Abstract: A method of an estimator of an inner control loop controlling a three-to-single-phase converter connected to an AC power grid via a transformer includes obtaining a value of a voltage reference uRef produced by the inner control loop for the converter, obtaining a value of a secondary side current produced by the converter and measured between the converter and the transformer, obtaining a value of a primary side current produced by the converter and measured between the grid and the transformer, and obtaining a value of a primary side voltage measured between the grid and the transformer. The method also includes estimating a control current iCtrl component of the primary or secondary side current iMeas which results from the voltage reference, based on the obtained values of the voltage reference, the secondary side current, the primary side current and the primary side voltage, and feeding the estimated control current iCtrl* to the inner control loop.

Abstract: A method of an estimator of an inner control loop controlling a three-to-single-phase converter connected to an AC power grid via a transformer includes obtaining a value of a voltage reference uRef produced by the inner control loop for the converter, obtaining a value of a secondary side current produced by the converter and measured between the converter and the transformer, obtaining a value of a primary side current produced by the converter and measured between the grid and the transformer, and obtaining a value of a primary side voltage measured between the grid and the transformer. The method also includes estimating a control current iCtrl component of the primary or secondary side current iMeas which results from the voltage reference, based on the obtained values of the voltage reference, the secondary side current, the primary side current and the primary side voltage, and feeding the estimated control current iCtrl* to the inner control loop.

Abstract: A method of controlling a Modular Multilevel Converter (MMC) during a fault in a power grid to which the MMC is connected is disclosed. The MMC includes a plurality of phases, each including at least one phase leg. Each phase leg includes a plurality of series connected converter cells. The method includes determining that a fault has occurred in the power grid, the fault including a reduced voltage in the power grid. The method also includes, for each phase leg of the MMC, reducing the DC voltage of the phase leg by a predetermined amount from a nominal DC voltage to a reduced DC voltage, the DC voltage being the sum of all respective voltages over an energy storage of each cell of the phase leg. The method also includes determining that the fault has been cleared, whereby the voltage in the power grid is returned to nominal. The method also includes for each of the phase legs of the MMC, increasing the DC voltage back to the nominal DC voltage.

Abstract: A method of controlling a Modular Multilevel Converter (MMC) during a fault in a power grid to which the MMC is connected is disclosed. The MMC includes a plurality of phases, each including at least one phase leg. Each phase leg includes a plurality of series connected converter cells. The method includes determining that a fault has occurred in the power grid, the fault including a reduced voltage in the power grid. The method also includes, for each phase leg of the MMC, reducing the DC voltage of the phase leg by a predetermined amount from a nominal DC voltage to a reduced DC voltage, the DC voltage being the sum of all respective voltages over an energy storage of each cell of the phase leg. The method also includes determining that the fault has been cleared, whereby the voltage in the power grid is returned to nominal. The method also includes for each of the phase legs of the MMC, increasing the DC voltage back to the nominal DC voltage.