1. The generation and harm of island effect
The so-called islanding effect refers to the situation in distributed power generation systems where the grid is disconnected from the solar inverter grid-connected power generation system for some reason, and the distributed grid-connected power generation systems at each user end fail to detect the power outage in time, resulting in the power generation system directly supplying power to the connected load.
The causes of islanding effect:
(1) Man-made disconnection of the power grid and power generation system, such as deliberate disconnection during maintenance at a point on the grid side or at the inlet;
(2) Sudden interruption of normal power supply caused by equipment failure;
(3) The public power grid detects a fault, causing the disconnector to open, but the grid-connected power generation system or protection device does not detect the fault;
(4) Switching of power distribution system and load;
(5) The effect of natural forces (wind, rain, lightning, etc.).
When the main grid trips, the islanding effect will cause the following hazards to the power generation system and users:
(1) Some lines that are believed to have been disconnected are still live, endangering the safety of maintenance personnel;
If the load capacity is greater than the capacity of the inverter power supply, it will burn out the inverter power supply due to overload;
After the occurrence of an islanding event, the output of the inverter power supply loses synchronization with the grid, which will result in large inrush currents when the grid resumes power supply, causing the line to trip again;
(4) Unstable power supply voltage and frequency (the power grid cannot control the voltage and frequency in the islanding area).
2. Detection and prevention of islanding effect
Detection method of islanding effect:
Due to the relationship between the power grid and distributed generation involved in remote detection technology, it will improve the
The cost of the system is therefore not considered.This article mainly studies local detection technology, including: active detection and
Passive detection includes two parts.Active detection includes:
The active negative sequence component disturbance method is mainly used in three-phase systems containing negative sequence components, so it is not discussed in this article.
(2) Although the active current disturbance method is simple and easy to implement, there is no undetectable area (theoretically) and the transmission
It has the advantages of low harmonic and high efficiency, but it requires high performance of solar inverters.When connected to a multi-unit grid-connected system,
The quality of the power grid will be affected due to the superposition of multiple current disturbance sources, so this article will not focus on it.
The active frequency drift method is to use the fact that the frequency of the solar inverter in the islanding state loses the control of the phase-locked loop
The function of the valve is to expand the offset until it exceeds the OFP/UFP threshold range.
The principle of the adaptive frequency drift method is: Given an error range that allows the voltage frequency of the common node to vary, when the actual frequency is within the error range, a fixed offset disturbance is added to the current frequency;when the actual frequency exceeds the error range, the offset disturbance is automatically increased in a positive feedback manner, accelerating the voltage frequency drift beyond the frequency protection threshold.
① When the PCC (Point of common coupling) node voltage frequency is within the allowable error range, periodically introduce an output current frequency offset that is the same as the direction of the deviation:
② When the actual fp deviation exceeds δf:
Passive detection includes:
(1) Although the voltage harmonic detection method can successfully detect islands in a wide range, its protection threshold is difficult to determine, and transient voltage disturbance factors in the power grid can cause false actions, thus failing to achieve good development.
The phase jump detection method (PJD) detects an islanding condition by detecting whether there is a phase jump in the output voltage of the solar inverter.Its advantage is that since solar grid-connected inverters inherently require phase detection to synchronize with the grid, it is easy to implement.The disadvantage is that the size of the undetectable area varies with the system power factor, making it difficult to choose a reliable threshold value.
Note: The PJD must be detected within the first cycle after the occurrence of an islanding event, otherwise the phase difference will become smaller and smaller due to the effect of the PLL, resulting in detection failure.When the PJD threshold is set too small, it may cause false action of the PJD.
(3) For PCC, the instantaneous opening of switch S will determine the ΔP and ΔQ of the isolated power generation system.After switch S is opened, both ΔP and ΔQ will be zero.
3. Summary
A comprehensive introduction to the most important factor affecting the safety performance of solar inverters: islanding effect.The classification, occurrence, and harm of islanding effect, as well as how to detect and prevent it, are systematically explained. The detection of islanding effect includes remote detection (due to the relationship between power grid and distributed generation, which will increase the cost of power generation, it is not studied), local detection, which includes active detection and passive detection.Due to the low occurrence rate of islanding effect (the probability of failure in normal power grid is about 1/10^6, while the probability of islanding effect is about 1/10^9), only a brief introduction is given.The specific research on islanding effect will be introduced in a special article.
This dissertation on the research and implementation of single-phase photovoltaic grid-connected controllers mainly completes the following tasks:
(1) I have completed the query and reading of materials related to photovoltaic inverters, and have gained a comprehensive, specific, and in-depth understanding of photovoltaic inverter controllers.I know which modules make up the photovoltaic inverter controller, the electronic components required to form these modules, and the relationships between these modules.
(2) Completed the drawing of relevant circuit diagrams.This thesis mainly uses Altium designer 6.9 software to complete the drawing of relevant circuit diagrams.The drawn circuit diagrams mainly include the following four large modules: power module, interleaved flyback module, full bridge module, and feedback circuit.
(3) Completing the programming work for the main chip.This article uses CCS5.5 (Code Composer Studio) to complete the program programming work for the main chip TMS320F28035, mainly completing the related programming work for maximum power point tracking.The program written mainly includes the following four parts:
① Main program part: mainly completes the initialization of the application program, configuration of peripherals, management of the application program, background code looping, and task scheduling;
② Peripheral initialization: mainly completes the initialization of peripherals (such as clocks, phase-locked loops, general-purpose I/O interfaces, etc.) and enables the peripheral clock;
③ Definition of global variables;
④ Calling of subprograms.
(4) Completed relevant modeling, simulation, and experimental work.This article mainly uses MATLAB/SIMULINK to complete the modeling and simulation of the maximum power point tracking system.Including: modeling and simulation of solar panels, BOOST circuit, disturbance observation method, and pulse width modulation.Mainly using single-phase solar inverter prototypes, oscilloscopes, development boards, and other equipment to perform experiments on power connection for open-loop detection of solar inverters, detection of photovoltaic inverter systems with maximum power point tracking systems, and experimental waveforms and analysis of the main circuit.
(5) Proposed a key technology to make the solar inverter output current and grid voltage in phase: phase modulation method of PI control loop.It has been proven through experiments to be able to meet the phase requirements of the grid for input current.
A certain amount of research has been conducted on single-phase solar grid-connected inverters, which are a type of household clean energy conversion device that is efficient, relatively low-cost, and has a long service life.However, there are still many issues that need to be optimized before this solar inverter can be widely applied in practice, and further theoretical research and field testing are required.
(1) The efficiency of solar inverters and the use of components involved in this article are all carried out under ideal or near-ideal conditions. In practical engineering applications, the impact of harsh working conditions on components and solar inverter efficiency should be considered. Therefore, we should appropriately adjust the efficiency according to the different working conditions of solar inverters in practical engineering applications.
(2) Considering safety factors and limitations of experimental equipment, the solar inverter designed in this article does not directly connect to the large power grid for networked power supply.In the future, when large-scale applications are applied to practical applications and networked power supply is necessary, issues such as the pollution of the power grid caused by the generated electrical energy from the solar inverter must be considered.It is necessary to ensure that the generated electrical energy can be well connected to the large power grid.
Due to the limitations of the focus of this article and the low probability of islanding effects, this article only provides a brief introduction and explanation of the islanding effect, and does not conduct a systematic study.In practical engineering, systematic and comprehensive research on islanding effects should be conducted, and appropriate safety measures should be taken to avoid the occurrence of islanding effects.
(4) The efficiency of solar inverters described in this article only refers to the efficiency of converting DC power generated by solar panels into AC power (generally up to 95% or more).However, the overall power generation efficiency is limited by the efficiency of solar panels in converting solar energy into electrical energy in the entire photovoltaic industry chain (generally about 20% for single crystal silicon and much lower for polycrystalline silicon).Therefore, it is necessary to focus on how to improve the power generation efficiency of photovoltaic cells.This part involves the use of new materials, and in the future, interdisciplinary cooperation and innovation should be strengthened in the overall research of photovoltaic inverters.Only in this way can the overall efficiency of photovoltaic inverters be greatly improved.
In short, this single-phase solar grid-connected inverter has high conversion efficiency and low cost, and is particularly suitable for household miniaturization promotion.It will play an increasingly important role in the solar roof plan and the construction of photovoltaic building integration.