TWO CONVERTERS AND NESTING OF THEIR CELLS

TECHNICAL FIELD The present disclosure generally relates to power converters. In particular it relates to a method of connecting converter cells of a first power converter to converter cells of a second power converter, and to a power converter arrangement connected by means of the method.

BACKGROUND A direct current (dc) electrical machine may be connected to a power converter working as an electronic commutator. Two of these systems may be arranged in a back-to-back connection to create a continuously variable transmission which for example can be used to interconnect a diesel generator with a propeller, on a ship. In large vessels, traditionally diesel motors are used to drive the propellers. However, this direct connection does not allow operating the diesel in its highest efficiency point. Therefore, more and more the diesel(s) are connected to generators that generate medium voltage AC voltage. The propellers are then driven by medium voltage electromotors, using power converters to regulate their speed. Apart from allowing operating the diesels and the propellers much more efficiently, a further advantage of this system is that an on-board grid is created, from which auxiliaries can be supplied and which allows for flexibility in operating more or less diesels in parallel.

However, the motors, generators and power converters take up a significant amount of space, and are expensive. Furthermore, the used converters typically have either not optimal efficiency, e.g. IGBT-based pulse-width modulated (PWM) converters have an efficiency of only around 96%, or they have limited controllability, e.g. a line-commutated inverter (LCI) does not allow good control of the propeller at low speed and causes large ripple and harmonic losses in the machine.

An example of a back-to-back connection of a generator 1 and a motor 3 is shown in Fig. la. The generator 1 is connected to the motor 3 by means of two power converters 2a and 2b which according to the example is a rectifier and an inverter, respectively. The windings of the generator 1 are connected to a number of series-connected converter cells la-id of the power converter 2a. The converter cells la-id of the power converter 2a are series-connected with converter cells 4a-4d of the power converter 2b. In particular, all of the converter cells la-id and 4a-4d are connected in series. The converter cells 4a-4d of the power converter 2b are connected to the windings of the motor 3, thus forming the back-to-back connection.

Due to the series connection of the converter cells belonging to each of the power converters 2a, 2b, the induced voltages in each of the windings add up. Thus, for example for the system depicted in Fig. la, the voltage between the top conductor 5 and the bottom conductor 7 between the two power converters 2a and 2b is in the order of 4 times the voltage in each winding, as shown in Fig. lb.

Fig. ib shows the cascade connection, i.e. series connection, of converter cells la-id and 4a-4d of the back-to-back connection shown in Fig. la with the generator and motor removed for clarity, with example voltages shown at various points. According to the example shown in Fig. lb, each converter cell la-id, 4a-4d has a voltage rating of 1000 V. Although the exemplified system has a ground point, it can be seen that the voltage between the top conductor 5 and the bottom conductor 7 is 4000 V. The top conductor 5 and the bottom conductor 7 must hence be dimensioned for a voltage level of 4000 V.

It is however not only these two cables that need to be dimensioned for such a high voltage. Each machine winding, being close to the machine iron, i.e. ground, must be insulated to withstand at least half of this voltage assuming that the machine iron has a potential that is in the middle between the highest and the lowest potential in the system.

This causes a high cost of the windings, and also increases the size of the machine, since part of the slots is taken up by insulation material instead of conductors.

SUMMARY

In view of the above, an object of the present disclosure is to provide a method of series connecting converter cells of a first power converter to converter cells of a second power converter in a manner which reduces the overall requirements of the electrical insulation.

Hence, according to a first aspect of the present disclosure there is provided A method of connecting N converter cells of a first power converter with N converter cells of a second power converter to form a single chain of 2N converter cells, wherein the method comprises connecting alternatingly the converter cells of the first power converter with the converter cells of the second power converter.

By means of this connection scheme the maximal induced voltage in conductors connecting the first power converter with the second power converter may be reduced up to a factor N, N being the number of converter cells in each power converter. The insulation requirements on the machine windings as well as of the cables connecting the converter cells may thereby be significantly reduced. As a result electrical machine cost and size may be reduced.

Furthermore, beneficially, control of the first power converter and the second power converter does not have to be adapted in view of prior art solutions utilising cascaded converter cells (CCC).

The proposed "CCC" concept combines very good controllability with very high efficiency, as well as with low cost of the inverter. According to one embodiment the connecting comprises a) connecting for each converter cell of the first power converter a first terminal to a first terminal of a respective converter cell of the second power converter, wherein the converter cells of the first power converter are numbered according to their order of connection with a converter cell of the second power converter and wherein each converter cell of the second power converter has a number corresponding to the number of the converter cell of the first power converter to which it is connected, b) connecting for each of N-i subsequent numbered converter cells of the first power converter a second terminal to a second terminal of a respective converter cell of the second power converter, wherein each converter cell of the second power converter to which the second terminal of a converter cell of the first power converter is connected has a number that is directly preceding lower or directly subsequent higher than the number of the converter cell of the second power converter to which the first terminal of that converter cell of the first power converter is connected, and c) connecting a second terminal of the remaining converter cell of the first power converter to a second terminal of the remaining converter cell of the second power converter.

By means of this connection scheme the maximal induced voltage in conductors connecting the first power converter with the second power converter may be reduced by a factor N, N being the number of converter cells in each power converter.

According to one embodiment the converter cells of the first power converter are arranged subsequently one after the other. According to one embodiment the converter cells of the second power converter are arranged subsequently one after the other.

According to one embodiment in step c) the remaining converter cell of the first power converter is the highest numbered converter cell of the

subsequently arranged converter cells of the first power converter. According to one embodiment in step c) the remaining converter cell of the second power converter is the lowest numbered converter cell of the subsequently arranged converter cells of the second power converter.

According to one embodiment one of the first power converter and the second power converter is a rectifier and the other is an inverter.

According to a second aspect of the present disclosure there is provided a power converter arrangement comprising a first power converter comprising N converter cells, a second power converter comprising N converter cells, wherein the converter cells of the first power converter and the converter cells of the second power converter are connected alternatingly, forming a single chain of 2N converter cells.

According to one embodiment each converter cell of the first power converter has a first terminal connected to a first terminal of a respective converter cell of the second power converter, wherein each converter cell of the first power converter is associated with a number and each converter cell of the second power converter is associated with a number corresponding to the number associated with that converter cell of the first power converter to which it is connected via its first terminal, wherein each of N-i subsequent numbered converter cells of the first power converter has a second terminal connected to a second terminal of a respective converter cell of the second power converter, wherein each converter cell of the second power converter to which the second terminal of a converter cell of the first power converter is connected has a number that is directly preceding lower or directly

subsequent higher than the number of the converter cell of the second power converter to which the first terminal of that converter cell of the first power converter is connected, wherein a second terminal of the remaining converter cell of the first power converter is connected to a second terminal of the remaining converter cell of the second power converter.

According to one embodiment the converter cells of the first power converter are arranged subsequently one after the other. According to one embodiment the converter cells of the second power converter are arranged subsequently one after the other.

According to one embodiment the remaining converter cell of the first power converter is the highest numbered converter cell of the subsequently arranged converter cells of the first power converter.

According to one embodiment the remaining converter cell of the second power converter is the lowest numbered converter cell of the subsequently arranged converter cells of the second power converter.

According to one embodiment one of the first power converter and the second power converter is a rectifier and the other is an inverter.

According to one embodiment the first power converter and the second power converter are arranged within the same housing.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, etc., unless explicitly stated otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS The specific embodiments of the inventive concept will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. la depicts an example of a prior art back-to-back connection of a generator and a motor;

Fig. lb shows an example of voltage distribution across the converter cells of the power converters in Fig. la;

Figs 2a-b show examples of connections of converter cells of two power converters according to the present disclosure; Figs 3a-3b show circular diagrams of the connection of the power converters in Figs 2a- 2b, respectively; and

Fig. 4 is a flow chart of a method of series-connecting N converter cells of a first power converter with N converter cells of a second power converter to obtain 2N series-connected converter cells.

DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplifying

embodiments are shown. The inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description. In the following, a method of series-connecting a first set of converter cells forming part of a first power converter and a second set of converter cells forming part of a second power converter will be described. By means of this connection scheme, insulation requirements of the on the machine windings as well as of the cables connecting the converter cells may be significantly reduced while the control scheme of existing cascade-connected converter cell solutions may be maintained.

The method presented herein is based on alternatingly connecting 2*N converter cells, with every other converter cell, i.e. N converter cells, forming part of a first power converter and the remaining N converter cells forming part of a second power converter. All of the converter cells are thus cascade- connected, i.e. series-connected, with the advantage that the maximal induced voltage in conductors connecting the first power converter with the second power converter may be reduced by a factor N. The method may be applied to any type of electrical machine, e.g. a rotating electrical machine or a linear electrical machine, generator and motor.

The first power converter may be an inverter or a rectifier. The second power converter may be an inverter or a rectifier. In a typical variation, one of the power converters is a rectifier and the other one is an inverter. In this case, the rectifier is connectable to a generator and the inverter is connectable to a motor.

Fig. 2a depicts a first example of a power converter arrangement 9a. The power converter arrangement 9a comprises a first power converter 11a and a second power converter lib. The first power converter 11a comprises a plurality of converter cells 1-4. In Fig. 2a, the first power converter 11a is exemplified comprising four converter cells; it should however be noted that a first power converter according to the present disclosure may comprise any number N of converter cells. The second power converter 11b comprises a plurality of converter cells ι'-4'. In Fig. 2a, the second power converter lib is exemplified comprising four converter cells; it should however be noted that a second power converter according to the present disclosure may comprise any number N of converter cells. The number N is however the same for both the first power converter 11a and the second power converter 11b.

The converter cells 1-4 and ι'-4' may be any type of suitable converter cells, for example H-bridge cells, and they may utilise semiconductor switches e.g. IGBTs, IGCTs or thyristors, for switching operations.

Each converter cell 1-4 of the first power converter 11a has a first terminal and a second terminal. Each converter cell ι'-4' of the second power converter 11b has a first terminal and a second terminal. In Figs 2a and 2b, the first terminals are exemplified by the upper connection and the second terminals are exemplified by the lower connection for each converter cell 1-4, ι'-4'. Each converter cell 1-4 of the first power converter 11a is arranged to be connected by means of its first terminal to a first terminal of a converter cell i'-4' of the second power converter 11b. Each converter cell 1-4 of the first power converter 11b is arranged to be connected by means of its second terminal to a second terminal of a converter cell ι'-4' of the second power converter 11b.

Each converter cell 1-4 of the first power converter 11a is connected to two different converter cells ι'-4' of the second power converter 11b. Each converter cell 1-4 of the first power converter 11a is hence connected to one converter cell ι'-4' of the second power converter lib by means of its first terminal and to another converter cell ι'-4' of the second power converter 11b by means of its second terminal.

Each converter cell 1-4 of the first power converter 11a may be associated with a number. Each converter cell ι'-4' of the second power converter 11b may also be associated with a number. Two converter cells, i.e. one of the converter cells 1-4 of the first power converter 11a and one of the converter cells i'-4' of the second power converter 11b, which are connected by means of their respective first terminal may be associated with corresponding numbers. Thus, for example converter cell 1 of the first power converter 11a is connected via its first terminal to converter cell 1' of the second power converter lib, in particular to the first terminal of converter cell 1'. These two converter cells may thus for example be associated with the number 1. In the examples shown in both Fig. 2a and 2b, the converter cells are numbered in increasing order from top to bottom. Each of N-i subsequent numbered converter cells 1-4 of the first power converter 11a has a second terminal connected to a second terminal of a respective converter cell ι'-4' of the second power converter 11b. It should be noted that the first of the N-i converter cells 1-4 must not necessarily start with the lowest numbered or the highest numbered converter cell. Each converter cell ι'-4' of the second power converter lib to which the second terminal of a converter cell 1-4 of the first power converter 11a is connected has a number that is directly preceding lower or directly

subsequent higher than the number of the converter cell ι'-4' of the second power converter lib to which the first terminal of that converter cell of the first power converter 11a is connected.

Thus, according to the example in Fig. 2a, the second terminal of converter cell 1 of the first power converter 11a is connected to converter cell 2' of the second power converter 11b. The second terminal of converter cell 2 of the first power converter 11a is connected to converter cell 3' of the second power converter lib and so on until converter cell 3. This corresponds to N-i subsequent numbered converter cells 1-4 of the first power converter 11a having their second terminal connected to a power converter ι'-4' of the second power converter lib, having a number that is directly subsequent higher than the number of the converter cell i'-'4 of the second power converter lib to which the first terminal of that converter cell of the first power converter 11a is connected.

Fig. 2b illustrates a power converter arrangement 9b with the other alternative mentioned above, i.e. when each converter cell ι'-4' of the second power converter 11b to which the second terminal of a converter cell 1-4 of the first power converter 11a is connected has a number that is directly preceding lower than the number of the converter cell ι'-4' of the second power converter lib to which the first terminal of that converter cell of the first power converter 11a is connected. A second terminal of the remaining converter cell 4 of the first power converter 11a is connected to a second terminal of the remaining converter cell 1' of the second power converter 11b. According to the example in Fig. 2a, converter cell 4, i.e. the highest numbered converter cell of the first power converter 11a, has its second terminal connected to the second terminal of the lowest numbered converter cell 1' of the second power converter 11b. In both of Figs 2a and 2b, example voltage ratings of ιοοο V of the converter cells 1-4, i'-4' are shown, in a similar manner as in the prior art example shown Fig. lb. In Figs 2a and 2b, it can be seen that the maximum voltage level in any conductor is 1000 V, instead of 4000 V as in the prior art with four converter cells per power converter. As previously mentioned, the maximal induced voltage in conductors connecting the first power converter with the second power converter may be reduced by a factor N, which in the present example is 4.

The converter cells 1-4 of the first power converter 11a may be arranged subsequently one after the other, as shown in for example Figs 2a and 2b. It is however of course optional to arrange the converter cells 1-4 in this manner.

The remaining converter cell 4 of the first power converter 11a may for example be the highest numbered, as shown in Fig. 2a, or the lowest numbered, converter cell 1-4 of the subsequently arranged converter cells 1-4 of the first power converter 11a, as shown in Fig. 2b.

The converter cells ι'-4' of the second power converter 11b may be arranged subsequently one after the other, as shown in for example Figs 2a and 2b. It is however of course optional to arrange the converter cells 1-4 in this manner.

The remaining converter cell 1' of the second power converter may be the lowest numbered converter cell ι'-4' of the subsequently arranged converter cells i'-4' of the second power converter lib in case the remaining converter cell 4 of the first power converter 11a is the highest numbered of the converter cells 1-4 of the first power converter 11a. In case the remaining converter cell of the first power converter 11a is the lowest numbered of the converter cells 1-4 of the first power converter 11a, then the remaining converter cell 1' of the second power converter may be the highest numbered converter cell ι'-4' of the subsequently arranged converter cells ι'-4' of the second power converter 11b. Figs 3a and 3b show a circular diagram of the connections of Fig. 2a and Fig. 2b, respectively. It can be seen that these connections indeed present a cascade connection scheme of the converter cells 1-4 and ι'-4'. The converter cells 1-4 and ι'-4' are connected in an alternating manner. With reference to Fig. 4 a method of series-connecting N converter cells of a first power converter 11a with N converter cells of a second power converter 11b to form a single chain of 2N converter cells will now be described.

In a step a) the first terminal of each converter cell 1-4 of the first power converter 11a is connected to a first terminal to a first terminal of a respective converter cell ι'-4' of the second power converter 11b.

The converter cells 1-4 of the first power converter 11a are numbered according to their order of connection with a converter cell ι'-4' of the second power converter 11b. Each converter cell ι'-4' of the second power converter 11b has a number corresponding to the number of the converter cell 1-4 of the first power converter 11a to which it is connected.

Thus, for example converter cell 1 of the first power converter 11a may first be connected to converter cell 1' of the second power converter 11b. In this case, converter cell 1 of the first power converter 11a receives, or is associated with, a number 1, and converter cell 1' of the second power converter lib similarly receives, or is associated with, a number 1. It should be noted that the numbering provided is solely for being able to keep track of the connection scheme, and thus it is not important whether the converter cells are associated with numbers, letters or any other symbols for the purpose of performing the scheme presented herein. In a similar manner as described above, converter cell 2 of the first power converter 11a is next connected to converter cell 2' of the second power converter 11b, wherein both converter cells 2 and 2' are associated with a number 2. This process continuous for all converter cells 1-4, ι'-4'. In a step b) for each of N-i subsequent numbered converter cells of the first power converter a second terminal is connected to a second terminal of a respective converter cell of the second power converter. Each converter cell of the second power converter to which the second terminal of a converter cell of the first power converter is connected has a number that is directly preceding lower or directly subsequent higher than the number of the converter cell of the second power converter to which the first terminal of that converter cell of the first power converter is connected.

In a step c) a second terminal of the remaining converter cell 4 of the first power converter 11a is connected to a second terminal of the remaining converter cell 1' of the second power converter 11b.

According to one variation, high power density may be reached by integrating the converters with the machines. In case the two machines and their power converters can be constructed closely together or in an integrated fashion, it is especially advantageous to interconnect the converter cells as presented herein.

For the control of the system, as well as for the current in and voltage across each of the machine windings, there is no difference between this new concept and the prior art shown in Figure la. However, for the insulation required between the coils and the machine iron, and between the coils, the method and power converter arrangement according to the present disclosure has a significant advantage in that the insulation in each coil can be significantly reduced.

In case it is desired to limit the number of interconnections between the two converters, intermediate solutions may be used, in which for example an interconnection is made after every second cell.

Furthermore, it should be noted that similar configurations can be realized also for the case where the CCC converter consists of several branches in parallel. Parallel connection of branches, in which each branch contains winding coils from all of the phases, is another way of limiting the maximum potential occurring in the system.

The proposed system can be used in marine applications, but also in other applications in which two shafts rotating at different independent speeds are to be interconnected. The proposed system is specifically suited for high- power applications, typically in the range from 1-10 MW and upwards.

The inventive concept has mainly been described above with reference to a few examples. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.