Multi-degree-of-freedom general carrier modulation method and system for current source converter

Abstract

The invention belongs to the technical field of power electronics, and provides a multi-degree-of-freedom general carrier modulation method and system for a current source converter, and the method comprises the steps: constructing a modular cascade current source converter; a sine pulse width sequence is output through carrier modulation to analyze direct-current side voltage harmonic distribution in the constructed modular cascade current source type converter; determining the optimal phase shift angle of the modular cascade current source type converter by taking the sine pulse width sequence waveform quality, the switching frequency and the DC voltage performance optimization as optimization targets according to the distribution result of the DC side voltage harmonic waves; and adjusting the three-phase carrier wave of each current source module in the current source type converter based on the determined optimal phase shift angle, and completing multi-degree-of-freedom general carrier wave modulation of the current source type converter.

Description

A multi-degree-of-freedom universal carrier modulation method and system for current source converters Technical Field

[0001] This invention belongs to the field of power electronics technology, specifically relating to a multi-degree-of-freedom general carrier modulation method and system for a current source converter. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] Integrated gate commutated thyristors (IGCTs) are a type of power semiconductor device designed for medium- and high-voltage ultra-high-power applications. They retain the advantages of low conduction losses and high surge resistance of thyristor devices, while achieving forced turn-off capability through highly integrated gate drive and extremely low gate circuit inductance design. They are fully controllable devices capable of both turn-on and turn-off. In recent years, with the continuous improvement of the high-power, high-voltage, and low-conduction-loss characteristics of fully controllable IGCTs, research on fully controllable current source converters (CSCs) based on IGCTs in renewable energy power generation, medium-voltage motor drives, and offshore power transmission systems has received in-depth attention. Compared to traditional thyristor-based grid commutated converters (LCCs), which suffer from strong dependence on AC systems, inability to regulate reactive power, and susceptibility to commutation failure, IGCT-type current source converters offer advantages such as active turn-on and turn-off, reactive power regulation, bidirectional power control, avoidance of commutation failure, and higher voltage and current withstand capabilities. They also have advantages in lightweighting HVDC transmission systems for renewable energy transmission and STATCOM systems.

[0004] In conventional LCC-type HVDC systems, each valve group uses multiple thyristors connected in series to improve withstand voltage, and each thyristor is connected in parallel with a passive buffer circuit to achieve withstand voltage equalization. In addition, multiple LCC converters can be cascaded using phase-shifting transformers to further improve the system's withstand voltage capability at the converter level. The 12-pulse LCC-HVDC system uses two sets of 6-pulse bridges for each pole's converter, which is one of the most common configurations in engineering. This cascading method can be used to reduce harmonics, improve power quality, and reduce filter size. With the development of fully controllable IGCT devices, IGCT-HVDC will bring revolutionary development to DC transmission projects, but also pose challenges to phase-adaptive modulation control technology. Similar to the 12-pulse LCC-HVDC, the IGCT-HVDC system can also use modular cascading to withstand ultra-high voltages. Due to the carrier modulation strategy, its DC bus quality has a natural advantage over the multi-pulse method. Existing research has not systematically analyzed the impact of carrier modulation strategies on the DC-side voltage harmonic distribution, and the main influencing factors of DC-side voltage quality and corresponding improvement schemes are not yet clear.

[0005] In terms of modulation strategies for modular cascaded IGCT-type CSC structures, sinusoidal pulse width modulation (SPWM) is a suitable choice due to its inherent scalability, modularity, and ease of implementation. However, existing research mainly focuses on SPWM technology and system performance in parallel CSC scenarios, and research on the system performance of SPWM in modular cascaded CSC scenarios has not yet been conducted. Although some studies have fully elucidated the advantages of modular cascaded CSC structures, no research has yet focused on the potential of SPWM to improve DC bus power quality, and research on general-purpose multi-degree-of-freedom SPWM modulation is particularly lacking. Summary of the Invention

[0006] To address the aforementioned issues, this invention proposes a multi-degree-of-freedom universal carrier modulation method and system for current source converters. For modular cascaded current source converters, a multi-degree-of-freedom optimized module carrier phase shifting and inter-phase carrier phase shifting method is used to reduce the DC-side voltage harmonic content.

[0007] According to some embodiments, the first aspect of the present invention provides a multi-degree-of-freedom universal carrier modulation method for a current source converter, employing the following technical solution: A multi-degree-of-freedom universal carrier modulation method for a current source converter includes: constructing a modular cascaded current source converter; analyzing the DC-side voltage harmonic distribution in the constructed modular cascaded current source converter using a carrier modulation output sinusoidal pulse width sequence; determining the optimal phase shift angle of the modular cascaded current source converter based on the DC-side voltage harmonic distribution results, with the optimization objectives being the optimal sinusoidal pulse width sequence waveform quality, switching frequency, and DC voltage performance; and adjusting the three-phase carrier of each current source module in the current source converter based on the determined optimal phase shift angle to complete the multi-degree-of-freedom universal carrier modulation of the current source converter.

[0008] As a further technical limitation, in the process of constructing a modular cascaded current source converter, several IGCT devices are connected in series to form a valve group. The devices in each valve group use synchronous switching signals, and each valve group using synchronous switching signals is equivalent to a high-voltage valve group. A current source module is established through six switching valve groups, and several current source modules are then cascaded to obtain a modular cascaded current source converter. The number of cascaded current source modules is configured according to the voltage level requirements.

[0009] Furthermore, each of the six-switch valve group modules adopts carrier modulation to output a sinusoidal pulse width sequence. Based on the duality principle of voltage source converter and current source converter, the switching signals of the voltage source converter are converted into two-to-three logic and mapped to the current source module. The all-zero signals generated by the logic conversion are replaced with zero current vectors. The DC-side voltage harmonic distribution generated by each module is analyzed by the replaced zero current vectors to obtain the DC-side voltage harmonic distribution in the constructed modular cascaded current source converter.

[0010] Furthermore, through the aforementioned binary-to-three logic conversion, the binary logic modulation signal of the voltage source is converted into a ternary logic modulation signal of the current source. The expression for the binary-to-three logic conversion is as follows: Among them, I a_PWM ,I b_PWM ,I c_PWM These are the three-phase current pulse outputs of the current source converter, which are ternary logic values ​​of [1, 0, -1]; V a_PWM V b_PWM V c_PWM These are the three-phase voltage pulse outputs of the voltage source converter line; A, B, and C are the three-phase pulse width sequences generated by comparing the sinusoidal modulation wave under carrier modulation of the voltage source converter with the triangular carrier, which are binary logic signals that can directly drive the upper bridge arm of the voltage source converter. , , These are the inversions of three-phase binary logic signals modulated by a voltage source carrier, which can directly drive the lower bridge arm of the voltage source converter.

[0011] As a further technical limitation, the DC-side voltage harmonic distribution is analyzed using carrier-modulated harmonic spectrum analysis. The carrier-modulated harmonic spectrum used is... Among them, S x This represents the pulse width sequence generated by comparing a sinusoidal modulated wave with a triangular carrier wave, where x represents the three phases a, b, and c, and S... a =A,S b =B,S c =C;m a ω is the modulation ratio. o For output frequency; For a Bessel function of the first kind; ω c The carrier frequency is represented by θ; m and n represent the harmonic orders corresponding to the switching frequency and the fundamental frequency, respectively. cy θ is the carrier phase angle; ox For each phase fundamental wave, θ oa =0, θ ob =-2 / 3π,θ oc =2 / 3π.

[0012] As a further technical limitation, it also includes phase-shifting of the inter-phase carrier for each cascaded module of the modular cascaded current source converter, that is, the carriers corresponding to the three-phase modulation waves of each cascaded module are staggered by a certain angle.

[0013] As a further technical limitation, the constructed modular cascaded current source converter includes at least a rectifier side, which is a bipolar converter station composed of cascaded current source modules consisting of six switch valve groups. The DC current output from the DC side via the smoothing reactor is transmitted over a long distance through a bipolar overhead line.

[0014] As a further technical limitation, the constructed modular cascaded current source converter also includes a modular IGCT type current source inverter station, which expands the voltage level by connecting multiple modules in series, and realizes bipolar DC energy transmission between the two AC systems based on the connection between the transformer and the receiving end main grid.

[0015] As a further technical limitation, the multi-degree-of-freedom universal carrier modulation is to use traditional phase-shift carrier modulation for each cascaded current source module, substitute the frequency components of the primary carrier frequency and the secondary carrier frequency into the harmonic spectrum of the pulse width sequence, and then model and analyze the total DC-side voltage under phase-shift carrier modulation to obtain the total DC-side voltage of the cascaded module.

[0016] According to some embodiments, a second aspect of the present invention provides a multi-degree-of-freedom universal carrier modulation system for a current source converter, employing the following technical solution: A multi-degree-of-freedom universal carrier modulation system for a current source converter includes: a construction module configured to construct a modular cascaded current source converter; an analysis module configured to analyze the DC-side voltage harmonic distribution in the constructed modular cascaded current source converter using a carrier modulation output sinusoidal pulse width sequence; a determination module configured to determine the optimal phase shift angle of the modular cascaded current source converter based on the DC-side voltage harmonic distribution results, with the optimization objectives being the optimal sinusoidal pulse width sequence waveform quality, switching frequency, and DC voltage performance; and a modulation module configured to adjust the three-phase carrier of each current source module in the current source converter based on the determined optimal phase shift angle, thereby completing the multi-degree-of-freedom universal carrier modulation of the current source converter.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention improves the DC voltage level by cascading multiple current source modules, connecting the AC side to the main power grid through a multi-winding transformer, and connecting the DC side bus in series. The total DC bus voltage is the sum of the voltages of each sub-module. The present invention cascades N current source modules, and the carrier of each module is phase-shifted, and then the phase carriers of each phase are phase-shifted to stagger the switching actions of the cascaded modules in time, which can achieve the effect of harmonic cancellation.

[0018] This invention uses carrier phase shifting to make the DC voltage ripples of the series modules interleave in time. After they are superimposed, the ripples cancel each other out, and the ripple frequency of the total DC voltage is increased to N times that of a single module, thus reducing the DC voltage THD. This improves the performance of the bus voltage of the current source module cascade system, resulting in a significant reduction in THD and a significant suppression of major high-order harmonics.

[0019] This invention achieves optimized DC output performance through reasonable phase shift angle design, thereby reducing the size of the DC-side smoothing filter and lowering costs. The modular cascaded structure allows for fault-tolerant system operation via switches and module bypasses during fault conditions, effectively improving reliability.

[0020] This invention employs a multi-degree-of-freedom universal carrier modulation strategy. By simply designing the phase shift angle, the modulation strategy can be easily extended. As the number of cascaded modules increases, the system voltage level continuously improves, and the system capacity can be linearly expanded according to the number of modules. Attached Figure Description

[0021] The accompanying drawings, which form part of this embodiment, are used to provide a further understanding of this embodiment. The illustrative embodiments and their descriptions are used to explain this embodiment and do not constitute an improper limitation of this embodiment.

[0022] Figure 1 is a flowchart of the multi-degree-of-freedom universal carrier modulation method for a current source converter in Embodiment 1 of the present invention; Figure 2 is a structural diagram of a cascaded modular current source converter in Embodiment 1 of the present invention; Figure 3 is a block diagram of the implementation of modular multi-degree-of-freedom universal carrier modulation in a single current source converter in Embodiment 1 of the present invention; Figure 4 is a block diagram of the implementation of universal carrier modulation in two cascaded current source converters in Embodiment 1 of the present invention; Figure 5 shows the theoretical results of traditional phase-shift carrier modulation and the multi-degree-of-freedom universal carrier modulation proposed in the present invention under different modulation indices on two cascaded current source converters in Embodiment 1 of the present invention. Figure 6 is a schematic diagram showing the theoretical results of the traditional phase-shift carrier modulation in Embodiment 1 of the present invention and the multi-degree-of-freedom universal carrier modulation proposed in the present invention under different power factor angles and different modulation intensities on two cascaded current source converters; Figure 7 is a schematic diagram showing the verification results of the traditional phase-shift carrier modulation in Embodiment 1 of the present invention on two cascaded current source converters; Figure 8 is a schematic diagram showing the verification results of the multi-degree-of-freedom universal carrier modulation in Embodiment 1 of the present invention on two cascaded current source converters; Figure 9 is a block diagram of the multi-degree-of-freedom universal carrier modulation system of the current source converter in Embodiment 2 of the present invention. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0024] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0025] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0026] In this invention, terms such as "upper," "lower," "left," "right," "front," "back," "vertical," "horizontal," "side," and "bottom" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are used only to facilitate the description of the structural relationships of the various components or elements of this invention and do not specifically refer to any component or element in this invention. They should not be construed as limiting the invention.

[0027] In this invention, terms such as "fixed connection," "connected," and "linked" should be interpreted broadly, indicating a fixed connection, an integral connection, or a detachable connection; a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can determine the specific meaning of these terms in this invention based on the specific circumstances, and they should not be construed as limitations on the invention.

[0028] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0029] Example 1: This invention introduces a multi-degree-of-freedom general carrier modulation method for a current source converter.

[0030] Figure 1 illustrates a multi-degree-of-freedom universal carrier modulation method for a current source converter, comprising: constructing a modular cascaded current source converter; analyzing the DC-side voltage harmonic distribution in the constructed modular cascaded current source converter using a carrier-modulated sinusoidal pulse width sequence; determining the optimal phase shift angle of the modular cascaded current source converter based on the DC-side voltage harmonic distribution results, with the optimization objectives being the optimal quality of the sinusoidal pulse width sequence waveform, the number of switching operations, and the DC voltage performance; and adjusting the three-phase carrier of each current source module in the current source converter based on the determined optimal phase shift angle to complete the multi-degree-of-freedom universal carrier modulation of the current source converter.

[0031] As one or more implementation methods, this embodiment establishes a modular cascaded current source converter as shown in Figure 2; that is, multiple IGCT devices are connected in series to form a valve group, and the devices in each valve group use synchronous switching signals, which are equivalent to a high-voltage valve group; a current source module is established using a six-switch valve group, and then multiple current source modules are cascaded to form a modular cascaded current source converter system. The number of cascaded current source modules can be flexibly configured according to the system voltage level requirements.

[0032] The power valve group in Figure 2 consists of multiple fully controlled IGCT devices connected in series to achieve high withstand voltage. The left side of Figure 2 is the rectifier side, where two sets of six-switch valve groups are cascaded to form a bipolar converter station. S... 11 S 31 S 51 S 41 S 61 S 21 S represents the power valve group of rectifier-side module 1. 12 S 32 S 52 S 42 S 62 S 22 The power valve group representing rectifier module 2 consists of two sets of six-switch valve group modules. The AC side of these modules is connected to the main power grid at the sending end via a phase-shifting transformer. The DC side is connected in series to form the total DC voltage, where V... p V n V0 represents the positive terminal potential, negative terminal potential, and midpoint potential of the DC side, respectively. DC1 V DC2 V represents the DC-side voltage generated by rectifier modules 1 and 2 respectively. dc This represents the total DC-side voltage. A stable DC current I is output from the DC side via a smoothing reactor. dc It transmits power over long distances via bipolar overhead lines. The right side of Figure 2 shows a modular IGCT type current source inverter station. Similar to the rectifier side, multiple modules are connected in series to expand the voltage level. It is connected to the main power grid at the receiving end through a transformer to realize bipolar DC energy transmission between the AC systems at both ends.

[0033] According to the modular cascaded current source converter in this embodiment, each module is equivalent to a six-switch valve group. Each valve group consists of multiple IGCTs driven by synchronous signals connected in series. The six switching signals are given by a two-three logic carrier modulation strategy. During the two-three logic conversion process, all-zero signals are generated, which cannot directly drive the IGCT valve group. They need to be replaced with effective current zero vectors. The cascaded module connection method in step one is that multiple six-switch valve group modules are connected in series on the DC side, while the AC side is connected to the power grid through a phase-shifting transformer. The total DC bus voltage V of this cascaded modular system is...pn It is the sum of the voltages of each DC bus, which can be expressed as: .

[0034] As one or more implementation methods, each six-switch valve group module in this embodiment uses carrier modulation to output a sinusoidal pulse width sequence. To ensure the universality of the carrier type, the duality principle of voltage source converter and current source converter is utilized. The switching signals of the voltage source converter are mapped to the current source module by performing a two-to-three logic conversion. At the same time, the all-zero signal generated by the logic conversion is replaced with a zero current vector, and the DC-side voltage harmonic distribution generated by each module is analyzed.

[0035] According to the multi-degree-of-freedom general carrier modulation strategy of the modular current source converter system in this embodiment, the implementation method of general modular two-to-three logic conversion carrier modulation is as follows: A binary logic signal is generated by comparing the three-phase modulation wave in the voltage source converter with the carrier; then, through the duality principle of the modulation of the voltage source converter and the current source converter, the binary logic modulation signal of the voltage source is converted to the ternary logic modulation signal of the current source using two-to-three logic conversion. The expression for the two-to-three logic conversion is: Among them, I a_PWM ,I b_PWM ,I c_PWM V is the three-phase current pulse output of the current source converter, a ternary logic [1, 0, -1]; a_PWM V b_PWM V c_PWM A represents the line voltage pulse output of the voltage source converter; A, B, and C are three-phase pulse width sequences generated by comparing the sinusoidal modulated wave under carrier modulation with the triangular carrier wave. These are binary logic signals that can directly drive the upper bridge arm of the voltage source converter. , , Inverting the three-phase binary logic signal under voltage source carrier modulation allows it to directly drive the lower bridge arm of the voltage source converter.

[0036] The method for replacing all-zero switching states generated in the general modular two-to-three logic conversion in this embodiment is as follows: The switching signals of the current source converter after the two-to-three logic conversion are detected. If all are zero, it means that all switches of the current source are in the off state, thus failing to meet the requirement that the current of the current source converter must remain continuous at any time. Therefore, it is necessary to perform effective zero-vector replacement on these all-zero states, i.e., any one phase of the three-phase bridge arm is directly connected. The expression for the replaced switching states is as follows: Wherein, S1, S4, S3, S6, S5, and S2 are the six switching signals of the current source converter; S1 and S4 are the upper and lower bridge arm signals of phase A; S3 and S6 are the upper and lower bridge arm signals of phase B; and S5 and S2 are the upper and lower bridge arm signals of phase C. When the upper and lower bridge arm switching signal is set to 1, it indicates that the bridge arm is straight-through, which is an effective zero vector.

[0037] In this embodiment, the DC-side voltage of each module under the general modular two-three logic modulation is summarized and its expression is as follows: Among them, V dc V is the DC-side voltage of a single current source converter module. a V b V c These are the AC output phase voltages of the converter; V ab V bc V ca These are the AC output line voltages of the converter.

[0038] This embodiment analyzes the DC-side voltage harmonic distribution generated by each module, that is, firstly, the carrier modulation harmonic spectrum is mathematically characterized, and its expression is: Among them, S x This represents the pulse width sequence generated by comparing a sinusoidal modulated wave with a triangular carrier wave, where x represents the three phases a, b, and c. S... a =A,S b =B,S c =C. m a ω is the modulation ratio. o For output frequency, For a Bessel function of the first kind, ω c The carrier frequency is represented by m and n, which still represent the harmonic orders corresponding to the switching frequency and the fundamental frequency, respectively. θ cy θ is the carrier phase angle. In a conventional carrier modulation strategy... cy It is typically set to 0, but when there is a phase shift between the arms, the phase shift angle of each arm can be expressed relative to the reference arm. θ ox The term θ represents the fundamental phase of each phase, where θ is the fundamental phase of each phase. oa =0, θ ob =-2 / 3π,θ oc =2 / 3π.

[0039] Then, the three main frequency components—fundamental frequency, primary carrier frequency, and secondary carrier frequency—from each phase pulse width sequence are substituted into V. dc The formula allows for the calculation of the content of major frequency harmonics in the DC-side voltage, and its expression is as follows: .

[0040] As one or more implementation methods, this embodiment uses traditional phase-shifted carrier modulation for each current source module; the carrier is staggered between different current source modules according to a predetermined phase shift angle, and the optimal phase shift angle is determined through theoretical and simulation analysis. The optimization objectives include optimizing the output sinusoidal pulse width sequence waveform quality, the number of switching operations, and the DC voltage performance.

[0041] In this embodiment, traditional phase-shift carrier modulation is used for each cascaded current source module, and its pulse width sequence harmonic spectrum can be expressed as: Wherein, A1, B1, and C1 are the three-phase pulse width sequences generated by comparing the sinusoidal modulated wave with the triangular carrier wave under traditional phase-shifted carrier modulation in voltage source converter module 1, and A2, B2, and C2 are the three-phase pulse width sequences generated by comparing the sinusoidal modulated wave with the triangular carrier wave under traditional phase-shifted carrier modulation in voltage source converter module 2.

[0042] Substituting the primary frequency component, the first carrier frequency, into the equation, i.e., when m=1, we have the following relationship: That is, the first carrier frequency harmonic components of voltage source converter module 1 and voltage source converter module 2 in the DC side voltage can cancel each other out.

[0043] Similarly, substituting the primary frequency component, the secondary carrier frequency, into the equation, i.e., when m=2, we have the following relationship: It can be concluded that A1 and A2, B1 and B 2、 The components of the secondary carrier frequency in C1 and C2 are equal.

[0044] This embodiment models the total DC-side voltage under phase-shifted carrier modulation. Taking a cascaded two-module configuration as an example, the expression for its DC-side voltage is as follows: Among them, V dc_total V is the total DC-side voltage generated by the cascaded modules. dc1 V dc2 These are the DC-side voltages generated by cascade module 1 and cascade module 2, respectively.

[0045] Therefore, the main harmonic frequency components in the total DC-side voltage of the cascaded two modules can be obtained as follows: .

[0046] As one or more implementation methods, this embodiment performs inter-phase carrier phase shifting based on inter-module phase shifting modulation. For each current source module, the three-phase carriers are staggered according to a predetermined phase shift angle, and DC voltage performance is optimized through multi-degree-of-freedom carrier phase shifting modulation.

[0047] This embodiment performs inter-phase carrier phase shifting on the basis of phase-shifted carrier modulation of cascaded current source modules. That is, the corresponding carriers of the three-phase modulation waves of each cascaded module are staggered by a certain angle, θ. cyTaking 120° as an example, its pulse width sequence harmonic spectrum can be expressed as: Wherein, A1, B1, and C1 are the three-phase pulse width sequences generated by comparing the sinusoidal modulated wave with the triangular carrier under phase-shift modulation of the voltage source converter module 1, and A2, B2, and C2 are the three-phase pulse width sequences generated by comparing the sinusoidal modulated wave with the triangular carrier under phase-shift modulation of the voltage source converter module 2.

[0048] Substituting the primary frequency component, the first carrier frequency, into the equation (m=1) yields the following: Similarly, substituting m=2, i.e., the secondary carrier frequency, as the main frequency component, we get: That is, the frequency components of the first carrier frequency in A1 and A2, B1 and B2, and C1 and C2 cancel each other out, while the frequency components of the second harmonic frequency are equal and superimposed.

[0049] In this embodiment, the main harmonic frequency components in the DC-side total voltage of the cascaded interphase carrier phase-shift modulation module 1 and module 2 are: .

[0050] Figure 3 shows a block diagram of modular multi-degree-of-freedom universal carrier modulation implemented on a single current-source converter; by using the three-phase modulation wave V a * V b * V c * These correspond to three-phase modulation signals a, b, and c, with phase angles offset by 120°. The three-phase modulation signals are compared with a triangular carrier wave to generate three dual-logic signals A, B, and C, which are high and low level (0 and 1) signals used to characterize the comparison results between each phase modulation signal and the carrier wave. Logic signals A, B, and C are then subjected to two- and three-level logic operations to generate six candidate switching signals. , , , , , The overline indicates a logical inversion. The six signals mentioned above correspond to the conduction commands of the upper and lower bridge arms of the three-phase current source converter, respectively. However, these signals cannot be directly given to the current source switches because the logical operation will produce an all-zero state, that is, all six switch signals will be zero. This will cause the current of the current source converter to be intermittent, which is an invalid switching state.

[0051] Therefore, when the logic operation result is a fully off state of [000000], the all-zero vector needs to be replaced with the preset legal zero current vector states [100100], [010010], and [100100] to ensure the continuity of DC current. The final six drive signals S1, S3, S5, S4, S6, and S2 are formed, where S1, S3, and S5 are the upper bridge arm switch signals of phases a, b, and c, respectively, and S4, S6, and S2 are the lower bridge arm switch signals of phases a, b, and c, respectively. These switch signals can be applied to the six switch valve groups in the converter to control their opening or closing.

[0052] Figure 4 shows the block diagram of the universal carrier implemented on two cascaded current source converters; to fully improve the DC bus voltage waveform quality, compared with the three-phase modulation waveform V in traditional two-three logic carrier modulation... a * V b * V c * Sharing the same carrier signal, this embodiment first employs inter-phase carrier phase shifting. That is, the carriers compared with the three-phase modulation are no longer the same, but are staggered by a certain angle according to a certain pattern. The initial angles of each phase carrier are 0°, 120°, and 240°, respectively. Then, each phase modulation signal is compared with its respective phase carrier to obtain the switching signal. For cascaded module 1, the switching signals obtained by comparing the three-phase modulation wave with its respective carrier are A1, B1, and C1; for cascaded module 2, the same three-phase modulation wave V as cascaded module 1 is used. a * V b * V c * However, the carrier of each phase is shifted by 180° from the corresponding phase of module 1, which is the phase shift of the carrier between the converters. Thus, the initial angles of the carriers of each phase are 180°, 300°, and 60°, as shown by the dotted lines. The switching signals obtained by comparing the three-phase modulated waves with their respective carriers are A2, B2, and C2. After obtaining A1, B1, C1, A2, B2, and C2, they are subjected to two-to-three logic transformations and effective zero vector replacement, ultimately obtaining the six-channel drive signals S of cascaded module 1. 11 S 31 S 51 S 41 S 61 S 21 and the six drive signals S of cascade module 1 12 S 32 S 52 S 42 S 62 S 22 .

[0053] Figure 5 shows the theoretical results of traditional phase-shift carrier modulation and the multi-degree-of-freedom universal carrier modulation proposed in this invention under different modulation indices on two cascaded current source converters. The power factor angle is set to 0, i.e., the power factor is 1, the fundamental frequency is 10Hz, and the carrier frequency is 1kHz.

[0054] Figures 5(a) and 5(b) respectively show different modulation schemes m a =0.3, m a =0.6, m a The harmonic analysis results of the DC-side voltage at a value of 0.9 show that, using traditional phase-shifted carrier modulation, the DC components of the DC-side voltage are 0.9 pu, 1.8 pu, and 2.4 pu, while the second harmonic components are 1.58 pu, 2.01 pu, and 1.00 pu. When using the multi-degree-of-freedom universal carrier modulation in this embodiment, the DC components of the DC-side voltage remain unchanged, while the harmonic components are 0.61 pu, 0.72 pu, and 0.21 pu, indicating that the universal carrier modulation proposed in this invention can significantly suppress the high-frequency second harmonic components.

[0055] Figure 6 shows the theoretical results of traditional phase-shift carrier modulation and the multi-degree-of-freedom universal carrier modulation proposed in this invention on two cascaded current source converters under different power factor angles and different modulation intensities. The power factor angle ranges from 0° to 45°, and the modulation intensities range from 0.1 to 1.0.

[0056] Figures 6(a) and 6(b) show the amplitude of the second harmonic of the DC bus voltage under conventional phase-shifting carrier modulation and the multi-degree-of-freedom universal carrier modulation proposed in this invention, respectively. It can be seen that, under the power factor angle range of 0° to 45° and the modulation index range of 0.1 to 1.0, the multi-degree-of-freedom universal carrier modulation in this embodiment can significantly reduce the high-order harmonic components of the DC side of the cascaded current source converter.

[0057] Figure 7 shows the verification results of traditional phase-shifted carrier modulation on two cascaded current-source converters, illustrating the three-phase reference voltage, module 1 carrier, module 2 carrier, DC-side voltage, and DC-side voltage FFT harmonic analysis results. For each cascaded module, the three-phase modulation wave V... a * V b * V c *Sharing the same triangular carrier wave, the carrier waves of cascaded module 1 and module 2 are staggered by 180°. The reference voltage modulation is 0.8, the power factor is 1, the fundamental frequency is 10Hz, and the carrier frequency is 1kHz. That is, under traditional phase-shift carrier modulation, the main harmonic frequency of the DC bus voltage in each cascaded module is increased to twice the carrier frequency (2kHz). FFT analysis shows that the DC component of the DC-side voltage is 2.4 per unit, and the 2kHz high-frequency component is 1.45 per unit.

[0058] Figure 8 shows the verification results of multi-degree-of-freedom universal carrier modulation on two cascaded current source converters, illustrating the three-phase reference voltage, module 1 carrier, module 2 carrier, DC-side voltage, and DC-side voltage FFT harmonic analysis results. For each cascaded module, the three-phase modulation wave V a * V b * V c * Instead of using the same triangular carrier wave, each phase carrier wave is staggered by 120°, forming a series of interleaved carrier waveforms. The corresponding phase carrier waves of cascaded module 1 and module 2 are further staggered by 180°. The reference voltage modulation is 0.8, the power factor is 1, the fundamental frequency is 10Hz, and the carrier frequency is 1kHz. Thus, under the multi-degree-of-freedom universal carrier modulation of this embodiment, the main harmonic frequency of the DC bus voltage in each cascaded module is also increased to twice the carrier frequency (2kHz). FFT analysis results show that the DC component of the DC-side voltage is 2.4 per unit, and the 2kHz high-frequency component is 0.38 per unit. Compared with traditional phase-shifting carrier modulation, the multi-degree-of-freedom universal carrier modulation in this embodiment can significantly suppress the 2kHz high-frequency component and significantly improve the DC-side voltage waveform quality.

[0059] This embodiment addresses the shortcomings and deficiencies of existing modulation strategies in terms of versatility, scalability, and harmonic content in modular cascaded CSC systems. By establishing a simulation and experimental system for modular cascaded CSC systems, it combines series devices and cascaded converter structures to realize the application of a fully controlled CSC converter in medium- and high-voltage ultra-high-power scenarios. The embodiment studies the DC-side voltage harmonic distribution characteristics of modular cascaded CSC systems based on carrier modulation strategies, clarifying the impact of factors such as zero-sequence injection, cascaded converter phase shifting, and inter-phase carrier phase shifting on DC voltage performance, and designs a modulation strategy with optimal THD performance. A multi-degree-of-freedom general carrier modulation strategy for modular cascaded CSC converters is adopted. A two- or three-logic carrier modulation strategy is used to achieve simple expansion of cascaded modules, and multi-degree-of-freedom carrier phase shifting is used to optimize DC-side voltage harmonics, thereby improving system reliability.

[0060] Example 2: This invention introduces a multi-degree-of-freedom universal carrier modulation system for a current source converter.

[0061] Figure 9 illustrates a multi-degree-of-freedom universal carrier modulation system for a current source converter, comprising: a construction module configured to construct a modular cascaded current source converter; an analysis module configured to analyze the DC-side voltage harmonic distribution in the constructed modular cascaded current source converter using a carrier-modulated sinusoidal pulse width sequence; a determination module configured to determine the optimal phase shift angle of the modular cascaded current source converter based on the DC-side voltage harmonic distribution results, with the optimization objectives being the optimal sinusoidal pulse width sequence waveform quality, switching frequency, and DC voltage performance; and a modulation module configured to adjust the three-phase carrier of each current source module in the current source converter based on the determined optimal phase shift angle, thereby completing the multi-degree-of-freedom universal carrier modulation of the current source converter.

[0062] The detailed steps are the same as the multi-degree-of-freedom general carrier modulation method for the current source converter provided in Example 1, and will not be repeated here.

[0063] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code. The solutions in the embodiments of the present invention can be implemented using various computer languages, such as the object-oriented programming language Java and the interpreted scripting language JavaScript.

[0064] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in one or more blocks of the flowchart illustrations and / or one or more blocks of the block diagrams.

[0065] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means that implement the functions specified in one or more flowcharts and / or one or more block diagrams.

[0066] These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process, such that the instructions, which execute on the computer or other programmable apparatus, provide steps for implementing the functions specified in one or more flowcharts and / or one or more block diagrams.

[0067] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0068] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

[0069] The above description is merely a preferred embodiment of this practice and is not intended to limit the scope of this practice. Various modifications and variations can be made to this practice by those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this practice should be included within the protection scope of this practice.

Claims

1. A multi-degree-of-freedom universal carrier modulation method for a current source converter, characterized in that, include: Construct a modular cascaded current source converter; The DC-side voltage harmonic distribution in the constructed modular cascaded current source converter is analyzed using a carrier-modulated sinusoidal pulse width sequence. Based on the distribution results of the DC-side voltage harmonics, the optimal phase shift angle of the modular cascaded current source converter is determined with the optimization objectives of optimal sinusoidal pulse width sequence waveform quality, switching frequency, and DC voltage performance. Based on the determined optimal phase shift angle, the three-phase carrier of each current source module in the current source converter is adjusted to complete the multi-degree-of-freedom universal carrier modulation of the current source converter.

2. The multi-degree-of-freedom universal carrier modulation method for a current source converter as described in claim 1, characterized in that, In constructing a modular cascaded current source converter, several IGCT devices are connected in series to form a valve group. The devices in each valve group use synchronous switching signals, and each valve group using synchronous switching signals is equivalent to a high-voltage valve group. A current source module is established through six switching valve groups, and several current source modules are cascaded to obtain a modular cascaded current source converter. The number of cascaded current source modules is configured according to the voltage level requirements.

3. The multi-degree-of-freedom universal carrier modulation method for a current source converter as described in claim 2, characterized in that, Each of the six-switch valve group modules uses carrier modulation to output a sinusoidal pulse width sequence. Based on the duality principle of voltage source converter and current source converter, the switching signals of the voltage source converter are converted into two-to-three logic and mapped to the current source module. The all-zero signals generated by the logic conversion are replaced with zero current vectors. The DC-side voltage harmonic distribution generated by each module is analyzed by the replaced zero current vectors to obtain the DC-side voltage harmonic distribution in the constructed modular cascaded current source converter.

4. The multi-degree-of-freedom universal carrier modulation method for a current source converter as described in claim 3, characterized in that, The binary-to-three logic conversion transforms the binary logic modulation signal of the voltage source into a ternary logic modulation signal of the current source. The expression for this binary-to-three logic conversion is as follows: Among them, I a_PWM ,I b_PWM ,I c_PWM These are the three-phase current pulse outputs of the current source converter, which are ternary logic values ​​of [1, 0, -1]; V a_PWM V b_PWM V c_PWM These are the three-phase voltage pulse outputs of the voltage source converter; A, B, and C are the three-phase pulse width sequences generated by comparing the sinusoidal modulated wave under carrier modulation with the triangular carrier, which are binary logic signals that can directly drive the upper bridge arm of the voltage source converter. 、 、 These are the inversions of three-phase binary logic signals modulated by a voltage source carrier, which can directly drive the lower bridge arm of the voltage source converter.

5. The multi-degree-of-freedom universal carrier modulation method for a current source converter as described in claim 1, characterized in that, The DC-side voltage harmonic distribution is analyzed using carrier-modulated harmonic spectrum analysis. The carrier-modulated harmonic spectrum used is as follows: Among them, S x This represents the pulse width sequence generated by comparing a sinusoidal modulated wave with a triangular carrier wave, where x represents the three phases a, b, and c, and S... a =A,S b =B,S c =C;m a ω is the modulation ratio. o For output frequency; For a Bessel function of the first kind; ω c The carrier frequency is represented by θ; m and n represent the harmonic orders corresponding to the switching frequency and the fundamental frequency, respectively. cy θ is the carrier phase angle; θ is the fundamental phase of each phase. oa =0, θ ob =-2 / 3π,θ oc =2 / 3π.

6. The multi-degree-of-freedom universal carrier modulation method for a current source converter as described in claim 1, characterized in that, It also includes phase-shifting of the inter-phase carrier for each cascaded module of the modular cascaded current source converter, that is, the carriers corresponding to the three-phase modulation waves of each cascaded module are staggered by a certain angle.

7. The multi-degree-of-freedom universal carrier modulation method for a current source converter as described in claim 1, characterized in that, The constructed modular cascaded current source converter includes at least a rectifier side, which is a bipolar converter station composed of cascaded current source modules consisting of six switch valve groups. The DC current output from the DC side via the smoothing reactor is transmitted over a long distance through a bipolar overhead line.

8. The multi-degree-of-freedom universal carrier modulation method for a current source converter as described in claim 1, characterized in that, The constructed modular cascaded current source converter also includes a modular IGCT type current source inverter station, which expands the voltage level by connecting multiple modules in series, and realizes bipolar DC energy transmission between the two AC systems based on the connection between the transformer and the receiving end main grid.

9. The multi-degree-of-freedom universal carrier modulation method for a current source converter as described in claim 1, characterized in that, The multi-degree-of-freedom universal carrier modulation is achieved by using traditional phase-shift carrier modulation for each cascaded current source module. The frequency components of the primary and secondary carrier frequencies are substituted into the harmonic spectrum of the pulse width sequence, and then the total DC voltage under phase-shift carrier modulation is modeled and analyzed to obtain the total DC voltage of the cascaded module.

10. A multi-degree-of-freedom universal carrier modulation system for a current source converter, characterized in that, include: A building block, configured to build a modular cascaded current source converter; The analysis module is configured to analyze the DC-side voltage harmonic distribution in a modular cascaded current source converter using a carrier-modulated output sinusoidal pulse width sequence. The determination module is configured to determine the optimal phase shift angle of the modular cascaded current source converter based on the distribution of DC-side voltage harmonics, with the optimization objectives being the optimal quality of the sinusoidal pulse width sequence waveform, the number of switching operations, and the DC voltage performance. The modulation module is configured to adjust the three-phase carrier of each current source module in the current source converter based on the determined optimal phase shift angle, thereby completing the multi-degree-of-freedom universal carrier modulation of the current source converter.

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