Current limiting method, system, equipment and medium for network construction type converter

Abstract

The invention discloses a current limiting method, a current limiting system and current limiting equipment for a network-forming converter, and a medium. Relates to the technical field of flexible direct-current power transmission. An existing current limiting method may cause large-range offset of system frequency. The method comprises the following steps: calculating the corresponding maximum output power when the converter outputs the maximum overload current on the basis of a droop control architecture of a network-forming converter, and monitoring the actual output power of the converter in real time; when the system is subjected to large disturbance and the power exceeds the limit, current active and reactive power instantaneous values are latched and serve as feed-forward control parameters, and power reference values in a droop control equation are dynamically adjusted, so that active and reactive output is limited within the maximum value, and it is ensured that the output current of the converter does not exceed the maximum overload current amplitude. On the premise that the original droop frequency modulation characteristic is not changed, rapid and reliable current limitation is achieved, complex parameter measurement and setting are not needed, the potential operation scene of the flexible direct current system is effectively dealt with, and safe and reliable operation of the network construction type flexible direct current system is ensured.

Description

Technical Field

[0001] This invention relates to the field of flexible DC transmission technology, and in particular to current limiting methods, systems, equipment and media for grid-type converters. Background Technology

[0002] Against the backdrop of large-scale power grid development, on the one hand, it is necessary to transform energy production and consumption patterns, increase the proportion of renewable energy, optimize the energy structure, and achieve clean development. On the other hand, it is also necessary to utilize the transmission network more effectively and maximize the transmission capacity of lines while ensuring the reliable operation of the power system. Flexible transmission methods dominated by power electronic devices have emerged, greatly promoting the access of renewable energy and increasing the transmission power of lines. Among them, the flexible DC transmission system (voltage source converter based high voltage direct current system, VSC-HVDC) has attracted widespread attention from experts and scholars due to its advantages of low output voltage and current harmonic content, no risk of commutation failure, and rapid decoupling control of active and reactive power. It is expected to be widely used in various occasions such as renewable energy grid connection, multi-terminal DC construction, and weak system interconnection, and is a key technology for building future intelligent transmission networks. However, after large-scale renewable energy is fed into the receiving-end grid through the VSC-HVDC system, the proportion of synchronous generator units decreases, the inertia of the receiving-end grid weakens, resulting in a decrease in the system's anti-disturbance capability. At the same time, the output fluctuations of renewable energy power plants cause frequent grid fluctuations, posing a huge challenge to the safe and stable operation of the receiving-end grid. Grid-forming (GFM) control is an emerging converter control technology that can simulate the operating characteristics of synchronous generators and can be equivalent to a voltage source, providing reliable voltage and frequency support to the power grid. However, its voltage source characteristics mean that the output current is determined by external operating characteristics, which can easily lead to overcurrent risk when a frequency drop occurs, thereby damaging the converter.

[0003] To address overcurrent issues caused by frequency drops, experts have proposed various control methods. Existing current limiting methods primarily include mode switching and virtual impedance methods. When the current exceeds the converter's maximum allowable current, mode switching switches the converter's control mode from grid-based to grid-connected, changing its voltage source characteristics to current source characteristics, utilizing the grid-connected converter's inherent characteristics for current limiting. However, this method requires switching to an additional control loop, altering the original control architecture. This not only leads to abrupt phase angle changes but also disrupts the original frequency regulation characteristics of the grid-based converter, reducing the robustness of the control system. Virtual impedance methods limit the voltage reference value by adding virtual impedance, preventing the voltage controller from issuing excessive current commands, thereby limiting the output current of the GFM converter. However, the actual impedance of the system is difficult to obtain accurately, making accurate virtual impedance design challenging. While improved adaptive virtual impedance methods alleviate the virtual impedance design problem to some extent and achieve current limiting, the inability to accurately locate disturbances and precisely measure line impedance, along with the difficulty in properly tuning the virtual impedance, remains an unsolved problem, and the desired current limiting function is still difficult to achieve. While some methods exist to limit output current by restricting the converter's output voltage and controlling the converter's output voltage to match the grid connection point voltage, thus achieving current limiting, this approach is difficult to apply when dealing with system overload issues, as the grid connection point voltage remains almost unchanged and the converter cannot obtain an effective control signal. Therefore, although the aforementioned technologies achieve current limiting in specific scenarios, they introduce new control loops, altering the original frequency control characteristics of the grid-connected system, resulting in poor robustness of the control system. Furthermore, they present difficulties in the precise measurement and design of parameters, making it hard to effectively cover the potential operating scenarios of the grid-connected flexible DC system and limiting its applicability. Therefore, it is necessary to research current limiting methods that do not affect the frequency regulation control characteristics of the grid-connected converter, do not require complex parameter tuning, and have a wider range of applicability scenarios, thereby ensuring the safe and reliable operation of the grid-connected flexible DC system. Summary of the Invention

[0004] The technical problem to be solved and the technical task proposed by this invention is to improve and refine existing technical solutions, and to provide a current limiting method for grid-type converters that does not change the frequency regulation characteristics of flexible direct current transmission systems, thereby solving the problems of existing current limiting methods affecting system frequency regulation characteristics and the difficulty in parameter design. To this end, this invention adopts the following technical solution.

[0005] The first technical solution of the present invention is: a current limiting method for a grid-type converter that does not change the frequency regulation characteristics of a flexible direct current transmission system, comprising the following steps: Based on the droop control architecture of the grid-type converter, the voltage amplitude and maximum overload current amplitude at the converter output are obtained, and the maximum output power corresponding to the maximum overload current of the converter is calculated. The converter output terminal three-phase voltage and three-phase current are acquired in real time, and the instantaneous active power, instantaneous reactive power and instantaneous power are calculated. Using the maximum output power as a criterion, when the system is subjected to a large disturbance and the actual output power exceeds the maximum output power, the instantaneous values ​​of the current active power and reactive power are latched. The latched active power value is used as the maximum active power parameter of the feedforward control. The power reference value of the active-frequency droop equation is adjusted in real time through the feedforward active power to limit the active power output to the maximum active power. The latched reactive power value is used as the maximum reactive power parameter of the feedforward control. The power reference value of the reactive power-voltage droop equation is adjusted in real time through the feedforward reactive power to limit the reactive power output to the maximum reactive power. This ensures that the converter output power never exceeds the maximum output power, thereby limiting the output current to not exceed the maximum overload current amplitude. The feedforward control does not change the frequency modulation characteristics of the droop control, and automatically exits the current limiting mode after the system disturbance is removed.

[0006] This technical solution, while fully retaining the original droop control structure and frequency regulation function of the grid-connected converter, transforms the complex current limiting problem into a precise power limiting problem through a collaborative approach of 'real-time power calculation-latch-feedforward control'. When the system encounters a large disturbance leading to power over-limit, this method can instantaneously latch the current power value and quickly intervene, dynamically adjusting the power reference value of the droop control through feedforward, thereby strictly limiting the active and reactive power output of the converter within the safe capacity, fundamentally avoiding the risk of overcurrent damage to power devices. The entire process does not require switching control modes, eliminating phase angle abrupt changes and system impacts, and maintaining the grid-connected converter's ability to support the voltage and frequency of the power grid. In addition, this method does not rely on difficult-to-obtain parameters such as precise measurement of grid impedance, avoiding cumbersome parameter tuning, exhibiting strong robustness, ease of engineering implementation, and effective adaptation to various complex operating conditions of flexible DC systems, ensuring the safety of the converter itself while guaranteeing the overall stability and power supply reliability of the system.

[0007] As a preferred technical means, the maximum output power is the product of the converter terminal voltage amplitude and the maximum overload current amplitude, multiplied by a coefficient of 1.5.

[0008] By directly quantifying the maximum output power as the product of the "converter terminal voltage amplitude," the "maximum overload current amplitude," and a fixed coefficient (1.5), a clear, stable, and easily adjustable power overload threshold is established. This fully considers the characteristics of a three-phase system and accurately reflects the true capacity limit of the instantaneous power at the converter port. The current limiting method based on this criterion can achieve rapid and accurate judgment of overload conditions without relying on complex online calculations or difficult-to-measure system parameters, thus ensuring the timeliness and reliability of current limiting action and providing a solid guarantee for the safe and stable operation of the converter. The fixed coefficient originates from the phase coefficient of the apparent power calculation in a three-phase system, i.e., 3 / 2, corresponding to the instantaneous power calculation criterion under a three-phase three-wire system.

[0009] As a preferred technical means: the instantaneous active power is calculated using the three-phase instantaneous power theory, and is obtained by the sum of the products of the three-phase voltage and the three-phase current; the instantaneous reactive power is calculated using a combination of three-phase voltage difference and three-phase current; the instantaneous power is obtained by taking the square root of the sum of the squares of the instantaneous active power and the instantaneous reactive power.

[0010] This technical solution employs a power calculation system based on classical three-phase instantaneous power theory, capable of capturing dynamic power changes in the power grid in real time and with high accuracy. Instantaneous active power is directly calculated by multiplying the instantaneous values ​​of three-phase voltage and current, providing a clear physical meaning and no delay, thus offering a reliable data foundation for quickly determining the system's power status. Instantaneous reactive power is calculated using a special combination of three-phase voltage difference and current, effectively separating the reactive component of the system and exhibiting good compatibility with traditional definitions. Finally, instantaneous power is obtained by taking the square root of the sum of the squares of the calculated active and reactive power, ensuring the comprehensiveness and accuracy of the power calculation results. This accurately reflects the apparent power level at the converter ports, providing precise and time-free key criteria for subsequent overload judgment and current limiting protection, significantly improving the response speed and reliability of the entire control system.

[0011] As a preferred technical means: the active power-frequency droop control equation is used to adjust the converter angular frequency according to the active power deviation, and its expression is the system rated angular frequency plus the product of the droop coefficient and the difference between the active power reference value and the actual active power; the reactive power-voltage droop control equation is used to adjust the converter terminal voltage amplitude according to the reactive power deviation, and its expression is the system rated voltage plus the product of the droop coefficient and the difference between the reactive power reference value and the actual reactive power.

[0012] The core control strategy of this technical solution inherits the droop characteristics of traditional synchronous generators and enables their efficient implementation in power electronic converters. The active power-frequency droop control equation introduces a product term of active power deviation and droop coefficient, constructing a linear decreasing relationship between power and frequency. This allows the converter to autonomously respond to system frequency changes, simulating the inertia and primary frequency regulation function of a synchronous machine by increasing or decreasing active power, thus providing crucial frequency support for the power grid. Correspondingly, the reactive power-voltage droop control equation establishes a direct correlation between reactive power and voltage amplitude, enabling the converter to automatically adjust reactive power output according to the voltage level at the connection point, thereby stabilizing the grid voltage. The clear equation structure and well-defined physical meaning of the parameters (the droop coefficient determines the sensitivity of the adjustment) together form the cornerstone of grid-connected converters for stabilizing the power grid. Most importantly, this solution performs feedforward optimization based on this mature and reliable droop equation, rather than altering its inherent structure, thus fundamentally ensuring the continuity and stability of the system's frequency and voltage regulation characteristics.

[0013] As a preferred technical means, the method for determining the feedforward active power and the adjusted active power reference value is as follows: when the instantaneous output power of the system is greater than or equal to the maximum output power, the value of the feedforward active power is the difference between the current converter angular frequency and the system rated angular frequency divided by the active power-frequency droop coefficient, plus the difference between the maximum active power output power and the current active power reference value; at this time, the adjusted active power reference value is the difference between the current converter angular frequency and the system rated angular frequency divided by the active power-frequency droop coefficient, plus the maximum active power output power; when the instantaneous output power of the system is less than the maximum output power, the feedforward active power is zero, and the adjusted active power reference value remains unchanged as the current active power reference value.

[0014] This technical solution employs an adaptive feedforward mechanism tightly coupled with system state (angular frequency deviation) and operating boundary (maximum active power). When the system is overloaded, the feedforward is not a simple fixed value but is dynamically calculated, incorporating the correction for the current angular frequency deviation from the rated value and the adjustment of the power reference value. This allows the adjusted active power reference value to simultaneously respond to system frequency changes and power limiting requirements, fully utilizing the converter's overload capacity to provide frequency support to the grid while precisely limiting the output power within a safe upper limit, achieving a balance between support and protection. During normal operation, the feedforward automatically becomes zero, and the control loop seamlessly switches back to the original droop equation, ensuring a smooth transition of control modes without any disturbances. The entire strategy has clear logic, rapid response, and requires no additional mode switching logic, fundamentally maintaining the integrity of the droop control characteristics.

[0015] As a preferred technical means, the method for determining the feedforward reactive power and the adjusted reactive power reference value is as follows: when the instantaneous output power of the system is greater than or equal to the maximum output power, the value of the feedforward reactive power is the difference between the current converter terminal voltage amplitude and the system rated voltage divided by the reactive power-voltage droop coefficient, plus the difference between the maximum reactive power output power and the current reactive power reference value; at this time, the adjusted reactive power reference value is the difference between the current converter terminal voltage amplitude and the system rated voltage divided by the reactive power-voltage droop coefficient, plus the maximum reactive power output power; when the instantaneous output power of the system is less than the maximum output power, the feedforward reactive power is zero, and the adjusted reactive power reference value remains unchanged at the current reactive power reference value.

[0016] This technical solution combines the real-time voltage status of the system with the reactive power output limit to form an adaptive limiting mechanism. When the system is overloaded, the feedforward quantity is not simply fixed to the maximum reactive power, but dynamically incorporates the correction amount for the current voltage deviation from the rated value and the adjustment amount of the reactive power reference value. This allows the adjusted reactive power reference value to simultaneously respond to voltage fluctuations at the connection point and its own reactive power capacity limitations. This ensures that the converter can provide maximum reactive power to support the grid voltage during overload, while strictly constraining the reactive power output within a safe and permissible range, precisely achieving the optimal balance between voltage support and equipment self-protection. During normal operation, the feedforward channel automatically shuts down, and the control system seamlessly returns to the standard droop characteristic, ensuring a smooth and seamless switching of control modes. The entire mechanism is responsive, computationally accurate, requires no external intervention, and maintains the inherent reactive power-voltage regulation characteristics of the grid-connected converter.

[0017] As a preferred technical means, the process of automatically exiting the current limiting mode is as follows: when the system disturbance is cleared and the output power is less than the maximum allowable output power, the feedforward active power and feedforward reactive power are automatically set to zero, and the adjusted active power reference value and reactive power reference value are restored to the initial active power reference value and the initial reactive power reference value, respectively. The converter then exits the current limiting mode.

[0018] The core advantage of this technical solution in its exit mechanism lies in its ability to achieve disturbance-free, automatic, and smooth mode switching. Once the system disturbance is removed and the power does not exceed the limit, the feedforward active and reactive power can be instantly and automatically reset to zero, allowing the adjusted power reference value to seamlessly return to the initial setpoint. This process is entirely triggered autonomously by the system state (power not exceeding the limit), without any external detection, judgment, or switching commands, avoiding secondary impacts or phase angle changes that may be caused by control mode switching. The converter can therefore naturally recover to the normal operating state according to the original droop control equation. The entire process is smooth, rapid, and reliable, strictly ensuring the continuity of the network-type control characteristics and the overall stability of the system.

[0019] The second technical solution of the present invention is: a grid-type converter current limiting system for implementing the aforementioned method of current limiting of a grid-type converter without changing the frequency regulation characteristics of a flexible direct transmission system, the grid-type converter current limiting system comprising: The data acquisition module is used to acquire electrical quantity measurements at the converter ports, including three-phase voltage, three-phase current, system frequency, angular frequency, and converter port voltage amplitude. The power calculation module is used to calculate instantaneous active power, instantaneous reactive power and instantaneous power based on the three-phase voltage and three-phase current obtained by the data acquisition module. The judgment module is used to compare the instantaneous power calculated by the power calculation module with the preset maximum allowable output power, and output a judgment signal based on the comparison result; The latching module is used to record the instantaneous active power and instantaneous reactive power at the moment before the current overload, based on the judgment signal output by the judgment module, and output the latched active power value and reactive power value. The power feedforward module is used to generate feedforward active power and feedforward reactive power based on the latch value output by the latch module, so as to adjust the power reference value in the droop control. The current control module is used to generate the current reference value of the converter based on the difference between the DC capacitor voltage reference value and the actual value, and to generate the PWM control signal. The control module is used to control the output power of the converter according to the PWM control signal, thereby limiting the output current.

[0020] The data acquisition module ensures that control decisions are based on the real, real-time electrical state of the converter ports, providing a foundation for system perception. The power calculation module, employing classical theory, accurately calculates key power state quantities in real time, serving as the data source for all subsequent intelligent decisions. The judgment and latching module constitutes the system's "intelligent brain," capable of quickly diagnosing overload conditions and precisely capturing and locking the critical power value just before an overload occurs, providing the most critical and reasonable input parameters for feedforward control and avoiding the potential impact of directly using maximum values. The power feedforward module is the innovative core of the entire system; it receives latched values ​​and generates precise feedforward quantities, seamlessly embedding and fine-tuning the original droop control reference value to achieve precise and rapid limitation of output power without altering the original control structure. Finally, the current control and execution module ensures that all control decisions are accurately and quickly converted into the converter's PWM drive signal, ultimately achieving effective limitation of the output current.

[0021] The third technical solution of the present invention is: a computer device, the device including one or more processors and one or more memories, wherein the one or more memories store at least one piece of program code, and when the program code is executed by the one or more processors, it implements the aforementioned method for current limiting of a grid-type converter without changing the frequency regulation characteristics of a flexible direct current transmission system.

[0022] The fourth technical solution of the present invention is: a storage medium storing at least one piece of program code, wherein when the program code is executed by a processor, it implements the steps of the aforementioned method for current limiting of a grid-type converter without changing the frequency regulation characteristics of a flexible direct transmission system.

[0023] Beneficial Effects: This invention, while providing a stable voltage frequency in the grid-type converter, does not affect the original frequency regulation characteristics of the grid-type converter. It adds a power feedforward module to the power loop, using the converter's maximum output power as the criterion for current overload. Based on the stable output voltage of the grid-type converter, current limiting is achieved by limiting the converter's output power. Compared with existing current limiting methods, this method does not require changing the frequency regulation characteristics of the grid-type control, does not cause sudden phase angle changes in the system, and the control system has better robustness. Furthermore, this method does not require precise impedance parameter measurement and control parameter tuning, effectively addressing potential operating scenarios of grid-type flexible DC systems and achieving reliable and stable current limiting. Attached Figure Description

[0024] Figure 1 This is a diagram of a network-type flexible DC system and its control architecture.

[0025] Figure 2 This is a diagram of the control architecture for current overload hysteresis detection.

[0026] Figure 3 This is a block diagram of an improved droop control structure with the addition of a power feedforward module.

[0027] Figure 4 It is a current waveform diagram of a grid-type flexible DC system without current limitation under large and small disturbances.

[0028] Figure 5 It is a current waveform diagram of a grid-type flexible DC system without current limitation under small disturbances.

[0029] Figure 6 It is a current waveform diagram of a grid-type flexible DC system without current limitation under large disturbances.

[0030] Figure 7 This is the current waveform diagram of the mesh-type flexible DC system under this method when subjected to large and small disturbances.

[0031] Figure 8 This is the current waveform diagram of the mesh-type flexible DC system under this method when there is a small disturbance.

[0032] Figure 9 This is the current waveform diagram of the flexible DC-DC grid system under this method during large disturbances.

[0033] Figure 10 This is a comparison chart of the d-axis current components under conditions of no current limitation and control by this method.

[0034] Figure 11 yes Figure 10 A magnified view of a portion of the image.

[0035] Figure 12 This is a comparison chart of the q-axis current components under conditions of no current limitation and control by this method.

[0036] Figure 13 yes Figure 12 A magnified view of a portion of the image.

[0037] Figure 14 It is a voltage waveform diagram of a grid-type flexible DC system without current limitation under large and small disturbances.

[0038] Figure 15 yes Figure 14 A magnified view of a portion of the image.

[0039] Figure 16 This is a voltage waveform diagram of a grid-type flexible DC system under this method when subjected to large and small disturbances.

[0040] Figure 17 yes Figure 16 A magnified view of a portion of the image.

[0041] Figure 18 This is a comparison chart of the output active power under the control of the method without current limitation and under the control of this method.

[0042] Figure 19 yes Figure 18 A magnified view of a portion of the image.

[0043] Figure 20 This is a comparison chart of the output reactive power under conditions of no current limitation and control by this method.

[0044] Figure 21 yes Figure 20 A magnified view of a portion of the image.

[0045] Figure 22 This is a comparison chart of active power reference values ​​under the control of the method without current limitation and under the control of this method.

[0046] Figure 23 This is a comparison chart of reactive power reference values ​​under the control of the method without current limitation and under the control of this method.

[0047] Figure 24 This is a comparison graph of the frequency response under both large and small disturbances without current limitation and under the control of this method.

[0048] Figure 25 This is a flowchart of the present invention. Detailed Implementation

[0049] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings.

[0050] Example 1: like Figure 25 The method for limiting current in a grid-type flexible DC system that does not affect the frequency modulation control characteristics, namely the power feedforward current limiting method, includes the following steps: Obtain the voltage amplitude at the output terminal of the converter and maximum overload current amplitude Calculate the maximum output power corresponding to the maximum overload current of the converter. ; Real-time acquisition of three-phase voltage at the converter output terminal and actual output three-phase current Real-time calculation of the active power output of the converter and reactive power And calculate instantaneous power ; At maximum output power As a criterion, when a grid-type flexible DC system is subjected to a large disturbance, if the actual output power of the converter... The instantaneous value of the active power output of the latched converter and instantaneous value of reactive power ; Based on the active-frequency droop equation in droop control, the latched value of active power is... As the parameter of maximum active power Feedforward active power It functions to adjust the power reference value of the active-frequency droop equation in real time. The active power output of the converter is limited to the maximum active power. ; The reactive power-voltage droop equation based on droop control latches the reactive power value. As the parameter of maximum reactive power Feedforward reactive power It functions to adjust the power reference value of the reactive power-voltage droop equation in real time. Limit the reactive power output of the converter to the maximum reactive power. ; With the help of the power feedforward module, the converter output power Always less than or equal to maximum output power This ensures that the converter output current does not exceed the maximum overload current amplitude. ; The additional power feedforward circuit does not change the frequency regulation control characteristics of the droop control. When the system disturbance is removed, the active power output and reactive power output gradually recover, and the current limiting mode is automatically exited.

[0051] This embodiment enables current limiting by limiting the output power of the converter without altering the frequency regulation control characteristics of the grid-type converter. First, the maximum allowable output power of the converter is calculated based on its actual parameters and terminal voltage amplitude when the current is not overloaded. The three-phase voltage and actual three-phase current at the converter output are obtained. The active and reactive power output of the converter are calculated in real time, and the instantaneous power is calculated. The maximum output power is used as the basis for the action of the latching module. When the converter output power is greater than the maximum allowable output power, the active and reactive power values ​​at this time are latched and provided to the power feedforward module. At this time, the power feedforward module outputs feedforward power and adjusts the active and reactive power reference values ​​in the droop control equation in real time to ensure that the converter always outputs the maximum power, thereby limiting the maximum allowable overload current. When the disturbance is cleared, since this current limiting method does not change the original frequency regulation control characteristics of the grid-type converter, the active and reactive power output of the converter gradually recovers under the action of droop control, the current gradually decreases, and the converter automatically exits the current limiting mode and returns to the stable operating state before the disturbance.

[0052] The control method proposed in this invention is applicable to grid-type flexible DC systems. When the system is subjected to disturbances and converter current limiting is required, the proposed control strategy can adjust the reference values ​​of active and reactive power in the droop control equation in a timely manner through the power feedforward module without affecting the grid-type frequency regulation control characteristics, thereby limiting the power output of the grid-type converter and achieving converter current limiting. This method does not require complex parameter measurement and tuning, can effectively cope with the potential operating scenarios of flexible DC systems, and ensure the safe and reliable operation of grid-type flexible DC systems.

[0053] See Figure 1 This embodiment mainly consists of a new energy power source, a back-to-back converter, and a load section. The flexible DC transmission system delivers power to the load and can limit current when the system is subjected to load disturbances, thus preventing current overload. The specific method is as follows: S1. The machine-side converter adopts a constant voltage control strategy to generate a DC capacitor voltage command value. ; S2, D-axis current setpoint of machine-side converter The value is 0, which is the q-axis current command value. Based on the DC capacitor voltage command value in step S1 and DC capacitor voltage After the difference is calculated, the result is obtained through the proportional-integral control module. q Shaft current command value ; S3. Based on the current command value in step S2 and instruction value The PWM control signal for the machine-side converter is generated by the current loop; See Figure 1 The generator-side converter uses constant voltage control, and the DC capacitor voltage command value is... and DC capacitor voltage After the difference is calculated, the q-axis current command value is generated by the proportional-integral control module. d-axis current command value The value is set to 0, and finally a PWM control signal is generated through the current loop control.

[0054] S4. The grid-side converter adopts droop control and provides an active power reference value. The converter output power angle is given based on the active-frequency control equation of droop control. The active-frequency control equation for droop control is: (1) In the formula, The inverter's angular frequency. This is the rated value of the grid-side angular frequency. This is a reference value for active power. For converter output power, The active-frequency droop coefficient is the active-frequency droop factor. It is a coefficient that reflects the sensitivity of the converter's angular frequency adjustment to the active power deviation.

[0055] S5. Provide reactive power reference values. The reference value of the grid-side voltage is given based on the reactive power-voltage control equation of droop control. The reactive power-voltage control equation for droop control is: (2) In the formula, This represents the amplitude of the converter terminal voltage. This is the grid-side voltage rating. This is the reactive power-voltage droop factor. This is a reference value for reactive power. The reactive power output of the converter; reactive power-voltage droop factor It is a coefficient that reflects the sensitivity of reactive power deviation to the adjustment of converter terminal voltage amplitude.

[0056] S6. Based on the converter output power angle obtained in steps S4 and S5 and converter terminal voltage amplitude The PWM control signal for the grid-side converter is provided through voltage and current dual closed-loop control; See Figure 1 The grid side uses droop control and power command. and grid-side power The converter angular frequency is obtained by subtracting the active power and frequency droop equation. , The output power angle of the grid-side converter is obtained through the integration process. θ Voltage command value The signal is generated by equation (2) and finally the grid-side converter PWM control signal is obtained through the voltage and current dual closed-loop control module. Based on the PWM control signal of the machine-side converter obtained in step S3 and the PWM control signal of the grid-side converter obtained in step S6, the output power of the flexible DC system is controlled. 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 stable machine-side converter output voltage. S7, see reference Figure 2 Based on the actual parameters of the converter and the converter port voltage, calculate the maximum output power of the converter when the current is not overloaded. The calculation equation is as follows: (3) In the formula, This represents the maximum output power of the converter when the current is not overloaded. The amplitude of the converter port output voltage. This is the maximum overload current of the converter.

[0057] S8. Calculate the instantaneous active power, instantaneous reactive power, and instantaneous power output of the converter based on the three-phase voltage and current at the converter ports. The calculation equation is as follows: (4) (5) (6) In the formula, The voltage of phase a at the converter port. This refers to the phase b voltage at the converter port. This refers to the c-phase voltage at the converter port. The output current of phase a of the converter. This is the output current of phase b of the converter. This is the output current of phase c of the converter.

[0058] S9. When the flexible DC system is disturbed, the grid frequency drops. Under the action of grid-type frequency regulation control, the system output power increases to provide frequency and voltage support for the grid. S10. At this time, calculate the instantaneous output power of the converter. If the output power exceeds the maximum output power limit, latch the current active power output and reactive power output, and assign the latched value to the power feedforward module. S11, Reference Figure 3 The active power feedforward submodule of the power feedforward module plays a role in generating feedforward active power to adjust the active power reference value in the active power-frequency droop control equation in real time. The calculation equations for feedforward active power and active power reference value are as follows:

[0059] (7) In the formula, For feedforward active power, To provide the converter with the maximum active power output, This is the new active power reference value after the feedforward active power adjustment; S12. The reactive power feedforward submodule of the power feedforward module comes into play, generating feedforward reactive power to adjust the reactive power reference value in the reactive power-voltage droop control equation in real time. The calculation equations for feedforward reactive power and reactive power reference value are as follows:

[0060] (8) In the formula, For feedforward reactive power, To maximize the reactive power output of the converter, This is the new reactive power reference value after the feedforward reactive power adjustment; S13. Under the adjustment of feedforward power, the output power of the converter is always the maximum allowable output power, and the current is also limited to the maximum allowable overload current. S14. After the system disturbance is cleared, under the action of the droop control equation, the active power output and reactive power output gradually decrease, the converter output current decreases accordingly, and the feedforward active power... and feedforward reactive power Automatic zeroing, the converter automatically exits current limiting mode; See Figure 2When the flexible DC system is disturbed, the grid frequency drops. Under the action of grid-based frequency regulation control, the system output power increases to provide voltage and frequency support for the grid. The instantaneous output power of the converter is calculated by combining the three-phase voltage at the converter port and the three-phase output current. If the output power exceeds the maximum output power limit, the current active power output and reactive power output are latched, and the latched values ​​are assigned to the power feedforward module. It is then determined whether the angular frequency is less than the system's minimum angular frequency. If the condition is met, the power matching module is activated. At this time, the active power feedforward module plays a role in generating... The feedforward active power module adjusts the active power reference value in the active-frequency droop control equation in real time. Simultaneously, the reactive power feedforward module also functions, generating feedforward reactive power to adjust the reactive power reference value in the reactive-voltage droop control equation in real time. Under the adjustment of the feedforward power, the converter's output power remains at the maximum allowable output power, and consequently, the current is limited to the maximum allowable overload current. When the system disturbance is cleared, under the action of the droop control equation, the active and reactive power outputs gradually decrease, and the converter output current decreases accordingly. and feedforward reactive power Automatic zeroing, the converter automatically exits current limiting mode; In some embodiments of the present invention, the criteria for determining current overload are as follows: (9) in, This provides instantaneous power output from the converter.

[0061] When the above inequality holds true, it indicates that the system current has exceeded the predetermined safe range, and corresponding current limiting measures need to be taken to control the current.

[0062] This embodiment uses the MATLAB experimental platform to verify the effectiveness of the proposed current limiting method. Figure 1 The schematic diagram of the grid-type flexible direct current transmission system shown serves as the test object for this embodiment. To verify the technical solution, this embodiment compares the actual effects of no current limitation and the current limitation method proposed in this invention under varying degrees of disturbance, assuming droop control is adopted on both the grid side and the grid side.

[0063] like Figure 4 , Figure 5 , Figure 6As shown, the system experiences a small active power disturbance of 3% at 1.5s, increasing the active power output of the converter and consequently increasing the output current. However, the maximum output current of 4.6A does not exceed the converter's maximum allowable current of 4.82A. The active power disturbance is cleared at 2.5s, the current decreases, and the system gradually returns to normal. At 4s, the system experiences a large active power disturbance of 10%. Without current limiting, the converter's output current exceeds the maximum allowable current of 4.82A, eventually reaching 5A. If the active power deficit is not recovered in time, prolonged overcurrent will endanger the converter's safety, shorten its lifespan, and in severe cases, even cause its destruction.

[0064] like Figure 7 , Figure 8 , Figure 9 As shown in the figure, under the control of the proposed method, when a small disturbance occurs in the system at 1.5s, the maximum output current of the converter does not exceed the maximum allowable current of 4.82A, and the proposed current limiting method does not work. When a large disturbance of 10% active power occurs in the system at 4s, under the action of the proposed method, the output current of the system always remains at the maximum allowable current of 4.82A and does not exceed the limit. Meanwhile, from... Figure 9 As can be seen, when the system's active power deficit is replenished in time, the current naturally recovers to the normal operating value, and the current recovery process is the same as the recovery process when there is no current limit. The proposed method does not change the original droop control characteristics.

[0065] Figure 10 , Figure 11 and Figure 12 , Figure 13 The figure shows a comparison of the d-axis and q-axis current components under conditions of no current limitation and the proposed method. As can be seen from the figure, when the system experiences a large disturbance, the proposed method simultaneously limits both the d-axis and q-axis currents. Without current limitation, the maximum value of the d-axis current component exceeds 4.9A, and the maximum value of the q-axis current component exceeds 1.35A, ultimately causing the converter output current to exceed the maximum allowable value of 4.82A. Under the proposed control method, the d-axis current component is limited to 4.72A, the q-axis current component is limited to 1.32A, and the converter output current is ultimately limited to 4.82A, ensuring that the current does not exceed the limit and protecting the converter's own safety.

[0066] Figure 14 , Figure 15 and Figure 16 , Figure 17The voltage waveform changes of the grid-type flexible DC transmission system under large and small disturbances are shown in both the absence of current limitation and the proposed method. The results show that the large loss of active power has little impact on the converter output voltage. Under both the absence of current limitation and the proposed method, the output voltage changes of the grid-type converter are small, with only a voltage amplitude change of about 1V. At the same time, as shown in the figure, the proposed method does not change the voltage support performance of the original droop control, and the equipment can still provide stable and reliable voltage support without voltage oscillation instability.

[0067] Figure 18 , Figure 19 and Figure 20 , Figure 21 This paper compares the changes in active and reactive power under conditions of no current limitation and the proposed method. The results show that due to the active power deficit in the system, the system's equivalent frequency decreases. Under droop control, the converter increases its active power output, exceeding 2300W under no current limitation. This increase in active power also leads to an increase in reactive power, exceeding 470Var. Ultimately, the converter's output power exceeds the equipment's maximum capacity, resulting in overload operation. Figure 22 and Figure 23 The power feedforward module adjusts the reference values ​​of active power and reactive power in real time, so that the active power output of the converter is always limited to 2250W and the reactive power output is always limited to 480Var. The output power of the converter does not exceed the maximum capacity of the equipment, thus avoiding overload operation of the equipment.

[0068] Figure 24 The diagram compares the frequency response under small and large disturbances with and without current limiting, as well as under the proposed method. Under small disturbances, the proposed current limiting method is ineffective, and both methods exhibit the same frequency response characteristics. Under large disturbances, the converter without current limiting significantly increases its output power to support the system frequency and prevent a large frequency drop. However, the output power exceeds the converter's maximum capacity, placing the equipment in an overcurrent condition, which seriously endangers the converter's safety. Under the proposed method, the converter's output power also increases, similarly supporting the system frequency and preventing a large frequency drop. However, under the proposed method, the converter's output power is always limited to its maximum capacity, preventing overcurrent and ensuring the equipment's safety. However, due to the power limiting effect of the proposed method, the equipment's output power is lower than without current limiting, resulting in a greater system frequency drop. After the active power deficit recovers, the system frequency gradually recovers, the converter's output power decreases, the output current decreases, and it smoothly exits the current limiting mode without additional mode switching.

[0069] Analysis of comprehensive simulation results shows that, without current limiting, grid-type converters in flexible DC systems are prone to current overload when the system power is insufficient, endangering the safety of the equipment itself. The current limiting method proposed in this patent increases the power feedforward module and changes the reference values ​​of reactive power and active power in the active-frequency equation and reactive-voltage equation to limit the active and reactive output power of the converter, thereby achieving the purpose of current limiting. Simulations verify the superior performance of the proposed method, which limits the output current of the grid-type converter without changing the original frequency regulation control characteristics of the grid-type converter and maintaining the synchronization capability of the converter.

[0070] Example 2 This embodiment provides a grid-type converter current limiting system that does not change the frequency regulation characteristics of the flexible direct current transmission system. It adopts the method of Embodiment 1. The grid-type converter current limiting system includes: The data acquisition module is used to acquire the three-phase voltage at the ports of the grid-type converter. Three-phase current ,frequency angular frequency Voltage amplitude Feedforward active power Feedforward reactive power and reference power ; The power calculation module is used to calculate instantaneous active power. reactive power Instantaneous power ; The judgment module is used to determine the output power of the converter. Does it exceed the maximum allowable output power? And determine whether the latch module has been activated based on its size; The latch module records the instantaneous active and reactive power at the moment before the current overload, and feeds the latched value to the maximum active power parameter of the power feedforward module. and maximum reactive power parameters ; The power feedforward module is used to generate feedforward active power and feedforward reactive power based on the latch value output by the latch module, so as to adjust the power reference value in the droop control. The current control module is used to control the current based on the DC capacitor voltage reference value. and DC capacitor voltage The difference is used to obtain the current reference value of the flexible DC converter through the proportional-integral control module. And the current loop generates the converter PWM control signal; The control module is used to control the output power of the flexible DC converter according to the PWM control signal of the converter. By controlling the output power of the converter, the output current is limited to ensure that the current is not overloaded when the power grid is subjected to large load disturbances, thus protecting the converter safety.

[0071] The system achieves intelligent control of the grid-type flexible DC system through the coordinated operation of its various modules. By measuring the converter port voltage and output current in real time, and calculating the instantaneous active power, reactive power, and total power output of the converter, the system determines whether the converter is experiencing overcurrent based on whether the output power exceeds the maximum output power. It also latches the active and reactive power output values ​​in real time, generates feedforward power based on the latched values, and adjusts the active and reactive power reference values ​​in the droop control equation in real time. This limits the converter output power and thus the converter current, without altering the original droop grid-type frequency regulation characteristics. This ensures the safe operation of the converter while improving the reliability of the equipment.

[0072] Example 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, it implements a grid-type converter current limiting method that does not change the frequency modulation characteristics of a flexible direct current transmission system as described in any embodiment of the present invention.

[0073] Example 4 This embodiment provides a computer-readable storage medium storing a computer program thereon. When the computer program is executed by a processor, it implements a current limiting method for a grid-type converter that does not change the frequency regulation characteristics of a flexible direct current transmission system, as described in any embodiment of the present invention.

[0074] Those skilled in the art will recognize that the units and algorithm steps described in the embodiments disclosed in this invention can be implemented using electronic hardware, computer software, or a combination of electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0075] Those skilled in the art will understand that, for the sake of convenience and brevity, the specific working processes of the systems, devices, and units described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0076] In the several embodiments provided in this application, any function, if implemented as a software functional unit and sold or used as an independent product, can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this application, essentially, or the part that contributes to the prior art, or a portion of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of this application. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0077] The current limiting method, system, equipment, and medium for grid-type converters shown above are specific embodiments of the present invention, demonstrating the substantial features and progress of the present invention. Based on actual usage needs and under the guidance of the present invention, equivalent modifications in shape, structure, etc., can be made to them, all of which are within the scope of protection of this solution.

Claims

1. A current limiting method for a grid-type converter that does not change the frequency regulation characteristics of a flexible direct current transmission system, characterized in that, Includes the following steps: Based on the droop control architecture of the grid-type converter, the voltage amplitude and maximum overload current amplitude at the converter output are obtained, and the maximum output power corresponding to the maximum overload current of the converter is calculated. The converter output terminal three-phase voltage and three-phase current are acquired in real time, and the instantaneous active power, instantaneous reactive power and instantaneous power are calculated. Using the maximum output power as a criterion, when the system is subjected to a large disturbance and the actual output power exceeds the maximum output power, the instantaneous values ​​of the current active power and reactive power are latched. The latched active power value is used as the maximum active power parameter of the feedforward control. The power reference value of the active-frequency droop equation is adjusted in real time through the feedforward active power to limit the active power output to the maximum active power. The latched reactive power value is used as the maximum reactive power parameter of the feedforward control. The power reference value of the reactive power-voltage droop equation is adjusted in real time through the feedforward reactive power to limit the reactive power output to the maximum reactive power. This ensures that the converter output power never exceeds the maximum output power, thereby limiting the output current to not exceed the maximum overload current amplitude. The feedforward control does not change the frequency modulation characteristics of the droop control, and automatically exits the current limiting mode after the system disturbance is removed.

2. The current limiting method for a grid-type converter without changing the frequency regulation characteristics of a flexible direct current transmission system according to claim 1, characterized in that: The maximum output power is the product of the converter terminal voltage amplitude and the maximum overload current amplitude, multiplied by a coefficient of 1.

5.

3. The current limiting method for a grid-type converter without changing the frequency regulation characteristics of a flexible direct current transmission system according to claim 2, characterized in that: The instantaneous active power is calculated using the three-phase instantaneous power theory, obtained by summing the products of the three-phase voltage and the three-phase current; the instantaneous reactive power is calculated using a combination of three-phase voltage difference and three-phase current; the instantaneous power is obtained by taking the square root of the sum of the squares of the instantaneous active power and the instantaneous reactive power.

4. The current limiting method for a grid-type converter without changing the frequency regulation characteristics of a flexible direct current transmission system according to claim 3, characterized in that: The active power-frequency droop control equation is used to adjust the converter angular frequency according to the active power deviation. Its expression is the system rated angular frequency plus the product of the droop coefficient and the difference between the active power reference value and the actual active power. The reactive power-voltage droop control equation is used to adjust the converter terminal voltage amplitude according to the reactive power deviation. Its expression is the system rated voltage plus the product of the droop coefficient and the difference between the reactive power reference value and the actual reactive power.

5. The current limiting method for a grid-type converter without changing the frequency regulation characteristics of a flexible direct current transmission system according to claim 4, characterized in that: The method for determining the feedforward active power and the adjusted active power reference value is as follows: when the instantaneous output power of the system is greater than or equal to the maximum output power, the value of the feedforward active power is the difference between the current converter angular frequency and the system rated angular frequency divided by the active power-frequency droop coefficient, plus the difference between the maximum active power output power and the current active power reference value. At this point, the adjusted active power reference value is the difference between the current converter angular frequency and the system rated angular frequency divided by the active power-frequency droop coefficient, plus the maximum active power output. When the instantaneous output power of the system is less than the maximum output power, the feedforward active power is zero, and the adjusted active power reference value remains unchanged from the current active power reference value.

6. The current limiting method for a grid-type converter without changing the frequency regulation characteristics of a flexible direct current transmission system according to claim 5, characterized in that: The method for determining the feedforward reactive power and the adjusted reactive power reference value is as follows: when the instantaneous output power of the system is greater than or equal to the maximum output power, the value of the feedforward reactive power is the difference between the current converter terminal voltage amplitude and the system rated voltage divided by the reactive power-voltage droop coefficient, plus the difference between the maximum reactive power output power and the current reactive power reference value. At this point, the adjusted reactive power reference value is the difference between the current converter terminal voltage amplitude and the system rated voltage divided by the reactive power-voltage droop coefficient, plus the maximum reactive power output power. When the system output instantaneous power is less than the maximum output power, the feedforward reactive power is zero, and the adjusted reactive power reference value remains unchanged from the current reactive power reference value.

7. The current limiting method for a grid-type converter without changing the frequency regulation characteristics of a flexible direct current transmission system according to claim 5, characterized in that: The process of automatically exiting the current limiting mode is as follows: when the system disturbance is cleared and the output power is less than the maximum allowable output power, the feedforward active power and feedforward reactive power are automatically set to zero, and the adjusted active power reference value and reactive power reference value are restored to the initial active power reference value and the initial reactive power reference value, respectively. The converter then exits the current limiting mode.

8. A grid-type converter current limiting system for implementing the grid-type converter current limiting method without changing the frequency regulation characteristics of a flexible direct current transmission system as described in any one of claims 1 to 7, characterized in that, include: The data acquisition module is used to acquire electrical quantity measurements at the converter ports, including three-phase voltage, three-phase current, system frequency, angular frequency, and converter port voltage amplitude. The power calculation module is used to calculate instantaneous active power, instantaneous reactive power and instantaneous power based on the three-phase voltage and three-phase current obtained by the data acquisition module. The judgment module is used to compare the instantaneous power calculated by the power calculation module with the preset maximum allowable output power, and output a judgment signal based on the comparison result; The latching module is used to record the instantaneous active power and instantaneous reactive power at the moment before the current overload, based on the judgment signal output by the judgment module, and output the latched active power value and reactive power value. The power feedforward module is used to generate feedforward active power and feedforward reactive power based on the latch value output by the latch module, so as to adjust the power reference value in the droop control. The current control module is used to generate the current reference value of the converter based on the difference between the DC capacitor voltage reference value and the actual value, and to generate the PWM control signal. The control module is used to control the output power of the converter according to the PWM control signal, thereby limiting the output current.

9. A computer device, characterized in that: The device includes one or more processors and one or more memories, wherein at least one piece of program code is stored in the one or more memories, and when the program code is executed by the one or more processors, it implements the grid-type converter current limiting method that does not change the frequency regulation characteristics of the flexible direct transmission system as described in any one of claims 1-7.

10. A storage medium storing at least one line of program code, characterized in that, When the program code is executed by the processor, it implements the steps of a grid-type converter current limiting method that does not change the frequency regulation characteristics of a flexible direct transmission system as described in any one of claims 1-7.

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