This study presents a new method that overcomes the limitations of previous Thévenin equivalent methods used for detecting voltage instability. The new method determines the maximum deliverable power to a load by accounting for the nonlinearity of the Thévenin voltage seen from a load. The developed algorithm computes Thévenin equivalents seen from voltagecontrolled and non-controlled nodes to determine how much load changes influence the contributing generators to a load. The algorithm computes an active load margin which gives information about the distance to instability. Tests are performed on the modified Nordic32 system, where the on-load tap changer mechanism of transformers was the driving force causing voltage instability that leads to small-signal rotor angle instability. The results demonstrate that the proposed method accurately detects voltage instability compared to the method based on the Thévenin impedance matching criterion, which first detects instability when the system conditions have crossed the critical boundaries.