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The paper presents a new method to measure the active and reactive power in three¿¿phase systems. The method is based on the space vector formulation, which uses the instantaneous values of voltages and currents in a three-phase system. The work with instantaneous values allows the authors to obtain the instantaneous active and reactive power, because the measuring circuit requires no filters or integrators. The circuit is used in a transistor rectifier, which includes the control of the reactive component of the input currents. The convertor has sinusoidal input currents and these currents keep in phase with the source voltages independent of the amplitude and polarity of the load current, which means operation with optimum power factor without requiring reactive power. These results confirm that the measuring circuit can be used in high-quality reactive-power compensators for critical loads.