Grid-forming converter current limiting method, system, device, and medium

Abstract

The application discloses a grid-forming converter current limiting method, system, device and medium. It relates to the technical field of flexible direct current transmission. The current limiting method may cause large-scale deviation of system frequency. The application comprises a droop control framework based on a grid-forming converter, the maximum output power corresponding to the maximum overload current of the converter is calculated, and the actual output power is monitored in real time; when the system is subjected to a large disturbance and the power is over limited, the current active and reactive power instantaneous values are latched as feedforward control parameters, the power reference value in the droop control equation is dynamically adjusted, the active and reactive outputs are limited within the maximum value, and it is ensured that the output current of the converter does not exceed the maximum overload current amplitude. The application realizes fast and reliable current limiting without changing the original droop frequency modulation characteristics, does not need complex parameter measurement and setting, effectively deals with potential operation scenarios of the flexible direct current system, and ensures safe and reliable operation of the grid-forming flexible direct current system.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flexible direct current transmission, in particular to a grid-forming converter current limiting method, system, device and medium. BACKGROUND

[0002] Under the background of large power grid development, on the one hand, it is necessary to change the mode of energy production and consumption, improve the proportion of renewable energy, optimize the energy structure, and realize clean development. On the other hand, it is also necessary to more effectively utilize the power transmission network and maximize the transmission capacity of the line under the premise of ensuring the reliable operation of the power system. The flexible power transmission mode dominated by power electronic devices emerges as the times require, greatly promotes the access of renewable energy and the increase of line transmission power. Among them, the voltage source converter based high voltage direct current system (VSC-HVDC) has attracted the attention of experts and scholars due to its low output voltage and current harmonic content, no risk of commutation failure, and the advantages of active and reactive power fast decoupling control. It is expected to be widely used in renewable energy grid connection, multi-terminal DC construction, weak system interconnection and other occasions, and is the key technology to build future intelligent power transmission network. However, after large-scale renewable energy is fed into the receiving end power grid through the VSC-HVDC system, the proportion of synchronous generator units decreases, the inertia of the receiving end power grid weakens, and the anti-disturbance ability of the system decreases. At the same time, the output fluctuation of renewable energy power station causes frequent fluctuations of the power grid, and the safe and stable operation of the receiving end power grid faces great challenges. Grid-forming (GFM) is a new converter control technology emerging in recent years, which can simulate the operating characteristics of synchronous generators and can be equivalent to a voltage source to provide reliable voltage and frequency support for the power grid. However, the voltage source characteristics of the grid-forming control lead to the output current being determined by external operating characteristics, and when a frequency drop accident occurs, overcurrent risk is easy to occur, which in turn damages the converter.

[0003] To solve the problem of overcurrent caused by frequency drop, experts have proposed different control methods. The existing current limiting methods mainly include mode switching method and virtual impedance method; when the current exceeds the maximum current allowed by the converter, the mode switching method will convert the control mode of the converter from the original grid-forming mode to the grid-following mode, that is, from the voltage source characteristic to the current source characteristic, and uses the characteristics of the grid-following converter to limit the current. However, this method needs to switch to an additional control loop, which changes the original control architecture, not only causing a sudden change in phase angle, but also destroying the frequency regulation characteristics of the grid-forming converter, reducing the robustness of the control system. The virtual impedance method is to limit the voltage reference value by adding a virtual impedance to prevent the voltage controller from issuing a larger current instruction, thereby limiting the output current of the GFM converter. However, the actual impedance of the actual system is difficult to accurately obtain, making it difficult to accurately design the virtual impedance. Although the improved adaptive virtual impedance method can alleviate the design problem of virtual impedance to some extent and achieve the purpose of current limiting, due to the inability to accurately locate the disturbance and accurately measure the line impedance, the reasonable setting of virtual impedance is still a difficult problem, and the expected current limiting function is still difficult to achieve. Although there are methods to limit the output current of the converter by limiting the output voltage of the converter, the output voltage of the converter is the same as the grid point voltage, thereby achieving current limiting, but for system overload problems, the grid point voltage hardly changes, and the converter cannot obtain effective control signals. This method is difficult to apply, so although the above-mentioned technologies can achieve the effect of current limiting in specific scenarios, they introduce new control loops, change the frequency control characteristics of the grid-forming system, and the robustness of the control system is poor. There are difficulties in accurate measurement and design of parameters, and it is difficult to effectively cover the potential operating scenarios of the grid-forming flexible DC system, so the application scenarios are limited, and therefore it is necessary to study a current limiting method that does not affect the frequency regulation characteristics of the grid-forming converter and does not require complex parameter setting, and has a wider application scenario, so as to ensure the safe and reliable operation of the grid-forming flexible DC system. SUMMARY

[0004] The technical problems to be solved and the technical tasks proposed by the present application are to perfect and improve the prior art, and to provide a grid-forming converter current limiting method without changing the frequency regulation characteristics of the flexible DC transmission system, to solve the problem of the existing current limiting method affecting the system frequency regulation characteristics and the difficulty of parameter design. To this end, the present application adopts the following technical solutions.

[0005] The first technical solution of the present application is a grid-forming converter current limiting method without changing the frequency regulation characteristics of the flexible DC transmission system, comprising the following steps:

[0006] Based on the droop control architecture of the grid-forming converter, the output voltage amplitude and the maximum overload current amplitude of the converter are obtained, and the maximum output power corresponding to the maximum overload current of the converter is calculated;

[0007] Real-time acquisition of three-phase voltage and three-phase current at the output end of the converter, calculation of instantaneous active power, instantaneous reactive power and instantaneous power;

[0008] Taking the maximum output power as the criterion, when the system is subjected to a large disturbance and the actual output power exceeds the maximum output power, the current active power and reactive power instantaneous values are latched;

[0009] The latched active power value is taken as the maximum active power parameter of the feedforward control, and 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;

[0010] The latched reactive power value is taken as the maximum reactive power parameter of the feedforward control, and the power reference value of the reactive-voltage droop equation is adjusted in real time through the feedforward reactive power to limit the reactive power output to the maximum reactive power;

[0011] So that the output power of the converter does not exceed the maximum output power, thereby limiting the output current to not exceed the maximum overload current amplitude;

[0012] The feedforward control does not change the frequency regulation characteristics of the droop control, and the system disturbance is automatically exited after removal.

[0013] The technical solution completely retains the original droop control structure and frequency regulation function of the network-type converter, and through the cooperative mode of'real-time power calculation-latching-feedforward control', the complex current limiting problem is converted into a precise power limiting problem. When the system encounters a large disturbance leading to power over-limiting, the current power value is latched and quickly intervened, and the power reference value of the droop control is dynamically adjusted through the feedforward amount, so that the active and reactive outputs of the converter are strictly limited within the safe capacity, thereby fundamentally avoiding the risk of overcurrent damage to the power device. The whole process does not need to switch the control mode, eliminates the phase angle jump and system impact, and maintains the voltage and frequency support capability of the network-type converter to the power grid. In addition, the method does not depend on the accurate measurement of the grid impedance and other difficult-to-obtain parameters, avoids the tedious parameter setting, has strong robustness, is easy to implement in engineering, can effectively adapt to various complex operating conditions of the flexible and straight system, ensures the safety of the converter itself, and guarantees the stability and power supply reliability of the whole system.

[0014] As a preferred technical means: the maximum output power is the product of the voltage amplitude at the output end of the converter and the maximum overload current amplitude, and is multiplied by a coefficient of 1.5.

[0015] A clear, stable and easy-to-adjust power overload threshold is established by directly quantifying the maximum output power as the product of the "converter terminal voltage amplitude", "maximum overload current amplitude" and a fixed coefficient (1.5). The characteristics of the three-phase system are fully considered, and the real capacity limit of the instantaneous power at the converter port can be accurately reflected. Based on this criterion, the current limiting method can quickly and accurately judge the overload state without relying on complex online calculation or difficult-to-measure system parameters, thereby ensuring the timeliness and reliability of the current limiting action and providing a solid guarantee for the safe and stable operation of the converter. The fixed coefficient is derived from the phase coefficient of the three-phase apparent power calculation, i.e. 3 / 2, which corresponds to the instantaneous power calculation criterion under the three-phase three-wire system.

[0016] As a preferred technical means: the calculation of the instantaneous active power adopts the three-phase instantaneous power theory and is obtained by the sum of the products of three-phase voltage and three-phase current; the calculation of the instantaneous reactive power is obtained by the combination operation based on three-phase voltage difference and three-phase current; and the instantaneous power is obtained by square sum and square root of the instantaneous active power and the instantaneous reactive power.

[0017] The power calculation system adopted in the technical solution is based on the classical three-phase instantaneous power theory and can accurately capture the dynamic power change of the power grid in real time. The instantaneous active power is directly calculated by the sum of the products of three-phase voltage and current instantaneous values, which has clear physical meaning and no delay, and provides a real and reliable data basis for quickly judging the power state of the system. The instantaneous reactive power is calculated by a special combination operation based on three-phase voltage difference and current, which can effectively separate the reactive component of the system and has good compatibility with the traditional definition. Finally, the instantaneous power is calculated by square sum and square root of the calculated active and reactive power, which ensures the comprehensiveness and accuracy of the power calculation result and can truly reflect the apparent power level at the converter port, providing an accurate and delay-free key criterion for subsequent overload judgment and current limiting protection, greatly improving the response speed and reliability of the entire control system.

[0018] As a preferred technical means: the active-frequency droop control equation is used to adjust the angular frequency of the converter 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-voltage droop control equation adjusts the amplitude of the converter terminal voltage 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.

[0019] The core control strategy of the technical solution inherits the droop characteristics of the traditional synchronous generator and enables efficient implementation in the power electronic converter. The active-frequency droop control equation establishes a linear descending relationship between power and frequency by introducing a product term of active power deviation and droop coefficient, enabling the converter to autonomously respond to system frequency changes and simulate the inertia and primary frequency modulation function of the synchronous machine by increasing or reducing active power, thereby providing essential frequency support for the power grid. Correspondingly, the reactive-voltage droop control equation establishes a direct correlation between reactive power and voltage amplitude, enabling the converter to automatically adjust reactive output based on the voltage level at the access point, thereby stabilizing the grid voltage. The equation structure is clear, and the parameters have clear physical meaning (the droop coefficient determines the sensitivity of the adjustment), which together form the cornerstone of grid-connected converter stability. Most importantly, the scheme is based on the mature and reliable droop equation for feedforward optimization, rather than changing its inherent structure, thereby fundamentally ensuring the continuity and stability of the system frequency and voltage modulation characteristics.

[0020] As a preferred technical means: the determination method of the feedforward active power and the adjusted active power reference value is that when the system output instantaneous power 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 rated angular frequency of the system divided by the active-frequency droop coefficient, plus the difference between the maximum active 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 rated angular frequency of the system divided by the active-frequency droop coefficient, plus the maximum active output power; when the system output instantaneous power is less than the maximum output power, the feedforward active power is zero, and the adjusted active power reference value remains unchanged.

[0021] The technical solution adopts an adaptive feedforward mechanism that is closely coupled with the system state (angular frequency deviation) and the operating boundary (maximum active power). When the system is overloaded, the feedforward amount is not a simple fixed value, but is dynamically calculated, and its value includes the correction amount of the current angular frequency deviation from the rated value and the adjustment amount of the power reference value. The adjusted active power reference value can respond to both system frequency changes and power limiting requirements, fully utilizing the overload capacity of the converter to provide frequency support for the power grid, accurately limiting the output power within the safe upper limit, and achieving a balance between support and protection. During normal operation, the feedforward amount is automatically zero, and the control loop seamlessly switches back to the original droop equation, ensuring smooth transition of the control mode without any disturbance. The entire strategy logic is clear, the response is fast, and no additional mode switching logic is needed, thereby fundamentally maintaining the integrity of the droop control characteristics.

[0022] As a preferred technical means: the determination mode of the feedforward reactive power and the adjusted reactive power reference value is that when the system output instantaneous power 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 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; 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 as the current reactive power reference value.

[0023] The technical solution combines the real-time voltage state of the system with the reactive power output limit to form an adaptive limiting mechanism. When the system is overloaded, the feedforward amount is not simply fixed as the maximum reactive power, but dynamically includes the correction amount of the current voltage deviation from the rated value and the adjustment amount of the reactive power reference value. The adjusted reactive power reference value can respond to the voltage fluctuation of the access point and the self-reactive power capacity limit, ensuring that the converter can provide maximum reactive power to support the grid voltage when overloaded, and strictly limiting the reactive power output within the safe allowable range, accurately achieving the optimal balance between voltage support and device self-protection. In normal operation, the feedforward channel is automatically silenced, and the control system is restored to the standard droop characteristic without disturbance, ensuring smooth and seamless switching of the control mode. The entire mechanism responds quickly and accurately, without external intervention, maintaining the inherent reactive power-voltage regulation characteristics of the grid-forming converter.

[0024] As a preferred technical means: the process of automatically exiting the current limiting mode is that when the system disturbance is removed and the output power is less than the maximum allowed output power, the feedforward active power and the feedforward reactive power are automatically set to zero, and the adjusted active power reference value and the reactive power reference value are restored to the initial active power reference value and the initial reactive power reference value, respectively, and the converter exits the current limiting mode.

[0025] The core advantage of the technical solution in the exit mechanism is that it realizes non-disturbance, automatic, and smooth mode switching. Once the system disturbance is removed and the power is not over-limit, the feedforward active and reactive power can be automatically set to zero instantly, so that the adjusted power reference value seamlessly returns to the initial set value. This process is triggered autonomously by the system state (power not over-limit) without any external detection, judgment, or switching instruction, avoiding secondary impact or phase angle mutation caused by control mode switching. The converter can naturally return to the normal operating state according to the original droop control equation, and the entire process is smooth, rapid, and reliable, strictly ensuring the continuity of the grid-forming control characteristics and the overall stability of the system.

[0026] The second technical solution of the application is a grid-connected converter current limiting system for implementing the grid-connected converter current limiting method without changing the frequency modulation characteristics of the flexible and direct transmission system.

[0027] A data acquisition module is configured to acquire electrical quantity measurement values of the converter port, including three-phase voltage, three-phase current, system frequency, angular frequency, and converter port voltage amplitude.

[0028] A power calculation module is configured to calculate instantaneous active power, instantaneous reactive power, and instantaneous power according to the three-phase voltage and three-phase current obtained by the data acquisition module.

[0029] A judgment module is configured to compare the instantaneous power calculated by the power calculation module with a preset maximum allowed output power, and output a judgment signal according to the comparison result.

[0030] A latch module is configured to record the instantaneous active power and instantaneous reactive power at a time instant before current overload according to the judgment signal output by the judgment module, and output the latched active power value and reactive power value.

[0031] A power feedforward module is configured to generate feedforward active power and feedforward reactive power according to the latched value output by the latch module, so as to adjust the power reference value in droop control.

[0032] A current control module is configured to generate a current reference value of the converter through a control algorithm based on the difference between the DC capacitor voltage reference value and the actual value, and generate a PWM control signal.

[0033] A control module is configured to control the output power of the converter according to the PWM control signal, so as to limit the output current.

[0034] The data acquisition module ensures that the control decision is based on real and real-time electrical state of the converter port, and provides a sensing basis for the system. The power calculation module uses classical theory to accurately calculate the key power state quantity in real time, and is the data source of all subsequent intelligent decisions. The judgment and latch modules constitute the "intelligent brain" of the system, which can not only quickly diagnose the overload state, but also accurately capture and lock the critical power value at the moment before overload occurs, providing the most critical and reasonable input parameter for feedforward control and avoiding the impact that may be caused by directly using the maximum value. The power feedforward module is the innovation core of the whole system, which receives the latched value and generates accurate feedforward quantity, and realizes accurate and rapid limitation of the output power without changing the original control structure by seamlessly embedding and fine-tuning the original droop control reference value. Finally, the current control and execution module ensures that all control decisions can be accurately and quickly converted into PWM driving signals of the converter, and finally realizes effective limitation of the output current.

[0035] The third technical solution of the present application is a computer device, which comprises one or more processors and one or more memories, and at least one program code is stored in the one or more memories, and the program code is executed by the one or more processors to realize the grid-connected converter current limiting method without changing the frequency regulation characteristics of the flexible HVDC transmission system as described above.

[0036] The fourth technical solution of the present application is a storage medium, which stores at least one program code, and the program code is executed by a processor to realize the steps of the grid-connected converter current limiting method without changing the frequency regulation characteristics of the flexible HVDC transmission system as described above.

[0037] Beneficial effects: Based on the stable voltage frequency of the grid-connected converter, the frequency regulation characteristics of the original grid-connected converter are not affected, the power feedforward module is added in the power loop, the maximum output power of the converter is used as the basis for judging current overload, and the current limitation is realized by limiting the output power of the converter on the basis of the stable output voltage of the grid-connected converter. Compared with the existing current limiting method, the present method does not need to change the frequency regulation characteristics of the grid-connected control, will not cause the phase angle mutation of the system, and the control system has better robustness. At the same time, the present method does not need accurate impedance parameter measurement and control parameter setting, can effectively cope with the potential operation scenarios of the grid-connected flexible HVDC system, and realizes reliable and stable current limitation. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 is a grid-connected flexible HVDC system and control architecture diagram.

[0039] Figure 2 is a current overload hysteresis judgment control architecture diagram.

[0040] Figure 3 is an improved droop control structure diagram with power feedforward module.

[0041] Figure 4 is a current waveform diagram of the grid-connected flexible HVDC system without current limitation under large disturbance.

[0042] Figure 5 is a current waveform diagram of the grid-connected flexible HVDC system without current limitation under small disturbance.

[0043] Figure 6 is a current waveform diagram of the grid-connected flexible HVDC system without current limitation under large disturbance.

[0044] Figure 7 is a current waveform diagram of the grid-connected flexible HVDC system under the present method under large and small disturbances.

[0045] Figure 8is the current waveform diagram of the network-forming flexible system under the method of the present application when a small disturbance occurs.

[0046] Figure 9 is the current waveform diagram of the network-forming flexible system under the method of the present application when a large disturbance occurs.

[0047] Figure 10 is the comparison diagram of the d-axis current component under the condition of no current limit and the control of the method of the present application.

[0048] Figure 11 is Figure 10 a partial enlargement of Fig. 10.

[0049] Figure 12 is the comparison diagram of the q-axis current component under the condition of no current limit and the control of the method of the present application.

[0050] Figure 13 is Figure 12 a partial enlargement of Fig. 11.

[0051] Figure 14 is the voltage waveform diagram of the network-forming flexible system under no current limit when a small and large disturbance occurs.

[0052] Figure 15 is Figure 14 a partial enlargement of Fig. 12.

[0053] Figure 16 is the voltage waveform diagram of the network-forming flexible system under the method of the present application when a small and large disturbance occurs.

[0054] Figure 17 is Figure 16 a partial enlargement of Fig. 13.

[0055] Figure 18 is the comparison diagram of the output active power under the condition of no current limit and the control of the method of the present application.

[0056] Figure 19 is Figure 18 a partial enlargement of Fig. 14.

[0057] Figure 20 is the comparison diagram of the output reactive power under the condition of no current limit and the control of the method of the present application.

[0058] Figure 21 is Figure 20 a partial enlargement of Fig. 15.

[0059] Figure 22 is the comparison diagram of the active power reference value under the condition of no current limit and the control of the method of the present application.

[0060] Figure 23 is the comparison diagram of the reactive power reference value under the condition of no current limit and the control of the method of the present application.

[0061] Figure 24is a size perturbation without current limit and frequency response under the control of the method.

[0062] Figure 25 is a flow chart of the present invention. DETAILED DESCRIPTION

[0063] The technical solutions of the present invention are further described in detail below in combination with the accompanying drawings of the specification.

[0064] Example one:

[0065] As Figure 25 shown, a network type flexible DC system current limiting method without affecting the frequency control characteristics, namely the power feedforward current limiting method, includes the following steps:

[0066] Obtain the converter output voltage amplitude and the maximum overload current amplitude , calculate the maximum output power corresponding to the converter output maximum overload current ;

[0067] Real-time acquisition of three-phase voltage and actual output three-phase current at the converter output, real-time calculation of active power and reactive power output by the converter, and calculation of instantaneous power ;

[0068] With the maximum output power as the criterion, when the network type flexible DC system is subjected to a large disturbance, if the actual output power of the converter , then the active power instantaneous value and the reactive power instantaneous value output by the converter are latched;

[0069] Based on the active-power-frequency droop equation in droop control, the latched value of active power is taken as the maximum active power parameter , the active power is fed forward to play a role, the power reference value of the active-power-frequency droop equation is adjusted in real time , and the active power output by the converter is limited to the maximum active power ;

[0070] Based on the reactive-power-voltage droop equation in droop control, the latched value of reactive power is taken as the maximum reactive power parameter , the reactive power is fed forward to play a role, the power reference value of the reactive-power-voltage droop equation is adjusted in real time Limiting the reactive power output of the converter to the maximum reactive power ;

[0071] Under the action of the power feedforward module, the output power of the converter is always less than or equal to the maximum output power , thereby realizing that the output current of the converter does not exceed the maximum overload current amplitude ;

[0072] The additional power feedforward link does not change the frequency control characteristics of the droop control, and after the system disturbance is removed, the active output power and the reactive output power gradually recover, and the current limiting mode is automatically exited.

[0073] The embodiment can realize current limiting by limiting the output power of the converter without changing the frequency control characteristics of the grid-forming converter. 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 and the amplitude of the terminal voltage, the three-phase voltage at the output terminal of the converter and the actual output three-phase current are obtained, the active power and the reactive power output by the converter are calculated in real time, and the instantaneous power is calculated, the maximum output power is taken as the action basis of the latch module, when the output power of the converter is greater than the maximum allowed output power, the active power value and the reactive power value at this time are latched, and the active power and the reactive power latched values are provided to the power feedforward module, at this time the power feedforward module outputs the feedforward power, the active power reference value and the reactive power reference value in the droop control equation are adjusted in real time, the converter always outputs the maximum power is realized, thereby realizing the limitation of the maximum allowed overload current, when the disturbance is removed, since the current limiting method does not change the frequency control characteristics of the grid-forming converter, under the action of the droop control, the active power and the reactive power output by the converter gradually recover, the current gradually decreases, the converter automatically exits the current limiting mode, and returns to the stable working state before the disturbance.

[0074] The control method proposed by the application is suitable for a grid-forming flexible DC system, when the system is disturbed and the current of the converter needs to be limited, the control strategy proposed by the application can timely adjust the reference values of the active power and the reactive power in the droop control equation without affecting the frequency control characteristics of the grid-forming converter, limit the power output of the grid-forming converter, and thereby realize the current limiting of the converter. The method does not require complex parameter measurement and setting, can effectively cope with potential operating scenarios of the flexible DC system, and ensures the safe and reliable operation of the grid-forming flexible DC system.

[0075] Referring to Figure 1 , the embodiment mainly comprises a new energy power supply, a back-to-back converter, a load part and the like. The flexible DC power transmission system transmits power to the load and can limit the current when the system is disturbed by the load, so that the current is not overloaded, and the specific method is as follows:

[0076] S1, the machine-side converter adopts a constant voltage control strategy to generate a direct current capacitor voltage instruction value ;

[0077] S2, the machine-side converter d-axis current given value is 0, and the q-axis current instruction value According to the difference between the direct current capacitor voltage instruction value in step S1 and the direct current capacitor voltage , the d-axis current instruction value q is obtained through the proportional integral-based control module ;

[0078] S3, according to the current instruction value in step S2 and the instruction value , the PWM control signal of the machine-side converter is generated by the current loop;

[0079] Referring to Figure 1 , the machine-side converter adopts constant voltage control, and the difference between the direct current capacitor voltage instruction value and the direct current capacitor voltage is obtained through the proportional integral-based control module to generate the q-axis current instruction value , the d-axis current instruction value is 0, and finally the PWM control signal is generated through the current loop control.

[0080] S4, the grid-side converter adopts droop control to give the active power reference value , and the converter output power angle is given according to the active-frequency control equation of the droop control, and the active-frequency control equation of the droop control is:

[0081] (1)

[0082] In the formula, is the angular frequency of the converter, is the rated value of the angular frequency of the grid side, is the active power reference value, is the variable output power, is the active-frequency droop coefficient, and the active-frequency droop coefficient is a coefficient reflecting the adjustment sensitivity of the active power deviation to the angular frequency of the converter.

[0083] S5, the reactive power reference value is given, and the reference value of the grid-side voltage is given according to the reactive-voltage control equation of the droop control, and the reactive-voltage control equation of the droop control is:

[0084] (2)

[0085] wherein, is the amplitude of the converter terminal voltage, is the rated value of the grid-side voltage, is the reactive-voltage droop coefficient, is the reactive power reference value, is the converter output reactive power; the reactive-voltage droop coefficient is a coefficient reflecting the sensitivity of the adjustment of the converter terminal voltage amplitude to the reactive power deviation.

[0086] S6, the converter output power angle obtained according to steps S4 and S5 and the amplitude of the converter terminal voltage give the PWM control signal of the grid-side converter via the voltage-current double closed loop control;

[0087] Referring to Figure 1 , the grid-side adopts droop control, and the power instruction and the grid-side power are subtracted to obtain the converter angular frequency , the grid-side converter output power angle is obtained via an integral element, the voltage instruction value θ is generated by formula (2), and finally the PWM control signal of the grid-side converter is obtained via a voltage-current double closed loop control module; The PWM control signal of the machine-side converter obtained according to step S3 and the PWM control signal of the grid-side converter obtained according to step S6 control the output power of the flexible DC system. Since the machine-side converter adopts constant voltage control, on the basis of the stable machine-side converter output voltage, the system output power and output current are mainly determined by the grid-side converter;

[0088] S7, referring to

[0089] in combination with the actual parameters of the converter and the converter port voltage, the maximum power allowed to be output by the converter when the current is not overloaded is calculated, and the calculation equation is: Figure 2

[0090] (3)

[0091] wherein, is the maximum power output by the converter when the current is not overloaded, is the amplitude of the converter port output voltage, is the maximum overload current of the converter.

[0092] ​S8, the actual output of the converter is calculated by combining the three-phase voltage and three-phase current of the converter port, and the calculation equation is:

[0093] (4)

[0094] (5)

[0095] (6)

[0096] In the formula, is the a-phase voltage of the converter port, is the b-phase voltage of the converter port, is the c-phase voltage of the converter port, is the a-phase output current of the converter, is the b-phase output current of the converter, is the c-phase output current of the converter.

[0097] S9, when the flexible DC system is disturbed, the grid frequency decreases, and under the action of the network type frequency modulation control, the system output power increases to provide frequency and voltage support for the grid;

[0098] S10, at this time, the instantaneous output power of the converter is calculated, if the output power exceeds the maximum output power limit, the current active power output and reactive power output are latched, and the latched value is assigned to the power feedforward module;

[0099] S11, referring to Figure 3 the active power feedforward submodule of the power feedforward module plays a role, generates a feedforward active power, and adjusts the active power reference value in the active-frequency droop control equation in real time, and the calculation equation of the feedforward active power and the active power reference value is as follows:

[0100]

[0101] (7)

[0102] In the formula, is the feedforward active power, is the maximum active power of the converter output, is the new active power reference value after the feedforward active power adjustment;

[0103] S12, the reactive power feedforward submodule of the power feedforward module plays a role, generates a feedforward reactive power, and adjusts the reactive power reference value in the reactive-voltage droop control equation in real time, and the calculation equation of the feedforward reactive power and the reactive power reference value is as follows:

[0104]

[0105] (8)

[0106] wherein, is the feed-forward reactive power, is the maximum reactive power output of the converter, is the new reactive power reference value after adjustment of the feed-forward reactive power;

[0107] S13, under the adjustment of the feed-forward power, the output power of the converter is always the maximum allowed output power, and the current is also limited to the maximum allowed overload current;

[0108] S14, after the system disturbance is removed, the active output power and the reactive output power gradually decrease under the action of the droop control equation, the converter output current decreases accordingly, the feed-forward active power and the feed-forward reactive power are automatically set to zero, and the converter automatically exits the current limiting mode;

[0109] Referring to Figure 2 , when the flexible DC system is disturbed, the grid frequency decreases, under the action of the grid-forming type frequency modulation control, the system output power increases to provide voltage frequency support for the grid, the instantaneous output power of the converter is calculated in combination with the three-phase voltage at the port of the converter and the three-phase output current of the converter, if the output power exceeds the maximum output power limit, the current active power output and the current reactive power output are latched, and the latched values are assigned to the power feed-forward module, it is judged whether the angular frequency is less than the minimum angular frequency of the system, if the less than condition is met, the power matching module is enabled, at this time, the active power feed-forward module plays a role, generates the feed-forward active power, and adjusts the active power reference value in the active-frequency droop control equation in real time, at the same time, the reactive power feed-forward module also plays a role, generates the feed-forward reactive power, and adjusts the reactive power reference value in the reactive-voltage droop control equation in real time, under the adjustment of the feed-forward power, the output power of the converter is always the maximum allowed output power, and the current is also limited to the maximum allowed overload current, after the system disturbance is removed, the active output power and the reactive output power gradually decrease under the action of the droop control equation, the converter output current decreases accordingly, the feed-forward active power and the feed-forward reactive power are automatically set to zero, and the converter automatically exits the current limiting mode;

[0110] In some embodiments of the present application, the judgment basis of the current overload is:

[0111] (9)

[0112] wherein, is the instantaneous power output of the converter.

[0113] 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.

[0114] 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.

[0115] like Figure 4 , Figure 5 , Figure 6 As 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.

[0116] 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.

[0117] Figure 10 , Figure 11 and Figure 12 , Figure 13The comparison of d-axis current component and q-axis current component under no current limit and the proposed method is shown. It can be seen from the figure that the proposed method simultaneously realizes the limitation of d-axis current and q-axis current when the system suffers from large disturbance. The maximum value of d-axis current component exceeds 4.9A and the maximum value of q-axis current component exceeds 1.35A without current limit, which finally leads to the output current of the converter exceeding the maximum allowable value 4.82A. Under the action of 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 finally the output current of the converter is limited to 4.82A, realizing the current not exceeding the limit and protecting the safety of the converter itself.

[0118] Figure 14 、 Figure 15 and Figure 16 、 Figure 17 The voltage waveform changes of the network-forming flexible transmission system under large disturbance under no current limit and the proposed method are shown. The results show that the large shortage of active power has little effect on the output voltage of the converter. Under no current limit and the proposed method control, the output voltage of the network-forming converter changes little, only about 1V of voltage amplitude change. At the same time, it can be seen from the figure that the proposed method does not change the voltage support performance of the original droop control. The equipment can still provide stable and reliable voltage support and will not appear voltage oscillation instability.

[0119] Figure 18 、 Figure 19 and Figure 20 、 Figure 21 The comparison of active power and reactive power changes under no current limit and the proposed method is shown. The results show that due to the shortage of active power in the system, the equivalent frequency of the system is reduced. Under the action of droop control, the converter increases the active power output. When there is no current limit, the active power output exceeds 2300W. The increase of active power also causes the increase of reactive power. The reactive power output exceeds 470Var. Finally, the output power of the converter exceeds the maximum capacity of the equipment, which further leads to the overload operation of the equipment. Combined with Figure 22 and Figure 23 , the power feedforward module adjusts the reference value of active power and the reference value of 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, avoiding the overload operation of the equipment.

[0120] Figure 24The current-limiting method does not work under small disturbance, and the frequency response characteristics of the two methods are the same. When a large disturbance occurs, the output power of the current limiter without current limitation increases significantly, supports the system frequency, and avoids a large frequency drop of the system. However, at this time, the output power exceeds the maximum capacity of the current limiter, and the equipment is in an overcurrent working condition, which seriously endangers the safety of the current limiter itself. Under the proposed method, the output power of the current limiter increases, and also has the ability to support the system frequency to avoid a large frequency drop of the system. However, under the control of the proposed method, the output power of the current limiter is always limited to the maximum capacity of the equipment, avoiding equipment overcurrent and ensuring the safety of the equipment itself. However, due to the power limiting effect of the proposed method, the output power of the equipment is smaller than that without current limitation, so the system frequency drops more than that without current limitation. After the active power shortage is recovered, the system frequency gradually recovers, the output power of the current limiter decreases, the output current decreases, and the current limiting mode is smoothly exited without additional mode switching.

[0121] The simulation results show that under the condition of no current limitation, the network-forming converter in the flexible transmission system is prone to current overload when the system power is insufficient, which endangers the safety of the equipment itself. The current limiting method proposed in the patent limits the active output power and reactive output power of the converter by increasing the power feedforward module and changing the reference values of active power and reactive power in the active-frequency equation and the reactive-voltage equation, thereby achieving the purpose of current limitation. The simulation verifies the superior performance of the proposed method, which not only limits the output current of the network-forming converter, but also does not change the original frequency control characteristics of the network-forming converter, maintaining the synchronization ability of the converter.

[0122] Embodiment Two

[0123] The embodiment provides a network-forming converter current limiting system without changing the frequency modulation characteristics of the flexible transmission system, which adopts the method of embodiment one. The network-forming converter current limiting system comprises:

[0124] a data acquisition module configured to acquire three-phase voltage , three-phase current , frequency , angular frequency , voltage amplitude , feedforward active power , feedforward reactive power and reference power .

[0125] a power calculation module configured to calculate instantaneous active power , reactive power , and instantaneous power .

[0126] a judging module, configured to judge whether the output power of the converter exceeds the maximum allowed output power and determine whether the latch module acts according to the size;

[0127] a latch module, configured to record the instantaneous active power and the instantaneous reactive power at a time before the current overload, and provide the latch value to the maximum active power parameter and the maximum reactive power parameter of the power feedforward module ;

[0128] a power feedforward module, configured to generate the feedforward active power and the feedforward reactive power according to the latch value output by the latch module, so as to adjust the power reference value in the droop control;

[0129] a current control module, configured to obtain the current reference value of the flexible DC converter through a proportional-integral control module based on the difference between the DC capacitor voltage reference value and the DC capacitor voltage and generate the PWM control signal of the converter through a current loop;

[0130] a control module, configured to control the output power of the converter according to the PWM control signal of the flexible DC converter, so as to limit the output current and protect the converter from overloading when the power grid is subjected to a large load disturbance.

[0131] The system realizes intelligent control of the meshed flexible DC system through the cooperative work of the modules. The system realizes the intelligent control of the meshed flexible DC system through the cooperative work of the modules. Through real-time measurement of the converter port voltage and the output current, real-time calculation of the instantaneous active power, the instantaneous reactive power and the instantaneous power output by the converter, judgment of whether the converter is overcurrent according to whether the output power exceeds the maximum output power, real-time latching of the active power and the reactive power output, generation of the feedforward power in combination with the latched value, real-time adjustment of the active power reference value and the reactive power reference value in the droop control equation, realization of the limitation of the output power of the converter and the limitation of the current of the converter, and no change of the original droop meshing frequency modulation characteristic, the system improves the reliability of the equipment while ensuring the safe operation of the converter.

[0132] Embodiment Three

[0133] The embodiment provides an electronic device, which comprises a memory, a processor, and a computer program stored in the memory and executable on the processor, and the processor implements a meshed converter current limiting method without changing the frequency modulation characteristic of a flexible DC transmission system according to the computer program.

[0134] ​​​​Embodiment Four

[0135] The embodiment provides a computer readable storage medium, which stores a computer program, and the computer program is executed by a processor to implement a network configuration type current limiting method of a converter which does not change frequency modulation characteristics of a flexible direct transmission system.

[0136] Those skilled in the art can realize that the units and algorithm steps described in the embodiments of the present application can be realized in electronic hardware, computer software and a combination of electronic hardware and computer software. Whether the functions are realized 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 realize the described functions for each specific application, but such implementation should not be considered beyond the scope of the present application.

[0137] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working processes of the above-described system, device and unit can refer to the corresponding processes in the foregoing method embodiments, which will not be described here.

[0138] In several embodiments provided in the present application, any function realized in the form of a software function unit and sold or used as an independent product can be stored in a computer readable storage medium. Based on such understanding, the technical solutions of the present application essentially or the parts of the technical solutions that make contributions to the prior art or the parts of the technical solutions can be embodied in the form of a software product. The computer software product is stored in a storage medium and includes a plurality of instructions for causing a computer device (which can be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the methods described in the embodiments of the present application. The foregoing storage medium includes a U disk, a mobile hard disk, a read-only memory (ROM), a random access memory (RAM), a magnetic disk or an optical disk and various program code storage media.

[0139] The network configuration type current limiting method, system, device and medium shown above are specific embodiments of the present application, have embodied the essential characteristics and progress of the present application, and can be modified in shape, structure, etc. according to actual use needs under the guidance of the present application, which are all within the protection scope of the present application.

Claims

1. A network configuration type current limiting method for a converter which does not change the frequency modulation characteristics of a flexible AC transmission system, characterized in that, The method comprises the following steps: Based on the droop control architecture of the networked converter, the amplitude of the output voltage of the converter and the maximum overload current amplitude are obtained, and the maximum output power corresponding to the maximum overload current of the output of the converter is calculated; The three-phase voltage and three-phase current at the output end of the converter are obtained in real time, and the instantaneous active power, instantaneous reactive power and instantaneous power are calculated; When the system is subjected to a large disturbance and the actual output power exceeds the maximum output power, the current active power and reactive power instantaneous values are latched as the criterion of the maximum output power; The latched active power value is used as the maximum active power parameter of the feedforward control, and the power reference value of the active-frequency droop equation is adjusted in real time by 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, and the power reference value of the reactive-voltage droop equation is adjusted in real time by the feedforward reactive power to limit the reactive power output to the maximum reactive power; The output power of the converter is always limited to not exceed the maximum output power, and the output current is limited to not exceed the maximum overload current amplitude; The feedforward control does not change the frequency regulation characteristics of the droop control, and the system disturbance is automatically exited from the current limiting mode after the disturbance is removed.

2. The method of claim 1, wherein the method does not change the frequency modulation characteristics of the flexible AC transmission system. The maximum output power is the product of the amplitude of the voltage at the output end of the converter and the maximum overload current amplitude, and is multiplied by a coefficient of 1.

5.

3. The method of claim 2, wherein the method does not change the frequency modulation characteristics of the flexible AC transmission system. The calculation of the instantaneous active power uses the three-phase instantaneous power theory, and is obtained by the sum of the product of the three-phase voltage and the three-phase current; the calculation of the instantaneous reactive power uses the combined operation based on the three-phase voltage difference and the three-phase current; and the instantaneous power is obtained by the square root of the sum of the square of the instantaneous active power and the instantaneous reactive power.

4. The method of claim 3, wherein the method does not change the frequency modulation characteristics of the flexible AC transmission system. The active-frequency droop control equation is used to adjust the angular frequency of the converter according to the active power deviation, and its expression is the rated angular frequency of the system plus the product of the droop coefficient and the difference between the active power reference value and the actual active power; the reactive-voltage droop control equation is used to adjust the voltage amplitude at the output end of the converter according to the reactive power deviation, and its expression is the rated voltage of the system plus the product of the droop coefficient and the difference between the reactive power reference value and the actual reactive power.

5. The method of claim 4, wherein the method does not change the frequency modulation characteristics of the flexible AC transmission system. The determination method of the feedforward active power and the adjusted active power reference value is that when the system output instantaneous power is greater than or equal to the maximum output power, the value of the feedforward active power is the difference between the current angular frequency of the converter and the rated angular frequency of the system divided by the active-frequency droop coefficient, plus the difference between the maximum active output power and the current active power reference value; At this time, the adjusted active power reference value is the difference between the current angular frequency of the converter and the rated angular frequency of the system divided by the active-frequency droop coefficient, plus the maximum active output power; when the system output instantaneous power 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.

6. The method of claim 5, wherein the method does not change the frequency modulation characteristics of the flexible AC transmission system. The determination mode of the feed-forward reactive power and the adjusted reactive power reference value is that when the system output instantaneous power is greater than or equal to the maximum output power, the value of the feed-forward 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 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; when the system output instantaneous power is less than the maximum output power, the feed-forward active power and the feed-forward reactive power are zero, and the adjusted active power reference value and the reactive power reference value are restored to the initial active power reference value and the initial reactive power reference value respectively, and the converter exits the current limiting mode.

7. The method of claim 5, wherein the method does not change the frequency modulation characteristics of the flexible AC transmission system. The process of automatically exiting the current limiting mode is that when the system disturbance is removed and the output power is less than the maximum allowed output power, the feed-forward active power and the feed-forward reactive power are automatically set to zero, the adjusted active power reference value and the reactive power reference value are restored to the initial active power reference value and the initial reactive power reference value respectively, and the converter exits the current limiting mode.

8. A grid-connected converter current-limiting system for implementing the grid-connected converter current-limiting method of any one of claims 1 to 7, without changing the frequency modulation characteristics of the soft-flexible delivery system, characterized in that, It comprises: a data acquisition module for acquiring electrical quantity measurement values of a converter port, including three-phase voltage, three-phase current, system frequency, angular frequency and converter port voltage amplitude; a power calculation module for calculating instantaneous active power, instantaneous reactive power and instantaneous power according to the three-phase voltage and three-phase current obtained by the data acquisition module; a judgment module for comparing the instantaneous power calculated by the power calculation module with the preset maximum allowed output power, and outputting a judgment signal according to the comparison result; a latch module for recording the instantaneous active power and the instantaneous reactive power at a time point before current overload according to the judgment signal output by the judgment module, and outputting the latched active power value and the latched reactive power value; a power feed-forward module for generating feed-forward active power and feed-forward reactive power according to the latched values output by the latch module, so as to adjust the power reference value in droop control; a current control module for generating a current reference value of the converter through a control algorithm based on the difference between the DC capacitor voltage reference value and the actual value, and generating a PWM control signal; a control module for controlling the output power of the converter according to the PWM control signal, so as to limit the output current.

9. A computer device, comprising: The device comprises one or more processors and one or more memories, and 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 program code realizes the grid-connected converter current limiting method of claim 1-7.

10. A storage medium having stored therein at least one program code, characterized in that, The program code is executed by the processor to realize the steps of the grid-connected converter current limiting method of claim 1-7 without changing the frequency modulation characteristics of the flexible and straight sending out system. The program code is executed by the processor to realize the steps of the grid-connected converter current limiting method of claim 1-7 without changing the frequency modulation characteristics of the flexible and straight sending out system.

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