Power system maximum loadability is a crucial index. The direct interior point algorithm was introduced by several researchers to calculate power system maximum loadability. This paper simplifies the direct interior point model and applies it to various optimization problems. Zone-based maximum loadability problems are discussed. Weight factors and tie line flows are included. ATC (Available Transfer Capability) calculation is also investigated. The meaning and applications of 'shadow price' are explained and shown. The principle of oscillation for this algorithm is explained. The factors that influence the algorithm's speed and convergence are investigated. Our research proves the interior point algorithm is a powerful tool that can handle various kinds of maximum loadability problems.