Virtual droop control and virtual inertia control switching method based on logistic function

Abstract

The invention discloses a virtual droop control and virtual inertia control switching method based on a logistic function, and relates to the technical field of high-voltage flexible direct-current power transmission. The method comprises the following steps: obtaining the topology of a super-capacitor energy storage type modular multilevel converter and the topology of a double-end bipolar flexible DC system based on the super-capacitor energy storage type modular multilevel converter; the inertia response characteristic and the droop response characteristic of a receiving end power grid in the flexible direct current sending-out system based on the super capacitor energy storage type modular multilevel converter are analyzed; designing a virtual droop control and virtual inertia control switching strategy based on a logistic function; and performing simulation verification on a virtual droop control and virtual inertia control switching strategy. The method is simple and easy to implement, virtual droop control and virtual inertia control can be assisted to be switched in a smoother mode by introducing the logistic function, the impact of switching of a traditional control strategy on the power grid can be effectively avoided, and the transient stability of the power grid is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high-voltage flexible direct current power transmission, and particularly relates to a virtual droop control and virtual inertia control switching method based on a logistic function. BACKGROUND

[0002] With the increasing proportion of renewable energy power generation, power electronic converters bear the key energy conversion and grid-connected regulation functions in the new power system. However, the problems such as the loss of system inertia and the weakening of damping characteristics caused by the high proportion of power electronic equipment access are increasingly prominent, which seriously threatens the frequency stability of the power grid. Among them, virtual droop control and virtual inertia control as the core link correspond to the system steady-state frequency regulation and dynamic inertia support demand respectively. However, the traditional fixed parameter control strategy is difficult to adapt to the multi-objective optimization demand in the source-load random fluctuation scene, and how to realize the adaptive switching of the two control modes becomes a key technical problem to improve the system operation resilience.

[0003] The flexible direct current power transmission system based on the modular multilevel converter (MMC) has the advantages of independent adjustment of active power and reactive power, low output voltage harmonic content, and no reactive power compensation problem, and is widely used in the field of new energy power transmission. As a power type energy storage, super capacitor has the advantages of high power density, fast response speed and high cycle times, and is suitable for energy storage devices for short-time frequency support. Therefore, the combination of the two forms a super capacitor energy storage type MMC, which can not only be directly connected to the medium and high voltage power grid, but also actively participate in the frequency regulation of the receiving end, improving the stability and anti-interference ability of the power system.

[0004] The logistic function as a typical S-shaped nonlinear function has the characteristics of continuous derivability and adjustable parameters, and has been successfully applied in the fields of fuzzy control and adaptive systems. Its mathematical expression is:

[0005]

[0006] In the formula, K determines the output range, n controls the curve slope, and x0 is the center point. The function can realize the smooth transition of variables, and the switching rate can be dynamically changed by adjusting the parameter n.

[0007] The application provides a virtual droop control and virtual inertia control switching strategy based on a logistic function of a modular multilevel converter with supercapacitor-based energy storage system (MMC-SESS), by introducing the logistic function, the virtual droop control and the virtual inertia control can be switched in a more smooth manner, the impact of the switching of the traditional control strategy on the power grid can be effectively avoided, and the transient stability of the power grid is improved. Meanwhile, the virtual droop control and virtual inertia control switching strategy based on the logistic function provides a new idea for frequency disturbance optimization of a receiving-end power grid, and has certain engineering practical application prospect. SUMMARY

[0008] The application aims to provide a virtual droop control and virtual inertia control switching method based on a logistic function, so as to solve the problems that the traditional fixed parameter control strategy in the background art is difficult to adapt to the multi-objective optimization demand in the source-load random fluctuation scene, and is difficult to realize adaptive switching of the virtual droop control and the virtual inertia control two control modes.

[0009] To achieve the above-mentioned purpose, the application adopts the following technical solutions:

[0010] The application provides a virtual droop control and virtual inertia control switching method based on a logistic function, comprising the following steps:

[0011] S1, obtaining a supercapacitor energy storage type modular multilevel converter topology and a double-terminal bipolar flexible direct current system topology based on the supercapacitor energy storage type modular multilevel converter;

[0012] S2, analyzing the inertia response characteristics of a receiving-end power grid in a flexible direct current sending-out system based on the supercapacitor energy storage type modular multilevel converter;

[0013] S3, analyzing the droop response characteristics of the receiving-end power grid in the flexible direct current sending-out system based on the supercapacitor energy storage type modular multilevel converter;

[0014] S4, designing a virtual droop control and virtual inertia control switching strategy based on a logistic function;

[0015] S5, simulating the virtual droop control and virtual inertia control switching strategy based on the logistic function.

[0016] Preferably, the supercapacitor energy storage type modular multilevel converter topology in S1 is as follows:

[0017] The three-phase super capacitor energy storage type modular multilevel converter topology has two bridge arms in each phase, each bridge arm has N energy storage sub-modules containing super capacitors and is connected in series with a bridge arm inductor L0 and a resistor R0; the internal topology of the energy storage sub-module is composed of an energy storage part and an MMC half-bridge sub-module part; the energy storage part includes a switch tube T3, a switch tube T4, an inductor L SESS and a super capacitor C sc , wherein the capacitance C sc of the super capacitor is an equivalent value of a plurality of super capacitors connected in series and parallel; the MMC half-bridge sub-module part includes a switch tube T1, a switch tube T2 and a capacitor C sm .

[0018] Preferably, the S1 double-ended bipolar flexible direct current system based on the super capacitor energy storage type modular multilevel converter is as follows:

[0019] The double-ended bipolar flexible direct current system is divided into a sending end and a receiving end; the double-ended bipolar flexible direct current system adopts positive and negative bipolar wiring and contains two converter stations, each converter station includes two super capacitor energy storage type modular multilevel converters, which respectively constitute one pole of the bipolar, and the other pole can continue to operate when a single pole fault occurs.

[0020] Preferably, the inertia response characteristic in S2 is that when the load is disturbed, the output is automatically adjusted by the synchronous generator set speed regulation system to suppress the frequency deviation rate Δo(t) and slow down the dynamic characteristic of the frequency change rate, which is described as follows:

[0021]

[0022] In the formula: ΔP L and Δf(t) are the power load disturbance and the frequency deviation caused by the load disturbance, respectively; P sys and f0 are the rated power and the rated frequency of the system, respectively; M sys is the inertia time constant of the synchronous generator set;

[0023] The super capacitor energy storage type modular multilevel converter simulates the inertia response of the synchronous generator by controlling the power output of the super capacitor group; specifically, when the receiving end power grid has a frequency deviation rate Δo(t), the super capacitor group quickly injects or absorbs power to suppress the frequency change rate.

[0024] Preferably, the droop response characteristic in S3 is that after the inertia response, the frequency-dependent load of the frequency regulation unit adjusts the power output according to the frequency fluctuation to reduce the frequency deviation;

[0025] The super capacitor group adjusts the output according to the frequency deviation to simulate the droop characteristic of the traditional unit, and the virtual droop control is represented as:

[0026] ΔP SESS = KE ·Δf

[0027] Among them, K E Δf is the virtual droop coefficient of the supercapacitor bank, and Δf is the frequency deviation.

[0028] The droop response characteristics of the receiving-end grid in a flexible DC transmission system based on a supercapacitor energy storage modular multilevel converter are described as follows:

[0029]

[0030] Among them, K G*i For traditional generator units, the unit frequency regulation power, K D K is the load frequency regulation effect coefficient. D* K G* These are the frequency regulation effect coefficient of the load and the unit frequency regulation power of the synchronous generator set, respectively; P GN δ* represents the rated power of the generator; δ* represents the droop coefficient of the generator; P sys f0 and f0 represent the system's rated power and rated frequency, respectively.

[0031] The K-type modular multilevel converter based on supercapacitor energy storage introduces a feature into its original characteristics. E Increasing the total system regulation power reduces the steady-state frequency deviation Δf. ss .

[0032] Preferably, step S4 is as follows:

[0033] The control modes based on the supercapacitor energy storage modular multilevel converter include virtual inertial control and virtual droop control. Virtual inertial control is used to effectively suppress the rate of change of frequency deviation, while virtual droop control is used to effectively reduce the absolute value of frequency deviation. The control strategy formula is:

[0034]

[0035] Among them, M E K represents the virtual inertia coefficient, where a1 is the proportional coefficient for virtual inertial control. E a1 is the virtual droop coefficient, a2 is the distribution ratio coefficient of virtual droop control, where a1+a1=1; Δf is the frequency deviation;

[0036] During the inertial response phase, virtual inertial control is the primary method, supplemented by virtual droop control. During the inertial response phase, when a1≥0.5, suppressing the rate of change of frequency plays a dominant role.

[0037] During the droop response phase, virtual droop control is the primary method, supplemented by virtual inertial control. During the droop response phase, a2≥0.5, suppressing frequency deviation plays a dominant role.

[0038] During load disturbances, virtual inertial control is the primary method; during frequency recovery, virtual droop control is the primary method. Simultaneously, a smooth switching between virtual inertial control and virtual droop control is achieved, and a logistic function is introduced to construct an analytical model for the allocation coefficients.

[0039] Furthermore, the logistic function constructs an analytical model for the allocation coefficients, as follows:

[0040]

[0041] In the formula, parameters n1 and n2 are both positive numbers, affecting the specific values ​​and rate of change of the distribution coefficient; Δf th With (dΔf / dt) th These are the threshold values ​​for frequency deviation and the rate of change of frequency deviation, respectively.

[0042] dΔf / dt exceeds (dΔf / dt) th When Δf exceeds Δf, virtual inertial control is used; th When both virtual inertial control and virtual droop control exceed their respective thresholds, a control method that primarily uses virtual inertial control and secondarily uses virtual droop control is adopted.

[0043] Preferably, step S5 is as follows:

[0044] A simulation model was built in PSCAD / EMTDC to verify the switching strategy between virtual droop control and virtual inertial control based on the logistic function.

[0045] Compared with the prior art, the beneficial effects of the present invention are:

[0046] The method proposed in this invention is a virtual droop control and virtual inertial control switching strategy based on the logistic function. This strategy can autonomously switch the frequency modulation control mode according to the frequency change amount and frequency change rate, reduce the malfunction of the control mode during the frequency recovery period, and realize transient inertia support and short-term frequency support. Attached Figure Description

[0047] Figure 1 This is a flowchart of the method for switching between virtual droop control and virtual inertial control based on logistic functions in this invention;

[0048] Figure 2 This is a schematic diagram of the topology of the supercapacitor energy storage type modular multilevel converter (MMC-SESS) in this invention;

[0049] Figure 3 This is a schematic diagram of the topology of the dual-ended bipolar flexible DC system in this invention;

[0050] Figure 4This is a schematic diagram of the receiving-end MMC-SESS basic control strategy in this invention;

[0051] Figure 5 This is the coordination logic diagram of the receiving end MMC-SESS in the dual-ended bipolar flexible DC system of this invention;

[0052] Figure 6 This is a schematic diagram showing the values ​​of a1 and a2 during load disturbance in this invention;

[0053] Figure 7 This is a comparison diagram of the frequency deviation of the receiving-end AC grid under three strategies in the dual-end bipolar flexible DC system of this invention;

[0054] Figure 8 This is a comparison chart of the power released by the receiving-end MMC-SESS energy storage module under three strategies in this invention;

[0055] Figure 9 This is a comparison diagram of the pressure resistance of IGBT4 in the receiving-end MMC-SESS energy storage module under three strategies in this invention;

[0056] Figure 10 This is a comparison diagram of the current carrying capacity of IGBT4 in the receiving-end MMC-SESS energy storage module under three strategies in this invention;

[0057] Figure 11 This is a comparison diagram of the power output of the receiving-end frequency-modulated generator set under three strategies in this invention. Detailed Implementation

[0058] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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 are within the scope of protection of the present invention.

[0059] Example 1:

[0060] See Figure 1 The method for switching between virtual droop control and virtual inertial control based on logistic functions mainly includes the following steps:

[0061] Step 1: Analyze the topology of MMC-SESS and the topology characteristics of the MMC-SESS dual-ended bipolar flexible DC system.

[0062] In this step, the structure of MMC-SESS is as follows: Figure 2As shown, each phase of the three-phase MMC-SESS has two bridge arms, upper and lower. Each bridge arm contains N energy sub-modules (ESMs) with supercapacitors, connected in series with the bridge arm inductor L0 and resistor R0. The inductor is used to suppress the current between the bridge arms, and the resistor is equivalent to the loss on the bridge arm. The three phases together form a distributed energy storage MMC topology. The internal topology of the ESM consists of the energy storage section and the traditional MMC half-bridge sub-module section. The energy storage section includes switching transistors T3 and T4, and inductor L0. SESS and supercapacitor C sc The capacitance value C of the supercapacitor sc It is the equivalent value of multiple supercapacitors connected in series and parallel. The operating mode is switched by alternating the conduction of switching transistors T3 and T4, thus achieving the charging and discharging of the supercapacitors. The MMC half-bridge submodule includes switching transistors T1 and T2 and capacitor C. sm The operating mode is switched by controlling the on and off of switching transistors T1 and T2; this topology enables the energy storage part to be highly modularized, enhances the dynamic adjustment capability of the energy storage system, realizes bidirectional energy flow, and can quickly respond to system needs and perform charge and discharge regulation.

[0063] Based on the topology of the MMC-SESS dual-terminal bipolar flexible DC transmission system, as follows: Figure 3 As shown, the system is divided into a sending end and a receiving end. AC system 1 is the sending end system, and AC system 2 is the receiving end system. The system adopts a positive and negative bipolar connection and includes two converter stations. Each converter station has two MMCs with supercapacitor energy storage, forming one pole of the bipolar system. In the event of a single pole failure, the other pole can continue to operate, improving system reliability. The energy storage modules of the MMCs can respond quickly according to system needs, working with the MMCs to achieve dynamic power compensation, smoothing power fluctuations in the grid. Simultaneously, in the event of a fault, the MMCs' rapid blocking capability and the instantaneous power injection of the SESS (Support for Transient Power Response) can effectively cope with DC or AC side faults, improving the system's transient stability. Figure 4 As shown, this is the basic control strategy of the receiver-side MMC-SESS.

[0064] Step 2: Analyze the inertial response characteristics of the receiving-end power grid in the flexible DC transmission system based on MMC-SESS.

[0065] Inertial response is the dynamic characteristic of a power system in response to load disturbances, where the synchronous generator speed control system automatically adjusts the output to suppress the rate of change of frequency deviation Δo(t) and mitigate the rate of frequency change. Its mathematical description is:

[0066]

[0067] Where: ΔP L Δf(t) and Δf(t) represent the frequency deviations caused by grid load disturbance and load disturbance, respectively; Psys f0 and f0 represent the system's rated power and rated frequency, respectively; M sys M is the (mechanical) inertial time constant of the synchronous generator set. sys The larger the value, the stronger the ability to suppress the rate of frequency change, and the better the system frequency stability.

[0068] MMC-SESS can simulate the inertial response of a synchronous generator by controlling the power output of the energy storage system: when the frequency deviation rate Δo(t) occurs in the receiving-end grid, the SESS rapidly injects or absorbs power to suppress the rate of frequency change. This characteristic of smoothing frequency fluctuations effectively increases the system's inertial time constant M. sys This enhances the ability to suppress the rate of change of frequency.

[0069] Step 3: Analyze the droop response characteristics of the receiving-end grid in the flexible DC transmission system based on MMC-SESS.

[0070] The droop response follows the inertial response. Frequency-modulated units adjust their power output according to frequency fluctuations in relation to the frequency-dependent load, thereby reducing frequency deviation. The droop response characteristics of frequency-modulated units and loads as a function of frequency are as follows:

[0071]

[0072] In the formula: K D* K G* These are the frequency regulation effect coefficient of the load and the unit frequency regulation power of the synchronous generator set, respectively; P GN δ represents the rated power of the generator. * This is the generator's droop coefficient.

[0073] The corresponding load and generator output are:

[0074]

[0075] SESS can adjust its output based on frequency deviation, simulating the droop characteristics of traditional units. The virtual droop control can be expressed as:

[0076] ΔP SESS =K E ·Δf (5)

[0077] In the formula: K E Here, Δf represents the virtual droop factor of the SESS, and Δf represents the frequency deviation. The SESS directly participates in primary frequency modulation, reducing frequency deviation.

[0078] In summary, the droop response characteristics of the receiving-end grid in the MMC-SESS-based flexible DC transmission system can be described as follows:

[0079]

[0080] In the formula: K G*i For traditional generator units, the unit frequency regulation power, K D The load frequency regulation effect coefficient is used. Compared with the traditional system, MMC-SESS equivalently simulates the output of the traditional unit, introducing K into the original characteristics. E The total system regulation power was increased, thereby reducing the steady-state frequency deviation Δf. ss .

[0081] Step 4: Design and analyze the switching strategy between virtual droop control and virtual inertial control based on the logistic function.

[0082] The MMC-SESS control modes primarily include virtual inertial control and virtual droop control. The former effectively suppresses the rate of change of frequency deviation, while the latter effectively reduces the absolute value of frequency deviation. During the inertial response phase, virtual inertial control is the primary mode, supplemented by virtual droop control. This effectively suppresses the maximum rate of change of frequency deviation and reduces the maximum frequency deviation at different stages of the inertial response phase. The corresponding control strategy formula is:

[0083]

[0084] Where: M E K represents the virtual inertia coefficient, where a1 is the proportional coefficient for virtual inertial control. E a1 is the virtual droop coefficient, and a2 is the allocation ratio coefficient for virtual droop control, where a1+a1=1.

[0085] During the inertial response phase, a1 ≥ 0.5, suppressing the rate of change of frequency plays a dominant role. In the droop response phase, MMC-SESS primarily uses virtual droop control, supplemented by virtual inertial control. This effectively suppresses the maximum rate of change of frequency deviation and reduces the maximum frequency deviation at different stages of the droop response phase. The corresponding MMC-SESS output is shown in equation (7), where a1 + a1 = 1. During the droop response phase, a2 ≥ 0.5, suppressing the frequency deviation plays a dominant role.

[0086] To prioritize inertial control during load disturbances and droop control during frequency recovery, while simultaneously achieving smooth switching between virtual inertial and virtual droop control, a logistic function is introduced to construct an analytical model for the allocation coefficients, as follows:

[0087]

[0088] In the formula, parameters n1 and n2 are both positive numbers, affecting the specific values ​​and rate of change of the distribution coefficient; Δf th With (dΔf / dt) thThese are the threshold values ​​for frequency deviation and the rate of change of frequency deviation, respectively. dΔf / dt exceeds (dΔf / dt). th When Δf exceeds Δf, virtual inertial control is engaged; th At that time, virtual droop control is activated, such as... Figure 5 As shown. When both exceed their respective thresholds, a control method is adopted that primarily uses virtual inertial control and secondarily uses virtual droop control.

[0089] Step 5: Create a simulation example in PSCAD / EMTDC and perform a simulation to verify the proposed control strategy:

[0090] by Figure 3 Taking the system shown as an example, a 20% step active load disturbance is set at the PCC point of the receiving-end power grid, with a disturbance time range of 15-25 seconds, and a simulation model is built in PSCAD / EMTDC.

[0091] In this invention, n1 and n2 are set to values ​​of 500 and 0.3 respectively, and Δf th With (dΔf / dt) th Let the values ​​be 0.05Hz and 40Hz / s respectively. Then, the following formula can be derived:

[0092]

[0093] During load disturbances, the values ​​of a1 and a2 are as follows: Figure 6 As shown. During the short period of load application, a1 is 1 and a2 is 0, indicating that virtual inertial control is engaged; as the rate of change of frequency deviation is effectively controlled, the values ​​of a1 and a2 switch smoothly, and the system engages virtual droop control.

[0094] The frequency deviation of the receiving-end power grid proposed in this invention is compared with that of direct switching strategy and no frequency modulation strategy. Figure 7 As shown, when the load on the receiving-end power grid suddenly increases, compared with the strategy without frequency regulation and the direct switching strategy, the strategy proposed in this invention can effectively control the frequency deviation of the receiving-end power grid within 1%, and stabilize within 0.6 seconds. While the frequency deviation of the strategy proposed in this invention is slightly larger than that of the direct switching strategy, the error is within an acceptable range. Furthermore, the direct switching strategy requires pre-experimentation to determine the switching time point between virtual inertial control and virtual droop control, while the strategy proposed in this invention can achieve automatic and smooth switching between virtual inertial control and virtual droop control. Therefore, within the allowable error range, the strategy proposed in this invention can improve the frequency stability and automation level of the system, and has practical significance.

[0095] The power generated by the supercapacitor bank is as follows Figure 8As shown. During the initial short period after load is applied, the system's inertial response dominates, and the supercapacitor bank rapidly releases energy to generate inertia. When the rate of change of frequency deviation reaches its maximum, the system switches from a virtual inertial response to a virtual droop response. At this point, the supercapacitor bank autonomously adjusts its energy absorption and release according to frequency overshoot and recovery to help the system frequency recover. After the system frequency stabilizes, the supercapacitor bank releases energy at a stable power output to compensate for the power deficit.

[0096] Figure 9 and Figure 10 The voltage and current carrying capacity of the IGBT in the submodule are demonstrated. Without a strategy, the supercapacitor in the submodule is not engaged, and the DC component of the IGBT current is zero. Compared with the direct switching strategy, the voltage and current amplitude errors of the IGBT proposed in this invention are within 6% and 1%, respectively.

[0097] Figure 11 The output power of the receiving-end frequency-modulated generator set was demonstrated. During the frequency response period, compared with the direct switching method and no strategy, the proposed strategy significantly reduced the output of the frequency-modulated generator set because the supercapacitor group absorbed part of the power deficit caused by sudden load increases. Compared with the direct switching method, the error of the proposed strategy is controlled within 2%.

[0098] The above description is only for the purpose of helping to understand the method and core essence of the present invention, but the scope of protection of the present invention is not limited thereto. For those skilled in the art, any equivalent substitutions or modifications made to the technical solution and inventive concept disclosed in the present invention within the scope of the technology disclosed in the present invention should be covered within the scope of protection of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for switching between virtual droop control and virtual inertial control based on logistic functions, characterized in that, Includes the following steps: S1. Obtain the topology of the supercapacitor energy storage modular multilevel converter and the topology of the dual-terminal bipolar flexible DC system based on the supercapacitor energy storage modular multilevel converter; S2. Analyze the inertial response characteristics of the receiving-end grid in a flexible DC transmission system based on a supercapacitor energy storage modular multilevel converter; S3. Analyze the droop response characteristics of the receiving-end grid in a flexible DC transmission system based on a supercapacitor energy storage modular multilevel converter; S4. Design a switching strategy for virtual droop control and virtual inertial control based on logistic functions; S5. Simulation verification of the switching strategy between virtual droop control and virtual inertial control based on logistic functions.

2. The method for switching between virtual droop control and virtual inertial control based on logistic functions according to claim 1, characterized in that, The supercapacitor energy storage type modular multilevel converter topology in S1 is as follows: In the three-phase supercapacitor energy storage modular multilevel converter topology, each phase has two bridge arms, one upper and one lower. Each bridge arm contains N energy storage submodules with supercapacitors, connected in series with the bridge arm inductor L0 and resistor R0. The internal topology of each energy storage submodule consists of an energy storage section and an MMC half-bridge submodule section. The energy storage section includes switching transistors T3 and T4, and inductor L0. SESS and supercapacitor C sc The capacitance value C of the supercapacitor sc It is the equivalent value of multiple supercapacitor groups connected in series and parallel; the MMC half-bridge submodule includes switching transistor T1, switching transistor T2, and capacitor C. sm .

3. The method for switching between virtual droop control and virtual inertial control based on logistic functions according to claim 2, characterized in that, The dual-ended bipolar flexible DC system based on a supercapacitor energy storage modular multilevel converter in S1 is as follows: The dual-ended bipolar flexible DC system is divided into a sending end and a receiving end. The dual-ended bipolar flexible DC system adopts positive and negative bipolar wiring and includes two converter stations. Each converter station includes two supercapacitor energy storage type modular multilevel converters, which respectively constitute one pole of the bipolar system. When one pole fails, the other pole can continue to operate.

4. The method for switching between virtual droop control and virtual inertial control based on logistic functions according to claim 1, characterized in that, The inertial response characteristic in S2 is the dynamic characteristic of automatically adjusting the output through the synchronous generator set speed regulation system to suppress the frequency deviation rate of change Δo(t) and slow down the frequency change rate when the load is disturbed. The supercapacitor energy storage modular multilevel converter simulates the inertial response of a synchronous generator by controlling the power output of the supercapacitor bank; specifically, when the frequency deviation rate Δo(t) of the receiving-end grid occurs, the supercapacitor bank rapidly injects or absorbs power to suppress the frequency change rate.

5. The method for switching between virtual droop control and virtual inertial control based on logistic functions according to claim 1, characterized in that, The droop response characteristic in S3 refers to the adjustment of power output by the frequency-modulated unit and the frequency-related load according to frequency fluctuations after the inertial response, thereby reducing frequency deviation. The supercapacitor bank adjusts its output based on frequency deviation, simulating the droop characteristics of a traditional generator unit; a virtual droop coefficient K is introduced into the original characteristics of the supercapacitor energy storage modular multilevel converter. E Increase the total system regulation power to reduce the steady-state frequency deviation Δf. ss .

6. The method for switching between virtual droop control and virtual inertial control based on logistic functions according to claim 1, characterized in that, S4 is specifically as follows: The control modes based on the supercapacitor energy storage modular multilevel converter include virtual inertial control and virtual droop control. Virtual inertial control is used to effectively suppress the rate of change of frequency deviation, while virtual droop control is used to effectively reduce the absolute value of frequency deviation. The control strategy formula is: Among them, M E K represents the virtual inertia coefficient, where a1 is the proportional coefficient for virtual inertial control. E a1 is the virtual droop coefficient, a2 is the distribution ratio coefficient of virtual droop control, where a1+a1=1; Δf is the frequency deviation; During the inertial response phase, virtual inertial control is the primary method, supplemented by virtual droop control. During the inertial response phase, when a1≥0.5, suppressing the rate of change of frequency plays a dominant role. During the droop response phase, virtual droop control is the primary method, supplemented by virtual inertial control. During the droop response phase, a2≥0.5, suppressing frequency deviation plays a dominant role. During load disturbances, virtual inertial control is the primary method; during frequency recovery, virtual droop control is the primary method. Simultaneously, a smooth switching between virtual inertial control and virtual droop control is achieved, and a logistic function is introduced to construct an analytical model for the allocation coefficients.

7. The method for switching between virtual droop control and virtual inertial control based on logistic functions according to claim 6, characterized in that, The logistic function is used to construct an analytical model for the allocation coefficients, as follows: In the formula, parameters n1 and n2 are both positive numbers, affecting the specific values ​​and rate of change of the distribution coefficient; Δf th With (dΔf / dt) th These are the threshold values ​​for frequency deviation and the rate of change of frequency deviation, respectively. dΔf / dt exceeds (dΔf / dt) th When Δf exceeds Δf, virtual inertial control is used; th When both virtual inertial control and virtual droop control exceed their respective thresholds, a control method that primarily uses virtual inertial control and secondarily uses virtual droop control is adopted.

8. The method for switching between virtual droop control and virtual inertial control based on logistic functions according to claim 1, characterized in that, S5 is specifically as follows: A simulation model was built in PSCAD / EMTDC to verify the switching strategy between virtual droop control and virtual inertial control based on the logistic function.

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