CN111193287A - Photovoltaic grid-connected control method and system based on wave trap and proportional resonant controller - Google Patents
Photovoltaic grid-connected control method and system based on wave trap and proportional resonant controller Download PDFInfo
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Abstract
The invention discloses a photovoltaic grid-connected control method and a system based on a wave trap and a proportional resonant controller, wherein the photovoltaic grid-connected control method comprises a maximum power point tracking model, a positive and negative sequence separation model, a current control model and an inverter control model with harmonic suppression, wherein the maximum power point tracking model calculates the starting or stopping of maximum power point tracking according to an input voltage amplitude signal; calculating positive and negative sequence components of the power grid voltage by using a positive and negative sequence separation model; the current control model calculates an active and reactive current reference value of the power grid according to the voltage amplitude of the power grid; the inverter control model comprises a voltage outer loop control model with a wave trap and a current inner loop model adopting a proportional resonant controller added with harmonic suppression control, and calculates an inverter output voltage reference value according to positive and negative sequence components of the grid voltage and a grid active and reactive current reference value. The invention solves the interference of harmonic components to the system under the asymmetric fault and improves the electric energy quality of the whole photovoltaic grid-connected system in the low-voltage ride-through process.
Description
Technical Field
The invention belongs to the technical field of power systems, and particularly relates to a photovoltaic grid-connected control method and system based on a wave trap and a proportional resonant controller.
Background
At present, the problems of world climate environment change and resource shortage are increasingly prominent, and new energy power generation modes such as solar energy and the like are widely used. How to establish a photovoltaic grid-connected model with the same operation characteristics as an actual system and research the low-voltage ride through characteristics under the fault condition has important significance for researching the safe and stable operation of a power grid.
For a long time, a great deal of experts and scholars carry out a great deal of research work on control strategies of inverters in photovoltaic grid-connected systems under fault conditions, and provide various strategies for improving the photovoltaic low-voltage ride-through performance, so that the direct-current side voltage fluctuation is mainly caused by the fact that the direct-current voltage has a second harmonic component, and the network side current distortion is mainly caused by the fact that the network voltage is unbalanced and a negative sequence component is generated. According to different control targets, a plurality of experts and scholars propose different control strategies, but only optimize partial power quality problems such as direct current side voltage or power distortion and the like, and the problems that the quality of output power of an inverter is low, the voltage fluctuation of the direct current side is large, grid connection power distortion is serious and the like cannot be comprehensively solved. Aiming at the current situation that the permeability of new energy resources such as photovoltaic and the like is increased, the quality of electric energy output by photovoltaic is improved, and the method plays a vital role in large-scale photovoltaic grid connection and safety and stability of a power grid.
When the grid fails and the voltage of the grid drops, the photovoltaic power generation system has certain capability of running without being disconnected from the grid, even provides certain reactive power to help the grid recover, so that the inverter needs to be controlled to provide emergency reactive support for the grid, and the capability of continuous running of the photovoltaic system is improved. In order to maintain the waveform of the output current of the inverter and ensure the quality of the output electric energy, the inverter must be ensured to stably operate by adopting a corresponding control strategy. Particularly during asymmetric faults, problems of direct-current side overvoltage, grid-connected power distortion and the like exist, the problems can cause power fluctuation of a power grid, and even the condition of imbalance of the power grid is further aggravated. Therefore, it is desirable to provide a photovoltaic grid-connected control method and system that is practical, has a harmonic suppression effect, and can improve the quality of output power during a fault.
Disclosure of Invention
The invention aims to solve the technical problem that in order to overcome the defects of the prior art, the invention provides the photovoltaic grid-connected control method and the system based on the wave trap and the proportional resonant controller so as to improve the low-voltage ride through capability of the photovoltaic grid-connected system.
The technical scheme provided by the invention is as follows:
a photovoltaic grid-connected control method based on a wave trap and a proportional resonant controller is controlled based on a photovoltaic grid-connected control system, wherein the control system comprises a positive and negative sequence separation model, a current control model and an inverter control model with harmonic suppression;
calculating positive and negative sequence components of the power grid voltage by using a positive and negative sequence separation model;
the current control model outputs and calculates the power grid active and reactive current reference value according to the power grid voltage amplitude and the voltage outer ring active current reference value;
the inverter control model comprises a voltage outer loop control model with a wave trap and a current inner loop model based on a proportional resonant controller; the voltage outer ring control model obtains the output of the voltage outer ring active current reference value; the current inner loop model calculates an inverter output voltage (namely, inverter alternating-current side voltage) reference value according to the positive and negative sequence components of the grid voltage and the grid active reactive current reference value;
the reference value of the output voltage of the inverter is output by an SVPWM (voltage vector control) pulse generation module to output an SVPWM driving signal to control the inverter.
Further, the positive and negative sequence separation model calculates an active reactive current reference value by the following steps:
step 1.1, Clark conversion is carried out on the three-phase power grid voltage to obtain a two-phase static coordinate system, namely the power grid voltage under an αβ coordinate system;
step 1.2, after the grid voltage under the αβ coordinate system is filtered by a trap, a synchronization signal is extracted through a second-order generalized integrator (SOGI), and then positive-sequence and negative-sequence components of the grid voltage under the αβ coordinate system are calculated by a positive-sequence and negative-sequence component calculating module (PNSC);
step 1.3, carrying out Park conversion on the positive sequence and negative sequence components of the voltage under the αβ coordinate system to obtain a two-phase rotating coordinate system, namely the positive sequence and negative sequence components of the grid voltage under the dq coordinate system.
Further, the formula for calculating the grid active and reactive current reference value by the current control model is as follows:
wherein,
a reactive current reference value of the power grid;
the active current reference value of the power grid is obtained; u shapegIs the per unit value of the voltage amplitude of the power grid; i isnRated current of the photovoltaic power generation system; i ismaxThe maximum current allowed to be output by the inverter; i isdrefOutputting the reference value of the active current of the voltage outer ring; th1And Th2Two thresholds for controlling the magnitude of the reactive current reference, where Th2Is a judgment value of whether voltage drop occurs or not, Ug≤Th2Namely, the grid voltage drop is judged.
Further, the calculation formula of the voltage outer loop active current reference value output is as follows:
wherein, KPUAnd KIUProportional coefficients and integral coefficients of the voltage outer ring PI controller are respectively; u shapedcIs a dc bus voltage;
setting a direct current bus voltage value; p*Is the instantaneous power of the direct current side; s is a complex variable; g(s) is the transfer function of the wave trap.
Further, the current inner loop model is:
wherein, Ud、UqOutputting reference values of d and q axis components of the voltage for the inverter; u shaped +、Ud –、Uq +、Uq –The positive sequence components and the negative sequence components of the d and q axes of the grid voltage are obtained; gP′R(s) is the transfer function of the proportional resonant controller;
the reference values of active and reactive current of the power grid are obtained; i.e. id、iqOutputting current d and q axis components for the inverter; l is1、L2Is the inductance in the cross-decoupling term; and omega is the angular frequency of the grid voltage.
Further, the proportional resonance controller adopts a quasi-PR controller added with harmonic suppression control, and the transfer function of the quasi-PR controller is as follows:
wherein, KPIAnd KRIProportional coefficient and resonance coefficient respectively; kRhIs the h-order resonance coefficient; h is the harmonic frequency; omega0The angular frequency is the power frequency; omegacIs the fundamental cutoff frequency; omegachIs the h harmonic cut-off frequency.
Furthermore, the photovoltaic array is connected with a power grid through a booster circuit and an inverter in sequence; if the voltage drop of the power grid is not detected, MPPT (maximum power point tracking) control is carried out on the photovoltaic array, namely a PWM (pulse-width modulation) wave generated by the MPPT drives a booster circuit of the photovoltaic array to maintain the maximum power output of the photovoltaic array; and when the voltage drop of the power grid is detected, the MPPT control is disconnected, and the difference value between the real-time power output by the photovoltaic array and the maximum active power output by the photovoltaic array passes through the PI controller and then drives the booster circuit of the photovoltaic array through the PWM wave generated by the PWM pulse generation module.
A photovoltaic grid-connected control system based on a wave trap and a proportional resonant controller comprises a positive-negative sequence separation model, a current control model and an inverter control model with harmonic suppression;
calculating positive and negative sequence components of the power grid voltage by using a positive and negative sequence separation model;
the current control model outputs and calculates the power grid active and reactive current reference value according to the power grid voltage amplitude and the voltage outer ring active current reference value;
the inverter control model comprises a voltage outer loop control model with a wave trap and a current inner loop model based on a proportional resonant controller; the voltage outer ring control model obtains the output of the voltage outer ring active current reference value; the current inner loop model calculates an inverter output voltage reference value according to the positive and negative sequence components of the power grid voltage and the power grid active and reactive current reference value;
and the reference value of the output voltage of the inverter outputs an SVPWM driving signal to control the inverter through an SVPWM pulse generation module.
Further, the photovoltaic grid-connected control system also comprises a maximum power point tracking model, if the voltage drop of the power grid is not detected, MPPT control is carried out on the photovoltaic array based on the maximum power point tracking model, namely PWM waves generated by the maximum power point tracking model drive a booster circuit of the photovoltaic array, and the maximum power output of the photovoltaic array is maintained; and when the voltage drop of the power grid is detected, the MPPT control is disconnected, and the difference value between the real-time power output by the photovoltaic array and the maximum active power output by the photovoltaic array passes through the PI controller and then drives the booster circuit of the photovoltaic array through the PWM wave generated by the PWM pulse generation module.
The working principle of each module in the photovoltaic grid-connected control system is the same as that in the photovoltaic grid-connected control method.
Has the advantages that:
(1) on the basis of an actual photovoltaic grid-connected control method, the low-voltage ride-through control link is perfected, the problems of direct-current side voltage fluctuation, grid-connected power distortion and the like during an asymmetric fault are effectively solved, the interference of harmonic components to the system under the asymmetric fault is effectively solved, the electric energy quality of the whole photovoltaic grid-connected system in the low-voltage ride-through process is improved, and reactive support can be provided for a power grid. The low-voltage ride through capability of the photovoltaic grid-connected system is improved, and the operation safety and stability of the photovoltaic grid-connected system are improved;
(2) the model of each control link can more accurately reflect the real state of the equipment; the modeling process is quick, efficient and accurate, the parameters are easy to obtain, and the practicability is high;
(3) the maximum power point tracking model adopts different control strategies aiming at two working conditions of normal operation and fault operation, so that the photovoltaic power generation system can regulate the output power more quickly and effectively and reduce the voltage fluctuation of a direct-current bus;
(4) the current control model adopts different current reference value calculation methods aiming at two working conditions of fault occurrence and fault recovery to assist the power grid to stably transit between the fault working condition and the normal working condition;
(5) the inverter control model with the harmonic suppression combines the wave trap with the proportional resonance controller, effectively suppresses harmonic components in the network side current, eliminates negative sequence voltage components under asymmetric faults through a current inner loop control strategy of negative sequence voltage feedforward, and effectively solves the problems of voltage fluctuation of a direct-current bus and distortion of output power.
Drawings
The invention is further illustrated with reference to the following figures and examples.
FIG. 1 is a block diagram of an overall model for controlling a photovoltaic grid-connected inverter in an embodiment of the invention;
FIG. 2 is a flowchart of a control method according to an embodiment of the present invention
Fig. 3 is a block diagram of a maximum power point tracking model;
FIG. 4 is a block diagram of a positive and negative sequence separation model;
FIG. 5 is a characteristic curve of the trap for amplitude, frequency and phase frequency;
FIG. 6 is a block diagram of a current control model during a fault;
FIG. 7 is a plot of amplitude, frequency, phase and frequency characteristics of the transfer function of the proportional resonant controller;
FIG. 8 is a block diagram of a voltage outer loop model of a band trap;
FIG. 9 is a block diagram of a current inner loop model based on a proportional resonant controller;
FIG. 10 shows the operating characteristics and waveforms of the grid-connected point A phase voltage drop condition; wherein fig. 10(a) is an alternating-current side three-phase voltage; fig. 10(b) shows three-phase current on the ac side; FIG. 10(c) is a graph of DC bus side voltage when a conventional PI control strategy is employed; FIG. 10(d) is a DC bus side voltage when using a prior art control strategy; FIG. 10(e) is a plot of inverter output power when a conventional PI control strategy is employed; FIG. 10(f) is a plot of inverter output power when the control strategy of the present invention is employed; FIG. 10(g) is a graph of net side current harmonic distortion using a conventional PI control strategy; FIG. 10(h) is a graph of the net side current harmonic distortion rate using the control strategy of the present invention;
FIG. 11 shows the operating characteristics and waveforms of the grid-connected point under the condition of simultaneous dropping of the phase B and phase C voltages; wherein fig. 11(a) is an alternating side three-phase voltage; fig. 11(b) shows three-phase current on the ac side; FIG. 11(c) is a DC bus voltage when a conventional PI control strategy is employed; FIG. 11(d) is a DC bus voltage when using a prior art control strategy; FIG. 11(e) is a plot of inverter output power when a conventional PI control strategy is employed; FIG. 11(f) is a plot of inverter output power when the control strategy of the present invention is employed; FIG. 11(g) is a graph of net side current harmonic distortion using a conventional PI control strategy; fig. 11(h) shows the net side current harmonic distortion rate using the control strategy of the present invention.
Detailed Description
The present invention will be further described in detail with reference to the drawings and specific examples.
Example 1:
as shown in fig. 1, the embodiment discloses a photovoltaic grid-connected control system based on a wave trap and a proportional resonant controller, which includes a positive-negative sequence separation model, a current control model and an inverter control model with harmonic suppression;
the output ends of the positive and negative sequence separation model and the current control model are connected with the input end of the inverter control model with harmonic suppression;
the positive and negative sequence separation model calculates positive and negative sequence components of the grid voltage (namely grid-connected point voltage of the photovoltaic power generation system, which is a measured value);
the input end of the current control model is connected with the voltage amplitude signal output end; the current control model outputs and calculates the power grid active and reactive current reference value according to the power grid voltage amplitude and the voltage outer ring active current reference value;
the inverter control model comprises a voltage outer loop control model with a wave trap and a current inner loop model based on a proportional resonant controller; the voltage outer ring control model obtains the output of the voltage outer ring active current reference value; the current inner loop model calculates an inverter output voltage reference value according to the positive and negative sequence components of the power grid voltage and the power grid active and reactive current reference value;
and the reference value of the output voltage of the inverter outputs an SVPWM driving signal to control the inverter through an SVPWM pulse generation module.
Example 2:
in this embodiment, on the basis of embodiment 1, the photovoltaic grid-connected control system further includes a maximum power point tracking model; the photovoltaic array is connected with a power grid through a booster circuit and an inverter in sequence; the output port of the maximum power point tracking model is connected with a Boost circuit (Boost circuit) of the photovoltaic array;
if the voltage drop of the power grid is not detected, an active power priority control mode is adopted and based on the maximum powerThe point tracking model carries out MPPT control on the photovoltaic array, namely PWM waves generated by the maximum power point tracking model drive a booster circuit of the photovoltaic array to maintain the maximum power output of the photovoltaic array; obtaining an active current reference value I through a voltage outer ringdrefReference value of reactive current IqrefSet to zero, the reference value U of the output voltage of the inverter output by the current inner loopd、UqAn SVPWM (voltage vector control) pulse generation module outputs an SVPWM driving signal to control an inverter, so that the inverter only transmits active power to a power grid, and the unit power factor operation of a system is realized;
when the grid voltage drop (the grid voltage drops due to grid faults, namely the grid voltage is abnormal) is detected, the grid voltage is converted into a reactive power priority control mode, MPPT control is disconnected, the difference value between the real-time power output by the photovoltaic array and the maximum active power output by the photovoltaic array passes through the PI controller, and then drives a booster circuit of the photovoltaic array through PWM waves generated by the PWM pulse generation module, so that the power input into the grid by the photovoltaic array is adjusted, and the input end and the output end of the inverter quickly reach power balance in the low-voltage ride-through process. When the voltage of a power grid drops, the photovoltaic power generation system keeps running without being disconnected for a period of time according to grid connection requirements to continuously transmit active power to the power grid, so that the output current of the grid side can be increased rapidly, in order to prevent the output current of an inverter from overflowing during a fault period, the active current also has to be correspondingly controlled during a low-voltage ride-through period, the photovoltaic power generation system preferentially generates reactive power during the fault period, the residual current and the active current during a steady-state period are reduced to generate active power, and the d-axis component i of the grid side current during the fault period is ensured to generate thedNo overcurrent. And new active and reactive current reference values are obtained through the current control model, and active power and reactive power input into the power grid can be redistributed.
The method for judging whether the voltage of the power grid drops comprises the following steps: calculating a grid voltage amplitude signal U through a grid voltage amplitude calculation moduleg(ii) a The input ends of the maximum power point tracking model and the current control model are connected with the output end of the power grid voltage amplitude calculation module to obtain a per unit value U of the power grid voltage amplitude signalgIf U is presentg≤Th2If the grid voltage drops, the system determines that the grid voltage drops, and sets Th in this embodiment2＝0.9。
Example 3:
as shown in fig. 2, this embodiment discloses a photovoltaic grid-connected control method based on a wave trap and a proportional resonant controller, the control is performed based on a photovoltaic grid-connected control system, and in order to ensure the waveform of the output current of the inverter and the quality of the output electric energy, a voltage outer loop eliminates a voltage U at a direct current side by adding the wave trap to a fed-back direct current voltagedcAnd the intermediate second harmonic component feeds forward the instantaneous current on the direct current side in order to further reduce the voltage fluctuation of the direct current bus. Aiming at the power grid asymmetric fault (under the condition of unbalanced/asymmetric drop of the power grid voltage), the current inner loop model controls the negative sequence current component of the inverter to be zero so as to effectively inhibit the negative sequence component in the grid-connected current and ensure that the grid-connected current has good electric energy quality. Besides fundamental wave components, the grid-side current also contains a large amount of 3-order, 5-order, 7-order and other low-order harmonics, and a proportional resonance controller is adopted to inhibit the low-order harmonics, so that good electric energy quality of the grid-connected current is ensured. The specific working principle of each module in this example is as follows:
1.1 maximum power point tracking model
The maximum power point tracking model is shown in fig. 3, and adopts a smooth switching control strategy to calculate the starting or stopping of maximum power point tracking according to an input voltage amplitude signal; if the voltage drop of the power grid is not detected, carrying out MPPT (maximum power point tracking) control on the photovoltaic array to maintain the maximum power output of the photovoltaic array; when the voltage of a power grid is detected to drop, MPPT control is disconnected, the difference value between the output power of a photovoltaic array (instantaneous power output by the photovoltaic array) and the maximum active power output by the photovoltaic array passes through a PI controller, and generated PWM waves drive a Boost circuit of the photovoltaic array, so that the input end and the output end of an inverter rapidly reach power balance in the low-voltage ride-through process, and the voltage fluctuation of a direct-current bus is reduced. When the grid voltage is detected to be recovered to be normal, a stable value under the condition that the direct-current side bus voltage is normal is given to the MPPT reference voltage Udc_refSo as to realize the rapid tracking of the maximum power point,the MPPT dynamic and static response performance is improved, and the voltage of the direct current bus is enabled to realize stable transition.
1.2 Positive and negative sequence separation model
The voltage and current of the power grid are actually the sum of direct current and double frequency alternating current in a synchronous rotating coordinate system. The positive and negative sequence separation model is shown in fig. 4, wherein the positive sequence component is a direct current component, the negative sequence component is a double frequency alternating current relative to the positive sequence component, and if the double frequency component can be filtered, the fundamental frequency component can be used for feedback or feedforward of the regulator, so as to avoid causing system oscillation and overcurrent. The amplitude-frequency and phase-frequency characteristic curve of the wave trap is shown in fig. 5, and it can be seen from the amplitude-frequency characteristic curve of the wave trap that the second harmonic component of the voltage signal can be filtered through the wave trap, and because the dynamic response of the wave trap is superior to that of a low-pass filter, the positive-negative sequence separation model of the embodiment adopts the wave trap to filter the second harmonic component of the voltage, which is beneficial to the detection and control of the dynamic signal of the system.
The transfer function of the trap is:
in the formula: omeganRepresenting the harmonic frequency to be filtered by the characteristic angular frequency, and taking K to be 0.707; in the positive and negative sequence separation link, omeganTaking the angular frequency (100Hz) of the second harmonic of the voltage of the power grid; at the outer ring of voltage, ωnTaking the angular frequency of a double frequency component of the direct current bus voltage;
the mathematical derivation of the positive and negative sequence separation structure is as follows:
in the formula: u shapeabcIs the voltage of the power grid,
Ua、Uband UcFor the three-phase mains voltage instantaneous value,
respectively positive sequence component and negative sequence component separated from power network voltage,
wherein
Clark conversion is carried out on positive sequence and negative sequence components of the power grid voltage to obtain a two-phase static coordinate system, namely positive and negative sequence components of the voltage under αβ coordinate system
Comprises the following steps:
in the formula:
respectively, positive sequence and negative sequence components of the voltage in αβ coordinate system.
The formulas (4) and (5) are arranged to be related to UαβThe relation of (1):
in the formula:
q represents original letterThe signals are phase-shifted by 90,
therefore, the three-phase power grid voltage U can be firstly measuredabcClark conversion is carried out to obtain a two-phase static coordinate system, namely a power grid voltage U under αβ coordinate systemαβThen comparing the grid voltage U under αβ coordinate systemαβAfter being filtered by a trap filter, a second-order generalized integrator (SOGI) is used for extracting a synchronous signal, and a positive/negative-sequence component calculation module (PNSC) is used for calculating and obtaining a positive sequence component and a negative sequence component of the grid voltage under an αβ coordinate system
Finally, the positive sequence component and the negative sequence component of the voltage under the αβ coordinate system are paired
Performing Park conversion to obtain two-phase rotating coordinate system, namely, the positive sequence and negative sequence components of the grid voltage under the dq coordinate system
Wherein
Ud +、Ud -、Uq +、Uq -The positive sequence components and the negative sequence components of the d axis and the q axis of the grid voltage are respectively.
1.3 Current control model
The current control model is divided into a current reference value calculation link in a fault period as shown in fig. 6, and assists the power grid to smoothly transit between a fault working condition and a normal working condition. During the fault period, the photovoltaic power generation system must track the voltage change of the power grid in real time and output certain reactive current to the power grid to support the voltage of the fault point. When a power grid fails, the voltage of the power grid drops, the output current of the power grid side is increased sharply, in order to prevent the output current of an inverter from overflowing during the fault, active current must be controlled correspondingly during low-voltage ride-through, a photovoltaic power generation system preferentially sends reactive power during the fault, residual current and active current during a steady state are reduced to send active power, new active and reactive current reference values are obtained through an active and reactive current reference value calculating module, and active power and reactive power input into the power grid can be redistributed.
According to technical regulations for connecting photovoltaic power stations to an electric power system, a photovoltaic power generation system must track voltage changes of a power grid in real time during a fault period and output certain reactive current to the power grid to support voltage of a fault point, and reference values of active current and reactive current at the moment can be calculated according to equations (8) and (9):
in the formula: u shapegIs the per unit value of the voltage amplitude of the power grid; i isnIs the rated current of the photovoltaic power generation system,
a reactive current reference value of the power grid; th1And Th2Two thresholds for controlling the magnitude of the reactive current reference, where Th2Is a judgment value of whether voltage drop occurs or not, Ug≤Th2I.e. determining the grid voltage drop, Th is set in this embodiment10.2 and Th2＝0.9；
In the formula: i ismaxThe maximum current allowed to be output by the inverter is generally 1.1 times of rated current;
and the reference value is the active current reference value of the power grid.
1.4 inverter control model with harmonic suppression
(1) A voltage outer loop control model with a wave trap;
under the condition of asymmetric fault of a power grid, the direct-current bus voltage of the grid-connected inverter and the output power of the inverter have the following relation:
in the formula: p0For outputting an effective value of active power, P, for the inverterc2、Ps2Outputting cosine term amplitude and sine term amplitude of a double-frequency harmonic component of active power for the inverter respectively; u shapedcIs the dc bus voltage (i.e. the dc side voltage of the inverter, which is the measured value); c is the DC side capacitor of the inverter; pPVOutputting power for the photovoltaic array; omega is the power grid voltage angular frequency (the power grid voltage angular frequency acquired by the phase-locked loop in real time); t is time.
When the power grid fails and the voltage of the power grid drops asymmetrically, in order to ensure the waveform of the output current of the inverter and the quality of the output electric energy, the inverter needs to be ensured to operate stably by adopting a corresponding control strategy. Direct current bus voltage U fed back by voltage outer ring pairdcA trap is added to eliminate the second harmonic component of the DC bus voltage, and the instantaneous current on the DC side is fed forward to further reduce the voltage fluctuation of the DC bus. The output of the voltage outer ring active current reference value is characterized as follows:
in the formula: i.e. idrefOutputting the reference value of the active current of the voltage outer ring; kPUAnd KIUProportional coefficient and integral coefficient of voltage outer ring PI controller are calculated according to analytic method or obtained based on parameter setting method, in this embodiment, KPU＝6.7，KIU＝44.5；
Setting a direct current bus voltage value; p*/UdcFor instantaneous current feed-forward on the DC side, P*Instantaneous power (obtained by measuring the voltage and current of a direct current bus) on the direct current side of the inverter; s is a complex variable.
(2) Based on a current inner loop model of a proportional resonant controller.
In order to achieve the purpose, the separated negative sequence component U of the grid voltage is usedd –，Uq –Feedforward, feedback decoupling ω L from current state1id，ωL2iqPositive sequence component feedforward compensation U of power grid voltaged +，Uq +And combining to obtain the d and q axis component reference values of the alternating-current side voltage of the inverter. The gain of the inner ring PR controller at the resonance frequency is much larger than that of the PI controller, and the signal characteristic at the resonance frequency is easier to extract, so that the control of the steady-state error is realized. Since the grid side current contains a large amount of low harmonics of 3 rd order, 5 th order, 7 th order, etc. in addition to the fundamental component, the PR controller is used for harmonic suppression, and the amplitude-frequency-phase-frequency characteristic curve of the transfer function of the proportional resonance controller is shown in fig. 7. The transfer function of the PR controller with the harmonic suppression control added in the current inner loop is expressed as follows:
in the formula: kPIAnd KRIProportional coefficient and resonance coefficient of PR controller are calculated according to analytic method or obtained based on parameter setting method, in this embodiment, K isPI＝0.9，KRI＝10；ω0Is the power frequency angular frequency (50Hz), h omega0Is the resonant frequency of the PR controller, h is the harmonic order, KRhIs the h-order resonance coefficient; s is a complex variable.
In practice, due to the positive and negative deviations of the frequency or the uncertainty of the measurement sampling, a low-pass filter is usually used to replace a pure integration link in the PR controller, so that the anti-interference performance of the PR controller and the stability of the system are improved. The transfer function of a quasi-PR controller incorporating harmonic suppression control and its simplified transfer function are:
in the formula: omegacFor fundamental cutoff frequency, this example ωc＝3.14rad/s；ωchFor h-order harmonic cut-off frequency, the fluctuation range of the power grid frequency is considered to be +/-0.5 Hz (the fluctuation range of the h-order harmonic frequency is 0.5hHz), and omega is madech＝2π*0.5h＝πh，KRh＝113。
By adopting a proportional resonance controller and combining a control strategy of negative sequence voltage feedforward, a current inner ring model can be characterized as follows:
in the formula: u shaped、UqReference values of d and q axis components of the alternating-current side voltage of the inverter are obtained; i.e. id、iqThe components of the d and q axes of the alternating side current of the inverter are shown;
the reference values of active and reactive current of the power grid are obtained; l is1、L2For the inductance in the cross-decoupling term, L is an empirical parameter in this embodiment1＝L2＝0.185mH。
Based on equations (11) - (14), an inverter control outer loop model and an inverter control inner loop model with harmonic suppression can be established as shown in fig. 8 and 9.
2 model verification
2.1 simulation verification of photovoltaic grid-connected model
And establishing a 250KW photovoltaic grid-connected power generation system simulation model on a Matlab/Simulink simulation platform.
(1) Suppose that when the system stably operates to 0.1s, the voltage of the A phase drops, the voltage of the power grid drops to 0.4pu, and the fault is removed after 0.2 s. And carrying out comparative analysis on the PI control strategy under the same test condition. The operating characteristics of the system in this control mode are shown in fig. 10.
(2) Suppose that when the system stably runs to 0.1s, voltage drops occur in B, C two phases, the grid voltage drops to 0.4pu, and the fault is cut off after 0.2 s. And carrying out comparative analysis on the PI control strategy under the same test condition. The operating characteristics of the system in this control mode are shown in fig. 11.
From fig. 10(a), it can be seen that the voltage drop occurs in the phase a of the grid voltage within 0.1-0.2 s, and the grid voltage is unbalanced. It can be seen in fig. 10(b) that the output current is effectively limited to within 1.1pu, avoiding overcurrent impact; comparing and analyzing fig. 10(c) and fig. 10(d), it is found that the dc bus voltage has double frequency fluctuation under the conventional PI control strategy, the dc bus voltage is relatively stable and has small fluctuation under the combined action of the improved maximum power point tracking and the voltage outer loop, and the double frequency fluctuation is effectively suppressed; comparing and analyzing fig. 10(e) and fig. 10(f), it is found that the active power and the reactive power of the conventional PI control strategy fluctuate greatly when the voltage of the power grid is unbalanced, and the active power and the reactive power are relatively stable by using the control strategy of the present invention (the negative sequence voltage feedforward control strategy based on the wave trap and the proportional resonant controller), so as to achieve the purpose of control; the 3 rd harmonic in the grid side current of fig. 10(g) and 10(h) reaches 13.6%, while the grid-connected current 3, 5 and 7 th harmonic components in the control strategy of the invention are obviously reduced compared with the traditional PI control strategy, and the current harmonic distortion rate is controlled within the range of 5%.
From fig. 11(a), it can be known that voltage drop occurs when the phases B and C of the grid voltage are the same within 0.1-0.2 s, and the unbalance of the grid voltage is further increased. As can be seen from fig. 11(b), the amplitude of the three-phase current is limited within 1.1pu, and the three-phase current can be kept balanced; the comparison and analysis of fig. 11(c) and fig. 11(d) find that the dc bus voltage is rapidly increased at the moment of the power grid fault under the conventional PI control strategy, which threatens the safe and stable operation of the system. Under the combined action of improved maximum power point tracking and a voltage outer ring, the direct-current bus voltage is relatively stable, the system fault is immediately responded, and the overall fluctuation of the direct-current side bus voltage is small; fig. 11(e) and fig. 11(f) compare and analyze to find that the active power and the reactive power of the conventional PI control strategy fluctuate greatly when the voltage of the power grid is unbalanced, while the active power and the reactive power are relatively stable by using the control strategy of the present invention (the control strategy based on the negative sequence voltage feedforward of the trap and the proportional resonant controller), so as to achieve the purpose of control, fig. 11(g) and fig. 11(h) can find that the harmonic distortion rate of the grid-connected current is too large and the content of each harmonic is high under the conventional PI control strategy. 3-order harmonic in the grid-side current reaches 16%, and the grid-connected current 3, 5 and 7-order harmonic components in the control strategy are obviously reduced compared with the traditional PI control strategy, and the current harmonic distortion rate is controlled within a 5% range. When voltage drops, the photovoltaic array raises the voltage of a power grid by outputting certain reactive power, and the grid-connected control system has good low-voltage ride through performance. The overall control strategy improves the stability of the system and the quality of the output electric energy. The whole system is in stable transition before, during and after the fault occurs, and the dynamic control performance of the system is improved.
Claims (9)
1. A photovoltaic grid-connected control method based on a wave trap and a proportional resonant controller is characterized in that control is performed based on a photovoltaic grid-connected control system, and the control system comprises a positive-negative sequence separation model, a current control model and an inverter control model with harmonic suppression;
calculating positive and negative sequence components of the power grid voltage by using a positive and negative sequence separation model;
the current control model outputs and calculates the power grid active and reactive current reference value according to the power grid voltage amplitude and the voltage outer ring active current reference value;
the inverter control model comprises a voltage outer loop control model with a wave trap and a current inner loop model based on a proportional resonant controller; the voltage outer ring control model obtains the output of the voltage outer ring active current reference value; the current inner loop model calculates an inverter output voltage reference value according to the positive and negative sequence components of the power grid voltage and the power grid active and reactive current reference value;
and the reference value of the output voltage of the inverter outputs an SVPWM driving signal to control the inverter through an SVPWM pulse generation module.
2. The photovoltaic grid-connected control method based on the wave trap and the proportional resonant controller according to claim 1, wherein the positive and negative sequence separation model calculates an active reactive current reference value by the following steps:
step 1.1, Clark conversion is carried out on the three-phase power grid voltage to obtain a two-phase static coordinate system, namely the power grid voltage under an αβ coordinate system;
step 1.2, after the grid voltage under the αβ coordinate system is filtered by a trap, a synchronization signal is extracted through a second-order generalized integrator, and positive-sequence and negative-sequence components of the grid voltage under the αβ coordinate system are calculated by a positive-sequence and negative-sequence component calculating module;
step 1.3, carrying out Park conversion on the positive sequence and negative sequence components of the voltage under the αβ coordinate system to obtain a two-phase rotating coordinate system, namely the positive sequence and negative sequence components of the grid voltage under the dq coordinate system.
3. The photovoltaic grid-connected control method based on the wave trap and the proportional resonant controller according to claim 1, wherein the formula for calculating the grid active and reactive current reference value by the current control model is as follows:
wherein,
a reactive current reference value of the power grid;
the active current reference value of the power grid is obtained; u shapegIs the per unit value of the voltage amplitude of the power grid; i isnRated current of the photovoltaic power generation system; i ismaxThe maximum current allowed to be output by the inverter; i isdrefOutputting the reference value of the active current of the voltage outer ring; th1And Th2Two thresholds for controlling the magnitude of the reactive current reference, where Th2Is a judgment value of whether voltage drop occurs or not, Ug≤Th2Namely, the grid voltage drop is judged.
4. The grid-connected photovoltaic control method based on the wave trap and the proportional resonant controller according to claim 3, wherein the calculation formula of the voltage outer loop active current reference value output is as follows:
wherein, KPUAnd KIUProportional coefficients and integral coefficients of the voltage outer ring PI controller are respectively; u shapedcIs a dc bus voltage;
setting a direct current bus voltage value; p*Is the instantaneous power of the direct current side; s is a complex variable; g(s) is the transfer function of the wave trap.
5. The photovoltaic grid-connected control method based on the wave trap and the proportional resonant controller according to claim 1, wherein the current inner loop model is as follows:
wherein, Ud、UqOutputting reference values of d and q axis components of the voltage for the inverter; u shaped +、Ud –、Uq +、Uq –The positive sequence components and the negative sequence components of the d and q axes of the grid voltage are obtained; g'PR(s) is the transfer function of the proportional resonant controller;
the reference values of active and reactive current of the power grid are obtained; i.e. id、iqOutputting current d and q axis components for the inverter; l is1、L2Is the inductance in the cross-decoupling term; and omega is the angular frequency of the grid voltage.
6. The grid-connected photovoltaic control method based on the wave trap and the proportional resonant controller according to claim 5, wherein the proportional resonant controller adopts a quasi-PR controller with added harmonic suppression control, and the transfer function is as follows:
wherein, KPIAnd KRIProportional coefficient and resonance coefficient respectively; kRhIs the h-order resonance coefficient; h is the harmonic frequency; omega0The angular frequency is the power frequency; omegacIs the fundamental cutoff frequency; omegachIs the h harmonic cut-off frequency; s is a complex variable.
7. The photovoltaic grid-connected control method based on the wave trap and the proportional resonant controller according to any one of claims 1 to 6, characterized in that the photovoltaic array is connected with a power grid through a booster circuit and an inverter in sequence; if the voltage drop of the power grid is not detected, MPPT control is carried out on the photovoltaic array, namely a PWM wave generated by MPPT drives a booster circuit of the photovoltaic array to maintain the maximum power output of the photovoltaic array; and when the voltage drop of the power grid is detected, the MPPT control is disconnected, the difference value between the real-time power output by the photovoltaic array and the maximum active power output by the photovoltaic array passes through the PI controller, and then the PWM wave generated by the PWM pulse generation module drives the booster circuit of the photovoltaic array.
8. A photovoltaic grid-connected control system based on a wave trap and a proportional resonant controller is characterized by comprising a positive and negative sequence separation model, a current control model and an inverter control model with harmonic suppression;
calculating positive and negative sequence components of the power grid voltage by using a positive and negative sequence separation model;
the current control model outputs and calculates the power grid active and reactive current reference value according to the power grid voltage amplitude and the voltage outer ring active current reference value;
the inverter control model comprises a voltage outer loop control model with a wave trap and a current inner loop model based on a proportional resonant controller; the voltage outer ring control model obtains the output of the voltage outer ring active current reference value; the current inner loop model calculates an inverter output voltage reference value according to the positive and negative sequence components of the power grid voltage and the power grid active and reactive current reference value;
and the reference value of the output voltage of the inverter outputs an SVPWM driving signal to control the inverter through an SVPWM pulse generation module.
9. The photovoltaic grid-connected control system based on the wave trap and the proportional resonant controller according to claim 8, further comprising a maximum power point tracking model, wherein if the grid voltage drop is not detected, MPPT control is performed on the photovoltaic array based on the maximum power point tracking model, that is, a boost circuit of the photovoltaic array is driven by a PWM wave generated by the maximum power point tracking model to maintain maximum power output of the photovoltaic array; and when the voltage drop of the power grid is detected, the MPPT control is disconnected, and the difference value between the real-time power output by the photovoltaic array and the maximum active power output by the photovoltaic array passes through the PI controller and then drives the booster circuit of the photovoltaic array through the PWM wave generated by the PWM pulse generation module.
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