A high power quality inverter device with chopper-inverter multiplexing

Abstract

The present application belongs to the technical field of photovoltaic inverter compensation system, and particularly relates to a high power quality current device with chopping and inverting multiplexing, comprising: series and parallel APF, Z-type grounding transformer, three-phase bridge circuit, and photovoltaic power generation unit; one end of the transformer is connected to the AC measurement of the bridge circuit, and the other end is externally connected to the load power grid; the transformer leads out a grounding point to one end of the photovoltaic power generation unit while being connected to the AC side, and the other end of the photovoltaic power generation unit is connected to the bridge circuit; the series and parallel APF constitute a unified power quality controller, and are both connected between the transformer and the bridge circuit; the parallel APF is used for compensating voltage sinusoidal distortion and voltage imbalance in the presence of inverting conditions, and the series APF is used for filtering harmonic current and compensating reactive power in the presence of inverting conditions; the transformer is used for providing a grounding reference to the photovoltaic power generation unit in the presence of chopping conditions, and offsetting the zero sequence harmonic component of the AC side voltage in the presence of inverting conditions.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of photovoltaic inverter compensation systems, and more particularly to a high-power-quality current conversion device with chopping and inverting multiplexing. BACKGROUND

[0002] With the rapid development of photovoltaic power generation technology, photovoltaic grid-connected systems have gradually become an important way to realize large-scale access of renewable energy to the power grid. In photovoltaic power generation systems, the direct current power generated by photovoltaic cells needs to be converted by a current converter to adapt to the energy storage system and grid connection requirements. The existing photovoltaic grid-connected topology usually includes a direct current to direct current (DC / DC) converter, a direct current to alternating current (DC / AC) inverter, and an energy storage unit. The system mainly realizes the functions of power generation, energy storage, and grid connection. However, this traditional design generally has problems such as a large number of devices, high cost, and complex control. In particular, the power quality of small user-side systems is difficult to guarantee, and phenomena such as harmonic pollution, reactive power imbalance, and voltage distortion easily occur.

[0003] In order to optimize the structure of the photovoltaic power generation system and improve the power quality, a new power generation, energy storage, and grid connection unified system has been proposed in recent years. This system can realize the multiplexing of the same circuit in the chopping (transmission of photovoltaic power generation units to direct current side energy storage units) and inverting (direct current side energy storage units to grid power output) processes through the multiplexing of bridge circuits, thereby reducing the number of devices and the system cost. The bridge circuit in this chopping and inverting multiplexing system can not only serve as a chopping circuit to realize the step-up and regulation of direct current voltage, but also as an inverting circuit to convert the direct current of the energy storage unit into high-quality alternating current suitable for the grid, thereby realizing the bidirectional flow and efficient conversion of electric energy.

[0004] However, there are complex power quality problems in the process of grid connection of photovoltaic power generation units and energy storage units. First, the direct current after conversion by chopping and inverting may introduce harmonic currents, reactive power, and voltage fluctuations, which have an adverse effect on the stability and power quality of the grid. Second, the dynamic changes in system load during the charging and discharging process of the energy storage battery will cause voltage fluctuations and power factor reduction, further exacerbating the deterioration of power quality. SUMMARY

[0005] In view of the above defects or improvement needs of the prior art, the present application provides a high-power-quality current conversion device with chopping and inverting multiplexing, which aims to efficiently improve the power quality in the power generation, energy storage, and grid connection unified system.

[0006] To achieve the above object, according to one aspect of the present application, a high power quality converter device with chopper-inverter multiplexing is provided, comprising: a series APF, a parallel APF, a Z-type grounding transformer, a three-phase bridge circuit of 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] Wherein, 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 externally connected to a load grid, the grounding transformer is connected to one end of the photovoltaic power generation unit through a grounding point on the AC side, 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 voltage stabilizing capacitor to form a unified power quality controller, one side of the series APF and one side of the parallel APF are connected to the Z-type grounding transformer and the three-phase bridge circuit through series transformers and parallel transformers respectively.

[0008] The parallel APF is used to compensate for voltage sinusoidal distortion and voltage imbalance in the presence of inverter working condition, and the series APF is used to filter harmonic current and compensate for reactive power in the presence of inverter working condition; the Z-type grounding transformer is used to provide a grounding reference to the photovoltaic power generation unit in the presence of chopper working condition, and to offset the zero sequence harmonic component of the AC side voltage through its Z-type method in the presence of inverter working condition; the inverter working condition is that the power is from the DC side energy storage unit to the load grid, and the chopper working condition is that the power 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] Further, the control unit is specifically used to generate PWM signals using PI controllers to control the series APF, and to generate PWM signals using PI controllers and P controllers to control the parallel APF.

[0010] Further, the control unit is specifically used 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 using a direct control method.

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

[0012] Further, in the presence of chopper working condition, the Z-type grounding transformer in the high power quality converter device serves as a DC path of the chopper circuit, and the unified power quality controller in the high power quality converter device is controlled to be disconnected.

[0013] Further, in the presence of the inverter working condition, the Z-type grounding transformer in the high power quality converter device serves as an AC passage, the unified power quality controller in the high power quality converter device is controlled to access the AC passage, and the control of the power quality is realized.

[0014] Further, in the presence of the inverter working condition, the Z-type grounding transformer in the high power quality converter device serves as an AC passage, the unified power quality controller in the high power quality converter device is controlled to access the AC passage, and the control of the power quality is realized.

[0015] Overall, compared with the prior art, the technical scheme provided by the present application mainly has the following beneficial effects:

[0016] 1. The present application provides a high power quality converter device with chopper-inverter multiplexing. Considering that the converter device can work in multiple working conditions, the present application introduces a unified power quality regulating device, which is a power compensation device integrating series active filter and parallel active filter, can effectively compensate the harmonic current, reactive power and voltage distortion problem generated in the system operation, improve the power quality, and ensure the stability and reliability of the photovoltaic power generation system when connected to the grid. Therefore, the introduction of UPQC solves the problem of insufficient power quality control in the traditional photovoltaic power generation system. In addition, by combining the voltage regulation and reactive power compensation capability of APF with the DC bias magnetism suppression function of Z-type grounding transformer, the anti-interference capability of the system can be effectively improved, and stable operation under various complex conditions can be ensured. Therefore, the present application can efficiently improve the power quality in the power generation and energy storage grid-connected unified system.

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

[0018] 3. The present application further proposes a high power quality conversion method, which uses the above-mentioned high power quality conversion device to realize an efficient and stable conversion solution for photovoltaic power generation system. Preferably, the cooperative relationship between the Z-type grounding transformer and the UPQC under different working conditions is proposed, and the power quality management is realized efficiently and at low cost. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The overall topology of the high power quality converter device according to the present application is shown in the following figure:

[0020] Figure 2 The schematic diagram of the power quality compensation of the high power quality converter device in the grid-connected inverter operation mode is provided for the embodiment of the present application.

[0021] Figure 3 The schematic diagram of the power quality compensation of the high power quality converter device in the hybrid inverter and chopper operation mode is provided for the embodiment of the present application.

[0022] Figure 4 The equivalent mathematical model schematic diagram is provided for the embodiment of the present application.

[0023] Figure 5 The overall control block diagram of the series APF of the high power quality converter device is provided for the embodiment of the present application.

[0024] Figure 6 The overall control block diagram of the parallel APF of the high power quality converter device is provided for the embodiment of the present application. DETAILED DESCRIPTION

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

[0026] Embodiment one

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

[0028] 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 externally connected to the load grid. The grounding transformer is connected to the photovoltaic power generation unit at one end of the AC side, 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 by a DC voltage stabilizing capacitor to form a unified power quality controller. One side of the series APF and one side of the parallel APF are connected to the Z-type grounding transformer and the three-phase bridge circuit through series transformers and parallel transformers, respectively.

[0029] The parallel APF is used to compensate for voltage sinusoidal distortion and voltage imbalance under the inverter working condition, and the series APF is used to filter out harmonic currents and compensate for reactive power under the inverter working condition; the Z-type grounding transformer is used to provide a grounding reference to the photovoltaic power generation unit under the chopping working condition, and to offset the zero-sequence harmonic component of the AC side voltage through its Z-type connection under the inverter working condition. The inverter working condition is that the electric energy is transferred from the DC side energy storage unit to the load grid, and the chopping working condition is that the electric energy is transferred 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] Current mainstream power quality management solutions for photovoltaic inverters typically include active power filters (APFs) and unified power quality compensation devices. While APFs alone can effectively compensate for harmonics and reactive power, improving power quality, they are limited in addressing DC bias in power transformers. In contrast, Z-type grounding transformers effectively suppress DC bias, protecting the safety and stability of power transformers. Therefore, combining APFs with Z-type grounding transformers can achieve comprehensive improvements in power quality while enhancing the system's anti-interference capabilities and reliability.

[0031] In existing photovoltaic inverter systems, DC bias has long been a key factor affecting power quality and equipment life. The rectification and inversion operations of photovoltaic inverters can introduce DC components, which can cause the transformer's core to saturate, leading to a DC bias effect. While traditional APF solutions can effectively filter harmonics and provide reactive power compensation, they are unable to address DC bias. The Z-type grounding transformer provides a low-impedance path that effectively drains DC components from the system, preventing DC flux accumulation in the transformer and reducing the resulting transformer overheating and noise.

[0032] Furthermore, the combination of a Z-type grounding transformer and an APF significantly enhances the system's grid interference resistance. During PV system operation, voltage fluctuations, flicker, and unbalanced loads often compromise system stability and safety. By combining the APF's voltage regulation and reactive power compensation capabilities with the Z-type grounding transformer's DC bias suppression, the system's interference resistance is effectively enhanced, ensuring stable operation under a variety of complex operating conditions.

[0033] The embodiment introduces a unified power quality conditioner in a chopper inverter multiplexing system. The unified power quality conditioner is a power compensation device integrating series active filter and parallel active filter, which can effectively compensate for harmonic current, reactive power and voltage distortion in system operation, improve power quality and ensure stability and reliability of the grid-connected photovoltaic power generation system. Therefore, the introduction of the UPQC solves the problem of insufficient power quality control in the traditional photovoltaic power generation system. 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 between the power grid and the load, so as to isolate the harmonic current and 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 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 achieving compensation for the load voltage.

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

[0035] The Z-type grounding transformer has two windings on each core column, and the turns are equal and the phases are the same, and the opposite polarities are connected in series to form a zigzag winding. When current flows through the winding of the grounding transformer, the magnetic flux generated by the three-phase winding current is respectively Assuming that the three-phase core reluctance is the same and the core leakage reactance is ignored, then: wherein N is the number of winding turns, R m is the core reluctance.

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

[0037]

[0038] For the zero sequence current component, the phases of the three-phase currents are the same. Taking the A-phase winding as an example, it is assumed that the initial phase of the A-phase current is 0, and the magnetic motive force of this phase is: For the zero sequence current component, the magnetic flux generated by the three-phase winding current is mutually canceled in the grounding transformer core, and no magnetic motive 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, the DC side energy storage unit and the photovoltaic cell unit multiplexed by chopper inversion can realize the power generation, energy storage and grid connection linkage in the grid-connected photovoltaic power generation system. The high power quality converter device provided by the embodiment has the functions of chopper inversion multiplexing and comprehensive power quality management, and can realize the boost chopper conversion mode, the three-phase grid-connected inversion mode and the mixed chopper and inversion conversion mode, and can perform power quality management in each mode. Therefore, through the flexible switching among the three modes, the energy efficient conversion and comprehensive power quality management in the photovoltaic power generation system are realized.

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

[0041] In the three-phase grid-connected inversion mode, the high-voltage DC power in the DC side energy storage unit is inverted to AC power through the three-phase bridge circuit multiplexed by chopper inversion, and is connected to the grid through the Z-type grounding transformer. Since harmonics, voltage distortion and reactive power problems may occur during grid-connected inversion, the system adjusts the power quality through UPQC. The series APF adjusts to a sinusoidal current source through a PI controller, and its high impedance characteristic can effectively isolate voltage disturbances and harmonic currents 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 a P controller, and its low impedance characteristic allows harmonic currents and reactive currents to pass through the APF branch, avoiding their backflow to the grid, thereby compensating for voltage distortion and improving power factor. In this mode, the control unit accurately controls the inversion waveform through SPWM modulation to ensure the high quality of the grid-connected power, and adjusts the output voltage and frequency dynamically to enable the device to adapt to complex grid operating conditions.

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

[0043] Through the coordinated operation of the above three modes, the high-power quality converter device of the embodiment not only realizes 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 grid connection, ensuring the overall improvement of power quality and improving the efficiency and stability of the system.

[0044] As a preferred embodiment, the control unit directly controls the series APF with a PI controller, so that it always maintains a sinusoidal current source of the fundamental wave, thereby making the series APF have very high impedance to isolate the voltage disturbance caused by the nonlinear load on the grid side and the harmonic current and reactive current flowing into the grid, and realizing the compensation of harmonic current and reactive current in the grid connection link of the converter device.

[0045] The control unit directly controls the parallel APF with a PI controller and a P controller, so that it always maintains a sinusoidal voltage source of the fundamental wave, thereby making the parallel APF have very low impedance, allowing the harmonic current and reactive current in the grid side to flow through this branch, thereby isolating the harmonic current and reactive current on the grid side, preventing them from flowing into the grid, and realizing the compensation of voltage in the grid connection link of the converter device, ensuring the sinusoidal nature of the current waveform and the improvement of the power factor.

[0046] As another preferred embodiment, in view of the characteristics of multi-working-condition conversion of the converter device, 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 not needing to detect voltage and current distortion but directly compensating.

[0047] The converter device can work in multiple working conditions, and all need compensation, so a UPQC control link which can quickly and stably compensate is needed. The control unit for controlling UPQC adopts series APF direct control into a series sinusoidal current source in the AC side, and parallel APF direct control into a parallel sinusoidal voltage source in the AC side, to compensate the voltage and current in the AC side. Such control design does not need to detect the compensation amount of voltage and current, and reduces the complexity of the system, and can adapt to multiple working conditions, so that the converter device can output high power quality AC power.

[0048] The mathematical model and related control strategy of the converter device are analyzed below.

[0049] As Figure 1 is a UPQC topology structure with a Z-type grounding transformer. The AC side suppresses the DC magnetic flux in the transformer core through a Z-type grounding transformer, and the AC side adopts UPQC, which is composed 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 adopts three-phase three-wire system, the parallel APF adopts three-phase four-wire system, and the neutral point of the fourth bridge arm of the parallel APF is grounded.

[0050] It is assumed that the series transformer and the parallel transformer of the converter device are ideal transformers, and each switching device of the UPQC is an ideal switching device. The mathematical model of the parallel APF in the three-phase stationary coordinate system is obtained by KCL and KVL, and the mathematical model in the synchronous rotating coordinate system is obtained by Clark transformation and Park transformation. The current flow direction is as shown in Figure 4 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 inductance on the series side; R1 is the line resistance; U dc is the output voltage on the DC side; u cd , u cq is the component of the output voltage of the series APF in the d, q coordinate system.

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

[0054]

[0055] In the formula, L2 is the filter inductance 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] Where i Cd ,i Cq ,i C0 is the compensation current of the parallel APF in the d, q, and 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 and voltage control variables remain sinusoidal, the series APF is controlled as a fundamental sinusoidal current source with high impedance. This isolates voltage disturbances and spectral currents in nonlinear loads from entering the grid, ensuring that the grid input current is sinusoidal and in phase with the grid's fundamental positive-sequence voltage. The parallel APF is controlled as a fundamental sinusoidal voltage source with sufficiently low impedance to isolate harmonic and reactive currents in the load current from entering the grid. It also compensates for harmonic and unbalanced voltages, 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 APFs. In a synchronously rotating coordinate system, the decoupled series and parallel APFs are linear continuous systems with a single input and a single output. Therefore, compensation control of the series and parallel APFs is implemented in the synchronously rotating coordinate system. The phase control targets of the UPQC's input current and output voltage are the phase of the grid voltage's positive sequence component. Therefore, a phase-locked loop is used to obtain the phase of the grid voltage's fundamental positive sequence component, accurately determining its frequency and phase angle for coordinate transformation in controller design.

[0062] Step 3: According to the mathematical model of series APF, the output i of series APF is 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 between the d and q axes. After the introduction of feedforward, a dual-input dual-output system can be transformed into two independent single-input single-output systems, achieving decoupling of the d and q axes, as shown in the decoupling control diagram.

[0063] It can be seen that the DC side voltage and i d , i q related, can use PI controller to stabilize the DC side voltage, and through the d-axis current to track the DC bus voltage, 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 of 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 are the proportional coefficient and integral coefficient of the current PI controller; u 1d , u 1q are the outputs of the controller.

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

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

[0071] Step 4: According to the mathematical model of the shunt APF, the shunt APF system is divided into two subsystems, namely the cross-coupled d, q-axis system and the 0-axis system independent of the d, q-axis. In order to eliminate the coupling between the d, q-axis, a double-decoupling double-loop control strategy is introduced. After decoupling, the shunt 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 shunt APF adopts voltage-current double-loop control, in which the inner loop tracks the inductance current feedback and the outer loop controls the load voltage, and the voltage outer loop can be designed as:

[0072]

[0073] wherein K pu , K iu are the proportional coefficient and integral coefficient of the outer voltage loop PI controller, i cd , i cq , i c0 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 0-axis and can be set to 0. The error is amplified by using a P controller, and the inner loop current can be designed as:

[0075]

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

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

[0078] The device provided in the embodiment mainly comprises a series active power filter (series APF), a parallel active power filter (parallel APF), a Z-type grounding transformer, a three-phase bridge circuit of chopping and inverting 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 at the DC side and the photovoltaic cell at the AC side through the 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 a DC voltage stabilizing capacitor to form a unified power quality controller (UPQC) and are used for compensating voltage distortion and harmonic current, respectively. The device can work in a boosting and chopping mode, a three-phase grid-connected inverting mode, and a mixed mode of inverting and chopping to meet different energy conversion requirements. In the boosting and chopping mode, low-voltage DC power generated by the photovoltaic cell is converted into high-voltage DC power and stored in the energy storage unit. In the grid-connected inverting mode, the energy in the energy storage unit is converted into AC power and fed into the grid. In the mixed mode of inverting and chopping, boosting and chopping and grid-connected inverting are performed simultaneously. For the control of the above-mentioned conversion device, 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, to isolate the harmonic and reactive currents at the grid side and improve the power quality. In the grid-connected inverting mode and the mixed mode of inverting and chopping, the unified power quality conditioner (UPQC) is connected to regulate the power quality at the grid-connected AC side, compensate for the harmonic current, reactive current, and voltage drop at the grid side, and thus ensure that the conversion device can provide high-quality AC power when connected to the grid. Therefore, the above-mentioned device can realize dynamic regulation and comprehensive treatment of power quality and provide an efficient and stable conversion solution for the photovoltaic power generation system.

[0079] Embodiment two

[0080] A high power quality conversion method, which uses a high power quality conversion device with chopping and inverting multiplexing as described above for conversion.

[0081] As a preferred embodiment, in the presence of a chopping working condition, the Z-type grounding transformer in the high power quality conversion device serves as a DC path of the chopping circuit, and the unified power quality controller in the high power quality conversion device is controlled to be disconnected.

[0082] As a preferred embodiment, in the presence of an inverting working condition, the Z-type grounding transformer in the high power quality conversion device serves as an AC path, the unified power quality controller in the high power quality conversion device is controlled to be connected to the AC path, and the control of the power quality is realized.

[0083] As a preferred embodiment, in the mixed inverter and chopper current working condition, the Z-type grounding transformer in the high power quality inverter device is used as the AC passage and the DC passage of the chopper circuit; the unified power quality controller in the high power quality inverter device is connected to the AC passage to realize the control of the power quality.

[0084] The related technical solution is the same as that of the first embodiment, which will not be described herein.

[0085] Those skilled in the art can easily understand that the above description is only the preferred embodiment of the present application, and is not used to limit the present application, and any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A high power quality current 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 to a grounding point connected 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 working condition, and the series APF is used to filter out harmonic currents and compensate for reactive power under the inverter working condition; the Z-type grounding transformer is used to provide a grounding reference to the photovoltaic power generation unit under the chopping working condition, and to offset the zero-sequence harmonic component of the AC side voltage through its Z-type connection under the inverter working condition. The inverter working condition is that the electric energy is transferred from the DC side energy storage unit to the load grid, and the chopping working condition is that the electric energy is transferred 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 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.

3. The high power quality converter device 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 power 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 working 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 power quality control.

7. The high power quality conversion method according to claim 4, characterized in that: Under the hybrid inverter and chopper operation mode, 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 power quality control.

Images