CN111371120A - Micro-grid smooth switching control system and method based on LCL grid-connected inverter - Google Patents

Micro-grid smooth switching control system and method based on LCL grid-connected inverter Download PDF

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CN111371120A
CN111371120A CN202010320729.2A CN202010320729A CN111371120A CN 111371120 A CN111371120 A CN 111371120A CN 202010320729 A CN202010320729 A CN 202010320729A CN 111371120 A CN111371120 A CN 111371120A
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grid
controller
lcl
inverter
connected inverter
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张超
杜晓光
孙泉
张国超
刘兆栋
宋娜
吴璇璇
张术鹏
董新华
陈子健
候良宵
刘梦瑶
王怀宇
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Shandong University of Science and Technology
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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
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/28—Arrangements for balancing of the load in a network by storage of energy

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Abstract

The invention discloses a microgrid smooth switching control system and method based on an LCL grid-connected inverter, which are respectively connected to a grid-connected PQ controller and an island VF controller of the LCL grid-connected inverter; when the LCL grid-connected inverter is in a grid-connected mode, the PQ controller is switched on, the output of the VF controller can follow the output of the PQ controller, and the output of the VF controller is ensured to be in accordance with the output of the PQ controller so as to carry out a conversion process; when the LCL grid-connected inverter is switched into the island mode, the grid-connected inverter is switched to the VF controller by the PQ controller, the VF controller has the same output state before and after switching, and the LCL grid-connected inverter is smoothly switched to the off-grid mode in the mode of being connected with a power grid. The technical scheme disclosed by the invention can realize smooth transition of the micro-grid, and ensure that disturbance can be avoided in any operation mode, and simultaneously, the requirement that the voltage and the frequency must be kept in an allowable range at any time is met.

Description

Micro-grid smooth switching control system and method based on LCL grid-connected inverter
Technical Field
The invention belongs to the technical field of microgrid smooth switching, and particularly relates to a microgrid smooth switching control system and method based on an LCL grid-connected inverter.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
The micro grid has two modes of grid connection and island in operation: when the micro-grid is connected to the power grid, the large power grid can provide stable power support, the distributed power supply unit adopts PQ control, constant power output and efficient energy management can be guaranteed, and the energy storage device works in a charging state at the moment; and when the island detection system finds that the power grid can not work normally or is separated from the micro power grid in a planned mode, the micro power grid is switched to an island mode to work, the energy storage device makes up for the power shortage through an improved VF droop control strategy, the balance between energy supply and demand is realized, and frequency and voltage support is provided for the micro power grid.
The inventor finds in research that the problem of system smooth switching can be solved for the overall control strategy of the microgrid, and then when the load demand or the grid structure in the whole system is different from the previous one, how to distribute power to the distributed power supplies in the microgrid makes the system operate smoothly, and it is the key of the problem that the transient stability after the fault of the microgrid is maintained.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides the microgrid smooth switching control system based on the LCL grid-connected inverter, so that the microgrid inverter operation mode and the energy storage device control mode can be smoothly switched in different modes of island and grid connection.
In order to achieve the above object, one or more embodiments of the present invention provide the following technical solutions:
little electric wire netting smooth switching control system based on LCL grid-connected inverter includes:
the grid-connected PQ controller and the island VF controller are respectively connected to the LCL grid-connected inverter;
when the LCL grid-connected inverter is in a grid-connected mode, the PQ controller is switched on, the output of the VF controller can follow the output of the PQ controller, and the output of the VF controller is ensured to be in accordance with the output of the PQ controller so as to carry out a conversion process;
when the LCL grid-connected inverter is switched into an island mode, the grid-connected inverter is switched to a VF controller by a PQ controller, the VF controller has the same output state before and after switching, and the LCL grid-connected inverter is smoothly switched to an off-grid mode in a mode of being connected with a power grid.
On the other hand, in order to achieve the above object, one or more embodiments of the present invention provide the following technical solutions:
the microgrid smooth switching control method based on the LCL grid-connected inverter comprises the following steps:
when the LCL grid-connected inverter is in a grid-connected mode, the PQ controller is switched on, the output of the VF controller can follow the output of the PQ controller, and the output of the VF controller is ensured to be in accordance with the output of the PQ controller so as to carry out a conversion process;
when the LCL grid-connected inverter is switched into the island mode, the grid-connected inverter is switched to the VF controller by the PQ controller, the VF controller has the same output state before and after switching, and the LCL grid-connected inverter is smoothly switched to the off-grid mode in the mode of being connected with a power grid.
The above one or more technical solutions have the following beneficial effects:
when the LCL grid-connected inverter is in a grid-connected mode, the PQ controller is switched on and enables the output of the VF controller to follow the output of the PQ controller, so that the output of the VF controller is ensured to be in accordance with the output of the PQ controller, and a conversion process is carried out; when the grid-connected mode of the LCL grid-connected inverter is converted into the island mode, the PQ controller is switched to the VF controller, and because the output of the VF controller before switching follows the output of the PQ controller, the controllers before switching and after switching have the same output state, short-time oscillation caused by inconsistent states before and after switching of the controllers can not be caused, and the inverter is ensured to be smoothly converted into the off-grid mode in the grid-connected mode.
The working state of the microgrid of the technical scheme disclosed by the disclosure can be changed along with the running state of the microgrid, but the overall control strategy can be changed according to the working mode, and which control strategy is adopted by the system can be selected according to the working mode and the running state of the system, so that the smooth transition of the microgrid can be realized, the disturbance can be avoided in any running mode, and meanwhile, the requirement that the voltage and the frequency must be kept in the allowable range at any time is met.
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The accompanying drawings, which are incorporated in and constitute a part of this specification, are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and together with the description serve to explain the invention and not to limit the invention.
Fig. 1 is a topology schematic diagram of a three-phase LCL-based grid-connected inverter provided in embodiment 1 of the present disclosure;
fig. 2 is a schematic diagram of a grid-connected control system based on PQ and VF control according to embodiment 1 of the present disclosure;
fig. 3 is a schematic diagram of a bidirectional DC/DC converter based dual closed-loop control provided in embodiment 1 of the present disclosure;
FIG. 4 is a schematic diagram of a bidirectional DC/DC controller bode provided by an embodiment of the present disclosure;
fig. 5 is a schematic diagram of a Nyquist curve of a system after PI compensation according to an embodiment of the present disclosure;
fig. 6 is a schematic diagram of a voltage curve change when the grid is connected to an islanding mode according to the embodiment of the present disclosure;
fig. 7 is a schematic diagram of a voltage curve change in a re-connection process according to an embodiment of the present disclosure;
fig. 8 is a schematic diagram illustrating frequency changes during mode switching of a microgrid according to an embodiment of the present disclosure;
fig. 9 is a photovoltaic grid-connected topology structure diagram provided by an example of the present disclosure;
fig. 10 is a block diagram of the mode switching control of the PQ-and VF-based controller according to an example of the present disclosure.
Detailed Description
It is to be understood that the following detailed description is exemplary and is intended to provide further explanation of the invention as claimed. 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 invention 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 exemplary embodiments according to the invention. 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.
The embodiments and features of the embodiments of the present invention may be combined with each other without conflict.
Example one
The embodiment discloses a microgrid smooth switching control system based on an LCL grid-connected inverter, which is shown in figure 2 and comprises a maximum power point tracking controller and an inverter control circuit;
the maximum power point tracking controller is connected to a photovoltaic grid-connected system, the photovoltaic grid-connected system comprises a photovoltaic power generation module, a booster circuit, an energy storage unit and an LCL grid-connected inverter which are sequentially connected, and the LCL grid-connected inverter and the energy storage module are respectively connected to a direct-current bus.
The direct-current bus is further connected to a grid-connected inverter, an alternating-current load and a power grid in sequence, and the inverter control circuit is connected to the grid-connected inverter.
The maximum power point tracking controller controls the photovoltaic grid-connected system to achieve maximum power output, and the maximum power output is sent to the direct current bus after filtering.
The photovoltaic power generation module generates power and supplies power to a large power grid, the inverter maintains stable power when the photovoltaic power generation unit and the large power grid run in a grid-connected mode and the inverter runs in an off-grid mode, the energy storage unit charges when the photovoltaic power generation unit runs in the grid-connected mode, and discharges to the power grid when the photovoltaic power generation unit runs in the off-grid mode, so that bidirectional energy flow is achieved, and the stability of a power.
The direct current side of the LCL grid-connected inverter is connected to a direct current bus, the alternating current side of the LCL grid-connected inverter is connected to a local load and an alternating current grid after being filtered, voltage stabilization control of the direct current bus is achieved, direct current is inverted into alternating current to be supplied to the load and the grid, and in the implementation example, a specific circuit diagram of the LCL grid-connected inverter is shown in attached figure 1.
Taking a photovoltaic power generation unit as an example, power tracking and distribution are realized through a maximum power tracking system and an energy storage unit with a bidirectional DC/DC converter. The method comprises the following steps: due to the instability of the output of the photovoltaic cell, the maximum power output is realized through a maximum power tracking system, and the output is filtered by an LC filter and then sent to a direct current bus. Wherein the maximum power tracking is realized based on a Boost circuit of a previous stage. The inverter at the rear stage adopts a unidirectional full-bridge inverter, the direct current side is connected to the direct current bus, the alternating current side is connected to the local load and the alternating current power grid after being filtered by the inductance filter, the voltage stabilization control of the direct current bus is realized, and the direct current is inverted into alternating current to be provided for the load and the power grid. The energy storage unit consists of a Battery and a bidirectional DC/DC converter. The low-voltage side and the high-voltage side of the bidirectional DC/DC converter are respectively connected with the storage battery and the direct-current bus, so that bidirectional energy flow can be realized, and electric energy storage and control can be completed. Under the condition that the energy storage unit participates in system operation, when the voltage of the direct current bus is higher than a stable value, the bidirectional DC/DC converter works in a voltage reduction mode to charge the storage battery. When the voltage of the direct current bus is lower than a stable value, the direct current bus works in a boosting mode, and the storage battery discharges to maintain the voltage of the direct current bus and the power of the system to be stable.
It should be noted that the inverter control circuit in the embodiment of the present disclosure includes a grid-connected PQ controller and an island VF controller respectively connected to an LCL grid-connected inverter;
when the LCL grid-connected inverter is in a grid-connected mode, the PQ controller is switched on, the output of the VF controller can follow the output of the PQ controller, and the output of the VF controller is ensured to be in accordance with the output of the PQ controller so as to carry out a conversion process;
when the LCL grid-connected inverter is switched into the island mode, the grid-connected inverter is switched to the VF controller by the PQ controller, the VF controller has the same output state before and after switching, and the LCL grid-connected inverter is smoothly switched to the off-grid mode in the mode of being connected with a power grid.
In the example, the bidirectional DC/DC converter ensures the stability of the voltage of the direct current bus when the power grid fails, and adopts a double closed-loop control strategy to stabilize the voltage of the direct current bus and control the charging and discharging current of the storage battery. Let Gid(s) is the duty cycle d to the low side inductor current iLTransfer function of Gvi(s) is the inductor current iLDC bus voltage U to high sidedcThe transfer function of (c):
Figure RE-GDA0002508486620000051
Figure RE-GDA0002508486620000052
the model adopts PI controllers, UdcIs a value of a direct voltage, C2Is a capacitor, R is a resistor, L1The inductance is adopted, the duty ratio is D, the delay of sampling and calculation is considered, and a delay link G is added into the systemdelay(s) as shown in FIG. 3. GasAnd(s) is a feedback link, mainly a filtering link of inductive current. To make it bidirectionalThe DC/DC maintains the stability of the voltage of the direct current bus when the power grid fails, the voltage outer loop samples the voltage of the direct current bus, and the current inner loop samples the inductive current. The current inner ring and the voltage outer ring are compensated by PI controllers, three PI compensators are respectively designed to be compensators in a control circuit of the bidirectional DC/DC converter, and the three PI compensators are as follows:
PI1：
Figure RE-GDA0002508486620000061
PI2：
Figure RE-GDA0002508486620000062
PI3：
Figure RE-GDA0002508486620000063
the bode plots of the model with no compensation and after compensation of the three PI controllers are shown in FIG. 4. Taking PI in the present system1As a compensation, a Nyquist curve is made as shown in fig. 5. It can be seen that the system is stable after compensation by the PI controller.
In the model, a disturbance observation method is adopted, and maximum power tracking is realized through a Boost circuit. The system samples the output voltage and current of the photovoltaic cell and obtains U through a disturbance observation methodpv *And finally realizing maximum power tracking by controlling the duty ratio of the Boost circuit through a double closed-loop control strategy.
Figure RE-GDA0002508486620000064
Figure RE-GDA0002508486620000065
The current inner ring and the voltage outer ring are respectively compensated by PI controllers, three PI compensators are respectively designed, and the compensator which is an MPPT control circuit is as follows:
PI1：
Figure RE-GDA0002508486620000066
PI2：
Figure RE-GDA0002508486620000067
PI3：
Figure RE-GDA0002508486620000068
the unidirectional full-bridge inverter of the model of the embodiment adopts a direct-current bus voltage outer ring and a grid-connected current inner ring control strategy, and the inner ring and the outer ring are both controlled by PI. The system firstly samples the voltage of the direct current bus, compares the voltage obtained by sampling with the voltage reference value of the direct current bus, and obtains the current reference value amplitude of the current inner ring through the operation of the PI controller. And multiplying the amplitude by the sine value pointed by the sine table pointer of the digital phase-locked loop to obtain the current inner loop alternating current reference value. And comparing the current inner ring reference value with a current reference value obtained by system sampling, obtaining a modulation waveform by PI operation, and comparing the modulation waveform with a triangular wave to obtain four paths of PWM control signals.
In another embodiment, a PQ and VF controller-based switching system is provided that includes a grid-connected PQ controller, an island VF controller, an inner loop controller, a load, and a grid model. The controller topology is shown in fig. 2.
When in a grid-connected mode, the PQ controller is switched on, so that the output of the VF controller can follow the output of the PQ controller, and the output of the VF controller is ensured to accord with the output of the PQ controller so as to carry out a conversion process; when the grid-connected mode is changed into the island mode, the grid-connected mode is switched to the VF controller, and because the output of the VF controller before switching follows the output of the PQ controller, the controller before switching and after switching has the same output state, short-time oscillation caused by inconsistent states before and after switching of the controller can not be caused, and the inverter is ensured to be smoothly switched to the grid-disconnected mode under the grid-connected mode.
The energy storage device is in a charging standby state, the control strategy of the micro power supply adopts PQ control, if the load of the micro power grid changes, the large power grid can bear the change of voltage and frequency, and the micro power grid is not needed to consider the change. Adjusting the frequency and voltage of the power grid, controlling the inverter to output a reference power value, and keeping energy balance; when the module for detecting the island finds that the external power grid cannot normally operate or the micro-grid requires to operate in the island, the PCC is interrupted, and during the PCC, the batteries on the energy storage system are controlled by the improved VF droop so as to maintain the balance of the power supply and demand of the power grid.
The improved VF droop control can keep the voltage and the frequency stable even if the voltage and the frequency of the microgrid are not under the rated value, continuously change the output power of an inverter to the microgrid to meet the continuously changed load requirement, and also can ensure that the voltage and the frequency fluctuation do not exceed the operation allowable value.
Voltage and current control is added on the basis of traditional droop control, power is reasonably distributed to each distributed power supply, then a voltage reference value is used for inner ring fine adjustment, a received indication signal can be used for directly controlling an inverter, and the micro power supply power output keeps following the fluctuation of a load. However, in the droop control, when the load fluctuates, the voltage output of the inverter changes accordingly, and because the single-loop control is adopted, the voltage control is adopted, so that the PI controller reduces the voltage fluctuation in the output process of the inverter, so as to avoid the voltage fluctuation and ensure a smooth signal. Current control can maintain current tracking state and respond quickly. The method not only can evenly distribute the energy flow, but also can carry out secondary frequency modulation so as to enable the frequency in an island state to return to a rated value.
In this example, the building of the microgrid simulation model is completed, and the initial parameters adopted during simulation are as follows: the voltage of the power grid is 380V, and the frequency is 50 Hz; DG1 is P ref1＝1kW,QrefThe total line impedance R of DG1 is 0.68 Ω/km and the inductance L is 0.2H/km, with 1 equal to 600 Var. The terminal voltage of the storage battery is 380V, and the rated capacity is 8640 Ah. The constant power load parameters are: p1-22 kW, Q1-9 kVar; p2-18 kW, Q2-2 kVar.
According to the embodiment, when the micro-grid is specified to operate in an island mode, the output power of the distributed power generation unit meets the requirement of the local load consumption, and mode switching during the micro-grid operation is simulated.
The performance characteristics of the operation of the microgrid in the simulation process are provided, and it can be obtained from the graph that the voltage drops in a small range when the distributed power supply is switched from grid connection to island in 1s, the phenomenon is mainly because the energy storage device quickly supplements the power shortage after entering an island mode and quickly recovers to a normal state, and the voltage of the microgrid can be kept stable within a voltage fluctuation allowable value. When the island is switched to be connected to the grid in 2s, the voltage can be seen to be in stable transition.
The present embodiment is shown in fig. 8, when the microgrid is switched, the frequency oscillation of the system just starts to occur because the active power of the system is not enough to meet the load requirement, the system introduces disturbance at 0.5s, and the system changes from grid connection to island connection at 1s, it is easy to see that before and after the microgrid mode is switched and is disturbed, the frequency fluctuation is very small, the frequencies of the microgrid and the power distribution network are at similar values at the time of mode switching, and the microgrid also rapidly changes the frequency to work together with the frequency of the power distribution network.
In another embodiment, a control method based on a three-phase LCL grid-connected inverter is disclosed, which is used for regulating and controlling the output grid-connected current and ensuring that the phase and amplitude of the output grid-connected current are synchronous with the phase and amplitude of grid voltage on a grid side.
The three-phase voltage type bridge inverter circuit adopts a 180-degree conduction mode, namely the conduction angle of each bridge arm is 180 degrees. In the grid connection process, because a large amount of low-frequency harmonic waves exist in a weak power grid, a filter needs to be added and then the weak power grid is connected with the power grid.
The selected control mode ensures that the total harmonic content of the current transmitted to the power grid conforms to the international standards such as IEEE 929-2000 and the like.
The Clark transformation is utilized to transform a static a, b and c three-phase coordinate system into a static αβ two-phase coordinate system, and a three-phase voltage equation of the three-phase grid-connected inverter can be obtained through kirchhoff's law in a circuit:
Figure RE-GDA0002508486620000091
three-phase current equation:
Figure RE-GDA0002508486620000092
obtaining a mathematical model of the three-phase grid-connected inverter under a dq two-phase rotating coordinate system by using Park conversion, and obtaining a formula of the Park conversion according to a relation between a two-phase static αβ coordinate system and two-phase rotating d and q coordinate systems:
Figure RE-GDA0002508486620000093
obtaining a mathematical model expression of the three-phase grid-connected inverter in the two-phase rotating coordinate system:
Figure RE-GDA0002508486620000094
under a two-phase rotating d-axis and q-axis coordinate system, a system model of the three-phase grid-connected inverter becomes a double-input and double-output coupling system. According to the LCL grid-connected inverter model under the dq axis coordinate system, the three-phase voltage and the three-phase current can be obtained.
The coupling relation exists between the grid voltages, in order to realize decoupling control, the three-phase static coordinate system is converted into a two-phase rotating coordinate system (namely Park conversion), and the decoupling control is obtained through conversion:
Figure RE-GDA0002508486620000101
under the coupled system, the decoupling of the system is realized by designing the feedforward of the coupling quantity on the theoretical level, and the controller of each component in the d coordinate axis and the q coordinate axis is better controlled and designed.
The three-phase LCL inverter control system adopts a double closed loop mode, and the open loop transfer function is as follows:
Figure RE-GDA0002508486620000102
in the formula CfIs a filter capacitor, KPTo scale factor, KIFor the integral adjustment coefficient, K is a proportional controller coefficient, and as some possible implementations, in the double closed-loop control mode, when analyzing the single-closure control of the capacitor current, a transfer function is obtained as:
Figure RE-GDA0002508486620000103
when the capacitor current is inner loop controlled, a second order system is formed. The typical second-order system corresponding to it is:
Figure RE-GDA0002508486620000104
comparing the single closed loop control with a typical second order system,
Figure RE-GDA0002508486620000105
Figure RE-GDA0002508486620000106
it is clear from the above formula that the damping coefficient varies with the change of the proportional controller K. The natural frequency is determined entirely by the parameters of each element in the LCL filter in the grid-connected inverter.
In yet another embodiment, an inverter mode switching method based on PQ and VF control strategies is disclosed.
The micro grid can be operated off grid or on grid with the large grid, each distributed power supply in the micro grid operated together with the large grid can only ensure power output balance, because the total capacity of the micro grid is smaller than that of the grid, the fluctuation of voltage and frequency can be regulated by the large grid without being borne by the micro grid, and then the inverter control strategy is generally PQ control.
When the off-grid operation is performed, when the micro-grid is separated from the large power grid, the micro-grid needs to maintain stable voltage and frequency, so that the micro-power sources distributed in the micro-grid can provide energy to further maintain the voltage and the frequency unchanged, and the inverter adopts improved VF droop control.
The direct current side of the inverter is connected with the photovoltaic power generation unit and the storage battery, the alternating current side of the inverter is connected with the microgrid, so that the microgrid is a key interface for converting direct current into alternating current, the power output condition in the microgrid can be changed, PQ control means that the inverter can output constant power, and the reference power of the inverter is determined as the condition for controlling the power of the inverter.
PQ control outputs active power P when grid-connected operation of power gridrefTo output reactive power QrefThe control strategy ensures that the inverter outputs constant power. The PQ controller structure is double loop control, the outer loop is power control, and the inner loop is current control. In the dq0 coordinate system, the power injected into the AC network by the inverter can be expressed as:
Figure RE-GDA0002508486620000111
when the Park transformation is applied, the d-axis direction is selected to be the same as the vector direction of the voltage, and the voltage has no component on the q-axis. Simplifying the expression of output power, active power only related to d-axis current, reactive power only related to q-axis current, and available current reference value idrefAnd iqrefCan be expressed as:
Figure RE-GDA0002508486620000112
the time domain of the current inner loop controller can be expressed as:
Figure RE-GDA0002508486620000113
the connection of DG and microgrid depends on the inverter, the inverters can be regarded as being connected in parallel together in island mode, and the output power of each inverter unit is as follows:
Figure RE-GDA0002508486620000121
inverter mode switching based on PQ and VF control strategies, including islanding mode detection, PQ control, improved VF droop control.
As shown in fig. 10, the specific operation method is switched based on the PQ and VF controller modes:
in the grid-connected mode, the switch K1 is closed, the switch K2 is opened, the PQ controller is switched on, the switch K3 is opened, and the switch K4 is closed, so that the output of the VF controller can follow the output of the PQ controller, and the output of the VF controller is ensured to be in accordance with the output of the PQ controller, so that the conversion process is carried out; when the grid-connected mode is changed into the island mode, the switch K2 is closed, the switch K1 is opened, the switch K4 is opened, the switch K3 is closed, the controller is switched to the VF controller, and because the output of the VF controller before switching follows the output of the PQ controller, the controller before switching and after switching have the same output state, short-time oscillation caused by inconsistent states before and after switching of the controller can not be caused, and the inverter is ensured to be smoothly switched to the grid-disconnected mode in the grid-connected mode.
And (3) island detection and control variables, namely, disturbance is introduced by an active detection measure to influence the quality of electric energy output by a system, an over-voltage detection method, an under-voltage detection method, an over-frequency detection method and an under-frequency detection method in passive detection are selected as island detection of the microgrid, and when the output power of the microgrid operating in an island is not equal to the power required by a load, the voltage and the frequency in the system deviate.
When the voltage and the frequency exceed preset allowable values, it can be shown that an island mode will occur, and the micro-grid needs to change the operation mode into the island mode.
The following standards are available according to IEEE Std.2000.929 and UL 1741: when the voltage of the PCC is within the range that V is more than or equal to 0.8pu and less than or equal to 1.1pu (per unit value), or the voltage angular difference delta theta between the micro-grid and the large grid is smaller than delta theta min, the working state of the micro-grid is grid-connected operation: when the voltage and the frequency of the PCC exceed set thresholds, the large power grid is judged to be incapable of working normally, and the micro power grid needs to be separated from the large power grid island to operate.
The micro distributed power supply with small capacity can adopt a constant power control mode when being connected with a large power grid in a grid mode, the voltage and frequency fluctuation of the micro distributed power supply is regulated by the large power grid, the distributed power supply can only input or output power without considering the frequency and voltage transient fluctuation generated by grid connection, and thus the distributed power supply can indirectly regulate the feeder voltage in the power grid instead of directly regulating the feeder voltage in the power grid, and further, a power system is prevented from being damaged by certain disadvantages.
PQ control outputs active power P when grid-connected operation of power gridrefTo output reactive power QrefThe control strategy ensures that the inverter outputs constant power.
The PQ control is a dual loop control, the outer loop is a power control, and the inner loop is a current control. In the dq0 coordinate system, the power injected into the AC network by the inverter can be expressed as:
P＝udid+uqiq
Q＝uqid-udiq
DC voltage v of voltage source inverter on network sidedcAnd reactive power QmeasTo sum them with a given DC voltage VdcrefAnd reactive power QrefAfter the reference values are compared, a current reference signal generated by regulation is obtained, and the d-axis component and the q-axis component of the current reference value can be obtained through the difference value through a PI controller. Then idref、iqrefShaft current i converted from power grid side current through rotating coordinated、iqAfter comparison, the d-axis component and the q-axis component of the pulse width modulation signal are obtained through a PI controller, and finally u is obtainedsd、usqAnd obtaining a driving signal for driving the voltage source inverter through coordinate transformation.
The improved VF droop control can keep the voltage and the frequency stable even if the voltage and the frequency of the microgrid are not under the rated value, continuously change the output power of an inverter to the microgrid to meet the continuously changed load requirement, and also can ensure that the voltage and the frequency fluctuation do not exceed the operation allowable value.
The main circuit is provided with a circuit breaker for controlling connection and disconnection between the micro-grid and the large grid and a logic switch for controlling mode switching of a controller in the micro-grid, so that the micro-grid and the large grid can be smoothly switched.
The three-phase LCL grid-connected inverter disclosed by the invention has a good inhibition effect on high-frequency harmonic waves, the total harmonic distortion rate between the network access current and the power grid voltage after passing through the LCL filter is very low, and the effect that the frequency and the phase of the voltage and the current input into the power grid are consistent can be ensured.
The photovoltaic grid-connected system built by the method is divided into six working modes, referring to fig. 9, and switching among different modes is determined by the running state of a power grid, local load, photovoltaic cell power and the like. A Boost circuit and a bidirectional DC/DC converter are analyzed by a small signal analysis method, a double closed-loop controller is designed to control each part, and detailed control strategies of a maximum power tracking system, the bidirectional DC/DC converter and an inverter are respectively given, so that the system is always kept in a stable state in the operation process.
The bidirectional DC/DC converter ensures the stability of the voltage of the direct current bus when the power grid fails. As shown in fig. 3, a double closed-loop control strategy is adopted for the bidirectional DC/DC converter to stabilize the DC bus voltage and control the charging and discharging current of the storage battery.
And a maximum power tracking is realized through a Boost circuit by adopting a disturbance observation method. The system samples the output voltage and current of the photovoltaic cell and obtains U through a disturbance observation methodpv *And finally realizing maximum power tracking by controlling the duty ratio of the Boost circuit through a double closed-loop control strategy.
For the difference between two working states of island operation and grid-connected operation and the local load at different times in a day, six different working modes are respectively designed in the embodiment and described as follows:
let PloadRepresenting local load consumed power, PpvRepresenting the generation of power by the photovoltaic cell.
(1) Working mode 1: normal operation of the grid, K1Turn-off, K2Opening, PloadAnd (0), electric energy generated by the photovoltaic cell is merged into a power grid, and the photovoltaic cell controls the voltage of the direct current bus.
(2) The working mode 2 is as follows: normal operation of the grid, K1Turn-off, K2Opening, 0<Pload<＝PpvThe photovoltaic cell provides electric energy for the load and simultaneously integrates the residual electric energy into a power grid, and the photovoltaic cell controls the voltage of the direct-current bus.
(3) Working mode 3: normal operation of the grid, K1Turn-off, K2Opening, Pload>＝PpvThe photovoltaic cell and the power grid provide electric energy for the load at the same time, and the power grid controls the voltage of the direct-current bus.
(4) The working mode 4 is as follows: grid fault, K1Opening, K2Off, PloadAnd (5) when the voltage is equal to 0, the photovoltaic cell generates electric energy and inputs the electric energy into the storage battery pack, and the photovoltaic cell controls the voltage of the direct current bus.
(5) The working mode 5 is as follows: grid fault, K1Opening, K2Off, 0<Pload<＝PpvThe photovoltaic cell provides electric energy for the load and simultaneously inputs the residual electric energy into the storage battery pack, and the photovoltaic cell controls the voltage of the direct current bus.
(6) The working mode 6 is as follows: grid fault, K1Opening, K2Off, Pload>＝PpvThe photovoltaic cell and the storage battery pack simultaneously provide electric energy for the load, and the storage battery controls the voltage of the direct-current bus.
According to the inverter mode switching based on the PQ and VF control strategies, when the output power of the distributed power generation unit meets the requirement of the local load usage, the isolated island is detected by building a model according to over-voltage and under-voltage and over-frequency and under-frequency isolated island detection methods, and smooth switching among modes during micro-grid operation is realized.
According to the structural characteristics of the micro-grid and the characteristics of the grid-connected and island operation modes, a mode switching control strategy of the micro-grid is formulated. When the micro-grid is connected to the power grid, the large power grid can provide stable power support, the distributed power supply units are controlled by PQ, constant power output and efficient energy management can be guaranteed, and the energy storage device works in a charging state at the moment. And when the island detection system finds that the power grid can not work normally or is separated from the micro power grid in a planned mode, the micro power grid is switched to the island mode to work. The energy storage device compensates for power shortages through an improved VF droop control strategy, achieves a balance between energy supply and demand, and provides frequency and voltage support for the microgrid. The distributed power supply unit adopts PQ control to track the maximum power, and the energy utilization rate is improved. By adopting the control method, the smooth switching of the operation mode of the micro-grid can be realized, the control mode of the distributed power supply is optimized, and the stability of the voltage, the frequency and the output power of the distributed power supply can be ensured in an island mode and a grid-connected mode.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
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 (10)

1. Little electric wire netting smooth switch control system based on LCL grid-connected inverter, characterized by includes:
the grid-connected PQ controller and the island VF controller are respectively connected to the LCL grid-connected inverter;
when the LCL grid-connected inverter is in a grid-connected mode, the PQ controller is switched on, the output of the VF controller can follow the output of the PQ controller, and the output of the VF controller is ensured to be in accordance with the output of the PQ controller so as to carry out a conversion process;
when the LCL grid-connected inverter is switched into an island mode, the grid-connected inverter is switched to a VF controller by a PQ controller, the VF controller has the same output state before and after switching, and the LCL grid-connected inverter is smoothly switched to an off-grid mode in a mode of being connected with a power grid.
2. The microgrid smooth switching control system based on an LCL grid-connected inverter as claimed in claim 1, further comprising a maximum power point tracking controller, wherein the maximum power point tracking controller is connected to a photovoltaic grid-connected system, the photovoltaic grid-connected system comprises a photovoltaic power generation module, a booster circuit, an energy storage unit and the LCL grid-connected inverter which are connected in sequence, and the LCL grid-connected inverter and the energy storage module are respectively connected to a direct current bus.
3. The LCL grid-connected inverter-based microgrid smooth switching control system of claim 2, characterized in that a direct current bus is further connected to the grid-connected inverter, an alternating current load and a power grid in turn, said inverter control circuit is connected to the grid-connected inverter,
the maximum power point tracking controller controls the photovoltaic grid-connected system to realize maximum power output, and the maximum power output is transmitted to the direct current bus after filtering;
the direct current side of the LCL grid-connected inverter is connected to the direct current bus, and the alternating current side of the LCL grid-connected inverter is connected to the local load and the alternating current power grid after being filtered, so that the voltage stabilization control of the direct current bus is realized, and the direct current is inverted into alternating current to be supplied to the load and the power grid.
4. The microgrid smooth switching control method based on the LCL grid-connected inverter is characterized by comprising the following steps:
when the LCL grid-connected inverter is in a grid-connected mode, the PQ controller is switched on, the output of the VF controller can follow the output of the PQ controller, and the output of the VF controller is ensured to be in accordance with the output of the PQ controller so as to carry out a conversion process;
when the LCL grid-connected inverter is switched into the island mode, the grid-connected inverter is switched to the VF controller by the PQ controller, the VF controller has the same output state before and after switching, and the LCL grid-connected inverter is smoothly switched to the off-grid mode in the mode of being connected with a power grid.
5. The method as claimed in claim 4, wherein the LCL grid-connected inverter is controlled by PQ when the energy storage device is in a charging standby state, and when the load of the microgrid changes, the frequency and voltage of the microgrid do not need to be adjusted within the voltage and frequency changes which can be borne by the large power grid, otherwise, the frequency and voltage of the power grid are adjusted, and the inverter is controlled to output the reference power value, so as to keep energy balance.
6. The microgrid smooth switching control method based on an LCL grid-connected inverter as claimed in claim 5, characterized in that when an external power grid cannot normally operate or the microgrid requires to operate in an island, PCC is interrupted, and VF droop is used to control the battery power supply on the energy storage device so as to maintain the balance of the power supply and demand of the power grid.
7. The microgrid smooth switching control method based on an LCL grid-connected inverter as claimed in claim 4, characterized in that an island is detected by using over-voltage and under-voltage and over-frequency and under-frequency island detection methods, the island is detected and the microgrid is switched to an island working mode.
8. The microgrid smooth switching control method based on an LCL grid-connected inverter as claimed in claim 4, characterized in that the LCL grid-connected inverter adopts a 180-degree conduction mode, i.e. the conduction angle of each bridge arm is 180 degrees, and in the grid-connected process, a filter is added and then connected with a power grid.
9. The microgrid smooth switching control method based on an LCL grid-connected inverter as claimed in claim 4, characterized in that a three-phase voltage equation of a three-phase grid-connected inverter is obtained through kirchhoff's law in a circuit.
10. The microgrid smooth switching control method based on an LCL grid-connected inverter as claimed in claim 4, characterized in that the three-phase LCL inverter is controlled in a double closed loop manner.
CN202010320729.2A 2020-04-22 2020-04-22 Micro-grid smooth switching control system and method based on LCL grid-connected inverter Withdrawn CN111371120A (en)

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN112311006A (en) * 2020-10-14 2021-02-02 国网天津市电力公司营销服务中心 Photovoltaic integrated system
CN112636320A (en) * 2021-03-09 2021-04-09 中石大蓝天(青岛)石油技术有限公司东营分公司 N-source direct-current feed-compensation micro-grid structure and control method
WO2022183720A1 (en) * 2021-03-01 2022-09-09 株洲中车时代电气股份有限公司 Adaptive bidirectional dcdc charging and discharging control method and device

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN112311006A (en) * 2020-10-14 2021-02-02 国网天津市电力公司营销服务中心 Photovoltaic integrated system
WO2022183720A1 (en) * 2021-03-01 2022-09-09 株洲中车时代电气股份有限公司 Adaptive bidirectional dcdc charging and discharging control method and device
CN112636320A (en) * 2021-03-09 2021-04-09 中石大蓝天(青岛)石油技术有限公司东营分公司 N-source direct-current feed-compensation micro-grid structure and control method
CN112636320B (en) * 2021-03-09 2021-06-29 中石大蓝天(青岛)石油技术有限公司东营分公司 N-source direct current feed compensation micro-grid control method

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