Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
OPERATION OF AN AC/DC MODULAR MULTILEVEL CONVERTER (M2LC)
Document Type and Number:
WIPO Patent Application WO/2018/206074
Kind Code:
A1
Abstract:
It is presented a method for operating a power converter for converting power between a HVDC connection and an AC connection. The power converter comprises a plurality of converter cells arranged in converter arms between poles of the HVDC connection, wherein each converter cell comprises a switching element and an energy storage element. The method is performed in a power converter controller and comprises the steps of: determining an insertion index for each converter arm, the insertion index indicating how many converter cells, of the respective converter arm, should be active, wherein the insertion index varies over time based on both the voltage of the HVDC connection and the voltage amplitude of the AC connection, wherein the voltage of the HVDC connection differs from the voltage amplitude of the AC connection; and controlling the converter cells of the converter arms based on the respective insertion index.

Inventors:
RAY, Swakshar (102 Malligai Street, Ambal Nagar Porur,TAMIL NADU, CHENNAI 6, 600116, IN)
KASAL, Gaurav (EF2 Navin Jayaram Garden, ManapakkamTAMIL NADU, Chennai 6, 600116, IN)
KUMAR-NAYAK, Khirod (House no 59, CRRpuram 1st Main RoadManapakkam, TAMIL NADU, Chennai 5, 600125, IN)
Application Number:
EP2017/060910
Publication Date:
November 15, 2018
Filing Date:
May 08, 2017
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
ABB SCHWEIZ AG (Brown Boveri Strasse 6, 5400 Baden, 5400, CH)
International Classes:
H02M7/483; H02M1/00
Other References:
KALLE ILVES: "Modeling and design of modular multilevel converters for grid applications", 1 January 2014 (2014-01-01), Stockholm, XP055275727, ISBN: 978-91-7-595293-2, Retrieved from the Internet [retrieved on 20170101]
LI YALONG ET AL: "The Impact of Voltage-Balancing Control on Switching Frequency of the Modular Multilevel Converter", IEEE TRANSACTIONS ON POWER ELECTRONICS, INSTITUTE OF ELECTRICAL AND ELECTRONICS ENGINEERS, USA, vol. 31, no. 4, 1 April 2016 (2016-04-01), pages 2829 - 2839, XP011592279, ISSN: 0885-8993, [retrieved on 20151126], DOI: 10.1109/TPEL.2015.2448713
None
Attorney, Agent or Firm:
WALL, Christian (ABB AB, Intellectual PropertyForskargränd 7, Västerås, 721 78, SE)
Download PDF:
Claims:
CLAIMS

1. A method for operating a power converter for converting power between a high voltage direct current, HVDC, connection and an alternating current, AC, connection, the power converter comprising a plurality of converter cells arranged in converter arms between poles of the HVDC connection, wherein each converter cell comprises a switching element and an energy storage element, the method being performed in a power converter controller (14) and comprising the steps of:

determining (50) an insertion index for each converter arm, the insertion index indicating how many converter cells, of the respective converter arm, should be active, wherein the insertion index varies over time based on both the voltage of the HVDC connection and the voltage amplitude of the AC connection, wherein the voltage of the HVDC connection differs from the voltage amplitude of the AC connection; and

controlling (52) the converter cells of the converter arms based on the respective insertion index.

2. The method according to claim 1, wherein the step of determining (50) an insertion index comprises determining the insertion index with an upper limit of one and a lower limit according to:

where Ud represents half the voltage between the poles of the HVDC connection and Vs represents the voltage amplitude on the AC connection.

3. The method according to any one of the preceding claims, wherein the sum of the voltage of all energy storage elements of each converter arm is equal to the sum of the voltage of the HVDC connection and the voltage amplitude of the AC connection.

4. The method according to any one of the preceding claims, wherein the step of controlling the converter cells comprises controlling the voltage of each converter arm in accordance with:

^ (r + (l - / )) , where Uc represents the sum of the voltage of all energy storage elements of the converter arm, r = m * sin ωΐ, m represents a modulation index of switches of the converter cells of the converter arm, ω represents angular frequency and t represents time, k = 2(^c Ud and Ud represents half the voltage between the poles of the HVDC connection.

5. The method according to any one of the preceding claims, wherein each converter cell is a half bridge converter cell.

6. A power converter controller (14) for operating a power converter for converting power between a high voltage direct current, HVDC, connection and an alternating current, AC, connection, the power converter comprising a plurality of converter cells arranged in converter arms between poles of the HVDC connection, wherein each converter cell comprises a switching element and an energy storage element, the power converter controller (14) comprising:

a processor (60); and

a memory (64) storing instructions (67) that, when executed by the processor, cause the power converter controller to:

determine an insertion index for each converter arm, the insertion index indicating how many converter cells, of the respective converter arm, should be active, wherein the insertion index varies over time based on both the voltage of the HVDC connection and the voltage amplitude of the AC connection, wherein the voltage of the HVDC connection differs from the voltage amplitude of the AC connection; and

control the converter cells of the converter arms based on the respective insertion index.

7. The power converter controller (14) according to claim 6, wherein the instructions to determine an insertion index comprise instructions (67) that, when executed by the processor, cause the power converter controller to determine the insertion index with an upper limit of one and a lower limit according to: l8

ud+vs '

where Ud represents half the voltage between the poles of the HVDC connection and Vs represents the voltage amplitude on the AC connection.

8. The power converter controller (14) according to claim 6 or 7, wherein the sum of the voltage of all energy storage elements of each converter arm is equal to the sum of the voltage of the HVDC connection and the voltage amplitude of the AC connection.

9. The power converter controller (14) according to any one of claims 6 to 8, wherein the instructions to control the converter cells comprise

instructions (67) that, when executed by the processor, cause the power converter controller to control the voltage of each converter arm in

accordance with:

^ (r + (l - /c)) ,

where Uc represents the sum of the voltage of all energy storage elements of the converter arm, r = m * sin ωΐ, m represents a modulation index of switches of the converter cells of the converter arm, ω represents angular frequency and t represents time, k = 2(^c Ud and Ud represents half the voltage between the poles of the HVDC connection.

10. The power converter controller (14) according to any one of claims 6 to 9, wherein each converter cell is a half bridge converter cell.

11. A power converter (1) comprising the power converter controller (14) according to any one of claims 6 to 10, wherein the power converter controller (14) is configured to control the power converter (1).

12. A computer program (67, 91) for operating a power converter for converting power between a high voltage direct current, HVDC, connection and an alternating current, AC, connection, the power converter comprising a plurality of converter cells arranged in converter arms between poles of the HVDC connection, wherein each converter cell comprises a switching element and an energy storage element, the computer program comprising computer program code which, when run on a power converter controller (14) causes the power converter controller (14) to:

determine an insertion index for each converter arm, the insertion index indicating how many converter cells, of the respective converter arm, should be active, wherein the insertion index varies over time based on both the voltage of the HVDC connection and the voltage amplitude of the AC connection, wherein the voltage of the HVDC connection differs from the voltage amplitude of the AC connection; and

control the converter cells of the converter arms based on the respective insertion index.

13. A computer program product (64, 90) comprising a computer program according to claim 12 and a computer readable means on which the computer program is stored.

Description:
OPERATION OF AN AC/DC MODULAR MULTILEVEL CONVERTER (M2LC)

TECHNICAL FIELD

The invention relates to a method and a power converter controller for operating a power converter for converting power between a high voltage direct current, HVDC, connection and an alternating current, AC, connection.

BACKGROUND

High voltage power conversion between DC and AC are known in the art for a variety of different applications. One such application is related to HVDC (High Voltage DC). One way to implement power converters is using Modular Multilevel

Converter (M2C), being a form of Voltage Source Converter (VSC). The M2C comprises a number of converter cells, each comprising switching elements and an energy storage element. The converter cells are controlled to collectively generate a sinusoidal voltage on an AC connection. One major cost factor in such converters are the number of converter cells that are required. Any reduction in number of converter cells is of great benefit.

SUMMARY

It is an object to reduce the number of converter cells required in a power converter at least in some circumstances.

According to a first aspect, it is presented a method for operating a power converter for converting power between a high voltage direct current, HVDC, connection and an alternating current, AC, connection. The power converter comprises a plurality of converter cells arranged in converter arms between poles of the HVDC connection, wherein each converter cell comprises a switching element and an energy storage element. The method is performed in a power converter controller and comprises the steps of: determining an insertion index for each converter arm, the insertion index indicating how many converter cells, of the respective converter arm, should be active, wherein the insertion index varies over time based on both the voltage of the HVDC connection and the voltage amplitude of the AC connection, wherein the voltage of the HVDC connection differs from the voltage amplitude of the AC connection; and controlling the converter cells of the converter arms based on the respective insertion index.

The step of determining an insertion index may comprise determining the insertion index with an upper limit of one and a lower limit according to: u d +v s '

where Ud represents half the voltage between the poles of the HVDC connection and V s represents the voltage amplitude on the AC connection.

The sum of the voltage of all energy storage elements of each converter arm may be equal to the sum of the voltage of the HVDC connection and the voltage amplitude of the AC connection.

The step of controlling the converter cells may comprise controlling the voltage of each converter arm in accordance with:

^ (r + (l - /c)) ,

where Uc represents the sum of the voltage of all energy storage elements of the converter arm, r = m * sin ωΐ, m represents a modulation index of switches of the converter cells of the converter arm, ω represents angular frequency and t represents time, k = 2( ^ c Ud and Ud represents half the voltage between the poles of the HVDC connection.

Each converter cell may be a half bridge converter cell.

According to a second aspect, it is presented a power converter controller for operating a power converter for converting power between a high voltage direct current, HVDC, connection and an alternating current, AC, connection, the power converter comprising a plurality of converter cells arranged in converter arms between poles of the HVDC connection, wherein each converter cell comprises a switching element and an energy storage element. The power converter controller comprises: a processor; and a memory storing instructions that, when executed by the processor, cause the power converter controller to: determine an insertion index for each converter arm, the insertion index indicating how many converter cells, of the respective converter arm, should be active, wherein the insertion index varies over time based on both the voltage of the HVDC connection and the voltage amplitude of the AC connection, wherein the voltage of the HVDC connection differs from the voltage amplitude of the AC connection; and control the converter cells of the converter arms based on the respective insertion index.

The instructions to determine an insertion index may comprise instructions that, when executed by the processor, cause the power converter controller to determine the insertion index with an upper limit of one and a lower limit according to: u d +v s '

where Ud represents half the voltage between the poles of the HVDC connection and V s represents the voltage amplitude on the AC connection.

The sum of the voltage of all energy storage elements of each converter arm may be equal to the sum of the voltage of the HVDC connection and the voltage amplitude of the AC connection.

The instructions to control the converter cells may comprise instructions that, when executed by the processor, cause the power converter controller to control the voltage of each converter arm in accordance with:

^ (r + (l - /c)) ,

where Uc represents the sum of the voltage of all energy storage elements of the converter arm, r = m * sin ωΐ, m represents a modulation index of switches of the converter cells of the converter arm, ω represents angular frequency and t represents time, k = 2( ^ c Ud and Ud represents half the voltage between the poles of the HVDC connection.

Each converter cell may be a half bridge converter cell. According to a third aspect, it is presented a power converter comprising the power converter controller according to the second aspect, wherein the power converter controller is configured to control the power converter.

According to a fourth aspect, it is presented a computer program for operating a power converter for converting power between a high voltage direct current, HVDC, connection and an alternating current, AC, connection, the power converter comprising a plurality of converter cells arranged in converter arms between poles of the HVDC connection, wherein each converter cell comprises a switching element and an energy storage element. The computer program comprises computer program code which, when run on a power converter controller causes the power converter controller to: determine an insertion index for each converter arm, the insertion index indicating how many converter cells, of the respective converter arm, should be active, wherein the insertion index varies over time based on both the voltage of the HVDC connection and the voltage amplitude of the AC connection, wherein the voltage of the HVDC connection differs from the voltage amplitude of the AC connection; and control the converter cells of the converter arms based on the respective insertion index.

According to a fifth aspect, it is presented a computer program product comprising a computer program according to the fourth aspect and a computer readable means on which the computer program is stored.

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, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawings, in which:

Fig l is a schematic diagrams illustrating an environment where

embodiments of the present invention can be applied;

Fig 2 is a flow chart illustrating a method for controlling the power converter of Fig l;

Fig 3 is a schematic drawing illustrating a converter cell of Fig l when implemented as half bridge cell; and Fig 4 is a schematic drawing illustrating components of the power converter controller of Fig l according to one embodiment;

Fig 5 shows one example of a computer program product comprising computer readable means.

DETAILED DESCRIPTION

The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention 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 invention to those skilled in the art. Like numbers refer to like elements throughout the description.

Fig 1 is a schematic diagrams illustrating an environment where

embodiments of the present invention can be applied. Fig 1 shows a power converter 1 in the form of a Modular Multilevel Converter (M2C). The power converter 1 comprises one bridge leg for each phase. Fig 1 illustrates one such bridge leg 52. Additional phases (such as for a three phase system) are configured in the same way, connected to the same HVDC connection but for separate phases of the AC connection.

A High Voltage Direct Current (HVDC) connection 5 is provided to the DC power is provided using a positive DC bus DC+ and a negative DC bus DC-. The voltage between the two DC poles DC + and DC- is denoted 2 * Ud. A power converter controller 14 is connected to a plurality of converter cells 9a- h (connections between the power converter controller 14 and the converter cells are omitted din order not to obscure the figure). In some embodiments, the power converter controller 14 is implemented as a valve control unit (VCU).

An upper converter arm 11a of the bridge leg comprises a first set of converter cells 9a-9d, and a lower converter arm 11b of the bridge leg comprises a second set of converter cells 9e-h. All converter cells 9a-9h are controlled by control signal from the power converter controller 14. This allows the power converter controller 14 to e.g. effect pulse width modulation (PWM) to generate an alternating current on an AC terminal 40. The signal on the AC terminal is represented by V s *sin(a)t), whereby V s is the amplitude, ω is the angular frequency and t is time.

By placing several converter cells 9a-d, 9e-h in series, high voltage

applications can be supported. Moreover, the switching frequency of each converter cell can be reduced, thus reducing switching losses. It is to be noted that the number of converter cells shown here is only an example and any suitable number of converter cells can be used. Each converter cell can be a half bridge converter cell, in which case the energy storage of each converter cell can only contribute voltage of the same polarity.

An upper converter arm inductor 10a is provided between the converter cells 9a-d of the upper converter arm 11a and the AC terminal. Analogously, a lower converter arm inductor 10b is provided between the converter cells 9e- h of the lower converter arm lib and the AC terminal. According to embodiments presented herein, the cells in the converter arms are controlled based on both the voltage of the HVDC connection and the voltage amplitude of the AC connection. This is particularly applicable when these two voltages differ from each other. More details on how this can be performed is provided below. By controlling the converter cells in this manner, an implementation with fewer converter cells can be provided, thus reducing installation costs.

Fig 2 is a flow chart illustrating a method for controlling the power converter of Fig l. The method can be performed in the power converter controller of Fig l. The method is used for operating a power converter for converting power between a HVDC connection and an AC connection. As explained above, the power converter comprises a plurality of converter cells arranged in converter arms between poles of the HVDC connection. Each converter cell comprises a switching element, and an energy storage element. The switching elements can e.g. be implemented using an insulated gate bipolar transistor (IGBT), Integrated Gate-Commutated Thyristor (IGCT), a Gate Turn-Off thyristor (GTO), or any other suitable high power semiconductor component. The energy storage elements can e.g. be a capacitor, supercapacitor, etc.

In a determine insertion index step 50, an insertion index is determined for each converter arm. The insertion index indicates how many converter cells, of the respective converter arm, should be active. Significantly, the insertion index varies over time based on both the voltage of the HVDC connection and the voltage amplitude of the AC connection. In this case, the voltage of the HVDC connection differs from the voltage amplitude of the AC connection. In other words, the power converter controller requires access to both the voltage of the HVDC connection and the voltage amplitude of the AC connection.

The insertion index, 7, can e.g. be determined with an upper limit of one and a lower limit according to: where Ud represents half the voltage between the poles of the HVDC connection and V s represents the voltage amplitude on the AC connection.

In a control step 52, the converter cells of the converter arms are controlled based on the respective insertion index. The voltage of each converter arm can e.g. be controlled in accordance with:

^ (r + (l - /c)) ,

where Uc represents the sum of the voltage of all energy storage elements of the converter arm, r = m * sin ωΐ, m represents a modulation index of switches of the converter cells of the converter arm, k = 2( ^ c Ud and Ud represents half the voltage between the poles of the HVDC connection.

The sum of the voltage of all energy storage elements of each converter arm can be configured to be equal to the sum of the voltage of the HVDC connection and the voltage amplitude of the AC connection. When the voltage amplitude of the AC connection is lower than Ud, this implies a lower voltage requirement, thus reducing the number of required converter cells, as explained in more detail below.

The basis for the method will now be presented, with reference also to Fig 1, primarily with reference to the upper converter arm 11a. The suffix p in parameters indicate that the parameter in question relates to the positive converter arm, i.e. the upper converter arm 11a.

The insertion index γ ρ is a parameter which varies between o to 1, controlling how many converter cells should be connected. Hence, it can be deduced that the voltage u vp across the upper converter arm 11a can be obtained by:

Uy-p = U d —Yp U C p (l) where U cp represents the sum of all the voltage of all storage elements in the converter arm. It is to be noted that for a lower converter arm, U cn

corresponds to Ucp. A general notation covering either one of Ucn and Ucp is Uc. Any voltage drop across inductors is neglected in this calculation. Furthermore, the insertion index y p can be expressed according to: where -i<r p <i (2)

Using (2) in (1), we get where r p is m*sin ωί and m is modulation index and can be expressed as

The required converter bus voltage on the AC terminal 40 can be expressed by: u V p =V s *sin(a)t) (4) where V s < Ud. In the conventional case the arm voltage is rated to the value 2*Ud. However, according to embodiments herein, when the converter bus voltage is less than half of the DC link pole to pole voltage, i.e. V s <k, the sum voltage of the cell storage elements voltage Ucp does not need to be as high as 2*Ud. This operation can be achieved by rating U cp to Ud+V s , thereby reducing the total number of cells required in the power converter. In the following, the insertion index y p refers to a modified insertion index which is obtained in a new way compared to the prior art.

Hence, using U cp = U d + V s in (1), we can express the converter bus voltage according to: u .vp 2 U d +V s - 2Yp) (5)

Now, to obtain a sinusoidal voltage with peak value V s , the following condition is satisfied:

=>—V s < u vp < V s (6)

Now it will be presented how to achieve the required range of γ ρ . Let us suppose we go through the conventional limits of insertion index and modulation index. Using (2) in (5), we get:

_ (Ud+Vs)

v p 2 V p (u d +v s ) ) →u vp = ^ (r p + (l - k)) (7) which is not sinusoidal, where

Replacing Ud+V s with Ucpw get u vp = - U f {r p + {l - k)) (9)

The term (ι-k) is the offset which is applied at the output voltage. We can subtract the term in the modulation reference to obtain the desired output voltage.

Now after subtracting offset in equation (7) we get

As peak value of u vp is V s and r p =m sin ωΐ, Now replacing r p with [r p - (l - k) ] in (2), i.e. adding the offset, we end up with (6). Hence proved.

The same calculations hold true for the lower (negative) arm 11b. u vn = {r n - (1 - k)) (12) We can thus add the offset (1 - k) to obtain sinusoidal voltage

Now we will describe how U cp can be calculated.

The value of U cp depends upon the value of V s , i.e. the peak value of the converter bus voltage. Given the RMS (Root Mean Square) filter bus phase voltage (u/) is Ufrms and RMS converter bus phase voltage u vp is U vp rms. P and Q are the real and reactive power injected to the grid. P and Q are the real and reactive power, respectively, supplied by one converter arm and X is the arm impedance. We can then deduce: r pl = 1 u 1frms * I u J vprms χ Γι « 3

^- J

Where Χ=2πΡ ν , Lv is the inductance of the converter arm inductor and the suffix '1' denotes through any one converter arm.

Ql = Ufrms * Uvprms * ^ (l < )

When three phases are used, the active and reactive power in each converter arm are i/6 th of the total active and reactive power of the converter.

Using sin δ from (13): cos(S) = Vl - sm 2 (S) = 1(1 - Pj 2 * - . 2 ) (15)

v Ufrms * U vprms )

Replacing cos(5) in equation (14), we get:

Ufrms j Ufrms * Uy prms * X 2 — Q 1 * X (l6) u frms u vprms Γ 1 Λ u frms^ ~ ^ u frms VI Λ ~ VI vprms * + Vl 2 * x 2 + u frms + 2 * uj rms * * x)/ 2

(17)

3 (peafcj = 2 *ί/. vprms (18)

(19)

Hence, the sum-cell capacitor voltage is limited by the total active and reactive power of the system.

The inventors have simulated an example with the following values:

Rating of the converter: S = 850 MVA

Rated P = ±600 MW

Rated Q = -600 MVAr, i.e. Q is absorbed

Pole to pole DC bus voltage = 2 * U d =yoo kV

V L = 220 kV RMS Line to line grid voltage, i.e

C = 1.2 mF

Peak Valve current = 2.3 KA

Blocking current = 2.66 KA

Voltage rating for each converter cell U ce u = 17.77 kV Converter cell redundancy = 10% For a conventional method, the total number of cells required can be calculated according to: n = 2 * Ud * (1 + 10%) / Uceii = 700 kV * 1.1/17.77 kV = 43.

Using embodiments presented herein, reactive power is absorbed in this example, i.e. Q = -600MVAR and hence sing X = 2n *L v in (17), we will then get U vprms = 117 kV an kV.

Hence U cp iim = 165+ Ud = 515 kV. The total number of cells per arm is then calculated according to (U cp iim * i.i)/U C eii = 515*1.1/17.77 33.

In conclusion, using embodiments presented herein, the number of converter cells are reduced from 43 to 33, i.e. with 10/43 = 2 3 %· This is a very large reduction and implies significantly reduced cost for such an installation.

Fig 3 is a schematic drawing illustrating a converter cell of Fig 1 when implemented as half bridge cell. The converter cell 9 here comprises a leg of two serially connected switching elements 3oa-b. Each switching element 3oa-b can for example be implemented using an insulated gate bipolar transistor (IGBT), Integrated Gate-Commutated Thyristor (IGCT), a Gate Turn-Off thyristor (GTO), or any other suitable high power semiconductor component. Optionally, there is an antiparallel diode connected across each switching element 30a-b (not shown). An energy storage element 31 is also provided in parallel with the leg of switching elements 30a-b. The energy storage element 31 can e.g. be a capacitor, supercapacitor, etc. The voltage synthesised by the converter cell 2 can thus either be zero or the voltage of the energy storage element 31.

Fig 4 is a schematic drawing illustrating components of the power converter controller 14 of Fig 1 according to one embodiment. Optionally, the power converter controller 14 forms part of a host device, in which one or more components can be shared with the host device. A processor 60 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit etc., capable of executing software instructions 67 stored in a memory 64, which can thus be a computer program product. The processor 60 can be configured to execute the method described with reference to Fig 2 above. The memory 64 can be any combination of random access memory (RAM) and read only memory (ROM). The memory 64 also comprises persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid-state memory.

A data memory 66 is also provided for reading and/or storing data during execution of software instructions in the processor 60. The data memory 66 can be any combination of random access memory (RAM) and read only memory (ROM).

The power converter controller 14 further comprises an I/O interface 62 for communicating with other external entities. Other components of the power converter controller 14 are omitted in order not to obscure the concepts presented herein.

Fig 5 shows one example of a computer program product comprising computer readable means. On this computer readable means, a computer program 91 can be stored, which computer program can cause a processor to execute a method according to embodiments described herein. In this example, the computer program product is an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc. As explained above, the computer program product could also be embodied in a memory of a device, such as the computer program product 64 of Fig 4.

While the computer program 91 is here schematically shown as a track on the depicted optical disk, the computer program can be stored in any way which is suitable for the computer program product, such as a removable solid state memory, e.g. a Universal Serial Bus (USB) drive. The invention has mainly been described above with reference to a few embodiments. 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 invention, as defined by the appended patent claims