Power converter and control method and system of power converter

Abstract

The invention discloses a power converter and a control method and system of the power converter, and relates to the technical field of electronic power, and the power converter comprises a controller and at least one power conversion topology; the power conversion topology is a three-level active neutral point clamped (ANPC) topology or a three-level neutral point clamped (NPC) topology; the controller is configured to determine an adopted control method by using an actual half-bus voltage deviation value and a predicted half-bus voltage deviation value, and control a switching device in the at least one power conversion topology based on the control method so as to balance the potential of a bus midpoint of the power converter; wherein the control method is a double-modulation wave carrier control method or a single-modulation wave carrier control method. By means of the scheme, neutral-point potential balance under different working conditions can be achieved, and it is guaranteed that the power converter is in the optimal state.

Description

Technical Field

[0001] The present application relates to the field of electronic power technology, and in particular to a power converter, a control method and a system for the power converter. Background Art

[0002] In recent years, the penetration rate of distributed renewable energy generation, represented by wind and solar power, has continued to rise, and the installed capacity of energy storage has continued to expand. The grid connection of renewable energy is inseparable from power converters. Wind power converters, photovoltaic inverters, energy storage converters, etc. can be collectively referred to as power converters. Power converters are mainly three-level. Three-level neutral point clamped (NPC) converters and three-level active neutral point clamped (ANPC) converters are currently commonly used multi-level power converters. Compared with traditional two-level converters, three-level power converters have many advantages, such as high output power, low output waveform harmonic distortion, low device voltage stress and system electromagnetic interference (EMI), and are therefore widely used in the field of renewable energy.

[0003] To ensure safe and reliable operation of three-level converters, the midpoint potential must be half the DC side voltage. Increasing the DC support capacitor's capacity can be a hardware solution, but this increases cost and device size. Currently, software methods for balancing the midpoint potential generally use zero-sequence voltage injection based on a single modulation waveform. This solution offers low harmonics, minimal losses, and simple control.

[0004] Under normal voltage conditions and high power factors, low-frequency fluctuations and DC offsets in the midpoint voltage are minimal, and can be completely addressed with zero-sequence voltage injection. However, in voltage fault conditions, such as voltage sags, especially asymmetrical sags, negative-sequence current is generally required to suppress negative-sequence voltage. This negative-sequence current can cause the midpoint voltage to fluctuate at the power frequency, making it difficult to control midpoint voltage fluctuations with zero-sequence voltage injection. Consequently, this solution has poor control effectiveness for midpoint potential. Summary of the Invention

[0005] In order to solve the above-mentioned technical problems existing in the prior art, the present application provides a power converter, a control method and a system for the power converter, which can achieve midpoint potential balance under different working conditions and ensure that the power converter is in the optimal state.

[0006] In the first aspect, the present application provides a power converter, comprising: a controller and at least one power conversion topology; the power conversion topology is a three-level active neutral point clamped ANPC topology or a three-level neutral point clamped NPC topology; the controller is configured to determine a control method to be adopted using an actual half-bus voltage deviation value and a predicted half-bus voltage deviation value, and control a switching device in at least one power conversion topology based on the control method to balance the potential of the bus midpoint of the power converter; wherein the control method is a dual modulation wave carrier control method or a single modulation wave carrier control method.

[0007] The solution provided by this application can select the currently applicable control method from either the dual-modulation carrier control method or the single-modulation carrier control method based on the actual and predicted half-bus voltage deviation values ​​to maintain bus midpoint potential balance. This combines the advantages of both control methods, reducing harmonics, the number of DC capacitors, lowering costs, and increasing power density, while also taking into account the number of switching cycles to reduce losses, ultimately achieving optimal control of the power converter.

[0008] In one possible implementation, the controller is specifically configured as follows: when the power converter starts running, a single modulation wave carrier control method is first adopted; when the actual half-bus voltage deviation value is greater than or equal to a first preset value, a dual modulation wave carrier control method is adopted; otherwise, the control method to be adopted is determined based on the predicted half-bus voltage deviation value, the grid negative sequence voltage, the positive sequence reactive current set value, and the negative sequence reactive current set value; after switching from the single modulation wave carrier control method to the dual modulation wave carrier control method, when the maximum value of the actual half-bus voltage deviation value within a continuous power frequency cycle is not greater than the first preset value and the maximum value of the predicted half-bus voltage deviation value is not greater than the second preset value, switch to the single modulation wave carrier control method.

[0009] In one possible implementation, the controller is specifically configured as follows: when the predicted half-bus voltage deviation value is greater than or equal to the second preset value, or the grid negative sequence voltage is greater than or equal to the third preset value, or the positive sequence reactive current given value is greater than or equal to the fourth preset value, or the negative sequence reactive current given value is greater than or equal to the fifth preset value, the single modulation wave carrier control method is switched to the dual modulation wave carrier control method; when the maximum value of the actual half-bus voltage deviation value within a continuous power frequency cycle is not greater than the first preset value and the maximum value of the predicted half-bus voltage deviation value is not greater than the second preset value, the control method to be adopted is again determined based on the predicted half-bus voltage deviation value, the grid negative sequence voltage, the positive sequence reactive current given value, and the negative sequence reactive current given value.

[0010] In a possible implementation, the controller is specifically configured to: determine a maximum value of an actual half-bus voltage deviation value once, and determine a maximum value of a predicted half-bus voltage deviation value once in each control cycle.

[0011] In a possible implementation, the controller is configured to: when determining to switch from the dual modulation wave carrier control method to the single modulation wave carrier control method, terminate the dual modulation wave carrier control method after a first preset time delay.

[0012] In a second aspect, the present application provides a method for controlling a power converter, for controlling the power converter provided in the first aspect above, the method comprising:

[0013] Determine the control method to be used by using the actual half-bus voltage deviation value and the predicted half-bus voltage deviation value;

[0014] Controlling the switching devices in the at least one power conversion topology based on the control method to balance the potential of the busbar midpoints of the power converter;

[0015] Wherein, the control method is a dual modulation wave carrier control method or a single modulation wave carrier control method.

[0016] This method selects the currently applicable control method from either the dual-modulation carrier control method or the single-modulation carrier control method based on the actual and predicted half-bus voltage deviation values ​​to maintain bus midpoint potential balance. This combines the advantages of both control methods to reduce harmonics, the number of DC capacitors, lower costs, and increase power density, while also minimizing switching losses and ensuring optimal power converter control.

[0017] In one possible implementation, the actual half-bus voltage deviation value and the predicted half-bus voltage deviation value are used to determine the control method to be adopted, specifically including: when the power converter starts to run, first adopting the single modulation wave carrier control method; when the actual half-bus voltage deviation value is greater than or equal to the first preset value, adopting the dual modulation wave carrier control method, otherwise, the control method to be adopted is determined based on the predicted half-bus voltage deviation value, the grid negative sequence voltage, the positive sequence reactive current given value, and the negative sequence reactive current given value; after switching from the single modulation wave carrier control method to the dual modulation wave carrier control method, when the maximum value of the actual half-bus voltage deviation value within a continuous power frequency cycle is not greater than the first preset value and the maximum value of the predicted half-bus voltage deviation value is not greater than the second preset value, switching to the single modulation wave carrier control method.

[0018] In one possible implementation, the control method to be adopted is determined based on the predicted half-bus voltage deviation value, the grid negative-sequence voltage, the positive-sequence reactive current given value, and the negative-sequence reactive current given value, specifically including: when the predicted half-bus voltage deviation value is greater than or equal to the second preset value, or the grid negative-sequence voltage is greater than or equal to the third preset value, or the positive-sequence reactive current given value is greater than or equal to the fourth preset value, or the negative-sequence reactive current given value is greater than or equal to the fifth preset value, the single-modulation wave carrier control method is switched to the dual-modulation wave carrier control method; when the maximum value of the actual half-bus voltage deviation value within a continuous power frequency cycle is not greater than the first preset value and the maximum value of the predicted half-bus voltage deviation value is not greater than the second preset value, the control method to be adopted is again determined based on the predicted half-bus voltage deviation value, the grid negative-sequence voltage, the positive-sequence reactive current given value, and the negative-sequence reactive current given value.

[0019] In a possible implementation, the maximum value of the actual half-bus voltage deviation value and the maximum value of the predicted half-bus voltage deviation value are determined once in each control cycle.

[0020] In a possible implementation, when it is determined to switch from the dual modulation wave carrier control method to the single modulation wave carrier control method, the method further includes: terminating the dual modulation wave carrier control method after a first preset time delay.

[0021] In a third aspect, the present application further provides a storage medium for storing program instructions, which, when executed, implement the method described above.

[0022] In a fourth aspect, the present application also provides a new energy power generation system, which includes the power converter provided by the above first aspect and any implementation method of the first aspect, and also includes a DC side circuit, which is connected to the DC side of the power converter; the DC side circuit includes a new energy power generation end and / or an energy storage system.

[0023] In one possible implementation, the new energy power generation end is a photovoltaic power generation end, including photovoltaic modules.

[0024] In one possible implementation, the new energy power generation end is a wind power generation end. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A schematic diagram of a power converter provided in this application;

[0026] Figure 2 A schematic diagram of a power converter provided in an embodiment of the present application;

[0027] Figure 3 A flow chart of a method for controlling a power converter provided in an embodiment of the present application;

[0028] Figure 4 A flowchart of another power converter control method provided in an embodiment of the present application;

[0029] Figure 5 A schematic diagram of a new energy power generation system provided in an embodiment of the present application. DETAILED DESCRIPTION

[0030] In order to enable people skilled in the art to understand the present application more clearly, the relevant terms in the technical solution of the present application are first explained below.

[0031] Positive sequence and negative sequence: In a three-phase power grid, positive sequence and negative sequence are distinguished based on the phase order of phases A, B, and C. For example, positive sequence means that phase A leads phase B by 120 degrees, phase B leads phase C by 120 degrees, and phase C leads phase A by 120 degrees. Negative sequence means that phase A lags phase B by 120 degrees, phase B lags phase C by 120 degrees, and phase C lags phase A by 120 degrees.

[0032] Dynamic: refers to the changing operating state of the power grid, such as due to load changes, equipment being put into operation or taken out of operation, etc.

[0033] Reactive current: In an AC circuit, when there's a phase difference between current and voltage, this portion of the current is called reactive current. This current is not considered active current, but rather has other effects on the circuit. The ratio of reactive to active current determines the power factor; a low power factor means reduced grid efficiency.

[0034] See also Figure 1 , which is a schematic diagram of a power converter provided in this application.

[0035] The power converter shown is a three-level active neutral point clamped (ANPC) converter. Each phase has a three-level ANPC topology. Each three-level ANPC topology includes six switches: two outer switches, two inner switches, and two clamping switches. Each switch includes a body diode.

[0036] Taking phase A as an example, the two outer tubes are T A1 and T A4 , the two inner tubes are T A2 and T A3 , the two clamping tubes are T A5 and T A6 .

[0037] When the clamping tube T of each phase A5 、T A6 、T B5 、T B6 、T C5 、T C6When the IGBT is always off, only the anti-parallel diode is connected, and the topology becomes an NPC structure.

[0038] C1 and C2 are DC capacitors, U C1 、U C2 is the DC capacitor terminal voltage, and DC+, O, and DC- represent the three phase voltage states. In this topology, to ensure normal system operation, the DC side midpoint voltage should be maintained at UC1 = UC2. In actual operation, the midpoint current is not always zero, which will cause low-frequency fluctuations in the DC side midpoint voltage and midpoint potential offset, thereby causing DC capacitor voltage imbalance.

[0039] The zero-sequence voltage injection method based on a single modulation wave can control and achieve key voltage balance. However, in the case of voltage faults, such as voltage drops, especially asymmetric drops, it is generally required to generate negative-sequence current to suppress negative-sequence voltage. Negative-sequence current will cause the mid-point voltage to fluctuate at the power frequency. The zero-sequence voltage injection method is difficult to control mid-point voltage fluctuations. Therefore, this scheme has a poor control effect on the mid-point potential.

[0040] In order to solve the above problems, the embodiments of the present application provide a power converter, a control method and a system for a power converter. The midpoint potential balance control method is reasonably selected according to the control strategy and different operating conditions, which can reduce harmonics, reduce the number of DC capacitors, and thus reduce costs and increase power density. At the same time, the number of switching times is taken into account to reduce losses, so that the power converter is under optimal control.

[0041] In order to enable people skilled in the art to understand the present application more clearly, the technical solutions in the embodiments of the present application will be described below in conjunction with the drawings in the embodiments of the present application.

[0042] The terms "first" and "second" in this application description are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features.

[0043] In this application, unless otherwise clearly specified and limited, the term "connection" should be understood in a broad sense. For example, "connection" can be a fixed connection, a detachable connection, or an integrated connection; it can be a direct connection or an indirect connection through an intermediate medium.

[0044] An embodiment of the present application provides a power converter, which is described in detail below with reference to the accompanying drawings.

[0045] See also Figure 1 , which is a schematic diagram of a power converter provided in an embodiment of the present application.

[0046] The power converter 10 may be an energy storage converter, a photovoltaic inverter, a wind power converter, etc., and is not specifically limited in the embodiment of the present application.

[0047] The power converter 10 includes a controller 12 and at least one power conversion topology, which may be a three-level ANPC topology or a three-level NPC topology.

[0048] In a possible implementation, when the power converter outputs three-phase AC power, the power converter includes three three-level ANPC topologies, namely 111 , 112 and 113 , each of which is used to output one-phase AC power.

[0049] The controller 12 may be an application specific integrated circuit (ASIC), a programmable logic device (PLD), a digital signal processor (DSP), or a combination thereof. The PLD may be a complex programmable logic device (CPLD), a field programmable gate array (FPGA), a generic array logic (GAL), or any combination thereof, and is not specifically limited in the embodiments of the present application.

[0050] The switching tube of the power converter 10 can be an insulated gate bipolar transistor (IGBT), a metal oxide semiconductor field effect transistor (MOSFET), a silicon carbide metal oxide semiconductor (SiC MOSFET), etc., and is not specifically limited in the embodiment of the present application.

[0051] The controller 12 can be used to control the on and off of the switch tube in the power converter 10 .

[0052] For the power converter 10 shown in the figure, when one phase outputs zero level, current is drawn from or injected into the capacitor midpoint, resulting in an unbalanced midpoint potential. When the midpoint potential is unbalanced, the midpoint potential needs to be adjusted. The principle of adjustment is to draw current from or inject current into the capacitor midpoint.

[0053] For example, when the positive sequence component of the AC port voltage is between 50% and 90% of the nominal voltage, the power converter 10 should be able to output a positive sequence dynamic reactive current to support the positive sequence voltage recovery and absorb a negative sequence dynamic reactive current from the grid to suppress the increase in the negative sequence voltage. The dynamic reactive current increment of the power converter 10 should respond to the change in the AC port voltage and satisfy the following formula (1):

[0054]

[0055] in, The positive sequence dynamic reactive current increment injected into the power converter 10; is the negative sequence dynamic reactive current increment absorbed by the power converter 10; is the dynamic positive sequence reactive current proportional coefficient of the power converter 10, The value should not be less than 1.0; is the dynamic negative sequence reactive current proportional coefficient of the power converter 10, The value should not be less than 1.0; is the per-unit value of the positive sequence component of the AC port voltage of the power converter 10; is the per-unit value of the negative sequence component of the AC port voltage of the power converter 10; I N is the rated current of the power converter 10 .

[0056] The controller 12 is configured to reasonably select the midpoint potential balance control method according to the control strategy and different operating conditions, which can reduce harmonics, reduce the number of DC capacitors, thereby reducing costs and increasing power density, while taking into account the number of switching times to reduce losses, so that the power converter is under optimal control.

[0057] During most of its life cycle, the power converter controls active power with a high power factor. At this time, the midpoint voltage has small low-frequency fluctuations and DC offset. In special circumstances, such as grid faults, high- and low-pass devices will generate reactive power, and the positive- and negative-sequence reactive currents will cause the midpoint voltage to fluctuate at the industrial frequency.

[0058] Based on the above situation, the controller 12 in the embodiment of the present application is configured to use the zero-sequence voltage injection method to control the midpoint balance when the power factor is high, and to use dual modulation wave control in voltage fault conditions, such as voltage drops, especially asymmetric drop conditions.

[0059] In order to enable those skilled in the art to more clearly understand the implementation principle of the technical solution of the present application, the zero-sequence voltage injection method and the dual modulation wave control method are introduced below respectively.

[0060] First, the implementation of the zero-sequence voltage injection method is explained.

[0061] The zero-sequence voltage injection method of the embodiment of the present application is implemented based on a single modulation wave carrier and can include the following three implementation methods:

[0062] Calculate the zero-sequence voltage based on the direction of the load current;

[0063] Calculate the zero-sequence voltage based on the load current value, bus voltage value and capacitance value;

[0064] The zero-sequence voltage is calculated according to a proportional-integral (PI) regulator.

[0065] Taking the calculation of zero-sequence voltage according to the direction of load current as an example, continue to refer to Figure 2 , according to the DC side upper and lower capacitor voltage U C1 、U C2 The regulator is designed based on the deviation and direction of the load current, and then the zero-sequence voltage U that needs to be injected into the three phases is determined. com , thereby controlling the midpoint potential balance.

[0066] Among them, U com It can be determined by the following formula (2):

[0067] U com =-sgn(U y )·sgn(i y )·(U C2 -U C1 ).

[0068] Among them, U com is the calculated zero-sequence voltage; U y is the phase voltage with the opposite sign to the other two phase voltages, where y = a, b, c; i y is the corresponding phase current; sgn() is the sign function; U C1 is the upper half bus voltage; U C2 is the lower half bus voltage.

[0069] The following describes how to implement the dual modulation wave control method.

[0070] When using the dual modulation wave control method, each phase includes the upper modulation wave U xp And the down-modulation wave U xn .

[0071] Up-modulation wave U xp Compare with the upper carrier to control the first outer tube and the second inner tube of each phase, that is, to control the T of phase a A1 and T A3 , controlling the T of phase b B1 and T B3 , controlling the T of phase c C1 and T C3 .

[0072] Down-modulation wave U xn Compare with the download wave to control the first inner tube and the second outer tube of each phase, that is, control the T of phase a A2 and T A4 , controlling the T of phase b B2 and T B4 , controlling the T of phase c C2 and T C4 .

[0073] In the three-level ANPC topology, the first clamping tube is linked to the second outer tube, and the second clamping tube is linked to the first outer tube. A5 and T A4 Linkage, T A6 and T A1 linkage; for phase b, that is, T B5 and T B4 Linkage, T B6 and T B1 linkage; for phase c, that is, T c5 and T c4 Linkage, T c6 and T c1 linkage.

[0074] The first clamping transistor and the second clamping transistor in the NPC topology are always in the off state.

[0075] The upper modulation wave voltage and the lower modulation wave voltage of each phase can be determined by the following formula (3):

[0076]

[0077] Where x = a, b, c; U max , U min They are respectively the typing and minimum value of the three-phase modulation wave Ux in the single modulation wave carrier modulation strategy.

[0078] The dual modulation wave carrier control calculates the compensation amount of each phase based on the capacitor voltage difference, upper and lower modulation waves, phase current, etc. The calculation formula is as follows (4):

[0079] V x_com =k p ·|U C2 -U C1 |·sgn[(U C2 -U C1 )·i x ]·sgn(U xp -U xn ).

[0080] Where V x_com is the compensation amount of each phase; K pis the proportionality coefficient; i x is a phase current.

[0081] To minimize switching losses, compensation is applied only to regions where neither modulation wave is clamped to zero. For phase a, this corresponds to the regions [π / 3 to 2π / 3] and [4π / 3 to 5π / 3]; for phase b, this corresponds to the regions [0 to π / 3] and [π to 4π / 3]; and for phase c, this corresponds to the regions [2π / 3 to π] and [5π / 3 to π]. Therefore, at any instant, only one phase voltage is applied with compensation.

[0082] Based on Kirchhoff's Current Law (KCL), see Figure 2 For the topology shown, the average model of the midpoint potential fluctuation in the abc coordinate system is as follows:

[0083] C·(U′ C2 -U′ C1 )=-|V ra |·i a -|V rb |·i b -|V rc |·i c .

[0084] C is the capacitance value of the half bus capacitor; U′ C2 -U′ C1 is the difference between the upper and lower busbar voltage theoretical values; i a 、i b 、i c They are the instantaneous values ​​of the three-phase current, V ra 、V rb 、V rc From formula (5), we can know that the theoretical difference △U′ of the midpoint voltage can be calculated based on the three-phase modulation wave and the three-phase instantaneous current.

[0085] The following describes the meanings of the relevant parameters involved in this application:

[0086] △U=|U C2 -U C1 |, the actual difference of half-bus voltage, which can also be called the actual half-bus voltage deviation value;

[0087] △U′=|U′ C2 -U′ C1 |, the theoretical difference of half bus voltage, which can also be called the predicted half bus voltage deviation value;

[0088] △U M It is the maximum value of the actual half-bus voltage deviation within a power frequency cycle;

[0089] △U′ M It is the maximum value of the predicted half-bus voltage deviation within a power frequency cycle.

[0090] The controller 12 in the embodiment of the present application is configured to select a suitable control strategy by combining the actual half-bus deviation value ΔU and the predicted half-bus theoretical deviation value ΔU′.

[0091] When the power converter 10 is affected by changes in the external environment during operation, causing the actual half-bus voltage deviation value △U to be too large, it must be controlled. At this time, it switches to dual-modulation wave carrier control. The controller predicts the deviation of the half-bus in the next control cycle in real time based on equation (5), that is, obtains the predicted half-bus voltage deviation value △U′ for the next control cycle. Then, it can switch to dual-modulation wave carrier control in advance to ensure that the midpoint voltage fluctuation meets the control requirements.

[0092] During the power frequency cycle, the controller 12 determines △U once in each control cycle. M , and determine once △U′ M , the amount of calculation is small and the data is more accurate,

[0093] In the embodiment of the present application, the end condition of the dual-modulation wave carrier control is increased by a delay of one industrial frequency cycle, which can avoid frequent switching between the dual-modulation wave carrier control and the single-modulation wave carrier control (zero-sequence voltage injection method). Frequent switching will cause sudden changes in the modulation wave, and then cause problems such as harmonics, which will cause the control system to become unstable in severe cases.

[0094] The control process of the controller 12 will be described in detail below.

[0095] When the power converter starts operating, the controller 12 first performs single-modulation wave carrier control. When the actual half-bus voltage deviation value ΔU is greater than or equal to the first preset value K1, it indicates that the half-bus voltage difference is too large for some reason and the midpoint control strategy needs to be adjusted. At this time, the controller 12 switches to dual-modulation wave carrier control.

[0096] In the process of dual modulation wave carrier control, the actual half bus voltage deviation value △U and the predicted half bus voltage deviation value △U' are combined to select the appropriate control strategy to ensure that the midpoint voltage deviation meets the requirements. M and △U′ M Calculate the maximum value of .

[0097] The controller 12 calculates the current value ΔU and the maximum value ΔU M Compare and compare the current value △U′ with the maximum value △U′ M When the current value is greater than the maximum value, update the current value to the maximum value △U M (△U′M ), when the current value is less than the maximum value, the maximum value is maintained and the counter starts timing to ensure that it is calculated once in each control cycle and continuously runs to calculate one power frequency cycle (for example, 20 milliseconds).

[0098] In one power frequency cycle, as long as △U M are less than or equal to K1, and △U′ M If both are less than or equal to the second preset value K2, the system switches to single modulation wave control. Otherwise, dual modulation wave carrier control is continued. In addition, a delay of one power frequency cycle is added to the exit condition of dual modulation wave carrier control to avoid frequent switching between dual modulation wave carrier control and single modulation wave carrier control.

[0099] The controller 12 determines ΔU once in each control cycle. M , and determine once △U′ M , the amount of calculation is small and the data is more accurate. If the maximum value of the power frequency cycle is calculated within one control cycle, the amount of calculation will be large and the error will be large.

[0100] When the actual half bus voltage deviation value △U is less than the first preset value K1, the controller 12 calculates the half bus voltage deviation value △U′ and the grid negative sequence voltage U according to the predicted half bus voltage deviation value - , positive sequence reactive current given value I q+ , negative sequence reactive current given value I q- Predict the degree of deviation of the midpoint potential and switch to dual modulation wave control in advance.

[0101] Specifically, the controller 12 predicts that the half bus voltage deviation value ΔU′ is greater than or equal to the second preset value K2, or the grid negative sequence voltage U - Greater than or equal to the third preset value N1, or the positive sequence reactive current given value I q+ Greater than or equal to the fourth preset value N2, or the negative sequence reactive current given value I q- When any one of the items greater than or equal to the fifth preset value N3 is met, dual modulation wave carrier control is adopted; otherwise, single modulation wave carrier control is performed.

[0102] After that, in the process of executing dual modulation wave carrier control, the controller starts to start △U′ in the power frequency cycle M The maximum value of △U′ and △U′ M Comparison shows that the current value △U′ is greater than △U′ M When △U′ is updated to △U′ M Otherwise, the maximum value is maintained. The counter starts timing, and △U′ is calculated once in each control cycle. Continuous calculation of one power frequency cycle is performed. In one power frequency cycle, as long as △U M are all less than K1, and △U′ Mare both less than or equal to the second preset value K2, then the half bus voltage deviation value △U′ and the grid negative sequence voltage U - , positive sequence reactive current given value I q+ , negative sequence reactive current given value I q- The degree of midpoint potential deviation is predicted to determine whether to switch to dual-modulation control. Furthermore, a delay of one power frequency cycle is added to the exit condition for dual-modulation carrier control to avoid frequent switching between dual-modulation carrier control and single-modulation carrier control. The maximum value of a power frequency cycle is calculated once per control cycle, which reduces the computational effort. Calculating the maximum value within a control cycle is computationally intensive and can result in significant errors.

[0103] To sum up, the solution provided in the embodiment of the present application can select a suitable midpoint potential balance control method according to different trigger conditions, can take into account the effect of midpoint balance control and the current harmonic content, reduce the number of DC capacitors, reduce costs, increase power density, reduce losses, and put the power converter in the optimal operating mode.

[0104] It is understood that when it is determined to switch from the dual-modulation wave carrier control method to the single-modulation wave carrier control method, the dual-modulation wave carrier control method is terminated after a first preset delay. The above description uses the first preset time as one power frequency cycle as an example. In actual applications, the first preset time may also be other values, such as two power frequency cycles.

[0105] Based on the power converter provided in the above embodiments, an embodiment of the present application further provides a control method for a power converter, which is described in detail below with reference to the accompanying drawings.

[0106] See also Figure 3 , which is a flow chart of a control method for a power converter provided in an embodiment of the present application.

[0107] This method is used to achieve midpoint potential balance of power converters. For the specific implementation of power converters, please refer to Figure 2 The corresponding description is not repeated here in the embodiment of the present application. The method includes the following steps:

[0108] S11: Determine the control method to be adopted using the actual half-bus voltage deviation value and the predicted half-bus voltage deviation value.

[0109] Among them, the control method is a dual modulation wave carrier control method or a single modulation wave carrier control method.

[0110] The single-modulation wave carrier control method, also known as the zero-sequence voltage injection method, has the characteristics of low harmonics, low losses, and simple control, but the control effect is poor under special working conditions, such as low power factor and grid faults.

[0111] The dual-modulation wave carrier control method has more precise control, but it has large harmonics and high losses.

[0112] The advantages and disadvantages of both methods are obvious. The control method provided in this application can combine the advantages and disadvantages of the two methods and provide flexible control, thereby reducing harmonics, reducing the number of DC capacitors, reducing costs, and increasing power density. At the same time, it takes into account the number of switching times to reduce losses, so that the power converter is under optimal control.

[0113] S12: Controlling a switching device in at least one power conversion topology based on a control method to balance the potential of a busbar midpoint of the power converter.

[0114] The following describes the specific implementation method.

[0115] See also Figure 4 , which is a flow chart of another power converter control method provided in an embodiment of the present application.

[0116] The method comprises the following steps:

[0117] S21: Use single modulation wave carrier control method.

[0118] When the power converter starts running, the single modulation wave carrier control method is first adopted.

[0119] S22: Is △U greater than or equal to K1?

[0120] If so, execute S23; otherwise, execute S25.

[0121] When the actual half-bus voltage deviation value △U is greater than or equal to the first preset value K1, it indicates that the half-bus voltage difference is too large for some reason. At this time, the midpoint control strategy needs to be adjusted and switched to dual-modulation wave carrier control.

[0122] S23: Use dual modulation wave carrier control method.

[0123] In the process of dual modulation wave carrier control, the actual half bus voltage deviation value △U and the predicted half bus voltage deviation value △U' are combined to select the appropriate control strategy to ensure that the midpoint voltage deviation meets the requirements. M and △U′ M Calculate the maximum value of .

[0124] S24: Is △U within a continuous power frequency cycle? M ≤K1, and △U′ M ≤K2.

[0125] The current value △U and the maximum value △U M Compare and compare the current value △U′ with the maximum value △U′ MWhen the current value is greater than the maximum value, update the current value to the maximum value △U M (△U′ M ), when the current value is less than the maximum value, the maximum value is maintained and the counter starts timing to ensure that it is calculated once in each control cycle and continuously runs to calculate one power frequency cycle (for example, 20 milliseconds).

[0126] The maximum value of the actual half-bus voltage deviation value is not greater than the first preset value, and the maximum value of the predicted half-bus voltage deviation value is not greater than the second preset value.

[0127] If so, execute S21; otherwise, execute S23.

[0128] It can be understood that when it is determined to switch from the dual modulation wave carrier control method to the single modulation wave carrier control method, a delay of one industrial frequency cycle is added to the exit condition of the dual modulation wave carrier control to avoid frequent switching between dual modulation wave carrier control and single modulation wave carrier control.

[0129] Determine △U once in each control cycle M , and determine once △U′ M , the amount of calculation is small and the data is more accurate. If the maximum value of the power frequency cycle is calculated within one control cycle, the amount of calculation will be large and the error will be large.

[0130] S25: Determine △U′≥K2, or U - ≥N1, or I q+ ≥N2, or I q- ≥Whether at least one of N3 is true.

[0131] If so, execute S26; otherwise, execute S21.

[0132] In this step, when the actual half-bus voltage deviation value is less than the first preset value K1, based on the predicted half-bus voltage deviation value △U', the grid negative sequence voltage U - , positive sequence reactive current given value I q+ , negative sequence reactive current given value I q- Specifically, when the predicted half bus voltage deviation value △U′ is greater than or equal to the second preset value K2, or the grid negative sequence voltage U - Greater than or equal to the third preset value N1, or the positive sequence reactive current given value I q+ Greater than or equal to the fourth preset value N2, or the negative sequence reactive current given value I q- When any one of the items greater than or equal to the fifth preset value N3 is met, dual modulation wave carrier control is adopted; otherwise, single modulation wave carrier control is performed.

[0133] S26: Use dual modulation wave carrier control method.

[0134] In the process of executing dual modulation wave carrier control, △U′ in the power frequency cycle is started. M The maximum value of △U′ and △U′ M Comparison shows that the current value △U′ is greater than △U′ M When △U′ is updated to △U′ M Otherwise, the maximum value is maintained. The counter starts timing, calculating △U′ once in each control cycle, and continuously calculating one power frequency cycle.

[0135] S27: Is there a △U that lasts for one power frequency cycle? M are less than or equal to K1, and △U′ M Are less than or equal to K2.

[0136] If so, execute S25; otherwise, execute S26.

[0137] In one power frequency cycle, as long as △U M are all less than K1, and △U′ M are both less than or equal to the second preset value K2, then the half bus voltage deviation value △U′ and the grid negative sequence voltage U - , positive sequence reactive current given value I q+ , negative sequence reactive current given value I q- The degree of midpoint potential deviation is predicted to determine whether to switch to dual-modulation control. Similarly, a delay of one power frequency cycle is added to the exit condition for dual-modulation carrier control to avoid frequent switching between dual-modulation carrier control and single-modulation carrier control. The maximum value of a power frequency cycle is calculated once per control cycle, which is computationally efficient. Calculating the maximum value within a control cycle is computationally expensive and can result in significant errors.

[0138] In summary, the method provided by the embodiments of the present application enables selection of the currently applicable control method from either the dual-modulation carrier control method or the single-modulation carrier control method based on the actual and predicted half-bus voltage deviation values ​​to maintain bus midpoint potential balance. This allows the advantages of both control methods to be combined, reducing harmonics, the number of DC capacitors, lowering costs, and increasing power density, while also taking into account the number of switching cycles to reduce losses, ultimately ensuring optimal control of the power converter.

[0139] Based on the power converter provided in the above embodiments, the embodiments of the present application further provide a new energy power generation system, which will be described in detail below with reference to the accompanying drawings.

[0140] See also Figure 5 , which is a schematic diagram of a new energy power generation system provided in an embodiment of the present application.

[0141] The illustrated new energy power generation system 170 includes a DC side circuit 171 and a power converter 172 .

[0142] The specific implementation method and working principle of the power converter 172 can be found in the description of the above embodiments, and will not be repeated here in the embodiments of the present application.

[0143] The DC side circuit 171 is connected to the DC side of the power converter 172 .

[0144] In one possible implementation, DC-side circuit 171 includes a new energy generation terminal. For example, the new energy generation terminal is a photovoltaic generation terminal, including a photovoltaic module. The photovoltaic module is used to generate direct current (DC) using light energy and output it to the DC side of power converter 172. For another example, the new energy generation terminal is a wind power generation terminal, which is used to convert wind energy into electrical energy.

[0145] In another possible implementation, the DC side circuit 171 includes an energy storage system. The energy storage system may include a battery for storing energy. The energy storage system may output DC power to the DC side of the power converter 172 .

[0146] The power converter of the new energy power generation system provided by this application can select the currently applicable control method from the dual-modulation wave carrier control method and the single-modulation wave carrier control method based on the actual half-bus voltage deviation value and the predicted half-bus voltage deviation value to maintain the potential balance of the bus midpoint. Therefore, the respective advantages of the two control methods can be combined to reduce harmonics, reduce the number of DC capacitors, reduce costs, and increase power density. At the same time, the number of switching times is taken into account to reduce losses, so that the power converter is under optimal control and the reliability of the new energy power generation system is improved.

[0147] It should be understood that in this application, "at least one (item)" means one or more, and "plurality" means two or more. "And / or" is used to describe the association relationship of associated objects, indicating that three relationships may exist. For example, "A and / or B" can mean: only A exists, only B exists, and A and B exist at the same time, where A and B can be singular or plural. The character " / " generally indicates that the previous and next associated objects are in an "or" relationship. "At least one of the following items" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, at least one of a, b or c can mean: a, b, c, "a and b", "a and c", "b and c", or "a and b and c", where a, b, c can be single or multiple.

[0148] Each embodiment in this specification is described in a progressive manner, and the same or similar parts between the embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments. The device embodiments described above are merely illustrative, wherein the units and modules described as separate components may or may not be physically separated. In addition, some or all of the units and modules may be selected according to actual needs to achieve the purpose of the scheme of this embodiment. Those of ordinary skill in the art can understand and implement it without paying any creative work.

[0149] The above is only a specific implementation method of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present application. These improvements and modifications should also be regarded as the scope of protection of the present application.

Claims

1. A power converter, characterized in that: include: a controller and at least one power conversion topology; The power conversion topology is a three-level active neutral point clamped ANPC topology or a three-level neutral point clamped NPC topology; The controller is configured to determine a control method to be adopted using the actual half-bus voltage deviation value and the predicted half-bus voltage deviation value, and control the switching devices in the at least one power conversion topology based on the control method to balance the potential of the bus midpoint of the power converter; Wherein, the control method is a dual modulation wave carrier control method or a single modulation wave carrier control method.

2. The power converter according to claim 1, wherein: The controller is specifically configured to: When the power converter starts to operate, the single modulation wave carrier control method is first adopted; When the actual half-bus voltage deviation value is greater than or equal to the first preset value, the dual-modulation wave carrier control method is adopted; otherwise, the control method to be adopted is determined based on the predicted half-bus voltage deviation value, the grid negative sequence voltage, the positive sequence reactive current given value, and the negative sequence reactive current given value; After switching from the single modulation wave carrier control method to the dual modulation wave carrier control method, when the maximum value of the actual half-bus voltage deviation value within a continuous power frequency cycle is not greater than the first preset value and the maximum value of the predicted half-bus voltage deviation value is not greater than the second preset value, switch to using the single modulation wave carrier control method.

3. The power converter according to claim 2, wherein: The controller is specifically configured to: When the predicted half-bus voltage deviation value is greater than or equal to a second preset value, or the grid negative sequence voltage is greater than or equal to a third preset value, or the positive sequence reactive current given value is greater than or equal to a fourth preset value, or the negative sequence reactive current given value is greater than or equal to a fifth preset value, the single modulation wave carrier control method is switched to the dual modulation wave carrier control method; When the maximum value of the actual half-bus voltage deviation value within a continuous power frequency cycle is not greater than the first preset value and the maximum value of the predicted half-bus voltage deviation value is not greater than the second preset value, the control method to be adopted is determined again based on the predicted half-bus voltage deviation value, the negative-sequence voltage of the power grid, the positive-sequence reactive current given value, and the negative-sequence reactive current given value.

4. The power converter according to claim 2, wherein: The controller is specifically configured to: determine the maximum value of the actual half-bus voltage deviation value once, and determine the maximum value of the predicted half-bus voltage deviation value once in each control cycle.

5. The power converter according to any one of claims 1 to 4, characterized in that: The controller is configured to: When it is determined to switch from the dual modulation wave carrier control method to the single modulation wave carrier control method, the dual modulation wave carrier control method is terminated after a first preset time delay.

6. A method for controlling a power converter, characterized in that: The power converter includes at least one power conversion topology, wherein the power conversion topology is a three-level active neutral point clamped ANPC topology or a three-level neutral point clamped NPC topology. The control method of the power converter includes: Determine the control method to be used by using the actual half-bus voltage deviation value and the predicted half-bus voltage deviation value; Controlling the switching devices in the at least one power conversion topology based on the control method to balance the potential of the busbar midpoints of the power converter; Wherein, the control method is a dual modulation wave carrier control method or a single modulation wave carrier control method.

7. The method according to claim 6, characterized in that The control method used to determine the actual half-bus voltage deviation value and the predicted half-bus voltage deviation value specifically includes: When the power converter starts to operate, the single modulation wave carrier control method is first adopted; When the actual half-bus voltage deviation value is greater than or equal to the first preset value, the dual-modulation wave carrier control method is adopted; otherwise, the control method to be adopted is determined based on the predicted half-bus voltage deviation value, the grid negative sequence voltage, the positive sequence reactive current given value, and the negative sequence reactive current given value; After switching from the single modulation wave carrier control method to the dual modulation wave carrier control method, when the maximum value of the actual half-bus voltage deviation value within a continuous power frequency cycle is not greater than the first preset value and the maximum value of the predicted half-bus voltage deviation value is not greater than the second preset value, switch to using the single modulation wave carrier control method.

8. The method according to claim 7, characterized in that The control method to be adopted is determined based on the predicted half-bus voltage deviation value, the grid negative sequence voltage, the positive sequence reactive current given value, and the negative sequence reactive current given value, specifically including: When the predicted half-bus voltage deviation value is greater than or equal to a second preset value, or the grid negative sequence voltage is greater than or equal to a third preset value, or the positive sequence reactive current given value is greater than or equal to a fourth preset value, or the negative sequence reactive current given value is greater than or equal to a fifth preset value, the single modulation wave carrier control method is switched to the dual modulation wave carrier control method; When the maximum value of the actual half-bus voltage deviation value within a continuous power frequency cycle is not greater than the first preset value and the maximum value of the predicted half-bus voltage deviation value is not greater than the second preset value, the control method to be adopted is determined again based on the predicted half-bus voltage deviation value, the negative-sequence voltage of the power grid, the positive-sequence reactive current given value, and the negative-sequence reactive current given value.

9. The method according to claim 7, characterized in that The maximum value of the actual half-bus voltage deviation value and the maximum value of the predicted half-bus voltage deviation value are determined once in each control cycle.

10. The method according to any one of claims 6 to 9, characterized in that When it is determined to switch from the dual modulation wave carrier control method to the single modulation wave carrier control method, the method further includes: The dual modulation wave carrier control method is terminated after a first preset time delay.

11. A new energy power generation system, characterized in that: The new energy power generation system comprises the power converter according to any one of claims 1 to 5, and further comprises a DC side circuit connected to the DC side of the power converter; The DC side circuit includes a new energy power generation terminal and / or an energy storage system.

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