Control method of low-frequency differential-mode voltage injection bipolar frequency converter

Abstract

The invention relates to the technical field of power electronic converters and flexible power transmission, in particular to a control method of a low-frequency differential-mode voltage injection bipolar frequency converter, the frequency converter adopts an ABC three-phase bridge arm structure, each phase comprises an upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm are connected to a power frequency power grid through a middle connection point. The low-frequency power grid is connected with the frequency converter through the bipolar transformer; each of the upper bridge arm and the lower bridge arm comprises n full-bridge sub-modules connected in series and a bridge arm inductor, a primary winding in the bipolar transformer is connected to a low-frequency power grid, and a positive winding and a negative winding are connected to the upper bridge arm and the lower bridge arm respectively; and generating low-frequency differential mode compensation voltage according to the capacitor voltage difference of the upper and lower bridge arms, and injecting the low-frequency differential mode compensation voltage into the modulation wave in a differential mode form to realize capacitor voltage balance. The low-frequency disturbance caused by the inductance difference is effectively inhibited, the adverse effect on the system is reduced, the stable operation of the system under different working conditions is ensured, and the stability and reliability of the power conversion process are ensured.

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 control method for a bipolar inverter with low-frequency differential mode voltage injection. Background Art

[0002] Low-frequency transmission technology, an emerging solution to overcome the bottlenecks of traditional power frequency transmission, offers key advantages: when the frequency drops to 15-20 Hz, the line's equivalent capacitive reactance increases by 2-3 times, significantly reducing the need for reactive power compensation. Furthermore, line losses are reduced by approximately 30% compared to 50 Hz systems, making it particularly suitable for scenarios such as offshore wind power grid integration and land-based wide-area interconnection. As a key energy hub for frequency-splitting power transmission, AC-AC converters must simultaneously meet the following requirements: efficient power conversion between the low-frequency and power frequency sides, dynamic response to bidirectional energy flow, and stable operation under complex operating conditions.

[0003] The current mainstream modular multi-level bipolar inverter uses a symmetrical topology with upper and lower bridge arms. Because transformer parameters are difficult to achieve complete uniformity, the equivalent inductance parameters of the upper and lower bridge arms can differ significantly, causing capacitor voltage imbalances in the upper and lower bridge arms. This leads to different module operating states and, in turn, affects the overall lifespan of the inverter. A dynamic closed-loop control strategy is urgently needed to address this capacitor voltage imbalance in real time.

[0004] The information disclosed in this background technology section is only intended to deepen the understanding of the overall background technology of the present disclosure and should not be regarded as an admission or any form of suggestion that the information constitutes the prior art known to those skilled in the art. Summary of the Invention

[0005] The present invention provides a bipolar inverter with low-frequency differential-mode voltage injection 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 inverter with low-frequency differential-mode voltage injection comprises the following steps:

[0008] Monitor the voltage difference between the upper and lower bridge arm capacitors in real time, and generate a low-frequency current compensation reference value in the dq coordinate system through a PI controller;

[0009] Controlling the d-axis current to track the compensation reference value and returning the q-axis current to zero to obtain the dq components of the low-frequency compensation voltage;

[0010] The dq components are converted into a low-frequency differential mode reference voltage u by inverse Park transformation. ld_j ;

[0011] will u ld_j The modulation wave is injected into the upper and lower bridge arms in the form of differential mode to dynamically offset the capacitor voltage imbalance caused by the difference in inductor parameters.

[0012] Furthermore, the injection of the low-frequency differential mode reference voltage includes the following steps:

[0013] Take the reference voltage u on the power frequency side j_g and low-frequency side reference voltage u j_l Based on this, an optimized three-phase reference voltage combination is generated:

[0014]

[0015] Adjust u through closed-loop control j_l , so that the voltage difference between the upper and lower bridge arm capacitors approaches zero.

[0016] Furthermore, the proportional-integral parameters of the PI controller are dynamically adjusted according to the difference in bridge arm inductance to adapt to compensation requirements under different working conditions.

[0017] Furthermore, the injection frequency of the low-frequency differential mode voltage is 1 / 3 to 1 / 2 of the rated frequency of the low-frequency side of the system.

[0018] Furthermore, the rotational angular velocity of the inverse Park transformation is synchronized with the angular frequency of the low-frequency side of the system.

[0019] A bipolar inverter with low-frequency differential-mode voltage injection, used to implement the above-mentioned control method, comprises:

[0020] A three-phase bridge arm structure, where each phase bridge arm consists of an upper bridge arm and a lower bridge arm, each of which includes multiple full-bridge sub-modules and bridge arm inductors connected in series;

[0021] A bipolar transformer, whose primary winding is connected to the low-frequency grid and whose secondary winding is connected to the upper and lower bridge arms respectively;

[0022] The closed-loop control module is configured to perform the capacitor voltage difference detection, PI control, inverse Park transformation and differential mode voltage injection functions.

[0023] Furthermore, the secondary winding of the bipolar transformer includes a positive winding and a negative winding, the positive winding is connected to the upper bridge arm using a Y0 connection, and the negative winding is connected to the lower bridge arm using a Y6 connection.

[0024] Furthermore, the full-bridge submodule includes an IGBT and a supporting capacitor, and outputs a step voltage waveform through pulse width modulation.

[0025] The beneficial effects of the present invention are:

[0026] The optimized control strategy for bipolar inverter bridge arm inductance asymmetry based on low-frequency differential-mode voltage injection proposed in the present invention obtains a low-frequency differential-mode reference voltage through mutual balancing of the upper and lower bridge arm voltages, closed-loop control and Park inverse transformation. It can accurately compensate for the difference in inductance between the upper and lower bridge arms, achieve precise balance of capacitor voltage, and stabilize the operating voltage of each module in an ideal state.

[0027] Furthermore, the above-mentioned optimized control strategy for bipolar inverter bridge arm inductance asymmetry based on low-frequency differential-mode voltage injection can effectively suppress low-frequency disturbances caused by inductance differences, reduce their adverse effects on the system, ensure the stability and reliability of the power conversion process, and reduce the probability of system failures. It can also reduce the internal harmonic distribution of the bipolar inverter, thereby reducing unnecessary power losses within the bipolar inverter. It can adapt to changes in the upper and lower bridge arm inductance parameters caused by manufacturing processes, equipment aging, or failures, and effectively function under different operating conditions, maintaining stable system operation and enhancing the system's adaptability to complex environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] 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.

[0029] Figure 1 Schematic diagram of the topological structure of the bipolar inverter with low-frequency differential-mode voltage injection in the present invention. DETAILED DESCRIPTION

[0030] 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.

[0031] 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.

[0032] 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.

[0033] The present invention discloses a bipolar inverter with low-frequency differential mode voltage injection and a control method thereof. Figure 1 The bipolar inverter topology diagram shown uses an ABC three-phase bridge arm structure, with each phase consisting of an upper arm and a lower arm. The upper and lower arms are connected to the power grid through an intermediate connection point, that is, the midpoint of the arm is connected to the power grid through the AC busbar. Filter inductors and circuit breakers are configured to suppress harmonics. The low-frequency grid is connected to the inverter via a bipolar transformer. Both the upper and lower arms consist of n full-bridge submodules connected in series and a bridge inductor. The bipolar transformer includes a primary winding, a positive winding, and a negative winding. The primary winding is connected to the low-frequency grid, and the positive and negative windings are connected to the upper and lower arms, respectively. The closed-loop control module generates a low-frequency differential-mode compensation voltage based on the voltage difference between the upper and lower arm capacitors. This voltage is injected into the modulation wave in differential mode to achieve capacitor voltage balancing.

[0034] Furthermore, the positive winding adopts the Y0 connection method, and its three-phase output ends are respectively connected to the top of the three-phase upper bridge arm; the negative winding adopts the Y6 connection method, and its three-phase output ends are respectively connected to the bottom of the three-phase lower bridge arm; bidirectional conversion of industrial frequency and low-frequency electric energy is achieved through electromagnetic coupling.

[0035] Each bridge arm is composed of n full-bridge submodules connected in series. Each submodule contains four IGBTs (T1-T4) and a supporting capacitor (C), capable of outputting ±U_c voltages (where U_c is the capacitor's rated voltage). The bridge arm inductor (L_arm) is connected in series at the end of the bridge arm. Its equivalent inductance is the sum of the transformer's leakage inductance (L_tr) and the bridge arm's stray inductance (L_arm), i.e., Leq = Ltr + Larm.

[0036] The full-bridge submodule is set to an H-bridge topology, including the first IGBT, the second IGBT, the third IGBT, the fourth IGBT and an electrolytic capacitor. The CE poles of the first IGBT, the second IGBT, the third IGBT and the fourth IGBT are all anti-parallel connected with freewheeling diodes;

[0037] The first IGBT and the second IGBT are connected in series, the third IGBT and the fourth IGBT are connected in series, the positive pole of the full-bridge sub-module is the connection point of the first IGBT emitter and the second IGBT collector, and the negative pole of the full-bridge sub-module is the connection point of the third IGBT emitter and the fourth IGBT collector; the electrolytic capacitor is connected across the positive and negative poles of the full-bridge sub-module.

[0038] In the modulation mechanism of the full-bridge sub-module, the sub-module outputs a step wave through pulse width modulation (PWM), and the upper and lower bridge arms are complementary turned on to realize the frequency conversion function from industrial frequency to low frequency; the sub-module capacitor voltage needs to be kept balanced to avoid IGBT overvoltage or uneven loss due to voltage offset.

[0039] Furthermore, the phase difference between the positive winding and the negative winding is 180°, and the neutral points are both grounded.

[0040] The upper and lower bridge arms of a bipolar inverter within the same phase have different equivalent inductances. This inductance includes transformer leakage inductance and stray inductance, and the deviation range for the equivalent inductance between the upper and lower bridge arms is 10%-15%. This difference in inductance between the upper and lower bridge arms within the same phase is the root cause of capacitor voltage imbalance. This capacitor voltage difference arises from the uneven current distribution caused by the asymmetric inductance parameters of the upper and lower bridge arms. The control strategy directly influences the energy flow between the bridge arms by adjusting the differential-mode voltage component of the modulation wave between the upper and lower bridge arms.

[0041] The closed-loop control module includes a voltage difference detection unit for real-time acquisition of the capacitor voltage difference between the upper and lower bridge arms; a PI controller for converting the voltage difference into a low-frequency current compensation reference value in a dq coordinate system; and an inverse Park transform unit for converting the dq components of the compensation voltage into a three-phase low-frequency differential-mode reference voltage. The closed-loop control module has a real-time sampling period of less than 50 μs and integrates a capacitor voltage sensor and a current sensor.

[0042] The optimized control strategy for bipolar inverter bridge arm inductance asymmetry based on low-frequency differential-mode voltage injection proposed in the present invention obtains a low-frequency differential-mode reference voltage through mutual balancing of the upper and lower bridge arm voltages, closed-loop control and Park inverse transformation. It can accurately compensate for the difference in inductance between the upper and lower bridge arms, achieve precise balance of capacitor voltage, and stabilize the operating voltage of each module in an ideal state.

[0043] Furthermore, the above-mentioned optimized control strategy for bipolar inverter bridge arm inductance asymmetry based on low-frequency differential-mode voltage injection can effectively suppress low-frequency disturbances caused by inductance differences, reduce their adverse effects on the system, ensure the stability and reliability of the power conversion process, and reduce the probability of system failures. It also reduces the internal harmonic distribution of the bipolar inverter, thereby reducing unnecessary power losses within the bipolar inverter. It can adapt to changes in the upper and lower bridge arm inductance parameters caused by manufacturing processes, equipment aging, or failures, and effectively function under different operating conditions, maintaining stable system operation and enhancing the system's adaptability to complex environments.

[0044] The present invention further discloses a control method for a bipolar inverter with low-frequency differential mode voltage injection, which uses the above-mentioned bipolar inverter and includes the following steps:

[0045] Monitor the voltage difference between the upper and lower bridge arm capacitors in real time, and generate a low-frequency current compensation reference value in the dq coordinate system through the PI controller; control the d-axis current to track the compensation reference value, and return the q-axis current to zero to obtain the dq components of the low-frequency compensation voltage; convert the dq components into a low-frequency differential mode reference voltage u through inverse Park transformation ld_j ; Among them, the rotation angular velocity is synchronized with the low-frequency side angular frequency; u ld_j The modulation wave is injected into the upper and lower bridge arms in the form of differential mode to dynamically offset the capacitor voltage imbalance caused by the difference in inductor parameters.

[0046] In this embodiment, the difference in inductance between the upper and lower bridge arms leads to inconsistent di / dt during dynamic operation, causing capacitor voltage deviation. The submodule uses pulse width modulation (PWM) to output a step-wave waveform, with the upper and lower bridge arms conducting in a complementary manner to achieve frequency conversion from industrial frequency to low frequency. The submodule capacitor voltages must be balanced to avoid overvoltage or uneven IGBT losses caused by voltage deviation.

[0047] The injection of the low-frequency differential-mode reference voltage includes the following steps:

[0048] Take the reference voltage u on the power frequency side j_g and low-frequency side reference voltage u j_l Based on this, an optimized three-phase reference voltage combination is generated:

[0049]

[0050] Adjust u through closed-loop control j_l , so that the voltage difference between the upper and lower bridge arm capacitors approaches zero.

[0051] Among them, the low-frequency differential mode reference voltage u j_l It is generated through inverse Park transformation and injected into the upper and lower bridge arm modulation waves in differential mode to directly adjust the voltage difference between the bridge arms; the injection point is located in the bridge arm modulation signal synthesis link, and after superposition with the power frequency modulation signal, it drives the sub-module switching action.

[0052] Furthermore, the proportional-integral parameters of the PI controller are dynamically adjusted according to the difference in bridge arm inductance to adapt to the compensation requirements under different working conditions.

[0053] Furthermore, the injection frequency of the low-frequency differential mode voltage is 1 / 3 to 1 / 2 of the rated frequency of the low-frequency side of the system.

[0054] Furthermore, the rotational angular velocity of the inverse Park transform is synchronized with the angular frequency of the low-frequency side of the system. Those skilled in the art should understand that the present invention is not limited to the above-described embodiments. The above-described embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and improvements may be made to the present invention without departing from the spirit and scope of the present invention. These changes and improvements fall within the scope of the present invention as claimed. 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 inverter with low-frequency differential mode voltage injection, characterized in that: The following steps are involved: Monitor the voltage difference between the upper and lower bridge arm capacitors in real time, and generate a low-frequency current compensation reference value in the dq coordinate system through a PI controller; Controlling the d-axis current to track the compensation reference value and returning the q-axis current to zero to obtain the dq components of the low-frequency compensation voltage; The dq components are converted into a low-frequency differential mode reference voltage u by inverse Park transformation. ld_j ; will u ld_j The modulation wave is injected into the upper and lower bridge arms in the form of differential mode to dynamically offset the capacitor voltage imbalance caused by the difference in inductor parameters.

2. The control method for a bipolar inverter with low-frequency differential mode voltage injection according to claim 1, characterized in that: The injection of the low-frequency differential mode reference voltage comprises the following steps: Take the reference voltage u on the power frequency side j_g and low-frequency side reference voltage u j_l Based on this, an optimized three-phase reference voltage combination is generated: Adjust u through closed-loop control j_l , so that the voltage difference between the upper and lower bridge arm capacitors approaches zero.

3. The control method for a bipolar inverter with low-frequency differential-mode voltage injection according to claim 1, characterized in that: The proportional-integral parameters of the PI controller are dynamically adjusted according to the difference in bridge arm inductance to adapt to the compensation requirements under different working conditions.

4. The control method for a bipolar inverter with low-frequency differential mode voltage injection according to claim 1, characterized in that: The injection frequency of the low-frequency differential mode voltage is 1 / 3 to 1 / 2 of the rated frequency of the low-frequency side of the system.

5. The control method for a bipolar inverter with low-frequency differential mode voltage injection according to claim 1, characterized in that: The rotational angular velocity of the inverse Park transformation is synchronized with the angular frequency of the low-frequency side of the system.

6. A bipolar inverter with low-frequency differential mode voltage injection, characterized in that: A control method for implementing any one of claims 1 to 5, comprising: A three-phase bridge arm structure, where each phase bridge arm consists of an upper bridge arm and a lower bridge arm, each of which includes multiple full-bridge sub-modules and bridge arm inductors connected in series; A bipolar transformer, whose primary winding is connected to the low-frequency grid and whose secondary winding is connected to the upper and lower bridge arms respectively; The closed-loop control module is configured to perform the capacitor voltage difference detection, PI control, inverse Park transformation and differential mode voltage injection functions.

7. The bipolar inverter with low-frequency differential-mode voltage injection according to claim 6, characterized in that: The secondary winding of the bipolar transformer includes a positive winding and a negative winding. The positive winding is connected to the upper bridge arm using a Y0 connection method, and the negative winding is connected to the lower bridge arm using a Y6 connection method.

8. The bipolar inverter with low-frequency differential-mode voltage injection according to claim 6, characterized in that: The full-bridge submodule includes an IGBT and a supporting capacitor, and outputs a step voltage waveform through pulse width modulation.

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