Network side converter control method and device suitable for step wave collection system

Abstract

The invention discloses a network side converter control method and device suitable for a step wave collection system, and belongs to the technical field of power generation, power transformation or power distribution. According to the method and the device, the system voltage reference can be accurately obtained without communication by detecting the three-phase characteristic level of the alternating current side; the commutation phase-shifting control generates a commutation phase-shifting angle based on a current sampling result at the moment of voltage commutation, so that the current at the commutation stage is controllable and advanced; according to the double closed-loop control, step wave current is mapped to a direct current reference, control simplification is achieved, and the direct current bus voltage is kept constant; step waves are modulated and converted into two-level square waves through third harmonic injection, and the modulation utilization rate is effectively improved. The method can realize steady-state and commutation control without depending on system parameters such as line impedance and power level, is suitable for complex scenes such as multi-machine parallel connection and inconsistent parameters, and can effectively improve the grid-connected stability of the grid-side converter in the step wave collection system.

Description

Technical Field

[0001] This invention relates to power systems, specifically disclosing a grid-side converter control method and device suitable for stepped wave collection systems, belonging to the technical field of power generation, transformation, or distribution. Background Technology

[0002] my country has abundant reserves of new energy sources such as wind power and photovoltaics. Under the background of large-scale grid connection of new energy, traditional aggregation technology relying on AC sine waves has gradually exposed problems such as large switching losses, high system costs, and low transmission efficiency. In order to improve system efficiency and economy, in recent years, an AC aggregation method based on stepped waves has been proposed. By reducing the switching frequency of the converter, the DC voltage utilization rate is improved, and the overall efficiency and cost are significantly optimized.

[0003] However, existing research focuses primarily on the overall architecture and operational mechanism of stepped-wave collection systems, while neglecting the control methods for grid-side converters under stepped-wave voltage and current conditions. Grid-side converters in collection systems must perform functions such as grid-connected current regulation and power control. Existing solutions generally rely on the following approach: the grid-side converter needs to sample the standard four-level stepped-wave voltage at the AC port of the remote sending-end converter in real time, and transmit the sampled frequency and phase information back to the grid-side converter for phase-locking via GPS, fiber optic synchronization, or other means. Simultaneously, the commutation control of the grid-side converter requires numerical calculations based on precise system parameters such as line impedance and transmission power. In complex scenarios such as multi-unit parallel operation, inconsistent branch parameters, or practical engineering, these conditions are often difficult to meet, leading to problems such as unstable phase-locking, large commutation control deviations, and severe grid-connected current distortion, making it difficult to achieve the required grid-connected performance.

[0004] In summary, the present invention aims to propose a grid-side converter control method suitable for stepped wave collection systems to overcome the above-mentioned defects. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of the aforementioned background technology by providing a grid-side converter control method and device suitable for stepped wave collection systems. This solves the technical problems in the prior art, such as the difficulty of phase-locking under stepped wave conditions, strong dependence on system parameters, and complex control of multiple units in parallel. The invention aims to achieve stable phase-locking, high-quality current control, and reliable grid-connected operation of grid-side converters in stepped wave collection systems.

[0006] To achieve the above-mentioned objectives, the present invention employs the following technical solution:

[0007] A grid-side converter control method applicable to stepped wave collection systems.

[0008] Steady-state stage: Based on the per-unit phase-locked loop result of the current steady-state stage system stepped voltage reference, the output current of the AC port of the grid-side converter is preprocessed. A voltage and current dual closed-loop control strategy is used to control the preprocessed result with constant current to generate a modulation wave signal under the DC reference. Then, based on the per-unit phase-locked loop result of the current steady-state stage system stepped voltage reference, the modulation wave signal under the DC reference is restored to the modulation wave signal under the stepped voltage reference. When obtaining the per-unit phase-locked loop result of the next steady-state stage system stepped voltage reference, the output current of the AC port of the grid-side converter is controlled to switch to the corresponding amplitude of the next steady-state stage.

[0009] Commutation stage: At the beginning of the next steady-state stage, the output current of the AC port of the grid-side converter is sampled. Using the current amplitude reference of the inner current loop as a reference, the output current of the AC port of the grid-side converter at the beginning of the next steady-state stage under the DC reference is normalized. Based on the normalization result, the required advance phase shift angle for the commutation stage is obtained, and the per-unit phase-locked loop result of the system stepped wave voltage reference for the next steady-state stage is advanced phase-locked.

[0010] As a further optimization of the grid-side converter control method applicable to stepped wave collection systems, in the steady-state phase: the per-unit phase-locked loop result of the system stepped wave voltage reference in the current steady-state phase is obtained by the following method: obtaining the commutation times of the system stepped wave voltage reference by detecting the characteristic level of the AC port voltage of the grid-side converter, and then reconstructing the system stepped wave voltage reference based on the commutation times of the system stepped wave voltage reference; wherein, the commutation times of the A-phase stepped wave voltage reference are obtained by the following criterion: detecting the A-phase voltage u at the AC port of the grid-side converter. ao1 satisfy At that moment, the system's A-phase stepped wave voltage reference completed its commutation from 1 / 2 to 1, U in U is the DC bus capacitor voltage of the grid-side converter. d This refers to the DC bus capacitor voltage of the sending-end converter.

[0011] The voltage u of phase A at the AC port of the grid-side converter was detected. ao1 satisfy At that moment, the system's A-phase stepped wave voltage reference completed its commutation from -1 / 2 to -1.

[0012] The voltage u of phase B at the AC port of the grid-side converter was detected. bo1 satisfy At that moment, the system's A-phase stepped wave voltage reference completed its commutation from 1 / 2 to -1 / 2.

[0013] The voltage u of phase B at the AC port of the grid-side converter was detected. bo1 satisfy At that moment, the system's A-phase stepped wave voltage reference completed its commutation from -1 / 2 to 1 / 2.

[0014] The voltage u of phase C at the AC port of the grid-side converter was detected. co1 satisfy At that moment, the system's A-phase stepped wave voltage reference completed its commutation from -1 to -1 / 2.

[0015] The voltage u of phase C at the AC port of the grid-side converter was detected. co1 satisfy At that moment, the system's A-phase stepped wave voltage reference completes the commutation from 1 to 1 / 2.

[0016] As a further optimization of the grid-side converter control method applicable to stepped wave collection systems, in the steady-state stage: the AC port output current of the grid-side converter is preprocessed based on the per-unit phase-locked loop result of the stepped wave voltage reference of the system in the current steady-state stage. Specifically, the sampling result of the AC port output current of the grid-side converter in the current steady-state stage is divided by the per-unit phase-locked loop result of the stepped wave voltage reference of the system in the current steady-state stage.

[0017] As a further optimization of the grid-side converter control method applicable to stepped wave collection systems, in the steady-state stage: a voltage and current dual closed-loop control strategy is adopted to perform constant current control on the preprocessing results to generate a modulated wave signal under the DC reference. Specifically, the voltage outer loop performs PI adjustment on the difference between the DC bus capacitor voltage of the grid-side converter and its rated reference value to generate a current amplitude reference for the current inner loop. The difference between the preprocessing results and the current amplitude reference is then PI adjusted through the current inner loop to obtain the modulated wave signal under the DC reference.

[0018] As a further optimization scheme for the grid-side converter control method applicable to stepped wave collection systems, in the steady-state stage: the modulation wave signal under the DC reference is restored to the modulation wave signal under the stepped wave reference based on the per-unit phase-locked loop result of the stepped wave voltage reference of the system in the current steady-state stage. Specifically, the modulation wave signal under the DC reference is multiplied by the per-unit phase-locked loop result of the stepped wave voltage reference of the system in the current steady-state stage, or the modulation wave signal under the DC reference is multiplied by two-level signals that are in phase with the per-unit phase-locked loop result of the stepped wave voltage reference of the system in the current steady-state stage.

[0019] As a further optimization of the grid-side converter control method applicable to stepped wave collection systems, a two-level signal in phase with the per-unit phase-locked loop result of the stepped wave voltage reference of the current steady-state system is obtained by injecting a third harmonic into the per-unit phase-locked loop result of the stepped wave voltage reference of the current steady-state system; or, by detecting the moment when the stepped wave voltage reference of the system commutates from -1 / 2 level to 1 / 2 level and from 1 / 2 level to -1 / 2 level, and then obtaining the two-level signal from the commutation moment of the directly obtained two-level square wave signal.

[0020] As a further optimization of the grid-side converter control method applicable to stepped wave collection systems, during the commutation stage: at the beginning of the next steady-state stage, the output current of the grid-side converter's AC port is sampled. Using the current amplitude reference of the inner current loop as a benchmark, the output current of the grid-side converter's AC port at the beginning of the next steady-state stage under the DC benchmark is normalized. Specifically, the sampled output current of the grid-side converter's AC port at the beginning of the next steady-state stage is divided by the per-unit phase-locked loop result of the stepped wave voltage benchmark of the system in the next steady-state stage to obtain the output current of the grid-side converter's AC port at the beginning of the next steady-state stage under the DC benchmark. The output current of the grid-side converter's AC port at the beginning of the next steady-state stage under the DC benchmark is then divided by the current amplitude reference to obtain the normalized result.

[0021] As a further optimization scheme for the grid-side converter control method applicable to stepped wave collection systems, in the commutation stage: the required advance phase shift angle for the commutation stage is obtained based on the normalization processing result, and the per-unit phase-locked loop result of the stepped wave voltage reference of the system in the next steady-state stage is advanced phase-shifted. The specific method is as follows: the deviation error between the normalization processing result and unit 1 is obtained, the deviation error is subjected to PI control with the commutation time interval of the stepped wave voltage reference of the system as the discrete period, the required advance phase shift angle for the commutation stage is obtained, and the per-unit phase-locked loop result of the stepped wave voltage reference of the system in the next steady-state stage is advanced phase-shifted based on the advance phase shift angle.

[0022] As a further optimization scheme for the grid-side converter control method applicable to stepped wave collection systems, the deviation error between the normalized processing result and unit 1 is obtained, and the specific method is as follows:

[0023] The AC port output current of the grid-side converter is from I d1 / 2 Switch to I d1 And from -I d1 / 2 Switch to -I d1 When, the deviation error is ,

[0024] The AC port output current of the grid-side converter is from -I d1 Switch to -I d1 / 2 and from I d1 Switch to I d1 When / 2, the deviation error is ,

[0025] in, Let x be the deviation error, and I be the normalized result. d1 This refers to the output current amplitude at the AC port of the grid-side converter.

[0026] A grid-side converter control device suitable for a stepped-wave collection system is provided to implement the aforementioned control method. The control device includes: a stepped-wave voltage phase-locked loop (PLL) module, a commutation phase-shifting control module, and a voltage-current dual closed-loop control module. The stepped-wave voltage PLL module is used to obtain the per-unit PLL result of the stepped-wave voltage reference of the system in the current steady-state stage. The commutation phase-shifting control module is used to control the AC port output current of the grid-side converter to switch to the amplitude corresponding to the next steady-state stage. At the beginning of the next steady-state stage, the AC port output current of the grid-side converter is sampled. Using the current amplitude reference of the inner current loop as a reference, the AC port output current of the grid-side converter at the beginning of the next steady-state stage under the DC reference is normalized. Based on the normalization result, the commutation phase is obtained. A phase shift angle is required to perform a phase shift ahead of the per-unit phase-locked loop (PLL) result of the system stepped voltage reference for the next steady-state stage. The voltage and current dual closed-loop control module is used to preprocess the AC port output current of the grid-side converter based on the per-unit PLL result of the system stepped voltage reference for the current steady-state stage. The voltage and current dual closed-loop control strategy is used to perform constant current control on the preprocessed result to generate a modulation wave signal under the DC reference. Then, based on the per-unit PLL result of the system stepped voltage reference for the current steady-state stage, the modulation wave signal under the DC reference is restored to the modulation wave signal under the stepped voltage reference. When the per-unit PLL result of the system stepped voltage reference for the next steady-state stage is obtained, the output current of the AC port of the grid-side converter is controlled to switch to the corresponding amplitude of the next steady-state stage.

[0027] The present invention, by adopting the above technical solution, has the following beneficial effects:

[0028] (1) The grid-side converter control method proposed in this invention is suitable for stepped wave collection system. It breaks through the technical limitation that the traditional sine wave-based control strategy is difficult to apply directly in stepped wave scenarios, enabling the grid-side converter to generate stepped wave current in the same phase based on the system stepped wave voltage reference, thereby achieving stable and efficient power transmission.

[0029] (2) The grid-side converter control method proposed in this invention is applicable to the stepped wave collection system. Its stepped wave voltage phase-locked loop has significant advantages. By detecting the characteristic level of the three-phase voltage at the AC port of the grid-side converter, real-time and localized phase-locking of the stepped wave voltage reference of the system can be achieved, completely eliminating the dependence of existing schemes on communication methods such as GPS or optical fiber due to remote voltage sampling. This phase-locked loop method is not sensitive to line impedance, load changes and multi-machine parallel conditions, and can still achieve accurate phase-locking under complex operating conditions.

[0030] (3) The control strategy proposed in this invention is based entirely on local sampling and closed-loop regulation. It can achieve steady-state control and dynamic commutation without relying on any system parameters such as line impedance and power level. This avoids the dependence of traditional schemes on the accuracy of system modeling and makes the control strategy universal in complex scenarios such as multi-machine parallel connection and inconsistent parameters. In addition, the control method proposed in this invention adopts a preprocessing strategy to uniformly map the different current amplitudes of different steady-state stages of the stepped wave current to the same DC reference, simplifying the stepped wave current control into equivalent DC control and significantly reducing the control complexity. Finally, when generating the modulation wave signal, this invention converts the original four-level stepped wave modulation method into a two-level square wave, which significantly reduces the peak value of the modulation wave without changing the output of the grid-side converter and effectively alleviates the risk of overmodulation. Attached Figure Description

[0031] Figure 1 This is a block diagram of a stepped wave gathering system provided in one embodiment of the present invention.

[0032] Figure 2 This is a block diagram of a grid-side converter control method for a stepped wave collection system comprising two grid-side converters connected in parallel, provided in one embodiment of the present invention.

[0033] Figure 3 The waveforms are typical AC-side output current waveforms and corresponding system stepped wave voltage reference waveforms after the grid-side converter in the stepped wave collection system applies the control method proposed in this invention.

[0034] Figure 4 This invention relates to a grid-side converter control method applicable to a stepped wave collection system. When the modulation wave signal under the stepped wave reference is restored, the third harmonic injection module converts the four-level stepped wave into a two-level square wave. Detailed Implementation

[0035] 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, and 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] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0037] like Figure 1 As shown, the stepped-wave collection system comprises multiple grid-side converters connected in parallel, with its DC-side renewable energy source equivalent to a constant power source. Each grid-side converter is connected to the low-voltage side of the collection transformer via a box-type transformer, a step-up transformer, and a transmission line. The high-voltage side of the collection transformer is connected to the sending-end converter, which employs an existing power frequency constant duty cycle control strategy to generate a standard three-phase four-level stepped-wave voltage on its AC side, serving as the voltage reference for the collection system. The grid-side converter generates a stepped-wave current in phase on its AC side based on this voltage reference, transmitting power. Specifically, it can be divided into two stages: the steady-state stage and the commutation stage. The steady-state stage refers to the process where the current remains constant DC, while the commutation stage refers to the process where the current switches from the current steady-state stage to the next steady-state stage. The duration of the commutation stage is much shorter than the duration of the steady-state stage.

[0038] In an embodiment of the present invention, Figure 2 The diagram shows a block diagram of a grid-side converter control method applicable to a stepped wave converging system according to the present invention, and a simplified equivalent circuit of the stepped wave converging system. All parameters in the simplified circuit have been equivalently applied to the grid-side converter side by considering the transformer turns ratio. Taking a converging system containing two grid-side converters connected in parallel as an example, where C... in1 C in2 For the DC bus capacitors of grid-side converter 1 and grid-side converter 2; L f1 L f2 For the AC side filter inductor of the grid-side converter; L s1 L s2 These correspond to the sum of the leakage inductance of the box-type transformer, the equivalent line inductance, and the leakage inductance of the step-up transformer in the branches where grid-side converter 1 and grid-side converter 2 are located, respectively; R s1 R s2 These correspond to the equivalent resistances of the branches where grid-side converter 1 and grid-side converter 2 are located, respectively; L cTo collect the leakage inductance of the transformer; C d This refers to the DC bus capacitor of the sending-end converter. U in1 U in2 i represents the DC bus capacitor voltage of grid-side converter 1 and grid-side converter 2; a1 i a2 The output currents of phase A at the AC ports of grid-side converter 1 and grid-side converter 2 are I, with amplitudes of I and I, respectively. d1 I d2 i b1 i b2 For the B-phase output current of the AC port of grid-side converter 1 and grid-side converter 2; i c1 i c2 For the C-phase output current of the AC port of grid-side converter 1 and grid-side converter 2; u ao1 u bo1 u co1 U represents the three-phase voltage at the AC port of grid-side converter 1. d This is the DC bus capacitor voltage of the sending-end converter.

[0039] In this embodiment of the invention, each grid-side converter in the stepped wave collection system adopts the same control method, and it is necessary to sample its DC bus capacitor voltage, AC output port three-phase voltage, and three-phase current. Figure 2 The control method block diagram of grid-side converter 1 in the system is given in detail. It is implemented by a control device including a stepped-wave voltage phase-locked loop module, a commutation phase-shifting control module, a third harmonic injection module, and a voltage and current dual closed-loop control module. The grid-side converter can adopt a traditional three-phase two-level converter topology.

[0040] The stepped-wave voltage phase-locked loop (PLL) module detects the characteristic levels of the three-phase voltage at the AC port of the grid-side converter to obtain the per-unit PLL result of the stepped-wave voltage reference for the current steady-state stage within the system, thereby achieving precise phase-locking. The commutation phase-shift control module samples the three-phase current at the AC port of the grid-side converter at the end of commutation, i.e., the beginning of the next steady-state stage, to obtain the required phase shift angle for commutation and applies it to the per-unit PLL result of the stepped-wave voltage reference for the next steady-state stage. During the commutation stage, this ensures that the output current phase of the grid-side converter has a controllable lead relative to the system stepped-wave voltage reference, achieving stable commutation. The voltage and current dual closed-loop control module consists of an outer voltage loop and an inner current loop. The outer voltage loop generates a current reference value by sampling the DC bus capacitor voltage of the grid-side converter. The inner current loop adjusts the output current based on the per-unit PLL result of the system stepped-wave voltage reference for the current steady-state stage, ensuring that the output current at the AC port of the grid-side converter remains in phase with the stepped-wave voltage reference within the system and exhibits a four-level stepped waveform with the same frequency during the steady-state stage. The third harmonic injection module injects the third harmonic into the modulation wave signal generated by the voltage and current dual closed-loop control module, which effectively converts the original four-level stepped wave modulation mode into a two-level square wave without affecting the output current of the grid-side converter, thereby reducing the peak value of the modulation wave and improving the modulation utilization rate.

[0041] In this embodiment of the invention, taking phase A as an example, the phase-locked loop result that is in phase with the system phase A stepped wave voltage reference is as follows: Figure 3 middle As shown. Based on the control method proposed in this invention, the AC A-phase output current of grid-side converter 1 and grid-side converter 2 is as follows: Figure 3 in i a1 i a2 As shown. This current exhibits a clear commutation process, and the duration of the commutation phase is much shorter than the duration of the steady-state phase, as... Figure 3 The gray area is shown. Therefore, within the power frequency cycle, it can be divided into a steady-state stage and a commutation stage. During the steady-state stage, the current i... a1 i a2 Maintaining a constant DC current, the current i during the commutation phase a1 i a2 Switching from the current steady-state stage to the next steady-state stage.

[0042] In this embodiment of the invention, the constant current control during the steady-state phase is mainly implemented by the voltage and current dual closed-loop control module in the proposed control method. First, the stepped current is preprocessed, i.e., the sampling result i of the AC port current of the grid-side converter during the current steady-state phase is obtained. a1 i b1 i c1 Per-unit phase-locked loop result of the current steady-state system stepped voltage reference output by the phase-shifted phase-shifting control module. , , Divide them. Furthermore, the stepped wave current amplitudes at different steady-state stages can be converted to the same DC reference, i.e., in grid-side converter 1, in I... d1 The corresponding steady-state stage is converted to I by dividing by 1. d1 I d1 The steady-state phase corresponding to / 2 is also converted to I when divided by 1 / 2. d1 -I d1 The steady-state phase corresponding to / 2 is also converted to I when divided by -1 / 2. d1 -I d1 The corresponding steady-state stage, when divided by -1, is also converted to I. d1 Similarly, for grid-side converter 2, different steady-state stages can all be converted to I. d2 Therefore, after preprocessing, the control of the stepped wave current can be transformed into the control of the DC current. Throughout the entire power frequency cycle, it is only necessary to control the preprocessed current to be a constant DC current I. d1 with I d2 This ensures that during the steady-state phase, the grid-side converter can output a stepped wave current in phase with the system stepped wave voltage reference, thus transmitting power.

[0043] In an embodiment of the present invention, taking grid-side converter 1 as an example, the required current amplitude after preprocessing of the stepped wave current is referenced to I. m_ref The voltage is provided by the outer loop of the voltage-current dual closed-loop control module. The outer loop will sample the DC bus capacitor voltage U of the grid-side converter. in1 Its rated reference value U in_ref By taking the difference, and then controlling it via PI, the required amplitude reference I for the inner current loop can be obtained while maintaining a constant DC bus capacitor voltage of the grid-side converter. m_ref .

[0044] In an embodiment of the present invention, taking grid-side converter 1 as an example, the inner current loop in the voltage and current dual closed-loop control module references the pre-processed DC current and the current amplitude generated by the outer voltage loop to I. m_ref By performing a differential calculation and controlling it via a PI controller, the DC reference I can be obtained. d1 The modulation wave signals required for each phase in the current steady state are then determined. Subsequently, the DC reference I is... d1The modulation wave signals required for each phase under the current steady-state stage are multiplied by the per-unit phase-locked loop result of the current steady-state system stepped wave voltage reference output by the phase-shifting control module. This allows the reconstruction of the modulation wave signals for each phase under the stepped wave reference in the current steady state. The modulation wave signals for each phase under the stepped wave reference are then compared with a high-frequency triangular carrier wave. When the modulation wave signal of the next phase under the stepped wave reference is greater than that of the triangular carrier wave, the upper arm of the grid-side converter for that phase is turned on and the lower arm is turned off. Conversely, when the modulation wave signal of the next phase under the stepped wave reference is less than that of the triangular carrier wave, the upper arm of the grid-side converter for that phase is turned off and the lower arm is turned on. This process ultimately yields the required switching signals for each phase.

[0045] In embodiments of the present invention, when the modulation wave signals of each phase under the stepped wave reference are restored, they can be directly phase-locked with the per-unit phase-locked result of the current steady-state system stepped wave voltage reference output after phase shifting by the commutation phase-shifting control module. , , Multiplication can also be performed with the per-unit phase-locked loop result of the current steady-state system stepped voltage reference after phase shifting. , , Two-level signals with the same phase are multiplied to improve modulation efficiency. These two-level signals are generated by a third harmonic injection module, the specific principle of which is as follows: Figure 4 As shown.

[0046] Per-unit phase-locked loop results obtained from the stepped-wave voltage phase-locked loop module A four-level stepped wave signal can be represented as:

[0047] (1)

[0048] in, t is the fundamental angular frequency, j is a positive integer, and t is a time variable.

[0049] The per-unit phase-locked loop results generated by the third harmonic injection module Two-level signals in phase can be represented as:

[0050] (2)

[0051] Compared to Equation (1), Equation (2) retains the amplitude and phase information of all frequency components in Equation (1), only introducing a series of harmonics of the third order and multiples of 3. In a three-phase system, the harmonics of the third order and multiples of 3 in the modulating wave signal have no effect on the converter output, but the amplitude of the modulating wave signal is effectively reduced from 1 corresponding to the per-unit phase-locked loop result to 0.75 corresponding to the two-level signal, thus effectively increasing the modulation utilization rate.

[0052] In an embodiment of the present invention, when the per-unit phase-locked loop result is converted into a two-level square wave signal by the third harmonic injection module, it can be obtained by injecting the third and multiples of the third harmonic, as shown in equation (2). Alternatively, the moment when the per-unit phase-locked loop result commutates from -1 / 2 level to 1 / 2 level and from 1 / 2 level to -1 / 2 level can be directly detected to obtain the commutation moment of the two-level square wave signal, thereby obtaining the two-level square wave signal.

[0053] In an embodiment of the present invention, the AC output current of the grid-side converter leads the system stepped-wave voltage reference for commutation, such as... Figure 3 As shown, the lead time depends on the output power of each grid-side converter in the system and the parameters of its respective branch. The specific implementation method is as follows: First, the grid-side converter performs a phase-locked loop (PLL) shift on the phase-locked loop result of the system's stepped-wave voltage for the next steady-state stage, allowing the grid-side converter to obtain the PLL information for the next steady-state stage just before the current steady-state stage ends. Then, based on this PLL information, the inner current loop quickly controls the grid-side converter's output current to switch to the next steady-state stage to ensure that the pre-processed current remains constant DC. Finally, by setting a reasonable PLL shift time, the system's stepped-wave voltage reference undergoes phase commutation simultaneously with the grid-side converter's output current, ending the commutation stage and allowing the system to smoothly switch to the next steady-state stage.

[0054] In an embodiment of the present invention, the advance phase shift angle required during the commutation stage is generated by the commutation phase shift control module. Specifically, taking the grid-side converter 1 as an example, firstly, the per-unit phase-locked loop result based on the system stepped-wave voltage reference is... At the commutation moment of the system's stepped-wave voltage reference, the phase A current of the AC port of the grid-side converter is sampled and held. Subsequently, the sampling result i of the phase A current of the AC port of the grid-side converter at the end of the commutation phase, i.e., the start of the next steady state, is used. a1 Preprocessing is performed, i a1 Per-unit phase-locked loop results with the next steady-state system stepped voltage reference voltage after phase shift This is divided to obtain the phase A current at the AC port of the grid-side converter at the start of the next steady-state phase under DC reference. Then, the phase A current at the AC port of the grid-side converter at the start of the next steady-state phase under DC reference is compared with the current amplitude output from the voltage outer loop, using I as a reference. m_ref Divide and normalize to obtain the normalized result. The simplified commutation stage control objective is to determine whether the normalized result reaches unit 1 at the commutation time of the system's stepped-wave voltage reference, thereby adjusting the lead phase shift angle. Taking grid-side converter 1 as an example, considering that there are six commutation processes in the stepped-wave current, namely from -I... d1 Switch to -I d1 / 2, by -I d1 / 2 Switch to Id1 / 2, by I d1 / 2 Switch to I d1 , by I d1 Switch to I d1 / 2, by I d1 / 2 Switch to -I d1 / 2, by -I d1 / 2 Switch to -I d1 Excessively large or small lead phase shift angles exhibit different deviation errors from unit 1 in different commutation processes, requiring standardized handling of this error. (Using I...) d1 / 2 Switch to I d1 with -I d1 / 2 Switch to -I d1 Using this as a benchmark, its error from unit 1 is defined as:

[0055] (3)

[0056] Where x is the normalized result. Then in -I d1 Switch to -I d1 / 2 and I d1 Switch to I d1 When / 2, after unified processing, the error needs to be defined as:

[0057] (4)

[0058] in-I d1 / 2 Switch to I d1 / 2 and I d1 / 2 Switch to -I d1 When / 2, after unified processing, the error needs to be defined as:

[0059] (5)

[0060] Finally, this error is input to a PI controller with a discrete period equal to the commutation time interval of the system stepped wave voltage reference, which outputs the required lead phase shift angle for the commutation stage of the grid-side converter. Based on the required lead phase shift angle for the commutation stage of grid-side converter 1... The per-unit phase-locked loop result output by the stepped-wave voltage phase-locked loop module. Phase shifting is used to obtain the per-unit phase-locked loop result of the phase-shifted system stepped voltage reference. , , .

[0061] In an embodiment of the present invention, the collection system comprises two grid-side converters, 1 and 2, connected in parallel. Their output power and branch parameters are different. Let's assume that the output power of grid-side converter 1 is greater than that of grid-side converter 2, i.e., I... d1Greater than I d2 Controlled by the commutation phase shift control module, the leading phase shift angle of each grid-side converter is positively correlated with its output power. Grid-side converter 1 performs commutation first, followed by grid-side converter 2. Typical output current waveforms of the two grid-side converters during the commutation phase are shown below. Figure 3 in i a1 i a2 As shown. Taking the commutation stage where the system stepped voltage reference switches from 1 / 2 to 1 as an example, when grid-side converter 1 starts commutation, the current i a1 As the voltage increases linearly, the voltage of phase A at point PCC rises. At this time, grid-side converter 2 has not yet started commutation, and its output current i a2 Therefore, it shows a downward trend. Subsequently, grid-side converter 2 begins commutation, and the current i a1 With current i a2 Simultaneously, the current increases linearly until the system's stepped-wave voltage reference commutation moment, at which point both simultaneously reach the current amplitude I of the next steady-state stage. d1 with I d2 The commutation phase ends.

[0062] In an embodiment of the present invention, the stepped-wave voltage phase-locked module samples and detects the three-phase port voltage on the AC side of the grid-side converter and determines whether u occurs. ao1 u bo1 u co1 It then determines whether a characteristic level appears, thereby obtaining the step wave voltage reference u within the collection system. A u B u C By analyzing the frequency and phase information, the system's stepped-wave voltage reference commutation time can be determined, achieving real-time, localized, and precise phase-locked looping without relying on remote sampling or communication methods. Taking grid-side converter 1 as an example, the specific implementation method of phase-locking is as follows: the stepped-wave voltage phase-locked loop module detects the AC port voltage u of the three phases A, B, and C of the grid-side converter respectively. ao1 u bo1 u co1 The system checks whether characteristic levels corresponding to steady-state stages with voltage references of 1 and -1 appear. If they do, the system's stepped voltage reference is considered to have completed the corresponding commutation process. Taking phase A as an example, when the AC voltage of phase A in grid-side converter 1 is in the steady-state stage corresponding to voltage reference 1, the AC voltage of phase A in grid-side converter 2 is also in the steady-state stage corresponding to voltage reference 1. At this time, the input voltage of phase A on the AC side of the sending-end converter is 2U. d / 3, using the superposition theorem, the characteristic level corresponding to this steady-state stage is:

[0063] (6)

[0064] In equation (6), L is defined f1 With Ls1 The series inductance is L1, L f2 With L s2 The series inductance is L2, and K is the proportion of the inductance after L1 and L2 are connected in parallel. L1 and L2 are connected in parallel and then in series. c The ratio of sensitivity, K1 is L2 to L c The proportion of inductance in L2 and L after parallel connection c The ratio of the inductance of L1 after parallel connection and then series connection, K2 is the ratio of L1 to L c The proportion of inductance in L1 and L after parallel connection c Parallel connection followed by series connection L c The ratio of inductances, therefore K + K1 + K2 = 1, g1 is L f1 L f1 Series L s1 The ratio. In a pooling system, there is U. in ≈U d And U in >U d If this is true, substituting it into equation (6), we can obtain the characteristic level range corresponding to this steady-state stage as follows:

[0065] (7)

[0066] Therefore, once the phase A stepped wave voltage phase-locked module detects that the phase A voltage at the AC port of the grid-side converter satisfies the condition corresponding to equation (7), it can be considered that the phase A stepped wave voltage reference of the system has completed the commutation from 1 / 2 to 1 at that moment. Subsequently, the phase A stepped wave voltage phase-locked module is locked, waiting to detect the corresponding characteristic level. Similarly, the characteristic level range of the steady-state stage with a voltage reference of -1 is:

[0067] (8)

[0068] Once the phase A stepped wave voltage phase-locked module detects that the phase A voltage at the AC port of the grid-side converter meets the condition corresponding to equation (8), it can be considered that the phase A voltage reference of the system has completed the commutation from -1 / 2 to -1 at that moment. Subsequently, the phase A stepped wave voltage phase-locked module is locked and waits again to detect the corresponding characteristic level.

[0069] (9)

[0070] Once the phase B stepped wave voltage phase-locked module detects that the phase B voltage at the AC port of the grid-side converter meets the condition corresponding to equation (9), it can be considered that the phase A stepped wave voltage reference of the system has completed the commutation from 1 / 2 to -1 / 2 at that moment. Subsequently, the phase B stepped wave voltage phase-locked module is locked and waits again to detect the corresponding characteristic level.

[0071] (10)

[0072] Once the phase B stepped wave voltage phase-locked module detects that the phase B voltage at the AC port of the grid-side converter meets the condition corresponding to equation (10), it can be considered that the phase A stepped wave voltage reference of the system has completed the commutation from -1 / 2 to 1 / 2 at that moment. Subsequently, the phase B stepped wave voltage phase-locked module is locked and waits again to detect the corresponding characteristic level.

[0073] (11)

[0074] Once the C-phase stepped wave voltage phase-locked module detects that the C-phase voltage at the AC port of the grid-side converter satisfies the condition corresponding to equation (11), it can be considered that the A-phase stepped wave voltage reference of the system has completed the commutation from -1 to -1 / 2 at that moment. Subsequently, the C-phase stepped wave voltage phase-locked module is locked and waits again to detect the corresponding characteristic level.

[0075] (12)

[0076] Once the C-phase stepped wave voltage phase-locked loop module detects that the C-phase voltage at the AC port of the grid-side converter satisfies the condition corresponding to equation (11), it can be considered that the system stepped wave A-phase voltage reference has completed the commutation from 1 to 1 / 2 at that moment. Subsequently, the C-phase stepped wave voltage phase-locked loop module is locked and waits again to detect the characteristic level corresponding to the moment when the system stepped wave A-phase voltage reference commutates from -1 to -1 / 2.

[0077] Finally, by combining the characteristic level detection results of phases A, B, and C, the commutation times of all six phases of the system's stepped wave voltage reference can be obtained, and thus the stepped wave voltage reference of phase A can be reconstructed. The stepped wave voltage references of phases B and C can be obtained by the same method, or by directly delaying the stepped wave voltage reference of phase A by 120° and 240°.

[0078] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A grid-side converter control method suitable for a stepped wave converging system, characterized in that, Steady-state stage: Based on the per-unit phase-locked loop result of the current steady-state stage system stepped voltage reference, the output current of the AC port of the grid-side converter is preprocessed. A voltage and current dual closed-loop control strategy is used to control the preprocessed result with constant current to generate a modulation wave signal under the DC reference. Then, based on the per-unit phase-locked loop result of the current steady-state stage system stepped voltage reference, the modulation wave signal under the DC reference is restored to the modulation wave signal under the stepped voltage reference. When obtaining the per-unit phase-locked loop result of the next steady-state stage system stepped voltage reference, the output current of the AC port of the grid-side converter is controlled to switch to the corresponding amplitude of the next steady-state stage. Commutation stage: At the beginning of the next steady-state stage, the output current of the AC port of the grid-side converter is sampled. Using the current amplitude reference of the inner current loop as a reference, the output current of the AC port of the grid-side converter at the beginning of the next steady-state stage under the DC reference is normalized. Based on the normalization result, the required advance phase shift angle for the commutation stage is obtained, and the per-unit phase-locked loop result of the system stepped wave voltage reference for the next steady-state stage is advanced phase-locked.

2. The grid-side converter control method for a stepped wave converging system according to claim 1, characterized in that, In the steady-state phase: the per-unit phase-locked loop result of the system stepped wave voltage reference in the current steady-state phase is obtained by the following method: obtaining the commutation times of the system stepped wave voltage reference by detecting the characteristic level of the AC port voltage of the grid-side converter, and then restoring the system stepped wave voltage reference based on the commutation times of the system stepped wave voltage reference; wherein, the commutation times of the A-phase stepped wave voltage reference are obtained by the following criterion: detecting the A-phase voltage u at the AC port of the grid-side converter. ao1 satisfy At that moment, the system's A-phase stepped wave voltage reference completed its commutation from 1 / 2 to 1, U in U is the DC bus capacitor voltage of the grid-side converter. d This refers to the DC bus capacitor voltage of the sending-end converter. The voltage u of phase A at the AC port of the grid-side converter was detected. ao1 satisfy At that moment, the system's A-phase stepped wave voltage reference completed its commutation from -1 / 2 to -1. The voltage u of phase B at the AC port of the grid-side converter was detected. bo1 satisfy At that moment, the system's A-phase stepped wave voltage reference completed its commutation from 1 / 2 to -1 / 2. The voltage u of phase B at the AC port of the grid-side converter was detected. bo1 satisfy At that moment, the system's A-phase stepped wave voltage reference completed its commutation from -1 / 2 to 1 / 2. The voltage u of phase C at the AC port of the grid-side converter was detected. co1 satisfy At that moment, the system's A-phase stepped wave voltage reference completed its commutation from -1 to -1 / 2. The voltage u of phase C at the AC port of the grid-side converter was detected. co1 satisfy At that moment, the system's A-phase stepped wave voltage reference completes the commutation from 1 to 1 / 2.

3. The grid-side converter control method for a stepped wave converging system according to claim 2, characterized in that, In the steady-state phase: the output current of the AC port of the grid-side converter is preprocessed according to the per-unit phase-locked loop result of the system step wave voltage reference in the current steady-state phase. Specifically, the sampling result of the output current of the AC port of the grid-side converter in the current steady-state phase is divided by the per-unit phase-locked loop result of the system step wave voltage reference in the current steady-state phase.

4. The grid-side converter control method for a stepped wave converging system according to claim 3, characterized in that, In the steady-state phase: a voltage and current dual closed-loop control strategy is adopted to perform constant current control on the preprocessing results to generate a modulated wave signal under the DC reference. Specifically, the voltage outer loop is used to perform PI adjustment on the difference between the DC bus capacitor voltage of the grid-side converter and its rated reference value to generate the current amplitude reference of the current inner loop. The difference between the preprocessing results and the current amplitude reference is then performed PI adjustment through the current inner loop to obtain the modulated wave signal under the DC reference.

5. The grid-side converter control method for a stepped wave converging system according to claim 4, characterized in that, In the steady-state phase: the modulated wave signal under the DC reference is restored to the modulated wave signal under the step wave reference based on the per-unit phase-locked loop result of the current steady-state phase system step wave voltage reference. Specifically, the modulated wave signal under the DC reference is multiplied by the per-unit phase-locked loop result of the current steady-state phase system step wave voltage reference, or the modulated wave signal under the DC reference is multiplied by two-level signals that are in phase with the per-unit phase-locked loop result of the current steady-state phase system step wave voltage reference.

6. The grid-side converter control method for a stepped wave converging system according to claim 5, characterized in that, The two-level signal that is in phase with the per-unit phase-locked loop result of the current steady-state system stepped wave voltage reference is obtained by injecting a third harmonic into the per-unit phase-locked loop result of the current steady-state system stepped wave voltage reference; or, the two-level signal is obtained by detecting the moment when the system stepped wave voltage reference commutates from -1 / 2 level to 1 / 2 level and from 1 / 2 level to -1 / 2 level, and then obtaining the two-level signal from the commutation moment of the directly obtained two-level square wave signal.

7. The grid-side converter control method for a stepped wave converging system according to claim 6, characterized in that, During the commutation phase: at the start of the next steady-state phase, the output current of the grid-side converter AC port is sampled. Using the current amplitude reference of the inner current loop as a benchmark, the output current of the grid-side converter AC port at the start of the next steady-state phase under the DC benchmark is normalized. Specifically, the output current of the grid-side converter AC port sampled at the start of the next steady-state phase is divided by the per-unit phase-locked loop result of the system stepped wave voltage benchmark in the next steady-state phase to obtain the output current of the grid-side converter AC port at the start of the next steady-state phase under the DC benchmark. The output current of the grid-side converter AC port at the start of the next steady-state phase under the DC benchmark is then divided by the current amplitude reference to obtain the normalization result.

8. The grid-side converter control method for a stepped wave converging system according to claim 7, characterized in that, In the commutation stage: the required advance phase shift angle for the commutation stage is obtained based on the normalization processing result, and the per-unit phase-locked loop result of the system step wave voltage reference for the next steady-state stage is advanced phase-shifted. Specifically, the deviation error between the normalization processing result and unit 1 is obtained, and the deviation error is subjected to PI control with the commutation time interval of the system step wave voltage reference as the discrete period. The required advance phase shift angle for the commutation stage is obtained, and the per-unit phase-locked loop result of the system step wave voltage reference for the next steady-state stage is advanced phase-shifted based on the advance phase shift angle.

9. The grid-side converter control method for a stepped wave converging system according to claim 8, characterized in that, The specific method for obtaining the deviation error between the normalized result and unit 1 is as follows: The AC port output current of the grid-side converter is from I d1 / 2 Switch to I d1 And from -I d1 / 2 Switch to -I d1 When, the deviation error is , The AC port output current of the grid-side converter is from -I d1 Switch to -I d1 / 2 and from I d1 Switch to I d1 When / 2, the deviation error is , in, Let x be the deviation error, and I be the normalized result. d1 This refers to the output current amplitude at the AC port of the grid-side converter.

10. A grid-side converter control device suitable for a stepped wave converging system, used to implement the control method of claim 1, characterized in that, The control device includes: The stepped wave voltage phase-locked loop module is used to obtain the per-unit phase-locked loop result of the stepped wave voltage reference of the system in the current steady state stage; The commutation phase-shifting control module is used to control the switching of the AC port output current of the grid-side converter to the amplitude corresponding to the next steady-state stage. At the beginning of the next steady-state stage, it samples the AC port output current of the grid-side converter. Using the current amplitude reference of the inner current loop as a benchmark, it normalizes the AC port output current of the grid-side converter at the beginning of the next steady-state stage under the DC reference. Based on the normalization result, it obtains the required lead phase-shifting angle for the commutation stage and performs lead phase shifting on the per-unit phase-locked loop result of the system's stepped-wave voltage reference for the next steady-state stage; and... The voltage and current dual closed-loop control module is used to preprocess the AC port output current of the grid-side converter based on the per-unit phase-locked loop result of the system stepped voltage reference in the current steady-state stage. It uses a voltage and current dual closed-loop control strategy to perform constant current control on the preprocessed result to generate a modulation wave signal under the DC reference. Then, based on the per-unit phase-locked loop result of the system stepped voltage reference in the current steady-state stage, it restores the modulation wave signal under the DC reference to the modulation wave signal under the stepped voltage reference. When obtaining the per-unit phase-locked loop result of the system stepped voltage reference in the next steady-state stage, it controls the AC port output current of the grid-side converter to switch to the corresponding amplitude of the next steady-state stage.

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