ISLANDING DETECTION USING AN ACCUMULATED PHASE ANGLE DRIFT MEASUREMENT H.T. Yip*, G. Millar*, G. J. Lloyd*, A. Dysko†, G.M. Burt†, R. Tumilty‡ * AREVA T&D UK Ltd, UK, tony.yip@areva-td.com, gordon.millar@areva-td.com, graeme.lloyd@areva-td.com † University of Strathclyde, UK, a.dysko@strath.ac.uk, g.burt@eee.strath.ac.uk ‡ KEMA, UK, ryan.tumilty@kema.com Keywords: power system relaying, loss of grid, digital transient simulation, distributed generation. Abstract Distributed generation is increasingly likely to play a major role in electricity supply systems as recognition is made of its low carbon credentials. However, the integration of these units at distribution voltages is a major challenge for utilities. One particular issue is that generators may, unintentionally, continue to supply local demand when areas of the network are isolated from the main system. Reliably detecting this condition is regarded by many as an ongoing challenge as existing methods are not entirely satisfactory. This paper proposes a novel method based on accumulated phase angle drift that provides inherently enhanced stability without unduly sacrificing sensitivity. It is passive and thus requires no additional invasive hardware. Transient simulations are used to demonstrate its performance. 1 Introduction The connection of generation at distribution voltages is seen as one of the most important challenges facing modern electricity supply systems. These units offer the potential to take advantage of local renewable or sustainable energy sources, whilst avoiding the high carbon emissions and losses associated with large fossil fuel thermal stations and long distance transmission respectively. However, distribution networks were originally designed to support unidirectional power flows from high to low voltages meeting the demands of different consumers. Moreover, voltage regulation and operational practices were developed based on the specific assumption that the overwhelming majority of customers simply consume electrical energy and so do not possess their own parallel generating equipment. With ambitious and sometimes binding government connection targets in place [1], this passive nature is almost certainly likely to be challenged. Although the eventual capacity installed is open to debate, there is strong consensus that many issues need to be tackled before distributed generation can play a safe, reliable and profitable role in modern electricity supply systems. A specific area of concern for utilities is that distributed generation (DG) may continue to supply local demand when areas of the network are isolated from the main system [2]. This is a particularly undesirable condition and therefore protection is required for its detection and the subsequent tripping of DG. Although many protection methods have been developed for this task, concern still exists with regard to their performance in terms of the highly interrelated criteria of sensitivity and stability. This paper proposes the use of a method based on accumulated phase angle drift that provides inherently enhanced stability without unduly sacrificing sensitivity. This method continues with the prevailing practice of using only passive techniques and thus requires no additional invasive hardware. This paper firstly reviews the loss of grid problem in section 2 and, importantly, highlights the potential for growing utility concern surrounding the ever greater levels of generation being connected at distribution voltages. Leading on from this, section 3 discusses previously reported protection methods that can be used to detect a loss of grid condition. Comment is made on the effectiveness of both existing passive and several proposed active methods. The use of accumulated phase angle drift is introduced in section 4 and is followed by the reporting of a range of transient simulation tests in section 5. These are used to demonstrate the performance of this novel loss of grid method in terms of sensitivity to near balance conditions and stability during network faults. 2 The Loss of Grid Problem The term loss of grid (or islanding) is used to describe the condition wherein a generator is inadvertently isolated from the grid and continues to supply local demand. Such an undesirable eventuality could potentially occur due to circuit tripping by protection operation or, perhaps more rarely, accidentally due to network reconfiguration. Figure 1 illustrates these two possibilities: both a fault as shown on the substation busbar (circuit breaker opening) and the erroneous operation of the indicated switch would isolate the generator and local demand from the system. It is informative to note that as levels of automated network reconfiguration increase alongside DG connections, a similarly increased likelihood exists for the formation of unplanned islands in the future. An islanded condition is unacceptable for a number of reasons [3], including: the risk to utility operatives whilst reconfiguring a network that would formerly have not been