MMC rectifier and control method and control system thereof

Abstract

The invention discloses an MMC rectifier and a control method and system thereof. The MMC rectifier comprises a voltage outer loop, a current distributor, a current inner loop, a reverse Park conversion module and a PWM control module. The voltage outer ring is provided with a PI controller, the output end of the PI controller is connected with the input end of a current distributor, the current inner ring is provided with a passive controller, the output end of the current distributor is connected with the input end of the passive controller, and the output end of the passive controller is connected with the input end of a reverse Park conversion module. And the output end of the reverse Park conversion module is connected with the input end of the PWM control module.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, specifically relating to an MMC rectifier and its control method and control system. Background Technology

[0002] Regions rich in clean energy sources such as wind and solar power are often far from load centers, necessitating the establishment of large-capacity, inter-regional power transmission networks. High-voltage direct current (HVDC) transmission, compared to alternating current (AC) transmission, offers advantages such as flexible regulation, stable reactive power angle, and low power loss, making it an ideal transmission method for delivering green energy from clean energy bases to load centers. Modular multilevel converters (MMCs) are receiving increasing attention in the HVDC transmission field due to their superior technical performance, including high voltage levels, low harmonic content, and ease of expansion.

[0003] A suitable control strategy has a significant impact on the performance of MMCs. Typical MMC control employs a PI (proportional-integral) controller-based structure. In a synchronous rotating coordinate system, the PI controller's excellent tracking capability of DC quantities enables control of the MMC. However, due to the influence of circulating currents in the MMC, the PI controller struggles to achieve zero steady-state error regulation. Proportional-resonant (PR) control offers good suppression of harmonics at specific frequencies and can directly control AC quantities, but PR controller design is complex. Both PI and PR controllers are essentially linear controllers, and their parameter selection is based on small-signal stability analysis, making it difficult to provide satisfactory dynamic performance. Furthermore, the complex control structure increases the difficulty of tuning the linear controller parameters. Based on the shortcomings of traditional PI and PR controllers, some researchers have applied model predictive control to MMCs. By designing a suitable value function, multi-objective control can be achieved. However, the value function needs to be calculated in each operating cycle, resulting in excessive computational overhead.

[0004] The essence of passive control is to establish the passivity of the controlled system. By designing a passive controller, the closed-loop system can meet the passive condition and achieve asymptotic stability of the closed-loop system near the equilibrium point. It has been widely used in the field of power conversion.

[0005] Therefore, there is an urgent need for an MMC rectifier and its control method and control system. The MMC rectifier uses passive control to achieve better dynamic performance than traditional PI control. Summary of the Invention

[0006] The purpose of this invention is to overcome the problem of poor dynamic performance of existing MMC rectifiers using PI control, and to propose an MMC rectifier, its control method, and its control system.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] An MMC rectifier control system includes a voltage outer loop, a current distributor, a current inner loop, an inverse Park converter module, and a PWM control module. The voltage outer loop is equipped with a PI controller, the output of which is connected to the input of the current distributor. The current inner loop is equipped with a passive controller, the output of which is connected to the input of the passive controller. The output of the passive controller is connected to the input of the inverse Park converter module, and the output of the inverse Park converter module is connected to the input of the PWM control module.

[0009] An MMC rectifier control method, utilizing the aforementioned MMC rectifier control system, provides a given DC voltage reference value U. * dc The DC voltage reference value U * dc The input PI controller outputs the dq-axis current, which is then distributed by a current distributor to output a reference current. This reference current is input to the passive controller to output a voltage reference value. The voltage reference value is then input to the inverse Park converter and output as the PWM control signal for the MMC submodule of the MMC rectifier.

[0010] Furthermore, the PWM control signal of the MMC submodule of the MMC rectifier is input to the PWM control module to control the PWM conversion of the MMC rectifier.

[0011] Furthermore, the method for constructing a passive controller includes: writing single-phase voltage and current equations based on the single-phase equivalent circuit of the MMC; constructing a mathematical model of the MMC rectifier using the single-phase voltage and current equations; establishing an Euler-Lagarange model of the MMC rectifier based on the mathematical model of the MMC rectifier; and constructing the passive control law of the passive controller of the MMC rectifier based on the expected equilibrium point of the Euler-Lagarange model of the MMC rectifier.

[0012] Furthermore, the single-phase equivalent circuit of the MMC rectifier includes the AC neutral point, the DC neutral point, and the series inductance and equivalent resistance of the upper and lower bridge arms. The single-phase voltage and current equations of the MMC rectifier are obtained using Kirchhoff's laws based on the reference direction, and are shown below:

[0013]

[0014] Where g is the AC reference voltage potential at the neutral point on the AC side, o is the DC reference voltage potential at the neutral point on the DC side, and L s For AC side inductance, L p L n These are the series inductors for the upper and lower bridge arms, R. p R n These are the equivalent resistances of the upper and lower bridge arms, i pj i nj These are the currents flowing through the upper and lower bridge arms, U, respectively. dc For DC system voltage, u sj i j These represent AC voltage and current, respectively.

[0015] Furthermore, the passive control law of the passive controller is shown in the following equation:

[0016]

[0017] in, and For state variable i d and i q Reference value, R h =[R h1 0; 0 R h2 ].

[0018] Furthermore, the current distributor sets the power factor based on the AC system on the wind farm side, as shown in the following formula:

[0019] i * d =cos(θ),

[0020] i * q =sin(θ);

[0021] Among them, i * d Let i be the d-axis current. * q Let θ be the q-axis current and θ be the power factor angle, with a power factor of 1.

[0022] An MMC rectifier utilizes the MMC rectifier control method described above.

[0023] Furthermore, it includes an MMC main circuit, which consists of six bridge arms. Each bridge arm includes several MMC sub-modules, a resistor, and an inductor. Each MMC sub-module consists of a half-bridge rectifier unit connected in parallel with a capacitor. The AC side of the MMC main circuit is connected to one side of the transformer, and the other side of the transformer is connected to the AC bus. The DC side of the MMC main circuit is connected to the load.

[0024] Furthermore, the half-bridge rectifier unit includes a first switch V1 and a second switch V2, and the MMC submodule includes three states: latched state, engaged state, and disengaged state.

[0025] When both the first switch V1 and the second switch V2 are turned off, the MMC submodule is in a locked state.

[0026] When the first switch V1 is turned on and the second switch V2 is turned off, the MMC submodule is in the active state, and the capacitor C is charging or discharging.

[0027] When the first switch V1 is turned off and the second switch V2 is turned on, the MMC submodule is in the cut-off state.

[0028] An MMC rectifier utilizes the MMC rectifier control method described above.

[0029] Compared with the prior art, the present invention has the following beneficial technical effects:

[0030] This invention proposes a control method for an MMC rectifier, which selects the dq-axis component of the current as the state variable, derives the passive control law, and designs a passive current controller based on it. Based on the passive control strategy, it can achieve rapid tracking of current and power under different operating conditions, and has good harmonic characteristics and better dynamic characteristics. Attached Figure Description

[0031] The accompanying drawings are provided to further understand the invention and constitute a part of this invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0032] Figure 1 This is a block diagram of the MMC rectifier control of the present invention.

[0033] Figure 2 This invention relates to the passive controller structure for the MMC rectifier.

[0034] Figure 3 This is the main circuit topology of the three-phase MMC rectifier of the present invention.

[0035] Figure 4 This is the single-phase equivalent circuit of the MMC rectifier of the present invention. Detailed Implementation

[0036] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. 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 should fall within the scope of protection of the present invention.

[0037] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0038] Example 1

[0039] See Figure 1 An MMC rectifier control system includes a voltage outer loop, a current distributor, a current inner loop, an inverse Park converter module, and a PWM control module. The voltage outer loop is equipped with a PI controller, the output of which is connected to the input of the current distributor. The current inner loop is equipped with a passive controller, the output of which is connected to the input of the passive controller. The output of the passive controller is connected to the input of the inverse Park converter module, and the output of the inverse Park converter module is connected to the input of the PWM control module.

[0040] Example 2

[0041] See Figure 1 A method for controlling an MMC rectifier, utilizing an MMC rectifier control system as described in Embodiment 1, provides a given DC voltage reference value U. * dc The DC voltage reference value U * dc The input PI controller outputs the dq-axis current, which is then distributed by a current distributor to output a reference current. This reference current is input to the passive controller to output a voltage reference value. The voltage reference value is then input to the inverse Park converter and output as the PWM control signal for the MMC submodule of the MMC rectifier.

[0042] Preferably, the PWM control signal of the MMC submodule of the MMC rectifier is input to the PWM control module to control the PWM conversion of the MMC rectifier.

[0043] Preferably, the method for constructing the passive controller includes: writing single-phase voltage and current equations based on the single-phase equivalent circuit of the MMC, constructing a mathematical model of the MMC rectifier using the single-phase voltage and current equations, establishing an Euler-Lagarange model of the MMC rectifier based on the mathematical model of the MMC rectifier, and constructing the passive control law of the passive controller of the MMC rectifier based on the expected equilibrium point of the Euler-Lagarange model of the MMC rectifier.

[0044] Preferred, see Figure 4 The single-phase equivalent circuit of the MMC rectifier includes the AC neutral point, the DC neutral point, and the series inductors and equivalent resistances of the upper and lower bridge arms. The single-phase voltage and current equations of the MMC rectifier are obtained using Kirchhoff's laws based on the reference direction. The single-phase voltage and current equations are shown below:

[0045]

[0046] Where g is the AC reference voltage potential at the neutral point on the AC side, o is the DC reference voltage potential at the neutral point on the DC side, and L s For AC side inductance, L p L n These are the series inductors for the upper and lower bridge arms, R. p R n These are the equivalent resistances of the upper and lower bridge arms, i pj i nj These are the currents flowing through the upper and lower bridge arms, U, respectively. dc For DC system voltage, u sj i j These represent AC voltage and current, respectively.

[0047] Preferred, see Figure 2 The passive control law of the passive controller is shown in the following equation:

[0048]

[0049] in, and For state variable i d and i q Reference value, R h =[R h1 0; 0 R h2 ].

[0050] Preferably, the current distributor sets the power factor according to the AC system on the wind farm side, and the power factor setting is shown in the following formula:

[0051] i * d =cos(θ),

[0052] i * q =sin(θ);

[0053] Among them, i * d Let i be the d-axis current. * q Let θ be the q-axis current and θ be the power factor angle, with a power factor of 1.

[0054] Example 3

[0055] See Figure 3 An MMC rectifier, utilizing an MMC rectifier control method as described in Embodiment 1.

[0056] Preferably, it includes an MMC main circuit, which consists of six bridge arms. Each bridge arm includes several MMC sub-modules, a resistor, and an inductor. Each MMC sub-module consists of a half-bridge rectifier unit connected in parallel with a capacitor. The AC side of the MMC main circuit is connected to one side of the transformer, and the other side of the transformer is connected to the AC bus. The DC side of the MMC main circuit is connected to the load.

[0057] Preferably, the half-bridge rectifier unit includes a first switch V1 and a second switch V2, and the MMC submodule includes three states: latched state, engaged state, and disengaged state.

[0058] When both the first switch V1 and the second switch V2 are turned off, the MMC submodule is in a locked state.

[0059] When the first switch V1 is turned on and the second switch V2 is turned off, the MMC submodule is in the active state, and the capacitor C is charging or discharging.

[0060] When the first switch V1 is turned off and the second switch V2 is turned on, the MMC submodule is in the cut-off state.

[0061] Example 4

[0062] An MMC rectifier is provided, utilizing the MMC rectifier control method described in Embodiment 1. The main circuit topology of the MMC rectifier is as follows: Figure 3 As shown, the MMC consists of six bridge arms, each including several sub-modules, a resistor, and an inductor. Each sub-module consists of a half-bridge rectifier unit connected in parallel with a capacitor; U dc The voltage on the DC side of the MMC is given by R0 and L0, which are the resistance and inductance of the bridge arm, respectively. pi (i = a, b, c) represents the output voltage of the upper bridge arm, u ni(i = a, b, c) represents the output voltage of the lower bridge arm, u si (i = a, b, c) represents the AC voltage on the system side, L i (i = a, b, c) represents the system-side inductance, u i (i = a, b, c) represents the AC voltage on the offshore wind farm side, R w and L w For the resistance and inductance on the wind farm side, i pi (i = a, b, c) represents the current in the upper bridge arm, i ni (i = a, b, c) represents the lower bridge arm current, i i (i = a, b, c) represents the AC side current. The submodule has three states: latched, engaged, and disengaged. When both V1 and V2 are off, the submodule is in the latched state; when V1 is on and V2 is off, the submodule is in the engaged state, and capacitor C charges or discharges; when V2 is on and V1 is off, the submodule is in the disengaged state. By controlling the switching devices, the engagement or disengagement of each submodule is achieved, thereby outputting a multi-level modulated three-phase voltage wave on the MMC AC side.

[0063] MMC single-phase equivalent circuit, such as Figure 4 As shown. g is the AC side neutral point (AC voltage reference potential), o is the DC side neutral point (DC voltage reference potential), L s For AC side inductance, L p L n These are the series inductors for the upper and lower bridge arms, R. p R n These are the equivalent resistances of the upper and lower bridge arms, i pj i nj These are the currents flowing through the upper and lower bridge arms, U, respectively. dc For DC system voltage, u sj i j These are AC voltage and current, respectively. According to... Figure 4 Using Kirchhoff's laws, the reference direction shown in the figure can be used to derive the single-phase voltage and current equation as shown in equation (2-1).

[0064]

[0065] To simplify the discussion, assume that the inductance and resistance of the upper and lower bridge arms are equal, i.e.: L p =L n =L0,R p =R n =R0. Rearranging equation (2-1) gives:

[0066]

[0067] Remember u diffj =u pj -unj For differential mode voltage, Equation (2-2) can be rewritten as Equation (2-3), which is the MMC mathematical model (j = a, b, c).

[0068]

[0069] The Euler-Lagarange model of MMC is the foundation for proving the passivity of the system and designing passive controllers. The derivation process of the Euler-Lagarange model of the MMC rectifier is as follows:

[0070] The MMC mathematical model shown in equation (2-3) is rewritten in three-phase form as shown in equation (2-4).

[0071]

[0072] Rewrite equation (2-4) in a synchronously rotating coordinate system, taking u into account. go If the transformed fundamental component is 0, then the equation of equation (2-4) in the synchronous rotating coordinate system can be expressed as equation (2-5), where i d i q These are the dq-axis components of the output current, where ω is the fundamental angular frequency, and u... sd u sq These are the dq-axis components of the AC side voltage, u diffd u diffq These are the dq-axis components of the MMC differential-mode voltage, respectively.

[0073]

[0074] Choose X = (x1 x2) T =(i d i q ) T If the variables are state variables, then equation (2-5) can be rewritten as equation (2-6), which is the EL model of the MMC rectifier, where M = [L 0; 0 L] is a positive definite diagonal matrix, J = [0 –ωL; ωL 0] reflects the internal interconnection structure of the system, K = [R 0; 0 R] reflects the dissipation characteristics of the system, and U = [-u sd -u diffd -u sq -u diffq [ ] represents the energy exchange matrix.

[0075]

[0076] According to passive control theory, when a multi-input output system satisfies the dissipation inequality shown in equation (2-7), the system is strictly passive, where H(x) is the system energy storage function, Q(x) is a positive definite function, and u and y are the system input and output, respectively.

[0077]

[0078] Let the energy function stored in the MMC-HVDC be Equation (2-8).

[0079]

[0080] Differentiating and rearranging equation (2-8), we get:

[0081]

[0082] Substituting equation (2-6) into equation (2-9) and rearranging, we get:

[0083]

[0084] Let y = X and Q(x) = X in equation (10). T RX, then equation (10) can be written as:

[0085]

[0086] Comparing equations (2-11) and (2-7), it can be seen that the MMC system is passive.

[0087] Let the desired equilibrium point of the MMC system be... but and For state variable i d and i q The reference value, the error of the state variable can be expressed as equation (2-12).

[0088] X e =XX * (2-12)

[0089] Substituting equation (2-12) into equation (2-6) yields equation (2-13).

[0090]

[0091] The system's energy error function is Differentiating it yields equation (2-14).

[0092]

[0093] According to equation (13), we can obtain Substituting it into equation (2-14) yields equation (2-15).

[0094]

[0095] To ensure the system can quickly recover to the desired equilibrium point after a disturbance, the energy error should converge to zero as quickly as possible. Therefore, damping is injected to accelerate energy dissipation. The expression for the dissipation term after damping is shown in equation (2-16), where R... h =[R h1 0; 0 R h2 ].

[0096] R d X e = (R+R) h )X e (2-16)

[0097] Equations (2-15) and (2-16) can be combined to obtain equation (2-17).

[0098]

[0099] When the system has no steady-state error, the term on the right side of equation (2-17) equals 0, thus obtaining the passive control law of the system as shown in equation (2-18).

[0100]

[0101] Substituting equation (2-18) into equation (2-15) and rearranging, we obtain equation (2-19).

[0102]

[0103] From equation (2-19), we can see that The passive control law shown in equation (2-18) can effectively accelerate the convergence speed of the error energy function.

[0104] Given the expected value X * Since is a constant and its reciprocal is 0, equation (2-18) can be written in the following form:

[0105]

[0106] Solve for u using equation (2-20) diffd and u diffq :

[0107]

[0108] From equation (2-21), it can be seen that the injected damping R h Voltage coupling quantity ωLi d and ωLiq This linearizes the system's state equations, thereby achieving decoupled control of the system current. The MMC passive controller structure is as follows: Figure 2 As shown.

[0109] For MMC rectifiers, the current reference for the passive controller comes from the outer voltage loop, which uses a traditional PI controller. The MMC rectifier control block diagram based on passive theory is as follows: Figure 1 As shown, where U dc * k is the reference value for DC voltage. p and k i These are the parameters for the outer loop PI controller. (From...) Figure 1 It can be seen that the DC voltage, after passing through the outer voltage loop PI controller, outputs a dq-axis current. This dq-axis current, after being distributed, serves as the reference current for the inner current loop. This reference current is then output by the passive controller, and after inverse Park transform, the PWM control signals for each submodule are obtained. It is important to note that... Figure 1 The dq-axis current distributor shown is set according to the required power factor of the AC system on the wind farm side (when power factor control is required): i * d =cos(θ), i * q =sin(θ), or define the reference value of the q-axis current, where θ is the power factor angle. Usually, the power factor is taken as 1 for economic reasons, and the power generated by the wind farm is sent out as active power.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit its scope of protection. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that after reading the present invention, they can still make various changes, modifications or equivalent substitutions to the specific implementation of the invention, but these changes, modifications or equivalent substitutions are all within the scope of protection of the pending claims of the invention.

Claims

1. An MMC rectifier control system, characterized in that, It includes an outer voltage loop, a current distributor, an inner current loop, an inverse Park converter module, and a PWM control module. The outer voltage loop is equipped with a PI controller, the output of which is connected to the input of the current distributor. The inner current loop is equipped with a passive controller, the output of which is connected to the input of the passive controller. The output of the passive controller is connected to the input of the inverse Park converter module, and the output of the inverse Park converter module is connected to the input of the PWM control module.

2. A method for controlling an MMC rectifier, utilizing the MMC rectifier control system described in claim 1, characterized in that, Given DC voltage reference value U * dc The DC voltage reference value U * dc The input PI controller outputs the dq-axis current, which is then distributed by a current distributor to output a reference current. This reference current is input to the passive controller to output a voltage reference value. The voltage reference value is then input to the inverse Park converter and output as the PWM control signal for the MMC submodule of the MMC rectifier.

3. The MMC rectifier control method according to claim 2, characterized in that, The PWM control signal of the MMC submodule of the MMC rectifier is input to the PWM control module to control the PWM conversion of the MMC rectifier.

4. The MMC rectifier control method according to claim 2, characterized in that, The method for constructing a passive controller includes: writing single-phase voltage and current equations based on the single-phase equivalent circuit of the MMC; constructing a mathematical model of the MMC rectifier using the single-phase voltage and current equations; establishing an Euler-Lagarange model of the MMC rectifier based on the mathematical model of the MMC rectifier; and constructing the passive control law of the passive controller of the MMC rectifier based on the expected equilibrium point of the Euler-Lagarange model of the MMC rectifier.

5. The MMC rectifier control method according to claim 4, characterized in that, The single-phase equivalent circuit of the MMC rectifier includes the AC neutral point, the DC neutral point, and the series inductors and equivalent resistances of the upper and lower bridge arms. The single-phase voltage and current equations of the MMC rectifier are obtained using Kirchhoff's laws based on the reference direction. The single-phase voltage and current equations are shown below: Where g is the AC reference voltage potential at the neutral point on the AC side, o is the DC reference voltage potential at the neutral point on the DC side, and L s For AC side inductance, L p L n These are the series inductors for the upper and lower bridge arms, R. p R n These are the equivalent resistances of the upper and lower bridge arms, i pj i nj These are the currents flowing through the upper and lower bridge arms, U, respectively. dc For DC system voltage, u sj i j These represent AC voltage and current, respectively.

6. The MMC rectifier control method according to claim 1, characterized in that, The passive control law of the passive controller is shown in the following equation: in, and For state variable i d and i q Reference value, R h =[R h1 0; 0 R h2 ].

7. The MMC rectifier control method according to claim 1, characterized in that, The current distributor sets the power factor based on the AC system on the wind farm side, as shown in the following formula: I * d =cos(θ), I * q =sin(θ); Among them, i * d Let i be the d-axis current. * q Let θ be the q-axis current and θ be the power factor angle, with a power factor of 1.

8. An MMC rectifier, characterized in that, The method for controlling an MMC rectifier as described in any one of claims 2-7.

9. An MMC rectifier according to claim 8, characterized in that, It includes the MMC main circuit, which consists of six bridge arms. Each bridge arm includes several MMC sub-modules, a resistor and an inductor. Each MMC sub-module consists of a half-bridge rectifier unit connected in parallel with a capacitor. The AC side of the MMC main circuit is connected to one side of the transformer, and the other side of the transformer is connected to the AC bus. The DC side of the MMC main circuit is connected to the load.

10. An MMC rectifier according to claim 9, characterized in that, The half-bridge rectifier unit includes a first switch V1 and a second switch V2. The MMC submodule includes three states: latched state, engaged state, and disengaged state. When both the first switch V1 and the second switch V2 are turned off, the MMC submodule is in a locked state. When the first switch V1 is turned on and the second switch V2 is turned off, the MMC submodule is in the active state, and the capacitor C is charging or discharging. When the first switch V1 is turned off and the second switch V2 is turned on, the MMC submodule is in the cut-off state.

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