Contingency analysis is employed by system operators to estimate post-disturbance power system robustness. For large system like WECC or the Eastern Interconnection (EI) the computational burden and time consumed for full blown ac analysis is tremendous. Also, a recent upsurge in the area of electric energy markets and transmission/generation planning has created a niche for computationally efficient and yet reliable, simple and robust power flow models. This has intensified the inclination of researchers to come up with equivalent dc networks that match ac solutions as close as possible. This paper introduces a novel method of deriving dc model using PTDF approach. The performance of this model is then compared to the several other dc models for single branch outage contingencies. Furthermore, shortcomings of several dc models shall be analyzed.