Power matching-based current limiting method, system and equipment for network-constructing flexible direct current system

Abstract

The invention discloses a power matching-based current limiting method, system and equipment for a network construction type flexible direct current system. Relates to flexible direct current transmission. An existing current limiting method affects system stability and control accuracy. The current limiting condition is equivalently converted into the power limiting condition under the characteristic that the output voltage of the grid-forming converter is stable, the power limiting condition is converted into the angular frequency judgment condition under the characteristic of droop control, and when the angular frequency of the system is smaller than the lower limit of the angular frequency threshold value, it is judged that the system has overcurrent under large disturbance, and then the system is controlled. And the power matching module is connected, the output power of the converter is limited to be the maximum value, and then the current is limited to be the maximum value allowed to be output by the converter. According to the technical scheme, the current limiting effect of the converter can be achieved on the premise of not changing the network construction type frequency modulation control characteristics, complex parameter measurement and setting are not needed, the potential operation scene of the flexible direct current system can be effectively dealt with, and safe and reliable operation of the network construction type flexible direct current system is ensured.

Description

Technical Field

[0001] The present invention relates to flexible direct current transmission, and in particular to a current limiting method, system and equipment for a grid-type flexible direct current system based on power matching. Background Art

[0002] At present, the power system is developing in a greener and low-carbon direction. On the one hand, it is necessary to change the way of energy production and consumption, increase the proportion of renewable energy, and optimize the energy structure to achieve clean development. On the other hand, under the premise of ensuring the reliable operation of the power system, it is necessary to make more effective use of the existing transmission network and maximize the transmission capacity of the line. Flexible transmission technology dominated by power electronic devices has emerged as the times require, and it has played an important role in significantly improving the access of renewable energy and the transmission power of the line.

[0003] Among them, the flexible direct current transmission system (Voltage Source Converter-based High Voltage Direct Current System, VSC-HVDC) has received extensive attention due to its low harmonic content, no risk of commutation failure, and the ability to achieve rapid decoupling control of active and reactive power. The VSC-HVDC system is expected to achieve breakthroughs in a variety of applications such as renewable energy grid connection, multi-terminal DC interconnection, and large-scale system interconnection, and become a key technology for future intelligent transmission networks.

[0004] However, as large-scale renewable energy is connected to the grid through VSC-HVDC systems, the proportion of synchronous generators decreases, the inertia of the grid decreases, and the anti-interference ability of the system decreases. In addition, the output fluctuations of renewable energy power stations will cause frequent fluctuations in the grid, posing a huge challenge to the safe and stable operation of the grid. In this context, grid-forming (GFM) technology, as an emerging converter control method, can simulate the operating characteristics of synchronous generators and make the converter equivalent to a voltage source, thereby providing reliable voltage and frequency support for the grid. However, due to its voltage source characteristics, the output current of the GFM converter is affected by external operating characteristics. When a frequency drop accident occurs, it is easy to cause overcurrent risks, thereby damaging the converter.

[0005] In order to solve the overcurrent problem caused by frequency drop, experts have proposed a variety of current limiting control methods. At present, the mainstream current limiting methods include mode switching method and virtual impedance method. When the current exceeds the maximum allowable current of the converter, the mode switching method switches the control mode of the converter from the grid-forming mode to the grid-following mode, converts its voltage source characteristics into current source characteristics, and limits the current through the control characteristics of the grid-following converter. However, this method will cause the switching of the control mode, produce phase angle mutations, and change the original frequency regulation characteristics of the grid-forming system, thereby reducing the robustness of the control system.

[0006] The virtual impedance method limits the voltage reference value by introducing virtual impedance, thereby preventing the voltage controller from issuing excessive current commands, thereby limiting the output current of the GFM converter. However, in actual systems, it is very difficult to accurately measure the real impedance of the power grid, and the design of virtual impedance is therefore faced with great challenges. Although the improved adaptive virtual impedance method has alleviated this problem to a certain extent and achieved current limitation, the reasonable setting of virtual impedance is still an unsolved problem due to the inability to accurately locate disturbances and measure line impedance, and it is difficult to meet the current limitation requirements of the system.

[0007] In addition, some studies have proposed limiting the current by limiting the output voltage of the converter. The specific approach is to keep the output voltage of the converter consistent with the grid connection point voltage to achieve current limitation. However, considering that the system may be overloaded, the converter cannot obtain an effective control signal because the grid connection point voltage hardly changes. Therefore, this method is not applicable when dealing with system overload problems.

[0008] Although the existing technology has achieved the function of current limitation in certain specific scenarios, it has introduced an additional control loop, which changes the original frequency control characteristics of the grid-type system, resulting in poor system robustness. At the same time, many methods have difficulties in parameter design and precise measurement, which limits their applicability in a wider range of operating scenarios. In order to ensure the safe and reliable operation of the grid-type flexible DC system, it is urgent to study a new current limitation method that can avoid complex parameter setting without affecting the grid-type frequency modulation control characteristics, and has a wider range of applicable scenarios. Summary of the invention

[0009] The technical problem to be solved and the technical task proposed by the present invention are to improve and perfect the existing technical solutions, and provide a current limiting method, system and device for a grid-type flexible direct current system based on power matching, so as to effectively avoid the risk of overcurrent and ensure the safe and reliable operation of the system. To this end, the present invention adopts the following technical solutions.

[0010] A current limiting method for a grid-type flexible direct current system based on power matching comprises the following steps: Step 1) Calculate the maximum power allowed to be output by the converter in steady state according to the voltage rating of the grid connection point and the maximum overload current of the converter; Step 2) Based on the active power-frequency droop equation, combined with the difference between the maximum power and the grid-side power and the active power-frequency droop coefficient, the minimum angular frequency corresponding to the maximum power output of the converter is calculated; Step 3) When the system is disturbed, the real-time frequency of the system is obtained and converted into the real-time angular frequency; Step 4) When the real-time angular frequency is lower than the minimum angular frequency, the power matching module is enabled to adjust the power reference value in the active power-frequency droop equation to limit the converter output power to the maximum power, thereby limiting the system current to not exceed the maximum allowable value; Step 5) After the disturbance is removed, the system frequency is restored to the rated range through droop control. When the real-time angular frequency is greater than or equal to the minimum angular frequency, the power matching module is automatically removed to restore the original frequency modulation control characteristics.

[0011] This technical solution is compatible with frequency regulation characteristics and maintains grid stability. It achieves current limiting by dynamically adjusting the power reference value without modifying the original droop control structure, ensuring that the system can still provide frequency regulation support during the current limiting process and avoiding the risk of frequency instability caused by forced locking or derating in traditional current limiting methods.

[0012] This technical solution adopts dynamic power matching to achieve precise over-current protection; the power matching module is triggered based on the comparison between the real-time angular frequency and the minimum angular frequency, and the power is limited only when there is a risk of over-current, which not only avoids unnecessary intervention, but also can accurately limit the output current to the maximum allowable value, thereby improving system safety.

[0013] This technical solution adopts an adaptive recovery mechanism to reduce manual intervention; after the disturbance is removed, the power matching module is automatically removed and the original control characteristics are restored. The frequency is restored autonomously by relying on droop control without the need for additional control instructions, which reduces the complexity of operation and maintenance and enhances the self-healing ability of the system.

[0014] This technical solution has low parameter dependence and wide applicability. It only requires basic parameters such as grid connection point voltage and maximum overload current of the converter. It does not require complex external loop parameter setting or high-frequency measurement. It is suitable for grid-connected flexible direct current systems of different capacities, reducing the difficulty of project implementation.

[0015] This technical solution avoids control conflicts and improves reliability. Through the conditional trigger mechanism of the power matching module (acting only when the real-time angular frequency is lower than the minimum angular frequency), the decoupling of current limiting control and frequency modulation control is ensured, mutual interference between multiple control links is prevented, and the overall reliability of the system is improved.

[0016] This technical solution optimizes economic efficiency and helps extend the life of the equipment; it limits the current to the maximum allowable value rather than completely blocking it, maximizing power output while protecting the converter, reducing power generation losses, extending the service life of the equipment, and taking into account both safety and economy.

[0017] As a preferred technical means: In step 1), the maximum power By setting the grid-connected point voltage rating U pcc and the maximum overload current of the converter Multiply them together to get .

[0018] This technical solution calculates the maximum power by directly multiplying the grid connection point terminal voltage rating with the maximum overload current of the converter, making the power calculation process more concise and efficient and avoiding complicated parameter measurement and calculation steps.

[0019] This technical solution ensures that the output power of the converter is always within the maximum safe range, effectively limiting the system current to not exceed the maximum allowable value, thereby avoiding the risk of overcurrent.

[0020] The voltage and current-based calculation method enables the solution to flexibly adapt to the parameters of different converters and grid connection points, without relying on other complex external measurements or adjustments. Therefore, it has wide applicability and can cope with a variety of actual operating conditions. The calculation is performed directly through the existing voltage and current parameters of the system, without the need for complex setting and adjustment steps, so that this method can be quickly deployed and put into use in practical applications. By limiting the maximum output power of the converter, it helps to respond quickly when encountering disturbances, ensure stable operation of the system, and avoid equipment damage or system instability caused by excessive current. The simplified calculation method avoids the introduction of too many complex control links, reduces the potential risk of control failure, and enhances the robustness and safety of the system.

[0021] As a preferred technical means: In step 2), based on the active power-frequency droop equation , calculate the maximum output power of the converter The minimum angular frequency corresponding to ,in is the system rated angular frequency, is the active power-frequency droop coefficient, is the grid-side power.

[0022] By considering the rated angular frequency of the system, the active-frequency droop coefficient and the grid-side power, this technical solution can accurately calculate the minimum angular frequency corresponding to the maximum power output of the converter, ensuring that the system can respond to frequency changes in real time and avoid overcurrent or other abnormal conditions. The calculation of the minimum angular frequency provides a clear frequency threshold to ensure that the system can take current limiting measures in time when the frequency deviates from the rated frequency, which helps to maintain the stability of the grid frequency and prevent system instability caused by too low frequency. By setting the minimum angular frequency, it is ensured that when the system is disturbed, the converter can start the power matching module in time, optimize the frequency modulation response of the system, and improve the accuracy and effectiveness of the control system. Calculating the minimum angular frequency provides a more sensitive frequency response mechanism for the system, reduces the negative impact of frequency fluctuations on the grid, enhances the anti-disturbance ability of the grid, and improves the reliability of the system. By dynamically adjusting the minimum angular frequency, the system can flexibly adapt to changing grid conditions according to different operating states and external disturbances, further improving the flexibility and adaptability of the flexible DC system. The calculation of the minimum angular frequency helps to determine when to start the power matching module, ensure that the output power of the converter is always maintained within a safe range, effectively prevent overcurrent and ensure stable operation of the system.

[0023] As a preferred technical means: in step 4), the power reference value is adjusted as follows: , where the additional power The calculation meets the following conditions: ; In the formula, is the real-time angular frequency, It is the new power reference value in the active power-frequency control equation after the power matching module is connected.

[0024] This technical solution achieves precise power limitation through dynamic closed-loop feedback control. The formula is based on the real-time angular frequency With rated angular frequency Deviation, combined with the droop coefficient , dynamically calculate additional power , forming a closed-loop feedback control mechanism; by adjusting , forcing the converter output power to be limited to , to ensure that the current does not exceed the limit. When the power is restored to the original reference value, excessive intervention is avoided. Achieve millisecond-level dynamic response and accurately limit power to , avoiding the conservatism or lag of traditional current limiting methods.

[0025] Seamlessly compatible with droop control, retaining frequency modulation function; by adjusting the power reference value Rather than directly modifying the droop coefficient Or frequency reference value, to ensure that the frequency modulation control characteristics remain unchanged: When limiting current: through The power command is temporarily adjusted, but the droop equation structure remains intact and the system still participates in the grid frequency regulation; when recovering: Automatically restore the original reference value and seamlessly switch to normal frequency regulation mode; the current limiting process does not sacrifice the grid frequency support capability, which is especially suitable for the core functional requirements of grid-connected converters.

[0026] Adaptive power matching to reduce control conflicts; additional power The calculation integrates frequency deviation and power deviation to achieve dual adaptation, avoid the rigidity of single threshold control, and adapt to different disturbance intensities and operating scenarios; Frequency adaptation: Reflects the degree to which the system frequency deviates from the rated value. The lower the frequency ( The smaller the value is), the greater the additional power correction is, and the stronger the current limiting is. Power adaptation: To dynamically compensate for the difference between the power reference value and the actual grid-side power, ensuring the current limiting target Precise tracking.

[0027] Self-consistent equation design is used to ensure stability; in the formula The recursive definition of Include itself) can be decoupled into an explicit expression through mathematical derivation: ; In practical applications, stable convergence is achieved through iteration or closed-loop control to ensure the mathematical self-consistency of the power reference value adjustment process, eliminate the oscillation risk that may be caused by implicit equations, and improve the dynamic stability of the system.

[0028] After the disturbance, the system will recover without disturbance, thus enhancing the robustness of the system; automatic exit mechanism: when hour, , the power reference value is restored to , relying on droop control to restore the frequency autonomously without manual reset or external instructions. During the frequency recovery process, the power matching module gradually exits to avoid secondary shocks caused by reference value jumps. Reduce the complexity of operation and maintenance and improve the reliability of the system under continuous disturbances.

[0029] As a preferred technical means: in step 5), the frequency recovery process of the system after disturbance removal is realized by the active power-frequency droop equation, which is: angular frequency reference value ,in is the initial power reference value.

[0030] Maintain the consistency of frequency modulation control characteristics; frequency recovery is achieved by continuing to use the original active power-frequency droop equation, completely retaining the original droop control structure; the recovery process is seamless: after the disturbance is removed, the power reference value Restore to the initial value, the system directly returns to the original droop control logic, avoiding the stability risk caused by switching different control modes. Frequency modulation characteristics remain unchanged: droop coefficient , Rated angular frequency Parameters such as power are not adjusted to ensure that the system frequency response characteristics are consistent with those before the disturbance. Maintaining the dynamic stability of the power grid is especially suitable for high-proportion renewable energy scenarios that require continuous frequency support. The power reference value is reset naturally: After returning to the initial value, the system gradually adjusts the power output according to the original droop characteristics to eliminate the instantaneous overload risk when the current limiting is exited.

[0031] Mathematical robustness ensures global stability; stability of the droop equation: the droop control equation itself has small signal stability, and its linear feedback mechanism ( and The negative correlation between the frequency and the power) can suppress frequency oscillation and ensure the convergence of the recovery process. Decoupling from the current limiting logic: Frequency recovery only depends on the droop equation and is not coupled with the current limiting action of the power matching module, avoiding the risk of instability caused by the interaction of multiple control links. Enhance the robustness of the system under complex disturbances and is suitable for weak power grids or high volatility scenarios.

[0032] The second technical solution of the present invention is to provide a current limiting system for a grid-type flexible direct current system based on power matching, the system comprising: Data acquisition module: used to obtain the voltage rating of the grid connection point in real time , Maximum overload current of the converter , system real-time frequency and grid-side power ; Maximum power calculation module: Calculates the maximum power allowed to be output by the converter in steady state according to the voltage rating of the grid connection point and the maximum overload current of the converter. ; Minimum angular frequency calculation module: based on active power-frequency droop equation , calculate the minimum angular frequency when the converter outputs maximum power; Frequency monitoring and judgment module: monitors the system angular frequency in real time and triggers the power matching module when the angular frequency is lower than the minimum angular frequency; Power matching module: When triggered, the power reference value in the active power-frequency droop equation is adjusted to , where the additional power According to the real-time angular frequency dynamic calculation, the converter output power is limited to ; Recovery control module: After the disturbance is removed, the system frequency is restored through droop control. When the angular frequency is restored to be greater than or equal to the minimum angular frequency, the power matching module is automatically cut off.

[0033] The frequency monitoring and judgment module tracks the system angular frequency in real time and based on the minimum angular frequency threshold ( ) Dynamically trigger the power matching module to avoid overcurrent hysteresis problems; the power matching module Dynamic calculation, real-time adjustment of power reference value, forming a closed-loop control to ensure that the output power is strictly limited to . Millisecond-level response to disturbances and precise current limiting avoid the conservatism or overshoot risk of traditional open-loop current limiting.

[0034] Power matching modules are only available in The system intervenes in real time without modifying the original droop control equation, and retains the frequency modulation function of the grid-connected converter; after the disturbance is removed, the recovery control module relies on the droop control to autonomously restore the frequency without additional intervention, ensuring that the system always participates in the grid frequency regulation. The current limiting process does not affect the dynamic stability of the grid, and is particularly suitable for the frequency modulation needs of a high proportion of new energy grids.

[0035] The recovery control module monitors the angular frequency in real time ( )Automatically cut off the power matching module without manual reset; rely on the inherent characteristics of droop control to restore the frequency and avoid secondary control conflicts or oscillation risks. Reduce human operation errors, enhance the system's self-healing ability, and extend the service life of the equipment.

[0036] Only grid voltage is required , Converter overload current Basic parameters such as active power and frequency droop coefficient are not required, and high-frequency measurement or complex parameter setting is not required. , grid side power Parameters such as 100V and 100V are compatible with existing network control and can be directly integrated into mainstream converter platforms. It reduces engineering implementation costs and is suitable for flexible direct current systems of various capacities and topologies.

[0037] Limit output power to (rather than completely locked), maximize converter utilization and reduce power generation losses; through current hard constraint ( )Prevent equipment from being damaged by overload and avoid downtime and maintenance costs caused by overcurrent. Optimize the economic operation efficiency of the system while ensuring equipment safety.

[0038] At minimum angular frequency As the judgment criterion, the natural switching between current limiting and frequency modulation is realized, solving the control conflict problem in the traditional method. Dynamic calculation forms an adaptive power reference value adjustment closed loop to ensure the accuracy of current limiting. Relying on droop control to autonomously restore frequency, avoid secondary impact when current limiting is exited, and improve system reliability. This technical solution realizes multi-objective optimization of safe current limiting, frequency regulation support, and economic operation in the field of overcurrent protection of grid-type flexible direct current systems, and is suitable for complex operation scenarios of power grids with high penetration of new energy.

[0039] As a preferred technical means: the additional power of the power matching module The calculation formula is: .

[0040] This technical solution achieves: Closed-loop dynamic current limiting: Real-time and accurate power limiting is achieved through frequency-power dual variable feedback.

[0041] Frequency regulation compatibility: The current limiting action is embedded in the original droop framework to keep the grid support capacity unchanged.

[0042] Self-consistent stable design: The recursive formula is transformed into a stable control logic to avoid the risk of oscillation.

[0043] Adaptive recovery: Relying on droop control to autonomously exit the current limiting state to ensure system robustness.

[0044] In the current limiting control of the grid-forming flexible DC system, both safety protection and grid service functions are taken into account, providing a highly reliable and flexible overcurrent solution for a high-proportion new energy grid.

[0045] As a preferred technical means: the frequency recovery process of the recovery control module is achieved by the active power-frequency droop equation The power reference value is restored to the initial value. .

[0046] This technical solution implements a seamless recovery mechanism: relying on the inherent characteristics of droop control, a natural transition between current limiting exit and frequency recovery is achieved. Stability-first design: the global convergence of the recovery process is ensured through a mathematically robust droop equation. Economy and compatibility: maximize the use of the existing control architecture and reduce transformation costs. This technical solution takes into account efficiency, safety and economy during the disturbance recovery phase, providing a highly reliable and low-cost frequency recovery solution for the grid-type flexible direct current system.

[0047] The third technical solution of the present invention is to provide a computer device, which includes one or more processors and one or more memories, wherein at least one program code is stored in the one or more memories, and when the program code is executed by the one or more processors, the aforementioned current limiting method of a grid-type flexible direct current system based on power matching is implemented.

[0048] The fourth technical solution of the present invention is to provide a storage medium, wherein at least one program code is stored in the storage medium, and when the program code is executed by a processor, the steps of the aforementioned current limiting method of a grid-type flexible direct current system based on power matching are implemented.

[0049] Beneficial effects: This technical solution dynamically switches the power matching module through the minimum angular frequency threshold to achieve current limiting without interfering with frequency modulation. The feedback mechanism based on angular frequency monitoring ensures the rapidity and accuracy of the current limiting action. The inherent characteristics of droop control are used to automatically restore the system to avoid secondary shocks. While ensuring the safe operation of the grid-forming flexible direct current system, this technical solution retains its active support capability for the power grid to the greatest extent, and is suitable for overcurrent protection requirements in scenarios with a high proportion of new energy access. Specifically: 1. Compatible with frequency modulation characteristics and maintain grid stability. Current limiting is achieved by dynamically adjusting the power reference value without modifying the original droop control structure, ensuring that the system can still provide frequency modulation support during the current limiting process, avoiding the risk of frequency instability caused by forced locking or derating of traditional current limiting methods.

[0050] 2. Dynamic power matching and precise over-current protection trigger the power matching module based on the comparison between the real-time angular frequency and the minimum angular frequency, and limit the power only when there is an over-current risk, which not only avoids unnecessary intervention, but also accurately limits the output current to the maximum allowable value, thus improving system safety.

[0051] 3. Adaptive recovery mechanism to reduce manual intervention After the disturbance is removed, the power matching module is automatically removed and the original control characteristics are restored. The frequency is restored autonomously by relying on droop control without the need for additional control instructions, reducing the complexity of operation and maintenance and enhancing the self-healing capability of the system.

[0052] 4. Low parameter dependence and wide applicability It only requires basic parameters such as grid connection point voltage and maximum overload current of the converter. It does not require complicated external loop parameter setting or high-frequency measurement. It is suitable for grid-connected flexible direct current systems of different capacities, reducing the difficulty of project implementation.

[0053] 5. Avoid control conflicts and improve reliability Through the conditional trigger mechanism of the power matching module (only in ensure the decoupling of current limiting control and frequency modulation control, prevent mutual interference among multiple control links, and improve the overall reliability of the system.

[0054] 6. Economic optimization and extended equipment life Limiting the current to the maximum allowable value instead of completely blocking it can protect the converter while maximizing power output, reducing power generation losses, extending equipment life, and balancing safety and economy. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 It is a block diagram of the grid-connected structure of a grid-connected converter.

[0056] Figure 2 It is a structural block diagram of the power matching current limiting method.

[0057] Figure 3 This is the current waveform of the hardware-in-the-loop experiment.

[0058] Figure 4 It is the voltage waveform of the hardware-in-the-loop experiment.

[0059] Figure 5 It is the hardware-in-the-loop experimental frequency, output power, and reference power diagram.

[0060] Figure 6 It is a schematic diagram of the simulation model of four machines and two areas.

[0061] Figure 7 This is the current waveform of the four-machine and two-area simulation experiment.

[0062] Figure 8 This is the voltage waveform diagram of the four-machine and two-area simulation experiment.

[0063] Fig. 9 This is the frequency, output power, and reference power diagram of the four-machine and two-area simulation experiment.

[0064] Fig.10 It is a flow chart of the present invention. DETAILED DESCRIPTION

[0065] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings.

[0066] Embodiment 1: like Fig.10 As shown, this embodiment is a current limiting method for a grid-type flexible DC system based on power matching, which includes the following steps: S1: According to the rated voltage of the grid connection point U pcc and the maximum overload current of the converter , calculate the maximum power allowed to be output by the converter in steady state ; The calculation equation is: ; Maximum Power By setting the grid-connected point voltage rating U pcc and the maximum overload current of the converter Multiply them together to get .

[0067] S2: Based on the active power-frequency droop equation, combined with the difference between the maximum power and the grid-side power and the active power-frequency droop coefficient, the minimum angular frequency corresponding to the maximum power output of the converter is calculated; The active power-frequency droop equation is: ; Calculate the maximum output power of the converter by using the active power-frequency droop equation The minimum angular frequency corresponding to ,in is the system rated angular frequency, is the active power-frequency droop coefficient, is the grid-side power.

[0068] S3: Get the real-time frequency of the system when the system is disturbed And converted to real-time angular frequency .

[0069] S4: When the real-time angular frequency Below minimum corner frequency When , the power matching module is enabled to adjust the power reference value in the active power-frequency droop equation to limit the converter output power to the maximum power, thereby limiting the system current to not exceed the maximum allowable value; The power reference value adjustment method is , where the additional power The calculation meets the following conditions: ; In the formula, is the real-time angular frequency, It is the new power reference value in the active power-frequency control equation after the power matching module is connected.

[0070] S5: After the disturbance is removed, the system frequency is restored to the rated range through droop control. When the real-time angular frequency is greater than or equal to the minimum angular frequency, the power matching module is automatically removed to restore the original frequency modulation control characteristics.

[0071] The frequency recovery process of the system after disturbance removal is realized by the active power-frequency droop equation, which is: angular frequency reference value ,in is the initial power reference value.

[0072] The present invention is a current limiting method for a grid-type flexible direct current system that does not affect the frequency modulation control characteristics, that is, a power matching current limiting method can achieve current limiting by limiting the output power of the converter without changing the grid-type frequency modulation control characteristics. First, the maximum power allowed to be output by the converter when the current is not overloaded is calculated in combination with the actual parameters of the converter itself and the terminal voltage value of the grid connection point, and the minimum angular frequency corresponding to the maximum power output of the converter is calculated in combination with the active-frequency droop equation of the droop control. The minimum angular frequency is used as the basis for the control action. When the system angular frequency is lower than the minimum angular frequency, the power matching module is connected to limit the output power of the converter, so that the converter always outputs the maximum power, and then the system current is always limited to the maximum allowable overload current. After the disturbance is removed, since the current limiting method does not change the original frequency modulation control characteristics of the grid-type converter, under the action of the droop control, the system frequency gradually recovers, the current gradually decreases, the power matching module is automatically removed, and the converter returns to the stable working state before the disturbance.

[0073] The control method proposed in the present invention is suitable for a grid-type flexible DC system. When the system is disturbed and the converter current limitation is required, the proposed control strategy can limit the power output of the grid-type converter by accessing a power matching module without affecting the grid-type frequency modulation control characteristics, thereby achieving the purpose of current limitation. It does not require complex parameter measurement and setting, and can effectively cope with potential operating scenarios of the flexible DC system to ensure the safe and reliable operation of the grid-type flexible DC system.

[0074] This embodiment is mainly composed of a new energy power source, a back-to-back converter, a load part, etc. The flexible DC transmission system transmits power to the load and can limit the current when the load is disturbed to achieve current overload. Figure 1 As shown: The machine-side converter adopts a constant voltage control strategy to generate a DC capacitor voltage command value U ref ; The current command value of the machine-side converter is calculated by converting the DC capacitor voltage command value obtained in step S1 into U ref and DC capacitor voltage U dc After making the difference, the current command value is obtained through the control module based on proportional integral (PI); Current command value and DC side current i dc After making the difference, the PWM control signal of the machine-side converter is generated through the current loop; The machine-side converter adopts constant voltage control, and the DC capacitor voltage command value U ref and DC capacitor voltage U dcAfter the difference is made, the shaft current command value is generated through the proportional integral (PI) based control module. Finally, the current command value and the DC side current i dc After making the difference, the current loop is controlled based on proportional integral (PI) to generate a PWM control signal.

[0075] Figure 1 AC is alternating current, DC is direct current, and abc / dq transformation is to convert the three-phase AC system (abc coordinate system) into a two-phase rotating coordinate system (dq coordinate system). The grid-side converter adopts droop control to obtain the converter port voltage u abc and port current i abc , L f and C f They are the filter inductor and capacitor of the converter port, connected to the power grid via a transformer and a line. Z g is the line impedance, P L is the system load, the converter port voltage u abc and port current i abc After conversion, the d-axis current is generated i d , q-axis i q , d-axis u d and q-axis voltage u q , then the active power reference value is given P ref , according to the active power-frequency control equation of droop control, the grid-side power angle is given θ .

[0076] Give reactive power reference value Q ref , according to the reactive power-voltage control equation of droop control, the reference value of the grid-side voltage is given U ref , the reactive power-voltage control equation of droop control is: ; In the formula, U ref is the voltage command value, U 0 is the grid-side voltage rating, k q is the reactive power-voltage droop coefficient, Q refis the reactive power reference value, Q 0 is the grid-side reactive power.

[0077] According to the grid side power angle θ and grid voltage reference value U ref , the d-axis reference voltage is generated by coordinate transformation U dref and q-axis reference voltage U qref , and respectively with i d and i q The d-axis reference current is generated by voltage loop control. U dref and q-axis reference current U qref The PWM control signals of the grid-side converter are given out through the current loop control.

[0078] The grid side adopts droop control, power command P ref and grid-side power P 0 After making the difference, the grid-side angular frequency change is obtained through the active power-frequency droop equation , The grid side power angle is obtained through the integral link θ , voltage command value U ref The reactive power-voltage control equation of droop control is generated, and finally the grid-side converter PWM control signal is obtained through the voltage generator and the voltage-current dual closed-loop control module; The output power of the flexible DC system is controlled according to the PWM control signal of the machine-side converter and the PWM control signal of the grid-side converter. Since the machine-side converter adopts constant voltage control, the system output power and output current are mainly determined by the grid-side converter on the basis of the stable output voltage of the machine-side converter.

[0079] Combined with the actual parameters of the converter and the terminal voltage value of the grid-connected point, the maximum power allowed to be output by the converter when the current is not overloaded is calculated.

[0080] The minimum angular frequency corresponding to the maximum power output of the converter is calculated by combining the active power-frequency droop equation of the droop control.

[0081] When the flexible DC system is disturbed, the grid frequency drops. Under the action of grid-type frequency control, the system output power increases to provide frequency support for the grid.

[0082] At this time, the system frequency is obtained and the system angular frequency is calculated. If the angular frequency is less than the system minimum angular frequency, the power matching module is enabled. At this time, the power reference value in the active power-frequency control equation changes.

[0083] Under the action of the power matching module, the output power of the converter is always the maximum value allowed, and thus the current is also limited to the maximum value allowed.

[0084] After the disturbance is removed, the power matching current limiting method does not affect the characteristics of the meshing frequency modulation control. Under the action of droop control, the system frequency gradually recovers, and the power matching module no longer meets the access conditions and is automatically removed.

[0085] See also Figure 2 , P 0 is the rated output active power of the converter, P au To match the active power, P ref is the active power reference value, P is the actual output active power of the converter, k p is the active power-frequency droop coefficient, N is the rated angular velocity of the converter, 0 is the actual angular velocity of the converter, Q 0 is the rated output reactive power of the converter, k q is the reactive power-voltage droop coefficient, Q ref is the reactive power reference value, Q is the actual output reactive power of the converter, U N is the rated output voltage of the converter, U is the converter port output voltage, θ is the converter power angle; 1 / s is the integrator. When the flexible DC system is disturbed, the grid frequency decreases. Under the action of grid-type frequency modulation control, the system output power increases to provide frequency support for the grid, obtain the system frequency during the disturbance, and calculate the system angular frequency 0 , judge whether the angular frequency is less than the minimum angular frequency of the system. If it meets the condition of less than, the power matching module is enabled. Under the action of the power matching module, the matching power is generated. P au , change the active power reference value P ref , so that the output power of the converter PIt is always limited to the maximum value allowed, and thus the current is also limited to the maximum value allowed. After the disturbance is removed, the power matching current limiting method does not affect the grid-type frequency modulation control characteristics. Under the action of droop control, the system frequency gradually recovers, and the power matching module no longer meets the access conditions and is automatically removed.

[0086] This embodiment verifies the effectiveness of the proposed frequency control through hardware-in-the-loop experiments and MATLAB experimental platform. Figure 1 and Figure 6 The grid-type flexible direct current system structure block diagram and the four-machine two-area simulation schematic diagram are used as the test objects of this embodiment. In order to verify the technical solution, this embodiment compares the unlimited current, mode switching method and the power matching current limiting method proposed by the present invention under different working conditions under the premise that the grid side adopts droop control.

[0087] Figure 3 (a) is the current waveform of the hardware-in-the-loop experiment without current limitation under large disturbance. Figure 3 (b) is the current waveform of the hardware-in-the-loop experiment of mode switching current limiting. Figure 3 (c) is the current waveform of the hardware-in-the-loop experiment of power matching and current limiting. Figure 3 The experimental results shown show that when a disturbance occurs, the output current of the converter increases rapidly without current limitation. In this case, the current exceeds the maximum allowable current of the converter, 32.5 A, and eventually reaches 35.8 A. Long-term operation will pose a threat to the safety of the converter, shorten its service life, and may even cause the converter to be damaged in severe cases. In contrast, both the mode switching method and the power matching current limiting method of the present invention can effectively limit the output current, and the current is kept within a safe range of 1 pu. When the disturbance is removed, the system returns to the normal operating mode without current limitation, while the mode switching method cannot return to the normal operating mode due to the loss of synchronization of the grid-type converter, and the output current remains at the maximum value of 32.5 A. Under the power matching current limiting method, the current can be restored to a normal value without the need for additional control.

[0088] Figure 4 (a) is the voltage waveform of the hardware-in-the-loop experiment without current limit under large disturbance. Figure 4 (b) is the voltage waveform of the hardware-in-the-loop experiment of mode switching current limiting. Figure 4 (c) is the voltage waveform of the hardware-in-the-loop experiment of power matching and current limiting. Figure 4 As shown in the figure, the mode switching method loses its regulation function due to the loss of voltage loop, which causes the voltage to fluctuate after reaching the maximum output current, thereby causing the output power to fluctuate. However, the voltage of the power matching current limiting method remains stable. In addition, Figure 4As can be seen from (a) in the figure, the grid voltage remains stable under load surge conditions, further proving that the current limiting method based on voltage limiting is not applicable under overload conditions.

[0089] Figure 5 (a) is the frequency diagram of the hardware-in-the-loop experiment. Figure 5 (b) is the output power diagram of the hardware-in-the-loop experiment. Figure 5 (c) is the reference power diagram of the hardware-in-the-loop experiment. Figure 5 The experimental results shown in the figure show that without current limiting, the current gradually increases, the output power gradually increases, and finally stabilizes at 16.72 kW, which exceeds the maximum output power of the converter of 15 kW. Although this method maintains the synchronization of the converter, the overcurrent problem still occurs. For the mode switching method, after a large disturbance occurs, the output current remains at 1 pu, but the voltage fluctuation causes its output power to oscillate, the system frequency decreases rapidly, and eventually the converter loses synchronization. For the case where current limiting and mode switching methods are not considered, the reference power always remains at 10 kW, which fails to match the output power, resulting in overcurrent and instability problems. Under the power matching current limiting method, when the output power reaches the maximum value of 15 kW, the reference power is continuously adjusted to match the output power, so that the output power is always maintained at 15 kW and the current is maintained at 1 pu. In addition, after the disturbance is cleared, the reference voltage automatically recovers.

[0090] Figure 7 (a) is the current waveform of the four-machine two-area simulation experiment with no current limit under large disturbance. Figure 7 (b) is the current waveform of the four-machine two-region simulation experiment with mode switching current limiting. Figure 7 (c) in the figure is the current waveform of the four-machine two-area simulation experiment with power matching and current limiting. Figure 8 (a) is the voltage waveform of the four-machine two-area simulation experiment with no current limit under large disturbance. Figure 8 (b) is the voltage waveform of the four-machine two-region simulation experiment with mode switching current limiting. Figure 8 (c) is the voltage waveform of the four-machine two-area simulation experiment with power matching and current limiting. Fig. 9 (a) in the figure is the frequency diagram of the simulation experiment with four machines and two areas. Fig. 9 (b) is the output power diagram of the four-machine two-area simulation experiment. Fig. 9 (c) is the reference power diagram of the four-machine two-area simulation experiment. Figure 7 , Figure 8 , Fig. 9As shown in the figure, the simulation experimental results of the three control methods in the four-machine two-area system show that when current limitation is not adopted, the output power of the converter reaches up to 160 MW, far exceeding the maximum output power of the converter of 140 MW, resulting in an output current of 1186 A, which triggers an overcurrent accident and lasts for about 7 seconds, seriously threatening the safe operation of the converter. For the mode switching method, although the current is always maintained at 1050 A, the voltage control fails, and the output voltage begins to oscillate after 45 seconds, resulting in power oscillation, rapid decrease in system frequency, and eventually causing the converter to lose synchronization. Under the power matching current limiting method, when the output power reaches the maximum value, the reference power is corrected in real time, and the output power of the converter is maintained at the maximum output power of 140 MW and the current is maintained at the maximum value of 1050 A. This method can also smoothly exit the maximum output power without causing power mutation, thereby ensuring the safe operation of the system.

[0091] Comprehensive analysis of hardware-in-the-loop experimental results and simulation results shows that when current limitation is not performed, the grid-type flexible DC converter is prone to overcurrent problems when subjected to disturbances, which seriously endangers equipment safety. Although the existing mode switching method can effectively limit current, it fails due to the failure of the voltage loop and changes the original grid-type frequency modulation control characteristics, which can easily cause voltage and power oscillations and lead to system instability. In contrast, the power matching-based current limiting method proposed in the present invention successfully achieves current limitation by introducing a power matching module and dynamically adjusting the power reference value, while maintaining the original frequency modulation control characteristics of the grid-type converter, ensuring the converter's synchronization capability, and having obvious performance advantages.

[0092] Embodiment 2: A grid-type flexible direct current system current limiting system based on power matching, the system comprising: 1. Data acquisition module: used to obtain the voltage rating of the grid connection point in real time , Maximum overload current of the converter , system real-time frequency and grid-side power ; The data acquisition module collects key parameters (such as voltage, current, and frequency) in real time to ensure a reliable data base.

[0093] 2. Maximum power calculation module: Calculate the maximum power allowed to be output in the steady state of the converter according to the voltage rating of the grid connection point and the maximum overload current of the converter ; 3. Minimum angular frequency calculation module: based on active power-frequency droop equation , calculate the minimum angular frequency when the converter outputs maximum power; 4. Frequency monitoring and judgment module: monitors the system angular frequency in real time and triggers the power matching module when the angular frequency is lower than the minimum angular frequency; The frequency monitoring and judgment module tracks the system angular frequency in real time and dynamically triggers the power matching module based on the minimum angular frequency threshold to avoid overcurrent hysteresis problems.

[0094] 5. Power matching module: When triggered, the power reference value in the active power-frequency droop equation is adjusted to , where the additional power According to the real-time angular frequency dynamic calculation, the converter output power is limited to ; Additional power of power matching module The calculation formula is: .

[0095] Power matching module by adding power Dynamic calculation, real-time adjustment of power reference value, forming a closed-loop control to ensure that the output power is strictly limited to .

[0096] Power matching modules are only available in The original droop control equation is not modified, and the frequency modulation function of the grid-type converter is retained. The power matching module and the recovery control module dynamically cooperate to achieve accurate switching between current limiting and frequency modulation; reduce system complexity, facilitate upgrades and expansions, and adapt to the needs of different operating scenarios.

[0097] 6. Recovery control module: After the disturbance is removed, the system frequency is restored through droop control. When the angular frequency is restored to be greater than or equal to the minimum angular frequency, the power matching module is automatically cut off.

[0098] The frequency recovery process of the recovery control module is achieved through the active power-frequency droop equation The power reference value is restored to the initial value. .

[0099] After the disturbance is removed, the recovery control module relies on droop control to autonomously restore the frequency without additional intervention, ensuring that the system always participates in grid frequency regulation; the current limiting process does not affect the dynamic stability of the grid, and is especially suitable for the frequency regulation needs of power grids with a high proportion of new energy.

[0100] The recovery control module monitors the angular frequency in real time ( )Automatically cut off the power matching module without manual reset; rely on the inherent characteristics of droop control to restore the frequency and avoid secondary control conflicts or oscillation risks. Reduce human operation errors, enhance the system's self-healing ability, and extend the service life of the equipment.

[0101] It is understandable that the detailed functional implementation of each of the above modules can be found in the introduction of the aforementioned method embodiment, and no further elaboration is made here.

[0102] Embodiment 3 This embodiment provides an electronic device, which includes: a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, a current limiting method for a grid-type flexible direct current system based on power matching as in any embodiment of the present invention is implemented.

[0103] Embodiment 4 This embodiment provides a computer-readable storage medium having a computer program stored thereon. When the computer program is executed by a processor, a current limiting method for a grid-type flexible direct current system based on power matching as in any embodiment of the present invention is implemented.

[0104] Those of ordinary skill in the art will appreciate that the various units and algorithm steps described in the embodiments disclosed herein can be implemented in a combination of electronic hardware, computer software, and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0105] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working processes of the systems, devices and units described above can refer to the corresponding processes in the aforementioned method embodiments and will not be repeated here.

[0106] In several embodiments provided in the present application, if any function is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present application, or the part that contributes to the prior art or the part of the technical solution, can be embodied in the form of a software product, which is stored in a storage medium and includes several instructions for a computer device (which can be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the methods of each embodiment of the present application. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (Read-Only Memory; hereinafter referred to as: ROM), random access memory (Random Access Memory; hereinafter referred to as: RAM), disk or optical disk, and other media that can store program codes.

[0107] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A current limiting method for a grid-type flexible DC system based on power matching, characterized in that The following steps are involved: Step 1) Calculate the maximum power allowed to be output by the converter in steady state according to the voltage rating of the grid connection point and the maximum overload current of the converter; Step 2) Based on the active power-frequency droop equation, combined with the difference between the maximum power and the grid-side power and the active power-frequency droop coefficient, the minimum angular frequency corresponding to the maximum power output of the converter is calculated; Step 3) When the system is disturbed, the real-time frequency of the system is obtained and converted into the real-time angular frequency; Step 4) When the real-time angular frequency is lower than the minimum angular frequency, the power matching module is enabled to adjust the power reference value in the active power-frequency droop equation to limit the converter output power to the maximum power, thereby limiting the system current to not exceed the maximum allowable value; Step 5) After the disturbance is removed, the system frequency is restored to the rated range through droop control. When the real-time angular frequency is greater than or equal to the minimum angular frequency, the power matching module is automatically removed to restore the original frequency modulation control characteristics.

2. According to the power matching-based current limiting method for a grid-type flexible DC system according to claim 1, it is characterized by: In step 1), the maximum power By setting the grid connection point voltage rating and the maximum overload current of the converter Multiply them together to get .

3. According to claim 2, a current limiting method for a grid-type flexible DC system based on power matching is characterized in that: In step 2), based on the active power-frequency droop equation , calculate the maximum output power of the converter The minimum angular frequency corresponding to ,in is the system rated angular frequency, is the active power-frequency droop coefficient, is the grid-side power.

4. According to claim 3, a current limiting method for a grid-type flexible DC system based on power matching is characterized in that: In step 4), the power reference value is adjusted as follows: , where the additional power The calculation meets the following conditions: In the formula, is the real-time angular frequency, It is the new power reference value in the active power-frequency control equation after the power matching module is connected.

5. According to the power matching-based current limiting method for a grid-type flexible DC system according to claim 4, it is characterized by: In step 5), the frequency recovery process of the system after disturbance removal is realized by the active power-frequency droop equation, which is: angular frequency reference value ,in is the initial power reference value.

6. A grid-type flexible DC system current limiting system based on power matching, characterized by: A current limiting method for a grid-type flexible direct current system based on power matching as described in any one of claims 1 to 5 is applied; the system comprises: Data acquisition module: used to obtain the voltage rating of the grid connection point in real time , Maximum overload current of the converter , system real-time frequency and grid-side power ; Maximum power calculation module: Calculates the maximum power allowed to be output by the converter in steady state according to the voltage rating of the grid connection point and the maximum overload current of the converter. ; Minimum angular frequency calculation module: based on active power-frequency droop equation , calculate the minimum angular frequency when the converter outputs maximum power; Frequency monitoring and judgment module: monitors the system angular frequency in real time and triggers the power matching module when the angular frequency is lower than the minimum angular frequency; Power matching module: When triggered, the power reference value in the active power-frequency droop equation is adjusted to , where the additional power According to the real-time angular frequency dynamic calculation, the converter output power is limited to ; Recovery control module: After the disturbance is removed, the system frequency is restored through droop control. When the angular frequency is restored to be greater than or equal to the minimum angular frequency, the power matching module is automatically cut off.

7. According to claim 6, a grid-type flexible DC system current limiting system based on power matching is characterized by: The additional power of the power matching module The calculation formula is: 。 8. According to claim 7, a grid-type flexible DC system current limiting system based on power matching is characterized in that: The frequency recovery process of the recovery control module is achieved by the active power-frequency droop equation The power reference value is restored to the initial value. .

9. A computer device, characterized in that: The device includes one or more processors and one or more memories, and at least one program code is stored in the one or more memories. When the program code is executed by the one or more processors, a current limiting method for a grid-type flexible direct current system based on power matching is implemented as described in any one of claims 1-5.

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