System and method for providing N bipolar AC phase voltages

A system for providing N bipolar AC phase voltages UVj, with j=1 . . . N, wherein N is an integer number that is two or more, said system comprising:

• N modular energy storage direct converter systems (MESDCS) and a control system,
• wherein each MESDCS comprises a converter arm having a first end and a second end and comprising a plurality of sequentially interconnected modules, wherein each module comprises at least one first terminal and at least one second terminal, a storage element for electrical energy or an energy conversion element, and a plurality of module switches,
• wherein in each two adjacent modules the at least one first terminal of one module is connected either directly or via an intermediate component to the at least one second terminal of the other module,
• wherein under control of said control system, said plurality of module switches are configured to
• connect energy storage elements or energy conversion elements of adjacent modules in series, and
• for one or both of selectively deactivate or bypass the energy storage element or energy conversion element of each module and connect energy storage elements or energy conversion elements of adjacent modules in parallel,
• wherein the first ends of each MESDCS are connected to a common floating connection point, and wherein the j-th MESDCS is controllable, by said control system, to output a star voltage USj with respect to the floating connection point, with j=1, . . . , N, wherein said system is configured to provide each of said bipolar AC phase voltages UVj as voltage differences between two of said star voltages, such that UVj=USj+1−USj, or UVj=USj−USj+1 for each j between 1 and N−1, and
• UVN=US1−USN, or UVN=USN−US1, respectively,
• wherein said control system is configured to control each MESDCS to output a corresponding bipolar star voltage USj that can be decomposed into a periodic bipolar AC function Pj(t) and a bipolar offset Uoff(t) that is common to each star voltage USj, such that USj=Pj(t)+Uoff(t),
• wherein the periodic bipolar AC functions Pj(t) associated with different star voltages USj are phase-shifted copies of each other such that for each integers i, j chosen from [1, . . . , N] and k chosen from [1, . . . , N−1], Pi(t)=Pj(t+k·T/N), wherein T is a period of said periodic bipolar AC function Pj(t), and wherein preferably, Pi(t)=Pj(t+(i−j)·T/N), and
• wherein Uoff (t) is such that for each star voltage USj, a difference between the maximum and minimum values acquired during a full period T is less than without such bipolar offset.

A method for providing N bipolar AC phase voltages UVj, with j=1 . . . N using N modular energy storage direct converter systems (MESDCS), wherein N is an integer number that is two or more,

• wherein each MESDCS comprises a converter arm having a first end and a second end and comprising a plurality of sequentially interconnected modules, wherein each module comprises at least one first terminal and at least one second terminal, a storage element for electrical energy or an energy conversion element, and a plurality of module switches,
• wherein in each two adjacent modules, the at least one first terminal of one module is connected either directly or via an intermediate component to the at least one second terminal of the other module,
• wherein said method comprises operating said plurality of module switches for connecting energy storage elements or energy conversion elements of adjacent modules in series, and
• for one or both of selectively deactivating or bypassing the energy storage element or energy conversion element of each module and connecting energy storage elements or energy conversion elements of adjacent modules in parallel,
• wherein the first ends of each MESDCS are connected to a common floating connection point, and wherein the j-th MESDCS is controlled to output at its second end a star voltage USj with respect to the floating connection point, with j=1, . . . , N,
• wherein the method comprises providing each of said bipolar AC phase voltages UVj as voltage differences between two of said star voltages, such that UVj=USj+1−USj, or UVj=USj−USj+1 for each j between 1 and N−1, and UVN=US1−USN, or UVN=USN−US1, respectively,
• wherein each MESDCS is controlled to output a corresponding unipolar star voltage USj that can be decomposed into a periodic bipolar AC function Pj(t) and a unipolar offset Uoff(t) that is common to each star voltage USj, such that USj(t)=Pj(t)+Uoff(t), wherein the absolute value of said common unipolar offset Uoff(t) is at all times t sufficiently high that USj (t) is unipolar,
• wherein the periodic bipolar AC functions Pj(t) associated with different star voltages USj are phase-shifted copies of each other such that for each integers i, j chosen from [1, . . . , N] and k chosen from [1, . . . , N−1], Pi(t)=Pj(t+k·T/N), wherein T is the period of said periodic bipolar AC function Pj(t), wherein Pi(t)=Pj(t+(i−j)·T/N).