Control method for carrying out auxiliary frequency modulation by using energy of MMC (Modular Multilevel Converter) sub-module

Abstract

The invention discloses a control method for carrying out auxiliary frequency modulation by using MMC sub-module energy, and the method is based on the control of the energy storage level of an MMC sub-module, so that an MMC can provide virtual inertia for a system to reduce the grid-side frequency fluctuation when the output of a wind power plant fluctuates or the load of a user side fluctuates. According to the method, an MMC sub-module energy controller with frequency supporting capacity is designed by utilizing the energy storage characteristic of an MMC sub-module, and controller parameters are calculated and determined to be used for controlling a three-phase converter node to have a passive small gain characteristic; the method is simple and visual, parameters are easy to design, and the MMC can provide certain virtual inertia for the system, so that participation in power grid frequency adjustment is assisted, the stability and reliability of a high wind power permeability system are improved, and the method is suitable for a frequency modulation scene in a high-proportion wind power delivery grid-connected system.

Description

Technical Field

[0001] The present invention belongs to the technical field of wind power system control, and in particular relates to a control method for auxiliary frequency modulation using MMC submodule energy. Background Art

[0002] With the increase in wind power penetration, how to provide efficient and fast frequency support in wind farms (especially large wind farms) is a major problem. Since wind power generation has considerable uncertainty, when the wind force and wind direction change, the wind turbine output changes, and the system will experience frequency fluctuations; and the lack of rotating reserve and hot reserve in wind farms leads to low system inertia and weak ability to stabilize and adjust frequency.

[0003] To solve this problem, a common solution is to use virtual inertia control in the wind power transmission link to provide additional inertia support to the power grid and assist in power grid frequency regulation. The existing virtual inertia control idea is generally to control the DC side voltage V dc , thereby adjusting the energy storage of the DC-Link capacitor mounted on the DC bus to provide virtual inertia, such as references [W.Wu et al.,"AVirtual Inertia Control Strategy for DC Microgrids AnalogizedWith Virtual Synchronous Machines,"in IEEE Transactions on IndustrialElectronics,vol.64,no.7,pp.6005-6016,July 2017] and references [J.Li,B.Wen andH.Wang,"Adaptive Virtual Inertia Control Strategy of VSG for Micro-Grid Basedon Improved Bang-Bang Control Strategy,"in IEEE Access,vol.7,pp.39509-39514,2019]; the limitation of this method is that it will cause fluctuations in the DC bus voltage, which is not suitable for scenarios with strict requirements on the stability of the DC bus voltage. In addition, the DC-Link capacitor capacity is not large, and the inertia support it can provide is also relatively limited.

[0004] In recent years, with the development of flexible transmission technology, modular multilevel converters (MMC) have been widely used in medium and long-distance wind power transmission scenarios. Figure 1As shown in the figure, the MMC topology is similar to the traditional VSC, but its upper and lower bridge arms use cascaded submodules (SM) instead of single IGBTs, which greatly reduces the harmonic content of the AC side voltage. Considering the energy storage characteristics of the MMC submodule capacitor, we hope to use the energy of the MMC internal submodule to provide additional inertia for the system, thereby providing certain frequency support and frequency regulation capabilities for the power grid. Summary of the invention

[0005] In view of the above, the present invention provides a control method for auxiliary frequency regulation using the energy of MMC submodules. The method is based on the control of the energy storage level of the MMC submodules, so that when the output of the wind farm fluctuates or the load on the user side fluctuates, the MMC can provide virtual inertia to the system to reduce the frequency fluctuation on the grid side.

[0006] A control method for auxiliary frequency modulation using MMC submodule energy is designed for MMC-HVDC (high voltage direct current transmission) grid-connected system, and an MMC submodule energy controller with frequency support capability is designed, and the total energy storage reference value of the MMC submodule of the controller is calculated and determined. The MMC submodule energy controller is used to provide a three-phase circulating current reference value for the MMC circulating current controller, and then the MMC is controlled by the MMC circulating current controller, so that the MMC can absorb part of the redundant power caused by load disconnection to reduce grid frequency fluctuations.

[0007] Furthermore, the specific implementation of the MMC submodule energy controller is as follows:

[0008] (1) Calculate and obtain the energy storage difference W between the a-phase and b-phase bridge arm submodules of the MMC a-b , the energy storage difference between the a-phase and c-phase bridge arm submodules W a-c , the AC power difference between phase a and phase b P a-b , the AC power difference between phase a and phase c P a-c 、MMC submodule total energy storage W MMC And the total output power P of the MMC AC side g ;

[0009] (2) 0-W a-b , 0-W a-c as well as The results are passed through the notch filter and PI controller in turn to obtain the corresponding MMC phase a and phase b AC power increment ΔP a-b , AC power increment ΔP between phase a and phase c a-c And the total power increment ΔP of the MMC AC side output g ;

[0010] (3) P g +ΔP g, P a-b +ΔP a-b and P a-c +ΔP a-c The result forms the power vector P and is then combined with the system matrix M and the Clarke inverse transformation matrix C -1 Multiplication is C -1 MP, and obtain the three-phase circulating current reference value of MMC.

[0011] Furthermore, the transfer function N(s) of the notch filter is expressed as follows:

[0012]

[0013] Where: n is the center frequency, Q is the quality factor, and s is the Laplace operator.

[0014] Furthermore, the total energy storage reference value of the MMC submodule is The calculation expression is as follows:

[0015]

[0016] in: It indicates the maximum virtual inertia provided by MMC for the system, Δf indicates the frequency change, that is, Δf=f-f0, f is the actual grid frequency, f0 is the rated grid frequency, W MMC0 is the total energy storage rating of the MMC submodule.

[0017] Furthermore, the maximum virtual inertia The calculation expression is as follows:

[0018]

[0019] in: Indicates the upper limit of the total energy storage capacity of the MMC submodule, Δf max Indicates the maximum limit allowed for grid frequency fluctuation.

[0020] Furthermore, the expression of the system matrix M is as follows:

[0021]

[0022] Where: V dc is the DC bus voltage of the MMC.

[0023] Preferably, in order to filter out small frequency fluctuations to avoid frequent operation of the MMC submodule energy controller, a suitable frequency fluctuation start threshold Δf is set. min, the grid frequency f is monitored in real time through the Phase Measurement Unit (PMU). If the frequency change Δf is in the interval [-Δf min , Δf min ], the MMC submodule energy controller does not work; if the frequency change Δf exceeds the interval [-Δf min , Δf min ], then the MMC submodule energy controller is started, Δf = f-f0, f0 is the rated frequency of the power grid.

[0024] A computer device comprises a memory and a processor, wherein the memory stores a computer program, and the processor is used to execute the computer program to implement the control method for auxiliary frequency modulation using MMC submodule energy.

[0025] A computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the control method for auxiliary frequency modulation using MMC submodule energy is implemented.

[0026] The method of the present invention utilizes the energy storage characteristics of the MMC submodule to design an MMC submodule energy controller with frequency support capability, and calculates and determines controller parameters for controlling a three-phase converter node to have a passive small gain characteristic; the method is concise and intuitive, the parameters are easy to design, and the MMC can provide a certain virtual inertia to the system, thereby assisting in the regulation of the grid frequency, improving the stability and reliability of the high wind power penetration system, and is suitable for frequency regulation scenarios in a high-proportion wind power transmission grid-connected system. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram of the topological structure of a modular multilevel converter (MMC).

[0028] Figure 2 Schematic diagram of the classic MMC sub-module energy controller structure.

[0029] Figure 3 It is a schematic diagram of the structure of the MMC submodule energy controller with frequency support capability of the present invention.

[0030] Figure 4 Schematic diagram of the wind power grid-connected system structure based on back-to-back MMC-HVDC.

[0031] Figure 5 It is a schematic diagram of the system frequency change curve when the control method of the present invention is adopted and when the control method of the present invention is not adopted.

[0032] Figure 6 It is a schematic diagram of the internal energy change curve of the MMC when the control method of the present invention is adopted and when the control method of the present invention is not adopted. DETAILED DESCRIPTION

[0033] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0034] The traditional synchronous generator system provides inertia support for the power grid through the rotational inertia of the turbine and generator shaft system. Its physical expression is:

[0035]

[0036] Where: H is the generator inertia time constant, P M and P E are the mechanical power input through the generator shaft system and the electromagnetic power output by the generator, and f is the grid frequency. M , P E When unbalanced, it will cause the frequency f to fluctuate.

[0037] In the wind power transmission grid-connected system, according to the power conservation law, P E It can be expressed as follows:

[0038]

[0039] Where: W MMC is the total energy storage of the sub-module capacitors within the MMC, P L and P W They are grid-side load power and wind farm output power respectively.

[0040] Substituting into equation (1) and rearranging it, we can get:

[0041]

[0042] From formula (3), it can be seen that the change of the energy of the sub-module inside the MMC also contributes to the grid frequency fluctuation. Then, following the form of formula (1), let the virtual inertia provided by the MMC for the system be H MMC , and then we have:

[0043]

[0044] At this time, formula (3) can be rewritten as:

[0045]

[0046] Formula (5) reflects that MMC can provide additional inertia support for the system by adjusting the energy storage level of the submodule. By integrating and sorting both sides of formula (4), we can get:

[0047] W MMC =2(f-f0)H MMC +WMMC0 (6)

[0048] It can be seen that the energy storage capacity of the corresponding MMC submodule can be calculated through the frequency fluctuation value and the virtual inertia value provided by the MMC.

[0049] Literature [Eduardo Prieto-Araujo, et al., Control design of Modular Multilevel Converters in normal and AC fault conditions for HVDC grids [J], Electric Power Systems Research, Volume 152, 2017, Pages 424-437] pointed out that by adjusting the DC component in the circulating current of the MMC bridge arm, the total energy inside the MMC and the energy balance of the sub-modules between the three-phase bridge arms can be controlled. Based on this concept, a classic MMC internal energy controller was designed, such as Figure 2 As shown, is the energy reference value of the MMC submodule, W a-b ,W a-c are the energy storage differences between the a-phase and b-phase bridge arms and the a-phase and c-phase bridge arms of the MMC, respectively. g ,P a-b ,P a-c They are the total output power of the MMC AC side, the AC power difference between phase a and phase b, and the AC power difference between phase a and phase c. It is the control instruction of the DC component of the three-phase circulating current of the MMC; the controller can control the MMC energy to a given reference value.

[0050] The present invention improves the traditional MMC submodule energy controller, that is, based on formula (6), the frequency change Δf and the MMC virtual inertia H MMC Obtain the corresponding MMC submodule energy reference value Based on this, an MMC submodule energy controller with frequency support capability is built, such as Figure 3 shown.

[0051] Then, the dead zone of the MMC submodule energy controller is set to filter out small frequency fluctuations to avoid too frequent controller actions. The present invention selects a suitable start threshold Δf min The system frequency is monitored in real time through a phase measurement unit (PMU). When the frequency fluctuation Δf is within the range [-Δf min ,Δf min], the controller will not work, and it will be started only when it detects that Δf exceeds this range.

[0052] Finally, the present invention calculates and determines the parameters of the MMC frequency support energy controller and designs appropriate controller parameters to obtain the best frequency support effect and ensure that the frequency fluctuation does not exceed the limit, as follows:

[0053] By rearranging the terms in formula (6), we can obtain:

[0054]

[0055] It can be seen that when the frequency fluctuates, the larger ΔW MMC It can provide greater inertia, thus having a better frequency support effect. That is, the upper limit of the energy storage capacity of the MMC submodule The upper limit of the submodule capacitor voltage is The constraints of the grid frequency fluctuation are combined with the maximum allowable range of Δf max For MMC virtual inertia H MMC Design so that when the frequency fluctuation is the largest, the MMC can provide the system with the largest virtual inertia support, that is,

[0056]

[0057] And design W MMC The control law is:

[0058]

[0059] Therefore, MMC can assist the system in frequency stability regulation to the greatest extent, and can autonomously exit the virtual inertia control mode and return to the original working point after the frequency is restored.

[0060] Verification Example

[0061] To further verify the reliability and feasibility of the present invention, Figure 4 As shown, we establish an MMC-HVDC wind power transmission grid-connected system, and the DFIG wind farm transmits wind energy to the synchronous machine network through back-to-back MMC-HVDC.

[0062] At t=30s, a load with a power of 8.0MW is disconnected from the grid, resulting in grid-side frequency fluctuations. In this scenario, simulation experiments are carried out on the MMC-HVDC system using the control method of the present invention and the MMC-HVDC system not using the control method of the present invention, and the system frequency change curves and MMC internal energy change curves under the two conditions are obtained, as shown in FIG. Figure 5 and Figure 6 shown.

[0063] It can be seen that when the control method of the present invention is adopted, the MMC absorbs part of the redundant power caused by the load being disconnected from the grid, effectively reduces the frequency fluctuation, and keeps the system frequency within the prescribed range of 60±0.5 Hz.

[0064] The above description of the embodiments is to facilitate the understanding and application of the present invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments. Improvements and modifications made by those skilled in the art to the present invention based on the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A control method for auxiliary frequency modulation using MMC submodule energy, characterized in that: For the MMC-HVDC grid-connected system, an MMC submodule energy controller with frequency support capability is designed, and the total energy storage reference value of the MMC submodule of the controller is calculated and determined. The MMC submodule energy controller is used to provide a three-phase circulating current reference value for the MMC circulating current controller, and then the MMC is controlled by the MMC circulating current controller, so that the MMC can absorb part of the redundant power caused by load disconnection to reduce grid frequency fluctuations.

2. The control method for auxiliary frequency modulation using MMC submodule energy according to claim 1, characterized in that: The specific implementation of the MMC submodule energy controller is as follows: (1) Calculate and obtain the energy storage difference W between the a-phase and b-phase bridge arm submodules of the MMC a-b , the energy storage difference between the a-phase and c-phase bridge arm submodules W a-c , the AC power difference between phase a and phase b P a-b , the AC power difference between phase a and phase c P a-c 、MMC submodule total energy storage W MMC And the total output power P of the MMC AC side g ; (2) 0-W a-b , 0-W a-c as well as The results are passed through the notch filter and PI controller in turn to obtain the corresponding MMC phase a and phase b AC power increment ΔP a-b , AC power increment ΔP between phase a and phase c a-c And the total power increment ΔP of the MMC AC side output g ; (3) P g +ΔP g , P a-b +ΔP a-b and P a-c +ΔP a-c The result forms the power vector P and is then combined with the system matrix M and the Clarke inverse transformation matrix C -1 Multiplication is C -1 MP, and obtain the three-phase circulating current reference value of MMC.

3. The control method for auxiliary frequency modulation using MMC submodule energy according to claim 2, characterized in that: The transfer function N(s) of the notch filter is expressed as follows: Where: n is the center frequency, Q is the quality factor, and s is the Laplace operator.

4. The control method for auxiliary frequency modulation using MMC submodule energy according to claim 2, characterized in that: The total energy storage reference value of the MMC submodule The calculation expression is as follows: in: It indicates the maximum virtual inertia provided by MMC for the system, Δf indicates the frequency change, that is, Δf=f-f0, f is the actual grid frequency, f0 is the rated grid frequency, W MMC0 is the total energy storage rating of the MMC submodule.

5. The control method for auxiliary frequency modulation using MMC submodule energy according to claim 4, characterized in that: The maximum virtual inertia The calculation expression is as follows: in: Indicates the upper limit of the total energy storage capacity of the MMC submodule, Δf max Indicates the maximum limit allowed for grid frequency fluctuation.

6. The control method for auxiliary frequency modulation using MMC submodule energy according to claim 2, characterized in that: The expression of the system matrix M is as follows: Where: V dc is the DC bus voltage of the MMC.

7. The control method for auxiliary frequency modulation using MMC submodule energy according to claim 1, characterized in that: In order to filter out small frequency fluctuations and avoid frequent operation of the MMC submodule energy controller, a suitable frequency fluctuation start threshold Δf is set. min , the grid frequency f is monitored in real time through the phasor measurement unit. If the frequency change Δf is in the interval [-Δf min , Δf min ], the MMC submodule energy controller does not work; if the frequency change Δf exceeds the interval [-Δf min , Δf min ], then the MMC submodule energy controller is started, Δf = f-f0, f0 is the rated frequency of the power grid.

8. A computer device comprising a memory and a processor, characterized in that: The memory stores a computer program, and the processor is used to execute the computer program to implement the control method for auxiliary frequency modulation using MMC submodule energy as claimed in any one of claims 1 to 7.

9. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by the processor, the control method for auxiliary frequency modulation using MMC submodule energy is implemented as claimed in any one of claims 1 to 7.

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