A method for cooperative control of unidirectional energy flow three-level back-to-back converter

Abstract

The application discloses a kind of energy unidirectional flow three-level back-to-back converter collaborative control methods, it is related to high-power multilevel power electronic converter control technical field.The converter is with Vienna rectifier as front stage, with diode clamped three-level inverter as back stage.The application solves the zero-crossing distortion problem of grid-side current of front-stage Vienna rectifier by the method of reactive current compensation;Through the method of reconstructing target midpoint current and injecting optimal zero sequence voltage on the inverter side, the coupling problem between zero sequence voltage injection and zero-crossing distortion of rectifier-side overcurrent of front-stage converter is solved, and the collaborative control of midpoint potential balance of converter common DC bus is realized, which has practical value for stable control of energy unidirectional flow high-power converter in industrial industry.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high-power multi-level power electronic converter control, and particularly relates to a method for collaborative control of energy unidirectional flow three-level back-to-back converter. BACKGROUND

[0002] The back-to-back three-level neutral point clamped (NPC) topology is the most commonly used topology in the field of medium-voltage high-power converters. It is favored due to its low harmonic content, low electromagnetic interference, and mature control strategy. However, in many industrial applications, such as drive systems, wind turbine systems, and power supply links in telecommunications systems, bidirectional power flow is not required, and energy is strictly limited to unidirectional flow. Therefore, three-level back-to-back converters with Vienna rectifiers as the front stage have more application prospects due to their high energy transmission density, low switching loss, and excellent topology.

[0003] Vienna rectifier grid-side current zero-point distortion and midpoint voltage imbalance in back-to-back three-level converters are inherent problems of three-level converters. Vienna rectifier zero-point distortion can be modulated by space vector pulse width modulation (SVPWM), which reselects the vector when the grid-side current vector is at zero. This can eliminate current zero-point distortion, but it will also cause some harmonic pollution. The patent with publication number CN115864779 A clamps the reference voltage to zero when the current in each phase is at zero, which suppresses current zero-point distortion, but it also reduces the quality of the input current to some extent. In addition, for the control of midpoint potential balance in three-level back-to-back converters, research has mainly focused on modulation wave modulation, carrier modulation, and space vector control. The patent with publication number CN106533206 A uses mathematical coordinate rotation transformation to normalize voltage space vectors in other sectors to the first sector for unified calculation, and achieves midpoint potential balance through vector selection. However, this is essentially SVPWM modulation, which is complex to calculate and not conducive to engineering implementation and application. Furthermore, most current research on the inherent problems of three-level back-to-back converters focuses on one side of the converter, with little consideration given to simultaneous control strategies for both sides of the converter. From a holistic perspective, researching back-to-back collaborative control strategies can further reduce the harmonic content of the grid-side current and further reduce the voltage ripple content of the common DC bus voltage of the converter.

[0004] The present application proposes a method for collaborative control of energy unidirectional flow three-level back-to-back converters to solve the problems of input current zero-point distortion leading to high harmonic content on the grid side and midpoint potential imbalance leading to low output performance of the converter. This method addresses the difficulties present in existing technology and is a pressing problem for those skilled in the art. SUMMARY

[0005] Therefore, the application provides a method for cooperative control of energy unidirectional flow three-level back-to-back converter, which realizes cooperative control of midpoint potential balance of the converter common DC bus.

[0006] In order to achieve the above-mentioned purpose, the application adopts the following technical scheme:

[0007] A method for cooperative control of energy unidirectional flow three-level back-to-back converter, comprising the following steps:

[0008] S1. Analyzing the characteristics of the converter, and proposing to use a Vienna rectifier as a front stage and a diode clamped three-level inverter as a rear stage;

[0009] S2. Using the method of reactive current compensation to suppress current zero-crossing distortion of the Vienna rectifier in the front stage;

[0010] S3. According to the coupling relationship between the zero sequence voltage injected by the Vienna rectifier in the front stage and the zero-crossing distortion of the grid-side current, decoupling is performed by using the method of target midpoint current reconstruction, and the reconstructed target midpoint current is calculated;

[0011] S4. Based on the calculated reconstructed target midpoint current, injecting zero sequence voltage into the reference voltage of the diode clamped three-level inverter in the rear stage to obtain the midpoint current after injecting the zero sequence voltage;

[0012] S5. Calculating the current expected value, judging whether the zero sequence voltage and the midpoint current generated by the diode clamped three-level inverter in the rear stage are in a linear segmented relationship, if yes, calculating the zero sequence voltage required to be injected into the diode clamped three-level inverter in the rear stage by an interpolation function, if not, selecting the zero sequence voltage corresponding to the key point of the midpoint current expected value and injecting it into the diode clamped three-level inverter in the rear stage, so as to realize cooperative control of midpoint potential balance.

[0013] Optionally, in S2, the reactive current i rq is introduced into the double closed-loop control circuit of the Vienna rectifier in the front stage, so that the power factor angle δ is equal to the current distortion angle γ, and finally the grid-side current of the Vienna rectifier in the front stage is synchronized with the three-phase reference voltage, thereby suppressing the current zero-crossing distortion;

[0014] wherein, the expression of the reactive current i rq is as follows:

[0015]

[0016] wherein, ω1 is the grid angular frequency of the rectifier side in the front stage, L s is the inductance of the rectifier side in the front stage, and This represents the d-axis component of the output voltage on the rectifier side of the preamplifier.

[0017] Optionally, in S3, based on the coupling relationship between the zero-sequence voltage injected by the preceding Vienna rectifier and the zero-crossing distortion of the grid-side current, the target midpoint current reconstruction method is used for decoupling, and the expression for the reconstructed target midpoint current is calculated as follows:

[0018]

[0019] Among them, T c Let x be the carrier period, x be the rectifier phases a, b, and c, u1 and u2 be the upper and lower capacitor voltages respectively, and C be the common DC bus capacitance value. i is the three-phase reference voltage on the front-end rectifier side. x This is the actual three-phase input current to the front-end rectifier side.

[0020] Optionally, in S4, a zero-sequence voltage is injected into the reference voltage of the diode-clamped three-level inverter. The expression for the midpoint current after the zero-sequence voltage is injected is as follows:

[0021]

[0022] Where y represents the u, v, and w phases on the inverter side, and i y For the actual three-phase output current on the inverter side, u y * The three-phase reference voltage on the inverter side of the subsequent stage, u zi The zero-sequence voltage injected into the inverter side of the subsequent stage.

[0023] Optionally, there are three possible linear piecewise relationships between the zero-sequence voltage in S5 and the midpoint current generated by the subsequent diode-clamped three-level inverter:

[0024] Scenario 1: When v zmin ≤-v3 and -v1≤v zmax hour;

[0025] Scenario 2: When -v3 <v zmin ≤-v2 and -v2 <v zmax When ≤-v1;

[0026] Scenario 3: When -v2 <v zmin or v zmax When <-v2.

[0027] Optionally, in S5, the zero-sequence voltage required to be injected into the subsequent diode-clamped three-level inverter is calculated using the interpolation function as follows:

[0028]

[0029] in, is the current expected value, v zy is the right end point current value of the interval where the target midpoint current is located, i ox is the left end point current value of the interval where the target midpoint current is located, i oy is the zero sequence voltage value corresponding to the right end point current value of the interval where the target midpoint current is located, v zx is the zero sequence voltage value corresponding to the left end point current value of the interval where the target midpoint current is located.

[0030] Compared with the prior art, the energy unidirectional flow three-level back-to-back converter cooperative control method provided by the application has the following beneficial effects:

[0031] (1) The application solves the problem of grid-side current zero-crossing distortion and effectively reduces grid-side current harmonics.

[0032] (2) The application considers the phenomenon of coupling between zero sequence voltage injection and grid-side current zero-crossing distortion, solves the problem of converter common DC bus midpoint potential imbalance by reconstructing the target midpoint current and using a piecewise interpolation function to solve the optimal zero sequence voltage injection to the inverter side, and effectively enhances the good output performance of the converter. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are only embodiments of the application, and those skilled in the art can obtain other drawings according to the provided drawings without creative labor.

[0034] Figure 1 is a schematic diagram of a back-to-back converter provided by the application;

[0035] Figure 2 is a schematic diagram of the working characteristics of the current forced commutation inherent in the Vienna rectifier provided by the application; wherein 2a is a zero-crossing distortion principle diagram of the front-stage Vienna rectifier, and 2b is a reactive power compensation principle diagram of the front-stage Vienna rectifier;

[0036] Figure 3 is a double closed-loop control block diagram of reactive power compensation on the front-stage rectifier side provided by the application;

[0037] Figure 4Three cases of linear segmented relationship between the zero sequence voltage and the neutral point current generated by the diode clamped three-level inverter of the back stage are provided in the present application; wherein, 4a is case one of linear segmented relationship between the zero sequence voltage and the neutral point current generated by the diode clamped three-level inverter of the back stage, 4b is case two of linear segmented relationship between the zero sequence voltage and the neutral point current generated by the diode clamped three-level inverter of the back stage, and 4c is case three of linear segmented relationship between the zero sequence voltage and the neutral point current generated by the diode clamped three-level inverter of the back stage;

[0038] Figure 5 The overall control flow chart of the converter cooperative control method is provided in the present application.

[0039] Figure 6 The overall control block diagram of the converter cooperative control method is provided in the present application. DETAILED DESCRIPTION

[0040] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0041] Referring to Figure 1 The present application discloses a kind of energy unidirectional flow three-level back-to-back converter cooperative control method, comprising the following steps:

[0042] S1. analysis converter characteristics, propose Vienna rectifier as front stage, diode clamped three-level inverter as back stage;

[0043] S2. front stage Vienna rectifier adopts reactive current compensation method to suppress current zero point distortion;

[0044] S3. according to the coupling relationship between the zero sequence voltage injected by front stage Vienna rectifier and the zero point distortion of network side current, decoupling is carried out using target neutral point current reconstruction method, and reconstructed target neutral point current is calculated;

[0045] S4. based on the reconstructed target neutral point current calculated, inject zero sequence voltage in the reference voltage of back stage diode clamped three-level inverter, obtain the neutral point current after injecting zero sequence voltage;

[0046] S5. Calculate the current expectation value, determine whether the zero sequence voltage and the midpoint current generated by the latter diode clamped three-level inverter are linearly segmented, if so, calculate the zero sequence voltage required to be injected into the latter diode clamped three-level inverter by interpolation function, if not, select the zero sequence voltage corresponding to the key point of the midpoint current expectation value to be injected into the latter diode clamped three-level inverter, so as to realize the coordinated control of the midpoint potential balance.

[0047] Further, in S2, the reactive current i rq is introduced into the double closed-loop control circuit of the front-stage Vienna rectifier network side, so that the power factor angle δ is equal to the current distortion angle γ, and finally the front-stage Vienna rectifier network side current is synchronized with the three-phase reference voltage, thereby suppressing the current zero-crossing distortion.

[0048] Wherein, the expression of the reactive current i rq is:

[0049]

[0050] Wherein, ω1 is the front-stage rectifier side grid angular frequency, L s is the front-stage rectifier side inductance, is the component of the front-stage rectifier side output voltage in the d-axis.

[0051] Further, in S3, according to the coupling relationship between the zero sequence voltage injected by the front-stage Vienna rectifier and the zero-crossing distortion of the network side current, the target midpoint current reconstruction method is used for decoupling, and the reconstructed target midpoint current expression is calculated as:

[0052]

[0053] Wherein, T c is the carrier period, x is the rectifier side a, b, c phase, u1, u2 are the upper and lower capacitor voltages, C is the common DC bus capacitance value, is the front-stage rectifier side three-phase reference voltage, i x is the front-stage rectifier side actual input three-phase current.

[0054] Further, in S4, the zero sequence voltage is injected into the reference voltage of the latter diode clamped three-level inverter, and the expression of the midpoint current after injecting the zero sequence voltage is:

[0055]

[0056] Wherein, y is the inverter side u, v, w phase, i y is the actual output three-phase current of the latter inverter side, u y * is the three-phase reference voltage of the latter inverter side, u ziThe zero sequence voltage injected to the inverter side of the latter stage.

[0057] Further, the linear segmented relationship between the zero sequence voltage in S5 and the midpoint current generated by the diode clamped three-level inverter of the latter stage has three cases:

[0058] Case one: when v zmin ≤-v3 and -v1≤v zmax ;

[0059] Case two: when -v3<v zmin ≤-v2 and -v2<v zmax ≤-v1;

[0060] Case three: when -v2<v zmin or v zmax <-v2.

[0061] Further, the zero sequence voltage injected to the diode clamped three-level inverter of the latter stage calculated by the interpolation function in S5 is:

[0062]

[0063] wherein, is the expected current value, v zy is the right end current value of the interval where the target midpoint current is located, i ox is the left end current value of the interval where the target midpoint current is located, i oy is the zero sequence voltage value corresponding to the right end current value of the interval where the target midpoint current is located, v zx is the zero sequence voltage value corresponding to the left end current value of the interval where the target midpoint current is located.

[0064] In a specific embodiment, the following contents are included: Figure 1 The topology of the converter is that the former stage is a Vienna structure rectifier and the latter stage is a midpoint clamped three-level inverter. Energy flows from the former stage to the latter stage through a common DC bus and finally flows into a three-phase load, wherein e a , e b , e c are input voltages of the converter, i a , i b , i c and i u , i v , i w are input currents of the converter and output currents of the converter, respectively, L s and L m are inductance values of the reactors of the former stage and the latter stage, and L s =L mC1 and C2 are capacitors on the same DC bus, and C1 = C2. The voltages of the upper and lower capacitors are u1 and u2, respectively. Based on the three-level carrier-based pulse width modulation (CBPWM) modulation method, the three-phase reference voltage on the rectifier side... and the three-phase reference voltage on the inverter side It can be represented as:

[0065]

[0066] Where, m x Indicates the modulation coefficient (0 <m x ≤1), ω x Let x represent the angular frequencies of the rectifier side (x=1) and the inverter side (x=2). The voltage equations for the Vienna rectifier and the three-level inverter in the dq-axis coordinate system are as follows:

[0067]

[0068] Among them, i rd i rq and i id i iq Let e ​​be the components of the current on the rectifier side and the inverter side in the dq coordinate system. rd With e rq Let u be the component of the input voltage of the preceding stage in the dq coordinate system. rd u rq and u id u iq These are the components of the output voltage in the dq coordinate system for the front-stage rectifier side and the rear-stage inverter side, respectively. iod u ioq This represents the component of the input voltage on the load side of the subsequent inverter in the dq coordinate system.

[0069] Vienna rectifiers have inherent forced current commutation characteristics, such as... Figure 2 As shown in Figure 2a, where U Ls Let i be the voltage drop across the inductor of the converter's front-end, i be the current on the grid side of the front-end, and e be the voltage drop across the inductor of the converter's front-end. d The front-end grid-side voltage is u, and the three-phase output voltage is u. rx (x = a, b, c) is the angle relative to the grid-side current lag γ.

[0070] A strategy of using delayed reactive current compensation is employed to eliminate the influence of the lag angle γ on the grid-side three-phase current, thereby suppressing current zero-crossing distortion. For example... Figure 2 As shown in Figure 2b, the introduced reactive current i q The active current i satisfies δ=γ. rd and reactive current i rq satisfy:

[0071]

[0072] wherein,

[0073] is obtained:

[0074] The front-stage rectification side reactive power compensation double closed-loop control block diagram is shown in Figure 3 The current inner loop q-axis current reference value in the voltage and current double closed-loop control structure is changed from zero to rq The solving expression of

[0075] In the back-to-back converter shown in Figure 1 The current flowing out of the bus midpoint is defined as the positive direction, and oref represents the target midpoint current when the midpoint potential balance is achieved, and

[0076] i oref =C·(u2-u1) / T c

[0077] wherein, T c is the carrier period, x is the rectification side a, b, c phase, u1 and u2 are the upper and lower capacitor voltages respectively, and C is the common DC bus capacitor value.

[0078] The bus midpoint current is related to the phase current when the output of each phase is zero, and the actual midpoint average current of each carrier period can be calculated as:

[0079]

[0080] In the formula, x represents the rectification side a, b, c phase, and y represents the inverter side u, v, w phase.

[0081] In the operation process of the converter, the actual bus midpoint average current generated in each carrier period is:

[0082]

[0083] The method of reconstructing the target midpoint current decouples the zero sequence voltage injection and the zero crossing distortion of the front-stage grid side current, and the reconstructed target midpoint current is:

[0084]

[0085] Zero sequence electric u zi is injected at the back-stage inverter side, so that the midpoint current i oi approximates to wherein the expression of i oi is:

[0086]

[0087] From the above formula, we can know i oi with u zi The relationship is piecewise linear, with three segmentation points. Let v1, v2, and v3 be the minimum, intermediate, and maximum values ​​of the three-phase reference voltages. The three segment points for injecting zero-sequence voltage are -v1, -v2, and -v3. To ensure that overmodulation does not occur after injecting zero-sequence voltage on the inverter side, u zi The following constraints apply

[0088]

[0089] u zi with i oi The piecewise linear relationship has Figure 4 The three cases shown are:

[0090] Scenario 1: When v zmin ≤-v3 and -v1≤v zmax At that time; the midpoint current and the injected zero-sequence voltage u zi Relationship such as Figure 4 As shown in Figure 4a;

[0091] Scenario 2: When -v3 <v zmin ≤-v2 and -v2 <v zmax When ≤-v1; Midpoint current and injected zero-sequence voltage u zi Relationship such as Figure 4 As shown in Figure 4b;

[0092] Scenario 3: When -v2 <v zmin or v zmax When <-v2; Midpoint current and injected zero-sequence voltage u zi Relationship such as Figure 4 As shown in 4c.

[0093] When the expected current value is calculated At that time, first judge lie in Figure 4 Which two segment points v in the middle? x v y Then, the optimal zero-sequence voltage u to be injected on the inverter side is calculated using the interpolation function. zi

[0094]

[0095] If the target midpoint current Not belonging to Figure 4If the midpoint voltage is within the range of the segment point, the zero sequence voltage corresponding to the key point close to the midpoint current expectation value is selected to be injected into the later-stage inverter, so as to balance the midpoint potential. The injected zero sequence voltage u zi The revised reference voltage after the reversion is

[0096]

[0097] The control flow is shown in Figure 5 The carrier phase disposition pulse width modulation (PDPWM) is used to output the pulse signal of the switch tube of the rectifier side and the inverter side, so as to control the stable operation of the converter. The overall control block diagram of the collaborative control strategy is shown in Figure 6 .

[0098] The embodiments in the specification are described in a progressive manner, and each embodiment focuses on the difference from other embodiments. The same or similar parts of each embodiment can be referred to each other.

[0099] The above description of the disclosed embodiments enables those skilled in the art to carry out or use the present application. Various modifications to the embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for coordinated control of a three-level back-to-back converter with unidirectional energy flow, characterized in that, Includes the following steps: S1. Analyze the characteristics of the converter and propose to use the Vienna rectifier as the front stage and the diode-clamped three-level inverter as the back stage; S2. The front-end Vienna rectifier uses reactive current compensation to suppress current zero-crossing distortion. S3. Based on the coupling relationship between the zero-sequence voltage injected by the front-end Vienna rectifier and the zero-crossing distortion of the grid-side current, the target midpoint current reconstruction method is used to decouple them and calculate the reconstructed target midpoint current. S4. Based on the calculated reconstructed target midpoint current, inject zero-sequence voltage into the reference voltage of the subsequent diode-clamped three-level inverter to obtain the midpoint current after injecting the zero-sequence voltage. S5. Calculate the expected current value and determine whether the zero-sequence voltage and the midpoint current generated by the subsequent diode-clamped three-level inverter are linearly piecewise related. If so, calculate the zero-sequence voltage to be injected into the subsequent diode-clamped three-level inverter using the interpolation function. If not, select the zero-sequence voltage corresponding to the key point of the expected midpoint current value and inject it into the subsequent diode-clamped three-level inverter to achieve coordinated control of midpoint potential balance. In S2, reactive current is introduced into the double closed-loop control circuit on the grid side of the upstream Vienna rectifier. i rq This makes the power factor angle δ equal to the current distortion angle γ, ultimately synchronizing the grid-side current of the front-end Vienna rectifier with the three-phase reference voltage, thereby suppressing the current zero-crossing distortion. Among them, reactive current i rq The expression is: in, The angular frequency of the grid on the front-end rectifier side. For the front-end rectifier side inductor, The output voltage of the front-stage rectifier side is d The components of the axis; In S3, based on the coupling relationship between the zero-sequence voltage injected by the preceding Vienna rectifier and the zero-crossing distortion of the grid-side current, the target midpoint current reconstruction method is used for decoupling, and the expression for the reconstructed target midpoint current is calculated as follows: in, T c For carrier period, x For the rectifier side, phases a, b, and c are... u 1 , u 2 These are the voltages of the upper and lower capacitors, respectively. C This is the common DC bus capacitance value. This is the three-phase reference voltage on the front-end rectifier side. i x This is the actual three-phase input current to the front-end rectifier side.

2. The method for coordinated control of a three-level back-to-back converter with unidirectional energy flow according to claim 1, characterized in that, In S4, a zero-sequence voltage is injected into the reference voltage of the diode-clamped three-level inverter. The expression for the midpoint current after the zero-sequence voltage is injected is as follows: in, For the inverter side, the u, v, and w phases are... i y This refers to the actual three-phase output current on the inverter side of the subsequent stage. This is the three-phase reference voltage for the inverter side. u zi The zero-sequence voltage injected into the inverter side of the subsequent stage.

3. The method for coordinated control of a three-level back-to-back converter with unidirectional energy flow according to claim 1, characterized in that, There are three possible linear piecewise relationships between the zero-sequence voltage and the midpoint current generated by the subsequent diode-clamped three-level inverter in S5: Scenario 1: When v zmin ≤-v3 and -v1≤v zmax hour; Scenario 2: When -v3 <v zmin ≤-v2 and -v2 <v zmax When ≤-v1; Scenario 3: When -v2 <v zmin or v zmax When <-v2.

4. The method for coordinated control of a three-level back-to-back converter with unidirectional energy flow according to claim 1, characterized in that, The zero-sequence voltage required to be injected into the subsequent diode-clamped three-level inverter is calculated using the interpolation function in S5 as follows: in, The desired current value, v zy The value of the current at the right end of the interval containing the target midpoint current. i ox The value of the current at the left end of the interval containing the target midpoint current. i oy The zero-sequence voltage value corresponding to the right-end current value of the interval containing the target midpoint current. v zx The zero-sequence voltage value is the value corresponding to the current value at the left end of the interval where the target midpoint current is located.

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