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Brief introduction to the development trend of photovoltaic inverter technology

Nov 13, 2020

Brief introduction to the development trend of photovoltaic inverter technology

 

This paper briefly analyzes the technical development route of photovoltaic inverters, focuses on the future development and direction of inverter technology, and gives the technical characteristics and typical application cases of microgrid inverters.

 

  In the light V industry before the rise of the inverter or inverter technology it is mainly applied to such as rail transport, power and other industries. When the light V industry after the rise of the PV inverter into a new energy power generation system of core equipment, will be familiar to everyone. Especially in European and American countries, due to the energy saving concept deeply rooted, light V market earlier carried out, in particular a domestic light -volt systems are rapidly evolving in many countries, household-type inverters have been used as household appliances in use, penetration Higher.

 

  After photovoltaic inverter to convert the direct current generated by the photovoltaic modules into an alternating current fed into the grid, the performance and reliability of the inverter determining the power quality and power generation efficiency, and therefore a photovoltaic inverter in the whole core photovoltaic system status.

 

solar-power-inverter-for-home

  Generally speaking, from the perspective of the relationship between the inverter and the grid, photovoltaic inverters are divided into the following main categories

 

   Grid inverter

 

   Off-grid inverter

 

   Bidirectional inverter

 

   Microgrid inverter

 

  Among them, grid-connected inverters occupy a major market share in all categories and are also the beginning of the development of all inverter technologies. Compared with other types of inverters, grid-connected inverters are relatively simple in technology, focusing on photovoltaic input and grid output. Safe, reliable, efficient, and high-quality output power has become the key measure of this type of inverter. Technical indicators. Among the technical conditions for grid-connected photovoltaic inverters formulated in different countries, the above points have become standard common measurement points, of course, the details of the parameters are different. For grid-connected inverters, all technical requirements are centered on meeting the grid’s requirements for distributed power generation systems. More requirements come from the grid’s requirements for inverters, that is, top-down requirements. Such as voltage, frequency specifications, power quality requirements, safety, control requirements when failures occur. And how to connect to the grid, what voltage level to integrate into the grid and so on, so the grid-connected inverter is always to meet the requirements of the grid, it does not come from the internal requirements of the power generation system. And from a technical point of view, a very important point is that grid-connected inverters are "grid-connected power generation", that is, they generate power when they meet the grid-connected conditions. It does not bear the stability and safety of the grid itself, nor does it involve the light -volt system energy management issues within, so it is simple. It's as simple as the business model of the electricity it generates. According to statistics of foreign countries, has been the construction and operation of light -volt system , the approximately more than 90% of PV grid-connected system, it is used in a grid-connected inverter .

 


  One type of inverter that opposes the grid-connected inverter is the off-grid inverter . Off-grid inverter means that the inverter output is not connected to the grid, but connected to the load, directly driving the load to supply power. Off-grid inverter applications is less, mainly in remote areas, do not have the grid conditions and poor network conditions, or a self-generating, self-use electricity demand at the occasion, off-grid system is emphasized that "self-occupied" . Because of the few applications of off-grid inverters , there are very few technical researches and developments. In the country on the occasion on off-grid inverter technical conditions, standard seldom, which led to the kind of inverter research and development of less and less, showing a declining trend. However , the functions of off-grid inverters and the technologies involved are not simple, especially when combined with energy storage batteries, the control and management of the entire system are more complicated than those of grid-connected inverters. It should be said that the system consisting of off-grid inverters , photovoltaic panels, batteries, loads and other equipment is already a simple microgrid system. The only point is that the system is not connected to the grid and is an "island system".

 

  In fact, off-grid inverters are a basis for the development of bidirectional inverters. Bidirectional inverters actually integrate the technical characteristics of grid-connected inverters and off-grid inverters, and are used in local power supply networks or power generation systems. When used in parallel with the grid. Although there are not many such applications at present, because this type of system is the embryonic form of the development of microgrids and conforms to the infrastructure and business operation mode of future distributed power generation, it is a technically connected role, that is, in the current grid-connected application And the role of a bridge between local microgrid applications in the future. In fact, in some of the light -volt develop faster, more mature countries and markets, home and small regional units of micro-grid applications has slowly developed in the beginning. At the same time, the local government encourages the development of local power generation, storage, and power consumption networks with households as a unit, giving priority to new energy generation for self-use, and the shortfall is taken from the grid. Therefore, two-way inverters need to consider more control functions and energy management functions, such as battery charge and discharge control, grid-connected / off-grid operation strategies, and load reliable power supply strategies. All in all, bidirectional inverter will play more important from the perspective of the entire system of control and management functions, while not a single or only consider the power requirements of the load requirements.

 

As one of the development directions of power grids, local power generation, distribution, and power consumption networks built with new energy power generation as the core will be one of the main development methods for microgrids in the future. In this mode, the local micro-grid will form an interactive relationship with the large-scale grid. The micro-grid will no longer rely on the large-scale grid to operate, but will operate in an independent manner, that is, an island. In order to meet the safety of the region, and the priority of reliable electricity consumption, the grid-connected operation mode will be formed only when local power is abundant or when power needs to be taken from the external grid. At present, due to the immature conditions of various technologies and policies, the microgrid has not been applied on a large scale. Only a small number of demonstration projects are in operation, and most of these projects are connected to the grid. The microgrid inverter combines the technical characteristics of the two-way inverter, and at the same time acts as an important grid management function. It is a typical integrated control inverter integrated machine that integrates inverter, control and management. It is responsible for partial energy management, load control, battery management, inverter, protection and other functions. It will complete the management function of the entire microgrid together with the microgrid energy management system (MGEMS), and will be the core equipment for constructing the microgrid system . Compared with the first grid-connected inverter in the development of inverter technology, it has broken away from the pure inverter function, and carried the function of microgrid management and control, and paid attention to and solved some problems from the system level. The system has a certain degree of typicality, with diesel generators acting as the basic power supply unit, supplemented by renewable energy generation. Storage type provided by the company inverse variable, providing bi-directional inverter, converter, battery discharge, micro-grid management system manages the entire micro grid, contacts A, B, C are controlled by the micro network management system , Can cut off non-important loads according to the power supply during island operation, maintain the stability of the microgrid and the safe operation of important loads.

 

At present, various inverter manufacturers mainly supply grid-connected inverters, a small number of off-grid inverters and bidirectional inverters, and only a few large domestic and foreign manufacturers have introduced microgrid inverters for microgrid applications . Moreover, due to the lack of relevant technical standards and certification standards, this type of inverter still has the characteristics of large differences in product functions and technical levels. Especially for household microgrid products, only a few European technology-leading manufacturers such as Germany's SMA and KACO have a small number of products, and domestic manufacturers have not yet launched corresponding products.

 

 According to different applications, photovoltaic grid-connected inverters are usually divided into household type, commercial type and power station type. Wherein the type and household type are usually used for commercial light volts building light BIPV, BAPV power stations or roof volt systems having similar technical usage scenarios and common characteristics, it is often classified as a class, referred to household -use or String type grid-connected inverters are distinguished from large power station type or centralized inverters. The output power level range of household photovoltaic grid-connected inverters is usually 1kw~30kw or less than tens of kilowatts. This type of inverter usually meets the following requirements:

 

  Single-phase or three-phase low /medium voltage grid connection

 

  Transformerless design, conversion efficiency greater than 96%

 

  Wall-mounted installation, IP65 protection level, meeting outdoor use conditions

 

  The whole machine is light, friendly display interface, beautiful appearance, etc.

 

  These basic requirements actually meet the end users, system integrators, installers and other aspects of the demand requirement of the optical body volt systems or optical volt variant is able to generate more electricity, and capable of safe and reliable network operation, easy installation, maintenance , System monitoring and metering of electricity bills.

  In the light V market development, it is the earliest start of foreign household-type system. In Europe, Australia and other light -volt market development earlier and faster national government earlier issued a corresponding subsidy policy, the relevant agencies issued a grid technical specifications and standards in these policies and standards to encourage promotion, The residential photovoltaic market in these countries has started and developed very quickly. Photovoltaic inverters are subject to demands and constraints from two aspects. On the one hand, they are from users, whether direct users or indirect users, on the product functions and performance requirements of inverter manufacturers. On the other hand, grid-connected inverters have to deal with Relevant technical standards and specifications of the country that is connected to the grid meet the requirements of product certification to meet the grid requirements of the grid-connected area or the country. Light V market at the rate of development of these countries quickly, thus requiring more than two changes from the soon to promote the inverter technology, product innovation and upgrading. This article will comprehensively summarize and analyze some of the new features and requirements in technology and product functions of household grid-connected inverters that have recently appeared.

 

  Able to receive instructions from the power grid and adjust grid-connected electrical parameters

 

There are two important technical requirements in the "VDE-AR-N4105" standard issued by Germany   in 2012 for the integration of inverters into the low-voltage power grid. The first is to clearly require the power grid manager to reduce the active power. The main points are as follows:

 

  For safety reasons: the grid is overloaded.

 

  The grid manager can reduce the active power remotely.

 

  The inverter needs to be technically capable of reducing active power in steps less than 10% Pn.

 

  The usual power reduction levels are 100%, 60%, 30%, 0%

 

  Response time is within 1 minute.

 

  The other is a new requirement for reactive power control, the specific requirements are as follows:

 

  The maximum total apparent power of the system Σ Smax≤3.68KVA, without reactive power adjustable, the power factor range is ±0.95.

 

  The maximum total apparent power of the system is 3.68KVA<ΣSmax≤13.8KVA, and it runs on the characteristic curve provided by the grid manager within the range of ±0.95.

 

  The maximum total apparent power of the system is 13.8KVA< ΣSmax , and it runs according to the characteristic curve provided by the grid manager within the range of ±0.90.

 

  The VDE4105 standard described above clearly requires the regulation and control of reactive and active power, and at the same time proposes adjustment methods. These put forward new requirements for inverter manufacturers, system providers and other supporting product providers, especially Inverter. In order to meet the new standards, low-voltage grid-connected inverters must be upgraded technically. These upgrades contain specific details in several aspects. The first is to increase the function of adjustable output power in the control, and the second is the power factor. The cosθ can be adjusted within a certain range, that is, the reactive power adjustment function is added. Usually , the cost of adjusting the cosθ within ±0.9 is not great. Third, in order to satisfy that the inverter can accept grid commands to complete the above-required parameter adjustments, the inverter must provide an interface for receiving grid commands in communication, and the communication protocol can be compatible with some supporting monitoring equipment. Of course, the inverter manufacturer does not Such products are not necessarily available. Therefore, after the introduction of the VDE4105 standard, a new technical threshold was immediately created for small power inverter manufacturers at home and abroad. Failure to meet the new standard means that such products will not be sold in Germany, and other European countries will also refer to it. Germany’s new standards formulate national standards, and similar requirements will be issued on these technical points. Soon these standards will be promoted throughout European countries and even globally.

 

  As a domestic inverter manufacturer, the market for its household products has always been abroad. Therefore , the newly promulgated grid-connected technical standards including the VDE4105 standard pose new challenges to these companies. At present, some domestic companies, including Beijing Kunlan All of them have passed the certification, but many companies are holding a wait-and-see attitude and have not provided similar functions or have not done such certification. Considering short-term benefits, many countries in Europe and other countries have not explicitly proposed technical requirements similar to the VDE4105 standard. Therefore, these companies holding a wait-and-see attitude are driven by the current interests of not being affected or having little impact. These technical requirements were not considered seriously. However, from the analysis of future development trends, Germany, as the country with the most advanced photovoltaic technology, standards and market development , often plays a role as a weather vane. Its policies and standards have a great influence. Other European countries and countries outside Europe have Following the trend of imitating and following, once the standard is widely promoted, those companies that cannot meet the new requirements of the standard are bound to suffer setbacks and losses.

 

  On the other hand, the new standard introduced by Germany is technically a constraint and contribution requirement for distributed power generation. The constraint is reflected in the requirement that the grid manager will be more able to withstand the grid and the stability of the local grid will be considered. Distributed power generation devices reduce the power generation and dynamically adjust the output power. The contribution is reflected in the fact that in addition to outputting active power, the inverter can also output reactive power to the grid, and the output quota, whether it can output capacitive active or inductive reactive power Regulated, this requirement is completely based on the fact that modern power electronic technology can achieve reactive power compensation. In the earlier power electronic technology, uncontrollable and semi-controllable devices and backward control technology are restricted. A similar requirement was realized. Domestic inverter manufacturers should also fully understand the profound technical background behind the new standard requirements . Distributed power generation should be used as a supplement to the power grid. First, it must be safe, reliable, controlled and clean (cleanliness of the power grid refers to The harmonic content is as small as possible), so it is absolutely necessary and necessary to accept the instructions from the power grid and complete the corresponding parameter adjustment. One of the advantages of the micro power supply is that it is two-way controllable.

 

  Multi-channel MPPT tracking to improve system power generation efficiency

 

  In the first wave of technical competition for photovoltaic inverters, the pursuit of stand-alone conversion efficiency, MPPT tracking efficiency has become a technical point for everyone to compete, and inverters with higher conversion efficiency can undoubtedly win this type of competition. On the one hand, the current string inverter technology is still developing towards improving the overall efficiency of a single machine and the efficiency of a single MPPT. On the other hand, the fact must also attract our attention, that is, in power electronic devices, new and mature topologies In the reality that there is no major breakthrough in a long period of time, there is not much room for efficiency improvement. The products of domestic small-power inverter manufacturers and the products of mainstream foreign inverter manufacturers are on the same level in terms of efficiency parameters. The models even surpass foreign products. However, string type , overall efficiency of the inverter is transformerless design has been reached of more than 98%, the inverter vendors struggle on the "decimal point", the stand-alone efficiency improvement has reached a saturation line, to further enhance The space is not large, but the cost may be high. For example, more Level technology can improve a certain system efficiency, but the cost is increased use of single transistors. If the three-phase three-level adopts a traditional two-level full bridge and 6 IGBTs, if it is changed to a three-level topology, it must Increased to 12 IGBTs in the inverter part, adding devices means increasing the cost of the whole machine, but the efficiency space that can be improved is below single digits. These performance improvements often cannot be paid for by customers, and customers prefer cost-effective integration Better machine choice, instead of purely pursuing stand-alone efficiency.

   In fact, it is very difficult to increase the efficiency of solar inverters by 1%. However, it is not uncommon to reduce the power generation effect by 10% due to improper system design, and some may even cause the system to fail to operate normally for a long time. Proceeding from this fact, new requirements are put forward for inverter design, that is, to meet more flexible system configuration. In the case of single-channel MPPT tracking, all PV components are connected in series and connected to the inverter. If a PV component failure occurs in the string, the overall MPPT efficiency of the road will be affected, which may cause the system The efficiency of the inverter is greatly reduced, and no matter how high the conversion efficiency and tracking efficiency of the inverter are, it will not play a role. In order to reduce the risk of system efficiency reduction caused by such failures, household small-power inverters have emerged the technical trend of multi-channel independent MPPT tracking. Multi-channel MPPT can reduce the loss caused by the failure of a single MPPT and give the system The configuration brings greater flexibility.

Due to the new topology design and the provision of multiple MPPT tracking channels, the efficiency of a single channel can still be very high, but at the same time it provides users with more flexible system configuration and reduces the risk of potential system efficiency loss. There is no doubt that multi-channel MPPT tracking is a new feature popular with users.


  Multiple communication solutions to meet flexible monitoring needs


  The installation of household inverters is more complicated and can be used in residential, warehouse, public buildings, office buildings, carports and other environments. This also puts forward more flexible and changeable requirements for inverter monitoring. In the previous inverter design, the communication was mostly wired, such as the RS485-based communication interface. The customer inevitably had to wire during the installation. At the same time, the client monitoring method is mostly PC-based application software, which can realize one-to-many monitor. Now this model has been greatly challenged. Customers are no longer satisfied with the wired method, but more require the inverter to provide a wireless monitoring solution. This change requires the inverter itself to be compatible with wireless communication protocols. And interface, on the other hand, it can provide an overall monitoring solution, especially a wireless monitoring solution for the client. As far as inverter design is concerned, the technical route that breaks through the traditional industrial system communication interface and system monitoring scheme design will pose a challenge to many inverter companies that have transformed from the traditional power industry. New communication and monitoring solutions are more based on technology development in the IT communication field, which is completely different from traditional electrical technology development, and such products and solutions have a certain degree of independence and can be sold separately. Therefore, inverter manufacturers can cooperate with manufacturers that specialize in monitoring product development and provide overall solutions to customers. Inverter manufacturers only need to open some of their agreements to monitoring manufacturers. This cooperation mode is in the current inverter industry. Medium is the more common one.


Monitoring products are not limited to the monitoring of photovoltaic system products such as inverters. The previous analysis mentioned that inverters also have to receive instructions from the grid and complete some electrical control actions, that is, the communication between the monitoring system and the inverter. It is bidirectional, and the downstream commands (from the monitoring system to the inverter) have certain real-time requirements. Therefore, monitoring products gradually become more professional. Companies that provide such products should have a more extensive and in-depth understanding of new technical standards, and cooperate more closely with downstream products such as inverters to continuously improve products and solutions.


In recent PV exhibitions, fewer and fewer brand-new products have been launched. Inverter manufacturers tend to be the same in key factors such as product functions and performance. The difference in competition is not in core technology, but in turning Differentiation of non-core technologies such as product design, display panel, and mechanism design.

The end users of household inverters are individuals and families, so the appearance design of the inverter is closer to the product design of household appliances, beautiful, compact, easy to install, the whole machine is lightweight, exquisite workmanship, and the display panel is graphical , Intuitive. These features exactly meet people's needs for a household product that enters the home. In many cases, household inverters are used indoors, so the product itself should meet the requirements of low noise, beautiful appearance, small size, and intuitive display interface. Beautiful and low-weight characteristics. For inverter manufacturers, it is necessary to fully study the needs of target market users for such electrical products in terms of appearance, aesthetics, installation, etc., to meet the differentiated needs of customers, and make inverter products In addition to safety, high efficiency and other conditions, it can also meet user needs for appearance, interface display, installation, etc., to better meet customer needs, promote market development and product upgrades.


Domestic inverter manufacturers have reached the same starting line as their foreign counterparts in terms of product performance, but they lag behind foreign products in terms of product stability, especially in terms of product design. The product design lacks creativity and looks more Old-fashioned, the display panel is simple and simple, and the color and material are also very simple. This lacks an advantage in the competition with similar foreign products in the same market.


There are many reasons for this result, both subjective and objective. The subjective reason is that the domestic inverter products were developed after the foreign inverter products, and it was the stability of the products. In terms of performance, a lot of effort and attention has been invested, and the product features of non-core technologies are not paid much attention. On the other hand, the objective reason is that product design does not fully understand the needs of foreign customers for product appearance and other aspects. The lack of R&D talents in this area is not an example of inverter products. In fact, many domestic products are far from foreign products in appearance and customer experience, so this is also a reason for this result.


In the current situation of increasingly fierce competition in the international market for household inverter products, especially the convergence of product functions and performance, domestic inverter products should pay more attention to product design, cultivate and introduce products Professional talents in appearance and structural design, in-depth research and understanding of the needs of foreign users, learn from the experience of foreign counterparts in this area, and vigorously narrow this gap.


The above summarizes some new technical characteristics of household inverters. There is no doubt that the market is not only pursuing a high-efficiency power generation device at present and in the future, but also technical constraints and requirements from various aspects make this type of inverter It is necessary to focus on the appearance design of the whole machine, the volume and weight will be further reduced, the power density will increase, it has an advanced display panel and a graphical display interface, has an interface for wireless communication, has multiple MPPT tracking and a wider MPPT Voltage range, capable of receiving adjustment commands from the grid side for real-time adjustment of reactive and active power, and low voltage ride-through functions (required by the German BDEW technical standard). Domestic inverter products have a strong price advantage in foreign sales, but one’s pursuit of low prices is not a long-term development strategy for a company, nor is it conducive to the overall competition of domestic photovoltaic inverter companies abroad. The price will inevitably lead to the "double reverse" punishment similar to the battery modules encountered, and bring crisis to the development of the entire industry. Therefore, domestic inverter companies should do a good job of product details, actively adapt to the latest technical requirements and specifications, and work hard to enhance the comprehensive competitiveness of their products.