Abstract: A first converter station is part of a high voltage direct current transmission system that includes a DC transmission link connected to the first converter station a second converter station. A DC current and a DC voltage of the DC transmission link are sensed by the first converter station. It is determined that the sensed DC current is equal to or larger than a threshold current value, that the sensed DC current is equal to or larger than the threshold current value, and that at least a partial recovery of the sensed DC voltage has occurred. On the basis that it is determined that the at least a partial recovery of the sensed DC voltage has occurred, it is determined that a phase-to-ground fault has occurred. In response to determining that a phase-to-ground fault has occurred, a power delivered by the first converter station can be reduced.

Abstract: Methods of energizing converters, and converter stations thereof, based on a mix of half-bridge (HB) converter cells and full-bridge (FB) converter cells are provided. According to one embodiment, a method for energizing a converter may include establishing a direct electrical connection between DC terminals of a converter, electrically connecting the converter to an AC power source via AC connectors and obtaining, from the AC power source and for a certain period of time, an AC voltage between two electrical phase lines for charging capacitors of the FB cells connected to the two electrical phase lines. Accordingly, the FB cells will be charged to a higher value and they may be controlled for charging capacitors of the HB submodules to at least a minimum voltage required to start switching devices of the HB submodules by drawing power from the AC power source.

Abstract: A first voltage source converter and converter station including such a first voltage source converter, as well as a method and computer program product for controlling the first voltage source converter are disclosed. The first voltage source converter has a DC side for connection to a DC system, has an AC side for connection to an AC system and is interconnected with an AC side of a second voltage source converter, which has a DC side connected to the DC system. The first voltage source converter includes a number of converter valve pairs, each being connected to a corresponding AC phase of the AC system and a control unit controlling the converter valves to generate at least one AC waveform and to reduce oscillations between the converters.

Abstract: A first converter station is part of a high voltage direct current transmission system that includes a DC transmission link connected to the first converter station a second converter station. A DC current and a DC voltage of the DC transmission link are sensed by the first converter station. It is determined that the sensed DC current is equal to or larger than a threshold current value, that the sensed DC current is equal to or larger than the threshold current value, and that at least a partial recovery of the sensed DC voltage has occurred. On the basis that it is determined that the at least a partial recovery of the sensed DC voltage has occurred, it is determined that a phase-to-ground fault has occurred. In response to determining that a phase-to-ground fault has occurred, a power delivered by the first converter station can be reduced.

Abstract: Methods of energizing converters, and converter stations thereof, based on a mix of half-bridge (HB) converter cells and full-bridge (FB) converter cells are provided. According to one embodiment, a method for energizing a converter may include establishing a direct electrical connection between DC terminals of a converter, electrically connecting the converter to an AC power source via AC connectors and obtaining, from the AC power source and for a certain period of time, an AC voltage between two electrical phase lines for charging capacitors of the FB cells connected to the two electrical phase lines. Accordingly, the FB cells will be charged to a higher value and they may be controlled for charging capacitors of the HB submodules to at least a minimum voltage required to start switching devices of the HB submodules by drawing power from the AC power source.

Abstract: A first voltage source converter and converter station including such a first voltage source converter, as well as a method and computer program product for controlling the first voltage source converter are disclosed. The first voltage source converter has a DC side for connection to a DC system, has an AC side for connection to an AC system and is interconnected with an AC side of a second voltage source converter, which has a DC side connected to the DC system. The first voltage source converter includes a number of converter valve pairs, each being connected to a corresponding AC phase of the AC system and a control unit controlling the converter valves to generate at least one AC waveform and to reduce oscillations between the converters.

Abstract: An arrangement, method and computer program product are provided for limiting circulating currents in a converter converting between AC and DC. The converter has a number of AC and DC terminals and includes a number of converter arms, where a first and a second converter arm are connected in parallel between a first DC terminal and a first AC terminal. Each converter arm includes a string of series-connected converter cells. The arrangement includes a control unit that obtains a current of the first converter arm and a current of the second converter arm, forms an average of the two converter arm currents, forms a first and a second voltage control signal based on the average and uses the first and second voltage control signal in the control of the voltage provided by the first and second converter arm.

Abstract: An arrangement, method and computer program product are provided for limiting circulating currents in a converter converting between AC and DC. The converter has a number of AC and DC terminals and includes a number of converter arms, where a first and a second converter arm are connected in parallel between a first DC terminal and a first AC terminal. Each converter arm includes a string of series-connected converter cells. The arrangement includes a control unit that obtains a current of the first converter arm and a current of the second converter arm, forms an average of the two converter arm currents, forms a first and a second voltage control signal based on the average and uses the first and second voltage control signal in the control of the voltage provided by the first and second converter arm.

Abstract: A method for controlling a first and a second reverse-conducting insulated gate bipolar transistor (RC-IGBT), electrically connected in series, is disclosed. A collector of the first RC-IGBT is electrically connected to a positive pole of a direct current voltage source, and an emitter of the second RC-IGBT is electrically connected to a negative pole of the DC voltage source. Further, an emitter of the first RC-IGBT is electrically connected to a collector of the second RC-IGBT to form an alternating current terminal. A gate voltage is applied to respective gates of the first and second RC-IGBTs, wherein the gate voltage is controlled based on a magnitude and a direction of an output current on the AC terminal and on a command signal alternating between a first and a second value.

Abstract: A method and apparatuses for handling delayed zero crossing in fault current through a circuit breaker are disclosed. An interface arrangement is configured to couple an alternating current, AC, power system with a direct current, DC, power system, or vice versa. The interface arrangement includes at least one converter for conversion of AC power to DC power, or vice versa, which includes a DC side for coupling of the converter to the DC power system and an AC side for coupling of the converter to the AC power system. A circuit breaker is arranged in a current path between the AC side of the at least one converter and the AC power system. There may be a risk of delayed zero crossing in fault current occurring in case a fault occurs in a predefined portion of the interface arrangement.

Abstract: A method and apparatuses for handling delayed zero crossing in fault current through a circuit breaker are disclosed. An interface arrangement is configured to couple an alternating current, AC, power system with a direct current, DC, power system, or vice versa. The interface arrangement includes at least one converter for conversion of AC power to DC power, or vice versa, which includes a DC side for coupling of the converter to the DC power system and an AC side for coupling of the converter to the AC power system. A circuit breaker is arranged in a current path between the AC side of the at least one converter and the AC power system. There may be a risk of delayed zero crossing in fault current occurring in case a fault occurs in a predefined portion of the interface arrangement.

Abstract: A method to control a voltage source converter (CON1; CON2) in a HVDC system comprises the step of controlling a frequency and a voltage amplitude of an AC voltage (UV1; UV2) generated by the voltage source converter (CON1; CON2) independently of the conditions in an AC network (N1; N2) connected to the voltage source converter (CON1; CON2). This method is performed by a control unit of a HVDC system. In a special embodiment, the method forms the basis of a method to black start an AC network, where the AC network comprises transmission lines and is connected to at least two AC power stations, where one of the at least two AC power stations is connected via a HVDC system to the AC network.

Abstract: Cells in a phase leg of a multilevel converter are controlled through regulating, using tap-changer control, the modulation index of the converter to above 0.80, providing a group of carrier waves for the phase leg, where these carrier waves are identical but displaced in time from each other with a time delay, providing a voltage reference for the phase leg being separate from voltage references provided for other phase legs, stop switching of the cells when waveform values of a power transmission converter waveform are detected to be in a selected interval around a peak value of the power transmission converter waveform and control switching of the cells based on comparisons of the respective carrier waves with the voltage reference when the waveform values of the power transmission converter waveform are detected to be outside the selected interval.

Abstract: A method for controlling a first and a second reverse-conducting insulated gate bipolar transistor (RC-IGBT), electrically connected in series, is disclosed. A collector of the first RC-IGBT is electrically connected to a positive pole of a direct current voltage source, and an emitter of the second RC-IGBT is electrically connected to a negative pole of the DC voltage source. Further, an emitter of the first RC-IGBT is electrically connected to a collector of the second RC-IGBT to form an alternating current terminal. A gate voltage is applied to respective gates of the first and second RC-IGBTs, wherein the gate voltage is controlled based on a magnitude and a direction of an output current on the AC terminal and on a command signal alternating between a first and a second value.

Abstract: Cells in a phase leg of a multilevel converter are controlled through regulating, using tap-changer control, the modulation index of the converter to above 0.80, providing a group of carrier waves for the phase leg, where these carrier waves are identical but displaced in time from each other with a time delay, providing a voltage reference for the phase leg being separate from voltage references provided for other phase legs, stop switching of the cells when waveform values of a power transmission converter waveform are detected to be in a selected interval around a peak value of the power transmission converter waveform and control switching of the cells based on comparisons of the respective carrier waves with the voltage reference when the waveform values of the power transmission converter waveform are detected to be outside the selected interval.

Abstract: An interface arrangement for coupling between an AC system and a DC system. The arrangement includes a converter for conversion between AC and DC and having a DC side with a first and a second terminal for connection to the DC system and an AC side with a group of terminals for being coupled to the AC system, a transformer having a primary side with a first set of primary windings for being coupled to the AC system and a secondary side with a second set of secondary windings coupled to the converter, where the transformer is equipped with a third set of auxiliary windings and a short-circuiting device is connected between the third set of auxiliary windings and ground.

Abstract: A method is provided for controlling power flow within a wind park system for power transmission to a main power grid, the wind park system including two or more parallel-connected island wind park grids. The method includes the steps of: controlling a first voltage source converter as a swing bus in frequency control, the frequency control including controlling frequency to a steady state reference operating point for operating points within a power dead band and by frequency droop control when exceeding an end point of the power dead band, and controlling at least a second voltage source converter in power control, the power control including controlling power flow to a steady state reference operating point for operating points within a frequency dead band and by power droop control when exceeding an end point of the frequency dead band. A controller and computer program product are also provided.

Abstract: An interface arrangement for coupling between an AC system and a DC system. The arrangement includes a converter for conversion between AC and DC and having a DC side with a first and a second terminal for connection to the DC system and an AC side with a group of terminals for being coupled to the AC system, a transformer having a primary side with a first set of primary windings for being coupled to the AC system and a secondary side with a second set of secondary windings coupled to the converter, where the transformer is equipped with a third set of auxiliary windings and a short-circuiting device is connected between the third set of auxiliary windings and ground.

Abstract: A method and a device to discharge cell capacitors in a cell based voltage source converter. Each cell has switching elements in half bridge or full bridge configuration and a capacitor in parallel to the half or full bridge. Each cell has two terminals, whereof at least one is between two switching elements. The converter has AC and DC terminals, with the possibility to connect each of the terminals to ground, via a further switching element. A resistor is implemented into at least one of the ground connections. To discharge to capacitors, the switching elements in the respective cells are configured such, that the capacitor is in parallel connection to the terminals. The capacitors are thus discharged via the resistor to ground.

Abstract: The invention relates to a method for controlling power flow within a wind park system (100) for power transmission to a main power grid (90), the wind park system (100) comprising two or more parallel-connected island wind park grids. The method comprises the steps of: controlling a first voltage source converter (10) as a swing bus in frequency control, the frequency control comprising controlling frequency to a steady state reference operating point for operating points within a power dead band and by means of frequency droop control when exceeding an end point of the power dead band, and controlling at least a second voltage source converter (20), (30), (40) in power control, the power control comprising controlling power flow to a steady state reference operating point for operating points within a frequency dead band and by means of power droop control when exceeding an end point of the frequency dead band. The invention also relates to controllers, computer programs and computer program products.