Droop control method and system based on parameter self-adaption

Abstract

The invention discloses a parameter self-adaption-based droop control method and system. The method comprises the steps of obtaining 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 a critical power angle; the system power angle corresponding to the second critical voltage value is a rated power angle, and the reactive power droop coefficient of the droop converter is the minimum reactive power droop coefficient under normal operation; when the power grid voltage is smaller than the first critical voltage value and not smaller than the second critical voltage value, the reactive droop coefficient is adjusted in a self-adaptive mode with the purpose that the system power angle reaches the rated power angle; and when the voltage of the power grid is smaller than a second critical voltage value, adaptively adjusting the reactive power reference value by taking the system power angle reaching the rated power angle as a target, and constraining the active power reference value according to the capacity of the droop converter. By adopting the embodiment of the invention, the system can be ensured to stably operate at a balance point for a long time, and the grid-connected capacity in the transient process of the system is ensured at the same time.

Description

Technical Field

[0001] The present invention relates to the technical field of droop control, and in particular 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 increase in the popularity of photovoltaic power generation and wind power generation, converters based on grid-based control have been widely used to ensure the stability and reliability of the power system.

[0003] Droop control is an important control strategy in converters based on grid-based control. Since droop control ignores inertia characteristics, it can quickly achieve synchronous stability of the power angle when a minor fault occurs in the power grid. However, when faced with a more serious power grid fault, the system balance point will be lost, and traditional droop control will find it difficult to maintain transient stability, and the power angle will completely lose synchronous stability. Summary of the invention

[0004] The 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 make corresponding adaptive parameter adjustments according to the fault degree to ensure that the system operates stably at a balance point for a long time, while ensuring the grid-connected capacity during the transient process of the system.

[0005] Specifically, an embodiment of the present invention provides a droop control method based on parameter adaptation, comprising: Obtaining 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; 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.

[0006] As an improvement of the above solution, the method 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.

[0007] As an improvement of the above scheme, the phase trajectory of the grid-connected system is generated 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, including: 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.

[0008] As an improvement of the above solution, 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.

[0009] As an improvement of the above scheme, 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; With the system power angle reaching the rated power angle as the goal and the minimum reactive power droop coefficient as the constraint, the reactive power droop coefficient is adaptively adjusted.

[0010] As an improvement of the above scheme, 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.

[0011] As an improvement of the above scheme, 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, and the parameter-adaptive-based droop control method further 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.

[0012] As an improvement of the above solution, the dynamic response parameters of the grid-connected system before and after adding the PI controller are calculated respectively, including: 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.

[0013] As an improvement of the above solution, the step of setting the proportional coefficient and the integral coefficient of the PI controller according to the dynamic response parameter includes: 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.

[0014] The embodiment of the present invention further provides a droop control system based on parameter adaptation, comprising: 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; The power reference value adjustment module is used to adaptively adjust the reactive power reference value with the goal of achieving the rated power angle of the system 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.

[0015] Compared with the prior art, the present invention discloses a droop control method and system based on parameter adaptation, which obtains the first critical voltage value and the 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 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 system power angle reaching the rated power angle as the goal; 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. By adopting the embodiment of the present invention, the fault degree of the grid-connected system can be conveniently analyzed, and the corresponding adaptive parameter adjustment can be performed according to the fault degree to ensure that the system operates stably at the balance point for a long time, and the grid-connected capacity in the transient process of the system is guaranteed. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] 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; Figure 2 It is a structure and principle diagram of a new energy grid-connected system based on droop control provided by an embodiment of the present invention; Figure 3 is a curve diagram of phase trajectories under different grid voltages provided by an embodiment of the present invention; Figure 4 is a curve diagram of a phase trajectory under the influence of a reactive droop coefficient and a reactive power reference value provided by an embodiment of the present invention; Figure 5 This is an equivalent structure diagram of active power control before and after adding a PI controller provided by an embodiment of the present invention; Figure 6 It is a parameter feasible domain analysis diagram of a PI controller in frequency compensation provided by an embodiment of the present invention; 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. DETAILED DESCRIPTION

[0017] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0018] In the description of the specification and claims, it is to be understood that the terms first, second, etc. in the specification and claims are only used for the purpose of describing the same technical features, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated, nor necessarily describing the order or time sequence. The terms are interchangeable where appropriate. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include at least one of the features.

[0019] The embodiment of the present invention provides a droop control method based on parameter adaptation. Figure 1 In this embodiment, the droop control method based on parameter adaptation is specifically performed through steps S1 to S3: S1. Obtain the first critical voltage value and the 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.

[0020] 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 of the grid-connected system in a critical stable state under different working conditions is used as the threshold of the fault level.

[0021] S2. 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.

[0022] 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 slight fault condition, and the system steady state can be achieved only by adjusting the reactive power droop coefficient in the droop control.

[0023] S3. 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, and the active power reference value is constrained according to the capacity of the droop converter.

[0024] It should be noted that when the grid voltage is less than the second critical voltage value, it is considered that the voltage drop 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 droop coefficient will still lead to the loss of the balance point. Therefore, the reactive power reference value is adjusted to ensure the existence of the balance point. Taking into account that an excessively large reactive power reference value will reduce the grid-connected capacity of the system, the embodiment of the present invention further constrains the active power reference value to ensure the grid-connected capacity of the system during the transient process as much as possible.

[0025] In the above scheme, by obtaining the critical voltage value of the grid-connected system under different working conditions, the fault degree of the grid-connected system can be easily analyzed, and the corresponding adaptive parameters can be adjusted according to the fault degree to ensure that the system operates stably at the equilibrium point for a long time, while ensuring the grid-connected capacity during the transient process of the system.

[0026] As a preferred implementation, step S1, obtaining the first critical voltage value and the second critical voltage value of the grid-connected system in a critical stable state, is specifically performed through steps S11-S14: S11, generating a phase trajectory of the grid-connected system according to a relationship among a voltage output frequency, a voltage output amplitude and a system power angle of the grid-connected system based on droop control; S12, taking the voltage value when the phase trajectory is tangent to the equilibrium point as the first critical voltage value of the grid-connected system in a critical stable state; S13, setting the system power angle to the rated power angle under normal operation to obtain the minimum reactive power droop coefficient; S14. Obtaining a second critical voltage value of the grid-connected system in a critical stable state according to the phase trajectory and the minimum reactive power droop coefficient.

[0027] It should be noted that the change of the system power angle, reactive droop coefficient adjustment and reactive power adjustment of the grid-connected system will cause the phase trajectory to change. By setting the parameters, the influence of other parameters on the phase trajectory can be more clearly understood from the curve of the phase trajectory.

[0028] It should also be noted that in step S12 and step S13, the unmentioned parameters are all set to reference values, under which the grid-connected system operates in a stable state. Exemplarily, the reactive droop coefficient and reactive power are both set to reference values.

[0029] In some preferred embodiments, the fault level of the grid-connected system can be graded according to the first critical voltage value and the second critical voltage value. 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 a primary 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 a secondary fault level. The voltage drop degree of the primary fault level is lighter than the voltage drop degree of the secondary fault level.

[0030] Further, preferably, step S11, generating a 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, 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.

[0031] See also Figure 2 The droop control adjusts the voltage frequency through the active power output by the grid-connected inverter, and adjusts the voltage amplitude through the reactive power to ensure the synchronization between the grid-connected inverter and the power grid. The control rules are as follows: (1); (2); 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 reactive power droop coefficient, is the reactive power reference value, is the actual reactive power, It is the grid connection point voltage reference value.

[0032] The expressions of active power and reactive power output by the grid-connected inverter in steady-state operation are: (3); (4); in, is the grid voltage, is the system power angle, is the grid impedance.

[0033] Since the bandwidth of the inner control loop is much higher than that of the outer power loop, the inner loop can be equivalent to unity gain when performing transient analysis. Therefore, the voltage at the PCC point can be equivalent to the output voltage of the VSG. Combining equations (2) and (4), the grid connection point voltage can be obtained: and system power angle Relationship: (5); Further combining equation (3) and equation (5) can obtain the relationship between active power and system power angle, which will not be repeated here.

[0034] According to equations (1) and (3), the nonlinear differential equation that characterizes the droop control power angle synchronization characteristics can be derived: (6).

[0035] By solving the differential equation (6), the phase trajectory of the grid-connected system can be obtained.

[0036] In the stable operation of the power system, the system power angle is is a critical power angle. If the system is disturbed and the power angle exceeds , which may cause the system to lose synchronization, trigger power oscillations or even system crashes. Therefore, when designing and operating power systems, various control measures need to be taken to ensure that the power angle remains within a safe range and avoids approaching or exceeding the 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 operation of the system, but the value should not be greater than .

[0037] exist Figure 3 In the specific embodiment shown, taking grid voltages of 1 p.u., 0.7 pu, 0.5 pu and 0.3 pu as examples, it can be seen from the drawn phase trajectory that the grid voltage of 0.5 pu is a critical voltage value. If the grid voltage is greater than 0.5 pu, the system itself has a balance point; if the grid voltage is less than 0.5 pu, the system balance point is lost; when the grid voltage is equal to 0.5 pu, the system is just in a critical stable state, indicating that the fault condition of the system is not serious at this time, and adaptive control can be performed through droop control. The specific second critical voltage value needs to be further determined.

[0038] 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: 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.

[0039] It should be noted that the power angle under normal operation is generally within a range of values, such as When the system stability is high. Then the rated power angle is any value in the above range. When the adjustment range of the power angle is larger, that is, the value of the system power angle is smaller, the reactive power droop coefficient is correspondingly smaller. For example, the rated power angle is The value of is set to .

[0040] When the system power angle is the rated power angle, the minimum reactive power droop coefficient can be derived through equations (2) and (4): : (7).

[0041] The embodiment of the present invention selects the reactive power droop coefficient The minimum value that can be achieved As a constraint, if the reactive power droop factor is reduced to , if the system balance point has not been reshaped, it means that the method of only reducing the reactive power droop coefficient is no longer applicable. Otherwise, not only will a new balance point fail to be generated, but it will also lead to a larger voltage deviation.

[0042] In step S14, compared with the case of 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 also 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 pu.

[0043] As a preferred implementation, 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 adjusting the reactive power droop coefficient includes: 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; With the system power angle reaching the rated power angle as the goal and the minimum reactive power droop coefficient as the constraint, the reactive power droop coefficient is adaptively adjusted.

[0044] See also Figure 4 It can be known that the phase trajectory can be changed by reducing the reactive droop coefficient. 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 balance point can be reshaped by adjusting the reactive droop coefficient. However, it should be noted that in some fault conditions, the voltage may continue to drop or the system power angle may not reach the rated power angle during the adjustment process. At this time, if the reactive droop coefficient is less than the minimum reactive droop coefficient, it may be difficult to reshape the system balance point by adjusting the reactive droop coefficient. Instead, it will cause a greater voltage deviation. Therefore, the minimum reactive droop coefficient is used as a constraint.

[0045] As a preferred implementation, step S3, when the grid voltage is less than the second critical voltage value, taking the system power angle reaching the rated power angle as the goal, adaptively adjusting the reactive power reference value, and constraining the active power reference value according to the capacity of the droop converter, includes: 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.

[0046] See also Figure 4 Through experiments, it was found that when the system is operating normally and voltage drops occur, by reducing the reactive power droop coefficient Or increase the reactive power reference value can reshape the balance point of the system to improve the transient stability of the system. However, it should be noted that adjusting the reactive power droop coefficient It will not affect the grid-connected capacity of the system, but the reactive power droop coefficient The impact on the phase trajectory is limited. When the grid voltage has a more serious fault, that is, the voltage drops seriously, it will continue to decrease. The existence of the balance point cannot be guaranteed. The balance point can be reshaped at any voltage drop level to ensure system stability. The disadvantage is that the It will reduce the grid-connected capacity of the system.

[0047] In the embodiment of the present invention, the active power reference value is constrained by considering the maximum capacity limit of the converter, and the reactive power reference value is linearly adjusted. The value of reactive power is adjusted to achieve system stability while ensuring the maximum power output. The function of is shown in formula (8): (8).

[0048] For example, this paper divides the grid voltage drop into three stages: 0p.u.-0.3pu; 0.3pu-0.5pu; 0.5pu-1p.u. When the grid voltage drops below 0.5pu, the voltage is first reduced. Solve the problem of instability. When the grid voltage drops below 0.3pu, Reducing the load can no longer reshape the balanced operating point, and some grid-connected capacity must be sacrificed to adjust To achieve transient stability, in order to ensure the grid-connected capacity as much as possible, To constrain.

[0049] The above scheme implements an adaptive droop control method based on the degree of grid voltage drop, which can conveniently analyze the fault degree of the grid-connected system and make corresponding adaptive parameter adjustments according to the fault degree to ensure that the system operates stably at the equilibrium point for a long time, while ensuring the grid-connected capacity during the system transient process.

[0050] 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 of reactive power and active power, the system dynamic response performance is completely dependent on the active power droop coefficient, and the adaptive control of the active power droop coefficient will cause the system to have slow dynamic response under disturbance conditions and in the fault recovery phase.

[0051] Based on the above considerations, in some preferred embodiments, the droop control method based on parameter adaptation further includes steps S4-S6: S4. In the active power response control process in which 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.

[0052] See also Figure 5 , Figure 5The figure is a schematic diagram of the grid-connected system before and after adding the PI (Proportional Integral) controller. By adding the PI controller, the output accuracy of the angular frequency can be improved to improve the dynamic response of the system. It should be noted that Figure 5 In the diagram, since the PI controller is effective in the process of regulating reactive power and active power, the partial equivalent circuit for adjusting the reactive power coefficient is not drawn in the structural diagram of the grid-connected system after adding the PI controller.

[0053] S5. 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; S6. Setting the proportional coefficient and the integral coefficient of the PI controller according to the dynamic response parameters.

[0054] Further, preferably, step S5, 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.

[0055] Furthermore, based on the above embodiment, step S6, setting the proportional coefficient and the integral coefficient of the PI controller according to the dynamic response parameter, includes: 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.

[0056] In some preferred embodiments, the Figure 5 The grid-connected system structure shown in Figure 1 can be expressed as follows: (9); in, is the proportional coefficient of frequency compensation control, is the integral coefficient of frequency compensation control, is the Laplace operator.

[0057] The initial active power transfer function before improvement is expressed as: (10); The improved optimized active power transfer function is expressed as: (11).

[0058] According to formula (10), the initial natural oscillation angular frequency that characterizes the dynamic response of the system can be obtained: and initial damping ratio , expressed as: (12); (13); in, is the low-pass filter cutoff frequency.

[0059] According to equations (12) and (13), the dynamic response of the system is mainly subject to the active droop coefficient However, the functional requirements of the system low-pass filter make it impossible for droop control to change the cutoff frequency to improve the dynamic response of the system.

[0060] According to formula (11), the optimized natural oscillation angular frequency that characterizes the dynamic response of the system can be obtained: and optimized damping ratio , expressed as: (14); (15).

[0061] Based on equations (11)-(15), it can be seen that after adding the PI controller for frequency compensation control, the response speed and damping characteristics of the system are no longer completely restricted by the active droop coefficient. The natural oscillation angular frequency and damping ratio can be improved by adjusting the parameters of the PI controller, so that the system can improve the response speed and have better damping characteristics.

[0062] In order to improve the response speed and damping characteristics of the improved system, the following should be met: , Preferably, it is possible to obtain Figure 6 The feasible domain of the parameters of the PI controller is shown in Figure 6 The shaded part in is the optional PI controller parameter combination. For example, the proportional coefficient , integral coefficient .

[0063] In the above scheme, by adding a PI controller to the grid-connected system for frequency compensation, the response speed during fault clearing can be improved, and the problem of a long time required for system recovery during the fault clearing process can be optimized.

[0064] The droop control method based on parameter adaptation provided by the embodiment of the present invention can conveniently analyze the fault degree of the grid-connected system, and adjust the corresponding adaptive parameters 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. In some preferred embodiments, the response speed when the fault is cleared can be further improved.

[0065] The embodiment of the present invention also provides a droop control system based on parameter adaptation. Figure 7 The parameter-adaptive droop control system comprises a critical voltage value acquisition module 11, a reactive power droop coefficient adjustment module 12 and a power reference value adjustment module 13, wherein: The critical voltage value acquisition module 11 is used to obtain 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; The reactive power droop coefficient adjustment module 12 is used to adaptively adjust the reactive power droop coefficient with 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; The power reference value adjustment module 13 is used to adaptively adjust the reactive power reference value with 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.

[0066] As a preferred implementation, the critical voltage value acquisition module 11 includes: A phase trajectory drawing unit, for generating a phase trajectory of the grid-connected system according to a relationship among a voltage output frequency, a voltage output amplitude and a system power angle of the grid-connected system based on droop control; A first critical voltage value acquisition unit, used to 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 critical stable state; A minimum reactive power droop coefficient calculation unit is used to set the system power angle to the rated power angle under normal operation to obtain the minimum reactive power droop coefficient; The second critical voltage value acquisition unit is used to obtain a second critical voltage value of the grid-connected system in a critical stable state according to the phase trajectory and the minimum reactive power droop coefficient.

[0067] Further, preferably, the phase trajectory drawing unit is specifically used to: 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.

[0068] Preferably, the minimum reactive power droop coefficient calculation unit is specifically used for: 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.

[0069] As a preferred implementation, the reactive power droop coefficient adjustment module 12 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; With the system power angle reaching the rated power angle as the goal and the minimum reactive power droop coefficient as the constraint, the reactive power droop coefficient is adaptively adjusted.

[0070] As a preferred implementation, the power reference value adjustment module 13 is specifically used to: 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.

[0071] As a preferred implementation, the parameter adaptive droop control system further includes: A frequency compensation module for adding a PI controller to the grid-connected system for frequency compensation during an active response control process in which an active power reference value is constrained according to the capacity of a droop converter; A parameter calculation module, used to calculate the dynamic response parameters of the grid-connected system before and after the PI controller is added; the dynamic response parameters include the natural oscillation angular frequency and damping ratio of the system; The controller setting module is used to set the proportional coefficient and the integral coefficient of the PI controller according to the dynamic response parameters.

[0072] Further, preferably, the parameter calculation module is specifically used to: 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.

[0073] Furthermore, preferably, the controller setting module is specifically used to: 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.

[0074] The droop control system based on parameter adaptation provided by the embodiment of the present invention can conveniently analyze the fault degree of the grid-connected system, and adjust the corresponding adaptive parameters according to the fault degree to ensure that the system operates stably at the equilibrium point for a long time, while ensuring the grid-connected capacity during the transient process of the system. In some preferred embodiments, the response speed when the fault is cleared can be further improved.

[0075] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiments can be implemented by instructing the relevant hardware through a computer program, and the program can be stored in a computer-readable storage medium, and when the program is executed, it can include the processes of the embodiments of the above-mentioned methods. The storage medium can be a disk, an optical disk, a read-only memory (ROM) or a random access memory (RAM), etc.

[0076] The above is a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several improvements and modifications without departing from the principle of the present invention. These improvements and modifications are also considered to be within the scope of protection 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.

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 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; With the system power angle reaching the rated power angle as the goal and the minimum reactive power droop coefficient as the constraint, the reactive power droop coefficient is adaptively adjusted.

6. 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, 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.

7. 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.

8. The droop control method based on parameter adaptation according to claim 7, 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.

9. The droop control method based on parameter adaptation according to claim 8, 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.

10. 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; The power reference value adjustment module is used to adaptively adjust the reactive power reference value with the goal of achieving the rated power angle of the system 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.

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