CN113725921B - Control method for participating in grid frequency modulation at left side of maximum power point in photovoltaic operation - Google Patents
Control method for participating in grid frequency modulation at left side of maximum power point in photovoltaic operation Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/381—Dispersed generators
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
- H02J2300/20—The dispersed energy generation being of renewable origin
- H02J2300/22—The renewable source being solar energy
- H02J2300/24—The renewable source being solar energy of photovoltaic origin
- H02J2300/26—The renewable source being solar energy of photovoltaic origin involving maximum power point tracking control for photovoltaic sources
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/50—Photovoltaic [PV] energy
- Y02E10/56—Power conversion systems, e.g. maximum power point trackers
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Abstract
Aiming at the problem that the frequency adjusting capability of a system is weakened due to the fact that the installed capacity of new energy power generation is continuously increased, the invention provides a frequency modulation control strategy of a single-stage photovoltaic inverter working in a mode of the left side of a maximum power point. The strategy utilizes a variable power tracking technology to enable the photovoltaic to always operate in the left area of the maximum power point, and the change of dP/dU values of the photovoltaic performance curve in the left area is small, so that the photovoltaic power can be adjusted more conveniently. The strategy enables the photovoltaic system to operate at a non-maximum power point under the condition that the frequency of the power grid is not disturbed, and the photovoltaic system can provide rising power support for the power grid through the standby part of photovoltaic power. Meanwhile, in order to be capable of adjusting the photovoltaic power more rapidly and accurately, the strategy is based on a variable power tracking evaluation function, and an optimal step length setting method of photovoltaic operation in a mode of the left side of the maximum power point is provided. The verification of the proposed control strategy shows that the proposed control strategy is feasible and effective.
Description
Technical Field
The invention relates to the field of control of photovoltaic power generation micro-grid inverters, in particular to a control method for enabling photovoltaic operation to participate in grid frequency modulation at the left side of a maximum power point.
Background
Photovoltaic (PV) power generation is very rapid in development as a new energy power generation, is more flexible in power generation form compared with other clean energy sources, can be applied to various social scenes, such as rural roofs, illumination street lamps, office building walls and the like, and has great application potential. Photovoltaic power generation typically employs a maximum power point tracking (Maximum power point tracking, MPPT) mode of operation in which the photovoltaic cells are operated at a maximum power point (Maximum power point, MPP) at all times. At present, besides the commonly used MPPT algorithm such as a conductivity increment method, a disturbance observation method and the like, a lot of existing researches provide an MPPT algorithm based on an intelligent algorithm aiming at the problems of illumination mutation, partial shadow and the like. Although the photovoltaic utilization rate is highest in the MPPT mode, the photovoltaic power generation amount in the MPPT mode is influenced by natural factors such as illumination, temperature and the like and is not controlled by people. And the photovoltaic power generation takes the power electronic equipment as an interface and does not have the characteristics of a traditional generator, and the photovoltaic power generation cannot provide support for a power grid when the frequency of the power grid is disturbed, so that a great amount of access of the photovoltaic power generation brings certain challenges to the stable operation of the power grid.
At present, some countries and regions have new regulations for photovoltaic power generation, and the photovoltaic power generation is required to have primary frequency modulation capability. There are also some countries requiring large photovoltaic power plants to be grid-scheduled, the photovoltaic power plants being adapted to output power according to scheduling instructions. Many students have made a great deal of research on the problem of the participation of the photovoltaic power supply in the frequency modulation of the power grid. At present, two methods for actively participating in power grid frequency modulation are mainly adopted.
The first method is to configure an energy storage device for the photovoltaic unit, and utilize the energy stored by the energy storage device to provide electric energy for the photovoltaic unit to participate in grid frequency modulation. The method has the advantage that when the frequency of the power grid is reduced, the energy of the energy storage device can be utilized to provide certain power support for the power grid. In addition, the power of the energy storage device can be regulated by the control strategy to smooth out the power fluctuation of the photovoltaic. However, the cost and life of the energy storage device are still important factors limiting the development of the energy storage device, and further development of production and manufacturing technologies is still required to expand the application range of the energy storage device. In addition, besides the centralized photovoltaic power station, a part of the installed capacity of the current photovoltaic power generation, which cannot be ignored, is the distributed photovoltaic. The distributed photovoltaic has a large quantity and scattered installation sites, and the difficulty of additionally installing the distributed photovoltaic is high and the cost is high.
The second approach is a variable power tracking approach (flexible power point tracking, FPPT). The variable power tracking algorithm introduces a difference delta P between the target power and the current power on the basis of the MPPT algorithm, and adjusts the working point of the photovoltaic cell according to the delta P. The photovoltaic cell is operated at a non-MPP position by using a variable power tracking method, and the output power of the photovoltaic cell can be controlled by adjusting the working voltage of the photovoltaic cell according to the P-U characteristic curve of the photovoltaic cell. And reserving part of standby power on the basis of the maximum power of the current photovoltaic cell, namely providing power support for the power grid when the frequency of the power grid changes. Although this approach reserves a portion of the power of the photovoltaic, it is equivalent to increasing the cost of generating electricity from the photovoltaic. But the cost of the photovoltaic panel is lower than the cost of the energy storage device, and no additional energy storage device is required.
In short, based on the scene and the requirement, the two schemes are effectively combined or other schemes, and finally, an application scheme for searching more economy and benefit is one of the problems to be solved when large-scale new energy units participate in grid frequency modulation.
Disclosure of Invention
Based on the photovoltaic active standby method, the invention further provides a photovoltaic unit frequency modulation strategy based on variable power tracking, and the strategy enables the photovoltaic to operate at the left side of the maximum power point when participating in power grid frequency modulation. The specific technical scheme is as follows.
The control method is characterized in that the photovoltaic is enabled to work in an MPP left area when the power grid frequency is normal through an FPPT, and the change of a dP/dU value of the photovoltaic characteristic curve in the MPP left area is small, so that the power adjustment is facilitated; when the frequency of the power grid is normal, the variable power tracking control is utilized to enable the photovoltaic cell to work at a non-maximum power point, and a variable related to frequency deviation-power is introduced on the reference power to enable the photovoltaic cell to have frequency modulation capability; and based on the evaluation function of the variable power tracking, the calculation method of the optimal step length is provided, so that the tracking speed of the variable power tracking is improved, and the specific strategy is as follows:
1) Frequency normal mode: when the system frequency works normally, tracking and running between areas at the left side of the maximum power point with variable power; and at the current photovoltaic maximum power (P MPP ) On the basis of the reserve part photovoltaic power (P Delta ) For providing a rising power support when the grid frequency decreases; according to the scheme, the reserved standby mode is adopted, the photovoltaic is involved in power grid frequency modulation, and the advantage is that the photovoltaic can have frequency modulation capability without additionally installing an energy storage device.
2) Frequency boost mode: frequency deviation-power variable ΔP at increasing frequency f Will be greater than 0, at which time the reference power for the variable power tracking will become k r P MPP +△P f The method comprises the steps of carrying out a first treatment on the surface of the Since the photovoltaic is standby for P when the frequency of the power grid is normal Delta The photovoltaic power is increased according to the characteristics of the left area of the maximum photovoltaic power point, the photovoltaic working voltage is required to be increased to increase the photovoltaic output power, and the delta P is increased f Continuously riseHigh, when k r P MPP +△P f Exceeding P MPP The photovoltaic is ultimately operated at the point of maximum power.
3) Frequency reduction mode: the photovoltaic can cut down the output power when the frequency decreases, in which case the frequency deviation varies with the power ΔP f Will be less than 0; according to the characteristics of the left area of the maximum power point of the photovoltaic, the photovoltaic working voltage needs to be reduced to reduce the photovoltaic output power, but the DC voltage of the inverter is ensured to be larger than the lowest working voltage (U dc,limit ) I.e. the lowest normal operating voltage of the inverter; in order to ensure that the photovoltaic system can normally participate in the regulation of the frequency of the power grid, the situation that the direct current voltage is too low to participate in frequency modulation cannot occur is avoided, and the method adopts the method about U MPP To the lowest DC operating voltage (U) 0.8Pmax ) And (5) estimating.
The three frequency modulation modes: if the frequency deviation is greater than the primary frequency modulation dead zone of 0.02Hz, the primary frequency modulation control is started, and a power reference value delta P is obtained f ，k r The formula at 0.9 is:
further obtaining the variable power tracking reference power value P of the photovoltaic under different power grid frequencies ref The formula is: p (P) ref ＝k r P MPP +△P f And realizes the reference value P by adopting a variable power tracking technology ref Is a tracking of (a).
The voltage estimation method is implemented by comparing the estimated voltage with a minimum operating voltage (U dc,limit ) Comparing, when U Est >U dc,limit Photovoltaic work participates in grid frequency modulation at the left side of maximum power; when U is Est <U dc,limit When the photovoltaic works on the right side of the maximum power, the photovoltaic participates in power grid frequency modulation, so that the situation that the photovoltaic system cannot participate in frequency modulation due to too low direct current voltage is avoided; to simplify the computation, the policy employs a policy for U MPP The voltage is estimated by a second order polynomial of the formula:
the optimal step length calculation method is based on a variable power tracking evaluation function and comprises the following steps: the variable power tracking evaluation function can comprehensively evaluate the tracking speed and the tracking precision, and the expression is as follows:
wherein TE is fa And TE (TE) ga Tracking errors of the fast tracking and progressive tracking states respectively, beta being a weight coefficient, TE re Tracking errors for the arrival state are also tracking accuracy of the system; it is calculated with tracking error so that smaller values represent better tracking effect; the variable step strategy employs two tracking steps, wherein a large step (U) is employed when any of the following conditions is satisfied step,fa ) Tracking, when neither of the following conditions is satisfied, uses a small step size (U step,ga ) Tracking.
The calculation formula of the optimal step length is as follows, wherein I sc,max The maximum short-circuit current of the photovoltaic cell under all possible working conditions is selected, the optimal step length is influenced by the working conditions of the photovoltaic cell, and the local environment factors are comprehensively considered for selecting when the step length is actually selected.
The beneficial effects of the invention are as follows: the strategy provides a frequency modulation control strategy of an active standby mode aiming at a single-stage photovoltaic inverter, and enables a photovoltaic unit to operate in a left area of a maximum power point, and the change of dP/dU values of a photovoltaic characteristic curve of the left area is small, so that the power adjustment is facilitated. And when the grid frequency is normal, the photovoltaic works at a non-maximum power point, and the photovoltaic has primary frequency modulation capability by introducing a variable related to frequency deviation-power on the reference power. Based on the variable power tracking evaluation function, an optimal step length setting method in the left standby mode is provided. The invention has certain reference significance for technical upgrading and reconstruction of the single-stage photovoltaic power generation system.
Drawings
FIG. 1 is a graph of operating point changes when photovoltaic cells participate in grid frequency modulation;
FIG. 2 is a graph of operating point change when illumination changes;
FIG. 3 is a diagram of a photovoltaic unit participating in grid frequency modulation control;
FIG. 4 is a graph of k under different illumination pv And P pv Is a relationship diagram of (1);
FIG. 5 is a schematic diagram of a variable step tracking;
FIG. 6 is a block diagram of FPPT control considering photovoltaic run mode selection on the left and right sides of the MPP;
FIG. 7 shows the system DC voltage and photovoltaic power variation when the illumination is changed;
FIG. 8 is a graph showing the change in operating point of a photovoltaic cell when the illumination changes;
FIG. 9 is a single stage photovoltaic power generation experimental apparatus and system configuration;
FIG. 10 (a) is a graph of the frequency variation of the system under conditions 1, 2;
FIG. 10 (b) is a graph of the power change of the system under conditions 1, 2;
FIG. 10 (c) is a graph of DC voltage change of the system under conditions 1 and 2;
FIG. 11 is a graph of photovoltaic operating point change under conditions 1, 2;
FIG. 12 is a graph of the output power variation of a photovoltaic power supply under asynchronous long control;
fig. 13 is a graph showing the change of the system dc voltage under asynchronous long control.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.
According to fig. 1, when the output power of the photovoltaic output is sufficient, the photovoltaic operates at an active standby point when the frequency of the power grid is normal, and when the frequency of the power grid fluctuates, the output power is adjusted to respond to the fluctuation of the frequency by adjusting the operating voltage of the photovoltaic according to the characteristics of the photovoltaic. When the output power of the photovoltaic system is too low due to insufficient illumination, the photovoltaic system does not participate in the system frequency modulation and normally works at the MPP. It can be seen from fig. 1 that the system has two active standby mode options during normal operation, on the left and right sides of the MPP, respectively, corresponding to point A, B in fig. 1.
As shown in FIG. 2, when the FPPT reference power is unchanged, the illumination is changed from Ir 1 Changed into Ir 2 The output power of the photovoltaic will decrease, but the strategy will constantly adjust the operating point of the photovoltaic to bring the photovoltaic power back to the reference power. When the illumination continues to drop from Ir 2 Changed into Ir 3 At this time, the maximum power P of the photovoltaic max,3 Less than the reference power and therefore will ultimately operate at MPP3 under strategic control.
The working curve of the PV changes continuously along with the change of illumination and temperature in the day, and current MPP point information needs to be obtained in order to realize the standby of the active power. For the case of multiple PV strings, one of the PV is operated at the MPP to obtain maximum power information, and the rest is operated at the active standby point according to the MPP information. For the case of only a single PV string, the strategy may execute the MPPT algorithm at fixed intervals to obtain MPP information, where the control strategy of the photovoltaic unit participating in grid frequency modulation is as shown in fig. 3. Reference values for the output power of the photovoltaic herein are as follows:
the frequency modulation strategy is based on the grid deviation Δf=f-f N To adjust the photovoltaic power; when Deltaf>At 0.02Hz, the load decreases or the grid frequency increases; Δf<-at 0.02Hz, the load increases or the grid frequency decreases; when the delta f is less than or equal to 0.02Hz, the strategy considers the frequency of the power grid to be normal. Under the condition that the power grid frequency is normal, the photovoltaic works in the area at the left side of the maximum power point and the power transmission is k r P MPP . If the frequency deviation is greater than the primary frequency modulation dead zone of 0.02Hz, the primary frequency modulation control is startedAnd (5) moving.
K in this patent r 0.9, i.e. the output power of the photovoltaic inverter is 0.9P when the grid frequency is normal MPP . At the same time f N 50Hz, thus k in r And f N Have been replaced with 0.9 and 50. Wherein DeltaP f Is a quantity related to the grid frequency, the magnitude is related to the frequency modulation factor, and the relation between the magnitude and the grid frequency is as follows:
thus, according to the above formula, the output power of the photovoltaic is 0.9P when the grid frequency is normal MPP I.e. operating at points a or B in fig. 1. When the grid frequency rises, the FPPT strategy is utilized to lower the output power of the photovoltaic, but the output power is not lower than 0.8P MPP Namely (2 k) r -1)P MPP . Similarly, when the grid frequency is reduced, the FPPT strategy is utilized to increase the output power of the photovoltaic, but the output power of the FPPT strategy does not exceed P MPP 。
The frequency modulation strategy needs to change the output power of the photovoltaic system along with the change of the power grid frequency, so as to ensure that the photovoltaic system can normally participate in the power grid frequency adjustment during working, and the situation that the direct current voltage is too low to participate in frequency modulation cannot occur, and the minimum direct current working voltage (U 0.8Pmax ) And (5) estimating. To simplify the calculation, the information about U can be employed MPP Is used to estimate the voltage.
In order to verify the accuracy of the formula, the photovoltaic cell parallel connection number 88 series connection number 7 of SPR-415E-WHT-D type photovoltaic cells in simulation is used in the patent, and the lowest direct current voltage working point of the patent is 0.8P MPP Point, the data of which is subjected to linear regression to obtain the estimated voltage parameter a of 9.54 multiplied by 10 -5 B is 0.7156. R of fitting function 2 0.9967, error variance 0.4731, p 5.15X10 -20 Description of fitting model accurate andthe error is within an acceptable range.
TABLE 1 voltage estimation U Est And the actual value U 0.8Pmax Comparison
It can be seen from table 1 that the fitted function has a better accuracy for predicting the lowest dc operating voltage. In addition, taking into account the variation of the grid voltage, U Est Meet U Est >U dc,limit In the case of photovoltaic operation in the left standby mode at maximum power or in the right standby mode at maximum power, while the direct voltage reference value (U dc,ref ) The following formula is also satisfied:
in the above, U g Is the effective value of the voltage of the photovoltaic grid-connected point line, c m Is a DC voltage margin.
The variable step tracking strategy provided by the patent is based on a disturbance observation method and aims at the condition that the photovoltaic work is on the left side of the MPP. Tracking states have been studied in two ways, transient state and steady state, and are subdivided into three states, fast tracking state, progressive tracking state and reach state, respectively. The variable step strategy employs two tracking steps, wherein the fast tracking state employs a large step (U step,fa ) The two working states of tracking, progressive tracking state and reaching state adopt small step length (U) step,ga ) Tracking. The power deviation and the improved photovoltaic cell power voltage ratio are used herein to determine the operating condition at this time and which tracking step should be used.
By using k in the above pv The method has better normalization as a judgment basis. As shown in fig. 4, photovoltaic cells under different conditionsK corresponding to characteristic curve of pool pv The value trends are almost consistent, and the value trends are taken as judgment basis, so that the setting of the variable step strategy parameters can be facilitated.
And when the two formulas are satisfied, small step tracking is adopted, otherwise, large step tracking is adopted. To ensure that the system can transition from the fast tracking state to the progressive tracking state, a large step U of one fast tracking must be made step,fa Is smaller than the minimum value of the direct current voltage interval corresponding to the two formulas. The left half side of the P-U characteristic curve of the photovoltaic cell is gentle, and the corresponding interval can be approximately seen as a straight line, namely dP pv /dU pv ＝I sc 。
The duration of the system variable power tracking is affected by the tracking time in the fast tracking state and the progressive tracking state. Thus when U step,fa The greater the speed of systematically tracking the state too quickly, but with limited correspondence of ΔP thr The larger the same. And DeltaP thr The larger represents a larger progressive tracking and reach state range, because it takes longer tracking time in the progressive tracking state, or the tracking accuracy of the reach state is sacrificed to increase the tracking speed.
The goal of variable step tracking is to compromise tracking speed and accuracy. Therefore, when U is selected step,fa Then based on the evaluation function of tracking error, an optimal small step length, namely U, can be uniquely determined step,ga The Tracking Error (TE) is minimized.
Wherein TE is fa And TE (TE) ga The tracking errors of the fast tracking state and the gradual tracking state are respectively, beta is a weight coefficient, and the tracking error of the test is the arrival state is also the tracking precision of the system. Thus TE fa Is about U step,fa TE of (E) ga And TE (TE) re Is about U step,ga Is a function of (2).
Thus utilizing TE 2 To U step,ga Deriving and letting the derivative equal to 0 is available:
the tracking step length of the system has practical physical meaning, so U step,ga Must be positive, when the calculated optimal U step,ga Substitution intoObtaining new system tracking error TE and comparing the obtained TE with U step,fa Derivation can obtain the optimal U step,fa ：
The above is the in-evaluation functionThe optimal step size calculation formula can be seen from the formula that the selection of the optimal step size is influenced by the working condition of the photovoltaic cell, and the selection of the local environment factors should be comprehensively considered when the step size is actually selected. The variable power tracking control strategy herein is shown in fig. 6, taking into account the dc voltage and variable step tracking issues.
In order to verify the FPPT strategy and the stability of the system in the patent, a simulation model shown in FIG. 3 is built in Matlab-Simulink.
In order to verify the stability of the system, the illumination intensity in the simulation is changed to observe the change condition of the power and the voltage of the inverter. The initial illumination intensity of the system is 850W/m 2 The illumination gradually decreases to 500W/m between 1s and 3s 2 And last for 5s, then the light is gradually lifted to 1100W/m during 8s to 10s 2 . The reference power of the FPPT strategy during illumination change is always 4563W, and simulation results are shown in fig. 7 and 8.
In fig. 7, the power is the instantaneous power measured by the ac of the inverter, the two red curves are the dc side voltage and the illumination intensity, and the purple curve is the reference power, i.e. 781.6kW. Fig. 8 shows the change of the operating point of the photovoltaic cell corresponding to fig. 7 on different photovoltaic curves, wherein the points with different colors correspond to the operating points of the photovoltaic cell at different times, and the operating point corresponding to the initial operating state of the system is the point a in fig. 8. It can be seen from fig. 7 that the system remains stable throughout the illumination change, and the output power of the photovoltaic is much less than the reference power during 1s to 3s, so the tracking step changes from 2V to 16V. Meanwhile, since the output power of the photovoltaic cell is lowered by illumination and cannot reach the reference power, the system finally works at the MPP2 in FIG. 8, and the current maximum power is output to be about 500.4kW. When the illumination rises, the output power of the photovoltaic cell exceeds the reference power and thus the output power is reduced by reducing the dc voltage, eventually operating at point B in fig. 8 where the output power reaches the reference power.
Finally, the frequency regulation strategy and the step calculation formula are verified by using an experiment, the system structure of the experimental device is shown in fig. 9, the experiment adopts a monopole three-phase inverter and uses dSPACE as a controller, and the change of the power grid frequency can be simulated by a power grid simulator. Loads 1 and 2 in fig. 9 are only two ac loads added for the convenience of experimental implementation, and do not affect the experimental results. The variable power tracking strategy is based on a disturbance observation method, and the tracking period adopted in the experiment is 2s. The maximum power output by the actual photovoltaic inverter is about 1890W, so the power at the active standby point is 0.9P max 1701W and the standby power DeltaP 189W. According to the set photovoltaic simulation source parameters, the corresponding optimal step size can be calculated by utilizing a step size calculation formula, and the calculated step size is rounded to avoid decimal occurrence, so that the step sizes are 13V and 2V respectively.
In order to verify the proposed frequency modulation method, two working conditions are set for verification. Working condition 1: the grid frequency is first stepped from 50Hz to 50.15Hz. When the output power of the system is constant, the power grid frequency is further stepped from 50.15Hz to 50Hz. Working condition 2: the grid frequency is first stepped from 50Hz to 49.85Hz. When the output power of the system is constant, the power grid frequency is further stepped from 49.85Hz to 50Hz.
Fig. 10 (a), (b) and (c) show the system frequency, the output power of the inverter and the dc voltage under the working conditions 1 and 2, respectively. Fig. 11 is a graph of the change in position of the operating point of the photovoltaic of fig. 10 on the corresponding photovoltaic curve. It can be seen from fig. 11 that the system is running with the photovoltaic always operating to the left of the MPP and the position of the operating point is adjusted as the grid frequency changes. As can be seen from fig. 10, as the grid frequency drops under the control of the proposed method, the photovoltaic can increase its power by adjusting its operating point, providing power support for the grid. Also as the grid frequency rises, the photovoltaic may curtail power by reducing its operating voltage. According to experimental results, the method can enable the photovoltaic system to have primary frequency modulation capability, and the photovoltaic system can be effectively involved in the frequency modulation process of the power grid.
In order to verify the step length selection method of the patent, the output power of the photovoltaic system under 4 unsynchronized long control is controlled from 0.8P max Lifting to 0.9P max The tracking process under asynchronous long control is shown in fig. 12 and 13. It can be seen from the figure that the different tracking steps reach the target value at the end, when U step,ga When the power is increased, the steady-state error of the final output power of the photovoltaic power supply is increased, and when U is step,fa The number of tracking times required to track to the target value becomes larger as it becomes smaller. The asynchronous long tracking process was evaluated using an evaluation function, wherein the parameter β had a value of 50, and the results are shown in table 2.
TABLE 2 tracking error contrast under asynchronous Long control
The NO.3 experiment in the NO.1-4 experiments adopts the optimal step length obtained by the setting method of the patent, and the other three experiments adopt other step lengths. As can be seen from Table 2, the TE of the NO.3 experiment was the smallest of the 4 steps, demonstrating that the step employed in the NO.3 experiment is the optimal step.
The present invention has been described in detail, and it is apparent that modifications apparent to those skilled in the art without departing from the spirit and effect of the present invention are included in the scope of the present invention.
Claims (5)
1. The control method is characterized in that the method enables the photovoltaic to work in a region on the left side of the maximum power point when the frequency of the power grid is normal through variable power tracking, and the change of dP/dU values of the photovoltaic performance curve of the region on the left side of the maximum power point is smaller, so that the power adjustment is facilitated; when the power grid frequency is normal, the photovoltaic cell is operated at a non-maximum power point by utilizing variable power tracking control, and a power variable DeltaP generated by frequency change is superimposed on the reference power f To provide the photovoltaic with frequency modulation capability; and based on the evaluation function of the variable power tracking, the calculation method of the optimal step length is obtained, so that the tracking speed of the variable power tracking is improved, and the specific strategy is as follows:
1) Frequency normal mode: when the system frequency works normally, the variable power tracking (flexible power point tracking, FPPT) operates between areas at the left side of the maximum power point; and at the current photovoltaic maximum power P MPP Is based on standby P Delta Photovoltaic power of a magnitude to provide a rising power support when the grid frequency decreases, the photovoltaic output power being k r P MPP The method comprises the steps of carrying out a first treatment on the surface of the The scheme realizes that the photovoltaic participates in the frequency modulation of the power grid in a reserved standby mode, and has the advantage that the photovoltaic has the frequency modulation capability without additionally installing an energy storage device;
2) Frequency boost mode: at increasing frequency ΔP f Will be greater than 0, at which time the reference power for the variable power tracking will become k r P MPP +△P f The method comprises the steps of carrying out a first treatment on the surface of the Since the photovoltaic is standby for P when the frequency of the power grid is normal Delta The photovoltaic power of the magnitude is required to be increased according to the characteristics of the left area of the maximum power point of the photovoltaic, and the photovoltaic working voltage is required to be increased to increase the photovoltaic output power along with delta P f Is continuously raised when k r P MPP +△P f Exceeding P MPP The photovoltaic is finally operated at the maximum power point;
3) Frequency reduction mode: light at reduced frequencyVolt-cut output power, at this time ΔP f Will be less than 0; the output power of the photovoltaic will be down regulated but not lower than (2 k) r -1)P MPP The method comprises the steps of carrying out a first treatment on the surface of the According to the characteristics of the left area of the maximum power point of the photovoltaic, the photovoltaic working voltage needs to be reduced to reduce the photovoltaic output power, but the direct current voltage of the photovoltaic grid-connected inverter is ensured to be larger than the lowest working voltage U when the working voltage is reduced dc,limit Namely the lowest normal working voltage of the photovoltaic grid-connected inverter; in order to ensure that the photovoltaic system can normally participate in the regulation of the frequency of the power grid, the situation that the direct current voltage is too low to participate in frequency modulation cannot occur is avoided, and the method adopts a method related to U MPP The second order polynomial of (2) is used for estimating the photovoltaic voltage to obtain an estimated voltage U Est 。
2. The control method for participating in grid frequency modulation at the left side of the maximum power point by using photovoltaic operation according to claim 1, wherein the mode determination principle is as follows: judging disturbance type according to network side frequency fluctuation, wherein Δf=f-f N The method comprises the steps of carrying out a first treatment on the surface of the When Deltaf>At 0.02Hz, the load decreases or the grid frequency increases; Δf<-at 0.02Hz, the load increases or the grid frequency decreases; when the delta f is less than or equal to 0.02Hz, the strategy considers that the power grid frequency is normal; according to different disturbance conditions, different control modes are adopted; under the condition that the power grid frequency is normal, the photovoltaic works in the area at the left side of the maximum power point and the power transmission is k r P MPP 。
3. The control method for participating in grid frequency modulation at the left side of the maximum power point by photovoltaic operation according to claim 1, wherein if the frequency deviation is greater than the primary frequency modulation dead zone of 0.02Hz, the primary frequency modulation control is started and Δp is obtained f ，k r The formula at 0.9 is:
obtaining variable power tracking reference power value P of photovoltaic under different power grid frequencies ref The formula is: p (P) ref ＝k r P MPP +△P f And realizes the reference value P by adopting a variable power tracking technology ref Is a tracking of (a).
4. A control method for participating in grid frequency modulation at the left side of the maximum power point for photovoltaic operation according to claim 1, characterized by the fact that the estimated voltage U is calculated by Est And the lowest operating voltage U dc,limit Comparing, when U Est >U dc,limit Photovoltaic work participates in grid frequency modulation at the left side of maximum power; when U is Est <U dc,limit When the photovoltaic works on the right side of the maximum power, the photovoltaic participates in power grid frequency modulation, so that the situation that the photovoltaic system cannot participate in frequency modulation due to too low direct current voltage is avoided; to simplify the computation, the policy employs a policy for U MPP The voltage is estimated by a second order polynomial of the formula:
5. the control method for participating in grid frequency modulation at the left side of a maximum power point by photovoltaic operation according to claim 1, wherein the optimal step size calculation method is characterized in that based on a variable power tracking evaluation function, the method comprises the following steps: the variable power tracking evaluation function can comprehensively evaluate the tracking speed and the tracking precision, and the expression is as follows:wherein TE is fa And TE (TE) ga Tracking errors of the fast tracking and progressive tracking states respectively, beta being a weight coefficient, TE re Tracking errors for the arrival state are also tracking accuracy of the system; it is calculated with tracking error so that smaller values represent better tracking effect; the variable step strategy employs two tracking steps, wherein a large step U is employed when either condition of the following is satisfied step,fa Realizing rapid tracking; when the two conditions are not satisfied, a small step length U is adopted step,ga Gradual tracking is realized:
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