Method for improving dynamic performance of multi-terminal direct current system and related product

Abstract

The invention discloses a method for improving the dynamic performance of a multi-terminal direct current system and related products, relates to the technical field of direct current power transmission, and solves the problems that the stability, feasibility, economy and implementation difficulty of the system are affected by four inertia improving methods including an existing parameter optimization method, a current / power feed-forward method, an energy storage auxiliary method and an electrical spring method respectively. According to the technical scheme, the method is characterized in that an inertia control method of an extra degree of freedom acting on an MMC modulation part is superposed with an inertia control method of a virtual energy storage element acting on an outer loop power control part, the inertia time constant of a direct-current system is improved, and the dynamic performance is optimized while the steady-state value of the system is not changed.

Description

Technical Field

[0001] The present invention relates to the field of direct current transmission technology, and more specifically, to a method for improving the dynamic performance of a multi-terminal direct current system and related products. Background Art

[0002] The multi-terminal direct current transmission technology based on modular multilevel converter (MMC) has the advantages of low loss, flexible and controllable power, and the ability to receive power at multiple points over long distances. It has good application prospects in large-scale clean energy grid connection and transmission.

[0003] However, the inertia of the DC system is low. Any disturbance in the DC voltage or DC current of the converter will cause large fluctuations in the DC system power flow and voltage, thereby deteriorating the dynamic performance of the DC system, and even causing oscillation and instability of the DC system in severe cases. On the one hand, the power generation of new energy represented by wind and solar energy is volatile, and the inertia of the DC system is low, so the DC voltage of the entire multi-terminal system may fluctuate greatly. On the other hand, with the increase in the number of constant power inverter stations, their equivalent constant power load characteristics will weaken the damping of the DC system, and the multi-terminal DC system under disturbance may excite high-frequency or low-frequency power oscillations.

[0004] There are four existing methods to increase inertia: parameter optimization method, current / power feedforward method, energy storage auxiliary method, and electrical spring method, but they will affect the system stability, feasibility, economy, and difficulty of implementation. Therefore, more effective measures need to be taken to improve the dynamic performance of the DC system.

[0005] Therefore, the present application provides a method for improving the dynamic performance of a multi-terminal DC system and related products to solve the above problems. Summary of the invention

[0006] The purpose of this application is to provide a method for improving the dynamic performance of a multi-terminal DC system and related products, to solve the four existing methods of improving inertia, namely parameter optimization method, current / power feedforward method, energy storage auxiliary method, and electrical spring method, which will affect the system stability, feasibility, economy and implementation difficulty respectively; this application takes more effective measures to improve the dynamic performance of the DC system, and uses the inertia control method of the additional degree of freedom acting on the MMC modulation part, which is superimposed with the inertia control method of the virtual energy storage element acting on the outer loop power control part, to improve the inertia time constant of the DC system, optimize the dynamic performance while not changing the steady-state value of the system.

[0007] The present application first provides a method for improving the dynamic performance of a multi-terminal DC system, including: S1, obtaining the grid voltage amplitude and phase and valve-side current of the MMC grid-connected point in the DC system, and performing dq decomposition on the grid voltage and valve-side current of the MMC grid-connected point, respectively obtaining the d-axis and q-axis components of the grid-connected point voltage and valve-side current in the dq rotating coordinate system, obtaining the actual values ​​of the active power, reactive power, DC voltage and grid-connected point frequency of the MMC, and obtaining the reference values ​​of the active power, reactive power and DC voltage of the MMC; S2, calculating the value of the virtual capacitor and inductor in the outer loop power control according to the value of the virtual capacitor and inductor added. Reference value of active power or DC voltage; S3, calculate the phase reference value of the DC system converter; S4, calculate the d-axis current reference value and q-axis current reference value on the MMC valve side; S5, calculate the differential mode and common mode voltages in the dq coordinate system of the MMC, convert them into their values ​​on the a-axis, b-axis and c-axis in the abc stationary coordinate system, and calculate the instantaneous value of the modulation wave of the upper and lower bridge arms of the MMC; S6, calculate the instantaneous value of the sub-modules that need to be put into operation in the upper and lower bridge arms of the MMC; S7, based on the instantaneous value of the sub-modules that need to be put into operation, use the nearest level approximation modulation theory to generate corresponding control pulses to realize the control of the DC system.

[0008] In a possible implementation, S2, calculates the reference value of active power or DC voltage in the outer loop power control according to the value of the added virtual capacitor inductor; including: for the virtual capacitor, when the MMC converter operates in a fixed active power control or droop control state, the reference value of active power in the outer loop power control is calculated according to the reference value of active power in the stable state, the virtual capacitor value, and the difference between the actual value of the DC voltage and the reference value; when the MMC converter operates in a fixed DC voltage control state, the reference value of the DC voltage in the outer loop power control is calculated according to the reference value of the DC voltage in the stable state, the difference between the actual value of active power and the reference value, the virtual capacitor value and the Laplace operator.

[0009] In one possible implementation, S3, calculating the phase reference value of the DC system converter; including: calculating the phase reference value of the DC system converter based on the voltage reference value of the q-axis in the phase-locked loop, the actual voltage value of the q-axis in the phase-locked loop, the proportional parameter and integral parameter of the PI controller in the phase-locked loop, the rated power frequency angular frequency and the Laplace operator.

[0010] In a possible implementation, S4, calculating the d-axis current reference value and the q-axis current reference value on the MMC valve side; including: when the MMC converter operates in a fixed active power control or droop control state, the d-axis current reference value and the q-axis current reference value are calculated according to the active power reference value in the outer loop power control, the actual value of the AC active power in the MMC outer loop power control, the proportional parameter and the integral parameter of the active power PI controller, the Laplace operator and the d-axis AC voltage; when the MMC converter operates in a fixed DC voltage control state, the d-axis current reference value and the q-axis current reference value are calculated according to the DC voltage reference value in the outer loop power control, the actual value of the DC voltage in the MMC outer loop power control, the proportional parameter and the integral parameter of the active power PI controller, and the Laplace operator.

[0011] In a possible implementation, S5, calculate the differential mode and common mode voltages in the dq coordinate system of the MMC, convert them into the values ​​on the a-axis, b-axis and c-axis in the abc stationary coordinate system, and calculate the instantaneous value of the modulation wave of the upper and lower bridge arms of the MMC; including: according to the dq axis AC voltage, the actual value of the grid connection point voltage frequency, the inductance of the AC side, the bridge arm inductance, the actual value and reference value of the positive sequence current on the MMC valve side in the dq coordinate system, the proportional parameter and integral parameter of the PI controller corresponding to the dq axis based on the DDSRF technology, and the Laplace operator, calculate the MMC C; calculate the common-mode voltage in the dq coordinate system of MMC according to the actual value of the voltage frequency at the grid connection point, the inductance of the bridge arm, the actual value and reference value of the internal circulating current in the dq coordinate system of MMC, the proportional parameter and integral parameter of the dq-axis internal circulating current suppression controller, and the Laplace operator; convert the differential-mode voltage and common-mode voltage of the d-axis and q-axis to the abc stationary coordinate system, and calculate the instantaneous value of the modulation wave of the upper and lower bridge arms of MMC according to the differential-mode voltage in the abc stationary coordinate system of MMC and the common-mode voltage in the abc stationary coordinate system of MMC.

[0012] In one possible implementation, S6, calculates the instantaneous values ​​of the sub-modules that need to be put into operation in the upper and lower bridge arms of the MMC; including: regarding the number of sub-modules put into operation in each phase as an additional control degree of freedom of the MMC, and setting a function of the DC disturbance based on the additional control degree of freedom; according to the number of sub-modules put into operation in each phase, the reference value of the DC voltage in the stable state, the instantaneous value of the modulation wave of the upper and lower bridge arms of each phase, and the actual value of the DC voltage in the outer loop power control, calculates the instantaneous values ​​of the sub-modules that need to be put into operation in the upper and lower bridge arms under the MMC additional degree of freedom inertia control strategy.

[0013] In one possible implementation, S7, based on the instantaneous value of the sub-module to be put into operation, utilizes the nearest level approximation modulation theory to generate corresponding control pulses to realize DC system control, thereby achieving the effect of improving dynamic performance during disturbances; including: generating control pulses corresponding to each IGBT of the converter based on the instantaneous value of the sub-module to be put into operation in the upper and lower bridge arms, and controlling the switching of the sub-modules through the control pulses.

[0014] The present application provides a computer device, which includes: a memory and a processor, wherein the processor is used to execute a computer program stored in the memory, so that the computer device executes a method for improving the dynamic performance of a multi-terminal DC system as described above.

[0015] The present application provides a computer storage medium, characterized in that it includes computer instructions. When the computer instructions are executed on a computer device, the computer device executes the above-mentioned method for improving the dynamic performance of a multi-terminal DC system.

[0016] The present application provides a computer program product, characterized in that when the computer program product is run on a computer device, the computer device is enabled to execute the above-mentioned method for improving the dynamic performance of a multi-terminal DC system.

[0017] Compared with the prior art, the present application has the following beneficial effects: the present application can introduce virtual energy storage elements in the outer-loop power control of the MMC control strategy of the DC system. At the same time, in the modulation part, N is set as an additional control degree of freedom, which can greatly improve the inertia time constant of the DC system and improve the dynamic performance; the control method of the present application is suitable for various fluctuations in indicators such as active power or DC voltage in the DC system, and has low implementation difficulty and high feasibility. Through theoretical analysis and simulation examples, it is verified that the DC system using the control method of the present invention can improve the degree of disturbance suppression and has no effect on stability when the system disturbance ends and returns to a steady state. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0019] Figure 1 is a flow chart of a method for improving dynamic performance of a multi-terminal DC system provided in an embodiment of the present application;

[0020] Figure 2 It is a control block diagram of a method for improving dynamic performance of a multi-terminal DC system;

[0021] Figure 3 This is a simulation comparison diagram when the present application is used to smooth voltage fluctuations;

[0022] Figure 4 This is a simulation comparison diagram when the present application is used to smooth power fluctuations;

[0023] Figure 5 It is a simulation diagram to verify that the virtual energy storage element does not affect the steady state of the system;

[0024] Figure 6 It is a structural diagram of a computer device provided in an embodiment of the present application. DETAILED DESCRIPTION

[0025] Hereinafter, the term "include" or "may include" used in various embodiments of the present application indicates the presence of the function, operation or element applied for, and does not limit the increase of one or more functions, operations or elements. In addition, as used in various embodiments of the present application, the terms "include", "have" and their cognates are intended only to indicate specific features, numbers, steps, operations, elements, components or a combination of the foregoing items, and should not be understood as first excluding the presence of one or more other features, numbers, steps, operations, elements, components or a combination of the foregoing items or the possibility of increasing one or more features, numbers, steps, operations, elements, components or a combination of the foregoing items.

[0026] In various embodiments of the present application, the expression "or" or "at least one of B or / and C" includes any combination or all combinations of the words listed at the same time. For example, the expression "B or C" or "at least one of B or / and C" may include B, may include C, or may include both B and C.

[0027] The terms used in the various embodiments of the application are only used to describe the purpose of specific embodiments and are not intended to limit the various embodiments of the application. As used herein, the singular form is intended to also include the plural form, unless the context clearly indicates otherwise. Unless otherwise limited, all terms used here (including technical terms and scientific terms) have the same meaning as the meanings commonly understood by ordinary technicians in the field of the various embodiments of the application. Terms (such as terms defined in generally used dictionaries) will be interpreted as having the same meaning as the contextual meaning in the relevant technical field and will not be interpreted as having idealized meanings or too formal meanings, unless clearly defined in the various embodiments of the application.

[0028] In order to make the objectives, technical solutions and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with examples and drawings. The illustrative implementation scheme of the present application and its description are only used to explain the present application and are not intended to limit the present application.

[0029] See also Figure 1 and Figure 2 As shown, Figure 1A flowchart of a method for improving dynamic performance of a multi-terminal DC system provided in an embodiment of the present application, Figure 2 It is a control block diagram of a method for improving the dynamic performance of a multi-terminal DC system. The method includes: S1, obtaining the grid voltage amplitude and phase and valve-side current of the MMC grid-connected point in the DC system, and performing dq decomposition on the grid voltage and valve-side current of the MMC grid-connected point, respectively obtaining the d-axis and q-axis components of the grid-connected point voltage and valve-side current in the dq rotating coordinate system, obtaining the actual values ​​of the active power, reactive power, DC voltage and grid-connected point frequency of the MMC, and obtaining the reference values ​​of the active power, reactive power and DC voltage of the MMC; S2, calculating the parameters of the active power or DC voltage in the outer loop power control according to the values ​​of the added virtual capacitors and inductors. S3, calculate the phase reference value of the DC system converter; S4, calculate the d-axis current reference value and q-axis current reference value on the MMC valve side; S5, calculate the differential mode and common mode voltages in the dq coordinate system of the MMC, convert them into their values ​​on the a-axis, b-axis and c-axis in the abc stationary coordinate system, and calculate the instantaneous values ​​of the modulation waves of the upper and lower bridge arms of the MMC; S6, calculate the instantaneous values ​​of the sub-modules that need to be put into operation in the upper and lower bridge arms of the MMC; S7, based on the instantaneous values ​​of the sub-modules that need to be put into operation, use the nearest level approximation modulation theory to generate corresponding control pulses to realize the control of the DC system.

[0030] The improvement of the present application lies in that the inertia control method of the additional degrees of freedom acting on the MMC modulation part is superimposed with the inertia control method of the virtual energy storage element acting on the outer loop power control part, so as to improve the inertia time constant of the DC system, optimize the dynamic performance without changing the steady-state value of the system, and has good feasibility, economy and low implementation difficulty.

[0031] In a possible implementation, S2, calculates the reference value of active power or DC voltage in the outer loop power control according to the value of the added virtual capacitor inductor; including: for the virtual capacitor, when the MMC converter operates in a fixed active power control or droop control state, the reference value of active power in the outer loop power control is calculated according to the reference value of active power in the stable state, the virtual capacitor value, and the difference between the actual value of the DC voltage and the reference value; when the MMC converter operates in a fixed DC voltage control state, the reference value of the DC voltage in the outer loop power control is calculated according to the reference value of the DC voltage in the stable state, the difference between the actual value of active power and the reference value, the virtual capacitor value and the Laplace operator.

[0032] Specifically, because the inertia time constant of the AC system is much larger than that of the DC system, in addition to utilizing the capacitor energy of the converter itself, the corresponding energy can also be released or stored through the AC system, thereby creating a virtual capacitor C on the DC side of the converter station that is much larger than the actual parallel capacitor. vc, smoothing the strong interaction of the DC system and providing corresponding inertial support for the DC system. Taking virtual capacitors as an example, when the MMC converter works in fixed active power control or droop control, the active power reference value P in the outer loop power control is * The calculation formula is as follows:

[0033]

[0034] Among them, C vc Refers to the virtual capacitance value, ΔU dc It refers to the difference between the actual value of DC voltage and the reference value, ΔP refers to the difference between the actual value of active power and the reference value, P acref Refers to the reference value of active power in a steady state;

[0035] When the MMC converter operates in the constant DC voltage control state, the DC voltage reference value U dc * The calculation formula is as follows:

[0036]

[0037] Among them, s is the Laplace operator, U dcref Refers to the reference value of DC voltage in a steady state.

[0038] Through the above formula, the control reference value of the active type in the outer loop power control can be calculated, and the virtual energy storage element can be used to achieve the effect of inertia improvement. At this time, the DC system inertia H mmc As shown below:

[0039]

[0040] Among them, N is the number of sub-modules put into MMC, C0 is the size of the sub-module capacitance, U c is the submodule capacitor voltage, S mmc is the converter capacity.

[0041] In one possible implementation, S3, calculating the phase reference value of the DC system converter; including: calculating the phase reference value of the DC system converter based on the voltage reference value of the q-axis in the phase-locked loop, the actual voltage value of the q-axis in the phase-locked loop, the proportional parameter and integral parameter of the PI controller in the phase-locked loop, the rated power frequency angular frequency and the Laplace operator.

[0042] Specifically, the phase reference value of the DC system converter is calculated by the phase-locked loop, and the calculation formula is as follows:

[0043]

[0044] Among them, the voltage reference value u of the q axis in the phase-locked loop is *sq =0, k p and k i are the proportional and integral parameters of the PI controller in the phase-locked loop, and ω0 is the rated power angular frequency.

[0045] In a possible implementation, S4, calculating the d-axis current reference value and the q-axis current reference value on the MMC valve side; including: when the MMC converter operates in a fixed active power control or droop control state, the d-axis current reference value and the q-axis current reference value are calculated according to the active power reference value in the outer loop power control, the actual value of the AC active power in the MMC outer loop power control, the proportional parameter and the integral parameter of the active power PI controller, the Laplace operator and the d-axis AC voltage; when the MMC converter operates in a fixed DC voltage control state, the d-axis current reference value and the q-axis current reference value are calculated according to the DC voltage reference value in the outer loop power control, the actual value of the DC voltage in the MMC outer loop power control, the proportional parameter and the integral parameter of the active power PI controller, and the Laplace operator.

[0046] Specifically, the d-axis current reference value i is calculated according to the MMC outer loop power control law. dref , the q-axis current reference value is the same. When the MMC converter works in the state of fixed active power control or droop control, the calculation formula is as follows:

[0047]

[0048] Among them, u sd is the d-axis AC voltage, P acmeas is the actual value of AC active power in the MMC outer loop power control, K P and K I They are the proportional parameter and integral parameter of the active power PI controller respectively.

[0049] When the MMC converter works in the constant DC voltage control state, the calculation formula is as follows:

[0050]

[0051] Among them, U dcmeas It is the actual value of DC voltage in MMC outer loop power control.

[0052] After considering the limiting of the outer loop power control, the output of the outer loop power control after adding the virtual energy storage element can be obtained through the above formula.

[0053] In a possible implementation, S5, calculate the differential mode and common mode voltages in the dq coordinate system of the MMC, convert them into the values ​​on the a-axis, b-axis and c-axis in the abc stationary coordinate system, and calculate the instantaneous value of the modulation wave of the upper and lower bridge arms of the MMC; including: according to the dq axis AC voltage, the actual value of the grid connection point voltage frequency, the inductance of the AC side, the bridge arm inductance, the actual value and reference value of the positive sequence current on the MMC valve side in the dq coordinate system, the proportional parameter and integral parameter of the PI controller corresponding to the dq axis based on the DDSRF technology, and the Laplace operator, calculate the MMC C; calculate the common-mode voltage in the dq coordinate system of MMC according to the actual value of the voltage frequency at the grid connection point, the inductance of the bridge arm, the actual value and reference value of the internal circulating current in the dq coordinate system of MMC, the proportional parameter and integral parameter of the dq-axis internal circulating current suppression controller, and the Laplace operator; convert the differential-mode voltage and common-mode voltage of the d-axis and q-axis to the abc stationary coordinate system, and calculate the instantaneous value of the modulation wave of the upper and lower bridge arms of MMC according to the differential-mode voltage in the abc stationary coordinate system of MMC and the common-mode voltage in the abc stationary coordinate system of MMC.

[0054] Specifically, the output current tracking control is decomposed based on the decoupled dual synchronous reference frame (DDSRF) technology to calculate the differential mode voltage in the dq frame of the MMC:

[0055]

[0056] in,

[0057]

[0058] L ac is the inductance on the AC side, L0 is the bridge arm inductance, u + diffd * and u + diffq * is the positive sequence differential mode voltage in the dq coordinate system, ω is the actual value of the grid connection point voltage frequency, i + vd and i + vq is the actual value of the positive sequence current on the MMC valve side in the dq coordinate system, i + vd * and i + vq * is the reference value of the positive sequence current on the MMC valve side in the dq coordinate system, k' p1 , k' i1 , k' p2 and k' i2They are the proportional parameters and integral parameters of the output current tracking PI controller based on DDSRF technology corresponding to the dq axes respectively.

[0059] The common-mode voltage is calculated from the conventional internal circulating current suppression:

[0060]

[0061] Among them, i cird and i cirq are the actual values ​​of the internal circulation in the dq coordinate system of the MMC, i cird * and i cirq * are the reference values ​​of the internal circulation in the dq coordinate system of the MMC, k' p3 , k' i3 , k' p4 and k' i4 are the proportional parameter and integral parameter of the dq axis internal circulating current suppression controller, respectively. After obtaining the differential and common mode voltages in the dq rotating coordinate system, because the control signal ultimately required by the MMC converter is a voltage modulation wave in the abc stationary coordinate system, it is necessary to convert the differential and common mode voltages of the d-axis and q-axis to the abc stationary coordinate system.

[0062] The final control quantity is calculated to obtain the instantaneous value of the modulation wave of the upper and lower bridge arms of each phase:

[0063]

[0064] Among them, u * diffj is the differential voltage in the abc stationary coordinate system in MMC, u * comj is the common mode voltage in the abc stationary coordinate system in the MMC.

[0065] In one possible implementation, S6, calculates the instantaneous values ​​of the sub-modules that need to be put into operation in the upper and lower bridge arms of the MMC; including: regarding the number of sub-modules put into operation in each phase as an additional control degree of freedom of the MMC, and setting a function of the DC disturbance based on the additional control degree of freedom; according to the number of sub-modules put into operation in each phase, the reference value of the DC voltage in the stable state, the instantaneous value of the modulation wave of the upper and lower bridge arms of each phase, and the actual value of the DC voltage in the outer loop power control, calculates the instantaneous values ​​of the sub-modules that need to be put into operation in the upper and lower bridge arms under the MMC additional degree of freedom inertia control strategy.

[0066] Specifically, in order to maximize the system inertia and improve the dynamic performance, the number of sub-modules N put into operation in each phase is regarded as a controllable quantity, that is, N is an additional control degree of freedom of the MMC. This control degree of freedom can be set as a function of the DC disturbance, which conforms to the following formula:

[0067] N=f(ΔU dc ,ΔI dc )

[0068] Among them, ΔI dc Refers to the difference between the actual value and the reference value of DC current in MMC.

[0069] When the DC system is not disturbed, N is equal to the value under the conventional control mode; when the DC system is disturbed, this additional control degree of freedom can be used to calculate the instantaneous value of the sub-modules required to be put into use in the upper and lower bridge arms under the MMC additional degree of freedom inertia control strategy, including the number of sub-modules temporarily increased due to the disturbance. The calculation formula is as follows:

[0070]

[0071] Among them, n pjmmc The number of submodules temporarily added to the upper bridge arm of the MMC due to disturbance, n njmmc The number of submodules temporarily added to the lower bridge arm of the MMC due to disturbance.

[0072] In one possible implementation, S7, based on the instantaneous value of the sub-module to be put into operation, utilizes the nearest level approximation modulation theory to generate corresponding control pulses to realize DC system control; including: generating control pulses corresponding to each IGBT of the converter based on the instantaneous value of the sub-module to be put into operation in the upper and lower bridge arms, and controlling the switching of the sub-modules through the control pulses.

[0073] Specifically, after obtaining the instantaneous values ​​of the sub-modules that need to be put into use in the upper and lower bridge arms, the control pulses corresponding to each IGBT of the converter can be generated through the nearest level approximation modulation principle, thereby increasing the inertia in the control of the DC system converter and achieving the effect of improving dynamic performance during disturbances.

[0074] See also Figure 3-5 As shown, the embodiment of the present application also provides specific experiments to verify the effect of the above-disclosed method for improving the dynamic performance of a multi-terminal DC system. A three-terminal DC system is adopted, each converter is 101 level, MMC1 uses fixed DC voltage control and fixed reactive power control, MMC2 and MMC3 use fixed active power and fixed reactive power control, the rated capacity of the converter station is 400MVA, the grid-side rated voltage is 220kV, and the DC-side rated voltage is ±400kV. The system officially enters a stable operation state from 0.5s. When running for 1s, the active power reference value of MMC2 is increased from 0pu to 1pu. The virtual capacitance value used in this method is set to 0.005F, and the virtual inductance value is set to 1H. From the simulation comparison Figure 3 and Figure 4It can be seen that this method has the effect of delaying disturbances in both DC voltage and active power, and for active power, the overshoot has been controlled to a very small value, almost close to 0. It can be seen that this method can achieve the effect of increasing inertia, improving dynamic performance, and smoothing DC voltage and active power disturbances, proving the effectiveness of the control strategy. Figure 5 It can be seen that the power or voltage value added by this method will return to 0 after the disturbance ends, and will not affect the steady state of the system, which further indicates that it has no effect on the stability of the system.

[0075] It can be seen that compared with the prior art, the present application has the following advantages and effects: (1) The present application can introduce virtual energy storage elements in the outer loop power control of the MMC control strategy of the DC system. At the same time, in the modulation part, N is set as an additional control degree of freedom, which can greatly improve the inertia time constant of the DC system and improve the dynamic performance. (2) The control method of the present application is suitable for various fluctuations in indicators such as active power or DC voltage in the DC system. It is easy to implement and has high feasibility. Through theoretical analysis and simulation examples, it is verified that the DC system using the control method of the present invention can improve the degree of disturbance suppression and has no effect on stability when the system disturbance ends and returns to a steady state.

[0076] See also Figure 6 As shown, the embodiment of the present application also provides a computer device for implementing the above-disclosed method for improving the dynamic performance of a multi-terminal DC system. The computer device is specifically used to implement Figure 1 The method for improving the dynamic performance of a multi-terminal DC system in the corresponding embodiment; the computer device 600 may include one or more central processing units (CPU) 622 (for example, one or more processors) and a memory 632, and one or more storage media 630 (for example, one or more mass storage devices) storing application programs 642 or data 644. Among them, the memory 632 and the storage medium 630 can be short-term storage or permanent storage. The program stored in the storage medium 630 may include one or more modules (not shown in the figure), and each module may include a series of instruction operations in the computer device. Furthermore, the central processing unit 622 can be configured to communicate with the storage medium 630 and execute a series of instruction operations in the storage medium 630 on the computer device 600.

[0077] The computer device 600 may also include one or more power supplies 626, one or more wired or wireless network interfaces 650, one or more input and output interfaces 658, and / or one or more operating systems 641, such as Windows Server™, Mac OS X™, Unix™, Linux™, FreeBSD™, etc.

[0078] In the embodiment of the present application, the central processor 622 can be used to execute Figure 1 The method for improving the dynamic performance of a multi-terminal DC system in the corresponding embodiment.

[0079] An embodiment of the present application provides a computer storage medium, including computer instructions. When the computer instructions are executed on a computer device, the computer device executes the display method in any possible embodiment described above.

[0080] An embodiment of the present application provides a computer program product. When the computer program product is executed on a computer device, the computer device is enabled to execute the display method in any possible embodiment described above.

[0081] The above specific implementation methods further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above are only specific implementation methods of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for improving the dynamic performance of a multi-terminal DC system, characterized in that: include: S1. Obtain the grid voltage amplitude and phase and valve-side current of the MMC grid-connected point in the DC system, and perform dq decomposition on the grid voltage and valve-side current of the MMC grid-connected point, respectively obtain the d-axis and q-axis components of the grid-connected point voltage and valve-side current in the dq rotating coordinate system, obtain the actual values ​​of the active power, reactive power, DC voltage and grid-connected point frequency of the MMC, and obtain the reference values ​​of the active power, reactive power and DC voltage of the MMC; S2. Calculate the reference value of active power or DC voltage in the outer loop power control according to the value of the added virtual capacitor and inductor; S3, calculating the phase reference value of the DC system converter; S4, calculating the d-axis current reference value and the q-axis current reference value on the MMC valve side; S5, calculate the differential mode and common mode voltages in the dq coordinate system in the MMC, convert them into their values ​​on the a-axis, b-axis and c-axis in the abc stationary coordinate system, and calculate the instantaneous values ​​of the modulation waves of the upper and lower bridge arms of the MMC; S6, calculating the instantaneous value of the submodules that need to be put into use in the upper and lower bridge arms of the MMC; S7. According to the instantaneous value of the submodule to be put into operation, the nearest level approximation modulation theory is used to generate corresponding control pulses to realize the control of the DC system.

2. A method for improving dynamic performance of a multi-terminal DC system according to claim 1, characterized in that: S2. Calculate the reference value of active power or DC voltage in the outer loop power control according to the value of the added virtual capacitor and inductor; including: for the virtual capacitor, When the MMC converter operates in a fixed active power control or droop control state, the active power reference value in the outer loop power control is calculated according to the active power reference value, virtual capacitance value, and the difference between the actual value of the DC voltage and the reference value in the stable state; When the MMC converter operates in a constant DC voltage control state, the DC voltage reference value in the outer loop power control is calculated according to the DC voltage reference value in the stable state, the difference between the actual active power value and the reference value, the virtual capacitance value and the Laplace operator.

3. A method for improving dynamic performance of a multi-terminal DC system according to claim 1, characterized in that: S3. Calculate the phase reference value of the DC system converter; including: The phase reference value of the DC system converter is calculated according to the voltage reference value of the q-axis in the phase-locked loop, the actual voltage value of the q-axis in the phase-locked loop, the proportional parameter and the integral parameter of the PI controller in the phase-locked loop, the rated power frequency angular frequency and the Laplace operator.

4. The method for improving dynamic performance of a multi-terminal DC system according to claim 1, characterized in that: S4, calculating the d-axis current reference value and the q-axis current reference value on the MMC valve side; including: When the MMC converter works in a fixed active power control or droop control state, the d-axis current reference value and the q-axis current reference value are calculated according to the active power reference value in the outer loop power control, the actual value of the AC active power in the MMC outer loop power control, the proportional parameter and the integral parameter of the active power PI controller, the Laplace operator and the d-axis AC voltage; When the MMC converter works in a constant DC voltage control state, the d-axis current reference value and the q-axis current reference value are calculated according to the DC voltage reference value in the outer loop power control, the actual value of the DC voltage in the MMC outer loop power control, the proportional parameter and the integral parameter of the active power PI controller, and the Laplace operator.

5. The method for improving dynamic performance of a multi-terminal DC system according to claim 1, characterized in that: S5. Calculate the differential mode and common mode voltages in the dq coordinate system of the MMC, convert them into their values ​​on the a-axis, b-axis and c-axis in the abc stationary coordinate system, and calculate the instantaneous values ​​of the modulation waves of the upper and lower bridge arms of the MMC; including: According to the actual value of the dq axis AC voltage, the grid connection point voltage frequency, the inductance on the AC side, the bridge arm inductance, the actual value and reference value of the positive sequence current on the MMC valve side in the dq coordinate system, the proportional parameter and integral parameter of the output current tracking PI controller based on the DDSRF technology corresponding to the dq axis, and the Laplace operator, the differential mode voltage in the dq coordinate system of the MMC is calculated; Calculate the common mode voltage in the dq coordinate system of the MMC according to the actual value of the grid connection point voltage frequency, the bridge arm inductance, the actual value and reference value of the internal circulating current in the dq coordinate system of the MMC, the proportional parameter and integral parameter of the dq axis internal circulating current suppression controller, and the Laplace operator; The differential mode voltage and common mode voltage of the d-axis and q-axis are converted to the abc stationary coordinate system, and the instantaneous values ​​of the modulation waves of the upper and lower bridge arms of the MMC are calculated according to the differential mode voltage in the abc stationary coordinate system of the MMC and the common mode voltage in the abc stationary coordinate system of the MMC.

6. A method for improving dynamic performance of a multi-terminal DC system according to claim 1, characterized in that: S6. Calculate the instantaneous values ​​of the submodules that need to be put into operation in the upper and lower bridge arms of the MMC; including: The number of sub-modules put into operation in each phase is regarded as the additional control freedom of the MMC, and the function of the DC disturbance is set based on the additional control freedom. According to the number of sub-modules put into operation in each phase, the reference value of the DC voltage in the stable state, the instantaneous value of the modulation wave of the upper and lower bridge arms of each phase, and the actual value of the DC voltage in the outer loop power control, the instantaneous values ​​of the sub-modules that need to be put into operation in the upper and lower bridge arms under the MMC additional degree of freedom inertia control strategy are calculated.

7. A method for improving dynamic performance of a multi-terminal DC system according to claim 1, characterized in that: S7. According to the instantaneous value of the submodule to be put into operation, the nearest level approximation modulation theory is used to generate corresponding control pulses to realize DC system control, so as to improve the dynamic performance during disturbance; including: The control pulses corresponding to the respective IGBTs of the converter are generated according to the instantaneous values ​​of the submodules to be put into operation in the upper and lower bridge arms, and the switching of the submodules is controlled by the control pulses.

8. A computer device, characterized in that: The computer device comprises: a memory and a processor, wherein the processor is used to execute a computer program stored in the memory, so that the computer device executes a method for improving dynamic performance of a multi-terminal DC system as described in any one of claims 1 to 7.

9. A computer storage medium, characterized in that: The method comprises computer instructions, which, when executed on a computer device, enable the computer device to execute a method for improving dynamic performance of a multi-terminal DC system as described in any one of claims 1 to 7.

10. A computer program product, characterized in that When the computer program product is run on a computer device, the computer device executes a method for improving dynamic performance of a multi-terminal DC system as described in any one of claims 1 to 7.

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