CN113363985B - Dual-mode grid-connected control strategy for bidirectional power converter - Google Patents

Dual-mode grid-connected control strategy for bidirectional power converter Download PDF

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CN113363985B
CN113363985B CN202110306073.3A CN202110306073A CN113363985B CN 113363985 B CN113363985 B CN 113363985B CN 202110306073 A CN202110306073 A CN 202110306073A CN 113363985 B CN113363985 B CN 113363985B
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voltage
current
power
power converter
bidirectional power
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CN113363985A (en
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高乐
任远
常潇
王金浩
李�瑞
原敏宏
李亚国
李慧蓬
韩润东
赵军
王腾鑫
张定邦
孟润泉
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State Grid Electric Power Research Institute Of Sepc
Taiyuan University of Technology
State Grid Shanxi Electric Power Co Ltd
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State Grid Electric Power Research Institute Of Sepc
Taiyuan University of Technology
State Grid Shanxi Electric Power Co Ltd
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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
    • H02J3/02—Circuit arrangements for ac mains or ac distribution networks using a single network for simultaneous distribution of power at different frequencies; using a single network for simultaneous distribution of ac power and of dc power
    • 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
    • H02J3/001—Methods to deal with contingencies, e.g. abnormalities, faults or failures
    • 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
    • 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 relates to the technical field of control of an AC/DC bus interface converter. A dual-mode grid-connected control strategy of a bidirectional power converter is applicable to a circuit of an AC/DC bus interface converter of the strategy, and a positive electrode P, N of a DC bus is connected with a capacitor in parallel through a DC side of the bidirectional power converterC dc The bidirectional power converter is connected with the direct current side of the bidirectional power converter, the bidirectional power converter is composed of 6 IGBTs, the alternating current side of the bidirectional power converter is connected to a PCC point through a filtering inductor L at the alternating current side of the bidirectional power converter and a filtering capacitor C at the alternating current side of the bidirectional power converter, the PCC point supplies power for a local alternating current load on one hand and supplies power through the network side impedance of a power distribution network on the other handZ g And the power exchange is carried out by connecting the power distribution network bus. The invention has the beneficial effects that the invention considers the application of the bidirectional power converter in a weak power grid aiming at the control strategy of the bidirectional power converter under the condition that the power distribution network is a strong power grid generally.

Description

Dual-mode grid-connected control strategy for bidirectional power converter
Technical Field
The invention relates to the technical field of control of an AC/DC bus interface converter.
Background
The alternating current-direct current hybrid micro-grid can effectively integrate various distributed energy sources such as wind, light and storage, and the problem of consumption of renewable energy sources becomes a research hotspot at present. For an alternating current and direct current hybrid micro-grid, the research on the control strategy of the AC/DC bidirectional power converter for connecting the alternating current bus and the direct current bus has important significance for alternating the power of an alternating current and direct current system, maintaining the voltage and the frequency of the bus to be stable, realizing the control of the power quality and improving the stability of the system. When an alternating current-direct current hybrid microgrid is in grid-connected operation, the existing control strategies all design the microgrid by taking the power grid as an ideal model, and the voltage and the frequency of the power distribution network are basically unchanged. However, as the high-density distributed power supply is connected, the inertia level of the system continuously decreases, the problems of voltage and frequency stability of a Point of Common Coupling (PCC) are obvious, and the power grid gradually presents a weak power grid working condition. Therefore, it is necessary to research a control strategy when the AC/DC bidirectional power converter is in a grid-connected mode and weak grid conditions occur. At present, a great deal of literature is available for defining and analyzing weak grid conditions from different aspects. For example, the documents "Impact of Short-circuit ratio and phase-locked-loop parameters on the small-signal transformer of a VSC-HVDC converter" introduce a Short-circuit ratio (SCR) to define the variation of the strong and weak grid of the system for analyzing the relative strength and stability of the converter grid-connected system. The document 'a short-circuit ratio measuring method based on fundamental wave impedance identification' proposes a measuring method based on fundamental wave impedance, and the short-circuit ratio of a system is obtained by online measurement by utilizing the relationship between the fundamental wave impedance and the short-circuit ratio of a grid-connected system. The document "Stability Improvement for Three-Phase Grid-Connected Converters Through Impedance in the precision-Axis" shows that when the system operates in a current-type control strategy, the system pole still tends to be a unit circle under the condition of weak power Grid strength, and the Stability margin and the rapidity are insufficient. The document 'sequence impedance modeling and stability analysis of a virtual synchronous generator accessed to a weak power grid' verifies that a virtual synchronous control strategy can still stably run under the condition of a grid-connected system weak power grid or high-permeability new energy power generation without the constraint of a phase-locked loop. The literature 'study on the operation control and stability of the weak power grid of the battery energy storage converter' researches the relationship between the stability margin of two control modes of a voltage mode and a current mode and the strength of the power grid and draws a conclusion: the current mode control strategy easily causes system oscillation under the weak power grid environment, and the voltage mode control strategy can stably operate. The patent CN201910261338.5 provides a dual-mode control strategy based on short-circuit ratio identification, a current source type control strategy, a voltage source type control strategy and the like, can realize stable operation of an inverter under a grid-connected mode under a weak grid working condition, and is further improved in the patent CN 201911308959.0.
In view of the above documents, the existing bidirectional power converter has the following disadvantages:
when the problem of grid-connected operation of an alternating-current and direct-current hybrid micro-grid is considered, a power distribution grid side is regarded as an ideal model to carry out control strategy design, control strategy research when the rectification/inversion process is subjected to grid strength change in a grid-connected mode is lacked, and further system inertia loss problem is also lacked.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: when the grid-connected operation is carried out, the bidirectional power converter can stably operate under the working condition of a strong power grid or the working condition of a weak power grid, and the inertia compensation is carried out aiming at the problem of system inertia loss under the working condition of the weak power grid.
The technical scheme adopted by the invention is as follows: a dual-mode grid-connected control strategy of a bidirectional power converter is applicable to a circuit of an AC/DC bus interface converter of the strategy, and a positive pole P, N of a DC bus is connected with a capacitor C in parallel through a DC side of the bidirectional power converter dc The bidirectional power converter is connected with the direct current side of the bidirectional power converter, the bidirectional power converter is composed of 6 IGBTs, the alternating current side of the bidirectional power converter is connected to a PCC point through a filtering inductor L at the alternating current side of the bidirectional power converter and a filtering capacitor C at the alternating current side of the bidirectional power converter, the PCC point supplies power for a local alternating current load on one hand and supplies power through the network side impedance Z of a power distribution network on the other hand g Connected to a distribution network bus for power exchange, U dc Representing the DC bus voltage, S 1 ～S 6 For 6 IGBTs forming a bidirectional power converter, e gi (i = a, b, c) is the output voltage of the alternating current side of the bidirectional power converter; i.e. i Li (i = a, b, c) is the output current at the ac side of the bidirectional power converter, u PCC And i PCC Respectively PCC point voltage and current u gi (i = a, b, c) is the bus voltage of the power distribution network, n is a three-phase neutral point, and the specific control strategy is carried out according to the following steps
Step one, the system normally operates in a current source control mode and is divided into a rectification/inversion process, when the power grid strength of a power distribution network is weakened by some external factors (such as high-capacity distributed power access), disturbance is injected into a PCC point, and the voltage u of the PCC point is collected PCC And current i PCC Obtaining the harmonic content of the power distribution network and further carrying out impedance Z on the network side of the power distribution network g Measuring, calculating short circuit ratio after power conversion, wherein the short circuit ratio is mainly calculated according to a formula
Figure GDA0003194101900000021
Performing a calculation wherein K SCR To the magnitude of the short-circuit ratio, S ac For short-circuit capacity, S, of the distribution network B For accessing capacity, U, of a bidirectional power converter N Is the rated voltage of the PCC point and,rated voltage of PCC is known quantity, bidirectional power converter is connected with capacity S B The access capacity of (a) can be obtained from an upper scheduling department;
step two, obtaining the short-circuit ratio K according to the step one SCR Comparing the current value with a preset threshold value to judge the strength of the power grid, when the obtained short-circuit ratio is greater than a critical state, proving that the system is in a strong power grid working condition, wherein the system control strategy is a current source control mode, when the obtained short-circuit ratio is less than the critical state, proving that the system is in a weak power grid working condition, and wherein the system control strategy is a voltage source control mode;
step three, respectively designing a current source control mode and a voltage source control mode, wherein the current source control mode is specifically controlled according to the droop characteristic of the current source control mode
Figure GDA0003194101900000022
Design is made with U dc Is a dc bus voltage; k is a radical of dc The active power droop coefficient; p BIC Transmitting active power to the converter;
Figure GDA0003194101900000023
the voltage is rated for the voltage of the direct current bus;
the specific operation is divided into the following 3 cases: 1. when the active power in the DC sub-network is balanced, the DC bus voltage
Figure GDA0003194101900000024
At the moment, the transmission power of the bidirectional power converter is 0;2. an inversion mode: when the load of the DC sub-network is cut off, the power supply in the DC sub-network is greater than the demand, and the voltage of the DC bus
Figure GDA0003194101900000025
The bidirectional power converter works in an inversion mode, transmits redundant power at a direct current side to an alternating current side, and transmits power P at the time BIC Is greater than 0;3. a rectification mode: when the load of the DC sub-network increases, the internal power of the DC sub-network is suppliedNot to be required, DC bus voltage
Figure GDA0003194101900000026
The bidirectional power converter works in a rectification mode, the alternating-current power distribution network transmits power to the direct-current sub-network through the converter to maintain the voltage stability of the direct-current sub-network, and the bidirectional power converter transmits power P at the moment BIC < 0, obtaining P BIC As input reference quantity and actual feedback quantity P e The difference is input into a PI regulator to obtain a d-axis reference current i dref Considering the capacity problem of the bidirectional power converter, the q-axis reference current i is used for the purpose of transferring active power during design control qref Setting the initial value to 0, and giving a current command i dref And i qref Actual value i of output current of signal and interface converter d ，i q Respectively making difference and inputting the difference into a PI regulator; with the actual voltage feedback u d ，u q Adding and subtracting the decoupling amount i in d-axis control q ω L and adding a decoupling amount i in q-axis control d ω L, where ω L represents the filter reactance value; the obtained modulation signal under the dq0 coordinate system is subjected to (T) abc/dq0 ) -1 Converting the obtained product into an abc coordinate system and inputting the obtained product into an SPWM (sinusoidal pulse width modulation) link;
the voltage source control mode concrete control method simulates the inertia and primary frequency modulation characteristics of a synchronous generator through an active controller; the reactive controller simulates the primary voltage regulation characteristic of the synchronous generator according to a formula
Figure GDA0003194101900000027
Designing an active controller, wherein J is virtual moment of inertia; omega and omega n Respectively virtualizing the output angular frequency of the synchronous machine and the rated angular frequency of the power grid; t is ref And T e Respectively a given torque and an electromagnetic torque; d is an active damping coefficient; p ref Active power; p e Outputting active power for the moment; theta is the phase of the potential in the virtual synchronous machine according to the formula
Figure GDA0003194101900000031
Designing a reactive controller, wherein E is an effective value of an internal potential of the virtual synchronous machine; e 0 Is the no-load electromotive force of the virtual synchronous machine; delta E Q A reactive voltage droop adjusting part; d q Is a reactive damping coefficient; q ref And Q e Respectively a reactive power given value and an output value. Taking the obtained E and theta as input reference quantities through a formula
Figure GDA0003194101900000032
And converting the abc/dq0 coordinates to obtain voltage reference quantity e of a d axis and a q axis dref And e qref Reference quantity e dref And e qref And a voltage feedback quantity u d 、u q The difference is used as the voltage outer loop input value and input into the PI regulator and the current feedback value i d 、i q Adding and adding a decoupling quantity u in d-axis control q ω C and subtract the decoupling amount u in the q-axis control d ω C obtains a current reference command i dref And i qref Reference current to command i dref 、i qref And a current feedback value i Ld ，i Lq Respectively making difference and inputting the difference into a PI regulator; and a voltage feedback value u d 、u q Adding and subtracting the decoupling amount i in d-axis control Lq ω L and adding a decoupling amount i in q-axis control Ld ω L, where ω L represents the filter reactance value; the obtained modulation signal under the dq0 coordinate system is subjected to (T) abc/dq0 ) -1 Converting the obtained product into an abc coordinate system, and inputting the obtained product into an SPWM (sinusoidal pulse width modulation) link;
in addition, the equation for converting the amount in the three-phase stationary coordinate system into the amount in the two-phase rotating coordinate system by the equivalent transformation is as follows:
Figure GDA0003194101900000033
wherein, T abc/dq0 The field of power electronics control generally converts the abc coordinate system in which a variable signal to be controlled is located into a rotating dq0 coordinate system, converting a three-phase alternating current signal into a direct current signal, which makes it possible to more effectively control an electrical system. Theta represents an included angle between the abc coordinate system and the dq0 coordinate system; the A, B, C alternating current bus voltage is converted into a variable under a dq0 coordinate system, and the variable can be controlled more effectively. For example, the PCC point voltage is u pcc Obtaining voltage feedback quantity u of d and q axes through coordinate axis transformation d 、u q (ii) a Current i pcc Obtaining d-axis and q-axis current feedback quantity i through coordinate axis transformation d 、i q (ii) a Current i flowing through the inductor L L Obtaining current feedback quantity i of d and q axes through coordinate axis transformation Ld ，i Lq 。
The invention has the beneficial effects that: firstly, aiming at the control strategy of the bidirectional power converter under the condition that the power distribution network is a strong power network, the invention considers the application of the bidirectional power converter under the weak power network. Aiming at the actual situation of the power distribution network intensity change, the existing inverter dual-mode control strategy is improved into a dual-mode control strategy capable of bidirectional power flow by taking the reference of the existing inverter dual-mode control strategy, and the flexibility in energy flow is higher. And when power flows bidirectionally, the system can stably operate when encountering the change of the power grid strength, and in addition, aiming at the problem of inertia loss of the alternating current and direct current hybrid micro-grid system under the weak power grid condition, a virtual synchronous machine control strategy is adopted to increase the inertia of the system, thereby playing an important role in the stable operation of the alternating current and direct current hybrid micro-grid and the power distribution network.
Drawings
FIG. 1 is a schematic diagram of a circuit topology and control strategy of an AC/DC bus interface converter;
FIG. 2 is a flow chart of short circuit ratio calculation;
FIG. 3 is a schematic diagram of mode switching;
FIG. 4 is a diagram of a current mode control strategy;
FIG. 5 is a graph of droop characteristics;
FIG. 6 is a diagram of a voltage mode control strategy;
FIG. 7 is a diagram of an overall simulation process for weak grid conditions in an inversion mode;
FIG. 8 is a diagram of an overall simulation process for weak grid conditions under a rectification mode.
Detailed Description
In the specific embodiment, the power supply in the marginal mountain area is taken as the background, factors such as internal impedance of a power distribution network, line impedance, the number of inverters connected in parallel at a public connection point and the like are comprehensively considered, and the circuit topology and the control mode thereof in the grid-connected mode of the alternating current-direct current hybrid micro-grid are conveniently researched and simplified as shown in fig. 1. Fig. 1 is a schematic diagram of a circuit topology and a corresponding dual-mode control strategy of an ac/dc bus interface converter, and P, N are positive and negative poles of a dc bus respectively; u shape dc Represents a dc-side bus voltage; c dc Capacitors are connected in parallel at the direct current side of the bidirectional power converter; s 1 ～S 6 6 IGBTs form a bidirectional power converter; e.g. of the type gi (i = a, b, c) is the output voltage of the alternating current side of the bidirectional power converter; i.e. i Li (i = a, b, c) is the output current of the alternating current side of the bidirectional power converter; l and C are a filter inductor and a filter capacitor on the alternating current side of the bidirectional power converter; i.e. i i (i = a, b, c) is output current of the AC side of the bidirectional power converter after LC filtering; PCC is a common connection point; u. u PCC And i PCC PCC point voltage and current respectively; z g The network side impedance of the power distribution network; u. of gi (i = a, b, c) is the distribution network bus voltage; and n is a three-phase neutral point. See table 1 for specific parameters. The electrical connection process is as follows: DC bus positive and negative electrodes P, N pass through capacitor C dc Connected with the DC side of the bidirectional power converter via 6 IGBTs (S) 1 ～S 6 ) The bidirectional power converter and the alternating current side filter inductor L and the filter capacitor C are connected to a PCC point, the PCC point is connected with a local alternating current power grid for power supply on one hand, and power exchange is carried out on the other hand through connection of a power distribution network side impedance and the power distribution network.
Table 1 below sets forth system simulation data parameters
Parameter(s) Numerical value
Rated voltage U of DC sub-network dc /V 750
Rated voltage amplitude U of AC sub-network gabc /V 220
Frequency f/Hz of AC sub-network 50
DC side capacitor C dc /uF 470
Filter inductance L/mH 7
Filter capacitor C/uF 30
The specific control process comprises four steps:
step one, calculating a short-circuit ratio. Judging the strength of the power grid according to the calculated short circuit ratio, and if the power distribution network connected with the AC/DC hybrid micro-grid is in a high-power grid working condition, operating the system in a current source control mode; and if the power distribution network connected with the alternating current-direct current hybrid micro-grid is in a weak grid working condition, the system operates in a voltage source control mode. And step three, designing a current source control mode and a voltage source control mode respectively.
Step one, carrying out normal operation of the systemThe method is characterized in that the method is operated in a current source control mode and divided into a rectification/inversion process, when the power grid strength of a power distribution network connected with an alternating-current and direct-current hybrid micro-grid is weakened by some external factor (such as high-capacity distributed power access), disturbance is injected into a PCC point, and the voltage u of the PCC point is collected PCC Current i PCC Obtaining the harmonic content of the harmonic wave, measuring the fundamental wave impedance, calculating the short-circuit ratio after power conversion, wherein the short-circuit ratio is mainly calculated according to a formula
Figure GDA0003194101900000051
Performing a calculation in which K SCR To the magnitude of the short-circuit ratio, S ac For short-circuit capacity, S, of the distribution network B For switching in capacity, U, to the converter N Is the PCC point rated voltage. Typically, the rated voltage of the PCC is known, and the power electronic device S B The access capacity of (a) is available from the upper dispatch department. K SCR Can be measured by measuring the line impedance Z g Thus obtaining the product. The specific flow is shown in fig. 2.
And step two, comparing the short-circuit ratio obtained in the step one with a preset threshold value so as to judge the strength of the power grid, as shown in fig. 3. When the obtained short circuit ratio is larger than a critical state, the system is proved to be in a strong power grid working condition, and the system control strategy is a current source control mode at the moment; and when the obtained short-circuit ratio is smaller than a critical state, the system is proved to be in a weak power grid working condition, and the system control strategy is a voltage source control mode at the moment.
And step three, designing a current source control mode and a voltage source control mode respectively.
The first operation process is as follows:
the overall simulation process of the weak grid working condition PCC point voltage waveform when the bidirectional power converter operates in the inverter mode is shown in fig. 7 (a), the current waveform is shown in fig. 7 (b), the single-phase (taking a phase as an example) voltage-current simulation waveform is shown in fig. 7 (c), the transmission power of the bidirectional power converter is shown in fig. 7 (d), the transmission power of the PCC point is shown in fig. 7 (e), and the direct-current bus voltage is shown in fig. 7 (f).
And the power distribution network is in a strong power grid working condition at the initial moment, and the bidirectional power converter performs bidirectional power transmission according to the difference between the direct-current bus voltage and the rated value.
0-0.2 s, the direct current bus bears 10KW load operation, the bus voltage is about 749V, the active power shortage bidirectional power converter in the direct current sub-network is in a rectification mode, and the power distribution network provides active power for the direct current sub-network about 2KW.
And 0.2 s-0.3 s, cutting off the 10KW load in the direct current sub-network, increasing the voltage of a direct current bus to 752V, and enabling the bidirectional power converter to be in an inversion mode to provide about 4KW of active power for the local load and the power distribution network.
0.3 s-0.35 s, the alternating current system presents a weak power grid working condition due to the fact that a large number of distributed power sources are connected, system impedance is equivalently increased in a circuit, at the moment, a traditional current source type control strategy cannot normally transmit power, and waveform is distorted.
And 0.35 s-0.65 s, and switching the control strategy to a voltage source type control strategy by the detection loop through short-circuit ratio measurement and calculation. At this time, the converter provides stable voltage and frequency for the PCC point, and uses virtual synchronous control to increase the inertia of the system, and the rated output power is 5KW. It can be seen that the system gradually returns to normal, reaches steady state at 0.65s and can maintain stable operation under weak grid conditions. At the moment, the direct current voltage is relatively high, and if the direct current voltage is higher than the limit value, the storage battery can be started to maintain the stable bus voltage. After 0.65s, the converter is in a stable operation process.
And a second operation process:
the overall simulation process of weak grid condition PCC point voltage waveform when the bidirectional power converter operates in the rectification mode is shown in fig. 8 (a), the current waveform is shown in fig. 8 (b), the single-phase (taking phase a as an example) voltage-current simulation waveform is shown in fig. 8 (c), the transmission power of the bidirectional power converter is shown in fig. 8 (d), the transmission power of the PCC point is shown in fig. 8 (e), and the direct-current bus voltage is shown in fig. 8 (f).
And the power distribution network is in a strong power grid working condition at the initial moment, and the bidirectional power converter performs bidirectional power transmission according to the difference between the direct-current bus voltage and the rated value.
0-0.2 s, the voltage of the direct current bus is about 752V, active power in the direct current sub-network is redundant, the bidirectional power converter is in an inversion mode, and about 4KW power is transmitted to the alternating current sub-network.
0.2 s-0.3 s, 10KW load is added in the DC sub-network, the voltage of the DC bus is reduced to 749V, and the active power supplemented by the AC sub-network is about 2KW when the bidirectional power converter is in a rectification mode.
0.3 s-0.35 s, the alternating current system presents a weak power grid working condition due to the fact that a large number of distributed power supplies are connected, system impedance is equivalently increased in a circuit, at the moment, a traditional current source type control strategy cannot normally transmit power, wave forms are distorted and gradually dispersed, and system stability is reduced.
And 0.35 s-0.65 s, and switching the control strategy to a voltage source type control strategy by the detection loop through short-circuit ratio measurement and calculation. At the moment, the converter provides stable voltage and frequency for the PCC point, and uses virtual synchronous control to increase the inertia of the system, and the rated output power is 5KW. It can be seen that the system gradually returns to normal, reaches steady state at 0.65s and can maintain stable operation under weak grid conditions. At the moment, the direct current voltage is relatively low, and if the direct current voltage is lower than a limit value, the storage battery can be started to maintain the stable bus voltage. After 0.65s, the converter is in a stable operation process.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.

Claims (1)

1. A dual-mode grid-connected control strategy of a bidirectional power converter is characterized in that: in the circuit of the AC/DC bus interface converter suitable for the strategy, the anode and cathode of the DC bus P, N are connected with a capacitor C in parallel through the DC side of the bidirectional power converter dc The alternating current side of the bidirectional power converter is connected to a PCC point through a filtering inductor L at the alternating current side of the bidirectional power converter and a filtering capacitor C at the alternating current side of the bidirectional power converter, and the PCC point is a local alternating current negative one on the one handThe load is supplied, and on the other hand, the load is supplied through the network side impedance Z of the power distribution network g Connected to a distribution network bus for power exchange, U dc Representing the DC bus voltage, S 1 ～S 6 For 6 IGBTs forming a bidirectional power converter, e gi Outputting voltage for the AC side of the bidirectional power converter; i.e. i Li For the output current, u, of the AC side of a bidirectional power converter PCC And i PCC Respectively PCC point voltage and current u gi Is the bus voltage of the distribution network, n is the three-phase neutral point, e gi 、i Li 、u gi I = a, b, c, the specific control strategy is carried out as follows
Step one, the system normally operates in a current source control mode and is divided into a rectification/inversion process, when the power grid strength of the power distribution network is weakened by some external factor, disturbance is injected into a PCC point at the moment, and the voltage u of the PCC point is collected PCC And current i PCC Obtaining the harmonic content of the power distribution network and further carrying out impedance Z on the network side of the power distribution network g Measuring, calculating the short circuit ratio after power conversion, wherein the short circuit ratio is calculated according to a formula
Figure FDA0003940453540000011
Performing a calculation wherein K SCR To the magnitude of the short-circuit ratio, S ac For short-circuit capacity, S, of the distribution network B For accessing capacity, U, of a bidirectional power converter N The rated voltage of the PCC is a known quantity, and the bidirectional power converter is connected with a capacity S B The access capacity of (a) can be obtained from an upper scheduling department;
step two, obtaining the short-circuit ratio K according to the step one SCR Comparing the current value with a preset threshold value to judge the strength of the power grid, when the obtained short-circuit ratio is greater than a critical state, proving that the system is in a strong power grid working condition, and the system control strategy is a current source control mode at the moment, and when the obtained short-circuit ratio is less than the critical state, proving that the system is in a weak power grid working condition, and the system control strategy is a voltage source control mode at the moment;
thirdly, designing a current source control mode and a voltage source control mode respectively, wherein the specific control method of the current source control mode is based on the droop characteristic thereof
Figure FDA0003940453540000012
Design is made with U dc Is a dc bus voltage; k is a radical of dc The active power droop coefficient; p BIC Transmitting active power to the converter;
Figure FDA0003940453540000013
the voltage is rated for the voltage of the direct current bus;
the specific operation is divided into the following 3 cases: 1. when the active power in the DC sub-network is balanced, the DC bus voltage
Figure FDA0003940453540000014
At the moment, the transmission power of the bidirectional power converter is 0;2. an inversion mode: when the load of the DC sub-network is cut off, the power supply in the DC sub-network is greater than the demand, and the voltage of the DC bus
Figure FDA0003940453540000015
The bidirectional power converter works in an inversion mode, transmits redundant power at a direct current side to an alternating current side, and transmits power P at the time BIC Is greater than 0;3. a rectification mode: when the load of the DC sub-network increases, the power supply in the DC sub-network is not in demand, and the voltage of the DC bus
Figure FDA0003940453540000016
The bidirectional power converter works in a rectification mode, the alternating-current power distribution network transmits power to the direct-current sub-network through the converter to maintain the voltage stability of the direct-current sub-network, and the bidirectional power converter transmits power P at the moment BIC < 0, obtaining P BIC As input reference quantity and actual feedback quantity P e The difference is input into a PI regulator to obtain a d-axis reference current i dref Considering the capacity problem of the bidirectional power converter toFor the purpose of transferring active power, q-axis reference current i is used in design control qref Setting the initial value to 0, and giving a current command i dref And i qref Actual value i of output current of signal and interface converter d ，i q Respectively making difference and inputting the difference into a PI regulator; with the actual voltage feedback u d ，u q Adding and subtracting the decoupling amount i in d-axis control q ω L and adding a decoupling quantity i in the q-axis control d ω L, where ω L represents the filter reactance value; the obtained modulation signal under the dq0 coordinate system is subjected to (T) abc/dq0 ) -1 Converting the obtained product into an abc coordinate system and inputting the obtained product into an SPWM (sinusoidal pulse width modulation) link;
the voltage source control mode concrete control method simulates the inertia and primary frequency modulation characteristics of a synchronous generator through an active controller; the reactive controller simulates the primary voltage regulation characteristic of the synchronous generator according to a formula
Figure FDA0003940453540000021
Designing an active controller, wherein J is virtual moment of inertia; omega and omega n Respectively virtualizing the output angular frequency of the synchronous machine and the rated angular frequency of the power grid; t is a unit of ref And T e Respectively a given torque and an electromagnetic torque; d is an active damping coefficient; p ref Active power; p e Outputting active power for the moment; theta is the phase of the potential in the virtual synchronous machine according to the formula
Figure FDA0003940453540000022
Designing a reactive controller, wherein E is an effective value of an internal potential of the virtual synchronous machine; e 0 Is the no-load electromotive force of the virtual synchronous machine; delta E Q A reactive voltage droop adjusting part; d q Is a reactive damping coefficient; q ref And Q e Respectively setting reactive power and outputting a value; taking the obtained E and theta as input reference quantity to pass through a formula
Figure FDA0003940453540000023
And converting the abc/dq0 coordinates to obtain voltage reference quantity e of a d axis and a q axis dref And e qref Reference quantity e dref And e qref And a voltage feedback quantity u d 、u q The difference is used as the voltage outer loop input value and input into the PI regulator and the current feedback value i d 、i q Adding and adding a decoupling quantity u in d-axis control q ω C and subtract the decoupling amount u in the q-axis control d ω C obtains a current reference command i dref And i qref Reference current to command i dref 、i qref And a current feedback value i Ld ，i Lq Respectively making difference and inputting the difference into a PI regulator; and a voltage feedback value u d 、u q Adding and subtracting the decoupling amount i in d-axis control Lq ω L and adding a decoupling amount i in q-axis control Ld ω L, where ω L represents the filter reactance value; the obtained modulation signal under the dq0 coordinate system is subjected to (T) abc/dq0 ) -1 Converting the obtained product into an abc coordinate system, and inputting the obtained product into an SPWM (sinusoidal pulse width modulation) link;
in addition, the equation for converting the amount in the three-phase stationary coordinate system into the amount in the two-phase rotating coordinate system by the equivalent transformation is as follows:
Figure FDA0003940453540000024
wherein, T abc/dq0 The power electronic control field converts the abc coordinate system in which variable signals need to be controlled into a rotating dq0 coordinate system, and converts three-phase alternating current signals into direct current signals, so that an electrical system is controlled more effectively; theta represents an included angle between the abc coordinate system and the dq0 coordinate system; converting A, B, C AC bus voltage into variable in dq0 coordinate system, and comparingThe variable can be controlled more effectively; PCC point voltage of u pcc Obtaining voltage feedback quantity u of d and q axes through coordinate axis transformation d 、u q (ii) a Current i pcc Obtaining current feedback quantity i of d and q axes through coordinate axis transformation d 、i q (ii) a Current i flowing through the inductor L L Obtaining d-axis and q-axis current feedback quantity i through coordinate axis transformation Ld ，i Lq 。
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