Method and system for improving frequency supporting capability of net-forming flexible direct current converter

Abstract

The invention belongs to the field of active frequency support of power electronic equipment in a network construction type flexible direct-current power transmission system, and particularly discloses a method and a system for improving the frequency support capability of a network construction type flexible direct-current converter, and the method comprises the steps: carrying out the disturbance state recognition and anti-shake processing through the dynamic data of the frequency change rate of an alternating-current power grid collected in real time; a feed-forward power output amplitude is dynamically adjusted through adaptive power feed-forward control, and is sent to calculation of an output power reference value in a virtual synchronous machine control strategy, so that frequency response is realized through a short-time dynamic path, and the frequency supporting capability is improved. According to the method, the converter station can provide active frequency support matched with system requirements under complex working conditions of different load levels, new energy permeability, fault types and the like, and the frequency stability of a power grid is comprehensively improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of active frequency support of power electronic equipment in the network-forming flexible HVDC system, and more particularly, relates to a method and system for improving the frequency support capability of a network-forming flexible HVDC converter. BACKGROUND

[0002] With the large-scale access of high-proportion new energy to the power grid, the power system is facing the problems of reduced inertia level and aggravated frequency instability risk. The modular multilevel converter-based high voltage direct current transmission technology (MMC-HVDC) has become a key link for new energy grid connection due to its flexibility and controllability. Among them, the virtual synchronous generator (VSG) control technology simulates the electromechanical characteristics of synchronous generators to provide active inertia support capability for the system, and has received widespread attention in recent years.

[0003] The transient and steady-state performance of VSG control is influenced by the virtual inertia coefficient and the damping coefficient. A large number of studies have shown that there is a contradiction between the influence of the two parameters on the system performance: increasing the virtual inertia can enhance the frequency support capability, but will cause the dominant pole of the system to move to the virtual axis, reduce the damping ratio and thus reduce the system stability; while increasing the damping coefficient can effectively suppress power oscillation, but will slow down the dynamic response speed of the system. In order to cope with the above contradictions, new frequency support methods suitable for VSC-HVDC converter valves are continuously explored to cooperatively improve the stability and frequency support capability of the system. Existing improvement methods mainly focus on the inertia and damping parameters themselves, for example, an adaptive inertia and damping control method is proposed, however, the parameter setting of such method is complex and the engineering applicability is limited. Other methods attempt to improve the VSG control structure, such as introducing lead-lag correction links in the virtual inertia forward channel and damping feedback channel, or using an improved transient damping method based on active power differential feedback, the above methods can be classified as frequency domain correction of VSG control loop. Although the frequency domain correction method can alleviate and The contradiction is solved, but the frequency support capability is still limited by the setting of the frequency modulation coefficient. The setting of the frequency modulation coefficient is highly dependent on the load characteristics of the system, and in the case of multiple types of disturbance, the frequency instability risk is easily caused. The above-mentioned improvement method generally lacks in-depth analysis of the internal mechanism of the VSG participating in the system frequency response, and the fundamental restriction of the setting of the frequency modulation coefficient on the frequency support capability is not fully considered, so it is difficult to fundamentally improve the frequency support capability of the VSC-HVDC converter valve. SUMMARY

[0004] In view of the defects of the prior art, the purpose of the present application is to provide a frequency support capability improvement method and system for a network-type flexible DC converter, aiming at solving the problem that the existing frequency support method for the VSC-HVDC converter valve fails to fully consider the fundamental restriction of the setting of the frequency modulation coefficient on the frequency support capability, resulting in difficulty in improving the frequency support capability of the VSC-HVDC converter valve.

[0005] To achieve the above-mentioned purpose, in a first aspect, the present application provides a frequency support capability improvement method for a network-type flexible DC converter, specifically comprising the following steps: The disturbance state is identified and the anti-jitter processing is performed through the dynamic data of the real-time collected AC power grid frequency change rate, the output amplitude of the feedforward power is dynamically adjusted through the adaptive power feedforward control, and the reference power in the virtual synchronous machine control method is calculated, so that the frequency response is realized through a short-time dynamic path, and the frequency support capability is improved.

[0006] Further preferably, when the load power increases, the synchronous machine output frequency is increased through the common adjustment of the primary frequency modulation power and the feedforward power, the grid-side voltage phase is increased, and then the difference between the internal potential and the grid voltage phase is increased; on the contrary, when the load power decreases, the synchronous machine output frequency is reduced through the common adjustment of the primary frequency modulation power and the feedforward power, the grid-side voltage phase is reduced, and then the difference between the internal potential and the grid voltage phase is reduced.

[0007] Further preferably, the steady-state error of the grid-side synchronous machine frequency after the adaptive power feedforward control is For:

[0008] Among them, is the unbalance of the load power; is the feedforward power; is the frequency modulation coefficient of the virtual governor; is the damping of the synchronous machine; is the frequency modulation coefficient of the synchronous machine governor.

[0009] Further preferably, the feedforward power is:

[0010] in, The inertial time constant of the synchronous machine; Rated capacity of the power grid system; The rated frequency of the power grid system; This is the output frequency of the synchronizer; To reflect the control variable of the feedforward power sign; when it is necessary to add the reference power in the virtual synchronous machine control strategy, =1; When it is necessary to reduce the reference power in the virtual synchronous machine control strategy, =-1; When there is no need to change the reference power in the virtual synchronous machine control strategy. =0.

[0011] More preferably, the disturbance state identification method is as follows: Real-time detection of AC power grid frequency change rate, if and At that time, it is determined that the power grid system is in the steady-state operating range, and thus... =0; When the rate of change of AC power grid frequency exceeds the threshold value for disturbance identification, the anti-shake processing mechanism is activated; The image stabilization mechanism is as follows: Under conditions of sudden load increase, if the AC power grid frequency change rate is at its minimum... consistently below When, set =1, update the reference power in the virtual synchronous machine control method at the current moment to... ; Under conditions of sudden load reduction, if the maximum value of the AC power grid frequency change rate is... If the value remains consistently above the threshold for disturbance detection, set... =-1, update the reference power in the virtual synchronous machine control method at the current moment to ; in, This represents the minimum rate of change of AC power grid frequency. This is the threshold value for disturbance identification; This represents the maximum rate of change of AC power grid frequency. This is the reference power used in the virtual synchronous machine control method of the previous moment.

[0012] Secondly, this application provides a frequency support capability enhancement system for a grid-type flexible DC converter, comprising: The frequency response module is used for disturbance state identification and anti-jitter processing through dynamic data of the real-time collected AC power grid frequency change rate, dynamic adjustment of the feedforward power output amplitude through adaptive power feedforward control, and sending into the calculation of the reference power in the virtual synchronous machine control method, so that the frequency response is realized through a short-time dynamic path, and the frequency support capability is improved.

[0013] Further preferably, the short-time dynamic path is that when the load power increases, the synchronous machine output frequency increases through the common adjustment of the power and the feedforward power in the virtual synchronous machine control method, the grid-side voltage phase is increased, and then the difference between the internal electromotive force and the grid voltage phase is increased; conversely, when the load power decreases, the synchronous machine output frequency is reduced through the common adjustment of the power and the feedforward power in the virtual synchronous machine control method, the grid-side voltage phase is reduced, and then the difference between the internal electromotive force and the grid voltage phase is reduced.

[0014] Further preferably, the steady-state error of the grid-side synchronous machine frequency after the adaptive power feedforward control in the frequency response module is .

[0015] Among them, is an unbalanced amount of load power; is a feedforward power; is a frequency modulation coefficient of a virtual governor; is a damping of a synchronous machine; is a frequency modulation coefficient of a synchronous machine governor.

[0016] Further preferably, the feedforward power in the frequency response module is .

[0017] Among them, is an inertia time constant of a synchronous machine; is a rated capacity of a power grid system; is a rated frequency of a power grid system; is a synchronous machine output frequency; is a control variable reflecting the sign of the feedforward power; when it is needed to increase the reference power in the virtual synchronous machine control method, =1; when it is needed to reduce the reference power in the virtual synchronous machine control method, =-1; when it is not needed to change the reference power in the virtual synchronous machine control method, =0.

[0018] Further preferably, the frequency response module comprises a disturbance state identification unit and an anti-jitter processing unit. The disturbance state identification unit is used for real-time detection of the AC power grid frequency change rate, and if and At that time, it is determined that the power grid system is in the steady-state operating range, and thus... =0; When the AC power grid frequency change rate is detected to exceed the threshold value for disturbance identification, the anti-shake processing mechanism is activated; The anti-jitter processing unit is used to minimize the rate of change of AC grid frequency under conditions of sudden load increase. Persistently below When, set =1, update the reference power in the virtual synchronous machine control method at the current moment to... ; Under conditions of sudden load reduction, if the maximum value of the AC power grid frequency change rate is... If the value remains consistently above the threshold for disturbance detection, set... =-1, update the reference power in the virtual synchronous machine control method at the current moment to ; in, This represents the minimum rate of change of AC power grid frequency. This is the threshold value for disturbance identification; This represents the maximum rate of change of AC power grid frequency. This is the reference power used in the virtual synchronous machine control method of the previous moment.

[0019] Overall, the technical solutions conceived in this application have the following beneficial effects compared with the prior art: This application provides a method for improving the frequency support capability of a grid-connected flexible DC converter. This method involves analyzing the imbalance between the converter and the receiving-end grid synchronizing machine after a power imbalance occurs in the system. s Equation analysis under the domain yields the frequency expression of the receiving-end grid synchronous machine. Combined with the voltage phasor relationship analysis between the converter station and the synchronous machine in the VSG control architecture, a short-time dynamic path is constructed. From the perspective of the electromechanical transient physical essence of the power system, an analytical model of frequency response is built, providing a theoretical basis for the adaptive power feedforward control method.

[0020] This application provides a method for improving the frequency support capability of grid-type flexible DC converters. The proposed adaptive power feedforward control method relies on a fast-response short-time dynamic path to directly associate the feedforward power command with the fundamental physical quantity of the system frequency change rate. This quantity is highly observable and easy to obtain in real time, significantly reducing the complexity of control quantity generation. The frequency change rate essentially reflects the real-time operating status of the system, enabling the feedforward power to have dynamic adaptive adjustment capability. This ensures that the converter station can provide active frequency support that matches the system requirements under complex operating conditions such as different load levels, new energy penetration rates, and fault types, thereby comprehensively improving the frequency stability of the power grid.

[0021] The application provides a frequency support capacity improvement method of a network-constructed flexible direct current converter, integrates a disturbance state logical discrimination mechanism and a jitter prevention filtering method, identifies a frequency change rate feature in real time and suppresses measurement noise, avoids power instruction false triggering caused by short-time fluctuation, effectively prevents frequent fluctuation of output power of the converter station, significantly reduces power oscillation risk, improves frequency support response speed, guarantees system transient stability operation boundary, and provides a frequency support capacity improvement solution method with rapidity and reliability for a high-proportion new energy power grid. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 FIG. 1 is a flowchart of a frequency support capacity improvement method of a network-constructed flexible direct current converter provided by an embodiment of the application; Figure 2 FIG. 2 is a control block diagram of a typical frequency domain correction VSG provided by an embodiment of the application; Figure 3 FIG. 3 is a system topology and total control block diagram provided by an embodiment of the application; Figure 4 FIG. 4 is a block diagram of acquiring a system frequency change rate provided by an embodiment of the application; FIG. 5(a) is a comparison diagram of a VSG control output frequency and a synchronous machine output frequency after power disturbance of a system provided by an embodiment of the application; FIG. 5(b) is a voltage phasor diagram after power disturbance of a system provided by an embodiment of the application; Figure 6 FIG. 6 is a diagram of two paths of a frequency response after power disturbance of a system provided by an embodiment of the application; Figure 7 FIG. 7 is a flowchart of calculating a power feedforward provided by an embodiment of the application; FIG. 8(a) is a diagram of a maximum frequency change rate of a system under a continuously increasing load working condition provided by an embodiment of the application; FIG. 8(b) is a comparison diagram of a power feedforward and related power under a continuously increasing load working condition provided by an embodiment of the application; FIG. 9(a) is a diagram of a maximum frequency change rate of a system under a bidirectional load working condition provided by an embodiment of the application; FIG. 9(b) is a comparison diagram of a power feedforward and related power under a bidirectional load working condition provided by an embodiment of the application; FIG. 10(a) is a comparison diagram of a typical frequency domain correction and output power added with a power feedforward control under a continuously increasing load working condition provided by an embodiment of the application; FIG. 10(b) is a comparison diagram of a typical frequency domain correction and a synchronous machine output frequency added with a power feedforward control under a continuously increasing load working condition provided by an embodiment of the application; Fig. 11(a) is a comparison chart of typical frequency domain correction and output power with power feedforward control under bidirectional changing load conditions according to an embodiment of the present application; Fig. 11(b) is a comparison chart of typical frequency domain correction and output frequency of the synchronous machine with power feedforward control under bidirectional changing load conditions according to an embodiment of the present application; Fig. 12(a) is a comparison chart of output power with different frequency modulation coefficients and under typical frequency domain correction control under bidirectional changing load conditions according to an embodiment of the present application; Fig. 12(b) is a comparison chart of output frequency of the synchronous machine with different frequency modulation coefficients and under typical frequency domain correction control under bidirectional changing load conditions according to an embodiment of the present application; Fig. 13(a) is a comparison chart of output power with different frequency modulation coefficients and under power feedforward control under bidirectional changing load conditions according to an embodiment of the present application; Fig. 13(b) is a comparison chart of output frequency of the synchronous machine with different frequency modulation coefficients and under power feedforward control under bidirectional changing load conditions according to an embodiment of the present application. DETAILED DESCRIPTION

[0023] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and should not be used to limit the present application.

[0024] The term "and / or" used herein is used to describe the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B can mean that there are three cases of A alone, A and B together, and B alone. The symbol " / " in this paper represents the relationship of or, for example, A / B represents A or B.

[0025] The terms "first" and "second" and the like in the description and claims herein are used to distinguish different objects, and are not used to describe a specific order of the objects.

[0026] In the embodiments of the present application, the words "exemplary" or "for example" are used to mean serving as an example, instance, or illustration. Any embodiment or design presented as "exemplary" or "for example" in the embodiments of the present application should not be construed as being more preferred or advantageous than other embodiments or design solutions. Rather, the use of "exemplary" or "for example" is intended to present concepts in a concrete manner.

[0027] In the description of the embodiments of the present application, unless otherwise specified, the meaning of "a plurality of" is two or more.

[0028] The embodiments of the present application will be described below with reference to the accompanying drawings.

[0029] As shown in Figure 1 , the present application provides a method for improving the frequency support capability of a network-forming HVDC converter, comprising the following steps: Step S1: Based on a typical frequency domain compensation framework, analyze the path mechanism of the HVDC converter participating in the frequency response; Step S2: By adding adaptive power feedforward control, build a power compensation channel independent of the traditional frequency modulation coefficient; Step S3: The disturbance identification and anti-jitter processing module dynamically adjusts the feedforward power amplitude and sends it to the power reference calculation, so that the frequency response is realized through a fast short-time dynamic path, thereby improving the frequency support capability of the system.

[0030] Figure 2 For the VSG control block diagram after the application of the typical frequency domain correction, the subsequent VSG control of the present application is realized on this basis; Figure 3 The frequency support capability improvement system structure diagram and station-level control diagram of the network-forming HVDC converter provided by the embodiments of the present application are provided; the system topology is a two-end HVDC system, the sending-end converter valve generates a control pulse signal PWM through a first controller, and the receiving-end converter valve generates a control pulse signal PWM through a second controller; Specifically, the structure block diagram of the first controller is shown in the left dashed box, the control method adopted is vector control, including phase-locked loop, DC voltage and reactive power outer loop control, current inner loop control and circulating current suppression; the structure block diagram of the second controller is shown in the right dashed box, which adopts the network-forming control based on the virtual synchronous machine method after the typical frequency domain correction, including VSG control and voltage and current control links; wherein the reference frequency in the VSG control is jointly constituted by the frequency modulation link and the adaptive power feedforward calculation amount proposed by the present application; Figure 3 Figure 3 Figure 2 Embodiment 1 The present application provides a method for improving the frequency support capability of a network-forming HVDC converter, comprising the following steps: Step S1: Based on the VSG control typical frequency domain compensation framework, analyze the path mechanism of the traditional flexible HVDC converter participating in the system frequency response; In view of the problem that the selection of the virtual inertia and the damping coefficient of the VSG has a contradiction between the steady-state response and the dynamic response of the system, the typical improvement method is the lead compensation of the inertia link and the differential compensation of the damping link; The VSG control block diagram after the frequency domain correction is shown in Figure 2 ,​​​Figure 2 Mid is the virtual excitation power outputted; is the reference power, is the angular frequency of VSG output, is the reference angular frequency; is the angular to power conversion coefficient; , is the frequency modulation coefficient; From the automatic control principle, G The expression of (P) is: s

[0031] wherein, is the lead compensation coefficient; and are time constants; therefore, the expression of the virtual excitation power outputted at this time is:

[0032] wherein, , , , other coefficients do not work when the final value theorem is applied; therefore, The steady-state output power of is:

[0033] Therefore, the steady-state output power of the VSG control strategy after the frequency domain correction is only composed of the given value and the first frequency modulation power, The steady-state power difference brought by will be eliminated; however, according to the steady-state output power formula, the active support power of the system after being disturbed is greatly affected by the frequency modulation coefficient The selection of the frequency modulation coefficient is limited by the system capacity constraint condition and the frequency fluctuation threshold, and such parameters have significant system dependence, which restricts the compatibility of the converter to different systems; in order to break through the adaptability problem of the above frequency modulation parameters, the present application establishes a frequency response model under the disturbed condition to reveal the transmission path of the frequency response in the dynamic process of the system; From the power flow diagram of the receiving end shown in Figure 3 It can be seen that the system power balance equation when the system is running is: ; wherein, is the synchronous machine output power; is the load power; therefore, after the system power imbalance occurs, both the converter valve and the synchronous machine have to respond to the frequency fluctuation caused by the system power imbalance, and the imbalance amount in the frequency domain is: s ​​​

[0034] wherein, represents the unbalance; according to the steady-state output power of the receiving converter, after the frequency-domain correction control method is adopted, the steady-state error of the receiving converter output power is:

[0035] wherein, is the frequency deviation of the receiving converter; according to the typical synchronous machine model, when the system has power imbalance, the response power of the synchronous machine is:

[0036] wherein, is the time constant of the synchronous machine governor; , and are the coefficients of the reheat steam turbine; is the inertia time constant of the synchronous machine; is the damping of the synchronous machine; is the frequency modulation coefficient of the synchronous machine governor; is the frequency deviation of the synchronous machine; represents the transfer function of the synchronous machine; since in the power system transmission, when the system has a sudden load disturbance, the frequency output by the VSG control of the receiving converter and the frequency output by the grid-side synchronous machine will both respond, but there is a sequence between the two; according to the experimental waveform figure 5(a), after the disturbance, the VSG response lags behind the grid-side synchronous machine; accordingly, the phase relationship between the virtual potential and the synchronous machine potential is shown in figure 5(b); when the system is in steady-state operation, the output frequency of the receiving converter is equal to the output frequency of the synchronous machine, when the disturbance occurs, is slower than , the internal potential still runs at for a period of time, and the grid voltage runs at a slower speed; therefore, within a period of time after the disturbance, the difference between the phases of the two voltages will increase, thereby increasing the output of ; expresses as frequency:

[0037] combining the above formula, the can be converted into:

[0038] According to expression, and coexist in the system, the system in a certain time (usually s level) due to , , and coefficient is a fixed value, therefore, and between the interaction relationship; accordingly, the system frequency response of two paths as Figure 6 shown; Step S2: power feedforward method based on frequency response path Because of the disturbance after the system of synchronous machine response than VSG fast, so the traditional VSG control method of frequency response path are the traditional path, the following derivation according to the traditional path response system of synchronous machine frequency of steady-state error; Substitute expression can be obtained:

[0039] In order to get the direct relationship between the steady-state frequency error of synchronous machine and the load increase, can be expressed as:

[0040] Substitute expression into expression and after moving item can be obtained:

[0041] Therefore, expression is finally:

[0042] Among them,

[0043] Then according to the final value theorem, the steady-state error :

[0044] From the expression of the steady-state error , to reduce the steady-state error, since and are the parameters of the synchronous machine can not be changed, only to change the frequency coefficient of virtual governor ​​Or change the unbalanced amount of load When the frequency modulation coefficient is constantly increasing, the system can reduce the system frequency error by enhancing the frequency response sensitivity of the governor; when the load fluctuation decreases, the system frequency steady-state error is also reduced; however, both methods have limitations, first, the selection of the frequency modulation coefficient depends on the system capacity and operating conditions, and excessive selection will cause small grid oscillation, and insufficient selection will weaken the support capacity of the large grid, which is difficult to adapt to multiple scenarios; second, load fluctuation control depends on accurate prediction and real-time scheduling, and the increase of wind and light penetration rate expands the dynamic range and significantly increases the adjustment complexity; In view of the defects of frequency response adjustment under the above traditional path, attempts are made to find a solution from the following short-time dynamic path; the logic of the short-time dynamic path is: when the load fluctuates, the output power reference value of the VSG is increased in advance before the synchronous machine responds, so as to improve , according to At a certain time, increases will directly affect make it rise, realize frequency support; the specific principle is as follows: After adding the preset power, combined with expression, the power balance formula of the grid system becomes:

[0045] After transforming expression, we can get:

[0046] Combined with the power balance equation before power feedforward, we can get:

[0047] Therefore, the frequency response of the grid side synchronous machine after adding power feedforward is:

[0048] The steady-state error of the grid side synchronous machine frequency is:

[0049] Comparing and , it can be seen that as long as the value of is reasonably selected, the frequency error can be reduced, avoiding the need to calculate the parameter ; Step S3: adaptive power feedforward calculation According to The expression shows that to reduce the steady-state frequency error of the system, the most direct method is to set the power feedforward amount equal to the imbalance of the load power; however, in actual engineering applications, this method has significant technical obstacles. The user-side load changes have random and instantaneous characteristics, and dynamic detection requires not only high-precision real-time detection equipment but also complex signal transmission networks. Moreover, from the occurrence of load disturbance to the feedback of the measurement signal to the control end, there is an unavoidable time delay effect, which will directly cause the power compensation instruction to be misaligned with the actual system demand, and may even cause secondary frequency fluctuations. Therefore, the direct compensation method is difficult to achieve the expected control effect in actual engineering. When the system has a power fluctuation and a power shortage, the rotor of the synchronous machine will release kinetic energy to fill the power shortage of the system and suppress the rapid fluctuation of the frequency. At this time, the power relationship of the system is:

[0050] wherein, is the kinetic energy; is the moment of inertia of the synchronous machine, and the inertia time constant of the synchronous machine The expression is:

[0051] wherein, is the rated capacity of the system; and is substituted into The expression can be obtained as:

[0052] Since the angular frequency of the system usually does not change much, there is Therefore, The expression of

[0053] According to the above formula, the frequency change rate of the grid synchronous generator has a clear corresponding relationship with the system power imbalance. Since the synchronous machines in the grid are equipped with related measurement devices, and these measurement devices are directly installed at the generator outlet bus, this near measurement method can effectively shorten the signal acquisition path, and the measurement delay can be controlled within 50 milliseconds, fully meeting the basic requirement of real-time for power system dynamic regulation. It is worth noting that the power imbalance obtained by frequency differentiation is essentially an approximate solution based on the electromechanical transient equation, and its calculation accuracy is affected by multiple factors such as inertia parameter setting error and network topology simplification. Therefore, there is an inherent deviation between the theoretical value and the true value in actual application. That is, after adding power feedforward, the system frequency may still present a limited amplitude of instantaneous drop when the load suddenly changes. However, it is unnecessary to require the grid frequency to always remain at the rated value. During the operation of the power system, the frequency has a permitted fluctuation range, which means that in engineering practice, it is not necessary to pursue the absolute elimination of frequency deviation, but only to ensure that the system frequency fluctuation is within the frequency fluctuation range allowed by the grid standard when the system load fluctuates.

[0054] Generally speaking, the maximum frequency change rate after disturbance can reflect the disturbance amount. When the load suddenly increases, the instantaneous lack of generated power will cause the frequency to present a negative change rate ( ), at which time the system needs to be balanced by positive compensation through power feedforward; conversely, when the load suddenly decreases, the positive frequency change rate ( ) caused by the excess generated power needs to be negatively adjusted by power feedforward; therefore, a control variable representing the sign of power feedforward is needed, and the state variable state is defined here as the criterion for power regulation direction; when power needs to be increased state takes 1, when power needs to be reduced state takes -1, and when power does not need to be changed state takes 0; therefore, the power feedforward expression is:

[0055] In the power compensation control system based on frequency change rate, the accurate determination of the state variable state needs to consider the dynamic direction and amplitude characteristics of the frequency change rate. The core of this control strategy is to build a judgment mechanism with anti-interference ability, which not only accurately identifies whether the system is in power shortage or surplus state, but also avoids false actions caused by measurement noise or small fluctuations; the process can be divided into the following two parts: (1) State determination logic and threshold setting principle To prevent the system from frequently changing the output power of the converter when the frequency rate of change fluctuates slightly, causing unnecessary oscillation, a frequency rate of change threshold parameter T is set as the threshold value for disturbance identification; according to the standard of the frequency rate of change, T is taken as 0.1 Hz / s; in real-time monitoring, the system continuously tracks the extreme value characteristics of the frequency rate of change: when the minimum value of the frequency rate of change in the detection window and the maximum value , it is determined that the system is in a steady-state operation interval; at this time, the state flag output =1 is triggered, and the main control program determines state =0 by continuously monitoring the stability of the flag (such as keeping 1 for 5 consecutive sampling periods), and the converter maintains the current reference power (i.e. the reference power at the previous moment) unchanged; (2) State transition mechanism and anti-jitter design When the system detects that the frequency rate of change breaks through the threshold, a multi-level state transition process is started: Load sudden increase condition: if the minimum value of the frequency rate of change continuously below -T (such as satisfying for 5 consecutive sampling periods), state transition is triggered; at this time, if the original state is output =1 or 3, it is updated to output =2; the main control program captures the state jump (1 2 or 3 2), and finally sets state=1 after a 20ms delay confirmation; the converter reference power is updated to , ( is the reference power at the previous moment), and is positive at this time, providing dynamic support for the system through the power boost path (short-term dynamic path); Load sudden decrease condition: when the maximum value of the frequency rate of change continuously above T, if the original state is output =1 or 2, it jumps to output =3; after state confirmation, state is set to state =-1, and the converter executes the power reduction instruction (at this time is negative), and the converter output power decreases to suppress the rise of the grid-side synchronous machine frequency; Figure 4 The architecture diagram of the phase-locked loop module and the frequency dynamic monitoring module in the receiving-end converter of the flexible HVDC transmission system is shown; the phase-locked loop module is used to extract the grid fundamental frequency information and phase parameters, and then derive the grid frequency rate of change; the specific implementation process includes: collecting the output voltage of the receiving-end grid synchronous machine ; mapping it from the stationary three-phase coordinate system to the rotating two-phase coordinate system through Park coordinate transformation, and extracting the q-axis component The proportional integral regulator processing generates a real-time angular frequency of the power grid The angular frequency is integrated to obtain a phase of the grid-side voltage The angular frequency signal is differentiated and divided by to finally output a frequency change rate parameter of the power grid RoCoF (i.e. ); According to the above analysis, the system power feedforward flow is shown in Figure 7 ; To further illustrate the control effect of the adaptive power feedforward control method and system of the grid-forming VSC proposed in the present application, the following describes the specific embodiments: Based on the engineering application requirements of the flexible HVDC transmission technology in the field of long-distance large-capacity power transmission, a two-terminal flexible HVDC transmission system shown in Figure 3 is built on the MATLAB / Simulink platform; the DC voltage level is 400 kV, the rated transmission power is 1100 MW, and the initial load is 550 MW, which is borne by the receiving-end converter station and the receiving-end power grid, i.e. 275 MW; the main parameters of the system are shown in Table 1; Table 1

[0056] Embodiment 2 The present application provides a frequency support capability improvement system of a grid-forming VSC, comprising: A frequency response module is used for disturbance state identification and anti-jitter processing through dynamic data of the real-time collected AC grid frequency change rate, dynamic adjustment of the feedforward power output amplitude through adaptive power feedforward control, and sending into the calculation of the reference power in the virtual synchronous machine control method, so that the frequency response is realized through a short-time dynamic path, and the frequency support capability is improved. A response path construction module is used for adding adaptive power feedforward control based on the path of the traditional grid-forming VSC participating in frequency response under the virtual synchronous machine control, improving the output power reference value of the virtual synchronous generator, and constructing a short-time dynamic path for power compensation.

[0057] Further preferably, the short-time dynamic path is that when the load power increases, the synchronous machine output frequency increases through the combined adjustment of the primary frequency modulation power and the feedforward power, the grid-side voltage phase increases, and then the difference between the internal potential and the grid voltage phase increases; on the contrary, when the load power decreases, the synchronous machine output frequency decreases through the combined adjustment of the primary frequency modulation power and the feedforward power, the grid-side voltage phase decreases, and then the difference between the internal potential and the grid voltage phase decreases.

[0058] Further preferably, the steady-state error of the grid-side synchronous machine frequency after the adaptive power feedforward control in the frequency response module is:

[0059] wherein, is the unbalance of the load power; is the feedforward power; is the frequency modulation coefficient of the virtual governor; is the damping of the synchronous machine; is the frequency modulation coefficient of the synchronous machine governor.

[0060] Further preferably, the feedforward power in the frequency response module is:

[0061] wherein, is the inertia time constant of the synchronous machine; is the rated capacity of the grid system; is the rated frequency of the grid system; is the output frequency of the synchronous machine; is the control variable reflecting the sign of the feedforward power; when the reference power in the virtual synchronous machine control method needs to be increased, = 1; when the reference power in the virtual synchronous machine control method needs to be reduced, = -1; when there is no need to change the reference power in the virtual synchronous machine control method, = 0.

[0062] Further preferably, the frequency response module comprises a disturbance state identification unit and a jitter prevention processing unit. The disturbance state identification unit is used to detect the change rate of the AC grid frequency in real time, and if and , it is determined that the grid system is in a steady-state operating interval, and = 0; when the change rate of the AC grid frequency is detected to break through the threshold value of disturbance identification, the jitter prevention processing mechanism is started. The jitter prevention processing unit is used to, in the case of sudden load increase, if the minimum value of the change rate of the AC grid frequency continuously below , set = 1, and update the reference power in the virtual synchronous machine control method at the current time to . In the case of sudden load decrease, if the maximum value of the change rate of the AC grid frequency continuously above the threshold value of disturbance identification, set =-1, update the reference power in the virtual synchronous machine control method at the current moment to ; in, This represents the minimum rate of change of AC power grid frequency. This is the threshold value for disturbance identification; This represents the maximum rate of change of AC power grid frequency. This is the reference power used in the virtual synchronous machine control method of the previous moment.

[0063] Verification aspect 1: Tracking performance of power feedforward Scenario I: Continuously increasing load conditions Scenario II: Bidirectional Variable Load Condition In Scenario I, the simulation duration was set to 10 seconds, and the initial system load was set to 550MW. To verify the dynamics and performance of the power feedforward control proposed in this section under continuous load increase, a 20% load was suddenly increased at 3 seconds. The system then ran at this load for 3 seconds, followed by a 40% load increase at 6 seconds. Finally, the system continued to run at 160% of the original load for 10 seconds. The changes in power feedforward and related quantities during the system operation are shown in Figure 8(a). As can be seen from Figure 8(a), when the system is subjected to a 20% load disturbance, The value increases from 0 to 0.378, which corresponds to the solid line in Figure 8(b). It also grows synchronously; when the system reaches the 6th second, after another 40% increase in load, It rose further to 0.75, and accordingly, It also rose to 587MW, demonstrating Dynamic response characteristics to load fluctuations; the dashed line in Figure 8(b) represents the reference value of the converter without power feedforward, which remains at half of the original power, i.e., 275MW; however, if the reference value remains unchanged, it will lead to insufficient power support when the VSG performs frequency response; the dotted line represents the expected output power of the receiving-end converter, i.e. The figure represents the load power minus the reference power of the grid synchronous machine. As can be seen from the figure, although the composite power of the solid line does not completely coincide with the expected power of the dashed line, the two exhibit good synchronization characteristics during dynamic changes. Especially during the load surge phase, the two curves start almost simultaneously and maintain similar growth slopes, which indicates that the new control strategy has good target tracking capability and shows good tracking performance. Furthermore, the solid line is significantly higher than the dashed line, which intuitively verifies the actual effect of the power feedforward control strategy in improving the dynamic adjustment capability of the converter.

[0064] The simulation time under scenario II is also set to 10s, the system initially runs at 550MW base load state, at 2s, the first step disturbance is implemented, the load is suddenly reduced to 60% of the initial value (330MW), after running for 4s, the reverse disturbance is applied, the load is instantaneously increased to 140% of the initial value (770MW); after 2s of stable operation, at 6s, the 60% load reduction is implemented again, and finally stabilized at 80% of the initial value (440MW) until the end of the simulation; this "first load reduction-then load increase-then load reduction" three-stage disturbance mode can fully investigate the adaptability of the control strategy to the bidirectional power mutation.

[0065] Fig. 9(a) and Fig. 9(b) show the system dynamic response characteristics under the complex load fluctuation scenario of working condition two. As can be seen from Fig. 9(a), when the load suddenly decreases at 2s, also decreases to-0.68, and the corresponding Fig. 9(b) also decreases to 102MW; when the load suddenly increases at 4s, the system frequency change rate also responds quickly, so that the reference power of the converter rises to 460MW; finally, when the system load suddenly drops from 140% of the original load to 80% of the original load at 6s, the change of the system frequency change rate enables the reference frequency to respond in time, and through the adjustment of the power feedforward, synchronously reduces to 195MW; this fully proves the effectiveness of the power feedforward strategy proposed in the application in the scene of bidirectional power mutation.

[0066] Verification aspect two: power support ability improvement performance of the proposed scheme Scenario I: continuous load increase working condition Scenario II: bidirectional load change working condition Fig. 10(a) and Fig. 10(b) show the comparison of the typical frequency domain correction strategy under scenario I and the new method of superimposed power feedforward control in terms of converter output power and synchronous machine output frequency response. From the active power output characteristics of the converter in Fig. 10(a), after adding power feedforward control, the supporting power under the new control method is significantly higher than that without power feedforward when the system is disturbed by power fluctuation. Since the traditional VSG inertia supporting power is proportional to the frequency change rate, the supporting power will decrease when the system frequency change rate decreases slowly; while the power feedforward is equivalent to directly changing the reference value, the supporting power will not decrease with the system stability, but will stabilize to a new output power within 0.2s as shown in Fig. 10(a); in terms of frequency response characteristics, as shown in Fig. 10(b), when the first 20% load disturbance is applied, the new control scheme with power feedforward increases the frequency steady-state point by 0.16Hz; when the system is subjected to the second 40% load impact, the frequency of the new control scheme with power feedforward drops by only 0.05Hz, which is much smaller than 0.37Hz without power feedforward; this fully reflects the continuous regulation capability of power feedforward control under multiple disturbance conditions, and under the condition of 60% extreme load impact, the power grid synchronous machine using the power feedforward method can still maintain a frequency output of 49.93Hz, while the frequency without power feedforward collapses to 49.45Hz, which is beyond the stable operation range of the power system; this shows that the power feedforward method proposed in the present application can well solve the problem of frequency drop caused by extreme load changes, thereby ensuring the safe and stable operation of the power grid.

[0067] Fig. 11(a) and Fig. 11(b) show the difference between the two control methods under scenario II complex disturbance conditions through comparison simulation experiments. As can be seen from the comparison of the active power output of the receiving end converter under the two control strategies in Fig. 11(a), after the load suddenly decreases or increases, although the instantaneous supporting power of the typical frequency domain correction control strategy will exceed that of the power feedforward, but similar to scenario I, the supporting power of the typical frequency domain correction control method will decrease with the subsequent decrease of the frequency change rate, which will cause the frequency of the receiving end power grid synchronous machine to continuously rise or fall as shown by the dashed line in Fig. 11(b), affecting the frequency stability of the system; in contrast, the strategy with power feedforward is only related to the maximum value of the absolute value of the frequency change rate after the load mutation, which is a constant, so the power output of the converter will not decrease with the subsequent decrease of the frequency change rate, and a continuous and stable power support can be achieved, and the frequency change range with power feedforward is 49.92~50.08Hz, which is smaller than the typical frequency domain correction of 49.67~50.34Hz, showing stronger frequency stability.

[0068] ​Verification aspect 3: Sensitivity of the proposed strategy to different frequency modulation coefficients Scenario I: Typical Frequency Domain Correction Control Scenario II: Adding power feedforward control To compare in different The parameter sensitivity differences between typical frequency domain correction strategies and control methods incorporating power feedforward under complex disturbances are discussed. =1000, 5000, 10000, 50000 and continue the disturbance condition of condition two, and compare the active power output of the receiver converter and the frequency response curve of the grid synchronous machine side of the two control strategies.

[0069] As can be seen from the power output characteristics analysis of the typical frequency domain correction method in Figure 12(a), It has a dual effect; as its value increases from 1000 to 50000, the output power increases, indicating that... The increase in power enhances the system's response to frequency drops, but the output power exhibits stability defects, and different... Significant differences in power levels (highest and lowest) The average power difference reaches 63.47MW); at the same time, high The power fluctuation amplitude under operating conditions is significantly greater than that under low conditions. This characteristic necessitates the establishment of multi-objective optimization models in practical engineering. These models must satisfy both the rapidity of transient response and the precision and stability requirements of steady-state operation, resulting in numerous and complex constraints that are detrimental to practical operation. Correspondingly, Figure 12(b) shows different typical frequency domain correction control conditions... The synchronous machine side frequency is visible, although with Increasing the load can indeed reduce the frequency change (e.g., after the first load change in 2 seconds). At 1000, the frequency ultimately reached 50.62Hz, and At 50000, the final frequency is 50.06Hz, a decrease of 0.56Hz, but different The suppression effects showed significant differences, with an average difference of 0.44 Hz. And when... When the value is 1000, the maximum frequency of the synchronous machine is 50.62Hz and the minimum is 49.43Hz. The fluctuation is greater than ±0.2Hz of the power grid standard, which poses a risk of instability.

[0070] Compared to typical frequency domain correction control Due to the high sensitivity of parameters, the control system incorporating the power feedforward strategy exhibits strong robustness. As shown in Figure 13(a), under the same operating conditions, when When the power output is adjusted from 1000 to 50000, the converter employing the power feedforward strategy maintains a stable and maximum output active power. With the lowest The average output power difference was only 12.10MW, which is about 80.93% less than the 63.47MW difference of the pure frequency domain correction control method. This indicates that the power feedforward algorithm, by introducing a feedforward compensation mechanism, successfully eliminated the influence of active power output on the virtual speed governor coefficient. The strong dependency makes the system Stable power output can be achieved over a wide range. Therefore, after adopting the power feedforward method, it is no longer necessary to... The complex calculations required for selecting parameters have been greatly simplified. As shown in Figure 13(b), even with the addition of power feedforward, the system... Despite wide-ranging variations, the average output frequency deviation of the receiving-end grid synchronizer is only 0.035Hz, a 92.05% reduction compared to the 0.44Hz frequency deviation achieved with frequency domain correction control alone. Changes in frequency will lead to changes in the rate of frequency change, and the calculation of power feedforward is closely related to the rate of frequency change. Therefore, changes in system frequency do not follow the same rules as in coordinated control. The larger the value, the smaller the frequency fluctuation. However, generally speaking, when... Even with large-scale variations, the lowest output frequency of the synchronous machine is 49.858Hz, and the highest output frequency is only 50.131Hz, which fully meets the requirement of ±0.2Hz grid frequency fluctuation, and the frequency support improvement effect is significant.

[0071] Experimental data demonstrate that the adaptive power feedforward control method and system for the grid-type flexible DC converter proposed in this application not only analyzes the system's frequency response mechanism at the physical level, but also accelerates the system's response speed when power imbalance disturbances occur, and the frequency support performance is independent of the selection of the frequency modulation coefficient. This verifies the superiority of the proposed method in improving the system's frequency support performance.

[0072] The frequency support capability enhancement system for the grid-type flexible DC converter provided in this application is described below. The frequency support capability enhancement system for the grid-type flexible DC converter described in this application and the frequency support capability enhancement method for the grid-type flexible DC converter described in this application can be referred to each other.

[0073] In summary, this application has the following advantages compared with the prior art: This application provides a method and system for improving the frequency support capability of a grid-connected flexible DC converter. This is achieved by analyzing the imbalance between the converter and the receiving-end grid synchronizing machine after a power imbalance occurs in the system. sThe equation analysis under the domain obtains the frequency expression of the receiving end power grid synchronous machine, and the voltage phase relationship analysis of the converter station and the synchronous machine in the VSG control architecture, which first reveals the double-path mechanism of the frequency response after the system power disturbance, namely the slow dynamic frequency regulation path and the fast dynamic feedforward path, and constructs a frequency response analysis model from the electromechanical transient physical nature level of the power system, which provides a theoretical basis for the adaptive power feedforward control method.

[0074] The application provides a method for improving the frequency support capability of a network-forming type flexible direct current converter. The adaptive power feedforward control strategy relies on a short-time dynamic path with fast response, directly associates the feedforward power instruction with the basic physical quantity of system frequency change rate, which has strong observability and is easy to obtain in real time, significantly reduces the complexity of control quantity generation, and ensures that the converter station can provide active frequency support matching the system demand under different load levels, new energy penetration rates, and fault types, thereby comprehensively improving the frequency stability of the power grid.

[0075] The application provides a method for improving the frequency support capability of a network-forming type flexible direct current converter. The adaptive power feedforward control strategy relies on a short-time dynamic path with fast response, directly associates the feedforward power instruction with the basic physical quantity of system frequency change rate, which has strong observability and is easy to obtain in real time, significantly reduces the complexity of control quantity generation, and ensures that the converter station can provide active frequency support matching the system demand under different load levels, new energy penetration rates, and fault types, thereby comprehensively improving the frequency stability of the power grid.

[0076] The above is only a specific embodiment of the application, but the protection scope of the application is not limited thereto. Any person skilled in the art can easily think of changes or replacements within the technical range disclosed in the application, which should be covered within the protection scope of the application. Therefore, the protection scope of the application should be subject to the protection scope of the claims.

Claims

1. A method for improving frequency support capability of a meshed HVDC converter, characterized in that, The method comprises the following steps: The disturbance state identification and anti-jitter processing are performed through dynamic data of the AC grid frequency change rate, the adaptive power feedforward control is used to dynamically adjust the feedforward power output amplitude, and then the output power reference value of the virtual synchronous generator is regulated, so that the frequency response is realized through the short-time dynamic path.

2. The frequency support capability enhancement method of claim 1, wherein, The short-time dynamic path is that when the load power increases, the synchronous generator output frequency is increased through the combined adjustment of the primary frequency modulation power and the feedforward power, the grid-side voltage phase is increased, and then the difference between the internal electromotive force and the grid voltage phase is increased; conversely, when the load power decreases, the synchronous generator output frequency is reduced through the combined adjustment of the primary frequency modulation power and the feedforward power, the grid-side voltage phase is reduced, and then the difference between the internal electromotive force and the grid voltage phase is reduced.

3. The frequency support capability enhancement method according to claim 1 or 2, characterized by, Steady state error of grid side synchronous machine frequency after adaptive power feedforward control Is: wherein, is an amount of imbalance of the load power; is a feedforward power; is a frequency modulation coefficient of the virtual governor; is a damping of the synchronous machine; is a frequency modulation coefficient of the synchronous machine governor.

4. The frequency support capability enhancement method of claim 3, wherein, Feed forward power Is: wherein, is the inertia time constant of the synchronous machine; is the rated capacity of the power grid system; is the rated frequency of the power grid system; is the output frequency of the synchronous machine; is the control variable reflecting the sign of the feedforward power; when the reference power in the virtual synchronous machine control method needs to be increased, = 1 ; when the reference power in the virtual synchronous machine control method needs to be decreased, = -1 ; when there is no need to change the reference power in the virtual synchronous machine control method, = 0.

5. The frequency support capability enhancement method of claim 4, wherein, The disturbance state identification method is that: The rate of change of the frequency of the alternating current power grid is detected in real time. If the minimum value of the rate of change of the frequency of the alternating current power grid and the maximum value of the rate of change of the frequency of the alternating current power grid are determined, it is determined that the power grid system is in a steady state operation interval, and the following is determined = 0, wherein T is a threshold value for disturbance identification of the rate of change of the frequency of the alternating current power grid. When the AC grid frequency change rate is detected to break through the disturbance identification threshold value, the anti-jitter processing mechanism is started; The anti-jitter processing mechanism comprises: In the load surge working condition, if the minimum value of the AC power grid frequency change rate is less than , set =1, and the reference power in the virtual synchronous machine control method at the current time is updated to ; In the load sudden reduction working condition, if the maximum value of the AC power grid frequency change rate is higher than the threshold value of the disturbance identification for a long time, set =-1, and the reference power in the virtual synchronous machine control method at the current time is updated as ​ wherein, is the minimum value of the AC grid frequency rate of change; is a threshold value for disturbance recognition; is the maximum value of the AC grid frequency rate of change; is the reference power in the virtual synchronous machine control method of the previous time instant.

6. A system for improving frequency support capability of a meshed HVDC converter, the system comprising: The frequency response module is used to perform the disturbance state identification and anti-jitter processing through dynamic data of the AC grid frequency change rate, dynamically adjust the feedforward power output amplitude through the adaptive power feedforward control, and send the reference power calculation in the virtual synchronous machine control method, so that the frequency response is realized through the short-time dynamic path, and the frequency support capability is improved. The short-time dynamic path is that when the load power increases, the synchronous generator output frequency is increased through the combined adjustment of the primary frequency modulation power and the feedforward power, the grid-side voltage phase is increased, and then the difference between the internal electromotive force and the grid voltage phase is increased; conversely, when the load power decreases, the synchronous generator output frequency is reduced through the combined adjustment of the primary frequency modulation power and the feedforward power, the grid-side voltage phase is reduced, and then the difference between the internal electromotive force and the grid voltage phase is reduced.

7. The frequency support capability enhancement system of claim 6, wherein, The frequency response module comprises a disturbance state identification unit and an anti-jitter processing unit.

8. The frequency support capability enhancement system of claim 6 or 7, wherein, The steady-state error of the grid-side synchronous machine frequency after adaptive power feedforward control in the frequency response module is: wherein, is an amount of imbalance of the load power; is a feedforward power; is a frequency modulation coefficient of the virtual governor; is a damping of the synchronous machine; is a frequency modulation coefficient of the synchronous machine governor.

9. The frequency support capability enhancement system of claim 8, wherein, Frequency response module feed forward power is: wherein, is the inertia time constant of the synchronous machine; is the rated capacity of the power grid system; is the rated frequency of the power grid system; is the output frequency of the synchronous machine; is the control variable reflecting the sign of the feedforward power; when the reference power in the virtual synchronous machine control method needs to be increased, = 1 ; when the reference power in the virtual synchronous machine control method needs to be decreased, = -1 ; when there is no need to change the reference power in the virtual synchronous machine control method, = 0.

10. The frequency support capability enhancement system of claim 9, wherein, ​ The disturbance state identification unit is used to detect the AC power grid frequency change rate in real time. If and , it is determined that the power grid system is in a steady-state operation interval, and it is determined that = 0; when the AC power grid frequency change rate is detected to break through the threshold value of disturbance identification, the anti-shake processing mechanism is started. The anti-vibration processing unit is used for updating the reference power in the virtual synchronous machine control method at the current time to when the AC power grid frequency change rate is less than ; and setting =1. ; In the load sudden reduction working condition, if the maximum value of the AC power grid frequency change rate is higher than the threshold value of the disturbance identification for a long time, set =-1, update the reference power in the virtual synchronous machine control method at the current time to ; wherein is a minimum value of the AC grid frequency rate of change; is a threshold value for disturbance identification; is a maximum value of the AC grid frequency rate of change; is a reference power in the virtual synchronous machine control method of the previous time instant.

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