High-power-quality converter device with chopping inversion multiplexing function

Abstract

The invention belongs to the related technical field of photovoltaic inverter compensation systems, and particularly relates to a high-power-quality converter with chopping inversion multiplexing, which comprises series and parallel APFs (active power filters), a Z-type grounding transformer, a three-phase bridge circuit and a photovoltaic power generation unit, one end of the transformer is connected to the alternating current side of the bridge circuit, the other end of the transformer is externally connected with a load power grid, a grounding point is led out to be connected to one end of the photovoltaic power generation unit while the transformer is connected to the alternating current side, and the other end of the photovoltaic power generation unit is connected to the bridge circuit; the series APF and the parallel APF form a unified power quality controller and are connected between the transformer and the bridge circuit; the parallel APF is used for compensating voltage sine distortion and voltage imbalance in the presence of an inversion working condition, and the series APF is used for filtering harmonic current and compensating reactive power in the presence of the inversion working condition; and the transformer is used for providing a grounding reference for the photovoltaic power generation unit in the presence of a chopping working condition and offsetting a zero-sequence harmonic component of the alternating current side voltage in the presence of an inversion working condition.

Description

Technical Field

[0001] The present invention belongs to the technical field related to photovoltaic inverter compensation systems, and more specifically, relates to a high power quality current conversion device with chopper inverter multiplexing. Background Art

[0002] With the rapid development of photovoltaic power generation technology, photovoltaic grid-connected systems have gradually become an important way to achieve large-scale access to the power grid for renewable energy. In photovoltaic power generation systems, the DC power generated by photovoltaic cells needs to be converted into power through converters in order to adapt to energy storage systems and grid-connected requirements. Existing photovoltaic grid-connected topologies usually include DC to DC (DC / DC) converters, DC to AC (DC / AC) inverters, and energy storage units. The system mainly realizes power generation, energy storage, and grid-connected functions. However, this traditional design generally has problems such as a large number of components, high cost, and complex control. In particular, the power quality on the small user side is difficult to guarantee, and it is prone to harmonic pollution, reactive power imbalance, and voltage distortion.

[0003] In order to optimize the structure of photovoltaic power generation system and improve the quality of electric energy, a new unified system of power generation, energy storage and grid connection has been proposed in recent years. This system can reuse the same circuit in the process of direct chopping (transmission from photovoltaic power generation unit to DC side energy storage unit) and energy storage to grid connection (power output from DC side energy storage unit to grid) through the reuse of bridge circuit, thereby reducing the number of devices and reducing system costs. The bridge circuit in this chopper inverter multiplexing system can be used as a chopper circuit to achieve DC voltage boost and regulation, and can also be used as an inverter circuit to convert the DC power of the energy storage unit into high-quality AC power adapted to the grid, realizing the bidirectional flow and efficient conversion of electric energy.

[0004] However, there are more complex power quality issues in the process of photovoltaic power generation units and energy storage units being connected to the grid. First, after the DC power is chopped and inverted, it may introduce power quality issues such as harmonic current, reactive power, and voltage fluctuations, which have an adverse effect on the stability and power supply quality of the power grid. Secondly, during the charging and discharging process of the energy storage battery, the dynamic changes in the system load will cause voltage fluctuations and a decrease in power factor, further exacerbating the deterioration of power quality. Summary of the invention

[0005] In view of the above defects or improvement needs of the prior art, the present invention provides a high power quality converter with chopper inverter multiplexing, which aims to efficiently improve the power quality in a unified power generation, energy storage and grid-connected system.

[0006] To achieve the above object, according to one aspect of the present invention, a high power quality converter with chopper inverter multiplexing is provided, comprising: a series APF, a parallel APF, a Z-type grounding transformer, a three-phase bridge circuit with chopper inverter multiplexing, a DC side energy storage unit, a photovoltaic power generation unit in parallel with the AC side, and a control unit;

[0007] Among them, one end of the Z-type grounding transformer is connected to the AC side of the three-phase bridge circuit, and the other end is connected to the external load grid. The grounding transformer is connected to the AC side and at the same time leads out a grounding point to connect to one end of the photovoltaic power generation unit, and the other end of the photovoltaic power generation unit is connected to the three-phase bridge circuit; the series APF and the parallel APF are connected through a DC stabilizing capacitor to form a unified power quality controller, and one side of the series APF and one side of the parallel APF in the unified power quality controller are connected between the Z-type grounding transformer and the three-phase bridge circuit through a series transformer and a parallel transformer respectively;

[0008] The parallel APF is used to compensate for voltage sinusoidal distortion and voltage imbalance under the inverter condition, and the series APF is used to filter harmonic current and compensate reactive power under the inverter condition; the Z-type grounding transformer is used to provide a grounding reference to the photovoltaic power generation unit under the chopping condition, and to offset the zero-sequence harmonic component of the AC side voltage through its Z-type connection under the inverter condition. The inverter condition is that the electric energy is from the DC side energy storage unit to the load grid, and the chopping condition is that the electric energy is from the photovoltaic power generation unit to the DC side energy storage unit; the control unit is used to drive the series APF and the parallel APF to achieve dynamic power quality regulation.

[0009] Furthermore, the control unit is specifically configured to use a PI controller to generate a PWM signal to control the series APF, and use a PI controller and a P controller to generate a PWM signal to control the parallel APF.

[0010] Furthermore, the control unit is specifically used to adopt a direct control method to directly control the series APF to be a sinusoidal voltage source and the parallel APF to be a sinusoidal current source without detecting voltage and current distortion.

[0011] According to another aspect of the present invention, a high power quality current conversion method is provided, which uses the high power quality current conversion device with chopper inverter multiplexing as described above to perform current conversion.

[0012] Furthermore, under the chopping condition, the Z-type grounding transformer in the high power quality converter device serves as a DC path of the chopping circuit, and controls the unified power quality controller in the high power quality converter device to be disconnected.

[0013] Furthermore, under the inverter working condition, the Z-type grounding transformer in the high power quality converter device serves as an AC path, and controls the unified power quality controller in the high power quality converter device to access the AC path to achieve control of power quality.

[0014] Furthermore, under the inverter and chopper hybrid conversion conditions, the Z-type grounding transformer in the high power quality converter device serves as the AC path and the DC path of the chopper circuit; the unified power quality controller in the high power quality converter device is controlled to access the AC path to achieve control of power quality.

[0015] In general, compared with the prior art, the technical solution conceived by the present invention has the following beneficial effects:

[0016] 1. The present invention proposes a high power quality converter with chopper inverter multiplexing. Considering that the converter can work in a variety of working conditions, the present invention introduces a unified power quality adjustment device, which is an electric power compensation device integrating a series active filter and a parallel active filter. It can effectively compensate for the harmonic current, reactive power and voltage distortion problems generated during system operation, improve the power quality, and ensure the stability and reliability of the photovoltaic power generation system when it is connected to the grid. Therefore, the introduction of UPQC solves the problem of insufficient power quality control in traditional photovoltaic power generation systems; in addition, by combining the voltage regulation and reactive power compensation capabilities of APF with the DC bias suppression function of the Z-type grounding transformer, the anti-interference ability of the system can be effectively improved to ensure stable operation under various complex working conditions. Therefore, the present invention can efficiently improve the power quality in the unified system of power generation, energy storage and grid connection.

[0017] 2. The present invention further proposes that the control unit adopts a PI controller and a P controller to directly control the parallel APF so that it always maintains a fundamental sinusoidal voltage source, so that the parallel APF has a very low impedance, and the harmonic current and reactive current on the grid side flow through the branch, thereby isolating the harmonic current and reactive current on the grid side and preventing them from flowing into the grid, thereby realizing compensation for the voltage of the grid-connected link of the converter device and ensuring the improvement of the sinusoidality of the current waveform and the power factor.

[0018] 3. The present invention also proposes a high power quality conversion method, which is implemented by the above-mentioned high power quality conversion device, providing an efficient and stable conversion solution for the photovoltaic power generation system. The synergistic relationship between the Z-type grounding transformer and the UPQC under different working conditions is preferably proposed to achieve power quality management with high efficiency and low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The overall topological structure of the high power quality converter device of the present invention;

[0020] Figure 2 A schematic diagram of power quality compensation of a high power quality converter device in a grid-connected inverter working mode provided by an embodiment of the present invention;

[0021] Figure 3 A schematic diagram of power quality compensation of a high power quality converter provided by an embodiment of the present invention in an inverter and chopper hybrid converter working mode;

[0022] Figure 4 Schematic diagram of an equivalent mathematical model provided for an embodiment of the present invention

[0023] Figure 5 A series APF overall control block diagram of a high power quality converter device provided by an embodiment of the present invention;

[0024] Figure 6 This is an overall control block diagram of the parallel APF of the high power quality converter device described in the present invention. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0026] Embodiment 1

[0027] A high power quality current conversion device with chopper inverter multiplexing, comprising: a series APF, a parallel APF, a Z-type grounding transformer, a three-phase bridge circuit with chopper inverter multiplexing, a DC side energy storage unit, a photovoltaic power generation unit in parallel with the AC side, and a control unit;

[0028] Among them, one end of the Z-type grounding transformer is connected to the AC side of the three-phase bridge circuit, and the other end is connected to the external load grid. The grounding transformer is connected to the AC side and at the same time leads out a grounding point to connect to one end of the photovoltaic power generation unit, and the other end of the photovoltaic power generation unit is connected to the three-phase bridge circuit; the series APF and the parallel APF are connected through a DC stabilizing capacitor to form a unified power quality controller, and one side of the series APF and one side of the parallel APF in the unified power quality controller are connected between the Z-type grounding transformer and the three-phase bridge circuit through a series transformer and a parallel transformer respectively;

[0029] The parallel APF is used to compensate for voltage sinusoidal distortion and voltage imbalance under the inverter condition, and the series APF is used to filter harmonic current and compensate reactive power under the inverter condition; the Z-type grounding transformer is used to provide a grounding reference to the photovoltaic power generation unit under the chopping condition, and to offset the zero-sequence harmonic component of the AC side voltage through its Z-type connection under the inverter condition. The inverter condition is that the electric energy is from the DC side energy storage unit to the load grid, and the chopping condition is that the electric energy is from the photovoltaic power generation unit to the DC side energy storage unit; the control unit is used to drive the series APF and the parallel APF to achieve dynamic power quality regulation.

[0030] The current mainstream photovoltaic inverter power quality management solutions on the market usually include active power filters and unified power quality compensation devices. Among them, APF used alone can effectively compensate for harmonics and reactive power and improve power quality, but it has limitations in dealing with DC bias problems in power transformers. In contrast, the Z-type grounding transformer can effectively suppress the DC bias phenomenon and protect the safety and stability of the power transformer. Therefore, the combination of APF and Z-type grounding transformer can achieve a comprehensive improvement in power quality while enhancing the system's anti-interference ability and reliability.

[0031] In the existing photovoltaic inverter systems on the market, the DC bias problem has always been a key factor affecting power quality and equipment life. Since the rectification and inversion operations of the photovoltaic inverter may introduce DC components, these DC components will cause the core saturation of the transformer, thereby producing a DC bias effect. Although the traditional APF solution can effectively filter out harmonics and perform reactive power compensation, it is powerless against the DC bias problem. The Z-type grounding transformer provides a low-impedance path that can effectively discharge the DC component in the system, prevent the DC flux from accumulating in the transformer, and reduce the transformer overheating and noise problems caused by this.

[0032] In addition, the combination of Z-type grounding transformer and APF also significantly enhances the system's grid anti-interference ability. During the operation of the photovoltaic system, voltage fluctuations, flicker, and unbalanced loads often affect the stability and safety of the system. By combining the voltage regulation and reactive power compensation capabilities of the APF with the DC bias suppression function of the Z-type grounding transformer, the system's anti-interference ability can be effectively improved to ensure stable operation under various complex working conditions.

[0033] This embodiment proposes to introduce a unified power quality control device in the chopper inverter multiplexing system. As a power compensation device integrating a series active filter and a parallel active filter, the unified power quality control device can effectively compensate for the harmonic current, reactive power and voltage distortion problems generated during the operation of the system, improve the power quality, and ensure the stability and reliability of the photovoltaic power generation system when it is connected to the grid. Therefore, the introduction of UPQC solves the problem of insufficient power quality control in traditional photovoltaic power generation systems. Among them, the series APF includes a coupled series transformer, a filter unit and an active part. The series APF has a very high impedance by connecting a sinusoidal current source in series between the power grid and the load, so as to isolate the harmonic current reactive current caused by the nonlinear load from flowing into the power grid, thereby compensating for the harmonic voltage and reactive current; the series APF includes a parallel transformer, a filter unit and an active part. The parallel APF has a very low impedance by connecting a sinusoidal voltage source in parallel on the load side, so that the distorted current in the load flows through the branch, thereby preventing it from flowing into the power grid, and realizing compensation for the load voltage.

[0034] The Z-type grounding transformer provides an effective grounding reference through a unique wiring method, enhances the system's anti-interference ability and electrical safety, and works in coordination with the series APF and parallel APF in the UPQC system to improve the overall power quality of the system.

[0035] Z-type grounding transformer, each core column of the Z-type grounding transformer has two windings, with equal number of turns and the same phase, and opposite polarity connected in series to form a zigzag winding. When current passes through the grounding transformer winding, the magnetic flux generated by the transformer three-phase winding current is recorded as Assuming that the three-phase core magnetic resistance is the same and ignoring the influence of core leakage reactance, then: Where N is the number of winding coils, R m is the magnetic resistance of the core.

[0036] The magnetic flux generated by the winding current is superimposed in the iron core, and the induced magnetomotive force in the three-phase winding is:

[0037]

[0038] For the zero-sequence current component, the three-phase currents have the same phase. Taking the A-phase winding as an example, it is assumed that the initial phase of the A-phase current is 0, then the magnetomotive force of this phase is: For the zero-sequence current component, the magnetic flux generated by the three-phase winding current cancels each other in the grounding transformer core, and no magnetomotive force will be generated in the winding. The Z-type grounding transformer has high impedance for positive and negative sequence current components, and low impedance for zero-sequence current components.

[0039] The three-phase bridge circuit, DC side energy storage unit and photovoltaic cell unit of chopper inverter multiplexing can realize the conversion link of power generation, energy storage and grid connection linkage in the grid-connected photovoltaic power generation system. This embodiment provides a high power quality conversion device with chopper inverter multiplexing and comprehensive power quality management function, which can realize boost chopper conversion mode, three-phase grid-connected inverter mode and inverter and chopper mixed conversion mode, and power quality management can be performed in each mode. Therefore, by flexibly switching between the three modes, efficient energy conversion and comprehensive power quality management in the photovoltaic power generation system can be achieved.

[0040] Among them, in the boost chopper conversion mode, the low-voltage DC power generated by the photovoltaic power generation unit is boosted to the high-voltage DC power of the DC side energy storage unit through the three-phase bridge circuit with chopper inverter multiplexing. The control unit drives the IGBT switch in the bridge circuit through the PWM modulation signal, and the bridge circuit operates in three independent boost chopper paths, and accurate energy transmission control is achieved by adjusting the switching duty cycle of the IGBT. In this mode, the Z-type grounding transformer not only serves as a DC energy path, but also effectively suppresses the low-frequency harmonic components in the input voltage to ensure the output quality of the photovoltaic cell. Since the power flow in this mode is unidirectional, it flows directly from the photovoltaic cell to the energy storage unit, and does not involve the grid connection link, so there is no need to adjust the power quality, and the series and parallel active filters (APF) are in a non-working state. This mode ensures the operating efficiency of the device with a simplified energy flow design, and is optimized in terms of system safety and reliability.

[0041] In the three-phase grid-connected inverter mode, the high-voltage DC power in the DC side energy storage unit is inverted into AC power through a three-phase bridge circuit reused by chopper inverter, and then connected to the grid through a Z-type grounding transformer. Since harmonics, voltage distortion and reactive power problems may occur during the grid-connected inverter process, the system adjusts the power quality through UPQC. The series APF is adjusted to a sinusoidal current source through the PI controller. Its high impedance characteristics can effectively isolate the voltage disturbance and harmonic current caused by nonlinear loads, ensuring the stability of the grid-side voltage. At the same time, the parallel APF works as a sinusoidal voltage source through the P controller. Its low impedance characteristics allow harmonic current and reactive current to pass through the APF branch to prevent them from flowing back to the grid, thereby compensating for voltage distortion and improving the power factor. In this mode, the control unit accurately controls the inverter waveform through SPWM modulation to ensure the high quality of the grid-connected power, and dynamically adjusts the output voltage and frequency to enable the device to adapt to complex grid operating conditions.

[0042] In the hybrid inverter mode of inverter and chopper, the device realizes the reuse of boost chopper and grid-connected inverter functions at the same time. The photovoltaic power generation unit boosts the high-voltage DC power of the energy storage unit through the bridge circuit, and the energy storage unit simultaneously outputs AC power through the bridge circuit inverter and is connected to the grid. In this mode, the control unit comprehensively regulates the switching state of the IGBT, and realizes the dynamic reuse of the bridge circuit by superimposing the modulation waves of chopping and inverter. In this mode, the Z-type grounding transformer simultaneously assumes the functions of DC and AC paths to ensure the stable operation of the boost and inverter functions. Since grid-connected inverter may cause power quality problems such as harmonics and reactive power, UPQC works synchronously in this mode. The series APF isolates the harmonic current and voltage disturbances on the grid side, and the parallel APF compensates the reactive power and voltage distortion in the system to ensure the further improvement of the sinusoidality and power factor of the grid-connected current waveform. This mode effectively improves the overall energy utilization rate of the device, and through the real-time adjustment of the control unit, it ensures the seamless switching between the boost and inverter functions, thereby improving the flexibility and reliability of the system operation.

[0043] Through the coordinated operation of the above three modes, the high power quality converter of this embodiment not only achieves the goal of efficient energy conversion in the photovoltaic power generation system, but also comprehensively manages problems such as harmonic current, reactive current and voltage distortion during the grid connection process, ensuring the overall improvement of the power quality of the power grid while improving the operating efficiency and stability of the system.

[0044] As an optimal implementation method, the control unit uses a PI controller to directly control the series APF, so that it always remains a fundamental sinusoidal current source, so that the series APF has a very high impedance, isolating the voltage disturbance and harmonic current and reactive current caused by the nonlinear load on the grid side from flowing into the grid, thereby realizing compensation for the harmonic current and reactive current in the grid-connected link of the converter.

[0045] The control unit uses PI controller and P controller to directly control the parallel APF, so that it always remains a fundamental sinusoidal voltage source, so that the parallel APF has a very low impedance, and the harmonic current and reactive current on the grid side flow through the branch, thereby isolating the harmonic current and reactive current on the grid side and preventing them from flowing into the grid, realizing compensation for the voltage of the grid-connected link of the converter device, and ensuring the improvement of the sinusoidality of the current waveform and the power factor.

[0046] As another preferred implementation mode, in view of the multi-condition conversion characteristics of the converter, a direct control method is adopted to directly control the series APF as a sinusoidal voltage source and the parallel APF as a sinusoidal current source, thereby eliminating the need to detect voltage and current distortion and directly compensating for it.

[0047] Since the converter described in this embodiment can work in a variety of working conditions, all of which require compensation, it is necessary to design a UPQC control link that can quickly and stably compensate. The control unit used to control UPQC uses a series APF to directly control a sinusoidal current source connected in series on the AC side, and a parallel APF to directly control a sinusoidal voltage source connected in parallel on the AC side to compensate for the voltage and current on the AC side. Such a control design does not require the detection of the compensation amount of voltage and current, which reduces the complexity of the system to a limited extent and can adapt to a variety of working conditions, thereby enabling the entire converter to output AC power with high power quality.

[0048] The mathematical model and related control strategies of the converter of this embodiment are analyzed and explained below.

[0049] like Figure 1 The topology of a UPQC with a Z-type grounding transformer added. The AC side uses a Z-type grounding transformer to suppress the DC flux in the transformer core. The AC side uses UPQC, which consists of a three-phase three-bridge arm APF (series APF) and a three-phase four-bridge arm APF (parallel APF) through a shared capacitor DC bus. The UPQC is connected between the transformer and the bridge circuit. The series APF uses a three-phase three-wire system, and the parallel APF uses a three-phase four-wire system. The neutral point of the fourth bridge arm of the parallel APF is grounded.

[0050] Assume that the AC side series transformer and parallel transformer of the converter are ideal transformers, and each switch device of UPQC is an ideal switch device. The mathematical model of the parallel APF in the three-phase stationary coordinate system is obtained by KCL and KVL, and the Clark transformation and Park transformation are performed on it to obtain the mathematical model in the synchronous rotating coordinate system. The current flow direction is as follows Figure 4 As shown, the mathematical model of the series APF in the d, q coordinate system is:

[0051]

[0052] In the formula, i d ,i q is the input current in the d, q coordinate system; L1 is the filter inductor on the series side; R1 is the line resistance; U dc is the DC side output voltage; u cd ,u cq It is the component of the output voltage of the series APF in the d and q coordinate system.

[0053] The mathematical model of the parallel APF in the d, q, 0 coordinate system is:

[0054]

[0055] Where L2 is the filter inductor on the parallel side, R2 is the line resistance, id ,i q , i0 is the load side inductor current in the d, q, 0 coordinate system, u Ld ,u Lq , u0 is the component of the parallel APF output voltage in the d, q, 0 coordinate system.

[0056] The current of the parallel APF filter capacitor is:

[0057]

[0058] In the formula, i Cd ,i Cq ,i C0 is the compensation current of the parallel APF in the d, q, 0 coordinate system, and C2 is the filter capacitor.

[0059] The control strategy of UPQC is given in detail below.

[0060] Step 1: To ensure that the current control quantity and voltage control quantity are always sinusoidal, the series APF is controlled as a fundamental sinusoidal current source with a very high impedance to isolate the voltage disturbance and the spectrum current in the nonlinear load from flowing into the grid, and to control the input current of the grid to be a sinusoidal wave and in phase with the fundamental positive sequence voltage of the grid. The parallel APF is controlled as a fundamental sinusoidal voltage source, and its impedance must be small enough to isolate the harmonic current and reactive current in the load current from entering the grid, and to compensate for the harmonic voltage and unbalanced voltage, ensuring that the output load voltage is in phase with the fundamental positive sequence component of the grid voltage.

[0061] Step 2: Decouple the series-parallel APF. In the synchronous rotating coordinate system, the decoupled series-parallel APF is a linear continuous system with a single input and a single output. Therefore, the compensation control of the series-parallel APF is realized in the synchronous rotating coordinate system. Among them, the phase control target of the input current and output voltage of the UPQC is the phase of the positive sequence component of the grid voltage. Therefore, the phase of the fundamental positive sequence component of the grid voltage is obtained through the phase-locked loop, and the frequency and phase angle of the fundamental positive sequence component are accurately determined, and used for the coordinate transformation of the controller design.

[0062] Step 3: From the mathematical model of the series APF, we know that the output i of the series APF d ,i q In addition to being affected by the output of the current PI controller, it is also affected by the coupled inductor voltage +ωL1i q , -ωL1i d Therefore, a feedforward compensation is introduced to eliminate the coupling of the d and q axes. After the introduction of feedforward, a dual-input dual-output system can be converted into two independent single-input single-output systems to achieve the decoupling of the d and q axes, as shown in the decoupling control in the figure.

[0063] It can be seen that the DC side voltage and i d ,i q The PI controller can be used to stabilize the DC side voltage and track the DC bus voltage through the d-axis current. The DC voltage controller can be designed as:

[0064]

[0065] In the formula, K pdc and K idc are the proportional coefficient and integral coefficient of the DC side voltage PI controller, I dc is the compensation increment for system loss.

[0066] In the synchronous rotating coordinate system, the series side APF current controller can be designed as:

[0067]

[0068] In the formula, K dpi , K dii is the proportional coefficient and integral coefficient of the current PI controller; u 1d ,u 1q are the outputs of the controller respectively.

[0069] Since the series APF only synthesizes the positive sequence current component, the reference current of the q axis It can be set to 0 to obtain a balanced sinusoidal current. Then: In the formula, i dk for i Lk The d-axis component after passing through a low-pass filter (LPF).

[0070] Finally, the SVPWM control technology is used to control the on and off of the series APF bridge arm to obtain a three-phase symmetrical sinusoidal input current, such as Figure 5 This is the overall control block diagram of UPQC in series with APF.

[0071] Step 4: From the parallel APF mathematical model, it can be seen that the parallel APF system is divided into two subsystems, namely the cross-coupled d and q axis system and the 0 axis system independent of the d and q axes. In order to eliminate the coupling between the d and q axes, a double decoupling dual-loop control strategy is introduced. After decoupling, the parallel APF system is converted into three independent single-input point output systems. In order to obtain faster dynamic response speed and higher steady-state compensation accuracy, the parallel APF adopts voltage and current dual-loop control, in which the inner loop tracks the inductor current feedback and the outer loop controls the load voltage. The voltage outer loop can be designed as:

[0072]

[0073] In the formula, K pu , K iu is the proportional coefficient and integral coefficient of the voltage outer loop PI controller, i cd ,i cq ,i c0 It is the load end compensation current in the synchronous rotating coordinate system.

[0074] Since the d-axis coincides with the UPQC grid voltage vector direction, the voltage reference quantities of the q-axis and the 0-axis are and Both can be set to 0. Using the P controller to amplify the error, the inner loop current can be designed as:

[0075]

[0076] In the formula, K p is the proportional coefficient of the P controller of the inner loop current, i id ,i iq ,i i0 is the inductor current on the parallel side in the synchronous rotating coordinate system.

[0077] Finally, the SVPWM control technology is used to control the on and off of the parallel APF bridge arm to obtain a three-phase symmetrical sinusoidal output voltage, such as Figure 6 This is the overall control block diagram of UPQC parallel APF.

[0078] The device proposed in this embodiment mainly includes: a series active power filter (series APF), a parallel active power filter (parallel APF), a Z-type grounding transformer, a three-phase bridge circuit for chopping and inverter multiplexing, a DC side energy storage unit, a photovoltaic power generation unit, and a control unit. The device converts energy between the energy storage unit on the DC side and the photovoltaic cell on the AC side through a bridge circuit. The Z-type grounding transformer provides a grounding reference for the photovoltaic cell and eliminates the third harmonic of the AC side voltage. The series and parallel APFs are connected through DC voltage stabilizing capacitors to form a unified power quality controller (UPQC), which is used to compensate for voltage distortion and harmonic current respectively. The device can operate in boost chopping mode, three-phase grid-connected inverter mode, and inverter and chopping hybrid mode to meet different energy conversion requirements. In boost chopping mode, the low-voltage DC generated by the photovoltaic cell is converted into high-voltage DC and stored in the energy storage unit; in grid-connected inverter mode, the electric energy in the energy storage unit is converted into AC and connected to the grid; in the chopping and inverter hybrid conversion mode, boost chopping and grid-connected inverter are realized at the same time. Regarding the control of the above-mentioned converter, the control unit can use PI and P controllers to control the series APF and the parallel APF to ensure that they act as fundamental sinusoidal current sources and voltage sources, respectively isolating the harmonics and reactive currents on the grid side, and improving the power quality. In the grid-connected inverter mode and the chopping and inverter hybrid conversion mode, a unified power quality conditioner (UPQC) is connected to adjust the power quality of the grid-connected AC side, compensate for the harmonic current, reactive current and voltage drops on the grid side, and thus ensure that the converter can provide high-quality AC power when connected to the grid. Therefore, the above-mentioned device can realize dynamic adjustment and comprehensive management of power quality, and provide an efficient and stable conversion solution for photovoltaic power generation systems.

[0079] Embodiment 2

[0080] A high power quality current conversion method adopts the high power quality current conversion device with chopper inverter multiplexing as described above to perform current conversion.

[0081] As a preferred implementation, under the chopping condition, the Z-type grounding transformer in the high power quality converter device serves as a DC path of the chopping circuit and controls the unified power quality controller in the high power quality converter device to be disconnected.

[0082] As a preferred implementation, under the inverter working condition, the Z-type grounding transformer in the high power quality converter device is used as an AC path to control the unified power quality controller in the high power quality converter device to access the AC path to achieve control of power quality.

[0083] It can be used as a preferred implementation method that, under the inverter and chopper mixed conversion conditions, the Z-type grounding transformer in the high power quality converter device serves as the AC path and the DC path of the chopper circuit; the unified power quality controller in the high power quality converter device is controlled to access the AC path to achieve control of the power quality.

[0084] The relevant technical solution is the same as that in Embodiment 1 and will not be described in detail here.

[0085] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A high power quality converter with chopper inverter multiplexing, characterized in that: include: Series APF, parallel APF, Z-type grounding transformer, three-phase bridge circuit with chopper inverter multiplexing, DC side energy storage unit, photovoltaic power generation unit in parallel with AC side, and control unit; Among them, one end of the Z-type grounding transformer is connected to the AC side of the three-phase bridge circuit, and the other end is connected to the external load grid. The grounding transformer is connected to the AC side and at the same time leads out a grounding point to connect to one end of the photovoltaic power generation unit, and the other end of the photovoltaic power generation unit is connected to the three-phase bridge circuit; the series APF and the parallel APF are connected through a DC stabilizing capacitor to form a unified power quality controller, and one side of the series APF and one side of the parallel APF in the unified power quality controller are connected between the Z-type grounding transformer and the three-phase bridge circuit through a series transformer and a parallel transformer respectively; The parallel APF is used to compensate for voltage sinusoidal distortion and voltage imbalance under the inverter condition, and the series APF is used to filter harmonic current and compensate reactive power under the inverter condition; the Z-type grounding transformer is used to provide a grounding reference to the photovoltaic power generation unit under the chopping condition, and to offset the zero-sequence harmonic component of the AC side voltage through its Z-type connection under the inverter condition. The inverter condition is that the electric energy is from the DC side energy storage unit to the load grid, and the chopping condition is that the electric energy is from the photovoltaic power generation unit to the DC side energy storage unit; the control unit is used to drive the series APF and the parallel APF to achieve dynamic power quality regulation.

2. The high power quality converter device according to claim 1, characterized in that: The control unit is specifically used to use a PI controller to generate a PWM signal to control the series APF, and use a PI controller and a P controller to generate a PWM signal to control the parallel APF.

3. The high power quality converter according to claim 1, characterized in that: The control unit is specifically used to adopt a direct control method to directly control the series APF to be a sinusoidal voltage source and the parallel APF to be a sinusoidal current source without detecting voltage and current distortion.

4. A high power quality conversion method, characterized in that: The high power quality current conversion device with chopper inverter multiplexing as claimed in claim 1 is used for current conversion.

5. The high power quality conversion method according to claim 4, characterized in that: Under the chopping condition, the Z-type grounding transformer in the high power quality converter device serves as a DC path of the chopping circuit, and controls the unified power quality controller in the high power quality converter device to be disconnected.

6. The high power quality conversion method according to claim 4, characterized in that: Under the inverter working condition, the Z-type grounding transformer in the high power quality converter device serves as an AC path, and controls the unified power quality controller in the high power quality converter device to access the AC path to achieve control of power quality.

7. The high power quality conversion method according to claim 4, characterized in that: Under the inverter and chopper hybrid conversion conditions, the Z-type grounding transformer in the high power quality converter device serves as the AC path and the DC path of the chopper circuit; the unified power quality controller in the high power quality converter device is controlled to access the AC path to achieve control of power quality.

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