A voltage source converter has director valve phase legs in parallel with waveshaper phase legs between two DC terminals. The director valve and waveshaper phase legs include upper and lower phase arms alternately operated to form waveshapes on AC terminals of the converter, thereby allowing a number of waveshaper phase arms to be available for use for other purposes. At least one of the available phase arms is controlled to contribute to other aspects of converter operation than waveshaping.

A first voltage control circuitry controls a first representative value, i.e., an average-value corresponding value of DC capacitor voltages of all converter cells to follow an overall voltage command value. A phase balance control circuitry controls second representative values, i.e., average-value corresponding values of DC capacitor voltages of the converter cells in leg circuits for respective phases to follow the first representative value. A positive-negative balance control circuitry controls deviations of third representative values, i.e., average-value corresponding values of the DC capacitor voltages of the converter cells in the positive and negative arms of the leg circuits for respective phases to become zero between the positive and negative arms. An individual balance control circuitry controls DC capacitor voltages of all the converter cells to follow the third representative values.

A first voltage control circuitry controls a first representative value, i.e., an average-value corresponding value of DC capacitor voltages of all converter cells to follow an overall voltage command value. A phase balance control circuitry controls second representative values, i.e., average-value corresponding values of DC capacitor voltages of the converter cells in leg circuits for respective phases to follow the first representative value. A positive-negative balance control circuitry controls deviations of third representative values, i.e., average-value corresponding values of the DC capacitor voltages of the converter cells in the positive and negative arms of the leg circuits for respective phases to become zero between the positive and negative arms. An individual balance control circuitry controls DC capacitor voltages of all the converter cells to follow the third representative values.

A high-voltage direct-current (HVDC) transmission system includes an alternating current (AC) electrical source and a power converter channel that includes an AC-DC converter electrically coupled to the electrical source and a DC-AC inverter electrically coupled to the AC-DC converter. The AC-DC converter and the DC-AC inverter each include a plurality of legs that includes at least one switching device. The power converter channel further includes a commutating circuit communicatively coupled to one or more switching devices. The commutating circuit is configured to switch on one of the switching devices during a first portion of a cycle of the H-bridge switching circuits and switch off the switching device during a second portion of the cycle of the first and second H-bridge switching circuits.

A high-voltage direct-current (HVDC) transmission system includes an alternating current (AC) electrical source and a power converter channel that includes an AC-DC converter electrically coupled to the electrical source and a DC-AC inverter electrically coupled to the AC-DC converter. The AC-DC converter and the DC-AC inverter each include a plurality of legs that includes at least one switching device. The power converter channel further includes a commutating circuit communicatively coupled to one or more switching devices. The commutating circuit is configured to switch on one of the switching devices during a first portion of a cycle of the H-bridge switching circuits and switch off the switching device during a second portion of the cycle of the first and second H-bridge switching circuits.

An improved scheme to filter harmonics and damp resonance for multipulse converter systems using a single characteristic passive filter branch without a plurality of non-characteristic harmonic filters is disclosed. The filter includes a first capacitor, a first inductor and a frequency-dependent resistor block connected in series. The frequency-dependent resistor block includes a second inductor connected in parallel with a circuit which is assembled by a second capacitor in series with a resistor.

A high-voltage direct-current (HVDC) transmission system includes an alternating current (AC) electrical source and a power converter channel that includes an AC-DC converter electrically coupled to the electrical source and a DC-AC inverter electrically coupled to the AC-DC converter. The AC-DC converter and the DC-AC inverter each include a plurality of legs that includes at least one switching device. The power converter channel further includes a commutating circuit communicatively coupled to one or more switching devices. The commutating circuit is configured to switch on one of the switching devices during a first portion of a cycle of the H-bridge switching circuits and switch off the switching device during a second portion of the cycle of the first and second H-bridge switching circuits.