CN112086997B - Photovoltaic coordination frequency modulation control method based on variable power tracking and super capacitor storage - Google Patents

Photovoltaic coordination frequency modulation control method based on variable power tracking and super capacitor storage Download PDF

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CN112086997B
CN112086997B CN202010822391.0A CN202010822391A CN112086997B CN 112086997 B CN112086997 B CN 112086997B CN 202010822391 A CN202010822391 A CN 202010822391A CN 112086997 B CN112086997 B CN 112086997B
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CN112086997A (en
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颜湘武
王晨光
马宏斌
贾焦心
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North China Electric Power University
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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
    • 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
    • H02J3/28—Arrangements for balancing of the load in a network by storage of energy
    • 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
    • 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
    • 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
    • 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
    • Y02E70/00—Other energy conversion or management systems reducing GHG emissions
    • Y02E70/30—Systems combining energy storage with energy generation of non-fossil origin

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Abstract

Aiming at the problems that the frequency regulation capability of a power system is reduced due to large-scale grid connection of a photovoltaic unit and the existing frequency modulation strategy influences the power generation benefit and the stability of the system, the invention provides the frequency modulation strategy of the photovoltaic system based on variable power point tracking and energy storage unit coordination control, namely primary frequency modulation control is carried out in a mode of variable power tracking response frequency upper disturbance and energy storage device response frequency lower disturbance, so that the photovoltaic system can operate at the maximum power point under the disturbance-free condition. The strategy reduces half of the energy storage capacity without losing the power generation benefit. The method discloses the reason of the inertia support power drop phenomenon caused by the frequency change rate measured by the phase-locked loop, and accordingly provides a method for correcting the frequency change rate measured value by using a first-order high-pass link. The economical efficiency and the control effect of the proposed coordination control scheme and the active standby scheme are compared, and the result shows that the economical efficiency and the technical performance of the proposed coordination control scheme are superior to those of the active standby scheme.

Description

Photovoltaic coordination frequency modulation control method based on variable power tracking and super capacitor storage
Technical Field
The invention relates to the field of distributed generation microgrid inverter control, in particular to a photovoltaic coordination frequency modulation control method based on variable power tracking and super-capacitor energy storage.
Background
From the end of 2015 to the present, the installed capacities of photovoltaic and wind turbines in China are continuously located at the top of the world and still show a growing trend. By the end of 2019, the cumulative installed capacity of the photovoltaic grid-connected power generation in China reaches 204.68GW (accounting for 10.18% of the total installed capacity), the solar power generation will become a mainstream energy utilization form in the future, and the popularization of the photovoltaic power generation also brings negative effects on the stable operation of a power grid. Because the photovoltaic unit generally adopts a Maximum Power Point Tracking (MPPT) algorithm, the output power of the MPPT algorithm is determined by the illumination and temperature of the external environment, and the MPPT algorithm cannot participate in primary frequency modulation of the system and lacks the inertial damping characteristic of the conventional generator, and the power system is more easily affected by power fluctuation and system failure. Aiming at the problem, part of high-permeability countries/regions clearly require that the new energy generator set has certain capacity of participating in power grid frequency modulation.
In order to realize participation in grid frequency regulation, there are generally two types of ways. One type is to configure an energy storage device, which is mainly used to stabilize photovoltaic output fluctuation and respond to grid-side frequency disturbance. The types of electrochemical energy storage configured are mainly accumulator, super capacitor or hybrid energy storage. Compared with storage battery energy storage, the super capacitor has the advantages of high power density, short charging and discharging time, environmental protection and the like, has more outstanding performance in the aspect of stabilizing bus voltage, and is suitable for being used as an energy buffer unit in a photovoltaic grid-connected system. The scheme of configuring the energy storage device by the new energy machine set has been researched more, and the scheme is divided into a light storage integrated grid-connected structure and a photovoltaic and energy storage independent grid-connected structure. Both of these structures have applications in domestic demonstration projects. However, some problems still exist in application practice, such as unclear quantization indexes of the matching test specifications, and yet, the calculation setting of key parameters, the configuration of the energy storage unit and the economic benefit are to be optimized and improved.
The other method is to reserve a certain spare capacity for participating in frequency modulation by adopting active spare control. The original intention of proposing the active standby scheme is that the cost of configuration energy storage scheme and maintenance cost are comparatively expensive, and partial area's photovoltaic power plant still exists "abandons light" phenomenon moreover, but the photovoltaic power generation benefit has been reduced to the active standby scheme. The energy storage scheme has the characteristics of strong controllability, capability of multi-mode and multi-scene operation, and is particularly suitable for off-grid modes and night scenes.
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 this, the invention further provides a photovoltaic unit frequency modulation strategy based on Variable Power Point Tracking (VPPT) and energy storage unit coordination control on the basis of giving consideration to frequency response and power generation benefits, including inertia support and primary frequency modulation. The specific technical scheme is as follows.
A photovoltaic coordination frequency modulation control method based on variable power tracking and super capacitor energy storage is characterized in that primary frequency modulation control is performed in a mode of variable power tracking response frequency up-disturbance and energy storage device response frequency down-disturbance, so that a photovoltaic system can operate at a maximum power point under the condition of no disturbance, and the strategy reduces half of energy storage capacity while not losing power generation benefit; based on analysis of reasons of inertia support power drop caused by measuring frequency change rate by a phase-locked loop, a first-order high-pass link is provided to correct a frequency change rate measured value, so that the economical efficiency and the technical performance of the existing scheme are improved, and the specific strategy is as follows:
1) Frequency up-scrambling mode: when the system works normally, the MPPT algorithm is operated at the maximum power point; when the load is reduced or the frequency is increased, starting a VPPT algorithm to enable a photovoltaic system to start to operate from a maximum power point to a region on the right side of the maximum power point of a P-U characteristic curve of a photovoltaic unit; the scheme can not only increase the adjustable range of power, but also output the power with the maximum power when the power generator normally works, thereby optimizing the power generation benefit;
2) Frequency down-scrambling mode: when the frequency is reduced, the photovoltaic system operates at the maximum power point and has no available frequency modulation capacity, so that the reasonable configuration of the energy storage device is the key for improving the regulation capacity of the photovoltaic system; the energy storage device is connected with a photovoltaic direct-current side bus capacitor through a bidirectional DC-DC converter; on one hand, the super capacitor can output high power instantly due to high power density; on the other hand, the cycle times are more, and the requirement of frequent charge and discharge is met;
3) Inertia support control mode: the energy storage device has the characteristic of quick response, can provide quick inertia power support besides participating in the primary frequency modulation process, and generally adopts a frequency differential feedback scheme for inertia support control, wherein the reference value delta P of the inertia support control sc2 The formula is as follows: delta P sc2 ＝k H df/dt, wherein k H Is a virtual inertia coefficient; the rate of change of the grid frequency df/dt during a disturbance can be measured and recorded in real time by a Phase Locked Loop (PLL) or Frequency Locked Loop (FLL).
The upper interference mode specifically comprises three steps: if the frequency is higher than the primary frequency modulation dead zone of 0.033Hz, the primary frequency modulation control is started, and a power reference value delta P is obtained pv The formula is as follows:
Figure BSA0000215144480000021
further obtaining the photovoltaic reference power value P when the frequency is higher ref The formula is as follows: p ref ＝P M +ΔP pv The target value P is realized by adopting VPPT algorithm ref The tracking of (2).
The lower disturbance mode is to prevent overcharge or overdischarge, and the energy storage system restrains a state of charge (SOC) value, so that the method comprises the following steps: SOC min ≤SOC(t)≤SOC max Wherein, SOC min Is SOC lower limit value; SOC max Is an upper limit value; under the condition of no disturbance, the energy storage system is charged with constant power to keep the SOC at the SOC max (ii) a When frequency disturbance occurs, the super-capacitor energy storage system is charged and discharged according to the sum of the primary frequency modulation power and the inertia support power reference value, namely P scref ＝ΔP sc1 +ΔP sc2 Wherein the primary frequency-modulated power reference value Δ P sc1 The formula is as follows:
Figure BSA0000215144480000031
the inertia support control mode provides correction for the measurement of the frequency change rate of the PLL 1; the specific method comprises the following steps: when the df/dt measured by PLL1 is greater than a limit value and reaches a maximum value R max Then, a first-order high-pass segment R is selected max The response of Hs/(Hs + 1) as a measure of the rate of change of frequency; when df/dt is less than the limit or does not reach the maximum R max When the frequency is selected to be df/dt directlyA measure of rate of change; inertia power reference value delta P using corrected frequency change rate measurement results sc2 When df/dt is greater than a limit and reaches a maximum value R max Then, pass through a first-order high-pass link k H R max -k H R max Calculating reference value delta P of/Hs +1 sc2 (ii) a When df/dt is less than the limit or does not reach the maximum R max Then, the reference value Δ P is calculated by the original equation sc2 The calculation formula is as follows:
Figure BSA0000215144480000032
wherein, t R To reach a maximum value R for df/dt max Time of (t) off And ending the first-order high-pass link control when df/dt is small (set to be 0.1 Hz/s) at the moment of exiting the first-order high-pass link control instruction, and switching back to the frequency differential link control again.
The invention has the beneficial effects that: the strategy is based on MPPT, when the system is not disturbed, the photovoltaic operation is at the maximum power point, when the frequency of the network side is increased, the photovoltaic output is reduced through the variable power operation point, and when the frequency of the network side is reduced, the energy storage device provides the spare capacity to participate in the frequency regulation. The strategy can reduce half of the energy storage capacity without losing the power generation benefit. In addition, in the aspect of control effect, aiming at the problem that the phase-locked loop under the sudden power disturbance is difficult to accurately measure the actual frequency and the frequency change Rate (RoCoF), the improved method is analyzed and provided by establishing a small signal model of the frequency response of the optical storage system. The invention has certain reference significance for technical upgrading and reconstruction of a large-scale photovoltaic power generation system. .
Drawings
FIG. 1 is a P-U characteristic curve of a photovoltaic unit;
FIG. 2 is an active standby control block diagram;
FIG. 3 is a flow chart of a variable power tracking algorithm;
FIG. 4 is a diagram of a photovoltaic system energy storage configuration;
FIG. 5 is two PLL structures and their linearization models;
FIG. 6 is a simulation system;
FIG. 7 is a graph of the measurement and model responses of PLL1 and PLL 2;
FIG. 8 is a small signal model of the power frequency response of an exemplary system;
FIG. 9 shows the measurement result of the frequency change rate of PLL1 and the corrected measurement result;
FIG. 10 is a block diagram of coordinated control of variable power tracking and super-capacitor storage
FIG. 11 shows simulation results under different conditions
Fig. 12 is a waveform of the energy storage unit in case of sudden load change in case of scheme 3.
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.
The active standby control is based on the output characteristic curve of the photovoltaic unit. The stable operation area of the photovoltaic unit is a right area, and the left area is an unstable area. As shown in the P-U characteristic curve of FIG. 1, the point A is an active standby point of the photovoltaic unit at a load shedding rate of d%, and the output power is P A (ii) a M point is the maximum power point when the photovoltaic unit normally operates, and the output power is P M . According to the national standard "general rule of virtual synchronous machine technology", it can be known that: during the system frequency reduction, the photovoltaic virtual synchronous machine should increase the active output and always not lower than the active output before the primary frequency modulation, the maximum value of the increase of the active output is at least 10% N ，P N Is rated power; during the system frequency rising period, the photovoltaic virtual synchronous machine should reduce the active output and always not higher than the active output before primary frequency modulation, the maximum value of the reducible amount of the active output is at least 10% N . Therefore, the load shedding ratio d% of the system can be set to 10%, P B The power point is the lowest power point of the photovoltaic unit during the frequency rise. When the photovoltaic unit normally operates, if active standby control is adopted, a current maximum power point P is determined firstly M . Corresponding to a post-load shedding output power P' of
P′＝P A ＝P M -d％P N
In order to enable the photovoltaic system to participate in primary frequency modulation, a primary frequency modulation control link with droop characteristics is introduced, so that the photovoltaic system running at an active standby point A can autonomously respond to frequency change under network side load disturbance, as shown in an active standby control block diagram 2. The system initially runs at the maximum power point P by using the MPPT algorithm M (ii) a Then switching to an active standby algorithm according to the maximum power point P M The after-load operating point P' can be determined. MPPT was then resumed every 10 minutes. As can be seen from FIG. 2, the power reference in the active standby mode is given by the following formula, where P is ref Is a reference power value for the photovoltaic.
P ref ＝P M -d％P N +ΔP pv
When no frequency disturbance exists, the photovoltaic unit operates at an active standby point; when the dead zone with the frequency deviation larger than +/-0.033 Hz is detected, the primary frequency modulation loop acts, and the caused photovoltaic output deviation is delta P pv ：
Figure BSA0000215144480000051
In the above formula k f Is the primary frequency modulation coefficient, f is the actual frequency of the power grid, f N Is the nominal frequency.
Because the system constantly switches the operation between MPPT control and active standby control, easily influence the stability of system. And the photovoltaic system is in an active standby mode during normal operation, and the lost generating capacity is P under the condition that the load shedding rate is 10 percent M -P A The benefits of photovoltaic power generation will be greatly lost if the photovoltaic generator is operated for a long time. When a reduction in the output occurs as a result of an increase in the grid-side frequency, the photovoltaic direct voltage increases further and the limit is easily reached. Therefore, although the frequency fluctuation of the system is relieved to a certain extent by adopting the method, the actual power generation benefit of the photovoltaic power station is influenced, and the problem of power regulation depth exists. Therefore, the maximum power generation benefit of the maximum power tracking algorithm in normal operation needs to be further explored, namely a mode combining variable power tracking and super-capacitor energy storage.
For the coordination control of variable power tracking and super capacitor energy storage, which is provided by the patent, a working mode of combining a VPPT algorithm and super capacitor energy storage is adopted, and when a system works normally, an MPPT algorithm is operated at a point M; when the load is reduced or the frequency is increased, the VPPT algorithm is started to enable the photovoltaic system to run to the right area from the point M. The scheme can not only increase the adjustable range of power, but also output the power with the maximum power during normal operation, thereby optimizing the power generation benefit.
Judging the disturbance type according to the network side frequency fluctuation, wherein delta f = f-f N . When the delta f is more than 0.033Hz, the system load is reduced or the frequency is increased; when delta f is less than-0.033 Hz, the system load is increased or the frequency is reduced; when the | delta f | is less than or equal to 0.033Hz, the system has no disturbance. And adopting different control modes according to different disturbance conditions. In the disturbance-free case, the photovoltaic system operates in MPPT mode.
The VPPT algorithm flow is shown in fig. 3.
When the frequency is reduced, the photovoltaic system operates at the maximum power point and has no available frequency modulation capacity, so that the reasonable configuration of the energy storage device is the key for improving the regulation capacity of the photovoltaic system. As shown in fig. 4, the energy storage device is connected to the photovoltaic DC-side bus capacitor through the bidirectional DC-DC converter.
The energy storage device has the characteristic of quick response, and can provide quick inertia power support besides participating in the primary frequency modulation process. Inertia support control is usually performed by a frequency differential feedback scheme, with reference to a reference value Δ P sc2 ，k H Is the virtual inertia coefficient. The rate of change df/dt of the grid frequency during a disturbance can be measured and recorded in real time by a Phase Locked Loop (PLL) or Frequency Locked Loop (FLL). However, the PLL or FLL is difficult to accurately measure the actual frequency and frequency change rate response under sudden power disturbance, and the manufacturing effect is poor. Taking two typical phase-locked loops as an example, the structure is shown in fig. 5 (a) and 5 (b). The difference between PLL1 and PLL2 is the difference in the loop filter. The loop filters of PLL1 and PLL2 are a first-order inertia element and a PI (proportional-integral) element, respectively, as shown in the following formula. In the formula G LPi (i =1,2) is the loop filter transfer function of PLL1 and PLL 2. Linearity of two phase locked loopsThe chemical model is shown in FIG. 5 (c).
Figure BSA0000215144480000061
An exemplary system is shown in fig. 6. First, the dynamic response of two phase-locked loops under the condition of active load sudden-load is tested by a simple system consisting of a virtual synchronous machine (VSG) and a load without considering the access of the optical storage unit. The reason why the PLL is difficult to accurately measure the actual frequency and the frequency change rate under sudden power disturbance is that: the actual system has two processes of sudden phase angle change and frequency change under power disturbance; the frequency and frequency rate response of the PLL are caused by these two processes, and the abrupt change in phase angle makes it difficult to accurately measure the frequency and frequency rate. It is worth noting that no attention has been paid to the current research on the reasons why the frequency change rate is difficult to be measured accurately and the improved method. A linearization model is established below to verify the dynamic process of the PLL.
In the linearized model, the transfer function of the frequency response is given by the following equation, P load For load active power, the superscript ^ represents the small disturbance amount. The transfer function of the frequency rate of change response is the frequency response transfer function multiplied by the s operator.
Figure BSA0000215144480000062
The simulated measurements of frequency and rate of change of frequency and the response of the linearized model are shown in fig. 7, and it can be seen that: the frequency and the frequency change rate of the PLL2 are suddenly changed, the dynamic performance is poor, and the measurement accuracy of the frequency and the frequency change rate of the PLL1 is high. However, there is still an oscillation fluctuation caused by sudden phase angle change in the measurement result of the frequency change rate of the PLL1 at the initial stage of disturbance.
Further, a complete linearization model of the example system considering the access of the optical storage unit is constructed, as shown in fig. 8. The two types of responses are consistent, and the constructed linearized model can accurately describe the dynamic characteristics of the example system. The access of the optical storage unit is mainly to reduce the steady-state frequency deviation, and the oscillation fluctuation of the frequency change rate of the PLL1 caused by the sudden change of the phase angle still exists in the initial stage of disturbance. Correspondingly, this may cause the inertia supporting power provided by the energy storage system to fall back/drop, weakening the inertia control effect.
To this end, the patent proposes to correct the measurement of the rate of change of the frequency of the PLL 1. The specific method comprises the following steps: when the df/dt measured by PLL1 is greater than a limit value and reaches a maximum value R max Then, a first-order high-pass segment R is selected max The response of Hs/(Hs + 1) as a measure of the rate of change of frequency; when df/dt is less than the limit or does not reach the maximum R max Then, df/dt is directly chosen as a measure of the rate of change of frequency. The corrected frequency change rate measurement result is shown in fig. 9, and it can be seen that the corrected measurement value can approximately represent the frequency change rate of the actual system, thereby avoiding the oscillation fluctuation of the measurement result.
In summary, a frequency modulation strategy block diagram of the optical storage system based on variable power tracking and energy storage unit coordination control is shown in fig. 10.
With respect to the capacity configuration of the supercapacitor energy storage device, the capacity configuration of the energy storage unit needs to meet the energy requirements of inertia support and primary frequency modulation. If the capacity of the energy storage device is too small, sufficient spare capacity cannot be provided to participate in frequency adjustment; if the capacity of the energy storage device is too large, the cost of the energy storage system is increased, and certain capacity is wasted. The capacity of the energy storage device needs to be reasonably set. The photovoltaic system is configured 10% P, taking as an example a centralized photovoltaic inverter with a capacity of 500kW N The time of the stored energy participating in inertia frequency modulation and primary frequency modulation is 30s, the stored energy capacity W is reserved e 50kW X30 s. Comprehensively considering the problems of cost and discharge efficiency of the super capacitor bank, 2 series-5 parallel 10 super capacitors of 160V multiplied by 12F are adopted as the energy storage device of the photovoltaic unit, and the lowest working voltage U of the energy storage device is min 16.8V, the highest working voltage U max 320V, its discharge efficiency eta d ＝97.70％。
And (4) economic evaluation aspect: and comparing and analyzing the economical efficiency of the active standby scheme and the scheme of configuring the energy storage device. The photovoltaic panel model of the 500kW photovoltaic system is SPR-415E-WHT-D, and the photovoltaic array consists of 7 x 176 photovoltaic panels. Based on the illuminance and temperature data recorded in the Germany area, the sampling frequency of the data is 1Hz, and the total recording time is 365 days. And calculating the generated power of the photovoltaic system according to a photovoltaic array output model of Q/GDW 1994-2013. Table 1 lists the annual power loss and annual monetary loss for the photovoltaic system when active standby mode is employed. Therefore, the annual economic loss under the condition of no electricity limitation is 4.08 ten thousand yuan, and even if 50 percent of the annual economic loss is limited by electricity, the annual economic loss reaches 1.93 ten thousand yuan.
TABLE 1 photovoltaic System operating economics analysis with active standby mode
Figure BSA0000215144480000071
Taking a super-capacitor module and an energy storage converter product provided by a certain company as an example, table 2 shows the one-time investment details of a 500kW photovoltaic system configured with a 50kW × 30s super-capacitor energy storage unit. The one-time investment is about 15.8 ten thousand yuan, wherein the super capacitor is depreciated according to 10 years, the depreciation of other system equipment can be calculated according to 20 years, and the average annual investment is about 1.58 ten thousand yuan.
TABLE 2 energy storage system investment details
Figure BSA0000215144480000072
Table 3 compares the technical economics of the two frequency modulation schemes. It can be known that the frequency modulation scheme using active standby can provide power support for a long time, but the economic loss caused by abandoned light is huge; in contrast, only one-time investment is needed for configuring the energy storage device, and the annual average investment amount is only half of the loss caused by the reserved standby power. And because the super capacitor energy storage device participates in the frequency modulation process, the optical storage system can rapidly provide inertia support power, and the technical performance is more complete. Therefore, in general, it is an optimal frequency modulation scheme to configure the energy storage device in the photovoltaic power station.
TABLE 3 comparison of technical economics of two modes of application
Figure BSA0000215144480000081
The design simulation system verifies the proposed theoretical analysis.
The example system of fig. 6 simulates a weak grid with a 500kVA voltage source VSG series line impedance. The photovoltaic system has the capacity of 1MVA and comprises two light storage and power generation units. Taking the frequency modulation depth as delta f max And the frequency modulation capacity of the optical storage system accounts for 10 percent of the rated capacity by = 0.5 Hz. The sum of the virtual inertia power and the primary frequency modulation power is called the frequency modulation power, and its size is not more than 10% N Thus, the primary and inertia coefficients are taken as follows:
Figure BSA0000215144480000082
under the small disturbance of grid frequency reduction, the frequency domain relation of power and frequency is as follows:
Figure BSA0000215144480000083
in order to verify various performances of the coordination control scheme of the optical storage system, three frequency modulation schemes are compared, which are respectively as follows:
scheme 1, scheme/MPPT mode not participating in frequency modulation;
scheme 2, active standby primary frequency modulation;
and in scheme 3, inertia support and primary frequency modulation of energy storage and VPPT coordinated control are realized.
The simulation results of the three schemes are shown in FIG. 11, and the simulation situation is shown in Table 4. Waveforms of dc side voltage, total output active power of the optical storage system, VSG frequency and frequency change rate are shown in fig. 11.
TABLE 4 simulation scenarios
Figure BSA0000215144480000091
To verify the correction method for the PLL1 frequency change rate measurement proposed in this patent, the two inertia control methods are compared with scheme 3, as shown in fig. 11 (a). It can be seen that with conventional control, a power droop due to df/dt measurement can occur under sudden load increases, resulting in an energy storage unit that is not able to efficiently provide inertia support power. And adopt this patent scheme can avoid appearing the power condition of falling, and the energy storage unit can provide inertia effectively and support power and reduce the frequency rate of change.
The working condition setting of the load sudden change is shown in a table 4, and the simulation results of the three schemes are shown in a figure 11 (b). The waveform of the energy storage unit under the control of the scheme 3 is shown in figure 12. It can be seen that, in the scheme 2, as the load increases/decreases and the direct current voltage decreases/increases, the primary frequency modulation is performed through the active standby algorithm. Although an inertia control link can also be added in the active standby scheme, the response speed of the active standby tracking algorithm is slower than that of the super-capacitor energy storage system, and the effect of reducing the frequency change rate is weaker than that of the light storage system coordination control scheme 3. In the scheme 3, when the load suddenly increases, the energy storage unit is in a discharge state, and inertia support power and primary frequency modulation power are provided at the same time; and when the load is suddenly reduced, the energy storage unit is in a charging state and provides inertia support by absorbing less electric quantity, and primary frequency modulation is performed by the photovoltaic system in a VPPT (voltage potential transformer) mode.
Compared with the three schemes, the schemes 2 and 3 can reduce the steady-state frequency deviation, but only the scheme 3 can reduce the frequency change rate, so that the effect of restraining the rapid frequency fluctuation is achieved.
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 (3)

1. A photovoltaic coordination frequency modulation control method based on variable power tracking and super capacitor energy storage is characterized in that primary frequency modulation control is performed in a mode of variable power tracking response frequency up-disturbing and energy storage device response frequency down-disturbing, so that a photovoltaic system can operate at a maximum power point under the condition of no disturbance, and the energy storage capacity is reduced by half while the power generation benefit is not lost; based on analysis of reasons of inertia support power drop caused by measuring frequency change rate by a phase-locked loop, a first-order high-pass link is provided to correct a frequency change rate measured value, so that the economical efficiency and the technical performance of the existing scheme are improved, and the specific strategy is as follows:
1) Frequency disturbance mode: a frequency up-scrambling mode and a frequency down-scrambling mode;
2) Inertia support control mode: the energy storage device has the characteristic of quick response, can provide quick inertia power support besides participating in the primary frequency modulation process, and generally adopts a frequency differential feedback scheme for inertia support control, wherein the reference value delta of the frequency differential feedback scheme is controlledP sc2 The formula is as follows:
Figure DEST_PATH_IMAGE002
(ii) a Wherein the content of the first and second substances,k H is a virtual inertia coefficient; rate of change d of grid frequency during disturbancef/dtReal-time measurement and recording by a Phase Locked Loop (PLL) or Frequency Locked Loop (FLL); the inertia support control mode corrects the measurement of the frequency change rate of the PLL 1; the specific method comprises the following steps: d when PLL1 measuresf/dtIs greater than the limit value and reaches the maximum valueR max Then, a first-order high-pass link is selected
Figure DEST_PATH_IMAGE004
As a measure of the rate of change of frequency; when d isf/dtLess than a limit or not reaching a maximumR max When d is directly selectedf/dtAs a measure of the rate of change of frequency; inertia power reference value delta using corrected frequency rate of change measurementsP sc2 When d is calculatedf/dtIs greater than the limit value and reaches the maximum valueR max Then, through a first-order high-pass link
Figure DEST_PATH_IMAGE006
Calculating a reference value deltaP sc2 (ii) a When d isf/dtLess than a limit or not reaching a maximumR max When passing through the original meterCalculating a reference value deltaP sc2 The calculation formula is as follows:
Figure DEST_PATH_IMAGE008
wherein the content of the first and second substances,t R is df/dtReaches the maximum valueR max At the time of the day,t off at the moment of exiting the first-order high-pass link control command, at df/dtThe first-order high-pass link control is finished for 0.1Hz/s, and the frequency differential link control is switched back again.
2. The photovoltaic coordinated frequency modulation control method based on variable power tracking and super capacitor storage according to claim 1, wherein the frequency disturbance mode is as follows:
1) Frequency up-scrambling mode: when the system works normally, the maximum power point is operated by a Maximum Power Point Tracking (MPPT) algorithm; when the load is reduced or the frequency is increased, a variable power tracking (VPPT) algorithm is started to enable the photovoltaic system to start to operate to the photovoltaic unit from the maximum power pointP-UThe right area of the maximum power point of the characteristic curve; the scheme can not only increase the adjustable range of power, but also output the power with the maximum power when the power generator normally works, thereby optimizing the power generation benefit;
2) Frequency down-scrambling mode: when the frequency is reduced, the photovoltaic system operates at the maximum power point and has no available frequency modulation capacity, so that the reasonable configuration of the energy storage device is the key for improving the regulation capacity of the photovoltaic system; the energy storage device is connected with a photovoltaic direct-current side bus capacitor through a bidirectional DC-DC converter; on one hand, the super capacitor can output high power instantaneously due to high power density; on the other hand, the circulation frequency is more, and the requirement of frequent charge and discharge is met.
3. The photovoltaic coordinated frequency modulation control method based on variable power tracking and super capacitor storage as claimed in claim 1, wherein the mode determination principle is as follows: judging the disturbance type, delta, from the network side frequency fluctuationf = f – f N (ii) a When deltaf >0.033 At Hz, the system load is reduced or the frequency is increased; deltaf <At-0.033 Hz, the system load increases or the frequency decreases; | Δf When the | is less than or equal to 0.033Hz, the system is undisturbed; adopting different control modes according to different disturbance conditions;
the upper interference mode specifically comprises three steps:
the first step is as follows: if the frequency is higher than the primary frequency modulation dead zone of 0.033Hz, the primary frequency modulation control is started, and a power reference value delta is obtainedP pv The formula is as follows:
Figure DEST_PATH_IMAGE010
the second step: further obtaining the photovoltaic reference power value when the frequency is higherP ref The formula is as follows:
Figure DEST_PATH_IMAGE012
the third step: achieving target values using VPPT algorithmP ref Tracking of (2);
the lower disturbance mode is to prevent overcharge or overdischarge, and the energy storage system is to charge state (SOC) The values are constrained to have:
Figure DEST_PATH_IMAGE014
wherein, in the step (A),SOC min is composed ofSOCA lower limit value;SOC max is an upper limit value; under the condition of no disturbance, the energy storage system carries out constant-power charging so as to ensure thatSOCIs held atSOC max ；
When frequency disturbance occurs, the super capacitor energy storage system is charged and discharged according to the sum of the primary frequency modulation power and the inertia support power reference value, namelyP scref =ΔP sc1 +ΔP sc2 Wherein the primary frequency-modulated power reference value DeltaP sc1 The formula is as follows:
Figure DEST_PATH_IMAGE016
and under the disturbance-free condition, the photovoltaic system operates in the MPPT mode.
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