Abstract: A method performed by a control system of a power electronics converter. A first part of a grid-side current controller runs a first feedback control algorithm having a first control cycle time and includes at least proportional control using a proportional gain. A third part of the controller runs a third feedback control algorithm having the first control cycle time and acting on an output from the first control algorithm after SOA limits have been applied and includes counteracting the proportional control of the first feedback control algorithm. A second part of the controller runs a second feedback control algorithm having a second control cycle time, less than the first control cycle time, and acting on an output from the third control algorithm with the same polarity as the first control algorithm and includes proportional control using the proportional gain.

Abstract: A method performed by a control system of a power electronics converter. A first part of a grid-side current controller runs a first feedback control algorithm having a first control cycle time and includes at least proportional control using a proportional gain. A third part of the controller runs a third feedback control algorithm having the first control cycle time and acting on an output from the first control algorithm after SOA limits have been applied and includes counteracting the proportional control of the first feedback control algorithm. A second part of the controller runs a second feedback control algorithm having a second control cycle time, less than the first control cycle time, and acting on an output from the third control algorithm with the same polarity as the first control algorithm and includes proportional control using the proportional gain.

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: In a method of compensating for a DC offset of a high-voltage AC output from a Modular Multilevel Converter (MMC) including at least one phase leg, the MMC is connected to a three-phase high-voltage AC grid via a grid transformer. The method includes, in at least one DC offset correcting device, measuring the DC offset by in each of the at least one DC offset correcting device: obtaining a high-voltage AC signal in the MMC, removing high-voltage AC components from the obtained high-voltage AC signal by means of a passive higher-order filter to obtained an analogue filtered signal, converting the analogue filtered signal to a digital signal by means of an analogue-to-digital converter, removing remaining AC components from the digital signal by means of a digital filter to obtain the DC offset, and in a controller comparing the obtained offset with a reference value and forming a control signal based on said comparing.

Abstract: In a method of compensating for a DC offset of a high-voltage AC output from a Modular Multilevel Converter (MMC) including at least one phase leg, the MMC is connected to a three-phase high-voltage AC grid via a grid transformer. The method includes, in at least one DC offset correcting device, measuring the DC offset by in each of the at least one DC offset correcting device: obtaining a high-voltage AC signal in the MMC, removing high-voltage AC components from the obtained high-voltage AC signal by means of a passive higher-order filter to obtained an analogue filtered signal, converting the analogue filtered signal to a digital signal by means of an analogue-to-digital converter, removing remaining AC components from the digital signal by means of a digital filter to obtain the DC offset, and in a controller comparing the obtained offset with a reference value and forming a control signal based on said comparing.