CN114188990B - Distributed photovoltaic auxiliary voltage regulation control method based on small alternating current signals - Google Patents
Distributed photovoltaic auxiliary voltage regulation control method based on small alternating current signals 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
- H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
- H02J3/48—Controlling the sharing of the in-phase component
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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
- H02J3/12—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load
- H02J3/16—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load by adjustment of reactive power
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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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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- 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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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/30—Reactive power compensation
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Abstract
The invention discloses a distributed photovoltaic auxiliary voltage regulation control method based on a small alternating current signal, belonging to the technical field of new energy. Aiming at the serious overvoltage problem caused by the increase of the capacity of a distributed photovoltaic power supply connected into a low-voltage distribution network, an auxiliary voltage regulation control strategy aiming at the distributed photovoltaic power supply is provided in consideration of the characteristics of the distribution network, so that the voltage is regulated by reducing part of active power on the basis of reactive regulation of a photovoltaic system, the reduction control of the active power of the photovoltaic power supply is realized by utilizing a mode of maximum power estimation, and communication and coordination cooperation among the photovoltaic power supplies are realized by injecting small alternating current signals into the power network. The control strategy ensures that each photovoltaic inverter participates in the voltage regulation process of the power grid, simultaneously ensures the economic benefit of each photovoltaic power supply as much as possible, and aims to fully exert the voltage regulation capability of each photovoltaic power supply while ensuring the economic benefit of each photovoltaic power supply. The provided coordination control scheme is compared with the existing reactive power regulation scheme (RPC), and the result shows that the provided method has better voltage regulation effect than the RPC strategy when the illumination amplitude is larger.
Description
Technical Field
The invention relates to the field of control of distributed generation micro-grid inverters, in particular to a distributed photovoltaic auxiliary voltage regulation control method based on small alternating current signals.
Background
Since humans walk into the industrial society, fossil energy, which is non-renewable energy and causes serious pollution to the environment when burned, has been largely developed, so that various countries have developed new energy to reduce the dependency on fossil energy. The photovoltaic industry in China has rapid development in recent years, and the total capacity of the photovoltaic power generation installation in China reaches the first global place of 204.3GW at the end of 2019, and accounts for 10.18% of the total installation capacity in China. The newly-increased installed capacity in 2019 is 30.1GW, wherein the newly-increased distributed photovoltaic installed capacity is 12.19GW, and accounts for 40.5% of the newly-increased installed capacity in 2019. Photovoltaic power generation will become a mainstream energy utilization form in the future, but photovoltaic power generation itself has uncertainty and randomness. Current photovoltaic power generation is basically controlled by using a maximum power tracking (Maximum power point tracking, MPPT) algorithm, and the output power of the current photovoltaic power generation is determined by the illumination and the temperature of the external environment. However, as photovoltaic permeability in the grid continues to increase, new challenges are presented to the operation of the grid.
Most distributed photovoltaics are roof photovoltaics or small-scale building photovoltaics, are connected into a power grid through a low-voltage line and are not subjected to unified dispatching of the power grid. As the installed capacity of the photovoltaic is continuously increased, overvoltage problems begin to occur in the low voltage distribution network. The R/X value in the low-voltage distribution network is larger, and the power grid voltage is influenced by the reactive power distribution and the active power distribution. When the output of the photovoltaic power supply with sufficient illumination is large, the condition of power reversal occurs in the power distribution network, so that the voltage of the line terminal is increased. Excessive line voltage can reduce the stability of the grid and even cause the failure of the user load near the overvoltage node to operate properly, ultimately resulting in economic losses.
The problem of overvoltage in a power distribution network is solved, and two solutions are mainly available at present. The first is to solve the problem from the angle of the power grid, and add a voltage regulating device or a compensating device on the line of the power distribution network. The method can fundamentally solve the problem of overvoltage, but the implementation of the method requires the transformation of the power distribution network, and the large-scale transformation of the power distribution network in the areas is expensive.
Another is more economical to assist grid regulation through photovoltaic strategies. Aiming at the characteristics of the power distribution network, active power and reactive power of photovoltaic output are mainly controlled at present. First is a method of reactive power regulation voltage (Reactive power point control, RPC). However, in the distribution network, the reactive power has limited influence on the voltage of the power grid, so that the effect of adjusting the voltage by using the reactive power is not ideal. There is a voltage regulation (Active power control, APC) by reducing the active power, but reducing the active power affects the economics of the photovoltaic power supply. The main form of photovoltaic power generation in the low-voltage distribution network is small distributed photovoltaic power generation, and the large-scale reduction of photovoltaic output can cause certain economic loss for photovoltaic power generation users. The control strategy for jointly regulating the voltage by utilizing the active power and the reactive power has better regulating effect and economy compared with the APC control strategy.
In a word, whether add voltage regulating equipment or utilize photovoltaic to assist in regulating voltage through control strategy, the problem that high proportion photovoltaic access leads to the power distribution network voltage to rise is one of the problems that response "carbon peak, carbon neutralization" policy and comprehensive popularization of photovoltaic power generation need to be solved.
Disclosure of Invention
In view of the above, the invention further provides a distributed photovoltaic auxiliary voltage regulation control method based on small alternating current signals on the basis of considering voltage regulation effect and photovoltaic economic benefit, and the photovoltaic active power is regulated by utilizing a variable power tracking (Flexible Power Point Tracking, FPPT) technology. The specific technical scheme is as follows.
The distributed photovoltaic auxiliary voltage regulation control method based on the small alternating current signals is characterized in that the method reduces the power grid voltage by a method of combining active power reduction and reactive power absorption, so that when the power grid voltage is in a normal state, a photovoltaic power supply operates at a maximum power point in a unit power factor, and the voltage regulation capability of a photovoltaic system is exerted to the greatest extent; the power flow distribution of active power is controlled in the low-voltage distribution network, so that the regulation and control of the power grid voltage can be effectively realized; and small alternating current signals are injected into the power grid through the PV system, so that coordination among different PV systems is realized, and the photovoltaic system on the radial line achieves better regulation and control effects on the line voltage. The specific method comprises the following steps:
1) Increasing the absorption reactive mode: when the grid-connected point voltage is in the voltage regulation dead zone, each photovoltaic power supply operates in a unit power working mode and operates at the maximum power point by an MPPT algorithm; and when the voltage of the grid-connected point continues to rise and exceeds the voltage dead zone range, the photovoltaic power supply starts to absorb reactive power corresponding to the voltage of the grid-connected point, and the photovoltaic still operates in an MPPT mode.
2) Active power reduction mode: when the voltage of the grid-connected point continues to rise above a set value, reducing the photovoltaic output power by utilizing a variable power tracking technology; the variable power tracking can enable the photovoltaic to work on the right side of the maximum power point by adjusting the working voltage of the photovoltaic panel; according to the characteristics of the right area of the maximum power point of the photovoltaic P-U characteristic curve, the photovoltaic power is cut and subtracted by improving the photovoltaic working voltage; the scheme can exert the photovoltaic voltage regulating capability to the greatest extent, works at the maximum power point when the voltage of the power grid is lower than the limit value, and can ensure the power generation benefit when the photovoltaic participates in the voltage regulation of the power grid;
3) Pressure regulating limit mode: when the photovoltaic system reaches the regulation limit and the voltage of the power grid continues to rise, continuously injecting small alternating voltage signals into the power grid to send information of reaching the regulation limit to the upper-level photovoltaic; when the upper-level photovoltaic power supply detects the signal, the grid-connected point voltage at the moment is used as a voltage set value of a self voltage control strategy, and then the voltage regulation of the power grid is participated according to the new voltage set value.
4) Increasing the reactive mode: when the grid-connected point voltage is in the voltage regulation dead zone, each photovoltaic power supply operates in a unit power working mode and operates at the maximum power point by an MPPT algorithm; and when the voltage of the grid-connected point is reduced beyond the voltage dead zone range, the photovoltaic power supply starts to emit reactive power corresponding to the voltage of the grid-connected point.
The determination principle of the four modes is as follows: Δu=u according to the current grid voltage range g -U N The method comprises the steps of carrying out a first treatment on the surface of the When the delta U is less than or equal to 0.02U N When the photovoltaic system works in a normal mode, the voltage of the power grid is not regulated; when 0.02U N <ΔU<U set When the voltage of the power grid rises but does not reach the set voltage limit value, the photovoltaic system works in an absorption reactive power increasing mode. When DeltaU>U set When the grid voltage is too high and exceeds the limit value, the photovoltaic system works in a mode of reducing active power. When DeltaU>0.02U N When the voltage of the power grid is reduced beyond the dead zone range, the photovoltaic system works in a reactive mode.
The mode specifically comprises three steps: if the voltage is higher or lower than 0.02U N The voltage regulation strategy of the photovoltaic inverter is started and the voltage regulation dead zone of the photovoltaic inverter is obtainedThe reactive power reference value Q is expressed as follows:
the upper limit of reactive power regulation is limited by the minimum power factor of the system and the maximum capacity of the photovoltaic inverter, the reactive power reference value Q can be obtained according to the maximum reactive power value, and the calculation formula of the maximum reactive power is as follows:
the photovoltaic inverter cuts the photovoltaic power of a fixed percentage at most according to the grid-connected point voltage; in order to ensure the economic benefit of the photovoltaic, fitting the maximum power of the current photovoltaic according to the temperature T and irradiance G of the current environment, and limiting the active power range of the system participating in the voltage regulation of the power grid by utilizing the fitted maximum power; and P is generated according to the voltage regulating strategy ref Adjusting the working point of the photovoltaic to a curve on the right side of the maximum power according to dP/dU information of the current working point of the photovoltaic by using an FPPT technology, and keeping the output power of the photovoltaic consistent with the reference power; when the maximum power of the photovoltaic is smaller than the reference power P ref And when the photovoltaic power generation system is in operation, the MPPT technology is utilized to enable the photovoltaic power generation system to work at the maximum power point according to dP/dU information of the current working point of the photovoltaic.
The coordination between the multiple stages of photovoltaics is realized by injecting a small alternating current signal into the power distribution network; the small alternating voltage signal is a high-frequency signal which is superimposed on the reference voltage U output by the inner ring controller ref Finally, the alternating current signal is injected into the power grid through SPWM (Sinusoidal Pulse Width Modulation) modulation, and the modulation voltage formula of the SPWM is as follows:
U SPWM ＝U ref +U signal
when the voltage continues to rise after the kth photovoltaic power supply reaches the regulation limit, the photovoltaic power supply intermittently injects small alternating current signals for k times at regular intervals in a short time; when a short time is detectedThe k times of small alternating current signals are arranged in the photovoltaic power supply, and the kth-1 photovoltaic power supply can carry out U according to voltage regulation measures set,k-1 Setting the current grid-connected point voltage value U k-1 。
The beneficial effects of the invention are as follows: the method is used for assisting in regulating the power grid voltage in an active and reactive power cooperative mode, photovoltaic operation is performed at a maximum power point when the power grid voltage is normal, and photovoltaic output is reduced through a variable power operation point when the power grid voltage rises to exceed a limit value. The power flow distribution of active power is controlled in the low-voltage distribution network, so that the regulation and control of the power grid voltage can be effectively realized; and small alternating current signals are injected into the power grid through the PV system, so that coordination among different PV systems is realized, and the photovoltaic system on the radial line achieves better regulation and control effects on the line voltage. The method has certain reference significance for solving the problem of overhigh voltage caused by large-scale photovoltaic access in the power distribution network.
Drawings
FIG. 1 is a power distribution network topology including a photovoltaic power source;
FIG. 2 is a voltage drop phasor diagram of a grid line;
FIG. 3 is a graph of reactive power versus voltage generated by a photovoltaic power source;
FIG. 4 is a graph of the photovoltaic power operation interval during voltage regulation;
FIG. 5 is an active power control flow diagram;
FIG. 6 is a P-V graph of a photovoltaic cell;
FIG. 7 is a graph of maximum power estimation;
FIG. 8 is a schematic diagram of a process for transferring information between photovoltaic power sources;
FIG. 9 is a simplified diagram of a system including a small AC signal;
FIG. 10 (a) shows the grid-tie voltage and U for pv4 set,4 A variation map;
FIG. 10 (b) shows the grid-tie voltage and U of pv3 set,3 A variation map;
FIG. 11 is a graph of the active and reactive power variation of pv 4;
FIG. 12 is a graph of the active and reactive power variation of pv 3;
FIG. 13 is a small AC signal detected at pv 3;
FIG. 14 (a) is a graph showing the voltage change at node 1 under the control of the proposed control method and RPC method;
FIG. 14 (b) is a graph showing the voltage change at node 2 under the control of the proposed control method and RPC method;
FIG. 14 (c) is a graph showing the voltage change at node 3 under the control of the proposed control method and RPC method;
FIG. 14 (d) is a graph showing the voltage change at node 4 under the control of the proposed control method and RPC method;
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.
A typical structure of a low voltage distribution network containing distributed photovoltaic power sources is shown in fig. 1. Wherein U is k Representing the voltage value at the node, P k And Q k Representing the active power and the reactive power flowing into the k node respectively, P pv,k And Q pv,k Representing active power and reactive power emitted by a kth photovoltaic power supply, P load,k And Q load,k Representing the local load at the k node. The relationship between the lateral and longitudinal components of the voltage drop and the voltage is:
the graph of the relationship phasor of the node voltage and the transverse component and the longitudinal component of the voltage drop is shown in figure 2. The difference of smaller voltage amplitude of the phase angle difference between two node voltages in the low-voltage distribution network is mainly influenced by the longitudinal component of voltage drop, and the influence of the transverse component can be ignored. The voltage difference between the nodes is thus:
in a low-voltage distribution network, the R/X value is large, the reactive power is changed to have limited effect on voltage regulation, so that the active power is necessary to be used for regulating the voltage, but the economic benefit of the photovoltaic is affected by the reduction of the active power. From the above equation, it can be seen that the overvoltage condition at the line end is most serious when the illumination is particularly sufficient. If the voltage is controlled to cut the active power in accordance with the grid-connected voltage alone, the active output of the line-end photovoltaic power supply is cut down significantly and the active power of the front-end photovoltaic power supply is cut down very little, which compromises the benefits of the line-end photovoltaic users. Therefore, the photovoltaic power supplies are matched with each other, so that the voltage regulating capability of each photovoltaic inverter can be better exerted while the economic benefit of each user is ensured.
The reactive power control strategy adopts Q-U droop control, the voltage is the voltage amplitude at the grid-connected point, and the reactive power output by the photovoltaic inverter is determined according to the voltage at the grid-connected point. Wherein the relation between reactive power and grid-connected point voltage
Reactive power causes more loss, so the voltage dead zone of the Q-U control is set in consideration of economic efficiency. And when the grid-connected point voltage is in the dead zone range, the photovoltaic inverter operates with unit power. When the voltage exceeds U limit And outputting the maximum reactive power by the photovoltaic inverter. Dead zone voltage and U selected by this patent limit Reactive tuning Q-U curves of 1.02 and 1.05, respectively, are shown in fig. 3.
The upper limit of reactive power regulation is limited by the minimum power factor of the system and the maximum capacity of the photovoltaic inverter, and the control strategy selects 0.7 as the minimum power factor. Wherein F is min For the minimum power factor, P is the active power emitted by the current photovoltaic, and S is the rated capacity of the photovoltaic inverter.
When the voltage at the grid connection point is increased to U set And continues to rise, the active output needs to be cut down to control the rise in voltage. The illumination and the temperature in one day are continuously changed, the power output by the photovoltaic power supply is also changed along with the time, and the photovoltaic power supply has the following functions at any momentWith a maximum output power P max . Taking an active power control strategy at a certain moment as an example: detecting the voltage at the grid connection position, when the voltage exceeds U set In this case, the photovoltaic power supply reduces the output power. Active power curtailed to 0.9P max The photovoltaic inverter then reaches the regulation limit of the active power, corresponding to the left border of the hatched area in fig. 4.
The shaded area in fig. 4 is the operation area of the photovoltaic power supply under the control strategy of the present patent. When the regulation limit of the active power is reached, if the voltage continues to rise beyond U set The photovoltaic power supply will send a signal to the upper stage photovoltaic inverter. Take the example of node 4 in fig. 1.
Once the signal transmitted by the next stage is detected, the photovoltaic power supply of the next stage is indicated to reach the active power regulation limit and the grid-connected point voltage is out of limit. After detecting the signal, the photovoltaic power supply takes the voltage at the current grid-connected point as a new U set 。
The photovoltaic cell can adjust the output power by adjusting the operating voltage of the photovoltaic cell. As can be seen from fig. 6, when the output power of the photovoltaic cell is to be cut, the operating voltage is to be increased; when the output power of the photovoltaic cell is to be increased, the operating voltage of the photovoltaic cell is to be reduced. The overall flow chart of the active power regulation strategy of this patent is shown in fig. 5.
In order to ensure the stability of the system during normal operation, the photovoltaic power supply should work in the area II. When the system voltage is normal, the photovoltaic power supply is in the maximum power tracking operation state and operates at the point M in fig. 6. When the voltage exceeds U set The active power is continuously cut down until the active power reaches 0.9P max I.e., from point M to point a in fig. 6, the system reaches the regulation limit when operating to point a.
The larger dP/dU value at operating point a to prevent larger power fluctuations, two steps may be used in curtailing power and MPPT as in fig. 5. The photovoltaic cell can work at the point A, and the maximum power of the photovoltaic cell needs to be calculated in real time.
Wherein I and U are respectively the output current and voltage of the photovoltaic cell, I ph Is photo-generated current, I0 is diode reverse saturation current, n is diode management ideal factor, U th For temperature potential, R s Is of series internal resistance, R sh Is the parallel internal resistance. From the above, it can be seen that the direct solving of the maximum power formula is complex and has large calculation amount, and is difficult to directly solve on a small controller. To simplify the calculation, a quadratic polynomial with respect to illumination and temperature can be used to calculate the photovoltaic maximum power.
P in formula (12) max And the maximum output power of the photovoltaic cell under the corresponding temperature and illumination conditions is T and G, and the temperature and the illumination are respectively. The coefficients in the formula may be determined by P of the photovoltaic cell max Linear regression analysis was performed on the G and T data. A photovoltaic module consists of a number of cells connected in series and parallel, and the whole module can be estimated in the same way and the total power is equal to the sum of the powers of the small cells, provided that the array is not occluded.
P max ＝a 1 T+a 2 T 2 +b 1 G+b 2 G 2 +cTG+d
The data was subjected to linear regression using photovoltaic cells of the SPR-305E-WHT-D type to derive parameters for its maximum power estimation. The parameters are carried into an estimation formula, the estimated maximum power estimated value is compared with the actual maximum power of the photovoltaic cell, and the result is shown in table 1. Fig. 7 is a graph of power estimates for different illumination and temperatures, with points in the graph being data for 16 photovoltaic cells used in the regression calculation. It can be seen from fig. 8 and table 1 that this method has a high accuracy for the calculation of the maximum power.
Table 1 maximum power estimate vs. actual
The proposed auxiliary voltage regulation strategy uses a power line as a transmission line for communication, and realizes communication between photovoltaic inverters by injecting a small alternating current signal into the system.
In the communication process, as shown in fig. 8, when the voltage is out of limit after the k-th photovoltaic power supply reaches the regulation limit, the photovoltaic power supply k can intermittently inject k times of small alternating current signals at regular intervals in a short time. When a small alternating current signal with k times is detected in a short time, the kth-1 photovoltaic power supply can adjust U according to voltage regulation measures set,k-1 Setting the current grid-connected point voltage value U k-1 。
The communication strategy injects a signal into the system by superimposing a small alternating current signal on the original modulated wave, and only one photovoltaic power supply can output the small alternating current signal at the same time. In fig. 9, it can be seen that when any one photovoltaic power source outputs a small ac signal, the other power sources can detect the signal at the grid-connected point. The amplitude of the detected signal is the amplitude obtained by impedance voltage division after the attenuation of the filtering link of the small alternating current signal, so that the threshold value is comprehensively considered according to the filter and the network parameters when the threshold value is set.
In order to avoid erroneous judgment and false operation caused by insufficient accuracy of an extraction algorithm during detection, frequencies near a system fundamental wave are avoided during frequency selection of a small alternating current signal. And the grid connection point of the photovoltaic power supply is close to the load, so that harmonic frequencies which are easy to generate by civil and industrial loads are avoided when the frequencies are selected. In addition, the alternating current side of the photovoltaic inverter is provided with a filter, so that the selected small signal frequency cannot be too high. The signal frequency must be below the cut-off frequency of the inverter ac test filter at which it is set to prevent excessive attenuation of the signal as it passes through the filter. The frequency of the small alternating current signal is selected to be the fundamental wave doubling frequency of 100Hz, and meanwhile, the amplitude of the small alternating current signal is selected to be taken into consideration of the total harmonic content of the system. It is generally required that the grid-connected total harmonic content of the inverter is not more than 5% and that the single harmonic content is not more than 3%.
To verify the control strategy of this patent, the topology in fig. 1 was simulated on Matlab. pv 1-4 are four photovoltaic power sources with the same parameters, U of each photovoltaic power source set Initial values were 1.05. The voltage amplitude of the distribution network selected in the simulation is 311V, and the amplitude of the small alternating current signal is 5V.
As the illumination intensity is continuously enhanced near noon, the active power output by the photovoltaic power supply is continuously increased. According to fig. 2, as the photovoltaic power output increases, the voltage at each node in the grid increases. The control strategy of the patent is utilized for voltage regulation, and the output power of pv4 and pv3 are taken as examples and are shown in fig. 11 and 12. The photovoltaic power supply continuously rises along with the voltage U of the grid-connected point, and reactive power is absorbed from the power grid according to the method. P in the figure max The maximum power value estimated by the maximum power estimation method is the maximum power value estimated by the maximum power estimation method, and the photovoltaic power supply is always in the MPPT state when the system does not reach the regulation limit. It can be seen from FIGS. 10 and 11 that when U 4 When the voltage exceeds Uset, pv4 starts to cut down active power when reactive power is limited by the minimum power factor of the system and starts to cut down. When P pv,4 Equal to 0.9P max When pv4 reaches the regulation limit, a small ac signal is injected into the system. As shown in FIG. 12, when pv3 detects a small AC signal from pv4, U will be set,3 Set to current U 3 With a value of 1.046, pv3 begins to cut active power as the voltage continues to rise.
And each photovoltaic power supply carries out Fourier analysis on the grid-connected point voltage in real time, and extracts signals with the frequency of 100Hz in the grid-connected point voltage. As shown in FIG. 13, there are 4 small AC signals in a short time, at this time, it can be judged that pv4 has reached the regulation limit, and when this signal is detected, pv3 starts responding to set the voltage at this time to be a new U set,3 。
The control strategy of the active and reactive coordination of the power supply voltage regulation system is compared with the traditional RPC control strategy. Under the same working condition, the voltages of each node under the control of two different strategies at different moments are shown in fig. 14.
As can be seen in fig. 14, reactive power regulation can be effected to some extentThe voltage is regulated, but the voltage is quickly out of limit when the power of the photovoltaic power supply is high. From Table 2 in U 4 For example, the voltage of the power grid is 1.069 under the condition of no control, the voltage of the power grid is reduced to 1.059 after reactive power control is added, the node voltage is reduced to 1.054 after the active power is reduced by 10% by the control strategy of the photovoltaic power supply, and the regulating effect of the photovoltaic power supply on the voltage of the power grid is quite obvious.
Table 2 node voltages at 12 pm.
The feasibility of the proposed method was verified by simulation and compared with the conventional reactive voltage Regulation (RPC) method. Simulation results show that the method has better voltage regulating effect than the RPC method when the illumination amplitude is larger. Although the photovoltaic power source reduces the output active power to the point of reduced photovoltaic utilization when the proposed method is employed compared to RPC control strategies. However, the method balances the reduction of active power among a plurality of photovoltaic power sources, and ensures the economic benefit of each photovoltaic power source as much as possible.
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 distributed photovoltaic auxiliary voltage regulation control method based on the small alternating current signals is characterized in that the method reduces the power grid voltage by a method of combining active power reduction and reactive power absorption, so that when the power grid voltage is in a normal state, a photovoltaic power supply operates at a maximum power point in a unit power factor, and the voltage regulation capability of a photovoltaic system is exerted to the greatest extent; the power flow distribution of active power is controlled in the low-voltage distribution network, so that the regulation and control of the power grid voltage can be effectively realized; and small alternating current signals are injected into the power grid through the photovoltaic system, so that coordination among different photovoltaic systems is realized, the photovoltaic system on the radial line achieves a better regulation and control effect on the line voltage, and the specific strategies are as follows:
1) Increasing the absorption reactive mode: when the grid-connected point voltage is in the voltage regulation dead zone, each photovoltaic power supply operates in a unit power working mode and operates at a maximum power point by using a maximum power tracking (Maximumpower point tracking, MPPT) algorithm; when the voltage of the grid-connected point continues to rise and exceeds the voltage dead zone range, the photovoltaic power supply starts to absorb reactive power corresponding to the voltage of the grid-connected point, and the photovoltaic still operates in an MPPT mode at the moment;
2) Active power reduction mode: when the voltage of the grid-connected point continues to rise above a set value, reducing the photovoltaic output power by utilizing a variable power tracking (Flexible Power Point Tracking, FPPT) technology; the variable power tracking can enable the photovoltaic to work on the right side of the maximum power point by adjusting the working voltage of the photovoltaic panel; according to the characteristics of the right area of the maximum power point of the photovoltaic P-U characteristic curve, the photovoltaic power is cut and subtracted by improving the photovoltaic working voltage; the scheme can exert the photovoltaic voltage regulating capability to the greatest extent, works at the maximum power point when the voltage of the power grid is lower than the limit value, and can ensure the power generation benefit when the photovoltaic participates in the voltage regulation of the power grid;
3) Pressure regulating limit mode: when the photovoltaic system reaches the regulation limit and the voltage of the power grid continues to rise, continuously injecting small alternating voltage signals into the power grid to send information of reaching the regulation limit to the upper-level photovoltaic; when the upper-level photovoltaic power supply detects the signal, the grid-connected point voltage at the moment is used as a voltage set value of a self voltage control strategy, and then the voltage regulation of the power grid is participated according to the new voltage set value;
4) Increasing the reactive mode: when the voltage of the grid-connected point is in a voltage regulation dead zone, each photovoltaic power supply operates in a unit power working mode and operates at a maximum power point by a maximum power tracking algorithm; and when the voltage of the grid-connected point is reduced beyond the voltage dead zone range, the photovoltaic power supply starts to emit reactive power corresponding to the voltage of the grid-connected point.
2.The method for controlling the voltage regulation of the distributed photovoltaic auxiliary system based on the small alternating current signal according to claim 1, wherein the mode determining principle is as follows: Δu=u according to the current grid voltage range g -U N Wherein DeltaU is the power grid voltage deviation, U g For the grid voltage, U N Rated voltage of the power grid; when the delta U is less than or equal to 0.02U N When the photovoltaic system works in a normal mode, the voltage of the power grid is not regulated; when 0.02U N <ΔU<U set When the power grid voltage rises but does not reach the set voltage limit U set The photovoltaic system works in an absorption reactive mode; when DeltaU>U set When the grid voltage is too high, the limit value U is exceeded set The photovoltaic system works in a mode of reducing active power; when DeltaU>0.02U N When the voltage of the power grid is reduced beyond the dead zone range, the photovoltaic system works in a reactive mode.
3. The method for controlling the voltage regulation of the distributed photovoltaic assistance based on small alternating current signals according to claim 2, wherein the mode comprises three steps: if the voltage is higher or lower than 0.02U N The voltage regulation strategy of the photovoltaic inverter is started, and a reactive power reference value Q is obtained, wherein the formula is as follows:
the upper limit of reactive power regulation is limited by the minimum power factor F of the system min And the rated capacity S limit of the photovoltaic inverter according to the maximum reactive power value Q max The reactive power reference value Q can be obtained, and the calculation formula of the maximum reactive power is as follows:
where P is the active power emitted by the current photovoltaic.
4. The method for controlling the auxiliary voltage regulation of the distributed photovoltaic system based on the small alternating current signals according to claim 1, wherein the photovoltaic inverter cuts the photovoltaic power by a fixed percentage at most according to the voltage of the grid-connected point; in order to ensure the economic benefit of the photovoltaic, fitting the maximum power of the current photovoltaic according to the temperature T and irradiance G of the current environment, and limiting the active power range of the system participating in the voltage regulation of the power grid by utilizing the fitted maximum power; and P is generated according to the voltage regulating strategy ref Adjusting the working point of the photovoltaic to a curve on the right side of the maximum power according to dP/dU information of the current working point of the photovoltaic by using an FPPT technology, and keeping the output power of the photovoltaic consistent with the reference power; when the maximum power of the photovoltaic is smaller than the reference power P ref And when the photovoltaic power generation system is in operation, the MPPT technology is utilized to enable the photovoltaic power generation system to work at the maximum power point according to dP/dU information of the current working point of the photovoltaic.
5. The distributed photovoltaic auxiliary voltage regulation control method based on small alternating current signals according to claim 1, wherein coordination among multiple stages of photovoltaics is achieved by injecting a small alternating current signal into a power distribution network; small ac voltage signal U signal Is a high-frequency signal, which is superimposed on the reference voltage U output by the inner ring controller ref Finally, the alternating current signal is injected into the power grid through SPWM (Sinusoidal Pulse Width Modulation) modulation, and the modulation voltage U of SPWM thereof SPWM The formula is:
U SPWM ＝U ref +U signal
when the voltage continues to rise after the kth photovoltaic power supply reaches the regulation limit, the photovoltaic power supply intermittently injects k times of small alternating current signals into the power grid in a short time; when a small alternating current signal with k times is detected in a short time, the k-1 th photovoltaic power supply can control the voltage limit value U in the photovoltaic power supply control strategy according to the voltage regulating measure set,k-1 Setting the current grid-connected point voltage value U k-1 。
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