Transformerless unified power quality conditioner and control method and system thereof

Abstract

The application provides a transformerless unified power quality conditioner, AC ports of first and second AC / DC converters are connected with a power grid and a load respectively, a series structure is formed, by adjusting output voltages of the two AC / DC converters, series voltages between the power grid and the load can be adjusted, so that voltage quality on the load side can be managed; a CHB-STATCOM is connected with an AC port of a third AC / DC converter, or is directly connected with a negative end of a DC bus, a parallel part is formed, for providing reactive power support for the load side, so that the grid side can realize unit power factor operation; by adjusting the output voltage of the AC / DC converter, DC bus energy balance can be realized. The application also provides a control method and system of the conditioner. The application reduces system volume and cost, and avoids the influence of the nonlinear characteristics of a power frequency isolation transformer connected at the AC grid side of the series part on the performance of the conditioner.

Description

Technical Field

[0001] This invention relates to the fields of power electronics and power quality management technology, specifically to a transformerless unified power quality regulator and its control method and system. Background Technology

[0002] With the continuous advancement of science and technology and modernization, the gradual depletion of fossil fuels and the increasingly severe environmental problems such as the greenhouse effect and global warming have prompted the global society to increasingly favor the development of renewable energy and promote the transformation and upgrading of the energy structure in order to establish a green and low-carbon energy system. Therefore, green and clean renewable energy has become key to solving energy and environmental problems. Electricity, as a safe, fast, and efficient form of energy, has become the cornerstone of modern social development. It plays a crucial role in our daily lives and various industrial production processes. In the process of moving towards renewable energy, the large-scale application of microgrid technology and distributed energy systems has become widespread. While this transformation has brought many conveniences, it has also brought new problems and challenges to the operation of the power grid and placed higher demands on power quality. On the one hand, the power generation of new energy sources, such as distributed photovoltaic systems, exhibits characteristics such as volatility, randomness, and intermittency, leading to high 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 changed the load characteristics of the distribution network. The fast charging of electric vehicles requires high power output, which may cause instantaneous voltage drops in local power grids, i.e., voltage sags, affecting the operation of other electrical equipment. Since the 1980s, with technological advancements, more and more electrical devices have become increasingly sensitive to power quality. Even minor power quality issues can lead to performance degradation or even equipment malfunction, demanding that distribution networks possess enhanced adjustment and response capabilities. Therefore, improving power quality has become an indispensable requirement. Optimizing grid operation and improving power quality are not only crucial for economic benefits but also key to ensuring sustainable development.

[0003] In the future, the Unified Power Quality Controller (UPQC) will be of great significance for addressing power quality issues. However, the UPQCs used in existing demonstration projects usually need to be connected to the power grid via transformers, which results in high costs, bulky size, difficulty in expanding ports, and hinders widespread application.

[0004] The search revealed:

[0005] Chinese invention patent application CN117937490A, entitled "Transformerless Unified Power Quality Regulator and its Modulation and Control Methods," provides a topology for a transformerless unified power quality regulator. 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, this unified power quality regulator lacks reactive power compensation capability and, limited by the converter topology, is only suitable for low-voltage applications. Summary of the Invention

[0006] To address the aforementioned shortcomings in the prior art, this invention provides a transformerless unified power quality regulator, its control method, and a system thereof.

[0007] According to one aspect of the present invention, a transformerless unified power quality conditioner is provided, 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. By adjusting the output voltage of these two AC / DC converters, the series voltage between the power grid and the load can be adjusted, thereby improving 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 terminal of the DC bus to form a parallel section, which is used to provide reactive power support to 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, a control method for the transformerless unified power quality regulator described in any one of the above-mentioned embodiments is provided, comprising the following control strategy:

[0013] A constant load port voltage control strategy is used to compensate for the series voltage through the first AC / DC converter and the second AC / DC converter when the grid voltage fluctuates, so that the voltage at the load port is maintained at a set value.

[0014] The reactive power compensation control strategy based on CHB-STATCOM outputs the reactive current required by the load through CHB-STATCOM, thereby realizing the 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 output current of the CHB-STATCOM 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, a control system for the transformerless unified power quality regulator described in any one of the above-mentioned embodiments is provided, comprising any one or more of the following control modules:

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

[0018] The reactive power compensation control module based on CHB-STATCOM is used to output the reactive current required by the load through CHB-STATCOM, so as to realize the grid-side pure active power mode operation.

[0019] The 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 output current of the CHB-STATCOM when the CHB-STATCOM is connected to the device through the third AC / DC converter.

[0020] By adopting 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 regulator, its control method, and system provided by this invention use two AC / DC converters in a series connection with the grid side and the load side to achieve voltage quality compensation at the load port. This avoids the use of a power frequency transformer, 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 the performance of the unified power quality regulator.

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

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

[0024] The transformerless unified power quality conditioner, its control method, and system provided by this 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. Attached Figure Description

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

[0026] Figure 1 This is a schematic diagram of the topology of a transformerless unified power quality regulator in a preferred embodiment of the present invention.

[0027] Figure 2 This is a block diagram of the constant load port voltage control strategy of a transformerless unified power quality regulator in a preferred embodiment of the present invention.

[0028] Figure 3 This is a block diagram of the voltage outer loop control strategy for 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 for 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 This is a sample simulation topology diagram of a transformerless unified power quality regulator in a specific application example of the present invention.

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

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

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

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

[0036] Figure 11 In another specific application example of the present invention, the active power P on the grid side source Reactive power Q source Active power P on the load side load Reactive power Q load STATCOM outputs active power P STATCOM Output reactive power Q STATCOM Line graph. Detailed Implementation

[0037] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0038] One embodiment of the present invention provides a transformerless unified power quality conditioner. This conditioner 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 eliminates the series transformer, aiming to reduce system size and cost. It also avoids the impact of the nonlinear characteristics of the power frequency isolation transformer at the AC grid connection point on the UPQC performance, thus realizing an economical, convenient, high power density, efficient and feasible UPQC solution.

[0039] Specifically, as shown in Figure 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 and second AC / DC converters are connected to the power grid and the load, respectively, forming a series structure. By adjusting the output voltage of these two AC / DC converters, the series voltage between the power grid and the load can be adjusted, thereby improving the voltage quality on the load side.

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

[0042] The CHB-STATCOM can be connected to the AC port of the third AC / DC converter, or directly connected to the negative terminal of the DC bus to form a parallel section, which is used to provide reactive power support to the load side and enable the grid side to operate at unity power factor; the DC bus energy balance is achieved by adjusting the output voltage of the third AC / DC converter.

[0043] The filter inductor is connected to the three-phase output 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 a DC bus. The required bus voltage level can be designed according to the magnitude of the adjustable series voltage required in the actual application scenario and the level of the devices, and a suitable topology can be selected.

[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 multilevel structure.

[0046] In some preferred embodiments, CHB-STATCOM is composed of cascaded full-bridge submodules.

[0047] Based on the transformerless unified power quality regulator provided in the above embodiments of the present invention, an embodiment of the present invention also provides a control method for the transformerless unified power quality regulator.

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

[0049] A constant load port voltage control strategy is used to compensate for series voltage fluctuations by using a first AC / DC converter and a second AC / DC converter, thereby maintaining the load port voltage at a set value. Figure 2 As shown;

[0050] A reactive power compensation control strategy based on CHB-STATCOM is employed, which uses the reactive current required by the load output by the CHB-STATCOM to achieve pure active power mode operation on the grid side. Figure 3 and Figure 4 As shown;

[0051] A DC bus balancing control strategy is used when the CHB-STATCOM is connected to the device through a third AC / DC converter. This strategy adjusts the output voltage of the third AC / DC converter to interact with the output current of the CHB-STATCOM, thereby maintaining a constant DC bus voltage. Figure 5 As shown.

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

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

[0054] The actual current tracks the reference value of the current, and the actual current is controlled without net error by a PI controller; then, the actual current is decoupled by adding current cross decoupling feedforward; 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] To fully utilize the modulation of the first and second AC / DC converters, i.e., to ensure the series voltage is equally shared by both, the output voltage v of the two AC / DC converters will be... s1 and v s2 The following relationship must be satisfied:

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

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

[0058] The CHB-STATCOM in TL-UPQC can be composed of cascaded full-bridge submodules. Compared to half-bridge submodules, full-bridge submodules have more output voltage levels, capable of outputting '0', 'V', and 'V'. c 'and'-V c There are three voltage levels. Therefore, the CHB-STATCOM output voltage does not need to contain a DC component.

[0059] To achieve more output voltage levels and improve harmonic characteristics, CHB-STATCOM employs carrier phase-shifted pulse width modulation (CPS-PWM). Each phase submodule has the same modulation voltage, and the carrier phases differ by 2π / N, where N is the number of each phase submodule in CHB-STATCOM. Since the energy balance principle of CHB-STATCOM is the same for all three phases, the subsequent analysis will be based on phase A.

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

[0061]

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

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

[0064]

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

[0066]

[0067] Among them, i p (t) represents the instantaneous value of the CHB-STATCOM output current, I p The output current amplitude of CHB-STATCOM, ρ p This refers to the initial phase of the CHB-STATCOM output current.

[0068] Equation (4) shows that the submodule capacitor current of CHB-STATCOM contains a DC component and a second harmonic 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 must be 0, that is:

[0069]

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

[0071]

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

[0073] Therefore, ignoring losses, the overall energy balance of the CHB-STATCOM can be achieved by adjusting the phase of the CHB-STATCOM output voltage to be perpendicular to the output current. In addition, differences exist in the parameters of submodule capacitors, switching devices, and driver chips, leading to voltage imbalances between the capacitors of different submodules. Therefore, each submodule must be considered individually, employing equalization control to slightly adjust its voltage modulation ratio to further achieve energy balance.

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

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

[0076]

[0077] Among them, i lq This represents the q-axis component of the load-side current.

[0078] Ignoring the internal losses of the CHB-STATCOM, the output current of the CHB-STATCOM only has a q-axis component, meaning it only interacts with the TL-UPQC system for reactive power. In reality, the internal losses of the 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 within the CHB-STATCOM. Therefore, this strategy employs an overall DC voltage control strategy for the CHB-STATCOM, using the CHB-STATCOM capacitor voltage as the control target, i.e., the voltage outer loop, to track the sum of the submodule capacitor voltages. A PI controller is used to obtain the d-axis current reference value; for example... Figure 3 As shown. Where N is the number of submodules per phase, Set the capacitor voltage value for each submodule, v ckj This represents the capacitor voltage of the j-th phase and the k-th submodule;

[0079] like Figure 3 As shown, this is the outer voltage loop of CHB-STATCOM, with the capacitor voltage of CHB-STATCOM as the control target. Equation (7) and Figure 3The current reference values ​​for the d-axis and q-axis of the CHB-STATCOM are given. A PI controller is used to perform closed-loop control of the output current, resulting in the inner current loop of the CHB-STATCOM. The control block diagram is shown below. Figure 4 As shown. Figure 4 In the middle, a voltage feedforward compensation term v is added to the control loop. pd To counteract the voltage disturbance at the connection point between CHB-STATCOM and the third AC / DC converter; and This is a cross-coupling term used to decouple the output current control in the dq coordinate system. Reference values ​​for the CHB-STATCOM output voltage along the d and q axes are obtained. and Then, using the inverse matrix of the Park transformation, a coordinate system transformation of dq / abc is performed to obtain the CHB-STATCOM output voltage reference value in the abc coordinate system. and The output voltage reference value of each submodule is 1 / N of the total output voltage reference value, that is:

[0080]

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

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

[0083] In TL-UPQC, the third AC / DC converter connected to the DC bus uses SPWM modulation, ignoring the switching subharmonic components. Therefore, its output voltage contains both DC and AC components. To fully utilize the modulation capability of the third AC / DC converter, the DC component is generally half of the DC bus voltage. If only the fundamental frequency AC component is considered, the output voltages of the first and second AC / DC converters (serving as voltage regulation modules) and the third AC / DC converter (serving as a power balancing module) can be expressed as:

[0084]

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

[0086] Combining equation (1), the average switching functions of the voltage regulation module and the power balance module are obtained as follows:

[0087]

[0088] Among them, s s1 (t), s s2 (t) represents 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 for (t) is:

[0090]

[0091] Among them, I s I l and I p These represent the magnitudes of the grid-side current, load-side current, and CHB-STATCOM output current, respectively, ρ. s ρ l and ρ p These are the initial phases of the grid-side current, the load-side current, and the CHB-STATCOM output current, respectively.

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

[0093]

[0094] Equation (11) shows that the current flowing into the DC bus consists of a second harmonic component and a DC component. To maintain a constant DC bus voltage, the DC component in the current of equation (11) must be zero, i.e.:

[0095]

[0096] Equation (12) represents the energy balance constraint condition for the DC bus. Based on this constraint condition, the effective value V of the output voltage of the balancing module can be adjusted. sp or phase θ sp This constraint condition is met, achieving DC bus energy balance. To obtain the strongest power regulation capability and maximize the utilization of the DC bus voltage, the output voltage phase of the balancing module can be made to match the output current phase of the CHB-STATCOM, i.e.

[0097] θ sp =ρ p (13)

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

[0099]

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

[0101] First, the DC bus voltage is controlled overall. The deviation between the actual voltage value and the reference value is input to the PI regulator to obtain the output voltage reference value of the balancing module. Simultaneously, referring to the phase-to-phase capacitor voltage equalization control method of CHB-STATCOM, a zero-sequence voltage is superimposed on the power balancing module and interacts with the output current of CHB-STATCOM to achieve power transfer between the three phases, thereby realizing phase-to-phase voltage equalization of the DC bus. The phase-to-phase capacitor voltage equalization control method aims to achieve phase-to-phase balance of the DC bus and maintain a constant DC bus voltage; after maintaining the bus voltage stability, constant voltage control is then implemented.

[0102] It's important to note that the CHB-STATCOM's current primarily consists of a q-axis component, with a very small d-axis component. Therefore, the phase of the power balancing module's output voltage must be aligned with the CHB-STATCOM's q-axis direction. To ensure the balancing module's output voltage phase θ... sp The phase of the parallel branch current ρ p The current flowing into the DC bus is the same, but the direction of the current is opposite to the positive direction of the current in the parallel branch. Therefore, the direction of the current multiplied by the q-axis component and the zero-sequence component of the power balancing module's output voltage is -sign(ipq), where i pq This refers to the q-axis component of the CHB-STATCOM output current.

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

[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 constant load port voltage control module is used to compensate for the series voltage through a first AC / DC converter and a second AC / DC converter when the grid voltage fluctuates, so that the voltage at the load port is maintained at a set value.

[0106] The reactive power compensation control module based on CHB-STATCOM is used to output the reactive current required by the load through CHB-STATCOM, so as to realize the grid-side pure active power mode operation.

[0107] The 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 output current of the CHB-STATCOM 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. Those skilled in the art can refer to the technical solution of the control method to realize the composition of the control system. That is, the embodiments in the control method can be understood as preferred examples of constructing the control system, and will not be elaborated here.

[0109] The transformerless unified power quality conditioner and its control method and system provided in the above embodiments of the present invention achieve 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. This improves the power quality of the load when the grid voltage fluctuates, optimizes the operation of the power grid, and provides reactive power compensation for the load, enabling the grid side to operate at unity power factor. Depending on the adjustable series voltage required in the actual application scenario and the level of the components, the AC / DC converters in the series and parallel circuits can adopt different topologies, such as half-bridge, three-level, and modular multilevel structures.

[0110] The technical solutions provided by the above embodiments of the present invention will be further described in detail below with reference to specific application examples.

[0111] Specific application example 1

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

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

[0114]

[0115]

[0116] Before simulation time t = 0.8s, the grid voltage temporarily drops to 0.8 pu; from t = 0.8 to 1.4s, the grid voltage is 1.0 pu; and from t = 1.4 to 2.0s, the grid voltage temporarily rises to 1.2 pu. Under these simulation parameters, the theoretical result is that the grid voltage on the load side remains at 8165V.

[0117] Figure 7 Figures (a) and (b) show the output curves of the grid voltage and load voltage in this specific application example. During the simulation, the grid voltage on the load side remained at around 8165V.

[0118] Figure 8 The voltage of the CHB-STATCOM submodule capacitor in this specific application example is shown. During the simulation, the voltage of the STATCOM submodule capacitor is maintained at around 800V.

[0119] Figure 9 This refers to the DC bus voltage in this specific application example, where the DC bus voltage remains at around 3000V throughout the simulation period.

[0120] Figures 7-9 The simulation results shown are consistent with the theoretical results. Therefore, the designed load voltage controller can achieve rapid 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 TL-UPQC is as follows: Figure 6 As shown in the table below, the topology was simulated and verified using MATLAB / Simulink software. The simulation parameters are shown in the table below.

[0123]

[0124] Under the conditions shown in the table above, the simulation time is t = 0.2–2.0 s, and the given load is P. load=0.8pu; Q load A resistive-inductive load with a capacitance of 0.6 pu. Under these simulation parameters, the theoretical result is that the power grid only provides active power to the load.

[0125] Figure 10 The output curves of the grid voltage and current in this specific application example are shown. During the simulation time, the fundamental voltage and grid current are in phase.

[0126] Figure 11 In this specific application example two, the active power P on the grid side... source Reactive power Q source Active power P on the load side load Reactive power Q load STATCOM outputs active power P sTATCOM Output reactive power Q STATCOM The curves show that during the simulation period, the active power of the power grid remained stable at around 0.8 pu and the reactive power remained stable at around 0; the active power of the load remained stable at around 0.8 pu and the reactive power remained stable at around 0.6 pu; and the active power of the STATCOM remained stable at around 0 and the reactive power remained stable at around 0.6 pu.

[0127] Figure 10 and Figure 11 The simulation results shown are consistent with the theoretical results. Therefore, reactive current compensation can be performed through the designed STATCOM controller to achieve the grid voltage fundamental wave and grid current in phase.

[0128] The transformerless unified power quality conditioner provided in the above embodiments of the present invention comprises two AC / DC converters, the first and second, which are back-to-back AC / DC converters based on a low-voltage DC bus. The AC ports of these two AC / DC converters are connected to the AC grid and the load, respectively, forming a series structure between feeders. A cascaded H-bridge static synchronous compensator is connected via the AC port of a third AC / DC converter connected to the low-voltage DC bus, serving as a parallel component. Furthermore, the CHB-STATCOM can be directly connected to the negative terminal of the DC bus; in this case, DC bus energy balance is achieved by adjusting the output voltage of the AC / DC converter connected to the AC grid.

[0129] The control method and system for a transformerless unified power quality regulator provided in the above embodiments of the present invention utilize a constant load port voltage control strategy based on series voltage compensation. By adjusting the output voltage of the converter connected to the power grid and the load, the load port voltage can be controlled. Its reactive power compensation control strategy based on CHB-STATCOM injects the reactive current required by the load into the parallel-connected CHB-STATCOM, providing reactive power compensation for the system. Its low-voltage DC bus energy balance control strategy maintains the low-voltage DC bus energy balance 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 in the above embodiments of the present invention avoid the use of power frequency isolation transformers required by conventional solutions by using AC / DC converters and CHB-STATCOM, which significantly reduces costs and avoids the impact of the nonlinear characteristics of isolation transformers on UPQC performance.

[0131] The transformerless unified power quality regulator and its control method and system provided in the above embodiments of the present invention use two AC / DC converters in series with the grid side and the load side to achieve voltage quality compensation at the load port. This avoids the use of power frequency transformers, 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 the performance of the unified power quality regulator. The parallel part is a static synchronous compensator, which can provide reactive power support for the system, maintain pure active power 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 AC / DC converters to the DC bus, the AC grid interface and load interface can be expanded, ultimately forming a multi-port series-parallel transformerless unified power quality regulator structure, thereby managing the power quality of multiple loads.

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

Claims

1. A control method of a transformerless unified power quality conditioner, characterized by, The transformerless unified power quality conditioner comprises 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 with a power grid and a load respectively, and together constitute a series structure; by adjusting the output voltages of the two AC / DC converters, the series voltage between the power grid and the load can be adjusted, so as to realize the management of the voltage quality on the load side; the third AC / DC converter is connected with a DC bus; the AC port of the third AC / DC converter is connected with the CHB-STATCOM, constituting a parallel part, for providing reactive power support for the load side, so that the grid side realizes unit power factor operation; by adjusting the output voltage of the third AC / DC converter, the energy balance of the DC bus can be realized; the filter inductor is connected to the three-phase outlet of the CHB-STATCOM, for filtering out the harmonics in the output current of the CHB-STATCOM; The control method comprises the following control strategies: The load port voltage setting control strategy 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 kept at a set value; The reactive power compensation control strategy based on the CHB-STATCOM is used to output the reactive current required by the load through the CHB-STATCOM, so that the grid side realizes the pure active mode operation; The DC bus balance 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.

2. The control method of transformerless unified power quality conditioner according to claim 1, characterized by, The load port voltage setting control strategy comprises: In the synchronous rotating coordinate system, the voltage outer ring provides a load port voltage reference value, and the reference value of the current is obtained through a PI controller; The actual current tracks the reference value of the current, and the actual current is controlled without net difference through a PI controller; then, the actual current is d-q decoupling controlled by adding current cross-decoupling feedforward; by adding grid voltage feedforward, the disturbance influence of the 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 divided equally by the first AC / DC converter and the second AC / DC converter, then the two AC / DC converters output voltages v s1 and v s2 satisfy the following relationship: v s1 = -v s2 (1).

3. The control method of transformerless unified power quality conditioner according to claim 1, characterized by, The CHB-STATCOM comprises: The CHB-STATCOM adopts a carrier phase-shifted modulation method, the modulation voltages of the single-phase submodules are the same, the carrier phases are different by 2π / N, and N is the number of submodules per phase of the CHB-STATCOM; For energy balance of the A-phase sub-module, let the output voltage of the CHB-STATCOM be v ST (t), which has the expression: where V ST is the CHB-STATCOM output voltage amplitude, ω is the grid voltage angular frequency, t is the system time, θ ST is the CHB-STATCOM output voltage initial phase; Thus, the switching function S of the submodule output voltage ck (t) is represented as: wherein, is the DC capacitor voltage reference value for each full-bridge sub-module; further, the current i flowing into the sub-module capacitor is obtained ck (t) is: Wherein, i p (t) is the instantaneous value of CHB-STATCOM output current, I p is the amplitude of CHB-STATCOM output current, ρ p is the initial phase of CHB-STATCOM output current; Formula (4) shows that the submodule capacitor current of the 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 submodules and the C-phase submodules is the same as that of the A-phase submodules.

4. The control method of transformerless unified power quality conditioner according to claim 3, characterized by, Further comprising: Rewrite formula (5) as:

5. The control method of transformerless unified power quality conditioner according to claim 1, characterized by, The reactive power compensation control strategy based on the CHB-STATCOM comprises: The reactive component of the load current is taken as the reference value, and the CHB-STATCOM realizes real-time and accurate compensation of grid-side reactive control, so the q-axis reference value of the CHB-STATCOM current is Iqref=Qref+Iqref0 where i lq is the q-axis component of the load-side current; A small amount of active power is drawn from the AC power grid by adjusting the d-axis component of the output current of the CHB-STATCOM, so as to maintain the energy balance inside the CHB-STATCOM.

6. The control method of transformerless unified power quality conditioner according to claim 5, characterized by, The small amount of active power drawn from the AC power grid by adjusting the d-axis component of the output current of the CHB-STATCOM so as to maintain the energy balance inside the CHB-STATCOM comprises: An overall DC voltage control strategy of the CHB-STATCOM is adopted, and the capacitor voltage of the CHB-STATCOM is taken as a control target, that is, a voltage outer loop is adopted to track the sum of the capacitor voltages of the sub-modules, and a PI controller is adopted to obtain a d-axis current reference value; After the d-axis and q-axis current reference values of the CHB-STATCOM are obtained, a PI controller is adopted to perform closed-loop control on the output current, so as to obtain a current inner loop of the CHB-STATCOM. Adding a feed-forward compensation term v of the voltage in the control loop pd To offset the influence of voltage disturbance at the connection point of CHB-STATCOM and the third AC / DC converter, the output current control in dq coordinate system is decoupled through a cross-decoupling term, to obtain the reference value of the d-axis and q-axis of the output voltage of CHB-STATCOM and The reference value of the output voltage of the CHB-STATCOM in the abc coordinate system is obtained by performing dq / abc coordinate system conversion and The reference value of the output voltage of the CHB-STATCOM in the abc coordinate system is obtained by performing dq / abc coordinate system conversion and The output voltage reference value of each submodule is 1 / N of the total output voltage reference value, that is: wherein, is the output voltage reference value of the i-th phase STATCOM, is the output voltage reference value of the i-th phase STATCOM, N is the number of sub-modules per phase.

7. The control method of transformerless unified power quality conditioner according to claim 1, characterized by, The DC bus balance comprises: The output voltage of the first AC / DC converter and the second AC / DC converter as the voltage regulating module and the output voltage of the third AC / DC converter as the power balance module are expressed as: wherein the DC component is half of the DC bus voltage, the third AC / DC converter adopts SPWM modulation, and the switching sub-harmonic component is ignored. wherein 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, i.e. the output voltages of the voltage regulating module; v sp (t) is the output voltage of the third AC / DC converter connected to the CHB-STATCOM, i.e. the output voltage of the power balancing module; is the DC component of the output voltage, is the DC bus voltage rating; V s1 , V s2 and V sp are the RMS values of the AC components of the output voltage, θ1, θ2 and θ sp are the phase angles of the AC components of the output voltage; Combining the relationship v s1 = -v s2 , the average switching functions of the voltage regulation module and the power balance module are obtained as where s s1 (t) is the average switching function of the two voltage regulating modules, s s2 (t) is the average switching function of the two voltage regulating 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 i (t) is where I s , I l and I p are the amplitudes of grid-side current, load-side current and CHB-STATCOM output current, respectively, and s , p l and p p are the initial phases of grid-side current, load-side current and CHB-STATCOM output current, respectively. The current flowing into the capacitor on the DC side of the third AC / DC converter is equal to the output port current of the third AC / DC converter multiplied by the average switching function. Since the DC bus provides a channel for power interaction of the third AC / DC converter, the current flowing into the DC bus is equal to the sum of the currents flowing into the three AC / DC converters, that is: Formula (12) is a direct current bus energy balance constraint condition; according to the constraint condition, the output voltage effective value V sp or phase θ sp So that the constraint condition is established, and the direct current bus energy balance is realized; Formula (11) indicates that the current flowing into the DC bus is composed of a double-frequency component and a DC component. θ sp = p p (13) The output voltage of the balance module is consistent in phase with the output current of the CHB-STATCOM.

8. The control method of transformerless unified power quality conditioner according to claim 7, characterized by, At this time, the DC bus energy balance condition is: The DC bus balance control strategy comprises: The deviation between the actual value and the reference value of the DC bus voltage is input into a PI regulator to obtain the output voltage reference value of the balance module.

9. The control method of transformerless unified power quality conditioner according to claim 8, characterized by, The inter-phase capacitor voltage balance control method of the CHB-STATCOM is referred to, a zero-sequence voltage is superimposed on the power balance module, the output current of the CHB-STATCOM is interacted with, power transfer among the three phases is realized, and thus the inter-phase voltage balance of the DC bus is realized.

10. The control method of transformerless unified power quality conditioner according to claim 8, wherein The output voltage q-axis component and zero sequence component of the power balance module are multiplied by the current direction as -sign(i pq ); wherein i pq is the q-axis component of the output current of the CHB-STATCOM.

11. The control method of transformerless unified power quality conditioner according to claim 1, characterized by, The phase of the output voltage of the balance module is consistent with the q-axis direction of the CHB-STATCOM.

12. The control method of transformerless unified power quality conditioner according to claim 1, characterized by, 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 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 multilevel structure.

13. The control method of transformerless unified power quality conditioner according to claim 1, characterized by, The CHB-STATCOM is composed of full-bridge type sub-modules in cascade.

14. A control system for a transformerless unified power quality conditioner, characterized by The transformerless unified power quality conditioner comprises 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 with a power grid and a load respectively, and together constitute a series structure; by adjusting the output voltages of the two AC / DC converters, the series voltage between the power grid and the load can be adjusted, so as to realize the management of the voltage quality on the load side; the third AC / DC converter is connected with a DC bus; the CHB-STATCOM is connected with the AC port of the third AC / DC converter, and constitutes a parallel part, which is used for providing reactive power support for the load side, so that the grid side realizes unit power factor operation; by adjusting the output voltage of the third AC / DC converter, the DC bus energy balance is realized; the filter inductor is connected to the three-phase outlet of the CHB-STATCOM, and is used for filtering out the harmonics in the output current of the CHB-STATCOM; The control system comprises the following control modules: a constant load port voltage control module, which is used for compensating 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 kept at a set value; a reactive power compensation control module based on the CHB-STATCOM, which is used for outputting the required reactive current of the load through the CHB-STATCOM, so as to realize the grid side pure active mode operation; a DC bus balance control module, which is used for interacting with the CHB-STATCOM output current by adjusting the output voltage of the third AC / DC converter when the CHB-STATCOM is connected to the device through the third AC / DC converter, so as to maintain the DC bus voltage constant.

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