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The closed ring string voltage balancing circuit a is connected between the DC voltage source strings a, a and a. Also, the closed ring voltage balancing circuit a, in the embodiment of FIG. The first string voltage balancing circuit a balances the voltages generated by the DC voltage source strings a and a, the second string voltage balancing circuit a balances the voltages generated by the DC voltage source strings a and a and the third string voltage balancing circuit a balances the voltages generated by the DC voltage source strings a and a, similar to the string voltage balancing circuits a and b of FIGS.

The closed ring string voltage balancing circuit a can therefore be considered as an extension of the string voltage balancing circuits a and b of FIGS. Also, an embodiment of the closed loop string voltage balancing circuit a forms the closed loop to include six switches, such as six transistors, as can include various types of transistors, for example.

As described, the first string voltage balancing circuit a is connected between the DC voltage source strings a and a to balance the voltages generated by the DC voltage source strings a and a. The first string voltage balancing circuit a includes two reverse blocking switches e. The inductor a L 1 is connected between the switches a and a, and the switches a and a divide the inductor a L 1 current between the two lines, such as according to their operating operation point, for example.

As also described, the second string voltage balancing circuit a is connected between the DC voltage source strings a and a to balance the voltages generated by the DC voltage source strings a and a. The second string voltage balancing circuit a includes two reverse blocking switches e.

The inductor a L 2 is connected between the switches a and a, and the switches a and a divide the inductor a L 2 current between the two lines, such as according to their operating operation point, for example. Also, as described, the third string voltage balancing circuit a is connected between the DC voltage source strings a and a to balance the voltages generated by the DC voltage source strings a and a.

The third string voltage balancing circuit a includes two reverse blocking switches e. The inductor a L 3 is connected between the switches a and a, and the switches a and a divide the inductor a L 3 current between the two lines, such as according to their operating operation point, for example. In the string voltage balancing circuits a, a and a, the switches a and a, the switches a and a and the switches a and a can include any of various suitable switches, such as metal-oxide semiconductor field effect transistor MOSFET switches, insulated-gate bipolar transistor IGBT semiconductor switches, various types of transistor-type switches, or any another suitable components, as can depend on the use or application, and should not be construed in a limiting sense.

The closed ring string voltage balancing circuit b forms a closed loop ring, such as illustrated in FIG. The power generation system b includes three unbalanced DC voltage source strings b, b and b, such as of PV panels or wind turbines. For example, for purposes of illustration, the DC voltage source string b can generate a kV output voltage V 1 , the DC voltage source string b can generate an 80 kV output voltage V 2 and the DC voltage source string b can generate a 90 kV output voltage V 3.

As illustrated, the DC voltage sources strings b, b, and b are connected in parallel. The DC voltage source strings b, b, and b are also connected in parallel to a load b to which an output voltage is provided. The closed ring string voltage balancing circuit b is connected between the DC voltage source strings b, b and b. Also, the closed ring voltage balancing circuit b, in the embodiment of FIG.

The first string voltage balancing circuit b balances the voltages generated by the DC voltage source strings b and b, the second string voltage balancing circuit b balances the voltages generated by the DC voltage source strings b and b and the third string voltage balancing circuit b balances the voltages generated by the DC voltage source strings b and b, similar to the string voltage balancing circuits a and b of FIGS.

The closed ring string voltage balancing circuit b can likewise be considered as an extension of the string voltage balancing circuits a and b of FIGS. Also, an embodiment of the closed loop string voltage balancing circuit b forms the closed loop to include six switches, such as six transistors, as can include various types of transistors, for example.

As described, the first string voltage balancing circuit b is connected between the DC voltage source strings b and b to balance the voltages generated by the DC voltage source strings b and b. The first string voltage balancing circuit b includes two IGBT type semiconductor switches as can be reverse blocking switches e. The first string voltage balancing circuit b also includes an inductor b L 1 , and two capacitors b C 1 and b C 3.

The inductor b L 1 is connected between the switches b and b, and the switches b and b divide the inductor b L 1 current between the two lines, such as according to their operating operation point, for example. In some embodiments, the switches b and b have reverse blocking capability or, as illustrated, can be connected to series diodes b and b configured to block reverse current to provide a reverse blocking capability, for example.

As also described, the second string voltage balancing circuit b is connected between the DC voltage source strings b and b to balance the voltages generated by the DC voltage source strings b and b. The second string voltage balancing circuit b includes two IGBT type semiconductor switches as can be reverse blocking switches e.

The second string voltage balancing circuit b also includes an inductor b L 2 , and two capacitors C 1 and C 2. The inductor b L 2 is connected between the switches b and b, and the switches b and b divide the inductor b L 2 current between the two lines, such as according to their operating operation point, for example. Also, as described, the third string voltage balancing circuit b is connected between the DC voltage source strings b and b to balance the voltages generated by the DC voltage source strings b and b.

The third string voltage balancing circuit b includes two IGBT type semiconductor switches as can be reverse blocking switches e. The third string voltage balancing circuit b also includes an inductor b L 3 , and two capacitors b C 2 and b C 3. The inductor b L 3 is connected between the switches b and b, and the switches b and b divide the inductor b L 3 current between the two lines, such as according to their operating operation point, for example.

In the string voltage balancing circuits b, b and , the switches b and b, the switches b and b and the switches b and b, in addition to being IGBT type semiconductor switches, can also include any of various suitable switches, such as metal-oxide semiconductor field effect transistor MOSFET switches, various types of transistor-type switches, or any another suitable components, as can depend on the use or application, and should not be construed in a limiting sense.

Also, as illustrated in FIG. The inductors b, b, and b can act as a filter, such as to minimize or reducing a ripple current, for example, in the power generation system b. The voltage unbalance in the lines can be translated into a negative potential on a corresponding capacitor, a positive potential on a corresponding capacitor or a zero potential on a corresponding capacitor to provide the voltage balancing, for example.

The closed loop string voltage balancing circuits a and b respectively illustrated in FIGS. It is noted that the voltage balancing circuits and topologies as illustrated in FIGS. The complimentary string voltage balancing circuit a includes two string voltage balancing circuits a and a that together form the complimentary string voltage balancing circuit a, such as illustrated in FIG. The first and second string voltage balancing circuits a and a of the complimentary string voltage balancing circuit a are respectively connected between the DC voltage source strings a, a and a.

Also, the first and second string voltage balancing circuits a and a of the complimentary string voltage balancing circuit a are each similar to the string voltage balancing circuits a and b of FIGS. The first string voltage balancing circuit a balances the voltages generated by the DC voltage source strings a and a, and the second string voltage balancing circuit a balances the voltages generated by the DC voltage source strings a and a, similar to the string voltage balancing circuits a and b of FIGS.

The complimentary string voltage balancing circuit a can therefore be considered as an extension of the string voltage balancing circuits a and b of FIGS. In the first and second string voltage balancing circuits a and a, the switches a and a and the switches a and a can include any of various suitable switches, such as metal-oxide semiconductor field effect transistor MOSFET switches, insulated-gate bipolar transistor IGBT semiconductor switches, various types of transistor-type switches, or any another suitable components, as can depend on the use or application, and should not be construed in a limiting sense.

As is illustrated in the power generation system a, the first string voltage balancing circuit a is connected between the DC voltage source strings a and a, and the second string voltage balancing circuit a is connected between the DC voltage source strings a and a, with the capacitance of the middle line, i. The complimentary string voltage balancing circuit a including the first and second string voltage balancing circuits a and a can minimize the number of components for voltage balancing compared to the closed ring string voltage balancing circuit a illustrated in FIG.

Therefore, the first and second string voltage balancing circuits a and a of the complementary string voltage balancing circuit a can balance or substantially balance the DC voltage source strings a, a and a output voltage at 90 kV, for example. A possible drawback of the complimentary string voltage balancing circuit a including the first and second string voltage balancing circuits a and a illustrated in FIG.

The complimentary string voltage balancing circuit b includes two string voltage balancing circuits b and b that together form the complimentary string voltage balancing circuit b, such as illustrated in FIG. The first and second string voltage balancing circuits b and b of the complimentary string voltage balancing circuit b are respectively connected between the DC voltage source strings b, b and b.

Also, the first and second string voltage balancing circuits b and b of the complimentary string voltage balancing circuit b are each similar to the string voltage balancing circuits a and b of FIGS. The first string voltage balancing circuit b balances the voltages generated by the DC voltage source strings b and b, and the second string voltage balancing circuit b balances the voltages generated by the DC voltage source strings b and b, similar to the string voltage balancing circuits a and b of FIGS.

The complimentary string voltage balancing circuit b can therefore be considered as an extension of the string voltage balancing circuits a and b of FIGS. The first string voltage balancing circuit b includes an inductor b L 1 , and two capacitors b C 1 and b C 2. The second string voltage balancing circuit b also includes an inductor b L 2 , and two capacitors b C 3 and b C 4. In the first and second string voltage balancing circuits b and b, the switches b and b and the switches b and b, in addition to being IGBT type semiconductor switches, can also include any of various suitable switches, such as metal-oxide semiconductor field effect transistor MOSFET switches, various types of transistor-type switches, or any another suitable components, as can depend on the use or application, and should not be construed in a limiting sense.

As is illustrated in the power generation system b, the first string voltage balancing circuit b is connected between the DC voltage source strings b and b, and the second string voltage balancing circuit b is connected between the DC voltage source strings b and b, with the capacitance of the middle line, i. The complimentary string voltage balancing circuit b including the first and second string voltage balancing circuits b and b can minimize the number of components for voltage balancing compared to the closed ring string voltage balancing circuit b illustrated in FIG.

Therefore, the first and second string voltage balancing circuits b and b of the complementary string voltage balancing circuit b can balance or substantially balance the DC voltage source strings b, b and b output voltage at 90 kV, for example. A possible drawback of the complimentary string voltage balancing circuit b including the first and second string voltage balancing circuits b and b illustrated in FIG. The complementary string voltage balancing circuits a and b respectively illustrated in FIGS.

The single star string voltage balancing circuit a is respectively connected to the DC voltage source strings a, a and a. Also, the single star string voltage balancing circuit a is similar in operation to the string voltage balancing circuits a and b of FIGS. The single star string voltage balancing circuit a can therefore be considered as an extension of the string voltage balancing circuits a and b of FIGS.

The single star string voltage balancing circuit a is respectively connected to the DC voltage source strings a, a and a to balance the voltages generated by the DC voltage source strings a, a and a. The single star string voltage balancing circuit a is respectively connected between the DC voltage sources strings a, a, and a and includes one inductor a L and three switches forming a star configuration. Also, the single star string voltage balancing circuit a can overcome limitations of a relatively large number of components and physical location differences of circuit topology location associated with the closed loop string voltage balancing circuits a and b and the complementary string voltage balancing circuits a and b, such as illustrated in FIGS.

The single star string voltage balancing circuit a includes three reverse blocking switches e. The inductor a L can act to filter out ripples in the individual string currents respectively controlled by the switches a, a and a flowing in the corresponding DC voltage source strings a, a and a. The single star string voltage balancing circuit a also includes three capacitors a C 1 , a C 2 and a C 3.

The capacitor a C 1 is connected to the switch a and is associated with the first DC voltage source string a to balance the voltage generated by the first DC voltage source string a. The capacitor a C 2 is connected to the switch a and is associated with the second DC voltage source string a to balance the voltage generated by the second DC voltage source string a.

The capacitor a C 3 is connected to the switch a and is associated with the third DC voltage source string a to balance the voltage generated by the third DC voltage source string a. In some embodiments, the switches a, a and a have reverse blocking capability or, as illustrated, can be connected to series diodes a, a and a configured to block reverse current to provide a reverse blocking capability, for example.

In the single star string voltage balancing circuit a, the switches a, a and a can include any of various suitable switches, such as metal-oxide semiconductor field effect transistor MOSFET switches, insulated-gate bipolar transistor IGBT semiconductor switches, various types of transistor-type switches, or any another suitable components, as can depend on the use or application, and should not be construed in a limiting sense.

For example, for purposes of illustration, the first DC voltage source string a can generate a kV output voltage V 1 , the second DC voltage source string a can generate an 80 kV output voltage V 2 and the third DC voltage source string a can generate a 90 kV output voltage V 3 , Therefore, the single star string voltage balancing circuit a can balance or substantially balance the DC voltage source strings a, a and a output voltage at 90 kV, for example.

The power generation system b includes three unbalanced. DC voltage source strings b, b and b, such as of PV panels or wind turbines. The DC voltage source strings b, , and b are also connected in parallel to a load b to which an output voltage is provided. The single star string voltage balancing circuit b is respectively connected to the DC voltage source strings b, b and b.

Also, the single star string voltage balancing circuit b is similar in operation to the string voltage balancing circuits a and b of FIGS. The single star string voltage balancing circuit b can therefore be considered as an extension of the string voltage balancing circuits a and b of FIGS. The single star string voltage balancing circuit b is respectively connected to the DC voltage source strings b, b and b to balance the voltages generated by the DC voltage source strings b, b and b.

The single star string voltage balancing circuit b is respectively connected between the DC voltage sources strings b, b, and b and includes one inductor b L and three switches forming a star configuration. Also, the single star string voltage balancing circuit b can similarly overcome limitations of a relatively large number of components and physical location differences of circuit topology location associated with the closed loop string voltage balancing circuits a and b and the complementary string voltage balancing circuits a and b, such as illustrated in FIGS.

The single star string voltage balancing circuit b includes three IGBT type semiconductor switches as can be reverse blocking switches e. Use of IGBT type switches for the switches b, b and b can allow for relatively higher switching frequencies and can reduce harmonic content, for example. The single star string voltage balancing circuit b also includes the inductor b L respectively connected to each the switches b, b and b. The inductor b L can act to filter out ripples in the individual string currents respectively controlled by the switches b, b and b flowing in the corresponding DC voltage source strings b, b and b.

The single star string voltage balancing circuit b also includes three capacitors b C 1 , b C 2 and b C 3. The capacitor b C 1 is connected to the switch b and is associated with the first DC voltage source string b to balance the voltage generated by the first DC voltage source string b. The capacitor b C 2 is connected to the switch b and is associated with the second DC voltage source string b to balance the voltage generated by the second DC voltage source string b.

The capacitor b C 3 is connected to the switch b and is associated with the third DC voltage source string b to balance the voltage generated by the third DC voltage source string b. In some embodiments, the switches b, b and b have reverse blocking capability or, as illustrated, can be connected to series diodes b, b and b configured to block reverse current to provide a reverse blocking capability, for example. In the single star string voltage balancing circuit b, the switches b, b and b, in addition to being IGBT type semiconductor switches, can also include any of various suitable switches, such as metal-oxide semiconductor field effect transistor MOSFET switches, various types of transistor-type switches, or any another suitable components, as can depend on the use or application, and should not be construed in a limiting sense.

For example, for purposes of illustration, the first DC voltage source string b can generate a kV output voltage V 1 , the second DC voltage source string b can generate an 80 kV output voltage V 2 and the third DC voltage source string b can generate a 90 kV output voltage V 3. Therefore, the single star string voltage balancing circuit b can balance or substantially balance the DC voltage source strings b, b and b output voltage at 90 kV, for example.

In the simulation, for the three DC voltage source strings b, b and b, the voltage generated by the first DC voltage source string b was kilovolts kV as V 1 , the voltage generated by the second DC voltage source string b was 80 kV as V 2 and the voltage generated by the third DC voltage source string b was 90 kilovolts kV as V 3. The inductance of the inductors b, b, b and b were each equal to 5 millihenry ies mH and the capacitance of the three capacitors b C 1 , b C 2 and b C 3 were each equal to 1 millifarad mF , a switching frequency of the switches b, b and b was 1 kilohertz kHz and the load b was set to 10 ohms.

In the graph c it can be seen that the load voltage generated by power generation system b was 90 kV, indicating a voltage balancing for the DC voltage source strings b, b and b. In the graph d, it can be seen that the current I sc1 for the first DC voltage source string b, the current I sc2 for the second DC voltage source string b, and the current I sc3 for the third DC voltage source string b were substantially balanced with respect to each other.

The single star string voltage balancing circuits a and b respectively illustrated in FIGS. The double star string voltage balancing circuit a is respectively connected to the DC voltage source strings a, a and a. Also, the double star string voltage balancing circuit a is similar in operation to the string voltage balancing circuits a and b of FIGS.

The double star string voltage balancing circuit a can therefore be considered as an extension of the string voltage balancing circuits a and b of FIGS. The double star string voltage balancing circuit a is respectively connected to the DC voltage source strings a, a and a, in a generally redundant manner, to balance the voltages generated by the DC voltage source strings a, a and a.

The double star string voltage balancing circuit a is respectively connected between the DC voltage sources strings a, a, and a and includes two inductors a L 1 and a L 2 and six switches forming a double star configuration. Also, the double star string voltage balancing circuit a can overcome limitations of a relatively large number of components and physical location differences of circuit topology location associated with the closed loop string voltage balancing circuits a and b and the complementary string voltage balancing circuits a and b, such as illustrated in FIGS.

A first part of the double star string voltage balancing circuit a includes three reverse blocking switches e. The inductor a L 1 can act to filter out ripples in the individual string currents respectively controlled by the switches a, a and a flowing in the corresponding DC voltage source strings a, a and a. The first part of the double star string voltage balancing circuit a also includes three capacitors a C 1 , a C 2 and a C 3 which are also shared with and included in a second part of the double star string voltage balancing circuit a.

A second part of the double star string voltage balancing circuit a includes three reverse blocking switches e. The inductor a L 2 can act to filter out ripples in the individual string currents respectively controlled by the switches a, a and a flowing in the corresponding DC voltage source strings a, a, and a.

Also, the second part of the double star string voltage balancing circuit a also includes the three capacitors a C 1 , a C 2 and a C 3 , which are also shared with and included in the first part of the double star string voltage balancing circuit a.

In the double star string voltage balancing circuit a, the switches a, a and a and the switches a, a and a can include any of various suitable switches, such as metal-oxide semiconductor field effect transistor MOSFET switches, insulated-gate bipolar transistor IGBT semiconductor switches, various types of transistor-type switches, or any another suitable components, as can depend on the use or application, and should not be construed in a limiting sense. For example, for purposes of illustration, the first DC voltage source string a can generate a kV output voltage, the second DC voltage source string a can generate an 80 kV output voltage and the third DC voltage source string a can generate a 90 kV output voltage.

Therefore, the double star string voltage balancing circuit a can balance or substantially balance the DC voltage source strings a, a and a output voltage at 90 kV, for example. The double star string voltage balancing circuit b is respectively connected to the DC voltage source strings b, b and b. Also, the double star string voltage balancing circuit b is similar in operation to the string voltage balancing circuits a and b of FIGS.

The double star string voltage balancing circuit b can therefore be considered as an extension of the string voltage balancing circuits a and b of FIGS. The double star string voltage balancing circuit b is respectively connected to the DC voltage source strings b, b and b, in a generally redundant manner, to balance the voltages generated by the DC voltage source strings b, b and b. The double star string voltage balancing circuit b is respectively connected between the DC voltage sources strings b, b, and and includes two inductors b L 1 and b L 2 and six switches forming a double star configuration.

Also, the double star string voltage balancing circuit b can similarly overcome limitations of a relatively large number of components and physical location differences of circuit topology location associated with the closed loop string voltage balancing circuits a and b and the complementary string voltage balancing circuits a and b, such as illustrated in FIGS. A first part of the double star string voltage balancing circuit b includes three IGBT type semiconductor switches as can be reverse blocking switches e.

The double star string voltage balancing circuit b also includes the inductor b L 1 that can act to filter out ripples in the individual string currents respectively controlled by the switches b, b and b flowing in the corresponding DC voltage source strings b, b and b. The first part of the double star string voltage balancing circuit b also includes three capacitors b C 1 , b C 2 and b C 3 which are also shared with and included in a second part of the double star string voltage balancing circuit b.

In some embodiments, the switches b, b and b have reverse blocking capability or, as illustrated, can be connected to series diodes or to series type diode arrangements b, b and b configured to block reverse current to provide a reverse blocking capability, for example. A second part of the double star string voltage balancing circuit b includes three IGBT type semiconductor switches as can be reverse blocking switches e.

The double star string voltage balancing circuit b also includes the inductor b L 2 respectively connected to each of the switches b, b and b. The inductor b L 2 can act to filter out ripples in the individual string currents respectively controlled by the switches b, b and b flowing in the corresponding DC voltage source strings b, b, and b. Also the second part of the double star string voltage balancing circuit b also includes the three capacitors b C 1 , b C 2 and b C 3 , which are also shared with and included in the first part of the double star string voltage balancing circuit b.

In the double star string voltage balancing circuit b, the switches b, b and b and the switches b, b and b in addition to being IGBT type semiconductor switches, can also include any of various suitable switches, such as metal-oxide semiconductor field effect transistor MOSFET switches, various types of transistor-type switches, or any another suitable components, as can depend on the use or application, and should not be construed in a limiting sense.

Therefore, the double star string voltage balancing circuit b can balance or substantially balance the DC voltage source strings b, b and b output voltage at 90 kV, for example. The double star string voltage balancing circuits a and b respectively illustrated in FIGS.

Also, when compared with the single star string voltage balancing circuits a and b, respectively illustrated in FIGS. An advantage of the double star string voltage balancing circuits a and b is that, while the double star string voltage balancing circuits a and b each include double the number of switches and inductors used from that of the single star string voltage balancing circuits a and b, the double star string voltage balancing circuits a and b can reduce the switches current to half or substantially half of that of the single star string voltage balancing circuits a and b, as well as can reduce the capacitor sizes used, for example.

In some embodiments, such redundancy, such as in the double star string voltage balancing circuits a and b, can therefore lower the device rating as is illustrated in Table 1 e. As is illustrated in Table 1, a double star balancing converter topology can be relatively advantageous based on a comparison index that considers the number of components and device rating, for example.

The power generation system includes three unbalanced DC voltage source strings , and , such as of PV panels or wind turbines. As illustrated, the DC voltage sources strings , and are connected in parallel, and are also connected in parallel to a load to which an output voltage is provided. The open ring string voltage balancing circuit is connected between the DC voltage source strings , and Also, the open ring voltage balancing circuit , in the embodiment of FIG.

The open ring string voltage balancing circuit can therefore be considered as an extension of the string voltage balancing circuits a and b of FIGS. The open ring string voltage balancing circuit includes reverse blocking switches e. In some embodiments, the switches and have reverse blocking capability or, as illustrated, can be connected to series diodes or to series diode type arrangements and configured to block reverse current to provide a reverse blocking capability, for example.

The open ring string voltage balancing circuit also includes reverse blocking switches e. In some embodiments, the switches and have reverse blocking capability or, as illustrated, can be similarly connected to series diodes or to series diode type arrangements and configured to block reverse current to provide a reverse blocking capability, for example. The open ring string voltage balancing circuit is connected between the DC voltage source strings , and to balance the voltages generated by the DC voltage source strings , and The open ring string voltage balancing circuit also includes thee capacitors , and The capacitor is connected in series with the DC voltage source string and is also connected to the switch The capacitor is connected in series with the DC voltage source string and is also connected to the switch and the switch In the open ring string voltage balancing circuit , the switches , , and include any of various suitable switches, such as metal-oxide semiconductor field effect transistor MOSFET switches, insulated-gate bipolar transistor IGBT semiconductor switches, various types of transistor-type switches, or any another suitable components, as can depend on the use or application, and should not be construed in a limiting sense.

The resistors , and respectively in series with the inductors , and can act as a filter, such as to minimize or reducing a ripple current, for example, in the power generation system For example, for purposes of illustration, the DC voltage source string can generate a kV output voltage V 1 , the DC voltage source string can generate a 90 kV output voltage V 2 and the DC voltage source string can generate an 80 kV output voltage V 3.

However, in the power generation system , the open ring string voltage balancing circuit illustrates voltage balancing for the three DC voltage source strings , and with essentially two voltage balancing circuits that form the open ring string voltage balancing circuit , using an open ring type configuration.

While the open ring string voltage balancing circuit can provide voltage balancing of DC voltage source strings in a power generation system in some operating conditions, the open ring string voltage balancing circuit , due to its open ring configuration, does not necessarily provide voltage balancing for all operating conditions in a power generation system.

However, in this example, the open ring string voltage balancing circuit cannot operate to effectively balance the voltages unless the voltages of its groupings of two capacitors have an opposite sign. Another problem that can arise in voltage balancing using the open ring string voltage balancing circuit with the open right type configuration is that the ratings of the switches, such as transistors, are typically not equal.

In this regard, in the open ring string voltage balancing circuit the two middle switches, such as transistors, are feeding one DC voltage source line and the two outer switches, such as transistors, each feed one DC voltage source line, for example. The open ring string voltage balancing circuit respectively illustrated in FIG. It is noted that the voltage balancing circuits and topologies as illustrated in FIG. Embodiments of string voltage balancing converters can also be relatively superior to AC microinverters, particularly for use in higher voltage utility scale systems, for example.

Also, embodiments of string voltage balancing converters can be relatively superior to DC microinverters because, in some cases, power generation systems using DC micro inverters typically still require voltage balancing when more strings are connected in parallel.

In such cases, one solution can be to overrate each of the DC microinverters to account for any voltage drops in any string, such as is described for large scale wind farm architectures in S. In various embodiments using string balancing voltage converters can substantially reduce a need to overrate each individual inverter or groups of inverters in a power generation system. In this regard, embodiments of string voltage balancing converters can be arranged in topologies that can allow for integration into central inverter architectures or can serve as an add-on to such architectures, for example.

In some embodiments, using string voltage balancing converters can be cost effective. For example, Table 2 presents a cost analysis based on published figures from December for a 7 kW system, which can be considered a sizable rooftop PV installation. Although the total cost of central converter solution is relatively the lowest, the costs of the central converter solution do not account for any of the system accessories, wiring, protection equipment, etc.

As is illustrated in Table 2, a multiple string system that utilizes string voltage balancing converters can cost somewhat more than the central converter system, but less than a system that utilizes microinverters. In this regard, additional components configured to perform string voltage balancing are typically relatively less costly and less complex than another converter, and can also be integrated into central inverter architectures or can serve as an add-on to such architectures, for example.

Also, as described, embodiments of string voltage balancing converters can be applicable to various series connection of DC voltage sources that are placed in parallel combinations, such as for PV applications, wind energy applications e. As mentioned, embodiments of string voltage balancing power converters, as can include embodiments of voltage balancing circuits and topologies, can be combined with embodiments of current balancing circuits and topologies of various configurations for power generation electrical systems to balance DC voltage source strings that are placed in parallel.

For example, balancing the current in each of DC voltage sources of a DC voltage source string, such as a string of PV panels or wind turbines, etc. In this regard, series connection of PV panels or wind turbines, etc. One solution is to assist in operating the DC voltage source strings DC voltage sources, such as PV panels, at a maximum power point mpp is connect each DC voltage source, such as a PV panel to a DC converter to enhance the PV panel's ability to deliver a maximum available power, then connect the DC converters in a suitable way series, parallel, etc.

Another solution to assist in operating the DC voltage source strings DC voltage sources, such as PV panels, at a maximum power point mpp is to connect the DC voltage source strings DC voltage sources, such as PV panels, in series and attach them to a balancing circuit to bypass the required difference current, such as illustrated in FIG.

Such process is referred to as differential power processing DPP series balancing. Implementing embodiments of DPP series balancing to DC voltage sources in DC voltage source strings in a power generation system having a plurality of DC voltage source strings can enhance the DC voltage source's, such as a PV panel's ability, to deliver a maximum available power and, therefore, can enhance each DC voltage source strings ability to generate a maximum power.

For exemplary purposes, the power generation system a illustrates a single DC voltage source string having a plurality of DC voltage sources or modules, such as PV panels. The DPP current balancing circuit a includes a plurality of switches a, a and a respectively connected with the DC voltage source modules a, a and a, each of the switches a, a and a is respectively associated with a diode a, a and a, to control bypass of a difference current.

An inductor a is connected between the DC voltage source modules a and a and connected between the switches a and a to carry a difference current and induce a difference voltage. A DC voltage V c is provided across a capacitor a connected in parallel with the DC voltage source modules a, a and a.

The switches a, a and a can include any of various suitable switches, such as metal-oxide semiconductor field effect transistor MOSFET switches, insulated-gate bipolar transistor IGBT semiconductor switches, various types of transistor-type switches, or any another suitable components, as can depend on the use or application, and should not be construed in a limiting sense.

For exemplary purposes, the power generation system b illustrates a single DC voltage source string having a plurality of DC voltage source modules, such as PV panels. The DC voltage source string in the power generation system includes DC voltage source modules b, b and b. The DPP current balancing circuit b includes a plurality of switches b, b and b each respectively connected with the DC voltage source modules b, b and b to control bypass of a difference current.

Inductors b, b and b are respectively connected in series with the DC voltage source modules b, b and b to filter a ripple current. A DC voltage V c is provided across a capacitor b connected in parallel with the DC voltage source modules b, b and b.

The switches b, b and b can include any of various suitable switches, such as metal-oxide semiconductor field effect transistor MOSFET switches, insulated-gate bipolar transistor IGBT semiconductor switches, various types of transistor-type switches, or any another suitable components, as can depend on the use or application, and should not be construed in a limiting sense. For exemplary purposes, the power generation system c similarly illustrates a single DC voltage source string having a plurality of DC voltage source modules, such as PV panels.

The DC voltage source string in the power generation system c includes DC voltage source modules c, c and c. The DC voltage source module c generates a voltage V 1 and has a current i l , DC voltage source module c generates a voltage V 2 and has a current i 2 , and DC voltage source module c generates a voltage V 3 and has a current i 3.

The DPP current balancing circuit c includes a plurality of switches c, c and c respectively connected with the DC voltage source modules c, c and c, each of the switches c, c and c is respectively associated with diodes c, c and c, to control bypass of a difference current. An inductor c L 12 passing a current i 13 is connected between the DC voltage source modules c and c and connected between the switches c and c to pass a difference current and induce a difference voltage.

An inductor c L 23 passing a current i 23 is connected between the DC voltage sources modules c and c and connected between the switches c and c to pass a difference current and induce a difference voltage. A DC voltage V c is provided across a capacitor c connected in parallel with the DC voltage source modules c, c and c. A load c is connected across the capacitor c. The switches c, c and c similarly can include any of various suitable switches, such as metal-oxide semiconductor field effect transistor MOSFET switches, insulated-gate bipolar transistor IGBT semiconductor switches, various types of transistor-type switches, or any another suitable components, as can depend on the use or application, and should not be construed in a limiting sense.

Also, the switches are controlled so as to control values of the currents flowing in the inductances and, therefore, values of currents carried by the DC voltage source modules can differ from one another, for example. In the embodiment of the power generation system c, in modeling and control of the DPP current balancing circuit c for current balancing of the individual DC voltage source modules c, c and c, the DPP current balancing circuit c has three switching states S 1 , S 2 and S 3.

Continuing with reference to FIGS. Also, the voltages induced by and the currents flowing across the inductors c L 12 c L 23 for the switching states S 1 , S 2 and S 3 can be determined according to the following relations:. Further, for the switching states S 1 , S 2 and S 3 at steady state:. Also, for the switching states S 1 , S 2 and S 3 at transient state:.

The controller also includes proportional-integral PI controllers and to implement control of the switching states S 1 , S 2 and S 3 , as can be in conjunction with other processes, operations, systems and controllers, embodiments of the DPP current balancing processes and operations, such as can be implemented by or in conjunction with the generalized system of FIG.

Also, the duty cycles or duty ratios corresponding to the switching states of the switches in various embodiments of the DPP current balancing circuits and processes, can vary, as dependent on the use or application or on the number of switches or circuit configurations, for example, and should not be construed in a limiting sense.

Further, in controlling the switching states S 1 , S 2 and S 3 , the switches can be gated, such as the switches , c and c, as can be transistors. Also, the diodes respectively corresponding to the switches, such as the diodes c, c and c, can be correspondingly antiparallel gated for current balancing of the DC voltage source modules, such as DC voltage source modules c, c and c. The gating of the switches and the antiparallel gating of the corresponding diodes can be based on the following relations that indicate an on-state for the corresponding switches, such as the switches c, c and c, also thereby indicate an off-state for the diodes corresponding to the switches:.

Above, t 1 corresponds to the first switch c, t 2 corresponds to the second switch c and t 3 corresponds to the third switch c. Irrespective of the inequalities of the values of the difference currents, the switches, such as the switches c, c and c, as can be transistors, can be gated. Such gating of the switches can reduce the diode voltage drop synchronized rectification and can decrease conduction losses, for example. The voltage and current balancing circuit includes a plurality of DPP current balancing circuits , and , each similar to the DPP current balancing circuit c, as described, for differential power processing to balance the current between series connected DC voltage source modules, such as PV arrays, in a corresponding DC voltage source string.

The voltage and current balancing circuit also includes an integrated embodiment of a string voltage balancing circuit that is similar to and operates in a similar manner to the voltage balancing circuits a and b, as described, to balance the voltage of a plurality DC voltage source strings in a power generation system. Positive and negative potentials for the voltage and current balancing circuit are indicated at and , respectively. An advantage of the configuration and topology of the voltage and current balancing circuit is that it can provide expandability and flexibility in that the DC voltage source strings can be from different manufacturing sources and can be integrated in the circuit topology.

The DPP current balancing circuit is associated with a first DC voltage source string having DC voltage source modules a, b, c and d. The DPP current balancing circuit includes a plurality of switches a, b, c and d respectively connected with the DC voltage source modules a, b, c and d, each of the switches a, b, c and d is respectively associated with diodes a, b, c and d, to control bypass of a difference current.

An inductor a passing a difference current and inducing a difference voltage is connected between the DC voltage source modules a and b and connected between the switches a and b. An inductor b passing a difference current and inducing a difference voltage is connected between the DC voltage source modules b and c and connected between the switches b and c. An inductor c passing a difference current and inducing a difference voltage is connected between the DC voltage source modules c and d and connected between the switches c and d.

A DC voltage V c1 is provided across a capacitor connected in parallel with the DC voltage source modules a, b, c and d. Similarly, the DPP current balancing circuit is associated with a second DC voltage source string having DC voltage source modules a, b, c and d.

A DC voltage V c2 is provided across a capacitor connected in parallel with the DC voltage source modules a, 13 62 b, c and d. It can be concluded that, although the two-level and three-level topologies present a step-up transformer for the connection with the medium voltage grid, which means higher losses, they are still preferable due to their physical and control simplicity if compared with the MMC topologies.

However, due to the low losses and greater reliability, it is possible to verify a growing trend of using MMC topologies in BESS applications. Energy storage systems raise controversial opinions in the literature, and have been among the most discussed issues in recent works.

Besides, aspects related to optimization of BESS, impact the analysis of operating costs, power losses, energy quality and lifetime evaluation. Another important issue to determine the feasibility of the project is the BESS services, which can be used to obtain an efficient system, maximizing the investment payback. Recent studies show that BESS can contribute even more to the expansion of renewable sources in the electric system and reduce the impacts related to the intermittent generation of these sources.

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New York: Wiley; Book Google Scholar. Download references. Lucas S. Xavier, Allan F. Cupertino, Victor F. You can also search for this author in PubMed Google Scholar. LSX is the main author of this work. This paper provides a further elaboration of some of the results associated to his Ph. D, where he carried out studies regarding comparisons of topologies with transformers and transformerless applied in battery energy storage systems, as well as a review of services. WCB held a general supervision over the performed work.

All authors have been involved in the preparation of the manuscript. Besides, all authors read and approved the final manuscript. Correspondence to Heverton A. Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Reprints and Permissions. Xavier, L. Power converters for battery energy storage systems connected to medium voltage systems: a comprehensive review. BMC Energy 1, 7 Download citation.

Received : 05 December Accepted : 31 May Published : 16 July Anyone you share the following link with will be able to read this content:. Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative.

Skip to main content. Search all BMC articles Search. Download PDF. Review Open Access Published: 16 July Power converters for battery energy storage systems connected to medium voltage systems: a comprehensive review Lucas S. Xavier 1 , William C. Amorim 2 , Allan F. Cupertino 1 , 2 , Victor F. Mendes 1 , Wallace C. Abstract Recent works have highlighted the growth of battery energy storage system BESS in the electrical system.

Introduction Battery energy storage system BESS have been used for some decades in isolated areas, especially in order to supply energy or meet some service demand [ 1 ]. Full size image. Three level converter topologies.

Transformerless two-level converter connected directly to the grid of MV level. BESS control strategies. Current control example of BESS. Services performed by BESS. Table 1 Main parameters of the converter topologies for this case study Full size table. Results The results are comparatively quantified for power losses at various power levels, total harmonic distortion, device number and energy storage in the inductors and capacitors.

Comparison of the power losses for each converter topology at various power levels. Efficiency for each converter topology at various power levels. Availability of data and materials All data generated or analysed during this study are included in this published article. Article Google Scholar Horiba T.

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