Patent ID: 12231027

DETAILED DESCRIPTION

A portion of a chain-link converter according to a first embodiment of the invention is designated generally by reference numeral10, as shown inFIG.1.

In particular, although the chain-link converter10of the invention includes sixty-four, series-connected chain-link modules12, only four are shown inFIG.1. In other embodiments of the invention, however, the chain-link converter may include fewer than or more than sixty-four chain-link modules.

In any event, each chain-link module12has a module controller14that is programmed to control operation of the corresponding chain-link module12to selectively provide a voltage source, whereby the chain-link converter10is able to provide a stepped variable voltage source.

More particularly, each chain-link module12includes a number of switching elements16which are connected in parallel with an energy storage device18in the form of a capacitor20. Other types of energy storage device, i.e. any device that is capable of storing and releasing energy to selectively provide a voltage, e.g. a fuel cell or battery, may also be used however.

Each switching element16includes a semiconductor device in the form of an Insulated Gate Bipolar Transistor (IGBT)22, although other types of self-commutated semiconductor devices, 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 bimode insulated gate transistor (BIGT), or any other self-commutated switching device may be used instead. In addition, one or more of the semiconductor devices may instead include a wide-bandgap material such as, but not limited to, silicon carbide, boron nitride, gallium nitride and aluminium nitride.

The number of semiconductor devices in each switching element16may vary depending on the required voltage and current ratings of that switching element16.

Each of the switching elements16also includes a passive current check element, which in the embodiment shown takes the form of a diode24, that is connected in anti-parallel with a corresponding semiconductor device. In other embodiments of the invention the passive current check element may include another type of passive current check device, i.e. any device that is capable of limiting current flow in only one direction. In addition, the number of passive current check devices in each passive current check element may vary depending on the required voltage and current ratings of that passive current check element.

The chain-link module12shown is a first exemplary chain-link module which includes a first pair of switching elements16that is connected in parallel with a capacitor20in a known half-bridge arrangement to define a 2-quadrant unipolar module. Switching of the switching elements16selectively directs current through the capacitor20or causes current to bypass the capacitor20, such that the first exemplary chain-link module12can provide zero or positive voltage and can conduct current in two directions.

A second exemplary chain-link module (not shown) includes first and second pairs of switching elements and a capacitor that are connected in a known full bridge arrangement to define a 4-quadrant bipolar module. In a similar manner to the first exemplary chain-link module, switching of the switching elements again selectively directs current through the capacitor or causes current to bypass the capacitor such that the second exemplary chain-link module can provide zero, positive or negative voltage and can conduct current in two directions.

The chain-link converter10shown includes solely first exemplary chain-link modules12, but in other embodiments of the invention the chain-link converter may include solely second exemplary chain-link modules, or a combination of first and second exemplary chain-link modules.

In any event, the provision of a plurality of chain-link modules12means that it is possible to build up a combined voltage across the chain-link converter10, via the insertion of the energy storage devices, i.e. the capacitors20, of multiple chain-link modules12(with each chain-link module12providing its own voltage), which is higher than the voltage available from each individual chain-link module12.

Accordingly, each of the chain-link modules12work together to permit the chain-link converter10to provide a stepped variable voltage source. This permits the generation of a voltage waveform across the chain-link converter10using a step-wise approximation. As such the chain-link converter10is capable of providing a wide range of complex waveforms which, when combined with other chain-link converters to define a voltage source converter, permits a conversion between AC and DC power.

Returning to the embodiment shown, the chain-link converter10also includes a chain-link converter controller26that is arranged in communication with each module controller14, and more particularly is arranged in communication with each module controller14via a passive optical network28, although other communication conduits may be used.

The chain-link converter10still further includes a current transformer30, and more particularly a DC current transformer, which, in the embodiment shown, is mounted on a busbar32, i.e. mounted on a main electrical conduction conduit that, in use, interconnects the chain-link converter10with an external DC or AC network. In any event, the current transformer30measures a converter current IDCthat flows into (or out of), i.e. through, the chain-link converter10, and passes that measurement to the chain-link converter controller26. This may be via the aforementioned passive optical network28, but could also be via some other equipment such as a protection system or other control system.

In addition, each chain-link module12includes a current measurement transducer, which in the embodiment shown takes the form of a Rogowski coil34, that measures a rate of change of current IACflowing through the associated chain-link module12. Other types of current measurement transducer may, however, be used, and not all of the chain-link modules12need necessarily include such a transducer.

In use, the chain-link converter controller26is programmed to communicate to each of module controller14the measured converter current IDCthat it receives from the current transformer30. Such communication takes place across the passive optical network28, and represents only a small overhead increase in the usual individual control commands sent by the converter controller26to each module controller14over this medium. This is because the same measured converter current IDCis broadcast to each module controller14. The passive optical network28typically allows communication at a frequency of approximately 10 kHz and this, together with bandwidth of the current transformer30, means that in practical terms an updated measured converter current IDCcan be provided by the converter controller26to each module controller14approximately every 100 μs.

In other embodiments of the invention, the converter controller may not necessarily communicate the measured converter current to each module controller. For example, if the corresponding chain-link module within which one or more module controllers is located does not additionally include a current measurement transducer, e.g. a Rogowski coil, the converter controller may not communicate the measured converter current to those module controllers.

Meanwhile, returning to the embodiment shown, each of the module controllers14that receives the measured converter current IDC, i.e. all of the module controllers14in the embodiment shown, is further programmed to, in use, combine the measured converter current IDC(which it receives from the converter controller26) with the measured rate of change of current IACflowing through the chain-link module12within which it is located (which each module controller14receives from the corresponding Rogowski coil34), to establish an instantaneous module current Iithat is flowing through the said chain-link module12.

More particularly, each module controller14is programmed to establish such an instantaneous module current Iiby using the measured converter current IDCas a base line current measurement and adding to that an integrated current measurement that is derived from the measured rate of change of current IACthat is flowing through the corresponding chain-link module12. In other embodiments of the invention, one or more of the module controllers may be programmed to add a directly measured rate of change of current flowing through the corresponding chain-link module to the base line current measurement to establish an instantaneous module current.

In addition to the foregoing, each of the module controllers14is also programmed to compare a latest received measured converter current IDCwith the established instantaneous module current Iiand to adjust measurement of the rate of change of current IACflowing through the corresponding chain-link module if there is a discrepancy between the latest received measured converter current IDCand the established instantaneous module current Ii. One way in which each module controller14may adjust such measurement is by altering the calibration of the Rogowski coil34, e.g. by using proportional integral control.

It follows that the chain-link converter10of the invention is able to make use of a precise current measurement at a known time in the recent past, i.e. the measured converter current IDC, and a high bandwidth current measurement that is established very shortly after that reference point in the recent past and which does not suffer from delays because it is made locally at the chain-link module12, i.e. the integrated current measurement derived from the measured rate of change of current IACflowing through the chain-link module12, to establish, e.g. calculate, an extremely accurate instantaneous module current Ii.

Thereafter, the chain-link converter10of the invention is able to utilise that extremely accurate instantaneous module current Iito protect both itself and any voltage source converter within which it is, in use, located from a range of different fault conditions.

More particularly, the converter controller26is additionally programmed to take protective action if the measured converter current IDCexceeds a predetermined safe converter threshold. Such a predetermined safe converter threshold might typically be of the order of 2000 A (although this will vary according to specific chain-link converter designs), and the protective action the converter controller26might take in those circumstances is to block the whole of a voltage source converter in which the chain-link converter10is located, and disconnect the said voltage source converter from any energy sources with which it is connected.

Also, each module controller14is additionally programmed to take protective action if the instantaneous module current Iiit establishes deviates from one or more predetermined parameters. Such deviation from a predetermined parameter includes:if the instantaneous module current Iiexceeds a predetermined safe module threshold, which is greater than the aforementioned predetermined safe converter threshold; andif the instantaneous module current increases at a rate greater than a predetermined safe rate.

In this regard, the predetermined safe module threshold is typically of the order of 3000 A, while a predetermined safe rate of instantaneous module current Iiincrease is usually a few amps per microsecond, e.g. about 5 A, per microsecond (although again, this will vary according to the specific design of the chain-link converter). In each case, the protective action taken by a given module controller14, should such deviation arise, is similarly to instigate blocking the whole of a voltage source converter in which the chain-link converter10is located, and the disconnecting of the said voltage source converter from any energy sources with which it is connected.

It follows that, with careful selection of the associated predetermined parameters mentioned above, the protection provided by the converter controller26takes precedence over the protection provided by each module controller14, with the converter controller26continuing to provide coordinated protection of the chain-link converter10, e.g. in the case of modest overcurrent fault events, while the module controllers14react to more extreme and faster occurring fault events.

Each module controller14is still further programmed to report the need for it to take protective action to the converter controller26. In turn, the converter controller26is further programmed to monitor the number of module controllers14providing such a report, and to instruct the or each module controller14providing such a report to avoid taking protective action if fewer than a predetermined number of module controllers14provide such a report. A predetermined number of such module controllers14could be as low as only two or three, although this can vary depending on the operating environment and controller parameters of the voltage source converter in which the chain-link converter10is, in use, located. In other embodiments of the invention, the converter controller may be additionally further programmed to instigate blocking of the chain-link converter, and/or blocking of the whole of a voltage source converter in which the chain-link converter is located, if the predetermined number of module controllers or greater reports the need to take protective action.