CN113725921A - Control method for photovoltaic operation on left side of maximum power point to participate in power grid frequency modulation - Google Patents
Control method for photovoltaic operation on left side of maximum power point to participate in power grid frequency modulation Download PDFInfo
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Abstract
Aiming at the problem that the adjusting capacity of a system to frequency 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 in a mode that the single-stage photovoltaic inverter works at the left side of a maximum power point. The photovoltaic power control strategy utilizes a variable power tracking technology to enable the photovoltaic to always operate in a left area of a maximum power point, and the change of dP/dU value of a photovoltaic characteristic curve of the left area is small, so that the photovoltaic power is more favorably adjusted. The strategy enables the photovoltaic system to operate at a non-maximum power point under the condition of no disturbance of the grid frequency, and the rising power support is provided for the grid by using the standby partial photovoltaic power. Meanwhile, in order to adjust the photovoltaic power more quickly 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 maximum power point left side mode is provided. The control strategy is verified, and the result shows that the control strategy provided by the invention is feasible and effective.
Description
Technical Field
The invention relates to the field of photovoltaic power generation microgrid inverter control, in particular to a control method for participating in power grid frequency modulation when photovoltaic operation is carried out on the left side of a maximum power point.
Background
Photovoltaic (PV) power generation is developed rapidly as a new energy source, and compared with other clean energy sources, the Photovoltaic (PV) power generation device is more flexible in power generation form, can be applied to various social scenes such as rural roofs, illuminating street lamps, office building walls and the like, and has great application potential. Photovoltaic power generation generally adopts a Maximum Power Point Tracking (MPPT) operating mode, and in this mode, a photovoltaic cell always operates at a Maximum Power Point (MPP). At present, besides a commonly used MPPT algorithm such as a conductance incremental 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 sudden change, local shadow and the like. Although the photovoltaic utilization rate in the MPPT mode is highest, the photovoltaic power generation amount in the MPPT mode is not artificially controlled under the influence of natural factors such as illumination, temperature and the like. And photovoltaic power generation does not have the characteristics of the traditional generator by taking power electronic equipment as an interface, and cannot provide support for a power grid when the frequency of the power grid is disturbed, so that the stable operation of the power grid is challenged by the large access of photovoltaic power generation.
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. In other countries and regions, large photovoltaic power stations are required to be scheduled by a power grid, and the photovoltaic power stations need to adjust output power according to scheduling instructions. Many scholars have made a lot of research aiming at the problem that photovoltaic power supply participates in power grid frequency modulation. At present, two methods are mainly used for leading a photovoltaic unit to actively participate in power grid frequency modulation.
The first method is to configure an energy storage device for the photovoltaic unit, and provide electric energy for the photovoltaic unit to participate in grid frequency modulation by using energy stored by the energy storage device. 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 fluctuation of the photovoltaic can be smoothed by adjusting the power of the energy storage device through a control strategy. However, the cost and life of energy storage devices are still important factors limiting their development, and further development of production and manufacturing techniques is still needed to expand their application range. The other point is that apart from a centralized photovoltaic power station, a part which cannot be ignored in the installed capacity of the existing photovoltaic power generation is distributed photovoltaic. Distributed photovoltaic is in large quantity, the installation place is dispersed, and the difficulty of installing energy storage for it is big and with high costs additional.
The second method is a variable power point tracking (FPPT). The variable power tracking algorithm introduces a difference value delta P between the target power and the current power on the basis of the MPPT algorithm, and the working point of the photovoltaic cell is adjusted 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 partial 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 method reserves part of the power of the photovoltaic, it is equivalent to increase the power generation cost of the photovoltaic. However, the cost of the photovoltaic panel is lower than that of the energy storage device, and the energy storage device does not need to be additionally arranged.
In a word, based on the scene and the demand, the two schemes are effectively combined or other schemes are combined, and finally an application scheme with more economic and more beneficial effects is sought, which is one of the problems to be solved urgently when a large-scale new energy unit participates in the frequency modulation of the power grid.
Disclosure of Invention
In view of the above, the invention further provides a photovoltaic unit frequency modulation strategy based on variable power tracking on the basis of a photovoltaic active standby method, and the strategy enables the photovoltaic unit to operate on the left side of the maximum power point when the photovoltaic unit participates in power grid frequency modulation. The specific technical scheme is as follows.
A control method for enabling photovoltaic to run on the left side of a maximum power point to participate in power grid frequency modulation is characterized in that the method enables the photovoltaic to work in the MPP left side area when the power grid frequency is normal through FPPT, and the change of a photovoltaic characteristic curve dP/dU value of the MPP left side area is small, so that power adjustment is facilitated; when the frequency of a power grid is normal, a photovoltaic cell is enabled to work at a non-maximum power point by utilizing variable power tracking control, and a variable related to frequency deviation-power is introduced into reference power to enable the photovoltaic cell to have the frequency modulation capability; and based on the evaluation function of variable power tracking, a calculation method of the optimal step length is provided, so that the tracking speed of variable power tracking is improved, and the specific strategy is as follows:
1) frequency normal mode: when the system frequency normally works, the variable power tracking operation is carried out at the left of the maximum power pointBetween the lateral regions; and at the current maximum photovoltaic power (P)MPP) On the basis of the reserve part of the photovoltaic power (P)Delta) For providing a rising power support when the grid frequency decreases; according to the scheme, the photovoltaic is enabled to participate in the frequency modulation of the power grid in a reserved standby mode, and the advantage that the photovoltaic has the frequency modulation capability without additionally installing an energy storage device is achieved.
2) Frequency raising mode: variation Δ P of frequency deviation versus power at increasing frequencyfWill be greater than 0, and the reference power of the variable power tracking will become krPMPP+△Pf(ii) a When the frequency of the power grid is normal, the photovoltaic is standby for PDeltaThe magnitude of the photovoltaic power is that according to the characteristics of the area on the left side of the photovoltaic maximum power point, the photovoltaic working voltage needs to be increased along with the delta P to increase the photovoltaic output powerfConstantly rising when k isrPMPP+△PfExceeds PMPPThe photovoltaic eventually operates at the maximum power point.
3) Frequency reduction mode: the photovoltaic can cut the output power when the frequency is reduced, and the frequency deviation-power variable Delta PfWill be less than 0; according to the characteristics of the left area of the maximum photovoltaic power point, the photovoltaic working voltage needs to be reduced for reducing the photovoltaic output power, but when the working voltage is reduced, the direct current voltage of the inverter is ensured to be larger than the minimum working voltage (U)dc,limit) Namely the lowest normal working voltage of the inverter; in order to ensure that the photovoltaic system can normally participate in power grid frequency regulation and the condition that direct-current voltage is too low to participate in frequency modulation cannot occur, U is adoptedMPPTo the lowest direct current operating voltage (U)0.8Pmax) And (6) performing prediction.
The three frequency modulation modes are as follows: if the frequency deviation is larger 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 obtainedf，krAt 0.9 the formula is:
further obtaining photovoltaic variable power tracking reference power value P under different power grid frequenciesrefThe formula is as follows: pref＝krPMPP+△PfAnd the reference value P is realized by adopting a variable power tracking technologyrefThe tracking of (2).
The voltage estimation method is realized by comparing the estimated voltage with the lowest working voltage (U)dc,limit) Comparing when U isEst> Udc,limitWhen the photovoltaic works on the left side of the maximum power, the photovoltaic participates in the frequency modulation of the power grid; when U is turnedEst<Udc,limitWhen the photovoltaic works on the right side of the maximum power, the photovoltaic works and participates in the frequency modulation of the power grid, so that the condition that the photovoltaic system cannot participate in the frequency modulation due to the fact that the direct-current voltage is too low is avoided; to simplify the computation, the strategy takes care of UMPPThe second order polynomial of (a) to estimate the voltage is as follows:
the optimal step calculation method is based on a variable power tracking evaluation function and provides that: the variable power tracking evaluation function can comprehensively evaluate the tracking speed and the tracking precision, and the expression is as follows:
wherein TEfaAnd TEgaTracking errors for fast tracking and progressive tracking states, respectively, beta is a weight coefficient, TEreThe tracking error for the arrival state is also the tracking accuracy of the system; the tracking error is calculated, so that the smaller the numerical value is, the better the tracking effect is; the variable step size strategy employs two tracking steps, wherein a large step size (U) is employed when the following arbitrary condition is satisfiedstep,fa) Tracking, using small step size (U) when neither of the two conditions is satisfiedstep,ga) And (6) tracking.
The optimal step size is calculated as follows, wherein Isc,maxFor the maximum short-circuit current of the photovoltaic cell under all possible working conditions, the selection of the optimal step length is influenced by the working conditions of the photovoltaic cell, and the selection of the optimal step length should comprehensively consider local environmental factors when the step length is actually selected.
The invention has the beneficial effects that: the strategy provides an active standby frequency modulation control strategy for a single-stage photovoltaic inverter, the photovoltaic unit is operated in a left area of a maximum power point, and the change of dP/dU value of a photovoltaic characteristic curve in the left area is small, so that power can be adjusted. And when the grid frequency is normal, the photovoltaic works at a non-maximum power point, and the photovoltaic has the 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 method has certain reference significance for technical upgrading and modification of the single-stage photovoltaic power generation system.
Drawings
FIG. 1 is a diagram of operating point changes when a photovoltaic cell participates in power grid frequency modulation;
FIG. 2 is a graph showing the variation of operating points when the illumination is varied;
FIG. 3 is a structural diagram of a photovoltaic unit participating in power grid frequency modulation control;
FIG. 4 shows k under different illuminationpvAnd PpvA relationship diagram of (1);
FIG. 5 is a schematic diagram of variable step tracking;
FIG. 6 is a block diagram of FPPT control considering photovoltaic left and right MPP operating mode selection;
FIG. 7 shows the DC voltage and photovoltaic power of the system when the illumination changes;
FIG. 8 shows the change of the operating point of the photovoltaic cell when the illumination is changed;
FIG. 9 shows a single-stage photovoltaic power generation experimental apparatus and system structure;
FIG. 10(a) is a frequency variation diagram of the system under the working conditions 1 and 2;
FIG. 10(b) is a power variation diagram of the system under the working conditions 1 and 2;
FIG. 10(c) is a diagram of the DC voltage variation of the system under working conditions 1 and 2;
FIG. 11 is a diagram of changes in photovoltaic operating points under working conditions 1 and 2;
FIG. 12 is a graph of the output power variation of the photovoltaic power supply under different step size control;
FIG. 13 is a diagram of the DC voltage variation of the system under different step-size control.
Detailed Description
The technical solution 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 the frequency modulation strategy shown in fig. 1, when the good photovoltaic output power of the weather is sufficient, the photovoltaic works at an active standby point when the power grid frequency is normal, and when the power grid frequency fluctuates, the output power is adjusted to respond to the frequency fluctuation by adjusting the working voltage of the photovoltaic according to the photovoltaic characteristics. When the illumination is insufficient, the photovoltaic output power is too low, and at the moment, the photovoltaic system does not participate in 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, left and right of the MPP, respectively, corresponding to A, B points in fig. 1.
As shown in FIG. 2, when the FPPT reference power is constant, the illumination changes from Ir1Is changed into Ir2The output power of the photovoltaic system is reduced, but the strategy is to continuously adjust the operating point of the photovoltaic system to make the photovoltaic power reach the reference power again. As the illumination continues to drop from Ir2Is changed into Ir3At this time, the maximum power P of the photovoltaicmax,3Less than the reference power and therefore will eventually operate at MPP3 under strategic control.
The working curve of the PV changes along with the change of illumination and temperature in one day, and the current MPP point information is acquired in order to realize the standby of active power. And aiming at the condition of containing a plurality of PV strings, one PV is operated at the MPP to obtain maximum power information, and the rest PV is operated at an active standby point according to the MPP information. For the case of only containing a single PV string, the strategy may execute an MPPT algorithm at fixed time intervals to obtain MPP information, and the control strategy for participating in grid frequency modulation by the photovoltaic unit is shown in fig. 3. The reference values for the photovoltaic output power in this context are as follows:
the frequency modulation strategy is based on the power grid deviation delta f-fNTo adjust the photovoltaic power; when Δ f>When the frequency is 0.02Hz, the load is reduced or the frequency of the power grid is increased; Δ f<-at 0.02Hz, the load increases or the grid frequency decreases; and when the | delta f | is less than or equal to 0.02Hz, the strategy considers that the power grid frequency is normal. Under the condition that the frequency of the power grid is normal, the photovoltaic works in the area on the left side of the maximum power point, and the output power is krPMPP. And if the frequency deviation is larger than the primary frequency modulation dead zone of 0.02Hz, starting primary frequency modulation control.
In this patent krIs 0.9, namely when the grid frequency is normal, the output power of the photovoltaic inverter is 0.9PMPP. At the same time fNIs 50Hz, so k in the followingrAnd fNBoth have been replaced with 0.9 and 50. Wherein Δ PfIs a quantity related to the grid frequency, and the magnitude is related to the frequency modulation coefficient, and the relation between the quantity and the grid frequency is as follows:
thus, according to the above formula, the photovoltaic output is 0.9P when the grid frequency is normalMPPI.e., operating at point a or B in fig. 1. When the frequency of the power grid rises, the output power of the photovoltaic system is adjusted downwards by using the FPPT strategy, but the output power is not lower than 0.8PMPPI.e. (2 k)r-1)PMPP. Similarly, when the grid frequency is reduced, the output power of the photovoltaic can be increased by utilizing the FPPT strategy, but the output power of the photovoltaic cannot exceed PMPP。
Frequency modulation strategy requires that the photovoltaic system be disabledThe output power changes along with the change of the power grid frequency, and the lowest direct current working voltage (U) is needed to ensure that the photovoltaic system can normally participate in the power grid frequency regulation when in work and the condition that the direct current voltage is too low to participate in the frequency regulation can not occur0.8Pmax) And (6) performing prediction. To simplify the calculation, information about U can be employedMPPTo estimate the voltage.
In order to verify the accuracy of the formula, the simulation method uses the SPR-415E-WHT-D type photovoltaic cell parallel number 88 serial number 7 in simulation, and the lowest direct-current voltage working point of the simulation method is 0.8PMPPThe parameter a of voltage estimation obtained by linear regression of the data is 9.54 multiplied by 10-5And b is 0.7156. R of the fitting function20.9967, variance of error 0.4731, p 5.15X 10-20And the fitting model is accurate and the error is within an acceptable range.
TABLE 1 estimated U of VoltageEstWith the actual value U0.8PmaxComparison of
It can be seen from table 1 that the fitted function has better accuracy for predicting the lowest dc operating voltage. In addition, taking into account changes in the network voltage, UEstSatisfy UEst>Udc,limitThe photovoltaic operation adopts a left standby mode at maximum power or a right standby mode at maximum power, and a direct current voltage reference value (U)dc,ref) The following equation is also satisfied:
in the above formula, UgIs the effective value of the photovoltaic grid-connected point line voltage, cmIs a dc voltage margin.
The variable step tracking strategy provided by the patent is based on a disturbance observation method and aims at the situation that photovoltaic work is on the left side of the MPP. In the prior art, the tracking state is divided into a transient state (transient state) and a steady state (steady state), and the tracking state is subdivided into three states, namely a fast tracking state, a gradual tracking state and an achievement state. The variable step size strategy adopts two tracking step sizes, wherein the fast tracking state adopts a large step size (U)step,fa) Small step length (U) is adopted for two working states of tracking, progressive tracking state and reaching statestep,ga) And (6) tracking. The power deviation and the improved photovoltaic cell power-to-voltage ratio are used herein to determine the operating state at that time and which tracking step size should be employed.
Using k in the above formulapvThe method has better normalization as a judgment basis. As shown in fig. 4, the characteristic curve of the photovoltaic cell corresponds to k under different working conditionspvThe value trends are almost consistent, and the setting of the variable step size strategy parameters can be facilitated by taking the value trends as judgment basis.
And when the two formulas are met, small-step tracking is adopted, otherwise, large-step tracking is adopted. In order to ensure that the system can transit from the fast tracking state to the gradual tracking state, a large step U of one-time fast tracking is requiredstep,faIs smaller than the minimum value of the direct current voltage interval corresponding to the above two formulas. The left half side of the P-U characteristic curve of the photovoltaic cell is gentle, and the P-U characteristic curve can be approximately seen as a straight line in a corresponding interval, namely dPpv/dUpv＝Isc。
The time length of the variable power tracking of the system is influenced by the tracking time in a fast tracking state and a progressive tracking state. Therefore when U isstep,faThe larger the system, the faster it will pass through the fast tracking state, but the limited pairCorresponding delta PthrThe larger the size. And Δ PthrThe larger the tracking time, the larger the range of the reached state, the longer the tracking time is, or the tracking precision of the reached state is sacrificed to increase the tracking speed.
The aim of the variable step tracking is to give consideration to the tracking speed and the tracking precision. Therefore, when U is selectedstep,faThen according to the evaluation function of the tracking error, an optimal small step length, namely U, can be uniquely determinedstep,gaThe Tracking Error (TE) is minimized.
Wherein TEfaAnd TEgaThe tracking errors of the fast tracking state and the progressive tracking state are respectively, beta is a weight coefficient, and the tracking error of TEre in the arrival state is also the tracking precision of the system. Thus TEfaIs about Ustep,faFunction of, TEgaAnd TEreIs about Ustep,gaAs a function of (c).
Thus using TE2To Ustep,gaTaking the derivative and making the derivative equal to 0 yields:
the tracking step of the system has practical physical meaning, so Ustep,gaMust be positive, when calculating the optimal Ustep,gaSubstitution into
Obtaining new system tracking error TE, and comparing the obtained TE with Ustep,faThe optimum U can be obtained by derivationstep,fa：
The above formula isTo be in the evaluation function
According to the optimal step size calculation formula, the selection of the optimal step size is influenced by the working condition of the photovoltaic cell, and the selection of the optimal step size should be comprehensively considered when the step size is actually selected. Considering the problems of dc voltage and variable step tracking, the variable power tracking control strategy herein is shown in fig. 6.
In order to verify the stability of the FPPT strategy and 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 change of the power and the voltage of the inverter is observed by changing the illumination intensity in the simulation. The initial illumination intensity of the system is 850W/m2The light illumination gradually decreases to 500W/m between 1s and 3s2And last for 5s, then the illumination is gradually increased to 1100W/m in the period from 8s to 10s2. The reference power of the FPPT strategy was 4563W during the illumination variation, and the simulation results are shown in fig. 7 and 8.
The power in fig. 7 is the instantaneous power measured by the inverter ac, the two red curves are the dc side voltage and the illumination intensity, respectively, and the purple curve is the reference power, i.e., 781.6 kW. Fig. 8 is a variation of the operating point of the photovoltaic cell corresponding to fig. 7 on different photovoltaic curves, where 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 all the time when the illumination changes, the output power of the photovoltaic is much smaller 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 cannot reach the reference power due to the low illumination, the system finally works at the MPP2 in fig. 8, and the current maximum power is output to be about 500.4 kW. 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 direct-current voltage, and finally the output power reaches the reference power at the point B in fig. 8.
Finally, the frequency modulation strategy and the step length calculation formula are verified by utilizing experiments, and the method is usedThe system structure of the experimental device is shown in fig. 9, a unipolar three-phase inverter is adopted in the experiment, dSPACE is used as a controller, and the change of the power grid frequency can be simulated through a power grid simulator. Loads 1 and 2 in fig. 9 are only two ac loads added for convenience of experiment implementation, and do not affect the experiment result. The variable power tracking strategy is based on a disturbance observation method, and the tracking period adopted in the experiment is 2 s. The maximum power output by the actual photovoltaic inverter is about 1890W, so the power at the active standby point is 0.9Pmax1701W and 189W of standby power Δ P. According to the set photovoltaic simulation source parameters, the corresponding optimal step length can be obtained by using a step length calculation formula, and the obtained step length is rounded to avoid decimal occurrence, so that the obtained step length is respectively 13V and 2V.
In order to verify the proposed frequency modulation method, two conditions were set for verification. Working condition 1: the grid frequency is first stepped from 50Hz to 50.15 Hz. And when the output power of the system is constant, the frequency of the power grid is stepped from 50.15Hz to 50 Hz. Working condition 2: the grid frequency is first stepped from 50Hz to 49.85 Hz. And when the output power of the system is constant, the frequency of the power grid is stepped from 49.85Hz to 50 Hz.
Fig. 10(a), (b), and (c) are curves of the system frequency, the output power of the inverter, and the dc voltage under operating conditions 1 and 2, respectively. Fig. 11 is a graph showing the change of the position of the operating point of the photovoltaic device in fig. 10 on the corresponding photovoltaic curve. It can be seen from fig. 11 that the photovoltaic system always operates on the left side of the MPP during operation of the system, and the position of the operating point is adjusted along with the change of the grid frequency. As can be seen from fig. 10, when the grid frequency decreases under the control of the proposed method, the photovoltaic can increase the power by adjusting its operating point, providing power support for the grid. Also when the grid frequency increases, the photovoltaic can cut power by lowering its operating voltage. According to experimental results, the method disclosed by the patent can enable the photovoltaic system to have primary frequency modulation capability, and can effectively enable the photovoltaic system to participate in the frequency modulation process of a power grid.
In order to verify the step length selection method, the output power of the photovoltaic system is controlled to be 0.8P under 4 different step lengthsmaxIs lifted to 0.9PmaxProcess for producingBy comparison, the tracking process under different step sizes is shown in fig. 12 and 13. It can be seen from the figure that different tracking steps reach the target value at the end when Ustep,gaWhen the voltage is increased, the steady-state error of the final output power of the photovoltaic power supply is increased, and when the voltage is U, the steady-state error of the final output power of the photovoltaic power supply is increasedstep,faThe number of tracings required to track to the target value becomes more as the size becomes smaller. And evaluating the tracking process of different step lengths by utilizing an evaluation function, wherein the value of the parameter beta is 50, and the result is shown in table 2.
TABLE 2 tracking error comparison under different step controls
In the experiments No.1-4, the experiment No.3 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, TE of the NO.3 experiment is the smallest among the 4 steps, which proves that the step adopted by the NO.3 experiment is the optimal step.
As described above, the present invention has been described in detail, and it is apparent that modifications thereof which are obvious to those skilled in the art without substantially departing from the point and effect of the present invention are also included in the scope of the present invention.
Claims (5)
1. A control method for enabling a photovoltaic to run on the left side of a maximum power point to participate in power grid frequency modulation 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 power grid frequency is normal through variable power tracking, and the change of a photovoltaic characteristic curve dP/dU value in the region on the left side of the maximum power point is small, so that power adjustment is facilitated; when the frequency of a power grid is normal, a photovoltaic cell is enabled to work at a non-maximum power point by utilizing variable power tracking control, and a variable related to frequency deviation-power is introduced into reference power to enable the photovoltaic cell to have the frequency modulation capability; and based on the evaluation function of variable power tracking, a calculation method of the optimal step length is provided, so that the tracking speed of variable power tracking is improved, and the specific strategy is as follows:
1) frequency normal mode: when the system frequency normally works, a Flexible Power Point Tracking (FPPT) is operated between areas on the left side of a maximum power point; and at the current maximum photovoltaic power (P)MPP) On the basis of PDeltaPhotovoltaic power of a magnitude to provide a rising power support when grid frequency decreases; according to the scheme, the photovoltaic is enabled to participate in the frequency modulation of the power grid in a reserved standby mode, and the advantage that the photovoltaic has the frequency modulation capability without additionally installing an energy storage device is achieved.
2) Frequency raising mode: variation Δ P of frequency deviation versus power at increasing frequencyfWill be greater than 0, and the reference power of the variable power tracking will become krPMPP+△Pf(ii) a When the frequency of the power grid is normal, the photovoltaic is standby for PDeltaThe magnitude of the photovoltaic power is that according to the characteristics of the area on the left side of the photovoltaic maximum power point, the photovoltaic working voltage needs to be increased along with the delta P to increase the photovoltaic output powerfConstantly rising when k isrPMPP+△PfExceeds PMPPThe photovoltaic eventually operates at the maximum power point.
3) Frequency reduction mode: the photovoltaic can cut the output power when the frequency is reduced, and the frequency deviation-power variable Delta PfWill be less than 0; according to the characteristics of the left area of the maximum photovoltaic power point, the photovoltaic working voltage needs to be reduced for reducing the photovoltaic output power, but when the working voltage is reduced, the direct current voltage of the inverter is ensured to be larger than the minimum working voltage (U)dc,limit) Namely the lowest normal working voltage of the inverter; in order to ensure that the photovoltaic system can normally participate in power grid frequency regulation and the condition that direct-current voltage is too low to participate in frequency modulation cannot occur, U is adoptedMPPTo the lowest direct current operating voltage (U)0.8Pmax) And (6) performing prediction.
2. The method for controlling the participation of photovoltaic power operation on the left side of the maximum power point in grid frequency modulation according to claim 1, wherein the mode determination principle is as follows: judging the disturbance type according to the network side frequency fluctuation, wherein delta f is f-fN(ii) a When Δ f>At 0.02HzLoad reduction or grid frequency increase; Δ f<-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; adopting different control modes according to different disturbance conditions; under the condition that the frequency of the power grid is normal, the photovoltaic works in the area on the left side of the maximum power point, and the output power is krPMPP。
3. The method for controlling the participation of photovoltaic power generation in grid frequency modulation on the left side of the maximum power point according to claim 1, wherein the three frequency modulation modes are as follows: if the frequency deviation is larger 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 obtainedf，krAt 0.9 the formula is:
further obtaining photovoltaic variable power tracking reference power value P under different power grid frequenciesrefThe formula is as follows: pref＝krPMPP+△PfAnd the reference value P is realized by adopting a variable power tracking technologyrefThe tracking of (2).
4. The method for controlling participation in grid frequency modulation during photovoltaic operation on the left side of the maximum power point as claimed in claim 1, wherein the voltage estimation method is implemented by comparing the estimated voltage with the lowest operating voltage (Umax)dc,limit) Comparing when U isEst>Udc,limitWhen the photovoltaic works on the left side of the maximum power, the photovoltaic participates in the frequency modulation of the power grid; when U is turnedEst<Udc,limitWhen the photovoltaic works on the right side of the maximum power, the photovoltaic works and participates in the frequency modulation of the power grid, so that the condition that the photovoltaic system cannot participate in the frequency modulation due to the fact that the direct-current voltage is too low is avoided; to simplify the computation, the strategy takes care of UMPPThe second order polynomial of (a) to estimate the voltage is as follows:
5. the control method for the photovoltaic operation to participate in the grid frequency modulation on the left side of the maximum power point according to claim 1, wherein the optimal step calculation method is based on a variable power tracking evaluation function and provides: the variable power tracking evaluation function can comprehensively evaluate the tracking speed and the tracking precision, and the expression is as follows:
wherein TEfaAnd TEgaTracking errors for fast tracking and progressive tracking states, respectively, beta is a weight coefficient, TEreThe tracking error for the arrival state is also the tracking accuracy of the system; the tracking error is calculated, so that the smaller the numerical value is, the better the tracking effect is; the variable step size strategy employs two tracking steps, wherein a large step size (U) is employed when the following arbitrary condition is satisfiedstep,fa) Tracking, using small step size (U) when neither of the two conditions is satisfiedstep,ga) And (6) tracking.
The calculation formula of the optimal step length contains Isc,maxFor the maximum short-circuit current of the photovoltaic cell under all possible working conditions, the selection of the optimal step length is influenced by the working conditions of the photovoltaic cell, and the selection of the optimal step length should comprehensively consider local environmental factors when the step length is actually selected.
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