A method of controlling output levels of an MMC converter to reduce fluctuation in a power grid frequency, which adjusts an output level of the MMC converter in response to a change in a power grid frequency of a power grid system in the MMC converter connected to a grid system, is proposed. The method includes a detection step of detecting a power grid frequency of a grid connected to the MMC converter in real time, a comparison step of comparing the detected power grid frequency with a preset reference power grid frequency, and an adjustment step of adjusting a number of output levels of the MMC converter to reduce a difference between the detected power grid frequency and the reference power grid frequency when the detected power grid frequency and the reference power grid frequency are different from each other.

A method of controlling output levels of an MMC converter to reduce fluctuation in a power grid frequency, which adjusts an output level of the MMC converter in response to a change in a power grid frequency of a power grid system in the MMC converter connected to a grid system, is proposed. The method includes a detection step of detecting a power grid frequency of a grid connected to the MMC converter in real time, a comparison step of comparing the detected power grid frequency with a preset reference power grid frequency, and an adjustment step of adjusting a number of output levels of the MMC converter to reduce a difference between the detected power grid frequency and the reference power grid frequency when the detected power grid frequency and the reference power grid frequency are different from each other.

An electronic system includes a plurality of switching elements (T) and a plurality of energy storage elements (L; C). The energy storage elements (L; C) are connected to one another by the switching elements (T). The energy storage elements (L; C) can be selectively switched to a first, a second or a third state by switching the switching elements (T). In the first state, the energy storage elements (L; C) are connected in series with one another. In the second state, the energy storage elements (L; C) are connected in parallel with one another. In the third state, the energy storage elements (L; C) are bypassed, wherein two of the energy storage elements (L; C) are each connected by no more than three of the switching elements (T).

A loss optimization control method for modular multilevel converters (MMCs) under fault-tolerant control is disclosed. The method includes the following steps: when a fault of a SM in a MMC occurs, bypassing the faulty SM to achieve fault-tolerant control; suppressing the fundamental circulating current using a fundamental circulating current controller; respectively calculating the loss of each SM in faulty arms and healthy arms by using loss expressions of different switching tubes in SMs of the MMC; aiming at the loss imbalance between the arms of the MMC, taking the loss of a healthy SM as the reference, adjusting the period of capacitor voltage sorting control in the faulty SMs, achieving the loss control over the working SMs in the faulty SMs, and finally achieving the loss balance of each SM in the faulty arms and the healthy arms. Compared with the conventional methods, the proposed method is easier to implement and does not increase the construction cost of MMCs.

A cascaded multilevel converter is disclosed. The converter comprises a plurality of modules coupled to form a branch, each of the modules comprising a switching circuit and a DC link for supplying DC voltage to the switching circuit. The converter further comprises a controller for controlling the switching circuit of each module to generate an AC voltage in the branch, wherein the controller is configured to: determine for each module a voltage across a capacitor of the DC link of the module, determine for each module a reference power value for charging the capacitor of the DC link of the module to a reference voltage value for the module, determine, from the reference power values of the modules, a common reference AC current value for AC current in the branch, determine, from the common reference AC current value, a common reference AC voltage value for AC voltage in the branch.

An MMC-type power conversion device includes a failure detection unit that detects presence or absence of failure of each of n upper arm current detectors and n lower arm current detectors. The failure detection unit makes a first determination based on comparison between a sum of detection values of n upper arm current detectors and the sum of detection values of n lower arm current detectors, a second determination based on comparison between a current command value and the sum of detection values of n upper arm current detectors, a third determination based on comparison between a current command value and the sum of detection values of n lower arm current detectors, and a fourth determination of comparing, for each phase, the sum of detection values of the current detectors of an upper arm and a lower arm of the same phase.

An MMC-type power conversion device includes a failure detection unit that detects presence or absence of failure of each of n upper arm current detectors and n lower arm current detectors. The failure detection unit makes a first determination based on comparison between a sum of detection values of n upper arm current detectors and the sum of detection values of n lower arm current detectors, a second determination based on comparison between a current command value and the sum of detection values of n upper arm current detectors, a third determination based on comparison between a current command value and the sum of detection values of n lower arm current detectors, and a fourth determination of comparing, for each phase, the sum of detection values of the current detectors of an upper arm and a lower arm of the same phase.

A power converter for transforming electrical power between direct current power and alternating current power is provided, as well as related methods and systems. The power converter comprises: a half-bridge inverter, a switching cell, and a connection branch connecting the half-bridge inverter to the switching cell. The half-bridge inverter comprises: first and second switches connected in parallel, and a first capacitor connected between the first and second switches. The switching cell comprises: a first pair of switches forming a first branch, a second pair of switches and a second capacitor forming a second branch; and a third capacitor connected between the first and second branches. The connection branch is coupled to the half-bridge inverter at a first point intermediate the first capacitor and the second switch, and coupled to the switching cell at a second point intermediate the first branch and the second capacitor.

A dual multi-level inverter topology with reduced switch count and small DC-link capacitor is provided. The inverter topology provides multi-level inverter operation without requiring a neutral point connection that is commonly present in a stacked capacitor topology (for example, a topology including two capacitors).

Aspects are described for hybrid modular multilevel converters that include half-bridge submodules. In some embodiments, a hybrid modular multilevel converter can include a direct current (DC) bus and an alternating current (AC) node. A first arm of the hybrid modular multilevel converter includes a first submodule chain link and a first arm inductor and a second arm includes a second submodule chain link and a second arm inductor. A capacitor connects between a first side of the first arm and a first side of the second arm.