Large signal synchronization stability control method and device for flexible DC power transmission system

Abstract

The invention discloses a large-signal synchronous stability control method and device for a flexible direct-current power transmission system, and the method comprises the steps: building a current reference value adaptive adjustment strategy based on synchronous stability enhancement for the adaptive adjustment of a current reference value; aiming at phase-locked loop integral controller input adaptive switching, constructing a phase-locked loop integral controller input adaptive switching strategy based on large disturbance stage detection; and according to the current reference value adaptive adjustment strategy based on synchronous stability enhancement and the phase-locked loop integral controller input adaptive switching strategy based on large disturbance stage detection, large signal synchronous stability control is carried out on the flexible DC power transmission system. The technical problem that an existing flexible direct current power transmission system control method is poor in stability under a large disturbance transient event of a high-proportion new energy weak power grid is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flexible direct current power transmission system, and particularly relates to a large signal synchronous stability control method and device for a flexible direct current power transmission system. BACKGROUND

[0002] Flexible direct current power transmission system is widely used in power system due to its high control flexibility, support for weak grid interconnection, and no risk of commutation failure. Flexible direct current power transmission system plays a key role in large-scale new energy long-distance transmission and grid connection. However, with the continuous deepening of power electronic level, the voltage fluctuation of alternating current side becomes more and more frequent, and the system presents nonlinear, multi-time scale coupling and strong complexity characteristics, which significantly increases the difficulty of synchronous stability analysis of flexible direct current power transmission. Especially when the alternating current grid encounters voltage dip and other strong disturbances, how to ensure that the flexible direct current power transmission system and the grid remain synchronous, maintain transient power transmission, avoid unnecessary system shutdown as much as possible, and quickly recover grid operation after disturbance elimination, has become a technical problem to be solved in the field of large signal synchronous stability control of flexible direct current power transmission.

[0003] At present, in the field of steady-state operation control of flexible direct current power transmission system, the double closed loop vector control strategy based on synchronous reference coordinate system phase-locked loop (SRF-PLL) and proportional integral (PI) control has been widely applied in engineering practice. The strategy converts the alternating current in abc three-phase coordinate system into direct current in dq two-axis through synchronous rotating coordinate system (dq coordinate system) transformation technology, and then realizes direct regulation and control of the alternating current side of the modular multilevel converter (MMC), so as to achieve the decoupling tracking control goal of active power and reactive power.

[0004] During transient events such as power grid faults, the flexible HVDC system will switch to fault ride-through control to support the grid point voltage and maintain power transmission. The traditional fault ride-through strategy sets the dq-axis current reference value according to the grid code, which is a function of the grid point voltage amplitude, and does not consider the large signal synchronous stability problem of the system during large disturbances. That is, when the grid voltage drops severely in a weak grid scenario, the system may not have a stable equilibrium point or insufficient deceleration area due to inappropriate setting of the dq-axis current reference value, causing the flexible HVDC system to lose synchronization and oscillate. In addition, the state of the system will change sharply after a large disturbance, and the existing SRF-PLL structure and parameter design have only been proven to be effective in dealing with small disturbances. However, in the case of large disturbances, the conventional phase-locked loop may not be conducive to the transition of the flexible HVDC system from the pre-fault steady state to the new equilibrium point, thereby affecting the smooth recovery of the system after the large disturbance disappears, and may cause the flexible HVDC system to be locked out and interrupt power transmission. Therefore, the traditional flexible HVDC control method has poor stability in the case of large disturbance transient events in a weak grid with a high proportion of new energy. SUMMARY

[0005] The present application provides a flexible HVDC system large signal synchronous stability control method and device, which is used to solve the technical problem of poor stability of the existing flexible HVDC system control method in the case of large disturbance transient events in a weak grid with a high proportion of new energy.

[0006] Therefore, the first aspect of the present application provides a flexible HVDC system large signal synchronous stability control method, comprising:

[0007] A grid-connected control model of a direct-drive wind turbine is established to obtain PI parameters of a DC voltage controller and PI parameters of a phase-locked loop controller.

[0008] A low voltage ride-through control trigger condition is set, and the low voltage ride-through control stage is configured as a fault initial stage, a fault duration stage and a fault recovery stage. The reactive current reference value for controlling the direct-drive wind turbine to remain connected to the grid during the fault is calculated in the fault initial stage and the fault duration stage, and the active current reference value for controlling the direct-drive wind turbine to remain connected to the grid during the fault is calculated in the fault recovery stage.

[0009] A target function and constraint condition of the low voltage ride-through adaptive control are constructed.

[0010] The particle swarm optimization algorithm is used to solve the target function, and the PI parameters of the DC voltage controller and the PI parameters of the phase-locked loop controller are optimized to obtain the optimal PI parameters of the DC voltage controller and the PI parameters of the phase-locked loop controller.

[0011] The optimal PI parameters of the DC voltage controller and the PI parameters of the phase-locked loop controller are written into the DC voltage controller and the phase-locked loop controller respectively, so that adaptive control is realized in the initial, continuous and recovery stages of the fault.

[0012] Optionally, the equation of the dq-axis component of the grid-connected point voltage and the phase-locked loop output phase is:

[0013]

[0014]

[0015] wherein, is the d-axis component of the grid-connected point voltage, is the q-axis component of the grid-connected point voltage, is the amplitude of the grid voltage, is the phase of the grid voltage, is the amplitude of the grid impedance, is the phase angle of the grid impedance, and φ is the phase difference between the grid-connected point voltage and the grid current, is the amplitude of the grid current, is the rated angular frequency of the grid, is the proportional coefficient of the PI controller of the SRF-PLL, is the integral coefficient of the PI controller of the SRF-PLL, is the phase-locked loop output phase.

[0016] Optionally, the differential equation of the AC side dynamic characteristic is:

[0017]

[0018] wherein, is the second derivative of the power angle, is the first derivative of the power angle, is the amplitude of the grid resistance, is the amplitude of the grid reactance, is the d-axis component of the grid current, is the q-axis component of the grid current.

[0019] Optionally, the input error direct current flow expression is:

[0020]

[0021] wherein, err is the input error direct current flow, is the d-axis component of the grid current, is the q-axis component of the grid current, is the amplitude of the grid voltage, is the amplitude of the grid resistance, is the state variable.

[0022] Optionally, the grid-connected point voltage criterion is:

[0023]

[0024] wherein Rms is the output level signal, is the grid-connected point phase voltage effective value unit, and is the threshold value.

[0025] Optionally, the SRF-PLL stability time and damping ratio are:

[0026]

[0027] wherein, is the damping ratio, is the stability time.

[0028] Optionally, the frequency change amount expression during the transient state is:

[0029]

[0030] wherein, is the phase-locked loop proportional controller output angular frequency change amount, is the phase-locked loop integral controller output angular frequency change amount.

[0031] Optionally, the phase-locked loop integral controller input adaptive switching strategy based on the large disturbance stage detection is:

[0032]

[0033]

[0034]

[0035] wherein, is the grid-connected point voltage q-axis component, Switch is the switching logic level, Rate_decr is the grid-connected point phase voltage effective value unit change amount u PCC_L2G_rms_rate compared with the corresponding threshold value Decr set to obtain the level signal, Rate_incr is the grid-connected point phase voltage effective value unit change amount u PCC_L2G_rms_rate compared with the corresponding threshold value Incr set to obtain the level signal, Decr set is the change rate threshold value when the grid-connected point voltage drops, and Incr set is the change rate threshold value when the grid-connected point voltage rises.

[0036] Optionally, the dq-axis current adaptive adjustment strategy is:

[0037]

[0038]

[0039]

[0040] wherein, is a d-axis component of the grid current, is a q-axis component of the grid current, is an adaptive corrected d-axis component of the grid current, is an adaptive corrected q-axis component of the grid current, is a transfer function, is a phase-locked loop output angular frequency variation.

[0041] The second aspect of the present application provides a flexible direct current transmission system large signal synchronous stability control device, comprising:

[0042] A first equation construction module is configured to, for a grid-connected type flexible direct current transmission converter synchronized by a phase-locked loop, based on model order reduction principles and considering SRF-PLL dynamic equations and circuit KVL equations, establish equations of grid-connected point voltage dq-axis components and phase-locked loop output phases;

[0043] A second equation construction module is configured to, according to the equations of grid-connected point voltage dq-axis components and phase-locked loop output phases, establish an expression of power angle, and based on the power angle being a physical quantity varying with time, establish a differential equation of alternating current side dynamic characteristics, wherein the angle difference between the phase-locked loop output phase and the phase of the grid voltage is defined as the power angle;

[0044] An adaptive adjustment strategy construction module is configured to, according to the input error direct current expression of the PI controller of the SRF-PLL and the expressions of the favorable area and the unfavorable area, construct a dq-axis current adaptive adjustment strategy for introducing the phase-locked loop output angular frequency variation into the dq-axis current, and determine whether to put into the dq-axis current adaptive adjustment strategy by using the grid-connected point voltage criterion;

[0045] A strategy enhancement module is configured to, according to the phase-locked loop output angular frequency fluctuation principle during the transient state, establish an integral controller of the dq-axis current adaptive adjustment, obtain an improved current reference value adaptive adjustment expression, and establish a current reference value adaptive adjustment strategy based on synchronous stability enhancement;

[0046] A first calculation module is configured to, according to a large signal model of the phase-locked loop, establish a stable time and a damping ratio of the SRF-PLL;

[0047] A second calculation module is configured to, according to the SRF-PLL dynamic equation, establish an expression of the frequency variation during the transient state.

[0048] An adaptive switching strategy construction module is configured to establish an adaptive switching strategy of an input of an SRF-PLL integral controller between a q-axis component of a grid-connected point voltage and 0 according to influencing factors of dynamic performance of the HVDC system during a transient state, and to establish a large disturbance stage identification criterion based on an effective value of a PCC phase voltage and a rate of change thereof, so as to construct an adaptive switching strategy of an input of a phase-locked loop integral controller based on large disturbance stage detection;

[0049] A control module is configured to perform large signal synchronous stability control on the HVDC system according to the adaptive adjustment strategy of the current reference value based on synchronous stability enhancement and the adaptive switching strategy of the input of the phase-locked loop integral controller based on large disturbance stage detection.

[0050] As can be seen from the above technical solutions, the large signal synchronous stability control method for the HVDC system has the following advantages:

[0051] The large signal synchronous stability control method for the HVDC system is provided, the adaptive adjustment of the current reference value is based on model order reduction theory, a large signal model for transient analysis of the HVDC system is constructed according to a phase-locked loop structure of the grid-connected HVDC system, the influence of the current reference value on the existence of an equilibrium point and transient stability after a large disturbance of the system is determined, an adaptive adjustment strategy of the current reference value based on synchronous stability enhancement is established by using output characteristics and feedback control of an integral controller in combination with fluctuation principles of a change amount of an angular frequency of an output of the phase-locked loop during the large disturbance, the adaptive switching of the input of the phase-locked loop integral controller is determined according to transient stability mechanisms and equivalent damping of the HVDC converter during the large disturbance, an adaptive switching strategy of the input of the phase-locked loop integral controller based on large disturbance stage detection is constructed in combination with dynamic characteristics of the phase-locked loop itself, and the large signal synchronous stability control is performed on the HVDC system according to the adaptive adjustment strategy of the current reference value based on synchronous stability enhancement and the adaptive switching strategy of the input of the phase-locked loop integral controller based on large disturbance stage detection, so that when a voltage of an AC side of the HVDC system is subjected to a large symmetrical drop and the converter enters a fault ride-through, the dq-axis current reference value of the current inner loop is adaptively adjusted, the system not only has a stable equilibrium point, but also can smoothly transit to a new steady state, transient instability is avoided, the adaptive switching of the input of the phase-locked loop integral controller is performed according to an identification result of a transient process when the voltage of the AC side is subjected to a large symmetrical drop, accurate tracking of a phase by the phase-locked loop is ensured, the control system of the HVDC has good dynamic performance and accuracy, and the system can avoid oscillation and divergence of the voltage and current during a recovery process of the HVDC system after the large voltage drop, so that the system can be restored to normal operation in a short time. The technical problem of poor stability of an existing control method for the HVDC system under a large disturbance transient event of a weak grid with a high proportion of new energy is solved. BRIEF DESCRIPTION OF DRAWINGS

[0052] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other related drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0053] Figure 1 A flowchart of a flexible HVDC power transmission system large signal synchronous stability control method provided in an embodiment of the present application;

[0054] Figure 2 A principle block diagram of a current reference value adaptive adjustment strategy based on synchronous stability enhancement provided in an embodiment of the present application;

[0055] Figure 3 A principle block diagram of a phase-locked loop integral controller input adaptive switching strategy based on large disturbance stage detection provided in an embodiment of the present application;

[0056] Figure 4 A structure schematic diagram of a two-terminal MMC-HVDC system simulation model provided in an embodiment of the present application;

[0057] Figure 5 A voltage, current, angular frequency and power angle response waveform diagram of an MMC2 grid-connected point provided in an embodiment of the present application;

[0058] Figure 6 A DC current response waveform diagram of an MMC2 provided in an embodiment of the present application;

[0059] Figure 7 Another voltage, current, angular frequency and power angle response waveform diagram of an MMC2 grid-connected point provided in an embodiment of the present application;

[0060] Figure 8 Another DC current response waveform diagram of an MMC2 provided in an embodiment of the present application;

[0061] Figure 9 A structure schematic diagram of a flexible HVDC power transmission system large signal synchronous stability control device provided in an embodiment of the present application. DETAILED DESCRIPTION

[0062] In order to make the person skilled in the art better understand the technical scheme of the present application, the technical scheme in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0063] For the convenience of understanding, please refer to Figure 1 The present application provides an embodiment of a large signal synchronous stability control method for a flexible DC power transmission system, comprising:

[0064] Step 101, for a grid-following flexible DC power transmission converter synchronized by a phase-locked loop, based on model order reduction theory and considering SRF-PLL dynamic equation and circuit KVL equation, equations of grid-connected point voltage dq-axis components and phase-locked loop output phase are established.

[0065] It should be noted that, in the present application, for a grid-following flexible DC power transmission converter synchronized by a phase-locked loop, based on model order reduction theory, equations of grid-connected point voltage dq-axis components and phase-locked loop output phase are established by considering SRF-PLL dynamic equation while establishing AC side circuit KVL equation:

[0066]

[0067]

[0068] wherein, is the grid-connected point voltage d-axis component, is the grid-connected point voltage q-axis component, is the amplitude of grid voltage, is the phase of grid voltage, is the amplitude of grid impedance, is the phase angle of grid impedance, and φ is the phase difference between grid-connected point voltage and grid current, also known as power factor angle, is the amplitude of grid current, is the rated angular frequency of grid, is the proportional coefficient of SRF-PLL PI controller, is the integral coefficient of SRF-PLL PI controller, is the phase-locked loop output phase.

[0069] Step 102, based on the equations of grid-connected point voltage dq-axis components and phase-locked loop output phase, an expression of power angle is established, and based on the fact that power angle is a physical quantity varying with time, a differential equation of AC side dynamic characteristics is established, wherein the angle difference between phase-locked loop output phase and the phase of grid voltage is defined as power angle.

[0070] It should be noted that the phase difference between the phase-locked loop output phase and the phase of the grid voltage is the power angle δ, considering , wherein ], and , the grid point voltage dq-axis component and SRF-PLL dynamics equation is established:

[0071]

[0072]

[0073] wherein, is the grid point voltage d-axis component, is the grid point voltage q-axis component, is the amplitude of the grid voltage, is the phase of the grid voltage, is the amplitude of the grid impedance, is the phase angle of the grid impedance, φ is the phase difference between the grid point voltage and the grid current, is the amplitude of the grid current, is the rated angular frequency of the grid, is the proportional coefficient of the PI controller of the SRF-PLL, is the integral coefficient of the PI controller of the SRF-PLL, is the amplitude of the grid resistance, is the amplitude of the grid reactance, is the d-axis component of the grid current, is the q-axis component of the grid current, is the phase of the grid impedance.

[0074] Considering the power angle is a physical quantity related to time, twice the derivative of both sides with respect to time is taken to establish the differential equation of the AC side dynamic characteristics:

[0075]

[0076] wherein, is the second derivative of the power angle, is the first derivative of the power angle.

[0077] Step 103, according to the input error direct current expression of the PI controller of the SRF-PLL and the expression of the favorable area and the unfavorable area, a dq axis current adaptive adjustment strategy is constructed, which introduces the output angular frequency variation of the phase-locked loop into the dq axis current, and a grid point voltage criterion is used to determine whether to put into the dq axis current adaptive adjustment strategy.

[0078] It should be noted that the input error direct current expression of the PI control of the SRF-PLL is established as follows:

[0079]

[0080] Wherein, err is the input error direct current, is the d-axis component of the grid current, is the q-axis component of the grid current, is the amplitude of the grid voltage, is the amplitude of the grid resistance, is a state variable, which is equal to the power angle δ.

[0081] If the amplitude of the grid voltage The severe drop makes When the PLL does not have a balance point, the input error direct current err cannot be controlled to zero.

[0082] The expression of the favorable area is established as follows:

[0083]

[0084] Wherein, is the favorable area, is the system power angle before the large disturbance, is the grid voltage per unit value after the large disturbance.

[0085] The expression of the unfavorable area is established as follows:

[0086] Wherein, is the unfavorable area.

[0087] In order to ensure that there is a balance point when the voltage drops severely and the favorable area is greater than the unfavorable area, so as to finally maintain the transient stability of the flexible AC / DC system, the output angular frequency variation of the phase-locked loop (the feedback term) is introduced into the dq axis current, and the voltage identification criterion is used, and the strategy takes effect only when the voltage drops severely, and the specific expression thereof is established as follows:

[0088]

[0089]

[0090] Wherein, For transfer functions, The adaptively corrected d-axis component of the grid current. Rms represents the adaptively corrected q-axis component of the grid current, and u is the per-unit value of the effective phase voltage at the grid connection point. PCC_L2G_rms With threshold value Rms set The output level signal after comparison.

[0091] The specific discrimination logic of RMS is as follows:

[0092]

[0093] Step 104: Based on the principle of angular frequency fluctuation of the phase-locked loop output during transient periods, establish an integral controller for adaptive adjustment of the dq-axis current, obtain an improved adaptive adjustment expression for the current reference value, and establish an adaptive adjustment strategy for the current reference value based on enhanced synchronous stability.

[0094] It should be noted that, considering have Given the characteristic that it still has a stable output value even when the control is set to 0, considering the simplest case, we establish... For an integral controller, its expression is:

[0095]

[0096] in, Let be the integral coefficient of the integral controller, and s be the Laplace operator.

[0097] During the transient process, The fluctuations made Constantly changing, thus and Continuously adapt and adjust, when When the current is set to 0, the system stabilizes at the new current reference value, achieving transient stability.

[0098] Establish an adaptive adjustment strategy for the current reference value based on synchronous stability enhancement, such as... Figure 2 As shown, Figure 2 The principle is: to label the effective value of the phase voltage at the grid connection point as a per-unit value. With threshold value The comparison is performed, and a high or low level is output based on the comparison result. If... Greater than The output is a low-level signal Rms=0. Since the input to the integral controller is 0, the output is also 0. , The adaptive current reference value method does not start. If Less than The input to the integral controller switches from 0 to , The fluctuations make and Continuously adapt and adjust, when When the current is set to 0, the system stabilizes at the new current reference value, and the system reaches transient stability.

[0099] Step 105: Based on the large-signal model of the phase-locked loop, establish the settling time and damping ratio of the SRF-PLL.

[0100] It should be noted that, based on the inertia and damping terms in the synchronous machine rotor motion equations, the equivalent inertia and damping terms in the dynamic differential equations of the flexible DC transmission system are as follows:

[0101]

[0102] in, For equivalent inertia, This is the equivalent damping.

[0103] Establishing the settling time and damping ratio of the SRF-PLL:

[0104]

[0105] in, For the damping ratio, For stable time.

[0106] Step 106: Based on the SRF-PLL dynamic equation, establish an expression for the frequency change during the transient period.

[0107] It should be noted that, based on the SRF-PLL dynamic equations, the expression for the frequency change during the transient period is established as follows:

[0108]

[0109] in, This refers to the change in angular frequency output by the phase-locked loop proportional controller. This represents the change in angular frequency output by the phase-locked loop integral controller.

[0110] Step 107: Based on the factors affecting the dynamic performance of the flexible DC transmission system during transient periods, establish an adaptive switching strategy for the input of the SRF-PLL integral controller between the q-axis component of the grid connection point voltage and 0, and establish a large disturbance stage identification criterion based on the effective value of the PCC phase voltage and its rate of change, and construct an adaptive switching strategy for the input of the phase-locked loop integral controller based on the detection of large disturbance stages.

[0111] It should be noted that according to the main influencing factors of the dynamic performance of the flexible HVDC transmission system during the transient period, in order to improve the dynamic performance of the flexible HVDC transmission system during the transient period, reduce the fluctuation of the angular frequency change, avoid negative damping of the system, shorten the stable time and reduce the overshoot, the input of the SRF-PLL integral controller is adaptively switched between 0 and 1 according to the large disturbance transient stage of the system in the present application. The specific logic is established as follows: if the system is in the transient process of large disturbance occurrence and end, the input of the SRF-PLL integral controller is switched to 0, if the system is in the large disturbance period or after the steady state, the input of the SRF-PLL integral controller is switched back to 1. .

[0112] The logic of adaptive switching of the input of the SRF-PLL integrator is established as follows:

[0113]

[0114] Switch is the switching logic level, and the specific expression is as follows:

[0115]

[0116] Where, Rate_decr is the level signal obtained by comparing the change amount u PCC_L2G_rms_rate of the effective value of the grid point phase voltage with the corresponding threshold value Decr set , Rate_incr is the level signal obtained by comparing the change amount u PCC_L2G_rms_rate of the effective value of the grid point phase voltage with the corresponding threshold value Incr set , Decr set is the change rate threshold value when the grid point voltage drops, and Incr set is the change rate threshold value when the grid point voltage rises. The specific comparison logic of Rate_decr and Rate_incr is as follows:

[0117]

[0118] Where, u PCC_L2G_rms_rate is the differential value of . The differential operation adopts a washout filter (WF) to realize, and the expression is as follows:

[0119]

[0120] Where, T is a low time constant, is the response function of the washout filter, and s is the Laplace operator.

[0121] The adaptive switching strategy of the phase-locked loop integral controller input based on large disturbance stage detection is constructed as follows: Figure 3As shown, Figure 3 In the middle, the block diagram of the adaptive adjustment method of current reference value based on synchronous stability enhancement is shown in the figure. Figure 2 same. Figure 3 The principle of the adaptive switching block diagram for the phase-locked loop (PLL) integral controller is as follows: The input switching of the integrator in the PLL PI controller is such that when the Switch signal is low, the PLL is operating normally, and the integrator input is... When the Switch signal is high, the input of the phase-locked loop integrator switches to 0, meaning the phase-locked loop degenerates from a second-order to a first-order phase-locked loop. Figure 3 The principle of the large disturbance phase detection module is as follows: Its differential value is obtained by rinsing the filter. .like Greater than Decr set If the condition is met, Rate_decr outputs a high level; otherwise, it outputs a low level. Less than Incr set If the AND logic result of Rate_decr and Rms is high, it indicates that the system is in a voltage drop phase where the voltage is below the threshold value. If the AND logic result of Rate_incr and Rms is high, it indicates that the system is in a voltage recovery phase where the voltage is below the threshold value. Performing an OR logic operation on the results of these two AND logic operations yields the Switch signal. The Switch signal outputs a high level when the voltage drops below the threshold value or recovers and is below the threshold value.

[0122] Step 108: Perform large-signal synchronous stability control on the flexible DC transmission system according to the adaptive adjustment strategy of the current reference value based on synchronous stability enhancement and the adaptive switching strategy of the phase-locked loop integral controller input based on large disturbance stage detection.

[0123] It should be noted that, according to Figure 3 The adaptive adjustment strategy of current reference value based on synchronous stability enhancement and the adaptive switching strategy of phase-locked loop integral controller input based on large disturbance stage detection are shown to perform large signal synchronous stability control on flexible DC transmission system.

[0124] The application provides a flexible HVDC system large signal synchronous stability control method, which is adaptive to current reference value, based on model order reduction theory, and a large signal model for transient analysis of the flexible HVDC system is constructed according to the phase-locked loop structure of the grid-connected flexible HVDC system, the influence of the current reference value on the existence of the equilibrium point and the transient stability after a large disturbance is determined, the output characteristics and feedback control of the integral controller are used to establish an adaptive current reference value adjustment strategy based on synchronous stability enhancement, according to adaptive switching of the input of the phase-locked loop integral controller, the transient stability mechanism and equivalent damping of the flexible HVDC converter during a large disturbance are determined, and an adaptive switching strategy of the input of the phase-locked loop integral controller based on large disturbance stage detection is constructed according to the dynamic characteristics of the phase-locked loop itself, and the flexible HVDC system is controlled in large signal synchronous stability according to the adaptive current reference value adjustment strategy based on synchronous stability enhancement and the adaptive switching strategy of the input of the phase-locked loop integral controller based on large disturbance stage detection, when a large amplitude symmetric voltage drop occurs on the AC side of the flexible HVDC system and the converter enters fault ride-through, the dq-axis current reference value of the current inner loop is adaptively adjusted, so that the system not only has a stable equilibrium point, but also can smoothly transition to a new steady state, avoiding transient instability, when a large amplitude symmetric voltage drop occurs on the AC side, adaptive switching of the input of the phase-locked loop integral controller is performed according to the identification result of the transient process, the phase-locked loop is accurately tracked, the control system of the flexible HVDC has good dynamic performance and accuracy, and after the voltage drop disappears, the voltage and current of the flexible HVDC system can be prevented from oscillating and diverging during the recovery process, so that the system can recover to normal operation in a short time. The technical problem of poor stability of the existing flexible HVDC system control method under large disturbance transient events of high proportion of new energy and weak power grid is solved.

[0125] In order to verify the effect of the flexible HVDC system large signal synchronous stability control method provided by the application, a specific embodiment is provided: based on a two-terminal MMC-HVDC system as shown in Figure 4 The system parameters and the control parameters of MMC2 are shown in Table 1.

[0126] Table 1

[0127]

[0128] Two large disturbance signals, i.e. AC system voltage drop (current setting strategy with reactive power priority) and AC system voltage drop (current setting strategy with active power priority), are set, and the flexible HVDC system large signal synchronous stability control method provided by the application is used for example test:

[0129] Large disturbance signal 1: AC system voltage drop (current setting strategy with reactive power priority)

[0130] The flexible DC transmission system is set to operate according to the control command reference values ​​in Table 1. During periods of large disturbances, the current reference value is set with reactive power priority: =0, =-1. At 4.5s, the AC system voltage connected to MMC2 drops to 0.026pu, lasting for 2s. The voltage, current, angular frequency, and power angle response waveforms at the MMC2 grid connection point, and the DC current response waveform of MMC2 are shown below. Figure 5 , Figure 6 As shown. By Figure 5 As can be seen, when the AC system voltage drops to 0.026 pu, the d-axis voltage at the grid connection point drops from 1 p.u. before the large disturbance to 0.29 pu, a drop of 71% of the pre-disturbance level. Due to the adaptive adjustment of the current reference value, MMC2 ultimately injects 0.066 pu of active current and -0.93 pu of reactive current into the AC system, primarily to transmit reactive power, thereby supporting the grid connection point voltage. The system maintains transient stability, and the power angle δ eventually stabilizes at 0.582°. After the large disturbance disappears in 6.5 seconds, the system returns to normal. Figure 6 It can be seen that because the AC side of the converter can maintain transient stability, the DC current also remains stable, and the system can smoothly deliver power.

[0131] Large disturbance signal 2: AC system voltage drop (using active power priority current setting strategy)

[0132] The flexible DC transmission system is set to operate according to the control command reference values ​​in Table 1. During periods of large disturbances, the active power priority current reference values ​​are set as follows: =0.8, =-0.6. At 4.5s, the AC system voltage connected to MMC2 drops to 0.213pu, lasting for 2s. The voltage, current, angular frequency, and power angle response waveforms at the MMC2 grid connection point, and the DC current response waveform of MMC2 are shown below. Figure 7 , Figure 8 As shown. By Figure 7 It can be seen that when the AC system voltage drops to 0.213 pu, the d-axis voltage at the grid connection point drops from 1 p.u. before the large disturbance to 0.43 pu, a drop of 57% of the pre-disturbance value. Due to the adaptive adjustment of the current reference value, MMC2 ultimately injects 0.523 pu of active current and -0.852 pu of reactive current into the AC system, supporting the voltage while generating a significant amount of active power. The system can maintain transient stability, and the final power angle... The temperature stabilized at 38.8°C, and the system returned to normal after the large disturbance disappeared in 6.5 seconds. Figure 8 It can be seen that because the AC side of the converter can maintain transient stability, the DC current also remains stable, and the system can smoothly deliver power.

[0133] For ease of understanding, please refer to Figure 9 An embodiment of a large signal synchronous stability control device for a flexible DC power transmission system is provided in the present application, comprising:

[0134] A first equation construction module is configured to, for a grid-connected flexible DC power transmission converter synchronized by a phase-locked loop (PLL), establish equations of dq-axis components of a point of common coupling (PCC) voltage and an output phase of the PLL based on a model order reduction principle and considering SRF-PLL dynamic equations and circuit KVL equations;

[0135] A second equation construction module is configured to, according to the equations of the dq-axis components of the PCC voltage and the output phase of the PLL, establish an expression of a power angle, and based on the power angle being a physical quantity varying with time, establish a differential equation of dynamic characteristics of an alternating current (AC) side, wherein an angle difference between the output phase of the PLL and a phase of the grid voltage is defined as the power angle;

[0136] An adaptive adjustment strategy construction module is configured to, according to an input error direct current (DC) expression of a PI controller of the SRF-PLL and expressions of a favorable area and an unfavorable area, construct a dq-axis current adaptive adjustment strategy for introducing an output angular frequency variation of the PLL into the dq-axis current, and determine whether to put into the dq-axis current adaptive adjustment strategy by using a PCC voltage criterion;

[0137] A strategy enhancement module is configured to, according to an output angular frequency fluctuation principle of the PLL during a transient state, establish an integral controller of the dq-axis current adaptive adjustment, obtain an improved current reference value adaptive adjustment expression, and establish a current reference value adaptive adjustment strategy based on synchronous stability enhancement;

[0138] A first calculation module is configured to, according to a large signal model of the PLL, establish a stable time and a damping ratio of the SRF-PLL;

[0139] A second calculation module is configured to, according to the SRF-PLL dynamic equations, establish an expression of a frequency variation during a transient state;

[0140] An adaptive switching strategy construction module is configured to, according to influencing factors of dynamic performance of the flexible DC power transmission system during the transient state, establish an adaptive switching strategy of an input of an integral controller of the PLL between a q-axis component of the PCC voltage and 0, and establish a large disturbance stage identification criterion based on an effective value of a phase voltage of the PCC and a rate of change thereof, and construct an adaptive switching strategy of the input of the integral controller of the PLL based on detection of the large disturbance stage;

[0141] A control module is configured to, according to the current reference value adaptive adjustment strategy based on synchronous stability enhancement and the adaptive switching strategy of the input of the integral controller of the PLL based on detection of the large disturbance stage, perform large signal synchronous stability control on the flexible DC power transmission system.

[0142] In one embodiment, the equation of the grid-connected point voltage dq-axis component and the phase locked loop output phase is:

[0143]

[0144]

[0145] wherein, is the grid-connected point voltage d-axis component, is the grid-connected point voltage q-axis component, is the amplitude of the grid voltage, is the phase of the grid voltage, is the amplitude of the grid impedance, is the phase angle of the grid impedance, φ is the phase difference between the grid-connected point voltage and the grid current, is the amplitude of the grid current, is the rated angular frequency of the grid, is the proportional coefficient of the PI controller of the SRF-PLL, is the integral coefficient of the PI controller of the SRF-PLL, is the phase locked loop output phase.

[0146] In one embodiment, the differential equation of the AC side dynamic characteristic is:

[0147]

[0148] wherein, is the power angle, is the second derivative of the power angle, is the first derivative of the power angle, is the amplitude of the grid resistance, is the amplitude of the grid reactance, is the d-axis component of the grid current, is the q-axis component of the grid current.

[0149] In one embodiment, the input error direct current flow expression is:

[0150]

[0151] wherein, err is the input error direct current flow, is the d-axis component of the grid current, is the q-axis component of the grid current, is the amplitude of the grid voltage, is the amplitude of the grid resistance, is the state variable.

[0152] In one embodiment, the grid-connected point voltage criterion is:

[0153]

[0154] wherein Rms is the level of the output signal, is the RMS value of the grid point phase voltage, and is the threshold value.

[0155] In one embodiment, the settling time and damping ratio of the SRF-PLL are:

[0156]

[0157] wherein, is the damping ratio, is the settling time.

[0158] In one embodiment, the expression of the frequency variation amount during the transient state is:

[0159]

[0160] wherein, is the angular frequency variation amount output by the proportional controller of the phase-locked loop, is the angular frequency variation amount output by the integral controller of the phase-locked loop.

[0161] In one embodiment, the input adaptive switching strategy of the integral controller of the phase-locked loop based on large disturbance stage detection is:

[0162]

[0163]

[0164]

[0165] wherein, is the q-axis component of the grid point voltage, Switch is the switching logic level, Rate_decr is the variation amount u of the RMS value of the grid point phase voltage PCC_L2G_rms_rate compared with the corresponding threshold value Decr set to obtain the level signal, Rate_incr is the variation amount u of the RMS value of the grid point phase voltage PCC_L2G_rms_rate compared with the corresponding threshold value Incr set to obtain the level signal, Decr set is the change rate threshold value when the grid point voltage drops, Incr set is the change rate threshold value when the grid point voltage rises.

[0166] In one embodiment, the dq-axis current adaptive adjustment strategy is:

[0167]

[0168]

[0169]

[0170] wherein, is a d-axis component of the grid current, is a q-axis component of the grid current, is an adaptive corrected d-axis component of the grid current, is an adaptive corrected q-axis component of the grid current, is a transfer function, is a phase-locked loop output angular frequency variation.

[0171] The flexible HVDC power transmission system large signal synchronous stability control device provided in the present application is used to execute the flexible HVDC power transmission system large signal synchronous stability control method provided in the present application, and the principle and the technical effects obtained are the same as those of the flexible HVDC power transmission system large signal synchronous stability control method provided in the present application, and will not be repeated here.

[0172] Those skilled in the art can clearly understand that, for the convenience and brevity of the description, the specific working processes of the above-described system, device and unit can refer to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0173] The above-described embodiments are only used to illustrate the technical solutions of the present application, rather than limiting them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacements for some technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for controlling large signal synchronous stability of a flexible direct current power transmission system, characterized by, Comprising: For the grid-following type flexible DC power transmission converter using phase-locked loop synchronization, based on model order reduction principle and considering SRF-PLL dynamic equation and circuit KVL equation, the equation of grid-connected point voltage dq-axis component and phase-locked loop output phase is established; According to the equation of grid-connected point voltage dq-axis component and phase-locked loop output phase, the expression of power angle is established, and based on the fact that the power angle is a physical quantity related to time, the differential equation of AC side dynamic characteristics is established, wherein the angle difference between the phase-locked loop output phase and the phase of the grid voltage is defined as the power angle; According to the input error direct current expression of the PI controller of the SRF-PLL and the expressions of the favorable area and the unfavorable area, the dq-axis current adaptive adjustment strategy is constructed, which introduces the phase-locked loop output angular frequency variation into the dq-axis current, and the grid-connected point voltage criterion is used to determine whether to put into the dq-axis current adaptive adjustment strategy; According to the principle of phase-locked loop output angular frequency fluctuation during transient period, the integral controller of the dq-axis current adaptive adjustment is established, the improved current reference value adaptive adjustment expression is obtained, and the current reference value adaptive adjustment strategy based on synchronous stability enhancement is established; According to the large signal model of the phase-locked loop, the stable time and damping ratio of the SRF-PLL are established; According to the SRF-PLL dynamic equation, the frequency variation expression during transient period is established; According to the influencing factors of the dynamic performance of the flexible DC power transmission system during transient period, the adaptive switching strategy of the input of the SRF-PLL integral controller between the q-axis component of the grid-connected point voltage and 0 is established, the large disturbance stage detection criterion based on the effective value and its rate of change of the PCC phase voltage is established, and the phase-locked loop integral controller input adaptive switching strategy based on large disturbance stage detection is constructed; According to the current reference value adaptive adjustment strategy based on synchronous stability enhancement and the phase-locked loop integral controller input adaptive switching strategy based on large disturbance stage detection, the large signal synchronous stability control of the flexible DC power transmission system is carried out.

2. The method of flexible HVDC system large signal synchronous stability control according to claim 1, characterized in that, The equation of grid-connected point voltage dq-axis component and phase-locked loop output phase is: wherein, is a grid point voltage d-axis component, is a grid point voltage q-axis component, is a magnitude of a grid voltage, is a phase of a grid voltage, is a magnitude of a grid impedance, is a phase angle of a grid impedance, φ is a phase difference between a grid point voltage and a grid current, is a magnitude of a grid current, is a grid rated angular frequency, is a proportional coefficient of a PI controller of a SRF-PLL, is an integral coefficient of a PI controller of a SRF-PLL, is a phase of a phase-locked loop output.

3. The method of flexible HVDC power transmission system large signal synchronous stability control according to claim 2, characterized in that, The differential equation of AC side dynamic characteristics is: wherein is the power angle, is the second derivative of the power angle, is the first derivative of the power angle, is the magnitude of the grid resistance, is the magnitude of the grid reactance, is the d-axis component of the grid current, is the q-axis component of the grid current.

4. The method for flexible HVDC system large signal synchronous stability control according to claim 1, characterized in that, The input error direct current expression is: where err is the input error direct current, is the d-axis component of the grid current, is the q-axis component of the grid current, is the amplitude of the grid voltage, is the amplitude of the grid resistance, is the state variable.

5. The method for flexible HVDC system large signal synchronous stability control according to claim 2, characterized in that, The grid-connected point voltage criterion is: wherein Rms is the level of the output signal, Vth is the threshold value of the inverter, and Vth is the threshold value of the inverter, and 6. The method for flexible HVDC system large signal synchronous stability control according to claim 2, characterized in that, The stable time and damping ratio of the SRF-PLL are: wherein, is the damping ratio, is the settling time.

7. The method of flexible HVDC system large signal synchronous stability control according to claim 3, characterized in that, The frequency variation expression during transient period is: wherein is the angular frequency variation output by the proportional controller of the phase-locked loop, is the angular frequency variation output by the integral controller of the phase-locked loop.

8. The method for flexible HVDC system large signal synchronous stability control according to claim 5, characterized in that, The phase-locked loop integral controller input adaptive switching strategy based on large disturbance stage detection is: wherein, is the q-axis component of the grid point voltage, Switch is the switching logic level, Rate_decr is the change amount u of the grid point phase voltage effective value per unit PCC_L2G_rms_rate is compared with the corresponding threshold value Decr set The level signal obtained by comparison, Rate_incr is the change amount u of the grid point phase voltage effective value per unit PCC_L2G_rms_rate is compared with the corresponding threshold value Incr set The level signal obtained by comparison, Decr set is the change rate threshold value when the grid point voltage drops, Incr set is the change rate threshold value when the grid point voltage rises.

9. The method for flexible HVDC system large signal synchronous stability control according to claim 5, characterized in that, The dq-axis current adaptive adjustment strategy is: wherein is a d-axis component of the grid current, is a q-axis component of the grid current, is an adaptive corrected d-axis component of the grid current, is an adaptive corrected q-axis component of the grid current, is a transfer function, is a phase-locked loop output angular frequency variation.

10. A device for controlling the large signal synchronous stability of a flexible DC power transmission system, characterized in that Comprising: The first equation construction module is used for, for the grid-following type flexible DC power transmission converter using phase-locked loop synchronization, based on model order reduction principle and considering SRF-PLL dynamic equation and circuit KVL equation, the equation of grid-connected point voltage dq-axis component and phase-locked loop output phase is established; The second equation construction module is used for, according to the equation of grid-connected point voltage dq-axis component and phase-locked loop output phase, the expression of power angle is established, and based on the fact that the power angle is a physical quantity related to time, the differential equation of AC side dynamic characteristics is established, wherein the angle difference between the phase-locked loop output phase and the phase of the grid voltage is defined as the power angle; An adaptive adjustment strategy construction module is configured to construct a dq-axis current adaptive adjustment strategy for introducing a phase-locked loop output angular frequency variation into the dq-axis current according to an input error direct current expression of a PI controller of the SRF-PLL and expressions of the favorable area and the unfavorable area, and determine whether to put into the dq-axis current adaptive adjustment strategy by using a grid-connected point voltage criterion; A strategy enhancement module is configured to establish an integral controller of the dq-axis current adaptive adjustment according to a phase-locked loop output angular frequency fluctuation principle during a transient state, obtain an improved current reference value adaptive adjustment expression, and establish a current reference value adaptive adjustment strategy based on synchronous stability enhancement; A first calculation module is configured to establish a stable time and a damping ratio of the SRF-PLL according to a large signal model of the phase-locked loop; A second calculation module is configured to establish a frequency variation expression during a transient state according to a dynamic equation of the SRF-PLL; An adaptive switching strategy construction module is configured to establish an adaptive switching strategy for input of an integral controller of the SRF-PLL between a grid-connected point voltage q-axis component and 0 according to influencing factors of dynamic performance of the flexible HVDC power transmission system during a transient state, establish a large disturbance stage identification criterion based on an effective value of a PCC phase voltage and a change rate thereof, and construct an adaptive switching strategy for input of the phase-locked loop integral controller based on large disturbance stage detection; A control module is configured to perform large signal synchronous stability control on the flexible HVDC power transmission system according to the current reference value adaptive adjustment strategy based on synchronous stability enhancement and the adaptive switching strategy for input of the phase-locked loop integral controller based on large disturbance stage detection.

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