Nine-switch UPQC and distributed wind power integrated system

Abstract

The present invention relates to the field of wind power generation technology, and specifically to a nine-switch UPQC and distributed wind power integrated system. The nine-switch UPQC and distributed wind power integrated system comprises: a nine-switch UPQC, a permanent magnet direct-drive wind turbine generator, and a box transformer; the wiring mode of the box transformer is Dyn11; the current compensation unit of the UPQC is an active power filter (APF), and the voltage compensation unit is a dynamic voltage restorer (DVR); the power quality and voltage fault ride-through capability of the distributed wind turbine are improved by connecting the voltage compensation unit in series and the current compensation unit in parallel.

Description

Technical field:

[0001] The present invention relates to the technical field of wind power generation, and in particular to a nine-switch UPQC and distributed wind power integrated system. Background technology:

[0002] Distributed wind power has the advantages of multi-point access, local consumption, and relieving the power shortage in the load center. Connecting distributed wind power to the distribution network according to local conditions can make full use of wind resources and effectively solve the problem of wind power abandonment and power restriction. However, the high penetration rate of distributed wind power access will affect the stable operation and power quality of the distribution network. Therefore, it is of great significance to study the power quality management of distributed wind power access to the distribution network.

[0003] Distributed wind power construction is close to the load, the voltage level of the distribution network is low, and the grid structure is relatively weak. Due to the randomness and volatility of wind power and even grid failures, it will lead to power quality problems such as voltage increase, drop, imbalance, harmonics, etc. in the distribution network. Distributed wind turbines with high wind power penetration power require higher fault ride-through capabilities during operation. Direct-drive permanent magnet synchronous wind turbines have the characteristics of long life, high reliability and high power generation efficiency in low wind speed scenarios. Therefore, permanent magnet direct-drive wind turbines are suitable for distributed wind power generation in low wind speed scenarios.

[0004] Stable control of DC bus voltage is one of the important links to ensure the normal operation and performance of the converter. The traditional proportional-integral controller has disadvantages such as overshoot, slow response speed and large static error. Autodisturbance rejection control (ADRC) is a control method based on process error to reduce the overall control error. It has the characteristics of high control accuracy, strong anti-disturbance ability and fast response speed. Therefore, the use of autodisturbance rejection control of DC bus voltage can effectively improve the working performance of the converter.

[0005] The nine-switch converter is developed from the traditional back-to-back converter, with the number of switches reduced by 25% and a more compact topology. The nine-switch converter can independently connect a single DC bus to power two AC loads, such as electric vehicle traction, or transmit power from one AC system to another through the DC bus, such as uninterruptible power supply, distributed generation, power conditioner, etc. Summary of the invention:

[0006] In order to improve the power quality of distributed wind power connected to 10kV distribution network under low wind speed and enhance the anti-interference ability of the system, this paper proposes a nine-switch UPQC suitable for 10kV distribution network to improve the operating performance of the distributed wind power grid-connected system. The specific technical solution is as follows:

[0007] The integrated system of nine-switch UPQC and distributed wind power includes: nine-switch UPQC, permanent magnet direct-drive wind turbine generator, and box-type transformer;

[0008] The wiring mode of the box-type transformer is Dyn11;

[0009] The current compensation unit of the UPQC is an active power filter (APF), and the voltage compensation unit is a dynamic voltage restorer (DVR); the power quality and voltage fault ride-through capability of the distributed wind turbine are improved by connecting the voltage compensation unit in series and the current compensation unit in parallel;

[0010] The nine-switch UPQC uses SPWM modulation to ensure that the amplitude of the modulation reference signal does not exceed the carrier U X Amplitude and meet the upper channel modulation reference signal u rA Not less than the lower channel modulation reference signal u rU The method used is to add a DC bias M to the up and down modulation reference signals. APF and M DVR , when the voltage drop is less than 20% of the rated value, configure M DVR =0.5, M APF =-0.5; When the voltage drop is greater than or equal to 20% of the rated value, M APF =-1, M DVR =0, the current compensation unit is not put into operation;

[0011] The nine-switch UPQC current compensation unit APF adopts the DC side voltage outer loop LADRC control and current feedback PR closed-loop control; the voltage compensation unit DVR adopts the outer loop voltage and inner loop current double closed-loop control combined with feedforward control, and adopts proportional resonance (PR) controller;

[0012] The DC side voltage auto-disturbance rejection control adopts the linear auto-disturbance rejection control method, which consists of a tracking differential unit (TD), an extended state observer (ESO) and a state error feedback control rate.

[0013] The expression of the linear tracking differential unit is:

[0014]

[0015] Where U dc2 For U dc1 The generalized derivative of; the larger k0 is, the faster the tracking speed is;

[0016] The expression of the linear extended state observer is:

[0017]

[0018] In the formula, Z1 and Z2 are the estimated value of the DC side voltage state and the estimated value of the total disturbance w(t), respectively; k1 and k2 are adjustable parameters of the control; b1 is the feedback coefficient of the control quantity;

[0019] The expression of linear state error feedback control rate is:

[0020]

[0021] Where k3 is the adjustable parameter of control.

[0022] This patent proposes a nine-switch UPQC for 10kV distribution network to improve the operating characteristics of distributed wind power systems. The following conclusions are drawn through simulation analysis:

[0023] Through simulation and comparative analysis of PI control, nonlinear anti-disturbance control and linear anti-disturbance control on the DC side voltage of the wind turbine grid-side converter, it is found that the linear anti-disturbance control has strong anti-disturbance capability, no overshoot and few adjustment parameters;

[0024] The nine-switch UPQC coordinated control strategy based on linear active disturbance rejection control can effectively improve the operating characteristics of the distributed wind power system under various grid voltage fault conditions and harmonic conditions at low wind speeds, achieve flexible voltage fault ride-through, and effectively improve the system's anti-disturbance capability. Description of the drawings:

[0025] Figure 1 It is a topological structure diagram of the nine-switch UPQC and distributed wind power integrated system in the embodiment.

[0026] Figure 2 It is the main circuit model of the current compensation side of the nine-switch UPQC.

[0027] Figure 3 It is the basic structure of DC side voltage self-disturbance rejection control.

[0028] Figure 4a It is the equivalent circuit of the nine-switch UPQC voltage and current compensation unit.

[0029] Figure 4b It is the DC side equivalent circuit of the nine-switch UPQC converter.

[0030] Figure 5 It is the SPWM modulation method of the nine-switch converter.

[0031] Figure 6 This is the block diagram of nine-switch UPQC coordinated control.

[0032] Figure 7 It is the control block diagram of the grid-side converter of the wind power system.

[0033] Figure 8It is a simulation analysis of different controls of grid-side converters in wind power systems.

[0034] Figure 9a The vertical axis directions of each waveform in the sequence are: U pcc,abc Grid voltage, U DVR,abc Compensation voltage, U g,abc Wind turbine terminal voltage, U dc1 Wind turbine DC voltage, U dc2 DC voltage of the nine-switch converter. The horizontal axis is time and the scale is the same.

[0035] Figure 9b The vertical axis directions of each waveform in the sequence are: I pcc,abc Grid current, I g,abc Wind turbine terminal current, P grid \Q grid Active and reactive power sent to the grid, P Crowbar Unloading circuit power, P g \Q g Active and reactive power of wind turbines. The horizontal axis is time and the proportions are the same.

[0036] Fig.10a The vertical axis directions of each waveform in the sequence are: U pcc,abc Grid voltage, U DVR,abc Compensation voltage, U g,abc Wind turbine terminal voltage, U dc1 Wind turbine DC voltage, U dc2 DC voltage of the nine-switch converter. The horizontal axis is time and the scale is the same.

[0037] Fig.10b The vertical axis directions of each waveform in the sequence are: I pcc,abc Grid current, I g,abc Wind turbine terminal current, P grid \Q grid Active and reactive power sent to the grid, P g \Q g Active and reactive power of wind turbines. The horizontal axis is time and the proportions are the same.

[0038] Fig.11a The vertical axis directions of each waveform in the sequence are: U pcc,abc Grid voltage, U DVR,abc Compensation voltage, U g,abc Wind turbine terminal voltage, U dc1 Wind turbine DC voltage, U dc2 DC voltage of the nine-switch converter. The horizontal axis is time and the scale is the same.

[0039] Fig.11bThe vertical axis directions of each waveform in the sequence are: I pcc,abc Grid current, I g,abc Wind turbine terminal current, P grid \Q grid Active and reactive power sent to the grid, P g \Q g Active and reactive power of wind turbines. The horizontal axis is time and the proportions are the same. Specific implementation method:

[0040] Example:

[0041] Nine-switch UPQC and distributed wind power integrated system Figure 1 The system consists of nine UPQC switches, five permanent magnet direct-drive wind turbines and a box transformer; the box transformer connection method is Dyn11; the wind turbine capacity is 2MW and the rated wind speed is 9m / s.

[0042] The nine-switch converter is evolved from the back-to-back converter, and each phase bridge arm has three switch IGBTs. This patent uses the nine-switch converter to realize the UPQC function. The current compensation unit is the active power filter APF, and the voltage compensation unit is the dynamic voltage restorer DVR. The power quality and voltage fault ride-through capability of the distributed wind turbine are improved through series voltage compensation and parallel current compensation.

[0043] Nine-switch UPQC current compensation side converter topology and mathematical model:

[0044] The topology and mathematical model of the nine-switch UPQC current compensation side converter are as follows: Figure 2 As shown. Among them, Rc is the equivalent resistance of the DC side, L is the connection inductance between the grid and the converter, C is the DC side capacitance, Udc is the DC side voltage, Usa, Usb, and Usc are the three-phase AC power supply voltages, Ia, Ib, and Ic are the three-phase AC currents, and Ua, Ub, and Uc are the output phase-to-phase voltages of the converter ports.

[0045] U a , U b , U c The switch function is expressed as:

[0046] U i =S i U dc ,i=a,b,c

[0047] According to the main circuit model, the KVL loop equation is obtained as follows:

[0048]

[0049] By Park transformation we can get:

[0050]

[0051] In the formula, i d 、i q are the d-axis and q-axis components of the three-phase AC side current respectively; U sd , U sq are the d-axis and q-axis components of the three-phase AC power supply voltage respectively; S d , S q is the switching function; ω is the grid angular frequency. The formula shows that when current compensation is in effect, the filter current output by the converter is affected by the DC side voltage U dc The influence of the DC side voltage U dc Stability is one of the important factors in the current compensation link.

[0052] The internal loss of the converter during operation is expressed as an equivalent resistance R c Instead, if the active power is balanced, the AC side active power P during stable operation ac With the DC side power P dc equal.

[0053] From the mathematical model, we can get:

[0054]

[0055] Combined, we get:

[0056]

[0057] by is the variable w, in the nine-switch UPQC current compensation, U sq =0,U s is the effective value of the AC voltage, so it can be deduced that:

[0058]

[0059] If you Then we get:

[0060]

[0061] During system operation, the ESO estimates and compensates for the total system disturbance w(t), which can suppress the fluctuation caused by the state change during the current compensation of the nine-switch UPQC.

[0062] Principle and design of first-order active disturbance rejection control of DC side voltage:

[0063] There are two control methods for DC side voltage auto-disturbance rejection control: nonlinear auto-disturbance rejection control and linear auto-disturbance rejection control. Both methods are composed of tracking differential unit TD, extended state observer ESO and state error feedback control rate. The basic structure is as follows: Figure 3 As shown, U is the reference value for the DC side voltage; dc0 For a given reference value Tracking signal; i d is the system control quantity; Z1 and Z2 are the observed values ​​of output and input respectively; b0 is the controller gain.

[0064] The expression of the nonlinear tracking differential unit is:

[0065]

[0066] In the formula, k0 is the adjustable parameter of control;

[0067] fal(e,α,δ) is a nonlinear function, and its expression is

[0068]

[0069] sign(x) is the sign function;

[0070] The expression of the linear tracking differential unit is:

[0071]

[0072] Where U dc2 For U dc1 The generalized derivative of ; the larger the k0, the faster the tracking speed.

[0073] The expression of the nonlinear extended state observer is:

[0074]

[0075] The expression of the linear extended state observer is:

[0076]

[0077] Where Z1 and Z2 are the estimated value of the DC side voltage state and the estimated value of the total disturbance w(t), respectively; k1 and k2 are adjustable parameters of the control; and b1 is the feedback coefficient of the control quantity.

[0078] The expression of nonlinear state error feedback control rate is:

[0079]

[0080] The expression of linear state error feedback control rate is:

[0081]

[0082] Where k3 is the adjustable parameter of control.

[0083] Mathematical model of nine-switch UPQC:

[0084] Without considering the internal impedance of the grid and assuming that the injection transformer is ideal, the parameters of the injection transformer are converted to the secondary side. When the transformation ratio n = 1, the single-phase equivalent operation circuit of the nine-switch UPQC is as follows: Figure 4a , Figure 4b As shown. Among them, u1 is the output voltage of the voltage compensation side; u2 is the output voltage of the current compensation side; l1 、u l2 are the filter inductor voltages on the voltage compensation side and the current compensation side, i l1 、i APF are their currents; u C is the voltage compensation side filter capacitor voltage, i C Its current; u g is the coupling point voltage of the distributed wind turbine, i g Its current; u pcc is the grid connection point voltage, i pcc Its current; i DVR is the current flowing from the DC side to the voltage compensation unit; u dc is the DC side voltage, i dc for its current.

[0085] According to the equivalent circuit and Kirchhoff's law, the mathematical model of the voltage compensation unit is:

[0086]

[0087] The mathematical model of the DC side and the current compensation unit is:

[0088]

[0089] Common connection point voltage u pcc When voltage distortion occurs, the voltage compensation unit DVR injects compensation voltage into the power grid in real time, so that the voltage at the coupling point of the distributed wind turbine group remains basically constant; when harmonic current is detected in the distributed wind turbine group, the current compensation unit APF acts to compensate the current; the voltage and current compensation unit of the nine-switch UPQC can improve the power quality of the distributed wind power grid-connected system and enhance the system's fault ride-through capability.

[0090] Nine-switch UPQC modulation mode:

[0091] The nine-switch UPQC uses SPWM modulation to ensure that the amplitude of the modulation reference signal does not exceed the carrier U XAmplitude and meet the upper channel modulation reference signal u rA Not less than the lower channel modulation reference signal u rU The method used is to add a DC bias M to the up and down modulation reference signals. APF and M DVR , the modulation method of the nine-switch converter is as follows Figure 5 shown.

[0092] This paper adopts the dynamic modulation ratio to improve the DC voltage utilization of the nine-switch converter. When the voltage drop is less than 20% of the rated value, the M DVR =0.5, M APF =-0.5; When the voltage drop is greater than or equal to 20% of the rated value, M APF =-1, M DVR =0, the current compensation unit is not put into operation.

[0093] Nine-switch UPQC coordinated control strategy:

[0094] The nine-switch UPQC coordinated control structure is shown in the figure below: Figure 6 As shown. The nine-switch UPQC current compensation unit APF adopts the DC side voltage outer loop LADRC control and current feedback PR closed-loop control; the voltage compensation unit DVR adopts the outer loop voltage and inner loop current double closed-loop control combined with feedforward control, and adopts the proportional resonant PR controller, which can effectively suppress the symmetrical and asymmetrical faults of the grid voltage, and at the same time improve the stability and anti-interference ability of the voltage compensation unit and reduce its steady-state error. The purpose of adding feedforward control is to enhance the system's anti-interference ability and stability and improve the system's dynamic performance.

[0095] Coordinated control strategy of grid-side converter in distributed wind power system:

[0096] The mathematical model of the grid-side converter of the distributed wind power system is consistent with the nine-switch UPQC current compensation side converter, which adopts the DC side voltage outer loop ADRC control and current feedback PI closed loop control. The simulation system block diagram is shown in Figure 7 This paper conducts simulation and comparative analysis on the DC side voltage of the grid-side converter of distributed wind turbines using PI control, nonlinear active disturbance rejection control and active disturbance rejection control.

[0097] Simulation analysis:

[0098] In order to verify the proposed nine-switch UPQC and distributed wind power system coordinated control to achieve voltage and current harmonic control and fault ride-through, four typical working conditions of grid voltage mild surge and drop fault, severe drop fault, continuous ride-through fault and grid voltage harmonic are set for simulation verification, and the DC side voltage control of the grid-side converter of the distributed wind power system is simulated and compared. The distributed wind power system is set to consist of 5 permanent magnet direct-drive wind turbines with a capacity of 2MW and a rated wind speed of 9m / s. The DC side voltage of the back-to-back converter of the wind power system is 1.2KV, and the DC side voltage of the nine-switch UPQC is 17.4KV.

[0099] Simulation analysis of different control of DC side voltage of grid-side converter:

[0100] In order to verify that the active disturbance rejection control can improve the DC side voltage control effect of the grid-side converter of the distributed wind power system, PI control, nonlinear active disturbance rejection control and linear active disturbance rejection control are used for simulation analysis. The simulation results are shown in Figure 2. Figure 8 shown.

[0101] As can be seen from the figure, the DC side capacitor is pre-charged with 1000V. When PI control is used, the DC voltage has an overshoot of about 200V and stabilizes at 1200V at 0.12s, while the active disturbance rejection control stabilizes at 1200V at 0.07s without overshoot. Linear active disturbance rejection control and nonlinear active disturbance rejection control can achieve almost the same effect, but the number of parameters to be adjusted is greatly reduced. Therefore, this paper adopts linear active disturbance rejection control for the DC side voltage.

[0102] Simulation analysis of severe voltage drop conditions:

[0103] In order to verify that the nine-switch UPQC can improve the operating performance of distributed wind turbines under severe grid voltage drop conditions, a grid connection point voltage U pcc,abc A severe fault condition with a drop of 80% occurs between 0.3s and 0.925s. Under this condition, current compensation is disabled and the DC unloading circuit of the nine-switch converter is enabled. The simulation results are as follows: Figure 9a , Figure 9b shown.

[0104] Depend on Figure 9a It is known that the grid voltage drops to 20% during 0.3s-0.9s; during the grid voltage drop period, the DVR generates a compensation voltage to keep the wind turbine terminal voltage stable. dc1 There was a slight oscillation at the beginning and end of the fault, and it quickly recovered and stabilized at 1200V. The DC side voltage Udc2 of the nine-switch UPQC had an overvoltage of less than 5V at the moment of the grid voltage drop, and quickly returned to normal when the unloading circuit was turned on. The voltage overvoltage occurred at the moment of voltage recovery after the unloading circuit was cut off, and recovered to 17.4KV after about 27ms. Figure 9bIt is known that at the moment of grid voltage drop and recovery, the grid current and wind turbine current remain stable after a slight transient. During the fault, the distributed wind turbine generated 10MW of active power, of which about 2MW was sent to the grid and 8MW was consumed by the unloading circuit (Crowbar circuit). The wind turbine achieved ride-through operation under severe voltage fault conditions. The system uses traditional PI control and coordinated control for comparative analysis. It can be seen from the DC side voltage waveform of the nine-switch converter that at the moment of severe voltage drop and recovery, the DC side voltage overshoot of the nine-switch UPQC using coordinated control is reduced by about 50%. When the voltage at the grid connection point drops severely, the nine-switch UPQC effectively improves the severe asymmetric fault ride-through operation capability and anti-interference capability of the distributed wind turbine.

[0105] Simulation analysis of asymmetric voltage drop conditions:

[0106] In order to verify that the nine-switch UPQC and the distributed wind power system can achieve fault ride-through through coordinated control when the grid voltage experiences asymmetric surges and asymmetric drops, the system grid-connected point voltage is designed to experience a slight asymmetric surge at 0.3-0.8s, in which phase A surges to 130%, phase B surges to 120%, and phase C surges to 110%; a slight asymmetric drop occurs at 1.0-1.3s, in which phase A drops to 70%, phase B drops to 80%, and phase C drops to 90%. Under this condition, current compensation is enabled and the DC unloading circuit of the nine-switch converter is disabled. The simulation results are as follows: Fig.10a , Fig.10b shown.

[0107] Depend on Fig.10a It is known that the grid voltage asymmetrically increases to 130% at 0.3s-0.8s, and asymmetrically drops to 70% at 1.0s-1.3s; during the grid voltage drop, the DVR generates a compensation voltage to keep the wind turbine terminal voltage stable. dc1 Due to the voltage asymmetry fault, the double frequency fluctuation occurs, and the DC side voltage U dc2 Slightly increased. Fig.10b It is known that the amplitude change of the grid current is opposite to the grid voltage change, which keeps the grid power stable. At the moment of grid voltage drop and recovery, the wind turbine current remains stable after a slight transient. g 、Reactive Q g And the grid-connected active power P grid 、Reactive Q grid The wind turbine generator system has achieved ride-through operation under asymmetric voltage rise and drop fault conditions.

[0108] Voltage symmetry fault condition simulation analysis:

[0109] In order to verify that the nine-switch UPQC improves the symmetrical fault ride-through performance of distributed wind turbines when a symmetrical voltage fault occurs in the grid voltage, the grid connection point voltage is designed to drop to 0.3pu within 0.3~0.5s and rise to 1.3pu within 0.7s~0.9s. Under this condition, current compensation is enabled and the DC unloading circuit of the nine-switch converter is disabled. The system simulation results are as follows Fig.11a , Fig.11b shown.

[0110] Depend on Fig.11a It is known that the grid voltage drops symmetrically to 70% at 0.3s-0.5s, and rises symmetrically to 130% at 0.7s-0.9s; during the grid voltage drop, the DVR generates a compensation voltage to keep the voltage at the wind turbine terminal stable. The DC side voltage Udc1 of the wind turbine oscillates slightly at the beginning and end of the fault, and quickly recovers to 1200V. The DC side voltage Udc2 of the nine-switch UPQC changes slightly during the grid voltage fault. Fig.11b It is known that the amplitude change of the grid current is opposite to the grid voltage change, which keeps the grid power stable. At the moment of grid voltage drop and recovery, the wind turbine current remains stable after a slight transient. g 、Reactive Q g And the grid-connected active power P grid 、Reactive Q grid The wind turbine generator system has achieved ride-through operation under asymmetric voltage rise and drop fault conditions.

Claims

1. Nine-switch UPQC and distributed wind power integrated system, characterized in that: include: Nine switch UPQC, permanent magnet direct drive wind turbine, box transformer; The wiring mode of the box-type transformer is Dyn11; The current compensation unit of the UPQC is an active power filter (APF), and the voltage compensation unit is a dynamic voltage restorer (DVR); the power quality and voltage fault ride-through capability of the distributed wind turbine are improved by connecting the voltage compensation unit in series and the current compensation unit in parallel; The nine-switch UPQC uses SPWM modulation to ensure that the amplitude of the modulation reference signal does not exceed the carrier U X Amplitude and meet the upper channel modulation reference signal u rA Not less than the lower channel modulation reference signal u rU The method used is to add a DC bias M to the up and down modulation reference signals. APF and M DVR , when the voltage drop is less than 20% of the rated value, configure M DVR =0.5, M APF =-0.5; When the voltage drop is greater than or equal to 20% of the rated value, M APF =-1, M DVR =0, the current compensation unit is not put into operation; The nine-switch UPQC current compensation unit APF adopts the DC side voltage outer loop LADRC control and current feedback PR closed-loop control; the voltage compensation unit DVR adopts the outer loop voltage and inner loop current double closed-loop control combined with feedforward control, and adopts proportional resonance (PR) controller; The DC side voltage auto-disturbance rejection control adopts the linear auto-disturbance rejection control method, which consists of a tracking differential unit (TD), an extended state observer (ESO) and a state error feedback control rate. The expression of the linear tracking differential unit is: Where U dc2 For U dc1 The generalized derivative of ; U is the reference value for the DC side voltage; dc0 For a given reference value The tracking signal; the larger the k0, the faster the tracking speed; The expression of the linear extended state observer is: Where U dc is the DC side voltage, Z1 and Z2 are the state estimation value of the DC side voltage and the estimated value of the total disturbance w(t), respectively; k1 and k2 are the adjustable parameters of the control; b1 is the feedback coefficient of the control quantity; i d is the system control quantity; The expression of linear state error feedback control rate is: Where k3 is the adjustable parameter of control.

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