System for reducing capacitance value of DC bus capacitor of AC / DC converter under unbalanced power grid

Abstract

The invention discloses an AC / DC converter DC bus capacitor capacitance value reduction system under an unbalanced power grid, which comprises a three-phase power grid, an LCL filter, a DC bus capacitor, a load, a three-phase two-level converter main circuit and a Buck power decoupling circuit, wherein the main circuit is used for power conversion; the Buck decoupling circuit is used for absorbing double frequency pulse power formed by a product of positive sequence voltage and negative sequence current on an alternating current side, so that the capacitance value of a direct current bus capacitor is reduced. The Buck decoupling circuit is added to the DC bus of the AC / DC converter under the unbalanced power grid, the double-frequency power pulsation on the DC side is effectively filtered out, the capacitance value of the DC bus capacitor is reduced, meanwhile, the thin-film capacitor with the small capacitance value and the long service life is used for replacing a traditional electrolytic capacitor, the reliability and stability of the system are improved, and the service life of the system is prolonged. The grid-side current balance positive sequence and the direct-current side voltage stability are realized, and the new energy power generation utilization rate and the overall efficiency of a power system are remarkably improved.

Description

Technical Field

[0001] The invention relates to a three-phase two-level converter control problem, and in particular to a DC bus capacitor value reduction system of an AC / DC converter under an unbalanced power grid. Background Art

[0002] With the steady advancement of the new power system, the large-scale grid connection of new energy with volatility, and the power electronics transformation of the core equipment of "source-grid-load", the power system is forming a dual-high development trend of high proportion of renewable energy and high proportion of power electronic equipment. The new power system with "dual-high" characteristics is often accompanied by the problem of over / under voltage grid voltage imbalance, which brings great challenges to the normal operation of the converter. The negative sequence voltage generated under the unbalanced grid condition will cause harm to the electrical equipment and the power system. For example, the negative sequence current generated by the negative sequence voltage in the motor will cause the generator set to vibrate and generate additional losses in the rotor, resulting in local overheating and reducing the efficiency and service life of the generator. The grid-connected inverter generates negative sequence current under the action of unbalanced grid voltage, and transmits unbalanced three-phase current to the grid, causing the inverter to no longer be connected to the grid and reducing the utilization rate of new energy power generation. The negative sequence voltage and positive sequence current cause the system to have double frequency power pulsation. The double frequency ripple voltage on the DC side will affect the DC side energy storage battery, causing serious heating of the battery and shortening its service life.

[0003] In response to the above hazards, traditional converter DC busbars often use large-capacity electrolytic capacitors to maintain a constant voltage on the DC side. Electrolytic capacitors are relatively weak electronic components in the entire system. According to statistics, 30% of power converter failures are caused by electrolytic capacitor failure. The system adds a method to detect the life of the capacitor, and replaces the capacitor in advance before the electrolytic capacitor fails, but it cannot extend the life of the capacitor. Adding a decoupling circuit to the converter to filter out the double-frequency pulsating power on the DC side, and using film capacitors instead of electrolytic capacitors will not reduce the system power density and can improve the reliability and stability of the system. Summary of the invention

[0004] In order to solve the multi-objective control problem of simultaneously achieving grid-side current balance of positive-sequence and negative-sequence voltages and double-frequency power pulsation on the DC side caused by positive-sequence current under unbalanced grid conditions, the present invention adds a Buck decoupling circuit on the DC side to increase the control freedom and effectively filter out the double-frequency power pulsation on the DC side.

[0005] To achieve the above object, the present invention provides a system for reducing the capacitance of a DC bus capacitor of an AC / DC converter under an unbalanced power grid, comprising: a three-phase power grid, an LCL filter, a DC bus capacitor and a load, and also comprising: a three-phase two-level converter main circuit and a Buck power decoupling circuit;

[0006] Wherein, the main circuit is used for power conversion;

[0007] The Buck decoupling circuit absorbs the double frequency pulsating power formed by the product of the positive sequence voltage and the negative sequence current on the AC side, thereby reducing the capacitance of the DC bus capacitor.

[0008] Preferably, a thin-film capacitor with a small capacitance and a long life is used to replace the electrolytic capacitor on the DC side of the traditional converter; a control design based on a stationary coordinate system is adopted to solve the problem of separating the positive and negative sequences of the voltage; and a PR controller is used to filter out harmonic components to improve the current quality on the grid side.

[0009] Preferably, since the zero-sequence component of the voltage in an unbalanced power grid is zero, the three-phase voltage u a 、u b 、u c Respectively expressed as:

[0010]

[0011] Among them, U P and θ P Respectively represent the peak value and phase of the positive sequence component; U N and θ N They are respectively represented as the peak value and phase of the negative sequence component; ω is the system angular frequency, and t is the time;

[0012] After the above equation is transformed into Clark coordinates, the voltage expression in the axis stationary coordinate system is obtained:

[0013]

[0014] Among them, u α 、u β Represents the voltage component in the stationary coordinate system.

[0015] Preferably, under symmetrical sinusoidal voltage conditions, U N = 0, the voltage vector modulus is without double frequency fluctuation, and the grid-side current is balanced with positive sequence; when the grid voltage is unbalanced, the negative sequence component U N ≠0, the reference current base value contains high-order harmonics, and the harmonic amplitude decreases with the increase of the order. The fundamental component and harmonic component of the current are controlled separately. The two control branches use a proportional resonant controller. The expression is:

[0016]

[0017] Among them, G f (s) represents the transfer function of the fundamental resonance controller; G h (s) represents the transfer function of the harmonic resonance controller; s represents the Laplace operator; ω c represents the cut-off frequency; ω represents the system angular frequency; ω hIndicates the angular frequency of the harmonic; K P1 and K r1 Represents the proportional coefficient and resonant gain of the fundamental resonant controller; K Ph and K rh Represents the proportional coefficient and resonant gain of the harmonic resonance controller.

[0018] Preferably, the control flow of the PR controller is as follows:

[0019]

[0020] Among them, G i (s) represents the transfer function of the PR controller; K r represents the double frequency resonance coefficient; s represents the Laplace operator; K p Indicates the proportionality coefficient; K r Represents the double frequency resonance coefficient.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The present invention adds a Buck decoupling circuit to the DC bus of the AC / DC converter under an unbalanced power grid, effectively filtering out the double frequency power pulsation on the DC side, reducing the capacitance of the DC bus capacitor, and using small-capacitance and long-life film capacitors to replace traditional electrolytic capacitors, thereby improving the reliability and stability of the system, achieving a balanced positive sequence current on the grid side and stable voltage on the DC side, and significantly improving the utilization rate of new energy power generation and the overall efficiency of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0024] Figure 1 This is a topology diagram of a Buck-type power decoupling LCL three-phase two-level converter according to an embodiment of the present invention;

[0025] Figure 2 This is a block diagram of power decoupling control based on a stationary coordinate system according to an embodiment of the present invention;

[0026] Figure 3 This is a waveform diagram of grid-side current and DC-side voltage without a decoupling circuit under an unbalanced power grid according to an embodiment of the present invention;

[0027] Figure 4 This is a waveform diagram of grid-side current and DC-side voltage with a decoupling circuit under an unbalanced power grid according to an embodiment of the present invention;

[0028] Figure 5 : is the three-phase grid-side current THD of an embodiment of the present invention; wherein, (a) represents phase A; (b) represents phase B; and (c) represents phase C. DETAILED DESCRIPTION

[0029] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] Embodiment 1

[0032] like Figure 1 As shown in FIG. 1 , a three-phase two-level converter based on Buck decoupling topology LCL is provided, including: a three-phase power grid, an LCL filter, a three-phase two-level converter main circuit, a Buck power decoupling circuit, a DC bus capacitor, and a load. The main circuit is responsible for power conversion, and the Buck decoupling circuit absorbs the double frequency pulsating power formed by the product of the positive sequence voltage and the negative sequence current on the AC side, thereby reducing the capacitance of the DC bus capacitor.

[0033] In this embodiment, the three-phase power grid is a 220V AC power grid. The LCL three-phase two-level converter based on the Buck decoupling topology has two parts: DC and AC. The connection relationship of the AC part is: the AC power grid is connected to the LCL filter, and then connected to the AC side of the three-phase four-bridge arm two-level converter. The connection relationship of the DC part is: the DC side of the three-phase four-bridge arm two-level converter is connected to the voltage stabilizing capacitor, and then connected to the resistive load. The structural details of the simulation platform are as follows: Figure 1 shown.

[0034] This embodiment uses a thin-film capacitor with a small capacitance and a long life to replace the electrolytic capacitor on the DC side of the traditional converter, which effectively improves the reliability of the system. In order to solve the problem of separating the positive and negative sequences of the grid voltage when the main circuit is controlled based on a rotating coordinate system, which requires complex rotating coordinate transformation and reference current reference value calculation, this embodiment adopts a control design based on a stationary coordinate system. Under asymmetric grid conditions, the harmonic components of the grid-side current are mainly the 3rd, 5th, and 7th harmonics. The PR controller is used to effectively filter out the above harmonic components and improve the quality of the grid-side current.

[0035] The working principle of this embodiment is as follows:

[0036] The zero-sequence component of the voltage in an unbalanced power grid is zero, so the three-phase power grid voltage ua 、u b 、u c They can be expressed as:

[0037]

[0038] Among them, U P and θ P Respectively represent the peak value and phase of the positive sequence component; U N and θ N They are respectively represented as the peak value and phase of the negative sequence component; ω is the system angular frequency, and t is the time;

[0039] After Clark coordinate transformation of equation (1), the voltage expression in the axis stationary coordinate system can be obtained as follows:

[0040]

[0041] Among them, u α 、u β Represents the voltage component in the stationary coordinate system.

[0042] Simplifying equation (2), we can get the positive and negative sequence components of the grid voltage in the stationary coordinate system as follows:

[0043]

[0044] in, is the positive sequence component of the grid voltage in the stationary coordinate system; is the negative sequence component of the grid voltage in the stationary coordinate system.

[0045] According to the instantaneous power theory, the instantaneous active power p and reactive power q can be expressed as:

[0046]

[0047] Among them, i α 、i β Represents the grid-side current component in the stationary coordinate system.

[0048] Assuming that the active power transmitted by the converter is P* and the reactive power is Q*, the current reference value can be obtained from equation (4):

[0049]

[0050] Among them, i αref and i βref Indicates the current reference value; and represents the positive sequence component of the current in the stationary coordinate system; and Represents the negative sequence component of the current in the stationary coordinate system.

[0051] When the power factor of the converter is 1, the reactive component Q*=0, and equation (5) can be simplified as:

[0052]

[0053] Voltage vector magnitude|u 2 Calculated as:

[0054]

[0055] It can be seen from formula (7) that under the condition of symmetrical sinusoidal voltage, U N = 0, the voltage vector modulus is no double frequency fluctuation, and the grid-side current is balanced positive sequence. When the grid voltage is unbalanced, the negative sequence component U N ≠0, the reference current base value contains high-order harmonics, the harmonic amplitude decreases with the increase of the number, and the main harmonic components are the 3rd, 5th, and 7th harmonics. The current controller is designed as follows Figure 2 As shown, the fundamental component and harmonic component of the current are controlled respectively, and the two control branches adopt the proportional resonant controller expression as follows:

[0056]

[0057] Among them, G f (s) represents the transfer function of the fundamental resonance controller; G h (s) represents the transfer function of the harmonic resonance controller; s represents the Laplace operator; ω c represents the cut-off frequency; ω represents the system angular frequency; ω h Indicates the angular frequency of the harmonic; K P1 and K r1 Represents the proportional coefficient and resonant gain of the fundamental resonant controller; K Ph and K rh Represents the proportional coefficient and resonant gain of the harmonic resonance controller.

[0058] In an unbalanced power grid, the grid voltage has a negative sequence component. The product of the grid balanced positive sequence current and the negative sequence voltage component will cause double frequency pulsating power on the DC side. Under the premise of not reducing the power density of the converter, in order to use film capacitors instead of electrolytic capacitors, an active power decoupling circuit is used to absorb the double frequency pulsating power on the DC side. Two power devices and inductors and capacitors are connected in series to form a Buck type power decoupling circuit, such as Figure 1 shown. Figure 1 The Kirchhoff voltage equation for the decoupled circuit is:

[0059]

[0060] Among them, u apd Represents the output voltage of the decoupling circuit; iLr Indicates the inductor current flowing through the decoupling circuit; u r Represents the voltage across the capacitor of the decoupling circuit; m r represents the modulation wave of the Buck decoupling circuit; u dc Indicates DC voltage; L r Represents the decoupling circuit inductance.

[0061] Perform Laplace transformation on equation (10) to obtain the closed-loop transfer function of the decoupling circuit and the reference value of the capacitor voltage of the decoupling circuit: Subtract the actual sampling value u of the capacitor voltage r The inductor current reference value is obtained through the PI controller As shown in formula (11):

[0062]

[0063] Among them, K p Indicates the proportionality coefficient; K i represents the integral coefficient; s represents the Laplace operator.

[0064] In formula (10), the inductor current i Lr is a sinusoidal quantity with double frequency, and is controlled by a proportional resonant controller. The PR controller is shown in equation (12), and the decoupling circuit control block diagram is shown in Figure 2 The decoupling control branch is shown.

[0065]

[0066] Among them, G i (s) represents the transfer function of the PR controller; K r represents the double frequency resonance coefficient; s represents the Laplace operator; K p Indicates the proportionality coefficient; K r Represents the double frequency resonance coefficient.

[0067] Embodiment 2

[0068] To verify the feasibility of the present invention, a 15kW three-phase two-level Buck decoupling topology based on stationary coordinate system control was built on the Simulink simulation platform. The simulation uses a 180μF decoupling capacitor and a 1mH decoupling inductor to form an LC branch, and the bus capacitor uses 560μF. The detailed simulation parameters are shown in Table 1.

[0069] Table 1

[0070]

[0071] Figure 3 , Figure 4 Comparison of waveforms with and without decoupling circuits under unbalanced power grids with the same DC bus capacitance. Figure 3 It can be seen that under unbalanced power grid, the current on the control grid side is balanced and positive sequence, there is no low-order harmonics, there is no decoupling circuit on the DC side of the converter, there is a large double frequency pulsation, and the DC side voltage pulsation is 14V. Figure 4 This is a waveform diagram of the grid-side current and DC-side voltage under the control method of the present invention. It can be seen from the figure that the grid-side current is balanced in positive sequence, the DC-side voltage has basically no double frequency pulsation, and the pulsation amplitude is 2.2V. Figure 5 is the total harmonic distortion value of the grid-side current. It can be seen from the figure that the three-phase grid current THD is less than 5%. Under the unbalanced grid voltage, it can control the high-quality grid current to flow into the grid. In summary, it is proved that the present invention can achieve a balanced positive sequence of grid-side current based on power decoupling control in a stationary coordinate system under an unbalanced grid voltage, and use film capacitors instead of electrolytic capacitors on the DC side to improve system stability.

[0072] The embodiments described above are only descriptions of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A system for reducing the DC bus capacitance of an AC / DC converter under an unbalanced power grid, including: A three-phase power grid, an LCL filter, a DC bus capacitor and a load, characterized in that it also includes: a three-phase two-level converter main circuit and a Buck power decoupling circuit; Wherein, the main circuit is used for power conversion; The Buck decoupling circuit absorbs the double frequency pulsating power formed by the product of the positive sequence voltage and the negative sequence current on the AC side, thereby reducing the capacitance of the DC bus capacitor.

2. The system for reducing the DC bus capacitance of an AC / DC converter under an unbalanced power grid according to claim 1, characterized in that: Small-capacitance and long-life film capacitors are used to replace the electrolytic capacitors on the DC side of the traditional converter. The control design based on the stationary coordinate system is used to solve the problem of separating the positive and negative sequence of voltage. At the same time, the PR controller is used to filter out harmonic components and improve the current quality on the grid side.

3. The system for reducing the DC bus capacitance of an AC / DC converter under an unbalanced power grid according to claim 2, characterized in that: Since the zero-sequence component of the voltage in an unbalanced power grid is zero, the three-phase voltage u a 、u b 、u c Respectively expressed as: Among them, U P and θ P Respectively represent the peak value and phase of the positive sequence component; U N and θ N They are respectively represented as the peak value and phase of the negative sequence component; ω is the system angular frequency, and t is the time; After the above equation is transformed into Clark coordinates, the voltage expression in the axis stationary coordinate system is obtained: Among them, u α 、u β Represents the voltage component in the stationary coordinate system.

4. The system for reducing the DC bus capacitance of an AC / DC converter under an unbalanced power grid according to claim 3, characterized in that: Under symmetrical sinusoidal voltage conditions, U N = 0, the voltage vector modulus is without double frequency fluctuation, and the grid-side current is balanced with positive sequence; when the grid voltage is unbalanced, the negative sequence component U N ≠0, the reference current base value contains high-order harmonics, and the harmonic amplitude decreases with the increase of the order. The fundamental component and harmonic component of the current are controlled separately. The two control branches use a proportional resonant controller. The expression is: Among them, G f (s) represents the transfer function of the fundamental resonance controller; G h (s) represents the transfer function of the harmonic resonance controller; s represents the Laplace operator; ω c represents the cut-off frequency; ω represents the system angular frequency; ω h Indicates the angular frequency of the harmonic; K P1 and K r1 Represents the proportional coefficient and resonant gain of the fundamental resonant controller; K Ph and K rh Represents the proportional coefficient and resonant gain of the harmonic resonance controller.

5. The system for reducing the DC bus capacitance of an AC / DC converter under an unbalanced power grid according to claim 4, characterized in that: The control flow of the PR controller is as follows: In the formula, G i (s) represents the transfer function of the PR controller; K r represents the double frequency resonance coefficient; s represents the Laplace operator; K p Indicates the proportionality coefficient; K r Represents the double frequency resonance coefficient.

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