A modular power flow control system having early detection and reporting of transmission line faults is described. The response time for closing a bypass switch and reporting the fault is less than 200 microseconds for hard faults, longer for soft faults. Reprogramming of distance relays is not required. Transmission line faults are characterized using a fault detection sensor suite, normally including at least a current sensor such as a current transformer and a rate of current change sensor such as a Rogowski coil, and in some embodiments, a temperature sensor. Other embodiments are disclosed.

Provided is an active power filter-based modular multilevel converter, relating to the field of power electronics. According to the converter, an active power filter circuit is provided between upper and lower arms of each phase in a modular multilevel converter. The active power filter circuit includes two switch power devices, two submodules, a capacitor, and an inductor. The upper and lower bridge-arms are connected in series by means of two submodules, the two switch power devices are connected in series and then connected in parallel to ends of intermediate submodules which are connected in series, and the capacitor and the inductor are connected in series and then connected in parallel to two ends of the switch power device connected to the lower bridge-arm. The defect of large capacitance of submodule in conventional modular multilevel topology is overcome.

A power conditioner that includes a voltage detection unit that detects a voltage value of output voltage of the power conditioner, a current detection unit that detects a current value of output current of the power conditioner, and a constant power factor control unit that performs a constant power factor control process of a power factor of output power of the power conditioner to become a preset power factor command value. The constant power factor control unit performs the constant power factor control process using the detected voltage and current values at a second moment in past, the second moment in past being earlier than a first moment by a preset reference time period, and the first moment being a moment at which the voltage and current values are acquired for performing the constant power factor control process.

In a three-phase, four-wire electrical distribution system, a zig-zag transformer and at least one Cascade Multilevel Modular Inverter (CMMI) is coupled between the distribution system and the neutral. A controller modulates the states of the H-bridges in the CMMI to build an AC waveform. The voltage is chosen by the controller in order to control an equivalent impedance that draws an appropriate neutral current through the zig-zag transformer. This neutral current is generally chosen to cancel the neutral current sensed in the line. In other embodiments, the chosen neutral current may be based on a remotely sensed imbalance, rather than on a local value, determined by the power utility as a critical load point in the system. The desired injection current is then translated by the controller into a desired zero-sequence reactive impedance, based on measurement of the local terminal voltage, allowing the controller to regulate the current without generating or consuming real power. In some embodiments, the zig-zag transformer is omitted.

A power control system includes: a first AC/DC converter; a second AC/DC converter; a first switch connected between a first transmission line of a first power system having a first system frequency and the first AC/DC converter; a second switch connected between the first transmission line and the second AC/DC converter; a third switch connected between a second transmission line of a second power system having a second system frequency and the first AC/DC converter; a fourth switch connected between the second transmission line and the second AC/DC converter; a fifth switch connected between the first AC/DC converter and the second AC/DC converter; and a control device. When the first and second AC/DC converters are caused to operate as AC/DC converters in a BTB (Back to Back) method, the control device controls at least the fifth switch to be in a closed state.

In a power conversion device, an inverter (10) of each of three arms (A1 to A3) is controlled such that circulating current (Iz) of three arms (A1 to A3) follows a reference (Izrt) in a test period (times t3 to t4) in which a power system (1) is cut off from the three arms (A1 to A3), and whether the power conversion device is normal is determined based on circulating current (Iz) in the test period. Whether the power conversion device is normal therefore can be determined without affecting the power system (1).

A power conditioner that includes a voltage detection unit that detects a voltage value of output voltage of the power conditioner, a current detection unit that detects a current value of output current of the power conditioner, and a constant power factor control unit that performs a constant power factor control process of a power factor of output power of the power conditioner to become a preset power factor command value. The constant power factor control unit performs the constant power factor control process using the detected voltage and current values at a second moment in past, the second moment in past being earlier than a first moment by a preset reference time period, and the first moment being a moment at which the voltage and current values are acquired for performing the constant power factor control process.

A transmission device transmits energy between multiple electrical energy networks, each providing a multi-phase connection voltage at a network frequency for electrical energy supply. The transmission device contains multi-phase multi-stage rectifiers and a control device, which adjusts the transmission of energy into at least one energy-receiving energy network according to an input by controlling the multi-stage rectifiers. The multi-stage rectifiers are each connected to one of the energy networks and to one another via at least one multi-phase transformer. Electrical energy flows via the transformer at a predefined transmission frequency from at least one energy-emitting energy network into at least one energy-receiving energy network. The transmission frequency is, in particular, multiple times the network frequency.

A method of discharging a Modular Multilevel Converter (MMC) includes a plurality of phase legs connected in delta configuration. Each leg includes a plurality of series connected submodules, each submodule including an energy storage. The method includes disconnecting the MMC from an electrical grid, discharging the energy storages by means of a circulating current, and reconnecting the MMC to the electrical grid. The discharging includes, for each phase leg, setting a voltage reference, and sequentially selecting submodules in zero state by means of a sorting algorithm for switching each of the selected submodules to plus or minus state until the voltage deviation from the set voltage reference of the energy storage of each submodule in the phase leg is within a predefined range.

A modular multilevel converter (MMC) converter including an auxiliary sub-module provided between an upper converter arm and a lower converter arm of the MMC converter is provided. An MMC converter includes an auxiliary sub-module including: an energy storage unit storing a DC voltage in the sub-module; a first semiconductor switch connected to the energy storage unit; a second semiconductor switch connected to the first semiconductor switch; a third semiconductor switch connected to the second semiconductor switch; and a switching controller turning ON/OFF the first to third semiconductor switches, wherein a mid-point between the first and second semiconductor switches is connected to the sub-module of the upper converter arm, a mid-point between the second and third semiconductor switches is connected to a load connection terminal, and a mid-point between the third semiconductor switch and the energy storage unit is connected to the sub-module of the lower converter arm.