Linear power flow models are widely used in power systems to simplify the nonlinear power flow equations. The DC power flow model is one of the representatives. There are many other linear power flow models that improve the DC power flow model with the inclusion of

$\boldsymbol{Q}$

and

$\boldsymbol{v}$

. However, existing linear models are derived based on empirical mathematical approximation without a general methodology guidance. In this paper, we found that the fundamental difference among different linear power flow models lies in the formulation of “independent variables.” Based on this finding, a general formulation of linear power flow models is proposed. The linearization error is theoretically analyzed. In particular, the case when

$\boldsymbol{ \theta }$

and

$\boldsymbol{v^{\scriptscriptstyle k}}$

are regarded as independent variables is thoroughly investigated. Method for finding the linear power flow with the minimum error is presented. The formulation of the independent variables associated with the minimum linearization error is determined by the distribution of state variables

$\boldsymbol{v}$

and

$\boldsymbol{\theta }$

. It is shown that the linearization error when

$\boldsymbol{v^{\scriptscriptstyle 2}}$

is regarded as an independent variable is normally smaller than that for

$\boldsymbol{v}$

because of the special properties of the distribution of

$\boldsymbol{v}$

in power grids.

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