A high voltage/ultrahigh voltage direct current transmission inverter side frequency control implementing method includes: transmitting a deviation between the inverter side power grid frequency and rated frequency to the inverter side frequency controller, wherein the frequency controller regulates and outputs a modulation quantity by adopting self-adaptive parameters according to different operation conditions; when the interstation communication is normal, the modulation quantity output of the inverter side frequency controller causes the rectifier side and the inverter side to form a new power/current order through the interstation communication; when the interstation communication is abnormal, converting the inverter side to current control from voltage control and converting the rectifier side to voltage control from current control; superposing the modulation quantity output of the inverter side frequency controller to the power/current order of the inverter side, changing the size of the transmission power to realizing the inverter side frequency control.

The present invention discloses a stereoscopic DC-DC converter for power transfer between two DC grids, the converter comprises a first converter, a second converter and a third converter, a positive terminal of the first converter is connected to a positive terminal of a second DC grid, a negative terminal of the first converter is connected to a positive terminal of the second converter, a negative terminal of the second converter is connected to a positive terminal of the third converter, a negative terminal of the third converter is connected to a negative terminal of the second DC grid, in the meantime, a positive terminal of the second converter is also connected to a positive terminal of a first DC grid, and the negative terminal of the second converter is also connected to a negative terminal of the first DC grid. Compared with the conventional DC-DC converter employing the DC-AC-DC converting technology, the DC-DC converter of the invention makes full use of existing DC voltage of the first DC grid, which significantly reduces overall power of converters that are used, and thus cost and power loss caused thereby.

A method for power conversion includes coupling a first string to a second string via a first connecting node and a second connecting node to form at least one leg of a power converter. The first string is operatively coupled across a first bus and a second bus and comprises a first branch and a second branch coupled via a third connecting node. The first branch and the second branch include a plurality of controllable semiconductor switches. Furthermore, the second string comprises a first chain link and a second chain link coupled via an alternating current phase bus and includes a plurality of switching units. The first chain link and/or the second chain link are controlled to generate a negative voltage across at least one of the plurality of controllable semiconductor switches during a switch turn off process.

A high voltage direct current soft-start circuit is provided in which a first end of a first switch is connected to a negative electrode of a high voltage direct current, a first end of a second switch is connected to the negative electrode of the high voltage direct current, and a drive control unit is connected separately to the first switch, the second switch, and a load, where a first part of a connector is connected to the drive control unit, and upon power-on, the first part of the connector communicates with a second part of the connector, to trigger the drive control unit to drive the first switch to turn on. The drive control unit delays a preset time after driving the first switch to turn on, drives the second switch to turn on, and drives the load to start after the second switch is turned on.

An electric power transmission system and method that provides a power electronic interface system for connecting a plurality of alternating current transmission lines by one or more direct current transmission lines. The system includes dc-link converters, frequency transformers connected to the converters, and multilevel converters are connected to a direct current transmission line.

A high voltage/ultrahigh voltage direct current transmission inverter side frequency control implementing method includes: transmitting a deviation between the inverter side power grid frequency and rated frequency to the inverter side frequency controller, wherein the frequency controller regulates and outputs a modulation quantity by adopting self-adaptive parameters according to different operation conditions; when the interstation communication is normal, the modulation quantity output of the inverter side frequency controller causes the rectifier side and the inverter side to form a new power/current order through the interstation communication; when the interstation communication is abnormal, converting the inverter side to current control from voltage control and converting the rectifier side to voltage control from current control; superposing the modulation quantity output of the inverter side frequency controller to the power/current order of the inverter side, changing the size of the transmission power to realizing the inverter side frequency control.

An HVDC power increase controller includes a command output unit for outputting a current command value according to a disturbance signal to a main controller; a voltage drop determiner receiving an alternating current (AC) voltage and comparing a level of the AC voltage to a lowest level of a voltage causing a rectification failure; and a power tracking determiner receiving a direct current (DC) power and comparing a level of the DC power to an estimated power level corresponding to the current command value. The command output unit adjusts the current command value according to a comparison result of the voltage drop determiner and the power tracking determiner.

A method for measuring electric power loss of a high voltage direct current (HVDC) system comprises the steps of measuring the amount of transmission electric power and the amount of receiving electric power; calculating a first electric power loss amount based on a difference value between the measured amount of the transmission electric power and the measured amount of the receiving electric power; calculating the amount of loss generated in each of positions in the HVDC system based on an impedance value of each of the positions; calculating a second electric power loss amount based on a sum value of the calculated loss amounts; and determining a correcting value for correcting the amount of electric power loss generated in the HVDC system based on a difference value between the first electric power loss amount and the second electric power loss amount.

A semiconductor switching circuit, for use in a HVDC power converter, includes a main current branch including a main semiconductor switching element through which current flows when the main semiconductor switching element is switched on; and an auxiliary current branch connected in parallel or inverse-parallel with the main current branch. The auxiliary current branch includes an auxiliary circuit having a plurality of active auxiliary semiconductor switching elements connected to form an active switching bridge, and the auxiliary current branch further includes an energy storage device and/or an impedance device. The active switching bridge has a control unit operatively connected therewith, and the control unit is configured to switch the active switching bridge to connect the auxiliary circuit into and out of circuit with the main current branch and thereby selectively create an alternative current path whereby current flowing through the main current branch is diverted to flow through.

The method of converting high voltage AC lines into bipolar high voltage DC systems makes use of the three transmission lines (referred to as the positive pole, the negative pole, and the modulating pole) in an existing high voltage AC system as transmission lines in a bipolar high voltage DC system. When current from the power source is up to the thermal current limit of the transmission lines, the transmission lines operate in two-wire mode, where current is delivered in the positive pole and returned in the negative pole, the modulating pole being open. When power source current exceeds the thermal current limit, operation is in three-wire mode, alternating for predetermined periods between parallel configuration of the positive pole and the modulating pole to divide current for delivery to the load, and parallel configuration of the negative pole and the modulating pole, dividing the return current.