CN113241753B - Improved virtual generator control method for direct-current micro-grid - Google Patents
Improved virtual generator control method for direct-current micro-grid Download PDFInfo
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- CN113241753B CN113241753B CN202110642391.7A CN202110642391A CN113241753B CN 113241753 B CN113241753 B CN 113241753B CN 202110642391 A CN202110642391 A CN 202110642391A CN 113241753 B CN113241753 B CN 113241753B
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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
- H02J1/00—Circuit arrangements for dc mains or dc distribution networks
- H02J1/10—Parallel operation of dc sources
- H02J1/102—Parallel operation of dc sources being switching converters
Abstract
The invention provides an improved virtual generator control method for a direct current micro-grid. According to the invention, by combining the average current PI controller with the virtual direct current generator, the accurate distribution of current is realized while the inertia and damping of the system are improved, the traditional droop control is not required to be introduced, and the problem of secondary drop of bus voltage caused by the droop control is further avoided. Meanwhile, an adaptive adjusting equation of an inertia coefficient and a damping coefficient is designed aiming at the problem of poor dynamic characteristic of the traditional virtual motor control, and the dynamic characteristic of the system is improved. The method considers the line impedance of each converter unit, can be applied to the multi-source parallel direct current micro-grid, and all controllers and calculation are completed locally, so that the current sharing speed is high.
Description
Technical Field
The invention relates to the technical field of operation control of a direct-current micro-grid, in particular to an improved virtual generator control method for the direct-current micro-grid.
Background
Distributed power generation technology based on new energy sources is receiving a great deal of attention due to the increasing shortage of fossil energy sources. Compared with an alternating-current micro-grid, the direct-current micro-grid is more suitable for flexible access of various distributed energy sources, can improve the power generation efficiency and reduce the loss and the cost, and the development of the direct-current micro-grid is also of great concern. However, in the direct current micro-grid, most of the distributed power generation units are connected with the power electronic converter, inertia and damping are lacked, and when the power generation units fluctuate or the load power is suddenly changed, the direct current bus voltage is greatly affected. How to realize accurate current sharing among converters on the premise of ensuring stable bus voltage is an important problem in research of direct-current micro-grids.
By simulating the external characteristics of the DC motor, a virtual DC motor (VDCM) controlled inverter may provide additional inertial and damping support, thereby improving the stability of the DC micro grid. The document A virtual DC machine control strategy for dual active bridge DC-DC converter and the document Virtual DC machine: an inertia emulation and control technique for a bidirectional DC-DC converter in a DC microgrid apply virtual direct current motor control to converters on the load and energy storage sides to suppress bus voltage fluctuations. However, the above documents do not flexibly control parameters of the VDCM, resulting in poor system dynamics. In addition, conventional VDCM control cannot achieve accurate current distribution, and droop control is currently the most widely studied current sharing control strategy for current sharing by introducing virtual impedance to regulate current distribution. It has an inherent contradiction between pursuing smaller voltage deviation and higher current sharing accuracy.
Therefore, improving the traditional VDCM control to enable accurate distribution of current to be achieved while improving the stability of a direct current micro-grid and having good dynamic characteristics is a key of research.
Disclosure of Invention
According to the technical problem that the accurate distribution of current can not be realized while the stability of the direct-current micro-grid is improved and the direct-current micro-grid has good dynamic characteristics, the improved virtual generator control method for the direct-current micro-grid is provided. According to the invention, by combining the average current PI controller with the virtual direct current generator, the accurate distribution of current is realized while the inertia and damping of the system are improved, the traditional droop control is not required to be introduced, and the problem of secondary drop of bus voltage caused by the droop control is further avoided.
The invention adopts the following technical means:
an improved virtual generator control method for a direct current micro-grid comprises a plurality of parallel power supplies, wherein any power supply is connected with a bidirectional DC/DC converter through a current regulating branch, the output end of the bidirectional DC/DC converter is connected with a direct current bus, and line impedance is arranged between the DC/DC converter and the direct current bus;
the current regulating branch circuit comprises an average current controller, a virtual direct current generator controller, a current controller, a limiter and a pulse width modulation module which are sequentially arranged;
the method comprises the following steps:
acquiring a current proportion distribution coefficient to be realized;
adjusting the armature current of each virtual direct current generator to be equal to the output current of the corresponding bidirectional DC/DC converter;
obtaining a reference value of each current regulating branch according to armature current and current proportion distribution coefficients of each virtual direct current generator;
calculating the average value of the reference values of each current regulating branch circuit as the input of an average current controller;
and inputting the output signal of the average current controller into a virtual direct current generator controller.
Further, the method further comprises the following steps: and feeding back an input current signal of the bidirectional DC/DC converter to an input end of a current controller.
Further, the inertia coefficient J and the damping coefficient D of the virtual direct current generator are adaptively adjusted according to the following formula:
wherein J is 0 Represents an initial value of an inertia coefficient, D 0 Represents the initial value of the damping coefficient, A and B are respectively regulating coefficients obtained in advance according to the small signal stability analysis result, |dU o The/dt| represents the change rate of the output voltage of the bidirectional DC/DC converterAbsolute value of k 0 Indicating the set threshold.
Further, the bi-directional DC/DC converter operates in a boost mode.
Further, the load side of the direct current micro-grid is connected with a parallel resistive load and a constant power load.
Compared with the prior art, the invention has the following advantages:
1. according to the invention, by combining the average current PI controller with the virtual direct current generator, the accurate distribution of current is realized while the inertia and damping of the system are improved, the traditional droop control is not required to be introduced, and the problem of secondary drop of bus voltage caused by the droop control is further avoided.
2. Aiming at the problem of poor dynamic characteristics of the control of the traditional virtual motor, the invention designs an adaptive adjustment equation of an inertia coefficient and a damping coefficient, and improves the dynamic characteristics of the system.
3. The method considers the line impedance of each converter unit, can be applied to the multi-source parallel direct current micro-grid, and all controllers and calculation are completed locally, so that the current sharing speed is high.
Based on the reasons, the invention can be widely popularized in the technical field of DC micro-grid operation control.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, and it is obvious that the drawings in the following description are some embodiments of the present invention, and other drawings may be obtained according to the drawings without inventive effort to a person skilled in the art.
Fig. 1 is an overall control block diagram of the system of the present invention.
Fig. 2 is a schematic diagram of a virtual dc generator of the present invention.
Fig. 3 is a graph of the absolute value of the voltage change rate.
Fig. 4 is a closed loop master polar plot of the system as the coefficient of inertia J changes.
Fig. 5 is a closed loop main pole diagram of the system as the damping coefficient D changes.
Fig. 6 is a simulation diagram of the bus voltage at the time of load abrupt change.
Fig. 7 is a simulation diagram of the current sharing of two converters.
In the figure: 1. a first bidirectional DC/DC converter; 2. a second bidirectional DC/DC converter; 3. a first line impedance; 4. a first line impedance.
Detailed Description
In order that those skilled in the art will better understand the present invention, a technical solution in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in which it is apparent that the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the present invention without making any inventive effort, shall fall within the scope of the present invention.
It should be noted that the terms "first," "second," and the like in the description and the claims of the present invention and the above figures are used for distinguishing between similar objects and not necessarily for describing a particular sequential or chronological order. It is to be understood that the data so used may be interchanged where appropriate such that the embodiments of the invention described herein may be implemented in sequences other than those illustrated or otherwise described herein. Furthermore, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.
The invention provides an improved virtual generator control method for a direct current micro-grid, which comprises a plurality of parallel power supplies, wherein any power supply is connected with a bidirectional DC/DC converter through a current regulating branch, the output end of the bidirectional DC/DC converter is connected with a direct current bus, and line impedance is arranged between the DC/DC converter and the direct current bus; the current regulating branch circuit comprises an average current controller, a virtual direct current generator controller, a current controller, a limiter and a pulse width modulation module which are sequentially arranged. The method comprises the following steps: acquiring a current proportion distribution coefficient to be realized; adjusting the armature current of each virtual direct current generator to be equal to the output current of the corresponding bidirectional DC/DC converter; obtaining a reference value of each current regulating branch according to armature current and current proportion distribution coefficients of each virtual direct current generator; calculating the average value of the reference values of each current regulating branch circuit as the input of an average current controller; and inputting the output signal of the average current controller into a virtual direct current generator controller. Further, the method also comprises the step of feeding back an input current signal of the bidirectional DC/DC converter to an input end of a current controller.
The present invention will be further described with reference to the accompanying drawings and specific examples of application.
Fig. 1 is a block diagram of the overall control of the system in this embodiment, comprising two parallel bi-directional DC/DC converters, through which the power supply is connected to the DC bus, the circuit part mainly comprising a first converter 1, a second converter 2, a first line impedance 3, a second line impedance 4, said converters operating in boost mode, a load side being connected to a resistive load and to a constant power load (constant power load, CPL). The control part mainly comprises an average current controller PI c Voltage controller PI u Current controller PI i And a virtual direct current generator (virtual DC generator, VDCG). The average current controller is used for realizing the proportion distribution of load current; the voltage-current PI controller is used for performing double-loop control to realize the required output voltage and current of the converter; VDCG control is used for increasing inertia and damping of the system, and further suppressing fluctuation of bus voltage after disturbance.
FIG. 2 is a schematic diagram of VDCG used in the present invention, where J is moment of inertia, D is damping coefficient, T m Is mechanical torque, T e Is electromagnetic torque, ω is actual mechanical angular velocity, ω 0 For nominal mechanical angular velocity, C T Is torque coefficient, phi is magnetic flux, E is armature electromotive force, I a For armature current，R a Is armature resistance, I ref A current reference is input to the converter. The converter can simulate the external characteristics of a direct-current generator by adopting VDCG control, thereby having the converter output current I o Equal to armature current I a 。
Due to I o ＝I a The armature currents of the VDCG controller in fig. 1 are transmitted to each other through a low bandwidth communication (Low Bandwidth Communication, LBC) network. k (k) 1 And k 2 Representing the current proportional division coefficient to be realized, reference value I a1 /k 1 And I a2 /k 2 Is a feedback variable from the VDCG controller, average value thereofAnd comparing the current value with each local reference current value, and inputting the current value into an average current PI controller, so that an accurate proportional current sharing effect can be realized. The output quantity of the average current PI controller is used as part of the input quantity of the VDCG controller, so that the output quantity of the average current PI controller and the VDCG controller are tightly combined to replace the traditional droop control, and the accurate distribution of current is realized while the inertia and damping of the system are improved.
Further, since the fixed inertia factor J and damping factor D in the conventional VDCG result in poor system dynamics, increasing J and D can provide additional inertia and damping to the system, thereby suppressing the influence of the disturbance signal on the bus voltage. Thus, during the initial ripple phase of the bus voltage, larger J and D are used to provide greater inertial and damping support for the system. In the bus voltage recovery phase, smaller J and D are used to obtain faster dynamic response speed. Based on the control idea, an adaptive adjustment equation is designed as follows:
the [ (x) ray ]1) J in (2) 0 And D 0 Initial values representing the inertia coefficient and damping coefficient, A and B representing the adjustment coefficient, selected based on the small signal stability analysis result, |dU o The absolute value of the change rate of the output voltage of the converter is shown by the ratio of the absolute value of the change rate of the bus voltage, and the arc tangent function is used for limiting the amplitude, k 0 Indicating the set threshold.
FIG. 3 is a graph of absolute value of voltage change rate, t 0 To t 1 Is the phase of bus voltage fluctuation, t 1 To t 2 Is a bus voltage recovery stage. Will threshold k 0 Set to slightly higher than the |dU of the recovery phase o Dt |. At a determined t 3 To t 4 In the range, the coefficients J and D are adaptively adjusted, and inertia and damping of the system are increased, so that bus voltage fluctuation caused by external interference is restrained. Selecting smaller fixed value J in other time periods 0 And D 0 To accelerate dynamic response speed.
Taking the first bi-directional DC/DC converter as an example for modeling, as shown in FIGS. 1-2, a transfer function G of the converter output-input current is obtained ii (s) control-output transfer function G ud (s), transfer function of control-inductor current G id (s) open loop output impedance Z oo (s)：
Where C, L is the capacitance and inductance of the converter, d' =1-d, d representing the duty cycle.
Further, a closed loop transfer function Guc(s) after adding VDCG control can be obtained:
A 1 ＝U ref G PIi (s)G ud (s) (8)
A 2 ＝G PIc (s)/2k i (9)
A 3 ＝U in G PIi (s)G id (s) (10)
fig. 4 and 5 are plots of closed loop main poles of the system with changes in the inertia factor J and damping factor D, respectively, and it can be seen that the stability of the system is relatively good when J is 0.05, D is 1.4 to 3.4, and D is 1 and J is 0.05 to 0.15. According to the value range, combining the dynamic characteristics of the system, and finally taking J 0 0.05, D 0 1.5, A0.06 and B1.3.
In order to verify the effectiveness of the invention, simulation verification is carried out by MATLAB/Simulink software, FIG. 6 is a bus voltage dynamic characteristic simulation diagram, and when load suddenly changes, the fluctuation condition of bus voltage under three control strategies is shown, and it can be seen that VDCG control can effectively inhibit the amplitude of bus voltage fluctuation compared with droop control, but the dynamic recovery characteristic is poor, and the improved VDCG can accelerate the dynamic recovery speed while effectively inhibiting the voltage fluctuation; fig. 7 is a current sharing simulation diagram of two converters when there is a difference in line impedance, and it can be seen that after average current control is enabled, accurate load current distribution is achieved in a short time, and then good current sharing effect can be maintained regardless of sudden load increase and sudden load decrease.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present invention, and not for limiting the same; although the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some or all of the technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit of the invention.
Claims (5)
1. The improved virtual generator control method for the direct-current micro-grid is characterized in that the direct-current micro-grid comprises a plurality of power supplies which are arranged in parallel, any power supply is connected with a bidirectional DC/DC converter through a current regulating branch, the output end of the bidirectional DC/DC converter is connected with a direct-current bus, and line impedance is arranged between the DC/DC converter and the direct-current bus;
the current regulating branch circuit comprises an average current controller, a virtual direct current generator controller, a current controller, a limiter and a pulse width modulation module which are sequentially arranged;
the method comprises the following steps:
acquiring a current proportion distribution coefficient to be realized;
adjusting the armature current of each virtual direct current generator to be equal to the output current of the corresponding bidirectional DC/DC converter;
obtaining a reference value of each current regulating branch according to armature current and current proportion distribution coefficients of each virtual direct current generator;
calculating the average value of the reference values of each current regulating branch, and comparing the average value with each reference current value to be used as the input of an average current controller;
and inputting the output signal of the average current controller into a virtual direct current generator controller.
2. The improved virtual generator control method for a direct current micro-grid of claim 1, further comprising: and feeding back an input current signal of the bidirectional DC/DC converter to an input end of a current controller.
3. The improved virtual generator control method for a dc micro grid according to claim 1, wherein the inertia coefficient J and damping coefficient D of the virtual dc generator are adaptively adjusted according to the following formula:
wherein J is 0 Represents an initial value of an inertia coefficient, D 0 Represents the initial value of the damping coefficient, A and B are respectively regulating coefficients obtained in advance according to the small signal stability analysis result, |dU o The absolute value of the change rate of the output voltage of the bidirectional DC/DC converter, k, is expressed by dt% 0 Indicating the set threshold.
4. The improved virtual generator control method for a direct current micro-grid of claim 1, wherein the bi-directional DC/DC converter operates in boost mode.
5. The improved virtual generator control method for a direct current micro-grid of claim 1, wherein the load side of the direct current micro-grid is connected to a resistive load and a constant power load in parallel.
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