WO2019170375A1

WO2019170375A1 - Ac-ac converter and method of operation

Info

Publication number: WO2019170375A1
Application number: PCT/EP2019/053513
Authority: WO
Inventors: Michael Zagrodnik; Xiao Lei Hu
Original assignee: Rolls-Royce Plc
Priority date: 2018-03-09
Filing date: 2019-02-13
Publication date: 2019-09-12
Also published as: GB201803765D0

Abstract

A three-phase multi-level AC-AC converter and method of operating the same. The converter including: a transformer having plural secondary windings, each of which provides a three-phase or single-phase source of AC power; three sets of single-phase AC-AC converter cells, each cell receiving AC power from a respective winding of the plurality of windings, the converter cells within a given set being connected in series, and the three sets of converter cells being configured to supply power to a load; and a three-phase cell, connected to the three sets of single-phase AC-AC converter cells; wherein the three-phase cell includes: an active front-end converter which is connected to a winding of the plurality of windings; a three-phase converter; and a DC-link which links the active front-end converter to the three-phase converter; wherein the three-phase converter is configured to receive and rectify regenerative three-phase AC power from the load; and wherein the multi-level AC-AC converter further includes a load-dump resistor connected in parallel with the DC- link.

Description

AC-AC CONVERTER AND METHOD OF OPERATION

Field of the Present Disclosure

The present disclosure relates to a three-phase multi-level AC-AC converter and its method of operation, and particularly to such a converter for use in an electrical drive for a marine vessel.

Background

There is an increasing trend in the deployment of medium voltage propulsion systems for large vessels and offshore platform rigs having a total electrical capacity of more than 10MW. Medium voltage multilevel converters are highly suited for this application.

A conventional cascaded H-bridge converter topology is shown in Figure 1A. These converters are operable as multi-level AC-AC converters, for example in marine vehicles which utilize electric drives. The converter topology has three sets of series-connected converter cells A1-A2, B1-B2, and C1-C2, and Figure 1 B shows the topography of just one of the converter cells C2. The cell includes a rectifier 103, which is provided by three branches of diodes. The rectifier converts received AC power into DC power. Generally the received AC power is three-phase AC power provided from a transformer 101 having multiple sets of phase shifted three-phase windings on the secondary. Whilst a three-phase transformer with Y primary windings and multiple extended-delta secondary windings is shown, it is possible to drive the system from a transformer with single-phase secondary windings. The DC power, which results from the rectification of the received AC power, is provided to a DC-link 104, which connects the rectifier 103 to an inverter 105. The inverter 105 converts the received DC power to AC power which is provided to an output of the cell. Generally the AC power provided is single phase AC power. As there are three sets of cells, and each set of cells provides a single phase AC power supply, the load 102 receives a three-phase AC power supply from the combination of the three sets of cells.

In a drive mode of the converter, three-phase AC power or single-phase AC power is supplied from the transformer 101 to each of the cells. Whilst two cells are shown in each set of series-connected cells, more may be provided. Each of the cells converts a three- phase or single-phase supply of AC power to a single-phase AC supply. As a consequence of having three sets of cells, the load 102 therefore receives three-phase AC power which has been converted in, for example, voltage and/or frequency relative to that provided by the transformer 101.

In a regeneration mode of the converter, where regenerative braking is used to draw power from the load 102, each of the cells receives single-phase AC power from the load 102. A load-dump resistor, present in the DC-link 104 of each of the cells, is activated to dissipate this energy.

However, the implementation of a load-dump resistor in each of the cells adds substantially to the cost of the converter. Moreover, the heat generated by each load-dump resistor in each cell can place thermal stresses on the other components within the cell.

On the other hand, providing load-dump resistors outside of each of the cells is expensive, and can result in significant electromagnetic interference.

Summary

The three-phase multi-level AC-AC converter of the present disclosure addresses the problems above by provision of a dedicated three-phase cell, which is provided with a high- specification load-dump resistor and means for providing regenerated AC power back to an AC source.

Accordingly, in a first aspect, the present disclosure provides a three-phase multi-level AC- AC converter, including:

a transformer having plural secondary windings, each of which provides a three- phase or single-phase source of AC power;

three sets of single-phase AC-AC converter cells, each cell receiving AC power from a respective winding of the plurality of windings, the converter cells within a given set being connected in series, and the three sets of converter cells being configured to supply power to a load; and

a three-phase cell, connected to the three sets of single-phase AC-AC converter cells;

wherein the three-phase cell includes:

an active front-end converter which is connected to a winding of the plurality of windings;

a three-phase converter; and

a DC-link which links the active front-end converter to the three-phase converter;

wherein the three-phase converter is configured to receive and rectify regenerative three-phase AC power from the load; and

wherein the multi-level AC-AC converter further includes a load-dump resistor connected in parallel with the DC-link.

The active front-end converter allows regenerated AC power from the load to be provided to the transformer and returned to the AC mains distribution network. Regenerated AC power from the load may also be provided to a distribution network different from the AC mains and may be transferred further to, for example, other AC loads, battery banks or other power storage means.

When the three-phase converter rectifies the regenerative AC power from the load it may provide DC power to the load-dump resistor. However, the three-phase converter may operate as an inverter to circulate reactive power between its respective phases, whilst using a relatively low capacity capacitor. In such a mode of operation, voltage ripple experienced by the single-phase cells may be reduced as compared to a conventional AC-AC converter.

Further optional features of the three-phase multi-level AC-AC converter of the first aspect will now be set out. These are applicable singly or in any combination.

The load-dump resistor can be located external to a casing of the three-phase cell. This can allow a load-dump resistor of high specification to be used, without influencing the size of the cell casing. Furthermore, as only a single load-dump resistor is required, cost-effective shielding can be implemented to minimise the electromagnetic interference caused by it.

The load-dump resistor may be connected in parallel across a diode of the DC-link and in series with a switch. By providing such a topology, the function of the load-dump resistor to dissipate power can be controlled. For example, the voltage across the DC-link may be allowed to initially increase (by, for example, charging of a capacitor within the DC-link). When the voltage passes a threshold, the switch may be closed so that excess power is dissipated by the load-dump resistor.

Each of the single-phase AC-AC converter cells may include a load-dump resistor. This may increase the redundancy of the system. Beneficially, the specification of the load-dump resistors located within the single-phase AC-AC converters cells can be reduced in comparison to conventional converter cells. The active front-end converter may be connected to windings of the plurality of windings that couple to the transformer, in which case regenerated energy is fed back to the AC distribution system that feeds the three-phase multi-level AC-AC converter. The active front- end converter may alternatively be connected to windings that are isolated from the plurality of windings in which case regenerated energy is fed back to a separate AC distribution system. Alternatively the active front-end converter may be absent in which case

regenerated energy is dissipated within the load-dump resistor.

In a second aspect, the present disclosure provides a marine propulsion drive including a three-phase multi-level AC-AC converter as set out in respect to the first aspect.

In a third aspect, the present disclosure provides a method of controlling a three-phase multi-level AC-AC converter, the converter including:

three sets of single-phase AC-AC converter cells, each cell including two pairs of switches for converting a respective phase of three-phase AC power; and

a three-phase cell which includes three pairs of switches, each pair of switches of the three-phase cell being configured for converting a respective phase of regenerative three-phase AC power;

wherein the method includes, in a driving mode of a load:

operating the two pairs of switches in each of the single-phase AC-AC converter cells such that a voltage vector contributed by each of the single-phase AC-AC converter cells is aligned with a corresponding phase of a three-phase driving current provided to the load so as to transfer real power from the three sets of single-phase AC-AC converter cells to the load; and

operating the three pairs of switches in the three-phase cell such that each voltage vector contributed by a respective pair of switches of the three-phase cell is at an angle of 90° from the corresponding phase of the driving current so as to transfer reactive power to the load; and

wherein the method further includes, in a regenerative mode of the load:

operating the two pairs of switches in each of the single-phase AC-AC converter cells such that a voltage vector contributed by each of the single-phase AC-AC converter cells is at an angle of 90° with a corresponding phase of a three-phase regenerative current provided from the load; and

operating the three pairs of switches in the three-phase cell such that each voltage vector contributed by a respective pair of switches of the three-phase AC-AC converter is aligned anti-parallel with the corresponding phase of the regenerative current so as to transfer real power from the load to the three-phase cell.

The three-phase multi-level AC-AC converter of the method of the third aspect may be the three-phase multi-level AC-AC converter of the first or the second aspect. The three-phase cell as set above may be operated to circulate reactive power between its respective phases, whilst using a relatively low capacity capacitor. In such a mode of operation, voltage ripple experienced by the single-phase cells may be reduced as compared to a conventional AC-AC converter.

Further optional features of the method of controlling a three-phase multi-level AC-AC converter of the third aspect will now be set out. These are applicable singly or in any combination.

The method may include a step of operating an active front-end converter of the three-phase cell so as to provide regenerative current from the load to a transformer. This can allow regenerative power to be transferred back to an AC supply or to power storage units, and therefore can increase the efficiency of a drive utilizing the method.

The method may include a step of operating an active front-end converter of the three-phase cell so as to regulate the charge on a capacitor located in a DC-link of the three-phase cell. This can simplify the operation of the converter, particularly making up for losses when the three-phase cell is circulating reactive power. The method may include a step of monitoring a DC voltage of a DC-link provided in the three-phase cell, so that if the DC voltage exceeds a reference value a connection is formed to allow power to flow into a load-dump resistor which is connected to the DC-link. The load- dump resistor may be located external to a casing of the three-phase cell.

Brief Description of the Drawings Embodiments of the present disclosure will now be described by way of example with reference to the accompanying drawings in which:

Figure 1A shows a conventional multi-level AC-AC converter and Figure 1 B shows the topology of a conventional single-phase converter cell; Figure 2A shows a multi-level AC-AC converter according to the present disclosure, and Figure 2B shows corresponding voltage vector components;

Figures 3A, 3B and 3C show variant topologies for a three-phase cell according to the present disclosure; and Figure 4 shows a schematic of a marine propulsion drive including an electrical system. Detailed Description and Further Optional Features

Figure 2A shows a multi-level AC-AC converter 200 according to the present disclosure. A transformer 201 includes plural secondary windings 202. Each of the windings receives a transformed AC power which may be either three-phase or single-phase, and provides the AC power to a respective single-phase AC-AC cell; for example, A1. Three sets of single- phase cells are shown: A1 & A2, B1 & B2, and C1 & C2. Each of the cells has a topology as shown in Figure 1 B. The single-phase cells provide AC power to a load 203. The load may be, for example, an electric motor. A three-phase cell, Cell 3, is connected to each of the sets of cells. The topology of the three-phase cell is discussed below. The phase voltage V provided by the cells is the summation of two components with vectors V1 and V2. The three-phase cell provides the voltage component with vector V1 , and the sets of single-phase cells provide the voltage component with vector V2. When the load is a motor, an induction-motor field-oriented control scheme can be used. This is based on the independent control of torque and rotor flux. Stator currents are selected as the controlled variables in such a scheme. Current regulation can be achieved by selecting the desired voltage vector such that the measured currents track the desired reference values. Insofar as motor control is concerned, it is the total phase voltage V which is important.

When operating in quadrants 1 and 3, i.e. in four-quadrant-control driving mode of the motor, a current is provided into the load 203. As can be seen in Fig. 2B, when operating in the driving mode the voltage component with vector V1 is orthogonal to the current I provided to the load, i.e. it is at a phase angle of 90° to the current. This means that the three-phase cell does not provide any real power to the load. Instantaneous reactive power is exchanged between the three phases of the three-phase cell, but as all phases in the three-phase cell share a DC-link (as discussed below) there is no net change in energy. When operating in quadrants 2 and 4, i.e. in a regenerative mode of the motor, a current is provided from the load 203. This can be in the form of regenerative braking of a motor. In such an operational mode, the voltage component with vector V2, i.e. that provided by the sets of single-phase cells, is orthogonal to the current I provided from the load. This means that there is no net transfer of power from the load to the dc-link of the single-phase cells. There is an oscillatory power flow in and out of the single-phase cells at twice the

fundamental frequency of the current from the load. This can be advantageous as the single- phase cells may be unable to absorb sustained regenerative power. In contrast, the voltage component with vector V1 is aligned with the current. This alignment may be anti-parallel to the current. As a result, real power is transferred from the load into the three-phase cell, Cell 3. The power transferred into the three-phase cell can then be either dissipated or returned to the transformer 201.

Whilst not necessary, load-dump resistors can be retained in each of the single-phase cells in case of a loss of control which results in power being passed into the single-phase cells during regenerative mode.

Figures 3A - 3C show example topologies of the three-phase cell, Cell 3 described above. In the three-phase cell 300 of Figure 3A, a three-phase converter 301 is provided which is connected to each of the sets of single-phase cells and can receive three-phase power from the load. The three-phase converter, in the regeneration mode discussed above, rectifies the received three-phase AC power into DC power. The DC power is provided to a DC link 302. The DC link includes a capacitor, and a load-dump resistor 303 connected in parallel with the capacitor. If the voltage across the capacitor reaches a reference value, a switch connected in series with the load-dump resistor is closed so that power can flow from the three-phase converter 301 into the load-dump resistor 303 to be dissipated. The load-dump resistor 303 is located outside of a casing of the three-phase cell 300.

The three-phase cell 310 shown in Figure 3B is similar to that shown in Figure 3A, and so like reference numerals are used for like features. However, a difference between the three- phase cells 310 and 300, is that three-phase cell 310 includes a passive front-end rectifier 304. This front-end rectifier is connected to at least one winding of the transformer, and so receives an AC current. This AC current can be used to charge the capacitor in the DC link 302 to a predetermined value, before the AC-AC converter is used. The rectifier may also stabilize the DC-link voltage and make up for losses when the three-phase cell is simply recirculating reactive energy. As the passive front-end rectifier 304 in this example is only used for charging the capacitor, the diodes comprising the rectifier can be of a much lower specification relative to those used in the single-phase cells.

The three-phase cell 320 shown in Figure 3C, is similar to those shown in Figures 3A and 3B, and so like reference numerals are used for like features. However, a difference between the three-phase cells 320 and 300, is that three-phase cell 320 includes an active front-end converter 305. The active front-end converter is connected to at least one winding of the transformer. The active front-end converter comprises three sets of switches, which are operable as either an inverter, for providing power from the DC link to the transformer during regeneration, or as a rectifier, for providing power from the transformer to the DC link (to regulate the charge of the capacitor and make up for losses).

Figure 4 is a schematic of a marine propulsion drive including a three-phase multi-level AC- AC converter 200 as discussed above. A three-phase AC power supply 401 is connected to the converter. When operating in a drive operational mode, the converter receives the three- phase AC as an input and outputs an AC supply to the load 203 e.g. in the form of an electric motor. This electric motor is connected in turn to a drive 403, e.g. a propeller. When operating in a regeneration mode, the converter receives regenerated three-phase AC from the load 203.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Moreover, in determining extent of protection, due account shall be taken of any element which is equivalent to an element specified in the claims. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

Claims

1. A three-phase multi-level AC-AC converter (200), including:

a transformer (201 ) having plural secondary windings (202), each of which provides a three-phase or single-phase source of AC power;

a three-phase cell (320), connected to the three sets of single-phase AC-AC converter cells;

wherein the three-phase cell includes:

an active front-end converter (305) which is connected to a winding of the plurality of windings;

a three-phase converter (301 ); and

a DC-link (302) which links the active front-end converter to the three-phase converter;

wherein the multi-level AC-AC converter further includes a load-dump resistor (303) connected in parallel with the DC-link.

2. The three-phase multi-level AC-AC converter of claim 1 , wherein the load-dump resistor is located external to a casing of the three-phase cell.

3. The three-phase multi-level AC-AC converter of claim 1 or 2, wherein the load-dump resistor is connected in parallel across a diode of the DC-link and in series with a switch.

4. The three-phase multi-level AC-AC converter of any preceding claim, wherein each of the single-phase AC-AC converter cells includes a load-dump resistor.

5. The three-phase multi-level AC-AC converter of any preceding claim, wherein the active front-end converter is connected to windings of the plurality of windings that couple to the transformer, so as to allow the regenerative three-phase AC power to be fed back to an AC distribution system that feeds the three-phase multi-level AC-AC converter.

6. The three-phase multi-level AC-AC converter of any of claims 1 - 4, wherein the active front-end converter is connected to further windings, which are isolated from the plurality of windings, so as to allow the regenerative three-phase AC power to be fed back to a separate AC distribution system.

7. A marine propulsion drive including a three-phase multi-level AC-AC converter of any preceding claim.

8. A method of controlling a three-phase multi-level AC-AC converter (200), the converter including:

a three-phase cell (300, 310, 320) which includes three pairs of switches, each pair of switches of the three-phase cell being configured for converting a respective phase of regenerative three-phase AC power;

wherein the method includes, in a driving mode of a load (203):

wherein the method further includes, in a regenerative mode of the load:

9. The method of claim 8, further including a step of:

operating an active front-end converter of the three-phase cell so as to provide regenerative current from the load to a transformer.

10. The method of claim 8, further including a step of:

operating an active front-end converter of the three-phase cell so as to regulate the charge on a capacitor located in a DC-link of the three-phase cell.

1 1. The method of any of claims 8 - 10, further including a step of:

monitoring a DC voltage of a DC-link provided in the three-phase cell, wherein if the

DC voltage exceeds reference value a connection is formed to allow power to flow into a load-dump resistor which is connected to the DC-link.

12. The method of claim 1 1 , wherein the load-dump resistor is located external to a casing of the three-phase cell.

13. The method of any of claims 8 - 12, wherein the three-phase multi-level AC-AC converter is part of a marine propulsion drive.