FORM 2
THE PATENTS ACT, 1970
(39 of 1970) As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
A Cascaded H-bridge Multilevel Inverter
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed:

FIELD OF THE INVENTION
This invention relates to a cascaded H-bridge multilevel inverter and, more particularly, to pre-charging circuit for direct current capacitors of such cascaded H-bridge multilevel inverter
BACKGROUND OF THE INVENTION
The application of power converter circuits which enhance the overall performance, efficiency, and reliability of industrial processes is common in all industry. There has been wide use of single and three phase diode/ thyristor rectifiers, for direct current (DC) power supplies. Adjustable Speed Drives (ASD), Uninterruptible Power Supplies (UPS), and other industrial appliances are common examples where power converters are generally used. Basically, ASDs consists of AC/DC converter connected to DC/AC inverter. However, in the current industrial sector, the demand for technologies to realize larger capacity and higher voltage power has increased. In response to such a challenge, cascaded H-bridge multilevel inverter, also known as medium voltage inverters, are currently being used in the industry with different topologies. Cascaded H-bridge multilevel inverter consist of plurality of'power cells' generally termed as 'power modules' arranged in different levels according to the requirement.
The basic components of the power cell of a voltage source inverter include a three phase diode rectifier, such as a number of rectifier bridge assemblies, associated direct current (DC) bus capacitor and a single phase inverter. The DC bus capacitor store energy and are the voltage source for the inverter. It is well known in the art that an inverter performs the opposite function of a rectifier and converts a DC voltage into a

variable voltage, variable frequency AC voltage. After the rectification process, the energy is available in the DC link capacitor for voltage source inverter. The semiconductor devices such as thyristors of the inverter bridge have the function to take this energy in DC mode and deliver it to the drive in an AC mode, as sinusoidal as possible.
A bypass device in a form of bypass switch is generally provided in all power cells to ensure that a single component failure of a diode, capacitor, or semiconductor devices, in case of cascaded power cells, can imitate closing the bypass switch effectively removing that power cell from that phase allowing continued although reduced power output.
In order to turn on the voltage inverter, the DC bus capacitor must first be charged. This process is called "pre-charge". Without pre-charge, the inrush current to power cell or medium voltage inverters is relatively very large and therefore may damage number of rectifier bridge assemblies, and may also cause upstream protective relays to operate and trip main circuit breakers.
As stated in hereinbefore, power cells are generally used in a cascaded fashion in case of medium voltage inverters, primarily to meet the power and voltage requirements. Such cascaded power cells are generally also known as cascaded H-bridge multilevel inverter or medium voltage inverters or modular multilevel converters. When a medium voltage inverter is connected to a power system, an excessive current or an inrush current is flown into the DC capacitor via diodes configuring the bidirectional chopper circuit or full bridge circuit of the power cell if the DC capacitor in every

power cell is not charged up. Various pre charging circuits are known in the art. One of the known initial charge system uses reactors (or resistors) in series with the input isolation transformers to limit the inrush current. Alternatively, resistor(s) can be placed in series with the output of the transformers, or inductor(s) can be placed in series with the transformers to limit inrush. When the DC bus capacitors are sufficiently charged, the reactors (or resistors) are removed from the circuit by a medium voltage rated contactor. A disadvantage of such known methods of reducing inrush current is the relatively large size and cost of the reactors (or resistors) and the medium voltage rated contactor.
US 7965529 discloses a pre-charge circuit for a direct current bus of a voltage source inverter, said pre-charge circuit comprising: a ferro-resonant transformer circuit comprising a primary winding structured to input a low voltage and a secondary winding structured to output a medium voltage and provide, a constant current source; and a medium voltage diode bridge circuit.
JP 3535477 discloses an initial charging method of an inverter device by providing a third winding connected to the commercial power to the main transformer, applying the commercial power to each smoothing capacitor which constitutes two or more aforementioned unit inverters via the third winding and carrying out initial charging.
The proposals described in the cited patent literature forces to modify the design of transformer. Accordingly, such arrangement will not be suitable for modular designs. Furthermore, power ratings of the external pre-charging power supply as used by JP 3535477 will required to be changed in accordance with the addition of DC bus

capacitors, i.e. with increase in the number of power cell. There has thus been a persistent need to develop a pre-charging circuit for direct current capacitors of a cascaded medium voltage inverter with reduced complexity and which eliminates the need of using a transformer for pre-charging the direct current capacitors.
SUMMARY OF THE INVENTION
The present invention provides a cascaded H-bridge multilevel inverter comprising: a plurality of groups of power cells, each power cell comprising a three phase diode rectifier; a direct current bus capacitor; a single phase inverter; and a bypass switch on output side of the power cell; wherein said power cells of each group are connected in series by bypass switches of respective power cell; and a pre-charging circuit for charging said direct current bus capacitors of each power cell; wherein said pre-charging circuit comprises a power source connected to the bypass switches on output side of the power cells through a common resistor; a plurality of switches, each connected to each group of power cells; and a microcontroller based control circuitry configured to control the switching sequence of the plurality of switches; wherein during a charging operation of the direct current bus capacitors of the power cells, an electric connection between the power source and bypass switches of the power cells is established by the switches thereby charging direct current capacitor one after another.
BRIEF DESCRIPTION OF THE DRAWINGS
The above-mentioned and other features and advantages of the various embodiments of the invention, and the manner of attaining them, will become more apparent and better understood by reference to the accompanying drawings, wherein:

Fig. 1 is a schematic circuit diagram of a power cell of a cascaded H-bridge multilevel
inverter.
Fig. 2 is a schematic diagram of an illustrative embodiment of cascaded H-bridge
multilevel inverter according to the present invention
DETAILED DESCRIPTION
Fig.l shows a circuit diagram of a power cell 1 of a cascaded H-bridge multilevel inverter. The components of the power cell 1 of a voltage source inverter include a three phase diode rectifier or a converter, such as a number of rectifier bridge assemblies 10, associated direct current (DC) bus capacitor 11 and a single phase inverter 12. The DC bus capacitors 11 store energy and are voltage source for inverter 12. The inverter such as 12, is an electronic circuit that converts DC to AC. After the rectification of AC voltage to DC by rectifier 10, the energy is available in the DC link capacitor for voltage source inverter. A bypass switch 13 is provided in the power cell 1. In order to turn on the voltage inverter 1, the DC bus capacitor 11 must first be charged. This process is called "pre-charge". As stated earlier, without pre-charge, the inrush current to power cell or medium voltage inverters is relatively very large and therefore may damage number of rectifier bridge assemblies 10, and may also cause upstream protective relays (not shown) to operate and trip main circuit breakers (not shown).
Referring to Fig.2, it represents the power circuit of a 7-level cascaded H-bridge multilevel inverter 40, which has 9 power cells altogether. Power cells 25, 26 and 27 forms the first group or level, power cells 28, 29 and 30 forms the second group or

level and power cells 31, 32 and 33 forms the third group or level. It will be appreciated that for the matter of simplicity and clarity, three level configuration has been discussed here. It may be noted that cascaded configurations with N number of power cells may also be implemented in accordance with the present invention. The cascaded H-bridge multilevel inverter 40 has 9 power cells. Each cell is fed by isolated three-phase voltages to generate a non-controlled dc-link. Each phase of the power supply output is fed by a group of series-connected power cells.
The bypass switches of each power cell of individual levels are connected to each other thereby connecting the power cells of one individual in series connection. For example, bypass switches of power cells 25, 26 and 27 of first group are interconnected to each other in series.
Further, in accordance with the present invention, power source 20 has been shown. The power source 20 may be a DC power source such as a battery, or an assembly of batteries, DC derived from AC or an AC power source such as an uninterrupted power supply (UPS).
The power source 20 is connected with a resistor 21 for limiting the voltage supplied form the power source 20. A plurality of switches 22, 23 and 24 have been connected to the external voltage source so that each switch may establish a connection between the external power source 20 and each level of the cascaded H-bridge multilevel inverter 40. The switches connect and disconnect said power supply 20 to power cells connected in series in each of levels wherein the power cells of each said levels 41, 42, 43 are connected in series with each other by bypass switch of respective power

cells. It should be noted here that power source 20 is connectable to bypass switch on output side of said power cells. A microprocessor or microcontroller based control circuitry may also be implemented to control the switching actions of bypass switch and switches connected to the power source 20. Such microcontroller based control circuitry is configured to control the switching sequence of the plurality of switches.
As discussed above, before connecting the cascaded H-bridge multilevel inverter 40 to 3 phase power supply, it is necessary to charge the direct current capacitor to avoid the inrush current to damage the inverter 40. Accordingly, during charging operation of the direct current capacitors of respective power cells, the cascaded H-bridge multilevel inverter 40 is disconnected from the main power supply. Further during a charging operation of the direct current capacitors of respective power cells, a connection between power source 20 and bypass switch of one particular power cell is established applying a charging voltage across direct current capacitor of said power cell thereby charging said capacitor and thereafter bypassing the charging voltage to another capacitor of another power cell. Since the power cells in one particular level is connected in a series fashion, therefore once the direct current capacitor of one particular cell is charged up to desired voltage then the charging voltage from the external power source is bypassed to next power cell of same level.
Once charging of direct current capacitors of one particular level of power cells is finished, switch connecting the external power supply 20 to that particular level is turned off and switch concerned with the next level of power cells is turned on. Accordingly, such switching action may continue to take place until all the direct current capacitors of each and every power cells of cascaded H-bridge multilevel

inverter is charged to desired voltage. Upon charging of all direct current capacitors of cascaded H-bridge multilevel inverter 40 is concluded, the external power source 20 is disconnected and connection to main power supply is made.
It should be noted here that at a time, the external power source 20 is connected to charge only one capacitor; accordingly the power rating of power source 20 is constant is not required to be changed when there is an addition of power cells in the cascaded H-bridge multilevel inverter. Furthermore, since the thyristor for initial charging control in each power cell can be deleted, a part of the power loss generated from the voltage drop of the thyristor element may also be deleted. Furthermore, it is apparent from the above discussion that the present invention eliminates the need for changing the power rating of external source. Additionally, the present invention does not require the re-designing of the transformer windings.
The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. Accordingly, the protection sought herein is as set forth in the claims below. Although the present invention is shown in a limited number of forms, it is not limited to just these forms, but is amenable to various changes and modifications.

We claim :
1. A cascaded H-bridge multilevel inverter comprising:
a plurality of groups of power cells, each power cell comprising a three phase diode rectifier (10); a direct current bus capacitor (11); a single phase inverter (12); and a bypass switch (13) on output side of the power cell; wherein said power cells of each group are connected in series by bypass switches of respective power cell; and
a pre-charging circuit for charging said direct current bus capacitors (11) of each power cell;
wherein said pre-charging circuit comprises a power source connected to the bypass switches on output side of the power cells through a common resistor; a plurality of switches, each connected to each group of power cells; and a microcontroller based control circuitry configured to control the switching sequence of the plurality of switches; wherein during a charging operation of the direct current bus capacitors of the power cells, an electric connection between the power source and bypass switches of the power cells is established by the switches thereby charging direct current capacitor one after another.
2. A cascaded H-bridge multilevel inverter as claimed in claim 1, wherein said power source (20) is DC.
3. A cascaded H-bridge multilevel inverter as claimed in claim 2, wherein said power source (20) is a battery or an assembly of batteries.

4. A cascaded H-bridge multilevel inverter as claimed in claim 2, wherein said power source (20) is DC derived from AC.
5. A cascaded H-bridge multilevel inverter as claimed in claim 1, wherein said power source (20) is AC.
6. A cascaded H-bridge multilevel inverter as claimed in claim 5, wherein said power source is an uninterrupted power supply (UPS).