In recent years, the increasing population and technological development have led to a sharp increase in demand for energy.The limited nature of non-renewable resources and the infinite nature of human needs form a major contradiction.In the 21st century, the world is calling for the use of clean renewable green energy to replace fossil fuels. Photovoltaic grid-connected power generation is one of the important green energy sources.The Chinese government has also invested a lot of energy in the development of new energy, provided more material support, and formulated corresponding legal norms, “Renewable Energy Law”, which has promoted the research and development of clean energy power generation technology including solar energy.Among all renewable energy sources, solar energy is the most abundant, and the energy absorbed by the land from the sun is about 21840TW.Solar energy has shown great potential in technological research and sustainable development.
Nowadays, all countries have begun to adopt renewable portfolio standards (RPS), which stipulate the share of clean renewable energy power in the electricity supply structure by a certain date in the future.It is expected that by the middle of the 21st century, solar photovoltaic power generation will account for 15% to 20% of the world’s total power generation.The US government authority took the lead in proposing the “Million Solar Roof Plan”, Japan proposed the “New Sun Plan” in promoting photovoltaic power generation, and the Netherlands also proposed the “Dutch Million Solar Photovoltaic Roof Plan” with pertinence. The Chinese government also vigorously advocates the development of photovoltaic industry. The government has successively proposed relevant policies conducive to the development of photovoltaic grid-connected systems, such as “Several Opinions on Promoting the Healthy Development of Photovoltaic Industry” and “Notice of the National Energy Administration on Further Implementing Policies Related to Distributed Photovoltaic Power Generation”, and it is urgent to increase research on photovoltaic grid-connected technology.The most important part of photovoltaic grid-connected system is solar inverter, so many scientific researchers and university teachers and students take solar inverter as a research topic.
1. Overview of research and technological development at home and abroad
1.1 Research status of solar inverters
In the past few years, photovoltaic grid-connected systems have shown great promise.Photovoltaic costs have fallen from 5 euros/watt in 2006 to 1 dollar/watt, and continue to decline, mainly due to the development of semiconductor materials and solar inverter technology. Solar energy has become the third largest renewable resource after hydrogen and wind energy in terms of global new energy installation capacity.Its installed capacity has increased from 1.4GW in 2000 to 100GW in 2012, and will continue to grow significantly, as shown in Figure .It is estimated that by 2030, solar power generation will increase to 402TWh, of which photovoltaic systems will account for 280TWh.
With the strong support of European countries for the photovoltaic industry, European countries dominate the global market in photovoltaic power generation, and they have a large number of excellent solar inverter manufacturers.Germany’s SMA and KACO are the world’s largest solar inverter suppliers, with a large number of mature technologies and products in solar inverter manufacturing. Spain’s Ingetear, Denmark’s Danfoss, Switzerland’s Sputink, etc. also have a large share of the market.Japan and the United States also have a strong industrial foundation and leading semiconductor technology. Relying on their own strength in circuit design, electrical and automatic control technology, they are also highly competitive in solar inverter production, such as Satcon, Power-one in the United States and Sharp in Japan.Some developing countries have also increased their investment in solar inverters.
The research of solar inverters in China originated in the 1980s, which was relatively late compared to foreign countries.In terms of solar inverter manufacturing technology, there is still a certain gap between domestic solar inverters and world-class enterprises in terms of structure, process and stability. However, a number of excellent manufacturers represented by Hefei Guangyuan Power have emerged and have entered the European market.Nanjing Guanya Power and Beijing Kenuo Technology both have large-scale solar inverter production capacity.In terms of market demand for photovoltaics, the 2011 European debt crisis, the US double-reverse policy, fluctuations in raw material prices, and other comprehensive factors led to a decline in the photovoltaic market. Subsequently, the country vigorously adjusted photovoltaic subsidies and introduced stimulus policies. In 2013, the photovoltaic market began to pick up, and the installed capacity reached 4.4GW in 2014.The implementation of the national “Golden Sun Demonstration Project” and a large number of photovoltaic grid-connected subsidies and preferential policies will stimulate the research and development of solar inverters.
1.2 Key technologies of solar inverters
The key technologies of single-phase solar inverters include the optimization of solar inverter topology, maximum power point tracking (MPPT) of photovoltaic arrays, power decoupling transformation, islanding detection, etc. The following describes the corresponding technologies, of which the maximum power point tracking technology will be detailed in Chapter 2.
(1) Optimization of topology structure of micro-inverter
The topology structure of solar inverters directly determines the efficiency and cost of the system. Choosing a high-efficiency, low-cost solar inverter is the guarantee for designing a well-designed photovoltaic inverter system.The single-phase solar inverter structure can be divided into two types according to whether it has an electrical isolation transformer or not.The transformer with power frequency can eliminate leakage current, achieve electrical isolation, but it will lead to an increase in the size of the solar inverter, a decrease in output efficiency, and high-frequency interference.The transformerless structure has no such disadvantages, but it has electrical safety hazards caused by leakage current and no isolation.Currently, the commonly used inverter topologies include H-bridge inverter structure, H5 topology structure, H6 structure, HERIC structure, CHB structure, etc. In practical operation, it is necessary to choose whether to use a transformer according to the project requirements, and to cooperate with the structure and control technology of the solar inverter to improve system performance.
(2) Grid-connected current control technology
Common current controllers include PI control, hysteresis control, repetitive control, and quasi proportional resonant (QPR) control.PI control is simple to implement and widely used in engineering, but it always has static errors when tracking sinusoidal signals and has weak anti-interference ability.Current hysteresis control has fast response speed and simple hardware circuit, but the switching frequency is not fixed, resulting in low control reliability and high output harmonics.Repetitive control can achieve zero steady-state error tracking of given signals, but there is a delay of one power frequency cycle in the control, resulting in poor dynamic performance.Quasi proportional resonant control can generate sufficient gain at the grid fundamental frequency in the control signal to achieve zero steady-state error control under sinusoidal given conditions, while reducing the impact of grid voltage frequency fluctuations and having good dynamic performance.
(3) Island detection technology
Islanding detection technology is designed to prevent the photovoltaic inverter system from failing to detect grid power loss and not automatically disconnecting from the grid network in time when the power grid is in power failure for maintenance or abnormal factors, which may cause damage to the power distribution system, loads, and maintenance personnel.Currently, islanding detection technology can be divided into passive and active detection methods according to the detection method.Passive detection methods do not introduce disturbances to the detection point, and only monitor whether the monitored quantity at the common coupling point exceeds the normal range to determine whether an islanding occurs.Common passive detection methods include overvoltage/undervoltage protection, frequency change rate detection, and harmonic detection.Active detection methods inject disturbances into the monitored parameters according to certain rules, detect the frequency, amplitude, and phase changes caused by the disturbances to the parameters, and then determine whether an islanding occurs.Active detection methods have a smaller blind area and a wider application range compared to passive detection methods.Currently, active detection and passive detection are often combined in practical use.
1.3 Research on solar inverters with auxiliary functions
As a branch of photovoltaic grid-connected systems, building-integrated photovoltaic grid-connected systems are widely used in distributed power generation systems.Building-integrated photovoltaic grid-connected systems are mainly used to meet the needs of small users.In current solar inverter control technology, most single-phase photovoltaic inverter systems focus only on the control of solar inverter output active power, which maximizes the efficiency of grid connection by ensuring that the solar inverter outputs a unity power factor.To maximize the acceptance of photovoltaic systems by the power system, it is particularly necessary to add auxiliary functions to photovoltaic inverters, such as low-voltage ride-through and reactive power compensation technology.
Due to the fact that most building-integrated systems are connected near the user, this makes it easier for photovoltaic systems to compensate for load reactive power.The effectiveness of the regulation of active and reactive power in solar inverters depends on the control technology of the inverter current and the accuracy of the detection of load reactive current.For single-phase reactive current detection, it is first necessary to construct a virtual two-phase orthogonal circuit. The most common method is to directly delay the output current of the load by 90 degrees to construct it, but the biggest drawback of this method is the slow dynamic response caused by the delay.An approach based on discrete Fourier transform is introduced, which has the advantage of a relatively more accurate algorithm for reactive power compensation, but inevitably increases the complexity of the operation and requires high performance from the main control chip.A fundamental current separation detection method is proposed, which mainly separates the fundamental components of the active and reactive currents through formula transformation. However, when the grid frequency fluctuates, its detection may produce errors.
2. Main research content and organizational structure
The power control strategy of solar inverters is the main research content of this article. The theoretical analysis mainly includes the principle of maximum power point tracking for photovoltaic power generation, the active control strategy based on voltage and current double closed-loop, and the reactive compensation control strategy for solar inverters.The experimental verification mainly simulates solar output through the programmable DC power supply 62150H-600s of CHROMA company, and verifies the control performance of solar inverters on the 3KW single-phase solar inverter platform.
The full text is divided into six chapters, and the main contents of each chapter are presented below.
1). This paper mainly introduces the active power control strategy of a single-phase two-stage photovoltaic inverter system.Firstly, the circuit topology of the single-phase two-stage inverter is analyzed to determine the basic structure of the photovoltaic inverter system.Then, based on the mathematical model analysis of solar cell modules, a variable step-size maximum power point tracking algorithm is proposed.Finally, a detailed theoretical analysis of the active power control scheme of the system is made.
2). The main analysis is on the control strategy of reactive power compensation for single solar inverters.Firstly, the principle of reactive power compensation for solar inverters is analyzed, and the control block diagram of the system is obtained.Then, a reference current generation method based on a sine signal integrator is proposed to construct a virtual two-phase current. Through the harmonic current detection ip-iq method, the reactive DC component of the load current is separated, and the grid-connected current instruction containing the active instruction and the reactive instruction is obtained through inverse coordinate transformation. The active control and reactive compensation of the solar inverter are realized by using a double closed-loop control, achieving the effect of a power compensator.
3). This article mainly analyzes and explains the hardware and software design of the solar inverter system.Firstly, it provides a detailed description of the technical specifications of the system and clarifies the design requirements.Then, it introduces the overall hardware design structure of the system and analyzes the design principles of each module.Finally, it introduces the software design of the system and illustrates it with flowcharts.Following these steps, the construction of the photovoltaic inverter system can be completed.
4). A simulation platform for photovoltaic inverter systems was built on the power electronics simulation software saber to verify the feasibility and effectiveness of methods such as maximum power point tracking, active power control for solar inverters, and reactive power compensation for solar inverters.
5). For the experimental analysis part, firstly, an overview of the hardware platform of the solar grid-connected inverter was presented.Then, experiments and analysis were conducted on the active control and reactive compensation of the photovoltaic system. The experimental results were generally consistent with theoretical and simulation analysis.