The presence of injected DC has harmful consequences for a power grid system. A piecewise sinusoidal ripple voltage wave at the line-frequency that rides on the main capacitor bank of the power converter is observed. This observation leads to a new detection method and mitigation method. A two-stage control circuit is added to the operation of a power converter that controls power line impedance in order to mitigate the injected DC and to block DC circulation. This control computes a correction angle to adjust the timing of generated pulsed square waves to counter-balance the ripple. A functional solution and the results of experiments are presented. Furthermore, an extraction method and three elimination methods for this ripple component are presented to allow dissipation of DC energy through heat and/or electronic magnetic wave, or to allow transformation of this energy into usable power that is fed back into the power grid.

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. The method also includes transmitting the control signal from each of the at least one DC offset correcting device to a control device of the MMC. The method also includes, the control device mapping the control signal(s) from the at least one DC offset correcting device to the at least one phase leg. The method also includes, based on the mapping, the control device sending switching commands to the semiconductor switches of MMC cells in each of the at least one phase leg to compensate for the DC offset.

A grid access current control method without current sensor applicable to a grid-connected inverter relates to a system including a main circuit of the grid-connected inverter and a control circuit of the grid-connected inverter. The control circuit of the grid-connected inverter includes a grid access current open-loop control module and a PWM generation module; the grid access current open-loop control module includes a first proportional regulator, a second proportional regulator, a delayer, and an adder; input ends of the first proportional regulator and the second proportional regulator each are led out as an input end of a grid access current reference signal; and an output end of the first proportional regulator is connected to an input end of the adder; an output end of the second proportional regulator is connected to an input end of the delayer.

A power supply includes: a rectifier circuit which has a plurality of rectifier switching elements, and separately extracts a positive voltage and a negative voltage for every phase from a primary power source of three-phase alternating current, respectively; a smoothing circuit which has a pair of smoothing capacitors connected in series to each other to be charged by the rectifier circuit, and a plurality of smoothing inductors respectively arranged between the rectifier circuit and the smoothing capacitors; a inverter circuit which has a plurality of inverter switching elements and inverts output of the smoothing circuit into alternating current; and a control circuit which controls switching of the plurality of rectifier switching elements so that output voltage of the smoothing circuit becomes a desired voltage, and electrical current flowing to each phase of the rectifier circuit becomes a desired electrical current.

A method for operating an inverter and an inverter configured to convert DC power supplied from a DC power source into AC power supplied to an AC network by applying operational parameter limits which define limits for allowed operating points of the inverter, collect, during the converting, data of operational conditions related to the inverter, determine on the basis of the collected data whether the set operational parameter limits can be optimized with regard to one or more optimization criteria, and in response to determining that the set operational parameter limits can be optimized, adapt one or more of the set operational parameter limits applied in the converting on the basis of the collected data.

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. The method also includes transmitting the control signal from each of the at least one DC offset correcting device to a control device of the MMC. The method also includes, the control device mapping the control signal(s) from the at least one DC offset correcting device to the at least one phase leg. The method also includes, based on the mapping, the control device sending switching commands to the semiconductor switches of MMC cells in each of the at least one phase leg to compensate for the DC offset.

A DC-DC converter includes a first full-bridge circuit and a second full-bridge circuit isolated by a transformer. The first full-bridge circuit includes switching elements, a first floating capacitor, and a second floating capacitor. The first full-bridge circuit operates in at least one of a full-bridge operation mode and a half-bridge operation mode. In switching of the operation mode, switching phases of the first full-bridge circuit are shifted in two portions in one cycle of a drive frequency, and shift amounts of the phases are determined such that positive and negative output voltages of the first full bridge circuit are balanced before and after the operation mode is switched.

A DC-DC converter includes a first full-bridge circuit and a second full-bridge circuit isolated by a transformer. The first full-bridge circuit includes switching elements, a first floating capacitor, and a second floating capacitor. The first full-bridge circuit operates in at least one of a full-bridge operation mode and a half-bridge operation mode. In switching of the operation mode, switching phases of the first full-bridge circuit are shifted in two portions in one cycle of a drive frequency, and shift amounts of the phases are determined such that positive and negative output voltages of the first full bridge circuit are balanced before and after the operation mode is switched.

The present disclosure provides an isolated multi-phase DC/DC converter with a reduced quantity of blocking capacitors. In one aspect, the converter includes a multi-phase transformer having a primary circuit and a secondary circuit magnetically coupled to the primary circuit, the primary circuit having a first quantity of terminals, and the secondary circuit having a second quantity of terminals; a third quantity of blocking capacitors, each being electrically connected in series to a respective one of the terminals of the primary circuit; and a fourth quantity of blocking capacitors, each being electrically connected in series to a respective one of the terminals of the secondary circuit. The third quantity is one less than the first quantity. The fourth quantity is one less than the second quantity.

The present disclosure provides an isolated multi-phase DC/DC converter with a reduced quantity of blocking capacitors. In one aspect, the converter includes a multi-phase transformer having a primary circuit and a secondary circuit magnetically coupled to the primary circuit, the primary circuit having a first quantity of terminals, and the secondary circuit having a second quantity of terminals; a third quantity of blocking capacitors, each being electrically connected in series to a respective one of the terminals of the primary circuit; and a fourth quantity of blocking capacitors, each being electrically connected in series to a respective one of the terminals of the secondary circuit. The third quantity is one less than the first quantity. The fourth quantity is one less than the second quantity.