This article first introduced the working principle of off-grid inverters and grid-connected inverters, followed by a description of how to distinguish grid-connected inverters and off-grid inverters. Finally, we introduced how grid-connected inverters are used off-grid. Follow the specific Xiao Bian together to find out. Grid inverter Grid-connected inverters are generally classified into photovoltaic grid-connected inverters, wind-power grid-connected inverters, power generation grid-connected inverters, and other power generation equipment. The most important feature of grid-connected inverters is their high power and low cost. Grid-connected inverters are generally used in systems with large-scale photovoltaic power stations. Many parallel PV strings are connected to the DC input of the same centralized inverter. Generally, high-power three-phase IGBT power modules are used, and the power is relatively low. The small use of field effect transistors, while using a DSP switching controller to improve the quality of the power produced, makes it very close to a sine wave current. Grid-connected inverter schematic Off-grid inverter Off-grid Inverter The complete system solution using modular components Xantrex XW consists of several manageable building blocks: XW inverter/charger, solar charge controller, automatic generator start-up module, and system control board. Off-grid inverter working principle diagram How to distinguish grid-connected inverters and off-grid inverters First, photovoltaic inverters, also known as photovoltaic inverter power regulators, can be divided into two types: stand-alone (off-grid) and grid-connected, depending on the use of inverters in the distributed blue-sky photovoltaic power generation system. The types of classification are mainly divided by the use of photovoltaic power generation systems. There is no need to investigate them. It is only necessary to know that Jiaotong University’s blue sky photovoltaic power generation can be divided into two types according to different uses of photovoltaic systems. The second is based on the inverter's own waveform modulation method can be divided into square wave inverters, step-wave inverters, sine wave inverters and combined three-phase inverters, which is the production of inverters in the National Tsinghua University The waveform of the device itself is divided. The third is divided into transformer-type inverters and transformer-less inverters based on the photovoltaic grid-connected systems used in Jiaotong University. This is mainly a classification of photovoltaic inverters from different requirements of grid-connected systems. How to use off-grid inverter off-grid Grid-connected inverters can be used directly as off-grid inverters Grid-connected inverters send energy directly to the grid, so tracking the frequency and phase of the grid is equivalent to a current source. Of course, there are also some inverters that have low-voltage ride-through capability and can do PQ adjustment. The off-grid inverter is equivalent to establishing an independent small grid, mainly controlling its own voltage, which is a voltage source. Grid-connected inverters do not require energy storage, but their energy is not regulated. How much of the PV is sent to the Internet will not depend on people's needs. The grid does not like it. Off-grid energy storage is generally required, not to send energy online. The power grid has no right to interfere.

On June 21, the first grid-side energy storage project in Jiangsu Province, the Jianshan Energy Storage Power Station, was successfully connected to the grid in Danyang, Zhenjiang, marking the official landing of grid-side energy storage project technology application in Jiangsu. The project is located in the 110 kV Jianshan Substation in Danyang City, with a power of 5 MW and a battery capacity of 10 MWh, covering an area of 1.8 acres. It is fully prefabricated and is connected to Jianshan through a 10 kV cable line. 10kV power grid side of the substation. In September last year, due to the decommissioning of three 330 MW coal-fired units of Jianbi Power Plant, Zhenjiang East Power Grid will face a power shortage situation during the peak of summer electricity consumption this year. As one of the measures to alleviate the power supply pressure during the summer peak season in eastern Zhenjiang in 2018, Jiangsu Electric Power Co., Ltd. combined the advantages of short construction cycle and flexible layout of electrochemical energy storage power stations to implement the construction of energy storage projects in the eastern area of Zhenjiang. As one of the eight grid-side energy storage power station projects in Zhenjiang, the energy power plant has been intensively implemented since it started construction in May. “Danyang occupies most of the load in the eastern part of Zhenjiang. The grid-connected operation of the Jianshan energy storage power station will not only help alleviate the power supply pressure of Danyang Power Grid this summer, but also provide support for the scale development of renewable energy sources. Regional power grids have peak and frequency modulation capabilities, improve power grid voltage levels, improve power supply quality, and ensure safe power grid operation. "State Grid Danyang Power Supply Company Development and Construction Department Director Zhang Jiangqun introduced. This energy storage power station is an important application practice of Jiangsu Electric Power to promote comprehensive energy service business in depth. The project is equipped with intelligent network load interactive terminals, anti-island protection and frequency and voltage emergency control systems and other equipment, which can quickly remove faults and ensure the safety of regional power grids. 

The integration of new energy sources is a relatively sensitive issue. It is also a problem often encountered in the design of power systems. It is as small as a single project access system, and as large as a region of new energy grid consumption, both There is a lot of attention. The impact of new energy grid-connected on the grid New energy actually contains a lot of things. Here we mainly discuss wind power, photovoltaic and distributed power. Of course, distributed power sources may contain small photovoltaics, etc. The impact on the power grid is actually divided into two parts. One is the new energy source that accesses the main network, and the other is the new energy source that is connected to the distribution network. The impact of the two is different. . 1) Accessing the new energy of the main network, taking wind power as an example, mainly has the following effects: Increase the peaking and frequency adjustment difficulty: wind power is random and intermittent. The fluctuation range is large and the fluctuation frequency is irregular. The anti-peak characteristics of wind power increase the difficulty of peak shaving. According to the statistics of the Northeast, Mengxi and Jilin power grids, the probability of wind power reverse peaking is 60%, 57% and 56% respectively. Due to the access of wind power in Jilin Power Grid, the peak-to-valley difference during the year has reached 210 days. Due to insufficient peaking capacity, Jilin and Mengxi power grids have experienced abandonment of wind during low-load periods. The figure below shows that the wind power output and the grid load show strong anti-regulation characteristics. (North China Power Grid Zhangjiakou Area) Increasing the difficulty of grid voltage control: Wind farm operation relies too much on system reactive power compensation, which limits the flexibility of grid operation. According to statistics, affected by wind power: the 220 kV bus voltage of the substation along the Ximeng ashengliang wind power base in Mengxi Power Grid is maintained at 1.1 times of the rated voltage throughout the year; when the 500 kV reactive power compensation equipment in the Mengxi Tala area is out of service, 220 The kilovolt system voltage rises to a maximum of 257 kV. Insufficient local grid access capability: Most wind farms are at the end of the power grid. After large-scale access, a large number of Internet access during the wind power boom period, the power grid transmission trend is increasing, the heavy-duty operation lines are increasing, and the thermal stability problem is becoming increasingly prominent. Since 2007, the wind power and small hydropower in Gansu Jiuquan area have developed rapidly, and the contradiction has been intensified. Despite the use of overload cutting machines and substation split operation to improve the transmission capacity, the wind field abandonment problem still exists for a long time. Increasing the stability risk of the grid: The intermittent nature of wind power increases the potential risk of stable operation of the grid. First, the trend caused by wind power is changeable, which increases the difficulty of operation control of the transmission section with stable limits. Second, the increase of wind power generation component causes the inertia of the system to decrease and affect the dynamic stability of the grid under the same load level. After the system fails, the wind turbine may not be able to re-establish the terminal voltage and lose stability, which will cause the voltage stability of the regional power grid to be stable. 2) Access to the new energy of the distribution network, taking distributed power as an example, mainly has the following effects: Protection problem: The change of the current will cause the sensitivity of the line protection to decrease and refuse to move; the second will cause the line protection to malfunction; the third will cause the instantaneous quick-break protection of the adjacent line to malfunction and lose selectivity; the four times will cause coincidence The gate was not successful. Countermeasures: First, limit the grid-connected capacity of the distributed power supply; second, increase the impedance of the distributed power supply isolation transformer; third, increase the direction power protection of the distributed power supply outlet; the most critical point is that before the distributed power supply is connected to the network, it must be fully Based on investigating the impedance of the power generation equipment, calculate the increase in short-circuit current that may be caused by the distributed power source to determine the grid-connected capacity of the distributed power source. Voltage problems: First, the impact of distributed power supply start and stop, and second, the intermittent impact of distributed power supply. Countermeasures: First, from the perspective of voltage support, distributed power sources have a significant positive effect. This depends on the reasonable choice of access location, reasonable selection of capacity and proper scheduling; second, under normal circumstances, distributed power supply should be more active and less reactive, maintain high power factor operation; reduce line voltage dependence on distributed generation; For distributed power access locations, appropriate reactive voltage support equipment should be installed and put into operation when the distributed power supply is out of operation. Power quality problem: distributed power generation is connected to the grid through power electronic inverters, which is easy to generate harmonics and three-phase voltage/current imbalance. The randomness of output power is easy to cause grid voltage fluctuation and flicker; distributed power supply is directly connected to the user. Into the grid, power quality issues directly affect the safety of users' electrical equipment. Countermeasures: First, the problem of distributed power supply and DC injection, in the grid-connected mode, through the distributed power control function, the isolation transformer is installed at the outlet of the larger-capacity inverter-type distributed power unit. The second is the voltage imbalance problem. For a distributed power supply unit with single-phase grid connection, it can be connected to a phase with more load, thus reducing the voltage imbalance. There are still many problems in accessing the distribution network: short-circuit current problem, communication measurement problem, island problem, etc., but the fundamental lies in: distributed power supply access distribution network, power flow distribution change, power distribution system from radial to multi-power structure .

Researchers at Stanford University have recently developed a new battery cathode using nanocomposites made of copper compounds that can be recharged 40,000 times. Professor Cui Yi, associate professor of materials science and engineering at Stanford University, said: "Because the technology is inexpensive and durable, it can meet the large-scale energy storage needs of the power grid." Cui Yi said: "This research provides solutions for wind power and solar energy that cause sudden drop in power generation due to the weather. Because we can't guarantee that every day is a sunny day, and we can't guarantee the daily wind, so if we want to develop on a large scale Optoelectronics, intermittent is its main obstacle.If we can have an efficient, durable and reusable battery, then we can store excess power generated by wind energy and solar energy. At the same time, the cost of the battery should not be too expensive. Otherwise, no commercial expansion can be achieved." Reaching 40,000 Charges and Discharges At present, researchers at Stanford University have partially realized the idea that a new electrode using copper compound nanomaterials can maintain its battery capacity of 83% after repeated charging 40,000 times. However, the number of charge and discharge times of conventional lithium-ion batteries is 400, and then the capacity drops rapidly. Stanford University's Graduate School of Materials Science and Engineering Colin Wells said: "Through several charge-discharge experiments a day, we expect the electrode to have a useful life of 30 years." The current research has been published in Nature Communications. In the magazine, Kline Wales is the main author. The co-author of the study, Cui Wei's tutor, Cui Yi, said: "Battery can be recharged so many times, performance did not diminish, this is a breakthrough." Researchers first used Prussian blue (ie, ferrocyanide). They then replaced one half of the iron with copper, used the resulting compound to make crystalline nanoparticles, and applied the particles to a cloth-like carbon matrix. Then, they immersed this electrode in a potassium nitrate electrolyte solution. Colin Wells said: "Because potassium ions can move freely, the charge and discharge cycles of the electrodes are very fast, which is very important." According to reports, the new battery uses cheap materials for chemical reactions. The principle is the same as that of lithium ion. Sodium or potassium ions move between the electrodes for charge and discharge. Cui Yi said: "To connect storage power, the battery will be very large, and sodium and potassium are attractive, because they are more productive and cheaper." Stronger than lithium-ion batteries Cui Yi led the research group There is a lot of research is based on lithium ions, lithium-ion batteries have high energy density, which means that they are relatively small, more suitable for portable electronic products such as notebooks Computer and so on. However, when it comes to energy storage in the grid, energy density is not as important. You can have a battery that is as big as a house. It doesn't need to be portable. In addition, the cost is also a big consideration. Some components of lithium-ion batteries are expensive, and it is unclear how large the cost of building a large-scale lithium-ion battery for grid energy storage is. Wells said: "At that time, we thought that if we want to develop batteries for energy storage in the grid, we should consider raw materials other than lithium ions. The materials we choose, such as iron, copper and nitrogen, are very cheap. We use aqueous electrolyte instead of organic electrolyte, and the cost is reduced." The main limitation of the new electrode is its chemical properties that make it suitable only as a high voltage electrode. However, each battery requires two electrodes - a high-voltage cathode and a low-voltage anode, to generate electricity by a voltage difference. Researchers need to find another kind of material to make an anode before they can make a battery. Cui Yi said: "At present, they are testing various substances to make anodes, and there are already some potential suitable materials." Robert Huggins, emeritus professor of the School of Materials Science and Engineering at Stanford University, said that although no complete battery has been formed at present, the performance of the new electrode has exceeded that of any existing battery. This discovery provides a good solution for energy storage in wind power systems. Cui Yi said: “The electrode materials that have been developed have great prospects in the laboratory stage, but there are difficulties in commercialization. There is no such problem for this new electrode. Commercialization of new electrodes is not difficult. We will chemical substances. Put in the flask to get the electrode material, you can get more raw materials. We don't have great technical challenges to produce this kind of battery." The new electrode has a lower charge capacity. In addition, there are also industry insiders who have presented this technology with deficiencies. Jay Whitaker, professor of materials science and engineering at Carnegie Mellon University, said: "This electrode has a good cycle time compared to other electrodes, but it also has its own shortcomings. Its charge capacity is low, per gram. When the material has a capacity of only 60 mAh, compared to the manganese oxide cathode of 100 mAh, although the cost is low, the cost of using copper instead of iron has increased. Donald Shadevi, professor of materials science and engineering at the Massachusetts Institute of Technology, said: "The most important indicator for large-scale grid-connected storage is the price of energy generated by each charging cycle. This new material has a clear advantage because it can be realized. With tens of thousands of cycles of charge and discharge, the cost will also be reduced. The overall performance will be better than sodium-sulfur batteries." According to Battery Researcher Christophe Johnson of Argonne National Laboratories, “In addition to cost and cycle time, the round-trip energy efficiency is also very important for grid energy storage so that no energy is wasted during the charging process. The cost of the electrode, however, is excellent for its efficiency and cycle life.” Researchers also need to develop an anode to form a complete battery cell. ”