CN116316814A - VSG-based interconnected converter control method in AC/DC hybrid micro-grid - Google Patents

VSG-based interconnected converter control method in AC/DC hybrid micro-grid Download PDF

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CN116316814A
CN116316814A CN202310105310.9A CN202310105310A CN116316814A CN 116316814 A CN116316814 A CN 116316814A CN 202310105310 A CN202310105310 A CN 202310105310A CN 116316814 A CN116316814 A CN 116316814A
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frequency
current bus
alternating current
direct current
ilc
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杜燕
高春然
杨世友
杨向真
苏建徽
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Hefei University of Technology
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Hefei University of 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
    • H02J3/002—Flicker reduction, e.g. compensation of flicker introduced by non-linear load
    • 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
    • H02J3/241—The oscillation concerning frequency
    • 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
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • 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
    • 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]

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Abstract

The invention discloses a VSG-based interconnected converter control method in an AC/DC hybrid micro-grid, which comprises the steps of setting an interconnected converter frequency control loop and a voltage/current loop; the frequency control loop comprises transient damping and inertia, the inertia is the integral of the difference value between the active power reference value and the output power actual value of the interconnected converter, and the active power reference value is obtained by calculating according to the per unit value of the alternating current bus frequency plus the variation thereof, the per unit value of the direct current bus voltage plus the variation thereof, and the steady damping coefficient; the setting of the dynamic virtual impedance contained in the voltage-current loop aims at the coincidence of the per-unit value of the alternating current bus frequency change rate and the maximum value of the direct current bus voltage change rate. The invention enables the interconnection converter to transmit power according to the set precision in the steady state by setting the steady state damping coefficient so as to coordinate the power between the AC sub-networks and the DC sub-networks; the dynamic characteristics of the alternating current bus frequency and the direct current bus voltage when the load suddenly changes are optimized by adding transient damping and setting virtual impedance.

Description

VSG-based interconnected converter control method in AC/DC hybrid micro-grid
Technical Field
The invention relates to the technical field of AC/DC hybrid micro-grid control, in particular to an interconnected converter control method based on VSG, which is applied to an AC/DC hybrid micro-grid.
Background
In order to meet the requirements of green environmental protection, more and more distributed power sources (distributed generator, DG) are connected into a power system in the form of micro-grids. Because distributed power DG is mostly direct current type power, more and more loads need direct current power to supply power simultaneously, and the energy loss in the direct current-alternating current conversion process can be reduced through direct current micro-grid connection. However, the existing power distribution network mainly comprises an alternating current system, and it is not practical to comprehensively reconstruct a direct current micro-grid. Therefore, to fully utilize the existing ac distribution network while reducing energy loss, an ac-dc hybrid micro-grid is one of the currently accepted viable solutions.
In an ac/dc hybrid microgrid, the selection of a suitable interconnected converter (interlinking converter, ILC) control strategy is critical to the stable operation of the system. The control strategy of the interconnection converter ILC is usually a bi-sagging control method, in which the ac frequency and the dc voltage are unified through a formula, and the unified value is compared to determine the flow direction of power. However, the dynamic response time of the droop control strategy is very short, and as the distributed power supply in the micro-grid increases, the virtual inertia of the hybrid micro-grid system can be reduced, and the stability of the system is affected. Meanwhile, the traditional control strategy only focuses on the ILC steady-state characteristic, and ignores the dynamic characteristic. To address the lack of inertia in hybrid microgrid systems, ILCs are often controlled with virtual synchronous generators (Virtual Synchronous Generator, VSG) in order to provide inertial support for ac busbar frequencies. However, the AC/DC sub-network has certain inertia, and when the two inertia are not matched, the problem of frequency oscillation and even instability can be generated. Moreover, the VSG control of the ILC, while improving dynamics, may also cause errors and inaccuracies in the power delivered by the ILC at steady state. Meanwhile, after the subnetworks are interconnected and operated, the dynamic characteristics of alternating current frequency and direct current voltage can be coupled through ILC, when the load is suddenly changed, the frequency change rate of an alternating current bus and the voltage change rate of a direct current bus under the independent operation of the atomic network can meet the constraint, but the threshold value can be exceeded after the subnetworks are interconnected.
Disclosure of Invention
The invention provides a VSG-based interconnection converter control method in an AC/DC hybrid micro-grid, which adopts VSG control and virtual impedance design containing AC bus frequency variation and DC bus voltage variation and combines transient damping control, so that ILC can accurately transmit power in steady state and AC/DC subnetwork power coordination is realized; but also can optimize the dynamic process of the frequency of the alternating current bus and the voltage of the direct current bus.
The invention adopts the following technical scheme for solving the technical problems:
the interconnected converter control method based on VSG in the AC/DC hybrid micro-grid is characterized by comprising the following steps:
setting an interconnection converter frequency control loop and an interconnection converter voltage current loop;
the interconnected converter frequency control loop comprises transient damping and inertia, the inertia is the integral of the difference value between an active power reference value and an output power actual value of the interconnected converter, and the active power reference value is based on the per unit value of the alternating current bus frequency plus the variation thereof, the per unit value of the direct current bus voltage plus the variation thereof, and a steady-state damping coefficient K P Calculating to obtain;
the voltage-current loop of the interconnection converter comprises dynamic virtual impedance, and the setting of the dynamic virtual impedance is based on the inertia of an alternating current/direct current sub-network, the frequency change rate threshold of an alternating current bus and the voltage change rate threshold of a direct current bus, and aims to make the per unit value of the frequency change rate of the alternating current bus and the maximum value of the voltage change rate of the direct current bus consistent;
the interconnection converter transmits power according to the set precision in the steady state by setting the steady state damping coefficient so as to coordinate AC/DCInter-flow subnet power; by adding transient damping and setting virtual impedance X v And overshoot and oscillation of output power during abrupt load change are reduced, so that the frequency of an alternating current bus and the voltage of a direct current bus are optimized.
The control method of the interconnected converter based on the VSG in the AC/DC hybrid micro-grid is carried out according to the following procedures:
step 1, calculating to obtain an active power reference value P of the interconnected converter by the step 1 ILC_ref ：
Figure BDA0004074543860000021
In the formula (1):
K P is the steady state damping coefficient, (. Cndot. p.u. Representing per unit;
U dc for detecting the obtained DC bus voltage in real time, deltaU dc The voltage variation of the direct current bus is used as the voltage variation of the direct current bus;
w ilc for real-time detection of the ac busbar frequency, Δw ilc The frequency variation of the alternating current bus is used as the frequency variation of the alternating current bus;
and has the following components:
Figure BDA0004074543860000022
wherein:
P ILC the transmission power of the interconnected converter is obtained for real-time detection;
m w for AC subnet sag factor, m v Is the droop coefficient of the direct current sub-network;
step 2: the transmission power P of the interconnected converter obtained by real-time detection ILC The frequency control loop is introduced through negative feedback and is connected with the active power reference value P of the interconnected converter ILC_ref Comparing, and eliminating P by integrating ILC And P ILC_ref Realizes the error of the active power reference value P of the interconnected converter ILC_ref Is a track of (2);
step 3, obtaining each line parameter through detection, including: virtual inertia of AC sub-network J ac Virtual inertia J of DC subnetwork dc Line impedance X of AC sub-network ac Line impedance X of interconnected converter ILC The frequency change rate of the alternating current bus and the voltage change rate of the direct current bus; obtaining each given parameter according to the grid-connected standard, including: calculating to obtain virtual impedance X according to the line parameters and given parameters v The virtual impedance X v Adding the voltage and current loop of the interconnected converter as dynamic virtual impedance;
step 4, obtaining the frequency w generated under the control of the alternating current sub-network VSG in a communication mode ac The frequency w ac Introducing negative feedback into a frequency control loop of the interconnected converter to perform transient damping control, and calculating by a formula (2) to obtain transient power P generated by the transient damping control e The method comprises the following steps:
P e ＝K e (w ilc -w ac ) (2)
in the formula (2): k (K) e Is a transient damping coefficient;
transient power P e The frequency control loop is added in a positive feedback mode, so that the dynamic characteristic of the AC/DC hybrid micro-grid is improved, and the suppression of power oscillation is realized.
The interconnected converter control method based on VSG in the AC/DC hybrid micro-grid is also characterized in that: the inertia contained in the interconnected converter frequency control loop has an inertia form shown in a formula (3):
Figure BDA0004074543860000031
in the formula (3):
J 1 virtual inertia constants of the interconnected converters; w (w) N Is the ac bus frequency rating.
The interconnected converter control method based on VSG in the AC/DC hybrid micro-grid is also characterized in that: steady-state damping coefficient K in frequency control loop of interconnected converter P Is calculated from formula (4):
Figure BDA0004074543860000032
in the formula (4):
P dcN rated power for DC sub-network, P acN Is the rated power of the AC sub-network.
The interconnected converter control method based on VSG in the AC/DC hybrid micro-grid is also characterized in that: the virtual impedance X v Calculated from formula (5):
Figure BDA0004074543860000033
in formula (5):
f ac the frequency of an alternating current bus in an alternating current-direct current hybrid micro-grid;
Figure BDA0004074543860000034
the frequency change rate threshold value of the alternating current bus in the alternating current-direct current hybrid micro-grid is set;
Figure BDA0004074543860000035
the voltage change rate threshold value of the direct current bus in the alternating current-direct current hybrid micro-grid is obtained.
The interconnected converter control method based on VSG in the AC/DC hybrid micro-grid is also characterized in that: in the formula (1), the formula (1) is represented by the formula (I) p.u. The per unit value is calculated according to expression (6):
Figure BDA0004074543860000036
in formula (6): by gamma pu A per unit value representing the characteristic quantity of the AC/DC hybrid micro-grid; by gamma max And gamma min The one-to-one correspondence represents the allowable maximum value and the allowable minimum value of the characteristic quantity of the AC/DC hybrid micro-grid.
Compared with the prior art, the invention has the beneficial effects that:
1. according to the invention, the variable quantity of the characteristic quantity of the AC/DC hybrid micro-grid is added in the VSG control link of the interconnected converter, so that inertia can be provided for the AC frequency, and accurate transmission of power in a steady state can be realized;
2. according to the invention, virtual impedance is designed according to the constraint, so that when the load suddenly changes, the frequency change rate of the alternating current bus and the voltage change rate of the direct current bus do not exceed a threshold value; meanwhile, transient damping is added in the frequency control link, so that overshoot in the dynamic process is effectively reduced, and oscillation is restrained.
Drawings
Fig. 1 is an ac/dc hybrid micro-grid structure topology according to the present invention;
fig. 2 is a control block diagram of the VSG-based interconnection current transformer of the present invention;
FIG. 3 is a graph showing the output power of an AC/DC sub-network under VSG control with variable amounts according to the present invention;
FIG. 4 is a graph showing the output power of an AC/DC sub-network under VSG control without variation in the present invention;
FIG. 5 is the output power of an AC/DC sub-network with virtual impedance and transient damping added in the present invention;
FIG. 6 is a graph showing the characteristic change rate of the AC/DC hybrid micro-grid when virtual impedance and transient damping are added in the invention;
FIG. 7 is a graph showing the rate of change of the characteristics of an AC/DC hybrid micro-grid without adding virtual impedance and transient damping in the present invention;
table 1 shows the parameters given for the ac/dc hybrid microgrid model of the present invention.
Detailed Description
Referring to fig. 1 and fig. 2, the control method for the VSG-based interconnection converter in the ac/dc hybrid micro-grid in this embodiment is as follows: setting an interconnection converter frequency control loop and an interconnection converter voltage current loop; the interconnected converter frequency control loop comprises transient damping and inertia, the inertia is the integral of the difference between the active power reference value and the output power actual value of the interconnected converter, the active power reference value is based on the per unit value of the alternating current bus frequency plus the variation thereof, the per unit value of the direct current bus voltage plus the variation thereof, andsteady state damping coefficient K P Calculating to obtain; the interconnected converter voltage-current loop comprises dynamic virtual impedance, and the setting of the dynamic virtual impedance is based on the inertia of an alternating current/direct current sub-network, the frequency change rate threshold of an alternating current bus and the voltage change rate threshold of a direct current bus, and aims to make the per unit value of the frequency change rate of the alternating current bus and the maximum value of the voltage change rate of the direct current bus consistent; setting a steady-state damping coefficient to enable the interconnection converter to transmit power according to set precision in steady state so as to coordinate power between alternating current and direct current sub-networks; by adding transient damping and setting virtual impedance X v And overshoot and oscillation of output power during abrupt load change are reduced, so that the frequency of an alternating current bus and the voltage of a direct current bus are optimized.
In specific implementation, the control method of the interconnected converter based on the VSG in the AC/DC hybrid micro-grid is carried out according to the following procedures:
step 1, calculating to obtain an active power reference value P of the interconnected converter by the step 1 ILC_ref ：
P ILC_ref ＝K P ((U dc +ΔU dc ) p.u. -(w ilc +Δw ilc ) p.u. ) (1)
In the formula (1):
K P is the steady state damping coefficient, (. Cndot. p.u. Representing per unit;
U dc for detecting the obtained DC bus voltage in real time, deltaU dc The voltage variation of the direct current bus is used as the voltage variation of the direct current bus;
w ilc for real-time detection of the ac busbar frequency, Δw ilc The frequency variation of the alternating current bus is used as the frequency variation of the alternating current bus;
and has the following components:
Figure BDA0004074543860000051
wherein: p (P) ILC The transmission power of the interconnected converter is obtained for real-time detection; m is m w For AC subnet sag factor, m v Is the droop coefficient of the direct current sub-network;
step 2: the transmission power P of the interconnected converter obtained by real-time detection ILC Introducing frequency by negative feedbackControl loop, interconnected with active power reference value P of converter ILC_ref Comparing, and eliminating P by integrating ILC And P ILC_ref Realizes the error of the active power reference value P of the interconnected converter ILC_ref Is a track of (2);
step 3, obtaining each line parameter through detection, including: virtual inertia of AC sub-network J ac Virtual inertia J of DC subnetwork dc Line impedance X of AC sub-network ac Line impedance X of interconnected converter ILC The frequency change rate of the alternating current bus and the voltage change rate of the direct current bus; obtaining each given parameter according to the grid-connected standard, including: the frequency change rate threshold value of the alternating current bus and the voltage change rate threshold value of the direct current bus are calculated according to the line parameters and given parameters to obtain virtual impedance X v Virtual impedance X v Adding the voltage and current loop of the interconnected converter as dynamic virtual impedance;
step 4, obtaining the frequency w generated under the control of the alternating current sub-network VSG in a communication mode ac Will frequency w ac Introducing negative feedback into a frequency control loop of the interconnected converter to perform transient damping control, and calculating by a formula (2) to obtain transient power P generated by the transient damping control e The method comprises the following steps:
P e ＝K e (w ilc -w ac ) (2)
in the formula (2): k (K) e Is a transient damping coefficient;
transient power P e The frequency control loop is added in a positive feedback mode, so that the dynamic characteristic of the AC/DC hybrid micro-grid is improved, and the suppression of power oscillation is realized.
The corresponding technical measures in the embodiment also comprise:
the inertia contained in the interconnected converter frequency control loop has the form of inertia shown in formula (3):
Figure BDA0004074543860000052
in the formula (3): j (J) 1 Virtual inertia constants of the interconnected converters; w (w) N For the frequency of the ac busRated value.
In steady state condition
Figure BDA0004074543860000053
Then formula (3) is simplified to formula (3-1):
Figure BDA0004074543860000054
DeltaU to be characterized by formula (1-1) dc And Deltaw ilc Is substituted into formula (3-1) and depends on m w Rated power sum m of AC sub-network v The relation with the rated power of the dc sub-network yields the formula (3-2):
Figure BDA0004074543860000061
in the formula (3-2):
P dcN rated power for DC sub-network, P acN Is the rated power of the AC sub-network.
When the output power of the AC/DC sub-network is ideally distributed according to the respective capacity, the per unit values of the characteristic quantities of the AC/DC hybrid micro-network are the same, as shown in the formula (3-3):
(U dc ) p.u. ＝(w ilc ) p.u. (3-3)
thus substituting equation (3-3) into equation (3-2) to obtain the steady state damping coefficient K in the frequency control loop of the interconnected converter under ideal conditions P Characterized by the formula (4), and obtaining K by calculating the formula (4) P ：
Figure BDA0004074543860000062
Virtual impedance X v Calculated from formula (5):
Figure BDA0004074543860000063
in formula (5):
f ac the frequency of an alternating current bus in an alternating current-direct current hybrid micro-grid;
Figure BDA0004074543860000064
the frequency change rate threshold value of the alternating current bus in the alternating current-direct current hybrid micro-grid is set;
Figure BDA0004074543860000065
the voltage change rate threshold value of the direct current bus in the alternating current-direct current hybrid micro-grid is obtained.
In the event of abrupt load change, the VSG control interconnecting the converter and the ac subnetwork has not been effected, and thus the instantaneous output power distribution relationship of the ac subnetwork is as shown in equation (5-1):
Figure BDA0004074543860000066
in the formula (5-1):
P ac_trans for instantaneous output power of AC sub-network, P dc_trans And outputting power instantly for the direct current sub-network.
As can be seen from equation (5-1), the virtual impedance X is changed v The instantaneous output power allocation relationship can be changed. And P can be obtained according to an AC/DC subnet VSG control equation dc_trans And the voltage change rate of the direct current bus and P ac_trans The relation with the frequency change rate of the alternating current bus is shown in the formula (5-2):
Figure BDA0004074543860000067
in the formula (5-2):
Figure BDA0004074543860000071
the frequency change rate of an alternating current bus in an alternating current-direct current hybrid micro-grid is set; />
Figure BDA0004074543860000072
The method is the voltage change rate of the direct current bus in the alternating current-direct current hybrid micro-grid.
Substituting the equation (5-2) into the equation (5-1) is performed according to the virtual impedance design target so that the per unit value of the frequency change rate of the ac bus is the same as the per unit value of the voltage change rate of the dc bus, and the calculation equation of the virtual impedance represented by the equation (5) can be derived.
In the formula (1), the formula (1) is represented by the formula (I) p.u. The per unit value is calculated according to expression (6):
Figure BDA0004074543860000073
in formula (6): by gamma pu A per unit value representing the characteristic quantity of the AC/DC hybrid micro-grid; by gamma max And gamma min The one-to-one correspondence represents the allowable maximum value and the allowable minimum value of the characteristic quantity of the AC/DC hybrid micro-grid.
In order to verify the effectiveness of the method, an AC/DC hybrid micro-grid model of island operation is built in Matlab/simulink, and given parameters of the model are shown in table 1:
table 1 given parameters of the ac/dc hybrid microgrid model:
parameter description Numerical value Parameter description Numerical value
DC sub-network rated power P dcN 20kW Rated power P of AC sub-network acN 20kW
DC bus rated voltage U dcN 700V Rated frequency w of AC bus N 100πrad/s
Direct current subnet sag factor m v 30V/20kW Sag factor m of AC subnetwork w 1Hz/20kW
DC subnet virtual inertia J dc 0.038kg·m 2 Virtual inertial constant J of ILC 1 0.8kg·m 2
Ac subnet line impedance 0.1+j0.3π ILC line impedance 0.1+j0.3π
The topology of the AC/DC hybrid microgrid is shown in FIG. 1, and consists of a DC microgrid, an AC microgrid and an interconnection converter connected with an AC/DC bus. The direct current sub-network consists of a DC/DC converter, is controlled by a virtual direct current motor, and the alternating current sub-network is equivalently controlled by a DC/AC inverter and is controlled by a virtual synchronous generator. At the initial moment, the direct current sub-network is empty, the side load of the alternating current sub-network is 10kW, and the capacities of the alternating current sub-network and the direct current sub-network are the same and are 20kW.
The interconnected converters employ control as shown in fig. 2In the control strategy, the embodiment adds the alternating current bus frequency variation and the direct current bus voltage variation into the power reference value of the VSG control link of the ILC, so that the balance of the alternating current and direct current subnetwork during load fluctuation can be realized. FIG. 3 shows the output power P of the AC sub-network under VSG control with variation ac And DC sub-network output power P dc At this time, the two output powers are equally divided according to the capacity of the sub-network, and are 5kW, so that no error exists. Fig. 4 shows the output power of the ac/dc sub-network under the VSG control without the variable amount, where the output power of the ac sub-network is 2.5kW and the output power of the dc sub-network is 7.5kW, the power of the ac sub-network is not output according to the sub-network capacity to support the load on the ac sub-network side on average, the power coordination of the ac/dc sub-network cannot be achieved, and the case where the output power of the ac/dc sub-network is greater than the capacity thereof may occur in severe cases, which is not allowed. Therefore, the variable quantity of the characteristic quantity of the AC/DC hybrid micro-grid is added in the VSG control link, so that errors existing in the coordination distribution of the AC/DC sub-grid power can be eliminated, and the accurate power transmission of the interconnected converter in a steady state can be realized.
Fig. 5 shows the output power of the ac/dc sub-network when virtual impedance and transient damping control are added under VSG control, and the system does not generate overshoot and oscillation at this time, the ac/dc sub-network quickly becomes stable, and the output power is 5kW at a theoretical value at a steady state. Fig. 6 shows the per unit value of the ac bus frequency change rate and the per unit value of the dc bus voltage change rate when the virtual impedance and the transient damping control are added under the VSG control, where the two per unit values of the ac bus frequency change rate threshold and the two per unit values of the dc bus voltage change rate threshold are both-1, so that the two change rates do not exceed the threshold, and the virtual impedance design goal is achieved, i.e., the two change rate per unit values are the same. Fig. 7 shows a per unit value of the frequency change rate of the ac bus and a per unit value of the voltage change rate of the dc bus when the virtual impedance and the transient damping are not added under the VSG control, and at this time, the per unit value of the frequency change rate of the ac bus exceeds a threshold value-1, which is about-1.25, which indicates that the frequency change rate of the ac bus does not meet the grid-connected standard. Comparing the output power of the ac/dc sub-network in fig. 3 and fig. 5, it can be found that when the virtual impedance and the transient damping control are not added in fig. 3, the overshoot and the oscillation exist in the system, the overshoot is greater, about 100%, and the output power of the ac sub-network appears to be less than 0 in the transient process, which indicates that the system generates a circulation, which is unfavorable for the stable operation of the ac/dc hybrid micro-network. Therefore, virtual impedance and transient damping control are added into the interconnection converter based on the VSG, system oscillation during load abrupt change can be restrained, and dynamic performance of the system during load abrupt change is optimized.
The method overcomes the defect that the VSG control of the existing interconnection converter only pays attention to steady state characteristics, can ensure that ILC accurately transmits power in steady state, and the AC/DC sub-network supports loads together according to respective capacities; and the dynamic process of the alternating current bus frequency and the direct current bus voltage during load abrupt change can be optimized. The effectiveness of the method is verified through simulation.

Claims (6)

1. A VSG-based interconnected converter control method in an AC/DC hybrid micro-grid is characterized by comprising the following steps:
setting an interconnection converter frequency control loop and an interconnection converter voltage current loop;
the interconnected converter frequency control loop comprises transient damping and inertia, the inertia is the integral of the difference value between an active power reference value and an output power actual value of the interconnected converter, and the active power reference value is based on the per unit value of the alternating current bus frequency plus the variation thereof, the per unit value of the direct current bus voltage plus the variation thereof, and a steady-state damping coefficient K P Calculating to obtain;
the voltage-current loop of the interconnection converter comprises dynamic virtual impedance, and the setting of the dynamic virtual impedance is based on the inertia of an alternating current/direct current sub-network, the frequency change rate threshold of an alternating current bus and the voltage change rate threshold of a direct current bus, and aims to make the per unit value of the frequency change rate of the alternating current bus and the maximum value of the voltage change rate of the direct current bus consistent;
setting a steady-state damping coefficient to enable the interconnection converter to transmit power according to set precision in steady state so as to coordinate power between alternating current and direct current sub-networks; by adding transient damping and setting virtual impedance X v And overshoot and oscillation of output power during abrupt load change are reduced, so that the frequency of an alternating current bus and the voltage of a direct current bus are optimized.
2. The method for controlling the interconnected converters based on the VSG in the AC/DC hybrid micro-grid according to claim 1, wherein the method is characterized by comprising the following steps:
step 1, calculating to obtain an active power reference value P of the interconnected converter by the step 1 ILC_ref ：
P ILC_ref ＝K P ((U dc +ΔU dc ) p.u. -(w ilc +Δw ilc ) p.u. ) (1)
In the formula (1):
K P is the steady state damping coefficient, (. Cndot. p.u. Representing per unit;
U dc for detecting the obtained DC bus voltage in real time, deltaU dc The voltage variation of the direct current bus is used as the voltage variation of the direct current bus;
w ilc for real-time detection of the ac busbar frequency, Δw ilc The frequency variation of the alternating current bus is used as the frequency variation of the alternating current bus;
and has the following components:
Figure FDA0004074543850000011
wherein:
P ILC the transmission power of the interconnected converter is obtained for real-time detection;
m w for AC subnet sag factor, m v Is the droop coefficient of the direct current sub-network;
step 2: the transmission power P of the interconnected converter obtained by real-time detection ILC The frequency control loop is introduced through negative feedback and is connected with the active power reference value P of the interconnected converter ILC_ref Comparing, and eliminating P by integrating ILC And P ILC_ref Realizes the error of the active power reference value P of the interconnected converter ILC_ref Is a track of (2);
step 3, obtaining each line parameter through detection, including: virtual inertia of AC sub-network J ac Virtual inertia J of DC subnetwork dc Line impedance X of AC sub-network ac Line impedance X of interconnected converter ILC The frequency change rate of the alternating current bus and the voltage change rate of the direct current bus; obtaining each given parameter according to the grid-connected standard, including: calculating to obtain virtual impedance X according to the line parameters and given parameters v The virtual impedance X v Adding the voltage and current loop of the interconnected converter as dynamic virtual impedance;
step 4, obtaining the frequency w generated under the control of the alternating current sub-network VSG in a communication mode ac The frequency w ac Introducing negative feedback into a frequency control loop of the interconnected converter to perform transient damping control, and calculating by a formula (2) to obtain transient power P generated by the transient damping control e The method comprises the following steps:
P e ＝K e (w ilc -w ac ) (2)
in the formula (2): k (K) e Is a transient damping coefficient;
transient power P e The frequency control loop is added in a positive feedback mode, so that the dynamic characteristic of the AC/DC hybrid micro-grid is improved, and the suppression of power oscillation is realized.
3. The method for controlling the interconnected converters based on the VSG in the AC/DC hybrid micro-grid according to claim 2, wherein the method comprises the following steps: the inertia contained in the interconnected converter frequency control loop has an inertia form shown in a formula (3):
Figure FDA0004074543850000021
in the formula (3):
J 1 virtual inertia constants of the interconnected converters; w (w) N Is the ac bus frequency rating.
4. The method for controlling the interconnected converters based on the VSG in the AC/DC hybrid micro-grid according to claim 2, wherein the method comprises the following steps: steady-state damping coefficient K in frequency control loop of interconnected converter P Is calculated from formula (4):
Figure FDA0004074543850000022
in the formula (4):
P dcN rated power for DC sub-network, P acN Is the rated power of the AC sub-network.
5. The method for controlling the interconnected converters based on the VSG in the AC/DC hybrid micro-grid according to claim 2, wherein the method comprises the following steps: the virtual impedance X v Calculated from formula (5):
Figure FDA0004074543850000023
in formula (5):
f ac the frequency of an alternating current bus in an alternating current-direct current hybrid micro-grid;
Figure FDA0004074543850000024
the frequency change rate threshold value of the alternating current bus in the alternating current-direct current hybrid micro-grid is set;
Figure FDA0004074543850000025
the voltage change rate threshold value of the direct current bus in the alternating current-direct current hybrid micro-grid is obtained.
6. The method for controlling the interconnected converters based on the VSG in the AC/DC hybrid micro-grid according to claim 2, wherein the method comprises the following steps: in the formula (1), the formula (1) is represented by the formula (I) p.u. The per unit value is calculated according to expression (6):
Figure FDA0004074543850000031
in formula (6): by gamma pu Representing ac/dc mixingA per unit value of the micro-grid characteristic quantity; by gamma max And gamma min The one-to-one correspondence represents the allowable maximum value and the allowable minimum value of the characteristic quantity of the AC/DC hybrid micro-grid.
CN202310105310.9A 2023-02-13 2023-02-13 VSG-based interconnected converter control method in AC/DC hybrid micro-grid Pending CN116316814A (en)

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