阅读说明：本技术 一种极低开关频率下的电流源变流器特定谐波抑制方法 (Current source converter specific harmonic suppression method under extremely low switching frequency ) 是由 张昌浩 何晋伟 于 2019-09-10 设计创作，主要内容包括：本发明公开一种极低开关频率下的电流源变流器特定谐波抑制方法,其特征在于：采用三相电流源型并网整流器拓扑结构,利用不对称特定谐波消除与旁路脉冲注入相结合的新型混合调制方法,主动控制输出PWM电流中的5次、7次谐波分量,对线电流中的5次、7次谐波分量进行主动补偿。这种控制策略使得大功率电流源变流器在极低开关频率下具备同时主动补偿5次、7次谐波的能力,在电网电压畸变的情况下得以有效降低线电流的低次谐波,大幅提升了系统的线电流电能质量,保证了系统的安全稳定运行。(The invention discloses a method for suppressing specific harmonic waves of a current source converter under an extremely low switching frequency, which is characterized by comprising the following steps of: a three-phase current source type grid-connected rectifier topological structure is adopted, 5-order and 7-order harmonic components in PWM current are actively controlled and output by a novel mixed modulation method combining asymmetric specific harmonic elimination and bypass pulse injection, and the 5-order and 7-order harmonic components in the line current are actively compensated. The control strategy enables the high-power current source converter to have the capability of actively compensating 5-order and 7-order harmonics at the same time under the extremely low switching frequency, so that the low-order harmonics of the line current can be effectively reduced under the condition of the voltage distortion of a power grid, the line current electric energy quality of the system is greatly improved, and the safe and stable operation of the system is ensured.)

1. a specific harmonic suppression method for a current source converter under an extremely low switching frequency is based on a three-phase current source grid-connected converter topological structure, wherein the current source grid-connected converter is a three-phase full-bridge topology formed by six gate pole commutation thyristors, and the alternating current side of the converter is connected to a Point of Common Coupling (PCC) through a filter and then exchanges power with a three-phase power grid; DC side bus inductor L for series connection and public use of DC side of converter_dcFor suppressing current fluctuations on the direct current side; the harmonic compensation method is characterized by comprising the following steps of:

(1) At the beginning of each sampling period, the converter collects the three-phase voltage V of the power grid by using the sampling circuit_PCCThree phase line current I_lineand a direct side direct current I_dc(ii) a Three-phase network voltage V_PCCGenerating a real-time phase V of a grid voltage by means of a phase-locked loop PLL_g；

(2) The DC current I_dcAnd a direct current reference I_{dc_ref}Subtracting the difference to obtain the output PWM current I of the converter through a proportional resonant (PI) controller as a feedback quantity_PWMThe direct current is directly controlled by the delay angle alpha relative to the real-time phase of the power grid voltage;

(3) The 5 th and 7 th harmonic current components in the three-phase grid line current are extracted by Sliding Discrete Fourier Transform (SDFT)Andmultiplying the harmonic components of line current 5 th order and 7 th order by the virtual impedance coefficientThen obtaining 5 th, 7 th harmonic component reference of output PWM currentAnd AndDivided by direct current I_dcObtaining 5 th and 7 th harmonic component references of output PWM current after per unitAnd

(4) Outputting PWM current 5 subharmonic component reference after per unit obtained in the step (3)Post-per-unit output PWM current 7 th harmonic component referenceThe delay angle alpha and the real-time phase V of the power grid voltage obtained in the step (1)_gGenerating switching signals G of six gate commutated thyristors by a mixed modulation strategy of bypass pulse injection and asymmetric specific harmonic elimination₁～G₆Finally, corresponding harmonic 5 th and 7 th components are generated in the converter output PWM current to suppress harmonics in the line current.

2. The novel current source converter specific harmonic suppression method according to claim 1, wherein the bypass pulse injection and asymmetric specific harmonic cancellation hybrid modulation strategy in step (4) comprises the following steps:

a) Output PWM current 7 th harmonic component reference after per unitThe width beta of the bypass pulse is calculated_BPAnd position theta_BP：

Wherein the content of the first and second substances,Are respectively asAmplitude and phase angle of.

b) By the width beta of the by-pass pulse_BPAnd position theta_BPCalculating to obtain a per-unit estimated value of parasitic 5 th harmonic current of bypass pulseAmplitude ofAngle of sum

c) The obtained estimated value is processedAnd the output PWM current 5 th harmonic component after per unitAdding to obtain 5 th harmonic reference of input asymmetric specific harmonic elimination modulation

d) according toamplitude ofAngle of sumSelecting corresponding asymmetric specific harmonic elimination modulation switch angle theta₁～θ₁₃；

e) Real-time phase V of power grid voltage_gdelay angle alpha, bypass pulse width beta_BPAnd position theta_BPAnd asymmetric specific harmonic cancellation modulation switch angle θ₁～θ₁₃Inputting a hybrid modulation method, and obtaining switching signals G of six gate commutated thyristors through asymmetric specific harmonic elimination modulation and bypass pulse injection₁～G₆。

Technical Field

The invention relates to the management of the power quality of a grid-connected converter, in particular to a specific harmonic suppression method for a current source converter under an extremely low switching frequency.

Background

In recent years, with increasingly prominent energy problems and increasing requirements for industrial production processes, a medium-voltage high-power converter serving as an energy-saving and emission-reducing means has become a common technical scheme in industries such as electric power, metallurgy and traffic. According to the difference of energy storage links at the direct current side, the converter is divided into a Voltage Source Converter (VSC) and a Current Source Converter (CSC). Although the VSC is dominant in the current medium-voltage high-power transmission system due to its higher efficiency and the advantages of the power switching devices, the CSC is one of the solutions of the high-power converter technology, and plays an important role in the current high-power medium-voltage transmission field due to the incomparable advantages of simple topology, convenient serial connection of the switching devices, high waveform quality of input/output current, reliable short-circuit protection, and the like.

In order to reduce switching loss and ensure efficiency, the switching frequency of a high-power current source converter is generally only a few hundred hertz, so that a Specific Harmonic Elimination (SHE) modulation method is widely applied due to excellent harmonic characteristics under extremely low switching frequency. However, the conventional specific harmonic elimination modulation does not have the capability of actively controlling the harmonic component of the output PWM current, so that only the minimum of the own output PWM current harmonic can be ensured, the harmonic current caused by the background harmonic of the grid voltage cannot be actively compensated, and the electric energy quality of the line current is difficult to ensure under the working condition of grid voltage distortion. Therefore, Ye Zhang is improved on the basis of the traditional specific harmonic elimination modulation, and an asymmetric specific harmonic elimination modulation method [1] is provided. The asymmetric specific harmonic elimination modulation enables the harmonic component of the output PWM current to be controllable, so that the high-power current source converter can actively compensate line current harmonic caused by power grid voltage distortion, and the electric energy quality of the high-power current source converter is greatly improved. However, since the switching angle of the specific harmonic cancellation modulation needs to be calculated off-line, although the switching angles with different harmonic contents can be calculated in advance and stored in the controller, if a plurality of harmonic components are controlled simultaneously, the required memory will increase exponentially. According to the size of the memory of the existing controller, the asymmetric specific harmonic elimination method can only control one specific subharmonic component, and cannot control more than 2 harmonic components at the same time. However, since the cut-off frequency of the ac-side filter of the high-power current source converter is generally 200 to 350Hz, the harmonics of 5 and 7 orders in the line current are all at risk of being amplified, and it is still impossible to satisfy the requirements in the actual system by only compensating one specific subharmonic component.

In addition, the proposal of the bypass pulse injection method also solves the inherent problem that the traditional specific harmonic elimination modulation method has no adjustable degree of freedom from another point of view. However, based on the current state of the art, the added freedom of the bypass pulse injection method is only used for dc current control [2] or parallel current source current sharing control [3], and is not used to compensate for specific sub-harmonics.

Reference documents:

[1]H.Zhou，Y.W.Li，N.R.Zargari，G.Cheng，R.Ni，and Y.Zhang，“Selective harmonic compensation(SHC)PWM for grid-interfacing high-power converters，”IEEE Trans.Power Electron.，vol.29，no.3，pp.1118-1127，Mar.2014.

[2]J.He，Q.Li，C.Zhang，J.Han，and C.Wang，“Quasi-Selective Harmonic Elimination(Q-SHE)Modulation-Based DC Current Balancing Method for ParallelCurrent Source Converters，”IEEE Trans.Power Electron.，vol.34，no.8，pp.7422-7436，Aug.2019.

[3]Y.Xiao，B.Wu，F.DeWinter，and R.Sotudeh，“High power GTO AC/DC current source converter with minimum switching frequency and maximum power factor，”Proceedings of 1996Canadian Conference on Electrical and ComputerEngineering，Calgary，Alberta，Canada，1996，pp.331-334vol.1.

disclosure of Invention

The invention aims to overcome the defects in the prior art, and provides a novel specific harmonic compensation method capable of simultaneously eliminating 5 th and 7 th harmonics in power grid background harmonics by combining an asymmetric specific harmonic elimination method and a bypass pulse injection method on the basis of a traditional specific harmonic elimination method.

In order to achieve the purpose, the technical scheme provided by the invention is as follows:

A specific harmonic suppression method for a current source converter under an extremely low switching frequency is based on a three-phase current sourceThe grid-connected converter comprises a grid-connected converter topological structure, a current source grid-connected converter and a grid-connected converter, wherein the current source grid-connected converter is a three-phase full-bridge topology formed by six gate pole converter thyristors, and the alternating current side of the converter is connected to a Point of Common Coupling (PCC) through a filter and then exchanges power with a three-phase power grid; DC side bus inductor L for series connection and public use of DC side of converter_dcFor suppressing current fluctuations on the direct current side; the harmonic compensation method comprises the following steps:

(1) At the beginning of each sampling period, the converter collects the three-phase voltage V of the power grid by using the sampling circuit_PCCThree phase line current I_lineAnd a direct side direct current I_dc(ii) a Three-phase network voltage V_PCCGenerating a real-time phase V of a grid voltage by means of a phase-locked loop PLL_g；

(2) the DC current I_dcAnd a direct current reference I_{dc_ref}Subtracting the difference to obtain the output PWM current I of the converter through a proportional resonant (PI) controller as a feedback quantity_PWMthe direct current is directly controlled by the delay angle alpha relative to the real-time phase of the power grid voltage;

(3) The 5 th and 7 th harmonic current components in the three-phase grid line current are extracted by Sliding Discrete Fourier Transform (SDFT)AndMultiplying the harmonic components of line current 5 th order and 7 th order by the virtual impedance coefficientThen obtaining 5 th, 7 th harmonic component reference of output PWM currentAnd anddivided by direct current I_dcObtaining 5 th and 7 th harmonic component references of output PWM current after per unitand

(4) outputting PWM current 5 subharmonic component reference after per unit obtained in the step (3)Post-per-unit output PWM current 7 th harmonic component referencethe delay angle alpha and the real-time phase V of the power grid voltage obtained in the step (1)_ggenerating switching signals G of six gate commutated thyristors by a mixed modulation strategy of bypass pulse injection and asymmetric specific harmonic elimination₁～G₆Finally, corresponding harmonic 5 th and 7 th components are generated in the converter output PWM current to suppress harmonics in the line current.

Further, the modulation strategy of mixing the bypass pulse injection and the asymmetric specific harmonic elimination in the step (4) comprises the following steps:

a) Output PWM current 7 th harmonic component reference after per unitThe width beta of the bypass pulse is calculated_BPAnd position theta_BP：

Wherein the content of the first and second substances,Are respectively asAmplitude and phase angle of.

b) By the width beta of the by-pass pulse_BPAnd position theta_BPCalculating to obtain a per-unit estimated value of parasitic 5 th harmonic current of bypass pulseAmplitude ofangle of sum

c) The obtained estimated value is processedAnd the output PWM current 5 th harmonic component after per unitAdding to obtain 5 th harmonic reference of input asymmetric specific harmonic elimination modulation

d) According toAmplitude ofAngle of sumSelecting corresponding asymmetric specific harmonic elimination modulation switch angle theta₁～θ₁₃；

e) real-time phase V of power grid voltage_gdelay angle alpha, bypass pulse width beta_BPAnd position theta_BPAnd asymmetric specific harmonic cancellation modulation switch angle θ₁～θ₁₃Inputting a hybrid modulation method, and obtaining switching signals G of six gate commutated thyristors through asymmetric specific harmonic elimination modulation and bypass pulse injection₁～G₆。

Compared with the prior art, the technical scheme of the invention has the following beneficial effects:

1. the method can simultaneously eliminate 5, 7 harmonics caused by power grid background harmonics; compared with a Space Vector Modulation (SVM) method in the prior art, the switching frequency in the method is only about 700Hz, and if the SVM wants to achieve the same control effect, the switching frequency needs at least 5 KHz.

2. The method can realize that one inverter can simultaneously eliminate 5 and 7 subharmonics, but the method for eliminating and modulating the specific subharmonics by using the asymmetric specific harmonics in the prior art can only control one specific subharmonic, and the same effect as the method can be realized only by mutually coordinating two inverters.

3. The invention adopts a novel specific harmonic modulation method combining bypass pulse injection and asymmetric specific harmonic elimination. The traditional SHE modulation strategy can only ensure that the PWM output current of the SHE modulation strategy does not contain specific subharmonics, but when the grid voltage contains background harmonics, the SHE modulation strategy cannot compensate the harmonics in the line current. In the technical scheme of the invention, the novel specific harmonic modulation method has the capability of actively controlling the specific subharmonic component in the PWM output current under the same switching frequency, and can be used for actively compensating the line current waveform distortion caused by the background harmonic of the power grid.

4. By applying the technical scheme of the invention, the three-phase current source type grid-connected converter can work under the severe operating condition with serious grid voltage distortion, so that the current and electric energy quality of a power grid line is obviously improved. The traditional high-power current source converter cannot actively suppress 5 and 7 th harmonics generated by the voltage distortion of a power grid, so that the cut-off frequency of a filter is set to be very low and is about 200Hz to avoid the amplification of 5 and 7 th harmonic currents by an alternating-current side filter, so that the filter is large in size and heavy in weight. After the method of the invention is applied, 5, 7 harmonic components in line current can be effectively compensated, and the cut-off frequency of the filter is not required to be very low, thus greatly reducing the volume and weight of the filter and saving the cost.

drawings

Fig. 1 is a topological structure of a three-phase current source grid-connected converter in an embodiment of the present invention.

Fig. 2 is a schematic diagram of a simplified topology and the proposed control method of the grid-connected converter according to the present invention.

FIG. 3 is a schematic diagram of the hybrid modulation gate control signal according to the present invention.

Fig. 4 is a graph showing the experimental results of applying the conventional specific harmonic cancellation method and the method of the present invention.

Detailed Description

The invention is described in further detail below with reference to the figures and specific examples. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

The control method is based on a three-phase current source type grid-connected converter topological structure, as shown in fig. 1, the structure is as follows: the current source type grid-connected converter is a three-phase full-bridge topology formed by six gate pole converter thyristors, and the alternating current side of the converter is connected to a Point of Common Coupling (PCC) through a filter and then exchanges power with a three-phase power grid; DC side bus inductor L for series connection and public use of DC side of converter_dcFor suppressing current fluctuations on the dc side.

The circuit and control block diagram of the control method provided by the invention are shown in fig. 2, and the specific method is as follows:

(1) At the beginning of each sampling period, the converter collects the three-phase voltage V of the power grid by using the sampling circuit_PCCThree phase line current I_lineAnd a direct side direct current I_dc(ii) a Three-phase network voltage V_PCCGenerating a real-time phase V of a grid voltage by means of a phase-locked loop PLL_g；

(2) The DC current I_dcAnd a direct currentCurrent reference I_{dc_ref}the difference is used as a feedback quantity, and the output PWM current I of the converter is obtained through a proportional resonant controller_PWMThe direct current is directly controlled by the delay angle alpha relative to the real-time phase of the power grid voltage;

(3) The 5 th and 7 th harmonic current components in the three-phase grid line current are extracted by Sliding Discrete Fourier Transform (SDFT)andMultiplying the harmonic components of line current 5 th order and 7 th order by the virtual impedance coefficientThen obtaining 5 th, 7 th harmonic component reference of output PWM currentAnd AndIs coated with direct current I_dcObtaining 5 th and 7 th harmonic component references of output PWM current after per unitAnd

(4) Outputting PWM current 5 subharmonic component reference after per unit obtained in the step (3)Post-per-unit output PWM current 7 th harmonic component referenceThe delay angle alpha and the real-time phase V of the power grid voltage obtained in the step (1)_ggenerating switching signals G of six gate commutated thyristors by a mixed modulation strategy of bypass pulse injection and asymmetric specific harmonic elimination₁～G₆Finally, corresponding harmonic 5 th and 7 th components are generated in the converter output PWM current to suppress harmonics in the line current.

The following are more specific embodiments of the present invention:

The mixed modulation strategy of bypass pulse injection and asymmetric specific harmonic elimination in the step (4) specifically comprises the following steps:

a) Output PWM current 7 th harmonic component reference after per unitThe width beta of the bypass pulse is calculated_BPAnd position theta_BP：

Wherein the content of the first and second substances,Are respectively asAmplitude and phase angle of.

b) By the width beta of the by-pass pulse_BPAnd position theta_BPCalculating to obtain a per-unit estimated value of parasitic 5 th harmonic current of bypass pulseamplitude ofAngle of sum

c) the obtained estimated value is processedAnd the output PWM current 5 th harmonic component after per unitAdding to obtain 5 th harmonic reference of input asymmetric specific harmonic elimination modulation

d) According toAmplitude ofangle of sumSelecting corresponding asymmetric specific harmonic elimination modulation switch angle theta₁～θ₁₃；

e) Real-time phase V of power grid voltage_gDelay angle alpha, bypass pulse width beta_BPAnd position theta_BPAnd asymmetric specific harmonic cancellation modulation switch angle θ₁～θ₁₃Inputting a hybrid modulation method, and obtaining switching signals G of six gate commutated thyristors through asymmetric specific harmonic elimination modulation and bypass pulse injection₁～G₆。

the specific implementation manner of the proposed novel hybrid modulation method of the asymmetric specific harmonic cancellation modulation and the bypass pulse injection is shown in fig. 3. FIG. 3 shows a gate control signal G₁～G₆Schematic representation. Wherein G is₁、G₃、G₅For upper bridge arm switching tube gateControl signal, G₂、G₄、G₆Is a gate control signal of a switching tube of a lower bridge arm. According to the known rule of the current source converter switch tube, the gate pole control signal G₁To G₆The waveforms are identical, leading by π/3 in phase only. With G₁Is exemplified according to reference [1]]If the asymmetric specific harmonic cancellation modulation method follows the principle of quarter-cycle inverse mirror symmetry, the corresponding switching angle relationship can be expressed as:

Wherein, theta₁～θ₁₃Is a free angle varying from 0 to pi/3, and tau₁～τ₁₃To fix the angle, once theta₁～θ₁₃Determination of τ₁～τ₁₃it is determined accordingly.

And with G₄By way of example, the pulse signal s is bypassed_BPAfter injection, G is based on the basic principle that the upper bridge arm switching tube and the lower bridge arm switching tube of the current source converter can only be conducted at each moment₆The corresponding position in the signal needs to go low. Wherein, the bypass pulse signal can be completely controlled by the width beta of the bypass pulse_BPand position theta_BPAnd (6) determining. The change in the control signal waveform caused by the bypass pulse signal also follows the principle of the current source leading by pi/3 in turn.

Fig. 4 is a comparative experimental diagram of the harmonic compensation method of the present invention and the conventional specific harmonic cancellation modulation method, wherein the dc current is controlled at 11A. It can be seen from the figure that the grid voltage is distorted significantly by the presence of the 5, 7 th harmonic. At the moment, the traditional specific harmonic elimination modulation method cannot actively inhibit the harmonic in the line current, so that the waveform of the line current is seriously distorted, and the THD is 16.8 percent; after the harmonic suppression method provided by the invention is adopted, 5 and 7 harmonics in the line current are obviously suppressed, and the electric energy quality is obviously improved to 4.9%.

To sum up: the specific harmonic compensation method for the current source converter under the extremely low switching frequency can be conveniently applied to a three-phase high-power current source type grid-connected converter, and a good power grid background harmonic compensation effect is achieved. The method can ensure that the line current has better electric energy quality under the condition of the voltage waveform distortion of the power grid, and greatly improves the reliability of the high-power current source converter. Meanwhile, the guarantee of the line current electric energy quality also enables the size of a filter at the alternating current side to be reduced, so that the cost of the converter is reduced, the size and the weight are reduced, and the method is a novel converter control method which is worth popularizing.

The present invention is not limited to the above-described embodiments. The foregoing description of the specific embodiments is intended to describe and illustrate the technical solutions of the present invention, and the above specific embodiments are merely illustrative and not restrictive. Those skilled in the art can make many changes and modifications to the invention without departing from the spirit and scope of the invention as defined in the appended claims.