Influence of different contingency scenarios on the allocation of harmonic emission limits in transmission systems

IEC TR 61000-3-6, all relevant contingency scenarios need to be considered during the planning stage. This might lead to very conservative emission limits for most of the operation time. In this article, the influence of contingencies on the allocation process is comprehensively analysed using a generic model of a transmission system. 472 contingencies can be defined for the grid. For each contingency, the harmonic limits for each customer installation are calculated according to IEC TR 61000-3-6. The variation in the harmonic impedance and the resulting variation in harmonic emission limits are analysed. It becomes clear that the contingencies have a significant influence on the resulting limits for certain nodes and certain harmonic orders. The allocated limits are then used to calculate the utilization of the planning level at each node and for each contingency assuming that each installation emits at its (minimal allocated) limit. The utilization for all node voltages is then evaluated using complementary cumulative distribution functions (CCDF). It becomes clear that for most of the contingency cases at most of the nodes, and hence most of the operation time of the grid, the system is underutilized. Only for a very few node voltages does the utilization reach around 80 %. Consequently, allocated emission limits based on the minimal values (i.e. considering all contingencies) tend to underutilize the planning levels for most contingencies. It is further shown that ignoring the most critical 1 % of the contingency cases leads to significantly higher utilization for most of the cases.