  One of the methodologies that carry out the division of the electrical grid
into zones is based on the aggregation of nodes characterized by similar Power
Transfer Distribution Factors (PTDFs). Here, we point out that satisfactory
clustering algorithm should take into account two aspects. First, nodes of
similar impact on cross-border lines should be grouped together. Second,
cross-border power flows should be relatively insensitive to differences
between real and assumed Generation Shift Key matrices. We introduce a
theoretical basis of a novel clustering algorithm (BubbleClust) that fulfills
these requirements and we perform a case study to illustrate social welfare
consequences of the division.
