Bipolar AC-AC frequency converter and control method thereof

Abstract

The invention relates to the technical field of converters, in particular to a bipolar AC-AC frequency converter and a control method thereof.The power frequency side of an AC-AC frequency converter body is connected to a power frequency power grid, the low frequency side of the AC-AC frequency converter body is connected to a low frequency power grid through a three-phase three-winding bipolar transformer, an A-B-C three-phase structure is adopted, and each phase comprises an upper bridge arm and a lower bridge arm; each bridge arm is formed by connecting n full-bridge sub-modules and a bridge arm inductor in series, but the inductance values of the three-phase bridge arm inductors are different. The phase three-winding bipolar transformer comprises a positive pole transformer and a negative pole transformer, the phase difference of corresponding port voltages of the positive pole transformer and the negative pole transformer is 180 degrees, and a symmetrical bipolar structure is formed; and neutral points of the two are interconnected to form a bipolar output loop. The unbalanced capacitor voltage caused by the inductance difference of the three-phase bridge arm is accurately adjusted, so that the working voltage of each module tends to be consistent, and the balanced operation of the system is ensured. Harmonic generation and electric energy fluctuation possibly caused by voltage imbalance are reduced, the electric energy quality is improved, and stable power supply of a power system is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronic converters and flexible power transmission, and in particular to a bipolar AC-AC converter and a control method thereof. Background Art

[0002] As a new flexible AC transmission solution, low-frequency AC transmission technology significantly improves line transmission capacity and reactive power characteristics by reducing transmission frequency. It has become a highly effective solution for offshore wind power grid integration and long-distance power transmission. As the core component of frequency-divided transmission systems, AC-AC converters must enable bidirectional energy conversion between the industrial frequency grid and the low-frequency transmission network. Bipolar AC-AC converters based on modular multilevel converters have become the mainstream choice due to their modular scalability and low harmonic output.

[0003] However, in engineering practice, the asymmetry of the three-phase bridge arm inductance parameters will cause the problem of unbalanced three-phase capacitor voltage. The inductance difference causes the amplitude and phase offset of the fundamental frequency component of the three-phase circulating current, and transfers unbalanced energy through capacitor charging and discharging, resulting in differences in the upper / lower bridge arm capacitor voltage ripple, aggravating capacitor stress and threatening insulation safety; asymmetric circulating current introduces additional resonance points in the low-frequency band, which may excite broadband oscillations and reduce fault ride-through capability; traditional decoupling control relies on the assumption of bridge arm symmetry. Parameter differences lead to the accumulation of model errors in the dq coordinate system, and the dynamic response delay of the PI controller makes it difficult to achieve voltage balance and precise power distribution.

[0004] Referring to the technical solution disclosed in patent number CN116191896B, while it mentions that the bridge arm of a modular multilevel inverter includes inductors and full-bridge submodules, it does not consider the differences in the inductance parameters of the three-phase bridge arms during actual manufacturing or operation. This asymmetry can lead to uneven capacitor voltages in each phase, which in turn causes submodule capacitor voltage drift, affecting system stability and output power quality. Secondly, while it mentions voltage control through a modulation wave formula, it does not address how to deal with voltage imbalance caused by inductance differences, and does not clearly design a harmonic suppression strategy. Therefore, there are deficiencies in addressing actual three-phase inductance asymmetry, dynamic voltage balance control, harmonic suppression, and system stability. Summary of the Invention

[0005] The present invention provides a bipolar cycloconverter and a control method thereof, which can effectively solve the problems in the background technology.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A control method for a bipolar AC-AC converter comprises the following steps:

[0008] Real-time acquisition of three-phase capacitor voltage u ca 、u cb 、ucc , and transform it to the αβ coordinate system through Clark transformation to obtain the voltage component u cα and u cβ ;

[0009] A PI controller is designed for the αβ components to generate a compensation reference voltage:

[0010]

[0011] will u α_ref and u β_ref Generate zero-sequence reference voltage u by polar coordinate transformation 0_ref , its amplitude U0 and phase θ0 satisfy:

[0012]

[0013] The modulation waves of the three-phase upper and lower bridge arms are adjusted according to the zero-sequence reference voltage, and the switching of the full-bridge sub-modules is controlled based on the modulation waves to achieve dynamic balance of the three-phase capacitor voltage.

[0014] Furthermore, synthesizing the three-phase upper and lower bridge arm modulation wave signals includes the following steps:

[0015] Generate j phase low frequency modulation wave component y respectively j_l and the power frequency modulation wave component y j_g :

[0016]

[0017] Superimposed zero-sequence reference voltage u 0_ref , get the upper arm modulation wave y ju and the lower bridge arm modulation wave y jl :

[0018]

[0019] Among them, m g is the power frequency modulation index, m l is the low frequency modulation index, ω g is the power frequency angular frequency, ω l is the low-frequency angular frequency, θ jg is the power frequency phase shift angle of phase j, θ jl is the low-frequency phase shift angle of phase j.

[0020] Furthermore, the proportional coefficient K of the PI controller p and the integral coefficient K i Determined by optimizing system impedance and dynamic response requirements.

[0021] Furthermore, the zero-sequence reference voltage u 0_ref Set to power frequency differential mode or low frequency common mode.

[0022] A bipolar AC-AC converter, applied to the above control method, comprises:

[0023] The main body of the AC-AC inverter adopts an ABC three-phase structure. Each phase bridge arm consists of an upper and a lower bridge arm. Each bridge arm contains n full-bridge sub-modules and bridge arm inductors.

[0024] A three-phase three-winding bipolar transformer, wherein the secondary port of the positive transformer T1 is connected to the upper bridge arm of each phase, the secondary port of the negative transformer T2 is connected to the lower bridge arm of each phase, and the neutral points of T1 and T2 are interconnected;

[0025] The inductance values ​​of the bridge arm inductors are allowed to be asymmetrically distributed among the three phases.

[0026] Furthermore, the transformer port voltage relationship between the positive transformer T1 and the negative transformer T2 in the three-phase three-winding bipolar transformer is set to:

[0027] The voltage phases of the X and x ports are 180° apart, the voltage phases of the Y and y ports are 180° apart, and the voltage phases of the Z and z ports are 180° apart.

[0028] Furthermore, the full-bridge submodule includes:

[0029] The emitters and collectors of the first to fourth IGBTs are cross-connected to form a bridge arm path;

[0030] an electrolytic capacitor, with its positive electrode connected to the collectors of the first and third IGBTs, and its negative electrode connected to the emitters of the second and fourth IGBTs;

[0031] Each IGBT is connected in parallel with an anti-parallel diode.

[0032] The beneficial effects of the present invention are:

[0033] The optimized control strategy for the three-phase inductance asymmetry of the bipolar AC-AC converter based on power frequency zero-sequence voltage compensation proposed in the present invention can accurately adjust the capacitor voltage imbalance caused by the difference in the inductance of the three-phase bridge arms. Compared with the problem of uneven capacitor voltages in each phase caused by the difference in the inductance parameters of the three-phase bridge arms in the patent CN116191896B scheme, the scheme of the present invention makes the operating voltage of each module tend to be consistent, ensuring balanced operation of the system.

[0034] In the existing technical solutions, voltage control is achieved only through the modulation wave formula. The solution of the present invention reduces the harmonic generation and power fluctuations that may be caused by voltage imbalance by balancing the capacitor voltage. However, the existing technical solutions do not clearly design a harmonic suppression strategy and rely on topological symmetry, which has weak adaptability to actual asymmetric working conditions. Therefore, the solution of the present invention improves the power quality by balancing the voltage and reducing harmonics, so that the output power is more in line with the standards of the power system and the requirements of the load, ensuring the stable power supply of the power system, and has high industrial value. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0036] Figure 1 Schematic diagram of the topological structure of the bipolar AC-AC converter in the present invention. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0038] It should be noted that when an element is referred to as being "fixed to" another element, it may be directly attached to the other element or there may be an intermediate element. When an element is referred to as being "connected to" another element, it may be directly connected to the other element or there may be an intermediate element. The terms "vertical," "horizontal," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only implementation methods.

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this invention pertains. The terms used in this specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0040] The present invention discloses a control method for a bipolar AC-AC converter. Figure 1The bipolar AC-AC inverter topology diagram shown in the figure includes an AC-AC inverter body and a three-phase three-winding bipolar transformer. The AC-AC inverter body adopts an ABC three-phase structure, where each phase includes an upper bridge arm and a lower bridge arm, and each bridge arm is composed of n full-bridge sub-modules connected in series with a bridge arm inductor. The three-phase three-winding bipolar transformer includes a positive transformer T1 and a negative transformer T2. The phase difference between the corresponding port voltages of the positive transformer T1 and the negative transformer T2 is 180 degrees, forming a symmetrical bipolar structure. The neutral points of the two are interconnected. The power frequency side of the AC-AC inverter body is connected to the power frequency grid, and the low frequency side is connected to the low frequency grid via the three-phase three-winding bipolar transformer.

[0041] Furthermore, the three-phase ports X, Y, and Z of the secondary star winding of the positive transformer T1 are respectively connected to the upper ends of the upper bridge arms of the A, B, and C phases of the AC-AC converter body; the three-phase ports x, y, and z of the secondary star winding of the negative transformer T2 are respectively connected to the lower ends of the lower bridge arms of the A, B, and C phases of the AC-AC converter body; the neutral point o of the positive transformer T1 and the neutral point o' of the negative transformer T2 are interconnected to form a bipolar output circuit.

[0042] In a three-phase, three-winding bipolar transformer, the voltage relationship between the positive transformer T1 and the negative transformer T2 is set to: 180° phase difference between the X and x ports, 180° phase difference between the Y and y ports, and 180° phase difference between the Z and z ports. Bridge inductors are connected in series with each bridge arm to suppress circulating current and balance power distribution. Differences in the three-phase inductor parameters are a key factor in capacitor voltage imbalance.

[0043] The full-bridge submodule consists of the first, second, third, and fourth IGBTs connected to anti-parallel diodes and an electrolytic capacitor. The emitter of the first IGBT is connected to the collector of the second IGBT, serving as the positive terminal of the submodule, while the emitter of the third IGBT is connected to the collector of the fourth IGBT, serving as the negative terminal. The positive terminal of the electrolytic capacitor is connected to the collectors of the first and third IGBTs, while the negative terminal is connected to the emitters of the second and fourth IGBTs. The bridge arm voltage is adjusted by switching the submodules, enabling bidirectional power flow.

[0044] The present invention further discloses a control method for a bipolar AC-AC inverter. Considering that the asymmetric inductance of the three-phase bridge arm leads to capacitor voltage imbalance, which affects system stability, a zero-sequence voltage is injected to compensate for the inductance difference, thereby achieving dynamic balance of the three-phase capacitor voltage. The control method includes the following steps:

[0045] Real-time acquisition of three-phase capacitor voltage u ca 、u cb 、u cc , and transform it to the αβ coordinate system through Clark transformation to obtain the voltage component u cα and u cβ; Convert the three-phase capacitor voltage into an orthogonal αβ coordinate system to eliminate the coupling effect caused by three-phase asymmetry.

[0046]

[0047] A PI controller is designed for the αβ components to generate a compensation reference voltage. The αβ components are forced to zero through PI control, thereby indirectly eliminating the three-phase voltage difference:

[0048]

[0049] The PI controller design needs to adjust K according to the system dynamic response requirements. p (proportional gain) and K i (integral gain) to ensure fast convergence and stability.

[0050] will u α_ref and u β_ref Generate zero-sequence reference voltage u by polar coordinate transformation 0_ref ,

[0051] u 0_ref =U0cos(ω g t+θ);

[0052] Its amplitude U0 and phase θ0 satisfy:

[0053]

[0054] The modulation waves of the three-phase upper and lower bridge arms are adjusted according to the zero-sequence reference voltage, and the switching of the full-bridge sub-modules is controlled based on the modulation waves to achieve dynamic balance of the three-phase capacitor voltage.

[0055] The synthesis of three-phase upper and lower bridge arm modulation wave signals includes the following steps:

[0056] Generate j phase low frequency modulation wave component y respectively j_l and the power frequency modulation wave component y j_g :

[0057]

[0058] Superimposed zero-sequence reference voltage u 0_ref , get the upper arm modulation wave y ju and the lower bridge arm modulation wave y jl :

[0059]

[0060] Among them, m g is the power frequency modulation index, m l is the low frequency modulation index, ω g is the power frequency angular frequency, ω lis the low-frequency angular frequency, θ jg is the power frequency phase shift angle of phase j, θ jl is the low-frequency phase shift angle of phase j.

[0061] Power frequency differential mode compensation:

[0062]

[0063] Low frequency common mode compensation:

[0064]

[0065] Phase j upper arm voltage u au and the lower bridge arm voltage u al The expressions are:

[0066]

[0067] Among them, n is the number of submodules, u c is the average value of the submodule capacitor voltage.

[0068] Proportional coefficient K of PI controller p and the integral coefficient K i Determined by optimizing system impedance and dynamic response requirements.

[0069] Zero-sequence reference voltage u 0_ref Set to power frequency differential mode or low-frequency common mode, applicable to scenarios with power quality sensitivity on the power frequency side and harmonic suppression requirements on the low-frequency side, respectively. This offsets the voltage offset caused by power frequency circulating current or suppresses the voltage fluctuation caused by low-frequency harmonics. The differential mode voltage only affects the voltage difference between the upper and lower bridge arms, while the common mode voltage acts on both the upper and lower bridge arms.

[0070] In the differential-mode injection scheme, the power-frequency zero-sequence voltage acts in reverse on the upper and lower bridge arms, forming differential-mode compensation to offset the voltage offset caused by inductor asymmetry. In the common-mode injection scheme, the low-frequency zero-sequence voltage is superimposed in the same direction on the three-phase bridge arms to adjust the overall capacitor voltage reference.

[0071] The optimized control strategy proposed in the present invention for a bipolar AC-AC inverter with three-phase inductance asymmetry based on power frequency zero-sequence voltage compensation can accurately adjust the capacitor voltage imbalance caused by the inductance difference of the three-phase bridge arms, so that the operating voltage of each module tends to be consistent, ensuring balanced operation of the system.

[0072] By balancing the capacitor voltage, the generation of harmonics and power fluctuations that may be caused by voltage imbalance is reduced, thereby improving the quality of power, making the output power more in line with the standards of the power system and the requirements of the load, ensuring the stable power supply of the power system, and having high industrial value. It should be understood by those skilled in the art that the present invention is not limited to the above-mentioned embodiments. The above-mentioned embodiments and descriptions are only for illustrating the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements all fall within the scope of the invention to be protected. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A control method for a bipolar AC-AC converter, characterized in that: The following steps are involved: Real-time acquisition of three-phase capacitor voltage u ca 、u cb 、u cc , and transform it to the αβ coordinate system through Clark transformation to obtain the voltage component u cα and u cβ , respectively set the proportional coefficient K p and the integral coefficient K i ; A PI controller is designed for the αβ components to generate a compensation reference voltage: will u α_ref and u β_ref Generate zero-sequence reference voltage u by polar coordinate transformation 0_ref , its amplitude U0 and phase θ0 satisfy: The modulation waves of the three-phase upper and lower bridge arms are adjusted according to the zero-sequence reference voltage, and the switching of the full-bridge sub-modules is controlled based on the modulation waves to achieve dynamic balance of the three-phase capacitor voltage.

2. The control method of the bipolar cycloconverter according to claim 1, characterized in that: The synthesis of three-phase upper and lower bridge arm modulation wave signals includes the following steps: Generate j phase low frequency modulation wave component y respectively j_l and the power frequency modulation wave component y j_g : Superimposed zero-sequence reference voltage u 0_ref , get the upper arm modulation wave y ju and the lower bridge arm modulation wave y jl : Among them, m g is the power frequency modulation index, m l is the low frequency modulation index, ω g is the power frequency angular frequency, ω l is the low-frequency angular frequency, θ jg is the power frequency phase shift angle of phase j, θ jl is the low-frequency phase shift angle of phase j.

3. The control method of the bipolar cycloconverter according to claim 2, characterized in that: Proportional coefficient K of PI controller p and the integral coefficient K i Determined by optimizing system impedance and dynamic response requirements.

4. The control method of a bipolar cycloconverter according to claim 1, wherein: The zero-sequence reference voltage u 0_ref Set to power frequency differential mode or low frequency common mode.

5. A bipolar AC-AC converter, characterized in that: The control method for implementing any one of claims 1 to 4 above comprises: The main body of the AC-AC inverter adopts an ABC three-phase structure. Each phase bridge arm consists of an upper and a lower bridge arm. Each bridge arm contains n full-bridge sub-modules and bridge arm inductors. A three-phase three-winding bipolar transformer, wherein the secondary port of the positive transformer T1 is connected to the upper bridge arm of each phase, the secondary port of the negative transformer T2 is connected to the lower bridge arm of each phase, and the neutral points of T1 and T2 are interconnected; The inductance values ​​of the bridge arm inductors are allowed to be asymmetrically distributed among the three phases.

6. The bipolar cycloconverter according to claim 5, characterized in that: The full-bridge submodule includes: The emitters and collectors of the first to fourth IGBTs are cross-connected to form a bridge arm path; Electrolytic capacitor, the positive electrode is connected to the collector of the first and third IGBTs, and the negative electrode is connected to the second and the emitter of the fourth IGBT; Each IGBT is connected in parallel with an anti-parallel diode.

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