Flexible DC power distribution network voltage control system based on active disturbance rejection algorithm

Abstract

The invention discloses a flexible DC power distribution network voltage control system based on an active disturbance rejection algorithm. The method comprises the following steps: establishing a flexible DC power distribution network converter model; establishing a differential link, and arranging a transition process for the voltage instruction value; establishing a linear expansion observer; establishing a nonlinear feedback link, and carrying out nonlinear combination on errors; a controlled object, a differential link, a linear expansion observer and a nonlinear feedback link are combined to form the active disturbance rejection control system of the flexible direct current power distribution network. The system can enhance the suppression capability of the system on voltage fluctuation, and has better engineering stability, tracking convergence and anti-interference superiority.

Description

Technical Field

[0001] The invention belongs to the field of flexible DC distribution network control, and relates to a flexible DC distribution network voltage control system based on an active disturbance rejection algorithm. Background Art

[0002] Compared with the traditional AC power grid, the flexible DC distribution network has the advantages of convenient access to renewable energy, high power supply efficiency, large power supply capacity, small power supply corridor area, high power quality, etc., and is widely used in the low-voltage field. And voltage, as an important criterion for the stability of the power grid, it is of great practical significance to study its control method.

[0003] At present, the research on the voltage control strategy of the flexible DC distribution network is relatively imperfect. Many basic control strategies come from the engineering experience of the high-voltage DC transmission system. The main ones that can be applied to the flexible DC distribution network are: master-slave control, voltage droop control, and voltage margin control. A significant disadvantage of the master-slave control system is that a single point failure of the master station may cause the voltage control of the entire distribution network to fail. This is especially important for the flexible DC distribution network because the distribution network needs to have higher robustness to cope with faults and accidents; in addition, since the master-slave control essentially relies on a central control point, the flexibility of each node or terminal device in the distribution network is poor. The voltage droop control assumes that there is a certain degree of linear relationship between voltage and power, but in actual applications, due to factors such as load changes, instability of power sources, and cable resistance, this linear relationship may be affected, resulting in inaccurate and untimely system response. In the case of rapid load changes or large disturbances, the stability of the voltage droop control may be poor, easily causing violent voltage fluctuations or system instability. The voltage margin control strategy mainly focuses on the gap between the voltage and the rated voltage, while ignoring the dynamic changes of the load. Especially in the case of complex load changes and energy fluctuations, the system may not be able to adjust the voltage margin in time, resulting in the voltage exceeding the allowable range. In a flexible DC distribution network with a large number of distributed energy accesses, the voltage margin control strategy may not be able to fully consider the volatility and mutual influence of each distributed power source, resulting in too large or too small voltage margin.

[0004] At present, DC distribution networks can be divided into single-ended, double-ended, and multi-ended DC distribution networks according to the number of connection ports to the external AC power grid, and each port is configured with a flexible commutation device. The more the number of ports, the larger the number of converter stations, and the higher the control requirements for the converter stations to cooperate with each other to jointly control the DC voltage stability, and the more difficult it is to formulate the control strategy. Summary of the Invention

[0005] To solve the above technical problems, the present invention provides a flexible DC distribution network voltage control system based on an active disturbance rejection algorithm, which can enhance the system's ability to suppress voltage fluctuations and has better engineering stability, tracking convergence, and anti-interference superiority.

[0006] The technical solution adopted by the present invention is as follows:

[0007] A flexible DC distribution network voltage control system based on an active disturbance rejection algorithm, comprising:

[0008] Establish a flexible DC distribution network converter model;

[0009] Establish a differential link to arrange a transition process for the voltage command value;

[0010] Establish a linear extended observer;

[0011] Establish a non-linear feedback link to perform non-linear combination of the errors;

[0012] Combine the controlled object, the differential link, the linear extended observer, and the non-linear feedback link to form an active disturbance rejection control system for the flexible DC distribution network.

[0013] Establish a grid-connected converter model, specifically including:

[0014] Let u gx be the voltage of phase x of the three-phase AC power grid, i x be the current of phase x of the three-phase AC power grid, u cx_n be the voltage of the midpoint of the x-phase bridge arm on the AC side of the converter relative to the midpoint n of the AC power supply, u cx_o represent the voltage of the midpoint of the x-phase bridge arm on the AC side relative to the midpoint o of the DC power supply, and R and L are the series resistance and inductance respectively;

[0015] For a three-phase two-level voltage source converter, the upper and lower bridge arms of each phase are equivalent to a single-pole double-throw switch, and the switching function is defined as:

[0016]

[0017] In the formula: S x is the switching function; a, b, c represent phase A, phase B, and phase C.

[0018] Assume that the DC voltage is constant. According to the above switching function, the transformation relationships between the voltage of the AC side of the converter relative to the bus midpoint o, the DC voltage, the AC current, and the DC current can be obtained:

[0019]

[0020] In the above formula: u ca_o is the voltage of phase A of the AC side of the converter relative to the midpoint o of the DC bus; ucb_o is the voltage of phase B on the AC side of the converter relative to the midpoint o of the DC bus; u cc_o is the voltage of phase C on the AC side of the converter relative to the midpoint o of the DC bus; S a is the switching function of phase A; S b is the switching function of phase B; S c is the switching function of phase C; i dc is the current on the DC side; i a is the current of phase A of the converter; i b is the current of phase B of the converter; i c is the current of phase C of the converter.

[0021] Therefore, the voltage of the AC side of the converter relative to the midpoint n of the AC power supply:

[0022]

[0023] In the above formula: u ca_n is the voltage of phase A on the AC side of the converter relative to the midpoint n of the AC power supply; u cb_n is the voltage of phase B on the AC side of the converter relative to the midpoint n of the AC power supply; u cc_n is the voltage of phase C on the AC side of the converter relative to the midpoint n of the AC power supply; u on is the voltage of the midpoint o of the DC bus relative to the midpoint n of the three-phase AC power supply.

[0024] Three-phase circuit equations of the AC side of the converter:

[0025]

[0026] In the above formula: i a represents the current of phase A of the converter; i b represents the current of phase B of the converter; i c represents the current of phase C of the converter; u ga is the voltage of the AC power supply of phase A; u gb is the voltage of the AC power supply of phase B; u gc is the voltage of the AC power supply of phase C;

[0027] In a three-phase three-wire system, there are:

[0028] u ga +u gb +u gc = 0;

[0029] i a +i b +i c = 0;

[0030] From the above equations, we get:

[0031]

[0032] Converter switch function model:

[0033]

[0034] In the above formula: R load is the DC load.

[0035] According to the fastest control synthesis function, a tracking differentiator is established:

[0036] X 1 (k + 1) = X 1 (k) + hX 2 (k);

[0037] X 2 (k + 1) = X 2 (k) + hfhan(X 1 (k) - r, X 2 (k), a, h 0 );

[0038] Where: X 1 (k + 1), X 2 (k + 1) are the state variables at the (k + 1)-th moment; X 1 (k), X 2 (k) are the state variables at the k-th moment; h is the integration step size; fhan(x 1 , x 2 , r, h) is the fastest control synthesis function; r is the speed factor, which determines the tracking speed of the tracking differentiator; h 0 is the filtering factor, whose value is slightly larger than the integration step size h and can prevent overshoot; the function

[0039] fhan = -r(a / d)S a -rsign(a)(1 - S a ), in the formula:

[0040] d = rh 2 , a 0 = hx 2 , y = x 1 + a 0 ;

[0041] In the above formula: d, a 0 , y are intermediate variables; x 1 , x 2 are the independent variables of the function fhan(x 1 , x 2 , r, h).

[0042]

[0043] In the above formula: a 1 and a 2 are both intermediate variables.

[0044] S y = (sign(y + d) - sign(y - d)) / 2;

[0045] In the above formula: S y is an intermediate variable.

[0046] a = (a 0 + y)S y + a 2 (1 - S y );

[0047] In the above formula: a is an intermediate variable.

[0048] S a = (sign(a + d) - sign(a - d)) / 2.

[0049] Build a linear extended observer:

[0050] Z 1 (k + 1) = Z 1 (k) + h(Z 2 (k) - β 1 ε);

[0051] Z 2 (k + 1) = Z 2 (k) + h(Z 3 (k) - β 2 ε + bu(k));

[0052] Z 3 (k + 1) = Z 3 (k) - hβ 3 ε;

[0053] Where: Z 1 (k + 1), Z 2 (k + 1) are the estimated values of the system state variables at the (k + 1)-th moment; Z 3 (k + 1) is the estimated value of the system lumped disturbance at the (k + 1)-th moment; Z 1 (k), Z 2 (k) are the estimated values of the system state variables at the k-th moment; Z 3 (k) is the estimated value of the system lumped disturbance at the k-th moment; u(k) is the control input of the controller at time k; h is the sampling step size, ε is the difference between the output estimate and the system output; β 1 and β 2 are both observer gain coefficients, which are tuned according to the bandwidth method: β1 = 3ω, β 2 = 3ω 2 , β 3 = ω 3 , where ω is the observer bandwidth.

[0054] Establish a non - linear feedback link and use the fal function to perform non - linear combination on the deviation;

[0055] e 1 = X 1 (k) - Z 1 (k);

[0056] Where: e 1 is the observer estimation error; X 1 (k) is the system state variable at the k - th moment; Z 1 (k) is the estimated value of the system state variable by the observer at the k - th moment.

[0057] e 2 = X 2 (k) - Z 2 (k);

[0058] Where: e 2 is the observer estimation error; X 2 (k) is the system state variable at the k - th moment; Z 2 (k) is the estimated value of the system state variable by the observer at the k - th moment.

[0059] u 0 = β 1 fal(e 1 , α 1 , σ) + β 2 fal(e 2 , α 2 , σ);

[0060] Where:

[0061]

[0062] In the above formula: fal(ε, α, σ) is a non - linear function; when |ε| < σ, the function fal(ε, α, σ) becomes the linear form ε / σ 1-α , avoiding instability caused by excessive non - linearity in the small error interval. When |ε| ≥ σ, the function fal(ε, α, σ) is the non - linear form |ε| α sign(ε), enhancing the adjustment ability for large errors. u 0 is the equivalent output of the controller; α 1 , α 2 are non - linear coefficients; α 1 , α 2Less than 1 and satisfying α 1 ≤α 2 ; σ is the threshold parameter; fal(e 1 ,α 1 ,σ) is to perform a non - linear combination on e using the fal(ε,α,σ) function; is to perform a non - linear combination on e using the fal function 1 2

[0063] Combine the controlled object, the differential link, the linear extended observer, and the non - linear feedback link to form an active disturbance rejection control system for a flexible DC distribution network.

[0064] Input the command signal r into the differential link, subtract the output of the differential link from the state estimate value of the linear extended observer, and input the obtained deviation value into the non - linear feedback link to output u 0 ;

[0065] u 0 Subtract the disturbance estimate value Z 3 , and after passing through a 1 / b 0 gain, obtain the control rate u;

[0066]

[0067] In the above formula: u is the input of the state - space model of the controlled object and also the output of the active disturbance rejection control system. b 0 is the control - input gain of the state - space model of the controlled object. u 0 Subtract the disturbance estimate value Z 3 , and then divide by the control - input gain to be used as the control quantity input to the controlled - system model.

[0068] The voltage control system for a flexible DC distribution network based on the active disturbance rejection algorithm of the present invention has the following technical effects:

[0069] 1) The main advantage of the differential transition link of the present invention is that by smoothing the differential signal, it effectively suppresses the interference of high - frequency noise on the system, enhancing...; at the same time, it optimizes the dynamic performance of the controller, reduces oscillation and overshoot, improves the stability of the system, and avoids the instability or divergence problems caused by direct differentiation, thus achieving a more stable control effect.

[0070] 2) The linear extended observer of the present invention has the advantages of simple design, fast dynamic response, strong robustness, clear stability analysis, and convenient parameter tuning. It can efficiently estimate the system state and disturbance, has strong adaptability to model uncertainty and external disturbances, is especially suitable for linear or weakly non - linear systems, and is easy to implement and widely used in engineering.

[0071] ​​3) The non-linear feedback link of the present invention uses the fal function to perform non-linear combination on the error. It can dynamically adjust the gain according to the magnitude of the error, accelerate convergence and improve the dynamic response speed when the error is large, and smooth the control and reduce jitter when the error is small. In addition, it can effectively suppress the influence of noise on the system, improve the robustness and stability of the system, making it particularly suitable for dealing with the uncertainties and non-linear problems of complex systems in active disturbance rejection control.

[0072] 4) The present invention combines the controlled object, the differential link, the linear extended observer, and the non-linear feedback link to form an active disturbance rejection control system for a flexible DC distribution network. When a voltage deviation occurs at the DC side bus, it realizes the restoration of the DC bus voltage and the coordinated power distribution of each converter, solving the problems of excessive DC bus voltage deviation caused by the randomness of new energy generation and load changes, which affect the stable operation of the system and even cause oscillations. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] The present invention will be further described below in conjunction with the drawings and embodiments;

[0074] Figure 1 It is a schematic diagram of the system structure of a flexible DC distribution network.

[0075] Figure 2 It is a schematic diagram of the structure of a three-phase two-level voltage source converter.

[0076] Figure 3 It is a schematic diagram of the voltage control structure of a flexible DC distribution network based on the active disturbance rejection algorithm.

[0077] Figure 4 It is an effect diagram of the DC bus voltage control performance.

[0078] Figure 5 It is an effect diagram of the converter power control performance. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0079] The model parameters of this embodiment are as follows:

[0080] The main station is denoted as VSC1, the slave station is denoted as VSC2, the three-phase voltage of the AC power grid is 380V, the inductive reactance of the AC line is 1mH, the maximum output powers of VSC1 and VSC2 are 25kW and 40kW respectively, the DC side capacitors of VSC1 and VSC2 are both 1.8mF, the initial load is set to 20kW, the resistance of the distribution line of VSC1 is 0.01Ω, and the resistance of the distribution line of VSC2 is 0.01Ω.

[0081] A method for establishing a voltage control system for a flexible DC distribution network based on the active disturbance rejection algorithm includes the following steps:

[0082] (1) Establish a converter model:

[0083]

[0084] (2) Establish a tracking differentiator link:

[0085] X 1 (k + 1) = X 1 (k) + hX 2 (k);

[0086] X 2 (k + 1) = X 2 (k) + hfhan(X 1 (k) - r, X 2 (k), a, h 0 );

[0087] The sampling step is taken as 0.01, the speed factor a is taken as 30, and the filtering factor h 0 is taken as 5h.

[0088] (3) Establish a LESO model according to the system model:

[0089] Z 1 (k + 1) = Z 1 (k) + h(Z 2 (k) - β 1 ε);

[0090] Z 2 (k + 1) = Z 2 (k) + h(Z 3 (k) - β 2 ε + bu(k));

[0091] Z 3 (k + 1) = Z 3 (k) - hβ 3 ε;

[0092] Set the observer gain according to the bandwidth method as: β 1 = 100, β 2 = 300, β 3 = 10000.

[0093] (4) Use the fal function to perform nonlinear combination on the deviation:

[0094] Let u 0 = β 1 fal(e 1 , α 1 , σ) + β 2 fal(e 2 , α 2 , σ), and the gains are all taken as 50.

[0095] (5) Take the control law as: Combined controlled object, differential link, feedback link, and observer.

[0096] At 0.2 s, the in-network load increases by 10 kW; at 0.5 s, the in-network load decreases by 10 kW; at 0.8 s, the in-network load decreases by 10 kW again.

[0097] Figure 4 、 Figure 5 They are respectively the control effects of the control system used in the present invention on the DC bus voltage and the power of the converter. It can be seen from the Figure 4 data analysis that when the load suddenly changes (increases or decreases by 10 kW) in the system using active disturbance rejection control, the fluctuation range of the DC bus voltage is always controlled within ±7.5 V, only accounting for 0.95% of the rated voltage, and the recovery time is about 0.05 s, showing fast dynamic response ability and strong anti-interference performance. This indicates that active disturbance rejection control can observe and compensate for disturbances in real time, ensure voltage stability and robustness, and significantly improve the control effect of the flexible DC distribution network under complex operating conditions.

[0098] From Figure 5 it can be seen that when the load suddenly changes (increases or decreases by 10 kW), the powers of the two voltage source converters (VSC1 and VSC2) are quickly adjusted to maintain the system power balance. After the load increases by 10 kW at 0.2 s, the power of VSC1 increases from about 18 kW to nearly 20 kW while the power of VSC2 briefly drops from about 5 kW to 3 kW and then quickly rises to 10 kW. Similarly, when the load decreases at 0.5 s and 0.8 s, the powers of VSC2 drop to 5 kW and nearly 0 kW respectively, and the power of VSC1 is gradually adjusted to about 18 kW and 16 kW. The system power fluctuation range is small and the adjustment time is short (about 0.05 s), reflecting the high efficiency and reliability of the control strategy and meeting the requirements of rapid load changes.

[0099] The results show that the present invention provides a flexible DC distribution network voltage control system based on the active disturbance rejection algorithm, which can stabilize the bus voltage and improve the power quality of the system.

Claims

1. A flexible DC distribution network voltage control system based on an active disturbance rejection algorithm is characterized by include: Establish a converter model for a flexible DC distribution network; Establish differential link; Establish a linear expansion observer; Establish a nonlinear feedback link to perform nonlinear combination of errors; The controlled object, differential link, linear expansion observer and nonlinear feedback link are combined to form an anti-disturbance control system for the flexible DC distribution network.

2. The flexible DC distribution network voltage control system based on the active disturbance rejection algorithm according to claim 1 is characterized in that: The established grid-connected converter model includes: Let u gx is the x-phase voltage of the three-phase AC grid, i x is the x-phase current of the three-phase AC grid, u cx_n is the voltage of the midpoint of the x-phase bridge arm on the AC side of the converter relative to the midpoint n of the AC power supply, u cx_o It represents the voltage at the midpoint of the AC side x-phase bridge arm relative to the midpoint o of the DC power supply. R and L are the series resistance and inductance respectively. For the three-phase two-level voltage source converter, the upper and lower bridge arms of each phase are equivalent to single-pole double-throw switches, and the switching function is defined as: Where: S x is the switching function; a, b, c represent phase A, phase B, phase C; Assuming that the DC voltage is constant, according to the above switching function, the conversion relationship between the voltage and DC voltage, and the AC current and DC current on the AC side of the converter relative to the bus midpoint o can be obtained: In the above formula: u ca_o is the voltage of phase A on the AC side of the converter relative to the DC bus midpoint o; u cb_o is the voltage of phase B on the AC side of the converter relative to the DC bus midpoint o; u cc_o is the voltage of phase C on the AC side of the converter relative to the DC bus midpoint o; S a is the switching function of phase A; S b is the switching function of phase B; S c is the C-phase switching function; i dc is the DC side current; i a is the current of phase A of the converter; i b is the converter B phase current; i c is the converter C phase current; Therefore, the voltage on the AC side of the converter relative to the midpoint n of the AC power supply is: In the above formula: u ca_n is the voltage of the converter AC side A relative to the midpoint n of the AC power supply; u cb_n is the voltage of the converter AC side B relative to the midpoint n of the AC power supply; u cc_n is the voltage of the converter AC side C relative to the midpoint n of the AC power supply; u on is the voltage of the DC bus midpoint o relative to the three-phase AC power midpoint n; The three-phase circuit equation of the converter AC side: In the above formula: i a Indicates the current of phase A of the converter; i b Indicates the current of phase B of the converter; i c Indicates the converter C phase current; u ga is the A-phase AC power supply voltage; u gb is the B-phase AC power supply voltage; u gc is the C-phase AC power supply voltage; In a three-phase three-wire system there are: in ga +in gb +in gc =0; i a +i b +i c =0; From the above equation we get: Converter switching function model: In the above formula: R load For DC load.

3. The flexible DC distribution network voltage control system based on the active disturbance rejection algorithm according to claim 2 is characterized in that: According to the fastest control comprehensive function, establish the tracking differential link: X1(k+1)=X1(k)+hX2(k); X2(k+1)=X2(k)+hfhan(X1(k)-r,X2(k),a,h0); Among them: X1(k+1) and X2(k+1) are the state variables at the k+1th moment; X1(k) and X2(k) are the state variables at the kth moment; h is the integration step; fhan(x1,x2,r,h) is the fastest control comprehensive function; r is the speed factor, which determines the tracking speed of the tracking differential link; h0 is the filter factor, whose value is slightly larger than the integration step h, which can prevent overshoot; function fhan=-r(a / d)S a -rsign(a)(1-S a ), where: d=rh 2 ,a0=hx2,y=x1+a0; In the above formula: d, a0, y are intermediate variables; x1, x2 are independent variables of the function fhan(x1, x2, r, h); In the above formula: a1 and a2 are intermediate variables; S y =(sign(y+d)-sign(y-d)) / 2; In the above formula: S y is an intermediate variable; a=(a0+y)S y +a2(1-S y ); In the above formula: a is an intermediate variable; S a =(sign(a+d)-sign(a-d)) / 2。 4. The flexible DC distribution network voltage control system based on the active disturbance rejection algorithm according to claim 1 is characterized in that: The linear expansion observer is established as follows: Z1(k+1)=Z1(k)+h(Z2(k)-β1ε); Z2(k+1)=Z2(k)+h(Z3(k)-β2ε+bu(k)); Z3(k+1)=Z3(k)-hβ3ε; Where: Z1(k+1) and Z2(k+1) are the estimated values ​​of the system state variables at the k+1th moment; Z3(k+1) is the estimated value of the system lumped disturbance at the k+1th moment; Z1(k) and Z2(k) are the estimated values ​​of the system state variables at the kth moment; Z3(k) is the estimated value of the system lumped disturbance at the kth moment; u(k) is the control input of the controller at the moment k; h is the sampling step, ε is the difference between the output estimate and the system output; β1 and β2 are both observer gain coefficients, which are adjusted according to the bandwidth method: β1=3ω, β2=3ω 2 , β3=ω 3 , ω is the observer bandwidth.

5. The flexible DC distribution network voltage control system based on the active disturbance rejection algorithm according to claim 1 is characterized in that: Establish a nonlinear feedback link and use the fal function to perform nonlinear combination of deviations; e1=X1(k)-Z1(k); Where: e1 is the observer estimation error; X1(k) is the system state variable at the kth moment; Z1(k) is the observer's estimate of the system state variable at the kth moment; e2=X2(k)-Z2(k); Where: e2 is the observer estimation error; X2(k) is the system state variable at the kth moment; Z2(k) is the observer's estimate of the system state variable at the kth moment; u0=β1fal(e1,α1,σ)+β2fal(e2,α2,σ); in: In the above formula: fal(ε,α,σ) is a nonlinear function; when |ε|σ, the function fal(ε,α,σ) becomes a linear form ε / σ 1-α ; When |ε|≥σ, the function fal(ε,α,σ) is a nonlinear form |ε| α sign(ε); u0 is the equivalent output of the controller; α1 and α2 are nonlinear coefficients; α1 and α2 are less than 1 and satisfy α1≤α2; σ is the threshold parameter; fal(e1,α1,σ) is the nonlinear combination of e1 using the fal(ε,α,σ) function; is the nonlinear combination of e2 using the fal function.

6. The flexible DC distribution network voltage control system based on the active disturbance rejection algorithm according to claim 1 is characterized in that: The controlled object, differential link, linear expansion observer and nonlinear feedback link are combined to form an automatic disturbance rejection control system for the flexible DC distribution network; The command signal r is input into the differential link, the output of the differential link is subtracted from the state estimate of the linear expansion observer, and the obtained deviation value is input into the nonlinear feedback link output u0; u0 minus the disturbance estimate Z3, and after 1 / b0 gain, the control rate u is obtained; In the above formula: u is the input of the state space model of the controlled object, and it is also the output of the anti-disturbance control system. b0 is the control input gain of the state space model of the controlled object. u0 minus the disturbance estimate Z3 and then divided by the control input gain can be used as the control variable input into the controlled system model.

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