1. Introduction
Photovoltaic power generation technology originated in the early 1980s, but its development was relatively slow due to limitations in technology, cost, and efficiency. With the application and development of modern power electronics and semiconductor technologies, photovoltaic power generation has experienced rapid growth in the last 30 years. Currently, the leading trend in new energy sources is based on power-level converters, making the design and application of converters the most fundamental and crucial aspects.
This paper mainly studies the hardware and software design of the inverter and the design considerations of its LCL filter. Based on this, the theoretical basis for selecting the parameters of the LCL filter in a three-phase photovoltaic inverter is investigated, the parameter calculation formula is derived, and the design method and specific implementation steps of the dual closed-loop control algorithm (voltage outer loop, current inner loop) are given. Simulation verification using Matlab shows that the above parameter selection method and control algorithm are feasible. Based on the above theoretical and algorithmic research, the main hardware circuit and parameter selection scheme of a 500kW three-phase photovoltaic grid-connected inverter are designed, a hardware experimental test platform is built, and grid-connected tests are conducted. The test results show that the system design and the application of the above algorithm in the system are feasible, providing practical guidance for the design and performance improvement of high-power photovoltaic grid-connected inverters.
2. Converter principle and model establishment
2.1 Converter Topology
Figure 2-1 Structure diagram of photovoltaic power generation system
A photovoltaic grid-connected power generation system is a power generation device that converts solar energy into electrical energy through photovoltaics and then inverts it into AC energy to be connected to the power grid. It generally consists of a photovoltaic array, a grid-connected inverter, metering, a display platform, a detection system, etc.
This paper is based on an experimental topology of a three-phase single-stage power frequency isolated 500kW inverter. The three-phase inverter bridge is the core of a photovoltaic grid-connected system. Three-phase grid-connected inverters mainly have several typical topologies, including two-level inverter bridges, three-level inverter bridges, and parallel H-bridges. The laboratory used in this paper employs the most common two-level topology, as shown in Figure 2-2.
Figure 2-2 Topology of a three-phase single-stage power frequency isolated inverter
The voltage Udc output from the photovoltaic string is inverted by a three-phase inverter bridge and fed into the three-phase power grid. The control system includes grid voltage synchronization phase-locked loop, three-phase command current generation, grid-connected current closed-loop tracking control, maximum power point tracking (MPPT), and islanding detection. MPPT is achieved by directly adjusting the amplitude of the grid-connected current. Since there is only one stage of conversion and no voltage control loop, MPPT control causes significant fluctuations in the DC voltage Udc. This makes voltage matching between the PV string and the inverter switches more complex. A power frequency isolation transformer can achieve electrical isolation and alleviate voltage matching issues. Electrical isolation significantly improves system safety, but power frequency transformers significantly increase the cost of the inverter.
2.2 Mathematical Model Analysis of the Converter
In deriving the mathematical model of the SVPWM rectifier in a three-phase three-wire balanced system, we assume that the AC input inductance L is linear, and its capacity and saturation meet the requirements; and that the power devices have no losses. The main circuit of the SVPWM rectifier is shown in Figure 2-3:
Figure 2-3 Main circuit of PWM rectifier
Figure 2-4 Spatial Vector Coordinate Transformation
For a given reference voltage, an SVPWM three-phase inverter can determine the sector containing the synthesized vector and the rotation angle of the reference vector U in the αβ coordinate system by coordinate transformation and according to the sector number formula. It can also calculate the action time of the basic vector and then calculate the corresponding action time of each switch based on the selected combination switches.
Table 2-1 Basic Vector Action Time for Each Sector
Figure 2-5 shows the simulation model of the basic vector action time and the corresponding action time of each switch. It is mainly generated using Matlab functions and the derived formulas above. Figure 2-6 shows the line voltage model of the three-phase inverter circuit output under ideal SVPWM modulation technology. Figure 2-7 is the simulation model of the SVPWM three-phase inverter circuit system, based on the two models established above.
Figure 2-5 Switching Time Calculation
Figure 2-6 SVPWM line voltage output
Figure 2-7 Simulation of SVPWM Three-Phase Inverter Circuit
Figure 2-8 Overall waveform diagram of SVPWM inverter
3. Converter Design Based on LCL Filter
3.1 Selection of Main Circuit Components
This experiment designs a 500kW grid-connected inverter, whose main circuit consists of a voltage regulator, inverter power unit, DC bus support capacitor, and AC filter LCL circuit.
Different functional zones in the main circuit should have corresponding switch isolation. This allows for the debugging of individual functional modules and functional zones, and enables the switching of DC or AC under overcurrent and overvoltage protection functions. Based on the effective protection of the control circuit, the main circuit can be disconnected for protection.
When designing, consider whether the power, voltage, and current capacity of the devices are redundant. Check the relevant datasheets. Also, pay attention to the power consumption of the selected devices, because when testing efficiency at low power output, the percentage of losses will be prominent and needs to be taken seriously.
3.1.1 Power Unit Selection and Design
The main function of the power unit is to track command targets and achieve active power output. Its internal components primarily consist of IGBTs. According to the power formula, a 500kW AC 380V system has a rated output active current of 76A. We control the DC bus voltage to around 500~700V.
4. System Simulation
A dual-closed-loop simulation model of a 500kW three-phase photovoltaic grid-connected inverter was established using SVPWM modulation, as shown in Figure 4-1. The DC voltage was set to 500V, the fundamental frequency to 50Hz, the switching frequency to 3kHz, the inductance to 0.1mH, and the capacitor to be connected in a delta configuration with a capacitance of 300μF. The simulation mode was discrete, and the simulation time was 1s. The waveform diagram shown in Figure 4-2 was obtained after the simulation was completed.
Figure 4-1 System simulation model
(a) Grid-side voltage and current waveforms
(b) Harmonic analysis of grid-side current
Figure 4-2 System simulation waveform
The simulation results above show that the grid-side voltage and current waveforms are relatively stable and sinusoidal, indicating that the system can operate stably and reliably. The active power id value fluctuates around the command value of 1.414 * 1070 = 1512.98 A during the simulation, while the reactive power iq value hovers around 0, verifying the effectiveness of the decoupled control of active and reactive power in the rotating coordinate system. Harmonic analysis of the grid-connected current shows a THD of 2.28%, indicating that the control strategy using an outer voltage loop and an inner current loop, along with the space vector modulation algorithm, effectively controls the system to maintain a low total harmonic distortion (THD) of the grid-connected current under unity power factor operation.
in conclusion
This paper mainly studies the design, modeling, and application of converters based on LCL filters. After model analysis, the application classification of converters is given, and MATLAB models of the rectifier and inverter are established for verification in simulation. After overall simulation and experimentation, the LCL filtering requirements are basically achieved, and the device reaches the rated output and power factor requirements. An experimental platform is built, and the simulation waveforms of the system are given, verifying the effectiveness of the design and providing reference for the design of high-power photovoltaic inverters.
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