Modulation method and system for neutral point potential balance of three-level inverter connected to new energy grid

Abstract

The present invention discloses a modulation method and system for balancing the midpoint potential of a three-level inverter connected to a new energy grid. The present invention comprises: a 、v b and v c Perform sector judgment and reference voltage vector mapping, according to the mapped three-phase reference voltage v a1 、v b1 and v c1 As well as the sampled midpoint potential deviation V1‑V2 and three-phase current i abc Calculate the optimal switching waveform and its time parameter d a1 d b1 and d c1 , and then the time parameter d a1 d b1 and d c1 The reverse mapping is the time parameter d of each phase waveform of the original voltage sector a d b and d c , the time parameter d of each phase waveform of the original voltage sector a d b and d c The input modulator generates a modulation signal to control the three-level inverter. The present invention aims to suppress the midpoint potential fluctuation of the three-level inverter for new energy grid connection and reduce current harmonics and switching losses at the same time.

Description

Technical Field

[0001] The present invention relates to the technical field of grid connection of a new energy power station, and in particular to a modulation method and system for midpoint potential balance of a new energy grid-connected three-level inverter. Background Art

[0002] In recent years, the scale of renewable energy power generation has grown rapidly, and the construction of a new power system based on microgrids of renewable energy is an inevitable trend in the current power system. At present, the microgrids using renewable energy power generation are connected to the grid mainly by power electronic converters. The power electronic converters convert the electric energy generated by the microgrids using renewable energy power generation into AC power and then integrate it into the grid. The three-level inverter has the advantages of high withstand voltage level, low harmonic content, and high working efficiency, and has been widely used in renewable energy grid-connected systems. However, there are low-frequency fluctuations in the midpoint potential of the three-level inverter. The fluctuation of the midpoint potential will cause the voltage on the output side of the inverter to be distorted, generate low-order harmonics, shorten the working life of the DC side capacitor, and cause the switching devices to withstand excessive voltage, and even damage the switching devices. Therefore, the midpoint potential balance problem has always been the focus of the three-level topology research.

[0003] At present, the midpoint potential algorithms for diode-clamped three-level inverters mainly include the charge balance method and the hysteresis comparison method. Both of them are based on adjusting the action time of the redundant small vector to adjust the current injected into the DC midpoint. However, these methods are subject to the limitations of the modulation coefficient and the load power factor, and are not suitable for renewable energy grid-connected power generation systems with adjustable power factor. In addition, some scholars have proposed a virtual vector modulation algorithm, which strictly ensures that the average value of the current injected into the midpoint in each switching cycle is zero, thereby suppressing the fluctuation of the midpoint potential. However, its current harmonics are large and the number of switching times is large, resulting in increased losses in power transmission, and is also not conducive to the stability of the system. Summary of the invention

[0004] Technical problem to be solved by the present invention: In view of the above-mentioned problems in the prior art, a modulation method and system for the neutral point potential balance of a three-level inverter connected to the renewable energy grid are provided. The present invention aims to suppress the fluctuation of the neutral point potential of the three-level inverter connected to the renewable energy grid, and at the same time reduce current harmonics and switching losses.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0006] A modulation method for neutral point potential balance of a new energy grid-connected three-level inverter comprises the following steps: S1, converting a three-phase reference voltage from a controller , and Perform sector judgment and reference voltage vector mapping to obtain the mapped three-phase reference voltage , and ; S2, according to the mapped three-phase reference voltage , and And the midpoint potential deviation of the sample and three-phase current Calculate the optimal switching waveform and its timing parameters , and ; S3, the time parameter , and Reverse mapping to the time parameters of each phase waveform of the original voltage sector , and ; S4, the time parameters of each phase waveform of the original voltage sector , and The input modulator generates a modulation signal to control the three-level inverter.

[0007] Optionally, step S1 includes:

[0008] S1.1, according to the three-phase reference voltage from the controller , and The phase angle is used to determine the sector in the three-level inverter space vector diagram, and the three-level inverter space vector diagram is evenly divided into six large sectors S1-S6;

[0009] S1.2, based on the determined sectors in the three-level inverter space vector diagram and the preset mapping rules, the three-phase reference voltage from the controller , and Perform reference voltage vector mapping to obtain the mapped three-phase reference voltage , and .

[0010] Optionally, in step S1.2, the three-phase reference voltage from the controller is converted based on the determined sectors in the three-level inverter space vector diagram and the preset mapping rule. , and Perform reference voltage vector mapping to obtain the mapped three-phase reference voltage , and When the sector in the three-level inverter space vector diagram is the large sector S1, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and ; If the sector in the three-level inverter space vector diagram is the large sector S2, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and ; If the sector in the three-level inverter space vector diagram is the large sector S3, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and ; If the sector in the three-level inverter space vector diagram is the large sector S4, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and ; If the sector in the three-level inverter space vector diagram is the large sector S5, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and ; If the sector in the three-level inverter space vector diagram is the large sector S6, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and .

[0011] Optionally, step S2 includes:

[0012] S2.1, an extended virtual voltage vector is introduced into sector S1, and the extended virtual voltage vector is composed of two basic vectors to ensure that the output midpoint current value is 0. Based on the extended virtual voltage vector, the reference voltage vector is synthesized to obtain four switching waveforms located in the S1 voltage sector, which are recorded as: SVPWM-P, SVPWM-N, EVSVPWM-P, EVSVPWM-N;

[0013] S2.2, establish the relationship between the four switching waveforms that satisfy the volt-second balance principle, and the midpoint currents of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N About the relationship between three-phase current;

[0014] S2.3, sampling the midpoint potential deviation of the three-level inverter connected to the new energy grid ,in is the capacitor voltage on the DC side, is the voltage of the lower capacitor; the reference value of the midpoint current required to balance the midpoint potential is calculated according to the following formula: :

[0015] ,

[0016] in, Indicates the DC side capacitance value, represents the switching cycle;

[0017] S2.4, according to the relationship between the four switching waveforms satisfying the volt-second balance principle, and the midpoint current of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, EVSVPWM-N Regarding the relationship between the three-phase currents, solve the midpoint currents of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N under the condition of satisfying the volt-second balance principle. The reference value of the midpoint current closest to the balanced midpoint potential The optimal switching waveform and its time parameters , and The optimal switching waveform is one of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N. The time parameter , and Defines the switching time parameters of phases A, B, and C in sector S1.

[0018] Optionally, in the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N in step S2.1, the switching waveforms of the three phases A, B, and C of the switching waveform SVPWM-P are OPO, OPO, and NON, respectively; the switching waveforms of the three phases A, B, and C of the switching waveform SVPWM-N are OPO, NON, and NON, respectively; the switching waveforms of the three phases A, B, and C of the switching waveform EVSVPWM-P are P, NOPON, and NON, respectively; and the switching waveform EVSVPWM- The switching waveforms of the A, B and C phases of N are OPO, NOPON and N respectively; the switching state O in the switching waveform indicates that S1 and S3 are closed, and S2 and S4 are opened; the switching state P in the switching waveform indicates that S3 and S4 are closed, and S1 and S2 are opened; the switching state N in the switching waveform indicates that S1 and S2 are closed, and S3 and S4 are opened; the “-” in the switching waveform is used to connect multiple switching states of a phase in a switching cycle to indicate the switching state switching order of the phase, and S1~S4 are four switching tubes of the new energy grid-connected three-level inverter.

[0019] Optionally, when establishing the relationship between the four switching waveforms satisfying the volt-second balance principle in step S2.2, the relationship between the switching waveform SVPWM-P satisfying the volt-second balance principle is:

[0020] ,

[0021] The switching waveform SVPWM-N satisfies the volt-second balance principle as follows:

[0022] ,

[0023] The switching waveform EVSVPWM-P satisfies the volt-second balance principle as follows:

[0024] ,

[0025] The switching waveform EVSVPWM-N satisfies the volt-second balance principle as follows:

[0026] ,

[0027] in, Indicates the time ratio of the A phase switch state P; Indicates the time ratio of the B phase switch state P; It is the DC bus voltage of the three-level inverter for renewable energy grid connection; , and is the reference voltage of the three phases A, B and C before mapping; Indicates the time ratio of the B phase switch state N, Indicates the time ratio of the C phase switch state N, , , and The value is between 0 and 1; the midpoint current of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N in step S2.2 The functional expression of the relationship between the three-phase current is:

[0028] ,

[0029] in, , and The three-phase current The A, B, and C phase currents in , and They are the time ratios of the three-phase A, B, and C in the O state respectively.

[0030] Optionally, in step S3, the time parameter , and Reverse mapping to the time parameters of each phase waveform of the original voltage sector , and Including: If the sector in the three-level inverter space vector diagram is a large sector S1, then the time parameter , and Mapped in order , and As the time parameter after mapping , and ; If the sector in the three-level inverter space vector diagram is the large sector S2, then the time parameter , and Mapped in order , and As the time parameter after mapping , and ; If the sector in the three-level inverter space vector diagram is the large sector S3, then the time parameter , and Mapped in order , and As the time parameter after mapping , and ; If the sector in the three-level inverter space vector diagram is the large sector S4, then the time parameter , and Mapped in order , and As the time parameter after mapping , and ; If the sector in the three-level inverter space vector diagram is the large sector S5, then the time parameter , and Mapped in order , and As the time parameter after mapping , and ; If the sector in the three-level inverter space vector diagram is the large sector S6, then the time parameter , and Mapped in order , and As the time parameter after mapping , and .

[0031] In addition, the present invention also provides a modulation system for neutral point potential balance of a new energy grid-connected three-level inverter, comprising a microprocessor and a memory connected to each other, wherein the microprocessor is programmed or configured to execute the modulation method for neutral point potential balance of the new energy grid-connected three-level inverter.

[0032] In addition, the present invention also provides a computer-readable storage medium, which stores a computer program or instruction, and the computer program or instruction is programmed or configured to execute the modulation method for midpoint potential balance of the new energy grid-connected three-level inverter through a processor.

[0033] In addition, the present invention also provides a computer program product, including a computer program or instructions, which are programmed or configured to execute the modulation method for neutral point potential balance of the new energy grid-connected three-level inverter through a processor.

[0034] Compared with the prior art, the present invention mainly has the following advantages: the modulation method for balancing the midpoint potential of the three-level inverter connected to the new energy grid of the present invention eliminates the midpoint vector in the space vector diagram of the three-level inverter, introduces an extended virtual voltage vector, and within one switching cycle, the average value of the midpoint current generated by the extended virtual voltage vector is zero, and the reference voltage vector is synthesized only by the extended virtual voltage vector, thereby suppressing the midpoint voltage fluctuation problem caused by the use of the midpoint vector in the traditional modulation method; and by selecting the appropriate extended virtual voltage vector and its action sequence in each area, the current harmonics are optimized and the switching loss is minimized; the present invention can effectively suppress the midpoint potential fluctuation, is not limited by the modulation coefficient and the load power factor, and effectively increases the system reliability and stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 Schematic diagram of the basic flow of the method of the embodiment of the present invention.

[0036] Figure 2 The figure is a flow chart of the control principle of the method according to the embodiment of the present invention.

[0037] Figure 3 Schematic diagram of the topological structure of a three-level inverter for grid-connected renewable energy in an embodiment of the present invention.

[0038] Figure 4 1 is a three-level space vector diagram in an embodiment of the present invention.

[0039] Figure 5 Schematic diagram of the S1 sector extended virtual voltage vector in an embodiment of the present invention.

[0040] Figure 6 Graph showing the midpoint potential simulation results of three modulation algorithms in an embodiment of the present invention.

[0041] Figure 7 Graph showing the simulation results of phase A current for three modulation algorithms in an embodiment of the present invention. DETAILED DESCRIPTION

[0042] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0043] The modulation method for neutral point potential balance of the three-level inverter connected to the new energy grid of the present invention introduces the concept of extended virtual voltage vector by sampling the neutral point voltage deviation and the three-phase current, and adopts the extended virtual voltage vector to synthesize the reference voltage vector, thereby suppressing the fluctuation of the neutral point potential; and adopts a simple implementation method to avoid the complex large and small sector judgment and voltage vector selection of the three-level. Specifically, Figure 1 and Figure 2 As shown, the modulation method for neutral point potential balance of a three-level inverter connected to a new energy grid in this embodiment includes the following steps: S1, sector judgment and reference voltage vector mapping: converting the three-phase reference voltage from the controller , and Perform sector judgment and reference voltage vector mapping to obtain the mapped three-phase reference voltage , and ; S2, calculate the optimal switching waveform and its time parameters: according to the mapped three-phase reference voltage , and And the midpoint potential deviation of the sample and three-phase current Calculate the optimal switching waveform and its timing parameters , and ; S3, reverse mapping: transform the time parameter , and Reverse mapping to the time parameters of each phase waveform of the original voltage sector , and ; S4, modulation: the time parameters of each phase waveform of the original voltage sector , and The input modulator generates a modulation signal to control the three-level inverter.

[0044] Figure 3 Schematic diagram of the topological structure of the three-level inverter for connecting the new energy grid to the grid in this embodiment. The new energy power generation / energy storage equipment is connected to the grid through the three-level inverter for connecting the new energy grid to the grid. Figure 3 middle Measure the DC bus voltage of the three-level inverter for new energy grid connection. and They are the upper and lower arm voltages of the three-level inverter for new energy grid connection, ~ The four switching devices are in one of the bridge arms of the three phases A, B and C. , and is the output voltage of the three phases A, B and C, , and is the output current of the three phases A, B and C, For the reactor, is the resistance, For the power grid.

[0045] In this embodiment, step S1 includes:

[0046] S1.1, according to the three-phase reference voltage from the controller , and The phase angle is used to determine the sector in the three-level inverter space vector diagram, and the three-level inverter space vector diagram is evenly divided into six large sectors S1-S6;

[0047] S1.2, based on the determined sectors in the three-level inverter space vector diagram and the preset mapping rules, the three-phase reference voltage from the controller , and Perform reference voltage vector mapping to obtain the mapped three-phase reference voltage , and .

[0048] In this embodiment, in step S1.2, the three-phase reference voltage from the controller is converted based on the sectors determined in the three-level inverter space vector diagram and the preset mapping rules. , and Perform reference voltage vector mapping to obtain the mapped three-phase reference voltage , and When the sector in the three-level inverter space vector diagram is the large sector S1, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and ; If the sector in the three-level inverter space vector diagram is the large sector S2, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and ; If the sector in the three-level inverter space vector diagram is the large sector S3, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and ; If the sector in the three-level inverter space vector diagram is the large sector S4, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and ; If the sector in the three-level inverter space vector diagram is the large sector S5, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and ; If the sector in the three-level inverter space vector diagram is the large sector S6, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and , as shown in Table 1.

[0049] Table 1: Mapping relationship table of six large sectors S1~S6 converted to large sector S1

[0050]

[0051] In this embodiment, step S2 includes:

[0052] S2.1, an extended virtual voltage vector is introduced into sector S1, and the extended virtual voltage vector is composed of two basic vectors to ensure that the output midpoint current value is 0. Based on the extended virtual voltage vector, the reference voltage vector is synthesized to obtain four switching waveforms located in the S1 voltage sector, which are recorded as: SVPWM-P, SVPWM-N, EVSVPWM-P, EVSVPWM-N;

[0053] S2.2, establish the relationship between the four switching waveforms that satisfy the volt-second balance principle, and the midpoint currents of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N About the relationship between three-phase current;

[0054] S2.3, sampling the midpoint potential deviation of the three-level inverter connected to the new energy grid ,in is the capacitor voltage on the DC side, is the voltage of the lower capacitor; the reference value of the midpoint current required to balance the midpoint potential is calculated according to the following formula: :

[0055] ,

[0056] in, Indicates the DC side capacitance value, represents the switching cycle;

[0057] S2.4, according to the relationship between the four switching waveforms satisfying the volt-second balance principle, and the midpoint current of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, EVSVPWM-N Regarding the relationship between the three-phase currents, solve the midpoint currents of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N under the condition of satisfying the volt-second balance principle. The reference value of the midpoint current closest to the balanced midpoint potential The optimal switching waveform and its time parameters , and The optimal switching waveform is one of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N. The time parameter , and Defines the switching time parameters of phases A, B, and C in sector S1.

[0058] Figure 4The three-level space vector diagram in this embodiment is evenly divided into six large sectors S1 to S6. The space vector diagram has obvious symmetry, so the six sectors can be uniformly converted to the first sector for calculation, and then converted to the actual sector during modulation, which simplifies the calculation process. The introduction of the extended virtual voltage vector is based on the principle that the average midpoint current is zero when the voltage vector acts, and 8 extended virtual voltage vectors in sector S1 can be obtained, as shown in Table 2 and Figure 5 As shown in the figure, the midpoint current generated when each voltage vector acts is marked. Among them, , , The current , , It is determined by the size of . When the current changes, its position on the three dotted lines will also change.

[0059] Table 2: Virtual voltage vector composition table

[0060]

[0061] Among them, the action time It represents the ratio of the action time of vector 1 to the action time of the extended virtual voltage vector.

[0062] In this embodiment, in the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N in step S2.1, the switching waveforms of the three phases A, B, and C of the switching waveform SVPWM-P are OPO, OPO, and NON, respectively; the switching waveforms of the three phases A, B, and C of the switching waveform SVPWM-N are OPO, NON, and NON, respectively; the switching waveforms of the three phases A, B, and C of the switching waveform EVSVPWM-P are P, NOPON, and NON, respectively; and the switching waveform EVSVPWM -N's switching waveforms of the three phases A, B, and C are OPO, NOPON, and N respectively; the switching state O in the switching waveform indicates that S1 and S3 are closed, and S2 and S4 are open; the switching state P in the switching waveform indicates that S3 and S4 are closed, and S1 and S2 are open; the switching state N in the switching waveform indicates that S1 and S2 are closed, and S3 and S4 are open; the "-" in the switching waveform is used to connect multiple switching states of a phase in a switching cycle to indicate the switching sequence of the switching state of the phase, and S1 to S4 are four switch tubes of the new energy grid-connected three-level inverter. The concept of the extended virtual voltage vector is relative to the traditional virtual voltage vector. The traditional virtual voltage vector uses three vectors to synthesize a virtual voltage vector, while the extended virtual voltage vector uses two vectors to synthesize an extended virtual voltage vector. The average value of the midpoint current generated by the extended virtual voltage vector is zero. Using the extended virtual voltage vector to synthesize the reference voltage vector ensures that the average value of the midpoint current in each switching cycle is always zero, thereby suppressing the fluctuation of the midpoint potential. According to the voltage synthesis principle of the “nearest three vectors” and the principle of minimum switching times, it is not difficult to determine the order of action of the extended virtual voltage vector of each small area, and the four switching waveforms of sector S1 are obtained. In sector S1, one switching waveform can always be found from the four switching waveforms to meet the volt-second balance while ensuring that the average value of the midpoint current is zero. The four switching waveforms in sector S1 are shown in Table 3.

[0063] Table 3: Switching waveforms of each phase of the four switching waveforms

[0064]

[0065] In Table 3, the switch state O means that S1 and S3 are closed, and S2 and S4 are open; the switch state P means that S3 and S4 are closed, and S1 and S2 are open; the switch state N means that S1 and S2 are closed, and S3 and S4 are open; the switch waveform OPO represents the switching sequence of a certain phase in a switching cycle. It can be seen from Table 3 that the number of switching state switching times per switching cycle for each switching waveform is 6 times, which is 3 / 4 less than the number of switching state switching times per switching cycle of the traditional virtual voltage vector modulation algorithm, and the switching loss is effectively reduced.

[0066] When establishing the relationship between the four switching waveforms satisfying the volt-second balance principle in step S2.2 of this embodiment, the relationship between the switching waveform SVPWM-P satisfying the volt-second balance principle is:

[0067] ,

[0068] The switching waveform SVPWM-N satisfies the volt-second balance principle as follows:

[0069] ,

[0070] The switching waveform EVSVPWM-P satisfies the volt-second balance principle as follows:

[0071] ,

[0072] The switching waveform EVSVPWM-N satisfies the volt-second balance principle as follows:

[0073] ,

[0074] in, Indicates the time ratio of the A phase switch state P; Indicates the time ratio of the B phase switch state P; It is the DC bus voltage of the three-level inverter for renewable energy grid connection; , and is the reference voltage of the three phases A, B and C before mapping; Indicates the time ratio of the B phase switch state N, Indicates the time ratio of the C phase switch state N, , , and The value is between 0 and 1; the midpoint current of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N in step S2.2 The functional expression of the relationship between the three-phase current is:

[0075] ,

[0076] in, , and The three-phase current The A, B, and C phase currents in , and They are the time ratios of the three-phase A, B, and C in the O state respectively.

[0077] In step S2.4 of this embodiment, the relationship between the four switching waveforms satisfying the volt-second balance principle and the midpoint currents of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N are as follows: Regarding the relationship between the three-phase currents, solve the midpoint currents of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N under the condition of satisfying the volt-second balance principle. The reference value of the midpoint current closest to the balanced midpoint potential The optimal switching waveform and its time parameters , and Time parameter , and They are:

[0078] ,

[0079] ,

[0080] ,

[0081] in, Indicates the time ratio of the A phase switch state P; Indicates the time ratio of the B phase switch state P; Indicates the time ratio of the C phase switch state P; Indicates the time ratio of the A phase switch state O; Indicates the time ratio of the B phase switch state O; Indicates the time ratio of the C phase switch state O; Indicates the time ratio of the A phase switch state N; Indicates the time ratio of the B phase switch state N; Indicates the time ratio of the C phase switching state N.

[0082] In this embodiment, the four switching waveforms are calculated to obtain a switching waveform and time parameters whose midpoint current is closest to the reference midpoint current under the condition that the volt-second balance principle is satisfied and the sum of the duty cycles of each phase is 1, and a specific switching waveform is obtained. According to the volt-second balance calculation formula and the midpoint current calculation formula, the matrix equation is established as follows:

[0083] ,

[0084] in, is the solution vector, containing the components of the voltage vector; is a coefficient matrix used to establish the relationship between current and voltage; is the inverse matrix of the coefficient matrix; is a known vector, including the voltage amplitude and the midpoint current.

[0085] For SVPWM-P type switching waveforms:

[0086] ,

[0087] in, ~ They are the mapped three-phase currents ABC, and their rules are the same as those of the voltage vector. is the reference line voltage of AB phase after mapping, , is the AC phase reference line voltage after mapping, , is the reference value of the midpoint current;

[0088] For SVPWM-N type switching waveforms:

[0089] ,

[0090] For EVSVPWM-P type switching waveform:

[0091] ,

[0092] For EVSVPWM-N type switching waveform:

[0093] ,

[0094] Notice If the matrix is ​​non-singular, its inverse matrix must exist, so the equation must have a solution. After the value is obtained, it should also be processed to avoid a phase duty cycle greater than 1 or less than 0. The value is then substituted into the calculation formula for the mid-point current to obtain the absolute value of the difference between the mid-point current generated by the four switching waveforms and the reference mid-point current. The switching waveform with the smallest absolute value is the optimal switching waveform.

[0095] The switching waveforms are reallocated to each phase according to the sector where the reference voltage vector is actually located. The allocation method is the inverse mapping of the reference voltage mapping mentioned above. In this way, the switching waveforms of each phase and the time parameters that determine the waveforms are obtained, which can be conveniently applied to the three-level inverter through the modulator. Specifically, in step S3 of this embodiment, the time parameters are , and Reverse mapping to the time parameters of each phase waveform of the original voltage sector , and Including: If the sector in the three-level inverter space vector diagram is a large sector S1, then the time parameter , and Mapped in order , and As the time parameter after mapping , and ; If the sector in the three-level inverter space vector diagram is the large sector S2, then the time parameter , and Mapped in order , and As the time parameter after mapping , and ; If the sector in the three-level inverter space vector diagram is the large sector S3, then the time parameter , and Mapped in order , and As the time parameter after mapping , and ; If the sector in the three-level inverter space vector diagram is the large sector S4, then the time parameter , and Mapped in order , and As the time parameter after mapping , and ; If the sector in the three-level inverter space vector diagram is the large sector S5, then the time parameter , and Mapped in order , and As the time parameter after mapping , and ; If the sector in the three-level inverter space vector diagram is the large sector S6, then the time parameter , and Mapped in order , and As the time parameter after mapping , and .

[0096] In step S4 of this embodiment, the time parameters of each phase waveform of the original voltage sector are , and When the input modulator generates a modulation signal to control the three-level inverter, , and Each is a 1*3 vector composed of switching time parameters, which is used to determine the corresponding phases in a cycle. Inner front half 0~ The length of time that the four switching devices are turned on or off in the three sub-intervals of the switching waveform, where:

[0097] ,

[0098] ,

[0099] .

[0100] Taking phase A as an example, the function expression of the modulation signal generated by the modulator in this embodiment is:

[0101] exist hour:

[0102] Phase A: Sa1 = 1, Sa2 = 1, Sa3 = 0, Sa4 = 0;

[0103] exist hour:

[0104] Phase A: Sa1 = 0, Sa2 = 1, Sa3 = 1, Sa4 = 0;

[0105] exist hour:

[0106] Phase A: Sa0 = 1, Sa2 = 0, Sa3 = 1, Sa4 = 1;

[0107] in, is a sawtooth carrier, ,in is the remainder function, is the time. Sa1~Sa4 are the four switch devices of phase A from top to bottom, 1 means on, 0 means off. Since the switching waveform is symmetrical, only the first half, i.e. 0~0.5 .

[0108] In order to verify the modulation method for neutral point potential balance of the new energy grid-connected three-level inverter of this embodiment, under the same working conditions, the control strategy based on voltage vector oriented control in this embodiment is used to simulate the SVPWM and VSVPWM methods of the prior art and the modulation method of this embodiment in MATLAB / Simulink simulation software. In order to ensure the consistency of the number of switching times, the switching period of VSVPWM is 4 / 3 times of the switching period of SVPWM and the modulation method of this embodiment. The simulation parameters are shown in Table 4.

[0109] Table 4: Simulation parameters

[0110]

[0111] Finally, the simulation results of the SVPWM and VSVPWM methods of the prior art and the modulation method of this embodiment are as follows: Figure 6 and Figure 7 As shown, Figure 6 and Figure 7 From top to bottom are SVPWM, VSVPWM and the modulation method of this embodiment. Figure 6 It can be seen that both VSVPWM and the modulation method of this embodiment can effectively suppress the fluctuation of the midpoint potential, and the midpoint potential fluctuation is between 0.7V and -0.7V. Due to the limitations of power factor and modulation coefficient, SVPWM cannot effectively suppress the fluctuation of the midpoint potential at certain moments, and the midpoint potential fluctuation is between 3V and -3V. Figure 5 In the figure, at the same switching frequency, VSVPWM has the worst current waveform and the largest current THD (total harmonic distortion), while SVPWM and the modulation method of this embodiment have significantly smaller current THD. It can be seen that the modulation method of this embodiment effectively suppresses the fluctuation of the midpoint potential while ensuring that the switching loss and current harmonics do not increase, taking into account the midpoint current regulation capability and the current harmonic suppression capability, and effectively improving the quality of power transmission and system stability.

[0112] In summary, the modulation method for balancing the midpoint potential of the new energy grid-connected three-level inverter in this embodiment includes obtaining a three-phase reference voltage from a controller, converting it into a sector S1, and defining four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N in the sector S1; establishing a relationship between the four switching waveforms that satisfy the volt-second balance principle; sampling the three-phase current value, converting it into the sector S1, and establishing a relationship between the midpoint current of the four switching waveforms and the three-phase current; sampling the midpoint potential deviation, and calculating the reference value of the midpoint current required to balance the midpoint potential; solving the switching waveform whose midpoint current is closest to the reference midpoint current among the four switching waveforms under the condition of satisfying the volt-second balance principle, and obtaining the time parameters that determine the shape of its switching waveform; redistributing the phases of the switching waveform according to the sector where the reference voltage vector is actually located, and acting on the three-level inverter through the modulator. Compared with the space vector modulation algorithm, the midpoint potential fluctuation is significantly suppressed after adopting the method of this embodiment, and the switching loss is not increased; compared with the virtual voltage vector modulation algorithm, the current harmonics are smaller and the switching loss is reduced; the algorithm is not limited by the load power factor and modulation coefficient, and can suppress the midpoint potential fluctuation in real time under various working conditions, effectively increasing the quality of power transmission and system stability.

[0113] In addition, this embodiment also provides a modulation system for neutral point potential balance of a new energy grid-connected three-level inverter, comprising a microprocessor and a memory connected to each other, wherein the microprocessor is programmed or configured to execute the modulation method for neutral point potential balance of the new energy grid-connected three-level inverter.

[0114] In addition, this embodiment also provides a computer-readable storage medium, which stores a computer program or instruction, and the computer program or instruction is programmed or configured to execute the modulation method for neutral point potential balance of the new energy grid-connected three-level inverter through a processor.

[0115] In addition, this embodiment also provides a computer program product, including a computer program or instructions, which are programmed or configured to execute the modulation method for neutral point potential balance of the new energy grid-connected three-level inverter through a processor.

[0116] Those skilled in the art should understand that the technical solutions provided by the embodiments of the present application may be in the form of methods, systems, or computer program products. Therefore, the present application may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present application may take the form of a computer program product implemented on one or more computer-readable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program codes. The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the process Figure 1 A process or multiple processes and / or boxes Figure 1 These computer program instructions can also be stored in a computer-readable memory that can guide a computer or other programmable data processing device to work in a specific way, so that the instructions stored in the computer-readable memory produce a product including an instruction device, which implements the functions specified in the process. Figure 1 A process or multiple processes and / or boxes Figure 1 These computer program instructions can also be loaded onto a computer or other programmable data processing device, so that a series of operation steps are executed on the computer or other programmable device to produce a computer-implemented process, so that the instructions executed on the computer or other programmable device provide for implementing the process in the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0117] The above is only a preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this technical field, some improvements and modifications without departing from the principle of the present invention should also be regarded as the protection scope of the present invention.

Claims

1. A modulation method for neutral point potential balance of a three-level inverter connected to a new energy grid, characterized in that: The following steps are included: S1, the three-phase reference voltage from the controller , and Perform sector judgment and reference voltage vector mapping to obtain the mapped three-phase reference voltage , and ; S2, according to the mapped three-phase reference voltage , and And the midpoint potential deviation of the sample and three-phase current Calculate the optimal switching waveform and its timing parameters , and ; S3, the time parameter , and Reverse mapping to the time parameters of each phase waveform of the original voltage sector , and ; S4, the time parameters of each phase waveform of the original voltage sector , and An input modulator generates a modulation signal to control a three-level inverter; Step S2 includes: S2.1, an extended virtual voltage vector is introduced into sector S1, and the extended virtual voltage vector is composed of two basic vectors to ensure that the output midpoint current value is 0. Based on the extended virtual voltage vector, the reference voltage vector is synthesized to obtain four switching waveforms located in the S1 voltage sector, which are recorded as: SVPWM-P, SVPWM-N, EVSVPWM-P, EVSVPWM-N; S2.2, establish the relationship between the four switching waveforms that satisfy the volt-second balance principle, and the midpoint currents of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N About the relationship between three-phase current; S2.3, sampling the midpoint potential deviation of the three-level inverter connected to the new energy grid ,in is the capacitor voltage on the DC side, is the voltage of the lower capacitor; the reference value of the midpoint current required to balance the midpoint potential is calculated according to the following formula: : ， in, Indicates the DC side capacitance value, represents the switching cycle; S2.4, according to the relationship between the four switching waveforms satisfying the volt-second balance principle, and the midpoint current of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, EVSVPWM-N Regarding the relationship between the three-phase currents, solve the midpoint currents of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N under the condition of satisfying the volt-second balance principle. The reference value of the midpoint current closest to the balanced midpoint potential The optimal switching waveform and its time parameters , and The optimal switching waveform is one of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N. The time parameter , and Defines the switching time parameters of phases A, B, and C in sector S1.

2. The modulation method for neutral point potential balance of a new energy grid-connected three-level inverter according to claim 1, characterized in that: Step S1 includes: S1.1, according to the three-phase reference voltage from the controller , and The phase angle is used to determine the sector in the three-level inverter space vector diagram, and the three-level inverter space vector diagram is evenly divided into six large sectors S1-S6; S1.2, based on the determined sectors in the three-level inverter space vector diagram and the preset mapping rules, the three-phase reference voltage from the controller , and Perform reference voltage vector mapping to obtain the mapped three-phase reference voltage , and .

3. The modulation method for neutral point potential balance of a new energy grid-connected three-level inverter according to claim 2 is characterized in that: In step S1.2, the three-phase reference voltage from the controller is converted based on the sectors determined in the three-level inverter space vector diagram and the preset mapping rules. , and Perform reference voltage vector mapping to obtain the mapped three-phase reference voltage , and When the sector in the three-level inverter space vector diagram is the large sector S1, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and ; If the sector in the three-level inverter space vector diagram is the large sector S2, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and ; If the sector in the three-level inverter space vector diagram is the large sector S3, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and ; If the sector in the three-level inverter space vector diagram is the large sector S4, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and ; If the sector in the three-level inverter space vector diagram is the large sector S5, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and ; If the sector in the three-level inverter space vector diagram is the large sector S6, the three-phase reference voltage from the controller , and Mapped in order , and As the mapped three-phase reference voltage , and .

4. The modulation method for neutral point potential balance of a new energy grid-connected three-level inverter according to claim 1, characterized in that: In step S2.1, among the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N, the switching waveforms of the three phases A, B, and C of the switching waveform SVPWM-P are OPO, OPO, and NON, respectively; the switching waveforms of the three phases A, B, and C of the switching waveform SVPWM-N are OPO, NON, and NON, respectively; the switching waveforms of the three phases A, B, and C of the switching waveform EVSVPWM-P are P, NOPON, and NON, respectively; the switching waveforms of the three phases A, B, and C of the switching waveform EVSVPWM-N are The switching waveforms of phases A, B and C are OPO, NOPON and N respectively; the switching state O in the switching waveform means that S1 and S3 are closed, and S2 and S4 are opened; the switching state P in the switching waveform means that S3 and S4 are closed, and S1 and S2 are opened; the switching state N in the switching waveform means that S1 and S2 are closed, and S3 and S4 are opened; the "-" in the switching waveform is used to connect multiple switching states of a phase in a switching cycle to indicate the switching state switching order of the phase, and S1~S4 are four switching tubes of the new energy grid-connected three-level inverter.

5. The modulation method for neutral point potential balance of a new energy grid-connected three-level inverter according to claim 1, characterized in that: When the relationship between the four switching waveforms satisfying the volt-second balance principle is established in step S2.2, the relationship between the switching waveform SVPWM-P satisfying the volt-second balance principle is: ， The switching waveform SVPWM-N satisfies the volt-second balance principle as follows: ， The switching waveform EVSVPWM-P satisfies the volt-second balance principle as follows: ， The switching waveform EVSVPWM-N satisfies the volt-second balance principle as follows: ， in, Indicates the time ratio of the A phase switch state P; Indicates the time ratio of the B phase switch state P; It is the DC bus voltage of the three-level inverter for renewable energy grid connection; , and is the reference voltage of the three phases A, B and C before mapping; Indicates the time ratio of the B phase switch state N, Indicates the time ratio of the C phase switch state N, , , and The value is between 0 and 1; the midpoint current of the four switching waveforms SVPWM-P, SVPWM-N, EVSVPWM-P, and EVSVPWM-N in step S2.2 The functional expression of the relationship between the three-phase current is: ， in, , and The three-phase current The A, B, and C phase currents in , and They are the time ratios of the three-phase A, B, and C in the O state respectively.

6. The modulation method for neutral point potential balance of a new energy grid-connected three-level inverter according to claim 3, characterized in that: In step S3, the time parameter , and Reverse mapping to the time parameters of each phase waveform of the original voltage sector , and Including: If the sector in the three-level inverter space vector diagram is a large sector S1, then the time parameter , and Mapped in order , and As the time parameter after mapping , and ; If the sector in the three-level inverter space vector diagram is the large sector S2, then the time parameter , and Mapped in order , and As the time parameter after mapping , and ; If the sector in the three-level inverter space vector diagram is the large sector S3, then the time parameter , and Mapped in order , and As the time parameter after mapping , and ; If the sector in the three-level inverter space vector diagram is the large sector S4, then the time parameter , and Mapped in order , and As the time parameter after mapping , and ; If the sector in the three-level inverter space vector diagram is the large sector S5, then the time parameter , and Mapped in order , and As the time parameter after mapping , and ; If the sector in the three-level inverter space vector diagram is the large sector S6, then the time parameter , and Mapped in order , and As the time parameter after mapping , and .

7. A modulation system for neutral point potential balance of a new energy grid-connected three-level inverter, comprising a microprocessor and a memory connected to each other, characterized in that: The microprocessor is programmed or configured to execute the modulation method for neutral point potential balance of a new energy grid-connected three-level inverter as described in any one of claims 1 to 6.

8. A computer-readable storage medium having a computer program or instruction stored therein, characterized in that: The computer program or instruction is programmed or configured to execute, through a processor, the modulation method for neutral point potential balance of a three-level inverter connected to a new energy grid as claimed in any one of claims 1 to 6.

9. A computer program product comprising a computer program or instructions, characterized in that The computer program or instruction is programmed or configured to execute, through a processor, the modulation method for neutral point potential balance of a three-level inverter connected to a new energy grid as claimed in any one of claims 1 to 6.

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