Method of operating a-c drive with parallel connected d-c link power converters

Claim 1

A method of operating an a-c drive comprising a plurality of parallel connected polyphase d-c link power converters having load side, converters with controlled rectifier switches which are cyclically fired to generate component alternating load currents which are summed by a direct parallel connection without isolation of said power converters to produce a composite load current, said method comprising: operating said d-c link power converters to generate substantially constant d-c link currents; and cyclically firing the controlled rectifier switches in said load side converters of the respective power converters at a repetition rate which effects load commutation between said controlled rectifier switches and with the firing of said controlled rectifier switches within each load side converter phase shifted with respect to the firing of the controlled rectifier switches in the other load side converters to generate from said substantially constant d-c link currents, component alternating load currents which are phase shifted by an amount which reduces the magnitude of selected harmonics in the composite load current produced through the direct parallel connection without isolation of said power converters.

Claim 2

A method of operating an a-c drive comprising a plurality of parallel connected polyphase d-c link power converters having load side, converters with controlled rectifier switches which are cyclically fired to generate component alternating load currents which are summed by a direct parallel connection without isolation of said power converters to produce a composite load current, said method comprising: operating said d-c link power converters to generate substantially constant d-c link currents; and cyclically firing the controlled rectifier switches in said load side converters of the respective power converters at a repetition rate which effects load commutation between said controlled rectifier switches and with the firing of said controlled rectifier switches within each load side converter phase shifted with respect to the firing of the controlled rectifier switches in the other load side converters to generate from said substantially constant d-c link currents, component alternating load currents which are phase shifted by an amount which reduces the magnitude of selected harmonics in the composite load current produced through the direct parallel connection without isolation of said power converters.

Claim 5

A method of operating in a load commutated mode an a-c drive comprising a pair of directly parallel connected without isolation, three-phase, d-c link power converters, each having load commutated controlled rectifier switches in a three-phase output inverter, said method comprising the steps of: operating aid d-c link power converters to generate essentially constant d-c link current; and cyclically firing said load commutated controlled rectifier switches in one of said output inverters phase shifted with respect to the firing of the controlled rectifier switches in the other output inverter to generate three-phase component load currents from each power converter which component load currents are summed by said directly parallel connected without isolation power converters to generate a composite pseudo twelve-pulse three-phase load current, and which three-phase component load currents are phase shifted with respect to one another by a phase angle which reduces the magnitude of at least a selected harmonic in said composite pseudo twelve-pulse three-phase load current.

Claim 6

A method of operating in a load commutated mode an a-c drive comprising a pair of directly parallel connected without isolation, three-phase, d-c link power converters, each having load commutated controlled rectifier switches in a three-phase output inverter, said method comprising the steps of: operating aid d-c link power converters to generate essentially constant d-c link current; and cyclically firing said load commutated controlled rectifier switches in one of said output inverters phase shifted with respect to the firing of the controlled rectifier switches in the other output inverter to generate three-phase component load currents from each power converter which component load currents are summed by said directly parallel connected without isolation power converters to generate a composite pseudo twelve-pulse three-phase load current, and which three-phase component load currents are phase shifted with respect to one another by a phase angle which reduces the magnitude of at least a selected harmonic in said composite pseudo twelve-pulse three-phase load current.