CN103869872A - Method for intelligent control of maximum power point tracking of cascade multi-level photovoltaic grid-connected system - Google Patents
Method for intelligent control of maximum power point tracking of cascade multi-level photovoltaic grid-connected system Download PDFInfo
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Abstract
The invention relates to a method for intelligent control of maximum power point tracking of a cascade multi-level photovoltaic grid-connected system. The method is suitable for the cascade multi-level photovoltaic grid-connected system composed of a photovoltaic cell panel group, a cascade inverter, a signal detection module, a control module and a power grid. The method comprises a first step of establishing a mathematical model according to a topological structure of the cascade multi-level photovoltaic grid-connected system; a second step of determining the value range of the voltage modulation ratio of every inversion unit in the cascade inverter according to the mathematical model; a third step of utilizing the control module to simultaneously perform PWM control on every inversion unit in the cascade inverter; a fourth step of controlling voltage balance of inversion unit direct-current side capacitors according to the mathematical model to achieve maximum power output of the cascade multi-level photovoltaic grid-connected system. Compared with the prior art, the cascade multi-level photovoltaic grid-connected system can remarkably improve conversion efficiency, and intelligent control of the maximum power output is achieved.
Description
Technical field
The present invention relates to a kind of new energy technology, especially relate to a kind of intelligent control method of cascade multilevel photovoltaic grid-connected system MPPT maximum power point tracking.
Background technology
The energy is the important substance basis that the mankind depend on for existence and development, is the power of world economy, is also the key factor that affects national security.Along with the mankind have caused these limited energy reserves to be petered out to the continuous exploitation utilization of ore fuel, force every country in the world all be devoted to the development and utilization of new forms of energy.Greatly develop regenerative resource, replace fossil energy by regenerative resource comprehensively, carry out a new industrial revolution, be not only the needs for human survival, especially the inexorable trend of world economy sustainable development.
The plurality of advantages such as distribution of solar energy is extensive, reserves are abundant, and the long and maintenance of clean, the safety of photovoltaic generating system, life-span is little, photovoltaic generation is considered to most important, most active new forms of energy of 21 century.Photovoltaic industry and the market demand have obtained fast development in recent years, and sun power has started the role transforming to " substitute energy " by " supplementing the energy ".It is estimated, the solar system can also exist 4,500,000,000 years, and the energy that the annual sun provides is 1.5 ten thousand times of world population commodity consumption amount.The exploitation of sun power will obtain significant progress, in world's energy structure shifts, will take on heavy responsibilities, and will certainly become the leading energy from now on.
In the situation that photovoltaic power generation technology is subject to extensive concern, it is very outstanding that the technical research of the core component of parallel network power generation---combining inverter has seemed.Wherein, cascaded multilevel inverter is as the one of multi-electrical level inverter, have advantages of other versions incomparable.Concrete manifestation is as follows:
(1) to bear voltage low for single power device, without dynamic voltage-balancing;
(2) output level number is many, and waveform improves;
(3) switching frequency is low, and electronic switching device loss is little;
(4), without output transformer, the structural volume of system and the loss of itself have greatly been reduced;
(5), based on full-bridge circuit structure, easily adopt modular combination to carry out extension system structure.
But must be pointed out, in cascade connection multi-level input circuit, the mutual imbalance of spread voltage on each capacitor of its DC side is sort circuit version " congenital " deficiency, the phenomenon certainly existing, is called for short " DC capacitor voltage imbalance "." DC capacitor voltage imbalance " can cause device withstand voltage inhomogeneous, also can make MPPT maximum power point tracking encounter difficulties.Therefore, make cascade multilevel photovoltaic grid-connected system can efficiently realize MPPT maximum power point tracking, just need a kind of method that can guarantee DC capacitor voltage balance to realize power stage and maximize, power factor that simultaneously can also the output of improvement system.
Described " MPPT maximum power point tracking ", is the simple saying that photovoltaic maximum power point is followed the tracks of, English referred to as MPPT.Its essential meaning is: in the time that photovoltaic array output voltage is smaller, along with the variation of voltage, output current changes very little, and photovoltaic array is similar is a constant current source; In the time that voltage exceedes certain critical value and continues to rise, electric current sharply declines, and photovoltaic array is now similar is a constant pressure source.The output power of photovoltaic array is along with the rising of output voltage has an output power maximum point.The effect of maximal power tracing device is in the environment all changing in temperature and radiation intensity, by change photovoltaic array with equivalent load, regulate the working point of photovoltaic array, photovoltaic array is operated on output power maximum point.In other words, the output characteristics of photovoltaic array has nonlinear characteristic, and its output is subject to the impact of intensity of illumination, environment temperature and loading condition.But under certain intensity of illumination and environment temperature, when photovoltaic array is only operated in a certain magnitude of voltage, the output power of photovoltaic array just can reach maximal value, therefore in photovoltaic generating system, improve the whole efficiency of system, an important approach is exactly to adjust in real time the working point of photovoltaic array, makes it to always work near maximum power point.
The variation major effect solar cell output current size of solar radiation intensity of illumination, temperature variation major effect output voltage size.When temperature is identical, along with the increase of intensity of illumination, the open-circuit voltage of solar-energy photo-voltaic cell is almost constant, and because short-circuit current increases to some extent, corresponding peak power output increases; When intensity of illumination is identical, along with the rising of temperature, the open-circuit voltage of solar-energy photo-voltaic cell declines, although short-circuit current also increases to some extent, system peak power output reduces.The rising that the output power of photovoltaic array is accompanied by output voltage has a process of falling after rising.At certain temperature and intensity of illumination, solar cell has unique maximum power point, in the time that photovoltaic cell is operated in this, can export the peak power under Current Temperatures and intensity of illumination condition.The intersection point of load curve and solar cell current-voltage characteristic curve is the working point of solar cell.In order to obtain maximum output power, be necessary to make solar cell and load to match.The maximum power output of solar-energy photo-voltaic cell array is mainly to realize by adjusting load curve under the environmental baseline changing, and this is also the fundamental basis of current all maximum power point tracing methods.Along with the variation of temperature and intensity of illumination, must constantly adjust output load working point, photovoltaic array is on corresponding peak power output point always.Described " MPPT maximum power point tracking of cascade multilevel photovoltaic grid-connected system ", just refers to technical matters how to go to realize MPPT maximum power point tracking in cascade multilevel photovoltaic grid-connected system.
For cascaded multilevel inverter, owing to there being multiple full bridge inverters (being called for short " inverter bridge "), can consider to realize MPPT by two kinds of methods.General traditional method is that output current and the voltage sample to whole inverter carries out MPPT control, and this method control is simple, and realization is easier to; But because cascaded inverter adopts multipath independent direct current source input, the working condition of each inverter bridge has difference, adopts the MPPT of this traditional approach can produce power loss, can not guarantee that each inverter bridge can both export with maximal efficiency.Therefore, must use respectively MPPT to control to each inverter bridge, regulate the output voltage of each inverter bridge, could realize the maximum power output of each inverter bridge, thereby could improve the efficiency of whole inverter.
The regulation technology of bridge-type inverter output voltage is had to two kinds of modes: a kind of is the PWM dutycycle that regulates inverter, and another kind is the voltage swing that regulates input end DC bus capacitor.The former regulative mode is more convenient, but dutycycle is too high or when too low, wave level can reduce, and then can cause waveform distortion.The input side of cascade connection multi-level photovoltaic DC-to-AC converter is photovoltaic array, because the output of photovoltaic array is subject to the impact of temperature, intensity of illumination, can cause output voltage unstable, will inevitably there is difference in the DC input voitage of each inverter bridge, thereby make the voltage at DC bus capacitor device two ends occur imbalance problem, the voltage that causes electronic switching device to bear is inconsistent, when serious, also can make portions of electronics switching device damage because bearing high voltage stress, MPPT control stiffness be caused to further negative effect simultaneously.Therefore, the maximum power output of cascade connection multi-level photovoltaic DC-to-AC converter be realized, the balance of voltage problem of DC side input capacitor must be first solved.
Find by prior art documents, Zhou Jiaqi, week dimension are come, " control method of generating electricity by way of merging two or more grid systems of photovoltaic generation current transformer " (Chinese Patent Application No.: the control method of generating electricity by way of merging two or more grid systems that 201110341180.6) a kind of photovoltaic generation current transformer is provided of Zhang Zhe etc.It comprises line voltage software phlase locking, outer voltage foundation, Current Control and the control of line voltage directional vector, line voltage software phlase locking adopts the method for software three-phase phase-locked loop, phaselocked loop is made up of phase detector, loop filter and oscillator, the control of line voltage directional vector adopts two closed loop tandem type control structures: outer voltage, current inner loop, Voltage loop control DC bus-bar voltage; Electric current loop is controlled AC input current.This invention can be accomplished accurately directed, controls DC bus-bar voltage stable, realizes MPPT maximum power point tracking control.But, because this invention is only carried out dead beat control design for electric current loop, the current-order that electric current loop provides according to Voltage loop is controlled AC input current, DC capacitor voltage is not regulated accordingly, is therefore in fact difficult to realize unity power factor operation.
Also find by prior art documents again " energy-saving type cascade multilevel photovoltaic grid-connected generating control system " (Chinese Patent Application No.: 201010234866.0) disclose a kind of energy-saving type cascade multilevel photovoltaic grid-connected generating control system of Ge Baoming.Wherein, multi-electrical level inverter is made up of multiple accumulation energy type photovoltaic generating modules, and multiple module series connection are the phase in three-phase inverter, and each module single-stage power conversion realizes buck, inversion and energy storage, and the wide region that adapts to photovoltaic cell voltage changes.Photovoltaic cell and energy-storage battery are distributed in respective modules, and independent control of each module generating, realizes distributed maximal power tracing.Because " each module generating independent control " strengthened the complicacy of circuit structure, certainly will increase Technological Economy cost, and reduce system reliability of operation.
Summary of the invention
Object of the present invention is exactly the intelligent control method that a kind of cascade multilevel photovoltaic grid-connected system MPPT maximum power point tracking is provided in order to overcome the defect that above-mentioned prior art exists, the method can make cascade multilevel photovoltaic grid-connected generating system significantly improve conversion efficiency, and reaches the intellectuality control of maximum power output.
Object of the present invention can be achieved through the following technical solutions:
A kind of intelligent control method of cascade multilevel photovoltaic grid-connected system MPPT maximum power point tracking, the method is applicable to the cascade multilevel photovoltaic grid-connected system being made up of photovoltaic battery panel group, cascaded inverter, signal detection module, control module and electrical network, comprises the following steps:
Step 1, sets up mathematical model according to the topological structure of cascade multilevel photovoltaic grid-connected system;
Step 3, control module is carried out PWM control to the each inversion unit in cascaded inverter simultaneously;
Step 4, the balance of voltage control according to mathematical model to inversion unit DC bus capacitor, reaches the maximum power output of cascade multilevel photovoltaic grid-connected system.
Cascade multilevel photovoltaic grid-connected system comprises: photovoltaic battery panel group, cascaded inverter, signal detection module, control module, electrical network.Photovoltaic battery panel group output interface is connected with the power input interface of cascaded inverter, the signal output interface of signal detection module is connected with the feedback signal input interface of control module, the steering order output interface of control module is connected with the steering order input interface of cascaded inverter, and the power output interface of cascaded inverter is connected with electrical network input interface.Photovoltaic battery panel group is under the irradiation of sunshine, there is photoelectric effect and produce the output of corresponding d. c. voltage signal and be delivered to the power input interface of cascaded inverter by its output interface, signal detection module detects the feedback signal of obtaining transports to the feedback signal input interface of control module by its signal output interface, control module is transported to cascaded inverter steering order input interface by steering order through its steering order output interface according to feedback signal computing, and the power output signal of cascaded inverter is delivered to the input interface of electrical network through its power output interface.
Described photovoltaic battery panel group, is made up of multiple photovoltaic battery panels.Each photovoltaic battery panel is formed by multiple photovoltaic cell unit combination, forms a kind of flat matrix structure.In each photovoltaic battery panel, first form photovoltaic cell group by multiple photovoltaic cells unit, then form a photovoltaic battery panel by multiple photovoltaic cell groups.Each photovoltaic cell unit output voltage signal in each photovoltaic cell group is connected in series one by one successively mutually, be that the anodal termination of first photovoltaic cell unit and the negative pole termination of second photovoltaic cell unit are connected, the like, the last anodal termination of last photovoltaic cell unit by each photovoltaic cell group and the negative pole end capitiform of first photovoltaic cell unit become the positive and negative output termination of each photovoltaic cell group, in the time of operation, can obtain the addition of each photovoltaic cell unit output voltage signal.Corresponding by the positive and negative output termination of multiple photovoltaic cell groups and fetch the positive and negative output termination as each photovoltaic battery panel again, the positive and negative output termination of each photovoltaic cell group is connected with each other, can form and have enough electric power output in the time of operation.Multiple photovoltaic battery panels are arranged together just forms photovoltaic battery panel group, separate between each photovoltaic battery panel, be them does not mutually directly connect on electric, but the positive and negative output termination of each photovoltaic battery panel respectively with the corresponding connection in positive and negative input termination of each inversion unit in cascaded inverter.The output interface of photovoltaic battery panel group is to be formed side by side by the positive and negative output termination of each photovoltaic battery panel.
Described cascaded inverter, is made up of multiple inversion units.The positive and negative input termination of each inversion unit respectively independently and connect a capacitor, with the corresponding connection in positive and negative output termination of photovoltaic battery panel, formed side by side independently of each other the power input interface of cascaded inverter by the positive and negative input termination of each inversion unit again.The positive and negative input of each inversion unit termination and the capacitor that connects be referred to as cascaded inverter DC bus capacitor, the voltage producing on the each capacitor of cascaded inverter DC side is referred to as cascaded inverter DC capacitor voltage, or is referred to as cascaded inverter input side capacitance voltage.Connect mutually in the output termination of each inversion unit, it is the right output termination of second inversion unit of left output termination connection of first inversion unit, the left output termination of second inversion unit connects the right output termination of the 3rd inversion unit, by that analogy, the left output termination of penultimate inversion unit connects the right output termination of last inversion unit, finally take the left output termination of last inversion unit and the right output termination of first inversion unit as two of cascaded inverter export terminations.Because cascaded inverter is to be formed by multiple inversion unit cascades, so be referred to as cascaded multilevel inverter, be called for short cascaded inverter.Described inversion unit is a kind of bridge circuit being made up of 4 power electronic devices, claims again H bridge.
Described signal detection module, comprising: input interface, front end signal processor and output interface.The photovoltaic voltage signal input part head of input interface is connected with each photovoltaic battery panel positive and negative electrode output termination, the inverter output voltage signal input part head of input interface is connected with each inversion unit output termination, and the mains voltage signal input termination of input interface is connected with grid side voltage measuring point; The output termination of input interface is connected with the input termination of front end signal processor; The output termination of front end signal processor is connected with the input termination of output interface, and the output termination of output interface is connected with the feedback signal input interface of control module.That is to say, signal detection module detects the voltage signal of the positive and negative output termination of each photovoltaic battery panel, the output voltage signal of each inversion unit simultaneously, and the voltage signal of grid side.
In the mathematical model of step 1, set up cascade multilevel photovoltaic grid-connected system output end current/voltage equation as follows:
Wherein, L is electrical network equivalent inductance value; R is electrical network equivalent resistance; V
gfor line voltage value; V
nbe n inversion unit output voltage values, it and n inversion unit input voltage value
it is the magnitude of voltage on corresponding DC bus capacitor device
have inversion unit input/output voltage equation as follows:
Wherein, d
nfor the voltage modulated ratio of inversion unit.
Magnitude of voltage on DC bus capacitor device, that is n inversion unit input voltage value
with inversion unit input end electric current
constituent relation equation is again:
Wherein, C is the capacitance of inversion unit DC bus capacitor device;
for appearing at n the magnitude of voltage on inversion unit DC bus capacitor device, c represents inversion unit DC bus capacitor device;
be n inversion unit input end electric current and n photovoltaic battery panel output current value, p represents photovoltaic battery panel; I
nbe n inversion unit output current value.N=1,2 ..., N, N is the included photovoltaic battery panel quantity of photovoltaic battery panel group.
N inversion unit output current value I
nwhen grid-connected with cascade connection multi-level photovoltaic system again, the series electrical flow valuve I of output forms inversion unit two ends power-balance relation equation and is:
In the time of I=0,
and then can obtain following equation from formula () and formula (two):
Because
can be expressed as:
Wherein,
be n inversion unit output voltage amplitude; ω is n inversion unit output voltage signal angular frequency;
be n inversion unit output voltage signal initial phase.
(formula six) substitution (formula five) is obtained:
Can be found out by (formula seven),
limited size in photovoltaic effect, belong to uncontrollable parameter, therefore only have by d
nwith
adjusting just can reach cascade multilevel photovoltaic grid-connected system to grid side voltage V
gcritical balance, both errors approach minimum.
Because V
gfor fixed value, in the time pursuing the critical balance of (formula seven), d
ncan not be excessive, otherwise can cause the minimizing of level and the distortion of waveform, therefore need to determine d
nrange of adjustment, | d
n|≤D.The value of D is definite by testing, as: D=1, now ,-1≤d
n≤ 1.
In step 3, adopt voltage type PWM or current mode PWM, and controlled the output voltage signal of each inversion unit by same pwm control signal simultaneously.
The control of step 4 DC capacitor voltage balance, comprises step by step following:
Step by step 1, regulate the size of inversion unit voltage modulated ratio;
Adopt voltage dicyclo to add the control method of electric current loop, by the stack of multiple error signals, obtain inversion unit voltage modulated than signal d
n.
Step by step 2, regulate the magnitude of voltage of the DC bus capacitor of inversion unit: first adopt voltage close loop control that the average voltage of the magnitude of voltage of each inversion unit DC bus capacitor and all inversion unit DC bus capacitors is compared, the error signal of acquisition is used for adjusting to the initial phase of corresponding inversion unit, the magnitude of voltage of the DC bus capacitor of this inversion unit is changed, then according to the distribution of power proportions, inversion unit is redistributed, then regulated the output valve of each inversion unit; Need by relatively producing of each inversion unit output current value and meritorious electrical reference signal flow valuve the current value compensating, for keeping magnitude of voltage and the compensation power loss of DC bus capacitor simultaneously;
Step by step 3, the relatively size of the magnitude of voltage of inversion unit DC bus capacitor and the average voltage of all inversion unit DC bus capacitors, when
time, return to step by step 1; When
time, carry out step by step 4; In formula,
the magnitude of voltage of each inversion unit DC bus capacitor,
for the mean value of all inversion unit DC capacitor voltages, ε serves as reasons and tests the enough little parameter of determining;
Step by step 4, process and obtain critical equilibrium point, calculate
value, and with electrical network in the actual measurement magnitude of voltage of voltage measuring point
compare, when
time, return to step by step 1; When
time, return to step 4, continue the maximum power point of cascade multilevel photovoltaic grid-connected system to follow the tracks of; Wherein, σ is the enough little parameter by electric system standard.
The value of parameter ε can be taken as 0.01, and the value of parameter σ can be taken as 0.05.
Compared with prior art, the present invention carries out MPPT maximum power point tracking control to each inversion unit of each cascaded inverter simultaneously, by each inversion unit input/output voltage modulation ratio and DC capacitor voltage being regulated on the basis regulating at PWM, make each inversion unit output waveform degree of distortion drop to minimum, make the output power of photovoltaic battery panel group reach maximal value, finally obtain the MPPT maximum power point tracking of cascade multilevel photovoltaic grid-connected system.
Accompanying drawing explanation
Fig. 1 is the process flow diagram of each step of the present invention;
Fig. 2 is the structural drawing of cascade multilevel photovoltaic grid-connected system.
Embodiment
Below in conjunction with the drawings and specific embodiments, the present invention is described in detail.
Embodiment
As shown in Figure 1, a kind of intelligent control method of cascade multilevel photovoltaic grid-connected system MPPT maximum power point tracking, the method is applicable to by photovoltaic battery panel group 1, cascaded inverter 2, signal detection module 3, the cascade multilevel photovoltaic grid-connected system that control module 4 and electrical network 5 form, the structure of this system as shown in Figure 2, photovoltaic battery panel group 1 output interface is connected with the power input interface of cascaded inverter 2, the signal output interface of signal detection module 3 is connected with the feedback signal input interface of control module 4, the steering order output interface of control module 4 is connected with the steering order input interface of cascaded inverter 2, the power output interface of cascaded inverter 2 is connected with electrical network 5 input interfaces.Photovoltaic battery panel group 1 is under the irradiation of sunshine, there is photoelectric effect and produce the output of corresponding d. c. voltage signal and be delivered to the power input interface of cascaded inverter 2 by its output interface, signal detection module 3 detects the feedback signal of obtaining transports to the feedback signal input interface of control module 4 by its signal output interface, control module 4 is through to after the computing of feedback signal, steering order is transported to cascaded inverter 2 steering order input interfaces through its steering order output interface, the power output signal of cascaded inverter 2 is delivered to the input interface of electrical network 5 through its power output interface.
Described " feedback signal ", comprising: each photovoltaic battery panel positive and negative electrode output voltage signal, each inversion unit output voltage signal and grid side voltage signal.
Wherein, photovoltaic battery panel group 1 is made up of multiple photovoltaic battery panel 11～1N, the photovoltaic battery panel number that N comprises for photovoltaic battery panel group 1.Each photovoltaic battery panel is formed by multiple photovoltaic cell unit combination, forms a kind of flat matrix structure.In each photovoltaic battery panel, first form photovoltaic cell group by multiple photovoltaic cells unit, then form a photovoltaic battery panel by multiple photovoltaic cell groups.Each photovoltaic cell unit output voltage signal in each photovoltaic cell group is connected in series one by one successively mutually, be that the anodal termination of first photovoltaic cell unit and the negative pole termination of second photovoltaic cell unit are connected, the like, the last anodal termination of last photovoltaic cell unit by each photovoltaic cell group and the negative pole end capitiform of first photovoltaic cell unit become the positive and negative output termination of each photovoltaic cell group.Corresponding by the positive and negative output termination of multiple photovoltaic cell groups and connect using the positive and negative output termination as each photovoltaic battery panel again.Multiple photovoltaic battery panel 11～1N are arranged together just forms photovoltaic battery panel group 1, separate between each photovoltaic battery panel 11～1N, be them does not mutually directly connect on electric, but the positive and negative output termination of each photovoltaic battery panel 11～1N respectively with the corresponding connection in positive and negative input termination of each inversion unit 21～2N in cascaded inverter 2.The output interface of photovoltaic battery panel group 1 is to be formed side by side by the positive and negative output termination of each photovoltaic battery panel 11～1N.
Signal detection module 3 comprises: input interface, front end signal processor and output interface.The photovoltaic voltage signal input part head of input interface is connected with each photovoltaic battery panel 11～1N positive and negative electrode output termination, the inverter output voltage signal input part head of input interface is connected with each inversion unit 21～2N output termination, and the mains voltage signal input termination of input interface is connected with electrical network 5 side voltage measuring points; The output termination of input interface is connected with the input termination of front end signal processor, and the output termination of front end signal processor is connected with the input termination of output interface, and the output termination of output interface is connected with the feedback signal input interface of control module 4.By this connection signal detection module 3 except detecting the voltage signal, the output voltage signal of each inversion unit of positive and negative output termination of each photovoltaic battery panel, and the voltage signal of grid side.
The concrete steps of the intelligent control method of MPPT maximum power point tracking of the present invention as shown in Figure 1, comprise
Step 1, sets up mathematical model according to the topological structure of cascade multilevel photovoltaic grid-connected system.
Set up cascade connection multi-level photovoltaic system output end current/voltage equation by inversion unit 21～2N input end current/voltage equation, input/output voltage equation, two ends power-balance relation:
Because cascade multilevel photovoltaic grid-connected system output end current/voltage equation is as follows:
Wherein, L is electrical network equivalent inductance value; R is electrical network equivalent resistance; V
gfor line voltage value; V
nbe n inversion unit output voltage values, it and n inversion unit input voltage value
it is the magnitude of voltage on corresponding DC bus capacitor device
have inversion unit input/output voltage equation as follows:
Wherein, d
nfor the voltage modulated ratio of inversion unit.
Magnitude of voltage on DC bus capacitor device, that is n inversion unit input voltage value
with inversion unit input end electric current
constituent relation equation is again:
Wherein, C is the capacitance of inversion unit DC bus capacitor device;
for appearing at n the magnitude of voltage on inversion unit DC bus capacitor device, c represents inversion unit DC bus capacitor device;
be n inversion unit input end electric current and n photovoltaic battery panel output current value, p represents photovoltaic battery panel; I
nbe n inversion unit output current value.N=1,2 ..., N, N is the included photovoltaic battery panel quantity of photovoltaic battery panel group.
N inversion unit output current value I
nwhen grid-connected with cascade connection multi-level photovoltaic system again, the series electrical flow valuve I of output forms inversion unit two ends power-balance relation equation and is:
In the time of I=0,
and then can obtain following equation from (formula one) and (formula two):
Because
can be expressed as:
Wherein,
be n inversion unit output voltage amplitude; ω is n inversion unit output voltage signal angular frequency;
be n inversion unit output voltage signal initial phase.
(formula six) substitution (formula five) is obtained:
Can be found out by (formula seven),
limited size in photovoltaic effect, belong to uncontrollable parameter, therefore only have by d
nwith
adjusting just can reach cascade multilevel photovoltaic grid-connected system to grid side voltage V
gcritical balance, both errors approach minimum.
Step 3, control module adopts dsPIC33FJ64MC506 chip, and each inversion unit is carried out to the duty cycle adjustment under same pwm signal simultaneously.
DsPIC33FJ64MC506 chip is the Novel 16-bit high performance digital signal controller that Microchip company releases.It combines the control advantage of single-chip microcomputer and the high-speed computation characteristic of digital signal processor (DSP), for embedded system provides one chip solution.It has inherited Harvard's bus structure and reduced instruction set computer (RISC) technology of PIC single-chip microcomputer series, and addressing mode is simple, travelling speed is fast, low in energy consumption, kinetic force is strong, the simultaneously integrated DSP function of mainboard level, can provide powerful digital signal processing capability.In addition, also provide as abundant peripheral interfaces such as UART, CAN, SPI, can communicate interconnected with other equipment easily.Meanwhile, dsPIC33F adopts the Harvard structure processor of 16 and strengthens instruction set, comprising the support to digital signal processing.Its CPU has 24 bit instruction words, the opcode field of a variable-length.Programmable counter PC is 24 bit wides, and least significant bit (LSB) is 0, except some special instructions, in the normal execution of program, always ignores its highest significant position.Therefore, the user memory space that programmable counter PC can addressing 4M, the highest addressing can reach the user storage space of 4M × 24.But cycles per instruction prefetch mechanisms maintains handling capacity and predictable execution is provided with helping.Except instruction, double word move and the table meter reading write command of reprogramming stream, all instructions all complete execution within the single cycle.Use DO and REPEAT instruction to support the program loop structure without expense, at any time can interrupt the execution of these two instructions.
Step 4, the control of DC capacitor voltage balance, comprising: step by step 1, regulate inversion unit voltage modulated to compare d
nsize; Step by step 2, regulate inversion unit DC capacitor voltage
balance; Step by step 3, relatively
with
size; Step by step 4, seek grid-connected critical balance.Wherein:
Step by step 1, equally by dsPIC33FJ64MC506 chip as software carrier, adopt voltage dicyclo to add the control method of electric current loop, by the stack of multiple error signals, obtain inversion unit voltage modulated than signal d
n.
Step by step 2, first by voltage inter-loop control by each inversion unit input side capacitance voltage value
mean value with all inversion unit input side capacitance voltages
compare, the error signal obtaining for adjusting the initial phase of corresponding inversion unit
cause its DC capacitor voltage value generation respective change.Secondly, output voltage values is redistributed inversion unit according to the distribution of power proportions, then regulated the output of each inversion unit.Meanwhile, by the current value that relatively produces needs compensation of each inversion unit output current value and meritorious electrical reference signal flow valuve, for keeping capacitance voltage value and compensation power loss.
Step by step 3, when
time, return to step by step 1, continue to carry out corresponding computing; When
Time, carry out following step by step 4.
Step by step 4, calculate
numerical value, and with the actual measurement magnitude of voltage of power network
compare.
When
time, return to step by step 1, continue to carry out corresponding computing; When
time, return to step 4, continue to carry out the whole process control to cascade multilevel photovoltaic grid-connected system.
Result of implementation confirms:
(1) the present invention carries out MPPT control to each inversion unit simultaneously, can guarantee that the output voltage of each inversion unit has same dutycycle, therefore can set up further adjusting basis for the DC capacitor voltage balance of each inversion unit.
(2) by the adjusting to each inversion unit input/output voltage modulation ratio and DC capacitor voltage, make each inversion unit output waveform degree of distortion drop to minimum.
(3) final obtain " MPPT maximum power point tracking of cascade multilevel photovoltaic grid-connected system " reaches splendid effect, and the output power of photovoltaic battery panel group reaches maximal value.
Claims (7)
1. the intelligent control method of a cascade multilevel photovoltaic grid-connected system MPPT maximum power point tracking, the method is applicable to the cascade multilevel photovoltaic grid-connected system being made up of photovoltaic battery panel group, cascaded inverter, signal detection module, control module and electrical network, it is characterized in that, comprise the following steps:
Step 1, sets up mathematical model according to the topological structure of cascade multilevel photovoltaic grid-connected system;
Step 2, determines the span of the voltage modulated ratio of each inversion unit in cascaded inverter according to mathematical model;
Step 3, control module is carried out PWM control to the each inversion unit in cascaded inverter simultaneously;
Step 4, the balance of voltage control according to mathematical model to inversion unit DC bus capacitor, reaches the maximum power output of cascade multilevel photovoltaic grid-connected system.
2. the intelligent control method of a kind of cascade multilevel photovoltaic grid-connected system MPPT maximum power point tracking according to claim 1, it is characterized in that, described photovoltaic battery panel group comprises multiple photovoltaic battery panels, and each photovoltaic battery panel is formed by multiple photovoltaic cell unit combination, described cascaded inverter comprises the inversion unit of multiple cascades, the positive and negative input termination of each inversion unit respectively independently and connect a capacitor, then with the corresponding connection in positive and negative output termination of corresponding photovoltaic battery panel, described signal detection module, comprise input interface, front end signal processor and output interface, the photovoltaic voltage signal input part head of input interface and each photovoltaic battery panel are just, negative electrode output termination connects, the inverter output voltage signal input part head of input interface is connected with each inversion unit output termination, the mains voltage signal input termination of input interface is connected with the voltage measuring point of electrical network, the output termination of input interface is connected with the input termination of front end signal processor, the output termination of front end signal processor is connected with the input termination of output interface, the output termination of output interface is connected with the feedback signal input interface of control module, signal detection module connects each photovoltaic battery panel and each inversion unit, detects voltage signal, the output voltage signal of each inversion unit and the voltage signal of electrical network of the positive and negative output termination of each photovoltaic battery panel simultaneously.
3. the intelligent control method of a kind of cascade multilevel photovoltaic grid-connected system MPPT maximum power point tracking according to claim 2, is characterized in that, in the mathematical model described in step 1, thereby is derived and is obtained following formula and set up adjusting d by theory
nwith
can reach cascade multilevel photovoltaic grid-connected system to grid side voltage V
gthe theoretical foundation of critical balance:
In formula, V
gfor the magnitude of voltage of electrical network, N is the included photovoltaic battery panel quantity of photovoltaic battery panel group, d
nfor the voltage modulated ratio of inversion unit,
be n inversion unit output voltage amplitude, ω is n inversion unit output voltage signal angular frequency,
be n inversion unit output voltage signal initial phase;
The voltage equation of DC bus capacitor:
In formula,
be n inversion unit input end electric current or n photovoltaic battery panel output current value, p represents photovoltaic battery panel, I
nbe n inversion unit output current value, C is the capacitance of inversion unit DC bus capacitor device;
for appearing at n magnitude of voltage on inversion unit DC bus capacitor device or the magnitude of voltage of n inversion unit input end;
The power-balance relation equation of inversion unit input end and output terminal:
In formula, I is the series electrical flow valuve of the grid-connected time output of cascade connection multi-level photovoltaic system, V
nbe the output voltage values of n inversion unit,
for appearing at n magnitude of voltage on inversion unit DC bus capacitor device or the magnitude of voltage of n inversion unit input end.
4. the intelligent control method of a kind of cascade multilevel photovoltaic grid-connected system MPPT maximum power point tracking according to claim 3, is characterized in that, the span of step 2 voltage modulated ratio is | d
n|≤D, and D=1.
5. the intelligent control method of a kind of cascade multilevel photovoltaic grid-connected system MPPT maximum power point tracking according to claim 2, it is characterized in that, in step 3, adopt voltage type PWM or current mode PWM, and controlled the output voltage signal of each inversion unit by same pwm control signal simultaneously.
6. the intelligent control method of a kind of cascade multilevel photovoltaic grid-connected system MPPT maximum power point tracking according to claim 3, is characterized in that, step 4 comprises step by step following:
Step by step 1, regulate the size of inversion unit voltage modulated ratio;
Step by step 2, regulate the magnitude of voltage of the DC bus capacitor of inversion unit: first adopt voltage close loop control that the average voltage of the magnitude of voltage of each inversion unit DC bus capacitor and all inversion unit DC bus capacitors is compared, the error signal of acquisition is used for adjusting to the initial phase of corresponding inversion unit, the magnitude of voltage of the DC bus capacitor of this inversion unit is changed, then according to the distribution of power proportions, inversion unit is redistributed, then regulated the output valve of each inversion unit; Need by relatively producing of each inversion unit output current value and meritorious electrical reference signal flow valuve the current value compensating, for keeping magnitude of voltage and the compensation power loss of DC bus capacitor simultaneously;
Step by step 3, the relatively size of the magnitude of voltage of inversion unit DC bus capacitor and the average voltage of all inversion unit DC bus capacitors, when
time, return to step by step 1; When
time, carry out step by step 4; In formula,
the magnitude of voltage of each inversion unit DC bus capacitor,
for the mean value of all inversion unit DC capacitor voltages, ε serves as reasons and tests the enough little parameter of determining;
Step by step 4, process and obtain critical equilibrium point, calculate
value, and with electrical network in the actual measurement magnitude of voltage of voltage measuring point
compare, when
time, return to step by step 1; When
time, return to step 4, continue the maximum power point of cascade multilevel photovoltaic grid-connected system to follow the tracks of; Wherein, σ is the enough little parameter by electric system standard.
7. the intelligent control method of a kind of cascade multilevel photovoltaic grid-connected system MPPT maximum power point tracking according to claim 6, is characterized in that, the value of parameter ε is 0.01, and the value of parameter σ is 0.05.
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