SMART ALGORITHMS TO ACCOMMODATE DISTRIBUTED GENERATION IN THE GRID

1 SUMMARY The future electricity networks will have to accommodate large scale distributed generation (DG) with renewable energy resources such as windfarms. One of the problems facing the electricity utilities is the thermal ratings of components in the networks. The networks were designed in the past to transport power from centralised power stations to consumers, now they have to accommodate additional in-feeds from distributed generation. The utilities are under pressure to maximise the utilisation of their network to increase power flow. The overhead line conductors form the backbone of the power transmission and distribution systems. In order to maximise the power transfer capability, it is necessary to fully utilise the thermal ratings of the lines. This cannot be achieved by using the traditional static summer and winter line ratings as defined for example by the UK's Engineering Recommendation P27, which are based on worst case weather conditions. Dynamic line rating (DLR) which takes into account the actual weather and environmental conditions is therefore gaining importance. This paper provides details of a protection relay that has been developed which is able to compute the dynamic line rating based on real-time weather conditions including ambient temperature, wind speed, wind direction and solar radiation. By comparing the maximum of the three phase line currents with this dynamic line rating, the relay is able to determine whether the rating of the line is about to be exceeded. The paper describes all the innovative features of this relay. It also discusses the different applications of this new technology. Analysis of the data obtained from a site trial in the UK shows that wind speed has the most significant impact on the line rating of the weather parameters. The analysis also shows that compared to the static ratings dynamic line ratings can enable up to 30% or more wind generation to be connected to the grid. Another particular concern for distribution network operators who must now manage integration of DG into the network is that of reliably detecting the islanding of DG from the main network. The term islanding or loss of mains (LOM) refers to the condition in which a generator is accidentally isolated from the mains supply and continues to feed local demand. This condition is highly undesirable for a number of reasons, including: risk to utility staff, provision of a poor quality supply to demand and the risk of out of synchronism re-closing. The condition is currently detected using a range of different passive protection elements including: over/under frequency and voltage, rate-of-change-of-frequency and voltage vector shift. Unfortunately, there is a widespread view that these techniques do not offer the levels of sensitivity to near balance conditions and stability during network faults that will enable the reliable integration of distributed generation. This paper also describes a protection relay incorporating a novel islanding detection method that is based on an accumulated phase angle drift measurement. No additional invasive hardware is required and so the method retains the passive characteristic that is valued by network operators. The method has been tested extensively using a Real Time Digital Simulator (RTDS). The results obtained indicate