The interval nonlinear AC power flow equation

which can be transformed to the affine optimization model by means of affine arithmetic

overcomes the conservative characteristics of interval arithmetic

but contains the approximation of strong nonlinear elements such as the trigonometric function. Thereby

we proposed a novel kind of interval power flow calculation method based on mixed power flow equations. This method uses a nonlinear AC power flow equation to establish the affine formulation of the node voltage amplitude/phase angle

and then substitutes them into the linear power flow equation to form the linear optimization model about the noise elements

helping to inhibit interval extension. The simulation results of IEEE-30 nodes system with wind farms show that the interval boundaries obtained by the proposed method are closer to those obtained by the Monte Carlo method

and are narrower than the affine method

while the proposed method is more efficient than the Monte Carlo method.