Method for improving comprehensive stability margin of converter

Abstract

The invention relates to a method for improving the comprehensive stability margin of a converter, and relates to the technical field of converter control under a high-proportion new energy power system, and the method comprises the steps: obtaining the relation between root locus characteristics and frequency support and between control parameters and frequency support through building a small signal model; dynamically adjusting control parameters; and optimizing the parameter range. Based on bifurcation appearance and disappearance rules, a differential compensation control strategy is designed, control parameters are adjusted to eliminate a transient periodic solution, and the method is far away from a bifurcation point. Meanwhile, small signal model analysis is combined, control parameters and virtual inertia are dynamically optimized, system frequency support is enhanced, the stability range is expanded, and the comprehensive stability margin of the converter is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of converter control in a high-proportion new energy power system, and in particular to a method for improving the comprehensive stability margin of a converter by considering adding a differential compensation link. Background Art

[0002] With the increase in the penetration rate of new energy and power electronic equipment, the power system has a trend of decreasing inertia and weakening system strength, and the stability problem is becoming more and more serious. To address this problem, power equipment with power electronic converters as interfaces is required to have a certain inertia support capability. In power grids with weak system strength and low physical inertia, grid-type control is suitable. When the grid-type converter is connected to the grid, it is equivalent to a controlled voltage source. Its control variables are voltage amplitude and phase angle. Through a power synchronization strategy similar to that of a synchronous generator, the voltage and frequency are independently established. Without the intervention of a phase-locked loop, the output power can be controlled to achieve synchronization with the grid. When there are energy storage elements or reserved spare capacity, the grid-type converter can also provide system damping and virtual inertia, reduce frequency offset, improve system stability, and reduce dependence on system mechanical inertia. In view of the above advantages, the attention of grid-type converters in new power systems has gradually increased, and the application of grid-type converters has also been booming.

[0003] At present, a variety of grid-building control strategies have been proposed at home and abroad, and most of the research focuses on droop control and virtual synchronous machine control. In the research process of virtual synchronous machines, the main consideration is to introduce virtual inertia and damping into the control system so that the external output characteristics of the converter have similar rotational inertia, droop and damping characteristics as synchronous generators, which can provide the necessary frequency and voltage support for the power grid. There is no definite method for selecting virtual rotational inertia. Most of them set limiting conditions to obtain a selectable range; or do not consider the specific value of the parameter. At present, most methods for adaptively adjusting control parameters are based on the power angle relationship of the synchronous motor as the core idea, and then use various algorithms to change parameters to achieve control effects. In this context, based on the results of bifurcation analysis, a control strategy with additional constraints is designed to effectively improve the comprehensive stability margin of the grid-building converter, contributing a new solution to the safe and stable operation of the power system. Summary of the invention

[0004] The purpose of the present invention is to provide a method for improving the comprehensive stability margin of a converter, so as to increase the regulating capability of a grid-type converter in a high-proportion new energy power system.

[0005] In order to achieve the above object, the technical solution of the present invention provides a method for improving the comprehensive stability margin of a converter, comprising the following steps:

[0006] The root locus characteristics and the relationship between the control parameters and the frequency support are obtained by establishing a small signal model; the control parameters are adjusted dynamically; and the parameter range is optimized.

[0007] Preferably, the method specifically comprises the following steps:

[0008] The small signal model of VSG is established, the influence of active power-frequency link is considered, the influence of reactive power-voltage link is ignored, the output voltage of virtual synchronous machine is regarded as a constant value, the dynamic influence of voltage and current double loops is ignored during operation, and the small signal closed-loop transfer function between the active power output value and the command value of virtual synchronous machine is obtained.

[0009] The root locus diagram of the grid-connected VSG system is drawn according to the small signal closed-loop transfer function. The virtual damping D gradually increases from 0.001 to 0.3 with a step size of 0.001. The virtual inertia is 0.008 kg·m 2 , 0.02kg·m 2 and 0.03kg·m 2 ;

[0010] The differential compensation link is connected in series with the forward channel of the first-order link virtual inertia VSG active control to improve the dynamic characteristics of the system while ensuring the steady-state characteristics of the system;

[0011] The closed-loop transfer function of the small active signal of the VSG grid-connected output with differential compensation is obtained;

[0012] While ensuring the steady-state characteristics of power, reduce the power overshoot of the system, increase system stability, and speed up the active power response time;

[0013] For the characteristics of the virtual inertia of the differential compensation link during VSG island operation and the constraints during grid-connected operation, the corresponding value ranges are obtained.

[0014] Preferably, the small signal closed-loop transfer function is:

[0015]

[0016] Preferably, after adding the differential compensation term, the VSG grid-connected active closed-loop transfer function is:

[0017]

[0018] Preferably, the closed-loop transfer function of the differentially compensated VSG grid-connected output active small signal is:

[0019]

[0020] Where: K d is the coefficient of the differential compensation link, K ωis the speed regulator adjustment coefficient, δ is the phase angle difference between the VSG output voltage and the grid voltage, K p From δ to P out Expression of, ω0 is the rated voltage angular frequency.

[0021] Preferably, when the VSG is operating in an island mode based on virtual inertia of the differential compensation link, its P-ω characteristic is:

[0022]

[0023] K d The value must meet the following requirements:

[0024]

[0025] At the same time, D needs to satisfy the critical damping D cr :

[0026]

[0027] In the formula,

[0028] Preferably, when the VSG based on virtual inertia of the differential compensation link is connected to the grid, the damping ratio ξ and natural oscillation frequency ω of its output active small signal closed-loop transfer function are nG They are:

[0029]

[0030] Preferred, J and K d The value must meet the following conditions:

[0031] 1) Damping ratio, setting ξ must satisfy:

[0032] 0.8≤ξ≤1 (8)

[0033] 2) Stability margin, set the amplitude margin h and phase margin γ to meet:

[0034]

[0035] 3) Adjust the time and set the closed-loop pole constraints as follows:

[0036] R e (s)=-ω nG ξ≤-10 (10)

[0037] 4) Decouple the inner and outer loops, set the voltage inner loop bandwidth frequency rad / s, then the power loop bandwidth frequency must satisfy ω fb ≤ω ub / 4, that is:

[0038]

[0039] The beneficial effects of the embodiments of the present invention are as follows:

[0040] By adding a differential compensation link, adding additional items and determining their values ​​based on the root locus diagram and other key constraints of the system, a grid-connected converter control strategy is designed to maintain stable operation of the system. This strategy can not only effectively deal with various disturbances and uncertainties inside and outside the system, but also significantly improve the comprehensive performance of the converter in different operating scenarios. This innovative control strategy design idea provides a strong technical guarantee for the safe, stable and efficient operation of a high-proportion new energy power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a root locus diagram of active closed-loop transfer function under the change of J and D parameters in the embodiment of the present invention;

[0042] Figure 2 It is a control block diagram of an active loop including differential compensation of a system in an embodiment of the present invention;

[0043] Figure 3 is the frequency response under different damping D;

[0044] Figure 4 In the embodiments of the present invention, different K d Differential compensation virtual inertia when ;

[0045] Figure 5 J and K satisfy the virtual inertia VSG constraint condition of the differential compensation link in the embodiment of the present invention d . DETAILED DESCRIPTION

[0046] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0047] An embodiment of the present invention provides a method for improving the comprehensive stability margin of a converter by considering adding a differential compensation link, comprising the following steps:

[0048] By establishing a small signal model of VSG, the influence of the virtual inertia coefficient of the virtual synchronous machine on the system stability can be analyzed. Then, through the reasonable design of parameters, the stability of the system operation can be increased within a certain range.

[0049] When establishing the small signal model of the system, the influence of the active power-frequency link is mainly considered, and the influence of the reactive power-voltage link is ignored. The output voltage of the virtual synchronous machine is regarded as a constant value, and the dynamic influence of the voltage and current double loops during operation is ignored. The small signal closed-loop transfer function between the active power output value and the command value of the virtual synchronous machine is obtained as shown in (1).

[0050]

[0051] When the active power command value of the VSG system is constant, its dynamic characteristics will be jointly determined by J, D and K. Without considering the influence of the droop control coefficient K on the system, the change trend of the system stability when the virtual inertia J and virtual damping D change is mainly discussed.

[0052] According to (1), the root locus of the grid-connected VSG system is drawn as follows: Figure 1 As shown in the figure, the virtual damping D gradually increases from 0.001 to 0.3 in steps of 0.001, and the virtual inertia J is 0.008 kg·m 2 , 0.02kg·m 2 and 0.03kg·m 2 , it can be seen that the transfer function has two characteristic roots λ1 and λ2. The direction indicated by the arrow in the figure is the direction in which the virtual damping D increases, and λ1 and λ2 change along the direction indicated by the arrow.

[0053] It can be seen from the figure that when D remains unchanged, as J increases, the characteristic root gets closer and closer to the imaginary axis, which means that the stability of the system is developing in a bad direction, so the value of J cannot be too large; when J remains unchanged, the smaller D is, the closer the characteristic root is to the imaginary axis. Within a certain range, as D increases, the characteristic root gradually moves away from the imaginary axis, which means that the system damping becomes larger and the system stability is gradually increasing; when D increases to a certain extent, λ1 still maintains a trend away from the imaginary axis, while λ2 begins to approach the imaginary axis, and the system stability decreases. Therefore, the virtual damping D is not the larger the better.

[0054] According to the principle of automatic control, the differential link can correct the output signal in advance to increase the system damping, increase the closed-loop zero point, speed up the system response speed, and do not affect the steady-state characteristics of the system. Therefore, a differential compensation link is connected in series with the forward channel of the first-order virtual inertia VSG active control to improve the dynamic characteristics of the system while ensuring the steady-state characteristics of the system. Add a differential compensation link to the forward channel of the active control. The way to add a differential compensation link to the forward channel of the active control is as follows: Figure 2 shown.

[0055] After adding the differential compensation term, the VSG grid-connected active closed-loop transfer function is shown in (2):

[0056]

[0057] The closed-loop transfer function of the small active signal of the VSG grid-connected output with differential compensation obtained by (2) is shown in (3):

[0058]

[0059] Where: K d is the coefficient of the differential compensation link, K ω is the speed regulator adjustment coefficient, δ is the phase angle difference between the VSG output voltage and the grid voltage, K p From δ to P out Expression of, ω0 is the rated voltage angular frequency.

[0060] The steady-state characteristics of the active power of the VSG based on virtual inertia of the differential compensation link remain unchanged during grid-connected operation, and the steady-state power error is still related to D. The addition of the differential term increases the damping ratio of the system and adds a closed-loop zero point. The closed-loop pole and closed-loop zero point positions can be determined by the differential coefficient K. d Adjustment is performed, thereby increasing the control freedom of dynamic characteristics and realizing decoupling regulation of active power steady-state and dynamic characteristics.

[0061] While ensuring the steady-state characteristics of power, the power overshoot of the system is reduced, the system stability is increased, and the active power response time is accelerated. When the VSG based on the virtual inertia of the differential compensation link is operating in an isolated island, its P-ω characteristic is:

[0062]

[0063] Compared with the first-order link virtual inertia, an additional zero point s1 = -1 / K d , by adjusting K d The position of the zero point can be changed to change its dynamic characteristics, and K d No change in G P-ω The steady-state operating point. However, due to G P-ω Still need to maintain inertia, so K d The value must satisfy (5):

[0064] According to Newton's laws of motion, simulating the synchronous generator rotor motion equation, the VSG mechanical equation can be obtained as formula (6):

[0065]

[0066] There are two equilibrium points in the above model, one is the stable equilibrium point (SEP) The other is the Unstable Equilibrium Point (UEP) According to the model Jacobian matrix evaluated at the SEP point, the following eigenvalues ​​can be obtained:

[0067]

[0068] in,

[0069] To keep the system stable, the eigenvalue is set below zero and the critical damping can be derived as:

[0070]

[0071] When D≤D cr , SEP behaves as a focus. When the damping is D>D cr When the damping increases to 100 (D>D cr ), the frequency response performance degrades, such as Figure 3 As shown in the figure. Although the frequency overshoot is greatly reduced, it takes more time to stabilize. The simulation results prove the determination of critical damping and verify the correctness of the bifurcation analysis.

[0072] In the range of values ​​that satisfy (5), if K d Small, the zero point is far away from the origin, the effect can be ignored, the dynamic characteristics are mainly determined by the poles, G P-ω Approximately first-order inertia; if K d Larger, stronger zero point effect, G P-ω Approximate linear droop characteristics, small inertia, such as Figure 4 As shown, K d2 >K d1 > 0. If you choose a suitable K d The virtual inertia of the lead-lag link can realize both the fast response of droop control and the larger inertia of the first-order link.

[0073] When the VSG based on virtual inertia of differential compensation link is connected to the grid, the damping ratio ξ and natural oscillation frequency ω of its output active small signal closed-loop transfer function nG They are:

[0074]

[0075] In order to reduce the steady-state power error, J and K are used to comprehensively consider the requirements of dynamic power overshoot, adjustment time and stability margin. d The value must meet the following conditions:

[0076] 1) Damping ratio. For VSG grid-connected systems, large active overshoot will cause strong power shocks to the energy storage unit during the dynamic process, and will also cause output current overcurrent. Therefore, in order to avoid excessive power overshoot, the system damping ratio needs to be increased, and the setting ξ needs to meet the following requirements:

[0077] 0.8≤ξ≤1 (10)

[0078] 2) Stability margin. Changes in system parameters and the influence of other factors may lead to errors in stability analysis. Therefore, when selecting control parameters, it is necessary to retain appropriate amplitude margin h and phase margin γ, and set the amplitude margin and phase margin to meet:

[0079]

[0080] 3) Adjustment time. The adjustment time of an underdamped second-order system is given by the real part of the closed-loop pole R e (s) decision, R e The smaller (s), the shorter the adjustment time. e (s) can also be used as a measure of stability to a certain extent. In summary, the constraint conditions for setting the closed-loop poles are:

[0081] R e (s)=-ω nG ξ≤-10 (12)

[0082] 4) Decoupling of inner and outer loops. To ensure the stability of the system and make the VSG and synchronous motor have consistent low-frequency characteristics, the power outer loop and the voltage inner loop need to be decoupled. Set the voltage inner loop bandwidth frequency ω ub =240×2πrad / s, then the power loop bandwidth frequency must satisfy ω fb ≤ω ub / 4, that is:

[0083]

[0084] According to (7) to (10) for the P-ω characteristics of the VSG in the virtual inertia of the differential compensation link during island operation and the constraints during grid-connected operation, J and the corresponding K are obtained: d The value range is, for example Figure 5 shown.

[0085] The VSG based on the virtual inertia of the first-order link needs to improve the dynamic characteristics of the grid-connected power by adjusting J and D, and the increase of D value also increases the steady-state active error. The VSG based on the virtual inertia of the differential compensation link increases the control freedom of the dynamic characteristics of the grid-connected power. By adjusting K d While improving the dynamic characteristics of the system, the D value can be kept unchanged to ensure that the steady-state error is minimized, thereby improving the comprehensive stability margin of the grid-connected converter in different operating scenarios.

[0086] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for improving the comprehensive stability margin of a converter, characterized in that: The following steps are involved: The root locus characteristics and the relationship between the control parameters and the frequency support are obtained by establishing a small signal model; the control parameters are adjusted dynamically; and the parameter range is optimized.

2. A method for improving the comprehensive stability margin of a converter according to claim 1, characterized in that: The specific steps include: The small signal model of VSG is established, the influence of active power-frequency link is considered, the influence of reactive power-voltage link is ignored, the output voltage of virtual synchronous machine is regarded as a constant value, the dynamic influence of voltage and current double loops is ignored during operation, and the small signal closed-loop transfer function between the active power output value and the command value of virtual synchronous machine is obtained. The root locus diagram of the grid-connected VSG system is drawn according to the small signal closed-loop transfer function. The virtual damping D gradually increases from 0.001 to 0.3 with a step size of 0.

001. The virtual inertia is 0.008 kg·m 2 , 0.02kg·m 2 and 0.03kg·m 2 ; The differential compensation link is connected in series with the forward channel of the first-order link virtual inertia VSG active control to improve the dynamic characteristics of the system while ensuring the steady-state characteristics of the system; The closed-loop transfer function of the small active signal of the VSG grid-connected output with differential compensation is obtained; While ensuring the steady-state characteristics of power, reduce the power overshoot of the system, increase system stability, and speed up the active power response time; For the characteristics of the virtual inertia of the differential compensation link during VSG island operation and the constraints during grid-connected operation, the corresponding value ranges are obtained.

3. A method for improving the comprehensive stability margin of a converter according to claim 2, characterized in that: The small signal closed-loop transfer function is:

4. A method for improving the comprehensive stability margin of a converter according to claim 3, characterized in that: After adding the differential compensation term, the VSG grid-connected active closed-loop transfer function is:

5. A method for improving the comprehensive stability margin of a converter according to claim 4, characterized in that: The closed-loop transfer function of the small active signal of the VSG grid-connected output with differential compensation is: Where: K d is the coefficient of the differential compensation link, K ω is the speed regulator adjustment coefficient, δ is the phase angle difference between the VSG output voltage and the grid voltage, K p From δ to P out Expression of, ω0 is the rated voltage angular frequency.

6. A method for improving the comprehensive stability margin of a converter according to claim 5, characterized in that: When the VSG is operating in an island mode based on virtual inertia of the differential compensation link, its P-ω characteristic is: K d The value must meet the following requirements: At the same time, D needs to satisfy the critical damping D cr :

7. A method for improving the comprehensive stability margin of a converter according to claim 6, characterized in that: When the VSG based on virtual inertia of differential compensation link is connected to the grid, the damping ratio ξ and natural oscillation frequency ω of its output active small signal closed-loop transfer function nG They are:

8. A method for improving the comprehensive stability margin of a converter according to claim 7, characterized in that: J and K d The value must meet the following conditions: 1) Damping ratio, setting ξ must satisfy: 0.8≤ξ≤1 (8) 2) Stability margin, set the amplitude margin h and phase margin γ to meet: 3) Adjust the time and set the closed-loop pole constraints as follows: R e (s)=-ω nG ξ≤-10 (10) 4) Decouple the inner and outer loops. Set the voltage inner loop bandwidth frequency to 400πrad / s. Then the power loop bandwidth frequency must satisfy ω fb ≤ω ub / 4, that is:

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