A power conversion device includes a power conversion circuit unit including a plurality of leg circuits, and a control device. Each of the leg circuits includes a plurality of first converter cells each having a capacitor and connected in series to each other and a plurality of second converter cells each having the capacitor and connected in series to each other. The plurality of first converter cells are controlled not based on a circulating current circulating between the plurality of leg circuits, and the plurality of second converter cells are controlled based on the circulating current. The control device executes control processing for increasing a current flowing through the second converter cell when a voltage at the capacitor in the second converter cell is less than a first threshold value.

A smart cell, comprising: a positive terminal; a negative terminal; a switching circuit which is arranged to select between a first switching state in which an energy storage device is connected between the positive terminal and the negative terminal and a second switching state which bypasses said energy storage device; an inductor provided between the positive terminal and the output of the switching network; and a controller arranged to monitor the voltage across the inductor and arranged to control a duty cycle of the switching circuit based on the magnitudes of voltage changes detected across the inductor. By monitoring and analysing the magnitude of voltage changes across the inductor, the controller determines the states of charge of other series connected smart cells without any communication between cells. None of the smart cells need to transmit information on their states of charge to other smart cells in the string as each cell can sense information about the other cells from the voltage changes on the inductor. By analysing the voltage across the local sense inductor, the average state of charge of a series string of smart cells can be obtained and compared to the state of charge of the local smart cell to determine how the duty cycle of the local smart cell should be modified to synchronize its state of charge with the series string. The magnitude of the voltage change across the inductor is related to the state of charge of the cell that just switched in or out of the string.

An inverter includes an upper unit comprises a first switch, a second switch and a third switch, wherein during a first half of a cycle of the inverter, the second switch is turned on before and turned off after the third switch, a lower unit comprising a fourth switch, a fifth switch and a sixth switch, wherein during a second half of the cycle of the inverter, the fifth switch is turned on before and turned off after the sixth switch, a flying capacitor connected between the upper unit and the lower unit, and a filter connected to a common node of the upper unit and the lower unit.

An inverter includes an upper unit comprises a first switch, a second switch and a third switch, wherein during a first half of a cycle of the inverter, the second switch is turned on before and turned off after the third switch, a lower unit comprising a fourth switch, a fifth switch and a sixth switch, wherein during a second half of the cycle of the inverter, the fifth switch is turned on before and turned off after the sixth switch, a flying capacitor connected between the upper unit and the lower unit, and a filter connected to a common node of the upper unit and the lower unit.

Systems and methods of this disclosure use low voltage energy storage devices to supply power at a medium voltage from an uninterruptible power supply (UPS) to a data center load. The UPS includes a low voltage energy storage device (ultracapacitor/battery), a high frequency (HF) bidirectional DC-DC converter, and a multi-level (ML) inverter. The HF DC-DC converter uses a plurality of HF planar transformers, multiple H-bridge circuits, and gate drivers for driving IGBT devices to generate a medium DC voltage from the ultracapacitor/battery energy storage. The gate drivers are controlled by a zero voltage switching (ZVS) controller, which introduces a phase shift between the voltage on the primary and secondary sides of the transformers. When the primary side leads the secondary side, the ultracapacitor/battery discharges and causes the UPS to supply power to the data center, and when the secondary side leads the primary side, power flows from the grid back to the UPS, thereby recharging the ultracapacitor/battery.

Ina power conversion system having a fixed pulse pattern modulation unit 2 that is configured to refer to tables storing therein pulse patterns that determine respective command voltage levels corresponding to phase information for each modulation ratio and to generate a gate signal g on the basis of a command modulation ratio d and a control phase θ and driving a power converter 3 on the basis of the gate signal g, the fixed pulse pattern modulation unit 2 is further configured to, when performing a pulse pattern transition, search for a proper post-transition table reference position and make a command voltage level follow a command voltage level of a post-transition pulse pattern. With this, the power conversion system that can perform the pulse pattern transition without current impulse and that can also be applied to a multi-level power converter having four levels or more can be provided.

A power converter for transforming electrical power between direct current (DC) power and alternating current (AC) power is provided, as well as a controller therefor and associated methods and systems. The power converter comprises: a first set of packed U-cell converters connectable between a first common connection point and a first terminal of an external circuit, the first common connection point connecting to a first terminal of a DC circuit element; a second set of packed U-cell converters connectable between a second common connection point and a second, opposite terminal of the external circuit, the second common connection point connecting to a second, opposite terminal of the DC circuit element; and a controller configured for controlling the operation of the first and second sets of packed U-cell converters.

A power converter for transforming electrical power between direct current (DC) power and alternating current (AC) power is provided, as well as a controller therefor and associated methods and systems. The power converter comprises: a first set of packed U-cell converters connectable between a first common connection point and a first terminal of an external circuit, the first common connection point connecting to a first terminal of a DC circuit element; a second set of packed U-cell converters connectable between a second common connection point and a second, opposite terminal of the external circuit, the second common connection point connecting to a second, opposite terminal of the DC circuit element; and a controller configured for controlling the operation of the first and second sets of packed U-cell converters.

A control device for an MMC is disclosed. The control device for an MMC including a plurality of converter arms that include a plurality of sub-modules connected in series and that are connected to a DC link includes: an arm controller, which detects the arm current of a converter arm so as to determine whether a DC failure has occurred, and, if it is determined that the DC failure has occurred, transmits a bypass control signal for protecting a sub-module and notifies of the DC failure; a sub-module controller for controlling the sub-module so as to bypass a DC failure current according to the bypass control signal received from the arm controller; and a main controller, which detects, in real-time, the arm current of the converter arm and a voltage (DC link voltage) of the DC link, determines whether the DC failure is a temporary DC failure or a permanent DC failure on the basis of the detected arm current and DC link voltage if the occurrence of the DC failure is notified by the arm controller, and transmits, to the arm controller, a normal operation control signal for normal operation of the sub-module or a bypass control signal for bypassing of the DC failure current.

A control device for an MMC is disclosed. The control device for an MMC including a plurality of converter arms that include a plurality of sub-modules connected in series and that are connected to a DC link includes: an arm controller, which detects the arm current of a converter arm so as to determine whether a DC failure has occurred, and, if it is determined that the DC failure has occurred, transmits a bypass control signal for protecting a sub-module and notifies of the DC failure; a sub-module controller for controlling the sub-module so as to bypass a DC failure current according to the bypass control signal received from the arm controller; and a main controller, which detects, in real-time, the arm current of the converter arm and a voltage (DC link voltage) of the DC link, determines whether the DC failure is a temporary DC failure or a permanent DC failure on the basis of the detected arm current and DC link voltage if the occurrence of the DC failure is notified by the arm controller, and transmits, to the arm controller, a normal operation control signal for normal operation of the sub-module or a bypass control signal for bypassing of the DC failure current.