Inertia damping adaptive VSG power frequency control method for optical storage grid-connected system

Abstract

The invention discloses an inertia damping self-adaptive VSG power frequency control method for an optical storage grid-connected system. A hybrid energy storage power control strategy is designed based on structural characteristics of an optical storage hybrid VSG system and hybrid energy storage working characteristics; and secondly, designing an inertia damping self-adaptive control strategy for improving the dynamic and steady state characteristics of the output power frequency of the inverter by leading out the output frequency deviation and the frequency change rate of the inverter, introducing an inertia link into the self-adaptive strategy to coordinate the dynamic characteristics of the inverter and the energy storage system, and finally providing a control parameter setting method.

Description

Technical Field

[0001] The present invention belongs to the technical field of electric power systems, and more specifically, relates to a power frequency control method for a VSG grid-connected photovoltaic storage system with adaptive inertia damping. Background Art

[0002] At present, the proportion of distributed photovoltaic grid-connected is expanding rapidly. Photovoltaic grid-connected with hybrid energy storage has become an effective means to reduce peak loads and fill valleys, improve system voltage and frequency stability, and enhance the high and low frequency energy absorption and support capacity of energy storage.

[0003] Distributed photovoltaic storage is only carried by power electronic components and cannot provide the necessary voltage and frequency support for the system. At high penetration, it will lead to poor power frequency stability and weak anti-interference ability of the power grid. Based on this virtual synchronous generator technology (VSG), the external characteristics of the synchronous machine are simulated to make the distributed energy show virtual inertia and damping, which can enhance the anti-interference ability of the system. In order to further improve the output active power and frequency stability of the distributed system under load mutations and dispatching instruction disturbances, the inertia damping adaptive control technology that takes advantage of the flexible and adjustable parameters of VSG has become a research hotspot.

[0004] However, the current adaptive VSG technology is mostly focused on improving the active frequency dynamic characteristics of photovoltaic grid-connected systems, lacking analysis of the system's active steady-state error under grid-side frequency disturbances, and lacking a coordinated adaptive control strategy that takes into account energy storage characteristics for new photovoltaic storage systems. Therefore, for the current new hybrid energy storage combined with distributed photovoltaic high-proportion grid-connected new scenarios, it is urgent to invent a VSG inertia damping adaptive friendly grid-connected control strategy that takes into account the dynamic characteristics and steady-state characteristics of the system under common transient conditions and takes into account energy storage characteristics. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide an inertia damping adaptive photovoltaic storage grid-connected system VSG power frequency control method to improve the active frequency dynamic characteristics of the photovoltaic grid-connected system under common transient disturbance conditions and reduce the output active power steady-state error.

[0006] To achieve the above-mentioned purpose of the invention, the present invention provides an inertia damping adaptive photovoltaic energy storage grid-connected system VSG power-frequency control method, characterized in that the inertia and damping in the VSG control strategy are adaptively adjusted through the output frequency deviation and frequency change rate of the inverter, thereby realizing inverter power and frequency control, so that the photovoltaic energy storage grid-connected system can have stable output under the scenarios of scheduling command changes, load disturbances and grid-side frequency disturbances.

[0007] The object of the invention of the present invention is achieved in this way:

[0008] The present invention discloses an inertia damping adaptive photovoltaic storage grid-connected system VSG power frequency control method. A hybrid energy storage power control strategy is designed based on the structural characteristics of the photovoltaic storage hybrid VSG system and the working characteristics of the hybrid energy storage. Secondly, an inertia damping adaptive control strategy for improving the dynamic and steady-state characteristics of the inverter output power frequency is designed by deriving the output frequency deviation and frequency change rate of the inverter. An inertia link is introduced into the adaptive strategy to coordinate the dynamic characteristics of the inverter and the energy storage system. Finally, a control parameter setting method is given.

[0009] At the same time, the VSG power frequency control method of the photovoltaic energy storage grid-connected system with inertia damping adaptive according to the present invention also has the following beneficial effects:

[0010] (1) Compared with the J and D fixed technologies, this technology can eliminate VSG power oscillation, reduce frequency fluctuations and effectively reduce active steady-state errors during transient response, thereby improving the friendliness of the distributed system.

[0011] (2) This technology can reduce the need to adjust the frequency, rate and amplitude of the energy storage system, improve the efficiency of energy storage and enhance the reliability of the photovoltaic storage system. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is a power frequency control structure diagram of a VSG photovoltaic energy storage grid-connected system with inertia damping self-adaptation of the present invention;

[0013] Figure 2 It is the energy management control strategy diagram of the hybrid energy storage module;

[0014] Figure 3 It is a topological diagram of a virtual synchronous generator;

[0015] Figure 4 This is the VSG active closed-loop small signal model diagram;

[0016] Figure 5 It is the root locus diagram under different J and D;

[0017] Figure 6 It is a comparison diagram of simulation results of the photovoltaic storage system using the present invention in a scenario where the active power dispatch instruction is changed.

[0018] Figure 7 It is a comparison chart of simulation results of the photovoltaic storage system of the present invention under load disturbance scenario.

[0019] Figure 8 It is a comparison chart of simulation results of the photovoltaic storage system of the present invention under the system frequency disturbance scenario. DETAILED DESCRIPTION

[0020] The specific implementation of the present invention is described below in conjunction with the accompanying drawings so that those skilled in the art can better understand the present invention. It should be noted that in the following description, when the detailed description of known functions and designs may dilute the main content of the present invention, these descriptions will be omitted here.

[0021] Example

[0022] Figure 1 This is a power frequency control structure diagram of a VSG photovoltaic energy storage grid-connected system with inertia damping self-adaptation according to the present invention.

[0023] In this embodiment, a single photovoltaic energy storage grid-connected system based on adaptive VSG is used for simulation verification. Figure 1 As shown in the figure, the photovoltaic energy storage grid-connected system includes an inverter control system, a photovoltaic power generation system and a hybrid energy storage system. The inverter output power rating is 100kW, which supplies power to local loads and the surplus power is connected to the grid. Among them, the inverter DC bus voltage U dc =1500V; inverter filter capacitor C f =10e-6; inverter filter inductor L f =4e-3; Line inductance L g =1.2e-3; rated output voltage of inverter U n =311V; inverter output rated frequency f n =50Hz;

[0024] Next, we will describe in detail a method for controlling power frequency of a VSG grid-connected photovoltaic energy storage system with adaptive inertia damping according to the present invention, as follows:

[0025] (1) Establish a photovoltaic energy storage system model based on virtual synchronous generators;

[0026] Introduce a battery and supercapacitor hybrid energy storage unit (HESS) on the DC side of the inverter to establish a hybrid energy storage power control strategy. The battery group takes advantage of its high energy density to supplement the low-frequency power shortage, and the supercapacitor takes advantage of its high power density to improve the energy peak dynamic rapid response capability of the energy storage system; the maximum power of the supercapacitor is usually higher than the rated power of the battery. Therefore, the HESS energy allocation scheme is set as follows: the battery usually only uses the power to be smoothed when the power exceeds its own rated power P bat-N Charge and discharge at rated power.

[0027] like Figure 2 As shown, the battery adopts constant power control, and the output power reference value P bat-ref with U bat The current reference value is obtained by taking the quotient, I bat-ref With the actual value I batAfter comparison, the control signal obtained by the PI controller and the PWM generator is used to complement the bidirectional Buck-Boost and track the output P bat-ref The supercapacitor adopts a constant voltage control strategy, which sets the DC bus voltage reference value V dc-ref With the actual value V dc After the difference is made, it is compensated by the PI controller and a complementary control signal is generated through the PWM module to ensure the dynamic stability of the DC bus voltage.

[0028] like Figure 3 The VSG control topology shown mainly includes active power control link, reactive power control link, virtual impedance link and voltage and current dual-loop control module. Since the present invention is mainly aimed at improving the dynamic and steady-state characteristics of the system's active frequency under small disturbances, and the VSG strategy can achieve active / reactive decoupling, only the construction principles of the virtual impedance link and the active frequency loop are introduced. The virtual impedance link introduces the rotor motion equation of the synchronous generator (the pole pair number is 1) to make the photovoltaic storage hybrid grid-connected system exhibit virtual inertia and damping characteristics.

[0029]

[0030] Where, J is the moment of inertia; T m With T e is the mechanical and electromagnetic torque of SG; T D is the damping torque; D is the damping coefficient; P m With P e is the mechanical and electromagnetic power; ω is the actual measured angular velocity; ω ref is the rated angular velocity; θ is the potential phase angle inside SG;

[0031] The active power-frequency droop control equation is introduced to simulate the synchronous machine speed regulator to provide frequency stability support for the system

[0032] P m -P ref =K f (ω ref -ω)

[0033] Where P ref is the active power reference value, K f is the active power-frequency droop coefficient;

[0034] (2) Establish a closed-loop small signal model of active power based on VSG;

[0035] Assume that U∠0 is the voltage at the common grid connection point, E∠δ is the inverter output voltage, and δ is the power angle of VSG. f Much larger than R f When the output power of VSG can be simplified as

[0036]

[0037] By combining the above virtual rotor motion equation, active loop droop equation and simplified output power equation, and using the rated value superimposed disturbance value mode to express the variables Pe, δ, ω, etc., the VSG active closed-loop small signal model can be obtained as follows: Figure 4 As shown;

[0038] (3) Analyze the influence mechanism of inertia damping on the dynamic characteristics and steady-state error of VSG output active power;

[0039] According to the VSG active closed-loop small signal model, the closed-loop transfer function between the reference power disturbance and the VSG output active power is obtained, and simplified into a typical structure of a second-order system.

[0040]

[0041] The time domain analysis method is used to analyze the inertia and damping response to the output active power σ% and t s The influence mechanism of K is summarized as follows. f When D is fixed and J increases, the damping degree decreases, the power overshoot σ% increases, and the adjustment time t s Lengthened; K f , J is fixed, when D increases, the damping ratio increases, σ% decreases, t s Become shorter.

[0042] Using the root locus analysis method, we set J = 0.3, J = 0.5, and J = 1.0, and plot the root change trajectory of the output active closed-loop characteristic equation in the s plane when the D step length is 1 and increases from 0 to 40. Figure 5 As shown. Figure 5 :J is fixed. As D increases (arrow direction), the two conjugate complex roots gradually move from the imaginary axis to the left half plane of s, and move to the opposite direction after converging on the real axis. When D is small, S 1 , S 2 Close to the imaginary axis, the stability is poor. When D increases, the system first enters the underdamped state, at which time the dynamic performance is better, there is a certain overshoot, and the adjustment time is short. When D continues to increase, the system becomes an overdamped state (two real poles) 2 As the system approaches the origin, the stability deteriorates, there is no overshoot but the response is slow. As J increases, the separation point of the two characteristic roots gradually approaches the imaginary axis, the system stability deteriorates, the oscillation is obvious, and the adjustment time is long.

[0043] Therefore, based on the above dynamic response characteristics and stability analysis, J should not be too large for a long time during the regulation process, and the damping matching J should be limited to a certain range. If D is too small, power oscillation cannot be suppressed. Figure 5The overdamping states are J = 0.3, D > 15; J = 0.5, D > 18; J = 1.0, D > 26. During the adaptive process, the system should be prevented from operating in an overdamping state as much as possible.

[0044] Based on the active closed-loop small signal model, the closed-loop transfer function between the system frequency disturbance and the VSG output active power is obtained, and the VSG active output response steady-state error is derived.

[0045]

[0046] From the above formula, we can deduce the output active steady-state error ΔP under grid-side frequency disturbance: e It is proportional to D. Increasing D will cause a larger steady-state error when the grid frequency is disturbed.

[0047] (4) Analyze the influence mechanism of inertia damping on the frequency dynamic characteristics of VSG;

[0048] Combining the virtual rotor motion equation and the VSG virtual speed regulator equation, we get

[0049]

[0050] In summary, if T m -T e -T D unchanged, J is inversely proportional to dω / dt, and the effect of D on dω / dt is affected by the signs of Δω and dω / dt and needs to be analyzed specifically; J, D, K f Therefore, when dω / dt is large during transient response, the rapid rise and fall of frequency can be avoided by increasing J, the frequency fluctuation can be suppressed and the frequency can be quickly restored to the rated value; and increasing J and D can reduce the frequency deviation.

[0051] (5) Design an inertia damping coordinated adaptive control strategy taking into account the dynamic characteristics of energy storage;

[0052] Drawing on the static regulation characteristics of synchronous machines, an inertial damping adaptive strategy is designed to effectively suppress transient power oscillations and frequency fluctuations under common working conditions such as changes in active dispatching instructions and load disturbances, and reduce the steady-state deviation of active power under grid frequency disturbance conditions; for the research object of the photovoltaic storage combined grid-connected system, the working characteristics of the energy storage link are considered, and a first-order inertia link is introduced before dω / dt of the J adaptive rule to match the exponential function, aiming to make full use of the characteristic that the output cannot mutate when the input of the inertia link mutates, coordinate the problem of mismatching the dynamic characteristics of VSG and the energy storage system, and at the same time improve the working efficiency of the energy storage equipment and extend its service life; the preset change rules are as follows

[0053] Interval Δω dω / dt Δω*dω / dt J D 1 ＞0 ＞0 ＞0 Larger Larger 2 ＞0 ＜0 ＜0 Smaller Smaller 3 ＜0 ＜0 ＞0 Larger Larger 4 ＜0 ＞0 ＜0 Smaller Smaller

[0054] The complete adaptive strategy is as follows:

[0055]

[0056]

[0057] The hyperbolic tangent function is introduced into the damping adaptive strategy to adaptively adjust the damping D according to the preset rules on the one hand, and to limit the range of variation of D on the other hand, so as to avoid excessive output active steady-state deviation caused by excessive D.

[0058] In the above formula, C J1 and C D is the regulation threshold of Δω change. J2 is the threshold of dω / dt change. The setting of the threshold represents the strictness of the adaptive algorithm. On the one hand, it is necessary to avoid frequent adjustments of the adaptive system, and on the other hand, it is necessary to ensure that the system can be adaptively adjusted in time when the fluctuation exceeds the limit. It can be flexibly adjusted according to the system design requirements. In this embodiment, the typical value C J1 =0.3, C J2 =1.5, C D =0.05. 0 , D 0 is the steady-state operating value of the system. According to the analysis in step 3, take the typical value J 0 =0.5, D 0 =15. Combined with the root locus analysis, the system stability should be better and not enter the over-damping state, so D max =30, D min =8.

[0059] The system simulation results under the active dispatch instruction change scenario and load disturbance scenario are shown in the following figure. Figure 6 and Figure 7 shown. Figure 6 Set the active dispatch instruction P ref The power increases from 100kW to 120kW in 0.5s, and the energy storage system works in charging mode. Figure 7 The local load is set to 100kW in the initial state, 30kW load is put into operation at 0.5s, and all loads are removed at 1s, and the energy storage system works in the discharge mode. Compared with the traditional fixed inertia damping design, the system using the present invention has no power oscillation and small power overshoot under transient disturbances, which helps to improve the power quality and enhance the stability of the system. At the same time, it can significantly reduce the frequency adjustment time, suppress fluctuations, and promote the rapid recovery of frequency to stability. In addition, the energy storage link of the grid-connected system using the present invention has a slower power rise rate and smaller fluctuations when providing energy, which reduces the need for energy storage equipment to adjust the frequency and amplitude, and improves the problem of mismatch between the dynamic characteristics of the energy storage system and the VSG system.

[0060] like Figure 8As shown, the grid frequency is set to drop by 0.1 Hz at 0.6s and recover to the rated value at 2.2s, and the power increase response of VSG participating in primary frequency modulation is compared. Compared with the traditional fixed inertia damping design, the system using the present invention can reduce the active steady-state error by about 27%, effectively improving the accuracy of control and avoiding the influence of excessive or insufficient response on the stability of the power system. At the same time, the system using the present invention exhibits better active frequency dynamic characteristics. Based on the above analysis, the present invention can take into account the optimization of transient and steady-state characteristics of the distributed system and optimize the output of the energy storage system, which meets the design requirements.

[0061] Although the above describes the illustrative specific embodiments of the present invention to facilitate those skilled in the art to understand the present invention, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations using the concept of the present invention are protected.

Claims

1. A power frequency control method for a VSG grid-connected photovoltaic energy storage system with adaptive inertia damping, characterized in that: By adaptively adjusting the inertia and damping in the VSG control strategy through the output frequency deviation and frequency change rate of the inverter, the inverter power and frequency control can be achieved, so that the output of the photovoltaic energy storage grid-connected system can be stable under the scenarios of scheduling command changes, load disturbances and grid-side frequency disturbances.

2. According to claim 1, a method for controlling power frequency of a VSG grid-connected photovoltaic energy storage system with adaptive inertia damping, characterized in that: The inertia J in the VSG control strategy satisfies: Where Δω is the output frequency deviation, dω / dt is the output frequency change rate; C J1 is the regulation threshold of Δω change; C J2 is the regulation threshold of dω / dt change; J0 is the steady-state operation value of photovoltaic energy storage grid connection, T is the inertia time constant of the inertia link, m and n are exponential regulation factors; k J1 ,k J2 ,k J3 is the adaptive coefficient.

3. According to the inertia damping adaptive photovoltaic energy storage grid-connected system VSG power frequency control method of claim 1, characterized in that: The damping D in the VSG control strategy satisfies: Among them, C D is the regulation threshold of Δω change, D0 is the damping steady-state operation value of the photovoltaic energy storage grid-connected system; D max With D min To adaptively adjust the maximum and minimum values ​​of damping, the tanh function is used to control the damping variation law and limit it to D max With D min within the range.

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