Improvements in or relating to chain-link converters

Abstract

In the field of chain-link converters operable to provide a step-variable voltage source, there is a need for an improved chain-link converter. A chain-link converter (10) includes a plurality of series-connected chain-link modules (12). Each chain-link module (12) has a module controller (14) programmed to control operation of the corresponding chain-link module (12) to selectively provide a voltage source whereby the chain-link converter (10) is capable of providing a step-variable voltage source. The chain-link converter (10) also includes a chain-link converter controller (26) arranged in communication with each module controller (14) and programmed to, in use, communicate a measured converter current (IDC) flowing through the chain-link converter (10) to the plurality of module controllers (14). The module controllers (14) receiving the measured converter current (IDC) are each also programmed to, in use, combine the measured converter current (IDC) with a measured rate of change of current (IAC) flowing through the corresponding chain-link module (14) to establish an instantaneous module current (L) flowing through the corresponding chain-link module (12).

Description

Technical Field

[0001] This invention relates to a chain link converter comprising a plurality of chain link modules connected in series, each chain link module having a module controller programmed to control the operation of the corresponding chain link module to selectively provide a voltage source, thereby enabling the chain link converter to provide a stepped variable voltage source. Additionally, this invention relates to a method of operating such a chain link converter. Background Technology

[0002] In HVDC power transmission networks, AC power is typically converted to DC power for transmission via overhead lines, submarine cables, and / or underground cables. This conversion eliminates the need to compensate for the AC capacitive load effect imposed by the power transmission medium (i.e., transmission lines or cables) and reduces the cost per kilometer of line and / or cable, thus becoming cost-effective when power needs to be transmitted over long distances. DC power can also be transmitted directly from offshore wind farms to onshore AC power transmission networks.

[0003] The conversion between DC and AC power is used where DC and AC networks must be interconnected. In any such power transmission network, a converter (i.e., a power converter) is required at each interface between AC and DC power to achieve the required conversion from AC to DC or from DC to AC.

[0004] One type of power converter is a voltage source converter, although other types of power converters are also possible. Summary of the Invention

[0005] Such a voltage source converter includes a first DC terminal and a second DC terminal, with at least one converter branch extending between the first DC terminal and the second DC terminal, and typically three converter branches, each of the three converter branches corresponding to a given phase of a three-phase power system.

[0006] Each or every converter branch includes a first branch portion and a second branch portion separated by AC terminals.

[0007] In use, the first DC terminal and the second DC terminal are connected to the DC network, and each AC terminal is connected to the corresponding phase of the AC network.

[0008] Each branch section includes a chain link converter extending between an associated AC terminal and a corresponding DC terminal in either the first or second DC terminal. Each chain link converter includes multiple chain link modules connected in series, and each chain link module includes multiple switching elements connected in parallel with an energy storage device typically in the form of a capacitor. However, other types of energy storage devices may also be used, i.e., any device capable of storing and releasing energy to selectively provide voltage, such as a fuel cell or battery.

[0009] Supplying multiple chain link modules means that it is possible to build up a combined voltage across each chain link converter via an energy storage device (i.e., a capacitor) that inserts multiple chain link modules (each of which provides its own voltage), the combined voltage being higher than the voltage that can be obtained from each individual chain link module.

[0010] Therefore, each of the chain link modules works together to allow the chain link converter to provide a step-variable voltage source. This allows for the generation of voltage waveforms using stepwise approximations across each chain link converter. Consequently, each chain link converter is capable of providing a wide range of complex waveforms.

[0011] For example, each chain link converter can be operated in the manner described above to generate an AC voltage waveform at the AC terminal or each AC terminal, thereby enabling the voltage source converter to provide the aforementioned power transfer functionality between the AC and DC networks.

[0012] According to a first aspect of the present invention, a chain link converter is provided, comprising:

[0013] Multiple series-connected chain link modules, each chain link module having a module controller programmed to control the operation of the corresponding chain link module to selectively provide a voltage source, thereby enabling the chain link converter to provide a step-variable voltage source; and

[0014] A chain-link converter controller is arranged to communicate with each module controller and is programmed to transmit measured converter current flowing through the chain-link converter to multiple module controllers during use.

[0015] Each of the module controllers receiving the measured converter current is further programmed to combine, in use, the measured converter current with the measured rate of change of the current flowing through the corresponding chain link module in order to establish the instantaneous module current flowing through the corresponding chain link module.

[0016] The supply of such a chain link converter controller and such a module controller allows the chain link converter of the present invention to utilize a single converter current measuring device and another measuring device, which is expensive and can only operate at relatively low frequencies but is still highly accurate, and the other measuring device can operate at higher measurement frequencies and is much cheaper, and therefore can be deployed on multiple chain link modules, but is only sensitive to rapid changes in current flow and therefore cannot provide an indication of the progress of current flow during steady-state conditions; and still combines the outputs of the two measuring devices to establish the instantaneous module current flowing through a given chain link module, which is in turn extremely useful for assisting in the control and protection of a given chain link module.

[0017] In particular, this arrangement allows each link module in a given chain to operate closer to its rated current operating characteristics because any increase in current flow through such a link module due to a fault occasion can be detected quickly, and the actual instantaneous module current flowing through the link module can be easily established so that protective measures can be deployed similarly quickly if necessary. Because such link modules can operate more safely and closer to their ratings, any voltage source converter (incorporating the link modules) needs to include fewer such link modules to provide a given power delivery capability, which in turn allows for more cost-effective provision of such voltage source converters.

[0018] Preferably, each module controller in the module controller that receives the measured converter current is programmed to establish the instantaneous module current by using the measured converter current as a baseline current measurement and adding an integral current measurement derived from the rate of change of the current flowing through the corresponding chain link module to the baseline current measurement.

[0019] In this way, the chain link converter can utilize precise current measurements at a recently known time (i.e., the measured converter current) and high-bandwidth current measurements established quickly after that recently known reference point without delay (because it is performed locally at the chain link module (i.e., an integral current measurement derived from the rate of change of the current flowing through the chain link module)) to establish (e.g., calculate) extremely accurate instantaneous module current.

[0020] In a preferred embodiment of the invention, each module controller in the module controller that receives the measured converter current is further programmed to compare the latest received measured converter current with the established instantaneous module current, and if there is a difference between the latest received measured converter current and the established instantaneous module current, to adjust the measurement of the rate of change of the current flowing through the corresponding chain link module.

[0021] Programming the module controller in the manner described above can, for example, improve the accuracy of the instantaneous module current over time by using an iterative process.

[0022] If the measured converter current exceeds a predetermined safe converter threshold, the chain link converter controller can be additionally programmed to take protective action.

[0023] Preferably, each module controller is further programmed to take protective action if its established instantaneous module current deviates from one or more predetermined parameters.

[0024] Deviation from the predetermined parameters may include at least one of the following:

[0025] The instantaneous module current exceeds a predetermined safety module threshold, and the predetermined safety module threshold is greater than the predetermined safety converter threshold; and

[0026] The instantaneous module current increases at a rate greater than the predetermined safe rate.

[0027] Such features help ensure that the chain link converter of the present invention is protected in the event of a fault current higher than normal, for example, by being blocked and disconnected from any energy source to which it is connected during use.

[0028] Furthermore, programming each module controller in the manner described above and carefully selecting the associated predetermined parameters means that the protection provided by the chain-link converter controller takes precedence over the protection provided by each module controller. This is desirable because, for example, in the event of a moderate overcurrent fault, the chain-link converter controller can thus continue to provide coordinated protection for the converter, while the module controllers can react to more extreme and faster-occurring fault events.

[0029] In another preferred embodiment of the invention, each module controller is also programmed to report the need for protective action to it to a chain link converter controller, which is further programmed to monitor the number of module controllers providing such reports.

[0030] In yet another preferred embodiment of the invention, the chain link converter controller (26) is further programmed to be at least one of the following:

[0031] If fewer than a predetermined number of module controllers provide such reports, then the aforementioned module controller is instructed to provide reports indicating the need for protective action, or each module controller is instructed to avoid taking protective action; and

[0032] If the predetermined number or more module controllers (14) provide such a report, then the blocking of the chain link converter and / or the blocking of the voltage source converter, which is located within the voltage source converter during use, is initiated.

[0033] Features such as these enable the chain link converter controller to determine whether one or more module controllers are spuriously operating, for example, when only one or a few module controllers report the need for protective action, and thereby avoid unnecessary and inconvenient shutdown of the chain link converter or voltage source converter, which is located within the voltage source converter during use, in the event of such failure by one or a few module controllers.

[0034] Optionally, the chain link converter further includes at least one current converter for providing a measured converter current flowing through the chain link converter.

[0035] Enabling the current converter to provide the measured converter current helps ensure that the measured current is accurate and precise enough for the associated operation of the module controller receiving this measurement.

[0036] Each chain link module with a module controller that receives measured converter current may also include a current measuring transducer for providing a measured rate of change of the current flowing through the corresponding chain link module.

[0037] Current measuring transducers have high bandwidth, meaning they can operate at high frequencies, and are therefore advantageously able to measure very rapid changes in current flow, such as those that may occur in certain types of faults.

[0038] Furthermore, supplying such a converter also means that fault currents flowing in paths that do not include the aforementioned current converters, and therefore would not typically be picked up by such current converters, can still be detected by one or more such converters, thereby allowing appropriate protective actions to be taken.

[0039] According to a second aspect of the invention, a method for operating a chain link converter is provided, the chain link converter comprising a plurality of chain link modules connected in series, each chain link module having a module controller programmed to control the operation of the corresponding chain link module to selectively provide a voltage source, thereby enabling the chain link converter to provide a step variable voltage source, and the chain link converter controller being arranged to communicate with each module controller, the method comprising the following steps:

[0040] (a) To cause the chain link converter controller to transmit the measured converter current flowing through the chain link converter to multiple module controllers; and

[0041] (b) The module controller receiving the measured converter current combines the measured converter current with the measured rate of change of the current flowing through the corresponding chain link module to establish the instantaneous module current flowing through the corresponding chain link module.

[0042] The method of the present invention shares the benefits of the corresponding features of the chain link converter of the present invention.

[0043] Within the scope of this application, it is expressly intended that the various aspects, embodiments, examples, and alternatives set forth in the foregoing paragraphs and claims and / or the following description and drawings, and in particular their individual features, may be employed independently or in any combination. That is, all embodiments and all features of any embodiment may be combined in any manner and / or combination unless such features are incompatible. The applicant reserves the right to accordingly modify any originally filed claim or to file any new claim, including the right to modify any originally filed claim to depend on any feature of any other claim and / or incorporate any feature of any other claim, although not originally claimed in that manner. Attached Figure Description

[0044] Now, the next step is to refer to... Figure 1 A brief description of preferred embodiments of the invention by way of non-limiting examples. Figure 1 A schematic diagram showing a portion of a chain link converter according to a first embodiment of the present invention is shown. Detailed Implementation

[0045] like Figure 1 As shown, a portion of the chain link converter according to the first embodiment of the present invention is generally designated by reference numeral 10.

[0046] In particular, although the chain link converter 10 of the present invention includes 64 chain link modules 12 connected in series, Figure 1Only four are shown in the diagram. However, in other embodiments of the invention, the chain link converter may include fewer or more than 64 chain link modules.

[0047] In any case, each chain link module 12 has a module controller 14 programmed to control the operation of the corresponding chain link module 12 to selectively provide a voltage source, thereby enabling the chain link converter 10 to provide a step variable voltage source.

[0048] More specifically, each chain link module 12 includes a plurality of switching elements 16 connected in parallel with an energy storage device 18 in the form of a capacitor 20. However, other types of energy storage devices may also be used, i.e., any device capable of storing and releasing energy to selectively provide voltage, such as a fuel cell or a battery.

[0049] Each switching element 16 includes a semiconductor device in the form of an insulated-gate bipolar transistor (IGBT) 22, although other types of self-commutating semiconductor devices may be used instead, such as a gate turn-off thyristor (GTO), a field-effect transistor (FET), a metal-oxide-semiconductor field-effect transistor (MOSFET), an injection-enhanced gate transistor (IEGT), an integrated gate-commutated thyristor (IGCT), a dual-mode insulated-gate transistor (BIGT), or any other self-commutating switching device. Additionally, one or more of the semiconductor devices may be replaced with a wide-bandgap material, such as, but not limited to, silicon carbide, boron nitride, gallium nitride, and aluminum nitride.

[0050] The number of semiconductor devices in each switching element 16 can vary depending on the required voltage and current ratings of that switching element 16.

[0051] Each of the switching elements 16 further includes a passive current sensing element, which in the illustrated embodiment takes the form of a diode 24 connected in antiparallel to the corresponding semiconductor device. In other embodiments of the invention, the passive current sensing element may include another type of passive current sensing device, i.e., any device capable of limiting current flow in only one direction. Furthermore, the number of passive current sensing devices in each passive current sensing element may vary depending on the required voltage and current ratings of that passive current sensing element.

[0052] The illustrated chain link module 12 is a first exemplary chain link module, which includes a first pair of switching elements 16 connected in parallel with a capacitor 20 in a known half-bridge arrangement to define a two-quadrant unipolar module. The switching of the switching elements 16 selectively directs current through the capacitor 20 or bypasses the current through the capacitor 20, such that the first exemplary chain link module 12 can provide zero voltage or a positive voltage and can conduct current in both directions.

[0053] The second exemplary chain link module (not shown) includes first and second pairs of switching elements and capacitors, which are connected in a known full-bridge arrangement to define a four-quadrant bipolar module. In a manner similar to the first exemplary chain link module, the switching of the switching elements again selectively directs current through the capacitors or bypasses the current through the capacitors, such that the second exemplary chain link module can provide zero voltage, positive voltage, or negative voltage and can conduct current in both directions.

[0054] The chain link converter 10 shown includes only the first exemplary chain link module 12, but in other embodiments of the invention, the chain link converter may include only the second exemplary link module or a combination of the first and second exemplary chain link modules.

[0055] In any case, supplying multiple chain link modules 12 means that it is possible to accumulate a combined voltage across the chain link converter 10 via an energy storage device (i.e., capacitor 20) into which multiple chain link modules 12 (each of which provides its own voltage) are inserted, the combined voltage being higher than the voltage that can be obtained from each individual chain link module 12.

[0056] Therefore, each of the chain link modules 12 works together to allow the chain link converter 10 to provide a step-variable voltage source. This allows for the generation of voltage waveforms using step-by-step approximations across the chain link converter 10. Thus, the chain link converter 10 is capable of providing a wide range of complex waveforms, which, when combined with other chain link converters to define a voltage source converter, allows for the conversion between AC and DC power.

[0057] Returning to the illustrated embodiment, the chain link converter 10 also includes a chain link converter controller 26 arranged to communicate with each module controller 14, and more specifically, the chain link converter controller 26 is arranged to communicate with each module controller 14 via a passive optical network 28, although other communication channels may be used.

[0058] The chain link converter 10 further includes a current converter 30, and more particularly a DC current converter, which, in the illustrated embodiment, is mounted on a bus 32, i.e., on a main conductive pipe that interconnects the chain link converter 10 to an external DC or AC network during use. In any case, the current converter 30 measures the converter current I flowing into (or out of) (i.e., through) the chain link converter 10. DC The measurement is then transmitted to the chain link converter controller 26. This can be via the aforementioned passive optical network 28, or via some other device such as a protection system or other control system.

[0059] Additionally, each chain link module 12 includes a current measuring transducer, which in the illustrated embodiment takes the form of a Rogowski coil 34, and measures the current I flowing through the associated chain link module 12. AC The rate of change. However, other types of current measuring transducers can be used, and not all chain link modules 12 need to include such transducers.

[0060] In use, the chain-link converter controller 26 is programmed to receive the measured converter current I from the current converter 30. DC This communication is transmitted to each module controller 14 within module controller 14. Such communication occurs across the passive optical network 28 and represents only a small increase in overhead in the typical individual control commands sent by the converter controller 26 to each module controller 14 via this medium. This is because the same measurement converter current I... DC Broadcast to each module controller 14. The passive optical network 28 typically allows communication at a frequency of approximately 10 kHz, and this, together with the bandwidth of the current converter 30, means that, in practical terms, the converter controller 26 can provide each module controller 14 with an updated measurement of the converter current I approximately every 100 μs. DC .

[0061] In other embodiments of the invention, the converter controller may not need to pass the measured converter current to each module controller. For example, if the corresponding chain link module does not additionally include a current-measuring transducer, such as a Rogowski coil, the converter controller may not pass the measured converter current to those module controllers, one or more of which are located within the corresponding chain link module.

[0062] Meanwhile, returning to the illustrated embodiment, the measured converter current I is received. DC Each module controller 14 in the module controller 14 (i.e., all module controllers 14 in the illustrated embodiment) is also programmed to measure the converter current I during use. DC (It receives the measured converter current I from the converter controller 26) DC The current I flowing through the chain link module 12 (which is located within the chain link module 12) and the current I AC The measured rate of change (each module controller 14 receives the current I from the corresponding Rogowski coil 34) AC The components are combined to establish the instantaneous module current I flowing through the chain link module 12. i .

[0063] More specifically, each module controller 14 is programmed to measure the converter current I... DC Used as a baseline current measurement and from the current I flowing through the corresponding chain link module 12 AC The integral current measurement derived from the rate of change of the measured current is added to the baseline current measurement to establish such an instantaneous module current I. i In other embodiments of the invention, one or more module controllers in the module controllers may be programmed to add the rate of change of a direct measurement of the current flowing through the corresponding chain link module to the baseline current measurement to establish the instantaneous module current.

[0064] In addition to the above, each module controller 14 in the module controller 14 is also programmed to receive the latest measured converter current I. DC With the established instantaneous module current I i Compare, and if the converter current I is measured in the latest received measurement DC With the established instantaneous module current I i If there are differences, adjust the current I flowing through the corresponding chain link module. AC The rate of change is measured. One way each module controller 14 can adjust such a measurement is by changing the calibration of the Rogowski coil 34, for example, by using proportional-integral control.

[0065] Therefore, the chain link converter 10 of the present invention can utilize accurate current measurements at a known time in the most recent past (i.e., the measured converter current I). DC And it is established quickly after the most recently passed reference point and will not suffer any delay (because it is done locally at the chain link module 12 (i.e., from the current I flowing through the chain link module 12)). AC The high-bandwidth current measurement (derived from the rate of change of the measured integral current measurement) is used to establish (e.g., calculate) extremely accurate instantaneous module current I. i .

[0066] Subsequently, the chain link converter 10 of the present invention can utilize that extremely precise instantaneous module current I i To protect itself and any voltage source converter from various fault conditions, the chain link converter 10 is located within the voltage source converter during use.

[0067] More specifically, the converter controller 26 is additionally programmed to detect if the measured converter current I... DCIf a predetermined safety converter threshold is exceeded, protective action is taken. Such a predetermined safety converter threshold can typically be around 2000A (although this will vary depending on the specific chain link converter design), and the protective action that the converter controller 26 may take in those cases is to block the entire voltage source converter to which the chain link converter 10 is located, and to disconnect the voltage source converter from any energy source to which it is connected.

[0068] Furthermore, each module controller 14 is additionally programmed to respond to the instantaneous module current I it establishes. i If the deviation from one or more predetermined parameters occurs, protective action will be taken. Such deviations from predetermined parameters include:

[0069] If the instantaneous module current I i The predetermined security module threshold is exceeded, and the predetermined security module threshold is greater than the aforementioned predetermined security converter threshold; and

[0070] If the instantaneous module current increases at a rate greater than the predetermined safe rate.

[0071] In this respect, the predetermined safety module threshold is typically around 3000A, while the instantaneous module current I... i The increased predetermined safety rate is typically a few amperes per microsecond, for example, about 5A per microsecond (although again, this will vary depending on the specific design of the chain link converter). In each case, if such a deviation occurs, the protective action taken by the given module controller 14 similarly begins to disrupt the entire voltage source converter where the chain link converter 10 is located, and disconnects the voltage source converter from any energy source to which it is connected.

[0072] Thus, by carefully selecting the aforementioned associated predetermined parameters, the protection provided by the converter controller 26 takes precedence over the protection provided by each module controller 14, wherein, for example, in the event of a moderate overcurrent fault event, the converter controller 26 continues to provide coordinated protection for the chain link converter 10, while the module controller 14 responds to more extreme and faster-occurring fault events.

[0073] Each module controller 14 is further programmed to report the need for protective action to the converter controller 26. Furthermore, the converter controller 26 is also programmed to monitor the number of module controllers 14 providing such reports, and if fewer than a predetermined number of module controllers 14 provide such reports, instruct the module controllers 14 providing such reports to avoid taking protective action. The predetermined number of such module controllers 14 can be as low as two or three, although this can vary depending on the controller parameters and operating environment of the voltage source converter to which the chain link converter 10 is located. In other embodiments of the invention, if a predetermined number or more module controllers report the need for protective action, the converter controller may be further programmed to initiate blocking of the chain link converter, and / or blocking of the entire voltage source converter to which the chain link converter is located.

Claims

1. A chain link converter (10), comprising: Multiple cascaded chain link modules (12), each chain link module (12) having a module controller (14) programmed to control the operation of the corresponding chain link module (12) to selectively provide a voltage source, thereby enabling the chain link converter (10) to provide a step-variable voltage source; and A chain link converter controller (26) is arranged to communicate with each module controller (14) and is programmed to measure the converter current (I) flowing through the chain link converter (10) during use. DC ) is passed to multiple module controllers (14). Receive the measured converter current (I) DC Each of the module controllers (14) is further programmed to, during use, transmit the measured converter current (I) DC ) and the current (I) flowing through the corresponding chain link module (12) and measured therein. AC The rate of change of ) is combined to establish the instantaneous module current (I) flowing through the corresponding chain link module (12). i ), It includes a converter current (I) that receives the measurement. DC Each chain link module (12) of the module controller (14) further includes a current measuring transducer, which is used to provide the current (I) flowing through the corresponding chain link module (12). AC The rate of change of ).

2. The chain link converter (10) according to claim 1, wherein, Receive the measured converter current (I) DC Each module controller (14) in the module controller (14) is programmed to measure the converter current (I) by passing the measured converter current (I) DC ) is used as a baseline current measurement and the current (I) flowing through the corresponding chain link module (12) is used as a baseline current measurement. AC The integral current measurement derived from the rate of change of the module current (I) is added to the baseline current measurement to establish the instantaneous module current (I). i ).

3. The chain link converter (10) according to any one of claims 1-2, wherein, Receive the measured converter current (I) DC Each of the module controllers (14) in the module controller (14) is additionally programmed to receive the latest measured converter current (I) DC ) and the established instantaneous module current (I i ) are compared, and if the latest received measurement of the converter current (I) is... DC ) and the established instantaneous module current (I i If there is a difference between the two, the current (I) flowing through the corresponding chain link module (12) is adjusted. AC Measurement of the rate of change of ).

4. The chain link converter (10) according to any one of claims 1-2, wherein, The chain link converter controller (26) is further programmed to respond if the measured converter current (I) DC If the threshold of the predefined safety converter is exceeded, protective action will be taken.

5. The chain link converter (10) according to any one of claims 1-2, wherein, Each module controller (14) is additionally programmed to respond to the instantaneous module current (I0) it establishes. i If the parameter deviates from one or more predetermined parameters, protective action will be taken.

6. The chain link converter (10) according to claim 5, wherein, Deviation from the predetermined parameters includes at least one of the following: The instantaneous module current (I) i The threshold value exceeds a predetermined security module threshold, which is greater than a predetermined security converter threshold. as well as The instantaneous module current (I) i It increases at a rate greater than the predetermined safe rate.

7. The chain link converter (10) according to claim 4, wherein, Each module controller (14) is also programmed to report to the chain link converter controller (26) the need for protective action, which in turn is programmed to monitor the number of module controllers (14) providing such reports.

8. The chain link converter (10) according to claim 5, wherein, Each module controller (14) is also programmed to report to the chain link converter controller (26) the need for protective action, which in turn is programmed to monitor the number of module controllers (14) providing such reports.

9. The chain link converter (10) according to claim 6, wherein, Each module controller (14) is also programmed to report to the chain link converter controller (26) the need for protective action, which in turn is programmed to monitor the number of module controllers (14) providing such reports.

10. The chain link converter (10) according to any one of claims 7-9, wherein, The chain link converter controller (26) is further programmed to be at least one of the following: If fewer than a predetermined number of module controllers (14) provide such reports, each module controller (14) is instructed to avoid taking protective action, and each module controller (14) provides a report on the need to take protective action; as well as If the predetermined number or more module controllers (14) provide such a report, then the blocking of the chain link converter (10) and / or the voltage source converter, which is located within the voltage source converter in use, is initiated.

11. The chain link converter (10) according to any one of claims 1-2, further comprising at least one current converter (30) for providing the measured converter current (I0) flowing through the chain link converter (10). DC ).

12. A method of operating a chain link converter (10), the chain link converter (10) comprising: Multiple cascaded chain link modules (12), each chain link module having a module controller (14) programmed to control the operation of the corresponding chain link module (12) to selectively provide a voltage source, thereby enabling the chain link converter (10) to provide a step-variable voltage source; and a chain link converter controller (26) arranged to communicate with each module controller (14). The method includes the following steps: (a) The chain link converter controller (26) causes the measured converter current (I) flowing through the chain link converter (10) to be transferred to the chain link converter (10). DC ) is passed to multiple module controllers (14); and (b) Make the converter current (I) that receives the measurement DC The module controller (14) of the module will measure the converter current (I) DC ) and the current (I) flowing through the corresponding chain link module (12) and measured therein. AC The rate of change of ) is combined to establish the instantaneous module current (I) flowing through the corresponding chain link module (12). i ), It includes a converter current (I) that receives the measurement. DC Each chain link module (12) of the module controller (14) further includes a current measuring transducer, which is used to provide the current (I) flowing through the corresponding chain link module (12). AC The rate of change of ).

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