A method for suppressing secondary power fluctuation under unbalanced network voltage

Abstract

The present invention relates to a method for suppressing secondary power fluctuations under unbalanced grid voltage, belonging to the field of power electronics. The unique flying capacitor of the rectifier topology is utilized to absorb the secondary fluctuating power caused by unbalanced grid voltage. The present invention can effectively suppress the secondary ripple component of the DC-side voltage, significantly improve the DC voltage quality; and there is no need to add external devices, which is beneficial to improving the system power density and reducing the hardware cost, and realizing stable operation under all working conditions.

Description

Technical Field

[0001] The present invention belongs to the field of power electronics, and particularly relates to a method for suppressing secondary power fluctuations under unbalanced grid voltage. Background Art

[0002] In a power system, due to reasons such as asymmetric grid faults, large-capacity single-phase load connection, transmission impedance mismatch, and renewable energy grid connection, the three-phase grid imbalance occurs frequently. When the grid is unbalanced, if the control strategy under balanced grid is still adopted, it will cause the abnormal working state of the rectifier, resulting in grid-side current distortion and DC-side voltage oscillation, and even burning out the rectifier device. Therefore, a large number of scholars have studied high-performance control strategies for unbalanced grid conditions to achieve the purpose of suppressing negative-sequence current and eliminating voltage ripple. According to the mathematical model under unbalanced grid, when ensuring the balance and symmetry of three-phase currents, the multiplication of negative-sequence voltage and positive-sequence current in the grid will generate secondary fluctuation power, which is transmitted to the DC side through the modulation of the rectifier, resulting in the DC-side voltage containing secondary voltage ripple and reducing the DC voltage quality. The traditional solution is to connect a large-capacity electrolytic capacitor in parallel on the DC side to suppress the amplitude of voltage pulsation ripple. However, the electrolytic capacitor has disadvantages such as large volume, short life, and low reliability, which affect the power density and operation reliability of the rectifier system.

[0003] The literature "Parallel Compensation Control of Power Decoupling Type Electrolytic Capacitor-less PFC Circuit" proposes a solution method of inserting a secondary power absorption circuit, and controls the energy storage capacitor in the absorption circuit to absorb the secondary fluctuation power to eliminate the secondary fluctuation component in the DC voltage. This method has good fluctuation suppression effect and can ensure a constant DC voltage output, but relatively increases the hardware cost of the system.

[0004] Therefore, it has practical application value to study a simple and effective secondary fluctuation power suppression strategy without increasing hardware cost. Summary of the Invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method for suppressing secondary power fluctuations under unbalanced grid voltage, which uses the unique flying capacitor of the rectifier topology to absorb the secondary fluctuation power caused by unbalanced grid voltage, thereby suppressing the secondary fluctuation component of the DC-side voltage and improving the DC voltage quality.

[0006] To achieve the above purpose, the technical solution of the present invention is: a method for suppressing secondary power fluctuations under unbalanced grid voltage, which uses the unique flying capacitor of the rectifier topology to absorb the secondary fluctuation power caused by unbalanced grid voltage and suppress the secondary fluctuation component of the DC-side voltage.

[0007] In an embodiment of the present invention, the rectifier is a flying capacitor type multilevel rectifier.

[0008] In an embodiment of the present invention, the rectifier is a flying capacitor type two-level rectifier with a double-frequency decoupling circuit, including three-phase bridge arms, and the three-phase bridge arms are connected in parallel to a DC bus composed of series capacitors C1 and C2; a single-phase bridge arm includes switching tubes S x2 , S' x2 , and series-connected switching tubes S x1 , S' x1 , where the subscript x represents bridge arms a, b, and c, the emitter of S x2 , the collector of S' x2 are connected to the emitter of S xf , the collector of S' x1 through a flying capacitor C x1 and an inductor L1, and are connected to a filter inductor.

[0009] In an embodiment of the present invention, the rectifier is a flying capacitor type three-level rectifier, including three-phase bridge arms, and the three-phase bridge arms are connected in parallel to a DC bus composed of series capacitors C1 and C2; a single-phase bridge arm includes switching tubes S x2 , S x1 , S' x1 , S' x2 connected in series in sequence between the positive and negative terminals of the DC bus, where the subscript x represents bridge arms a, b, and c, a flying capacitor C xf is connected in series between the emitter of S x2 and the collector of S' x2 , and the emitter of S x1 is connected to a filter inductor.

[0010] In an embodiment of the present invention, the rectifier is a flying capacitor type five-level rectifier, including three-phase bridge arms, and the three-phase bridge arms are connected in parallel to a DC bus composed of series capacitors C1 and C2; a single-phase bridge arm includes diodes D x1 , D x2 , D x3 , D x4 connected in series in sequence between the positive and negative terminals of the DC bus, where the subscript x represents bridge arms a, b, and c, the anode of D x2 is connected to the end point n between the capacitors C1 and C2, and there are switching tubes S x1 , S x3 , S' x2 , S' x1 connected in series in sequence between the anode of D x1 and the anode of D x2 , a flying capacitor C x2 is connected in series between the emitter of S x2 and the collector of S' xf , and the emitter of S x1 is connected to a filter inductor.

[0011] In an embodiment of the present invention, the method includes the following steps:

[0012] Step 1: Collect the grid voltage and current, control the rectifier to eliminate the negative sequence current component on the AC side, and calculate the three-phase modulation wave;

[0013] Step 2: Extract the second harmonic ripple component of the DC voltage and calculate the flying capacitor voltage command value;

[0014] Step 3: Calculate the relative adjustment amount of the modulation wave according to the deviation between the flying capacitor voltage command value and the flying capacitor voltage set value;

[0015] Step 4: Inject the zero-sequence compensation component to ensure the balance of the volt-second product of the bridge arm terminal voltage;

[0016] Step 5: Compare the obtained modulation wave with the carrier wave to obtain the PWM signal and control the operation of the rectifier.

[0017] In an embodiment of the present invention, Step 2 is specifically as follows:

[0018] Subtract the DC voltage command value V dc * from the DC voltage sampling value v dc to obtain the DC voltage ripple component v dc_ripple , and send it to a quasi-proportional-resonant controller with a resonant frequency twice that of the grid frequency to calculate the ripple component command value V 2ω ; the flying capacitor voltage command value V fc * under unbalanced grid voltage = 0.25V dc * + V 2ω .

[0019] In an embodiment of the present invention, Step 3 is specifically as follows:

[0020] According to the deviation value V fc * - v xf between the flying capacitor voltage command value and the flying capacitor voltage set value, calculate the current value i xf* required for controlling the flying capacitor voltage;

[0021] Combine with the current i x at present to calculate the relative adjustment amount Δm x of the modulation wave, satisfying Δm x = 0.5i xf* / i x .

[0022] In an embodiment of the present invention, in Step 4, the injected zero-sequence compensation component is m o = Δm x * (1 - 4v xf / V dc *).

[0023] In an embodiment of the present invention, in step 5, the modulation waves m x1 , S′ x1 ), (S x2 , S′ x2 ) corresponding to x1 , m x2 satisfy

[0024]

[0025] where m x is the three-phase modulation wave calculated in step 1;

[0026] By comparing m x1 , m x2 with the symmetric triangular carriers T r1 , T r2 with a phase difference of 180°, the PWM signals of the switching tubes (S x1 , S′ x1 ), (S x2 , S′ x2 ) are obtained to control the operation of the rectifier.

[0027] The present invention also provides a secondary power fluctuation suppression system under unbalanced grid voltage, including a memory, a processor, and computer program instructions stored on the memory and executable by the processor. When the processor executes the computer program instructions, the method steps as described above can be implemented.

[0028] The present invention also provides a computer-readable storage medium, on which computer program instructions executable by the processor are stored. When the processor executes the computer program instructions, the method steps as described above can be implemented.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] (1) This method can eliminate negative-sequence current and absorb secondary ripple power, while ensuring the symmetry of three-phase alternating current and the constancy of direct current voltage;

[0031] (2) This method only relies on the flying capacitors of the flying-capacitor multi-level rectifier itself to absorb secondary fluctuation power, without adding any external devices, which is beneficial to improving the system power density and reducing the hardware cost, and can achieve stable operation under all working conditions. Description of the Drawings

[0032] Figure 1 is the single-phase topology of the flying-capacitor five-level rectifier.

[0033] Figure 2 is the three-phase topology of the flying-capacitor five-level rectifier.

[0034] Figure 3 is the schematic diagram of carrier phase-shifted modulation.

[0035] Figure 4 is the single-phase topology of the flying-capacitor two-level rectifier with a double-frequency decoupling circuit.

[0036] Figure 5 is the three-phase topology of the flying-capacitor three-level rectifier. Specific implementation manners

[0037] The technical solution of the present invention will be specifically described below in conjunction with the accompanying drawings.

[0038] The present invention provides a method for suppressing secondary power fluctuation under unbalanced grid voltage, which uses the unique flying capacitors of the rectifier topology to absorb the secondary fluctuation power caused by unbalanced grid voltage and suppress the secondary fluctuation component of the DC-side voltage. The rectifier is a flying-capacitor multi-level rectifier.

[0039] Embodiment 1

[0040] As Figure 1 and 2 shown, in this embodiment, the rectifier adopted is a flying-capacitor five-level rectifier, which includes three-phase bridge arms, and the three-phase bridge arms are connected in parallel to the DC bus composed of series capacitors C1 and C2; a single bridge arm includes diodes D x1 , D x2 , D x3 , D x4 (the subscript x represents the bridge arms a, b, c), the anode of D x2 is connected to the end point n between the capacitors C1 and C2, and there are switching tubes S x1 , S x3 , S' x2 , S' x1 connected in series between the anode of D x1 and the anode of D x2 , and a flying capacitor C x2 is connected in series between the emitter of S x2 and the collector of S' xf , and the emitter of S x1 is connected to the filter inductor.

[0041] The method for suppressing secondary power fluctuation under unbalanced grid voltage in this embodiment is specifically as follows:

[0042] 1) Collect the grid voltage and current, control the rectifier to eliminate the negative-sequence current component on the AC side, and calculate the three-phase modulation signals v x_ref , with 0.5V dcThe per-unit value is normalized with the reference value to the range of 0 to 1 to obtain the three-phase modulation wave m x , expressed as

[0043]

[0044] 2) Subtract the DC voltage sampled value v dc from the DC voltage command value V dc , extract the second harmonic ripple component, and input it into a quasi-proportional-resonant controller with a resonant frequency twice that of the grid frequency to calculate the ripple component command value V 2ω of the flying capacitor voltage, expressed as

[0045]

[0046] The flying capacitor voltage command value V fc * includes a DC component and a second harmonic ripple component, expressed as

[0047] V fc * = 0.25V dc + V 2ω

[0048] 3) Adopt the carrier phase-shifted modulation strategy shown in Figure 3 , where the modulation waves m x1 , m x2 correspond to the carriers T r1 , T r2 respectively. Compare to obtain the pulse signals of S x1 , S x2 . The pulse signals of S′ x1 , S′ x2 are opposite to S x1 , S x2 . Adjust the amplitude between m x1 , m x2 to achieve flying capacitor voltage control.

[0049] Adopt a pure proportional controller K P , and calculate the current i fc * required for flying capacitor voltage control according to the deviation (V xf * - v xf *) between the flying capacitor voltage command value and the flying capacitor voltage set value, satisfying i xf * = K p (V fc * - v xf );

[0050] Let Δm x be the relative adjustment amount between m x1 , m x2 , then Δmx = 0.5i xf * / i x , where i x is the phase current;

[0051] 4) To maintain the balance of the volt-second product of the bridge arm voltage, inject the compensation component m o , satisfying m o = Δm x *(1 - 4v xf / V dc *);

[0052] 5) Then the obtained corresponding S x1 , S x2 of the modulation wave m x1 , m x2 satisfies

[0053]

[0054] Compare m x1 , m x2 with Figure 3 the carrier waves T r1 , T r2 shown to obtain the switch tube pulse signal and control the normal operation of the rectifier.

[0055] Embodiment 2

[0056] As Figure 4 shown, in this embodiment, the rectifier adopted is a flying capacitor type two-level rectifier with a double-frequency decoupling circuit (the filter inductor is not shown in the figure). The control idea is basically the same as that of Embodiment 1. Both use flying capacitors for power decoupling to achieve the absorption of power ripple and reduce the fluctuation of the secondary power when the grid voltage is unbalanced.

[0057] Embodiment 3

[0058] As Figure 5 shown, in this embodiment, the rectifier adopted is a flying capacitor type three-level rectifier (the series capacitors C1 and C2 are not shown in the figure). The control idea is basically the same as that of Embodiments 1 and 2. Both use flying capacitors for power decoupling to achieve the absorption of power ripple and reduce the fluctuation of the secondary power when the grid voltage is unbalanced.

[0059] The method of the present invention can also be extended to be used in circuit topologies with other voltage levels. The control idea is basically the same as that of Embodiments 1 - 3. Both use flying capacitors for power decoupling to achieve the absorption of power ripple and reduce the fluctuation of the secondary power when the grid voltage is unbalanced.

[0060] The present invention also provides a secondary power fluctuation suppression system under unbalanced network voltage, including a memory, a processor, and computer program instructions stored on the memory and capable of being run by the processor. When the processor runs the computer program instructions, the method steps as described above can be implemented.

[0061] The present invention also provides a computer-readable storage medium, on which computer program instructions capable of being run by the processor are stored. When the processor runs the computer program instructions, the method steps as described above can be implemented.

[0062] Those skilled in the art should understand that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk memories, CD-ROMs, optical memories, etc.) containing computer-usable program codes.

[0063] The present application is described with reference to the flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each flow and / or block in the flowcharts and / or block diagrams, and the combination of flows and / or blocks in the flowcharts and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, such that the instructions executed by the processor of the computer or other programmable data processing devices generate a device for implementing the functions specified in Figure 1 one or more flows and / or Figure 1 blocks or multiple blocks.

[0064] These computer program instructions can also be stored in a computer-readable memory capable of guiding the computer or other programmable data processing devices to work in a specific manner, such that the instructions stored in the computer-readable memory generate a manufactured article including an instruction device, and the instruction device implements the functions specified in Figure 1 one or more flows and / or Figure 1 blocks or multiple blocks.

[0065] These computer program instructions can also be loaded onto the computer or other programmable data processing devices, such that a series of operation steps are executed on the computer or other programmable devices to generate a computer-implemented process. Thus, the instructions executed on the computer or other programmable devices provide for implementing the functions specified in Figure 1 one or more flows and / or Figure 1Steps of functions specified in one or more boxes.

[0066] As described above, it is only the preferred embodiment of the present invention, and it is not a limitation to the present invention in other forms. Any person skilled in the art may use the technical content disclosed above to make changes or modifications into equivalent embodiments with equivalent changes. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the technical solution content of the present invention still belong to the protection scope of the technical solution of the present invention.

Claims

1. A method for suppressing secondary power fluctuations under unbalanced network voltage, characterized in that Utilize the unique flying capacitor of the rectifier topology to absorb the secondary fluctuating power caused by unbalanced grid voltage and suppress the secondary voltage fluctuation component on the DC side; the method includes the following steps: Step 1, collect the grid voltage and current, control the rectifier to eliminate the negative sequence current component on the AC side, and calculate the three-phase modulation wave; Step 2, extract the secondary ripple component of the DC voltage and calculate the command value of the flying capacitor voltage; Step 3, calculate the relative adjustment amount of the modulation wave according to the deviation between the command value of the flying capacitor voltage and the set value of the flying capacitor voltage; Step 4, inject the zero-sequence compensation component to ensure the balance of the volt-second product of the bridge arm terminal voltage; Step 5, compare the obtained modulation wave with the carrier wave to obtain the PWM signal and control the operation of the rectifier; Step 2 is specifically as follows: Subtract the DC voltage command value V dc * from the sampled DC voltage value v dc * to obtain the DC voltage ripple component v dc_ripple , and send it to a quasi-proportional-resonant controller with a resonant frequency twice the grid frequency to calculate the ripple component command value V 2ω ; The flying capacitor voltage command value V fc * under unbalanced grid voltage is 0.25V dc * + V 2ω ; Step 3 is specifically as follows: According to the deviation value V of the flying capacitor voltage command value and the flying capacitor voltage set value fc *-v xf , the current value i required for flying capacitor voltage control is calculated xf* , where V fc * is the flying capacitor voltage command value, and v xf is the flying capacitor voltage set value; Combined with the current current i x The relative adjustment amount Δm of the modulation wave is calculated x , satisfying Δm x = 0.5i xf* / i x ; In step 4, the injected zero-sequence compensation component is m o = Δm x (1 - 4v xf / V dc *), where v xf is the flying capacitor voltage set value, Δm x is the relative adjustment amount of the modulation wave, V dc * is the DC voltage command value.

2. A method for suppressing secondary power fluctuation under unbalanced network voltage according to claim 1, characterized in that The rectifier is a flying capacitor type multilevel rectifier.

3. A method for suppressing secondary power fluctuations under unbalanced line voltage, as claimed in claim 1, wherein The rectifier is a flying capacitor type two-level rectifier with a double-frequency decoupling circuit, including three-phase bridge arms, and the three-phase bridge arms are connected in parallel to a DC bus composed of series capacitors C1 and C2; a single-phase bridge arm includes switches S x2 , S' x2 , and series switches S x1 , S' x1 , where the subscript x represents the bridge arms a, b, c, the emitter of S x2 , the collector of S' x2 are connected to the emitter of S xf , the collector of S' x1 through a flying capacitor C x1 and an inductor L1, and are connected to a filter inductor.

4. A method for suppressing secondary power fluctuations under unbalanced network voltage according to claim 1, characterized in that, The rectifier is a flying capacitor type three-level rectifier, which includes three-phase bridge arms. The three-phase bridge arms are connected in parallel to a DC bus composed of series capacitors C1 and C2. The single-phase bridge arm includes switches S x2 , S x1 , S' x1 , S' x2 , where the subscript x represents the bridge arms a, b, and c. A flying capacitor C xf is connected in series between the emitter of S x2 and the collector of S' x2 . The emitter of S x1 is connected to the filter inductor.

5. A method for suppressing secondary power fluctuation under unbalanced network voltage, as claimed in claim 1, wherein The rectifier is a flying-capacitor five-level rectifier, which includes three-phase bridge arms. The three-phase bridge arms are connected in parallel to a DC bus composed of series capacitors C1 and C2. A single-phase bridge arm includes diodes D x1 , D x2 , D x3 , D x4 , where the subscript x represents the bridge arms a, b, and c. The anode of D x2 is connected to the endpoint n between capacitors C1 and C2. There are switching tubes S x1 and S x3 connected in series between the anode of D x2 and the anode of D x1 , and there are also switching tubes S′ x1 and S′ x2 . A flying capacitor C x2 is connected in series between the emitter of S x2 and the collector of S′ xf . The emitter of S x1 is connected to the filter inductor.

6. A method for suppressing secondary power fluctuation under unbalanced network voltage, according to any one of claims 3-5, characterized in that In step 5, the modulation waves m x1 , S' x1 ), (S x2 , S' x2 ) corresponding to x1 , m x2 satisfy where m x is the three-phase modulation wave calculated in step 1, Δm x is the relative adjustment amount of the modulation wave, and m o is the zero-sequence compensation component; Let m x1 and m x2 be compared with the symmetric triangular carrier waves T r1 and T r2 with a 180° phase difference respectively to obtain the PWM signals of the switching tubes (S x1 , S′ x1 ), (S x2 , S′ x2 ) to control the operation of the rectifier.

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