  Capacity expansion models for the electric power system often employ zonal
(rather than nodal) resolution, necessitating estimates of aggregate power
transfer limits across the interfaces between model zones. Interface limits
between planning areas are sometimes published, but they are not generalizable
to arbitrary zone shapes. There is thus a need for a reproducible method for
estimating interface transfer limits (ITLs) between user-defined zones directly
from nodal transmission system data. Here, we present a simple method for
estimating ITLs using a DC power flow approximation via the power transfer
distribution factor (PTDF) matrix. Linear optimization is performed to identify
the distribution of power flows that maximizes the total flow on
interface-crossing lines, subject to individual line ratings, limits on bus
injection/withdrawal, and the relationships among flows, injections, and
withdrawals imposed by the PTDF matrix. We demonstrate the application of the
method on a 134-zone ~65000-bus system, and we explore the influence of flow
direction, contingency level, and zone size on the estimated ITLs. There is
significant heterogeneity in the ratio of the ITL to the sum of
interface-crossing line ratings, which highlights the importance of accounting
for the physical constraints on power flows imposed by Kirchhoff's laws when
estimating zonal ITLs.
