SYNTHETIC TEST CIRCUIT

Abstract

A synthetic test circuit, comprising: a device under test including a chain-link converter under test, comprising a plurality of test modules, each test module including module switches connected with an energy storage device; a terminal connected to the device under test; at least one injection circuit operably connected to the terminal, the injection circuit including a source, the source including a source chain-link converter, which includes a plurality of source modules, each source module including a plurality of module switches connected with at least one energy storage device; a controller being configured to operate each source module to selectively bypass the corresponding energy storage device and insert the corresponding energy storage device into the corresponding source chain-link converter so as to generate a voltage across the source chain-link converter and thereby operate the injection circuit to inject a current waveform and/or a voltage waveform into the chain-link converter under test.

Claims

1. A synthetic test circuit, for performing an electrical test on a device under test, comprising: a device under test including a chain-link converter under test, the chain-link converter under test including a plurality of test modules, each test module including a plurality of module switches connected with at least one energy storage device; a terminal connected to the device under test; at least one injection circuit operably connected to the terminal, the or each injection circuit including a source, the source including a source chain-link converter, the source chain-link converter including a plurality of source modules, each source module including a plurality of module switches connected with at least one energy storage device; a controller being configured to operate each source module to selectively bypass the or each corresponding energy storage device and insert the or each corresponding energy storage device into the corresponding source chain-link converter so as to generate a voltage across the source chain-link converter and thereby operate the or each injection circuit to inject a current waveform and/or a voltage waveform into the chain-link converter under test.

2. A synthetic test circuit according to claim 1 wherein the or at least one injection circuit is a current injection circuit, the current injection circuit including a current source, the current source including the source chain-link converter, the source chain-link converter including a plurality of source modules.

3. A synthetic test circuit according to claim 1 wherein the or at least one injection circuit is a voltage injection circuit, the voltage injection circuit including a voltage source, the voltage source including the source chain-link converter, the source chain-link converter including a plurality of source modules.

4. A synthetic test circuit according claim 1 wherein the current waveform injected into the chain-link converter under test is selected from a group including: a part-sinusoidal or fully-sinusoidal current waveform; a harmonic modulated current waveform; a current waveform that includes one or more anti-phase current components, preferably a current waveform that includes one or more anti-phase harmonic or ripple components; a current waveform with a duty cycle of 180 or 240 electrical degrees; a bidirectional or unidirectional current waveform; or a combination thereof.

5. A synthetic test circuit according to claim 1 wherein the voltage waveform injected into the chain-link converter under test is selected from a group including: a part-sinusoidal or fully-sinusoidal voltage waveform; a harmonic modulated voltage waveform, preferably a triplen harmonic modulated voltage waveform; a voltage waveform including voltage ripple; a voltage waveform with a duty cycle of 180 or 240 electrical degrees; a bidirectional or unidirectional voltage waveform; or a combination thereof.

6. A synthetic test circuit according to claim 1 wherein the controller is configured to operate the or each injection circuit to inject the current waveform into the chain-link converter under test so as to control an energy level of the or each energy storage device of each test module and/or control the energy level of the chain-link converter under test to obtain a zero net change in energy level of the chain-link converter under test over an operating cycle.

7. A synthetic test circuit according to claim 1 wherein the controller is configured to operate each test module to selectively bypass the or each corresponding energy storage device and insert the or each corresponding energy storage device into the chain-link converter under test so as to generate a voltage waveform across the chain-link converter under test during the injection of the current waveform and/or the voltage waveform into the chain-link converter under test.

8. A synthetic test circuit according to claim 7 wherein the voltage waveform across the chain-link converter under test is selected from a group including: a part-sinusoidal or fully-sinusoidal voltage waveform; a harmonic modulated voltage waveform, preferably a triplen harmonic modulated voltage waveform; a voltage waveform including voltage ripple; a voltage waveform with a period of 180 or 240 electrical degrees; a bidirectional or unidirectional voltage waveform; or a combination thereof.

9. A synthetic test circuit according to claim 7 wherein the controller is configured to operate each test module to selectively bypass the or each corresponding energy storage device and insert the or each corresponding energy storage device into the chain-link converter under test so as to generate a voltage waveform across the chain-link converter under test during the injection of the current waveform and/or the voltage waveform into the chain-link converter under test so as to: combine the voltages across the or each source chain-link converter and the chain-link converter under test and thereby control the current waveform injected into the chain-link converter under test; control an energy level of the or each energy storage device of each test module; control the energy level of the chain-link converter under test to obtain a zero net change in energy level of the chain-link converter under test over an operating cycle; control the energy level of the chain-link converter under test to obtain a zero net change in energy level of each test module over an operating cycle; equalise or substantially equalise the voltage levels of the plurality of test modules; control the current loading of each test module; equalise or substantially equalise the current loading of the plurality of test modules; and/or control the injected current waveform to be a leading current, a lagging current, or in-phase with the voltage waveform across the chain-link converter under test.

10. A synthetic test circuit according to any of claims 7 wherein the controller is configured to control switching of each module switch of each test module to selectively bypass the or each corresponding energy storage device and insert the or each corresponding energy storage device into the chain-link converter under test so as to generate a voltage waveform across the chain-link converter under test.

11. A synthetic test circuit according to claim 10 wherein the controller is configured to control switching of each module switch of each test module so as to switch at a peak, non-zero, zero or substantially zero value of the current waveform injected into the chain-link converter under test and/or at a peak or non-zero value of the voltage waveform injected into the chain-link converter under test.

12. A synthetic test circuit according to claim 10 wherein the controller is configured to control switching of each module switch of each test module so as to block current from flowing in each test module and thereby inhibit current from flowing in the chain-link converter under test.

13. A synthetic test circuit according to claim 12 wherein each module switch of each test module includes an active switching device connected in parallel with an anti-parallel passive current check element, and the controller is configured to turn off each active switching device of each test module to allow the anti-parallel passive current check elements to form a plurality of series-connected passive current check element rectifiers with a combined internal voltage that is greater than a voltage waveform across the chain-link converter under test so as to block current from flowing in each test module and thereby inhibit current from flowing in the chain-link converter under test, wherein the combined internal voltage is provided by the or each energy storage device of each test module.

14. A synthetic test circuit according to claim 12 wherein the controller is configured to operate the or each injection circuit to inject a voltage waveform into the chain-link converter under test when the chain-link converter under test is controlled to inhibit current from flowing in the chain-link converter under test.

15. A synthetic test circuit according to any of claim 12 wherein the controller is configured to operate the or each injection circuit to inject an overcurrent waveform into the chain-link converter under test and to control the chain-link converter under test to inhibit the overcurrent waveform from flowing in the chain-link converter under test.

16. A synthetic test circuit according to claim 15 wherein the controller is configured to operate the or each injection circuit to inject an alternating voltage waveform into the chain-link converter under test when the overcurrent waveform is inhibited from flowing in the chain-link converter under test.

17. A synthetic test circuit according to claim 15 wherein the controller is configured to operate the or each injection circuit to: inject a first alternating voltage waveform into the chain-link converter under test; and operate each test module to selectively bypass the or each corresponding energy storage device and insert the or each corresponding energy storage device into the chain-link converter under test so as to generate a second alternating voltage waveform across the chain-link converter under test, subsequent to the overcurrent waveform being inhibited from flowing in the chain-link converter under test.

18. A synthetic test circuit according to claim 17 wherein the first and second alternating voltage waveforms are controlled to cause reactive power to circulate between the or each injection circuit and the chain-link converter under test.

19. A synthetic test circuit according to claim 1 further including a power supply unit, wherein the power supply unit is coupled to the chain-link converter of the injection circuit and/or the chain-link converter under test so as to permit the power supply unit to selectively charge each energy storage device.

20. A synthetic test circuit according to claim 19 wherein the power supply unit is directly coupled with the or each energy storage device of each module.

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Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Embodiments of the invention will now be described, by way of non-limiting examples, with reference to the accompanying drawings in which:

[0052] FIGS. 1a and 1b show schematically a synthetic test circuit according to an embodiment of the invention;

[0053] FIGS. 2a and 2b show schematically the structures of the 4-quadrant bipolar module and 2-quadrant unipolar module respectively;

[0054] FIGS. 3 and 4 illustrate the basic operations of the current and voltage injection circuits of FIGS. 1a and 1b;

[0055] FIG. 5 shows schematically an example of a voltage source converter for use in HVDC power transmission;

[0056] FIG. 6 illustrates the operation of the current injection circuit of FIG. 1a to inject both voltage and current waveforms into a chain-link converter under test;

[0057] FIG. 7 illustrates a first example of actual in-service current and voltage conditions experienced by a chain-link converter in an Alternate Arm Converter;

[0058] FIG. 8 illustrates the operation of the synthetic test circuit of FIG. 1a to create test current and voltage conditions corresponding to the actual in-service current and voltage conditions of FIG. 7;

[0059] FIG. 9 shows the results of a simulation model of the operation of the synthetic test circuit illustrated in FIG. 8;

[0060] FIG. 10 illustrates a second example of actual in-service current and voltage conditions experienced by a chain-link converter in an Alternate Arm Converter;

[0061] FIG. 11 shows the results of a simulation model of the operation of the synthetic test circuit of FIG. 1a to create test current and voltage conditions corresponding to the actual in-service current and voltage conditions of FIG. 10;

[0062] FIG. 12 illustrates the operation of a chain-link converter to control the energy level of the chain-link converter to obtain a zero net change in energy level of the chain-link converter over an operating cycle;

[0063] FIG. 13 illustrates the operation of a chain-link converter to enable voltage balancing of the modules of the chain-link converter;

[0064] FIG. 14 illustrates the operation of the synthetic test circuit of FIG. 1a to create test current and voltage conditions corresponding to the actual in-service current and voltage conditions that relate to actual in-service reactive power conditions;

[0065] FIG. 15 shows the results of a simulation model of the operation of the synthetic test circuit to create test current and voltage conditions shown in FIG. 14;

[0066] FIG. 16 shows exemplary voltage waveforms that may be injected into a chain-link converter under test when the chain-link converter under test is in a non-conducting state;

[0067] FIGS. 17 and 18 illustrate the operation of the synthetic test circuit of FIG. 1a to create test current and voltage conditions corresponding to the actual in-service current and voltage conditions during a DC fault;

[0068] FIG. 19 shows schematically a synthetic test circuit according to an embodiment of the invention;

[0069] FIG. 20 shows schematically a synthetic test circuit according to an embodiment of the invention;

[0070] FIG. 21 shows schematically a synthetic test circuit according to an embodiment of the invention;

[0071] FIG. 22 shows schematically a synthetic test circuit according to an embodiment of the invention; and

[0072] FIG. 23 shows schematically a synthetic test circuit according to an embodiment of the invention.

DETAILED DESCRIPTION

[0073] A synthetic test circuit according to an embodiment of the invention is shown in FIGS. 1a and 1b, and is designated generally by the reference numeral 30.

[0074] The synthetic test circuit 30 comprises first and second terminals 32,34, a current injection circuit 36, an isolation switch 38 and a voltage injection circuit 40. As shown in FIGS. 1a and 1b, the current injection circuit 36 is connected in series with the isolation switch 38 between the first and second terminals 32,34, and the voltage injection circuit 40 is connected between the first and second terminals 32,34, and is thereby connected in parallel with the series connection of the current injection circuit 36 and isolation switch 38.

[0075] The current injection circuit 36 includes a current source. The current source includes a series connection of an inductor 42 and a source chain-link converter 44.

[0076] The source chain-link converter 44 of the current injection circuit includes a plurality of series-connected source modules. Each source module includes two pairs of module switches 54 and an energy storage device in the form of a capacitor 56. In each source module, the pairs of module switches 54 are connected in parallel with the capacitor 56 in a full-bridge arrangement to define a 4-quadrant bipolar module that can provide negative, zero or positive voltages and can conduct current in two directions. FIG. 2a shows the structure of the 4-quadrant bipolar module.

[0077] The capacitor 56 of each source module of the current injection circuit is selectively bypassed and inserted into the source chain-link converter 44 by changing the states of the corresponding module switches 54. This selectively directs current through the capacitor 56 or causes current to bypass the capacitor 56 so that the source module provides a negative, zero or positive voltage.

[0078] The capacitor 56 of the source module is bypassed when the module switches 54 are configured to form a current path that causes current in the respective source chain-link converter 44 to bypass the capacitor 56, and so the source module provides a zero voltage, i.e. the source module is configured in a bypassed mode.

[0079] The capacitor 56 of the source module is inserted into the respective source chain-link converter 44 when the module switches 54 are configured to allow the current in the respective source chain-link converter 44 to flow into and out of the capacitor 56. The capacitor 56 then charges or discharges its stored energy so as to provide a non-zero voltage, i.e. the source module is configured in a non-bypassed mode. The full-bridge arrangement of the module switches permits configuration of the module switches 54 to cause current to flow into and out of the capacitor 56 in either direction, and so each source module can be configured to provide a negative or positive voltage in the non-bypassed mode.

[0080] The voltage injection circuit 40 includes a voltage source 46. The voltage source 46 includes a source chain-link converter 44, the structure and operation of which is identical to that of the source chain-link converter of the current source of the current injection circuit 36. The voltage source converter further includes an inductor (not shown) connected in series with the source chain-link converter 44.

[0081] The structure of each source chain-link converter 44 permits build-up of a combined voltage across each source chain-link converter 44, which is higher than the voltage available from each of its individual source modules, via the insertion of the capacitors 56 of multiple source modules, each providing its own voltage, into each source chain-link converter 44. In this manner each source chain-link converter 44 is capable of providing a stepped variable voltage source, which permits the generation of a voltage waveform across each source chain-link converter 44 using a step-wise approximation. As such each source chain-link converter 44 is capable of providing complex voltage waveforms.

[0082] Each module switch 54 constitutes an insulated gate bipolar transistor (IGBT) that is connected in anti-parallel with a diode. It is envisaged that, in embodiments of the invention, each IGBT may be replaced by a gate turn-off thyristor, a field effect transistor, an injection-enhanced gate transistor, an integrated gate commutated thyristor or any other self-commutated switching device.

[0083] It is envisaged that, in embodiments of the invention, each capacitor 56 may be replaced by another type of energy storage device that is capable of storing and releasing energy, e.g. a battery or fuel cell.

[0084] It is also envisaged that, in embodiments of the invention, each of the current and voltage injection circuits 36,40 may include a different number and/or arrangement of chain-link converters 44.

[0085] The controller 50 is configured to control switching of the module switches 54 of each source module to selectively bypass the corresponding capacitor 56 and insert the corresponding capacitor 56 into the corresponding source chain-link converter 44 so as to generate a voltage across the corresponding source chain-link converter 44.

[0086] The controller 50 is further configured to control switching of the isolation switch 38 to switch the current injection circuit 36 into and out of circuit with the first and second terminals 32,34 so as to selectively isolate the current injection circuit 36 from the device under test and the voltage injection circuit 40. The provision of the isolation switch 38 permits the current injection circuit 36 to be configured as a low voltage, high current injection circuit 36, and the voltage injection circuit 40 to be configured as a low current, high voltage injection circuit 40.

[0087] In use a chain-link converter 52 under test is connected between the first and second terminals 32,34.

[0088] The structure and operation of the chain-link converter 52 under test is identical to that of each of the aforementioned source chain-link converters. More specifically, the chain-link converter 52 under test includes a plurality of test modules, and the structure and operation of each test module is identical to that of each source module.

[0089] The controller 50 is configured to control switching of the module switches 54 of each test module to selectively bypass the corresponding capacitor 56 and insert the corresponding capacitor 56 into the chain-link converter 52 under test so as to generate a voltage across the corresponding chain-link converter 52 under test.

[0090] It will be appreciated that the number of modules in each chain-link converter 44,52 may vary depending on their respective requirements.

[0091] It is envisaged that, in embodiments of the invention, each module may be replaced by another module with a different configuration. For example, each module may include a pairs of module switches 54 and an energy storage device in the form of a capacitor 56, the pair of module switches 54 are connected in parallel with the capacitor 56 in a half-bridge arrangement to define a 2-quadrant unipolar module that can provide zero or positive voltages and can conduct current in two directions. FIG. 2b shows the structure of the 2-quadrant unipolar module.

[0092] The configuration of the synthetic test circuit 30 as set out above enables the current injection circuit 36 to be operated to inject a current waveform I into the chain-link converter 52 under test, as shown in FIG. 3, and enables the voltage injection circuit 40 to be operated to inject a voltage waveform V into the chain-link converter 52 under test, as shown in FIG. 4. A cycle of such injections of current and voltage waveforms may be repeated at a desired frequency (e.g. 50 Hz).

[0093] In some embodiments, the isolation switch 38 is closed when the current injection circuit 36 is operated to inject the current waveform I into the chain-link converter 52 under test, and the isolation switch 38 is opened when the voltage injection circuit 40 is operated to inject the voltage waveform V into the chain-link converter 52 under test. Also, when the chain-link converter 52 under test is conducting a positive or negative current injected by the current injection circuit, the controller 50 controls the switching of the module switches 54 of the voltage injection circuit 40 so as to block current from flowing through the voltage injection circuit 40 and thereby prevent the modules of the voltage injection circuit 40 from discharging into the chain-link converter 52 under test. This means that the synthetic test circuit 30 would not be required to supply high voltage and high current at the same time, thus minimising the amount of power used during electrical testing of the chain-link converter 52.

[0094] FIG. 5 show, in schematic form, an exemplary application of the chain-link converter 52 for use in HVDC power transmission.

[0095] In the exemplary application, the chain-link converter 52 forms part of an Alternate Arm Converter (AAC). The AAC includes a plurality of converter limbs 58, each of which extends between first and second DC terminals and includes first and second limb portions separated by a respective AC terminal. Each limb portion includes a chain-link converter 52 connected in series with a director switch 60. Each chain-link converter 52 includes a plurality of series-connected modules, each of which may be in the form of a 4-quadrant bipolar module or a 2-quadrant unipolar module. In use, each limb portion is operable to switch the corresponding limb portion into and out of circuit between the corresponding AC and DC terminals.

[0096] During such switching of each limb portion of the AAC, each limb portion starts and ends conduction at zero current on a repetition cycle, typically at a frequency of 50 Hz. Whilst each limb portion starts and ends conduction at zero current, each module of the chain-link converter 52 is switched in and out of circuit to generate a sinusoidal voltage waveform at the respective AC terminal. The IGBTs and diodes within each module are switched on and off, with each IGBT and diode experiencing hard voltage commutation at the capacitor voltage (which is typically 2 kV). The IGBTs and diodes switch on and off at different times and thus can switch at a current flowing in the corresponding limb portion ranging from zero current to a peak current value, e.g. 1500 A.

[0097] To enable voltage balancing of the modules within each limb portion, the capacitor 56 of each module is selectively bypassed and inserted into the corresponding chain-link converter such that each module and module switch 54 experience low current and high current switching in set amounts of time.

[0098] To enable DC filtering, the capacitor 56 of each module is selectively bypassed and inserted into the corresponding chain-link converter 52 so as to filter one or more harmonic or ripple components from a current waveform flowing therethrough.

[0099] The chain-link converter 52 and its components therefore experiences a wide range of actual in-service current and voltage conditions during its use in the AAC arrangement.

[0100] Typically the chain-link converter 52 must comply with various testing requirements, which are identical or closely similar to actual in-service current and voltage conditions, in order to be certified for service operation.

[0101] Therefore, in order to check whether the chain-link converter 52 complies with such testing requirements, the synthetic test circuit 30 is controlled to perform an electrical test on the chain-link converter 52 that involves creation of test conditions including one or more of, but are not limited to: sinusoidal current and voltage waveforms representative of actual in-service operation; operation of the chain-link converter 52 under test to generate sinusoidal and triplen harmonic modulated voltage waveforms; current and voltage waveforms representative of hard voltage and hard current switching of each module switch 54 of each test module at varying levels up to peak current rating; operation of the chain-link converter 52 under test to generate voltage waveforms with duty cycles of 180 and 240 electrical degrees and to conduct a current waveform with duty cycles of 180 and 240 electrical degrees; a current waveform that includes one or more anti-phase harmonic or ripple components; control over the overall energy level of the chain-link converter 52 under test and the individual energy levels of the test modules; control over the thermal loading of the test modules; current and voltage waveforms representative of actual in-service rectifier, inverter and leading/lagging reactive power operations; current and voltage waveforms representative of actual in-service current and voltage conditions of the chain-link converter 52 under DC fault conditions.

[0102] The capability of each chain-link converter 44,52 to provide complex voltage waveforms thereacross enables the current and voltage injection circuits 36,40 to inject a wide range of current and voltage waveforms into the chain-link converter 52 under test and enables the chain-link converter 52 under test to generate a wide range of voltage waveforms thereacross, and so enables the synthetic test circuit 30 to readily and reliably create test current and voltage conditions that are identical or closely similar to the above actual in-service current and voltage conditions.

[0103] It will be appreciated that the current and voltage waveforms in the figures are shown as continuous waveforms, but are actually stepwise approximated waveforms as constructed by the synthetic test circuit 30.

[0104] FIG. 6 illustrates the operation of the current injection circuit 36 to inject both voltage and current waveforms into the chain-link converter 52 under test.

[0105] In particular, the controller 50 operates the current injection circuit 36 to inject both voltage and current waveforms into the chain-link converter 52 under test so as to perform at least one cycle of sequential injections of the voltage and current waveforms into the chain-link converter 52 under test. Meanwhile the voltage injection circuit 40 is operated to block current from flowing therethrough.

[0106] FIG. 7 illustrates the actual in-service voltage and current conditions experienced by the chain-link converter 52 in both inverter and rectifier modes of the AAC arrangement, and their relationship to a sinusoidal voltage waveform formed at the AC terminal of the AAC arrangement. The actual in-service voltage and current conditions in FIG. 7 relate to the positive half-cycle of the sinusoidal voltage waveform at the AC terminal of the AAC arrangement, but it will be understood that the chain-link converter 52 experiences similar actual in-service voltage and current conditions in relation to the negative half-cycle of the sinusoidal voltage waveform at the AC terminal of the AAC arrangement.

[0107] In order to create test current and voltage conditions that are identical or closely similar to the actual in-service current and voltage conditions shown in FIG. 7, the current injection circuit 36 is operated to generate a bidirectional voltage waveform across the corresponding source chain-link converter 44. Meanwhile the chain-link converter 52 under test is operated to generate a bidirectional voltage waveform thereacross, whereby the generated bidirectional voltage waveform across the chain-link converter 52 under test is identical or closely similar to the actual in-service voltage conditions experienced by the chain-link converter shown in FIG. 7.

[0108] The voltages 86 across the source chain-link converter 44 and the chain-link converter 52 under test combine to control the voltage 88 across the inductor 42 and thereby control the current waveform injected into the chain-link converter 52 under test. Hence, in order to provide the voltage 88 across the inductor 42 to inject the current waveform into the chain-link converter 52 under test, the source chain-link converter 44 of the current injection circuit 36 is operated to generate a voltage waveform 86 thereacross that is the sum of the voltage 88 across the inductor and the voltage across the chain-link converter 52 under test, as shown in FIG. 8.

[0109] To create test current conditions that are identical or closely similar to the actual in-service current conditions in the rectifier mode of the AAC arrangement, the voltage across the inductor 42 is controlled to have a zero average component and to start with a positive section and end with a negative section so as to control the direction of current injected into the chain-link converter 52 under test, as shown in FIG. 8.

[0110] FIG. 9 shows the results of a simulation model of the operation of the synthetic test circuit 30 to create test current and voltage conditions that are identical or closely similar to the actual in-service current and voltage conditions in the rectifier mode of the AAC arrangement. It can be seen from FIG. 9 that the simulated test current and voltage conditions are comparable to the target test current and voltage conditions shown in FIG. 8, and so it follows the synthetic test circuit 30 is capable of creating test current and voltage conditions that are identical or closely similar to the actual in-service current and voltage conditions in the rectifier mode of the AAC arrangement.

[0111] Similarly, to create test current conditions that are identical or closely similar to the actual in-service current conditions in the inverter mode of the AAC arrangement, the voltage across the inductor 42 is controlled to have a zero average component and to start with a negative section and end with a positive section so as to control the direction of current injected into the chain-link converter 52 under test.

[0112] The operation of the synthetic test circuit 30 described with reference to FIGS. 7 and 8 are also applicable to electrical testing of the chain-link converter 52 under test on the basis of other complex waveforms, examples of which are described as follows.

[0113] In use, each chain-link converter 52 of the AAC arrangement may be required to track a triplen harmonic modulated voltage waveform, i.e. a voltage waveform that includes one or more triplen (3.sup.rd, 9.sup.th, 15.sup.th) harmonic components which are zero-phase sequence in nature. FIG. 10 shows an exemplary triplen harmonic modulated voltage waveform generated across a chain-link converter 52, whereby the voltage waveform includes a 3.sup.rd harmonic component. Accordingly, during use of the synthetic test circuit 30 to perform an electrical test of the chain-link converter 52 under test, the chain-link converter 52 may be operated to generate the triplen harmonic modulated voltage waveform thereacross whilst the current injection circuit 36 is operated to inject a current waveform into the chain-link converter 52 under test.

[0114] In use, each chain-link converter 52 of the AAC arrangement may be required to filter one or more harmonic or ripple components from a current waveform flowing therethrough. A consequence of this filtering process is that the current flowing through each chain-link converter 52 changes from a half-cycle sinusoid to a more complex waveform, as shown in FIG. 10. The filtering process may require a conduction overlap, e.g. a 60 electrical degrees overlap, between the limb portions of the AAC arrangement, whereby both limb portions are in simultaneous conduction to form a circulation path that includes the DC network and the limb portions. Such a conduction overlap requires the conduction of each limb portion to be extended above the 180 electrical degrees duty cycle associated with the half-cycle sinusoid, e.g. to 240 electrical degrees to achieve a 60 electrical degrees conduction overlap, as shown in FIG. 10.

[0115] Accordingly, during use of the synthetic test circuit 30 to perform an electrical test of the chain-link converter 52 under test, the current injection circuit 36 is operated to inject a bidirectional current waveform into the chain-link converter 52 under test. The bidirectional current waveform has a duty cycle of 240 electrical degrees, unlike a half-cycle sinusoid that has a duty cycle of 180 electrical degrees, and includes sections of an anti-phase 6th harmonic ripple current to simulate the aforementioned filtering process. More specifically, the bidirectional current waveform injected into the chain-link converter 52 under test begins with a negative, first current portion, continues with a positive, second current portion, and ends with a negative, third current portion. Meanwhile the chain-link converter 52 under test is operated to generate a voltage waveform thereacross, whereby the shape of the voltage waveform permits the formation of the conduction overlap.

[0116] FIG. 11 illustrates the results of a simulation model of the operation of the synthetic test circuit 30 to create test current and voltage conditions that are identical or closely similar to the actual in-service current and voltage conditions in FIG. 10. It can be seen from FIG. 11 that the simulated test current and voltage conditions are comparable to the target test current and voltage conditions shown in FIG. 10, and so it follows the synthetic test circuit 30 is capable of creating test current and voltage conditions that are identical or closely similar to the actual in-service current and voltage conditions in which the chain-link converter 52 tracks a triplen harmonic modulated voltage waveform and performs the filtering process.

[0117] FIG. 12 shows the operation of a chain-link converter 52 in which the voltage across the chain-link converter 52 and the current flowing through the chain-link converter 52 combine to control the energy level of the chain-link converter 52 to obtain a zero net change in energy level of the chain-link converter 52 over an operating cycle.

[0118] Accordingly, during use of the synthetic test circuit 30 to perform an electrical test of the chain-link converter 52 under test, the chain-link converter 52 under test is operated to generate a sinusoidal voltage waveform (or any other preferred voltage waveform) thereacross, while the current injection circuit 36 is operated to inject a current waveform into the chain-link converter 52 under test, whereby the current waveform is shaped to obtain a zero net change in energy level of the chain-link converter 52 over an operating cycle.

[0119] FIG. 13 illustrates the operation of a chain-link converter 52 to enable voltage balancing of the modules of the chain-link converter 52. In use, whilst the chain-link converter 52 is operated to generate a voltage waveform thereacross, the capacitor 56 of each module is selectively bypassed and inserted into the chain-link converter 52 such that each module experience low current and high current switching in set amounts of time required to balance the voltage levels of the capacitors 56 of the modules. The full-bridge arrangement of each module enables each module to be charged and discharged in equal amounts whilst being inserted into the chain-link converter 52, and so it becomes possible to operate the chain-link converter 52 to selectively bypass and insert each capacitor 56 into the chain-link converter 52 in order to control the energy level of the chain-link converter 52 to obtain a zero net change in energy level of each module over an operating cycle.

[0120] Accordingly, during use of the synthetic test circuit 30 to perform an electrical test of the chain-link converter 52 under test, the current injection circuit 36 is operated to inject a current waveform into the chain-link converter 52 under test, and the chain-link converter 52 under test is operated to generate a voltage waveform thereacross and to selectively bypass and insert each capacitor 56 into the chain-link converter 52 under test so as to enable voltage balancing of the test modules and to control the energy level of the chain-link converter 52 under test to obtain a zero net change in energy level of each test module over an operating cycle.

[0121] In addition, during use of the synthetic test circuit 30 to perform an electrical test of the chain-link converter 52 under test, the chain-link converter 52 under test can be operated to selectively bypass and insert each capacitor 56 into the chain-link converter 52 under test to equalise the current loading of the test modules.

[0122] In use, the Alternate Arm Converter may operate over a wide active real power-reactive power (P-Q) operating envelope. As such the chain-link converter 52 must be capable of operating with not only a wide range of real power flows but also a wide range of reactive power flows.

[0123] Accordingly, during use of the synthetic test circuit 30 to perform an electrical test of the chain-link converter 52 under test, the current injection circuit 36 is operated to generate a bidirectional voltage waveform across the corresponding source chain-link converter 44, while the chain-link converter 52 under test is operated to generate a bidirectional voltage waveform thereacross. The voltages across the source chain-link converter 44 and the chain-link converter 52 under test combine to control the voltage across the inductor 42 and thereby control the current waveform injected into the chain-link converter 52 under test, as shown in FIG. 14. The injected current waveform may be controlled to be a leading current or a lagging current to create test reactive power conditions.

[0124] When the injected current waveform is a leading or lagging current, the injected current waveform may be controlled to further include an additional current pulse at the end of the operating cycle to charge or discharge the chain-link converter 52 under test so as to control the energy level of the chain-link converter 52 under test. FIG. 14 shows the inclusion of an additional pulse in the injected current waveform to effect discharging of the chain-link converter 52 under test.

[0125] FIG. 15 shows the results of a simulation model of the operation of the synthetic test circuit 30 to create test current and voltage conditions shown in FIG. 14. It can be seen from FIG. 15 that the simulated test current and voltage conditions are comparable to the target test current and voltage conditions shown in FIG. 14, and so it follows the synthetic test circuit 30 is capable of creating test current and voltage conditions that are identical or closely similar to the actual in-service current and voltage conditions relating to actual in-service reactive power conditions.

[0126] In use, the chain-link converter 52 of the AAC arrangement may be operated to change from a conducting state to a non-conducting state. This is achieved by turning off each IGBT of each module to allow the anti-parallel diodes to form a plurality of series-connected diode rectifiers with a combined internal voltage that is greater than a voltage waveform across the chain-link converter 52 under test so as to block current from flowing in the module and thereby inhibit current from flowing in the chain-link converter 52 under test, wherein the combined internal voltage is provided by the capacitor 56 of each module.

[0127] Accordingly, during use of the synthetic test circuit 30 to perform an electrical test of the chain-link converter 52 under test, the chain-link converter 52 under test is operated to configure its test modules to form the plurality of series-connected diode rectifiers in order to block current from flowing therethrough and thereby inhibit current from flowing in the chain-link converter 52 under test.

[0128] When the plurality of series-connected diode rectifiers is formed, the voltage injection circuit 40 can be controlled to inject a voltage waveform into the chain-link converter 52 under test. This permits voltage testing of the chain-link converter 52 under test in its non-conducting state. For example, the voltage injection circuit 40 may be operated to control the magnitude, shape, rate of change and/or duration of the voltage waveform injected into the chain-link converter 52 under test that is being controlled to inhibit current from flowing therethrough. FIG. 16 shows exemplary voltage waveforms that may be injected into the chain-link converter 52 under test.

[0129] In use, a DC fault may occur in the DC network connected to the AAC. The occurrence of the DC fault may result in a high fault current flowing from the AC network to the DC network via the AAC, thus exposing the components of the AAC to the risk of damage.

[0130] When the DC fault occurs in the DC network, the chain-link converter 52 of the AAC arrangement may be operated to provide an opposing voltage to the fault current flowing through the AAC and thereby reduce the fault current to zero. In doing so the chain-link converter 52 is also required to absorb any inductive energy stored in the inductance of the AC network.

[0131] Accordingly, during use of the synthetic test circuit 30 to perform an electrical test of the chain-link converter 52 under test, the current injection circuit 36 is initially operated to inject an overcurrent waveform into the chain-link converter 52 under test, in order to expose the chain-link converter 52 under test to test overcurrent conditions. Subsequently the chain-link converter 52 under test is controlled to form the plurality of series-connected diode rectifiers so as to provide the opposing voltage and thereby drive the overcurrent waveform to zero and to absorb any inductive energy stored in the inductance of the synthetic test circuit 30. Finally the current injection circuit 36 is operated to inject a sinusoidal voltage waveform into the chain-link converter 52 under test when the overcurrent waveform is inhibited from flowing in the chain-link converter 52 under test, in order to simulate DC fault current extinction with both polarities of an alternating driving voltage.

[0132] In addition, the AAC may be operated as a static synchronous compensator during occurrence of a DC fault.

[0133] Accordingly, during use of the synthetic test circuit 30 to perform an electrical test of the chain-link converter 52 under test, the current injection circuit 36 is initially operated to inject an overcurrent waveform into the chain-link converter 52 under test, in order to expose the chain-link converter 52 under test to test overcurrent conditions. Subsequently the chain-link converter 52 under test is controlled to form the plurality of series-connected diode rectifiers so as to provide the opposing voltage and thereby drive the overcurrent waveform to zero and to absorb any inductive energy stored in the inductance of the synthetic test circuit 30. Finally the current injection circuit 36 is operated to inject a first sinusoidal voltage waveform into the chain-link converter 52 under test, and the chain-link converter 52 under test is operated to generate a second sinusoidal voltage waveform thereacross, whereby the magnitudes of the in-phase first and second sinusoidal voltage waveforms are controlled to cause reactive power to circulate between the current injection circuit 36 and the chain-link converter 52 under test. This permits simulation of test current and voltage conditions corresponding to actual in-service current and voltage conditions experienced by a chain-link converter 52 when the AAC is operated as a static synchronous compensator during occurrence of a DC fault.

[0134] Other electrical tests may be performed on the chain-link converter 52 under test. In one such electrical test, the synthetic test circuit 30 may be operated to inject a voltage waveform including voltage ripple into the chain-link converter 52 under test. In another such electrical test, each module switch 54 of each test module may be switched at a peak, non-zero, zero or substantially zero value of a current waveform injected into the chain-link converter 52 under test and/or at a peak or non-zero value of a voltage waveform injected into the chain-link converter 52 under test, in order to test the soft and hard current switching capabilities and hard voltage switching capabilities of the module switches 54 of the test modules.

[0135] In view of the foregoing the provision of the chain-link converters 44 in the current and voltage injection circuits 36,40 and the provision of the chain-link converter 52 under test therefore results in a synthetic test circuit 30 that is not only capable of performing high quality electrical testing, but also has the flexibility to perform an electrical test on a broad range of chain-link converters 52 with different ratings. This is because the capability of each chain-link converter 44,52 to generate a voltage waveform thereacross using a step-wise approximation allows the current and voltage injection circuits 36,40 to inject current and voltage waveforms of varying levels into the chain-link converter 52 under test and allows the chain-link converter 52 under test to generate a wide range of voltage waveforms thereacross, and thus renders the synthetic test circuit 30 capable of electrically testing various chain-link converters 52 across a wide range of ratings.

[0136] In addition the modular arrangement of each chain-link converter 44,52 means that the number of modules in each chain-link converter 44,52 can be readily scaled up or down to modify the voltage capability of each chain-link converter 44,52 to match the testing requirements of the chain-link converter 52 under test, without having to make significant changes to the overall design of the synthetic test circuit 30.

[0137] A synthetic test circuit according to an embodiment of the invention is shown in FIG. 19, and is designated generally by the reference numeral 130. The synthetic test circuit of FIG. 19 is similar in structure and operation to the synthetic test circuit 30 of FIGS. 1a and 1b, and like features share the same reference numerals.

[0138] The synthetic test circuit 130 of FIG. 19 differs from the synthetic test circuit 30 of FIGS. 1a and 1b in that the voltage rating of the source chain-link converter 44 of the current injection circuit 36 exceeds the voltage rating of the source chain-link converter 44 of the voltage injection circuit 40. This allows the current injection circuit 36 to be operated to selectively provide a blocking voltage to isolate the current injection circuit 36 from the voltage injection circuit 40 and chain-link converter 52 under test, thus obviating the need for the isolation switch 38.

[0139] There is provided a synthetic test circuit according to an embodiment of the invention, which is similar in structure and operation to the synthetic test circuit 30 of FIGS. 1a and 1b, and like features share the same reference numerals.

[0140] The synthetic test circuit according to the embodiment of the invention differs from the synthetic test circuit 30 of FIGS. 1a and 1b in that, in the synthetic test circuit according to the embodiment of the invention, a power supply unit 100 is directly coupled with the capacitor of each source module of the current injection circuit 36 and with the capacitor of each test module of the chain-link converter 52 under test, as shown in FIG. 20.

[0141] The power supply unit 100 includes a rectifier that connects an AC power bus to each capacitor to maintain the capacitor at a set voltage and offset losses. The use of each rectifier permits supply of power to and removal of energy from the corresponding capacitor. Since each rectifier may be operating at a different voltage with respect to the other capacitors and ground, a respective isolation transformer may be connected between each rectifier and the AC power bus.

[0142] There is provided a synthetic test circuit according to an embodiment of the invention, which is similar in structure and operation to the synthetic test circuit 30 of FIGS. 1a and 1b, and like features share the same reference numerals.

[0143] The synthetic test circuit according to the embodiment of the invention differs from the synthetic test circuit 30 of FIGS. 1a and 1b in that, in the synthetic test circuit according to the embodiment of the invention, a power supply unit 102 is connected in series with the current source in the current injection circuit 36 and with the chain-link converter 52 under test, as shown in FIG. 21.

[0144] The power supply unit 102 includes a DC power supply arranged to inject a direct voltage V.sub.DC into the current injection circuit 36. The DC power supply is further arranged to permit it to conduct a positive current I.sub.DC when injecting the direct voltage V.sub.DC into the current injection circuit 36. Such flow of positive current may be required when, for example, the chain-link converter 52 under test is required to conduct a positive current I.sub.DC, as shown in FIG. 8.

[0145] Referring to section (a) of FIG. 21, the DC power supply of the power supply unit 102 injects a direct voltage V.sub.DC which interacts with the direct current component I.sub.DC of the current waveform injected into the chain-link converter 52 under test in order to provide injection of real power into the synthetic test circuit. The source chain-link converter 44 of the current injection circuit 36 is operated to generate an alternating voltage waveform with a direct voltage component that is equal and opposite to that provided by the power supply unit 102. Since the power supply unit 102 and the source chain-link converter 44 conduct the same current waveform, the power exported from the power supply unit 102 is imported into the source chain-link converter 44. The imported power is then shared equally amongst the capacitors 56 of the source modules by selectively bypassing and inserting them into the source chain-link converter 44 so that each capacitor 56 receives the appropriate amount of energy to, for example, compensate for its respective power losses. It can be seen from FIG. 21 that the voltage applied across the respective inductor 42 is unaffected by the transfer of power from the power supply unit 102 to the source chain-link converter 44 of the current injection circuit 36.

[0146] The above use of the DC power supply of the power supply unit 102 may be similarly applied to the chain-link converter 52 under test. More specifically, referring to section (b) of FIG. 21, the chain-link converter 52 under test (instead of the source chain-link converter 44 of the current injection circuit 36) is operated to generate an alternating voltage waveform with a direct voltage component that is equal and opposite to that provided by the power supply unit 102. Since the power supply unit 102 and the chain-link converter 52 under test conduct the same current waveform, the power exported from the power supply unit 102 is imported into the chain-link converter 52 under test. The imported power is then shared equally amongst the capacitors 56 of the test modules by selectively bypassing and inserting them into the chain-link converter 52 under test so that each capacitor 56 receives the appropriate amount of energy to, for example, compensate for its respective power losses.

[0147] Furthermore, referring to section (c) of FIG. 21, the above use of the DC power supply of the power supply unit 102 may be applied to the source chain-link converter 44 of the current injection circuit 36 and the chain-link converter 52 under test at the same time. In this case both the source chain-link converter 44 of the current injection circuit 36 and the chain-link converter 52 under test are operated to generate respective alternating voltage waveforms with respective direct voltage components V.sub.DC1,V.sub.DC2, the sum of which is equal and opposite to the direct voltage V.sub.DC provided by the power supply unit 102, so that the power exported from the power supply unit 102 is imported into the source chain-link converter 44 of the current injection circuit 36 and the chain-link converter 52 under test.

[0148] As shown in FIG. 21, the power supply unit 102 includes an inductive-capacitive filter L,C arranged to filter the direct voltage V.sub.DC injected by the DC power supply, and also includes a control unit 106 programmed to control the DC power supply to inject the direct voltage V.sub.DC into the current injection circuit 36 and/or the chain-link converter 52 under test. The active control of the DC power supply may be used to maintain the injected direct voltage V.sub.DC at a desired voltage, and to damp or cancel at least one low-frequency oscillation in the injected direct voltage V.sub.DC arising from interactions between the power supply unit 102 and the rest of the synthetic test circuit. It will be appreciated that the inductive-capacitive filter L,C and the control unit 106 are optional features.

[0149] There is provided a synthetic test circuit according to an embodiment of the invention, which is similar in structure and operation to the synthetic test circuit of the embodiment of the invention depicted in FIG. 21, and like features share the same reference numerals.

[0150] The synthetic test circuit according to the embodiment of the invention differs from the synthetic test circuit according to the embodiment of the invention depicted in FIG. 21 in that, in the power supply unit 108 of the synthetic test circuit according to this embodiment of the invention, the DC power supply is arranged to permit it to conduct a negative current I.sub.DC when injecting the direct voltage V.sub.DC into the current injection circuit 36 and/or the chain-link converter 52 under test, as shown in FIG. 22. Such flow of negative current may be required when, for example, the chain-link converter 52 under test is required to conduct a negative current I.sub.DC, as shown in FIG. 7. Hence, the direct voltage V.sub.DC injected by the DC power supply in the synthetic test circuit according to this embodiment of the invention is opposite in polarity to the direct voltage V.sub.DC injected by the DC power supply in the synthetic test circuit according to the embodiment of the invention depicted in FIG. 21.

[0151] There is provided a synthetic test circuit according to an embodiment of the invention which combines the features of the synthetic test circuits of other embodiments of the invention, and like features share the same reference numerals.

[0152] More specifically, in the synthetic test circuit according to this embodiment of the invention, the power supply unit 110 includes first and second DC power supplies. The first DC power supply is similar in structure and operation to the DC power supply of the synthetic test circuit according to FIG. 21, and the second DC power supply is similar in structure and operation to the DC power supply of the synthetic test circuit according to FIG. 22.

[0153] FIG. 23 shows schematically the configuration of the power supply unit 110.

[0154] The first and second DC power supplies are connected between first and second selector terminals 112,114, and are separated by a ground connection. The first DC power supply is connected between the first selector terminal 112 and the ground connection, and the second DC power supply is connected between the second selector terminal 114 and the ground connection. A first inductive-capacitive filter L,C is arranged to filter the direct voltage V.sub.DC injected by the first DC power supply, and a second inductive-capacitive filter L,C arranged to filter the direct voltage V.sub.DC injected by the second DC power supply.

[0155] The power supply unit 110 further includes a selector switching element 116 connected in series with the current source in the current injection circuit 36 and with the chain-link converter 52 under test. In use, the control unit 106 switches the selector switching element 116 to connect to either the first or second selector terminal 112,114 so as to switch one of the first and second DC power supplies into circuit with the current injection circuit 36 and the chain-link converter 52 under test and at the same time switch the other of the first and second DC power supplies out of circuit with the current injection circuit 36 and the chain-link converter 52 under test.

[0156] The DC power supply switched into circuit with the current injection circuit 36 and the chain-link converter 52 under test can be controlled to inject a direct voltage V.sub.DC into either or both of the current injection circuit 36 and the chain-link converter 52 under test. The first DC power supply is arranged to permit it to conduct a positive current when injecting a first direct voltage V.sub.DC into the current injection circuit 36 and/or the chain-link converter 52 under test. The second DC power supply is arranged to permit it to conduct a negative current when injecting a second direct voltage V.sub.DC into the current injection circuit 36 and/or the chain-link converter 52 under test.

[0157] The configuration of the synthetic test circuit according to FIG. 23 permits the power supply unit 110 to selectively charge each capacitor of the chain-link converter of the current injection circuit 36 and/or the chain-link converter 52 under test in both directions of the current waveform to be injected into the chain-link converter 52 under test.

[0158] The use of the respective power supply unit 100,102,108,110 in the synthetic test circuit permits stable performance of the source chain-link converter 44 and the chain-link converter 52 under test to generate a voltage waveform thereacross, since the power supply unit 100,102,108,110 provides power to the capacitors of the source chain-link converter 44 and the chain-link converter 52 under test to offset the loss of energy as a result of, for example, conduction and switching losses. In practice the power supply units 100,102,108,110 may be configured to inject power into the current injection circuit 36 and the chain-link converter 52 under test to offset power losses in the synthetic test circuit as a whole, which in due course would lead to discharge of the capacitors of the chain-link converters 44,52.

[0159] It will be appreciated that the power supply unit 100,102,108,110 shown in FIGS. 20 to 23 may be applied to other embodiments of the invention.

[0160] In other embodiments of the invention, it is envisaged that the current injection circuit 36 may include a plurality of parallel-connected current sources. The number of parallel-connected current sources in the current injection circuit 36 may vary to adapt the current capability of the current injection circuit 36 for compatibility with the current rating and test current conditions of the chain-link converter 52 under test.

[0161] It will be appreciated that the above type of chain-link converter 52 and the above exemplary application of the chain-link converter 52 described in this specification are merely chosen to illustrate the working of the invention. Accordingly it will also be appreciated that the invention is intended to extend to the use of the synthetic test circuit 30,130 with other types of chain-link converters 52 that may be used in other types of power applications, which are not limited to the field of HVDC power transmission.

[0162] It will be also appreciated that the electrical tests and the shapes of the voltage and current waveforms described in this specification are merely chosen to illustrate the working of the invention. Accordingly it will also be appreciated that other electrical tests and other shapes of the voltage and current waveforms may be used with the synthetic test circuit 30,130 according to the invention.