WO2019025013A1

WO2019025013A1 - Converter

Info

Publication number: WO2019025013A1
Application number: PCT/EP2017/069857
Authority: WO
Inventors: Guillaume DE-PRÉVILLE; Stéphane BRÉHAUT; Wuxing LIANG
Original assignee: General Electric Technology Gmbh
Priority date: 2017-08-04
Filing date: 2017-08-04
Publication date: 2019-02-07
Also published as: EP3646456A1

Abstract

A converter (20) comprises: a DC side for connection to a DC network; an AC side for connection to an AC network; at least one module (22) connected between the AC and DC sides, the or each module (22) 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 in the or each module (22) arranged to be combinable to selectively provide a voltage source; and a controller (24) programmed to selectively provide a voltage reference signal (V_{be_TK}(t), V_{be_BK}(t)) to the or each module (22) so as to operate the or each module (22) to provide an output voltage so that a DC component of the current at the AC side is minimised or cancelled, the controller (30) including a plurality of sub-controllers (32, 34, 36), the plurality of sub-controllers (32, 34, 36) including: a first sub-controller (32) programmed to selectively generate a first voltage reference modification signal (40) to modify the voltage reference signal (V_{be_TK}(t), V_{be_BK}(t)) so that the or each module (22) is operated to provide the output voltage to control the current at the AC side so that a DC component of the current at the AC side is minimised or cancelled; a second sub-controller (34) programmed to selectively generate a second voltage reference modification signal (42) to modify the voltage reference signal (V_{be_TK}(t), V_{be_BK}(t)) so that the or each module (22) is operated to provide the output voltage to control the current at the DC side so that an AC component of the current at the DC side is minimised or cancelled; and a third sub-controller (36) programmed to selectively generate a third voltage reference modification signal (38) to modify the voltage reference signal (V_{be_TK}(t), V_{be_BK}(t)) so that the or each module (22) is operated to provide the output voltage to control the voltage at the DC side so that an AC component of the voltage at the DC side is minimised or cancelled.

Description

CONVERTER

This invention relates to a converter. In power transmission networks alternating current (AC) power is typically converted to direct current (DC) power for transmission via overhead lines and/or undersea cables. This conversion removes the need to compensate for the AC capacitive load effects imposed by the transmission line or cable, and thereby reduces the cost per kilometer of the lines and/or cables. Conversion from AC to DC thus becomes cost-effective when power needs to be transmitted over a long distance.

The conversion of AC to DC power is also utilized in power transmission networks where it is necessary to interconnect AC networks operating at different frequencies. Meanwhile conversion of DC to AC power is utilized in high voltage direct current (HVDC), static VAR compensator (SVC) and static synchronous compensator (STATCOM) applications.

Converters are required at each interface between AC and DC power to effect the required conversion.

According to a first aspect of the invention, there is provided a converter comprising: a DC side for connection to a DC network;

an AC side for connection to an AC network;

at least one module connected between the AC and DC sides, the or 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 in the or each module arranged to be combinable to selectively provide a voltage source; and

a controller programmed to selectively provide a voltage reference signal to the or each module so as to operate the or each module to provide an output voltage so that a DC component of the current at the AC side is minimised or cancelled, the controller including a plurality of sub-controllers, the plurality of sub-controllers including:

a first sub-controller programmed to selectively generate a first voltage reference modification signal to modify the voltage reference signal so that the or each module is operated to provide the output voltage to control the current at the

AC side so that a DC component of the current at the AC side is minimised or cancelled; a second sub-controller programmed to selectively generate a second voltage reference modification signal to modify the voltage reference signal so that the or each module is operated to provide the output voltage to control the current at the DC side so that an AC component of the current at the DC side is minimised or cancelled; and

a third sub-controller programmed to selectively generate a third voltage reference modification signal to modify the voltage reference signal so that the or each module is operated to provide the output voltage to control the voltage at the DC side so that an AC component of the voltage at the DC side is minimised or cancelled.

The presence of a DC component in the current at the AC side of the converter may have adverse consequences not only on the ability of the converter (e.g. a voltage source converter or a current source converter) to transfer high quality power between the AC and DC networks, but also on the performance and lifetime of any hardware associated with the converter, e.g. the presence of the DC component in the current at the AC side of the converter may result in saturation of any hardware or machine connected to the AC side of the converter. The provision of the aforementioned controller in the converter of the invention enables the operation of the or each module in the converter to protect the converter and any associated hardware/machine from the aforementioned adverse consequences. More particularly, the superposition of the operation of the three sub-controllers provides the converter with the capability to reliably cancel or compensate for a wide range of sources of disturbances which produce the DC component in the current at the AC side of the converter. This in turn facilitates a reduction or elimination of the adverse consequences caused by the presence of the DC component in the current at the AC side of the converter. The invention is applicable to steady-state operating conditions of the converter, and also to fault operating conditions of the converter (such as a strong grid disturbance in the form of an AC fault in the AC network or a DC fault in the DC network).

The DC component of the current at the AC side of the converter may be produced by various sources of disturbances, which may be internal or external to the converter. The DC component in the current at the AC side of the converter may be produced by one or more sources of disturbances internal to the converter, which may arise as a result of deficiencies in the converter control and/or hardware components of the converter. Examples of such internal sources of disturbances include, but are not limited to:

• imperfections in the converter control algorithm;

• imperfections in the converter control implementation, e.g. imperfections in sampling time and phase delay;

• inaccuracies or errors in current and voltage sensors associated with the converter control;

• deficiencies in passive components of the converter, such as an imbalance between reactors in different parts of the converter;

• converter component parameter inaccuracies;

• converter material tolerances;

• a limited number of modules in the converter which creates a natural DC bias voltage and distortion. The DC component in the current at the AC side of the converter may be directly produced by one or more sources of disturbances external to the converter, or may have arisen due to one or more sources of disturbances external to the converter which cause one or more AC components (e.g. a 50 Hz AC voltage or current component) to appear at the DC side of the converter. Examples of such external sources of disturbances include, but are not limited to:

• a fault in the AC network;

• another converter connected to the AC side of the converter, e.g. both converters are connected in parallel at their AC sides;

• a fault in the DC network;

· another converter connected to the same DC network, e.g. both converters are connected in parallel at their DC sides;

• the presence of a DC component in the current at the AC side of another AC-DC converter connected to the same DC network;

• electromagnetic induction and/or radiation caused by an electromagnetic field in the vicinity of the DC network, e.g. an electromagnetic field originating from AC hardware (such as an AC power transmission line/cable or a transformer) in the vicinity of the DC network.

In addition to minimising or cancelling a DC component of the current at the AC side of the converter, the or each module can be operated to facilitate transfer of power between the AC and DC networks, thus providing savings in terms of the cost, size and weight of the converter.

In embodiments of the invention, the converter may include a plurality of modules connected between the AC and DC sides, the plurality of modules arranged to form a chain-link converter.

The structure of the chain-link converter permits build-up of a combined voltage across the chain-link converter, which is higher than the voltage available from each of its individual modules, via the insertion of the energy storage devices of multiple modules, each providing its own voltage, into the chain-link converter. In this manner the chain-link converter is capable of providing a stepped variable voltage source, which permits the generation of a voltage waveform across the chain-link converter using a step-wise approximation. As such the chain-link converter is capable of providing a wide range of complex voltage waveforms, which is beneficial for minimising or cancelling a range of DC components of the current at the AC side of the converter.

The invention is applicable to any AC-DC or DC-AC converter which includes at least one module connected between the AC and DC sides, the or 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 in the or each module arranged to be combinable to selectively provide a voltage source. Examples of such converters include, but are not limited to, AC-DC voltage source converters such as the Modular Multilevel Converter (MMC), the Alternate Arm Converter (AAC) and the Series Bridge Converter (SBC), and DC-AC converters used in HVDC, SVC and STATCOM applications.

Preferably each sub-controller is implemented as a control loop.

The first sub-controller may be programmed to:

receive a measured or calculated DC component of the current at the AC side; and process the measured or calculated DC component to selectively generate the first voltage reference modification signal.

For example, the first sub-controller may be programmed to compare the measured or calculated DC component with a zero DC component reference signal in order to selectively generate the first voltage reference modification signal. Programming the first sub-controller in this manner provides a reliable means for minimising or cancelling a DC component of the current at the AC side of the converter.

The second sub-controller may be programmed to:

receive a measured or calculated AC component of the current at the DC side; and process the measured or calculated AC component to selectively generate the second voltage reference modification signal.

For example, the second sub-controller may be programmed to compare the measured or calculated AC component with a zero AC component reference signal in order to selectively generate the second voltage reference modification signal.

Programming the second sub-controller in this manner provides a reliable means for minimising or cancelling an AC component of the current at the DC side of the converter.

The third sub-controller may be programmed to:

receive a measured or calculated voltage of the module or modules; and process the measured or calculated voltage of the module or modules in order to selectively generate the third voltage reference modification signal.

For example, when the DC side of the converter includes first and second DC terminals and the AC side includes at least one AC terminal, the converter may include at least one converter limb extending between the DC terminals, the or each converter limb may include first and second limb portions separated by the or the respective AC terminal, the converter may include a plurality of modules connected between the AC and DC sides, and each limb portion may include one or more of the plurality of modules, wherein the third sub-controller may be programmed to:

receive a first measured or calculated voltage of the module or modules in the first limb portion;

receive a second measured or calculated voltage of the module or modules in the second limb portion; and

process the first and second measured or calculated voltages of the module or modules in the first and second limb portions to selectively generate the third voltage reference modification signal.

Programming the third sub-controller in this manner provides a reliable means for minimising or cancelling an AC component of the voltage at the DC side of the converter. The converter may form part of a converter assembly. The converter assembly may further include a transformer connected to the AC side of the converter. In relation to the transformer, the presence of the DC component of the current at the AC side of the invention has the adverse effects of

• saturating the transformer;

• causing the transformer to produce more noise (e.g. at least +10 dB);

• increasing the losses of the transformer (e.g. by 10%);

· requiring a larger transformer core to avoid saturation of the transformer, thus increasing the cost, size and weight of the transformer; and

• increasing the risk of saturation of any current protection device associated with the transformer, thus degrading the performance of the current protection device. The above adverse effects not only lower the performance and efficiency of the transformer, but also reduce the lifespan of the transformer and/or cause premature failure of the transformer.

In embodiments of the invention employing the use of the transformer, the controller may be programmed to selectively provide the voltage reference signal to the or each module so as to operate the or each module to provide the output voltage so that a DC component of the current at the transformer is minimised or cancelled. The ability of such a controller to minimise or cancel a DC component of the current at the transformer beneficially avoids the above adverse effects.

According to a second aspect of the invention, there is provided a method of controlling a converter, the converter comprising:

a DC side for connection to a DC network;

an AC side for connection to an AC network; and

at least one module connected between the AC and DC sides, the or 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 in the or each module arranged to be combinable to selectively provide a voltage source,

wherein the method comprises the steps of:

providing a voltage reference signal to the or each module so as to operate the or each module to provide an output voltage so that a DC component of the current at the AC side is minimised or cancelled; selectively generating a first voltage reference modification signal to modify the voltage reference signal so that the or each module is operated to provide the output voltage to control the current at the AC side so that a DC component of the current at the AC side is minimised or cancelled;

selectively generating a second voltage reference modification signal to modify the voltage reference signal so that the or each module is operated to provide the output voltage to control the current at the DC side so that an AC component of the current at the DC side is minimised or cancelled; and

selectively generating a third voltage reference modification signal to modify the voltage reference signal so that the or each module is operated to provide the output voltage to control the voltage at the DC side so that an AC component of the voltage at the DC side is minimised or cancelled.

The advantages of the converter of the first aspect of the invention and its embodiments apply mutatis mutandis to the method of the second aspect of the invention and its embodiments.

In the method of the invention, the converter may include a plurality of modules connected between the AC and DC sides, the plurality of modules arranged to form a chain-link converter.

The method of the invention may further include the steps of:

receiving a measured or calculated DC component of the current at the AC side; and

processing the measured or calculated DC component to selectively generate the first voltage reference modification signal.

The method of the invention may further include the step of comparing the measured or calculated DC component with a zero DC component reference signal in order to selectively generate the first voltage reference modification signal.

The method of the invention may further include the steps of:

receiving a measured or calculated AC component of the current at the DC side; and

processing the measured or calculated AC component to selectively generate the second voltage reference modification signal. The method of the invention may further include the step of comparing the measured or calculated AC component with a zero AC component reference signal in order to selectively generate the second voltage reference modification signal.

The method of the invention may further include the steps of:

receiving a measured or calculated voltage of the module or modules; and processing the measured or calculated voltage of the module or modules in order to selectively generate the third voltage reference modification signal.

When the DC side of the converter includes first and second DC terminals and the AC side includes at least one AC terminal, the converter may include at least one converter limb extending between the DC terminals, the or each converter limb may include first and second limb portions separated by the or the respective AC terminal, the converter may include a plurality of modules connected between the AC and DC sides, and each limb portion may include one or more of the plurality of modules. In such embodiments, the method of the invention may further include the steps of:

receiving a first measured or calculated voltage of the module or modules in the first limb portion;

receiving a second measured or calculated voltage of the module or modules in the second limb portion; and

processing the first and second measured or calculated voltages of the module or modules in the first and second limb portions to selectively generate the third voltage reference modification signal.

When each limb portion includes two or more of the plurality of modules, the method of the invention may include the steps of deriving the first measured or calculated voltage of the modules in the first limb portion from a sum of individual measured or calculated voltages of the modules in the first limb portion, and deriving the second measured or calculated voltage of the modules in the second limb portion from a sum of individual measured or calculated voltages of the modules in the second limb portion.

The method of the invention is applicable to a converter assembly comprising the converter and a transformer connected to the AC side of the converter. More particularly, when applied to the converter assembly, the method of the invention may include the step of providing the voltage reference signal to the or each module so as to operate the or each module to provide the output voltage so that a DC component of the current at the transformer is minimised or cancelled. It will be appreciated that the use of the terms "first" and "second", and the like, in this patent specification is merely intended to help distinguish between similar features (e.g. the first and second limb portions), and is not intended to indicate the relative importance of one feature over another feature, unless otherwise specified.

A preferred embodiment of the invention will now be described, by way of a non-limiting example, with reference to the accompanying drawings in which: Figure 1 shows schematically a converter according to an embodiment of the invention;

Figure 2 shows schematically the structure of a 2-quadrant unipolar module; Figure 3 shows schematically the structure of a 4-quadrant bipolar module;

Figure 4 shows a simplified equivalent circuit of a converter limb of the converter of Figure 1 ;

Figures 5 and 6 show schematically the layout of a controller of the converter of Figure 1 ; and

Figure 7 illustrates the measurement of voltages generated by valves of first and second limb portions in the converter limb of Figure 4.

A converter according to an embodiment of the invention is shown in Figure 1 , and is designated generally by the reference numeral 20. The converter 20 of Figure 1 is configured as a Modular Multilevel Converter (MMC). The converter 20 comprises a DC side and an AC side. The DC side includes first and second DC terminals, and the AC side includes a plurality of AC terminals. The converter 20 further includes a plurality of converter limbs.

Each converter limb extends between the first and second DC terminals. Each converter limb includes first and second limb portions separated by a respective one of the plurality of AC terminals. In each converter limb, the first limb portion extends between the first DC terminal and the AC terminal, and the second limb portion extends between the second DC terminal and the AC terminal. In use, the first and second DC terminals are respectively connected to positive and negative poles of a DC bus, the positive and negative terminals of the DC bus carrying voltages of VDc_BusT(t) and V_Dc_BusB(t) respectively, and the AC terminal of each converter limb is connected to a respective phase of a multi-phase AC network via a transformer (not shown). More specifically the AC terminal of each converter limb is connected to a respective transformer secondary winding (not shown), which in turn is mutually coupled with a respective transformer primary winding that is connected to the respective phase of the multi-phase AC network.

It will be understood that, where appropriate, the letters A, B and C are used to refer to the phases of the AC network, and are affixed to the reference numerals and reference terms throughout the description and drawings in order to differentiate between identical features of or associated with the respective phases A, B and C. It will also be understood that, where appropriate, the phrases Meas and Calculated are used to respectively indicate measured and calculated values of the associated feature.

It will be appreciated that the transformer may be, but is not limited to, a star-star transformer, a delta-star transformer, a delta-delta transformer, a zig zag transformer, a Scott transformer, a Le Blanc transformer or any combination based on a single phase transformer.

It is envisaged that, in other embodiments of the invention, the converter may have a single converter limb or a different plurality of converter limbs to match the number of phases of an AC network to which the converter is connected.

Each of the first and second limb portions includes a valve connected in series with a limb inductor. Each valve includes a plurality of series-connected modules 22. Each module 22 includes a pair of switching elements and an energy storage device in the form of a capacitor. In each module 22, the pair of switching elements are connected in parallel with the capacitor in a half-bridge arrangement to define a 2-quadrant unipolar module 22 that can provide zero or positive voltages and can conduct current in two directions, as shown in Figure 2.

Each switching element of each module 22 is constituted by a semiconductor device in the form of an Insulated Gate Bipolar Transistor (IGBT) which is connected in parallel with an anti-parallel diode. It is envisaged that, in other embodiments of the invention, each switching element of each module 22 may include a different switching device such as a gate turn-off thyristor, a field effect transistor such as JFET and MOSFET, an injection- enhanced gate transistor, an integrated gate commutated thyristor or any other self- commutated semiconductor device. It is envisaged that, in other embodiments of the invention, the capacitor may be replaced by another energy storage device that is capable of storing and releasing energy to selectively provide a voltage, e.g. a battery.

The capacitor of each module 22 is selectively bypassed or inserted into the corresponding valve by changing the state of the switching elements. This selectively directs current through the capacitor or causes current to bypass the capacitor, so that each module 22 provides a zero or positive voltage.

The capacitor of each module 22 is bypassed when the pair of switching elements in each module 22 is configured to form a short circuit in the module 22, whereby the short circuit bypasses the capacitor. This causes current in the valve to pass through the short circuit and bypass the capacitor, and so the module 22 provides a zero voltage, i.e. the module 22 is configured in a bypassed mode.

The capacitor of each module 22 is inserted into the valve when the pair of switching elements in each module 22 is configured to allow the current in the valve to flow into and out of the capacitor. The capacitor then charges or discharges its stored energy so as to provide a positive voltage, i.e. the module 22 is configured in a non-bypassed mode.

In this manner each module 22 is operable to selectively provide a voltage source.

It is possible to build up a combined voltage Vvai_ve_TK(t), Vvaive_BK(t) across each valve, which is higher than the voltage available from each of its individual modules 22, via the insertion of the capacitors of multiple modules 22, each providing its own voltage, into each valve.

It is envisaged that, in other embodiments of the invention, each module 22 may be a bidirectional voltage source that can provide negative, zero or positive voltages. Such a module 22 preferably can conduct current in two directions, i.e. each module 22 may be a 4-quadrant bipolar module 22. For example, each module 22 may include 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 22 that can provide negative, zero or positive voltages and can conduct current in two directions, as shown in Figure 3.

It is envisaged that, in still other embodiments of the invention, each valve may include a combination of 2-quadrant unipolar modules 22 and 4-quadrant bipolar modules 22. It will be appreciated that each module 22 may be replaced by a different type of module which includes at least one switching element and at least one energy storage device, in which the or each switching element and the or each energy storage device in the module are arranged to be combinable to selectively provide a voltage source.

During its operation the converter 20 may be exposed to one or more sources of disturbances that directly or indirectly produces a DC component in the current at the AC side of the converter 20. The or each source of disturbance may be internal or external to the converter 20. Examples of such sources of disturbances include, but are not limited to:

• an imbalance between the limb inductors of the first and second limb portions in each converter limb;

• imperfections in the converter control algorithm and implementation;

• a limited number of modules 22 in the converter 20 which creates a natural DC bias voltage and distortion;

• the presence of one or more AC voltage or current components on the DC bus.

Other possible sources of disturbances are described elsewhere in this patent specification.

As mentioned, the presence of the DC component in the current at the AC side of the converter 20 has adverse consequences on the performance and lifetime of not only the converter 20 but also any hardware/machine connected to the converter 20, such as the transformer. Accordingly it would be beneficial to minimise or cancel the DC component in the current at the AC side of the converter 20 in order to avoid the adverse consequences.

Cancellation or minimisation of the DC component in the current at the AC side of the converter 20 is achieved by configuring the converter 20 to further include a controller 24 programmed to operate each valve through control of the switching elements in each module 22.

The configuration of the controller 24 will be described with reference to Figure 4 which shows a representative converter limb 26 identical in structure and operation to each of the plurality of converter limbs, in which the letters A, B, C referring to the phases are replaced by the letter K referring to a generic phase. The following Table 1 sets out the nomenclature that is used throughout this patent specification.

Instantaneous measured phase current at AC side of converter 20

IPhaseKMeas(t) (the positive direction is from converter 20 to AC network, K = A, B,

C)

Measured voltage generated by valve of first limb portion (K = A, B,

Vvalve_TK eas(t)

C)

Measured voltage generated by valve of second limb portion (K = A,

Vvalve_BK eas(t)

B, C)

iTK eas(t) Measured current flowing in first limb portion (K = A, B, C) iBKMeas(t) Measured current flowing in second limb portion (K = A, B, C)

Valve voltage reference signal from controller 24 to valve of first limb portion (top, K = A, B, C)

Valve voltage reference signal from controller 24 to valve of second

limb portion (bottom, K = A, B, C)

Measured current on DC side of converter 20 (positive direction is from converter 20 to DC bus)

iDCMeas(t)

Measured current flowing between first DC terminal and DC bus iDCTMeas(t) (positive direction is from converter 20 to DC bus)

Measured current flowing between second DC terminal and DC bus iDCBMeas(t) (positive direction is from converter 20 to DC bus)

Measured voltage on DC side of converter 20 from positive pole of

VDC_BusTMeas(t)

DC bus to DC grounding

Measured voltage on DC side of converter 20 from negative pole of

VDC_BusBMeas(t)

DC bus to DC grounding

Measured total voltage on DC side of converter 20 from positive pole

VDC_BusMeas(t) to negative pole

^DC _BiisMeas{t) ⁼ ^DC _ BusTMecJj) + ^ DC _ BusBMeas^f) Summed voltage reference modification signal for modifying valve

VDistributedDC_TK(t)

voltage reference signal of valve of first limb portion

Summed voltage reference modification signal for modifying valve

VDistributedDC_BK(t)

voltage reference signal of valve of second limb portion

DC component reference signal for DC component in current flowing

IdcDemand(t)

in AC side of converter 20

AC component reference signal for AC component in current flowing lACDemand(t)

in DC bus

ephaseK_Ref(t) Voltage reference from converter-level control eValveK_Ref(t) Voltage reference from valve-level and module-level control

Lumb Value of limb reactor

Table 1: Nomenclature

The controller 24 is configured to generate a respective valve voltage reference signal Vbe_TK(t), Vb_e_BK(t) for operating each valve to generate a valve voltage Vvaivej-K(t), Vvai_ve_BK(t) thereacross. Each valve voltage reference signal Vbe jK, Vb_e_BK is constituted of a DC voltage component and an AC voltage component at an n multiple of the AC network frequency. The valve voltage reference signals Vb_e_TK(t), Vbe_BK(t) are generated by summing the DC bus voltage reference 28, the voltage reference eph_aseK_Ref(t) from the converter-level control, and the voltage reference evaiveK_Ref(t) from the valve-level and module-level control. Additionally the valve voltage reference signals Vbe_TK(t), Vbe_BK(t) can be modified by summed voltage reference modification signals VDistributedDC TK(t), VDi_StributedDc_Bt (t) which are generated by a control block 30, as shown in Figure 5.

The layout of the control block 30 is shown in more detail in Figure 6. The control block 30 includes a first sub-controller 32 implemented as a first internal control loop, a second sub-controller 34 implemented as a second internal control loop, and a third sub-controller 36 implemented as a third internal control loop, in which the three control loops are operated in parallel and fully decoupled.

Each of the first, second and third sub-controllers 32, 34, 36 are configured to respectively generate first, second and third voltage reference modification signals 40, 42, 38, which are then summed to generate the summed voltage reference modification signals

VDistributedDC JK(t), VDistributedDC_BK(t).

Referring to the first sub-controller 32, the instantaneous measured phase current iphaseK eas(t) at the AC side of the converter 20 can be directly measured using a current sensor. The instantaneous measured phase current iphaseKMeas(t) at the AC side of the converter 20 is then received by the first sub-controller 32. The presence of a DC component in the current at the AC side of the converter 20 can be quantified by extracting the DC component from the instantaneous measured phase current iphaseKMeas(t).

Alternatively the DC component in the current at the AC side of the converter 20 can be obtained by calculating the DC component from a calculated phase current iphaseKcaicuiated(t) derived from the measured currents i-rKMeas(t), iBKMeas(t) flowing in the first and second limb portions, as follows: iphaseKCalailatec t) ~ ^TKMeas^) ⁺ fCMeas{t)

The first sub-controller 32 compares the measured or calculated DC component with a zero DC component reference signal IdcDemand(t) in order to generate a first voltage reference modification signal 40. This enables the first voltage reference modification signal 40, when forming part of the summed voltage reference modification signals VDistributedDc_TK(t), VDistri utedDc_BK(t), to modify the valve voltage reference signal V_be_TK(t), V_be_BK(t) so that each valve is operated to provide an output voltage Vvai_ve_TK(t), Vvai_ve_BK(t) to control the current at the AC side so that the DC component of the current at the AC side is minimised or cancelled.

The first sub-controller 32 is advantageous for directly minimising or cancelling the DC component of the current at the AC side of the converter 20.

Referring to the second sub-controller 34, the presence of an AC component in the current on the DC side of the converter 20 can be quantified by extracting the AC component from the measured current iDCMeas(t) on the DC side of the converter 20, which can be obtained by using a current sensor to measure one of the currents iDCT eas(t), iDCBMeas(t) flowing between the DC terminals and the DC bus, as follows:

¹ DCMeas ( ) ⁼ ^DCTMeas ( ⁼ ^DCBMeas (0 Alternatively the AC component in the current at the DC side of the converter 20 can be obtained by calculating the AC component from a calculated current iDccaicuiated(t) at the DC side of the converter 20, which can be derived from both currents iDCTMeas(t), iDCB eas(t) in the DC bus that are measured using current sensors, as follows:

^■DCCalculated

The second sub-controller 34 compares the measured or calculated AC component with a zero AC component reference signal cDemandit) in order to generate a second voltage reference modification signal 42. This enables the second voltage reference modification signal 42, when forming part of the summed voltage reference modification signals V_Distrib_tedDC T (t), V_DistributedDc_BK(t), to modify the valve voltage reference signal \Λ_>β_τκ(ί), V_be_BK(t) so that each valve is operated to provide the output voltage V_Vaive_TK(t), V_Vaive_BK(t) to control the current at the DC side so that the AC component of the current at the DC side is minimised or cancelled, which in turn has the effect of minimising or cancelling the DC component at the AC side of the converter 20.

The second sub-controller 34 is advantageous for indirectly minimising or cancelling the DC component of the current at the AC side of the converter 20, when the DC component is indirectly produced by one or more sources of disturbances which produce an AC component in the current at the DC side of the converter 20.

Referring to the third sub-controller 36, the presence of an AC component in the voltage on the DC side of the converter 20 can be quantified by directly measuring, using voltage sensors, the voltages Vv_aive_TKMeas(t), Vvaive BKMeas(t) generated by the valves of the first and second limb portions, as illustrated in Figure 7.

Alternatively the presence of an AC component in the voltage on the DC side of the converter 20 can be calculated by calculating the voltages Vvaive_Ti (t), Vvai_ve_BK(t) generated by the valves of the first and second limb portions from the respective sums of individual measured voltages of the modules 22 in the first and second limb portions, as follows:

Vy Valve _ TKCalctilated

V Valve _ BKCalculated )^— Σ ^SM ' where VsM(t) is the individual measured voltage of each module 22, as shown in Figures 2 and 3, and n is the number of modules 22 that are operated to provide an output voltage.

The calculated valve voltages Vvai_ve_TKCaicuiated(t) and Vvaive_BKCaicuiated(t) are summed together. The AC voltage component from the summed calculated valve voltages is extracted with the help of a selected band filter on the n multiple of the AC network frequency.

This enables the third voltage reference modification signal 38, when forming part of the summed voltage reference modification signals V_DistrfbutedDc_TK(t), V_DistributedDc_Bi (t), to modify the valve voltage reference signal

so that each valve is operated to provide the output voltage Vvaivejn<(t), V_Vaive_BK(t) to control the current at the DC side so that the AC component of the current at the DC side is minimised or cancelled, which in turn has the effect of minimising or cancelling the DC component at the AC side of the converter 20.

The third sub-controller 36 is advantageous for indirectly minimising or cancelling the DC component of the current at the AC side of the converter 20, when the DC component is indirectly produced by one or more sources of disturbances which produce an AC component in the voltage at the DC side of the converter 20.

The superposition of the parallel operation of the three sub-controllers 32, 34, 36 therefore provides the converter 20 with the capability to reliably cancel or compensate for a wide range of sources of disturbances which produce the DC component in the current at the AC side of the converter 20. This in turn facilitates a reduction or elimination of the adverse consequences caused by the presence of the DC component in the current at the AC side of the converter 20.

The configuration of the control block 30 of the invention enables the DC component in each phase current to be minimised or cancelled independently of the DC components in the other phase currents. This in turn means that the control block 30 is compatible for use with a converter operating with a single phase or a different number of multiple phases. The control block 30 of the invention is applicable to both steady-state and fault operating conditions of the converter 20. The latter beneficially avoids saturation of the transformer, and any hardware/machine connected to the converter 20, during a fault in the AC network or the DC network.

Furthermore, in relation to the transformer and any hardware/machine connected to the converter 20, the configuration of the control block 30 to enable minimisation or cancellation of the DC component of the current at the AC side of the converter 20 has the beneficial effects of:

· reducing the risk of saturating the transformer/hardware/machine;

• reducing noise produced by the transformer/hardware/machine, thus reducing soundproofing requirements;

• reducing losses of the transformer/hardware/machine, thus reducing the size of the associated cooling system(s);

· reducing harmonic distortion generated by saturation of the transformer/hardware/machine, thus reducing pollution of the AC and DC networks;

• reducing the size of the transformer core required to avoid saturation of the transformer, thus reducing the cost, size and weight of the transformer;

· reducing the risk of saturation of any current protection device associated with the transformer, thus improving the performance of the current protection device;

• protecting the converter 20 from a 50 Hz voltage or current source on the DC side of the converter 20; and

• reducing the risk of propagation of the DC component in the current at the AC side of the converter 20, e.g. from the AC side of the converter 20 to the AC side of another converter 20 via the DC bus.

It will be appreciated that the embodiment of Figure 1 is applicable to other AC-DC voltage source converters with similar topologies, such as the Alternate Arm Converter (AAC). Additionally the control block 30 of the invention is applicable to other AC-DC or DC-AC converters with at least one module connected between the AC and DC sides, the or 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 in the or each module arranged to be combinable to selectively provide a voltage source. Examples of such converters include, but are not limited to, the Series Bridge Converter (SBC), and DC-AC converters used in HVDC, SVC and STATCOM applications.

Claims

1. A converter (20) comprising:

a DC side for connection to a DC network;

an AC side for connection to an AC network;

at least one module (22) connected between the AC and DC sides, the or each module (22) 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 in the or each module (22) arranged to be combinable to selectively provide a voltage source; and a controller (24) programmed to selectively provide a voltage reference signal

(VbejK(t), V_be_BK(t)) to the or each module (22) so as to operate the or each module (22) to provide an output voltage so that a DC component of the current at the AC side is minimised or cancelled, the controller (30) including a plurality of sub-controllers (32, 34, 36), the plurality of sub-controllers (32, 34, 36) including:

a first sub-controller (32) programmed to selectively generate a first voltage reference modification signal (40) to modify the voltage reference signal (V_be_TK(t), Vbe_BK(t)) so that the or each module (22) is operated to provide the output voltage to control the current at the AC side so that a DC component of the current at the AC side is minimised or cancelled;

a second sub-controller (34) programmed to selectively generate a second voltage reference modification signal (42) to modify the voltage reference signal (Vbe_TK(t), Vbe_BK(t)) so that the or each module (22) is operated to provide the output voltage to control the current at the DC side so that an AC component of the current at the DC side is minimised or cancelled; and

a third sub-controller (36) programmed to selectively generate a third voltage reference modification signal (38) to modify the voltage reference signal (Vb__TK(t), Vbe_BK(t)) so that the or each module (22) is operated to provide the output voltage to control the voltage at the DC side so that an AC component of the voltage at the DC side is minimised or cancelled.

2. A converter (20) according to Claim 1 including a plurality of modules (22) connected between the AC and DC sides, the plurality of modules (22) arranged to form a chain-link converter. 3. A converter (20) according to any one of the preceding claims wherein each sub- controller is implemented as a control loop.

4. A converter (20) according to any one of the preceding claims wherein the first sub- controller (32) is programmed to:

receive a measured or calculated DC component of the current at the AC side; and process the measured or calculated DC component to selectively generate the first voltage reference modification signal (40).

5. A converter (20) according to Claim 4 wherein the first sub-controller (32) is programmed to compare the measured or calculated DC component with a zero DC component reference signal (IdcDemand(t)) in order to selectively generate the first voltage reference modification signal (40).

6. A converter (20) according to any one of the preceding claims wherein the second sub-controller (34) is programmed to:

receive a measured or calculated AC component of the current at the DC side; and process the measured or calculated AC component to selectively generate the second voltage reference modification signal (42).

7. A converter (20) according to Claim 6 wherein the second sub-controller (34) is programmed to compare the measured or calculated AC component with a zero AC component reference signal (lACDemand(t)) in order to selectively generate the second voltage reference modification signal (42).

8. A converter (20) according to any one of the preceding claims wherein the third sub-controller (36) is programmed to:

receive a measured or calculated voltage (VS )) of the module (22) or modules

(22); and

process the measured or calculated voltage (VsM(t)) of the module (22) or modules (22) in order to selectively generate the third voltage reference modification signal (38). 9. A converter (20) according to Claim 8 wherein the DC side includes first and second DC terminals, the AC side includes at least one AC terminal, the converter (20) including at least one converter limb extending between the DC terminals, the or each converter limb including first and second limb portions separated by the or the respective AC terminal, the converter (20) including a plurality of modules (22) connected between the AC and DC sides, each limb portion including one or more of the plurality of modules (22), wherein the third sub-controller (36) is programmed to: receive a first measured or calculated voltage (V_SM(t)) of the module (22) or modules (22) in the first limb portion;

receive a second measured or calculated voltage (VsM(t)) of the module (22) or modules (22) in the second limb portion; and

process the first and second measured or calculated voltages (VsM(t)) of the module

(22) or modules (22) in the first and second limb portions to selectively generate the third voltage reference modification signal (38).

10. A converter (20) according to Claim 9 wherein each limb portion includes two or more of the plurality of modules (22), the first measured or calculated voltage of the modules (22) in the first limb portion is derived from a sum of individual measured or calculated voltages (VSMW) of the modules (22) in the first limb portion, and the second measured or calculated voltage of the modules (22) in the second limb portion is derived from a sum of individual measured or calculated voltages (VsM(t)) of the modules (22) in the second limb portion.

1 1. A converter assembly comprising a converter (20) according to any one of the preceding claims, the converter assembly further including a transformer connected to the AC side of the converter (20), wherein the controller (24) is programmed to selectively provide the voltage reference signal (V_be_TK(t), V_be_BK(t)) to the or each module (22) so as to operate the or each module (22) to provide the output voltage so that a DC component of the current at the transformer is minimised or cancelled.

12. A method of controlling a converter (20), the converter (20) comprising:

a DC side for connection to a DC network;

an AC side for connection to an AC network; and

at least one module (22) connected between the AC and DC sides, the or each module (22) 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 in the or each module (22) arranged to be combinable to selectively provide a voltage source, wherein the method comprises the steps of:

providing a voltage reference signal (\Λ>__τκ(ί), V_be_BK(t)) to the or each module (22) so as to operate the or each module (22) to provide an output voltage so that a DC component of the current at the AC side is minimised or cancelled;

selectively generating a first voltage reference modification signal (40) to modify the voltage reference signal (Vbe jK(t), V_be_BK(t)) so that the or each module (22) is operated to provide the output voltage to control the current at the AC side so that a DC component of the current at the AC side is minimised or cancelled;

selectively generating a second voltage reference modification signal (42) to modify the voltage reference signal (Vb_e_TK(t), Vbe_BK(t)) so that the or each module (22) is operated to provide the output voltage to control the current at the DC side so that an AC component of the current at the DC side is minimised or cancelled; and

selectively generating a third voltage reference modification signal (38) to modify the voltage reference signal (Vbe_TK(t), V_be_BK(t)) so that the or each module (22) is operated to provide the output voltage to control the voltage at the DC side so that an AC component of the voltage at the DC side is minimised or cancelled.