Single-phase UPQC topology without series isolation transformer and control method

Abstract

The application discloses a single-phase UPQC topology without series isolation transformer, two terminals of a primary winding of a three-winding step-down transformer are connected with a power transmission network, four terminals of two secondary windings (alpha winding and beta winding) are marked as V, Z, T and Y respectively, one end of a filter capacitor is connected with an output terminal O1 of a single-phase inverter and simultaneously connected with a load, the other end of the filter capacitor is connected with an output terminal O2 of the single-phase inverter through a filter inductor and simultaneously connected with the V terminal of the alpha winding of the three-winding step-down transformer; the Z terminal of the alpha winding is connected with the ground; the T terminal of the beta winding is connected with an output terminal O4 of a single-phase rectifier; the Y terminal of the beta winding is connected with an output terminal O3 of the single-phase rectifier through a reactor; the single-phase inverter and the single-phase rectifier share a DC support capacitor. The UPQC provided by the application replaces a power frequency isolation transformer in the series part of the prior art and connected with the AC power network, so that the influence of the non-linear characteristics of the isolation transformer on the performance of the UPQC is avoided.

Description

Technical Field

[0001] This invention belongs to the field of power quality management technology, and in particular relates to a single-phase UPQC topology and control method without series isolation transformer. Background Technology

[0002] To achieve the goal of building a low-carbon and environmentally friendly society, new energy sources, represented by wind power and photovoltaics, are gradually changing the power generation landscape, and new energy power generation technologies are becoming increasingly mature. However, due to the uncertainties and randomness of wind and solar energy, the output power of new energy power plants fluctuates significantly. Therefore, the grid connection of new energy power generation systems can cause problems such as grid voltage fluctuations and flicker. When sensitive loads draw power from the grid, fluctuating grid voltage can cause abnormal load operation or even damage the load, resulting in economic losses for users. At the same time, with the rapid development of power electronics technology, a large number of nonlinear, impulsive, and asymmetrical loads and devices are connected to the grid, leading to distortion of the grid current. The harmonic and reactive power content in the grid current increases, which, in severe cases, will endanger the production and development of modern industry and the stable operation of the power grid system. Therefore, improving power quality is a necessary trend of the times, and researching how to effectively improve power quality is of great significance.

[0003] A Unified Power Quality Controller (UPQC) is a comprehensive power quality regulation device. It combines the characteristics of series and parallel compensation devices, suppressing current harmonics while compensating for voltage fluctuations and reactive current. Whether power quality problems occur in the load or the power grid, the UPQC can provide effective compensation. Based on power electronics technology, the UPQC enables voltage control, balancing compensation, active filtering, and uninterrupted power supply, and possesses promising development prospects and unique research value. Therefore, researching UPQCs capable of improving multiple power quality problems in distribution systems is of great significance.

[0004] Since the series and parallel sections of a UPQC share a DC energy storage unit, isolation between the grid and the equipment is necessary; otherwise, problems such as capacitor shoot-through and phase-to-phase short circuits may occur. Currently, most research chooses to add a power frequency isolation transformer at the AC grid connection point of the series section. However, due to the nonlinear characteristics of the series transformer, its introduction also brings the following disadvantages. First, the short-circuit reactance of the series transformer reduces the voltage accuracy under open-loop control, thus affecting the device's performance. Second, the connection of the series transformer increases the device's power loss, reduces its efficiency, and increases system cost. Finally, the series transformer also suffers from saturation and inrush current during voltage dips. Therefore, there is an urgent need for a single-phase UPQC topology without a series isolation transformer, and to study the control method for this topology.

[0005] Chinese invention patent CN102593859B discloses a three-phase UPQC topology circuit based on MMC, and discloses the following structure: Parallel modules are respectively connected to the series module, the A-phase line of the transmission line, the B-phase line of the transmission line, and the C-phase line of the transmission line. The primary windings of the first isolation transformer, the second isolation transformer, and the third isolation transformer are respectively installed on the A-phase line, the B-phase line of the transmission line, and the C-phase line of the transmission line. One end of the secondary winding of the first isolation transformer is connected to the common node of the first and second reactors in the series module; one end of the secondary winding of the second isolation transformer is connected to the common node of the third and fourth reactors in the series module; and one end of the secondary winding of the third isolation transformer is connected to the common node of the fifth and sixth reactors in the series module. The other ends of the secondary windings of the first, second, and third isolation transformers are connected together. In this invention, the series section is connected to the system via an isolation transformer, which not only increases the system cost but also affects the performance of the UPQC due to the nonlinear characteristics of the series transformer.

[0006] Chinese utility model patent CN215498282U discloses a transformer and a UPQC system. The transformer includes: a transformer body; a series compensation tap led out from the high-voltage winding of the transformer body, used to adjust the output voltage of the transformer body to the voltage level required by the UPQC; the high-voltage side of the transformer body is Y-connected and connected to the series-side converter on the DC bus through a low-pass filter reactor of the series-side converter; the low-voltage side of the transformer body is Δ-connected and connected to the parallel-side converter on the DC bus in sequence through an isolation transformer and a low-pass filter reactor of the parallel-side converter. This patent enables the high-voltage side of a new type of transformer that can be directly connected to the power grid via the series side. By adjusting the tap of the high-voltage side of the new transformer, the voltage can be reduced to a voltage that the series converter can access. Compared to the traditional UPQC structure, this device replaces the three step-down transformers and series transformers in the traditional UPQC structure with one new type of transformer and three isolation transformers. The presence of the isolation transformer in this device can affect the performance of the UPQC due to its nonlinear characteristics. Summary of the Invention

[0007] To address the aforementioned problems, the purpose of this invention is to propose a single-phase UPQC topology and control method without a series isolation transformer. This eliminates the need for a power frequency isolation transformer at the AC grid connection point of the series section, achieving isolation through an additional transformer winding. This reduces system size and cost, and avoids the impact of the nonlinear characteristics of the power frequency isolation transformer at the AC grid connection point on UPQC performance.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A single-phase UPQC topology without a series isolation transformer, the single-phase UPQC topology includes a three-winding step-down transformer, a filter capacitor, a filter inductor, a single-phase inverter, a single-phase rectifier, a DC support capacitor, and a reactor;

[0010] The two terminals of the primary winding of the three-winding step-down transformer are connected to the power grid. The four terminals of the two secondary windings (α, β) are marked as V, Z, T, and Y, respectively. One end of the filter capacitor is connected to the output terminal O1 of the single-phase inverter and is also connected to the load. The other end of the filter capacitor is connected to the output terminal O2 of the single-phase inverter through the filter inductor and is also connected to the V terminal of the α winding of the secondary winding of the three-winding step-down transformer. The Z terminal of the α winding is connected to ground. The T terminal of the β winding is connected to the output terminal O4 of the single-phase rectifier. The Y terminal of the β winding is connected to the output terminal O3 of the single-phase rectifier through the reactor. The single-phase inverter and the single-phase rectifier share a DC support capacitor.

[0011] Furthermore, the single-phase inverter is a single-phase full-bridge circuit, with a bypass switch S1 connected in parallel across the filter capacitor to disconnect a faulty single-phase inverter. The single-phase rectifier is also a single-phase full-bridge circuit, with a bypass switch S2 connected in series at the connection point between the reactor and the Y terminal of the β winding to disconnect a faulty single-phase rectifier.

[0012] Furthermore, the power electronic devices in single-phase inverters and single-phase rectifiers are fully controlled switching devices.

[0013] This invention also provides a control method for a single-phase UPQC topology without a series isolation transformer. The purpose of the control method is to control the primary winding current of the three-winding step-down transformer to be in phase with the fundamental voltage, and the load voltage to be a sinusoidal AC voltage. The control method includes a phase-locked loop algorithm, a load current detection algorithm, a command current generation algorithm, a command voltage generation algorithm, an inverter voltage control algorithm, a rectifier current control algorithm, and a capacitor voltage control algorithm.

[0014] Furthermore, the control method includes the following steps:

[0015] S1: The phase-locked loop algorithm locks the grid voltage on the primary or secondary side of the three-winding step-down transformer to obtain the voltage phase reference sin(ωt), which provides a phase reference for the control system.

[0016] S2: The load current detection algorithm provides a basis for generating rectifier current commands by detecting the active, reactive and harmonic components in the load current;

[0017] S3: Command current generation algorithm generates current commands for the rectifier;

[0018] S4: Command voltage generation algorithm generates voltage commands for the inverter;

[0019] S5: The inverter voltage control algorithm is used to achieve closed-loop control of the inverter output voltage;

[0020] S6: The rectifier current control and capacitor voltage control algorithm is used to realize closed-loop control of the rectifier output current and maintain the stability of the DC side capacitor voltage.

[0021] Furthermore, the load current detection algorithm is as follows:

[0022] Load current i L Includes active component i p and reactive and harmonic components i qh The calculation method is as follows:

[0023]

[0024] The measured load current i L Multiplying this by the voltage phase reference sin(ωt) provided by the phase-locked loop converts the active current into a DC component, while simultaneously converting the reactive and harmonic currents into AC components, yielding:

[0025]

[0026] The above signal is filtered by a low-pass filter to remove the AC component, and the remaining DC component is multiplied by 2 to obtain the amplitude I of the active current. p This amplitude is multiplied by the voltage phase reference sin(ωt) provided by the phase-locked loop to obtain the load current i. L The active current component i in p Total load current i L Remove active components i p The sum of reactive and harmonic components, i, is obtained. qh Furthermore, we obtain:

[0027]

[0028] Furthermore, the instruction current generation algorithm is as follows:

[0029] The sum of reactive and harmonic components i obtained based on the above load current detection algorithm qh Combining the turns ratio (n) of the secondary α winding and β winding of the three-winding step-down transformer αβ n β This allows us to obtain the current command value i output by the single-phase rectifier. cref for:

[0030] i cref =-i qh n β / n α .

[0031] Furthermore, the command voltage generation algorithm is as follows:

[0032] Based on the voltage phase reference sin(ωt) provided by the phase-locked loop and the desired output voltage amplitude U o The output voltage command value u of the single-phase inverter can be obtained. cref for:

[0033] u cref =U o sin(ωt)-u α .

[0034] Furthermore, the inverter voltage control algorithm is as follows:

[0035] The single-phase inverter output voltage command value u cref The sampled single-phase inverter output voltage u c The sampled values ​​are compared, and the difference between them is processed by the closed-loop regulator to generate a modulation signal u1. The modulation signal is compared with the triangular carrier wave to obtain the switching signal g1.

[0036] Furthermore, the specific algorithms for single-phase rectifier current control and capacitor voltage control are as follows:

[0037] single-phase rectifier DC side capacitor voltage command value u dcref With DC side capacitor voltage u dc The difference is compared and processed by the closed-loop regulator to generate a fine-tuning active current command Δi. p Fine-tuning the active current component Δi p After being converted to the secondary β winding of a three-winding transformer, the current command value i is added. cref This is the final output current command value of the rectifier, which is the sum of the rectifier output current command value and the sampled rectifier output current i. c The difference is compared and processed by a closed-loop regulator to generate a modulation signal u2. The modulation signal is compared with a triangular carrier wave to obtain a switching signal g2.

[0038] Compared with the prior art, the single-phase UPQC topology and control method without series isolation transformer provided by the present invention has the following beneficial effects:

[0039] Research on power quality control technology has significant economic and social benefits and is a hot topic in the field of power research. The hazards of power quality problems such as voltage sags, voltage dips, three-phase unbalanced voltage, harmonic voltage, reactive current, harmonic current, and unbalanced current are becoming increasingly serious. However, most existing devices are connected to the system in parallel or series and can only solve some power quality problems.

[0040] The Unified Power Quality Controller (UPQC) consists of parallel and series sections. The series section addresses voltage issues and features DVR and DUPS functionality, while the parallel section addresses current issues and features D-STATCOM and APF functionality. The two sections can operate independently after decoupling to perform their respective functions, or they can operate together to achieve a unified, integrated function.

[0041] Because of the important role of UPQC in the power system, numerous technologies have emerged to optimize and improve UPQC, with varying starting points and objectives. For example, Chinese invention patent CN201210015325.8 discloses a three-phase UPQC topology circuit based on MMC, which mainly solves the problem of large equipment size and high production cost when a large-capacity unified power quality controller is connected to a high-voltage system, without considering replacing the isolation transformer with other components. Another example is Chinese invention patent CN103280798A, which discloses a UPQC topology circuit and control method for load capacity expansion, mainly solving the problem that the UPQC loses its function when the load increases to exceed the rated current of the isolation transformer. This patent also does not improve or remove the isolation transformer.

[0042] Therefore, it is evident that the presence of an isolation transformer within the UPQC is considered necessary by many skilled individuals. Although the presence of the isolation transformer will affect the performance of the UPQC to some extent, it is not easy for those skilled in the art to think of removing the isolation transformer. Thus, the inventor's proposal in this patent to "use an additional transformer winding to achieve isolation, reduce system volume, lower system cost, and avoid the impact of the nonlinear characteristics of the power frequency isolation transformer at the AC power grid connection point on the UPQC performance" is not something that would be easy to conceive.

[0043] Furthermore, the single-phase UPQC topology without series isolation transformer provided by this invention has advantages over traditional single-phase UPQC topologies, such as fewer power frequency transformers and lower voltage stress on the switching transistors. Regarding the number of power frequency transformers, this invention only requires an additional winding for isolation, eliminating the need for a bulky and costly power frequency isolation transformer. Regarding voltage stress on the switching transistors, by setting the turns ratio of the secondary β winding of the three-winding step-down transformer, the voltage stress on the single-phase rectifier switching transistors can be reduced.

[0044] In addition to the advantages mentioned above, the present invention also has the advantage of high reliability. When the converter fails and is disconnected from the circuit, the single-phase UPQC topology of the present invention can still supply power to the load through the secondary α winding of the three-winding step-down transformer, thus ensuring the normal operation of the load. Attached Figure Description

[0045] Figure 1The single-phase UPQC topology without series isolation transformer provided by the present invention is shown in the figure.

[0046] Figure 2 A block diagram of a single-phase UPQC control method without series isolation transformer provided by the present invention;

[0047] Figure 3 This is an integrated control block diagram for the load current detection algorithm and the command current generation algorithm;

[0048] Figure 4 This is an integrated control block diagram for the command voltage generation algorithm and the inverter voltage control algorithm.

[0049] Figure 5 This is a block diagram of the rectifier current control and capacitor voltage control algorithm.

[0050] Figure 6 , Figure 7 These are the simulation results of the present invention;

[0051] in, Figure 6 Subgraph (a) shows the grid voltage u g Subgraph (b) shows the grid current i g Subgraph (c) shows the load voltage u. L Subgraph (d) shows the load current i L Subgraph (e) shows the DC-side capacitor voltage u. dc ; Figure 7 Sub-diagram (a) shows the inverter command voltage u. cref Sub-figure (b) shows the inverter output voltage u. c Sub-diagram (c) shows the rectifier command current i. cref Subgraph (d) shows the rectifier output current i. c . Detailed Implementation

[0052] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0053] The present invention comprises two parts: a single-phase UPQC topology without series isolation transformer and a control method.

[0054] The single-phase UPQC topology without series isolation transformer proposed in this invention is as follows: Figure 1As shown, the system includes one three-winding step-down transformer, one filter capacitor, one filter inductor, one single-phase inverter, one single-phase rectifier, one DC support capacitor, and one connecting reactor. The two terminals of the primary winding of the three-winding step-down transformer are connected to the power grid, and the four terminals of the two secondary windings (α, β) are labeled V, Z, T, and Y, respectively. One end of the filter capacitor is connected to the output terminal O1 of the single-phase inverter and also to the load. The other end of the filter capacitor is connected to the output terminal O2 of the single-phase inverter through the filter inductor, and also to the V terminal of the α winding of the three-winding step-down transformer; the Z terminal of the α winding is connected to ground; the T terminal of the β winding is connected to the output terminal O4 of the single-phase rectifier; the Y terminal of the β winding is connected to the output terminal O3 of the single-phase rectifier through the reactor; the single-phase inverter and the single-phase rectifier share a single DC support capacitor.

[0055] A bypass switch S1 is connected in parallel across the filter capacitor to disconnect a faulty single-phase inverter. A bypass switch S2 is connected in series at the Y-terminal connection between the reactor and the β winding to disconnect a faulty single-phase rectifier.

[0056] In some preferred embodiments, the single-phase inverter and the single-phase rectifier are both single-phase full-bridge circuits, each including four switching devices connected in series and then in parallel to form a full bridge. A DC support capacitor C is connected in parallel with the full-bridge circuit. The power electronic switching devices in both the single-phase inverter and the single-phase rectifier are fully controlled switching devices. The switching devices are not limited to IGBTs; they can also be other fully controlled switching devices such as MOSFETs.

[0057] The reference directions for current and voltage are as follows: Figure 1 As shown. i g For grid-side current; u g This refers to the grid-side voltage; (u α i α (u) represents the voltage and current of the α winding on the secondary side of a three-winding step-down transformer; β i c (n) represents the voltage and current of the secondary β winding of a three-winding step-down transformer; α ,n β These represent the turns ratios of the α and β windings on the secondary side of a three-winding step-down transformer, respectively; u c The voltage across the filter capacitor is (uL,i) L ) represents the load voltage and current.

[0058] To facilitate understanding, the working principle of the topology provided in this invention will be explained in detail. The control objective of the single-phase UPQC is to control the grid-side current to be in phase with the grid-side voltage fundamental wave, and the load voltage to be a sinusoidal AC voltage. To achieve this objective, the single-phase inverter output voltage u c and the output current i of the single-phase rectifier cSpecific conditions must be met, and these conditions can be obtained using a reverse calculation method. Assume the grid-side current i... g With grid-side voltage u g Fundamental wave in phase, load voltage u L Given a sinusoidal AC voltage, the current i in the secondary β winding of the three-winding step-down transformer is... c The expression is:

[0059]

[0060] Wherein, the load current i L Includes active component i p and reactive and harmonic components i qh The calculation method is as follows:

[0061]

[0062] Therefore, the expected output voltage u of a single-phase inverter cref equal to load voltage u L The voltage u of the secondary α winding of the three-winding step-down transformer α The difference. Its expression is:

[0063] u cref =U o sin(ωt)-u α =U o sin(ωt)-n α u g

[0064] Desired output current i of single-phase rectifier cref Equal to load current i L The reactive and harmonic components that need to be compensated in i qh The current referred to the secondary β winding of the three-winding step-down transformer. Its expression is:

[0065] i cref =-i qh n β / n α .

[0066] The present invention also provides a control method for a single-phase UPQC without a series isolation transformer.

[0067] The block diagram of the single-phase UPQC control method without series isolation transformer proposed in this invention is as follows: Figure 2 As shown.

[0068] The purpose of the control method is to control the primary winding current of the three-winding step-down transformer to be in phase with the fundamental voltage, and the load voltage to be a sinusoidal AC voltage. The control method includes phase-locked loop algorithm, load current detection algorithm, command current generation algorithm, command voltage generation algorithm, inverter voltage control algorithm, rectifier current control algorithm, and capacitor voltage control algorithm.

[0069] The control method includes the following steps:

[0070] S1: The phase-locked loop algorithm locks the grid voltage on the primary or secondary side of the three-winding step-down transformer to obtain the voltage phase reference sin(ωt), which provides a phase reference for the control system.

[0071] S2: The load current detection algorithm provides a basis for generating rectifier current commands by detecting the active, reactive and harmonic components in the load current;

[0072] S3: Command current generation algorithm generates current commands for the rectifier;

[0073] S4: Command voltage generation algorithm generates voltage commands for the inverter;

[0074] S5: The inverter voltage control algorithm is used to achieve closed-loop control of the inverter output voltage;

[0075] S6: The rectifier current control and capacitor voltage control algorithm is used to realize closed-loop control of the rectifier output current and maintain the stability of the DC side capacitor voltage.

[0076] The working principles of the load current detection algorithm, command current generation algorithm, command voltage generation algorithm, inverter voltage control algorithm, rectifier current control algorithm, and capacitor voltage control algorithm will be explained in detail below.

[0077] 1) Load current detection algorithm

[0078] Load current i L Includes active component i p and reactive and harmonic components i qh Its detailed expression can be written as:

[0079]

[0080] To detect the active component, the sampled load current i L Multiplying the active current by the voltage phase reference sin(ωt) provided by the phase-locked loop converts the active current into a DC component, while simultaneously converting the reactive and harmonic currents into AC components. The detailed mathematical expression can be written as follows:

[0081]

[0082]

[0083] The above signal is filtered by a low-pass filter to remove the AC component, and the remaining DC component is multiplied by 2 to obtain the amplitude I of the active current. p This amplitude is multiplied by the voltage phase reference sin(ωt) provided by the phase-locked loop to obtain the load current i. L The active current component i in p Total load current i L Remove active components i p The sum of reactive and harmonic components, i, is obtained. qh The relevant expression can be written as:

[0084]

[0085] 2) Command Current Generation Algorithm

[0086] The sum of reactive and harmonic components i obtained based on the above load current detection algorithm qh Combining the turns ratio (n) of the secondary α winding and β winding of the three-winding step-down transformer α n β This yields the current command value i output by the single-phase rectifier. cref The expression is:

[0087] i cref =-i qh n β / n α .

[0088] 3) Command voltage generation algorithm

[0089] Based on the voltage phase reference sin(ωt) provided by the phase-locked loop and the desired output voltage amplitude U o The output voltage command value u of the single-phase inverter can be obtained. cref for:

[0090] u cref =U o sin(ωt)-u α .

[0091] 4) Inverter voltage control algorithm

[0092] Single-phase inverters require a closed-loop control algorithm to ensure the output voltage follows the command voltage. The inverter output voltage command value is u. cref With the sampled inverter output voltage u c The difference between the two signals is processed by a closed-loop regulator to generate a modulation signal u1. This modulation signal is then compared with a triangular carrier wave to obtain a switching signal g1.

[0093] 5) Rectifier current control and capacitor voltage control algorithms

[0094] A single-phase rectifier requires a closed-loop control algorithm to ensure the output current follows the commanded current while maintaining capacitor voltage stability. The commanded DC-side capacitor voltage u of the rectifier is then applied. dcref The DC-side capacitor voltage sampling value u dc The difference is compared and processed by the closed-loop regulator to generate a fine-tuning active current command Δi. p Fine-tuning the active current component Δi p After being converted to the secondary β winding of a three-winding transformer, the current command value i is added. cref Obtain the final rectifier output current command value. Compare the rectifier output current command value with the sampled rectifier output current i. c The difference is compared and processed by a closed-loop regulator to generate a modulation signal u2. The modulation signal is then compared with a triangular carrier wave to obtain a switching signal g2.

[0095] The feasibility and superiority of the present invention will be demonstrated through simulation of specific embodiments.

[0096] According to the single-phase UPQC topology without series isolation transformer proposed in this invention, a single-phase UPQC is constructed, comprising one three-winding step-down transformer, one filter capacitor, one filter inductor, one single-phase full-bridge inverter circuit, one single-phase full-bridge rectifier circuit, one DC support capacitor, and one reactor. (Refer to...) Figure 1 circuit structure and Figures 2 to 5 The control block diagram is used to construct the circuit and control algorithm.

[0097] Based on the above configuration, a simulation model was built in Matlab / Simulink. The effective value of the fundamental voltage on the grid side is... Simultaneously, 3rd, 5th, and 7th harmonic voltage sources are connected in series to simulate grid-side voltage harmonics, with the load consisting of a resistor and an inductor connected in series. Specific simulation parameters are shown in the table below.

[0098]

[0099]

[0100] Figure 6 , Figure 7 The simulation results of this invention are shown; wherein, Figure 6 Subgraph (a) shows the grid voltage u g Subgraph (b) shows the grid current i g Subgraph (c) shows the load voltage u. L Subgraph (d) shows the load current i L Subgraph (e) shows the DC-side capacitor voltage u. dc ; Figure 7 Sub-diagram (a) shows the inverter command voltage u.cref Subgraph (b) shows the inverter output voltage u. c Sub-diagram (c) shows the rectifier command current i. cref Subgraph (d) shows the rectifier output current i. c .

[0101] Depend on Figure 6 It can be seen that, under the grid voltage u g When waveform distortion exists, harmonic voltage compensation is performed using a single-phase inverter, and the load voltage u L It exhibits a good sinusoidal AC waveform. Reactive current compensation is achieved through a single-phase rectifier, ensuring that the fundamental voltage and current of the grid are in phase.

[0102] Depend on Figure 7 It can be seen that the detected harmonic voltage value and the actual compensation amount are equal, indicating that the inverter has a good compensation effect on grid voltage swells and drops and harmonic components. At the same time, the detected load reactive current value and the actual compensation amount are equal, indicating that the rectifier has a good compensation effect on the fundamental reactive component in the load current.

[0103] After 0.5 seconds, the grid voltage amplitude increased by 10%, but the load voltage remained well maintained near its rated value, demonstrating the model's good dynamic performance. Throughout the process, the DC bus voltage remained stable. Simulation results prove the feasibility and superiority of the proposed single-phase UPQC topology and control method without series isolation transformer.

Claims

1. A control method for a single-phase UPQC topology without a series isolation transformer, wherein the single-phase UPQC topology includes a three-winding step-down transformer, a filter capacitor, a filter inductor, a single-phase inverter, a single-phase rectifier, a DC support capacitor, and a reactor; characterized in that, The two terminals of the primary winding of the three-winding step-down transformer are connected to the power grid, and the two secondary windings... and The four terminals are labeled V, Z, T, and Y. One end of the filter capacitor is connected to the output terminal O1 of the single-phase inverter and also to the load. The other end of the filter capacitor is connected to the output terminal O2 of the single-phase inverter through the filter inductor and also to the secondary side of the three-winding step-down transformer. The V terminals of the winding are connected The Z terminal of the winding is connected to ground; The T terminal of the winding is connected to the output terminal O4 of the single-phase rectifier; The Y terminal of the winding is connected to the output terminal O3 of the single-phase rectifier through a reactor; the single-phase inverter and the single-phase rectifier share a DC support capacitor; The purpose of the control method is to control the primary winding current of the three-winding step-down transformer to be in phase with the fundamental voltage, and the load voltage to be a sinusoidal AC voltage. The control method includes a phase-locked loop algorithm, a load current detection algorithm, a command current generation algorithm, a command voltage generation algorithm, an inverter voltage control algorithm, a rectifier current control algorithm, and a capacitor voltage control algorithm. The control method includes the following steps: S1: The phase-locked loop algorithm locks the grid voltage on the primary or secondary side of the three-winding step-down transformer to obtain the voltage phase reference. This provides a phase reference for the control system; S2: The load current detection algorithm provides a basis for generating rectifier current commands by detecting the active, reactive and harmonic components in the load current; S3: The command current generation algorithm generates the current command for the rectifier; S4: The command voltage generation algorithm generates the voltage command for the inverter; S5: The inverter voltage control algorithm is used to realize closed-loop control of the inverter output voltage; S6: The rectifier current control and capacitor voltage control algorithm is used to realize closed-loop control of the rectifier output current and maintain the stability of the DC side capacitor voltage. The load current detection algorithm is as follows: Load current Contains active ingredients and reactive and harmonic components The calculation method is as follows: ,in, It is the load current. The amplitude of the reactive component of the mid-fundamental wave. It is the load current. The amplitude of the nth harmonic; The measured load current voltage phase reference provided by phase-locked loop Multiplying these components converts the active current into a DC component and the reactive and harmonic currents into AC components, resulting in: in, It is the load current. Middle harmonic current components and The amplitude of the corresponding harmonic components after multiplication; The AC component of the above signal is filtered out by a low-pass filter, and the remaining DC component is multiplied by two to obtain the amplitude of the active current. This amplitude is consistent with the voltage phase reference provided by the phase-locked loop. Multiply to obtain the load current. active current component Total load current Remove active components The sum of reactive and harmonic components is obtained. Furthermore, we obtain: 。 2. The control method for a single-phase UPQC topology without a series isolation transformer according to claim 1, characterized in that, The single-phase inverter is a single-phase full-bridge circuit, and a bypass switch S1 is connected in parallel across the filter capacitor to disconnect a faulty single-phase inverter; the single-phase rectifier is a single-phase full-bridge circuit, and the reactor and... A bypass switch S2 is connected in series at the Y terminal connection of the winding to disconnect a faulty single-phase rectifier.

3. The control method for a single-phase UPQC topology without a series isolation transformer according to claim 1, characterized in that, The power electronic devices in the single-phase inverter and the single-phase rectifier are fully controlled switching devices.

4. The control method for a single-phase UPQC topology without a series isolation transformer according to claim 1, characterized in that, The specific algorithm for generating the command current is as follows: The sum of reactive and harmonic components obtained based on the above load current detection algorithm Combined with the secondary side of the three-winding step-down transformer , The current command value output by the single-phase rectifier can be obtained. for: / 。 5. The control method for a single-phase UPQC topology without a series isolation transformer according to claim 4, characterized in that, The specific algorithm for generating the command voltage is as follows: Based on the voltage phase reference provided by the phase-locked loop and desired output voltage amplitude The output voltage command value of the single-phase inverter can be obtained. for: ,in, For the secondary side of a three-winding step-down transformer Voltage of the winding.

6. The control method for a single-phase UPQC topology without a series isolation transformer according to claim 5, characterized in that, The inverter voltage control algorithm is as follows: The output voltage command value of the single-phase inverter With the sampled single-phase inverter output voltage The sampled values ​​are compared, and the difference is processed by a closed-loop regulator to generate a modulated signal. The switching signal is obtained by comparing the modulated signal with the triangular carrier wave. .

7. The control method for a single-phase UPQC topology without a series isolation transformer according to claim 6, characterized in that, The specific algorithms for rectifier current control and capacitor voltage control are as follows: Command value of DC side capacitor voltage of single-phase rectifier With DC side capacitor voltage The difference is compared and processed by the closed-loop regulator to generate a fine-tuning active current command. Fine-tuning active current command Converted to the secondary side of a three-winding transformer Then, add the current command value. This is the final output current command value of the rectifier, which is the sum of the final output current command value of the rectifier and the sampled rectifier output current. The difference between the two signals is processed by a closed-loop regulator to generate a modulated signal. The modulated signal is compared with the triangular carrier wave to obtain the switching signal. .

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