  This letter addresses the issue of linearization of quadratic thermal limits
of transmission lines for linear OPF formulation. A new irregular polygon based
linearization is proposed. The approach is purely based on geometrical concepts
and does not introduce any optimization problem. Comparison of the number of
constraints is given with different errors for different branch capabilities
and systems. Test case analysis is also presented to validate the irregular
polygon linearization strategy with optimal value and computational time
results.
