VOLTAGE SOURCE CONVERTER

1 . A voltage source converter comprising:

a converter limb extending between first and second DC terminals and having first and second limb portions separated by an AC terminal, the first and second DC terminals being connectable to a DC electrical network and the AC terminal being connectable to an AC electrical network, each limb portion including at least one switching element;

a chain-link converter including a plurality of series-connected modules, each module including at least one switching element and at least one energy storage device, the or each switching element and the or each energy storage device of each module combining to selectively provide a voltage source, the chain-link converter being connected to the AC terminal, the or each switching element of each limb portion being switchable to switch the chain-link converter into and out of circuit with that limb portion and thereby switch the chain-link converter into and out of circuit with the corresponding DC terminal; and

a control unit which coordinates the switching of the switching elements of the limb portions and the or each switching element in each module of the chain-link converter to transfer power between the AC and DC electrical networks,

wherein the control unit controls the switching of the or each switching element in each module of the chain-link converter to generate an AC voltage waveform at the AC terminal, the AC voltage waveform including an AC voltage waveform portion between positive and negative peak values of the AC voltage waveform, the AC voltage waveform portion including at least two different voltage profiles to filter one or more harmonic components from the AC voltage waveform, at least one of the different voltage profiles being defined by a non-zero voltage slope.

2 . A voltage source converter according to claim 1 wherein at least two of the different voltage profiles are defined by different voltage slopes.

3 . A voltage source converter according to claim 1 wherein at least one of the different voltage profiles is defined by an instantaneous change in voltage.

4 . A voltage source converter according to claim 1 wherein the control unit is configured to control the switching of the or each switching element in each module of the chain-link converter to control the configuration of the AC voltage waveform portion at the AC terminal when the chain-link converter is switched out of circuit with both limb portions.

5 . A voltage source converter according to claim 1 wherein the control unit controls the switching of the or each switching element in each module of the chain-link converter to modify the value of each intercept angle (α 1 ,−α 1 ,α 2 ,−α 2 ) of the AC voltage waveform and thereby filter out one or more harmonic components from the AC voltage waveform, each intercept angle (α 1 ,−α 1 ,α 2 ,−α 2 ) defining a phase angle corresponding to a common point of intersection between two different voltage profiles of the AC voltage waveform.

6 . A voltage source converter according to claim 1 wherein the control unit controls the switching of the or each switching element in each module of the chain-link converter to modify the magnitude (k,−k) of the AC voltage waveform corresponding to each intercept angle (α 1 ,−α 1 ,α 2 ,−α 2 ) of the AC voltage waveform and thereby filter out one or more harmonic components from the AC voltage waveform, each intercept angle (α 1 ,−α 1 ,α 2 ,−α 2 ) defining a phase angle corresponding to a common point of intersection between two different voltage profiles of the AC voltage waveform.

7 . A voltage source converter according to claim 1 wherein the or each switching element and the or each energy storage device of each module combine to selectively provide a bidirectional voltage source.

8 . A voltage source converter according to claim 7 wherein each module includes two pairs of switching elements connected in parallel with an energy storage device in a full-bridge arrangement to define a 4-quadrant bipolar module that can provide negative, zero or positive voltage and can conduct current in two directions.

9 . A voltage source converter according to claim 1 wherein each limb portion includes a single switching element or a plurality of switching elements connected in series between the AC terminal and a respective one of the DC terminals.

10 . A voltage source converter according to claim 1 wherein at least one switching element includes a naturally-commutated or self-commutated switching device.

11 . A voltage source converter according to claim 1 wherein each limb portion includes at least one pair of switching elements connected in anti-parallel so that each limb portion can conduct current in two directions.

12 . A voltage source converter according to claim 1 wherein a first end of the chain-link converter is connected to the AC terminal and a second end of the chain-link converter is connectable to ground.

13 . A voltage source converter according to claim 1 including an inductor ( 48 ) that is connectable at its first end to the AC electrical network and is connected at its second end to the AC terminal, wherein the chain-link converter is connected to the AC terminal via the inductor.

14 . A voltage source converter according to claim 1 including a plurality of converter limbs and a plurality of chain-link converters, the AC terminal of each converter limb being connectable to a respective phase of a multi-phase AC network, each chain-link converter being connected to a respective one of the AC terminals.