CN112909919A - Neutral point potential balancing method and system for two-stage three-level three-phase four-wire system energy storage converter - Google Patents
Neutral point potential balancing method and system for two-stage three-level three-phase four-wire system energy storage converter Download PDFInfo
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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
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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
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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
- H02J3/26—Arrangements for eliminating or reducing asymmetry in polyphase networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/28—Arrangements for balancing of the load in a network by storage of energy
- H02J3/32—Arrangements for balancing of the load in a network by storage of energy using batteries with converting means
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
- H02M7/487—Neutral point clamped inverters
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
- H02M7/5387—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
- H02M7/53871—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
- H02M7/53875—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current with analogue control of three-phase output
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/50—Arrangements for eliminating or reducing asymmetry in polyphase networks
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Abstract
The invention discloses a method and a system for balancing the midpoint potential of a two-stage three-level three-phase four-wire system energy storage converter, which comprises the following steps: according to a low-voltage side inductive current reference value i in a three-level Buck/Boost circuit in the energy storage converterLrefControlling the three-level Buck/Boost circuit to work in a Boost mode or a Buck mode; reference Δ u from midpoint potentialrefAdjusting the difference between the current difference and the midpoint potential deviation delta u to adjust the voltage of each switching tube in the Buck/Boost circuit in a Boost modeThe duty ratio in the mode of formula or Buck is adopted, so that the total voltage of the direct current bus is constant and is balanced with the midpoint potential. The invention has the advantages of constant total voltage of the direct current bus, balanced midpoint potential, improved operation stability of the converter and the like.
Description
Technical Field
The invention mainly relates to the technical field of energy storage, in particular to a method and a system for balancing midpoint potential of a two-stage three-level three-phase four-wire system energy storage converter.
Background
The energy storage technology has wide application prospect in the fields of improving the reliability of a power grid, demand side management, new energy access and the like. The energy storage is connected into a power system, so that the safety stability of a power grid can be improved, the load peak-valley difference is relieved, the electric energy quality is improved, and the application of renewable energy sources is promoted; and the micro-grid system and the distributed power supply form an independent micro-grid, so that the stability of the micro-grid system can be improved. The electrochemical energy storage System mainly comprises a Battery Management System (BMS) and a bidirectional energy Conversion System (PCS). The utility model mainly has the following functions: 1) under the condition of grid connection, constant-power operation is realized according to a scheduling instruction; 2) under the off-grid condition, various loads (inductive, capacitive and asymmetric loads) are considered to realize constant-voltage constant-frequency operation.
Aiming at the wide-range input voltage (200-700V) of the direct current side of the PCS and the access requirement of the single-phase load of the alternating current side, the existing two-stage PCS adopts a three-level Buck/Boost and a T-type three-level three-bridge arm three-phase four-wire system (3-level T-type 3-leg 3-phase4-wire, 3 LT)23L3P 4W).
The unbalanced component of the midpoint potential in the three-level inverter circuit includes a dc component and an ac component, which may cause a number of problems: 1) the output voltage waveform is distorted, and the low harmonic content is increased; 2) the voltage stress of the device is increased, and the overvoltage damage of the device is easily caused. In order to ensure the long-term reliable operation of the equipment, a midpoint potential balancing method aiming at the two-stage PCS is required.
In the aspect of realizing balance control of a midpoint potential by an external circuit, document [1] adopts a double Buck/Boost circuit to compensate low-frequency current injected into a three-phase four-wire system NPC inverter direct-current bus, but the method needs more current sampling, so that the hardware cost is increased; document [2] realizes the balance of the midpoint potential of the three-phase three-wire system NPC inverter through PI phase shift control of the preceding-stage three-level Boost circuit, but the balance performance of the midpoint potential under the unbalanced load condition is not disclosed by the related technology.
The neutral point potential balancing method of the three-level three-phase four-wire system topology body is mainly based on 3D-SVPWM and carrier modulation under a-b-c and alpha-beta-0 coordinate systems. Document [3] selects a vector with strong midpoint potential balance capability according to the proposed midpoint control degree to synthesize a target vector based on a three-dimensional space vector modulation strategy under an alpha-beta-0 coordinate system, document [4] expands a modulation subspace according to a midpoint balance factor under an a-b-c coordinate system, document [5] decomposes a phase-zero level action time with midpoint potential balance capability under the a-b-c coordinate system, and document [6] [7] [8] decomposes the zero level action time based on carrier modulation. In the method, the introduction of a transition vector and the decomposition of a zero level lead to the increase of the switching times of a neutral point potential balancing phase in a control period, the switching loss is increased, and the inhibition capability of neutral point potential oscillation is limited aiming at the condition of larger load unbalance.
Reference [1] Li X, Zhang W, Li H, et al.Power Management Unit With Its controls for a Three-Phase Fuel Cell Power System With Large electronic devices [ J ]. IEEE Transactions on Power Electronics, 2011, 26 (12): 3766-3777.
[2]Xia C，Gu X，Shi T，et al.Neutral-Point Potential Balancing of Three-Level Inverters in Direct-Driven Wind Energy Conversion System[J].IEEE Transactions on Energy Conversion，2011，26(1)：0-29.
[3] Tang jia, zhou xudong, liu yang, etc. three-phase four-wire three-level three-arm active filter neutral-point balance control strategy [ J ] chinese electro-mechanical engineering report, 2009, 29 (24): 40-48.
[4]J.Tang，X Zou，Yun Xu，et al.Novel 3-DSVM scheme for three-phase four-wire tri-level APFs，2009IEEE 6th International Power Electronics and Motion Control Conference，Wuhan，2009，pp.882-886.
[5] Zhanghourong, wuyiming, pengbo, et al. optimization modulation strategy of three-phase four-wire system three-level converter [ J ] power system automation, 2018, 42 (22): 187-197.
[6]C Wang，Z Li，X Si，et al.Control of neutral-point voltage in three-phase four-wire three-level NPC inverter based on the disassembly of zero level，CPSS Transactions on Power Electronics and Applications，2018，3(3)：213-222.
[7]Li F，He F，Ye Z，et al.A Simplified PWM Strategy for Three-Level Converters on Three-phase Four-wire Active Power Filter[J].IEEE Transactions on Power Electronics，2018，33(5)：4396-4406.
[8]H Chen，M Tsai，Y Wang，et al.A Modulation Technique for Neutral Point Voltage Control of the Three-Level Neutral-Point-Clamped Converter[J].IEEE Transactions on Industry Applications，2018，54(3)：2517-2524.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: aiming at the technical problems in the prior art, the invention provides a method, a system, a medium and equipment for balancing the midpoint potential of a two-stage three-level three-phase four-wire system energy storage converter with constant total voltage and balanced midpoint potential of a direct current bus.
In order to solve the technical problems, the technical scheme provided by the invention is as follows:
a method for balancing the midpoint potential of a two-stage three-level three-phase four-wire system energy storage converter comprises the following steps:
according to a low-voltage side inductive current reference value i in a three-level Buck/Boost circuit in the energy storage converterLrefControlling the three-level Buck/Boost circuit to work in a Boost mode or a Buck mode;
reference Δ u from midpoint potentialrefAnd adjusting the duty ratio of each switching tube in the Buck/Boost circuit in a Boost mode or a Buck mode according to the difference between the deviation delta u and the midpoint potential so as to ensure that the total voltage of the direct current bus is constant and balanced with the midpoint potential.
As a further improvement of the above technical solution, when iLref>When the voltage is 0, the three-level Buck/Boost circuit works in a Boost mode; when i isLref<And when the voltage is 0, the three-level Buck/Boost circuit works in a Buck mode.
As a further improvement of the technical scheme, the three-level Buck/Boost circuit comprises a voltage outer loop controller GudcAnd voltage inner loop controller Gidc_buck/boost(ii) a The voltage outer loop controller GudcAnd voltage inner loop controller Gidc_buck/boostPI control is adopted; wherein
Wherein k ispudc、kiudc、kpidc_buck/boost、kiidc_buck/boostAre proportional gains; and s is the integration time.
As a further improvement of the technical scheme, the three-level Buck/Boost circuit comprises a midpoint potential balance controller GΔuSaid midpoint potential balance controller GΔuReferenced by midpoint potential Δ urefThe difference between the output value and the midpoint potential deviation delta u is used as input, the output value corrects the duty ratio of the switching tubes of the three-level Buck/Boost circuit, and the duty ratio of one switching tube is increased to reduce the duty ratio of the other switching tube, so that the inductive current charges the capacitor with smaller voltage for a long time, and the voltage balance of the two capacitors is realized.
As a further improvement of the above technical solution, the above GΔuComprises the following steps:
wherein k ispΔuTo proportional gain, krΔuFor the resonant gain, k, at power frequencyrΔu3Is triple of the resonant gain at the power frequency.
As a further improvement of the above technical solution, the closed loop transfer function of the midpoint potential balance controller can be expressed as:
midpoint potential disturbance Δ udistThe closed loop transfer function to midpoint potential shift Δ u can be expressed as:
as a further improvement of the technical scheme, each switching tube in the Buck/Boost circuit receives a duty ratio signal dup、ddownTriggering; wherein d isup、ddownThe direct current bus total voltage control duty ratio d and the midpoint potential balance control duty ratio delta d are synthesized to obtain the direct current bus total voltage control duty ratio delta d.
The invention also discloses a neutral point potential balancing system of the two-stage three-level three-phase four-wire system energy storage converter, which comprises
A first module for generating a reference value i of the low-voltage side inductor current in a three-level Buck/Boost circuit of the energy storage converterLrefControlling the three-level Buck/Boost circuit to work in a Boost mode or a Buck mode;
a second module for referencing Δ u according to the midpoint potentialrefAnd adjusting the duty ratio of each switching tube in the Buck/Boost circuit in a Boost mode or a Buck mode according to the difference between the deviation delta u and the midpoint potential so as to ensure that the total voltage of the direct current bus is constant and balanced with the midpoint potential.
The invention further discloses a computer-readable storage medium, on which a computer program is stored, which is characterized in that the computer program, when being executed by a processor, executes the steps of the method for balancing the midpoint potential of the two-stage three-level three-phase four-wire system energy storage converter.
The invention also discloses a computer device, which comprises a memory and a processor, wherein the memory is stored with a computer program, and the computer program is characterized in that when being executed by the processor, the computer program executes the steps of the method for balancing the midpoint potential of the two-stage three-level three-phase four-wire system energy storage converter.
Compared with the prior art, the invention has the advantages that:
(1) aiming at a two-stage three-level three-phase four-wire system energy storage converter, the constant total voltage and the balance of the midpoint potential of a direct-current bus are realized by controlling the working mode of a three-level Buck/Boost circuit and controlling the duty ratio of each switching tube in a corresponding control mode, the balance of the midpoint potential of the direct-current bus can be realized under the conditions of serious load imbalance and non-unit power factor, and the operation stability of the converter is improved; the complexity of a modulation algorithm caused by the fact that an inverter circuit realizes neutral point potential balance control is avoided, and meanwhile the neutral point potential balance capability under the conditions of unbalanced load and non-unit power factor load is improved.
(2) The method for controlling the midpoint potential of the three-level Buck/Boost chopper circuit based on the proportional resonant controller can effectively inhibit the main frequency oscillation of the midpoint potential generated by the T-shaped three-level inverter circuit, and is favorable for the long-term stable operation of the two-stage energy storage converter.
(3) The neutral point potential balancing method based on the preceding-stage three-level Buck/Boost circuit proportional resonance control is characterized in that resonance points are respectively arranged at the power frequency and the triple power frequency, so that the neutral point potential low-frequency oscillation is effectively inhibited, the direct-current component is eliminated, and the neutral point potential balancing method has good dynamic characteristics.
Drawings
FIG. 1 is a diagram of a two-stage PCS configuration according to the present invention.
FIG. 2 is a three-level Buck/Boost circuit topology diagram of the present invention.
Fig. 3 is a control block diagram of a three-level Buck/Boost circuit of the present invention.
FIG. 4 is a block diagram of a two-stage PCS midpoint balance control system of the present invention.
FIG. 5 is a Bode diagram of the closed loop transfer function of equation (6) in the present invention.
FIG. 6 is a characteristic root trace diagram of the midpoint potential balance control system of the present invention.
FIG. 7 is a step response diagram of the point potential balance control system according to the present invention.
FIG. 8 is a diagram of simulation results for the present invention.
FIG. 9 is a diagram of simulation results for the working condition five of the present invention.
Detailed Description
The invention is further described below with reference to the figures and the specific embodiments of the description.
As shown in fig. 1, the method for balancing the midpoint potential of the two-stage three-level three-phase four-wire system energy storage converter of the present embodiment includes the steps of:
according to a low-voltage side inductive current reference value i in a three-level Buck/Boost circuit in the energy storage converterLrefControlling the three-level Buck/Boost circuit to work in a Boost mode or a Buck mode;
reference Δ u from midpoint potentialrefAnd adjusting the duty ratio of each switching tube in the Buck/Boost circuit in a Boost mode or a Buck mode according to the difference between the deviation delta u and the midpoint potential so as to ensure that the total voltage of the direct current bus is constant and balanced with the midpoint potential.
Aiming at a two-stage three-level three-phase four-wire system energy storage converter, the constant total voltage and the balance of the midpoint potential of a direct-current bus are realized by controlling the working mode of a three-level Buck/Boost circuit and controlling the duty ratio of each switching tube in a corresponding control mode, the balance of the midpoint potential of the direct-current bus can be realized under the conditions of serious load imbalance and non-unit power factor, and the operation stability of the converter is improved; the complexity of a modulation algorithm caused by the fact that an inverter circuit realizes neutral point potential balance control is avoided, and meanwhile the neutral point potential balance capability under the conditions of unbalanced load and non-unit power factor load is improved.
As shown in fig. 1, in this embodiment, for the wide-range input voltage (200-700V) at the dc side of the PCS and the requirement for accessing the single-phase load at the ac side, the two-stage PCS adopts a three-level Buck/Boost circuit and a T-type three-level three-leg three-phase four-wire system (3-level T-type 3-leg 3-phase4-wire, 3 LT)23L3P 4W). The T-type three-level topology has the advantages of low common-mode voltage, low switching frequency and low loss, and also has the advantages of small conduction loss, uniform output current of the switching tube and less required devices compared with other forms of three-level topologies; the three-level three-phase four-wire system topology mainly comprises a three-bridge arm structure and a four-bridge arm structure, and compared with the four-bridge arm structure, the three-bridge arm structure is relatively weaker in zero-sequence voltage output capability but requires fewer switching devices.
Specifically, a three-level Buck/Boost circuit is shown in FIG. 2. Wherein C isdcA low-side capacitor; l isdcA low-voltage side filter inductor; r1、R2Respectively equivalent loads of a direct current positive bus and a direct current negative bus; q1、Q4The switching tube acts in a Buck running state; q2、Q3A switching tube acting in a Boost operation state; i.e. iLIs the low-voltage side inductor current; u. ofinIs the low side input voltage.
In this embodiment, the three-level Buck/Boost circuit takes the constant total voltage and the neutral-point potential balance of the dc bus as a control target, and a control block diagram is shown in fig. 3. Wherein u isC1C2_refIs a DC bus total voltage reference value, GudcIs a voltage outer loop controller (PI control, equation (1)), iLrefFor the low side inductor current reference value, according to iLrefJudging the working mode of the current circuit by the positive and negative judgment: when i isLref>When 0, the circuit works in Boost mode, and the current inner loop adopts a controller Gidc_boost(PI control, equation (2)), Q1、Q4Operating in diode state, Q2、Q3Receiving a duty ratio signal dup、ddownTriggering; when i isLref<When 0, the circuit works in Buck mode, and the current inner loop adopts a controller Gidc_buck(PI control, equation (2)), Q2、Q3Operating in diode state, Q1、Q4Receiving a duty ratio signal dup、ddownAnd (5) triggering. Wherein d isup、ddownThe direct current bus total voltage control duty ratio d and the midpoint potential balance control duty ratio delta d are synthesized to obtain the direct current bus total voltage control duty ratio delta d. Q1、Q4Carrier difference of 180 DEG, Q2、Q3The carrier waves are mutually different by 180 degrees;
in this embodiment, the neutral point potential balance controller GΔuReference Δ u from midpoint potentialrefAnd adjusting the duty ratio of each switching tube in the Buck/Boost circuit in a Boost mode or a Buck mode according to the difference between the current and the midpoint potential deviation delta u. Specifically, the neutral point potential balance controller GΔuReferenced by midpoint potential Δ urefThe difference between the output value and the midpoint potential deviation delta u is used as the input, the output value is Q1And Q4(Q2And Q3) The duty ratio of one switching tube is increased, and the duty ratio of the other switching tube is reduced, so that the capacitor with smaller voltage is charged by the inductive current for a longer time, and the purpose of voltage balance of the two capacitors is realized. Specifically, a midpoint potential balancing method based on proportional resonance control is adopted. Considering that the main frequency of the low-frequency oscillation of the midpoint potential is concentrated near the power frequency and three times of the power frequency, a resonance link G is designed at the two frequency pointsΔuCan be expressed as:
in the formula (3), kpΔuTo proportional gain, krΔuFor the resonant gain, k, at power frequencyrΔu3Is triple of the resonant gain at the power frequency.
Taking Boost operation mode as an example, when controlling the duty ratio d>At 0.5, there are three modes (Q) in one control cycle2Conducting, Q3Conducting and simultaneously conducting). Taking the DC positive and negative bus capacitor voltage uC1、uC2Low side inductor current iLThe constituent state vector x (t) ═ uC1(t)uC2(t)iL(t)]T(ii) a Low side input voltage uinThe DC bus total voltage control duty ratio d and the midpoint potential balance control duty ratio delta d form an input vector u (t) [ d (t) delta d (t) u ]in(t)]T. Let X ═ UC1 UC2 IL]TAnd U ═ D Δ D Uin]TThe steady state operating point vectors for x (t) and u (t), respectively. Approximating U at steady state operating points for x (t) and U (t)C1＝UC2Introducing small signal disturbance and neglecting a second-order disturbance product term to obtain a transfer function from a midpoint potential balance control duty ratio delta d small signal to a midpoint potential deviation delta u small signal, wherein the transfer function is as follows:
according to the superposition theorem, a block diagram of a two-stage PCS midpoint potential control system is shown in FIG. 4. 3LT can be controlled from the angle of controlling the neutral potential by three-level Buck/Boost2The effect of the 3L3P4W topology on midpoint potential is considered as a perturbation Δ udist. The closed loop transfer function of the midpoint potential balance control system can be expressed as:
midpoint potential disturbance Δ udistThe closed loop transfer function to midpoint potential shift Δ u can be expressed as:
and (3) drawing a Bode diagram of the closed-loop system according to the formula (6), and as shown in FIG. 5, obvious negative resonance peaks exist at 50Hz and 150Hz, which shows that the method can obviously inhibit power frequency and triple power frequency disturbance components.
Selecting a steady state operating point UC1＝UC2＝300V，IL＝50A，D＝0.56，ΔD＝0，Uin＝300V，C1＝C2＝2460μF，Ldc＝600μ，R1＝R2＝16.33Ω，Ts66.7 μ s. The distribution of the closed-loop characteristic root of the point potential balance control system in equation (5) as a function of the parameters of the PR controller is shown in fig. 6. Wherein 6a) is krΔu3At a certain timing, the feature root follows krΔuAnd k ispΔuVarying distribution, the dominant feature roots near the imaginary axis include Eig.1, Eig.2, and Eig.3, with k being the function ofpΔuThe attenuation coefficients sigma of Eig.1 and Eig.2 show a trend of increasing and then decreasing, and the oscillation frequency omega is dampeddDecrease; with krΔuIncrease of Eig.1 and Eig.2 natural frequencies ωnIncreasing, decreasing system damping; the distribution of Eig.3 is substantially unchanged. 6b) Is krΔuAt a certain timing, the feature root follows krΔu3And k ispΔuAccording to the changed distribution condition, Eig.1 shows a trend far away from the virtual axis, Eig.2 shows a trend close to the virtual axis, and the total damping of the system is reduced; eig.3 distribution is substantially unchanged (where the dark lines in the figure correspond to krΔuK corresponding to lighter linesrΔuSmall).
To verify the above conclusions, the step response of the midpoint potential control system was analyzed in MATLAB (midpoint potential reference Δ u)ref10V). As shown in FIG. 7a), kpΔuIncreasing within a certain range, reducing the overshoot of the system step response and reducing the damping oscillation frequency; k is a radical ofrΔuIncreasing, increasing the overshoot of the system step response and reducing the damping; k is a radical ofrΔu3And increasing the overshoot of the step response of the system and reducing the damping. In order to further verify the conclusion, the step response of the three-level Buck/Boost midpoint potential control system is simulated in MATLAB/Simulink according to the parameters. As shown in fig. 7b), the simulation result substantially coincided with the theoretical analysis result 7a) except for the high frequency component in the vicinity of the switching frequency, and it was verified thatAnd (3) the accuracy of the small signal model of the point potential control system.
In order to verify the midpoint potential balance method based on the preceding-stage three-level Buck/Boost proportional resonance control, MATLAB/Simulink software is used for constructing a two-stage PCS system shown in FIG. 1. Wherein 3LT2The 3L3P4W topology adopts constant voltage and constant frequency control and a three-level three-dimensional space vector modulation algorithm, and the three-level Buck/Boost adopts the DC bus total voltage and midpoint potential balance control method. Simulation verification is carried out for two typical working conditions (asymmetric load and inductive load), and the system structure and control parameters are shown in table 1:
TABLE 1 two stage PCS parameters
Simulation working condition-adopting three-phase symmetrical inductance-resistance load (Z)a＝Zb＝Zc＝24.2+j11.7Ω，
) The simulation results are shown in fig. 8. In the upper part of FIG. 8 is the AC three-phase load current iao、ibo、icoThe midpoint potential shift Δ u is provided below. When t is 0.04s, the midpoint potential balance control is enabled, and the Δ u peak-to-peak value decreases from 4.1V to 1V within 50 ms. Simulation results prove that the midpoint potential control method provided by the invention can effectively inhibit midpoint potential low-frequency oscillation and eliminate direct-current components under the condition of non-unit power factor load, and has good dynamic characteristics.
The simulation working condition II adopts asymmetric resistive load (Z)a＝16.13Ω,Zb＝Zc24.2 Ω), the simulation results are shown in fig. 9. In the upper part of FIG. 9 is the AC three-phase load current iao、ibo、icoThe midpoint potential shift Δ u is provided below. t is 0.04s enables midpoint potential balance control, and the peak-to-peak value of the delta u is reduced from 13.5V to 1.06V within 50 ms. Simulation results prove that the midpoint potential balancing method provided by the invention can effectively inhibit midpoint potential low-frequency oscillation and eliminate direct current components under the condition of asymmetric load, and has good dynamic characteristics.
The invention also discloses a neutral point potential balancing system of the two-stage three-level three-phase four-wire system energy storage converter, which comprises
A first module for generating a reference value i of the low-voltage side inductor current in a three-level Buck/Boost circuit of the energy storage converterLrefControlling the three-level Buck/Boost circuit to work in a Boost mode or a Buck mode;
a second module for referencing Δ u according to the midpoint potentialrefAnd adjusting the duty ratio of each switching tube in the Buck/Boost circuit in a Boost mode or a Buck mode according to the difference between the deviation delta u and the midpoint potential so as to ensure that the total voltage of the direct current bus is constant and balanced with the midpoint potential.
The system of the invention, for carrying out the method as described above, also has the advantages as described above for the method.
The embodiment of the invention also discloses a computer readable storage medium, wherein a computer program is stored on the computer readable storage medium, and when the computer program is executed by a processor, the computer program executes the steps of the method for balancing the midpoint potential of the two-stage three-level three-phase four-wire system energy storage converter. Meanwhile, the embodiment of the invention also discloses computer equipment which comprises a processor and a memory, wherein the memory is stored with a computer program, and the computer program executes the steps of the method for balancing the midpoint potential of the two-stage three-level three-phase four-wire system energy storage converter when being run by the processor. All or part of the flow of the method of the embodiments may be implemented by a computer program, which may be stored in a computer-readable storage medium and executed by a processor, to implement the steps of the embodiments of the methods. Wherein the computer program comprises computer program code, which may be in the form of source code, object code, an executable file or some intermediate form, etc. The computer readable medium may include: any entity or device capable of carrying computer program code, recording medium, U.S. disk, removable hard disk, magnetic disk, optical disk, computer Memory, Read-Only Memory (ROM), Random Access Memory (RAM), electrical carrier wave signals, telecommunications signals, software distribution media, and the like. The memory may be used to store computer programs and/or modules, and the processor may perform various functions by executing or executing the computer programs and/or modules stored in the memory, as well as by invoking data stored in the memory. The memory may include high speed random access memory, and may also include non-volatile memory, such as a hard disk, a memory, a plug-in hard disk, a Smart Media Card (SMC), a Secure Digital (SD) Card, a Flash memory Card (Flash Card), at least one magnetic disk storage device, a Flash memory device, or other volatile solid state storage device.
The above is only a preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above-mentioned embodiments, and all technical solutions belonging to the idea of the present invention belong to the protection scope of the present invention. It should be noted that modifications and embellishments within the scope of the invention may be made by those skilled in the art without departing from the principle of the invention.
Claims (10)
1. A method for balancing the midpoint potential of a two-stage three-level three-phase four-wire system energy storage converter is characterized by comprising the following steps of:
according to a low-voltage side inductive current reference value i in a three-level Buck/Boost circuit in the energy storage converterLrefControlling the three-level Buck/Boost circuit to work in a Boost mode or a Buck mode;
reference Δ u from midpoint potentialrefAnd adjusting the duty ratio of each switching tube in the Buck/Boost circuit in a Boost mode or a Buck mode according to the difference between the deviation delta u and the midpoint potential so as to ensure that the total voltage of the direct current bus is constant and balanced with the midpoint potential.
2. The method of claim 1The neutral point potential balancing method of the two-stage three-level three-phase four-wire system energy storage converter is characterized in that when i is equal toLref>When the voltage is 0, the three-level Buck/Boost circuit works in a Boost mode; when i isLref<And when the voltage is 0, the three-level Buck/Boost circuit works in a Buck mode.
3. The method for balancing the midpoint potential of the two-stage three-level three-phase four-wire energy storage converter according to claim 2, wherein the three-level Buck/Boost circuit comprises a voltage outer loop controller GudcAnd voltage inner loop controller Gidc_buck/boost(ii) a The voltage outer loop controller GudcAnd voltage inner loop controller Gidc_buck/boostPI control is adopted; wherein
Wherein k ispudc、kiudc、kpidc_buck/boost、kiidc_buck/boostAre proportional gains; and s is the integration time.
4. The method for balancing midpoint potential of the two-stage three-level three-phase four-wire energy storage converter according to claim 1, 2 or 3, wherein the three-level Buck/Boost circuit comprises a midpoint potential balance controller GΔuSaid midpoint potential balance controller GΔuReferenced by midpoint potential Δ urefThe difference between the output value and the midpoint potential deviation delta u is used as input, the output value corrects the duty ratio of the switching tubes of the three-level Buck/Boost circuit, and the duty ratio of one switching tube is increased to reduce the duty ratio of the other switching tube, so that the inductive current charges the capacitor with smaller voltage for a long time, and the voltage balance of the two capacitors is realized.
5. The method for balancing midpoint potential of the two-stage three-level three-phase four-wire system energy storage converter according to claim 4, wherein G is a positive integerΔuComprises the following steps:
wherein k ispΔuTo proportional gain, krΔuFor the resonant gain, k, at power frequencyrΔu3Is triple of the resonant gain at the power frequency.
6. The method for balancing midpoint potential of the two-stage three-level three-phase four-wire energy storage converter according to claim 4, wherein the closed loop transfer function of the midpoint potential balancing controller can be expressed as:
midpoint potential disturbance Δ udistThe closed loop transfer function to midpoint potential shift Δ u can be expressed as:
7. the method for balancing midpoint potential of the two-stage three-level three-phase four-wire system energy storage converter according to claim 1, 2 or 3, wherein each switch tube in the Buck/Boost circuit is subjected to a duty ratio signal dup、ddownTriggering; wherein d isup、ddownThe direct current bus total voltage control duty ratio d and the midpoint potential balance control duty ratio delta d are synthesized to obtain the direct current bus total voltage control duty ratio delta d.
8. A neutral point potential balance system of a two-stage three-level three-phase four-wire system energy storage converter is characterized by comprising
A first module for a first one of the modules,the method is used for obtaining a reference value i of low-voltage side inductive current in a three-level Buck/Boost circuit in an energy storage converterLrefControlling the three-level Buck/Boost circuit to work in a Boost mode or a Buck mode;
a second module for referencing Δ u according to the midpoint potentialrefAnd adjusting the duty ratio of each switching tube in the Buck/Boost circuit in a Boost mode or a Buck mode according to the difference between the deviation delta u and the midpoint potential so as to ensure that the total voltage of the direct current bus is constant and balanced with the midpoint potential.
9. A computer-readable storage medium, on which a computer program is stored, which, when being executed by a processor, performs the steps of the method for balancing the midpoint potential of a two-stage three-level three-phase four-wire energy storage converter according to any one of claims 1 to 7.
10. A computer arrangement comprising a memory and a processor, the memory having stored thereon a computer program, the computer program, when executed by the processor, performing the steps of the method of balancing midpoint potentials of a two-stage, three-level, three-phase, four-wire energy storage converter according to any of claims 1 to 7.
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