The equivalent load in phase coordinates is given by a constant active and reactive power. Based on the magnitude of the load power and voltage, a direct transfer matrix is calculated.
In fig. 1 is an equivalent constant load equivalent circuit. Resistance is determined only in a direct sequence and depends on the total load power.
Fig. 1. Equivalent load equivalent circuit
In phase coordinates, the equivalent load is described by the direct transfer matrix
where 
, 
, 
, 
, U – equivalent load voltage, S = P + jQ – equivalent load power.
The attached file shows the calculation of the equivalent load form A matrix in Matlab: A_load.m