Modulation and control method for improving short-time overload capacity of ANPC topology

Abstract

The application discloses a modulation and control method for improving short-time overload capacity of an ANPC topology. In view of the problem that the overload capacity improvement method of the existing grid-connected converter of the ANPC topology is high in cost and poor in universality, the application actively reduces the switching frequency, an operating parameter, at the software algorithm level, effectively reduces the output loss of the grid-connected converter in the overload operating state, and effectively improves the short-time overload operating capacity of the grid-connected converter by switching the modulation mode to effectively uniformly distribute the switching loss of each device without substantially increasing the cost and performance of the grid-connected converter.

Description

Technical Field

[0001] This invention belongs to the field of power electronic converter technology, specifically relating to a modulation and control method for improving the short-time overload capability of ANPC topology. Background Technology

[0002] With the increasing penetration rate of new energy sources, grid-connected converters, as the core unit connecting distributed generation and the main grid, have crucial dynamic characteristics. Traditional synchronous generators possess inherent overcurrent capabilities, providing short-circuit currents up to 5-10 times their rated value to withstand grid faults. However, converters based on power electronics technology, due to the inherent vulnerability of their switching devices, have their output current strictly limited to a low level when grid disturbances occur, forcing the system into current control mode, which often becomes a cause of system transient instability. Facing this challenge, significantly enhancing the overload operation capability of converters has become a critical technical issue that urgently needs to be addressed.

[0003] Existing overload operation methods for grid-connected converters primarily rely on hardware-level margin design or dynamic topology reconfiguration, requiring costly and customized hardware modifications. At the control level, while methods for adjusting switching frequencies exist, a multi-stage switching frequency adjustment strategy is lacking, significantly sacrificing power quality and causing substantial shocks during switching. At the modulation level, most methods switch to DPWM strategies, essentially reducing the equivalent switching frequency, which, when combined with frequency reduction, has limited effectiveness and fails to further improve overload capability. Furthermore, overload damage often occurs due to high-frequency components exceeding their safe junction temperatures, while low-frequency components are often still far from reaching their safe temperatures, meaning the overall temperature margin of the converter is not fully utilized. Summary of the Invention

[0004] To address the problems in existing technologies and fully utilize the overall temperature margin of the converter, this invention proposes a modulation and control method to improve the short-time overload capability of the ANPC topology. This invention effectively reduces the output loss of the grid-connected converter under overload conditions by actively reducing the switching frequency, an operating parameter, at the software algorithm level. Furthermore, it effectively evens out the switching losses of each device through modulation switching. Without significantly increasing the cost or performance of the grid-connected converter, and considering the characteristics of device loss changes during overload, this invention effectively improves its short-time overload capability.

[0005] To achieve the above-mentioned technical objectives, the present invention adopts the following technical solution:

[0006] This invention provides a modulation and control method for improving the short-time overload capability of an ANPC topology, comprising the following steps:

[0007] 1) For grid-connected converters that use LC or LCL filters on the AC side, determine the minimum switching frequency during overload operation; set the switching frequency level and corresponding output current during overload based on the minimum switching frequency and steady-state switching frequency.

[0008] 2) Based on the switching frequency range and converter hardware parameters, determine the controller parameters for maintaining stable operation at each range, write them into the storage unit of the digital controller, and set hysteresis control between each range.

[0009] 3) Under stable operating conditions, the converter operates in PWM-2 mode. In each control cycle, the voltage and current values ​​of each phase at the common grid connection point are sampled, and the overload operation state is determined based on the sampled information. If the overload operation state is entered, the current overload current multiple is determined based on the sampled information, and the switching frequency range is determined based on the current current multiple.

[0010] 4) If it is detected that the switching frequency needs to be switched, after the current control cycle is completed, the load count value of the PWM module in the next control cycle is updated to the load count value of the corresponding switch frequency to switch the switching frequency. At the same time, the controller integral value is kept unchanged, and the controller parameters of the corresponding level are switched to enter the corresponding level operation state in the next control cycle.

[0011] If the switching frequency is already at the lowest setting and the output current still needs to be increased, the modulation mode will be directly switched to PWM-1 modulation mode after the current control cycle calculation is completed.

[0012] According to a preferred embodiment of the present invention, in step 4), if the output current increases further or the overload time is too long, the system enters the current limiting mode or disconnects from the grid; if the grid fault has been cleared, the system exits the overload operation state.

[0013] The present invention also provides a modulation and control system for improving the short-time overload capability of an ANPC topology, comprising a power module of an ANPC topology, a digital controller, and a memory. The memory stores program instructions, and when the program instructions are executed by the digital controller, the modulation and control method described above is implemented.

[0014] Based on the above technical solution, the present invention has the following beneficial technical effects:

[0015] (1) By adjusting the switching frequency setting of the grid-connected converter, the present invention reduces the power semiconductor device loss in the ANPC grid-connected converter during grid faults, thereby improving the short-circuit overload current output capability of the grid-connected converter.

[0016] (2) By switching different modulation modes of the grid-connected converter, the present invention achieves uniform distribution of switching losses of power semiconductor devices in the ANPC grid-connected converter during grid faults. In steady state, the PWM-2 operating mode is used to ensure the minimum total loss. When the fault is deep, the PWM-1 is switched to distribute the switching losses of S2 and S3 power devices evenly to S1, S4, S5 and S6 power devices, making full use of the temperature margin of S1, S4, S5 and S6 power devices, thereby improving the short-circuit overload current output capability of the grid-connected converter.

[0017] (3) The control and modulation mode switching of the present invention can be achieved simply by adjusting the carrier frequency and modulation wave generation method of the PWM modulation stage through the digital controller, without changing the underlying hardware design, thus incurring zero additional cost. At the same time, it can be directly applied to traditional grid-connected converter equipment by updating the digital controller, thus this solution has good compatibility;

[0018] (4) The switching and adjustment of control and modulation methods in this invention does not change the converter control structure and operating state under normal operation. The multi-level switching achieves a balance between power quality and overcurrent capability under different fault depths, while the adjustment of controller parameters ensures loop stability. Attached Figure Description

[0019] Figure 1 The diagram shows a three-level ANPC topology using PWM-2 modulation and the switching waveform of the power device in one of the bridge arms.

[0020] Figure 2 The diagram shows a three-level ANPC topology using PWM-1 modulation and the switching waveform of the power device in one of the bridge arms.

[0021] Figure 3 A schematic diagram of the equivalent circuit of a grid-connected converter and its grid control structure;

[0022] Figure 4 The flowchart shows the design of a modulation and control method for improving the short-time overload capability of an ANPC topology proposed in this invention.

[0023] Figure 5 The results show the overload operation capability of grid-connected converters under traditional methods;

[0024] Figure 6 To reduce the overload operating capacity of grid-connected converters at switching frequencies;

[0025] Figure 7 The results show the overload operation capability of the grid-connected converter using the method proposed in this invention. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0027] Before introducing this invention, the technical terms used in this invention will be explained first:

[0028] ANPC topology grid-connected converters: such as Figure 1 , 2 As shown, ANPC is a three-level topology. Based on the traditional NPC three-level topology, it adds two switching transistors (S5 and S6) to dynamically control the neutral point voltage, thereby improving system efficiency and stability. Converters using the ANPC topology in each phase are called ANPC topology grid-connected converters. For each switching transistor, ANPC is generally recognized as S1, S2, S3, S4, S5, and S6 from top to bottom, from AC to DC.

[0029] PWM-2 modulation method: such as Figure 1 As shown, S1, S4, S5, and S6 operate at the power frequency switching frequency, while S2 and S3 operate at a high frequency switching frequency. The switching states of S1, S4, S5, and S6 are fixed in each power frequency half-cycle, while S2 and S3 continuously switch to output the required level.

[0030] PWM-1 modulation method: such as Figure 2 As shown, S2 and S3 operate at the power frequency switching frequency, while S1, S4, S5, and S6 operate at a high frequency switching frequency. The switching states of S2 and S3 are fixed in each half-cycle of the power frequency, and the output levels of S1, S4, S5, and S6 are continuously switched to the required levels.

[0031] Figure 1 This is a schematic diagram of the equivalent circuit and PWM-2 modulation of a grid-connected converter. Figure 2 The diagram shows the equivalent circuit of the grid-connected converter and the PWM-1 modulation scheme, as follows: Figure 3 As shown, the grid-connected converter collects data from the filter inductor L. f Current I c and filter capacitor C f voltage U f The coordinates are then transformed into a synchronous rotating coordinate system to achieve grid-type control. In this example, the grid control structure uses cascaded voltage control and current control as the inner loop control scheme to achieve closed-loop regulation and active support of the voltage at the grid connection point.

[0032] To demonstrate the effectiveness of the proposed method for enhancing the overload capacity of grid-connected converters based on switching frequency, the method was validated on a grid-connected converter simulation platform based on an ANPC three-level topology, using the FGHL50T65MQDTL4 power device. The following section provides a detailed explanation of the invention, using the example of a grid-connected converter operating under grid voltage dip faults with grid-type control, in conjunction with the accompanying drawings.

[0033] Figure 4 The flowchart illustrates the modulation and control method for improving the short-time overload capability of the ANPC topology according to the present invention. The method includes the following steps:

[0034] Step 1: Calculate the resonant frequency f based on whether an LC or LCL filter is used on the AC side. res Set the minimum switching frequency f during the overload period. sw_min = 2f res In PWM2 modulation mode, the losses are concentrated in S2 and S3. Based on the datasheet, the device loss P in S2 (or S3) can be obtained. loss2 The expression is:

[0035]

[0036] Among them, P sw and P cond2 For switching losses and conduction losses, a sw b sw and c sw To fit the polynomial coefficients of the switching energy versus collector current curve, U ref For the test voltage in the datasheet, U DC R is the voltage that the actual device withstands when it is turned off. ce V0 is the linearized dynamic resistance of the output characteristics near the operating point, V0 is the IGBT knee voltage, I is the current flowing through the power device, and f0 is the dynamic resistance. sw Where is the switching frequency, and D is the duty cycle.

[0037] Based on the above formula, the data in the table are used at the maximum temperature T. high With the lowest temperature T low The fitted data can be used to calculate S2 at the maximum temperature T. high With the lowest temperature T low The loss expression P loss2_H and P loss2_L The maximum junction temperature for safe operation of the device is specified as T. max (Often 10~20℃ lower than the maximum test temperature), then the maximum junction temperature T max The expression for the S2 loss is as follows:

[0038]

[0039] Assuming the steady-state current is I0, then typically at the maximum junction temperature T max The device can output a current of 1.2I0. Therefore, by substituting 1.2I0 as the current flowing through the device into the expression, the maximum power device loss P under safe operation can be calculated. loss2_limit .

[0040] The switching frequency level during overload is set according to the minimum switching frequency and steady-state switching frequency, and then the set switching frequency value is substituted into S2 and the maximum junction temperature T. max Lower loss expression P loss2_nax In the middle, according to the equation:

[0041]

[0042] The overload current corresponding to each switching frequency level can then be calculated. For example, in this embodiment, with a three-level design of 20kHz, 10kHz, and 4kHz, the calculated values ​​for the three switching frequency levels are steady-state, 1.5 times the overload current, and 2 times the overload current, respectively.

[0043] Step 2: Design the corresponding controller parameters based on the switching frequency of each gear and the converter hardware parameters (such as LCL filter parameters), for example, the proportional gain K of the voltage control loop. pv Integral gain K iv Current control loop proportional gain K pc Integral gain K ic Then, the controller parameters for each switching frequency level are directly written into the storage unit of the grid-connected converter digital controller. At the same time, hysteresis control is set between each level (for example, when the current reaches twice the rated current during overload operation, the switching frequency is switched to 4kHz; when the fault is recovered, the switching frequency is switched back to 10kHz when the current drops below 1.7 times the rated current, and switched back to 20kHz when the current drops below 1.2 times the rated current).

[0044] Step 3: Under normal operating conditions, the converter operates in PWM-2 mode, where the total converter loss is minimized. At the beginning of each cycle, the voltage and current of each phase at the common grid connection point are sampled. The sampled values ​​are used to determine whether the converter has entered an overload operating state. If it has entered an overload operating state, the current overload current multiple is determined based on the sampled information, and then it is determined whether the switching frequency range needs to be switched based on the current current multiple.

[0045] Step 4: When it is necessary to switch the switching frequency level, after the current control cycle is completed, update the load count value of the PWM module of the digital controller to the load count value corresponding to the operating switching frequency of the corresponding level in the next control cycle, so as to realize the switching frequency switching, keep the integral value of each controller unchanged, and switch to the corresponding level controller parameter value, and enter the corresponding level operation state in the next control cycle.

[0046] If the minimum switching frequency setting has been reached at this time, in order to further improve the overload capacity and overload duration, and to make full use of the temperature margin of the overall converter, the modulation mode is directly switched from PWM-2 to PWM-1 in the controller, thereby distributing the switching losses evenly throughout the entire ANPC topology.

[0047] If the output current increases further or the overload time is too long, it will enter the current limiting mode or operate offline; if the grid fault has been cleared, it will exit the overload operation mode.

[0048] for Figure 1 and Figure 2 The equivalent circuit of the grid-connected converter and the schematic diagrams of PWM-2 and PWM-1 modulation shown are verified using the following parameter settings: the base value of the AC voltage is 220V, the fundamental frequency of the AC grid is 50Hz, the rated capacity of the grid-connected converter is 27kVA, and the normal switching frequency is 20kHz. The rated value of the grid voltage amplitude E is 1.0 pu, and the equivalent impedance from the connection point to the grid is X. ge The value is 0.4 pu, and the filter inductance of the grid-connected converter is L. f = 0.05 pu, filter capacitor is C f = 0.06 pu, normal state voltage control loop proportional gain K pv = 0.07, Integral gain K iv = 32, Current control loop proportional gain K pc =14.44, integral gain K ic = 25.13. The grid voltage drops to 0.5 pu at 0.1s and outputs 3 times the overload current, recovering to 1 pu at 0.3s. The operating waveform is as follows. Figure 5 As shown, the steady-state operating current of the grid-connected converter is 1 pu, and the steady-state junction temperature of the internal power devices is 100℃. Because the grid-connected control algorithm maintains a constant output current at the converter port, its output current will increase after a voltage drop in the grid, causing the converter to enter an overload operating state. Setting the safe upper limit of the junction temperature of the internal power devices of the grid-connected converter to 125℃, it can be seen that under the traditional control method, the grid-connected converter can only output 3 times the overload current for 0.2s.

[0049] If only the switching frequency is reduced, the grid voltage drops to 0.5 pu in 0.1s and outputs 3 times the overload current, recovering to 1 pu in 0.4s. The operating waveform is as follows. Figure 6 As shown, the safe upper limit of the junction temperature of the power devices inside the grid-connected converter is set at 125℃. It can be seen that by simply reducing the switching frequency, the loss of the high-frequency tubes is effectively reduced, and the grid-connected converter can output an overload current of 3 times for 0.3s. Compared with the traditional control method, the overload capacity is improved by about 1.5 times.

[0050] If the method proposed in this invention is used, the grid voltage drops to 0.5 pu in 0.1s and outputs 3 times the overload current, and recovers to 1 pu in 0.5s. The operating waveform is as follows. Figure 7 As shown, the safe upper limit of junction temperature for the power devices inside the grid-connected converter is set at 125℃. It can be seen that by simply reducing the switching frequency, the switching losses of S2 and S3 are effectively evenly distributed to S1, S4, S5, and S6, effectively utilizing the temperature margin of the overall ANPC topology. The grid-connected converter can output a 3 times overload current for 0.4s. Compared to traditional control methods, this improves the overload capacity by approximately 2 times, while the control loop remains stable throughout the fault process.

[0051] In an embodiment of the present invention, a modulation and control system for improving the short-time overload capability of an ANPC topology is also provided, including a power module of the ANPC topology, a digital controller and a memory, wherein the memory stores program instructions, and when the program instructions are executed by the digital controller, the aforementioned modulation and control method is implemented.

[0052] For the system embodiments, since they basically correspond to the method embodiments, relevant details can be found in the descriptions of the method embodiments; the implementation methods of the remaining modules will not be repeated here. The system embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate, and the components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of the present invention according to actual needs. Those skilled in the art can understand and implement this without any creative effort.

[0053] The system embodiments of the present invention can be applied to any device with data processing capabilities, such as a computer or other similar device. The system embodiments can be implemented in software, hardware, or a combination of both. Taking software implementation as an example, as a logical device, it is formed by the processor of any data processing device loading the corresponding computer program instructions from non-volatile memory into memory for execution.

[0054] The above-described embodiments are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. Those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A modulation and control method for improving the short-time overload capability of an ANPC topology, characterized in that, Includes the following steps: 1) For grid-connected converters using LC or LCL filters on the AC side, determine the minimum switching frequency during overload operation; set the switching frequency level and corresponding output current during overload according to the minimum switching frequency and steady-state switching frequency; the switching frequency level is designed in three levels: 20kHz, 10kHz, and 4kHz, which correspond to steady-state, 1.5 times overload current, and 2 times overload current, respectively. 2) Based on the switching frequency range and converter hardware parameters, determine the controller parameters for maintaining stable operation at each range, write them into the storage unit of the digital controller, and set hysteresis control between each range; the controller parameters include the proportional gain of the voltage control loop, the integral gain, the proportional gain of the current control loop, and the integral gain. 3) Under stable operating conditions, the converter operates in PWM-2 mode. In each control cycle, the voltage and current values ​​of each phase at the common grid connection point are sampled, and the overload operation state is determined based on the sampled information. If the overload operation state is entered, the current overload current multiple is determined based on the sampled information, and the switching frequency range is determined based on the current current multiple. 4) If it is detected that the switching frequency needs to be switched, after the current control cycle is completed, the load count value of the PWM module in the next control cycle is updated to the load count value of the corresponding switch frequency to switch the switching frequency. At the same time, the controller integral value is kept unchanged, and the controller parameters of the corresponding level are switched to enter the corresponding level operation state in the next control cycle. If the lowest switching frequency setting is already in place and the output current still needs to be increased, the modulation mode will be directly switched to PWM-1 modulation mode after the current control cycle calculation is completed. The modulation mode switching is achieved by changing the modulation wave generation method without changing the hardware structure; if the output current increases further or the overload time is too long, it will enter the current limiting mode or operate offline; if the grid fault has been cleared, it will exit the overload operation state.

2. The modulation and control method for improving the short-time overload capability of an ANPC topology according to claim 1, characterized in that, In step 1), the lowest switching frequency ,in This is the resonant frequency of the AC-side filter.

3. The modulation and control method for improving the short-time overload capability of an ANPC topology according to claim 1, characterized in that, The PWM-2 modulation method minimizes the total loss during steady-state operation, while the PWM-1 modulation method evenly distributes the switching losses of power devices S2 and S3 to power devices S1, S4, S5, and S6 during overload operation.

4. A modulation and control system for improving the short-time overload capability of an ANPC topology, characterized in that, The system includes a power module with an ANPC topology, a digital controller, and a memory. The memory stores program instructions, which, when executed by the digital controller, implement the modulation and control method as described in any one of claims 1 to 3.

Images