A droop control method and system based on parameter adaptation

Abstract

A droop control method and system based on parameter adaptation disclosed by the present invention obtain a first critical voltage value and a second critical voltage value when the grid-connected system is in a critically stable state; the system power angle corresponding to the first critical voltage value is the critical power angle; the system power angle corresponding to the second critical voltage value is the rated power angle, and the reactive droop coefficient of the droop converter is the minimum reactive droop coefficient under normal operation; when the grid voltage is less than the first critical voltage value and not less than the second critical voltage value, the reactive droop coefficient is adaptively adjusted with the goal of the system power angle reaching the rated power angle; when the grid voltage is less than the second critical voltage value, the reactive power reference value is adaptively adjusted with the goal of the system power angle reaching the rated power angle, and at the same time, the active power reference value is constrained according to the capacity of the droop converter. By adopting the embodiment of the present invention, it is possible to ensure that the system operates stably at the equilibrium point for a long time, and at the same time, the grid connection capacity during the transient process of the system is ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of droop control, and particularly to a droop control method and system based on parameter adaptation. Background Art

[0002] The integration of renewable energy distributed generation and traditional power systems has reshaped the pattern of energy production and distribution. With the increasing popularity of photovoltaic power generation and wind power generation, in order to ensure the stability and reliability of the power system, converters based on grid-forming control are widely used.

[0003] Droop control is an important control strategy in converters based on grid-forming control. Since droop control ignores the inertial characteristics, it can quickly achieve the synchronization stability of the power angle when the power grid has a minor fault. However, when facing a more serious power grid fault, the system equilibrium point will be lost, and traditional droop control is difficult to maintain transient stability, and the power angle will completely lose synchronization stability. Summary of the Invention

[0004] Embodiments of the present invention provide a droop control method and system based on parameter adaptation, which can conveniently analyze the fault degree of the grid-connected system and perform corresponding adaptive parameter adjustment according to the fault degree to ensure that the system operates stably at the equilibrium point for a long time, and at the same time ensure the grid-connected capacity during the transient process of the system.

[0005] Specifically, embodiments of the present invention provide a droop control method based on parameter adaptation, including:

[0006] Obtain a first critical voltage value and a second critical voltage value of the grid-connected system in a critically stable state; the power angle of the system corresponding to the first critical voltage value is the critical power angle; the power angle of the system corresponding to the second critical voltage value is the rated power angle, and the reactive droop coefficient of the droop converter is the minimum reactive droop coefficient under the normal operation of the grid-connected system;

[0007] When the grid voltage is less than the first critical voltage value and not less than the second critical voltage value, adaptively adjust the reactive droop coefficient with the goal of the system power angle reaching the rated power angle;

[0008] When the grid voltage is less than the second critical voltage value, adaptively adjust the reactive power reference value with the goal of the system power angle reaching the rated power angle, and at the same time constrain the active power reference value according to the capacity of the droop converter.

[0009] As an improvement of the above solution, the obtaining of the first critical voltage value and the second critical voltage value of the grid-connected system in a critically stable state includes:

[0010] Generate a phase trajectory of the grid-connected system according to the relationship between the voltage output frequency, voltage output amplitude and system power angle of the grid-connected system based on droop control;

[0011] Take the voltage value when the phase trajectory is tangent to the equilibrium point as the first critical voltage value when the grid-connected system is in a critically stable state;

[0012] Set the system power angle to the rated power angle under normal operation to obtain the minimum reactive power droop coefficient;

[0013] According to the phase trajectory and the minimum reactive power droop coefficient, obtain the second critical voltage value when the grid-connected system is in a critically stable state.

[0014] As an improvement of the above solution, the generating the phase trajectory of the grid-connected system according to the relationship between the voltage output frequency, voltage output amplitude and system power angle of the grid-connected system based on droop control includes:

[0015] Construct a voltage-frequency control function according to the relationship between the voltage output frequency and the system power angle of the grid-connected system based on droop control , where ;

[0016] Construct a voltage-amplitude control function according to the relationship between the voltage output amplitude and the system power angle of the grid-connected system based on droop control , where ;

[0017] According to the voltage-frequency control function and the voltage-amplitude control function, generate the phase trajectory of the grid-connected system through ;

[0018] where is the active output frequency, is the active droop coefficient, is the active power reference value, is the actual active power, is the grid frequency reference value; is the grid voltage, is the grid connection point voltage, is the system power angle, is the grid impedance; is the reactive power droop coefficient, is the reactive power reference value, is the actual reactive power, is the grid connection point voltage reference value.

[0019] As an improvement of the above solution, the setting the system power angle to the rated power angle under normal operation to obtain the minimum reactive power droop coefficient includes:

[0020] Set the system power angle to the rated power angle under normal operation ;

[0021] By calculating the minimum reactive droop coefficient ; where is the grid impedance, is the grid connection point voltage reference value, is the grid voltage, is the active power reference value, is the reactive power reference value.

[0022] As an improvement to the above solution, when the grid voltage is less than the first critical voltage value and not less than the second critical voltage value, aiming at the system power angle reaching the rated power angle, adaptively adjusting the reactive droop coefficient includes:

[0023] When the grid voltage is less than the first critical voltage value and not less than the second critical voltage value, according to the rated power angle, obtain the minimum reactive droop coefficient under normal system operation;

[0024] Aiming at the system power angle reaching the rated power angle, and taking the minimum reactive droop coefficient as a constraint, adaptively adjust the reactive droop coefficient.

[0025] As an improvement to the above solution, when the grid voltage is less than the second critical voltage value, aiming at the system power angle reaching the rated power angle, adaptively adjust the reactive power reference value, and at the same time constrain the active power reference value according to the capacity of the droop converter, including:

[0026] When the grid voltage is less than the second critical voltage value, aiming at the system power angle reaching the rated power angle, through adaptive adjustment of the reactive power reference value ;

[0027] According to constrain and adjust the active power reference value ;

[0028] where is the grid impedance, is the grid connection point voltage reference value, is the reactive droop coefficient, is the maximum capacity of the droop converter, is the reactive power reference value, is the grid voltage, is the rated power angle under normal operation.

[0029] As an improvement to the above solution, after when the grid voltage is less than the second critical voltage value, aiming at the system power angle reaching the rated power angle, adaptively adjusting the reactive power reference value, and at the same time constraining the active power reference value according to the capacity of the droop converter, the droop control method based on parameter adaptation further includes:

[0030] During the active power response control process of constraining the active power reference value according to the capacity of the droop converter, a PI controller is added to the grid-connected system for frequency compensation;

[0031] Calculate the dynamic response parameters of the grid-connected system before and after adding the PI controller respectively; the dynamic response parameters include the natural oscillation angular frequency and damping ratio of the system;

[0032] Set the proportional coefficient and integral coefficient of the PI controller according to the dynamic response parameters.

[0033] As an improvement to the above solution, the step of calculating the dynamic response parameters of the grid-connected system before and after adding the PI controller respectively includes:

[0034] Obtain the initial active power transfer function and the optimized active power transfer function according to the grid-connected system before and after adding the PI controller;

[0035] Calculate the initial natural oscillation angular frequency and initial damping ratio of the grid-connected system according to the initial active power transfer function;

[0036] Calculate the optimized natural oscillation angular frequency and optimized damping ratio of the grid-connected system according to the optimized active power transfer function.

[0037] As an improvement to the above solution, the step of setting the proportional coefficient and integral coefficient of the PI controller according to the dynamic response parameters includes:

[0038] Taking the optimized natural oscillation angular frequency being greater than the initial natural oscillation angular frequency and the optimized damping ratio being greater than the initial damping ratio as the objective function, solve the range of the proportional coefficient and the range of the integral coefficient of the PI controller;

[0039] Set the proportional coefficient and integral coefficient of the PI controller according to the range of the proportional coefficient and the range of the integral coefficient.

[0040] An embodiment of the present invention further provides a droop control system based on parameter adaptation, including:

[0041] A critical voltage value acquisition module, configured to acquire a first critical voltage value and a second critical voltage value when the grid-connected system is in a critically stable state; the power angle of the system corresponding to the first critical voltage value is the critical power angle; the power angle of the system corresponding to the second critical voltage value is the rated power angle, and the reactive droop coefficient of the droop converter is the minimum reactive droop coefficient under normal operation of the grid-connected system;

[0042] A reactive droop coefficient adjustment module, configured to adaptively adjust the reactive droop coefficient with the goal of the system power angle reaching the rated power angle when the grid voltage is less than the first critical voltage value and not less than the second critical voltage value;

[0043] A power reference value adjustment module is used to adaptively adjust the reactive power reference value with the goal of making the system power angle reach the rated power angle when the grid voltage is less than the second critical voltage value, and at the same time, constrain the active power reference value according to the capacity of the droop converter.

[0044] Compared with the prior art, a droop control method and system based on parameter adaptation disclosed by the present invention obtain a first critical voltage value and a second critical voltage value when the grid-connected system is in a critically stable state; the system power angle corresponding to the first critical voltage value is the critical power angle; the system power angle corresponding to the second critical voltage value is the rated power angle, and the reactive droop coefficient of the droop converter is the minimum reactive droop coefficient under the normal operation of the grid-connected system; when the grid voltage is less than the first critical voltage value and not less than the second critical voltage value, the reactive droop coefficient is adaptively adjusted with the goal of making the system power angle reach the rated power angle; when the grid voltage is less than the second critical voltage value, the reactive power reference value is adaptively adjusted with the goal of making the system power angle reach the rated power angle, and at the same time, the active power reference value is constrained according to the capacity of the droop converter. By adopting the embodiments of the present invention, the fault degree of the grid-connected system can be conveniently analyzed, and corresponding adaptive parameter adjustment can be performed according to the fault degree to ensure that the system operates stably at the equilibrium point for a long time, and at the same time, the grid-connected capacity during the transient process of the system is ensured. Description of the Drawings

[0045] Figure 1 It is a schematic flow chart of the steps of a droop control method based on parameter adaptation provided by an embodiment of the present invention;

[0046] Figure 2 It is a structural and schematic diagram of a new energy grid-connected system based on droop control provided by an embodiment of the present invention;

[0047] Figure 3 It is a curve graph of the phase trajectory under different grid voltages provided by an embodiment of the present invention;

[0048] Figure 4 It is a curve graph of the phase trajectory under the influence of the reactive droop coefficient and the reactive power reference value provided by an embodiment of the present invention;

[0049] Figure 5 It is an equivalent active control structure diagram before and after adding a PI controller provided by an embodiment of the present invention;

[0050] Figure 6 It is an analysis diagram of the parameter feasible region of the PI controller in frequency compensation provided by an embodiment of the present invention;

[0051] Figure 7 It is a structural schematic diagram of a droop control system based on parameter adaptation provided by an embodiment of the present invention. Specific Embodiments

[0052] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts belong to the scope of protection of the present invention.

[0053] In the description of the specification and claims, it should be understood that the terms first, second, etc. in the specification and claims are only used for the purpose of distinguishing the description of the same technical features, and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features, nor necessarily describing the order or time sequence. The terms can be interchanged under appropriate circumstances. Thus, the features defined with "first" and "second" may explicitly or implicitly include at least one of the features.

[0054] The embodiment of the present invention provides a droop control method based on parameter adaptation. Please refer to Figure 1 In this embodiment, the droop control method based on parameter adaptation is specifically executed through steps S1 to S3:

[0055] S1. Obtain a first critical voltage value and a second critical voltage value when the grid-connected system is in a critically stable state; the system power angle corresponding to the first critical voltage value is the critical power angle; the system power angle corresponding to the second critical voltage value is the rated power angle, and the reactive droop coefficient of the droop converter is the minimum reactive droop coefficient under the normal operation of the grid-connected system.

[0056] It should be noted that in the embodiment of the present invention, the grid voltage of the grid-connected system is used as the basis for determining the severity of the fault. Specifically, the voltage value when the grid-connected system is in a critically stable state under different working conditions is used as the threshold of the fault level.

[0057] S2. When the grid voltage is less than the first critical voltage value and not less than the second critical voltage value, adaptively adjust the reactive droop coefficient with the goal of the system power angle reaching the rated power angle.

[0058] It should be noted that when the grid voltage is less than the first critical voltage value and not less than the second critical voltage value, the grid-connected system is in a slightly faulty condition, and the system steady state can be achieved only by adjusting the reactive droop coefficient in the droop control.

[0059] S3. When the grid voltage is less than the second critical voltage value, adaptively adjust the reactive power reference value with the goal of the system power angle reaching the rated power angle, and at the same time, constrain the active power reference value according to the capacity of the droop converter.

[0060] It should be noted that when the grid voltage is less than the second critical voltage value, it is considered that the voltage sag fault of the grid-connected system is serious. At this time, only relying on the droop control of the droop converter to adjust the reactive power droop coefficient will still cause the loss of the equilibrium point. Therefore, the reactive power reference value is adjusted to ensure the existence of the equilibrium point. And considering that too large a reactive power reference value will reduce the grid-connected capacity of the system, the embodiments of the present invention further constrain the active power reference value to ensure the grid-connected capacity of the system as much as possible during the transient process of the system.

[0061] In the above solution, by obtaining the critical voltage values of the grid-connected system under different working conditions, the fault degree of the grid-connected system can be conveniently analyzed, and corresponding adaptive parameter adjustments can be made according to the fault degree to ensure that the system operates stably at the equilibrium point for a long time, and at the same time ensure the grid-connected capacity of the system during the transient process of the system.

[0062] As a preferred implementation manner, in step S1, the obtaining of the first critical voltage value and the second critical voltage value when the grid-connected system is in a critically stable state is specifically executed through steps S11 - S14:

[0063] S11. Generate the phase trajectory of the grid-connected system according to the relationship between the voltage output frequency, the voltage output amplitude and the system power angle of the grid-connected system based on droop control;

[0064] S12. Take the voltage value when the phase trajectory is tangent to the equilibrium point as the first critical voltage value when the grid-connected system is in a critically stable state;

[0065] S13. Set the system power angle to the rated power angle under normal operation to obtain the minimum reactive power droop coefficient;

[0066] S14. According to the phase trajectory and the minimum reactive power droop coefficient, obtain the second critical voltage value when the grid-connected system is in a critically stable state.

[0067] It should be noted that the changes in the system power angle, the adjustment of the reactive power droop coefficient, and the adjustment of the reactive power of the grid-connected system will all cause changes in the phase trajectory. By setting parameters, the influence of other parameters on the phase trajectory can be more clearly understood from the curve of the phase trajectory.

[0068] It should also be noted that in steps S12 and S13, the parameters not mentioned are set to reference values. Under the reference values, the grid-connected system operates in a stable state. Exemplarily, both the reactive power droop coefficient and the reactive power are set to reference values.

[0069] In some preferred embodiments, according to the first critical voltage value and the second critical voltage value, the fault level of the grid-connected system can be classified. Exemplarily, when the grid voltage of the grid-connected system is less than the first critical voltage value and not less than the second critical voltage value, the grid-connected system is considered to be at the first-level fault level; when the grid voltage of the grid-connected system is less than the second critical voltage value, the grid-connected system is considered to be at the second-level fault level. The voltage drop degree at the first-level fault level is lighter than that at the second-level fault level.

[0070] Further, preferably, step S11 of generating the phase trajectory of the grid-connected system according to the relationship among the voltage output frequency, voltage output amplitude, and system power angle of the grid-connected system based on droop control includes:

[0071] Construct a voltage-frequency control function according to the relationship between the voltage output frequency and the system power angle of the grid-connected system based on droop control , where ;

[0072] Construct a voltage-amplitude control function according to the relationship between the voltage output amplitude and the system power angle of the grid-connected system based on droop control , where ;

[0073] According to the voltage-frequency control function and the voltage-amplitude control function, generate the phase trajectory of the grid-connected system through ;

[0074] where is the active output frequency, is the active droop coefficient, is the active power reference value, is the actual active power, is the grid frequency reference value; is the grid voltage, is the grid connection point voltage, is the system power angle, is the grid impedance; is the reactive droop coefficient, is the reactive power reference value, is the actual reactive power, is the grid connection point voltage reference value.

[0075] Please refer to Figure 2 . The droop control adjusts the frequency of the voltage by the active power output by the grid-forming grid-connected inverter and adjusts the amplitude of the voltage by the reactive power to ensure the synchronization of the grid-forming grid-connected inverter with the grid. Its control law is as follows:

[0076] (1);

[0077] (2);

[0078] Wherein, is the active output frequency, is the active droop coefficient, is the active power reference value, is the actual active power, is the grid frequency reference value; is the grid connection point voltage, is the reactive droop coefficient, is the reactive power reference value, is the actual reactive power, is the grid connection point voltage reference value.

[0079] The expressions for the active power and reactive power output by the network-forming inverter during steady-state operation are:

[0080] (3);

[0081] (4);

[0082] Wherein, is the grid voltage, is the system power angle, is the grid impedance.

[0083] Since the bandwidth of the inner control loop is much higher than that of the power outer loop, the inner loop can be equivalently regarded as a unity gain during transient analysis. Thus, the voltage at the PCC point can be equivalently regarded as the output voltage of the VSG. Combining Equation (2) and Equation (4), the relationship between the grid connection point voltage and the system power angle can be obtained:

[0084] (5);

[0085] Furthermore, by combining Equation (3) and Equation (5), the relationship between the active power and the system power angle can be obtained, which will not be elaborated here.

[0086] According to Equation (1) and Equation (3), a non-linear differential equation characterizing the power angle synchronization characteristics of droop control can be derived:

[0087] (6).

[0088] By solving the differential equation of Equation (6), the phase trajectory of the grid-connected system can be obtained.

[0089] During the stable operation of the power system, the system power angle is which is a critical power angle. If the system is disturbed and the power angle exceeds This may cause the system to lose synchronization, leading to serious problems such as power oscillations and even system collapse. Therefore, when designing and operating a power system, various control measures need to be taken to ensure that the power angle remains within a safe range and avoid approaching or exceeding this critical value. It should also be noted that the system power angle is a variable, which will restrict and interact with the voltage value. The specific value of the critical power angle described in step S1 will fluctuate with the system operation, but the value should not be greater than .

[0090] In Figure 3 the specific embodiment shown, taking the grid voltages of 1 p.u., 0.7 p.u., 0.5 p.u., and 0.3 p.u. as examples, it can be seen from the drawn phase trajectories that the grid voltage of 0.5 p.u. is a critical voltage value. If the grid voltage is greater than 0.5 p.u., the system itself has an equilibrium point; if the grid voltage is less than 0.5 p.u., the system equilibrium point is lost; when the grid voltage is equal to 0.5 p.u., the system is exactly in a critically stable state, indicating that the fault condition of the system is not serious at this time and can be adaptively controlled through droop control. The specific value of the second critical voltage value still needs to be further determined.

[0091] Preferably, step S13: setting the system power angle to the rated power angle under normal operation to obtain the minimum reactive power droop coefficient, includes:

[0092] Setting the system power angle to the rated power angle under normal operation ;

[0093] By calculating the minimum reactive power droop coefficient ; where is the grid impedance, is the grid-connected point voltage reference value, is the grid voltage, is the active power reference value, is the reactive power reference value.

[0094] It should be noted that the power angle under normal operation is generally a range value. For example, when, the system stability is relatively high. Then the rated power angle is any value within the above range. When the adjustment range of the power angle is larger, that is, the smaller the system power angle value, the smaller the reactive power droop coefficient correspondingly. Exemplarily, the rated power angle is set to .

[0095] When the system power angle is the rated power angle, the minimum reactive power droop coefficient can be derived through equations (2) and (4) as follows:

[0096] (7).

[0097] In the embodiment of the present invention, the reactive power droop coefficient the minimum value that can be achieved is taken as a constraint. If the reactive power droop coefficient is reduced to and the system equilibrium point has not been reshaped yet, it indicates that the method of only reducing the reactive power droop coefficient is no longer applicable at this time. Otherwise, not only a new equilibrium point cannot be generated, but also a larger voltage deviation will be caused.

[0098] In step S14, compared with obtaining the first critical voltage value, the parameters of the grid-connected system are further limited. In addition to setting the system power angle, the reactive power droop coefficient is set to the minimum reactive power droop coefficient to derive the second critical voltage value. Exemplarily, in the embodiment of the present invention, the second critical voltage value is 0.3 p.u.

[0099] As a preferred implementation manner, step S2: When the grid voltage is less than the first critical voltage value and not less than the second critical voltage value, with the system power angle reaching the rated power angle as the goal, adaptively adjust the reactive power droop coefficient, including:

[0100] When the grid voltage is less than the first critical voltage value and not less than the second critical voltage value, according to the rated power angle, obtain the minimum reactive power droop coefficient under normal operation of the system;

[0101] With the system power angle reaching the rated power angle as the goal and with the minimum reactive power droop coefficient as the constraint, adaptively adjust the reactive power droop coefficient.

[0102] See Figure 4 It can be known that by reducing the reactive power droop coefficient, the phase trajectory can be changed. And according to the above analysis, when the grid voltage is less than the first critical voltage value and not less than the second critical voltage value, the system equilibrium point can be reshaped by adjusting the reactive power droop coefficient. However, it should be noted that in some fault conditions, voltage continuous drop or the system power angle not reaching the rated power angle may still occur during the adjustment process. At this time, if the reactive power droop coefficient is less than the minimum reactive power droop coefficient, it may also be difficult to reshape the system equilibrium point by adjusting the reactive power droop coefficient, but instead a larger voltage deviation will be caused. Therefore, the minimum reactive power droop coefficient is used as the constraint.

[0103] As a preferred implementation manner, step S3: When the grid voltage is less than the second critical voltage value, with the system power angle reaching the rated power angle as the goal, adaptively adjust the reactive power reference value, and at the same time, constrain the active power reference value according to the capacity of the droop converter, including:

[0104] When the grid voltage is less than the second critical voltage value, aiming at the system power angle reaching the rated power angle, through adaptive adjustment of the reactive power reference value ;

[0105] According to constrain and regulate the active power reference value ;

[0106] Among them, is the grid impedance, is the grid connection point voltage reference value, is the reactive droop coefficient, is the maximum capacity of the droop converter, is the reactive power reference value, is the grid voltage, is the rated power angle under normal operation.

[0107] Please refer to Figure 4 , it is found through experiments that when the system is operating normally and a voltage dip occurs, by reducing the reactive droop coefficient or increasing the reactive power reference value can reshape the equilibrium point of the system to improve the transient stability of the system. However, it should be noted that adjusting the reactive droop coefficient will not affect the grid connection capacity of the system, but the influence of the reactive droop coefficient on the phase trajectory is limited. When a more serious fault occurs in the grid voltage, that is, the voltage dip is severe, continuing to reduce will also not be able to ensure the existence of the equilibrium point. And the reactive power reference value can reshape the equilibrium point at any voltage dip level to ensure system stability. The disadvantage is that too large will reduce the grid connection capacity of the system.

[0108] In the embodiment of the present invention, by considering the maximum capacity limit of the converter, the active power reference value is constrained, and by linearly adjusting the value of the reactive power reference value to achieve system stability while ensuring the maximum power output as much as possible. The specific function for adjusting the reactive power reference value is shown in Equation (8):

[0109] (8).

[0110] Exemplarily, this article divides the degree of grid voltage dip into three stages: 0 p.u. - 0.3 p.u.; 0.3 p.u. - 0.5 p.u.; 0.5 p.u. - 1 p.u. When the grid voltage drops below 0.5 p.u., first by reducing Solve the problem of instability. When the grid voltage drops below 0.3 p.u., it is impossible to reshape the balanced operating point, and it is necessary to sacrifice part of the grid-connected capacity to adjust to achieve transient stability. At the same time, in order to ensure the grid-connected capacity as much as possible, constraints are imposed.

[0111] In the above solution, an adaptive droop control method based on the degree of grid voltage drop is implemented. It can conveniently analyze the fault degree of the grid-connected system and perform corresponding adaptive parameter adjustment according to the fault degree to ensure that the system operates stably at the equilibrium point for a long time, and at the same time ensure the grid-connected capacity during the transient process of the system.

[0112] However, the applicant found that when the grid voltage drops severely, that is, when the grid voltage is less than the first critical voltage value and not less than the second critical voltage value, during the adjustment process of reactive power and active power, the dynamic response performance of the system completely depends on the active droop coefficient, and the adaptive control of the active droop coefficient will lead to slow dynamic response of the system during the disturbance situation and the fault recovery stage.

[0113] Based on the above considerations, in some preferred embodiments, the droop control method based on parameter adaptation further includes steps S4 - S6:

[0114] S4. During the active response control process of constraining the active power reference value according to the capacity of the droop converter, a PI controller is added to the grid-connected system for frequency compensation.

[0115] Please refer to Figure 5 , Figure 5 which is a schematic diagram of the structure of the grid-connected system before and after adding a PI (Proportional Integral) controller. By adding a PI controller, the output accuracy of the angular frequency can be improved to enhance the dynamic response of the system. It should be noted that in Figure 5 , since the PI controller takes effect during the adjustment process of reactive power and active power, the partial equivalent circuit for adjusting the reactive power coefficient is not drawn in the schematic diagram of the structure of the grid-connected system after adding the PI controller.

[0116] S5. Calculate the dynamic response parameters of the grid-connected system before and after adding the PI controller respectively; the dynamic response parameters include the natural oscillation angular frequency and damping ratio of the system;

[0117] S6. Set the proportional coefficient and integral coefficient of the PI controller according to the dynamic response parameters.

[0118] Furthermore, preferably, step S5. Calculate the dynamic response parameters of the grid-connected system before and after adding the PI controller respectively, including:

[0119] Based on the grid-connected system before and after adding a PI controller, the initial active power transfer function and the optimized active power transfer function are obtained;

[0120] According to the initial active power transfer function, calculate the initial natural oscillation angular frequency and the initial damping ratio of the grid-connected system;

[0121] According to the optimized active power transfer function, calculate the optimized natural oscillation angular frequency and the optimized damping ratio of the grid-connected system.

[0122] Furthermore, based on the above embodiments, step S6, setting the proportional coefficient and integral coefficient of the PI controller according to the dynamic response parameters, includes:

[0123] Taking the optimized natural oscillation angular frequency being greater than the initial natural oscillation angular frequency and the optimized damping ratio being greater than the initial damping ratio as the objective function, solve the range of the proportional coefficient and the range of the integral coefficient of the PI controller;

[0124] According to the range of the proportional coefficient and the range of the integral coefficient, set the proportional coefficient and the integral coefficient of the PI controller.

[0125] In some preferred embodiments, if the grid-connected system structure shown in Figure 5 is adopted, then the improved active power control can be expressed as:

[0126] (9);

[0127] Wherein, is the proportional coefficient of the frequency compensation control, is the integral coefficient of the frequency compensation control, is the Laplace operator.

[0128] The initial active power transfer function before improvement is expressed as:

[0129] (10);

[0130] The optimized active power transfer function after improvement is expressed as:

[0131] (11).

[0132] According to Equation (10), the initial natural oscillation angular frequency characterizing the dynamic response of the system and the initial damping ratio can be obtained, and are expressed as:

[0133] (12);

[0134] (13);

[0135] Wherein, is the cut-off frequency of the low-pass filter.

[0136] According to Equations (12) and (13), it can be seen that the dynamic response of the system is mainly restricted by the active power droop coefficient , and the functional requirements for the low-pass filter of the system make it impossible to change the cut-off frequency by droop control to improve the dynamic response of the system.

[0137] According to Equation (11), the optimized natural oscillation angular frequency and the optimized damping ratio characterizing the dynamic response of the system can be obtained, expressed as:

[0138] (14);

[0139] (15).

[0140] Based on Equations (11)-(15), it can be seen that after adding a PI controller for frequency compensation control, the response speed and damping characteristics of the system are no longer completely restricted by the active power droop coefficient. By adjusting the parameters of the PI controller, the improvement of the natural oscillation angular frequency and the damping ratio can be realized, so that the system can improve the response speed and have good damping characteristics at the same time.

[0141] To improve both the response speed and the damping characteristics of the improved system, it should satisfy , . Preferably, the feasible region of the parameters of the PI controller as shown in Figure 6 can be obtained. The shaded part in Figure 6 is the optional parameter combination of the PI controller. Exemplarily, the proportional coefficient and the integral coefficient are selected.

[0142] In the above solution, by adding a PI controller in the grid-connected system for frequency compensation, the response speed during fault clearing can be improved, and the problem that the system recovery time is relatively long during fault clearing can be optimized.

[0143] Adopting a droop control method based on parameter adaptation provided by the embodiments of the present invention can conveniently analyze the fault degree of the grid-connected system and perform corresponding adaptive parameter adjustment according to the fault degree to ensure that the system operates stably at the equilibrium point for a long time, and at the same time ensure the grid connection capacity during the transient process of the system. And in some preferred embodiments, it can further improve the response speed during fault clearing.

[0144] The embodiments of the present invention also provide a droop control system based on parameter adaptation. Please refer toFigure 7 , the parameter adaptive droop control system includes a critical voltage value acquisition module 11, a reactive power droop coefficient adjustment module 12, and a power reference value adjustment module 13, where:

[0145] The critical voltage value acquisition module 11 is used to acquire a first critical voltage value and a second critical voltage value when the grid-connected system is in a critically stable state; the system power angle corresponding to the first critical voltage value is the critical power angle; the system power angle corresponding to the second critical voltage value is the rated power angle, and the reactive power droop coefficient of the droop converter is the minimum reactive power droop coefficient under normal operation of the grid-connected system;

[0146] The reactive power droop coefficient adjustment module 12 is used to adaptively adjust the reactive power droop coefficient with the goal of the system power angle reaching the rated power angle when the grid voltage is less than the first critical voltage value and not less than the second critical voltage value;

[0147] The power reference value adjustment module 13 is used to adaptively adjust the reactive power reference value with the goal of the system power angle reaching the rated power angle when the grid voltage is less than the second critical voltage value, and at the same time, constrain the active power reference value according to the capacity of the droop converter.

[0148] As a preferred implementation, the critical voltage value acquisition module 11 includes:

[0149] A phase trajectory plotting unit, which is used to generate a phase trajectory of the grid-connected system according to the relationship between the voltage output frequency, voltage output amplitude, and system power angle of the grid-connected system based on droop control;

[0150] A first critical voltage value acquisition unit, which is used to use the voltage value when the phase trajectory is tangent to the equilibrium point as the first critical voltage value when the grid-connected system is in a critically stable state;

[0151] A minimum reactive power droop coefficient calculation unit, which is used to set the system power angle to the rated power angle under normal operation to obtain the minimum reactive power droop coefficient;

[0152] A second critical voltage value acquisition unit, which is used to obtain the second critical voltage value when the grid-connected system is in a critically stable state according to the phase trajectory and the minimum reactive power droop coefficient.

[0153] Furthermore, preferably, the phase trajectory plotting unit is specifically used for:

[0154] Construct a voltage-frequency control function according to the relationship between the voltage output frequency and the system power angle of the grid-connected system based on droop control , where ;

[0155] Construct a voltage amplitude control function according to the relationship between the voltage output amplitude and the system power angle of a grid-connected system based on droop control. , where ;

[0156] According to the voltage frequency control function and the voltage amplitude control function, generate the phase trajectory of the grid-connected system through ;

[0157] where is the active output frequency, is the active droop coefficient, is the active power reference value, is the actual active power, is the grid frequency reference value; is the grid voltage, is the grid connection point voltage, is the system power angle, is the grid impedance; is the reactive droop coefficient, is the reactive power reference value, is the actual reactive power, is the grid connection point voltage reference value.

[0158] Preferably, the minimum reactive droop coefficient calculation unit is specifically used for:

[0159] Set the system power angle to the rated power angle under normal operation ;

[0160] Calculate the minimum reactive droop coefficient through ; where is the grid impedance, is the grid connection point voltage reference value, is the grid voltage, is the active power reference value, is the reactive power reference value. As a preferred implementation manner, the reactive droop coefficient adjustment module 12 is specifically used for:

[0161] When the grid voltage is less than the first critical voltage value and not less than the second critical voltage value, obtain the minimum reactive droop coefficient under normal operation of the system according to the rated power angle;

[0162] With the goal that the system power angle reaches the rated power angle and with the minimum reactive droop coefficient as the constraint, adaptively adjust the reactive droop coefficient.

[0163]

[0164] ​As a preferred embodiment, the power reference value adjustment module 13 is specifically configured to:

[0165] When the grid voltage is less than the second critical voltage value, aiming at making the system power angle reach the rated power angle, by adaptive adjustment of the reactive power reference value ;

[0166] According to constrain and adjust the active power reference value ;

[0167] Wherein, is the grid impedance, is the grid connection point voltage reference value, is the reactive power droop coefficient, is the maximum capacity of the droop converter, is the reactive power reference value, is the grid voltage, is the rated power angle under normal operation.

[0168] As a preferred embodiment, the droop control system based on parameter adaptation further includes:

[0169] A frequency compensation module, which is used to add a PI controller to the grid-connected system for frequency compensation during the active response control process of constraining the active power reference value according to the capacity of the droop converter;

[0170] A parameter calculation module, which is used to calculate the dynamic response parameters of the grid-connected system before and after adding the PI controller; the dynamic response parameters include the natural oscillation angular frequency and damping ratio of the system;

[0171] A controller setting module, which is used to set the proportional coefficient and integral coefficient of the PI controller according to the dynamic response parameters.

[0172] Furthermore, preferably, the parameter calculation module is specifically configured to:

[0173] Obtain the initial active power transfer function and the optimized active power transfer function according to the grid-connected system before and after adding the PI controller;

[0174] Calculate the initial natural oscillation angular frequency and initial damping ratio of the grid-connected system according to the initial active power transfer function;

[0175] Calculate the optimized natural oscillation angular frequency and optimized damping ratio of the grid-connected system according to the optimized active power transfer function.

[0176] Even further, preferably, the controller setting module is specifically configured to:

[0177] Taking the fact that the optimized natural oscillation angular frequency is greater than the initial natural oscillation angular frequency and the optimized damping ratio is greater than the initial damping ratio as the objective function, solve the range of the proportional coefficient and the range of the integral coefficient of the PI controller;

[0178] Set the proportional coefficient and the integral coefficient of the PI controller according to the range of the proportional coefficient and the range of the integral coefficient.

[0179] By using the droop control system based on parameter self - adaptation provided by the embodiment of the present invention, the fault degree of the grid - connected system can be conveniently analyzed, and corresponding adaptive parameter adjustment can be carried out according to the fault degree to ensure that the system operates stably at the equilibrium point for a long time, and at the same time, the grid - connected capacity during the transient process of the system is ensured. And in some preferred embodiments, the response speed during fault clearing can be further improved.

[0180] Those of ordinary skill in the art can understand that all or part of the processes in the methods of the above - mentioned embodiments can be completed by instructing relevant hardware through a computer program. The program can be stored in a computer - readable storage medium. When the program is executed, it can include the processes of the embodiments of the above - mentioned methods. Among them, the storage medium can be a magnetic disk, an optical disk, a read - only memory (ROM), or a random access memory (RAM), etc.

[0181] The above is the preferred implementation manner of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements are also regarded as the protection scope of the present invention.

Claims

1. A droop control method based on parameter adaptation, characterized in that: include: Acquire a first critical voltage value and a second critical voltage value of the grid-connected system in a critical stable state; the system power angle corresponding to the first critical voltage value is the critical power angle; The system power angle corresponding to the second critical voltage value is the rated power angle, and the reactive power droop coefficient of the droop converter is the minimum reactive power droop coefficient under normal operation of the grid-connected system; When the grid voltage is less than the first critical voltage value and not less than the second critical voltage value, the reactive power droop coefficient is adaptively adjusted with the system power angle reaching the rated power angle; When the grid voltage is less than the second critical voltage value, the reactive power reference value is adaptively adjusted with the system power angle reaching the rated power angle as the goal, and the active power reference value is constrained according to the capacity of the droop converter; When the grid voltage is less than the first critical voltage value and not less than the second critical voltage value, the reactive power droop coefficient is adaptively adjusted with the system power angle reaching the rated power angle, including: When the grid voltage is less than the first critical voltage value and not less than the second critical voltage value, the minimum reactive power droop coefficient under normal system operation is obtained according to the rated power angle; Taking the system power angle reaching the rated power angle as the goal and taking the minimum reactive power droop coefficient as the constraint, adaptively adjusting the reactive power droop coefficient; When the grid voltage is less than the second critical voltage value, the reactive power reference value is adaptively adjusted with the system power angle reaching the rated power angle as the goal, and the active power reference value is constrained according to the capacity of the droop converter, including: When the grid voltage is less than the second critical voltage value, the system power angle reaches the rated power angle. Adaptive adjustment of reactive power reference value ; according to Active power reference value To constrain and regulate; in, is the grid impedance, is the grid connection point voltage reference value, is the reactive power droop coefficient, is the maximum capacity of the droop converter, is the reactive power reference value, is the grid voltage, is the rated power angle under normal operation.

2. A droop control method based on parameter adaptation as claimed in claim 1, characterized in that: The step of obtaining a first critical voltage value and a second critical voltage value of the grid-connected system in a critical stable state includes: According to the relationship between the voltage output frequency, voltage output amplitude and system power angle of the grid-connected system based on droop control, a phase trajectory of the grid-connected system is generated; The voltage value when the phase trajectory is tangent to the equilibrium point is used as the first critical voltage value of the grid-connected system in a critical stable state; The system power angle is set to the rated power angle under normal operation to obtain the minimum reactive power droop coefficient; According to the phase trajectory and the minimum reactive power droop coefficient, a second critical voltage value of the grid-connected system in a critical stable state is obtained.

3. A droop control method based on parameter adaptation as claimed in claim 2, characterized in that: The generating of the phase trajectory of the grid-connected system according to the relationship between the voltage output frequency, the voltage output amplitude and the system power angle of the grid-connected system based on the droop control comprises: According to the relationship between the voltage output frequency and the system power angle of the grid-connected system based on droop control, a voltage-frequency control function is constructed. ,in, ; According to the relationship between the voltage output amplitude and the system power angle of the grid-connected system based on droop control, the voltage amplitude control function is constructed ,in, ; According to the voltage frequency control function and the voltage amplitude control function, by Generate phase trajectories of grid-connected systems; in, is the active output frequency, is the active power droop coefficient, is the active power reference value, is the actual active power, is the grid frequency reference value; is the grid voltage, is the grid voltage, is the system power angle, is the grid impedance; is the reactive power droop coefficient, is the reactive power reference value, is the actual reactive power, It is the grid connection point voltage reference value.

4. The droop control method based on parameter adaptation according to claim 2, characterized in that: The system power angle is set to the rated power angle under normal operation to obtain the minimum reactive power droop coefficient, including: Set the system power angle to the rated power angle under normal operation ; pass Calculate the minimum reactive power droop factor ;in, is the grid impedance, is the grid connection point voltage reference value, is the grid voltage, is the active power reference value, is the reactive power reference value.

5. The droop control method based on parameter adaptation according to claim 1, characterized in that: When the grid voltage is less than the second critical voltage value, the reactive power reference value is adaptively adjusted with the system power angle reaching the rated power angle as the goal, and the active power reference value is constrained according to the capacity of the droop converter, the droop control method based on parameter adaptation also includes: In the active response control process where the active power reference value is constrained according to the capacity of the droop converter, a PI controller is added to the grid-connected system for frequency compensation; Calculating the dynamic response parameters of the grid-connected system before and after adding the PI controller; the dynamic response parameters include the natural oscillation angular frequency and damping ratio of the system; The proportional coefficient and the integral coefficient of the PI controller are set according to the dynamic response parameters.

6. A droop control method based on parameter adaptation as claimed in claim 5, characterized in that: The method of respectively calculating the dynamic response parameters of the grid-connected system before and after adding the PI controller comprises: According to the grid-connected system before and after adding the PI controller, the initial active power transfer function and the optimized active power transfer function are obtained; Calculating an initial natural oscillation angular frequency and an initial damping ratio of the grid-connected system according to the initial active power transfer function; According to the optimized active power transfer function, the optimized natural oscillation angular frequency and the optimized damping ratio of the grid-connected system are calculated.

7. A droop control method based on parameter adaptation as claimed in claim 6, characterized in that: The step of setting the proportional coefficient and the integral coefficient of the PI controller according to the dynamic response parameter comprises: Taking the optimized natural oscillation angular frequency being greater than the initial natural oscillation angular frequency and the optimized damping ratio being greater than the initial damping ratio as the objective function, solving the proportional coefficient range and the integral coefficient range of the PI controller; The proportional coefficient and the integral coefficient of the PI controller are set according to the proportional coefficient range and the integral coefficient range.

8. A droop control system based on parameter adaptation, characterized in that: include: A critical voltage value acquisition module is used to obtain a first critical voltage value and a second critical voltage value of a grid-connected system in a critical stable state; the system power angle corresponding to the first critical voltage value is the critical power angle; the system power angle corresponding to the second critical voltage value is the rated power angle, and the reactive power droop coefficient of the droop converter is the minimum reactive power droop coefficient under normal operation of the grid-connected system; A reactive power droop coefficient adjustment module is used to adaptively adjust the reactive power droop coefficient with the goal of the system power angle reaching the rated power angle when the grid voltage is less than the first critical voltage value and not less than the second critical voltage value; A power reference value adjustment module is used to adaptively adjust the reactive power reference value with the goal of the system power angle reaching the rated power angle when the grid voltage is less than the second critical voltage value, and at the same time constrain the active power reference value according to the capacity of the droop converter; The reactive power droop coefficient adjustment module is specifically used for: When the grid voltage is less than the first critical voltage value and not less than the second critical voltage value, the minimum reactive power droop coefficient under normal system operation is obtained according to the rated power angle; Taking the system power angle reaching the rated power angle as the goal and taking the minimum reactive power droop coefficient as the constraint, adaptively adjusting the reactive power droop coefficient; The power reference value adjustment module is specifically used for: When the grid voltage is less than the second critical voltage value, the system power angle reaches the rated power angle. Adaptive adjustment of reactive power reference value ; according to Active power reference value To constrain and regulate; in, is the grid impedance, is the grid connection point voltage reference value, is the reactive power droop coefficient, is the maximum capacity of the droop converter, is the reactive power reference value, is the grid voltage, is the rated power angle under normal operation.

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