An aircraft electrical power system comprising a power converter unit (Blocks A-F) comprising a single 6-pulse rectifier cell configured to receive an AC input in the range of 350-800 Hz from a 115V or 230V AC power source and an active power filter (Block B, C) connected as a shunt between the AC input and the 6-pulse rectifier cell; wherein the active power filter comprises a control algorithm configured to cause removal of current harmonic frequencies from the AC input but not of a current fundamental frequency of the AC input; wherein the control algorithm comprises: - a transformation stage configured to rotate sensed AC input currents from a natural reference frame to a synchronous reference frame; - a high pass filter configured to filter out a fundamental harmonic from a sensed load current and to provide only harmonics of the fundamental at an output; - adaptive current control means configured to provide a high control loop gain at the frequency of the harmonics from the output of the high pass filter regardless of AC power source fundamental frequency; a pulse width modulator or a space vector modulator configured to modulate the output of the adaptive current control means; and a synchronising algorithm configured to use Adaptive Lattice Synchronous Reference Frame Phase Locked Loop, ALSRF-PLL, synchronisation for synchronising the active power filter (Block B, C) with the AC power source comprising a single 6-pulse rectifier cell configured to receive an AC input in the range of 350-800 Hz from a 115V or 230V AC power source and an active power filter (Block B, C) connected as a shunt between the AC input and the 6-pulse rectifier cell; wherein the active power filter comprises a control algorithm configured to cause removal of current harmonic frequencies from the AC input but not of a current fundamental frequency of the AC input; wherein the control algorithm comprises: - a transformation stage configured to rotate sensed AC input currents from a natural reference frame to a synchronous reference frame; - a high pass filter configured to filter out a fundamental harmonic from a sensed load current and to provide only harmonics of the fundamental at an output; - adaptive current control means configured to provide a high control loop gain at the frequency of the harmonics from the output of the high pass filter regardless of AC power source fundamental frequency; - a transformation stage configured to rotate sensed AC input currents from a natural reference frame to a synchronous reference frame; - a high pass filter configured to filter out a fundamental harmonic from a sensed load current and to provide only harmonics of the fundamental at an output; - adaptive current control means configured to provide a high control loop gain at the frequency of the harmonics from the output of the high pass filter regardless of AC power source fundamental frequency; a pulse width modulator or a space vector modulator configured to modulate the output of the adaptive current control means; and a synchronising algorithm configured to use Adaptive Lattice Synchronous Reference Frame Phase Locked Loop, ALSRF-PLL, synchronisation for synchronising the active power filter (Block B, C) with the AC power source