CN111555343B - Universal distributed control method and system for cascading inverter - Google Patents
Universal distributed control method and system for cascading inverter Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/381—Dispersed generators
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
- H02M7/49—Combination of the output voltage waveforms of a plurality of converters
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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
- H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
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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
- H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
- H02J3/1821—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators
- H02J3/1835—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control
- H02J3/1842—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control wherein at least one reactive element is actively controlled by a bridge converter, e.g. active filters
- H02J3/1857—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control wherein at least one reactive element is actively controlled by a bridge converter, e.g. active filters wherein such bridge converter is a multilevel converter
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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
- H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
- H02J3/1892—Arrangements for adjusting, eliminating or compensating reactive power in networks the arrangements being an integral part of the load, e.g. a motor, or of its control circuit
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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
- H02J3/388—Islanding, i.e. disconnection of local power supply from the network
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
- H02M7/5387—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
- H02M7/53871—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
- H02J2300/40—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation wherein a plurality of decentralised, dispersed or local energy generation technologies are operated simultaneously
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/0003—Details of control, feedback or regulation circuits
- H02M1/0012—Control circuits using digital or numerical techniques
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/0067—Converter structures employing plural converter units, other than for parallel operation of the units on a single load
- H02M1/0074—Plural converter units whose inputs are connected in series
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/20—Active power filtering [APF]
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/30—Reactive power compensation
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Abstract
The invention provides a general distributed control method of a cascading inverter, which comprises the following steps: detecting instantaneous voltage and instantaneous current signals at the output ports of each individual cascaded inverter module; calculating active power and reactive power output by each cascade inverter module in a grid-connected mode or an island mode based on the instantaneous voltage and instantaneous current signals; calculating a current power factor angle according to the active power and the reactive power; controlling generation of angular frequencies of the single cascaded inverter module with power factor angle droop control based on a nominal angular frequency, a nominal voltage magnitude, and a nominal power factor angle, and the current power factor angle; forming a sinusoidal voltage reference according to the angular frequency and the rated voltage amplitude; based on the acquired instantaneous voltage signal, instantaneous current signal and the sinusoidal voltage reference at the port, a PWM signal is obtained for controlling the individual cascaded inverter modules using a voltage outer loop-current inner loop control.
Description
Technical Field
The invention relates to the technical field of power electronics, in particular to a general distributed control method and system of a cascading inverter.
Background
Parallel and cascade are two important ways to form a large-scale power system. In general, parallel operation is more widely used because of its plug-and-play and high reliability. For example, many micro-grids are made up of a large number of inverters connected in parallel. However, cascading operation is also an indispensable way to form high voltage devices or networks. For example, cascaded inverters are used for high voltage motor drives, STACOM, and energy storage systems.
In the past, most cascaded inverter control systems employed centralized control. However, centralized control relies on real-time communication networks and powerful centralized controllers, which however often lead to reduced reliability and higher once-in costs due to communication failures. Further, when the number of inverter modules is large and the distance between the modules is large, implementation thereof becomes more difficult.
Disclosure of Invention
In order to solve the above problems, the present invention provides a general distributed control method of a cascaded inverter, which includes the following steps:
detecting instantaneous voltage and instantaneous current signals at the output ports of each individual cascaded inverter module;
calculating the active power and the reactive power output by each cascading inverter module based on the instantaneous voltage and the instantaneous current signals;
calculating a current power factor angle according to the active power and the reactive power;
forming an angular frequency reference for the single cascaded inverter module using power factor angle droop control based on a nominal angular frequency, a nominal voltage magnitude, and a nominal power factor angle, and the current power factor angle;
forming a sinusoidal voltage reference according to the angular frequency reference and the rated voltage amplitude reference;
based on the acquired instantaneous voltage signal, instantaneous current signal and the sinusoidal voltage reference at the port, a PWM signal is obtained for controlling the individual cascaded inverter modules using a voltage outer loop-current inner loop control.
According to the general distributed control method of the cascade inverter, in the island mode, the active power and the reactive power output by the ith cascade inverter module are as follows:
wherein V is i And delta i Representing the instantaneous voltage and phase angle of the ith module; z's' load And θ' load To include the combined impedance and impedance angle of the transmission line and load.
According to the general distributed control method of the cascade inverter, in the grid-connected mode, the active power and the reactive power output by the ith cascade inverter module are as follows:
wherein Z is line ∠θ line Representing transmission line impedance; v (V) g And delta g Representing the voltage amplitude and phase angle of the grid, respectively.
According to the general distributed control method of the cascade inverter of the present invention, in the step of controlling the power factor angle of the current output power of the single inverter module based on the values of the active power and the reactive power:
the power factor angle in island mode is,
the power factor angle in grid-tie mode is,
wherein V is i And delta i Representing the voltage and phase angle of the ith module; z's' load And θ' load Is the combined impedance and impedance angle, Z, of the transmission line and the load, respectively line ∠θ line Representing transmission line impedance; v (V) g And delta g Representing the voltage amplitude and phase angle of the grid, respectively.
According to the general decentralized control method of the cascade inverter of the present invention, in the step of controlling the angular frequency of generating the single cascade inverter module using the power factor angle droop control based on the rated angular frequency, the rated voltage amplitude and the rated power factor angle, and the current power factor angle, the power factor angle droop control strategy is expressed as:
V i ＝V *
wherein omega i Is the angular frequency, ω, of the i-th module to be output * 、V * 、
Is the nominal angular frequency, the voltage amplitude and the power factor angle, m is the positive coefficient, +.>
Is the power factor angle of the i-th module.
According to another aspect of the present invention, there is also provided a universal decentralized control system for cascading inverters, the system comprising:
a detection unit to detect instantaneous voltage and instantaneous current signals at the output ports of each individual cascaded inverter module;
a power calculation unit to calculate active power and reactive power output by each cascaded inverter module based on the instantaneous voltage and instantaneous current signals;
a power factor angle calculation unit to calculate a current power factor angle from the active power and the reactive power;
a power factor angle droop control unit to form an angular frequency reference for the single cascaded inverter module based on a nominal angular frequency, a nominal voltage magnitude, and a nominal power factor angle, and the current power factor angle;
the double inner ring control unit is used for forming a sinusoidal voltage reference according to the angular frequency and the rated voltage amplitude, and obtaining PWM signals for controlling each cascade inverter module by utilizing voltage outer ring-current inner ring control based on the acquired instantaneous voltage signals, instantaneous current signals and the sinusoidal voltage reference at the ports.
According to the general distributed control system of the cascade inverter of the present invention, in the island mode, the power transfer characteristics (active power and reactive power) of the ith cascade inverter module are as follows:
wherein V is i And delta i Representing the instantaneous voltage and phase angle of the ith module; z's' load And θ' load To include the combined impedance and impedance angle of the transmission line and load.
According to the universal distributed control system of the cascade inverter of the invention, in the grid-connected mode, the power transmission characteristics (active power and reactive power) of the ith cascade inverter module are as follows:
wherein Z is line ∠θ line Representing transmission line impedance; v (V) g And delta g Representing the voltage amplitude and phase angle of the grid, respectively.
According to the general distributed control system of the cascade inverter of the present invention, in the power factor angle calculation unit:
the power factor angle in island mode is,
the power factor angle in grid-tie mode is,
wherein V is i And delta i Representing the voltage and phase angle of the ith module; z is Z l ′ oad And theta l ′ oad Is the combined impedance and impedance angle, Z, of the transmission line and the load, respectively line ∠θ line Representing transmission line impedance; v (V) g And delta g Representing the voltage amplitude and phase angle of the grid, respectively.
The universal decentralized control system for a cascaded inverter according to the invention is characterized in that in the power factor angle droop control unit, the power factor angle droop control strategy is expressed as:
V i ＝V *
wherein omega i Is the angular frequency, ω, of the i-th module to be output * 、V * 、
Is the nominal angular frequency, the voltage amplitude and the power factor angle, m is the positive coefficient, +.>
Is the power factor angle of the i-th module.
Compared with the existing control strategy, the invention has the following beneficial effects: the invention is suitable for both grid-connected mode and island mode; seamless switching can be performed in different modes; the stable condition in the grid-connected mode is independent of the transmission line impedance; suitable for various types of loads in island mode; each inverter can realize four-quadrant operation. The invention can realize the stable operation of the system through the small signal stability proof. Furthermore, the feasibility of the method provided by the invention can be verified through simulation.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
Drawings
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate the invention and together with the embodiments of the invention, serve to explain the invention, without limitation to the invention. In the drawings:
FIG. 1 shows a system block diagram of a cascaded inverter according to one embodiment of the invention;
FIG. 2 shows a control schematic of power factor angular droop control according to an embodiment of the present invention;
FIG. 3 shows a flow chart of an overall control method according to one embodiment of the invention;
FIG. 4a shows a plot of frequency versus time for a cascaded inverter system in two modes according to one embodiment of the invention;
FIGS. 4 b-4 c show graphs of active power and reactive power, respectively, of a cascaded inverter system over time in two modes according to one embodiment of the invention;
FIGS. 5a-5c show graphs of frequency, active power and reactive power of a cascaded inverter system over time under three load conditions, respectively, according to one embodiment of the invention;
FIGS. 6a-6c show graphs of frequency, active power and reactive power, respectively, of a cascaded inverter system over time in four quadrants, according to one embodiment of the invention;
figures 7a-7c show graphs of frequency, active power and reactive power, respectively, of a cascaded inverter system for various transmission line impedances over time, according to one embodiment of the invention, an
Fig. 8a and 8b show graphs of frequency versus power factor angle over time for a cascaded inverter system operating in four quadrants, respectively, according to one embodiment of the invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the following embodiments of the present invention will be described in further detail with reference to the accompanying drawings.
Recently, cascaded inverter decentralized control strategies have received widespread attention as they can overcome the drawbacks of the centralized control approach. For cascaded inverters in island mode, power factor droop control is first provided, which is suitable for resistive-inductive load (RL) only. In order to widen the application range of the method, an f-P/Q method is proposed. The control method is suitable for both resistive-inductive loads (RL) and resistive-capacitive loads (RC). However, this approach is still not viable for purely resistive loads. At the same time, it also has the problem of multiple equilibrium points, possibly leading to certain undesirable operating conditions.
In addition, the prior art provides an improved distributed control based on a unique point of equilibrium. The situation in grid-tie mode is completely different compared to island mode. Yet another prior art proposes to perform fully decentralized control, which first attempts to control the cascaded inverters in a decentralized manner. However, they are only applicable to specific transmission line types. In the prior art, distributed power control is performed for grid-connected photovoltaic power generation. However, PCC voltage information must be available for each module, which increases the implementation difficulty of the method. Existing decentralized control methods are only used in island mode or grid-tie mode, and it is also necessary to provide decentralized control that is applicable to both of them in practice.
In order to solve the problems, the invention provides a general distributed control method of a cascade inverter which works in grid-connected and island modes. Compared with the prior art, the provided power factor angle droop control has the following characteristics:
a unified control scheme. The method of the invention has a unified control scheme aiming at two modes, so that seamless switching between the two modes can be obtained;
a unique balance point. The invention is suitable for transmission lines of various impedance types. The prior art is suitable for either inductive or resistive transmission lines only. However, the invention may be used with any impedance type transmission line.
The inverter operates in four quadrants, and the method in the prior art can only work in a partial area of the four quadrants, while the invention can realize complete four quadrants operation.
As shown in fig. 1, a block diagram of the configuration of a cascaded inverter system according to one embodiment of the invention is shown.
The system configuration of a cascade inverter consisting of n DG units is shown in fig. 1. Unlike conventional cascaded inverter systems, DG units may be distributed over a wider area where real-time communication is not possible. The cascaded inverter system may be controlled to operate in a grid-tie mode or an island mode by controlling the opening and closing of a Static Transfer Switch (STS).
In the structure shown in fig. 1, the power transmission characteristics of the system are as follows.
In the island mode, the i-th DG unit outputs active power P i And reactive power Q i The method is as follows:
wherein V is i And delta i Representing the voltage and phase angle of the i-th cell. Z's' load And θ' load Is the impedance and impedance angle of the combined impedance, which includes the transmission line and the load, respectively. The power transfer characteristics in island mode can be obtained by decomposing the virtual-real part by the following equation:
active power P in grid-connected mode i And reactive power Q i Expressed as:
wherein Z is line ∠θ line Representing the transmission line impedance. V (V) g And delta g Representing the voltage amplitude and phase angle of the grid, respectively. The power transfer characteristics in grid-tie mode are:
as shown in fig. 2, a control schematic diagram of a power factor angle droop control strategy according to an embodiment of the invention is shown.
As can be seen from fig. 2, the system is based on the values of the active and reactive power for the power factor angle of the current output power of the individual inverter modules. The system then uses a power factor angle droop control strategy to control the angular frequency of the power output by the single inverter module at a next moment based on the nominal angular frequency, the nominal voltage magnitude, and the nominal power factor angle, and the power factor angle of the current output power.
The power factor angle droop control strategy of the cascaded inverter of the present invention can be expressed as:
V i ＝V * (8)
wherein omega i Is the angular frequency. Omega * 、V * 、
Is the rated angular frequency, voltage amplitude, and power factor angle. m is a positive coefficient, ">
Is the power factor angle.
As shown above, the scheme proposed in formulas (7) and (8) requires only local information of each module, and thus it is a decentralized method.
Finally, as shown in fig. 2, the system obtains PWM signals for controlling the output power of each cascaded inverter module through double inner loop control based on the angular frequency and the rated voltage amplitude.
As shown in fig. 3, a general method flow diagram according to one embodiment of the invention is shown.
In step S301, the system detects instantaneous voltage and instantaneous current signals at the output ports of each individual cascaded inverter module.
Next, in step S302, the active power and reactive power output by each cascaded inverter module in grid-tie mode or island mode is calculated based on the instantaneous voltage and instantaneous current signals. In step S303, the system calculates the current power factor angle of the inverter module according to the obtained active power and reactive power; in step S304, controlling the angular frequency at which the single cascaded inverter module is generated with power factor angle droop control based on the nominal angular frequency, the nominal voltage magnitude, and the nominal power factor angle, and the current power factor angle; in step S305, a sinusoidal voltage reference is formed according to the obtained angular frequency and the rated voltage amplitude; finally, in step S306, a PWM signal for controlling the respective cascaded inverter modules is obtained using a voltage outer loop-current inner loop control based on the acquired instantaneous voltage signal, instantaneous current signal and the sinusoidal voltage reference at the port.
Steady state analysis of the system of the present invention is performed as follows. In steady state, according to equation (7), it is possible to obtain:
where i, j e {1,2, …, n }.
If in island mode, according to formulas (2), (3), (8) and (9), the following equations are readily obtained:
likewise, if in grid-tie mode, similar conclusions can be drawn as to active and reactive power from the above.
The system of the present invention was further subjected to small signal stability analysis. In order to prove the stability of the method in the island mode and the grid-connected mode, small signal analysis near the balance point is carried out.
Let delta be s Is the synchronous phase angle of the cascaded inverter in steady state, and
due to->
Equation (7) can therefore be rewritten as:
linearizing equation (11) around the equilibrium point, one can get:
in the island mode, the formulas (2) and (3) are combined to obtain:
next, equation (13) is further linearized, which can result in:
combining equation (12) with equation (14) yields:
rewriting equation (15) into a matrix form yields:
wherein x= [ delta ] delta 1 …Δδ n ] T ，
L is a Laplacian matrix, which can be expressed as:
the eigen value of a is expressed as:
λ 1 (A)＝0,λ 2 (A)＝…＝λ n (A)＝-m (18)
it is apparent that the system of the present invention is stable in island mode. However, the stabilization is independent of the load parameters. This is a very important feature of the present invention.
In the grid-connected mode, combining the formula (5) with the formula (6) to obtain:
linearizing equation (19) around the equilibrium point, yields:
wherein, the liquid crystal display device comprises a liquid crystal display device,
substituting formula (20) into formula (12) to obtain:
wherein, the liquid crystal display device comprises a liquid crystal display device,
the eigen value of B is given by:
obviously, if
The system will also be stable in grid tie mode and the stability conditions are independent of transmission line impedance and load.
On the other hand, in order to prove the effectiveness of the power factor angle droop control, the technical scheme of the invention can be subjected to simulation test based on a Matlab/Simulink platform. As shown in table one, the relevant parameters of a test system comprising four DG modules are listed.
List one
Case one: unified control
In this example, the test is performed when the grid-tie mode is switched to the island mode. The inverter frequency is shown in fig. 4a, which shows that the system of the present invention can achieve seamless transitions between the two modes. The active and reactive power distribution is shown in fig. 4b and 4 c. Therefore, the scheme of the invention is a unified control method for the cascade inverter working in the grid-connected mode and the island mode.
And a second case: is suitable for various loads
In this case, performance in island mode was tested under pure resistive load, resistive-inductive load, and resistive-capacitive load. During [0 seconds, 6 seconds ], the system load is a pure resistive load; during [6 seconds, 12 seconds ], the system load is a resistive sense load; thereafter, the system load is changed to a resistive-capacitive load. Fig. 5a-5c show waveforms of frequency, active power and reactive power for all modes in sequence from top to bottom. As shown, the frequencies of all modules converge rapidly after start-up, it is apparent that the inverters are always synchronized, regardless of load variations. However, the frequency will vary with the load power factor. The frequency under resistive load is higher than the frequency under resistive load, which is consistent with the case indicated by equation (7). At the same time, the distribution of active and reactive power is very good. As shown, the proposed solution is suitable for various types of loads.
In case three, the cascaded inverter operates in island mode. The initial phase angles of the #1 inverter are set in the I, II, III, IV quadrants at [0 second, 5 seconds ], [5 seconds, 10 seconds ], [10 seconds, 15 seconds ], and [15 seconds, 20 seconds ], respectively, with the initial phase angles of the remaining inverters set to zero. Waveforms of frequency, active power and reactive power are shown in fig. 6a, 6b and 6c, respectively. As shown, the power factor angular droop control of the present invention always has a unique balance point, independent of the initial state.
In case four, the capacitive, inductive and resistive transmission lines are operated in grid-tie mode when fed in time intervals [0 seconds, 5 seconds ], [5 seconds, 10 seconds ], [10 seconds, 15 seconds ], and [15 seconds, 20 seconds ], respectively. Simulation results of frequency, active power and reactive power are shown in fig. 7a, 7b, 7 c. Based on these simulation results, the scheme of the invention is suitable for any impedance type transmission line.
To verify the ability of the inverter of the inventive method to operate in four quadrants, the inverter power factor angles were set to pi/4, 3 pi/4, -3 pi/4, and-pi/4, respectively. The waveform of the power factor angle is shown in fig. 8a, wherein the actual waveform can keep track of its reference value. The waveform of the frequency is shown in fig. 8b, which always converges to 50Hz. Therefore, the scheme of the invention can realize the four-quadrant operation of the inverter.
The invention aims to provide a universal distributed control for cascading inverter power factor angle droop control. Compared with the existing control strategy, the invention has the following beneficial effects: the invention is suitable for both grid-connected mode and island mode; seamless switching can be performed in different modes; the stable condition in the grid-connected mode is independent of the transmission line impedance; suitable for various types of loads in island mode; the method is suitable for four-quadrant operation of the inverter. The invention has proven to have controlled small signal stability. Furthermore, the feasibility of the method provided by the invention is verified through simulation.
It is to be understood that the disclosed embodiments are not limited to the specific structures, process steps, or materials disclosed herein, but are intended to extend to equivalents of these features as would be understood by one of ordinary skill in the relevant arts. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.
Reference in the specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrase "one embodiment" or "an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment.
Although the embodiments of the present invention are disclosed above, the embodiments are only used for the convenience of understanding the present invention, and are not intended to limit the present invention. Any person skilled in the art can make any modification and variation in form and detail without departing from the spirit and scope of the present disclosure, but the scope of the present disclosure is still subject to the scope of the appended claims.
Claims (7)
1. A universal decentralized control method for a cascaded inverter, the method comprising:
detecting instantaneous voltage and instantaneous current signals at the output ports of each individual cascaded inverter module;
calculating the active power and the reactive power output by each cascading inverter module based on the instantaneous voltage and the instantaneous current signals;
calculating a current power factor angle according to the active power and the reactive power;
forming an angular frequency reference for the single cascaded inverter module using a power factor angle droop control strategy based on a nominal angular frequency, a nominal voltage magnitude, and a nominal power factor angle, and the current power factor angle;
forming a sinusoidal voltage reference according to the angular frequency reference and the rated voltage amplitude reference;
based on the acquired instantaneous voltage signal, instantaneous current signal and sinusoidal voltage reference at the port, utilizing voltage outer loop-current inner loop control to obtain PWM signals for controlling the cascade inverter modules;
in the island mode, the power transfer characteristics of the i-th cascade inverter module are expressed as follows:
wherein V is i And delta i Representing the instantaneous voltage and phase angle of the ith module; z is Z ′ load And theta ′ load For a combined impedance and impedance angle comprising the transmission line and the load; v (V) j And delta j Representing the instantaneous voltage and phase angle of the j-th module;
in the grid-connected mode, the power transmission characteristics of the ith cascaded inverter module are as follows:
wherein Z is line And theta line Respectively representing transmission line impedance and transmission line impedance angle; v (V) g And delta g Respectively representing the voltage amplitude and the phase angle of the power grid;
in the step of generating a power factor angle based on the values of the active power and the reactive power for the current output power of the single inverter module:
the power factor angle in island mode is,
the power factor angle in grid-tie mode is,
wherein V is i And delta i Representing the voltage and phase angle of the ith module; z is Z ′ load And theta ′ load Is the combined impedance and impedance angle, Z, of the transmission line and the load, respectively line And theta line Respectively representing transmission line impedance and transmission line impedance angle; v (V) g And delta g Representing the voltage amplitude and phase angle of the grid, respectively.
2. The universal decentralized control method of a cascaded inverter according to claim 1, wherein in the step of controlling the angular frequency of generating the single cascaded inverter module using power factor angle droop control based on a nominal angular frequency, a nominal voltage magnitude and a nominal power factor angle and the current power factor angle, the power factor angle droop control strategy is expressed as:
wherein omega i Is the angular frequency, ω, of the i-th module to be output * 、V * 、
Is the nominal angular frequency, the voltage amplitude and the power factor angle, m is the positive coefficient, +.>
Is the power factor angle of the i-th module; v (V) i Representing the voltage of the ith module.
3. A universal decentralized control system for cascading inverters, characterized in that a method according to any one of claims 1-2 is performed, the system comprising:
a detection unit to detect instantaneous voltage and instantaneous current signals at the output ports of each individual cascaded inverter module;
a power calculation unit to calculate active power and reactive power output by each cascaded inverter module based on the instantaneous voltage and instantaneous current signals;
a power factor angle calculation unit to calculate a current power factor angle from the active power and the reactive power;
a power factor angle droop control unit to form an angular frequency reference for the single cascaded inverter module using a power factor angle droop control strategy based on a nominal angular frequency, a nominal voltage magnitude, and a nominal power factor angle, and the current power factor angle;
the double inner ring control unit is used for forming a sinusoidal voltage reference according to the angular frequency reference and the rated voltage amplitude reference, and obtaining PWM signals for controlling the cascade inverter modules by utilizing voltage outer ring-current inner ring control based on the acquired instantaneous voltage signals, instantaneous current signals and the sinusoidal voltage reference at the ports.
4. A universal decentralized control system for a cascaded inverter as claimed in claim 3, characterized in that in island mode the power transfer characteristics of the i-th cascaded inverter module are as follows:
wherein V is i And delta i Representing the instantaneous voltage and phase angle of the ith module; z is Z ′ load And theta ′ load For a combined impedance and impedance angle comprising the transmission line and the load; v (V) j And delta j Representing the instantaneous voltage and phase angle of the j-th module.
5. The universal decentralized control system for a cascaded inverter according to claim 4, wherein in the grid-tie mode, the power transfer characteristics of the i-th cascaded inverter module are as follows:
wherein Z is line And theta line Respectively representing transmission line impedance and transmission line impedance angle; v (V) g And delta g Representing the voltage amplitude and phase angle of the grid, respectively.
6. The universal decentralized control system for cascading inverters according to claim 5, wherein in the power factor angle calculation unit:
the power factor angle in island mode is,
the power factor angle in grid-tie mode is,
wherein V is i And delta i Representing the voltage and phase angle of the ith module; z is Z ′ load And theta ′ load Is the combined impedance and impedance angle, Z, of the transmission line and the load, respectively line And theta line Respectively representing transmission line impedance and transmission line impedance angle; v (V) g And delta g Representing the voltage amplitude and phase angle of the grid, respectively.
7. The universal decentralized control system for cascading inverters according to claim 6, wherein in the power factor angle droop control unit, the power factor angle droop control strategy is expressed as:
wherein omega i Is to be output by the ith moduleAngular frequency omega * 、V * 、
Is the nominal angular frequency, the voltage amplitude and the power factor angle, m is the positive coefficient, +.>
Is the power factor angle of the i-th module; v (V) i Representing the voltage of the ith module.
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