Transformerless unified power quality conditioner and control method and system thereof

Abstract

According to the transformer-free unified power quality regulator provided by the invention, the alternating current ports of the first AC / DC converter and the second AC / DC converter are respectively connected with the power grid and the load to form a series structure, and the series voltage between the power grid and the load can be regulated by regulating the output voltage of the two AC / DC converters, so that the voltage quality of the load side is managed; the CHB-STATCOM is connected with an alternating current port of the third AC / DC converter, or is directly connected to a negative end of a direct current bus to form a parallel part for providing reactive power support for a load side and enabling a network side to realize unity power factor operation; by adjusting the output voltage of the AC / DC converter, the energy balance of the DC bus is realized. Meanwhile, the invention provides a control method and system of the regulator. The system size is reduced, the system cost is reduced, and the influence of the nonlinear characteristics of the power frequency isolation transformer of which the series connection part is connected to the alternating current power grid on the performance of the regulator is avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics and power quality management, and in particular to a transformerless unified power quality conditioner and a control method and system thereof. Background Art

[0002] With the continuous advancement of science and technology and the modernization process, the gradual depletion of fossil energy and the increasingly severe environmental problems such as the greenhouse effect and global warming have prompted the global society to increasingly tend to develop renewable energy and promote the transformation and upgrading of the energy structure to establish a green and low-carbon energy system. Therefore, green and clean renewable energy has become the key to solving energy and environmental problems. Electric energy, as a safe, fast and efficient form of energy, has become the cornerstone of today's social development. It plays an extremely critical role in our daily life and various industrial production. In the process of moving towards renewable energy, the large-scale application of microgrid technology and distributed energy systems has begun to become popular. Although this change has brought many conveniences, it has brought new problems and challenges to the operation of the power grid and put forward higher requirements for power quality. On the one hand, the power generation power of new energy such as distributed photovoltaic systems has the characteristics of volatility, randomness and intermittency, resulting in a high degree of uncertainty in the voltage of the distribution network; on the other hand, the access of new DC loads, such as emerging DC load demands, especially electric vehicle charging stations, has brought changes to the load characteristics of the distribution network. The rapid charging of electric vehicles requires high power output, which may cause an instantaneous voltage drop in the local power grid, that is, a voltage sag, affecting the operation of other electrical equipment. Since the 1980s, with the advancement of technology, more and more electrical equipment has become more sensitive to power quality. Any minor power quality problem may cause performance degradation or even device failure, which requires the distribution network to have higher adjustment and response capabilities. Therefore, improving the quality of power has become a requirement that cannot be ignored. Optimizing power grid operation and improving power quality are not only related to economic benefits, but also the key to ensuring sustainable development.

[0003] In the future, the unified power quality controller (UPQC) will be of great significance for the management of power quality issues. However, the UPQC used in existing demonstration projects usually needs to be connected to the power grid through a transformer, which has problems such as high cost, bulky size, difficulty in expanding ports, and is not conducive to promotion and application.

[0004] After searching, we found:

[0005] The Chinese invention patent application "Transformerless unified power quality conditioner and modulation method and control method thereof" with publication number CN117937490A provides a topology structure of a transformerless unified power quality conditioner, wherein each phase of the regulator substructure may include: an ANPC three-level converter and an output half-bridge module connected to the ANPC three-level converter. However, the unified power quality conditioner does not have reactive power compensation capability and is limited by the converter topology structure, and is only applicable to low-voltage application scenarios. Summary of the invention

[0006] In view of the above-mentioned deficiencies in the prior art, the present invention provides a transformerless unified power quality conditioner and a control method and system thereof.

[0007] According to one aspect of the present invention, there is provided a transformerless unified power quality conditioner, comprising: a first AC / DC converter, a second AC / DC converter, a third AC / DC converter, a CHB-STATCOM and a filter inductor; wherein:

[0008] The AC ports of the first AC / DC converter and the second AC / DC converter are connected to the power grid and the load respectively, forming a series structure together; by adjusting the output voltages of the two AC / DC converters, the series voltage between the power grid and the load can be adjusted, thereby achieving control of the voltage quality on the load side;

[0009] The third AC / DC converter is connected to the DC bus;

[0010] The CHB-STATCOM is connected to the AC port of the third AC / DC converter, or directly connected to the negative end of the DC bus to form a parallel part, which is used to provide reactive power support for the load side so that the grid side can achieve unity power factor operation; by adjusting the output voltage of the third AC / DC converter, the DC bus energy balance is achieved;

[0011] The filter inductor is connected to the three-phase outlet of the CHB-STATCOM and is used to filter out harmonics in the output current of the CHB-STATCOM.

[0012] According to another aspect of the present invention, there is provided a control method for the transformerless unified power quality conditioner as described in any one of the above-mentioned embodiments of the present invention, comprising the following control strategy:

[0013] A load port voltage control strategy is used to maintain the voltage at a set value at the load port by compensating the series voltage through the first AC / DC converter and the second AC / DC converter when the grid voltage fluctuates;

[0014] Reactive power compensation control strategy based on CHB-STATCOM, which outputs reactive current required by load through CHB-STATCOM to achieve pure active power mode operation on the grid side;

[0015] A DC bus balancing control strategy is used to maintain a constant DC bus voltage by adjusting the output voltage of the third AC / DC converter and interacting with the CHB-STATCOM output current when the CHB-STATCOM is connected to the device through the third AC / DC converter.

[0016] According to a third aspect of the present invention, there is provided a control system of the transformerless unified power quality conditioner as described in any one of the above-mentioned embodiments of the present invention, comprising any one or more of the following control modules:

[0017] A load port voltage control module, which is used to compensate the series voltage through the first AC / DC converter and the second AC / DC converter when the grid voltage fluctuates, so that the voltage of the load port is maintained at a set value;

[0018] Reactive power compensation control module based on CHB-STATCOM, which is used to output the reactive current required by the load through CHB-STATCOM to achieve pure active power mode operation on the grid side;

[0019] A DC bus balancing control module is used to maintain a constant DC bus voltage by adjusting the output voltage of the third AC / DC converter and interacting with the CHB-STATCOM output current when the CHB-STATCOM is connected to the device through the third AC / DC converter.

[0020] Due to the adoption of the above technical solution, the present invention has at least one of the following beneficial effects compared with the prior art:

[0021] The transformerless unified power quality conditioner and its control method and system provided by the present invention have a topology using two AC / DC converters to form a series structure with the power grid side and the load side, so as to achieve voltage quality compensation for the load port, avoid the use of an industrial frequency transformer, reduce the system volume, reduce the system cost, and avoid the influence of the nonlinear characteristics of the industrial frequency isolation transformer at the place where the series part is connected to the AC power grid on the performance of the unified power quality conditioner.

[0022] The transformerless unified power quality conditioner and its control method and system provided by the present invention, wherein the parallel part is a static synchronous compensator, can provide reactive power support for the system, maintain pure active operation on the grid side, and also provide system-level voltage support for the load.

[0023] The transformerless unified power quality conditioner and its control method and system provided by the present invention can realize flexible and low-cost port expansion; by adding an AC / DC converter to the DC bus, the AC grid interface and the load interface can be expanded, and finally a multi-port series-parallel transformerless unified power quality conditioner structure is formed, thereby performing power quality management on multiple loads.

[0024] The transformerless unified power quality conditioner and control method and system provided by the present invention can be applied to scenarios with different voltage levels. By changing the topology of the AC / DC converter, power quality compensation can be performed on power grids with different voltage levels. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0026] Figure 1 Schematic diagram of the topological structure of a transformerless unified power quality conditioner in a preferred embodiment of the present invention.

[0027] Figure 2 A block diagram of a voltage control strategy for a fixed load port of a transformerless unified power quality conditioner in a preferred embodiment of the present invention;

[0028] Figure 3 This is a block diagram of the voltage outer loop control strategy of a cascaded H-bridge static synchronous compensator in a preferred embodiment of the present invention.

[0029] Figure 4 This is a block diagram of the current inner loop control strategy of a cascaded H-bridge static synchronous compensator in a preferred embodiment of the present invention.

[0030] Figure 5 This is a block diagram of a DC bus voltage control strategy in a preferred embodiment of the present invention.

[0031] Figure 6 The figure is a sample simulation topology diagram of a transformerless unified power quality conditioner in a specific application example of the present invention.

[0032] Figure 7 The following are grid and load voltage waveforms in a specific application example of the present invention; wherein (a) is the grid side voltage waveform, and (b) is the load side voltage waveform.

[0033] Figure 8 The figure is a capacitor voltage waveform diagram of a submodule of a cascaded H-bridge static synchronous compensator in a specific application example of the present invention.

[0034] Fig. 9 The figure is a DC bus voltage waveform diagram in a specific application example of the present invention.

[0035] Fig.10 This is a voltage and current waveform diagram on the grid side in another specific application example of the present invention.

[0036] Fig.11 is the grid side active power P in another specific application example of the present invention source , reactive power Q source , load side active power P load , reactive power Q load , STATCOM output active power P STATCOM , output reactive power Q STATCOM Graph. DETAILED DESCRIPTION

[0037] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements may be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0038] An embodiment of the present invention provides a transformerless unified power quality conditioner, which is a transformerless unified power quality conditioner (TL-UPQC) embedded in a cascaded H-bridge static synchronous compensator (CHB-STATCOM). It adopts a series-parallel structure and has the same performance as a conventional UPQC, but omits the series transformer, aiming to reduce the system volume and system cost, and avoids the influence of the nonlinear characteristics of the power frequency isolation transformer at the place where the series part is connected to the AC power grid on the UPQC performance, thereby realizing an economical, convenient, high-power-density, efficient and feasible UPQC solution.

[0039] Specifically, as shown in FIG. 1 , the transformerless unified power quality conditioner provided in this embodiment may include: a first AC / DC converter, a second AC / DC converter, a third AC / DC converter, a CHB-STATCOM and a filter inductor; wherein:

[0040] The AC ports of the first AC / DC converter and the second AC / DC converter are connected to the power grid and the load respectively, forming a series structure together; by adjusting the output voltages of the two AC / DC converters, the series voltage between the power grid and the load can be adjusted, thereby achieving control of the voltage quality on the load side;

[0041] The third AC / DC converter is connected to the DC bus;

[0042] The CHB-STATCOM is connected to the AC port of the third AC / DC converter, or directly connected to the negative end of the DC bus to form a parallel part, which is used to provide reactive power support for the load side so that the grid side can achieve unity power factor operation; by adjusting the output voltage of the third AC / DC converter, the DC bus energy balance is achieved;

[0043] The filter inductor is connected to the three-phase outlet of CHB-STATCOM and is used to filter out harmonics in the output current of CHB-STATCOM.

[0044] In some preferred embodiments, the first AC / DC converter, the second AC / DC converter and the third AC / DC converter form a back-to-back structure based on the DC bus. The required bus voltage level can be designed and a suitable topology can be selected based on the size of the adjustable series voltage and the device level required in the actual application scenario.

[0045] In some preferred embodiments, the first AC / DC converter, the second AC / DC converter and / or the third AC / DC converter may adopt a half-bridge structure, a three-level structure or a modular multi-level structure.

[0046] In some preferred embodiments, the CHB-STATCOM is composed of a cascade of full-bridge sub-modules.

[0047] Based on the transformerless unified power quality conditioner provided in the above-mentioned embodiment of the present invention, an embodiment of the present invention further provides a control method for the transformerless unified power quality conditioner.

[0048] Specifically, the control method of the transformerless unified power quality conditioner provided in this embodiment may include the following multiple control strategies:

[0049] A load port voltage control strategy is used to maintain the voltage at the load port at a set value by compensating the series voltage through the first AC / DC converter and the second AC / DC converter when the grid voltage fluctuates. Figure 2 As shown;

[0050] Based on the reactive power compensation control strategy of CHB-STATCOM, this strategy realizes the pure active power mode operation on the grid side by outputting the reactive current required by the load through CHB-STATCOM, such as Figure 3 and Figure 4 As shown;

[0051] The DC bus balancing control strategy is used to maintain the DC bus voltage constant by adjusting the output voltage of the third AC / DC converter and interacting with the output current of the CHB-STATCOM when the CHB-STATCOM is connected to the device through the third AC / DC converter, such as Figure 5 shown.

[0052] In some preferred embodiments, Figure 2 As shown, the load port voltage control strategy may further include:

[0053] In the synchronous rotating coordinate system, the voltage outer loop provides a fixed load port voltage reference value, and the current reference value is obtained through the PI controller;

[0054] The actual current tracks the reference value of the current, and the actual current is controlled without net error through the PI controller; then, by adding current cross-decoupling feedforward, the actual current is controlled by dq decoupling; by adding grid voltage feedforward, the disturbance effect of grid node voltage is eliminated and the dynamic response performance is improved, and finally the series voltage reference value is obtained;

[0055] In order to make full use of the modulation degree of the first AC / DC converter and the second AC / DC converter, that is, the series voltage is evenly divided by the two, the output voltage v of the two AC / DC converters is s1 and v s2 The following relations are satisfied:

[0056] v s1 =-v s2 (1).

[0057] In some preferred embodiments, the CHB-STATCOM may further include:

[0058] The CHB-STATCOM in TL-UPQC can be composed of a cascade of full-bridge sub-modules. Compared with the half-bridge sub-modules, the full-bridge sub-modules have more output voltage levels and can output '0', 'V c ' and '-V c Therefore, the CHB-STATCOM output voltage does not need to contain a DC component.

[0059] In order to achieve more output voltage levels and improve harmonic characteristics, CHB-STATCOM adopts carrier phase shifted pulse width modulation (CPS-PWM), where the modulation voltage of each phase submodule is the same, and the carrier phase differs by 2π / N, where N is the number of submodules per phase of CHB-STATCOM. Since the energy balance principle of CHB-STATCOM is the same for the three phases, the subsequent analysis is based on phase A.

[0060] For the energy balance of the A-phase submodule, let the overall output voltage of CHB-STATCOM be v ST (t), its expression is:

[0061]

[0062] Among them, V ST is the output voltage amplitude of CHB-STATCOM, ω is the grid voltage angular frequency, t is the system time, θ ST is the initial phase of the CHB-STATCOM output voltage;

[0063] Therefore, the switching function S of the submodule output voltage ck (t) can be expressed as:

[0064]

[0065] in, is the DC capacitor voltage reference value of each full-bridge submodule; further, the current i flowing into the submodule capacitor can be obtained ck (t) is:

[0066]

[0067] Among them, i p (t) is the instantaneous value of CHB-STATCOM output current, I p is the output current amplitude of CHB-STATCOM, ρ p It is the initial phase of CHB-STATCOM output current;

[0068] Formula (4) shows that the submodule capacitor current of CHB-STATCOM contains a DC component and a double frequency component. Similar to the DC bus capacitor, to maintain the energy balance of the CHB-STATCOM submodule, the DC component in the submodule capacitor current is 0, that is:

[0069]

[0070] Formula (5) can also be rewritten as:

[0071]

[0072] The energy balance of the B-phase submodule and the C-phase submodule is the same as that of the A-phase submodule.

[0073] Therefore, ignoring the loss, by adjusting the phase of the CHB-STATCOM output voltage to be perpendicular to the output current, the overall energy balance of the CHB-STATCOM can be achieved. In addition, there are differences in parameters such as sub-module capacitance, switching devices, and driver chips, which makes the capacitance and voltage of each sub-module unbalanced. Therefore, each sub-module should be considered separately, and the voltage modulation ratio of each sub-module should be slightly adjusted by balancing control to further achieve their own energy balance.

[0074] In some preferred embodiments, Figure 3 and Figure 4 As shown, the reactive power compensation control strategy based on CHB-STATCOM can further include:

[0075] Taking the reactive component of the load current as the reference value, CHB-STATCOM can realize real-time and accurate compensation of the reactive power control on the grid side. Therefore, the q-axis reference value of the CHB-STATCOM current is for:

[0076]

[0077] Among them, i lq is the q-axis component of the load-side current;

[0078] When the internal losses of CHB-STATCOM are ignored, the output current of CHB-STATCOM only has q-axis component, that is, it only interacts with the TL-UPQC system for reactive power. In actual situations, the internal losses of CHB-STATCOM cannot be ignored, but a small amount of active power can be drawn from the AC grid by adjusting the d-axis component of the CHB-STATCOM output current to maintain the energy balance inside the CHB-STATCOM. Therefore, this strategy adopts the overall DC voltage control strategy of CHB-STATCOM, with the capacitor voltage of CHB-STATCOM as the control target, that is, the voltage outer loop, to track the sum of the capacitor voltages of the sub-modules, and obtain the d-axis current reference value through the PI controller; Figure 3 As shown. Where N is the number of sub-modules per phase, Set the capacitor voltage value for each submodule, v ckj represents the capacitor voltage of the j-th phase and the k-th submodule;

[0079] like Figure 3 As shown in Figure 1, it is the voltage outer loop of CHB-STATCOM, with the capacitor voltage of CHB-STATCOM as the control target. Figure 3The current reference values ​​of the d-axis and q-axis of the CHB-STATCOM are given. The output current is closed-loop controlled by the PI controller to obtain the current inner loop of the CHB-STATCOM. The control block diagram is shown in Figure 4 shown. Figure 4 In the control loop, a voltage feedforward compensation term v is added pd , to offset the influence of voltage disturbance at the connection point between CHB-STATCOM and the third AC / DC converter; and is the cross decoupling term, which realizes the output current control decoupling in the dq coordinate system. The reference values ​​of the CHB-STATCOM output voltage d-axis and q-axis are obtained. and Then, the inverse matrix of Park transformation is used to transform the dq / abc coordinate system, and the reference value of the CHB-STATCOM output voltage in the abc coordinate system can be obtained as follows: and The output voltage reference value of each submodule is 1 / N of the total output voltage reference value, that is:

[0080]

[0081] in, is the output voltage reference value of the submodule of the i-phase STATCOM, is the output voltage reference value of the i-phase STATCOM

[0082] In some preferred implementations, the DC bus balancing may further include:

[0083] In TL-UPQC, the third AC / DC converter connected to the DC bus adopts SPWM modulation, ignoring the switching subharmonic components, so its output voltage contains DC and AC components. In order to make full use of the modulation capability of the third AC / DC converter, the DC component is generally half of the DC bus voltage. If only the base frequency AC component is considered, the output voltages of the first AC / DC converter and the second AC / DC converter as voltage regulation modules and the third AC / DC converter as power balancing module can be expressed as:

[0084]

[0085] Among them, v s1 (t) and v s2 (t) are the output voltages of the first AC / DC converter and the second AC / DC converter connected to the power grid and the load, respectively, that is, the output voltage of the voltage regulation module; v sp(t) is the output voltage of the third AC / DC converter connected to CHB-STATCOM, i.e., the output voltage of the power balancing module; is the DC component of the output voltage, is the rated voltage of the DC bus; V s1 、V s2 and V sp is the effective value of the AC component of the output voltage, θ1, θ2 and θ sp is the phase angle of the AC component of the output voltage;

[0086] Combining formula (1), the average switching function of the voltage regulation module and the power balancing module is obtained as follows:

[0087]

[0088] Among them, s s1 (t), s s2 (t) is the average switching function of the two voltage regulation modules, s sp (t) is the average switching function of the power balancing module;

[0089] Grid side current i s (t), load side current i l (t) and CHB-STATCOM output current i p The time domain expression of (t) is:

[0090]

[0091] Among them, I s ,I l and I p are the amplitudes of grid-side current, load-side current and CHB-STATCOM output current, ρ s , l and ρ p They are the initial phases of the grid-side current, load-side current and CHB-STATCOM output current respectively;

[0092] The current flowing into the DC side capacitor of the third AC / DC converter is equal to the current at its output port multiplied by the average switching function. Since the DC bus provides a power interaction channel for the third AC / DC converter, the current flowing into the DC bus is equal to the sum of the currents flowing from the three AC / DC converters, that is:

[0093]

[0094] Formula (11) shows that the current flowing into the DC bus consists of a double frequency component and a DC component. In order to keep the DC bus voltage constant, the DC component in the current in Formula (11) must be zero, that is:

[0095]

[0096] Formula (12) is the DC bus energy balance constraint. According to this constraint, the output voltage effective value V of the balancing module can be adjusted sp or phase θ sp This constraint condition is established to achieve DC bus energy balance. In order to obtain the strongest power regulation capability and maximize the use of the DC bus voltage, the output voltage phase of the balancing module can be made consistent with the output current phase of the CHB-STATCOM, that is,

[0097] θ sp =ρ p (13)

[0098] At this time, the DC bus energy balance condition is:

[0099]

[0100] In some preferred embodiments, Figure 5 As shown, the DC bus balancing control strategy may further include:

[0101] First, the DC bus voltage is controlled overall, and the deviation between the actual voltage value and the reference value is input into the PI regulator to obtain the output voltage reference value of the balancing module. At the same time, referring to the interphase capacitor voltage balancing control method of CHB-STATCOM, a zero-sequence voltage is superimposed on the power balancing module to interact with the output current of CHB-STATCOM to achieve power transfer between the three phases, thereby achieving DC bus interphase voltage balancing. Among them, the interphase capacitor voltage balancing control method is to achieve interphase balancing of the DC bus and maintain a constant DC bus voltage; after maintaining the bus voltage stable, constant voltage control is performed.

[0102] It should be noted that since the current of CHB-STATCOM mainly contains q-axis components and the d-axis components are very small, the phase of the output voltage of the power balancing module is consistent with the q-axis direction of CHB-STATCOM. sp The phase of the parallel branch current is p The current direction of the current actually flowing into the DC bus is opposite to the positive direction of the parallel branch current. Therefore, the current direction of the output voltage q-axis component and the zero-sequence component of the power balancing module multiplied by the zero-sequence component is -sign(ipq), where i pq is the q-axis component of the CHB-STATCOM output current.

[0103] Based on the same inventive concept, an embodiment of the present invention further provides a control system for a transformerless unified power quality conditioner.

[0104] Specifically, the control system of the transformerless unified power quality conditioner provided in this embodiment may include any one or more of the following control modules:

[0105] A load port voltage control module, which is used to compensate the series voltage through the first AC / DC converter and the second AC / DC converter when the grid voltage fluctuates, so that the voltage of the load port is maintained at a set value;

[0106] Reactive power compensation control module based on CHB-STATCOM, which is used to output the reactive current required by the load through CHB-STATCOM to achieve pure active power mode operation on the grid side;

[0107] A DC bus balancing control module is used to maintain a constant DC bus voltage by adjusting the output voltage of the third AC / DC converter and interacting with the CHB-STATCOM output current when the CHB-STATCOM is connected to the device through the third AC / DC converter.

[0108] It should be noted that the steps in the control method provided by the present invention can be implemented using corresponding modules in the control system, and those skilled in the art can refer to the technical scheme of the control method to implement the composition of the control system, that is, the embodiments in the control method can be understood as preferred examples for constructing a control system, which will not be elaborated here.

[0109] The transformerless unified power quality conditioner and its control method and system provided by the above-mentioned embodiment of the present invention realize constant voltage control on the load side by controlling the output voltage of the AC / DC converter connected to the power grid and the load, so that the power quality of the load can be improved when the power grid voltage fluctuates, and the optimized operation of the power grid can be realized. At the same time, reactive power compensation is provided for the load, so that the grid side can realize unity power factor operation. According to the size of the adjustable series voltage and the device level required in the actual application scenario, the AC / DC converters of the series circuit and the parallel circuit part can adopt different topological structures, such as half-bridge, three-level, modular multi-level structure, etc.

[0110] The technical solution provided by the above embodiment of the present invention is further described in detail below in conjunction with specific application examples.

[0111] Specific application example 1

[0112] When the AC / DC converter is a half-bridge (HB) structure, the topology of the TL-UPQC is as follows: Figure 6 As shown, Figure 6In the embodiment, the AC ports of the first and second half-bridge converters are connected to the grid side and the load side respectively, the AC port of the third half-bridge converter is connected to the CHB-STATCOM, and the three half-bridge converters are a back-to-back structure with a common DC bus.

[0113] The topology is simulated and verified using MATLAB / Simulink software, and the simulation parameters are shown in the following table.

[0114]

[0115]

[0116] Before the simulation time t = 0.8s, the grid voltage temporarily drops to 0.8pu, t = 0.8-1.4s, the grid voltage is 1.0pu, t = 1.4-2.0s, the grid voltage temporarily rises to 1.2pu. Under this simulation parameter, the theoretical result is: the load side grid voltage is always maintained at 8165V.

[0117] Figure 7 (a) and (b) are the grid voltage and load voltage output curves in the specific application example 1, where: during the simulation time, the load side grid voltage is always maintained at around 8165V.

[0118] Figure 8 is the capacitor voltage of the CHB-STATCOM submodule in the specific application example 1, wherein: during the simulation time, the capacitor voltage of the STATCOM submodule is always maintained at about 800V.

[0119] Fig. 9 is the DC bus voltage in the specific application example 1, wherein: during the simulation time, the DC bus voltage is always maintained at about 3000V.

[0120] Figure 7 to Figure 9 The simulation results shown are consistent with the theoretical results, so the designed load voltage controller can achieve fast compensation of the load voltage side voltage.

[0121] Specific application example 2

[0122] When the AC / DC converter is a half-bridge (HB) structure, the topology of the TL-UPQC is as follows: Figure 6 The topology is simulated and verified using MATLAB / Simulink software, and the simulation parameters are shown in the following table.

[0123]

[0124] Under the working conditions shown in the table above, the simulation time t = 0.2 ~ 2.0s, the given load is P load=0.8pu;Q load =0.6pu resistive inductive load. Under this simulation parameter, the theoretical result is: the power grid only provides active power for the load.

[0125] Fig.10 This is the grid voltage and current output curve in the specific application example 2, where: during the simulation time, the grid voltage fundamental wave is in phase with the grid current.

[0126] Fig.11 is the active power P on the grid side in the specific application example 2 source , reactive power Q source , load side active power P load , reactive power Q load , STATCOM output active power P sTATCOM , output reactive power Q STATCOM Curve, among which: during the simulation time, the active power of the power grid has been stable at about 0.8pu, and the reactive power has been stable at about 0; the active power of the load has been stable at about 0.8pu, and the reactive power has been stable at about 0.6pu; the active power of STATCOM has been stable at about 0, and the reactive power has been stable at about 0.6pu.

[0127] Fig.10 and Fig.11 The simulation results shown are consistent with the theoretical results. Therefore, the designed STATCOM controller can be used to compensate for reactive current and achieve the same phase between the grid voltage fundamental and the grid current.

[0128] The transformerless unified power quality conditioner provided by the above embodiment of the present invention has a first AC / DC converter and a second AC / DC converter as two back-to-back AC / DC converters based on a low-voltage DC bus, and the AC ports of the two AC / DC converters are respectively connected to the AC power grid and the load to form a series structure between feeders; the cascaded H-bridge static synchronous compensator is connected through the AC port of the third AC / DC converter connected to the low-voltage DC bus as a parallel part. In addition, the CHB-STATCOM can be directly connected to the negative end of the DC bus, and the DC bus energy balance is achieved by adjusting the output voltage of the AC / DC converter connected to the AC power grid.

[0129] The control method and system of the transformerless unified power quality conditioner provided by the above-mentioned embodiment of the present invention, which is based on the fixed load port voltage control strategy of series voltage compensation, can realize the control of the load port voltage by adjusting the output voltage of the converter connected to the power grid and the load; its reactive power compensation control strategy based on CHB-STATCOM, the parallel part of the CHB-STATCOM injects the reactive current required by the load to provide reactive power compensation for the system. Its low-voltage DC bus energy balance control strategy maintains the energy balance of the low-voltage DC bus by adjusting the output voltage of the AC / DC converter connected to the CHB-STATCOM and interacting with the current of the CHB-STATCOM.

[0130] The transformerless unified power quality conditioner and its control method and system provided by the above-mentioned embodiments of the present invention avoid the use of the industrial frequency isolation transformer required by the conventional solution by using AC / DC converter and CHB-STATCOM, significantly reduce the cost, and avoid the influence of the nonlinear characteristics of the isolation transformer on the UPQC performance.

[0131] The transformerless unified power quality conditioner and its control method and system provided by the above-mentioned embodiments of the present invention have a topology using two AC / DC converters, which form a series structure with the grid side and the load side to achieve voltage quality compensation for the load port, avoid the use of power frequency transformers, reduce the system volume, reduce the system cost, and avoid the influence of the nonlinear characteristics of the power frequency isolation transformer where the series part is connected to the AC power grid on the performance of the unified power quality conditioner; the parallel part is a static synchronous compensator, which can provide reactive support for the system, maintain pure active operation on the grid side, and also provide system-level voltage support for the load; flexible and low-cost port expansion can be achieved; by adding an AC / DC converter to the DC bus, the AC grid interface and the load interface can be expanded, and finally a multi-port series-parallel transformerless unified power quality conditioner structure is formed, thereby performing power quality management on multiple loads.

[0132] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A transformerless unified power quality conditioner, It is characterized in that include: A first AC / DC converter, a second AC / DC converter, a third AC / DC converter, a CHB-STATCOM and a filter inductor; wherein: The AC ports of the first AC / DC converter and the second AC / DC converter are connected to the power grid and the load respectively, forming a series structure together; by adjusting the output voltages of the two AC / DC converters, the series voltage between the power grid and the load can be adjusted, thereby achieving control of the voltage quality on the load side; The third AC / DC converter is connected to the DC bus; The CHB-STATCOM is connected to the AC port of the third AC / DC converter, or directly connected to the negative end of the DC bus to form a parallel part, which is used to provide reactive power support for the load side so that the grid side can achieve unity power factor operation; by adjusting the output voltage of the third AC / DC converter, the DC bus energy balance is achieved; The filter inductor is connected to the three-phase outlet of the CHB-STATCOM and is used to filter out harmonics in the output current of the CHB-STATCOM.

2. The transformerless unified power quality conditioner according to claim 1, It is characterized in that The first AC / DC converter, the second AC / DC converter and the third AC / DC converter form a back-to-back structure based on a DC bus.

3. The transformerless unified power quality conditioner according to claim 1, It is characterized in that The first AC / DC converter, the second AC / DC converter and / or the third AC / DC converter adopt a half-bridge structure, a three-level structure or a modular multi-level structure.

4. The transformerless unified power quality conditioner according to claim 1, It is characterized in that The CHB-STATCOM is composed of cascaded full-bridge submodules.

5. A control method for a transformerless unified power quality conditioner according to any one of claims 1 to 4, It is characterized in that The control strategies include: A load port voltage control strategy is used to maintain the voltage at a set value at the load port by compensating the series voltage through the first AC / DC converter and the second AC / DC converter when the grid voltage fluctuates; Reactive power compensation control strategy based on CHB-STATCOM, which outputs reactive current required by load through CHB-STATCOM to achieve pure active power mode operation on the grid side; A DC bus balancing control strategy is used to maintain a constant DC bus voltage by adjusting the output voltage of the third AC / DC converter and interacting with the CHB-STATCOM output current when the CHB-STATCOM is connected to the device through the third AC / DC converter.

6. The control method of the transformerless unified power quality conditioner according to claim 5, It is characterized in that The fixed load port voltage control strategy includes: In the synchronous rotating coordinate system, the voltage outer loop provides a fixed load port voltage reference value, and the current reference value is obtained through the PI controller; The actual current tracks the reference value of the current, and the actual current is controlled without net error through the PI controller; then, by adding current cross-decoupling feedforward, the actual current is controlled by dq decoupling; by adding grid voltage feedforward, the disturbance effect of grid node voltage is eliminated and the dynamic response performance is improved, and finally the series voltage reference value is obtained; The series voltage reference value is evenly divided by the first AC / DC converter and the second AC / DC converter, and the output voltage v of the two AC / DC converters is s1 and v s2 The following relations are satisfied: v s1 =-v s2 (1)。 7. The control method of the transformerless unified power quality conditioner according to claim 5, It is characterized in that The CHB-STATCOM comprises: The CHB-STATCOM adopts a carrier phase shift modulation method, and its single-phase submodules have the same modulation voltage, and the carrier phase differs by 2π / N, where N is the number of submodules per phase of the CHB-STATCOM; For the energy balance of the A-phase submodule, let the output voltage of CHB-STATCOM be v ST (t), its expression is: Among them, V ST is the output voltage amplitude of CHB-STATCOM, ω is the grid voltage angular frequency, t is the system time, θ ST is the initial phase of the CHB-STATCOM output voltage; Therefore, the switching function S of the submodule output voltage ck (t) is expressed as: in, is the DC capacitor voltage reference value of each full-bridge submodule; further, the current i flowing into the submodule capacitor is obtained ck (t) is: Among them, i p (t) is the instantaneous value of CHB-STATCOM output current, I p is the output current amplitude of CHB-STATCOM, ρ p It is the initial phase of CHB-STATCOM output current; Formula (4) shows that the submodule capacitor current of CHB-STATCOM contains a DC component and a double frequency component. To maintain the energy balance of the CHB-STATCOM submodule, the DC component in the submodule capacitor current is 0, that is: The energy balance of the B-phase submodule and the C-phase submodule is the same as that of the A-phase submodule.

8. The control method of the transformerless unified power quality conditioner according to claim 7, It is characterized in that Also includes: Rewrite equation (5) as:

9. The control method of the transformerless unified power quality conditioner according to claim 5, It is characterized in that The reactive power compensation control strategy based on CHB-STATCOM includes: Taking the reactive component of the load current as the reference value, CHB-STATCOM realizes real-time and accurate compensation of the reactive power control on the grid side. Therefore, the q-axis reference value of the CHB-STATCOM current is for: Among them, i lq is the q-axis component of the load-side current; By adjusting the d-axis component of the CHB-STATCOM output current, a small amount of active power is drawn from the AC grid to maintain the energy balance inside the CHB-STATCOM.

10. The control method of the transformerless unified power quality conditioner according to claim 9, It is characterized in that The method of adjusting the d-axis component of the CHB-STATCOM output current to extract a small amount of active power from the AC power grid to maintain the energy balance inside the CHB-STATCOM includes: The overall DC voltage control strategy of CHB-STATCOM is adopted, and the capacitor voltage of CHB-STATCOM is taken as the control target, that is, the voltage outer loop is adopted to track the sum of the capacitor voltages of the submodules, and the d-axis current reference value is obtained through the PI controller; After obtaining the current reference values ​​of the d-axis and q-axis of the CHB-STATCOM, the output current is closed-loop controlled through the PI controller to obtain the current inner loop of the CHB-STATCOM; Add voltage feedforward compensation term v in the control loop pd , in order to offset the influence of the voltage disturbance at the connection point between CHB-STATCOM and the third AC / DC converter, the output current control in the dq coordinate system is decoupled through the cross decoupling term, and the reference values ​​of the d-axis and q-axis of the output voltage of CHB-STATCOM are obtained. and Using the inverse matrix of Park transformation, the reference values ​​of the d-axis and q-axis of the output voltage of CHB-STATCOM are and After the dq / abc coordinate system conversion, the reference value of the output voltage of CHB-STATCOM in the abc coordinate system is obtained: and The output voltage reference value of each submodule is 1 / N of the total output voltage reference value, that is: in, is the output voltage reference value of the submodule of the i-phase STATCOM, is the output voltage reference value of the i-phase STATCOM, and N is the number of sub-modules per phase.

11. The control method of the transformerless unified power quality conditioner according to claim 5, It is characterized in that The DC bus balancing includes: Assuming that the third AC / DC converter adopts SPWM modulation, ignoring the switching subharmonic components, its output voltage includes a DC component and an AC component, wherein the DC component is half of the DC bus voltage, then the output voltages of the first AC / DC converter and the second AC / DC converter as the voltage regulation module and the third AC / DC converter as the power balancing module are expressed as: Among them, v s1 (t) and v s2 (t) are the output voltages of the first AC / DC converter and the second AC / DC converter connected to the power grid and the load, respectively, that is, the output voltage of the voltage regulation module; v sp (t) is the output voltage of the third AC / DC converter connected to CHB-STATCOM, i.e., the output voltage of the power balancing module; is the DC component of the output voltage, is the rated voltage of the DC bus; V s1 、V s2 and V sp is the effective value of the AC component of the output voltage, θ 1 ,θ 2 and θ sp is the phase angle of the AC component of the output voltage; Combined with the relationship between the output voltages of the first AC / DC converter and the second AC / DC converter, s1 =-v s2 , the average switching function of the voltage regulation module and the power balancing module is obtained as: Among them, s s1 (t), s s2 (t) is the average switching function of the two voltage regulation modules, s sp (t) is the average switching function of the power balancing module; Grid side current i s (t), load side current i l (t) and CHB-STATCOM output current i p The time domain expression of (t) is: Among them, I s ,I l and I p are the amplitudes of grid-side current, load-side current and CHB-STATCOM output current, ρ s , l and ρ p They are the initial phases of the grid-side current, load-side current and CHB-STATCOM output current respectively; The current flowing into the DC side capacitor of the third AC / DC converter is equal to the current at its output port multiplied by the average switching function; since the DC bus provides a power interaction channel for the third AC / DC converter, the current flowing into the DC bus is equal to the sum of the currents flowing from the three AC / DC converters, that is: Formula (11) shows that the current flowing into the DC bus consists of a double frequency component and a DC component. In order to keep the DC bus voltage constant, the DC component in the current of Formula (11) must be zero, that is: Formula (12) is the DC bus energy balance constraint condition. According to this constraint condition, by adjusting the output voltage effective value V sp or phase θ sp This constraint condition is established to achieve DC bus energy balance; Make the output voltage phase of the balancing module consistent with the output current phase of the CHB-STATCOM, that is: i sp =ρ p (13) At this time, the DC bus energy balance condition is:

12. The control method of the transformerless unified power quality conditioner according to claim 11, It is characterized in that The DC bus balancing control strategy includes: The DC bus voltage is controlled overall, and the deviation between the actual voltage value and the reference value is input into the PI regulator to obtain the output voltage reference value of the balancing module; Referring to the interphase capacitor voltage balancing control method of CHB-STATCOM, a zero-sequence voltage is superimposed on the power balancing module to interact with the output current of CHB-STATCOM to achieve power transfer between the three phases, thereby achieving DC bus interphase voltage balancing.

13. The control method of the transformerless unified power quality conditioner according to claim 12, It is characterized in that The phase of the output voltage of the balancing module is consistent with the q-axis direction of the CHB-STATCOM.

14. The control method of the transformerless unified power quality conditioner according to claim 12, It is characterized in that The direction of the current multiplied by the q-axis component and the zero-sequence component of the output voltage of the power balancing module is -sign (i pq ), where i pq is the q-axis component of the CHB-STATCOM output current.

15. A control system for a transformerless unified power quality conditioner according to any one of claims 1 to 4, It is characterized in that Includes any one or more of the following control modules: A load port voltage control module, which is used to compensate the series voltage through the first AC / DC converter and the second AC / DC converter when the grid voltage fluctuates, so that the voltage of the load port is maintained at a set value; Reactive power compensation control module based on CHB-STATCOM, which is used to output the reactive current required by the load through CHB-STATCOM to achieve pure active power mode operation on the grid side; A DC bus balancing control module is used to maintain a constant DC bus voltage by adjusting the output voltage of the third AC / DC converter and interacting with the CHB-STATCOM output current when the CHB-STATCOM is connected to the device through the third AC / DC converter.

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