TITLE Power systems

DESCRIPTION Technical Field

The present invention relates to power systems, and in particular to power systems that are well-suited for use onboard marine vessels for supplying power to one or more drive motors.

Background Art

A conventional power system for a marine vessel is shown in Figure 1. It includes four ac generators 2a to 2d that are synchronised in phase to supply power through individual switched connections (i.e. an electrical connection that incorporates a switch, circuit breaker, protective switchgear or the like) to an ac busbar 4. The ac busbar 4 is normally divided into two separate parts by a breaker section 6 with two of the ac generators 2a and 2b supplying power to a first part of the ac busbar 4a and the other two of the ac generators 2c and 2d supplying power to a second part of the ac busbar 4b.

The breaker section 6 allows the first and second parts 4a and 4b of the ac busbar to be electrically isolated from each other. This normally corresponds to equipment mounted in the port side of the marine vessel versus equipment mounted in the starboard side of the marine vessel. The normal and most practical implementation is for one of the switches of the breaker section 6 to be part of the ac busbar 4a with the other switch to be part of the ac busbar section 4b with an electric cable connecting the two switches. However if required the two switches can be integrated with the first part 4a of the ac busbar or the second part 4b of the ac busbar.

As described in more detail below, the design of the power system circuit is intended to incorporate the general principal of redundancy so that at least one drive motor will continue to operate in the event of a fault in one or more of the components, or if one or more of the components requires service or maintenance. For example, if there is a

fault on the first part 4a of the ac busbar then the switches of the breaker section 6 can be opened to enable the second part 4b of the ac busbar to operate normally or vice versa.

A first thruster drive 8a is connected to the first part 4a of the ac busbar. The first thruster drive 8a will normally use a circuit to assist its starting. The starting circuit is normally a conventional star/delta switching system but a soft starter using phase controlled thyristors or similar devices can also be used.

A second thruster drive 8b is connected to the second part 4b of the ac busbar and normally uses a starter circuit as previously described.

A first dual motor drive 10a includes two separate ac motors driving a common propeller shaft. A first power converter 12a is used to interface between one of the ac motors of the first dual motor drive 10a and the first part 4a of the ac busbar. The first power converter 12a is connected to the first part 4a of the ac busbar by an individual switched connection that includes a first transformer 14a. The first transformer 14a is shown as a design with four separate windings to enable a 24-pulse system to be produced in conjunction with the diode rectifier 24 that has four separate bridges.

A second power converter 12b is used to interface between the other one of the ac motors of the first dual motor drive 10a and the second part 4b of the ac busbar. The second power converter 12b is connected to the second part 4b of the ac busbar by an individual switched connection that includes a second transformer 14b. The second transformer 14b is shown as a design that is identical to the first transformer 14a.

The above arrangement means that if a fault occurs on the first part 4a of the ac busbar such that the first and second parts 4a and 4b are electrically isolated from each other by the breaker section 6, the first dual motor drive 10a will continue to receive power from the second part 4b of the ac busbar through the second transformer 14b and the second power converter 12b even though no power will be received from the first part 4a of the ac busbar through the first transformer 14a and

the first power converter 12a or vice versa. This particular arrangement means that the first dual motor drive 10a can continue to operate using one of the separate ac motors if there is a fault on one part of the ac busbar 4 but not on both parts.

An identical arrangement is used for the second dual motor drive 10b where third and fourth power converters 12c and 12d interface between one of the ac motors and the first part 4a of the ac busbar and between the other one of the ac motors and the second part 4b of the ac busbar, respectively. The third power converter 12c is connected to the first part 4a of the ac busbar by an individual switched connection that includes a third transformer 14c that is shown as a design that is identical to the first transformer 14a. The fourth power converter 12d is connected to the second part 4b of the ac busbar by an individual switched connection that includes a fourth transformer 14d that is shown as a design that is identical to the first transformer 14a.

The power system shown in Figure 1 is typical of the power system used for marine propulsion but it will be readily appreciated that many detailed variations are possible. For example, more or fewer ac generators may be provided depending on the required power. The power system may include more or fewer thruster drives or motor drives depending on the specific propulsion requirements. The motor drives may also use either two motors mounted as separate units, two motors physically built in the same housing (conventionally called a tandem motor), or a single motor with two different power converters supplying two independent windings. These all retain the redundancy features already described.

The ac motors can be of any type including induction, wound field synchronous, permanent magnet synchronous and superconducting motors.

A low voltage (LV) busbar 16 is divided into a first part 16a and a second part 16b by a breaker section 18. The implementation possibilities for the LV breaker section 16 are the same as those previously detailed for the breaker section 6.

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The first part 16a of the LV busbar is connected to the first part 4a of the ac busbar by an individual switched connection that includes a first transformer 20a and the second part 16b of the LV busbar is connected to the second part 4b of the ac busbar by an individual switched connection that includes a second transformer 20b. Two low voltage loads 22 are shown as being attached to the LV busbar 16, but any number of loads or circuit designs for the low voltage loads can be incorporated.

If the voltages of the ac busbar 4 and LV busbar 16 are the same then the transformers 20a and 20b may be omitted.

The use of individual switched connections throughout the power system circuit means that each component can be electrically isolated from the ac busbar 4 in the event of a fault or for servicing or maintenance. For example, if the first power converter 12a needs to be serviced, then the switched connection that includes the first transformer 14a can be broken to electrically isolate the first power converter 12a from the first part 4a of the ac busbar. In this situation, the first dual motor drive 10a can continue to operate using the separate ac motor that is interfaced to the second part 4b of the ac busbar through the second power converter 12b.

Each of the power converters 12a to 12d includes a diode rectifier 24 that is shown as a 24-pulse design (i.e. with four separate diode bridges) that receives an ac input voltage from the ac busbar 4 through the associated transformer. The dc output voltage of the diode rectifier 24 is fed via a dc link 26 to an inverter 28. The integrated assembly of the diode rectifier 24, dc link 26 and inverter 28 can be provided by an MV3000-DFE power converter supplied by Converteam Ltd of Boughton Road, Rugby, Warwickshire, CV21 IBU, United Kingdom. The diode rectifier 24 can also be configured with a 6-pulse design, with a 12-pulse design or an 18-pulse design instead of the 24-pulse design shown in Figure 1.

The ac output voltage of each inverter 28 is supplied to the associated ac motor of the dual motor drives 10a and 10b. The inverters 28 shown in Figure 1 are two-level

pulse width modulation (PWM) inverters but a variety of different inverters can be used with this power system such as a three-level neutral point clamped design.

The inverters 28 can be implemented as one or more modules in parallel depending on the required rating and can be arranged with or without a dynamic breaking chopper depending on whether or not there is a requirement to provide for braking of the propeller shaft.

As an alternative to a PWM voltage source design, the power converters 12a to 12d can be dc power converters, cycloconverter power converters or load commutated current source power converters.

A modified power system with active rectifiers for a marine vessel (sometimes called an "active front end" or "AFE" power system) is shown in Figure 2. The active rectifier power system of Figure 2 is essentially the same as the power system of Figure 1 and like parts have been given the same reference numerals. The difference lies in the construction of the power converters 12a to 12d where the diode rectifiers 24 of Figure 1 have been replaced with active rectifiers 30. For example, the active rectifiers 30 can be two- or three-level PWM rectifiers implemented using an MV3000-AFE or MV7000-AFE power converter, respectively. Both the MV3000- AFE or MV7000-AFE power converters are supplied by Converteam Ltd of Boughton Road, Rugby, Warwickshire, CV21 IBU, United Kingdom.

The active rectifiers 30 and the inverters 28 can have the same topology and may be implemented as one or more modules in parallel.

Each power converter 12a to 12d is still connected to the ac busbar 4 by a switched connection that includes a transformer 32a to 32d but the active rectifiers 30 produce essentially sinewave ac supply currents with a low level of PWM harmonics and this permits the use of standard three-phase power transformers 32a that are simpler and physically smaller than the transformers 14a to 14d shown in the power system of Figure 1.

With an active rectifier topology, power can flow in either direction and the power to stop the propellers that are driven by the first and second dual motor drives 10a and 10b can be returned to the ac busbar 4. The power converter circuits can be arranged with or without a dynamic breaking chopper.

Some filter circuits 34 are provided in the switched connection between each of the power converters 12a to 12d and the ac busbar 4. These filter circuits 34 are needed to filter the PWM voltage harmonics from the active rectifiers 30 before the active rectifiers are connected to the ac input voltage. The filters can be implemented on a per drive or on a group basis as part of the ac busbar loads.

The transformers 32a to 32d are used to ensure that if an earth fault occurs then its effects are constrained to a limited part of the power system and that the other parts of the power system can continue to operate without disruption. For example, if an earth fault occurs in an active rectifier 30 then the associated inverter 28 can be shutdown but the other parts of the power system will continue to operate as normal.

Although the transformers 32a to 32d are simpler and physically smaller than the transformers 14a to 14d shown in the power system of Figure 1, they must still be rated to at least the output power of each of the ac generators 2a to 2d. This means that they are still physically large. There is therefore a need for a simplified power system circuit that is capable of being implemented using transformers of a lower rating so that the overall cost and size of the transformers can be made as small as possible.

For marine vessels that have high power requirements, the current rating of all the parallel-connected ac generators can be very high for fault conditions and in some cases may exceed the rating of commercially available switching devices. A simplified power system circuit with a significantly lower fault current rating is therefore desirable as it would permit both the use of lower cost commercially available switching units and systems with increased power levels.

Summary of the Invention

The present invention addresses these problems by providing a power system (optionally for use in a marine vessel) comprising a first ac generator connected to a first ac busbar, a second ac busbar connected to the first ac busbar by a first mid-bus transformer, a first power converter connected to the first ac busbar, and a first ac motor connected to the first power converter, wherein the first mid-bus transformer does not have a phase shift.

The first mid-bus transformer can have a rating that is considerably lower (typically less than half) than the rating of the transformers 32a to 32d mentioned above for the same generator output power as it only has to transmit the difference in the power required between the first ac busbar and the second ac busbar. The first power converter and the first ac motor can receive power from either the first ac busbar directly or from the second ac busbar via the first mid-bus transformer.

In practice, the first ac busbar will be connected to a first winding of the first mid-bus transformer and the second ac busbar will be connected to a second winding of the first mid-bus transformer. There is no direct electrical connection between the first and second ac busbars and the distribution of power between the first and second ac busbars takes place by transformer action.

The first mid-bus transformer can be formed as part of the first ac busbar or as part of the second ac busbar. At least one of the first and second ac busbars may be connected to the first mid-bus transformer by a switched connection (i.e. an electrical connection that incorporates a switch, circuit breaker, protective switchgear or the like). For example, if the first mid-bus transformer is formed as part of the first ac busbar then the second ac busbar may be connected to the first mid-bus transformer by a switched connection. Alternatively, the first mid-bus transformer may be formed as a separate component and may be connected to the first ac busbar by a switched connection and to the second ac busbar by a switched connection.

Earth faults in the parts of the power system that are associated with the first ac busbar (i.e. on one electrical side of the first mid-bus transformer) will not affect the parts of the power system associated with the second ac busbar (i.e. on the opposite electrical side of the first mid-bus transformer) or vice versa. This means that the transformers 32a to 32d mentioned above are no longer required and can be omitted from the power system of the present invention. In other words, the power system of the present invention does not require a transformer to be located between the first power converter and the first ac busbar.

The first ac generator is preferably connected to the first ac busbar by a switched connection to selectively electrically isolate the first ac generator from the first ac busbar.

The power system may further include a second ac generator connected to the second ac busbar. The first mid-bus transformer does not require any phase shift for the power system to function. This is important as it simplifies the transformer design and also simplifies the control of the first and second ac generators, as they will operate at the same frequency and phase. The second ac generator is preferably connected to the second ac busbar by a switched connection to selectively electrically isolate the second ac generator from the second ac busbar.

The first power converter is preferably connected to the first ac busbar by a switched connection to selectively electrically isolate the first power converter from the first ac busbar. A filter circuit is preferably connected to, or included in, the switched connection to filter the ac input voltage to the first power converter, but may also be implemented as part of the first ac busbar loads. The first power converter preferably includes an active rectifier, a dc link and an inverter. The active rectifier and inverter may be of any suitable type but will preferably be operating using a pulse width modulation (PWM) strategy and will typically have a two-level or a three-level neutral point clamped design.

The power system may further include a second power converter connected to the second ac busbar and a second ac motor connected to the second power converter. The second power converter is preferably connected to the second ac busbar by a switched connection to selectively electrically isolate the second power converter from the second ac busbar. A filter circuit is preferably connected to, or included in, the switched connection to filter the ac input voltage to the second power converter, but may also be implemented as part of the second ac busbar loads. The second power converter preferably includes an active rectifier, a dc link and an inverter. The active rectifier and inverter may be of any suitable type but will preferably be operating using a pulse width modulation (PWM) strategy and will typically have a two-level or a three-level neutral point clamped design.

It will be readily appreciated that additional ac generators and power converters may be connected to the first ac busbar and the second ac busbar as necessary. An ac motor may be connected to each power converter. It is also possible for one or more thruster drives or other electrical loads to be connected to the first ac busbar and the second ac busbar.

Additional ac busbars may be connected in series to the first and second ac busbars using mid-bus transformers in the manner described above. Each additional ac busbar may be connected to one or more ac generators and/or one or more power converters and/or one or more thruster drives and/or other electrical loads as necessary.

For example, the power circuit may further include a third ac busbar connected to the second ac busbar by a second mid-bus transformer. The second mid-bus transformer can be formed as part of the second ac busbar or as part of the third ac busbar. At least one of the second and third ac busbars may be connected to the second mid-bus transformer by a switched connection. Alternatively, the second mid-bus transformer may be formed as a separate component and may be connected to the second ac busbar by a switched connection and to the third ac busbar by a switched connection.

A fourth ac busbar may be connected to the third ac busbar by a third mid-bus transformer. At least one of the third and fourth ac busbars may be connected to the third mid-bus transformer by a switched connection. Alternatively, the third mid-bus transformer may be designed as a separate component and may be connected to the third ac busbar by a switched connection and to the fourth ac busbar by a switched connection.

It is generally preferred that the power system includes a breaker section so that the series-connected ac busbars can be selectively divided into two parts, namely a first part on one electrical side of the breaker section and a second part on the opposite electrical side of the breaker section. The breaker section preferably includes one or more switches.

A third ac generator may be connected to the third ac busbar, preferably by a switched connection to selectively electrically isolate the third ac generator from the third ac busbar. A fourth ac generator may be connected to the fourth ac busbar, preferably by a switched connection to selectively isolate the fourth ac generator from the fourth ac busbar.

A third power converter may be connected to the third ac busbar by a switched connection to selectively electrically isolate the third power converter from the third ac busbar. A filter circuit is preferably connected to, or included in, the switched connection to filter the ac input voltage to the third power converter, but may also be implemented as part of the third ac busbar loads. The third power converter preferably includes an active rectifier, a dc link and an inverter. The active rectifier and inverter may be of any suitable type but will preferably be operating using a pulse width modulation (PWM) strategy and will typically have a two-level or a three-level neutral point clamped design.

A third ac motor is preferably connected to the third power converter.

A fourth power converter may be connected to the fourth ac busbar by a switched connection to selectively electrically isolate the fourth power converter from the fourth ac busbar. A filter circuit is preferably connected to, or included in, the switched connection to filter the ac input voltage to the fourth power converter, but may also be implemented as part of the fourth ac busbar loads. The fourth power converter preferably includes an active rectifier, a dc link and an inverter. The active rectifier and inverter may be of any suitable type but will preferably be operating using a pulse width modulation (PWM) strategy and will typically have a two-level or a three-level neutral point clamped design.

A fourth ac motor is preferably connected to the fourth power converter.

The ac motors can be of any suitable type (induction, synchronous etc.) and have any suitable phase (three-phase etc).

The following power flows are possible for the power system circuit described above depending on which one or more of the first, second, third and fourth ac generators are connected and supplying power to .their associated ac busbars:

(i) The first power converter and the first ac motor can receive power from either the first ac busbar directly; from the second ac busbar via the first mid-bus transformer; from the third ac busbar via the second mid-bus transformer, the second ac busbar, and the first mid-bus transformer; from the fourth ac busbar via the third mid-bus transformer, the third ac busbar, the second mid-bus transformer, the second ac busbar, and the first mid-bus transformer; or a combination thereof.

(ii) The second power converter and the second ac motor can receive power from either the second ac busbar directly; from the first ac busbar via the first mid-bus transformer; from the third ac busbar via the second mid-bus transformer; from the fourth ac busbar via the third

mid-bus transformer, the third ac busbar, and the second mid-bus transformer; or a combination thereof.

(iii) The third power converter and the third ac motor can receive power from either the third ac busbar directly; from the first ac busbar via the first mid-bus transformer, the second ac busbar, and the second mid- bus transformer; from the second ac busbar via the second mid-bus transformer; from the fourth ac busbar via the third mid-bus transformer; or a combination thereof, (iv) The fourth power converter and the fourth ac motor can receive power from either the fourth ac busbar directly; from the first ac busbar via the first mid-bus transformer, the second ac busbar, the second mid-bus transformer, the third ac busbar, and the third mid-bus transformer; from the second ac busbar via the second mid-bus transformer, the third ac busbar, and the third mid-bus transformer; from the third ac busbar via the third mid-bus transformer; or a combination thereof.

A pair of the first, second, third and fourth ac motors may form part of a dual motor drive using two separate motors that are adapted to drive a common shaft. In the case of a power system for a marine vessel the common shaft may be a propeller shaft, for example. A pair of the first, second, third and fourth ac motors may also form part of a dual motor drive with the two motors physically built or located in a common housing (e.g. a tandem motor) and adapted* to drive a common shaft. A pair of the first, second, third and fourth ac motors may also be combined to have two separate windings in the common stator of a single electrical machine.

It is generally preferred that the ac motors that are paired together to form part of a dual motor drive, or are combined to have two separate windings in the common stator of a single electrical machine, a re associated with ac busbars on opposite electrical sides of the previously described breaker section.

The use of first, second and third mid-bus transformers reduces the fault level so that more conventional and lower priced switchgear can be used for the connection between the series-connected ac busbars.

Drawings

Figure 1 is a schematic diagram of a first conventional power system;

Figure 2 is a schematic diagram of a second conventional power system; and

Figure 3 is a schematic diagram of a power system according to the present invention.

Figure 3 shows a power system of the present invention for use in a marine vessel. It is similar to the power systems of Figures 1 and 2 and like parts have been given the same reference numerals. The difference lies in the location of mid-bus transformers 38a, 38b and 38c between the various parts of the ac busbar 36 and the omission of the transformers 32a to 32d.

More particularly, the power system of Figure 3 has a modified ac busbar 36 that is broken up into four separate sections. The first ac generator 2a supplies power through an individual switched connection to a first ac busbar 36a. The second ac generator 2b supplies power through an individual switched connection to a second ac busbar 36b. The third ac generator 2c supplies power through an individual switched connection to a third ac busbar 36c. The fourth ac generator 2d supplies power through an individual switched connection to a fourth ac busbar 36d. The four ac generators 2a to 2d are synchronised in phase as there are no phase shifts in the mid- bus transformers 38a, 38b and 38c.

The first ac busbar 36a and the second ac bus 36b are connected together by a first mid-bus transformer 38a. More particularly, the first ac busbar 36a is connected to one winding of the first mid-bus transformer 38a and the second ac busbar 36b is connected to another winding of the first mid-bus transformer 38 a. There is no direct electrical connection between the first ac busbar 36a and second ac busbar 36b. The power system circuit includes a switch to allow the first ac busbar 36a and the second

ac busbar 36b to be electrically isolated from each other in case of a fault or for maintenance and repair.

The first mid-bus transformer 38a and the associated switch could be part of either the first ac busbar 36a or the second ac busbar 36b.

The second ac busbar 36b is connected to the third ac busbar 36c by a second mid-bus transformer 38b. More particularly, the second ac busbar 36b is connected to one winding of the second mid-bus transformer 38b and the third ac busbar 36c is connected to another winding of the second mid-bus transformer 38b. There is no direct electrical connection between the second ac busbar 36b and the third ac busbar 36c

The power system circuit includes a switch to allow the second ac busbar 36b and the third ac busbar 36c to be electrically isolated from each other in case of a fault or for maintenance and repair.

The second mid-bus transformer 38b and the associated switch could be part of either the second ac busbar 36b or the third ac busbar 36c.

The third ac busbar 36c also includes a breaker section 6 as described above to feed power to a LV busbar 16.

The LV busbar 16 is divided into a first part 16a and a second part 16b by a breaker section 18. The first part 16a of the LV busbar is connected to the third ac busbar 36c on one side of the breaker section 6 by an individual switched connection that includes a first transformer 20a. The second part 16b of the LV busbar is connected to the third ac busbar 36c on the other side of the breaker section 6 by an individual switched connection that includes a second transformer 20b. The LV busbar 16 will therefore continue to receive power from either the second ac generator 2b or the third ac generator 2c through the individual switched connection if the breaker section 6 is activated.

The third ac busbar 36c is connected to the fourth ac busbar 36d by a third mid-bus transformer 38c. More particularly, the third ac busbar 36c is connected to one winding of the third mid-bus transformer 38c and the fourth ac busbar 36d is connected to another winding of the third mid-bus transformer 38c. There is no direct electrical connection between the third ac busbar 36c and the fourth ac busbar 36d.

The power system circuit includes a switch to allow the third ac busbar 36c and the fourth ac busbar 36d to be electrically isolated from each other in case of a fault or for maintenance and repair.

The third mid-bus transformer 38c and the associated switch could be part of either the third ac busbar 36c or the fourth ac busbar 36d.

The first and second thruster drives 8a and 8b and the first and second dual motor drives 10a and 10b are as described above with reference to Figures 1 and 2. The power converters 12a to 12d are the same as those used in the AFE power system of Figure 2.

Moving the transformers 32a to 32d (Figure 2) from the individual switched connections between the various power converters 12a to 12d and the ac busbar 4 into the ac busbar 36 itself (Figure 3) means that the rating of each transformer can be considerably reduced. In fact, the rating of the mid-bus transformers 38a to 38c in the power system of the present invention can be less than half of the rating of the transformers 32a to 32d for the same generator output power. This is because the mid- bus transformers only have to carry differences in the required currents.

The transformers 38a to 38c are therefore cheaper and smaller. If the power system is implemented in a marine vessel this can mean increased space within the hull and weight reductions lead to better fuel economy. The transformers 38a to 38c can be of simple construction and do not require any phase shift capabilities. For most of the time during operation of the power system, and in particular when the generators 2a to

2d are at full power output, the transformers 38 do not carry any current and so losses are reduced.

The lack of any phase shift in the mid-bus transformers 38a to 38c means that the ac generators 2a, 2b, 2c and 2d can be synchronised to the same frequency and phase which simplifies the control of the generators. The movement of power between the series-connected ac busbar sections is easily implemented by small changes in the voltages of the generators. The lack of a phase shift in the mid-bus transformers 38a to 38c also means that each of the mid-bus transformers can be implemented by using a standard commercial product.

The second power converter 12c may receive power directly from the second ac generator 2b via the second ac busbar 36b. However, the second power converter 12c may also receive power from: (i) the first ac generator 2a via the first ac busbar 36a, the first mid-bus transformer 38a and the second ac busbar 36b; (ii) the third ac generator 2c via the third ac busbar 36c, the second mid-bus transformer 38b and the second ac busbar 36b; and (iii) the fourth ac generator 2d via the fourth ac busbar 36d, the third mid-bus transformer 38c, the third ac busbar 36c, the second mid-bus transformer 38b and the second ac busbar 36b. Equivalent power flows can be described for the first, third and fourth power converters 12a, 12c and 12d.

The provision of the breaker section 6 in the location shown in Figure 3 facilitates the operation of an LV power system circuit since when the switches are closed in the breaker section 6 the same voltage is applied to either transformer 20a and 20b. This also means that the power system voltage can be controlled to the required nominal value at the breaker section 6 and any voltage changes required to share current need only be made in the ac busbars.

The division of the ac busbar 36 into two or more sections offers increased flexibility for redundancy and will allow other parts to operate normally when one part of the ac busbar is experiencing a fault or is undergoing maintenance or repair.

The power system shown in Figure 3 also retains all of the proven benefits of the power systems shown Figures 1 and 2 such as low harmonics, constant LV levels and the ability to transfer power between sections and with a significantly smaller set of transformers.

The use of the mid-bus transformers 38a to 38c reduces the fault currents so that lower rated and cheaper switchgear can be used for all the switches connected to the ac bus.

This also permits the use of power systems with an overall higher power rating.

It will be readily appreciated that many variations to the power system circuit of Figure 3 are possible. Some typical examples for marine propulsion would include:

(i) The power system circuit may have four ac busbar sections each with one power converter that feeds a motor driving its own propeller, giving a marine vessel with four propellers.

(ii) The power system circuit may have four ac busbar sections each with one power converter that feeds a motor driving its own thruster, giving a marine vessel with four thrusters drives.

(iii) The power system circuit may have four ac busbar sections each with one power converter that feeds a motor. Two motors drive propellers and two motors drive thrusters, giving a marine vessel with two propellers and two thrusters. (iv) The power system circuit may have four ac busbar sections each with two power converters. Each power converter feeds a motor driving its own thruster, giving a marine vessel with eight thrusters.

(v) The power system circuit may have eight ac busbar sections each with one power converter. Each power converter feeds a motor driving its . own thruster, giving a marine vessel with eight thrusters.

(vi) The power system circuit may have two ac busbar sections each with one power converter. Each power converter feeds a motor driving its own thruster, giving a marine vessel with two thrusters.

(vii) The power system circuit may have two ac busbar sections each with one power converter. Each power converter feeds a motor driving its own propeller, giving a marine vessel with two propellers, (viii) The power system circuit may have two ac busbar sections each with one power converter. Each power converter feeds a motor driving a common propeller, giving a marine vessel with one propeller.

Although the power system of the present invention has been described with reference to marine propulsion, it will be readily appreciated that the same principles can be used in aircraft-based or land-based applications, for example in an oil production system located away from a conventional ac supply grid that is using several dedicated ac generators connected to a number of power converters and ac motors.