A power distribution system comprising: a converter-inverter arrangement having an input connectable to a three phase AC power source and a three phase output connectable to an input terminal of a load, the converter-inverter arrangement (14, 16) configured to control current flow and terminal voltages in the load; and a fault detection and protection system connected to the converter-inverter arrangement, the fault detection and protection system comprising: a plurality of current sensors (42, 44, 46) configured to measure a current on the three phase output of the converter-inverter arrangement; and characterised in comprising a controller configured to: measure the three phase current on the three phase output of the converter-inverter arrangement; extract a fundamental current component for each phase of the three phase current output of the converter-inverter arrangement; extract a third harmonic current component for each phase of the three phase current output of the converter-inverter arrangement; compare the fundamental current component and the third harmonic current component extracted from each phase to a first threshold and a second threshold, respectively; detect a ground fault on a phase of the three phase output based on the comparisons of the fundamental current component and the third harmonic current component to the first and second thresholds; and declare a ground fault when one phase of the three phase output has a fundamental current component that exceeds the first threshold and a third harmonic current component that exceeds the second threshold. a converter-inverter arrangement having an input connectable to a three phase AC power source and a three phase output connectable to an input terminal of a load, the converter-inverter arrangement (14, 16) configured to control current flow and terminal voltages in the load; and a fault detection and protection system connected to the converter-inverter arrangement, the fault detection and protection system comprising: a plurality of current sensors (42, 44, 46) configured to measure a current on the three phase output of the converter-inverter arrangement; and characterised in comprising a controller configured to: measure the three phase current on the three phase output of the converter-inverter arrangement; extract a fundamental current component for each phase of the three phase current output of the converter-inverter arrangement; extract a third harmonic current component for each phase of the three phase current output of the converter-inverter arrangement; compare the fundamental current component and the third harmonic current component extracted from each phase to a first threshold and a second threshold, respectively; detect a ground fault on a phase of the three phase output based on the comparisons of the fundamental current component and the third harmonic current component to the first and second thresholds; and declare a ground fault when one phase of the three phase output has a fundamental current component that exceeds the first threshold and a third harmonic current component that exceeds the second threshold. a plurality of current sensors (42, 44, 46) configured to measure a current on the three phase output of the converter-inverter arrangement; and characterised in comprising a controller configured to: measure the three phase current on the three phase output of the converter-inverter arrangement; extract a fundamental current component for each phase of the three phase current output of the converter-inverter arrangement; extract a third harmonic current component for each phase of the three phase current output of the converter-inverter arrangement; compare the fundamental current component and the third harmonic current component extracted from each phase to a first threshold and a second threshold, respectively; detect a ground fault on a phase of the three phase output based on the comparisons of the fundamental current component and the third harmonic current component to the first and second thresholds; and declare a ground fault when one phase of the three phase output has a fundamental current component that exceeds the first threshold and a third harmonic current component that exceeds the second threshold. measure the three phase current on the three phase output of the converter-inverter arrangement; extract a fundamental current component for each phase of the three phase current output of the converter-inverter arrangement; extract a third harmonic current component for each phase of the three phase current output of the converter-inverter arrangement; compare the fundamental current component and the third harmonic current component extracted from each phase to a first threshold and a second threshold, respectively; detect a ground fault on a phase of the three phase output based on the comparisons of the fundamental current component and the third harmonic current component to the first and second thresholds; and declare a ground fault when one phase of the three phase output has a fundamental current component that exceeds the first threshold and a third harmonic current component that exceeds the second threshold.