CN112242719A - Inverter improved droop control method based on virtual synchronous machine technology - Google Patents

Inverter improved droop control method based on virtual synchronous machine technology Download PDF

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CN112242719A
CN112242719A CN201910643150.7A CN201910643150A CN112242719A CN 112242719 A CN112242719 A CN 112242719A CN 201910643150 A CN201910643150 A CN 201910643150A CN 112242719 A CN112242719 A CN 112242719A
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droop
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CN112242719B (en
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颜湘武
崔扬
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North China Electric Power University
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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
    • H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
    • H02J3/48—Controlling the sharing of the in-phase component
    • 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
    • H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
    • H02J3/50—Controlling the sharing of the out-of-phase component

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Abstract

The invention discloses an inverter improved droop control method based on a virtual synchronous machine technology, and belongs to the field of low-voltage microgrid inverter control. The invention is based on the impedance of the connecting circuit and the collected three-phase current instantaneous valuei oObtaining the active loss DeltaP and reactive loss DeltaQ of the connection circuit and the active power of the head end of the connection circuitP VSGAnd reactive powerQ VSGMaking difference to obtain the active power of the end of the connecting circuitP UAnd reactive powerQ U(ii) a According to the impedance of the connecting circuit, the collected three-phase voltage instantaneous valueu oAnd instantaneous value of three-phase currenti oObtaining the voltage drop Deltau of the connection line, andu oobtaining effective value of voltage at head end of connecting line by RMS measurement after differenceU UWhile obtaining the actual angular frequencyω(ii) a Finishing control of the virtual synchronous machine according to a droop control equation and a rotor motion equationAnd obtaining a voltage control command. The invention realizes the decoupling of the steady-state power of the inverter, the controllable droop characteristic and voltage at the user side and the reasonable distribution of the power when a plurality of machines are connected in parallel.

Description

Inverter improved droop control method based on virtual synchronous machine technology
Technical Field
The application relates to an inverter droop control method based on a virtual synchronous machine technology, and belongs to the field of low-voltage microgrid inverter control. In particular to an inverter improved droop control method based on a virtual synchronous machine technology.
Background
To alleviate the conflict between distributed power and large power grids, the micro-grid concept has been proposed and extensively studied. The micro-grid can be operated in a grid-connected mode, and can be separated from a main grid when a large power grid fails or needs to be overhauled in a plan mode to be operated in an isolated island mode. The island operation control strategies which are applied more mainly comprise master-slave control and peer-to-peer control, wherein the peer-to-peer control mode based on droop control draws more attention by virtue of higher self-stability, autonomy and robustness. In consideration of the lack of damping and inertia characteristics of power electronic devices, in order to realize the coordinated operation with the traditional synchronous generator, the virtual synchronous machine control strategy is provided, wherein the rotor characteristic of the traditional synchronous machine is added on the basis of droop control.
The traditional droop control is suitable for a pure-inductive circuit environment, the voltage grade of a microgrid is generally low, and a connecting circuit is mostly resistive and inductive, even highly resistive. Under the influence of the line impedance, the output active power and reactive power are in a coupling state, namely the line impedance influences the droop control effect. Therefore, research is started from decoupling control, and power decoupling is performed through a control strategy, such as a virtual impedance method. In the literature, when verifying the effect of the decoupling method, the active and reactive powers at the virtual synchronizer port (i.e., the line head end) are mostly observed to be not affected by each other. In practice, however, the user side (i.e., the end of the line) is the power consumer, and the controlled effect thereof is more concerned. In addition, in addition to determining the decoupling effect from the state that whether the active power and the reactive power can be realized without influencing each other, in order to realize the controllability of the user side, the droop coefficient value at the tail end of the connecting line in actual operation also should be a control index.
An advantage of a microgrid operating in a peer-to-peer control mode is that it allows multiple virtual synchronous machines to be in operation simultaneously. The control strategy aims to realize that parallel coordination control can be realized among inverters based on the virtual synchronous machine technology when the load in the microgrid or the output of the distributed power supply suddenly changes and disturbance occurs in the power grid. Namely, the power of the demand side is reasonably distributed, and each inverter bears the load active power and reactive power in proportion according to the capacity. On the premise of realizing power decoupling, the traditional droop control can only ensure reasonable distribution of active power, and the reactive power distribution precision is obviously influenced by the impedance voltage drop of a connecting circuit.
Disclosure of Invention
Aiming at the problem of power coupling of a virtual synchronous machine caused by a high-resistance-inductance-ratio connecting circuit in a low-voltage micro-grid, the problem that droop coefficients at the tail ends of the connecting circuit are not controllable before and after power decoupling, the problem of voltage drop at the tail end of the connecting circuit and the problem of reasonable distribution of active power and reactive power when multiple machines are connected in parallel, the inverter improved droop control method based on the virtual synchronous machine technology is provided to solve the technical problem.
In order to achieve the purpose, the invention provides the following technical scheme:
an inverter improved droop control method based on a virtual synchronous machine technology comprises the following steps:
step 1: the instantaneous value of three-phase voltage is collected at the port of an inverter, namely the head end of a connecting line between a micro-grid and a public power gridu oInstantaneous value of three-phase currenti oObtaining instantaneous active power of the head end of the linep VSGAnd instantaneous reactive powerq VSGRespectively obtaining active power after filteringP VSGAnd reactive powerQ VSG；
Step 2: according to the impedance of the connecting circuit and the collected three-phase current instantaneous valuei oObtaining active power at the end of the connecting circuit after the link of line loss compensationP UAnd terminal reactive powerQ U；
And step 3: according to the impedance of the connecting circuit, the collected three-phase voltage instantaneous valueu oAnd instantaneous value of three-phase currenti oObtaining the effective value of the terminal voltage of the connection circuit after the circuit loss compensation linkU UWhile obtaining the actual angular frequencyω；
And 4, step 4: according to the active power of the terminalP UAnd the actual angular frequencyωFinishing the control of improving active/frequency droop and virtual synchronous machine to obtain voltage phase control instructionθThe step 4 comprises the following steps:
step 41: the method is used for finishing the improvement of active/frequency droop control according to the following formula and obtaining the mechanical power required by the control of the virtual synchronous machineP m：
Figure 426308DEST_PATH_IMAGE001
；
Wherein the content of the first and second substances,ωandω nrespectively an actual angular frequency and a rated angular frequency,P nis a reference value for the active power,k pactive/frequency droop coefficient and positive;
step 42: finishing the control of the virtual synchronous machine according to the following formula to obtain the phase control instructionθ：
Figure 446216DEST_PATH_IMAGE002
;
Wherein the content of the first and second substances,Jin order to be the moment of inertia,Din order to be a damping coefficient of the damping,ω gin order to obtain the angular frequency of the power grid,θis a phase control command;
and 5: according to the terminal reactive power QUAnd the effective value of the voltage at the end of the actual connection lineU UCompleting improved reactive/voltage droop control and obtaining voltage amplitude control instructionE refThe step 5 comprises the following steps:
step 51: finishing the improvement of reactive power/voltage droop control according to the following formula to obtain the reference value of the voltage at the tail end of the connecting lineU ref：
Figure 212922DEST_PATH_IMAGE003
；
Wherein the content of the first and second substances,Q nis a reference value for the reactive power,U nis a voltage of a rated voltage, and is,k qis a reactive/voltage droop coefficient and is positive;
step 52: obtaining a voltage amplitude control command according to the following formulaE ref：
Figure 329782DEST_PATH_IMAGE004
；
Wherein the content of the first and second substances,K PandK Iare respectively in proportionThe proportional coefficient and the integral coefficient of the integral controller,E refa voltage amplitude control command;
step 6: according to the voltage amplitude control instructionE refAnd the voltage phase control commandθForming a voltage reference value;
and 7: and obtaining a control signal through the voltage and current double closed-loop control module and the modulation pulse generation module, and sending the control signal to the inverter for inverter control.
In the above improved droop control method, the step 2 includes the following steps:
step 21: respectively acquiring an active loss instantaneous value deltap and a reactive loss instantaneous value deltaq of a connecting line according to the following formulas:
Figure 290785DEST_PATH_IMAGE005
；
in the formula (I), the compound is shown in the specification,R Lin order to connect the line resistors with each other,X Lis a reactance of a connecting circuit;
step 22: respectively acquiring active loss delta P and reactive loss delta Q of a connecting line according to the following formulas:
Figure 860307DEST_PATH_IMAGE006
;
wherein the content of the first and second substances,ω 0the low pass filter cuts off the angular frequency.
Step 23: respectively acquiring active power P at the tail end of the connecting circuit according to the following formulaUAnd terminal reactive power QU：
Figure 666589DEST_PATH_IMAGE007
；
In the above improved droop control method, the step 3 includes the following steps:
step 31: according to the collected three-phase current instantaneous value ioAnd obtaining the voltage drop Deltau of the connecting circuit by the following formula:
Figure 77104DEST_PATH_IMAGE008
；
step 32: according to the collected three-phase voltage instantaneous value uoAnd obtaining the terminal voltage u of the connection circuit according to the following formulaU：
Figure 271325DEST_PATH_IMAGE009
；
Step 33: the connection line end voltage uUObtaining the effective value U of the voltage of the terminal required by control after RMS measurementU；
Step 34: the connection line end voltage uUObtaining actual angular frequency after PLLω。
Compared with the prior art, the invention has the beneficial effects that:
1) the connection circuit between each micro source and the bus of the distributed low-voltage microgrid is considered, the power of the user side is directly controlled through control point migration, the output power of the tail end only establishes a one-to-one corresponding relation with the requirement of the user side and is not influenced by the connection circuit, so that the active power and the reactive power at the tail end of the circuit do not have a coupling relation; according to the method, power decoupling can be realized, and the influence of a connecting circuit does not exist, so that the droop coefficient at the tail end of the circuit can be equal to a given value by reasonably setting the parameters of the controller;
2) considering that the voltage drop of the line inevitably causes the voltage drop of the tail end, when the virtual impedance method is adopted for decoupling, the added virtual impedance brings extra voltage drop, and the voltage drop phenomenon of the tail end of the line is aggravated; the droop equation in the application extends the head end voltage required by control to the tail end voltage, and a PI controller is added to ensure that the tail end voltage of the line is not changed due to the impedance of a connecting line, so that the normal level of the voltage at the user side is effectively maintained;
3) on the premise of realizing power decoupling, when a plurality of inverters operate in parallel, the traditional droop control is adopted to only ensure the reasonable distribution of active power, and the reactive power distribution is influenced by the impedance voltage drop of a connecting circuit. The voltage in the droop control equation is the PCC point voltage, and the PCC point voltage value feedback is the PI controller, so that the voltage values in the multi-machine parallel reactive/voltage droop equation can be guaranteed to be equal, the reactive power distribution precision is guaranteed, and meanwhile, the reasonable distribution of the active power and the reactive power is realized;
4) the required interconnecting link head end data of the droop control method are obtained by compensating the line loss of the directly acquired interconnecting link head end data, the interconnecting link head end data are not acquired directly, and the locality of the control quantity and the robustness of the control are guaranteed.
Drawings
Fig. 1 is a schematic diagram illustrating overall control of a single inverter based on a virtual synchronous machine technology.
Fig. 2 is a schematic diagram of droop control based on virtual synchronous machine technology, fig. 2a is a schematic diagram of conventional droop control, and fig. 2b is a schematic diagram of improved droop control according to the present application.
Fig. 3 is a schematic structural diagram of a parallel system of multiple inverters based on a virtual synchronous machine technology.
Fig. 4 is a schematic diagram of output power at the end of a connection line of a single inverter based on a virtual synchronous machine technology, fig. 4a is a schematic diagram of output active power, and fig. 4b is a schematic diagram of output reactive power.
Fig. 5 is a schematic diagram of a sag curve fitting of the end of a connection line of a single inverter based on a virtual synchronous machine technology, fig. 5a is a schematic diagram of an active/frequency sag curve fitting, and fig. 5b is a schematic diagram of a reactive/voltage sag curve fitting.
Fig. 6 is a schematic diagram of active power distribution at the end of a connection line of a parallel system of two inverters based on a virtual synchronous machine technology, fig. 6a is a schematic diagram of active power distribution adopting droop control when line decoupling is not performed, fig. 6b is a schematic diagram of active power distribution adopting droop control based on a virtual impedance method, and fig. 6c is a schematic diagram of active power distribution adopting improved droop control of the present application.
Fig. 7 is a schematic diagram of reactive power distribution at the end of a connection line of a parallel system of two inverters based on a virtual synchronous machine technology, fig. 7a is a schematic diagram of reactive power distribution using droop control when no line decoupling is performed, fig. 7b is a schematic diagram of reactive power distribution using droop control based on a virtual impedance method, and fig. 7c is a schematic diagram of reactive power distribution using improved droop control according to the present application.
Detailed Description
The following detailed description of the embodiments of the invention is provided in connection with the accompanying drawings. It is to be understood that the specific embodiments are merely illustrative of the invention and are not to be construed as limiting the invention.
As shown in fig. 1-7, the improved droop control method comprises the following steps:
step 1: the instantaneous value of three-phase voltage is collected at the port of an inverter, namely the head end of a connecting line between a micro-grid and a public power gridu oInstantaneous value of three-phase currenti oObtaining instantaneous active power of the head end of the linep VSGAnd instantaneous reactive powerq VSGAre filtered and then respectively obtainedP VSGAndQ VSG：
obtaining instantaneous active power of the head end of the connecting circuit according to the following formulap VSGAnd instantaneous reactive powerq VSG：
Figure 531405DEST_PATH_IMAGE010
(1)
Wherein the content of the first and second substances,u odandu oqthree-phase instantaneous voltage at inverter portu oThe direct axis component and the quadrature axis component after Park transformation;i odandi oqthree-phase instantaneous current at inverter porti oAnd the direct axis component and the quadrature axis component are subjected to Park transformation.
Instantaneous active power at the head end of the linep VSGAnd instantaneous reactive powerq VSGActive power of fundamental component can be obtained through low-pass filterP VSGAnd reactive powerQ VSGThe following formula:
Figure 141378DEST_PATH_IMAGE011
(2)
wherein the content of the first and second substances,ω 0the low pass filter cuts off the angular frequency.
Step 2: according to the impedance of the connecting circuit and the collected three-phase current instantaneous valuei oObtaining active power at the end of the connecting circuit after the link of line loss compensationP UAnd terminal reactive powerQ U；
And step 3: according to the impedance of the connecting circuit, the collected three-phase voltage instantaneous valueu oAnd instantaneous value of three-phase currenti oObtaining the effective value of the terminal voltage of the connection circuit after the circuit loss compensation linkU UWhile obtaining the actual angular frequencyω；
And 4, step 4: according to the active power of the terminalP UAnd the actual angular frequencyωFinishing the control of improving active/frequency droop and virtual synchronous machine to obtain voltage phase control instructionθ；
And 5: according to the terminal reactive powerQ UAnd the effective value of the voltage at the end of the actual connection lineU UCompleting improved reactive/voltage droop control and obtaining voltage amplitude control instructionE ref；
Step 6: according to the voltage amplitude control instructionE refAnd the voltage phase control commandθForming a voltage reference value;
and 7: after the voltage reference value is different from the actual output voltage, the voltage reference value passes through a proportional integral controller to complete voltage outer loop control, and a filter inductance current reference value is obtained; and after the difference is made between the reference value of the filter inductance current and the actual filter inductance current, the current inner loop control is completed through the integral controller, the reference value of the potential in the inverter is obtained, a control signal is obtained through the modulation pulse generation module and is sent to the inverter to control the power tube to be switched on and off, and the inverter control is completed.
Further, step 2 may further comprise the following steps:
step 21: respectively acquiring an active loss instantaneous value deltap and a reactive loss instantaneous value deltaq of a connecting line according to the following formulas:
Figure 262524DEST_PATH_IMAGE005
(3)
in the formula (I), the compound is shown in the specification,R Lin order to connect the line resistors with each other,X Lis a reactance of a connecting circuit;
step 22: an active loss fundamental component DeltaP and a reactive loss fundamental component DeltaQ of an active loss instantaneous value DeltaP and a reactive loss instantaneous value Deltaq of a connecting line can be obtained through a low-pass filter, and the method comprises the following steps:
Figure 299750DEST_PATH_IMAGE006
(4)
step 23: head end active powerP VSGThe active power of the tail end required by control can be obtained by subtracting the active loss delta P of the lineP UHead end reactive powerQ VSGThe terminal reactive power required by control can be obtained by subtracting the reactive loss delta Q of the lineQ UThe following are:
Figure 781547DEST_PATH_IMAGE007
(5)
the step 3 may further comprise the following steps:
step 31: according to the collected three-phase current instantaneous value ioAnd obtaining the voltage drop Deltau of the connecting circuit by the following formula:
Figure 195211DEST_PATH_IMAGE008
(6)
step 32: according to the collected three-phase voltage instantaneous value uoAnd obtaining the terminal voltage u of the connection circuit according to the following formulaU：
Figure 78853DEST_PATH_IMAGE009
(7)
Step 33: the connection line end voltage uUObtaining the effective value U of the voltage of the terminal required by control after RMS measurementU；
Step 34: the connection line end voltage uUObtaining actual angular frequency after PLLω。
The step 4 may further comprise the following steps:
step 41: finishing the improvement of the active/frequency droop control according to the active/frequency droop equation to obtain the mechanical power required by the virtual synchronous machine controlP mAs in formula (8):
Figure 444238DEST_PATH_IMAGE001
(8)
wherein the content of the first and second substances,ωandω nrespectively an actual angular frequency and a rated angular frequency,P nis a reference value for the active power,k pactive/frequency droop coefficient and positive;
step 42: finishing virtual synchronous machine control according to a rotor motion equation to obtain the phase control instructionθAs in formula (9):
Figure 475648DEST_PATH_IMAGE002
(9)
wherein the content of the first and second substances,Jin order to be the moment of inertia,Din order to be a damping coefficient of the damping,ω gin order to obtain the angular frequency of the power grid,θis a phase control command.
The step 5 may further comprise the following steps:
step 51: finishing the improved reactive power/voltage droop control according to a reactive power/voltage droop equation to obtain a terminal voltage reference value of the connecting circuitU refAs in formula (10):
Figure 427423DEST_PATH_IMAGE003
(10)
wherein the content of the first and second substances,Q nbeing reactive powerThe reference value is set to be a reference value,U nis a voltage of a rated voltage, and is,k qis a reactive/voltage droop coefficient and is positive;
step 52: connecting the terminal voltage reference value and the terminal voltage U of the circuit according to the following formulaUAfter making difference, the difference is passed through proportional-integral regulator to obtain voltage amplitude control instructionE ref：
Figure 431151DEST_PATH_IMAGE004
(11)
Wherein the content of the first and second substances,K PandK Irespectively a proportional coefficient and an integral coefficient of the proportional-integral controller,E refis a voltage amplitude control command.
The present embodiment is verified below on the Matlab/Simulink simulation platform, and the conventional droop control, droop control based on virtual impedance, and droop control improved by the present application are compared. Selecting the DC side voltage of the systemU dc=800V, rated frequencyf n=50Hz, rated voltageU n= 380V; virtual inertiaJ=0.1kg·m2Coefficient of dampingD= 20; the impedance of the connecting line isZ line= (0.6+ j0.1) Ω, and the resistance is high; the virtual impedance is taken to be-0.58 omega; active/frequency droop coefficientk p=0.05, reactive/voltage droop coefficientk q= 0.5. Refer to fig. 4a, 4b, 5a and 5 b.
As shown in fig. 4a and 4b, wherein S1 and L1 respectively represent the active power and reactive power waveforms using droop control without line decoupling, S2 and L2 represent the active power and reactive power waveforms using droop control based on the virtual impedance method, and S3 and L3 represent the active power and reactive power waveforms using the improved droop control of the present application. Initially, the load side active demand was 10kW, increasing to 12kW at 1s and returning to 10kW at 2 s. As can be seen from the figure, when the un-decoupled conventional droop control is adopted, the coupling relationship exists between the active power and the reactive power, and the required load value cannot be reached; when droop control based on a virtual impedance method is adopted, the coupling degree between the power is reduced, but the value of the virtual impedance influences the output power; by adopting the method and the device, stable decoupling can be realized by improving droop control, and the load requirement can be met.
As shown in fig. 5a and 5b, the active demand on the load side increases from 10kW to 12kW and then to 14kW, the reactive demand increases from 2kVar to 4kVar and then to 5kVar, and the active/frequency droop curve and the reactive/voltage droop curve are obtained by fitting with the least square method. Wherein M1 and N1 represent droop curves with droop control without line decoupling, with slopes of 0.0549 and 1.9110, respectively; m2 and N2 represent droop curves using droop control based on the virtual impedance method, with slopes of 0.0547 and 0.7158, respectively; m3 and N3 represent droop curves with slopes of 0.0496 and 0.5011, respectively, for improved droop control using the present application. As can be seen from the figure, when the line decoupling is not performed, the droop coefficient at the tail end of the connection line deviates from a given value; after adopting sag control based on a virtual impedance method, the deviation degree is improved but is still not equal to a given value; by adopting the method and the device for improving the droop control, the droop coefficient of the tail end is basically equal to the given value, so that the droop coefficient of the user side is controllable. As can be seen from fig. 5b, the improved droop control of the present application effectively mitigates the voltage sag at the end of the connection line.
Refer again to fig. 6 and 7. The inverters A, B are operated in parallel, the former having twice the capacity of the latter, and the latter having twice the droop factor. The initial state is 30kW active and 3kVar reactive loads, 3kW active and 3kVar reactive loads are added at 2s, and the recovery is carried out at 3.5 s. As can be seen from the figure, when droop control is adopted without line decoupling, the active power of the two machines is not distributed according to the ratio of 2:1, and the reactive power condition is severe; after droop control based on a virtual impedance method is adopted, active power distribution basically meets 2:1, but reactive power distribution situation is contrary to setting; after the droop control is improved, active power and reactive power can be distributed according to the ratio of 2:1, and reasonable power distribution is achieved.
It should be noted that the above embodiments are only preferred embodiments of the present invention, and are not to be construed as limiting the technical solutions of the present invention, in order to clearly explain the principles and applications of the present invention. Any modification, equivalent replacement and improvement without departing from the principle of the present invention are within the protection scope of the present invention.

Claims (3)

1. An improved droop control method for an inverter based on a virtual synchronous machine technology is characterized by comprising the following steps:
step 1: the instantaneous value of three-phase voltage is collected at the port of an inverter, namely the head end of a connecting line between a micro-grid and a public power gridu oInstantaneous value of three-phase currenti oObtaining instantaneous active power of the head end of the linep VSGAnd instantaneous reactive powerq VSGRespectively obtaining active power after filteringP VSGAnd reactive powerQ VSG；
Step 2: according to the impedance of the connecting circuit and the collected three-phase current instantaneous valuei oObtaining active power at the end of the connecting circuit after the link of line loss compensationP UAnd terminal reactive powerQ U；
And step 3: according to the impedance of the connecting circuit, the collected three-phase voltage instantaneous valueu oAnd instantaneous value of three-phase currenti oObtaining the effective value of the terminal voltage of the connection circuit after the circuit loss compensation linkU UWhile obtaining the actual angular frequencyω；
And 4, step 4: according to the active power of the terminalP UAnd the actual angular frequencyωFinishing the control of improving active/frequency droop and virtual synchronous machine to obtain voltage phase control instructionθThe step 4 comprises the following steps:
step 41: the method is used for finishing the improvement of active/frequency droop control according to the following formula and obtaining the mechanical power required by the control of the virtual synchronous machineP m：
Figure 933700DEST_PATH_IMAGE002
；
Wherein the content of the first and second substances,ωandω nrespectively an actual angular frequency and a rated angular frequency,P nis a reference value for the active power,k pactive/frequency droop coefficient and positive;
step 42: finishing the control of the virtual synchronous machine according to the following formula to obtain the phase control instructionθ：
Figure 430540DEST_PATH_IMAGE004
;
Wherein the content of the first and second substances,Jin order to be the moment of inertia,Din order to be a damping coefficient of the damping,ω gin order to obtain the angular frequency of the power grid,θis a phase control command;
and 5: according to the terminal reactive power QUAnd the effective value of the voltage at the end of the actual connection lineU UCompleting improved reactive/voltage droop control and obtaining voltage amplitude control instructionE refThe step 5 comprises the following steps:
step 51: finishing the improvement of reactive power/voltage droop control according to the following formula to obtain the reference value of the voltage at the tail end of the connecting lineU ref：
Figure 80965DEST_PATH_IMAGE006
；
Wherein the content of the first and second substances,Q nis a reference value for the reactive power,U nis a voltage of a rated voltage, and is,k qis a reactive/voltage droop coefficient and is positive;
step 52: obtaining a voltage amplitude control command according to the following formulaE ref：
Figure 5058DEST_PATH_IMAGE008
；
Wherein the content of the first and second substances,K PandK Irespectively a proportional coefficient and an integral coefficient of the proportional-integral controller,E refa voltage amplitude control command;
step 6: according to the voltage amplitude control instructionE refAnd the voltage phase control commandθForming a voltage reference value；
And 7: and obtaining a control signal through the voltage and current double closed-loop control module and the modulation pulse generation module, and sending the control signal to the inverter for inverter control.
2. The improved droop control method of claim 1, wherein said step 2 comprises the steps of:
step 21: respectively acquiring an active loss instantaneous value deltap and a reactive loss instantaneous value deltaq of a connecting line according to the following formulas:
Figure 373723DEST_PATH_IMAGE010
；
in the formula (I), the compound is shown in the specification,R Lin order to connect the line resistors with each other,X Lis a reactance of a connecting circuit;
step 22: respectively acquiring active loss delta P and reactive loss delta Q of a connecting line according to the following formulas:
Figure 110472DEST_PATH_IMAGE012
;
wherein the content of the first and second substances,ω 0cut-off angular frequency for the low pass filter;
step 23: respectively acquiring active power P at the tail end of the connecting circuit according to the following formulaUAnd terminal reactive power QU：
Figure 880982DEST_PATH_IMAGE014
。
3. The improved droop control method of claim 1, wherein said step 3 comprises the steps of:
step 31: according to the collected three-phase current instantaneous value ioAnd obtaining the voltage drop Deltau of the connecting circuit by the following formula:
Figure 710398DEST_PATH_IMAGE016
；
step 32: according to the collected three-phase voltage instantaneous value uoAnd obtaining the terminal voltage u of the connection circuit according to the following formulaU：
Figure 831938DEST_PATH_IMAGE018
；
Step 33: the connection line end voltage uUObtaining the effective value U of the voltage of the terminal required by control after RMS measurementU；
Step 34: the connection line end voltage uUObtaining actual angular frequency after PLLω。
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