Based on the instantaneous d-q transformation theory,a voltage sags detection device is developed by Motorola DSP56F807.

The inductive parameters and models in the d-q circuit are derived firstly in the paper by a unified expression for calculating inductances of AC machines and double d-q transformations respectively.

By use of d-q transformation, in this control method a three phase AC system is transformed into rotating orthogonal coordinate system and under the transformed coordinate system the compensating current of active power filter is controlled by PI controller to make it trace the reference current.

A method of harmonic and reactive currents detection based on d-q coordinates transformation for three-phase voltage and current is proposed, which separates the responding components after d-q transformation to realize the current detection.

APF parallel operation based on d-q transform;

Recognition of transient voltage disturbances based on integrated mathematics morphology filters and non-delay d-q transform;

An improved d-q transform method without time delay is proposed.

Dummy three-phase voltages are gained by the derivative of any phase voltage, which is the improved d-q transform without delay.

Based on dual d-q coordinate transformation and trigonometric transform, an improved real-time detection method of random integer-order harmonic voltage is proposed.