A low-loss modulation method and system for a dc transformer

Abstract

The application provides a low-loss modulation method of a direct-current transformer, and the method comprises the following steps: generating a voltage modulation waveform according to a voltage modulation parameter; and modulating the direct-current transformer according to the voltage modulation waveform, so as to reduce the switching loss of the direct-current transformer. The application effectively reduces the switching loss of the direct-current transformer.

Description

Technical Field

[0001] This invention belongs to the field of DC transformer technology, and specifically relates to a low-loss modulation method and system for DC transformers. Background Technology

[0002] DC power grids are an important means of accommodating new energy sources. Compared with AC power grids, they can increase absorption capacity, reduce harmonics and stability issues, and naturally do not have reactive power, thus having broad application prospects.

[0003] The construction of DC power grids relies on DC transformers with voltage transformation capabilities. To transmit renewable energy over long distances, low-voltage DC needs to be stepped up to high-voltage DC to reduce line losses. This requires high-voltage DC transformers. Due to limitations in the development of power semiconductor devices, high-voltage DC transformers typically employ a modular multilevel converter structure to meet the demands of high-voltage, high-current applications.

[0004] Modular multilevel converter (MMC) DC transformers differ from those used in flexible DC transmission. They operate at medium to high frequencies to reduce the size, weight, and cost of isolation transformers. However, this increase in frequency leads to higher switching losses, causing components to overheat and fail. In high-voltage DC transformers, the low-voltage side submodules operate under high-current switching conditions, resulting in significantly higher switching losses than the high-voltage side modules. Consequently, they are highly susceptible to overheating and damage due to excessive switching losses.

[0005] Therefore, it is necessary to design a low-loss modulation method and system for DC transformers to solve the above-mentioned technical problems. Summary of the Invention

[0006] To address the aforementioned technical problems, the present invention provides a low-loss modulation method for a DC transformer, wherein the method includes: Based on the voltage modulation parameters, generate the voltage modulation waveform; The DC transformer is modulated based on the voltage modulation waveform to reduce its switching losses.

[0007] Furthermore, The voltage modulation parameters include the phase shift angle. δ Step width δ t, step voltage Ut, and gap voltage Us2.

[0008] Further, the generation of the voltage modulation waveform includes: Set the original square wave voltage on the primary / secondary side of the DC transformer; According to the phase shift angle δThe gap voltage Us2 causes the original square wave voltage of the primary / secondary side to form a first voltage waveform with gap voltage Us2; According to the step width δ The first voltage waveform is formed by the step voltage Ut and the step voltage Ut, so that the first voltage waveform is formed by the step voltage Ut on the rising / falling edge, as a voltage modulation waveform.

[0009] Furthermore, the statement based on the phase shift angle δ And the gap voltage, causing the original square wave voltage of the primary / secondary side to form a first voltage waveform with a gap voltage Us2, including: The waveforms of the first decomposed voltage, the second decomposed voltage, and the third decomposed voltage are superimposed to generate a voltage waveform with a gap voltage Us2, wherein, The first decomposition voltage is the original square wave voltage of the primary / secondary side of the DC transformer. The second decomposed voltage is the inverse of the original square wave voltage; The third decomposition voltage is the inverse of the second decomposition voltage, followed by phase shifting and phase angle shifting. δ The result.

[0010] Furthermore, the step width is used as a basis for... δ The step voltage Ut and the initial square wave voltage t on the primary / secondary side cause the original square wave voltage to form a second voltage waveform with a step voltage Ut on the rising / falling edge, including: Set the original square wave voltage on the primary / secondary side of the DC transformer; The original square wave voltage of the primary / secondary side is passed through a ramp circuit to limit the rate of change of the original square wave voltage of the primary / secondary side, so that the original square wave voltage of the primary / secondary side meets the trapezoidal wave voltage condition to form a trapezoidal wave voltage. The trapezoidal voltage is passed through a rounding stage based on the step voltage Ut, with the falling edge of the trapezoidal voltage waveform rounded down and the rising edge rounded up, to form a second voltage waveform with a step level.

[0011] Furthermore, the amplitude of the first decomposition voltage is Us, and the amplitude of the second decomposition voltage is 0.5(Us-Us2).

[0012] Furthermore, the trapezoidal wave voltage condition is as follows: The width of the voltage ramp is N times the step width. δ t, where N is the number of step voltages.

[0013] On the other hand, the present invention also provides a low-loss modulation system for a DC transformer, wherein the system comprises: The waveform generation module is used to generate voltage modulation waveforms based on voltage modulation parameters. The modulation module is used to modulate the DC transformer according to the voltage modulation waveform in order to reduce the switching losses of the DC transformer.

[0014] Furthermore, the voltage modulation parameters include the phase shift angle. δ Step width δ t, step voltage Ut, and gap voltage Us2.

[0015] Further, the generation of the voltage modulation waveform includes: Set the original square wave voltage on the primary / secondary side of the DC transformer; According to the phase shift angle δ The gap voltage, along with the original square wave voltage of the primary / secondary side, forms a first voltage waveform with a gap voltage Us2; According to the step width δ The first voltage waveform is formed by the step voltage Ut and the step voltage Ut, so that the first voltage waveform is formed by the step voltage Ut on the rising / falling edge, as a voltage modulation waveform.

[0016] This invention provides a low-loss modulation method and system for DC transformers, which modulates the DC transformer by generating a voltage modulation waveform, thereby reducing the switching losses of the DC transformer.

[0017] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures pointed out in the description and the drawings. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 A schematic diagram of a single-phase MMC type high-voltage DC transformer according to an embodiment of the present invention is shown.

[0020] Figure 2 A schematic diagram of the structure of a two-split MMC type high-voltage DC transformer according to an embodiment of the present invention is shown. Figure 2 Split MMC type high voltage DC transformer.

[0021] Figure 3 A schematic diagram of the structure of a half-bridge submodule according to an embodiment of the present invention is shown.

[0022] Figure 4 The waveform diagram of the original square wave voltage according to an embodiment of the present invention is shown.

[0023] Figure 5 A waveform diagram of a notched square wave voltage according to an embodiment of the present invention is shown.

[0024] Figure 6 A waveform diagram of a trapezoidal voltage with notches and steps according to an embodiment of the present invention is shown.

[0025] Figure 7 A waveform diagram of generating a notched square wave voltage according to an embodiment of the present invention is shown.

[0026] Figure 8 A waveform diagram of a stepped trapezoidal voltage is shown according to an embodiment of the present invention.

[0027] Figure 9 The diagram shows the voltage and current waveforms when power is transferred from the low-voltage side to the high-voltage side according to an embodiment of the present invention.

[0028] Figure 10 The diagram shows the voltage and current waveforms when power is transferred from the high-voltage side to the low-voltage side according to an embodiment of the present invention.

[0029] Figure 11 A bridge arm current waveform diagram according to an embodiment of the present invention is shown.

[0030] Figure 12 A DC transformer topology with voltage and current reference directions according to an embodiment of the present invention is shown.

[0031] Figure 13 The voltage and current waveforms are shown in one example according to an embodiment of the present invention.

[0032] Figure 14 The diagram shows the simulated voltage and current waveforms of the upper and lower bridge arms on the low-voltage side at a rated power of 100MW, from the low-voltage side to the high-voltage side, according to an embodiment of the present invention.

[0033] Figure 15 A flowchart of a low-loss modulation method for a DC transformer according to an embodiment of the present invention is shown.

[0034] Figure 16 A structural diagram of a low-loss modulation system for a DC transformer according to an embodiment of the present invention is shown. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] The low-loss modulation method and system for DC transformers of the present invention can be applied to the following two typical applications: 1. Single-phase MMC type high-voltage DC transformer; 2. Two-split MMC type high-voltage DC transformer.

[0037] The topologies of two types of DC transformers are described below.

[0038] like Figure 1 The diagram shows a single-phase MMC type high-voltage DC transformer topology. This topology comprises a high-voltage side single-phase MMC, an isolation transformer, and a low-voltage side single-phase MMC connected in sequence. The high-voltage side single-phase MMC includes two first bridge arms connected in parallel, and the midpoints of the two first bridge arms are respectively connected to the two ends of the high-voltage side winding of the isolation transformer. In this embodiment, the low-voltage side single-phase MMC includes two second bridge arms connected in parallel, and the midpoints of the two second bridge arms are respectively connected to the two ends of the low-voltage side winding of the isolation transformer.

[0039] In this embodiment, the first arm of a single-phase MMC on the high-voltage side is used as an example for illustrative purposes. Each first arm includes an upper arm and a lower arm connected in series. Each upper arm and lower arm of the first arm includes an inductor L and n half-bridge sub-modules. The n half-bridge sub-modules are cascaded, and each half-bridge sub-module has two cascade connection points, namely the first cascade connection point and the second cascade connection point, wherein: For the upper arm of the first bridge arm: among the n half-bridge submodules, the first cascade connection point of the first half-bridge submodule serves as the connection end of the upper bridge arm. The second cascade connection point of the nth half-bridge submodule is connected to one end of the inductor L of the upper bridge arm; among the remaining half-bridge submodules, the first cascade connection point of each half-bridge submodule is connected to the second cascade connection point of the previous half-bridge submodule, and the second cascade connection point of each half-bridge submodule is connected to the first cascade connection point of the next half-bridge submodule.

[0040] For the lower arm of the first bridge arm: among the n half-bridge submodules, the first cascade connection point of the first half-bridge submodule is connected to one end of the inductor L of the lower arm, and the second cascade connection point of the nth half-bridge submodule serves as the connection end of the lower arm; among the remaining half-bridge submodules, the first cascade connection point of each half-bridge submodule is connected to the second cascade connection point of the previous half-bridge submodule, and the second cascade connection point of each half-bridge submodule is connected to the first cascade connection point of the next half-bridge submodule.

[0041] In the first bridge arm, the other end of the inductor L in the upper bridge arm is connected to the other end of the inductor L in the lower bridge arm, and the connection point between the two inductors L is the midpoint of the first bridge arm.

[0042] For the entire high-voltage side single-phase MMC, the connection terminals of the two upper arms of the high-voltage side single-phase MMC are interconnected to form the first port of the high-voltage side single-phase MMC, and the connection terminals of the two lower arms of the high-voltage side single-phase MMC are interconnected to form the second port of the high-voltage side single-phase MMC. The first port and the second port of the high-voltage side single-phase MMC constitute the DC port of the high-voltage side single-phase MMC, and this input terminal is the DC port of the high-voltage side of the entire DC transformer.

[0043] In this embodiment, the second bridge arm of a low-voltage side single-phase MMC is used as an example for illustrative purposes. Each second bridge arm includes an upper bridge arm and a lower bridge arm connected in series. Each upper bridge arm and lower bridge arm of the second bridge arm includes an inductor L and m half-bridge sub-modules. The m half-bridge sub-modules are cascaded, and each half-bridge sub-module has two cascade connection points, namely the first cascade connection point and the second cascade connection point, wherein: For the upper arm of the second bridge arm: among the m half-bridge submodules, the first cascade connection point of the first half-bridge submodule serves as the connection end of the upper bridge arm. The second cascade connection point of the m-th half-bridge submodule is connected to one end of the inductor L of the upper bridge arm; among the remaining half-bridge submodules, the first cascade connection point of each half-bridge submodule is connected to the second cascade connection point of the previous half-bridge submodule, and the second cascade connection point of each half-bridge submodule is connected to the first cascade connection point of the next half-bridge submodule.

[0044] For the lower arm of the second bridge arm: among the m half-bridge submodules, the first cascade connection point of the first half-bridge submodule is connected to one end of the inductor L of the lower arm, and the second cascade connection point of the m-th half-bridge submodule serves as the connection end of the lower arm; among the remaining half-bridge submodules, the first cascade connection point of each half-bridge submodule is connected to the second cascade connection point of the previous half-bridge submodule, and the second cascade connection point of each half-bridge submodule is connected to the first cascade connection point of the next half-bridge submodule.

[0045] In the second bridge arm, the other end of the inductor L in the upper bridge arm is connected to the other end of the inductor L in the lower bridge arm, and the connection point between the two inductors L is the midpoint of the first bridge arm.

[0046] For the entire low-voltage side single-phase MMC, the connection terminals of the two upper bridge arms of the low-voltage side single-phase MMC are interconnected to form the first port of the low-voltage side single-phase MMC, and the connection terminals of the two lower bridge arms of the low-voltage side single-phase MMC are interconnected to form the second port of the low-voltage side single-phase MMC. The first port and the second port of the low-voltage side single-phase MMC constitute the DC port of the low-voltage side single-phase MMC, which is the DC port of the entire DC transformer on the low-voltage side.

[0047] In addition, such as Figure 3 As shown, Figure 1 and Figure 2 The structural diagram at point A is the structural diagram of the half-bridge submodule. For both the low-voltage side single-phase MMC and the high-voltage side single-phase MMC, each half-bridge submodule includes a capacitor C and two fully controlled devices T. The cathode of one fully controlled device T is connected to the anode of the other fully controlled device T through a connection point, which serves as the first cascade connection point of the half-bridge submodule. One end of the capacitor C is connected to the anode of one fully controlled device T, and the other end of the capacitor C is connected to the cathode of the other fully controlled device T. Each fully controlled device T has a diode D connected in anti-parallel. The cathode of the other fully controlled device T in the half-bridge submodule serves as the second cascade connection point of the half-bridge submodule.

[0048] In this invention, the configuration of the capacitor C, the two fully controlled devices T, and the anti-parallel diodes on each fully controlled device T in the high-voltage side single-phase MMC are different from or the same as those in the low-voltage side single-phase MMC I and low-voltage side single-phase MMC II.

[0049] like Figure 2 The diagram illustrates a two-phase MMC (Multi-phase CNC) HVDC transformer topology. Unlike single-phase MMC topologies, this topology features two low-voltage side single-phase MMCs and a two-phase isolation transformer. The isolation transformer has one high-voltage side winding and two low-voltage side windings. The two midpoints of one low-voltage side single-phase MMC are connected to the two ends of one low-voltage side winding of the isolation transformer, and the two midpoints of the other low-voltage side single-phase MMC are connected to the two ends of the other low-voltage side winding. The DC ports of the two low-voltage side single-phase MMCs are connected in parallel.

[0050] The low-loss modulation method and system for DC transformers of the present invention can be applied to, but is not limited to, the two typical applications mentioned above. For example, it can also be applied to three-split and more split topologies. The key feature of MMC-type high-voltage DC transformers is that both the high-voltage and low-voltage sides use MMC-type converters. The low-loss modulation method and system for DC transformers of the present invention are applicable to DC transformers with this feature.

[0051] For a two-split MMC type high-voltage DC transformer topology, the AC output voltage of the two single-phase MMCs on the low-voltage side is the same, so it can also be treated as a single-phase MMC.

[0052] The following is a detailed description of a low-loss modulation method and system for a DC transformer according to the present invention.

[0053] like Figure 15 As shown, the present invention provides a low-loss modulation method for a DC transformer, wherein the method includes: Based on the voltage modulation parameters, generate the voltage modulation waveform; The DC transformer is modulated based on the voltage modulation waveform to reduce its switching losses.

[0054] The present invention provides a detailed description of a low-loss modulation method for a DC transformer.

[0055] In one embodiment of the present invention, the voltage modulation parameters include a phase shift angle. δ Step width δ t, step voltage Ut, and gap voltage Us2.

[0056] Based on the voltage modulation parameters described above, in this embodiment, generating a voltage modulation waveform includes: Step 1: As Figure 4 As shown, the original square wave voltages on the primary / secondary sides (primary or secondary) of a DC transformer are set, where one period of the original square wave voltages on the primary / secondary sides is T. Figure 5 The horizontal axis represents the magnitude of the voltage, and the vertical axis represents time t, with the original square wave voltage (i.e., ...) on the primary side. Figure 4 The primary-side voltage is ±Up. The original square-wave voltage on the secondary side is ±Us, where the original square-wave voltage on the secondary side is... Figure 4 The voltage on the secondary side is converted to the primary side.

[0057] Step 2: As Figure 5 As shown, based on the phase shift angle δ The gap voltage Us2 causes the original square wave voltage of the primary / secondary side to form a first voltage waveform with gap voltage Us2; Step 3: As Figure 6 As shown, based on the step width δ t and step voltage Ut, such that the first voltage waveform is formed on the rising / falling (rising and falling) edges with step voltage Ut (and the width of step voltage Ut is δ The second voltage waveform of t) is used as a voltage modulation waveform.

[0058] The process of steps 2-3 above will be described in detail below.

[0059] In step 2, the step of determining the phase shift angle... δ And the gap voltage, causing the original square wave voltage of the primary / secondary side to form a first voltage waveform with a gap voltage Us2, including: like Figure 7 As shown, the waveforms of the first decomposed voltage, the second decomposed voltage, and the third decomposed voltage are superimposed to generate a voltage waveform with a gap voltage Us2 (as shown). Figure 7 In the middle, the voltage waveform at the top), The first decomposition voltage is the original square wave voltage of the primary / secondary side of the DC transformer, where the amplitude of the first decomposition voltage is Us; The second decomposed voltage is the inverse of the original square wave voltage, and the amplitude of the second decomposed voltage is 0.5 (Us-Us2). The third decomposition voltage is the inverse of the second decomposition voltage, followed by phase shifting and phase angle shifting. δ The result.

[0060] In step 3, the step width is used as a reference. δ The step voltage Ut and the initial square wave voltage t on the primary / secondary side cause the original square wave voltage to form a second voltage waveform with a step voltage Ut on the rising / falling edge, including: Step 31: As Figure 8 As shown, the original square wave voltage (the waveform of the square wave voltage is a rectangular wave) is set in the primary / secondary side of the DC transformer. Step 32: Pass the original square wave voltage on the primary / secondary side through a ramp circuit, that is, limit the rate of change of the original square wave voltage on the primary / secondary side, so that the original square wave voltage on the primary / secondary side satisfies the trapezoidal wave voltage condition to form a trapezoidal wave voltage (the waveform of the trapezoidal wave voltage is a trapezoidal wave). The trapezoidal wave voltage condition is: the width of the voltage ramp is N times the step width. δ t, where N is the number of step voltages; Step 33: The trapezoidal voltage from step 32 is passed through a rounding process based on the step voltage Ut. The falling edge of the trapezoidal voltage waveform is rounded down, and the rising edge of the trapezoidal voltage waveform is rounded up, to form a second voltage waveform with a step level (the second voltage waveform is a trapezoidal wave with a step level).

[0061] The first voltage waveform obtained through step 2 and the second voltage waveform obtained through step 3 are superimposed to form the voltage modulation waveform in step 4.

[0062] On the other hand, such as Figure 16As shown, the present invention also provides a low-loss modulation system for a DC transformer, wherein the system includes: The waveform generation module is used to generate voltage modulation waveforms based on voltage modulation parameters. The modulation module is used to modulate the DC transformer according to the voltage modulation waveform in order to reduce the switching losses of the DC transformer.

[0063] In this invention, the module implementation steps and functions of a DC transformer low-loss modulation system correspond one-to-one with a DC transformer low-loss modulation method, therefore, they will not be described again here.

[0064] The principle of modulating a DC transformer based on a voltage modulation waveform in this invention will be explained below.

[0065] by Figure 1 Taking the topology as an example for modulation, the AC side (isolation transformer side) output voltage of the high-voltage side single-phase MMC is a stepped trapezoidal wave voltage with an amplitude equal to the original square wave voltage Up of the primary side. The AC side (isolation transformer side) output voltage of the low-voltage side single-phase MMC is a stepped trapezoidal wave voltage with an amplitude equal to the secondary side voltage Us (the secondary side voltage referred to the primary side). The gap voltage width is the phase shift angle. δ .

[0066] The trapezoidal wavefront is composed of multiple steps of equal width and height. The step voltage is Ut, and the step width is... δ In an ideal scenario, Up = Us, and there is no voltage difference between the high-voltage side (primary side) and the low-voltage side (secondary side) after being converted to the primary side of the DC transformer, so the current remains constant. In the phase shift angle δ interval, there is a voltage difference of Up - Us2 between the primary and secondary sides, and the winding current increases in the reverse direction, eventually forming a trapezoidal current.

[0067] The main feature of this invention is that the gap voltage Us2 ≠ Us, and simultaneously, Us2 ≠ 0. Therefore, as... Figure 9 As shown, the secondary voltage is not directly inverted, but rather decreases first. Only after the secondary winding current Iw inverts does the secondary voltage invert, thus avoiding high-current turn-off of the power switching device (fully controlled device T, which can be IGCT, IGBT, etc.). Figure 11 As shown, the arm current = half of the winding current + half of the DC port current. For example, if it is the second arm on the secondary side, then the second arm current = half of the secondary winding current (i.e., the low-voltage side winding of the isolation transformer) + half of the low-voltage side single-phase MMC DC port current. If it is the first arm on the primary side, then the first arm current = half of the primary winding current + half of the high-voltage side single-phase MMC DC port current. The reference direction for arm voltage and current can be found at [reference needed]. Figure 13For example, the voltage direction of the first bridge arm can be referenced to the positive (+) and negative (-) directions of the primary winding, the voltage direction of the second bridge arm can be referenced to the positive (+) and negative (-) directions of the secondary winding, and the current direction of the second bridge arm can be referenced to the positive (+) direction of the secondary winding flowing to the negative (-) direction, i.e. Figure 12 The direction of the arrow in the image.

[0068] The reason for the low switching loss is explained below using the low-voltage side single-phase MMC as an example.

[0069] (1) Upper arm of the second bridge arm of the low-voltage side single-phase MMC: During the period when the secondary voltage drops to the gap voltage, the upper bridge arm current is at its maximum value, and the power switching devices on the secondary bridge arm turn off with a large current, resulting in high turn-off losses. However, since the gap voltage is relatively small, only a portion of the sub-modules (half-bridge sub-modules) are switched on, and their power switching devices switch. At this time, the overall turn-off loss of the secondary side is significantly reduced compared to the method where all power switching devices are turned off. After the current reverses, the remaining power sub-modules are switched on. At this time, the upper bridge arm current is relatively small, and the switching loss is small.

[0070] (2) The lower arm of the second bridge arm of the low-voltage side single-phase MMC: During the period when the secondary voltage drops to the gap voltage, the bridge arm current is at its minimum value, and the secondary bridge arm power switching device turns off with a small current, resulting in low loss. However, due to the small gap voltage, only a portion of the sub-modules are switched on. After the current reverses, the remaining power sub-modules are switched on. At this time, the bridge arm current is large, resulting in high loss. However, the current has already reversed and flows through the anti-parallel diode of the power switching device. Therefore, the switching of the sub-module at this time generates the conduction loss of the fully controlled power switching device and the reverse recovery loss of the diode. However, since the conduction loss of IGBT, IGCT and other power switching devices is less than the turn-off loss, the overall loss and temperature rise of a single device are reduced.

[0071] Based on the above analysis, it can be seen that the modulation method of the present invention can reduce the loss and temperature rise of a single power switching device, which is beneficial to its reliable operation and also to the improvement of the switching frequency.

[0072] The following example illustrates a low-loss modulation method for a DC transformer according to the present invention.

[0073] against Figure 1 The single-phase MMC type high-voltage DC transformer shown has a high-voltage side DC voltage of 400kV, a rated power of 100MW, and an AC frequency of 500Hz. A case study is conducted on this transformer. The AC side voltage waveform of the single-phase MMC type high-voltage DC transformer applying the modulation strategy of this invention is as follows. Figure 13 As shown.

[0074] Among them, the voltage amplitude Up=Us=400kV, the gap voltage Us2=300kV, the step voltage=50kV, and the rated operating phase shift angle is... δ=50°, step width δ t=2us, period T=2ms, number of step voltages N=16.

[0075] like Figure 13 As shown, at the voltage gap, there are initially two voltage steps, meaning that 2 / 16 = 1 / 8 of the sub-modules participate in switching. At this point, a large current is turned off, resulting in significant turn-off losses. 7 / 8 of the sub-modules are switched off after current reversal. Therefore, at a frequency of 500Hz, the average large current turn-off frequency of the sub-modules decreases to 500 / 8 = 62.5Hz, significantly reducing turn-off losses.

[0076] like Figure 14 The diagram shows the lower arm voltage on the low-voltage side at a rated power of 100MW, from the low-voltage side to the high-voltage side. Figure 14 (The above diagram) Current ( Figure 14 The simulation waveforms (voltage unit kV, current unit kA, time unit s) shown in the image below are divided into the following four cases: 1) When the voltage drops from 400kV to 300kV, 1 / 8 of the submodules are shut down, and their power switching devices are turned off with a small current of 250A, resulting in a small turn-off loss. 2) When the voltage drops from 300kV to 0kV, 5 / 8 of the submodules are shut down, and their power switching devices conduct with a high current of 1500A, resulting in significant conduction losses and diode reverse recovery losses. 3) When the voltage rises from 0kV to 100kV, 1 / 8 of the sub-modules are put into operation, and their power switching devices are turned off with a high current of 1500A, resulting in large conduction losses and diode reverse recovery losses. 4) When the voltage rises from 100kV to 400kV, the 5 / 8 sub-modules are put into operation, and their power switching devices conduct with a small current of 250A, resulting in small conduction losses and reverse recovery losses of the headphone tube.

[0077] It is evident that the larger losses are primarily conduction losses and diode reverse recovery losses, rather than turn-off losses. Since the single-cycle conduction loss of a device is less than its turn-off loss under the same current, the modulation employed in this invention helps reduce device losses, thereby lowering temperature rise and facilitating frequency increases.

[0078] Figure 14 middle( Figure 14 In the four graphs, the horizontal axis represents time (s), and the vertical axis represents voltage (KV), current (KA), voltage (KV), and current (KA) respectively from top to bottom. E1s is the output voltage of the upper bridge arm on the low-voltage side; Iarm1ps is the current of the upper bridge arm on the low-voltage side; E2s is the output voltage of the lower bridge arm on the low-voltage side; and Iarm1ns is the current of the lower bridge arm on the low-voltage side.

[0079] By rationally allocating turn-on and turn-off losses according to this invention, the losses and temperature rise of individual power switching devices in DC transformers are effectively reduced, which is conducive to their reliable operation, avoids thermal damage to the devices, and also facilitates the improvement of switching frequency.

[0080] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A low-loss modulation method for a DC transformer, wherein, The method includes: A voltage modulation waveform is generated based on voltage modulation parameters, wherein the voltage modulation parameters include a phase shift angle. δ Step width δ t, step voltage Ut, and gap voltage Us2; Based on the voltage modulation waveform, the DC transformer is modulated to reduce the switching losses of the DC transformer; The generation of the voltage modulation waveform includes: Set the original square wave voltage on the primary / secondary side of the DC transformer; According to the phase shift angle δ The gap voltage Us2 causes the original square wave voltage of the primary / secondary side to form a first voltage waveform with gap voltage Us2; According to the step width δ The first voltage waveform is formed by the step voltage Ut and the step voltage Ut, so that the first voltage waveform is formed by the step voltage Ut on the rising / falling edge, as a voltage modulation waveform.

2. The low-loss modulation method for a DC transformer according to claim 1, wherein, According to the phase shift angle δ And the gap voltage, causing the original square wave voltage of the primary / secondary side to form a first voltage waveform with a gap voltage Us2, including: The waveforms of the first decomposed voltage, the second decomposed voltage, and the third decomposed voltage are superimposed to generate a voltage waveform with a gap voltage Us2, wherein, The first decomposition voltage is the original square wave voltage of the primary / secondary side of the DC transformer. The second decomposed voltage is the inverse of the original square wave voltage; The third decomposition voltage is the inverse of the second decomposition voltage, followed by phase shifting and phase angle shifting. δ The result.

3. The low-loss modulation method for a DC transformer according to claim 2, wherein, According to the step width δ The step voltage Ut and the initial square wave voltage t on the primary / secondary side cause the original square wave voltage to form a second voltage waveform with a step voltage Ut on the rising / falling edge, including: Set the original square wave voltage on the primary / secondary side of the DC transformer; The original square wave voltage of the primary / secondary side is passed through a ramp circuit to limit the rate of change of the original square wave voltage of the primary / secondary side, so that the original square wave voltage of the primary / secondary side meets the trapezoidal wave voltage condition to form a trapezoidal wave voltage. The trapezoidal voltage is passed through a rounding stage based on the step voltage Ut, with the falling edge of the trapezoidal voltage waveform rounded down and the rising edge rounded up, to form a second voltage waveform with a step level.

4. A low-loss modulation method for a DC transformer according to claim 2 or 3, wherein, The amplitude of the first decomposition voltage is Us, and the amplitude of the second decomposition voltage is 0.5(Us-Us2).

5. A low-loss modulation method for a DC transformer according to claim 3, wherein, The trapezoidal wave voltage condition is: The width of the voltage ramp is N times the step width. δ t, where N is the number of step voltages.

6. A low-loss modulation system for a DC transformer, wherein, The system includes: The waveform generation module is used to generate a voltage-modulated waveform based on voltage modulation parameters, wherein the voltage modulation parameters include a phase shift angle. δ Step width δ t, step voltage Ut, and gap voltage Us2; A modulation module is used to modulate a DC transformer according to a voltage modulation waveform to reduce the switching losses of the DC transformer. The generation of the voltage modulation waveform includes: setting the original square wave voltage on the primary / secondary sides of the DC transformer; and adjusting the phase shift angle... δ The gap voltage, along with the original square wave voltage of the primary / secondary side, forms a first voltage waveform with a gap voltage Us2; based on the step width... δ The first voltage waveform is formed by the step voltage Ut and the step voltage Ut, so that the first voltage waveform is formed by the step voltage Ut on the rising / falling edge, as a voltage modulation waveform.

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