CN117691654A - Power conversion system, neutral-point potential balancing method and photovoltaic system - Google Patents
Power conversion system, neutral-point potential balancing method and photovoltaic system Download PDFInfo
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Abstract
The application provides a power conversion system, a midpoint potential balancing method and a photovoltaic system, wherein the power conversion system comprises a plurality of power converters, each power converter comprises two bus capacitors, an inverter circuit and a controller, the two bus capacitors are connected in series between a positive direct current bus and a negative direct current bus, the midpoints of the two bus capacitors are connected with a neutral line, and the inverter circuit is used for converting direct current provided by the positive direct current bus and the negative direct current bus into alternating current and outputting the alternating current to a power grid. The controller can switch between an even harmonic modulation signal and a common mode modulation signal according to the active power output by the power converter, and the total modulation signal obtained by superposing the even harmonic modulation signal or the common mode modulation signal and the initial modulation signal is used for generating a driving signal for controlling the on-off of a switching tube in the inverter circuit so as to reduce the voltage difference between a positive direct current bus and a negative direct current bus and balance the midpoint potential of the two bus capacitors.
Description
Technical Field
The invention relates to the technical field of new energy, in particular to a power conversion system, a neutral point potential balancing method and a photovoltaic system.
Background
The three-level topological power converter is widely applied to new energy power conversion occasions due to the characteristics of low output voltage distortion rate, small stress of a switching tube, high operation efficiency and the like. The three-level topological power converter adopts two bus capacitors to be connected in series on the direct current side to generate three levels, and the two bus capacitors have the problem of uneven voltage due to factors such as inconsistent characteristics of switching devices in the power converter, different driving delay and the like, namely the problem of unbalanced midpoint potential of the two bus capacitors. The unbalanced neutral point potential can cause the stress of a switching tube in the power converter to be increased, reduce the reliability of a system and the service life of a device, and even directly damage the device when serious. Therefore, neutral point potential balancing is a precondition for stable and reliable operation of the three-level power converter.
With the continuous increase of the capacity of new energy power stations, the capacity of power converters in the new energy power stations is continuously increased, and the parallel connection of a plurality of power converters in the new energy power stations becomes a technical trend. In addition, in the face of various voltage large disturbances of the power grid, the new energy power station needs to have voltage ride through capability, such as high voltage ride through, low voltage ride through, zero voltage ride through, and the like, and the new energy power station needs to provide auxiliary services to the power grid. Both of the above points present challenges to the neutral point potential balancing of a three level topology power converter. Therefore, how to balance the midpoint potential in the system of parallel power converters and realize stable, reliable and efficient operation of the power conversion system under the working condition of the full power grid and the full power section is a technical problem to be solved in the field.
Disclosure of Invention
The application provides a power conversion system, a midpoint potential balancing method and a photovoltaic system, which are used for balancing midpoint potential of a parallel power converter.
In a first aspect, the present application provides a power conversion system comprising a plurality of parallel power converters, each power converter comprising a first bus capacitor, a second bus capacitor, an inverter circuit, and a controller. The first bus capacitor and the second bus capacitor are connected in series between the positive direct current bus and the negative direct current bus, specifically, one end of the first bus capacitor is connected with the positive direct current bus, and the other end of the first bus capacitor is connected with the negative direct current bus after being connected with the second bus capacitor in series. The series point of the first bus capacitor and the second bus capacitor is used as the midpoint of the power converter to be connected with the N line. The inverter circuit is used for converting direct current provided by the positive direct current bus and the negative direct current bus into alternating current and outputting the alternating current to the power grid. In the present application, the controller may calculate the active power output by the power converter according to the detected output voltage and output current of the power converter, and switch between the even harmonic modulation signal and the common mode modulation signal according to the active power output by the power converter. The even harmonic modulation signal is generated based on the difference between the absolute values of the voltage of the positive DC bus and the voltage of the negative DC bus and the phase of the output voltage of the power converter, and the common mode modulation signal is generated based on the difference between the absolute values of the voltage of the positive DC bus and the voltage of the negative DC bus. The total modulation signal obtained by overlapping the even harmonic modulation signal or the common mode modulation signal and the initial modulation signal is used for generating a driving signal for controlling the on-off of a switching tube in the inverter circuit so as to reduce the difference between the voltage of the positive direct current bus and the voltage of the negative direct current bus. The initial modulation signal is generally a sine wave signal calculated according to relevant parameters such as output voltage and output current required by the power converter, and the sine wave signal carries parameter information such as amplitude, frequency and phase.
In the application, the controller switches the modulation signal between the even harmonic modulation signal and the common mode modulation signal according to the active power output by the power converter, so that the failure of the common mode modulation signal during no-load and pure reactive operation can be avoided, the failure problem of the even harmonic modulation signal during zero voltage crossing can be avoided, and the effective operation working conditions of the common mode modulation signal and the even harmonic modulation signal can be combined to support the stable, reliable and efficient operation of the power conversion system under the full-power grid working condition and the full-power section.
In the application, when the controller determines that the active power is smaller than the first preset value, the power converter is considered to operate in an idle mode or in a pure reactive power mode, and in order to effectively balance the midpoint potential of the power converter, the power converter can be switched to generate a modulation signal by adopting an even harmonic current injection method. Specifically, the controller can generate an even harmonic modulation signal based on the difference value of the absolute value of the voltage of the positive direct current bus and the voltage of the negative direct current bus and the phase of the output voltage of the power converter, and the total modulation signal obtained after the even harmonic modulation signal is overlapped with the initial modulation signal is used for generating a driving signal for controlling the on-off of a switching tube in the inverter circuit so as to reduce the difference value between the voltage of the positive direct current bus and the voltage of the negative direct current bus and effectively realize midpoint potential balance.
In the application, when the controller determines that the active power output by the power converter is greater than the second preset value, in order to prevent the balance midpoint potential from being invalid when the even harmonic current injection method passes through zero voltage, the controller may switch the power converter to generate the modulation signal by adopting the common mode voltage injection method. Specifically, the controller can generate a common-mode modulation signal based on the difference value of the absolute value of the voltage of the positive direct current bus and the voltage of the negative direct current bus, and the total modulation signal obtained after the common-mode modulation signal is overlapped with the initial modulation signal is used for generating a driving signal for controlling the on-off of a switching tube in the inverter circuit so as to reduce the difference value between the voltage of the positive direct current bus and the voltage of the negative direct current bus and effectively realize neutral-point potential balance.
In the application, the controller can also generate a modulation signal by adopting a common-mode voltage injection method when the power converters are powered on to work, namely in an initial state, namely under the working condition that each power converter in the power conversion system operates in most of time, the common-mode voltage injection method is adopted to balance the midpoint potential. The common-mode voltage injection method can fail in idle and pure reactive operation, so when the controller judges that the active power of the power converter is reduced to be lower than a first preset value (including the case of pure reactive operation of the power converter), the common-mode injection method is adopted to balance the midpoint potential and the controller can switch the power converter to balance the midpoint potential by adopting the even harmonic current injection method, and the power conversion system can stably, reliably and efficiently operate under the working condition of a full power grid and the full power section. When the controller judges that the active power output by the power converter is increased to be larger than a second preset value, the power converter can be switched to balance the midpoint potential by adopting a common mode voltage injection method so as to prevent the midpoint potential from being unbalanced due to failure of the even harmonic current injection method during zero voltage crossing. For example, the second preset value may be different from the first preset value, for example, the second preset value may be slightly higher than the first preset value, so that a return difference is left between the two parameters, so as to prevent frequent switching of the control method at the critical point of the judgment condition.
After the controller generates a modulation signal by adopting a common mode voltage injection method or an even harmonic current injection method, the modulation signal and a carrier signal can be compared to obtain a command signal, the command signal is sent to a driver, and the driver can generate a driving signal for controlling the on-off of a switching tube in an inverter circuit according to the command signal.
In the present application, since a plurality of power converters in a power conversion system cannot achieve absolute consistency of switching timings when switching control methods. Therefore, in the switching process of the mechanism, a common-mode voltage injection method is adopted for part of the power converters, and an even-order current injection method is adopted for part of the power converters, so that a large common-mode direct current circulation is generated between the power converters, and further protection shutdown occurs, and even hardware damage is caused. In order to solve this problem, a virtual resistor may be added to the power converter to generate a virtual impedance, so that a common-mode dc current circulating generated during switching can be suppressed.
In some embodiments of the present application, common mode voltage injection may be used in conjunction with virtual impedance. Specifically, a damping voltage of the power converter may be determined based on an output current of the power converter and a virtual impedance value of the power converter, and the common mode modulation signal may be adjusted based on the damping voltage.
In some embodiments of the present application, even harmonic current injection methods may be used in conjunction with virtual impedance. Specifically, a damping voltage of the power converter may be determined based on an output current of the power converter and a virtual impedance value of the power converter, and the even harmonic modulation signal may be adjusted based on the damping voltage.
In a second aspect, the present application provides a photovoltaic system comprising a photovoltaic array and a power conversion system as may be devised in any of the first aspects of the present application, the power conversion system being adapted to convert dc power provided by the photovoltaic array into ac power for output to a power grid.
In a third aspect, the present application provides a midpoint potential balancing method of a power conversion system, for use in the power conversion system, the method being executable by a controller in the power conversion system. The method provided by the embodiment of the application specifically comprises the following steps: the controller switches between an even harmonic modulation signal and a common mode modulation signal according to active power output by the power converter, wherein the even harmonic modulation signal is generated based on the difference value of the absolute value of the voltage of the positive direct current bus and the absolute value of the voltage of the negative direct current bus and the phase of the output voltage of the power converter, the common mode modulation signal is generated based on the difference value of the absolute value of the voltage of the positive direct current bus and the absolute value of the voltage of the negative direct current bus, and the even harmonic modulation signal or the total modulation signal obtained by overlapping the common mode modulation signal and the initial modulation signal is used for generating a driving signal for controlling the on-off of a switching tube in the inverter circuit so as to reduce the difference value between the voltage of the positive direct current bus and the voltage of the negative direct current bus. The initial modulation signal is generally a sine wave signal calculated according to relevant parameters such as output voltage and output current required by the power converter, and the sine wave signal carries parameter information such as amplitude, frequency and phase.
In the application, the controller switches the modulation signal between the even harmonic modulation signal and the common mode modulation signal according to the active power output by the power converter, so that the failure of the common mode modulation signal during no-load and pure reactive operation can be avoided, the failure problem of the even harmonic modulation signal during zero voltage crossing can be avoided, and the effective operation working conditions of the common mode modulation signal and the even harmonic modulation signal can be combined to support the stable, reliable and efficient operation of the power conversion system under the full-power grid working condition and the full-power section.
In some embodiments of the present application, when the active power output by the power converter is smaller than the first preset value, the power converter is considered to operate in an idle mode or in a pure reactive power mode, and in order to effectively balance the midpoint potential of the power converter, the power converter may be switched to generate a modulation signal by adopting an even harmonic current injection method. Specifically, the controller can generate an even harmonic modulation signal based on the difference value of the absolute value of the voltage of the positive direct current bus and the voltage of the negative direct current bus and the phase of the output voltage of the power converter, and the total modulation signal obtained after the even harmonic modulation signal is overlapped with the initial modulation signal is used for generating a driving signal for controlling the on-off of a switching tube in the inverter circuit so as to reduce the difference value between the voltage of the positive direct current bus and the voltage of the negative direct current bus and effectively realize midpoint potential balance.
In some embodiments of the present application, when the active power output by the power converter is greater than the second preset value, in order to prevent the balance midpoint potential from being lost during zero voltage crossing by the even harmonic current injection method, the power converter may be switched to generate the modulation signal by adopting the common mode voltage injection method. Specifically, the controller can generate a common-mode modulation signal based on the difference value of the absolute value of the voltage of the positive direct current bus and the voltage of the negative direct current bus, and the total modulation signal obtained after the common-mode modulation signal is overlapped with the initial modulation signal is used for generating a driving signal for controlling the on-off of a switching tube in the inverter circuit so as to reduce the difference value between the voltage of the positive direct current bus and the voltage of the negative direct current bus and effectively realize neutral-point potential balance.
In the application, the controller can also generate a modulation signal by adopting a common-mode voltage injection method when the power converters are powered on to work, namely in an initial state, namely under the working condition that each power converter in the power conversion system operates in most of time, the common-mode voltage injection method is adopted to balance the midpoint potential. The common-mode voltage injection method can fail in idle and pure reactive operation, so when the controller judges that the active power of the power converter is reduced to be lower than a first preset value (including the case of pure reactive operation of the power converter), the common-mode injection method is adopted to balance the midpoint potential and the controller can switch the power converter to balance the midpoint potential by adopting the even harmonic current injection method, and the power conversion system can stably, reliably and efficiently operate under the working condition of a full power grid and the full power section. When the controller judges that the active power output by the power converter is increased to be larger than a second preset value, the power converter can be switched to balance the midpoint potential by adopting a common mode voltage injection method so as to prevent the midpoint potential from being unbalanced due to failure of the even harmonic current injection method during zero voltage crossing. For example, the second preset value may be different from the first preset value, for example, the second preset value may be slightly higher than the first preset value, so that a return difference is left between the two parameters, so as to prevent frequent switching of the control method at the critical point of the judgment condition.
In the present application, since a plurality of power converters in a power conversion system cannot achieve absolute consistency of switching timings when switching control methods. Therefore, in the switching process of the mechanism, a common-mode voltage injection method is adopted for part of the power converters, and an even-order current injection method is adopted for part of the power converters, so that a large common-mode direct current circulation is generated between the power converters, and further protection shutdown occurs, and even hardware damage is caused. In order to solve this problem, a virtual resistor may be added to the power converter to generate a virtual impedance, so that a common-mode dc current circulating generated during switching can be suppressed.
In some embodiments of the present application, common mode voltage injection may be used in conjunction with virtual impedance. Specifically, a damping voltage of the power converter may be determined based on an output current of the power converter and a virtual impedance value of the power converter, and the common mode modulation signal may be adjusted based on the damping voltage.
In some embodiments of the present application, even harmonic current injection methods may be used in conjunction with virtual impedance. Specifically, a damping voltage of the power converter may be determined based on an output current of the power converter and a virtual impedance value of the power converter, and the even harmonic modulation signal may be adjusted based on the damping voltage.
The technical effects that any one of the second aspect and the third aspect may be designed to achieve are referred to the technical effects that any one of the foregoing first aspect may achieve, and the detailed description is not repeated here. These and other aspects of the present application will be more readily apparent from the following description of the embodiments.
Drawings
FIG. 1 is a schematic diagram of a frame structure of a photovoltaic system;
FIG. 2 is a schematic circuit diagram of a power conversion system;
FIG. 3 is a schematic diagram of a circuit configuration of a power converter;
fig. 4 is a schematic circuit diagram of a power conversion system according to an embodiment of the present disclosure;
FIG. 5 is a schematic diagram of a power converter according to an embodiment of the present application using a common mode voltage injection method;
FIG. 6 is a schematic diagram of a power converter according to an embodiment of the present application using an even harmonic current injection method;
fig. 7 is a schematic diagram of synthesis of control signals of a power converter according to an embodiment of the present disclosure;
FIG. 8 is a schematic diagram of a common mode voltage injection method using virtual impedance for a power conversion system according to an embodiment of the present disclosure;
fig. 9 is a schematic diagram of an even harmonic current injection method using virtual impedance for a power conversion system according to an embodiment of the present application.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the present application more apparent, the present application will be described in further detail with reference to the accompanying drawings. The specific method of operation in the method embodiment may also be applied to the device embodiment or the system embodiment. In the description of the present application, "at least one" means one or more, wherein a plurality means two or more. In view of this, the term "plurality" may also be understood as "at least two" in embodiments of the present invention. "and/or", describes an association relationship of an association object, and indicates that there may be three relationships, for example, a and/or B, and may indicate: a exists alone, A and B exist together, and B exists alone. The character "/", unless otherwise specified, generally indicates that the associated object is an "or" relationship. In addition, it should be understood that in the description of this application, the words "first," "second," and the like are used merely for distinguishing between the descriptions and not for indicating or implying any relative importance or order.
It should be noted that in the embodiments of the present application, "connected" is understood to mean electrically connected, and two electrical components may be connected directly or indirectly between two electrical components. For example, a may be directly connected to B, or indirectly connected to B through one or more other electrical components, for example, a may be directly connected to B, or directly connected to C, and C may be directly connected to B, where a and B are connected through C. In some scenarios, "connected" may also be understood as coupled, such as electromagnetic coupling between two inductors. In summary, the connection between a and B may enable the transfer of electrical energy between a and B.
The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. First, an application scenario of the power conversion system provided in the embodiment of the present application will be described. The power conversion system provided by the embodiment of the application can be applied to the scenes such as photovoltaic power stations, energy storage power stations or optical storage power stations.
Fig. 1 shows an exemplary schematic diagram of a frame structure of a photovoltaic system. Referring to fig. 1, the photovoltaic system includes a photovoltaic array 110, a power conversion device 120, an energy storage device 150, and a grid 130/external load 140. The power conversion device 120 may integrate a DC/DC conversion circuit and a DC/AC conversion circuit, the energy storage device 150 may be located between the DC/DC conversion circuit and the DC/AC conversion circuit, the energy generated by the photovoltaic array 110 may be boosted or down-converted by the DC/DC conversion circuit and then may be charged into the energy storage device 150, and the energy generated by the photovoltaic array 110 may be transmitted to the grid 130/the external load 140 through the DC/DC conversion circuit and the DC/AC conversion circuit. When the electrical energy generated by the photovoltaic array 110 is insufficient to power the grid 130/external load 140, the electrical energy stored in the energy storage device 150 may be transferred to the grid 130/external load 140 via the DC/AC conversion circuit within the power conversion device 120. On the other hand, the energy storage device 150 may also receive power from the power grid 130, where the AC power output by the power grid 130 is converted into DC power by the DC/AC conversion circuit and then transferred to the energy storage device 150, so as to charge the energy storage device 150.
Fig. 2 schematically shows a circuit configuration of a power conversion system, and fig. 3 schematically shows a circuit configuration of a power converter. Referring to fig. 2, in practice, a photovoltaic system will typically include a power conversion system comprised of a plurality of parallel power converters 160. A power converter 160 may be coupled to a group of photovoltaic panels in the photovoltaic array 110 for converting dc power output from the photovoltaic panels for transmission to the grid 130. A power converter 160 may also be connected to a set of battery clusters in the energy storage device 150, for converting dc power output from the set of battery clusters in the pair and transmitting the converted dc power to the power grid 130. Thus, the photovoltaic array 110 and the energy storage device 150 may be collectively referred to as a direct current source. Each power converter 160 includes an inverter circuit 01 and an output filter 02; the input end of the inverter circuit 01 is connected with a direct current source through a direct current bus, the output end of the inverter circuit 01 is connected with an output filter 02, and the output filter 02 is connected with the power grid 130. The inverter circuit 01 is used for converting direct current input by the photovoltaic panel (or the battery cluster) into alternating current and outputting the alternating current to the power grid 130, and the output filter 02 is used for filtering the alternating current output by the inverter circuit 01, so that the alternating current transmitted to the power grid 130 meets the waveform requirement. When the dc source connected to the inverter circuit 01 is a photovoltaic panel, the power converter 160 may specifically be a photovoltaic inverter; when the dc source connected to the inverter circuit 01 is a battery cluster, the power converter 160 may be an energy storage converter. The power converter 160 may be provided with a first BUS capacitor Cm1 and a second BUS capacitor Cm2 connected in series on the dc side of the inverter circuit 01, the first BUS capacitor Cm1 and the second BUS capacitor Cm2 being connected between the positive dc BUS bus+ and the negative dc BUS-, and a connection point of the first BUS capacitor Cm1 and the second BUS capacitor Cm2 being connected to a neutral line (N line) as a midpoint of the power converter 160. The power converters 160 may be arranged in parallel on the dc side, i.e. the positive dc BUS bus+ to which each power converter 160 is connected to each other, and the negative dc BUS to which each power converter 160 is connected to each other; the power converters 160 may also be arranged in parallel on the ac side, i.e. each power converter 160 is integrated into the power grid 130 via the same ac bus. Referring to fig. 3, in practical application, the inverter circuit 01 in the power converter 160 may be a three-phase four-wire inverter circuit, for example, a neutral point clamped (neutral point clamped, NPC) inverter circuit, and the inverter circuit 01 may specifically include three switch legs connected in parallel between a positive dc BUS bus+ and a negative dc BUS-, each of the switch legs may include four power tubes arranged in series and two clamp tubes arranged in series, where a connection point of the middle two power tubes is used as a bridge leg midpoint for connection with one of three phases (a phase, B phase, and C phase) in an ac BUS, and a midpoint of the two clamp tubes is used for connection with a neutral line (N line). The output filter 02 may have various structures, for example, may be an LCL filter, and may specifically be composed of an inductor, a capacitor, and an inductor sequentially connected to an ac bus; the capacitor mainly plays a role of filtering, one end of the capacitor can be connected with the alternating current bus, and the other end of the capacitor can be connected with the N line.
In an ideal situation, the amount of charge flowing into and out of the midpoint of the power converter 160 is the same, i.e., the voltages of the two bus capacitors of the power converter 160 are equal, during one power supply cycle of the power converter 160. However, in practical applications, since the operating states of the switching transistors in the switching bridge arm of the inverter circuit 01 are generally asymmetric, the amounts of charges flowing into and out of the midpoint of the power converter 160 are not the same in one power supply period, that is, the amounts of charges (or discharges) of the two bus capacitors in the power converter 160 are not equal, resulting in unequal voltages of the two bus capacitors of the power converter 160, which may distort the output voltage of the power converter 160 and even damage devices in the system.
One way of balancing the midpoint potential is to realize midpoint potential balance by adopting a common-mode voltage injection method, and specifically, the energy stored by two series capacitors at the direct current side can be balanced by injecting the common-mode voltage into a modulation wave for generating a driving signal of a switching tube, so that midpoint potential balance is realized. The common-mode voltage injection method can fail in idle and pure reactive operation, and for a framework in which a plurality of power converters are connected in parallel on both a direct current side and an alternating current side, when the power converters work in a discharge mode, huge direct current circulation can be formed between the power converters, so that the common-mode voltage injection method fails.
The other mode of the existing neutral point potential balancing is to realize neutral point potential balancing by adopting an even harmonic current injection method, and specifically, the neutral point potential balancing can be realized by injecting even harmonic current into a modulation wave of a driving signal of a generated switching tube to balance energy stored by two series capacitors at a direct current side. Even harmonic current injection methods fail at zero voltage crossing.
Based on the above, the embodiment of the application provides a power conversion system, a midpoint potential balancing method and a photovoltaic system, which are used for providing a midpoint potential balancing mode, and can support the power conversion system to stably, reliably and efficiently operate under the working condition of a full power grid and the full power section.
Fig. 4 is a schematic circuit diagram schematically illustrating a power conversion system according to an embodiment of the present application. Referring to fig. 4, in the present application, the power conversion system includes a plurality of parallel power converters 160, and each power converter 160 includes a first bus capacitor Cm1, a second bus capacitor Cm2, an inverter circuit 01, and a controller (not shown in the figure). The first BUS capacitor Cm1 and the second BUS capacitor Cm2 are connected in series between the positive direct current BUS bus+ and the negative direct current BUS-, specifically, one end of the first BUS capacitor Cm1 is connected with the positive direct current bus+ and the other end of the first BUS capacitor Cm1 is connected with the negative direct current BUS after being connected with the second BUS capacitor Cm2 in series. The series point of the first bus capacitor Cm1 and the second bus capacitor Cm2 is connected to the N line as the midpoint of the power converter 160. The power converters 160 may be arranged in parallel on the dc side, i.e. the positive dc BUS bus+ to which each power converter 160 is connected to each other, and the negative dc BUS to which each power converter 160 is connected to each other; the power converters 160 may also be arranged in parallel on the ac side, i.e. each power converter 160 is integrated into the power grid 130 via the same ac bus. The inverter circuit 01 is used for converting direct current provided by the positive direct current BUS BUS+ and the negative direct current BUS BUS-into alternating current and outputting the alternating current to the power grid. Illustratively, the inverter circuit 01 may include three switch legs connected in parallel, each of the switch legs including four power transistors and two clamp transistors, wherein a connection point of the middle two power transistors is used as a bridge leg midpoint for connection with one of three phases (a phase, B phase, and C phase) in the ac bus, and a midpoint of the two clamp transistors is used for connection with the N line. Each power converter may further include an output filter 02, where the output filter 02 is configured to filter the ac power output by the inverter circuit, and the output filter 02 may include a filter capacitor and/or a filter inductor, and in fig. 4, taking the output filter 02 as an LCL filter for example, the output filter 02 may specifically include an inductor, a capacitor, and an inductor sequentially connected to the ac bus; the capacitor mainly plays a role of filtering, one end of the capacitor can be connected with the alternating current bus, and the other end of the capacitor can be connected with the N line.
In this application, the controller may calculate the active power output by the power converter 160 based on the detected output voltage and output current of the power converter 160, and switch between the even harmonic modulation signal and the common mode modulation signal based on the active power output by the power converter. The even harmonic modulation signal is generated based on the difference between the absolute values of the voltage of the positive DC bus and the voltage of the negative DC bus and the phase of the output voltage of the power converter, and the common mode modulation signal is generated based on the difference between the absolute values of the voltage of the positive DC bus and the voltage of the negative DC bus. The total modulation signal obtained by overlapping the even harmonic modulation signal or the common mode modulation signal and the initial modulation signal is used for generating a driving signal for controlling the on-off of a switching tube in the inverter circuit so as to reduce the difference between the voltage of the positive direct current bus and the voltage of the negative direct current bus. The initial modulation signal is generally a sine wave signal calculated according to relevant parameters such as output voltage and output current required by the power converter, and the sine wave signal carries parameter information such as amplitude, frequency and phase.
In the application, the controller switches the modulation signal between the even harmonic modulation signal and the common mode modulation signal according to the active power output by the power converter, so that the failure of the common mode modulation signal during no-load and pure reactive operation can be avoided, the failure problem of the even harmonic modulation signal during zero voltage crossing can be avoided, and the effective operation working conditions of the common mode modulation signal and the even harmonic modulation signal can be combined to support the stable, reliable and efficient operation of the power conversion system under the full-power grid working condition and the full-power section.
In this application, when the controller determines that the active power is less than the first preset value, the power converter 160 may be considered to operate in an idle mode or in a pure reactive power mode, and in order to effectively balance the midpoint potential of the power converter 160, the power converter 160 may be switched to generate the modulation signal by using an even harmonic current injection method. Specifically, the controller can generate an even harmonic modulation signal based on the difference value of the absolute value of the voltage of the positive direct current bus+ and the voltage of the negative direct current BUS-and the phase of the output voltage of the power converter, and the total modulation signal obtained by superposing the even harmonic modulation signal and the initial modulation signal is used for generating a driving signal for driving the on-off of a switching tube in the inverter circuit so as to reduce the difference value between the voltage of the positive direct current BUS and the voltage of the negative direct current BUS, effectively realize midpoint potential balance and effectively realize midpoint potential balance.
In this application, when it is determined that the active power output by the power converter 160 is greater than the second preset value, the controller may switch the power converter 160 to generate the modulation signal by using the common mode voltage injection method in order to prevent the balance midpoint potential from being invalid during the zero voltage crossing. Specifically, the controller can generate a common-mode modulation signal based on the difference value between the absolute values of the voltage of the positive direct current bus+ and the voltage of the negative direct current BUS-, and the total modulation signal obtained by superposing the common-mode modulation signal and the initial modulation signal is used for generating a driving signal for controlling the on-off of a switching tube in the inverter circuit so as to reduce the difference value between the voltage of the positive direct current BUS and the voltage of the negative direct current BUS and effectively realize midpoint potential balance.
In this application, the controller may also generate the modulation signal by using a common mode voltage injection method when the power converters 160 are powered up, i.e. in an initial state, i.e. under a working condition that each power converter 160 in the power conversion system operates most of the time, the common mode voltage injection method is used to balance the midpoint potential. The common mode voltage injection method fails during idle load and during pure reactive power operation, so when the controller determines that the active power of the power converter 160 is reduced to be lower than a first preset value (including the case of pure reactive power operation of the power converter 160), the common mode injection method is adopted to balance the midpoint potential, and the controller can switch the power converter 160 to balance the midpoint potential by adopting the even harmonic current injection method, so that the power conversion system can stably, reliably and efficiently operate under the full power grid working condition and the full power section. When the controller determines that the active power output by the power converter 160 increases to be greater than the second preset value, the power converter 160 may be switched to balance the midpoint potential by using the common mode voltage injection method, so as to prevent the midpoint potential from being unbalanced due to failure of the even harmonic current injection method during zero voltage crossing. For example, the second preset value may be different from the first preset value, for example, the second preset value may be slightly higher than the first preset value, so that a return difference is left between the two parameters, so as to prevent frequent switching of the control method at the critical point of the judgment condition.
Fig. 5 is a schematic circuit diagram of a power converter according to an embodiment of the present application using a common mode voltage injection method. Referring to fig. 5, in some embodiments of the present application, when the controller generates the modulation signal by using the common-mode voltage injection method, specifically, the positive dc bus voltage Ucp and the negative dc bus voltage Ucn may be collected, and the difference Ucp-Ucn between the absolute values of the positive dc bus voltage and the negative dc bus voltage may be obtained, where the difference Ucp-Ucn may be passed through a low-pass filter (LPF), and then passed through a Proportional Integrator (PI) to obtain the common-mode voltage Ucom, and the common-mode modulation signal may be determined by the common-mode voltage Ucom.
Fig. 6 is a schematic circuit diagram of a power converter according to an embodiment of the present application using an even harmonic current injection method. Referring to fig. 6, in some embodiments of the present application, when the controller generates the modulation signal by adopting the even harmonic current injection method, specifically, the positive dc bus voltage Ucp and the negative dc bus voltage Ucn may be collected, and the difference Ucp-Ucn between the absolute values of the positive dc bus voltage Ucp and the negative dc bus voltage Ucn may be obtained, where the difference Ucp-Ucn may be passed through a Low Pass Filter (LPF) and then passed through a Proportional Integrator (PI) to generate the even harmonic amplitude signal i hm . The phase theta of the output voltage Umcc of the power converter can be obtained by utilizing a phase-locked loop (PLL), the even harmonic type can be judged according to the phase of the output voltage of the power converter and the phase of the output current, wherein the initial phase angle of the output current of the second harmonic is in the same direction as the initial phase angle of the output voltage Umcc, the initial phase angle of the output current of the fourth harmonic is opposite to the initial phase angle of the output voltage Umcc, and an even harmonic phase signal i can be generated according to different even harmonic type weight distribution ratios hamc . Based on even harmonic phase signal i hamc And even harmonic amplitude signal i hm An even harmonic modulation signal may be generated.
Fig. 7 is a schematic diagram illustrating the synthesis of control signals of the power converter according to the embodiment of the present application. Referring to fig. 7, after the controller generates a modulation signal by using a common mode voltage injection method or an even harmonic current injection method, the modulation signal and a carrier signal may be superimposed to obtain a command signal and sent to the driver, and the driver may generate a driving signal for controlling on/off of a switching tube in the inverter circuit according to the command signal.
In the present application, since a plurality of power converters in a power conversion system cannot achieve absolute consistency of switching timings when switching control methods. Therefore, in the switching process of the mechanism, a common-mode voltage injection method is adopted for part of the power converters, and an even-order current injection method is adopted for part of the power converters, so that a large common-mode direct current circulation is generated between the power converters, and further protection shutdown occurs, and even hardware damage is caused. To solve this problem, a virtual resistor R can be added to the power converter vir Generating virtual impedanceThereby, the common-mode direct current circulation generated in the switching process can be suppressed.
In some embodiments of the present application, common mode voltage injection may be used in conjunction with virtual impedance. Specifically, a damping voltage of the power converter may be determined based on an output current of the power converter and a virtual impedance value of the power converter, and the common mode modulation signal may be adjusted based on the damping voltage.
In some embodiments of the present application, even harmonic current injection methods may be used in conjunction with virtual impedance. Specifically, a damping voltage of the power converter may be determined based on an output current of the power converter and a virtual impedance value of the power converter, and the even harmonic modulation signal may be adjusted based on the damping voltage.
FIG. 8 is a schematic diagram illustrating a circuit diagram of a power conversion system using a virtual impedance common mode voltage injection method according to an embodiment of the present application; fig. 9 is a schematic circuit diagram of an even harmonic current injection method of a power conversion system using virtual impedance according to an embodiment of the present application. Referring to fig. 8 and 9, the virtual impedance is generated by: the output current on each phase inductance of the output filter in the sampling power converter is superposed to obtain a common-mode circulation i cir Common mode circulation i cir Filtering with Low Pass Filter (LPF) with filter cut-off frequency smaller than 10Hz, and multiplying the filtered quantity by a virtual resistor R vir The obtained voltage Uv can be used as damping voltage. Referring to fig. 8, when the controller generates a common mode modulation signal using a common mode voltage injection method, a positive dc bus voltage Ucp and a negative dc bus voltage Ucn are collected, a difference Ucp-Ucn between absolute values of the positive dc bus voltage and the negative dc bus voltage is obtained, the difference Ucp-Ucn passes through a Low Pass Filter (LPF), a common mode voltage Uc is obtained through a Proportional Integrator (PI), the voltage Uv is removed from the common mode voltage Uc, a final common mode voltage Ucom is obtained, and the common mode modulation signal is determined by the final common mode voltage Ucom. Referring to fig. 9, when the controller generates a modulation signal using a common mode voltage injection method, a positive dc bus voltage Ucp and a negative dc bus voltage Ucn are collected, and a difference Ucp-Ucn between absolute values of the two is obtained, and the difference Ucp-Ucn is subjected to a Low Pass Filter (LPF) and then to a ratioAn integrator (PI) generates an even harmonic amplitude signal i hm The method comprises the steps of carrying out a first treatment on the surface of the The phase theta of the output voltage Umcc of the power converter can be obtained by utilizing a phase-locked loop (PLL), the even harmonic type can be judged according to the phase of the output voltage of the power converter and the phase of the output current, wherein the initial phase angle of the output current of the second harmonic is in the same direction as the initial phase angle of the output voltage Umcc, the initial phase angle of the output current of the fourth harmonic is opposite to the initial phase angle of the output voltage Umcc, and an even harmonic phase signal i can be generated according to different even harmonic type weight distribution ratios h Based on even harmonic phase signal i h And even harmonic amplitude signal i hm The voltage Uv is removed from the generated even harmonic modulation signal, and a final even harmonic modulation signal can be obtained.
In the present application, since the damping voltage and the output current of the power converters are positively correlated, and the modulation signal generated by the controller and the output current are negatively correlated, it is possible to balance the difference in the output currents of the power converters and suppress the generation of the common-mode dc circulating current.
Based on the same conception, the application also provides a photovoltaic system which comprises a photovoltaic array and the power conversion system, wherein the power conversion system is used for converting direct current provided by the photovoltaic array into alternating current and outputting the alternating current to a power grid.
Based on the embodiment of the power conversion system, the application also provides a neutral point potential balancing method of the power conversion system, which is applied to the power conversion system and can be executed by a controller in the power conversion system. The method provided by the embodiment of the application specifically comprises the following steps: the controller switches between an even harmonic modulation signal and a common mode modulation signal according to active power output by the power converter, wherein the even harmonic modulation signal is generated based on the difference value of the absolute value of the voltage of the positive direct current bus and the absolute value of the voltage of the negative direct current bus and the phase of the output voltage of the power converter, the common mode modulation signal is generated based on the difference value of the absolute value of the voltage of the positive direct current bus and the absolute value of the voltage of the negative direct current bus, and the even harmonic modulation signal or the total modulation signal obtained by overlapping the common mode modulation signal and the initial modulation signal is used for generating a driving signal for controlling the on-off of a switching tube in the inverter circuit so as to reduce the difference value between the voltage of the positive direct current bus and the voltage of the negative direct current bus. The initial modulation signal is generally a sine wave signal calculated according to relevant parameters such as output voltage and output current required by the power converter, and the sine wave signal carries parameter information such as amplitude, frequency and phase.
In the application, the controller switches the modulation signal between the even harmonic modulation signal and the common mode modulation signal according to the active power output by the power converter, so that the failure of the common mode modulation signal during no-load and pure reactive operation can be avoided, the failure problem of the even harmonic modulation signal during zero voltage crossing can be avoided, and the effective operation working conditions of the common mode modulation signal and the even harmonic modulation signal can be combined to support the stable, reliable and efficient operation of the power conversion system under the full-power grid working condition and the full-power section.
In some embodiments of the present application, when the active power output by the power converter is smaller than the first preset value, the power converter is considered to operate in an idle mode or in a pure reactive power mode, and in order to effectively balance the midpoint potential of the power converter, the power converter may be switched to generate a modulation signal by adopting an even harmonic current injection method. Specifically, the controller can generate an even harmonic modulation signal based on the difference value of the absolute value of the voltage of the positive direct current bus and the voltage of the negative direct current bus and the phase of the output voltage of the power converter, and the total modulation signal obtained after the even harmonic modulation signal is overlapped with the initial modulation signal is used for generating a driving signal for controlling the on-off of a switching tube in the inverter circuit so as to reduce the difference value between the voltage of the positive direct current bus and the voltage of the negative direct current bus and effectively realize midpoint potential balance.
In some embodiments of the present application, when the active power output by the power converter is greater than the second preset value, in order to prevent the balance midpoint potential from being lost during zero voltage crossing by the even harmonic current injection method, the power converter may be switched to generate the modulation signal by adopting the common mode voltage injection method. Specifically, the controller can generate a common-mode modulation signal based on the difference value of the absolute value of the voltage of the positive direct current bus and the voltage of the negative direct current bus, and the total modulation signal obtained after the common-mode modulation signal is overlapped with the initial modulation signal is used for generating a driving signal for controlling the on-off of a switching tube in the inverter circuit so as to reduce the difference value between the voltage of the positive direct current bus and the voltage of the negative direct current bus and effectively realize neutral-point potential balance.
In the application, the controller can also generate a modulation signal by adopting a common-mode voltage injection method when the power converters are powered on to work, namely in an initial state, namely under the working condition that each power converter in the power conversion system operates in most of time, the common-mode voltage injection method is adopted to balance the midpoint potential. The common-mode voltage injection method can fail in idle and pure reactive operation, so when the controller judges that the active power of the power converter is reduced to be lower than a first preset value (including the case of pure reactive operation of the power converter), the common-mode injection method is adopted to balance the midpoint potential and the controller can switch the power converter to balance the midpoint potential by adopting the even harmonic current injection method, and the power conversion system can stably, reliably and efficiently operate under the working condition of a full power grid and the full power section. When the controller judges that the active power output by the power converter is increased to be larger than a second preset value, the power converter can be switched to balance the midpoint potential by adopting a common mode voltage injection method so as to prevent the midpoint potential from being unbalanced due to failure of the even harmonic current injection method during zero voltage crossing. For example, the second preset value may be different from the first preset value, for example, the second preset value may be slightly higher than the first preset value, so that a return difference is left between the two parameters, so as to prevent frequent switching of the control method at the critical point of the judgment condition.
In the present application, since a plurality of power converters in a power conversion system cannot achieve absolute consistency of switching timings when switching control methods. Therefore, in the switching process of the mechanism, a common-mode voltage injection method is adopted for part of the power converters, and an even-order current injection method is adopted for part of the power converters, so that a large common-mode direct current circulation is generated between the power converters, and further protection shutdown occurs, and even hardware damage is caused. In order to solve this problem, a virtual resistor may be added to the power converter to generate a virtual impedance, so that a common-mode dc current circulating generated during switching can be suppressed.
In some embodiments of the present application, common mode voltage injection may be used in conjunction with virtual impedance. Specifically, a damping voltage of the power converter may be determined based on an output current of the power converter and a virtual impedance value of the power converter, and the common mode modulation signal may be adjusted based on the damping voltage.
In some embodiments of the present application, even harmonic current injection methods may be used in conjunction with virtual impedance. Specifically, a damping voltage of the power converter may be determined based on an output current of the power converter and a virtual impedance value of the power converter, and the even harmonic modulation signal may be adjusted based on the damping voltage.
It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention also include such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
Claims (13)
1. A power conversion system comprising a plurality of parallel power converters, each of said power converters comprising a first bus capacitor, a second bus capacitor, an inverter circuit, and a controller;
the first bus capacitor and the second bus capacitor which are connected in series are connected between a positive direct current bus and a negative direct current bus, and the connection point of the first bus capacitor and the second bus capacitor is connected with a neutral line;
the inverter circuit is used for converting direct current provided by the positive direct current bus and the negative direct current bus into alternating current and outputting the alternating current to a power grid;
the controller is used for: according to the active power output by the power converter, switching is performed between an even harmonic modulation signal and a common mode modulation signal, wherein the even harmonic modulation signal is generated based on the difference value of the absolute value of the voltage of the positive direct current bus and the absolute value of the voltage of the negative direct current bus and the phase of the output voltage of the power converter, the common mode modulation signal is generated based on the difference value of the absolute value of the voltage of the positive direct current bus and the absolute value of the voltage of the negative direct current bus, and the even harmonic modulation signal or the total modulation signal obtained by superposing the common mode modulation signal and the initial modulation signal is used for generating a driving signal for controlling the on-off of a switching tube in the inverter circuit so as to reduce the difference value between the voltage of the positive direct current bus and the voltage of the negative direct current bus.
2. The power conversion system of claim 1, wherein the controller is to: and when the active power output by the power converter is smaller than a first preset value, generating the even harmonic modulation signal based on the difference value of the absolute values of the voltage of the positive direct current bus and the voltage of the negative direct current bus and the phase of the output voltage of the power converter.
3. The power conversion system of claim 1 or 2, wherein the controller is configured to: and when the active power output by the power converter is larger than a second preset value, generating the common mode modulation signal based on the difference value of the absolute values of the voltage of the positive direct current bus and the voltage of the negative direct current bus.
4. The power conversion system of claim 3, wherein the second preset value is greater than the first preset value.
5. The power conversion system of any of claims 1-4, wherein the controller is further configured to: a damping voltage of the power converter is determined based on an output current of the power converter and a virtual impedance value of the power converter, and the even harmonic modulation signal is adjusted based on the damping voltage.
6. The power conversion system of any of claims 1-5, wherein the controller is further to: a damping voltage of the power converter is determined based on an output current of the power converter and a virtual impedance value of the power converter, and the common mode modulation signal is adjusted based on the damping voltage.
7. A photovoltaic system comprising a power conversion system according to any one of claims 1 to 6 for converting dc power provided by the photovoltaic array into ac power for output to a grid, and a photovoltaic array.
8. A method of neutral point potential balancing of a power conversion system, the power conversion system comprising a plurality of power converters, each of the power converters comprising a first bus capacitor, a second bus capacitor, an inverter circuit, and a controller; the first bus capacitor and the second bus capacitor which are connected in series are connected between a positive direct current bus and a negative direct current bus, and the connection point of the first bus capacitor and the second bus capacitor is connected with a neutral line; the inverter circuit is used for converting direct current provided by the positive direct current bus and the negative direct current bus into alternating current and outputting the alternating current to a power grid; the neutral point potential balancing method comprises the following steps:
The controller switches between an even harmonic modulation signal and a common mode modulation signal according to active power output by the power converter, wherein the even harmonic modulation signal is generated based on a difference value between an absolute value of a voltage of the positive direct current bus and an absolute value of a voltage of the negative direct current bus and a phase of an output voltage of the power converter, the common mode modulation signal is generated based on a difference value between the absolute value of the voltage of the positive direct current bus and the absolute value of the voltage of the negative direct current bus, and the even harmonic modulation signal or a total modulation signal obtained by overlapping the common mode modulation signal and an initial modulation signal is used for generating a driving signal for controlling on-off of a switching tube in the inverter circuit so as to reduce the difference value between the voltage of the positive direct current bus and the voltage of the negative direct current bus.
9. The midpoint potential balancing method of claim 8, wherein the midpoint potential balancing method comprises:
when the active power output by the power converter is smaller than a first preset value, the controller generates the even harmonic modulation signal based on the difference value of the absolute values of the voltage of the positive direct current bus and the voltage of the negative direct current bus and the phase of the output voltage of the power converter.
10. The midpoint potential balancing method of claim 8 or 9, wherein the midpoint potential balancing method comprises:
and when the active power output by the power converter is larger than a second preset value, the controller generates the common mode modulation signal based on the difference value of the absolute values of the voltage of the positive direct current bus and the voltage of the negative direct current bus.
11. The midpoint potential balancing method of claim 10, wherein the second preset value is greater than the first preset value.
12. The midpoint potential balancing method of any one of claims 8 to 11, further comprising:
the controller determines a damping voltage of the power converter based on an output current of the power converter and a virtual impedance value of the power converter, and adjusts the even harmonic modulation signal based on the damping voltage.
13. The midpoint potential balancing method of any one of claims 8 to 12, further comprising:
the controller determines a damping voltage of the power converter based on an output current of the power converter and a virtual impedance value of the power converter, and adjusts the common mode modulation signal based on the damping voltage.
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