) to calculate bespoke ratings or for the application of RTTR. We recommend that a separate application guide covering the rating of all distribution network asset types is produced, to assist engineers in deriving safe yet economical bespoke ratings and in using RTTR systems within control schemes.
An area of consideration for updating DNO thermal rating policies is based around our findings that application of site specific parameters within thermal models can have a significant impact on the capacity of distribution network assets. This has led to the definition of three distinct approaches to assess the capacity of distribution network assets:
- Update generic ratings: to gather better data for off-line analysis to change the parameters used in thermal models to derive a new generic rating. We have identified no case to change the generic ratings or the approach to rating assessment in the three Engineering Recommendations.
- Create bespoke ratings: to use site specific parameters to derive a more accurate rating for specific sites. For overhead lines and transformers we have identified that 10-15% additional capacity can be released, however for cables, measurements at our cable sites have led to 10% de-rating due to high soil resistivity values.
- Apply RTTR: to deploy RTTR equipment to calculate a rating in near real-time for input into a control scheme. This has been shown to release substantial capacity for overhead lines and transformers but this can only be realised where it is coupled with powerflow management.
It is therefore recommended that the Engineering Recommendations (P15, P17 and P27) are updated in line with the bespoke ratings approach and consideration is given to the production of an application guide for RTTR schemes
