Patent ID: 8374810

Claim:
A method for calculating insertion indices for a phase leg of a DC to AC modular multilevel converter via a control apparatus, said control apparatus having at east one controller, the converter comprising one phase leg between upper and lower DC source common bars for each phase, each phase leg comprising two serially connected arms, wherein an AC output for each phase leg is connected between the two serially connected arms, wherein each arm comprises a number of submodules, wherein each submodule can be in a bypass state or a voltage insert mode, the insertion index comprising data representing a portion of available submodules that should be in the voltage insert mode for a particular arm, the method comprising the steps of: calculating, in said controller, a desired arm voltage for an upper arm connected to the upper DC source common bar and a lower arm connected to the lower DC source common bar, obtaining, in said controller, values representing actual total arm voltages in the upper arm and lower arm, respectively, and calculating, in said controller, insertion indices for the upper and lower arm, respectively, using the respective desired arm voltage and the respective value representing the total actual arm voltage; wherein the step of calculating desired arm voltages for a phase leg comprises calculating u CU ( t )= u D /2 −e V ( t )− u diff ( t ) for the upper arm, and calculating u CL ( t )= u D /2 +e V ( t )− u diff ( t ) for the lower arm, where u CU (t) represents desired upper arm voltage where u CL (t) represents desired lower arm voltage, u D represents a voltage between the upper and lower DC source common bars, e V (t) represents a reference inner AC output voltage and u diff (t) represents a control voltage to control a current passing through the whole phase leg, and calculating u diff ( t )= u diff1 ( t )+ u diff2 ( t ) where u diff1 (t) represents a voltage obtained by summing energy in the arms of the leg and u diff2 (t) represents a voltage obtained by calculating a difference in energy between the arms of the leg.