An arrangement for changing current return path in a bipole direct current power transmission system includes a converter station having an active and a passive converter connected in series between an active and a passive pole line via a first neutral bus, a metallic return transfer switch in an electrode line, a ground return transfer switch in a current redirecting path between the passive pole line and the neutral bus and a control unit which in case of change to the passive pole line for providing a return current path is configured to, upon control of power related to the active converter from steady-state to zero, close the ground return transfer switch and thereafter open the metallic return transfer switch, whereupon power related to the active converter may be controlled back to steady state.

Embodiments of the disclosure include an electrical assembly. The electrical assembly can include a converter including a DC side and an AC side, the DC side configured for connection to a DC network, the AC side configured for connection to an AC network, the converter including at least one switching element; a circuit interruption device operably connected to the AC side of the converter; a DC voltage modification device operably connected to the DC side of the converter, the DC voltage modification device including a DC chopper; and a controller configured to selectively control the or each switching element, the circuit interruption device and the DC voltage modification device, wherein the controller is configured to be responsive to a converter internal fault by carrying out a fault operating mode.

A power transmission network comprises, an AC electrical system AC transmission link from the AC electrical system to at least one AC-DC converter which includes: an AC connecting point connected to the AC transmission link and a DC connecting point for connection to a DC transmission link and a control system configured to operate each AC-DC converter in an AC voltage control mode as an AC slack bus to control a magnitude and/or a frequency of an AC voltage of the AC transmission link at a or a respective steady-state value and facilitating a power transfer between its connecting points to accommodate power generated or supplied to the AC electrical system. The control system is further configured to operate each first AC-DC converter in a protection mode to protect each AC-DC converter from an overvoltage and/or an overcurrent.

An objective of the invention to provide a hybrid converter station for HVDC system and the method operating the same. The hybrid rectifier station for high voltage direct current system includes: at least one AC bus; at least one line commutated converter configured to convert a portion of AC power supplied from the at least one AC bus to DC power transmitted on HVDC transmission line of the high voltage direct current system thereby generating reactive power demand; and at least one voltage source converter; wherein: the at least one line commutated converter and the at least one voltage source converter are coupled in parallel to the HVDC transmission line; and the at least one voltage source converter is configured to compensate the reactive power demand via the parallel coupling while converting another portion of the AC power supplied from the at least one AC bus to DC power transmitted on the HVDC transmission line. By reusing the VSC supplying both of the active power for power transmission and reactive power for LCC reactive power compensation, it is helpful for raising the total active AC power rating of the HVDC transmission system without incorporating extra power conversion device or changing the design of LCC. Besides, the nominal DC voltage of LCC and VSC is the same and the power flow shifting process is not needed.

A semiconductor switching string including a series-connected switching assemblies. Each assembly has a main switching element including first and second connection terminals which current flows between when the main switching element is on. The main element has an auxiliary element between the connection terminals. The string includes a local control unit connected with each auxiliary element which are programmed to switch an auxiliary element to create an alternative current path between the connection terminals that diverts current through to reduce the voltage across the corresponding main switching element. The local unit is programmed to control switching an auxiliary element to a fully-on mode in which the auxiliary element is at maximum rated base current, a pulsed mode which turns the auxiliary element on and off and/or an active mode operating the auxiliary element with a continuously variable base current. The string includes a higher level control unit programmed to implement the modes.

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.

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 power transmission network includes a plurality of converters. Each converter includes first and second electrical terminals, each first electrical terminal being connectable to an electrical network, each second electrical terminal being operatively connected to at least one other second electrical terminal. A first converter includes a first controller, the first controller configured to selectively operate the first converter to modulate a power flow at its first or second electrical terminal, and a second converter includes a second controller, the second controller being configured to selectively operate the second converter to modulate a power flow at its second electrical terminal to generate a power to compensate for an imbalance in power within the power transmission network caused by the modulation of power flow by the first converter, and thereby inhibit the modulation of power flow by the first converter from modifying a power flow in a third converter.

A wind power converter device is provided. The wind power converter device includes grid side converters, generator side converters and a DC bus module. Each of the grid side converters includes grid side outputs electrically coupled to a grid and a first and a second DC inputs. Each two of the neighboring grid side converters are connected in series at the second and the first DC inputs. Each of the generator side converters includes generator side inputs electrically coupled to a generator device and a first and a second DC outputs. Each two of the neighboring generator side converters are coupled in series at the second and the first DC outputs. The DC bus module is electrically coupled between the grid side converters and the generator side converters.

A bipole power transmission network includes a first upstream power converter which has a first DC terminal that is connected with a first transmission conduit which extends, in-use, to a first downstream power converter. The first upstream power converter has a second DC terminal that is connected with a return conduit which extends, in-use, to the first downstream power converter and a second downstream power converter. The first upstream power converter has at least one first AC terminal electrically connected with a first AC power source. The bipole power transmission network includes a second upstream power converter which has a third DC terminal that is connected with the return conduit, and a fourth DC terminal which is connected with a second transmission conduit that extends, in-use, to the second downstream power converter, and at least one second AC terminal electrically connected with a second AC power source.