Full-wind-speed low-carrier-to-signal ratio hybrid modulation method for direct-drive permanent magnet wind power system

Abstract

The application discloses a full-wind-speed low-carrier-wave-ratio hybrid modulation method for a direct-drive permanent-magnetic wind power system, which comprises the following steps: first, determining a hybrid modulation strategy; second, deducing a switching angle; third, re-dividing a sector; fourth, deducing an over-modulation strategy based on the switching angle; and fifth, formulating a switching strategy between SVPWM modulation strategies with different frequency division numbers. The application has the following beneficial effects: the switching principle of the switching between the switching angles of the full-wind-speed low-carrier-wave-ratio hybrid modulation strategy for the direct-drive permanent-magnetic wind power system based on the stator flux linkage is continuously given; and no matter whether the switching is from a high pulse number to a low pulse number or from a low pulse number to a high pulse number, the phase continuity of the line voltage and the non-impact of the phase current are realized when the switching is between the different frequency division numbers.

Description

Technical Field

[0001] This invention relates to the field of wind turbine generator control system technology, specifically to a hybrid modulation method for low carrier ratio at all wind speeds in a direct-drive permanent magnet wind power system. Background Technology

[0002] Currently, in the field of wind turbine control system technology, achieving effective control of wind turbines across the entire wind speed range has significant application value for the development of wind farms. Due to the considerable randomness and fluctuation of natural wind, simply studying the operating state of wind turbines within a specific wind speed range has significant limitations. Based on existing research literature, the operating modes of wind turbines can be categorized into three types: maximum power point tracking (MPPT) operation, constant speed operation, and constant power operation. Among the published technologies, some have proposed the idea of ​​interval modulation of the wind turbine based on its rotational speed. Asynchronous modulation is used at low speeds, while different synchronous modulation strategies are employed at medium and high speeds depending on the input AC frequency. Theoretically, this strategy can ensure efficient and stable operation of the wind turbine across the entire wind speed range. In practical implementation, synchronous modulation strategies can be divided into two categories. The first category is optimized synchronous modulation, such as specific harmonic elimination methods, minimum pulse width modulation for current harmonics, and optimal pulse width modulation for synchronization. Optimized synchronous modulation primarily involves reducing specific low-order harmonics by establishing a transcendental equation with the switching angle as the variable. The switching angle is then obtained by optimizing and solving this equation, but online solution is difficult and not easily implemented digitally. The second type is non-optimized synchronous modulation, such as 60° center modulation and synchronous space vector pulse width modulation (SVPWM). While 60° center modulation is easier to implement digitally than optimized synchronous modulation, its harmonic performance is poor. Synchronous SVPWM, based on space vector synthesis, has advantages such as ease of digital implementation and good harmonic performance, making it more widely used.

[0003] However, the synchronous SVPWM modulation strategy uses fixed sampling points at fixed positions within each large sector of the 0°–360° range to synthesize the reference voltage vector, thus its carrier ratio is fixed. When switching between different modulation strategies, abrupt changes in the carrier ratio can occur. In actual control, these changes cause abrupt changes in the carrier frequency, which in turn affect the modulation delay. This situation can adversely affect the control of the permanent magnet synchronous generator (PMSG).

[0004] The inventors of this patent application, through a search, found an invention patent entitled "A Deadbeat Control Method for Permanent Magnet Synchronous Motors under Low Carrier Ratio" (application number: CN202010557688.9), which discloses a deadbeat control method for permanent magnet synchronous motors under low carrier ratios, including the following steps: 1) Designing a deadbeat current controller under low carrier ratios using an accurate discrete model of the d and q axes; 2) Designing a hybrid modulation scheme of SVPWM and SHEPWM, using SVPWM modulation under high carrier ratios and SHEPWM modulation under low carrier ratios. This invention solves the problem of increased motor model error caused by using the forward Euler approximation under low carrier ratios, improves current control accuracy, compensates for delays under low carrier ratios, and effectively improves the voltage harmonic performance of inverters and permanent magnet synchronous motors under low carrier ratios. Meanwhile, an invention patent titled "Method for Eliminating Low-Carrier Hybrid Harmonics Based on Active Front-End Converter of New Energy Microgrid System" (application number CN202111101973.0) was also found. This patent discloses a method for eliminating low-carrier hybrid harmonics based on active front-end converter of new energy microgrid system. It eliminates third-order harmonic values ​​through a three-phase AFE converter, ensuring that the 17th and 19th harmonics are eliminated by a low-pass filter, leaving only the 5th, 7th, 11th, and 13th lower harmonic values. The THD operation achieves different amplitude ratios after passing through the low-pass filter, thus affecting THD5,7 and THD5,7... Comparing 11 and 13, based on the suppression of THD5, 7, 11, and 13 harmonics, the amplitude modulation ratio region is obtained through the SVPWM scheme at frequency multiples of 9 and 11. The modulation ratio of 9 times the frequency is used in the low amplitude modulation ratio region, and the modulation ratio of 11 times the frequency is used in the high amplitude modulation ratio region. Combining the SPWM and SVPWM methods, a hybrid method of 9 times and 11 times the lower carrier ratio is applied. In the theoretical design, the low harmonic bands of SPWM and SVPWM are found and switched for control. The application of the low carrier ratio PWM scheme improves the life of the inverter converter, reduces power consumption, and realizes the greening of power inverter equipment.

[0005] However, it still lacks flexibility in carrier ratio for synchronous SVPWM modulation strategies with different frequency divisions. It cannot guarantee that the voltage phase is continuous and the modulation delay is not abrupt when switching between different frequency divisions, thus ensuring that the control effect of PMSG will not decrease due to the switching of modulation strategies.

[0006] To address the aforementioned issues, some researchers have analyzed the pulse distribution patterns of synchronous SVPWM modulation strategies based on bus clamping and proposed a modulation strategy based on calculating the switching angle. This strategy eliminates the need for trigonometric function calculations, resulting in lower computational complexity. Furthermore, to address the frequent abrupt changes in carrier ratio between different modulation strategies, a re-division of sectors ensures phase continuity and non-abrupt modulation delay during switching between different frequency divisions of synchronous SVPWM modulation strategies, preventing a decline in motor current control performance due to modulation strategy switching. While guiding strategies have been proposed, in practical applications within wind turbine generator control systems, a modulation method that guarantees voltage phase continuity and non-abrupt modulation delay during switching between different frequency divisions, ensuring that PMSG control performance does not degrade due to modulation strategy switching, is still lacking. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a hybrid modulation method for low carrier ratio at all wind speeds in a direct-drive permanent magnet wind power system based on switching angle calculation, which can ensure that the voltage phase is continuous and the modulation delay does not change abruptly when switching between different frequency divisions, and ensure that the control effect of PMSG does not decrease due to the switching of modulation strategy, based on the symmetry of the pulse width modulation (PWM) pulse of the synchronous SVPWM modulation strategy.

[0008] To solve the above-mentioned technical problems, the technical solution provided by the present invention is: a hybrid modulation method for low carrier ratio at all wind speeds in a direct-drive permanent magnet wind power system, which includes the following steps:

[0009] The first step is to determine the hybrid modulation strategy;

[0010] The second step is to derive the switching angle;

[0011] The third step is to re-divide the sectors;

[0012] The fourth step is to derive the overmodulation strategy based on the switching angle;

[0013] The fifth step is to develop a strategy for switching between SVPWM modulation strategies with different frequency division numbers.

[0014] As a preferred option, the specific operation of determining the hybrid modulation strategy in the first step is as follows: set the highest switching frequency of the rectifier to 450Hz, and select a synchronous modulation strategy with 15 pulses; use a modulation mode with a larger pulse number at the highest allowed switching frequency to reduce harmonic distortion. According to this design principle, the combination of hybrid modulation strategies for direct-drive permanent magnet wind power generation at all wind speeds and low carrier ratios is set as follows: asynchronous modulation, CSVS-15, BBCS-11_30°, CSVS-9, BBCS-7_60°, BBCS II-5_30°, and CSVS-3. The operating range corresponding to each modulation mode is segmented according to the rotational speed, which are 0-300r / min, 300-450r / min, 450-600r / min, 600-750r / min, 750-900r / min, 900-1350r / min, and 1350-1500r / min respectively.

[0015] As a preferred option, the specific steps for deriving the switching angle in the second step are as follows:

[0016] 1) Define the n switching angles corresponding to the moment of the setting switch action in the first 1 / 4 cycle as α1, α2, ..., αn, where the relationship between the number of switching angles n and the number of pulses P in the first 1 / 4 cycle is: P = 2n + 1;

[0017] 2) Then, by calculating the switching angle within the first 1 / 4 of the cycle, the three-phase PWM pulses for the entire fundamental cycle can be obtained;

[0018] 3) In the synchronous SVPWM modulation strategy based on space vector synthesis, when the number of sampling points in sector I is N, the entire fundamental period is divided into 6N small sectors, where the angle of each small sector is π / (3N). Since the sampling points and voltage switching sequences of the synchronous SVPWM modulation strategy are fixed, the switching action time in each fundamental period is only related to the modulation ratio m. Taking CSVS-15 as a derivation example, the voltage switching sequence of the 6° sampling points in sector I is 0127, which is a rising edge. Therefore, the turn-off duration of the switch is 0.5T0, which is the switching action time, expressed in the form of angles as follows:

[0019]

[0020] 4) It is found that the synchronous SVPWM modulation strategy based on space vector synthesis can be derived from the corresponding switching angle based on the location of its sampling point and its voltage switching sequence.

[0021] 5) Based on the derivation principle of the switching angle obtained in step 4), the switching angles of the modulation algorithms CSVS-15, BBCS-11_30°, CSVS-9, BBCS-7_60°, BBCS-II-5_30° and CSVS-3 in the first 1 / 4 cycle are derived respectively.

[0022] As a preferred option, the specific operation method for re-dividing the sectors in the third step is as follows: the six large sectors corresponding to the modulation algorithms CSVS-15, BBCS-11_30°, CSVS-9, BBCS-7_60°, BBCS-II-5_30°, and CSVS-3 are re-divided into 3k small sectors (k is an integer) according to the requirements. When re-dividing the small sectors, it is necessary to ensure that there are at most two switching angles in each small sector.

[0023] As a preferred approach, the modulation principle of the overmodulation strategy based on the switching angle is derived in the fourth step as follows: as the modulation ratio increases, two adjacent switching angles approach each other until they coincide. During this process, the narrow pulse between the two switching angles will disappear. When all adjacent switching angles coincide, all the narrow pulses will disappear. At this time, it is square wave modulation. Based on this principle, various overmodulation strategies for synchronous SVPWM modulation are derived.

[0024] Preferably, the switching in step five is based on the following: asynchronous modulation and 15-pulse CSVS-15 are switched when the voltage phase is 26π / 15; the 15-pulse CSVS-15 switches to the 11-pulse BBCS-11-30° selected sector number 13; and the switching between other synchronous SVPWM modulation strategies with different pulse numbers is all selected in sector number 11.

[0025] By adopting the above structure, the present invention has the following beneficial effects: By analyzing the variation range and distribution law of the switching angle, the present invention re-divides the sectors of the direct-drive permanent magnet wind power generation full-wind-speed low carrier ratio hybrid modulation strategy based on switching angle, adhering to the principle of no more than two switching angles within a sector. This re-division of sectors allows for flexible setting of the carrier ratio of synchronous SVPWM modulation strategies with different frequency divisions, thereby ensuring continuous voltage phase and non-abrupt modulation delay when switching between different frequency divisions, and ensuring that the control effect of PMSG does not decrease due to the switching of modulation strategies.

[0026] Based on the continuity of stator flux linkage, a hybrid modulation strategy for direct-drive permanent magnet wind power generation with low carrier ratio across all wind speeds, based on switching angle, is presented, demonstrating the switching principle when switching between different frequency division numbers. Regardless of whether switching from a high pulse number to a low pulse number or vice versa, continuous line voltage phase and shock-free phase current are achieved during switching between different frequency division numbers. Furthermore, simulation analysis using the MATLAB / Simulink platform verifies the correctness and feasibility of the proposed method. Attached Figure Description

[0027] Figure 1 This invention relates to a hybrid modulation strategy for direct-drive permanent magnet wind power generation with low carrier ratio at all wind speeds.

[0028] Figure 2 This is a schematic diagram of the A-phase pulse and switching angle of the first 1 / 4 cycle of the CSVS-15 involved in this invention.

[0029] Figure 3 This invention relates to a comparison diagram of the relationship between the A-phase pulse and the switching angle within the first 1 / 4 cycle of six synchronous SVPWM modulations after sector re-division.

[0030] Figure 4 This invention relates to a curve showing the switching angle α1 as a function of the modulation ratio m, using CSVS-15 as an example.

[0031] Figure 5 This invention relates to a curve showing the switching angles α2 and α3 as a function of the modulation ratio, using CSVS-15 as an example.

[0032] Figure 6 This invention relates to a curve showing the switching angles α4 and α5 as a function of the modulation ratio, using CSVS-15 as an example.

[0033] Figure 7 This invention relates to curves showing the changes in switching angles α6 and α7 with modulation ratio under two states: no overmodulation and overmodulation.

[0034] Figure 8 This invention relates to the stator flux linkage trajectory diagram of synchronous SVPWM under different pulses after sector re-division. Detailed Implementation

[0035] The present invention will now be described in further detail with reference to the accompanying drawings.

[0036] Combined with appendix Figure 1 To the attached Figure 8 A hybrid modulation method for low carrier ratio at all wind speeds in a direct-drive permanent magnet wind power system, comprising the following steps:

[0037] The first step is to determine the hybrid modulation strategy;

[0038] The second step is to derive the switching angle;

[0039] The third step is to re-divide the sectors;

[0040] The fourth step is to derive the overmodulation strategy based on the switching angle;

[0041] The fifth step is to develop a strategy for switching between SVPWM modulation strategies with different frequency division numbers.

[0042] The preferred implementation method for each step in the specific implementation is as follows:

[0043] The first step is to determine the hybrid modulation strategy.

[0044] The rectifier's maximum switching frequency is set to 450Hz. When the pulse count is 15 or 13, the corresponding frequency ranges are quite close. To prevent frequent switching between the two modulation modes, a synchronous modulation strategy with a pulse count of 15 is chosen. For high-power wind turbine generators, the prominent features are high voltage and high current. The key to designing a full-wind-speed, low-carrier-ratio hybrid modulation strategy is to use a modulation mode with a larger pulse count as much as possible within the allowable maximum switching frequency to reduce harmonic distortion. Based on this design principle, the following combinations of full-wind-speed, low-carrier-ratio hybrid modulation strategies for direct-drive permanent magnet wind power generation are defined: asynchronous modulation, CSVS-15, BBCS-11_30°, CSVS-9, BBCS-7_60°, BBCS II-5_30°, and CSVS-3. The corresponding operating ranges for each modulation mode are detailed in the attached figure. Figure 1 As shown in the image.

[0045] The second step is to derive the switching angle.

[0046] Optimized synchronous modulation, while ensuring that the three-phase PWM pulses have half-wave odd symmetry and 1 / 4 even symmetry, allows the switching action to be set at any position in the fundamental cycle. Therefore, optimized synchronous modulation has no carrier wave and no modulating wave, and based on this, the concept of switching angle is proposed. In optimized synchronous modulation, the n switching angles corresponding to the time when the switching action is set in the first 1 / 4 of the cycle are defined as α1, α2, ..., α... n The relationship between the number of switching angles n and the number of pulses P in the first 1 / 4 of the cycle is: P = 2n + 1. Based on the symmetry of the synchronously modulated three-phase PWM pulses, the three-phase PWM pulses for the entire fundamental cycle can be obtained simply by calculating the switching angles in the first 1 / 4 of the cycle.

[0047] In the synchronous SVPWM modulation strategy based on space vector synthesis, when the number of sampling points in sector I is N, the entire fundamental period can be divided into 6N small sectors, where the angle of each small sector is π / (3N). Since the sampling points and voltage switching sequences of the synchronous SVPWM modulation strategy are fixed, the switching action time in each fundamental period is only related to the modulation ratio m. Taking CSVS-15 as an example, the voltage switching sequence of the 6° sampling points in sector I is 0127, which is a rising edge. Therefore, the duration of the switch being turned off is 0.5T0, which is the switching action time, expressed in angular form as:

[0048]

[0049] The above analysis shows that the synchronous SVPWM modulation strategy based on space vector synthesis can derive the corresponding switching angle based on the location of its sampling point and its voltage switching sequence. The following section will analyze the switching angles of the CSVS-15, BBCS-11_30°, CSVS-9, BBCS-7_60°, BBCS-II-5_30°, and CSVS-3 modulation algorithms in the first quarter of the cycle, based on the derivation principle of the switching angle, and derive the hybrid modulation strategy for low carrier ratio at all wind speeds in direct-drive permanent magnet wind power generation.

[0050] As attached Figure 2 The diagram shows the A-phase pulse and switching angle during the first quarter of the CSVS-15 cycle. From this, we can deduce the seven switching angles of CSVS-15 within the first quarter of the cycle, i.e., from 0° to 90°. The first switching angle is located in sector 1 of the I-th large sector. The sampling point of this sector is at 6°, and the voltage switching sequence is 0127. At this time, the position of the switching angle is the rising edge, and we can deduce that the switching angle α1 is:

[0051]

[0052] The second switching angle is located in sector 2 of the I-th large sector. The sampling point position of this sector is 18°, and the voltage switching sequence is 7210. At this time, the position of the switching angle is the falling edge, and the switching angle α2 can be derived as follows:

[0053]

[0054] The third switching angle is located in sector 3 of the I-th large sector. The sampling point of this sector is at 30°, and the voltage switching sequence is 0127. At this time, the position of the switching angle is the rising edge, and the switching angle α3 can be derived as follows:

[0055]

[0056] The fourth switching angle is located in sector 4 of the I-th large sector. The sampling point of this sector is at 42°, and the voltage switching sequence is 7210. At this time, the position of the switching angle is the falling edge, and the switching angle α4 can be derived as follows:

[0057]

[0058] The fifth switching angle is located in sector 5 of the I-th large sector. The sampling point of this sector is at 54°, and the voltage switching sequence is 0127. At this time, the position of the switching angle is the rising edge, and the switching angle α5 can be derived as follows:

[0059]

[0060] The sixth switching angle is located in sector 6 of the second major sector. The sampling point of this sector is 6°, and the voltage switching sequence is 7230. At this time, the switching angle is at the falling edge, and the switching angle α6 can be derived as follows:

[0061]

[0062] The seventh switching angle is located in sector 7 of sector II. The sampling point of this sector is at 18°, and the voltage switching sequence is 0327. At this time, the position of the switching angle is the rising edge, and the switching angle α7 can be derived as follows:

[0063]

[0064] Following the same derivation process, the switching angles of BBCS-11_30°, CSVS-9, BBCS-7_60°, BBCS-II-5_30°, and CSVS-3 can be obtained.

[0065] The five switching angles of BBCS-11_30° in the first quarter cycle are:

[0066]

[0067] The four switching angles of CSVS-9 in the first quarter cycle are:

[0068]

[0069] The three switching angles of BBCS-7_60° in the first quarter cycle are:

[0070]

[0071] The two switching angles of BBCS-II-5_30° in the first quarter cycle are:

[0072]

[0073] The switching angle of CSVS-3 in the first quarter cycle is:

[0074]

[0075] The third step is to re-divide the sectors.

[0076] Unlike asynchronous modulation, synchronous modulation defines the carrier ratio as the ratio of the carrier frequency to the fundamental frequency of the input voltage. This ratio is related to the number of sampling points within one fundamental cycle, and the ratio of the equivalent switching frequency to the input fundamental frequency is defined as the frequency division factor. Taking CSVS-15 as an example, CSVS-15 has 15 pulses within one fundamental cycle, resulting in 15 on / off operations for the switching device. The ratio of the equivalent switching frequency to the fundamental cycle is 15, i.e., the frequency division factor is 15. Simultaneously, CSVS-15 has 5 sampling points in sector I, resulting in 30 sampling points within one fundamental cycle. Therefore, this algorithm requires 30 carrier interruptions within one fundamental cycle to generate a complete pulse, i.e., the carrier ratio is 30, and the carrier frequency T... pwm for:

[0077]

[0078] In the formula, f s Let be the fundamental frequency of the input voltage. Based on the definition of carrier ratio for synchronous modulation, the table below shows the correspondence between the number of sampling points and the carrier ratio in the I-th sector for various synchronous SVPWM modulation strategies used in the full-wind-speed low-carrier-ratio hybrid modulation strategy.

[0079]

[0080] Table 1. Number of sampling points and carrier ratio under different frequency divisions for the low carrier ratio hybrid modulation strategy at all wind speeds.

[0081] As shown in Table 1, when designing a hybrid modulation strategy with low carrier ratio across all wind speeds, frequent carrier ratio abrupt changes occur during switching between different modulation modes. For example, when switching from BBCS-7_60° to BBCS-II-5_60°, the carrier ratio abruptly changes from 18 to 12. In actual control, these abrupt changes in carrier ratio cause abrupt changes in carrier frequency, which in turn affects the modulation delay. This situation can adversely affect the control of the PMSG.

[0082] From equations (2) to (13), it can be seen that the switching angle of each synchronous SVPWM modulation has a range of variation. Taking CSVS-15 as an example, the range of its switching angle variation does not exceed 12°, and there are at most two switching angles within a range of 24°. Therefore, the sectors can be re-divided, and the six large sectors can be re-divided into 3k small sectors (k is an integer) according to the requirements. Dividing into multiples of 3 sectors is to better utilize the three-phase symmetry. Moreover, when re-dividing the sectors, it must be ensured that there are at most two switching angles in each small sector, that is, at most two switching actions in each small sector.

[0083]

[0084] Table 2. Number of sampling points and carrier ratio at different frequency divisions after sector re-division.

[0085] By rationally dividing the sectors, the carrier ratio change problem mentioned above can be effectively solved. Table 2 shows the relationship between the number of sampling points and the carrier ratio of various modulation modes in sector I after re-dividing the sectors.

[0086] Based on the derivation of the switching angle above, the relationship between the switching angle and the modulation ratio in the first quarter of the cycle under six different synchronous SVPWM modulation strategies was obtained. The switching angles and A-phase pulses after re-dividing the sectors for these six synchronous SVPWM modulation strategies were plotted on the same graph, with the horizontal axis representing the switching angle and the vertical axis representing the modulation ratio, as shown in the attached figure. Figure 3 As shown, the x-coordinate of the intersection point of the switching angle curve and the A-phase pulse is the switching angle, and the y-coordinate is the corresponding modulation ratio. This makes the relationship between the switching angle and the pulse waveform more intuitive and clear.

[0087] The fourth step is the overmodulation strategy.

[0088] From the appendix Figure 3 The overmodulation principle of the hybrid modulation strategy for direct-drive permanent magnet wind power generation with low carrier ratio at all wind speeds, based on switching angles, can be summarized as follows: As the modulation ratio increases, two adjacent switching angles continuously approach each other until they coincide. During this process, the narrow pulse between the two switching angles disappears. When all adjacent switching angles coincide, all narrow pulses disappear, resulting in square wave modulation. Based on this principle, various overmodulation strategies for synchronous SVPWM modulation can be derived. Taking CSVS-15 as an example, an overmodulation strategy based on switching angles is derived.

[0089] (1) As attached Figure 4 The graph shows the switching angle α1 as a function of the modulation ratio m. It can be seen from the graph that as the modulation ratio increases, the switching angle α1 gradually decreases. When α1 decreases to 0, α1 coincides with 2π-α1, and π-α1 coincides with π+α1, and the narrow pulses between them disappear. Setting α1 = 0, M1 can be calculated as:

[0090]

[0091] Therefore, for α1, when m>M1, it will enter the overmodulation region, and its overmodulation algorithm is to keep α1=0 unchanged.

[0092] (2) As attached Figure 5The graph shows the switching angles α2 and α3 as a function of the modulation ratio. It can be seen from the graph that as the modulation ratio m gradually increases, the switching angles α2 and α3 gradually approach each other. When they are equal, α2 coincides with α3, 2π-α2 with 2π-α3, π-α2 with π-α3, and π+α2 with π+α3, at which point the narrow pulse between them disappears. Let α2 = α3, then M2 can be calculated as:

[0093]

[0094] Therefore, for α2 and α3, when m>M2, they will enter the overmodulation region, and the overmodulation algorithm is to keep the values ​​of the switching angles α2 and α3 unchanged when m=M2.

[0095] (3) As attached Figure 6 The graph shows the switching angles α4 and α5 as a function of the modulation ratio. It can be seen from the graph that as the modulation ratio m gradually increases, the switching angles α4 and α5 gradually approach each other. When they are equal, α4 coincides with α5, 2π-α4 coincides with 2π-α5, π-α4 coincides with π-α5, and π+α4 coincides with π+α5. At this point, the narrow pulse between them disappears. Let α4 = α5, then M3 can be calculated as:

[0096]

[0097] Therefore, for α4 and α5, when m>M3, they will enter the overmodulation region, and the overmodulation algorithm is to keep the values ​​of the switching angles α4 and α5 unchanged when m=M3.

[0098] (4) As attached Figure 7 Figure (a) shows the curves of switching angles α6 and α7 as a function of the modulation ratio. It can be seen from the figure that as the modulation ratio m gradually increases, switching angles α6 and α7 gradually approach each other, until they no longer overlap even when m increases to 1. Without an overmodulation algorithm, the narrow pulse between switching angles α6 and α7 cannot disappear. Therefore, when m > M2, it is necessary to linearly increase the slope of the increase in switching angle α6 and the slope of the decrease in switching angle α7 to bring these two switching angles closer to the sector boundary. (See Appendix...) Figure 7 From (a), we know that α6 and α7 should be gradually made closer to 2π / 5. Let the values ​​of α6 and α7 be a6 and a7 respectively when m = M2. Then we can find the slope of the linear increase of the switching angle α6 after m > M2 and the slope of the decrease of the switching angle α7, k6 and k7:

[0099]

[0100] Therefore, when m > M2, the curves of switching angles α6 and α7 as a function of modulation ratio m are redrawn, as follows: Figure 7As shown in (b), when m>M2, the switching angles α6 and α7 can be expressed as:

[0101]

[0102] Based on the above derivation, the formula for calculating the switching angle of CSVS-15 across the entire modulation ratio range can be summarized as follows:

[0103]

[0104] Equation (22) shows that all switching angles are linear functions of the modulation ratio m. Compared with the traditional synchronous SVPWM overmodulation algorithm, the switching angle-based overmodulation algorithm does not require trigonometric function calculations, making it simpler to implement and requiring less computation. The disappearance process of each narrow pulse in the CSVS-15 overmodulation algorithm is as follows:

[0105] (1) When m>0.916918, the switching angles α2 and α3, 2π-α2 and 2π-α3, π-α2 and π-α3, and π+α2 and π+α3 coincide, changing from 15 pulses to 11 pulses;

[0106] (2) When m>0.958823, the switching angles α4 and α5, 2π-α4 and 2π-α5, π-α4 and π-α5, and π+α4 and π+α5 coincide, changing from 11 pulses to 7 pulses;

[0107] (3) When m>0.9927, the switching angle α1 coincides with 2π-α1 and π-α1 coincides with π+α1, and changes from 7 pulses to 5 pulses;

[0108] (4) When m>1, the switching angles α6 and α7, 2π-α6 and 2π-α7, π-α6 and π-α7, and π+α6 and π+α7 coincide, changing from 5 pulses to single pulses.

[0109] Step 5: Switch strategies.

[0110] In direct-drive permanent magnet wind power systems, the fundamental frequency is low at low wind speeds, so asynchronous SVPWM modulation with a fixed carrier frequency is used. As wind speed increases, the fundamental frequency gradually rises, and the carrier ratio gradually decreases. When the carrier ratio reaches a certain value, it needs to switch to synchronous SVPWM modulation. Before and after the switch, the harmonic characteristics of asynchronous SVPWM modulation and CSVS-15 modulation strategies are relatively close, so there is basically no current surge during the switch. At this time, it is necessary to ensure that the carrier ratio does not change abruptly before and after the switch. After re-dividing the sectors, the carrier ratio of the CSVS-15 modulation strategy is 15. Therefore, if switching from asynchronous SVPWM modulation strategy to CSVS-15 modulation strategy at a frequency greater than 20Hz, the carrier frequency of asynchronous SVPWM modulation strategy should be set to 300Hz.

[0111] When switching between SVPWM modulation strategies with different frequency division numbers, the sectors are redefined because the direct-drive permanent magnet wind power generation full-wind-speed low-carrier-ratio hybrid modulation strategy based on switching angle re-divides the sectors. Therefore, the stator flux linkage trajectories of each synchronous SVPWM modulation strategy are mapped to the sectors of the voltage space vector, and the sectors are re-divided and numbered. The stator flux linkage trajectories of the two modulation strategies involved in the switching are plotted on the same graph, as shown in the attached figure. Figure 8 The diagram shows the stator flux linkage trajectories of synchronous SVPWM under different pulses after sector re-division. CSVS-15 and BBCS-11 have the same stator flux linkage trajectory, so neither strategy will generate a large current surge during switching between any small sector.

[0112] (1) By Figure 8 (a) It can be seen that when the carrier ratios of BBCS-11_30° (11-division) and CSVS-9 (9-division) are both set to 12, the stator flux trajectories of the two modulation modes are continuous at the beginning of each large sector. Therefore, a smooth transition between BBCS-11_30° (11-division) and CSVS-9 (9-division) can be achieved when switching between the first small sector (odd-numbered small sector) of each large sector.

[0113] (2) By Figure 8 (b) It can be seen that when the carrier ratios of 9-division CSVS-9 and 7-division BBCS-7_60° are both set to 12, the stator flux trajectories of the two modulation modes are continuous at the beginning of each small sector. Therefore, a smooth transition between 9-division CSVS-9 and 7-division BBCS-7_60° can be achieved when switching in any small sector.

[0114] (3) By Figure 8 (c) It can be seen that when the carrier ratios of 7-division BBCS-7_60° and 5-division BBCS-5_30° are both set to 12, the stator flux trajectories of the two modulation modes are continuous at the beginning of each large sector. Therefore, a smooth transition between 7-division BBCS-7_60° and 5-division BBCS-II-5_30° can be achieved when switching between odd-numbered small sectors.

[0115] (4) By Figure 8 (d) It can be seen that when the carrier ratios of 5-division BBCS-II-5_30° and 3-division CSVS-3 are both set to 12, the stator flux trajectories of the two modulation modes are continuous at the beginning of each large sector. Therefore, a smooth transition between 5-division BBCS-II-5_30° and 3-division CSVS-3 can be achieved when switching between odd-numbered small sectors.

[0116] In summary, the switching basis of the modulation strategy described in this invention is as follows: asynchronous modulation and 15-pulse CSVS-15 are switched when the voltage phase is 26π / 15. The 15-pulse CSVS-15 switches to the 11-pulse BBCS-11-30°, selecting sector number 13. Switching between other synchronous SVPWM modulation strategies with different pulse counts is also performed in sector number 11. Furthermore, verification has shown that based on this basis, a hybrid modulation method for low carrier ratio at all wind speeds in a direct-drive permanent magnet wind power system, based on switching angle calculation, can be derived that ensures continuous voltage phase and non-abrupt modulation delay when switching between different frequency divisions, and ensures that the control effect of PMSG does not decrease due to the switching of modulation strategies.

[0117] The present invention and its embodiments have been described above. This description is not restrictive, and the accompanying drawings are only one embodiment of the present invention; the actual structure is not limited thereto. In conclusion, if those skilled in the art are inspired by this description and design similar structures and embodiments without departing from the spirit of the present invention, such designs should fall within the protection scope of the present invention.

Claims

1. A full wind speed low carrier ratio hybrid modulation method for a direct drive permanent magnet wind power system, characterized in that: It comprises the following steps: The first step is to determine the hybrid modulation strategy; The second step is to derive the switching angle, and the specific operation is as follows: 1) Define the n switching angles corresponding to the switching action time in the first 1 / 4 cycle as α1, α2, … αn, wherein the number n of switching angles in the first 1 / 4 cycle and the number P of pulses have the following relationship: P = 2n + 1; 2) Then the switching angles in the first 1 / 4 cycle are obtained, and the three-phase PWM pulses in the entire fundamental cycle are obtained; 3) In the synchronous SVPWM modulation strategy based on space vector synthesis, when the number of sampling points in the Ith sector is N, the whole fundamental period is evenly divided into 6N small sectors, and the angle of each small sector is π / (3N). Since the sampling points and their voltage switching sequences of the synchronous SVPWM modulation strategy are fixed, the time of switch action in each fundamental period is only related to the modulation ratio m ; taking CSVS-15 as an example, the voltage switching sequence of the 6° sampling point in the Ith sector is 0127, and it is an upward edge, so the duration of the switch-off of the switch tube is 0.5 T 0, that is, the switch action time, which is expressed in the form of angle is: (1)； 4) The switching angle derivation principle based on the space vector synthesis synchronous SVPWM modulation strategy is derived according to the position of the sampling point and the voltage switching sequence; 5) According to the switching angle derivation principle obtained in step 4), the switching angles of CSVS-15, BBCS-11_30°, CSVS-9, BBCS-7_60°, BBCS-II-5_30°, and CSVS-3 modulation algorithms in the first 1 / 4 cycle are derived; The third step is to redivide the sectors, and the specific operation mode is as follows: the six large sectors corresponding to the CSVS-15, BBCS-11_30°, CSVS-9, BBCS-7_60°, BBCS-II-5_30°, and CSVS-3 modulation algorithms are redivided into 3k small sectors according to the requirements, k is an integer, and it is required to ensure that there are at most two switching angles in each small sector during the redivision of the small sectors; The fourth step is to derive the overmodulation strategy based on the switching angle; The modulation principle is: as the modulation ratio increases, the adjacent two switching angles approach each other until they coincide, in this process, the narrow pulses between the two switching angles will disappear, and when all adjacent switching angles coincide, all narrow pulses will disappear, at this time, it is square wave modulation; According to this principle, the overmodulation strategy of various synchronous SVPWM modulation is derived; The fifth step is to develop a switching strategy between different SVPWM modulation strategies with different frequency division numbers. 2.The full-wind-speed low-carrier-wave-ratio hybrid modulation method of a direct-drive permanent-magnet wind power system according to claim 1, wherein: The specific operation of determining the hybrid modulation strategy in the first step is as follows: the highest switching frequency of the rectifier is set to 450Hz, and the synchronous modulation strategy with pulse number 15 is selected; In the allowable highest switching frequency, the modulation mode with larger pulse number is used to reduce harmonic distortion, according to this design principle, the combination mode of the full-speed low-carrier ratio hybrid modulation strategy of the direct-drive permanent magnet wind power generator is set as follows: asynchronous modulation, CSVS-15, BBCS-11_30°, CSVS-9, BBCS-7_60°, BBCS II-5_30°, and CSVS-3; The working interval corresponding to each modulation mode is segmented according to the speed, and corresponds to 0-300r / min, 300-450r / min, 450-600r / min, 600-750r / min, 750-900r / min, 900-1350r / min, and 1350-1500r / min, respectively. 3.The full-wind-speed low-carrier-wave-ratio hybrid modulation method of a direct-drive permanent-magnet wind power system according to claim 1, wherein: The switching in the fifth step is based on: asynchronous modulation and 15-pulse CSVS-15 switching at the voltage phase of 26π / 15, 15-pulse CSVS-15 switching to 11-pulse BBCS-11-30° to select the small sector numbered 13, and the switching between other different pulse number synchronous SVPWM modulation strategies is selected in the small sector numbered 11.

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