Grounding scheme for modular embedded multilevel converter

A power converter includes at least one leg with a first string including a plurality of controllable semiconductor switches, a first connecting node, and a second connecting node, wherein the first string is operatively coupled across a first bus and a second bus. The at least one leg also includes a second string operatively coupled to the first string via the first connecting node and the second connecting node, wherein the second string includes a plurality of switching units. The first string includes a first branch and a second branch, wherein the second branch is operatively coupled to the first branch via a third connecting node and the third connecting node is coupled to a ground connection.

BACKGROUND

Embodiments of invention relates to power converters and more specifically to a multilevel converter.

In the last few decades, the field of power conversion has grown tremendously due to its imminent advantages in motor drives, renewable energy systems, high voltage direct current (HVDC) systems, and the like. The multilevel converter is emerging as a promising power conversion technology for various medium and high voltage applications.

Multilevel converters offer several advantages over an ordinary two-level converter. For example, the power quality of the multilevel converter is better than that of two level converters. Also, the multilevel converters are ideal for interface between a grid and renewable energy sources such as photovoltaics (PV), fuel cells, wind turbines, and the like. In addition, the efficiency of the multilevel converter is relatively higher as a result of its minimum switching frequency.

In the recent times, the multilevel converters having a modular structure and without transformers have been designed. The modular structure of the converters, allows stacking of these converters to an almost unlimited number of levels. Also, the modular structure aids in scaling up to different power and voltage levels. One example of such type of multilevel converters is a modular multilevel converters (MMC) which employees a large number of fully controllable semiconductor switches, such as insulated gate bipolar transistors (IGBTs).

Grounding is an important aspect of multilevel converters. A ground point or earth point refers to a node in the multilevel converter from which various node voltages are measured. Generally, the ground point determines voltage insulation ratings of various components in multilevel converter. Furthermore, the voltage insulation ratings are determined based on maximum voltage respect to search ground a particular component may observe during normal conditions and faults.

BRIEF DESCRIPTION

In accordance with an embodiment of the present technique, a power converter including at least one leg is provided. The at least one leg includes a first string comprising a plurality of controllable semiconductor switches, a first connecting node, and a second connecting node, wherein the first string is operatively coupled across a first bus and a second bus. The at least one leg further includes a second string operatively coupled to the first string via the first connecting node and the second connecting node, wherein the second string includes a plurality of switching units. Furthermore, the first string includes a first branch and a second branch, wherein the second branch is operatively coupled to the first branch via a third connecting node and the third connecting node is coupled to a ground connection.

In accordance with another embodiment of the present technique, a system for power conversion is provided. The system includes a power source, a load and a first power converter. The first power converter includes one or more legs, wherein each of the one or more legs includes a first string comprising a plurality of controllable semiconductor switches, a first connecting node, and a second connecting node, wherein the first string is operatively coupled across a first bus and a second bus. A second string is operatively coupled to the first string via the first connecting node and the second connecting node, wherein the second string comprises a plurality of switching units. Furthermore, the first string comprises a first branch and a second branch, and the second branch is operatively coupled to the first branch via a third connecting node; the third connecting node being coupled to a ground connection. The first power converter further includes a controller configured to control switching of the plurality of controllable semiconductor switches and the plurality of switching units.

DETAILED DESCRIPTION

As will be described in detail hereinafter, various embodiments of an exemplary system for power conversion and method for power conversion are presented. By employing the power converter and the method for power conversion described hereinafter, a multilevel converter with a grounding scheme is provided. In one example, the power converter may include a modular multilevel embedded converter. The term multilevel converter, as used herein, is used to refer to a converter that converts one form of input voltage/current to another form of output voltage/current with very low distortion.

Turning now to the drawings, by way of example inFIG. 1, a system100for converting power is depicted. In one embodiment, the system100for converting power may include a source102, a power converter104, and a grid/utility/load106. The term source, as used herein, is used to refer to a renewable power source, a non-renewable power source, a generator, a grid, and the like. Also, the term load, as used herein, may be used to refer to a grid, an electrical appliance, and the like. In addition, the power converter104may be a multilevel converter. In one embodiment, the source102may be operatively coupled to a first terminal (not shown) of the power converter104. A second terminal (not shown) of the power converter104may be operatively coupled to the load106. The first terminal and the second terminal may be alternatively employed as an input terminal or an output terminal of the power converter104.

Also, the system100may include a controller108. The controller108may be configured to control the operation of the power converter104, in one embodiment. By way of example, the controller108may be configured to control the operation of the power converter104by controlling switching of a plurality of semiconductor switches of the power converter104. Furthermore, in one embodiment, the system100may also include other circuit components (not shown) such as, but not limited to, a circuit breaker, an inductor, a compensator, a capacitor, a rectifier, a reactor, a filter, and the like.

Referring now toFIG. 2, a diagrammatical representation of an exemplary embodiment of a modular embedded multilevel converter (MEMC)300for use in the system ofFIG. 1according to aspects of the present disclosure is depicted. In one embodiment, MEMC300includes three legs301,303and305respectively. Furthermore, each leg301,303and305of the MEMC may include a first string302and a second string304. It should be noted that even though inFIG. 2, certain referral numerals are shown for only one leg301, they can be equally applicable to other two legs303and305. More particularly, the first string302may be operatively coupled to the second string304to form the leg301. Furthermore, the first string302may be operatively coupled between a first bus306and a second bus308. In one embodiment, the first bus306may include a positive DC bus and the second bus308may include a negative DC bus. The second string304may be operatively coupled to the first string302via a first connecting node310and a second connecting node312. Also, the first string302may include a first branch314operatively coupled to a second branch316via a third connecting node318. Similarly, the second string304may include a first portion320operatively coupled to a second portion322via an AC phase326and an inductor324. In an embodiment, inductor324is a split inductor i.e., inductor324is split into two parts. The third connecting node318may be operatively coupled to a third bus328.

In addition, the first leg301may be operatively coupled to the second leg303via the third connecting node318. Furthermore, in one example, the third connecting nodes318of each of the three first strings302may be operatively coupled to each other to form a third bus328. As noted hereinabove, the third bus328may be a middle or center DC bus. However, in another embodiment, for applications in machine drives, the third connecting nodes318of each of the three first strings302may be operatively coupled to a neutral bus. Moreover, the three legs301,303,305may be operatively coupled between the first bus306and the second bus308.

In one embodiment, the third bus328may be at a negative potential with respect to the first bus306and at a positive potential with respect to the second bus308. Also, the first string302may include a plurality of controllable semiconductor switches330. In the example ofFIG. 2, the plurality of controllable semiconductor switches may include partially controllable semiconductor switches. However, in another embodiment, the plurality of controllable semiconductor switches may include fully controllable semiconductor switches. Moreover, the plurality of controllable semiconductor switches may include a combination of partially controllable semiconductor switches and fully controllable semiconductor switches. By way of a non-limiting example, the first string302may include partially controllable semiconductor switches, fully controllable semiconductor switches, or a combination of partially controllable semiconductor switches and fully controllable semiconductor switches. Furthermore, in one example, the first branch314of the first string302may include two controllable semiconductor switches330. Similarly, the second branch316of the first string302may include two controllable semiconductor switches330. The controllable semiconductor switches330may include a power diode332in combination with a thyristor333or a silicon controlled rectifier, a gate turnoff thyristor, an IGBT, and the like.

The inductors324in each leg301,303and305are operatively coupled to at least one alternating current (AC) phase (e.g., A, B, and C). In addition, the first portion320and the second portion322of the second string304may include a plurality of switching units334connected in series to each other. The switching unit334may be a combination of a plurality of fully controllable semiconductor switches and an energy storage device. The fully controllable semiconductor switches may include an insulated gate bipolar transistor (IGBT), a metal oxide semiconductor field effect transistor (MOSFET), a field effect transistor (FET), a gate turn-off thyristor, an insulated gate commutated thyristor (IGCT), an injection enhanced gate transistor (IEGT), a silicon carbide based switch, a gallium nitride based switch, a gallium arsenide based switch, or equivalents thereof.

Referring now toFIG. 3, diagrammatical representation400of an exemplary embodiment of a switching unit such as the switching unit334ofFIG. 2is depicted. In the presently contemplated configuration, the switching unit400may include fully controllable semiconductor switches402and404, an energy storage device406, a first connector408, and a second connector410. As previously noted, the fully controllable semiconductor switches402,404may include an IGBT, a MOSFET, a FET, an IEGT, a gate turn-off thyristor, an IGCT, a silicon carbide based switch, a gallium nitride based switch, a gallium arsenide based switch, or equivalents thereof. Moreover, each of the fully controllable semiconductor switches402,404, may also include a power diode412that may be inbuilt and antiparallel to the fully controllable semiconductor switches402and404. The inbuilt power diodes412may provide a freewheeling path. These power diodes412may also be referred to as freewheeling diodes.

Also, in one non-limiting example, the energy storage device406may include a capacitor. In the example ofFIG. 3, the fully controllable semiconductor switch402may be operatively coupled in series to the energy storage device406to form a first limb414. Also, the other fully controllable semiconductor switch404forms a second limb416. The second limb416may be operatively coupled in parallel to the first limb414. Additionally, the first limb414and the second limb416may be operatively coupled between the first connector408and the second connector410. Although the example ofFIG. 3depicts the switching units400in a half bridge converter configuration as including two fully controllable semiconductor switches, and one energy storage device, use of other numbers of fully controllable semiconductor switches402,404, and energy storage devices406is also contemplated. In one embodiment, some or all of the switching units may be arranged to form a full bridge converter configuration.

Furthermore, in one non-limiting example, when the fully controllable semiconductor switch402is activated and the fully controllable semiconductor switch404is deactivated, the energy storage device406may appear across the first connector408and the second connector410. Consequently, the charge across the energy storage device406appears as a voltage across the first connector408and the second connector410. Alternatively, when the fully controllable semiconductor switch404is activated and the fully controllable semiconductor switch402is deactivated, the first limb414is bypassed, thereby providing zero voltage across the first connector408and the second connector410. Hence, by controlling the switching of the fully controllable semiconductor switches402and404in the plurality of switching units334on the second string304ofFIG. 2, the voltage developed across the second string304may be regulated.

Referring toFIG. 4(a), a diagrammatical representation of a leg502, such as the leg301ofFIG. 2, in a first state of switching of the controllable semiconductor switch is presented. The first state may also be referred to as a positive state. The leg502may include a first string504and a second string506. Also, the leg502may be operatively coupled between a first bus508and a second bus510. As noted hereinabove, the first bus508may include a positive DC bus and the second bus510may include a negative DC bus. Furthermore, the first string504may be operatively coupled to the second string506via a first connecting node512and a second connecting node514. Controllable semiconductor switches330(FIG. 2) of first string are all labeled as S1, S2, S3 and S4 respectively.

In addition, a first portion, such as the first portion320ofFIG. 2of the second string506and a second portion, such as the second portion322ofFIG. 2of the second string506may be represented by voltage sources Vp516and Vn518, respectively. As noted hereinabove, the second string506may include a plurality of switching units (not shown). The first portion of the second string506and the second portion of the second string506may be operatively coupled via an alternating current phase520. Also, the first string504may include a third connecting node522, which may be operatively coupled to a third bus524. Also, in the presently contemplated configuration the first string504includes four controllable semiconductor switches represented as S1, S2, S3and S4. Additionally, the voltage at the first bus508may be represented as +Vdcand the voltage at the second bus510may be represented as −Vdc. By way of example, the voltage of +Vdcat the first bus508and the voltage of −Vdcat the second bus510may be with respect to a virtual ground. Also, the voltage at the third bus524may be represented as Vmid, and the voltage at the alternating current phase may be represented as Vac.

As depicted inFIG. 4(a), during the first state of switching, the controllable semiconductor switches S1and S3are activated, while the controllable semiconductor switches S2and S4are maintained in a deactivated state. The activation of controllable semiconductor switches S1and S3provides a first current flow path526between the first bus508and the third bus524via a corresponding second string506. Consequently, the second string506may be operatively coupled between the first bus508and the third bus524in the positive state. Furthermore, while the first current flow path526is established, the voltage across the first bus508and the third bus524may depend on the switching of the fully controllable semiconductor switches corresponding to the plurality of switching units in the second string506, such as the switching units334ofFIG. 2. The current flowing through the first current flow path526is represented as Idc.

In a similar fashion,FIG. 4(b)is a diagrammatical representation528of a leg in a second state of switching of the controllable semiconductor switches. The second state of switching of the controllable semiconductor switches may also be referred to as a negative state. For ease of understanding,FIG. 4(b)is explained with reference toFIG. 4(a). In the second state, the controllable semiconductor switches S2and S4may be activated, while controllable semiconductor switches S1and S3are deactivated. The activation of the controllable semiconductor switches S2and S4may result in providing a second current flow path530between the third bus524and the second bus510. Accordingly, the second string506may be operatively coupled between the second bus510and the third bus524in the negative state.

Similarly,FIG. 4(c)is a diagrammatical representation532of a leg in a third state of switching of the controllable semiconductor switches. The third state of switching of the controllable semiconductor switches may also be referred to as a zero state. For ease of understanding,FIG. 4(c)is explained with reference toFIG. 4(a). Furthermore, in the third state, the controllable semiconductor switches S2and S3may be activated, while the controllable semiconductor switches S1, and S4are deactivated. The activation of the controllable semiconductor switches S2and S3may result in providing a third current flow path534. Subsequently, the current flows in the third current flow path534. This third current flow path534may also be referred to as a freewheeling path. In addition, both ends of the second string506may be operatively coupled to each other via the activated controllable semiconductor switches S2and S3and the third bus524. Although,FIGS. 4(a)-4(c)represent the three states of switching with reference to a single leg, these three states of switching may be employed simultaneously for a plurality of legs in a two phase power converter, a three phase power converter, and the like.

It should be noted that any power converter system needs a grounding point to reduce the insulation level requirement. For example, for a conventional modular multilevel converter, additional passive components are needed to create a grounding point either at alternating current (AC) or direct current (DC) side and these passive components need to be rated for the full system voltage. In accordance with an embodiment of the present technique, a grounding scheme for a MEMC is disclosed.

Referring toFIG. 5, a diagrammatical representation600of an exemplary embodiment of a grounding scheme for a MEMC ofFIG. 2, according to aspects of the present disclosure, is depicted. In the example ofFIG. 5, the MEMC600includes three legs301,303,305. As inFIG. 2, each leg may include a respective first and second string302and304. Furthermore, the first string302includes the first branch314and the second branch316.

The mid-point326of the second string304may be operatively coupled to a fourth bus which may be an alternating current (AC) phase. In particular, each of the three legs301,303,305may be associated with at least one AC phase. In a non-limiting example, a three phase AC system may include an AC phase-A, an AC phase-B, and an AC phase-C. Additionally, a first terminal (not shown) may be formed by a combination of the first bus306and the second bus308. The first terminal may also be referred to as a DC terminal. Also, the AC phases, AC phase-A, AC phase-B, and AC phase-C in combination may form a second terminal (not shown). The second terminal may also be referred to as an AC terminal.

In addition, the first leg301may be operatively coupled to the second leg303via the third connecting node318. In one embodiment, the third connecting node318may be the mid-point node or center point node of the first string302. Furthermore, in one example, the third connecting nodes318of each of the three first strings302may be operatively coupled to each other to form the third bus328. In one embodiment the third bus328is connected to an earth or ground connection604via grounding impedance602. This results in each of the third connecting nodes318being connected to ground connection604via grounding impedance602. The design of grounding impedance602depends on various parameters such as but not limited to an allowable ground current, soil conditions, and radio interference with surrounding instruments or even voltage across MEMC600.

The design of grounding impedance602affects voltage insulation ratings of various components of MEMC600. To achieve different design criteria, the impedance of the grounding network may have different impedance values at different system frequencies. In one embodiment, a value of the grounding impedance602may be about zero ohms for dc current and very high impedance for high frequency currents, i.e., there may an inductive grounding impedance between the third bus328and the ground connection604. In another embodiment, the third bus328may be connected to the ground connection604directly, i.e., without any grounding impedance. In such a case, the third bus328is always at near zero voltage, which results in easy design of voltage blocking levels for first branch314and second branch316.

As depicted inFIG. 4at any instant in time, the second string304is operatively coupled between the first bus306and third bus328, between the third bus328and the second bus308, or both ends of the second string304may be operatively coupled to a third bus328. Now as shown inFIG. 5, if the third bus328is connected to ground connection604, the second string304may have to withstand a maximum voltage of value Vdc assuming a voltage across first bus206and third bus328to be equal to Vdc. Accordingly, for effective control of the power converter, the first portion of the second string304and the second portion of the second string304may each have to withstand a maximum voltage of Vdc. Accordingly, the rating of each switching unit of the second string304may be only Vdc/N1, where N1is the number of switching units in each of the first and second portions of the second string304. Hence, the rating of each switching unit may be 2Vdc/N, where N is the number of switching units in the second string304and N=2N1. Furthermore, controllable semiconductor switches330in first string302may each be rated at Vdc/2.