CN110138013B - Micro-grid structure of parallel cascade converters and control method - Google Patents
Micro-grid structure of parallel cascade converters and control method Download PDFInfo
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Abstract
The invention discloses a micro-grid structure of parallel cascade converters and a control method, wherein the micro-grid structure comprises the following steps: the distributed sub-systems are connected in parallel, and each distributed sub-system supplies power to a regional load through line impedance connected with the distributed sub-system; the distributed subsystem comprises a plurality of H bridge inverters in cascade connection, each H bridge inverter is connected with a slave controller, and all the slave controllers are connected with the master controller. The main controller with a double-layer structure mainly generates a modulation wave signal of a cascade H-bridge unit in the distributed subsystem. The slave controller mainly realizes zero crossing point judgment of grid-connected current and carrier wave staggered modulation so as to obtain a PWM signal of each H-bridge unit. The system structure of the invention has higher networking flexibility, improves the overall stability of the system, reduces the calculation burden of the main controller, improves the equivalent switching frequency of the distributed subsystems, improves the reliability of the system and improves the electric energy quality of the system.
Description
Technical Field
The invention belongs to the technical fields of distributed energy generation, power electronic control technology and microgrid, and particularly relates to a microgrid structure of a parallel cascade converter and a control method.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
The distributed power generation of renewable energy sources is more dispersed, and a micro-grid can reliably connect a large amount of distributed power generation systems of renewable energy sources with medium and small capacities into the power grid, so that safe, reliable, stable and economic operation is realized, and the full utilization of the renewable energy sources is facilitated. However, renewable energy sources have problems of low voltage level, small capacity, unstable power, limited ability to independently provide reliable power to loads, and fluctuation of the power grid, which have limited the development thereof. The utilization capacity of the micro-grid for renewable energy can be fully exerted by using a proper power electronic control technology.
The micro-grid generally operates in two operation modes of grid connection and off-grid connection, and under the grid connection mode, the micro-power source is generally controlled to be a current source; in the off-grid mode, the micro-power source is generally controlled as a voltage source to maintain the voltage and frequency stability of the off-grid system. In the off-grid mode of operation, all power sources in the microgrid must be capable of independently providing sufficient power to local loads, so a parallel topology is typically used in the microgrid to increase system capacity. However, with the increase of the parallel subsystems, the individual centralized controllers will need to deal with the exponentially increasing sampling variables, control variables and control targets, and the design and configuration of the centralized controllers will become extremely complex, which seriously affects the feasibility of the system.
The problem of low voltage level still exists in the distributed subsystem based on renewable energy and power electronic interface, and the common solution is to adopt a structure that a series or cascade inverter is used as the distributed energy subsystem. However, in the series structure, a large number of inductance elements are used as system filtering equipment, and the series structure is more advantageous in order to reduce the system cost and the system occupied area. In a distributed energy subsystem with a cascaded structure, H-bridges are typically used as sub-modules to boost the voltage level of the system. However, when the cascade H-bridge operates at a low frequency, a large amount of harmonics may be injected into the system, which seriously harms the power quality of the microgrid and causes adverse effects on the load of the user.
Disclosure of Invention
In order to solve the problems, the invention provides a micro-grid structure of a parallel cascade converter and a control method, which realize reasonable distribution of micro-grid power, decentralized control of distributed subsystems and improvement of electric energy quality of the micro-grid.
In some embodiments, the following technical scheme is adopted:
a microgrid structure of parallel cascaded converters comprising: the distributed sub-systems are connected in parallel, and each distributed sub-system supplies power to a regional load through line impedance connected with the distributed sub-system; the distributed sub-system comprises a plurality of H-bridge inverters connected in cascade.
In other embodiments, the following technical solutions are adopted:
a control method of a micro-grid structure of a parallel cascade converter adopts a centralized and distributed control method of a main controller and a plurality of sub-controllers for each distributed subsystem, wherein the main controller adopts a double-layer control structure, and one layer adopts droop control to realize reasonable distribution of power of the distributed subsystems; the other layer is based on the current tracking control of a voltage-current double closed loop so as to realize grid-connected current unsteady state static error; the slave controller adjusts the control signal from the master controller through voltage equalization to respectively obtain the modulation wave signal of each cascade H-bridge inverter, and then compares the modulation wave signal with the respective carrier signal of the H-bridge inverter to obtain the PWM control signal of each H-bridge inverter.
Compared with the prior art, the invention has the beneficial effects that:
compared with the traditional microgrid, the microgrid structure has higher networking flexibility, improves the stability of the whole system, greatly reduces the number of filter inductance elements and reduces the development and operation cost of the microgrid system.
The distributed main controller adopts a double-layer control structure, and the upper layer adopts a distributed control method based on droop control, so that reasonable power distribution and stable voltage and frequency control of an off-grid system are realized under the condition of no communication; the lower layer adopts a double closed-loop control structure based on a PI controller to realize current tracking control, and steady-state ripples are reduced under the condition of no static error.
In the master controller, the modulation wave signals of the cascaded H-bridge units in the distributed subsystem are mainly generated, and the carrier signals and PWM signals of the cascaded H-bridge units are mainly generated by the corresponding slave controllers, so that the calculation burden of the master controller is greatly reduced.
The carrier staggered modulation mode based on current orientation is adopted in the slave controller, so that the equivalent switching frequency of the cascade system is improved, the problem of high harmonic content of a cascade H-bridge inverter in a distributed subsystem is solved, and the electric energy quality of the system is improved.
Drawings
Fig. 1 is a block diagram illustrating a micro-grid structure and control of a parallel cascade inverter according to a first embodiment of the present invention;
FIG. 2 is a block diagram of a main control structure according to a first embodiment of the present invention;
FIG. 3 is a block diagram of a slave control architecture according to a first embodiment of the present invention;
fig. 4 is a schematic view of carrier interleaving modulation according to an embodiment of the present invention.
Detailed Description
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
Example one
In one or more embodiments, a microgrid structure of parallel cascaded converters and a control method thereof are disclosed, and the microgrid structure is shown in fig. 1 and includes: the distributed sub-systems are connected in parallel, and each distributed sub-system supplies power to a regional load through line impedance connected with the distributed sub-system; the distributed sub-system comprises a plurality of H-bridge inverters connected in cascade.
The distributed subsystem is formed by cascading a plurality of H-bridge inverters, so that access of a single low-voltage distributed module is facilitated, and the voltage grade and the power capacity can be improved. Compared with the traditional microgrid structure, the system has higher networking flexibility, improves the stability of the whole system, greatly reduces the number of filter elements and reduces the development and operation cost of the microgrid system.
Aiming at the proposed micro-grid structure, a distributed control mode is adopted. Each H bridge inverter is connected with one slave controller, and all the slave controllers are connected with the master controller; the master controller mainly realizes droop control and current tracking, and the slave controller is mainly used for generating PWM signals of the H-bridge unit. Compared with the traditional microgrid structure, the system has higher networking flexibility, improves the stability of the whole system, improves the equivalent switching frequency of the cascade system, improves the equivalent switching frequency of the subsystem by the corresponding control method, and improves the electric energy quality of the system.
The distributed main controller adopts a double-layer control structure, the upper layer adopts a distributed control method based on droop control, and reasonable power distribution and stable voltage and frequency control of an off-grid system are realized under the condition of no communication; the lower layer adopts a double closed-loop control structure based on a PI controller to realize current tracking control, and steady-state ripples are reduced under the condition of no static error. In the main controller, a modulated wave signal of a cascade H-bridge unit in the distributed subsystem is mainly generated.
In order to reduce the calculation burden of the master controller, the carrier signal and the PWM signal of the cascade H-bridge unit are mainly generated by the corresponding slave controller. In the slave controller, firstly, zero crossing point judgment is carried out on grid-connected current, phase zero points are set for carrier signals of a first H-bridge inverter based on the zero crossing points of the current, then, carrier signals of all H-bridge units are determined by adopting a carrier interleaving modulation method, and finally, modulation wave signals transmitted in the master controller are compared with respective carrier signals of the H-bridge units, so that PWM signals of all the H-bridge units are obtained.
Fig. 2 is a block diagram of a main controller according to the present invention. The main controller adopts a double-layer control structure, the upper layer is a distributed subsystem power control layer, and the reasonable power distribution is realized by adopting droop control; and the lower layer is current tracking control based on a voltage and current double closed loop so as to realize grid-connected current unsteady state static error. The droop control loop calculates active power and reactive power of the subsystem by measuring outlet voltage and current of the distributed subsystem, and gives a voltage reference value as input of the voltage-current double closed loop by utilizing droop characteristics. And the voltage outer ring and the current inner ring realize the rapid and stable control of the voltage and the current of the subsystem through the PI controller.
In order to ensure power balance and frequency synchronization among distributed subsystems, a distributed droop control method designed according to the power transmission characteristics of an off-grid parallel inverter is as follows:
and omega, E, P and Q are the angular frequency of the outlet of the subsystem, the voltage amplitude, the active power and the reactive power respectively. GP(s) and GQ(s) respective transfer functions, in the typical droop characteristic, GP(s)＝m，GQAnd(s) n, wherein m and n are the slopes of the active power and reactive power droop characteristics, respectively.
Since the types and capacities of the distributed energy resources are different among the distributed subsystems, the droop characteristics of the distributed subsystems are different. In the subsystem parallel system, the frequency and the voltage amplitude are used as invisible communication traffic of the system, and reasonable power distribution among subsystems is realized.
Fig. 3 is a block diagram of a distributed controller as a slave controller. And each distributed controller correspondingly controls the H-bridge unit in the cascade structure of one distributed subsystem. In order to reduce the calculation burden of the main controller, the decentralized controller is used for carrying out the functions of current zero crossing point detection and carrier wave interleaved modulation generation of PWM signals. Firstly, judging the zero crossing point of grid-connected current, setting phase zero point for a carrier signal of a first H-bridge inverter based on the zero crossing point of the current, then determining the carrier signals of all H-bridge units by adopting a carrier interleaving modulation method, and finally comparing the modulation wave signal transmitted in the main controller with the respective carrier signal of the H-bridge units to obtain the PWM signal of each H-bridge unit. The cascade H-bridge structure adopts a carrier staggered modulation method to reduce harmonic components of grid-connected current and improve the power quality of a system. The modulation mode of carrier interleaving adopts a current orientation method, a grid-connected current zero crossing point is used as a carrier zero crossing point of a first cascade H bridge in a distributed subsystem, carrier zero points of other H bridges are delayed by an angle phi from the carrier zero point of the previous H bridge, and the expression is as follows:
wherein N is the number of the distributed subsystems cascaded with the H bridge.
Fig. 4 is a schematic diagram of carrier cross modulation, which is illustrated by taking three carrier signals of cascaded H bridges as an example. Firstly, the triangular carrier is adjusted according to the detection of the zero crossing point of the fundamental wave of the grid-connected current, namely the initial reference point of the first H-bridge carrier signal. And according to the formula (2), calculating that when the distributed subsystem is formed by cascading three H-bridge inverters, the difference of carrier signals is 60 degrees, so that the carrier of the second H-bridge inverter is delayed by 60 degrees compared with the carrier of the first H-bridge. Similarly, the carrier signal of the third H-bridge inverter is delayed by 60 ° compared to the second one.
In the slave controller, the modulation signal from the master controller is adjusted through voltage equalization to obtain a modulation wave signal of each cascaded H-bridge unit respectively, and then the modulation wave signal is compared with a carrier signal to obtain a PWM control signal of each H-bridge unit. Wherein the carrier signal is coupled to the output signal u in the main controllerabThe relationship is as follows:
uabi＝uab/N (3)
wherein u isabiModulated waves, u, of the ith cascaded H-bridge, respectivelyabAnd N is the number of cascaded H bridges of the distributed subsystem.
Although the embodiments of the present invention have been described with reference to the accompanying drawings, it is not intended to limit the scope of the present invention, and it should be understood by those skilled in the art that various modifications and variations can be made without inventive efforts by those skilled in the art based on the technical solution of the present invention.
Claims (5)
1. A microgrid structure of parallel cascaded converters, comprising: the distributed sub-systems are connected in parallel, and each distributed sub-system supplies power to a regional load through line impedance connected with the distributed sub-system; the distributed subsystem comprises a plurality of H bridge inverters in cascade connection, each H bridge inverter is connected with one slave controller, and all the slave controllers are connected with the master controller;
the slave controller is configured to realize the synchronization of H bridge inverter control signals in all distributed subsystems and generate PWM signals of the H bridge inverters;
the control process of the slave controller is specifically as follows:
judging zero crossing points of grid-connected current, setting phase zero points for carrier signals of a first H-bridge inverter based on the zero crossing points of the current, and then determining the carrier signals of all the H-bridge inverters by adopting a carrier interleaving modulation method;
comparing the modulation wave signal transmitted in the main controller with respective carrier signals of the H-bridge inverters to obtain a PWM signal of each H-bridge inverter;
the main controller adopts a double-layer control structure, wherein one layer adopts droop control to realize reasonable distribution of the power of the distributed subsystems; and the other layer is based on the current tracking control of a voltage-current double closed loop so as to realize grid-connected current unsteady state static error.
2. The microgrid structure of a parallel cascade converter of claim 1, wherein the droop control is specifically: calculating active power and reactive power of the distributed subsystems by measuring outlet voltage and current of the distributed subsystems; the frequency and the voltage amplitude of the distributed subsystems are used as invisible communication traffic among the distributed subsystems, and reasonable power distribution among the distributed subsystems is achieved;
providing a voltage reference value as the input of the voltage-current double closed loop by using the droop characteristic; and the voltage outer ring and the current inner ring realize the rapid and stable control of the voltage and the current of the distributed subsystem through the PI controller.
3. The microgrid structure of claim 1, wherein the carrier interleaving modulation method adopts a current orientation method, grid-connected current zero-crossing points are used as carrier zero-crossing points of a first cascaded H-bridge inverter in a distributed subsystem, and carrier zero-crossing points of the rest H-bridge inverters are all carrier zero-crossing pointsIs delayed by an angle compared with the zero crossing point of the carrier wave of the previous H-bridge inverter
And N is the number of the cascaded H-bridge inverters of the distributed subsystem.
4. A control method of a micro-grid structure of a parallel cascade converter is characterized in that a master controller and a plurality of slave controllers are used for each distributed subsystem to realize distributed control, the master controller adopts a double-layer control structure, and one layer adopts droop control to realize reasonable distribution of power of the distributed subsystems; the other layer is based on the current tracking control of a voltage-current double closed loop so as to realize grid-connected current unsteady state static error; the slave controller adjusts a control signal from the master controller through voltage equalization to respectively obtain a modulation wave signal of each cascaded H-bridge inverter, and then the modulation wave signal is compared with a respective carrier signal of the H-bridge inverter to obtain a PWM signal of each H-bridge inverter;
the droop control specifically comprises: calculating active power and reactive power of the distributed subsystems by measuring outlet voltage and current of the distributed subsystems; the frequency and the voltage amplitude of the distributed subsystems are used as invisible communication traffic among the distributed subsystems, and reasonable power distribution among the distributed subsystems is achieved;
providing a voltage reference value as the input of the voltage-current double closed loop by using the droop characteristic; the voltage outer ring and the current inner ring realize the rapid and stable control of the voltage and the current of the distributed subsystem through the PI controller;
the control process of the slave controller is specifically as follows:
judging zero crossing points of grid-connected current, setting phase zero points for carrier signals of a first H-bridge inverter based on the zero crossing points of the current, and then determining the carrier signals of all the H-bridge inverters by adopting a carrier interleaving modulation method;
and comparing the modulated wave signals with respective carrier signals of the H-bridge inverters to obtain PWM signals of each H-bridge inverter.
5. The method according to claim 4, wherein the carrier interleaving modulation method adopts a current orientation method, the zero-crossing point of the grid-connected current is used as the carrier zero-crossing point of the first cascaded H-bridge inverter in the distributed subsystem, and the carrier zero-crossing points of the remaining H-bridge inverters are delayed by an angle from the carrier zero-crossing point of the previous H-bridge inverter
And N is the number of the cascaded H-bridge inverters of the distributed subsystem.
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