Circulating current control in a modular multilevel voltage source converter

A modular multilevel voltage source converter comprising:

• a number of phase legs, each of the number of phase legs comprising an upper and a lower phase arm comprising cells with energy storage elements;
• and
• a control unit operative to:
• model a circulating current in a phase leg using a first expression (ΔVPR1, ΣVcap-PR1), a second expression (ΔVR2N, ΣVcap-R2N) and at least one modulation signal (mR, mcm), where the first expression comprises the sum of cell voltages in the upper phase arm ΣVcap-PR1 and the second expression comprises the sum of cell voltages in the lower phase arm ΣVcap-R2N;
• and
• determine a circulating current control signal (ER1−ER2+2VCM) for the phase leg through said circulating current modelling and combine the circulating current control signal with a modulation signal used for one of the phase arms thereby obtaining a modified modulation signal controlling the phase arm to reduce or introduce a circulating current while a waveshape is formed, wherein the at least one modulation signal comprises said modulation signal used for one of the phase arms.

A method of handling circulating currents in a modular multilevel voltage source converter comprising a number of phase legs, each comprising an upper and a lower phase arm comprising cells with energy storage elements, the method being performed in a control unit of the voltage source converter and comprising:

• modelling a circulating current in a phase leg using a first expression (ΔVPR1, ΣVcap-PR1), a second expression (ΔVR2N, ΣVcap-R2N) and at least one modulation signal, where the first expression comprises the sum of cell voltages in the upper phase arm ΣVcap-PR1 and the second expression comprises the sum of cell voltages in the lower phase arm ΣVcap-R2N;
• determining a circulating current control signal (ER1−ER2+2VCM) for the phase leg through said circulating current modelling;
• and
• combining the circulating current control signal with a modulation signal used for one of the phase arms thereby obtaining a modified modulation signal controlling the phase arm to reduce or introduce a circulating current while a waveshape is formed,
• wherein the at least one modulation signal comprises said modulation signal used for one of the phase arms.

A computer program product for handling circulating currents in a modular multilevel voltage source converter comprising a number of phase legs, each comprising an upper and a lower phase arm comprising cells with energy storage elements, the computer program product comprising a data carrier with computer program code configured to cause a control unit of the voltage source converter to:

• model a circulating current in a phase leg using a first expression (ΔVPR1, ΣVcap-PR1), a second expression (ΔVR2N, ΣVcap-R2N) at least one modulation signal, where the first expression comprises the sum of cell voltages in the upper phase arm ΣVcap-PR1 and the second expression comprises the sum of cell voltages in the lower phase arm ΣVcap-R2N;
• determine a circulating current control signal (ER1−ER2+2VCM) for the phase leg through said circulating current modelling;
• and
• combine the circulating current control signal with a modulation signal used for one of the phase arms thereby obtaining a modified modulation signal controlling the phase arm to reduce or introduce a circulating current while a waveshape is formed, wherein the at least one modulation signal comprises said modulation signal used for one of the phase arms.