CN109412182B - Modularized photovoltaic energy system without electrolytic capacitor and modulation method thereof - Google Patents
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Abstract
The invention relates to a modularized photovoltaic energy system without an electrolytic capacitor and a modulation method thereof, wherein the system comprises an integrated photovoltaic conversion module and a photovoltaic system main body module connected through a fluctuating direct current bus; the integrated photovoltaic conversion module is plug-and-play, and consists of a plurality of photovoltaic cells and a plurality of single-phase DC/DC converters, and each photovoltaic cell is independently connected with one single-phase DC/DC converter; the photovoltaic system main body module is composed of a high-power-density energy storage module and a single-phase inversion module, the input end of the single-phase inversion module is connected with the input end of the fluctuation direct-current bus, and the output end of the fluctuation direct-current bus is connected to a power grid. The system and the modulation method can prolong the service life of the photovoltaic system and improve the system efficiency.
Description
Technical Field
The invention relates to the technical field of new energy power generation, in particular to a modularized photovoltaic energy system without an electrolytic capacitor and a modulation method thereof.
Background
Since the twelve-five and thirteen-five planning, the nation pays more and more attention to the development and application of renewable energy technology, in the existing energy power generation system, solar power generation has a great proportion in renewable energy, and has a wide prospect due to the characteristics of no pollution and abundant and unlimited resources, so that a photovoltaic cell also becomes a new energy device, but due to the characteristics of volatility, randomness, intermittence and the like of the photovoltaic power generation, the output power is unstable and is greatly influenced by environmental factors.
In order to solve the above problems, an energy storage system is usually introduced into a photovoltaic power generation system, and the fluctuation of the output power of the photovoltaic cell is stabilized through a reasonable energy storage modulation and control method, so as to be used as a backup power supply and an energy buffer device. The photovoltaic cell outputs power mainly by generating direct current to be output, collecting the direct current to the same direct current bus through a corresponding converter, and then converting the direct current to alternating current through a DC/AC or DC/DC converter with unified amplification capacity to be transmitted to a power grid. In a typical photovoltaic inverter system, two-stage conversion is generally required to convert a variable direct-current voltage of a photovoltaic array into an alternating-current voltage with a fixed frequency of a power grid, and a first-stage configuration converts an unstable photovoltaic direct-current voltage into a stable direct-current voltage through a boost or buck DC/DC converter; the second stage configuration converts the regulated dc voltage through an inverter to an ac voltage that can be injected into the grid. In the existing photovoltaic system, an electrolytic capacitor is usually arranged between two stages to realize power decoupling between the two stages and generate stable direct-current voltage; generally, the service life of a photovoltaic cell assembly is as long as 25 years, but an electrolytic capacitor is added between two stages, and the electrolytic capacitor can only be used for 5 years or even lower under high-temperature working conditions, so that the service life of a photovoltaic energy system is seriously reduced.
Meanwhile, in most photovoltaic capacity systems in the prior art, a high-frequency switch is arranged between two stages of conversion, an inverter high-frequency inverter of the high-frequency switch cannot be connected with a full-load inductive load, the overload capacity difference can influence the efficiency of the whole two-stage conversion photovoltaic inverter system, the switching loss is large, and the utilization rate of photovoltaic solar energy is reduced.
In addition, most photovoltaic energy devices at the present stage are combined with a system technology, and are mostly concentrated on the reverse side of a topological structure and power flow directional control of a combined power generation system, and a photovoltaic maximum power tracking and grid-connected control strategy. Under the modulation of the traditional HPWM method, no matter whether the voltage of a direct current bus is a constant value or a sinusoidal change, one bridge arm of the single-phase inverter works in a high-frequency switching state, the other bridge arm works in a power frequency switching state, and the switching loss is high. In the symmetrical design of the single-phase inverter, the switching losses of the switching tubes of the two bridge arms are greatly different due to the difference of the switching states, so that thermal imbalance is further caused, the modulation control circuit is relatively complex, the control efficiency is low, and the operation of the system is difficult to effectively optimize and stabilize.
Disclosure of Invention
Aiming at the defects or shortcomings in the prior art, the invention provides a photovoltaic energy system without an electrolytic capacitor and a modulation method thereof.
In order to achieve the above object, the present invention adopts a technical solution of providing a photovoltaic energy system without an electrolytic capacitor, and is characterized in that: the photovoltaic conversion module comprises an integrated photovoltaic conversion module and a photovoltaic system main body module connected through a fluctuating direct current bus; the integrated photovoltaic conversion module is plug-and-play, and consists of a plurality of photovoltaic cells and a plurality of single-phase DC/DC converters, and each photovoltaic cell is independently connected with one single-phase DC/DC converter; the photovoltaic system main body module is composed of a high-power-density energy storage module and a single-phase inversion module, the input end of the single-phase inversion module is connected with the input end of the fluctuation direct-current bus, and the output end of the fluctuation direct-current bus is connected to a power grid.
As a further improvement of the invention, the high power density energy storage module comprises a high power density energy storage unit and a bidirectional DC/DC converter, wherein the bidirectional DC/DC converter is connected with the fluctuating direct current bus; the high-power-density energy storage unit is connected with the bidirectional DC/DC converter in parallel.
As a further improvement of the invention, the single-phase inversion module consists of 4 switching tubes (M)1，M2，M3，M4) Inductor LFCapacitor CFAnd a resistance RLForming; wherein, the switch M1And switch M2Series, switch M3And switch M4In series, and 4 switching tubes (M)1，M2，M3，M4) Each connected in parallel with a diode; inductor LFIs connected to the switch M1And switch M2Between the two ends are respectively connected with a capacitor CFAnd a resistance RLOne terminal of (C), a capacitorFAnd a resistance RLIs connected to the switch M at the other end3And switch M4Between, capacitance CFAnd a resistance RLAre connected in parallel.
Another objective of the present invention is to provide a modulation method for a modular photovoltaic energy system without electrolytic capacitors, in which only one set of bridge arms of a full bridge switch is on/off at a high frequency in a constant dc time (4 θ), the other set of bridge arm switches is on or off, and all switches are commutated within a low current range during high frequency operation; in sine fluctuation direct current time (2 pi-4 theta), the full bridge switch is completely in a conducting or a switching-off state, and theta is a modulation angle of the controllable system.
As a further improvement of the present invention, the modulation method is characterized in that: the method comprises the following steps:
(1)t0-t1interval, DC voltage is a constant value, switch tube M3Cut-off, switch tube M4Switching on and off tube M1And a switching tube M2Complementary conduction at higher frequency, switching tube M1The modulation signal of (2) is sin (ω t)/sin θ.
(2)t1-t2Interval, DC voltage is sine wave, switch tube M3In the off state, M4In the on state, the switch tube M1Off, M2And (4) opening.
(3)t2-t3Interval, switching tube on condition and t0-t1The interval state is the same.
(4)t3-t4Interval, constant DC voltage, switch tube M3Switching on and off tube M4Turn-off, switch tube M1And a switching tube M2Complementary conduction at higher frequency, switching tube M1The modulation signal of (2) is 1+ sin (ω t)/sin θ.
(5)t4-t5Interval, DC voltage is sine wave, switch tube M3In the on state, M4In the off state, the switch tube M1Turn-off, switch tube M2And (4) opening.
(6)t5-t6Interval, switching tube on condition and t3-t4The interval state is the same. (ii) a
Wherein, t0-t1In the interval 0 to theta, t1-t2Is in the interval 0-pi-theta, t2-t3Is the interval (pi-theta) -pi; t is t3-t4Is the interval of pi to (pi + theta); t is t4-t5Is in the interval of (pi + theta) to (2 pi + theta); t is t5-t6Is the interval of (2 pi-theta) -2 pi; omegaAnd theta is the power frequency angular velocity, and theta is the modulation angle of the controllable system.
The invention has the beneficial effects that:
1. the photovoltaic energy system provided by the invention cancels the electrolytic capacitor at the photovoltaic cell side in the photovoltaic conversion module and cancels the electrolytic capacitor between the photovoltaic conversion module and the single-phase inverter, thereby solving the problem that the service life of the photovoltaic system is influenced due to the electrolytic capacitor and being beneficial to prolonging the service life of the photovoltaic system.
2. The bidirectional DC/DC converter of the high-power-density energy storage module of the system can realize energy storage and can meet more requirements; further, photovoltaic modules in a multi-photovoltaic module system can be configured to have different ratings than other photovoltaic modules, maintaining interchangeability of photovoltaic modules with changes to the photovoltaic energy system, such that the photovoltaic energy system can maximize use of available roof space. Further, the plurality of photovoltaic modules may be configured to provide a plurality of photovoltaic arrays having different directional orientations, enhancing the sun-tracking and energy-collecting capabilities of the photovoltaic energy system.
3. In the system, the output power of the photovoltaic conversion module is constant, so that the tracking of the photovoltaic maximum power point is facilitated; the grid-connected single-phase inverter circuit adopts an improved Hybrid Pulse Width Modulation (HPWM) method, and the switching loss of the single-phase inverter can be reduced by more than 50%.
4. The modulation method of the system uses improved Hybrid Pulse Width Modulation (HPWM), which is composed of pulsating direct current modulation in a low frequency range and secondary modulation in a high frequency range, wherein the secondary modulation is used for realizing soft switching of a high frequency down converter, so that the switching loss can be further reduced;
drawings
Fig. 1 is a schematic diagram of a novel photovoltaic energy system.
Fig. 2 is a topological structure diagram of a single-phase inverter circuit of a photovoltaic energy system.
Fig. 3 is a modified Hybrid Pulse Width Modulation (HPWM) waveform diagram for an inverter circuit of a photovoltaic energy system.
Fig. 4 is a simulation waveform diagram of a photovoltaic energy system under the modulation of the improved HPWM.
FIG. 5 is a block diagram of DC bus voltage control.
Detailed Description
The following detailed description of the present invention is provided in conjunction with the accompanying drawings, but it should be understood that the scope of the present invention is not limited to the specific embodiments.
Throughout the specification and claims, unless explicitly stated otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element or component but not the exclusion of any other element or component.
Fig. 1 shows a schematic structural diagram of a modular electrolytic capacitor-less photovoltaic energy system according to a preferred embodiment of the present invention.
A circuit block diagram of a photovoltaic energy system according to an embodiment of the present invention is shown in fig. 1, and includes an integrated photovoltaic conversion module and a photovoltaic system main body module connected via a fluctuating DC bus, wherein the integrated photovoltaic conversion module is plug-and-play type and is composed of a plurality of photovoltaic cells and a plurality of single-phase DC/DC converters; each photovoltaic cell is individually connected with a single-phase DC/DC converter, and preferably, no electrolytic capacitor is arranged between each photovoltaic cell and the single-phase DC/DC converter; the photovoltaic system main body module of this embodiment comprises high power density energy storage module and single-phase contravariant module, and the input and the undulant direct current bus-bar of single-phase contravariant module are connected, and the output of undulant direct current bus-bar inserts the electric wire netting. The high-power-density energy storage module comprises a high-power-density energy storage unit and a bidirectional DC/DC converter which are connected in parallel, the preferable bidirectional DC/DC converter adopts a design without an electrolytic capacitor, and the bidirectional DC/DC converter is connected with a fluctuating direct current bus.
Further preferably, in this embodiment, the integrated photovoltaic conversion modules and the photovoltaic system main body module can adopt a plug-and-play interface, and the number and the grade of the integrated photovoltaic conversion modules are determined by application scenarios of different conditions. In the embodiment, preferably, the plurality of photovoltaic cells and the plurality of single-phase DC/DC converters are in a modular design, that is, the photovoltaic cells and the single-phase DC/DC converters are integrally designed and combined into a whole, so as to facilitate the rapid design and installation of the photovoltaic energy system. The single-phase DC/DC converter adopts a converter with common knowledge in the invention, preferably in the embodiment, the single-phase DC/DC converter adopts the Maximum Power Point Tracking (MPPT) which can realize the corresponding photovoltaic cell power supply, and feeds low-frequency changing power into the fluctuating direct current bus.
In the photovoltaic energy system of the invention, the direct current bus voltage of the fluctuating direct current bus is selected from a composite variable waveform consisting of constant direct current and sine fluctuating direct current, as shown in fig. 3. In each power frequency period, constant direct current occupies 4 theta, and sine fluctuation direct current occupies 2 pi-4 theta. In the isolated grid mode, the fluctuating direct current bus is controlled by a high-power-density energy storage module in the photovoltaic system main body module. In a grid-connected mode, a sine wave moving part is power grid voltage, and a direct current part is controlled by a high-power-density energy storage module. The isolated grid output current or the grid-connected feed current is determined by the single-phase inversion module and the high-power-density energy storage module together, for example, the most conventional control method is decoupling control, so that the power decoupling between the photovoltaic output power and the single-phase inverter can be borne by the high-power-density energy storage module.
In this embodiment, the single-phase inverter circuit of the single-phase inverter module adopts a full-bridge topology as shown in fig. 2, and includes 4 switching tubes (M)1，M2，M3，M4) An inductor LFCapacitor CFAnd a resistance RLFormed by the output side being incorporated in the network, in particular the switch M1And switch M2Series, switch M3And switch M4In series, and 4 switching tubes (M)1，M2，M3，M4) Each connected in parallel with a diode; inductor LFIs connected to the switch M1And switch M2Between the two ends are respectively connected with a capacitor CFAnd a resistance RLOne terminal of (C), a capacitorFAnd a resistance RLIs connected to the switch M at the other end3And switch M4Between, capacitance CFAnd a resistance RLAre connected in parallel.
The invention adopts the HPWM modulation method to control the on and off of the switching tube on the circuit shown in figure 2. During modulation, the full-bridge switches are switched on and off in constant direct current time (4 theta), only one group of bridge arms is switched on and off at a high frequency, the other group of bridge arm switches are in a conducting or switching-off state, and all the switches are commutated in a low current range during high-frequency working, so that the switching loss of the DC/AC inverter is greatly reduced. In the sine wave DC time (2 pi-4 theta), the full bridge switch is in the on or off state. Therefore, only half of the switching tubes in the single-phase inverter circuit work at high frequency within a short time (4 theta), and other switching tubes are kept in an on or off state for most of the time, so that the switching loss is remarkably reduced compared with a single-phase inverter adopting the traditional HPWM modulation.
The specific HPWM modulation process of the present invention is illustrated by way of example as shown in fig. 3:
(1)t0-t1interval, DC voltage is a constant value, switch tube M3Disconnection, M4Opening, M1、M2Complementary conduction at a higher frequency, M1The modulation signal of (2) is sin (ω t)/sin θ.
(2)t1-t2Interval, DC voltage is sine wave, switch tube M3In the off state, M4In the on state, the switch tube M1Off, M2And (4) opening.
(3)t2-t3Interval, switching tube on condition and t0-t1The interval state is the same.
(4)t3-t4Interval, constant DC voltage, switch tube M3Opening, M4Off, M1、M2Complementary conduction at a higher frequency, M1The modulation signal of (2) is 1+ sin (ω t)/sin θ.
(5)t4-t5Interval, DC voltage is sine wave, switch tube M3In the on state, M4In the off state, the switch tube M1Off, M2And (4) opening.
(6)t5-t6Interval, switching tube on condition and t3-t4The interval state is the same.
From FIG. 3, it can be seen that M is present for a period of time1&M2Operating in a high-frequency switching state, M3&M4Working in a power frequency switch state. After a period of time, M1&M2Operating in power-frequency switching mode, M3&M4Operating in a high frequency switching state. Through the rotation mechanism, the balance of switching tube switching loss and the thermal balance of response are realized.
According to the HPWM method, the DC/AC single-phase inverter can also adopt an asymmetric design, namely, the switching tubes of two bridge arms can be selected differently. For example, M1&M2Selecting devices suitable for high-frequency switching, and M3&M4Selecting devices suitable for low-frequency switching; cancel the rotation mechanism at the same time, let M1&M2Always operating in a high frequency regime, M3&M4The low-frequency switch device always works in a power frequency state, and due to the fact that the low-frequency switch device is low in price, the reasonable asymmetric design can achieve the purpose of considering both low cost and heat balance of the system to a certain extent.
The photovoltaic energy system simulation oscillogram shown in fig. 4 is obtained by modulating by the type HPWM modulation method. In FIG. 4, VinFor system bus voltage, Vg1Is a switch tube M1Drive signal of VoutFor a single phase output voltage, IoutThe simulation theta is 30 degrees for single-phase output current. As can be seen from fig. 4, under the control of the high power density energy storage module, i.e. the bus voltage control outer ring set value is set to a combination of constant dc and sine wave dc; the inner loop adopts a control mode of controlling inductor current to realize maximum power point tracking, specifically as shown in fig. 5, it can be known from the control circuit diagram of fig. 5 that the system bus voltage is composed of constant direct current and sine fluctuation direct current, and a drive signal Vg1 of a switching tube M1 shows that the switching tube part works in a high-frequency state in time under the modulation of HPWM, thereby reducing the switching loss of the system and improving the efficiency of the system. Thus, the number of the first and second electrodes,the whole photovoltaic energy system adopts a design without electrolytic capacitors, so that the service life and the reliability of the system are greatly improved. The single-phase output voltage and current of the system are low-harmonic sine waveforms, and the feasibility of the improved HPWM and system design is verified.
In summary, the invention has the following advantages:
1. the photovoltaic energy system provided by the invention cancels the electrolytic capacitor at the photovoltaic cell side in the photovoltaic conversion module and cancels the electrolytic capacitor between the photovoltaic conversion module and the single-phase inverter, thereby solving the problem that the service life of the photovoltaic system is influenced due to the electrolytic capacitor and being beneficial to prolonging the service life of the photovoltaic system.
2. The bidirectional DC/DC converter of the high-power-density energy storage module of the system can realize energy storage and can meet more requirements; and the photovoltaic modules in a multi-photovoltaic module system can be configured to have different ratings than other photovoltaic modules, maintaining interchangeability of photovoltaic modules with changes to the photovoltaic energy system, allowing the photovoltaic energy system to maximize the use of available roof space. Further, the plurality of photovoltaic modules may be configured to provide a plurality of photovoltaic arrays having different directional orientations, enhancing the sun-tracking and energy-collecting capabilities of the photovoltaic energy system.
3. In the system, the output power of the photovoltaic conversion module is constant, so that the tracking of the photovoltaic maximum power point is facilitated; the grid-connected single-phase inverter circuit adopts an improved Hybrid Pulse Width Modulation (HPWM) method, and the switching loss of the single-phase inverter can be reduced by more than 50%.
4. The modulation method of the system uses improved Hybrid Pulse Width Modulation (HPWM) which consists of pulsating direct current modulation in a low-frequency range and secondary modulation in a high-frequency range, and the secondary modulation is used for realizing soft switching of a high-frequency down converter and can further reduce switching loss;
the foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. It is not intended to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and its practical application to enable one skilled in the art to make and use various exemplary embodiments of the invention and various alternatives and modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims and their equivalents.
Claims (4)
1. A modulation method of a modularized photovoltaic energy system without an electrolytic capacitor is characterized in that: in constant direct current time, only one set of bridge arms of a full-bridge switch is switched on and off at a higher frequency, the other set of bridge arm switch is in a conducting or switching-off state, and all switches are commutated in a low current range during high-frequency working; the full-bridge switches are all in a conducting or a switching-off state within sine wave direct current time, wherein the constant direct current time is 4θThe sine wave DC time is 2 pi-4θ，θModulating the angle for the controllable system;
the control system comprises an integrated photovoltaic conversion module and a photovoltaic system main body module connected through a fluctuating direct current bus;
the integrated photovoltaic conversion module is plug-and-play, and consists of a plurality of photovoltaic cells and a plurality of single-phase DC/DC converters, and each photovoltaic cell is independently connected with one single-phase DC/DC converter;
the photovoltaic system main body module is composed of a high-power-density energy storage module and a single-phase inversion module, the input end of the single-phase inversion module is connected with the input end of the fluctuation direct-current bus, and the output end of the fluctuation direct-current bus is connected to a power grid.
2. The method of modulating a modular electrolytic capacitor-less photovoltaic energy system of claim 1, wherein: the method comprises the following steps:
（1）t0-t1interval, DC voltage is a constant value, switching tubeM 3Cut-off, switch tubeM 4Switching-on and switching-off tubeM 1Switch tubeM 2At a high frequency with respect to each otherComplementary conducting, switching tubeM 1The modulation signal of (c) is sin (ω t)/sin θ;
（2）t1-t2interval, DC voltage is sine wave, switching tubeM 3In the off-state of the circuit, the circuit is switched on,M 4in the on state, the switch tubeM 1The power is turned off and the power is turned off,M 2opening;
（3）t2-t3interval, switching tube on condition and t0-t1The interval state is the same;
（4）t3-t4interval, constant DC voltage, switching tubeM 3Switching-on and switching-off tubeM 4Switch-off and switch tubeM 1Switch tubeM 2Switch tube with high frequency complementary conductionM 1The modulation signal of (a) is 1+ sin (ω t)/sin θ;
（5）t4-t5interval, DC voltage is sine wave, switching tubeM 3In the on-state of the device,M 4in the off state, the switch tubeM 1Switch-off and switch tubeM 2Opening;
（6）t5-t6interval, switching tube on condition and t3-t4The interval state is the same;
wherein, t0-t1In the interval 0 to theta, t1-t2Is in the interval 0-pi-theta, t2-t3Is the interval (pi-theta) -pi; t is t3-t4Is the interval of pi to (pi + theta); t is t4-t5Is in the interval of (pi + theta) to (2 pi + theta); t is t5-t6Is the interval of (2 pi-theta) -2 pi; omega is the power frequency angular velocity, theta is the controllable system modulation angle.
3. The method of modulating a modular electrolytic capacitor-less photovoltaic energy system of claim 1, wherein: the high-power-density energy storage module comprises a high-power-density energy storage unit and a bidirectional DC/DC converter, and the bidirectional DC/DC converter is connected with the fluctuating direct current bus; the high-power-density energy storage unit is connected with the bidirectional DC/DC converter in parallel.
4. The method of modulating a modular electrolytic capacitor-less photovoltaic energy system of claim 1, wherein: the single-phase inversion module consists of 4 switching tubesM 1，M 2，M 3，M 4InductorL F Capacitor and method for manufacturing the sameC F And a resistorR L Forming; wherein, the switchM 1And switchM 2Series, switchM 3And switchM 4In series, and 4 switching tubesM 1，M 2，M 3，M 4Each connected in parallel with a diode; inductanceL F Is connected to the switch at one endM 1And switchM 2Between the other end of the capacitor and the other end of the capacitor are respectively connected with a capacitorC F And a resistorR L One terminal of (1), a capacitorC F And a resistorR L Is connected to the switch at the other endM 3And switchM 4Capacitance betweenC F And a resistorR L Are connected in parallel.
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| KR20110053795A (en) * | 2009-11-16 | 2011-05-24 | 영남대학교 산학협력단 | Capacitance estimation system of dc link capacitor for single phase pulse width modulation converter |
| CN103378761A (en) * | 2012-04-24 | 2013-10-30 | 丰郅(上海)新能源科技有限公司 | Three-stage topology photovoltaic inverter |
| CN103580032A (en) * | 2012-07-20 | 2014-02-12 | 比亚迪股份有限公司 | Power grid compensating system and control method thereof |
| CN104113082A (en) * | 2014-07-17 | 2014-10-22 | 西安交通大学 | Modular full direct current photovoltaic system and control method thereof |
| CN106059356A (en) * | 2016-06-02 | 2016-10-26 | 燕山大学 | Electrolytic capacitor-free photovoltaic inverter capable of suppressing leakage current and control method for photovoltaic inverter |
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