ELECTRICAL ASSEMBLY

Abstract

There is provided an electrical assembly comprising a converter (20) for connection to an electrical network (40), the converter (20) comprising at least one module (44) including at least one switching element (46) and at least one energy storage device (48), the or each switching element (46) and the or each energy storage device (48) in the or each module (44) arranged to be combinable to selectively provide a voltage source, the electrical assembly including a controller (54) configured to selectively control the switching of the or each switching element (46) in the or each module (44), wherein the electrical assembly includes a sensor (56a) configured for measuring a current of the electrical network (40), wherein the controller (54) and sensor (56a) are configured to operate in coordination to carry out a characterisation of an electrical parameter of the electrical network (40) so that, in use: the controller (54) selectively controls the switching of the or each switching element (46) in the or each module (44) to modify an electrical parameter of the converter (20) so as to modify the current of the electrical network (40); the sensor (56a) measures a resultant modified current of the electrical network (40); and the controller (54) processes the measured resultant modified current of the electrical network (40) so as to characterise the electrical parameter of the electrical network (40).

Claims

1-15. (canceled)

16. An electrical assembly comprising a converter for connection to an electrical network, the converter comprising at least one 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, the electrical assembly including a controller configured to selectively control the switching of the or each switching element in the or each module, wherein the electrical assembly includes a sensor configured for measuring a current of the electrical network, wherein the controller and sensor are configured to operate in coordination to carry out a characterisation of an electrical parameter of the electrical network so that, in use: the controller selectively controls the switching of the or each switching element in the or each module to modify an electrical parameter of the converter so as to modify the current of the electrical network; the sensor measures a resultant modified current of the electrical network; and the controller processes the measured resultant modified current of the electrical network so as to characterize the electrical parameter of the electrical network.

17. The electrical assembly of claim 16, wherein the controller is configured to process the characterized electrical parameter of the electrical network to control the switching of the or each switching element in the or each module so as to adapt control of the converter.

18. The electrical assembly of claim 16, wherein the controller is configured to process the characterized electrical parameter of the electrical network to control the switching of the or each switching element in the or each module so as to adapt control of the converter by carrying out at least one of: modifying an equivalent electrical impedance of the converter; modifying a control parameter of the converter; providing or increasing active damping of one or more oscillations in the electrical network; and providing active filtering.

19. The electrical assembly of claim 16, wherein the controller is configured to process the measured resultant modified current of the electrical network so as to characterize an electrical impedance of the electrical network and/or an operating voltage of the electrical network.

20. The electrical assembly of claim 16, wherein the controller is configured to selectively control the switching of the or each switching element in the or each module to provide a variable electrical impedance to modify an electrical impedance of the converter so as to modify the current of the electrical network.

21. The electrical assembly of claim 16, wherein the controller is configured to selectively control the switching of the or each switching element in the or each module to inject the perturbation voltage so as to modify the current of the electrical network.

22. The electrical assembly of claim 16, wherein the controller and sensor are configured to operate in coordination to carry out the characterization of the electrical parameter of the electrical network over a range of frequencies.

23. The electrical assembly of claim 16 including at least one first module and at least one second module, wherein the controller is configured to selectively control the switching of the or each switching element in the or each first module to carry out the characterization of the electrical parameter of the electrical network, and wherein the controller is configured to selectively control the switching of the or each switching element in the or each second module to carry out a normal operating mode of the converter.

24. The electrical assembly comprising a converter for connection to an electrical network, the converter comprising at least one first module and at least one second module, each of the first and second modules 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, the electrical assembly including a controller configured to selectively control the switching of the or each switching element in the or each module, wherein the controller is configured to selectively control the switching of the or each switching element in the or each second module to carry out a normal operating mode of the converter, wherein the controller is configured to, in response to receiving a characterised electrical parameter of the electrical network, process the characterized electrical parameter of the electrical network to control the switching of the or each switching element in the or each first module so as to adapt control of the converter.

25. The electrical assembly of claim 24, wherein the controller is configured to, in response to receiving a characterized electrical parameter of the electrical network, process the characterized electrical parameter of the electrical network to control the switching of the or each switching element in the or each first module so as to adapt control of the converter by carrying out at least one of: modifying an equivalent electrical impedance of the converter; modifying a control parameter of the converter; providing or increasing active damping of one or more oscillations in the electrical network; and providing active filtering.

26. The electrical assembly of claim 23, wherein the or each first module is configured to be bypassable during a normal operating mode of the converter.

27. The electrical assembly of claim 23, wherein the or each switching element of the or each first module has a higher switching frequency capability than the or each switching element of the or each second module.

28. A method of operating an electrical assembly to characterize an electrical parameter of an electrical network, the electrical assembly comprising a converter for connection to the electrical network, the converter comprising at least one 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: controlling the switching of the or each switching element in the or each module to modify an electrical parameter of the converter so as to modify the current of the electrical network; measuring a resultant modified current of the electrical network; and processing the measured resultant modified current of the electrical network so as to characterise the electrical parameter of the electrical network.

29. (canceled)

30. (canceled)

Description

[0096] 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:

[0097] FIG. 1 shows an electrical assembly according to an embodiment of the invention;

[0098] FIGS. 2a and 2b show exemplary half-bridge and full-bridge chain-link module configurations of a first module of the electrical assembly of FIG. 1;

[0099] FIGS. 3a and 3b show exemplary half-bridge and full-bridge chain-link module configurations of a second module of the electrical assembly of FIG. 1;

[0100] FIG. 4 shows an operation of a chain-link module to provide a variable electrical impedance;

[0101] FIG. 5 shows an equivalent circuit of a voltage source converter and an AC network when a chain-link module is operated to provide a variable electrical impedance;

[0102] FIG. 6 shows an operation of a chain-link module as a controlled perturbation voltage source;

[0103] FIG. 7 shows an equivalent circuit of a voltage source converter and an AC network when a chain-link module is operated as a controlled perturbation voltage source over a range of frequencies; and

[0104] FIG. 8 shows a flow chart of the characterisation process carried out by the electrical assembly of FIG. 1.

[0105] The figures are not necessarily to scale, and certain features and certain views of the figures may be shown exaggerated in scale or in schematic form in the interests of clarity and conciseness.

[0106] The following embodiment of the invention is used primarily in HVDC applications, but it will be appreciated that the following embodiment of the invention is applicable mutatis mutandis to other applications operating at different voltage levels.

[0107] An electrical assembly according to an embodiment of the invention is shown in FIG. 1 and is designated generally by the reference numeral 20.

[0108] The electrical assembly comprises a voltage source converter 20.

[0109] The voltage source converter 20 includes first and second DC terminals 24,26 and a plurality of converter limbs 28. Each converter limb 28 extends between the first and second DC terminals 24,26 and includes first and second limb portions 30,32 separated by a respective AC terminal 34. In each converter limb 28, the first limb portion 30 extends between the first DC terminal 24 and the AC terminal 34, while the second limb portion 32 extends between the second DC terminal 26 and the AC terminal 34.

[0110] In use, the first and second DC terminals 24,26 of the voltage source converter 20 are respectively connected to a DC network 36. In use, the AC terminal 34 of each converter limb 28 of the voltage source converter 20 is connected to a respective AC phase of a three-phase AC network 40 via a star-delta transformer arrangement 42. The three-phase AC network 40 is an AC power grid 40. It is envisaged that, in other embodiments of the invention, the transformer arrangement may be a different type of transformer arrangement, such as a star-star transformer arrangement.

[0111] Each limb portion 30,32 includes a switching valve, which includes a chain-link converter that is defined by a plurality of series-connected modules 44.

[0112] Each module 44 may vary in topology, examples of which are described as follows.

[0113] FIG. 2a shows schematically the structure of an exemplary first module 44 in the form of a half-bridge module 44a. The half-bridge module 44a includes a pair of switching elements 46 and a capacitor 48. Each switching element 46 of the half-bridge module 44a is in the form of a silicon carbide (SiC) based MOSFET which is connected in parallel with an anti-parallel diode. The pair of switching elements 46 are connected in parallel with the capacitor 48 in a half-bridge arrangement to define a 2-quadrant unipolar module 44a that can provide zero or positive voltage and can conduct current in both directions.

[0114] FIG. 2b shows schematically the structure of an exemplary first module 44 in the form of a full-bridge module 44b. The full-bridge module 44b includes two pairs of switching elements 46 and a capacitor 48. Each switching element 46 of the full-bridge module 44b is in the form of a SiC based MOSFET which is connected in parallel with an anti-parallel diode. The pairs of switching elements 46 are connected in parallel with the capacitor 48 in a full-bridge arrangement to define a 4-quadrant bipolar module 44b that can provide negative, zero or positive voltage and can conduct current in both directions.

[0115] FIG. 3a shows schematically the structure of an exemplary second module 44 in the form of a half-bridge module 44c. The half-bridge module 44c includes a pair of switching elements 46 and a capacitor 48. Each switching element 46 of the half-bridge module 44c is in the form of a silicon (Si) semiconductor based IGBT which is connected in parallel with an anti-parallel diode. The pair of switching elements 46 are connected in parallel with the capacitor 48 in a half-bridge arrangement to define a 2-quadrant unipolar module 44c that can provide zero or positive voltage and can conduct current in both directions.

[0116] FIG. 3b shows schematically the structure of an exemplary second module 44 in the form of a full-bridge module 44d. The full-bridge module 44d includes two pairs of switching elements 46 and a capacitor 48. Each switching element 46 of the full-bridge module 44d is in the form of a Si semiconductor based IGBT which is connected in parallel with an anti-parallel diode. The pairs of switching elements 46 are connected in parallel with the capacitor 48 in a full-bridge arrangement to define a 4-quadrant bipolar module 44d that can provide negative, zero or positive voltage and can conduct current in both directions.

[0117] In each switching valve, the modules 44 comprise a plurality of first modules 44 and a plurality of second modules 44. Each switching valve is configured so that, in each switching valve, the second modules 44 comprise a large majority, e.g. 95% to 99%, of the plurality of modules 44 while the first modules comprise a small minority, e.g. 1% to 4%, of the plurality of modules 44. The structure of each first module 44 is identical to the structure of each second module 44, except that the switching elements 46 of each first module 44 are SiC based switching elements and the switching elements 46 of each second module 44 are Si semiconductor based switching elements.

[0118] The structure of a given module 44 includes the arrangement and type of switching elements 46 and energy storage device 48 used in the given module 44. It will be appreciated that it is not essential for the structure of each first module 44 to be identical to the structure of each second module 44. For example, the plurality of modules 44 may comprise a combination of half-bridge first modules 44a and full-bridge second modules 44d or a combination of full-bridge first modules 44b and half-bridge second modules 44c. In addition, it is not essential for all of the first modules 44 to have the same module structure, and it is not essential for all of the second modules 44 to have the same module structure.

[0119] It will be understood that the materials of the switching elements 46 may vary. Preferably each switching element 46 of each first module 44 has a higher switching frequency capability than each switching element 46 of each second module 44. For example, each SiC based MOSFET may be replaced by another wide-bandgap material based switching element. Examples of other wide-bandgap materials include, but are not limited to, boron nitride, gallium nitride and aluminium nitride.

[0120] It is envisaged that, in other embodiments of the invention, each switching element 46 of each module 44 may be replaced by a gate turn-off thyristor (GTO), another field effect transistor (FET), an injection-enhanced gate transistor (IEGT), an integrated gate commutated thyristor (IGCT), bimode insulated gate transistor (BIGT) or any other self-commutated semiconductor device. It is also envisaged that, in other embodiments of the invention, each diode may be replaced by a plurality of series-connected diodes.

[0121] The capacitor 48 of each module 44 is selectively bypassed or inserted into the corresponding chain-link converter by changing the states of the switching elements 46. This selectively directs current through the capacitor 48 or causes current to bypass the capacitor 48, so that the module 44 provides a zero or positive voltage.

[0122] The capacitor 48 of the module 44 is bypassed when the switching elements 46 in the module 44 are configured to form a short circuit in the module 44, whereby the short circuit bypasses the capacitor 48. This causes current in the corresponding chain-link converter to pass through the short circuit and bypass the capacitor 48, and so the module 44 provides a zero voltage, i.e. the module 44 is configured in a bypassed mode.

[0123] The capacitor 48 of the module 44 is inserted into the corresponding chain-link converter when the switching elements 46 in the module 44 are configured to allow the current in the corresponding chain-link converter to flow into and out of the capacitor 48. The capacitor 48 then charges or discharges its stored energy so as to provide a positive voltage, i.e. the module 44 is configured in a non-bypassed mode.

[0124] In this manner the switching elements 46 in each module 44 are switchable to control flow of current through the corresponding capacitor 48.

[0125] It is possible to build up a combined voltage across each chain-link converter, which is higher than the voltage available from each of its individual modules 44, via the insertion of the capacitors of multiple modules 44, each providing its own voltage, into each chain-link converter. In this manner switching of the switching elements 46 in each module 44 causes each chain-link converter to provide a stepped variable voltage source, which permits the generation of a voltage waveform across each chain-link converter using a step-wise approximation. Hence, the switching elements 46 in each limb portion 30,32 are switchable to selectively permit and inhibit flow of current through the corresponding capacitor 48 in order to control a voltage across the corresponding limb portion 30,32.

[0126] It is envisaged that, in other embodiments of the invention, each module 44 may be replaced by another type of module which includes a plurality of switching elements and at least one energy storage device, the plurality of switching elements and the or each energy storage device in each such module arranged to be combinable to selectively provide a voltage source.

[0127] It is also envisaged that, in other embodiments of the invention, the capacitor 48 in each module 44 may be replaced by another type of energy storage device which is capable of storing and releasing energy to provide a voltage, e.g. a battery or a fuel cell.

[0128] Each switching valve includes a plurality of bypass devices 50. Each bypass device 50 is connected in parallel with a respective one of the modules 44. More specifically, each bypass device 50 includes a bypass switching element 52 connected across the terminals of the corresponding module 44. Each bypass switching element 52 may be in the form of a semiconductor switching element that comprises one or more semiconductor switching devices.

[0129] The electrical assembly further includes a controller 54 programmed to control the switching of the switching elements 46 and the bypass devices 50.

[0130] For the purposes of simplicity, the controller 54 is exemplarily described with reference to its implementation as a single control unit. In other embodiments, the controller 54 may be implemented as a plurality of control units. The configuration of the controller 54 may vary depending on specific requirements of the voltage source converter 20. For example, the controller 54 may include a plurality of control units, each of which is programmed to control the switching of the switching elements 46 of a respective one of the modules 44 and the corresponding bypass device 50. Each control unit may be configured to be internal to, or external of, the corresponding module 44. Alternatively, the controller may include a combination of one or more control units internal to the corresponding module 44 and one or more control units external of the corresponding module 44. Each control unit may be configured to communicate with at least one other control unit via telecommunications links.

[0131] The electrical assembly further comprises a current sensor 56a that is configured to measure an alternating current i.sub.ac of the AC network 40, and a voltage sensor 56b that is configured to measure a voltage v.sub.pcc at the point of common coupling (pcc). Each sensor 56a,56b may be configured to communicate with the controller 54 via telecommunications links.

[0132] The controller 54 and the sensors 56a,56b may be separate from or form part of the voltage source converter 20.

[0133] Operation of the electrical assembly is described as follows with reference to FIGS. 4 to 8.

[0134] In order to transfer power between the DC and AC networks 36,40, the controller 54 controls the switching of the switching elements 46 of the second modules 44 to switch the capacitors of the respective limb portions 30,32 into and out of circuit between the respective DC and AC terminals 24,26,34 to interconnect the DC and AC networks 36,40. The controller 54 switches the switching elements 46 of the second modules 44 of each limb portion 30,32 to provide a stepped variable voltage source between the respective DC and AC terminals 24,26,34 and thereby generate a voltage waveform so as to control the configuration of an AC voltage waveform at the corresponding AC terminal 34 to facilitate the transfer of power between the DC and AC networks 36,40.

[0135] Preferably the power transfer function is carried out using only the second modules 44 while the first modules 44 are bypassed by configuring their bypass devices 50 in order to improve the efficiency of the power transfer. Furthermore, the second modules 44 may be operated to carry out protection functions for the voltage source converter 20 and the DC and AC networks 36,40.

[0136] The first modules 44 provide the voltage source converter 20 with module redundancy in that the first modules 44 may be operated to carry out the power transfer function and protection functions in certain situations, such as one or more second modules 44 going offline and operating circumstances requiring a higher number of modules 44 than can be provided by the second modules 44.

[0137] The electrical impedance Z.sub.g of the AC network 40 may vary with time as a result of changes in its operating conditions that may arise as a result of a black start, varying power generation equipment due to selective connection and disconnection, varying loads, and energisation sequences for electrical network equipment or components such as transformers, cables and switchgear. Dynamics of the AC network 40 can be difficult to control in a reliable manner. This is in part because variation in the electrical impedance Z.sub.g of the AC network 40 can be volatile due to increasing penetration of renewable energy systems and can be unpredictable due to increasing numbers of distributed power sources and loads. Such variation in the electrical impedance Z.sub.g of the AC network 40 creates the risk of system instability that in turn could lead to undesirable power oscillations, equipment damage and/or the voltage source converter 20 and the AC network 40 tripping to an offline state.

[0138] To reduce the risk of system instability, the controller 54 controls the switching of the switching elements 46 of the first modules 44 to carry out a characterisation process in coordination with the sensors 56a,56b.

[0139] A first non-limiting example of the characterisation process is described as follows.

[0140] The controller 54 controls the switching of at least one selected first module 44 to vary its output voltage waveform so that the or each selected first module 44 in combination with an inductor 58 in the same limb portion 30,32 provides a variable electrical impedance Z.sub.L. The SiC based MOSFETs of the or each selected first module 44 are switched at higher frequencies than the Si semiconductor based IGBTs of the second modules 44. FIG. 4 shows the operation of a first module 44 in combination with an inductor 58 to provide the variable electrical impedance Z.sub.L. This in turn has the effect of modifying the equivalent electrical impedance Z.sub.C of the voltage source converter 20 to be variable. The equivalent electrical impedance Z.sub.C of the voltage source converter 20 is given by the following equation:

Z.sub.c=(½)Z.sub.L+jωL.sub.t

where ω is the angular frequency and L.sub.t is the transformer inductance.

[0141] As a result of modifying the equivalent electrical impedance Z.sub.C of the voltage source converter 20, the alternating current i.sub.ac of the AC network 40 is modified.

[0142] FIG. 5 shows an equivalent circuit of the voltage source converter 20 and the AC network 40 when one or more selected first modules 44 are operated as a variable electrical impedance so as to modify the equivalent electrical impedance Z.sub.C of the voltage source converter 20.

[0143] By varying the equivalent electrical impedance Z.sub.C of the voltage source converter 20, an estimation of the electrical impedance Z.sub.g and the equivalent operating voltage v.sub.g of the AC network 40 can be performed by using the Kirchhoff equations as follows:

v.sub.pcc(1)=v.sub.g+i.sub.ac(1)×Z.sub.g

v.sub.pcc(2)=v.sub.g+i.sub.ac(2)×Z.sub.g

v.sub.pcc(n)=v.sub.g+i.sub.ac(n)×Z.sub.g

where v.sub.pcc(k) is the voltage at the point of common coupling measured by the voltage sensor 56b at the instant of time k, and i.sub.ac(k) is the measured alternating current of the AC network 40 measured by the current sensor 56a at the instant of time k.

[0144] The controller processes the measured voltages v.sub.pcc(k) and the measured currents i.sub.ac(k) by applying a least squares algorithm, such as a recursive least squares algorithm, to the above equations in order to obtain estimated values for the electrical impedance Z.sub.g and the equivalent operating voltage v.sub.g of the AC network 40. It will be appreciated that there are other methods for solving the above equations in order to obtain estimated values for the electrical impedance Z.sub.g and the equivalent operating voltage v.sub.g of the AC network 40.

[0145] Variation of the equivalent electrical impedance Z.sub.C of the voltage source converter 20 therefore enables the estimation of the electrical impedance Z.sub.g and the equivalent operating voltage v.sub.g of the AC network 40, which can be done without demanding new active power (P) and reactive power (Q) operating points.

[0146] A second non-limiting example of the characterisation process is described as follows.

[0147] The controller 54 controls the switching of at least one selected first module 44 to vary its output voltage waveform so that the or each selected first module 44 acts as a variable voltage source to inject a perturbation harmonic voltage v.sub.p at the point of common coupling over a range of different frequencies that are multiples of an angular frequency ω.sub.0. The SiC based MOSFETs of the or each selected first module 44 are switched at higher frequencies than the Si semiconductor based IGBTs of the second modules 44. FIG. 6 shows the operation of a first module 44 as a controlled perturbation voltage source. The injection of the perturbation harmonic voltage v.sub.p in turn results in injection of a harmonic current component into the alternating current i.sub.ac of the AC network 40. Since the electrical impedance Z.sub.c(n) of the voltage source converter 20 at a given frequency ω(n) at the instant of time k is known, the alternating current i.sub.ac(n) of the AC network 40 at the instant of time k is given by:

i.sub.ac(n)=v.sub.p(n)/(Z.sub.g(n)+Z.sub.c(n))

[0148] The sensor 56 measures the alternating current i.sub.ac of the AC network 40 at a given frequency ω(n) and provides the measured alternating current i.sub.ac to the controller 54, which then estimates the electrical impedance Z.sub.g(n) of the AC network 40 at the given frequency ω(n) using the above equation.

[0149] If the voltage v.sub.g of the AC network 40 contains one or more harmonics, then the one or more harmonics can be measured from the alternating current i.sub.ac(n) when v.sub.p(n) is equal to zero using the superposition principle and subtracted from the above equation to estimate the electrical impedance Z.sub.g(n) of the AC network 40 at different frequencies.

[0150] An online frequency sweep can be performed over a range of frequencies that are multiples of the angular frequency wo in order to characterise the AC network electrical impedance Z.sub.g at each frequency. FIG. 7 shows an equivalent circuit of the voltage source converter 20 and the AC network 40 when one or more selected first modules 44 are operated as a controlled perturbation voltage source to inject the perturbation harmonic voltage over a range of frequencies so as to inject a harmonic current component into the alternating current i.sub.ac of the AC network 40.

[0151] The voltage source converter 20 therefore can be used as an online integrated impedance analyser to estimate the electrical impedance Z.sub.g of the AC network 40.

[0152] In the characterisation process, the injected perturbation voltage may be 1%, or about 1%, of the output voltage of the voltage source converter 20.

[0153] After the characterisation process is completed, the controller 54 uses the characterised electrical impedance Z.sub.g to estimate the stability of the system defined by the voltage source converter 20 and the AC network 40 as given by the ratio Z.sub.c/Z.sub.g. If the system is estimated to be at risk of being unstable i.e. the ratio Z.sub.c/Z.sub.g is approaching a value of −1, the controller 54 controls the switching of the switching elements of one or more of the first modules 44 to vary its output voltage waveform, or their output voltage waveforms, so as to modify an electrical impedance Z.sub.c of the voltage source converter 20 to control the ratio Z.sub.c/Z.sub.g and thereby increase a stability of the system. The Nyquist stability criterion, which explores the range of frequencies at which Z.sub.c/Z.sub.g=−1, can be used to help determine and configure the stability of the system.

[0154] As a result, the configuration of the electrical assembly to carry out the characterisation process enables the voltage source converter 20 to make real-time adjustments to its electrical impedance Z.sub.c in response to any change in electrical impedance Z.sub.g of the AC network 40. This not only enhances the control performance of the voltage source converter 20 under varying operating conditions of the AC network 40 but also can be used to provide an increased stability margin under weak AC power grids and time-varying parameters.

[0155] Furthermore, after the characterisation process is completed, the control of the voltage source converter 20 can be adapted in various ways to enhance its performance and/or enhance system stability.

[0156] The characterisation process and subsequent responsive adaptation of the control of the voltage source converter 20 are summarised as a series of steps in the flowchart of FIG. 8 with reference to the second example of the characterisation process. It will be appreciated that the flowchart of FIG. 8 applies mutatis mutandis to the first example of the characterisation process.

[0157] In a first step 100, one or more first modules 44 are operated to inject the perturbation voltage v.sub.p. This is followed by a second step 102 of measuring the modified alternating current i.sub.ac of the AC network 40, which is a consequence of the injection of the perturbation voltage v.sub.p.

[0158] In a third step 104, the electrical impedance Z.sub.g and the equivalent operating voltage v.sub.g of the AC network 40 are estimated, for example, using a recursive least-squares algorithm.

[0159] The fourth step 106 involves a decision process based on a cost function J. The cost function J is defined based on a weighting combination of several variables associated with the voltage source converter 20 such as the switching frequency of the SiC based MOSFETs of the first modules 44, the switching frequency of the Si semiconductor based IGBTs of the second modules 44, conduction losses, the number of modules that are switched into circuit in the voltage source converter 20, module voltage ripple, module energy levels and so on. In general, the cost function J will be non-linear. A decision threshold value J.sub.0 is defined.

[0160] As mentioned above, after the characterisation process is completed, the controller 54 uses the characterised electrical impedance Z.sub.g to estimate the stability of the system defined by the voltage source converter 20 and the AC network 40 as given by the ratio Z.sub.c/Z.sub.g. If it is necessary to then modify the electrical impedance Z.sub.c of the voltage source converter 20, then the electrical impedance Z.sub.c of the voltage source converter 20 can be varied to achieve one or more purposes such as system stability, meeting performance criteria, oscillation damping and so on.

[0161] Either or both of the first and second modules 44 may be controlled by the controller 54 to vary their output voltage waveforms in order to modify the electrical impedance Z.sub.c of the voltage source converter 20. If the value of the cost function J exceeds the decision threshold value J.sub.0, then the voltage source converter impedance Z.sub.c is modified in a fifth step 108 using the first modules 44 in order to benefit the voltage source converter's overall performance as defined by the cost function J. However, if the value of the cost function J is equal to or lower than the decision threshold value J.sub.0, then the voltage source converter impedance Z.sub.c is modified in a sixth step 110 using the second modules 44.

[0162] In addition, after the characterisation process is completed, the controller 54 may process the characterised electrical impedance Z.sub.g of the AC network 40 to control either or both of the first and second modules 44 so as to adapt control of the voltage source converter 20 by carrying out at least one of: [0163] modifying a control parameter of the voltage source converter 20; [0164] providing or increasing active damping of one or more oscillations in the AC network 40; and [0165] providing active filtering.

[0166] The voltage source converter 20 of FIG. 1 therefore combines the first modules 44 based on SiC based switching elements 46 and the second modules 44 based on Si semiconductor based switching elements 46 in a hybrid topology to provide the functionality of online integrated frequency characterisation of the AC network 40 performed by the first modules 44 with faster switching capabilities alongside the normal operating mode of the voltage source converter 20 performed by the second modules 44. The configuration of the electrical assembly of FIG. 1 therefore obviates the need to utilise a separate and dedicated impedance analyser that incurs additional installation and operating costs, adds to the overall size and weight of the electrical assembly, and is not as easily scalable for use with voltage source converters of different ratings and for different characterisation processes.

[0167] In addition, the use of the first modules 44 in the characterisation process provides the electrical assembly with the ability to carry out online frequency characterisation of the AC network 40 without the voltage source converter 20 and the AC network 40 having to go offline and without having to change the steady-state operating conditions of the voltage source converter 20 and the AC network 40.

[0168] Moreover, the use of the first modules 44 in the characterisation process advantageously permits scaling of the characterisation process to be compatible for use with converters of different ratings having different numbers of modules, and also permits variation in the number of modules used in the characterisation process to alter parameters of the characterisation process without having to incur significant cost in redesigning and building a new converter structure.

[0169] In addition to carrying out the characterisation process, the first modules 44 may be further configured to provide one or more additional functions such as active power damping and filtering with increased dynamic bandwidth, electrical fault management, module redundancy, and electrical impedance emulation in steady-state operating conditions, without interfering with the normal operating mode of the voltage source converter 20 carried out by the second modules 44. The higher switching frequency capability of the SiC based switching elements of the first modules 44 enables the first modules 44 to provide better active damping in comparison to the second modules 44 that has Si semiconductor based switching elements with lower switching frequency capability. Hence, the first modules 44 are capable of actively damping higher frequency oscillations in the AC network 40 that cannot be handled by the second modules 44.

[0170] Preferences and options for a given aspect, feature or parameter of the invention should, unless the context indicates otherwise, be regarded as having been disclosed in combination with any and all preferences and options for all other aspects, features and parameters of the invention.