CN113964858B - Three-phase inverter grid-connected control system based on dual synchronization principle - Google Patents
Three-phase inverter grid-connected control system based on dual synchronization principle Download PDFInfo
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Classifications
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/381—Dispersed generators
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- 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
- H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
- H02J2203/10—Power transmission or distribution systems management focussing at grid-level, e.g. load flow analysis, node profile computation, meshed network optimisation, active network management or spinning reserve management
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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
- H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
- H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
Abstract
The invention discloses a three-phase inverter grid-connected control system based on a dual synchronization principle. A synchronous control loop and a voltage stabilizing outer loop are arranged; the synchronous control loop is connected to the input end of the integrator, and the voltage stabilizing outer loop is connected to the input end of the current limiter; the synchronous control ring adopts a synchronous control function to carry out synchronous control according to the active power and the active power reference value to obtain angular frequency, establishes a brake auxiliary ring to correct the angular frequency according to the q-axis voltage component and the power factor angle, and integrally outputs the phase; and the voltage stabilizing outer ring is used for comparing the three-phase voltage or the power amplitude with a reference value, selecting a current reference value and combining the current reference value to output a d-axis current reference value after the voltage control function. The invention provides networking capability and inertia support for the current source three-phase inverter, improves the synchronization performance of the current source three-phase inverter under a weak network, avoids the problem of weak synchronization capability, and avoids the problem of transient instability caused by current limiting, so that the three-phase inverter is completely coupled with the synchronous machine.
Description
Technical Field
The invention relates to a grid-connected control system and a method in the field of control of grid-connected three-phase inverters, in particular to a dual synchronous machine type three-phase inverter grid-connected control system.
Background
With the development of power electronics technology, more and more renewable energy sources generate electricity and direct current transmission are integrated into the power grid through three-phase inverters. This brings convenience to the grid and also presents new challenges for its safe operation.
The current common grid-connected three-phase inverter mainly comprises a current source type three-phase inverter and a virtual synchronous machine type voltage source type three-phase inverter which are synchronous by a phase-locked loop. The current source type three-phase inverter synchronized by the phase-locked loop does not have voltage and frequency supporting capability, can not provide the needed inertia for a power grid, and is easy to cause stability problems due to the coupling of an outer ring and a synchronous control ring under a weak network. The virtual synchronous machine type voltage source type three-phase inverter simulates the characteristics of a synchronous machine, provides the capability of voltage and frequency support, and is not easy to generate small-interference instability.
However, when the three-phase inverter simulates a synchronous machine, external characteristics are taken as a voltage source and can be changed into a current source due to current limiting when overload occurs, so that a power angle curve is switched, stability analysis becomes complex, stability margin is reduced, and transient instability is more likely to occur. Therefore, the grid-connected three-phase inverter can provide the required inertia for the power system while ensuring the small interference stability and the transient stability under the weak grid, and the related technology is lacking in the prior art.
Disclosure of Invention
In order to solve the problems, the invention provides a dual synchronous machine type three-phase inverter grid-connected control system. The three-phase inverter under the control has the advantages of being good in stability due to small interference, free of transient state power angle curve switching, capable of providing inertia and frequency support for a power grid and the like, and capable of achieving stable operation of grid connection of the three-phase inverter. The invention adopts active power synchronization, controls the amplitude of the outer ring maintaining voltage and current, and controls the inner ring to adopt current dq decoupling control. Compared with a virtual synchronous machine, the virtual synchronous machine has no voltage outer ring, the power angle curve is always a current power angle curve, the problem of curve switching under the current saturation of the virtual synchronous machine is avoided, and the transient stability is improved by adopting the brake auxiliary ring.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:
the invention comprises a three-phase inverter, a power grid side structure, a space vector pulse width modulator, a park converter, an integrator and a current inner loop; the three-phase inverter is connected with the power grid side structure, external input signals are sequentially connected to the space vector pulse width modulator after passing through the integrator, and the external input signals are simultaneously connected to the space vector pulse width modulator after passing through the current limiter and the current inner loop, and the space vector pulse width modulator is connected to the three-phase control end of the three-phase inverter; the method is characterized in that: a synchronous control ring and a voltage stabilizing outer ring are also arranged; the synchronous control loop is connected to the input of the integrator and the voltage stabilizing outer loop is connected to the input of the current limiter.
The power grid side structure comprises an LCL filter, a transformer, a circuit and a power grid; the power grid is connected with the three-phase input end of the three-phase inverter after passing through the circuit, the transformer and the LCL filter in sequence.
Thus the invention is provided with a total of three control loops: and the synchronous control loop, the voltage stabilizing outer loop and the current inner loop. The current inner loop is controlled in the prior art to ensure the tracking performance of the current. The synchronous control loop is active power synchronization, adopts dual swing equation synchronization to provide inertia for the inverter grid-connected control system, and can also construct other transfer functions to realize quick synchronization. The voltage stabilizing outer ring maintains the amplitude of the voltage and the current by adopting a first-order inertial control or sagging proportional control mode of (1/1+Ts).
The synchronous control loop receives the active power P of the three-phase inverter E And the Q-axis voltage component V of the reactive power Q, three-phase inverter q A preset active power reference value P ref The synchronous control loop is used for controlling the synchronous control loop according to the active power PE and the active power reference value P ref Synchronous control is carried out by adopting a synchronous control function to obtain the angular frequency of the three-phase inverter, and meanwhile, the angular frequency is controlled according to the q-axis voltage component V of the three-phase inverter q Power factor angle And establishing a braking auxiliary ring to correct the angular frequency of the three-phase inverter, and then integrating the angular frequency of the three-phase inverter through an integrator to obtain the output phase of the three-phase inverter, and inputting the output phase of the three-phase inverter to the space vector pulse width modulator.
The active power PE and the reactive power Q of the three-phase inverter are obtained by calculating and converting three-phase currents and three-phase voltages acquired from the three-phase inverter. Q-axis voltage component V of three-phase inverter q Is obtained by the park conversion processing of a three-phase voltage through a park converterObtaining the product.
In the synchronous control loop, a dual rocking equation with the following formula is adopted as a synchronous control function:
wherein J is an inertia coefficient, D is a damping coefficient, ω is a per unit value of an angular frequency of the three-phase inverter, ω 0 For angular frequency reference value, P E Active power of three-phase inverter, P ref The reference value is the active power reference value of the three-phase inverter, and t represents time; the formula shows that the dual swing equation has opposite signs of active terms compared with the dual swing equation of the synchronous machine, and meets the requirement that the three-phase inverter operates at the current power angle of 0-90 degrees.
Performing pull-type transformation on the dual swing equation to a frequency domain expression, and performing active power P E Subtracting the active power reference value P ref Obtaining a power error, processing the power error by using a frequency domain expression to obtain a per unit value omega of the angular frequency of the three-phase inverter, and multiplying the per unit value omega of the angular frequency of the three-phase inverter by a reference value omega basic Obtaining an actual value of the angular frequency of the three-phase inverter; and then the actual value of the angular frequency of the three-phase inverter is integrated by an integrator to obtain the output phase of the three-phase inverter, the output phase is input into a space vector pulse width modulator and a park converter, and the output phase is used for controlling park conversion in the park converter.
The synchronous control function adopts inertial control, and the synchronous control function is one of the inertial control and adopts active power P E Subtracting the active power reference value P ref And synchronizing, and constructing inertial control to provide frequency and inertia support for the power system.
A braking auxiliary ring is added in the synchronous control ring, and the braking auxiliary ring is used for controlling the synchronous control ring according to the power factor angleThe following reactive criteria are established:
wherein,a limit value indicating a preset power factor angle, sign being a flag for inputting a brake assist ring;
and the braking auxiliary ring judges whether the power factor angle exceeds an unstable limit or not, if the power factor angle exceeds the stable limit and enters an unstable region, the braking auxiliary ring is put into braking control to manufacture a transient balance point, so that transient instability is prevented.
When sign=1, a brake auxiliary ring is put in, and the q-axis voltage component V of the three-phase inverter is used q Adding the synchronous control function, the synchronous control function becomes:
wherein K is q Is the braking coefficient of the auxiliary braking ring, V q Representing the q-axis voltage component of the three-phase inverter.
When sign=0, the brake assist ring is not put in, and the synchronization control function is kept unchanged.
The above formula shows that when the active reference value is greater than the active limit due to voltage drop or overload, the current power angle (the phase angle difference between the two current sources) gradually increases to an area exceeding 90 degrees, the brake control is triggered to construct virtual power, and a transient balance point is constructed, so that the current power angle is not increased continuously. When the braking control is triggered, the corresponding angle curve becomes fig. 5.
According to the invention, the transient stability is improved by the synchronous control ring and the additional braking auxiliary ring.
The stable operation range of the dual synchronous machine type three-phase inverter is 0-90 degrees, and the operation point is unstable when the operation point goes over 90 degrees to 90-180 degrees due to voltage drop or overload and the like. A brake auxiliary ring is added into the synchronous control ring to prevent transient instability and ensure the transient stability of the three-phase inverter.
The voltage stabilizing outer ring receives a preset voltage reference value V 0 Preset current reference value I 0 Preset reactive power reference value Q 0 The three-phase voltage amplitude V and the reactive power amplitude Q, the voltage stabilizing outer ring compares the three-phase voltage amplitude V or the reactive power amplitude Q with the corresponding reference value, and then the current reference value I is selected and preset after the control of the voltage control function G(s) 0 D-axis current reference value I of combined output three-phase inverter dref * Q-axis current reference value I of three-phase inverter qref * Taking to be zero.
In the voltage stabilizing outer ring, according to the three-phase voltage amplitude V and the corresponding voltage reference value, the three-phase voltage amplitude V is subtracted and then is processed by a voltage control function, and then the current reference value I of the three-phase inverter is added 0 Obtaining d-axis current reference value I of three-phase inverter dref * The method is specifically expressed as follows:
I dref * ＝I 0 +G(s)(V 0 -V)
wherein G(s) represents a voltage control function, s represents a Laplacian, V represents a three-phase voltage amplitude of the three-phase inverter, V 0 Representing the voltage reference value of a three-phase inverter, I 0 Indicating the current reference value of the three-phase inverter, I dref * Representing the d-axis current reference value of the three-phase inverter.
In the voltage stabilizing outer ring, according to the three-phase power amplitude Q and the corresponding power reference value, the three-phase power amplitude Q and the corresponding power reference value are subtracted and then are processed by a voltage control function, and then the current reference value I of the three-phase inverter is added 0 Obtaining d-axis current reference value I of three-phase inverter dref * The method is specifically expressed as follows:
I dref * ＝I 0 +G(s)(Q 0 -Q)
wherein G(s) represents a voltage control function, s represents a Laplacian, Q represents a three-phase power amplitude of the three-phase inverter, Q 0 Representing the power reference value of a three-phase inverter, I 0 Indicating the current reference value of the three-phase inverter, I dref * D-axis electricity representing three-phase inverterStream reference value.
D-axis current reference value I of three-phase inverter dref * And q-axis current reference value I qref * The common input is input into a current inner loop after current limiting, d-axis and q-axis modulation voltage components of the three-phase inverter are obtained through processing and output of the current inner loop, and then the d-axis and q-axis modulation voltage components are input into a space vector pulse width modulator; the current amplitude limiting is to cut off an input value by adopting a preset threshold value, wherein a value larger than the preset threshold value is directly assigned to the preset threshold value, and a value smaller than or equal to the preset threshold value is kept unchanged.
The space vector pulse width modulator processes and outputs pulse width modulation waveforms of the three-phase inverter according to d-axis and q-axis modulation voltage components of the three-phase inverter output by the current inner loop and output phases of the three-phase inverter output by the synchronous control loop, and then sends the pulse width modulation waveforms to the three-phase inverter to control the work of the three-phase inverter.
According to the invention, through the voltage stabilizing outer ring, according to the given voltage reference value and current reference value of the three-phase inverter, the three-phase inverter is ensured to maintain the output voltage and current amplitude.
The current inner loop adopts dq decoupling control. According to the invention, dq decoupling control is adopted by the current inner loop, and because no coupling exists between the current inner loop and other control loops on a time scale, the passive design of parameters can be realized, and meanwhile, the tracking performance and stability of the current inner loop are ensured.
Existing controls typically cause the three-phase inverter external characteristics to appear as a constant voltage source. The external characteristics of the three-phase inverter are represented as constant current sources, so that the impedance of a power grid and a grid side can be equivalent to two parallel current sources through Norton during analysis, and the impedance is equivalent to two voltage sources when a synchronous machine is connected in series to form external characteristic pairs, and an equivalent circuit diagram of the two voltage sources is shown in figure 2.
Because its external characteristics form a pair with the synchronous machine, its power equation form is identical with that of synchronous machine. In the synchronous machine, the power angle is defined as the angle that the voltage of the machine end of the synchronous machine leads the voltage of the power grid, the current power angle of the three-phase inverter is defined as the angle that the current of the three-phase inverter lags the current of the power grid (figure 3), and the pair of the power angles is formed. Because of the definition dual of the power angle, PE-Pref is opposite to Pref-PE in the synchronous link of the synchronous machine in the synchronous control of the three-phase inverter.
The power angle operation area of the three-phase inverter is the same as that of the synchronous machine, when the current power angle of the three-phase inverter is 0-90 degrees, the power characteristic of the three-phase inverter is the same as that of the synchronous machine at the power angle of 0-90 degrees, the light load absorbs reactive power, and the heavy load emits reactive power.
The invention designs the external characteristic of the three-phase inverter as a current source, namely, the external loop tracking control is canceled, only the tracking control of the current internal loop is reserved, and the traditional dq decoupling control is still adopted, and the control block diagram is shown in figure 1. The three-phase inverter is connected in parallel as a current source equivalent as shown in fig. 2. The line impedance and an infinite power grid are equivalently transformed into an infinite current source parallel line admittance B (the inverse of line reactance X) through Norton. This is equivalent to synchronization of synchronous machines in that voltage sources are connected in series to form a dual.
Defining the angle of the phase angle lagging reverse net side current phase angle of the three-phase inverter current as the current power angle delta I In comparison with the synchronous machine power angle, as shown in fig. 3, the power equation is as follows:
wherein delta I The power angle of the dual synchronous machine is defined as the angle of the current lag reverse equivalent network side current of the three-phase inverter, I 0 Representing the current amplitude of a three-phase inverter, I g Representing the magnitude of the net side current after the noon equivalence, and B represents the equivalent admittance equal to the inverse of net side reactance X.
The invention is in phase diagram with the form of power equation of the synchronous machine.
The three-phase inverter is designed to run in the area with the current power angle of 0-90 degrees, so that the requirements of the three-phase inverter on not only generating reactive power but also absorbing reactive power can be met, and the three-phase inverter is in dual with the characteristic that the synchronous machine runs at the power angle of 0-90 degrees.
In order for a three-phase inverter to have inertia providing and frequency supporting capabilities, it is necessary to construct transfer functions of the inertia. For the purpose of operating region dual, the transfer function of inertiaThe number is input as P-P 0 . P is the active power of the three-phase inverter, P 0 Is the active power reference value. It can be seen that the synchronous input of the pair is opposite in sign of the active term as compared with the synchronous machine pair swinging equation, and the requirement that the three-phase inverter operates at the current power angle of 0-90 degrees is met. The system is in a steady operating region between 0-90 deg., and in an unstable operating region between 90-180 deg..
Additional brake assist rings are employed to improve transient stability. The stable operation range of the dual synchronous machine type three-phase inverter is 0-90 degrees, and the operation point is unstable when the operation point is over 90 degrees and enters 90-180 degrees due to voltage drop or overload and the like. Therefore, in order to prevent transient instability, a brake auxiliary ring is added into the synchronous control ring, so that the transient stability of the three-phase inverter is ensured.
And giving a machine end voltage reference value (or reactive power reference value) and an output current reference value of the three-phase inverter, so as to ensure that the three-phase inverter can maintain the output voltage and current amplitude.
The current inner loop still adopts the traditional dq decoupling control, and because the coupling does not exist on a time scale with other control loops, the passive design of parameters can be realized, and meanwhile, the tracking performance and the stability of the current inner loop are ensured.
The beneficial effects of the invention are as follows:
the invention provides inertia for the three-phase inverter in the current source form, improves the small interference stability of the current source three-phase inverter under a weak network, simultaneously avoids the problem of instability caused by current saturation of the virtual synchronous machine type three-phase inverter, improves the transient stability of the virtual synchronous machine type three-phase inverter through the brake auxiliary ring, ensures that the three-phase inverter has good inertia, frequency supporting capability and larger stability margin, and can completely refer to the theory of synchronous machines during multi-machine analysis.
The control structure of the invention provides networking capability and inertia support for the current source three-phase inverter, improves the synchronization performance of the current source three-phase inverter under the weak network, and avoids the problem of weak synchronization capability of the traditional phase-locked loop current source three-phase inverter under the weak network. Meanwhile, compared with a virtual synchronous machine, the problem of transient instability caused by current amplitude limiting is avoided.
The control structure of the invention enables the three-phase inverter and the synchronous machine to be completely dual: the external characteristics of the voltage source and the current source are coupled, the power equation is coupled, the dual swing equation is coupled, and the power angle operation area is coupled.
Drawings
FIG. 1 is a control block diagram of the present invention;
FIG. 2 is a diagram of a dual equivalent circuit of the present invention and a synchronous machine;
FIG. 3 is a graph comparing the power angle vector diagram of the present invention with that of the synchronous machine;
FIG. 4 is a control block diagram of an embodiment of the present invention
FIG. 5 is a graph of the angle of force curve of an embodiment of the present invention for activating a brake assist ring during a transient state;
FIG. 6 is a waveform diagram of active power, reactive power, and frequency at different power references according to an embodiment of the present invention;
FIG. 7 is a waveform diagram of active power, reactive power and frequency under different inertia coefficients when the frequency of the network side fluctuates in the embodiment of the invention;
fig. 8 is a waveform diagram of active power, reactive power, and frequency with and without a brake assist ring when the grid side voltage drops substantially in accordance with an embodiment of the present invention.
Detailed Description
The invention will be described in further detail with reference to the accompanying drawings and specific examples.
Specific embodiments of the invention are as follows:
as shown in fig. 1, the system comprises a three-phase inverter, a power grid side structure, a space vector pulse width modulator, a park converter, an integrator and a current inner loop; the three-phase inverter is connected with the power grid side structure, external input signals are sequentially connected to the space vector pulse width modulator after passing through the integrator, the external input signals are simultaneously connected to the space vector pulse width modulator after passing through the current limiter and the current inner loop, and the space vector pulse width modulator is connected to the three-phase control end of the three-phase inverter; a synchronous control ring and a voltage stabilizing outer ring are also arranged; the synchronous control loop is connected to the input of the integrator and the voltage stabilizing outer loop is connected to the input of the current limiter.
The power grid side structure comprises an LCL filter, a transformer, a circuit and a power grid; the power grid is connected with the three-phase input end of the three-phase inverter after passing through the circuit, the transformer and the LCL filter in sequence.
As shown in fig. 4, the control loop includes a synchronous control loop, a reactive-current (Q-I) control loop (or a voltage-current (U-I) control loop), and a current inner loop.
The synchronous control loop adopts a power dual swing equation and transient stability auxiliary control, the current inner loop adopts traditional dq decoupling control, and the control outer loop can be inertial link 1/(js+D) or droop control.
In the synchronous control loop, a dual rocking equation of the following formula is adopted as a synchronous control function:
wherein J is an inertia coefficient, D is a damping coefficient, ω is a per unit value of an angular frequency of the three-phase inverter, ω 0 For angular frequency reference value, P E Active power of three-phase inverter, P ref The reference value is the active power reference value of the three-phase inverter, and t represents time; the formula shows that the dual swing equation has opposite signs of active terms compared with the dual swing equation of the synchronous machine, and meets the requirement that the three-phase inverter operates at the current power angle of 0-90 degrees.
Performing pull-type transformation on the dual swing equation to a frequency domain expression, and performing active power P E Subtracting the active power reference value P ref Obtaining a power error, processing the power error by using a frequency domain expression to obtain a per unit value omega of the angular frequency of the three-phase inverter, and multiplying the per unit value omega of the angular frequency of the three-phase inverter by a reference value omega basic Obtaining an actual value of the angular frequency of the three-phase inverter; and then the actual value of the angular frequency of the three-phase inverter is integrated by an integrator to obtain the output phase of the three-phase inverter, the output phase is input into a space vector pulse width modulator and a park converter, and the output phase is used for controlling park conversion in the park converter.
This example takes the synchronous mode of dual rocking equations as an example, and is called dual rocking equation synchronization in the present invention, and is opposite to the dual rocking equation of the synchronous machine. Providing a three-phase inverter with inertia providing and frequency supporting capabilities allows for the use of inertia transfer functions in a synchronous control loop.
The method comprises the steps that a braking auxiliary ring is added in a synchronous control ring, and the braking auxiliary ring establishes the following reactive criteria according to reactive power Q:
wherein I is 0 The current reference value of the three-phase inverter is represented, X represents the equivalent reactance of the power grid side, sign is a mark for inputting a brake auxiliary ring;
and the braking auxiliary ring judges whether the braking auxiliary ring crosses an unstable limit through reactive power characteristics, and if the braking auxiliary ring crosses the stable limit and enters an unstable area, the braking auxiliary ring is put into braking control to manufacture a transient balance point so as to prevent transient instability.
When sign=1, a brake auxiliary ring is put in, and the q-axis voltage component V of the three-phase inverter is used q Adding the synchronous control function, the synchronous control function becomes:
wherein K is q Is the braking coefficient of the auxiliary braking ring, V q Representing the q-axis voltage component of the three-phase inverter.
When sign=0, the brake assist ring is not put in, and the synchronization control function is kept unchanged.
The above formula shows that when the active reference value is greater than the active limit due to voltage drop or overload, the current power angle is gradually reduced to a 0-90 degree area, the brake control is triggered to construct virtual power, and a transient balance point is constructed, so that the current power angle is not reduced continuously. When the braking control is triggered, the corresponding angle curve becomes fig. 5.
According to the invention, the transient stability is improved by the synchronous control ring and the additional braking auxiliary ring.
The stable operation range of the dual synchronous machine type three-phase inverter is 0-90 degrees, and the operation point is unstable when the operation point goes over 90 degrees to 90-180 degrees due to voltage drop or overload and the like. A brake auxiliary ring is added into the synchronous control ring to prevent transient instability and ensure the transient stability of the three-phase inverter.
In this embodiment, the control outer loop uses U-I droop as an example to maintain the voltage and current amplitude values of the three-phase inverter, and the control expression is a formula.
I dref * ＝I 0 +K V (V 0 -V)
Wherein K is V Is the sagging coefficient of U-I, V represents the capacitance voltage amplitude of the three-phase inverter, V 0 A given reference value representing the three-phase inverter voltage, I 0 A given reference value representing three-phase inverter current, I ref * Representing the three-phase inverter current reference value of the U-I droop output to the current inner loop.
The current inner loop still adopts dq decoupling control, and because no coupling exists between the current inner loop and other control loops on a time scale, the passive design of parameters can be realized, and meanwhile, the tracking performance and the stability of the current inner loop are ensured.
And constructing a single machine infinity system shown in the figure 1 in Matlab/Simulink software for simulation analysis. The definition and physical meaning of some variables are shown in table 1 below.
Table 1 example simulation verifies parameter values for part of the system variables
Fig. 6 is a waveform diagram of the active power, reactive power, frequency and current power angle of the three-phase inverter in different power reference values in simulation verification according to the embodiment of the invention. Fig. 6 shows the three-phase inverter absorbing reactive power at light load; and when the load is heavy, the three-phase inverter emits reactive power, and the reactive power accords with theoretical analysis. The three-phase inverter has good dynamic response capability, and when the active power reference value changes, the three-phase inverter can track the change of the active power in real time and ensure the stability of frequency.
Fig. 7 shows waveforms of active power, reactive power and frequency of the three-phase inverter when the frequency of the power grid is changed under different inertia coefficients J. As can be seen from FIG. 6, the rate of change of frequency and active power can be delayed after increasing the inertia coefficient, so that the invention can provide inertia support for the system
Fig. 8 shows the power and frequency curves for the case of a severe network-side voltage drop with and without auxiliary brake rings. As can be seen from fig. 8, when voltage drop occurs, the conventional control does not have an auxiliary brake ring, and when a fault of voltage drop occurs in the power grid, the three-phase inverter is subjected to transient instability; after the control with the braking auxiliary ring is adopted, the three-phase inverter can still be kept stable when the power grid voltage drops. The change and the stabilization mechanism of the current power angle accord with theoretical analysis, and the auxiliary braking ring can improve the temporary state stability of the system.
Therefore, the dual synchronous control three-phase inverter can provide inertia and frequency support for the system, has good small interference stability, and has good transient stability after the auxiliary brake ring is adopted.
The invention is limited only by the following modifications and changes, which are made within the spirit of the invention and the scope of the appended claims.
Claims (4)
1. A three-phase inverter grid-connected control system based on a dual synchronization principle comprises a three-phase inverter, a power grid side structure, a space vector pulse width modulator, a park converter, an integrator and a current inner loop; the method is characterized in that: a synchronous control ring and a voltage stabilizing outer ring are also arranged; the synchronous control loop is connected to the input end of the integrator, and the voltage stabilizing outer loop is connected to the input end of the current limiter;
the synchronous control loop receives the active power P of the three-phase inverter E And the Q-axis voltage component V of the reactive power Q, three-phase inverter q A preset active power reference value P ref The synchronous control loop is based on the active power PE and the active power reference value P ref Synchronous control is carried out by adopting a synchronous control function to obtain the angular frequency of the three-phase inverter, and meanwhile, the angular frequency is controlled according to the q-axis voltage component V of the three-phase inverter q Power factor angleEstablishing a braking auxiliary ring to correct the angular frequency of the three-phase inverter, and then integrating the angular frequency of the three-phase inverter through an integrator to obtain an output phase of the three-phase inverter, and inputting the output phase of the three-phase inverter to the space vector pulse width modulator;
in the synchronous control loop, a dual rocking equation with the following formula is adopted as a synchronous control function:
wherein J is an inertia coefficient, D is a damping coefficient, ω is a per unit value of an angular frequency of the three-phase inverter, ω 0 For angular frequency reference value, P E Active power of three-phase inverter, P ref The active power reference value is the active power reference value of the three-phase inverter, and t represents time;
performing pull-type transformation on the dual swing equation to a frequency domain expression, and performing active power P E Subtracting the active power reference value P ref Obtaining a power error, processing the power error by using a frequency domain expression to obtain a per unit value omega of the angular frequency of the three-phase inverter, and multiplying the per unit value omega of the angular frequency of the three-phase inverter by a reference value omega basic Obtaining an actual value of the angular frequency of the three-phase inverter; then the actual value of the angular frequency of the three-phase inverter is integrated through an integrator to obtain the output phase of the three-phase inverter, and the output phase is input into a space vector pulse width modulator and a park converter;
the synchronous control ring is added with a braking auxiliary ring to prepareThe dynamic auxiliary ring is based on the power factor angleThe following reactive criteria are established:
wherein,a limit value indicating a preset power factor angle, sign being a flag for inputting a brake assist ring;
when sign=1, a brake auxiliary ring is put in, and the q-axis voltage component V of the three-phase inverter is used q Adding the synchronous control function, wherein the synchronous control function becomes as follows:
wherein K is q Is the braking coefficient of the auxiliary braking ring, V q Representing the q-axis voltage component of the three-phase inverter;
when sign=0, the brake auxiliary ring is not put in, and the synchronous control function is kept unchanged;
the voltage stabilizing outer ring receives a preset voltage reference value V 0 Preset current reference value I 0 Preset reactive power reference value Q 0 The three-phase voltage amplitude V and the reactive power amplitude Q, the voltage stabilizing outer ring compares the three-phase voltage amplitude V or the reactive power amplitude Q with the corresponding reference value, and then the current reference value I is selected and preset after the control of the voltage control function G(s) 0 D-axis current reference value I of combined output three-phase inverter dref *；
D-axis current reference value I of three-phase inverter dref * And q-axis current reference value I qref * The common input is input into a current inner loop after current amplitude limiting, d-axis and q-axis modulation voltage components of the three-phase inverter are obtained through processing output of the current inner loop,and then input to a space vector pulse width modulator; the space vector pulse width modulator processes and outputs the pulse width modulation waveform of the three-phase inverter according to d-axis and q-axis modulation voltage components of the three-phase inverter output by the current inner loop and the output phase of the three-phase inverter output by the synchronous control loop, and then sends the pulse width modulation waveform to the three-phase inverter to control the work of the three-phase inverter.
2. The three-phase inverter grid-connected control system based on the dual synchronization principle as claimed in claim 1, wherein:
in the voltage stabilizing outer ring, according to the three-phase voltage amplitude V and the corresponding voltage reference value, the three-phase voltage amplitude V is subtracted and then is processed by a voltage control function, and then the current reference value I of the three-phase inverter is added 0 Obtaining d-axis current reference value I of three-phase inverter dref * The method is specifically expressed as follows:
I dref *＝I 0 +G(s)(V 0 -V)
wherein G(s) represents a voltage control function, s represents a Laplacian, V represents a three-phase voltage amplitude of the three-phase inverter, V 0 Representing the voltage reference value of a three-phase inverter, I 0 Representing the current reference value of the three-phase inverter, I dref * Representing the d-axis current reference value of the three-phase inverter.
3. The three-phase inverter grid-connected control system based on the dual synchronization principle as claimed in claim 1, wherein: in the voltage stabilizing outer ring, according to the three-phase power amplitude Q and the corresponding power reference value, the three-phase power amplitude Q and the corresponding power reference value are subtracted and then are processed by a voltage control function, and then the current reference value I of the three-phase inverter is added 0 Obtaining d-axis current reference value I of three-phase inverter dref * The method is specifically expressed as follows:
I dref *＝I 0 +G(s)(Q 0 -Q)
wherein G(s) represents a voltage control function, s represents a Laplacian, Q represents a three-phase power amplitude of the three-phase inverter, Q 0 Representing the power reference value of a three-phase inverter, I 0 Representing current reference values of a three-phase inverter，I dref * Representing the d-axis current reference value of the three-phase inverter.
4. The three-phase inverter grid-connected control system based on the dual synchronization principle as claimed in claim 1, wherein: the current inner loop adopts dq decoupling control.
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