New features of household photovoltaic grid-connected inverter technology development

Photovoltaic grid-connected inverters are usually divided into several categories according to different application scenarios, such as household, commercial and power station types. Among them, household and commercial types are usually used in photovoltaic systems such as photovoltaic buildings BIPV, BAPV or rooftop power stations. They have similar usage scenarios and common technical characteristics, so they are usually classified into one category, called household or string grid-connected inverters, to distinguish them from large power station or centralized inverters. The 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

● Transformer-free design, conversion efficiency greater than 96%

● Wall-mounted installation, IP65 protection level, meets outdoor use conditions

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

The above basic requirements actually meet the demands of end users, system integrators, installers and other parties, such as the photovoltaic system or photovoltaic inverter to generate more electricity, to be able to connect to the grid safely and reliably, and to facilitate installation, maintenance, system monitoring and electricity bill measurement. Figure 1 shows the application scenario of the household system

Figure 1 Application scenario of residential photovoltaic system

In the development of the photovoltaic market, the earliest to start were foreign household systems. In countries such as Europe and Australia where the photovoltaic market developed earlier and faster, the government introduced corresponding subsidy policies earlier, and relevant institutions introduced grid-connected technical specifications and standards. Driven by the encouragement of these policies and standards, the household photovoltaic market in these countries started and developed rapidly. Photovoltaic inverters are subject to demands and constraints from two aspects. On the one hand, they come from users, whether direct users or indirect users, who put forward product functions and performance requirements for inverter manufacturers. On the other hand, grid-connected inverters must respond to the relevant technical standards and specifications of the grid-connected countries and meet the requirements of product certification to adapt to the grid requirements of the grid-connected area or country. The photovoltaic market is developing very fast in these countries, so the requirements from the above two aspects are also changing rapidly, which has promoted the innovation and replacement of inverter technology and products. This article will comprehensively summarize and analyze some new features and requirements that have recently appeared in terms of technology, product functions, etc. for household grid-connected inverters.

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

In the latest German standard VDE-AR-N 4105 for connecting inverters to low-voltage power grids, which was issued in 2012, there are two important technical requirements. The first is to explicitly require grid managers to reduce active power. The key points are as follows:

◆ For safety reasons: the power grid is overloaded.

◆Grid managers can reduce active power remotely.

◆The inverter should be able to reduce active power in steps less than 10% Pn.

◆ Typical 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 ΣSmax≤3.68KVA, no reactive power adjustment is required, and the power factor range is ±0.95.

◆The system's maximum total apparent power is 3.68KVA《ΣSmax ≤13.8KVA, and operates within the range of ±0.95 according to the characteristic curve provided by the grid manager.

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

The VDE4105 standard described above explicitly requires the regulation and control of reactive and active power, and proposes the regulation method. These put forward new requirements for inverter manufacturers, system providers and other supporting product providers, especially inverters. In order to meet the new standards, low-voltage grid-connected inverters must be upgraded in technology. These upgrades include several specific details. The first is to add the function of adjustable output power quota in control. The second is to make the power factor cosθ adjustable within a certain range, that is, to add the function of reactive power adjustment. Usually, the price paid to achieve the adjustable cosθ within the range of ±0.9 is not large. Third, in order to meet the inverter's ability to accept grid instructions to complete the parameter adjustment required above, the inverter must be able to provide an interface for accepting grid instructions in communication, and the communication protocol must be compatible with some supporting monitoring equipment. Of course, inverter manufacturers do not necessarily provide such products. 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 no longer be sold in Germany. At the same time, other European countries will also refer to the new German standard to formulate their own standards and will introduce similar requirements on these technical points. Soon these standards will be promoted throughout Europe 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, have posed new challenges to these companies. At present, some domestic companies, including Beijing Kunlan, have passed the certification, but many companies are taking a wait-and-see attitude and have not provided similar functions or have not done such certification. Considering short-term interests, many European countries and other countries have not clearly put forward technical requirements similar to the VDE4105 standard. Therefore, these companies holding a wait-and-see attitude have not seriously considered these technical requirements under the drive that their current interests have not been affected or have been affected very little. However, from the perspective of future development trends, Germany, as the country with the most advanced photovoltaic technology, standards and market development, often plays the role of a weather vane. Its policies and standards have a great influence. Other European countries and countries outside Europe have a tendency to imitate and follow. 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 issued by Germany is a constraint and contribution requirement for distributed generation from a technical point of view. The constraint is reflected in the fact that grid managers will be more concerned about the grid's bearing capacity, and the stability of the local grid requires distributed generation devices to reduce the power generation and dynamically adjust the output power. The contribution is reflected in the fact that the inverter can output reactive power to the grid in addition to the active power output, and the output amount, the output capacitive active power or the inductive reactive power can be adjusted. This requirement is entirely based on the fact that modern power electronics technology can achieve reactive power compensation. In the earlier power electronics technology, uncontrollable and semi-controllable devices and backward control technology have restricted the realization of similar requirements. Domestic inverter manufacturers should also fully realize the profound technical background behind the new standard requirements. Distributed generation should be a supplement to the grid. First of all, it must be safe, reliable, controlled and clean (clean grid means that the harmonic content is as small as possible). Therefore, it is completely necessary and necessary to accept instructions from the grid and complete the corresponding parameter adjustments. One of the advantages of micro power sources is precisely bidirectional control.

●Multi-channel MPPT tracking to improve system power generation efficiency

In the first wave of technology competition for photovoltaic inverters, the pursuit of single-machine conversion efficiency and MPPT tracking efficiency have become the technical points of competition. Inverters with higher conversion efficiency will undoubtedly win in this type of competition. At present, on the one hand, string inverters are still developing in terms of technology to improve the overall efficiency of single machines and single-channel MPPT efficiency. On the other hand, the fact that there has been no major breakthrough in power electronic devices and new mature topological structures for a long time has also attracted our attention. There is not much room for improving efficiency. The products of domestic small-power inverter manufacturers and foreign mainstream inverter manufacturers have been on par in terms of efficiency parameters, and even surpassed foreign products in some models. However, the overall efficiency of string-type inverters without transformer design has reached more than 98%. Inverter manufacturers have begun to struggle with the "decimal point". The improvement of single-machine efficiency has reached a saturation line. There is not much room for further improvement, but the cost may be great. For example, multi-level technology can improve system efficiency to a certain extent, but at the cost of increasing the use of single tubes. If the three-phase three-level adopts the traditional two-level full-bridge and uses 6 IGBTs, if it is changed to a three-level topology, the inverter part must be increased to 12 IGBTs. Adding devices means increasing the cost of the entire machine, but the efficiency improvement space is below single digits. These performance improvements are often not paid for by customers. Customers prefer machines with better overall cost-effectiveness, rather than simply pursuing single-machine efficiency.

In fact, it is very difficult to increase the efficiency of solar inverters by 1%. However, it is not uncommon for the power generation effect to be reduced by 10% due to improper system design, and some may even cause the system to be unable to operate normally for a long time. Based on 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 modules are connected in series and connected to the inverter. If a PV module fails in the string, the overall MPPT efficiency of the channel will be affected, which may greatly reduce the efficiency of the system. No matter how high the conversion efficiency and tracking efficiency of the inverter are, it will not work. In order to reduce the risk of reduced system efficiency caused by such failure, there is a technical trend of multi-channel independent MPPT tracking in household small-power inverters. Multi-channel MPPT can reduce the loss caused by the failure of a single MPPT and bring greater flexibility to the system configuration. Figure 2 is a circuit topology diagram of a three-phase three-level grid-connected inverter with two-channel MPPT independent tracking

Figure 2 A two-way independent MPPT inverter topology circuit

Due to the adoption of a new topology design, multiple MPPT tracking channels are provided. The efficiency of a single channel can still be very high, but at the same time it provides users with a 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 that is popular with users.

●Multiple communication solutions to meet flexible monitoring needs

The installation and use of household inverters are relatively complex. They can be used in residential, warehouse, public building, office building, carport and other environments. This also puts forward more flexible and changeable requirements for inverter monitoring. In the previous inverter design, communication was mostly wired, such as RS485-based communication interface. Customers had to do wiring during installation. At the same time, the means of client monitoring were mostly PC-based application software, which could achieve one-to-many monitoring. Now this model has been greatly challenged. Customers are no longer satisfied with the wired method, but more require the inverter to provide wireless monitoring solutions. This change requires the inverter itself to be compatible with wireless communication protocols and interfaces on the one hand, and to provide an overall monitoring solution, especially the wireless monitoring solution for the client on the other hand. For the design of the inverter, breaking through the technical route of the traditional industrial system communication interface and system monitoring solution design will pose a challenge to many inverter companies that have transformed from the traditional power industry. The new communication and monitoring solutions are more based on the technical development of IT communication field, which is completely different from the traditional electrical technology development. In addition, such products and solutions have a certain degree of independence and can be sold separately. Therefore, inverter manufacturers can fully cooperate with manufacturers who specialize in the development of monitoring products and provide overall solutions to customers. Inverter manufacturers only need to open some of their protocols to monitoring manufacturers. This cooperation model is relatively common in the current inverter industry.

Monitoring products are not limited to monitoring inverters and other photovoltaic system products. As mentioned in the previous analysis, the inverter also needs to receive instructions from the power grid and complete some electrical control actions. That is to say, the communication between the monitoring system and the inverter is two-way, and the downlink instructions (monitoring system to inverter) have certain real-time requirements. Therefore, monitoring products are gradually becoming more professional. Companies that provide such products should have a broader and deeper understanding of new technical standards, cooperate more closely with downstream products such as inverters, and continuously improve products and solutions. Figure 3 shows the household system monitoring solution provided by KLNE.

Figure 3 Household system monitoring solution

● Pay more attention to the design of appearance and structure

In the recent photovoltaic exhibitions, fewer and fewer new products have been launched. Inverter manufacturers are becoming more and more similar in key factors such as product functions and performance. The differentiation of competition does not lie in core technology, but has shifted to the differentiation of non-core technologies such as product appearance design, display panels, and mechanical design.

Figure 4: New household inverter design

The end users of household inverters are individuals and families, so the appearance design of the inverter is close to the design of household appliances, beautiful, compact, easy to install, lightweight, fine workmanship, and graphical and intuitive display panels. These features exactly meet people's needs for a daily product that enters the home. In many cases, household inverters are placed indoors for use, so the product itself should meet the characteristics of low noise, beautiful, small size, intuitive display interface, beautiful, and light weight. For inverter manufacturers, they should fully study the target market users' needs for this type of electrical products in terms of appearance, aesthetics, installation, etc., meet the differentiated needs of customers, and make the inverter products not only have safety and high efficiency, but also meet the user's needs for appearance, interface display, installation, etc., so as to better meet customer needs and promote market development and product upgrades.

Domestic inverter manufacturers have already stood on the same starting line as their foreign counterparts in terms of product performance, but they are still lagging behind foreign products in terms of product stability, especially product appearance design. The product appearance design lacks creativity, the appearance is relatively old-fashioned, the display panel is simple and plain, and the color and material are also single, so they lack advantages in competing with similar foreign products in the same market. There are many reasons for this result, both subjective and objective. The subjective reason is that domestic inverter products are developed after foreign inverter products. At the beginning, a lot of efforts and attention were invested in product stability and performance, and no great attention was paid to product features in non-core technologies. On the other hand, the objective reason is that product design does not understand the needs of foreign customers for product appearance and other aspects, and there is also a lack of R&D talents in this area. This is not an individual case of inverter products. In fact, many domestic products have a big gap with foreign products in terms of appearance and customer experience, so this is also one of the reasons for this result. As the competition in the international market for household inverter products becomes increasingly fierce, especially as product functions and performance become more similar, domestic inverter products should pay more attention to product appearance design, cultivate and introduce professionals in product appearance and structural design, conduct in-depth research and understand the needs of foreign users, learn from the experience of foreign counterparts in this regard, and vigorously narrow the gap.

The above summarizes some new technical features 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. Technical constraints and requirements from all aspects make this type of inverter pay more attention to the appearance design of the whole machine. The volume and weight will be further reduced, the power density will be increased, and it will have an advanced display panel and a graphical display interface. It has an interface for wireless communication, multi-channel MPPT tracking and a wider MPPT voltage range. It can accept the adjustment instructions from the grid side to adjust the reactive power, active power and other real-time, and low voltage ride-through functions (German BDEW technical standards). Domestic inverter products have a strong price advantage in foreign sales, but the pursuit of low prices is not a long-term development strategy for an enterprise, nor is it conducive to the overall competition of domestic photovoltaic inverter companies abroad. Blindly low prices will inevitably lead to "double anti-" penalties similar to those encountered by battery components, bringing crises to the development of the entire industry. Therefore, domestic inverter companies should do a good job in product details, actively adapt to the latest technical requirements and specifications, and work hard to improve the comprehensive competitiveness of products.

Beijing Kunlan New Energy Technology Co., Ltd. is an international photovoltaic inverter company that has mastered the core technology of photovoltaic grid-connected inverters. It has always been in a leading position in the domestic market for household grid-connected inverter technology. The inverter brand KLNE Sunteams series and Solatec series inverters have successively won the A-level energy efficiency evaluation results of Photon Laboratory, and the overall performance of the machine is excellent. The newly launched Sunteams 10-15Kw series of medium-power inverters have passed the VDE4105 standard certification, with multi-channel MPPT tracking, and a graphical display interface that allows customers to view key operating data intuitively. The SolartecD series household inverters launched based on the advanced digital technology platform have 2 independent MPPT tracking, meet the VDE4105 standard, graphical display interface, support Bluetooth communication, provide mobile phone monitoring solutions and various wireless monitoring solutions, and the product has a novel and beautiful appearance design. It is a heavyweight product launched by Kunlan based on a good grasp of the market, technology, and customer needs. Beijing Kunlan will continue to provide customers with high-quality and cost-effective products on the premise of ensuring product quality and maintaining technological advancement, and take a long-term and steady development path.

Figure 5 KLNE Solartec household grid-connected inverter