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Historically defence plans and more specifically load shedding schemes, have always been designed nationally. After the severe disturbance in Europe in 2006, it became clear that the schemes used in the different control zones were not aligned. As a consequence there was an uneven distribution of the load shed in the different zones. A first step in achieving a more unified approach to withstand a large under frequency incidents has, been taken by ENTSO-E. But starting from policy 5 concerning emergency operations, it became clear that still a lot of variation to the proposed scheme is possible. This can be variations in the amount of load shed in each step, the frequency settings at which the shedding takes place and total amount of load that is shed at 48Hz. An overview of the different schemes used in Western Europe in 2006 is given and a comparison is made with some of the scheme currently in use. Starting from this comparison, the question rises of how these schemes would interact. To investigate the consequences of different approaches in designing an under frequency load shedding schemes in an interconnected system, simulations on a two zone system have been performed. From the different combinations simulated, it became clear that different approaches not necessarily lead to a higher total amount of load shed, but that the shedding is more located in specific zones. A unified approach results in similar amounts of load shed but more evenly spread over the zones. It should also be noted that this harmonisation does not imply that exactly the size for the steps for frequency setting for the tripping of the relay should be used. As this could result in a big load disconnection which in turn could lead to an overcompensation. Therefore the band around the linear scheme proposed by ENTSO-E should be defined more clearly. Also the minimum total amount of load that should be shed at 48 Hz should be defined. ispartof: CIGRE Innovation for secure and efficient transmission grids location:Brussels, Belgium date:12 Mar - 14 Mar 2014 status: published |
