New Features of Household Grid-connected Photovoltaic Grid-connected Inverters

Photovoltaic grid-connected inverters are generally classified into household type, commercial type, and power plant type, depending on the application. Among them, the household type and commercial type are usually used in photovoltaic systems such as BIPV, BAPV or rooftop power stations. They have similar usage scenarios and common technical features. Therefore, they are generally classified into a single type, which is called household type or string. Grid-connected inverters are distinguished from large-scale power plants or centralized inverters. Household-type photovoltaic grid-connected inverter output power level range is usually 1kw ~ 30kw or tens of kilowatts or less, this type of inverter usually meet the following requirements:

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

Transformerless design, greater than 96% conversion efficiency

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

Lightweight, friendly display interface, beautiful appearance, etc.

The above basic requirements actually satisfy the requirements of the end users, system integrators, installers and other aspects of the main requirements of the photovoltaic system or photovoltaic generator can generate more electricity, safe and reliable grid-connected operation, easy installation, maintenance, system The monitoring and measurement of electricity bills and other aspects of the appeal.


Figure 1 Residential PV System Application Scenario

In the development of the photovoltaic market, the earliest start was the foreign household-type system. In Europe, countries such as Australia, where the photovoltaic market has developed earlier and faster, the government has earlier introduced a corresponding subsidy policy. The relevant agencies have introduced grid-connected technical specifications and standards, which are encouraged by these policies and standards. The domestic household photovoltaic market started and developed very quickly. Photovoltaic inverters are subject to demand and constraints from two aspects. On the one hand, they are from users, whether they are direct users or indirect users, and they demand the functions and performance of the inverter manufacturers. On the other hand, grid-connected inverters have to deal with The relevant technical standards and specifications of the accessing countries meet the requirements for product certification to meet the grid requirements of the grid-connected regions or countries. The development of photovoltaic market in these countries is very fast, so the requirements from the above two aspects have also changed rapidly, which has promoted the innovation and replacement of inverter technology and products. This article will comprehensively summarize and analyze some of the new features and requirements that have emerged recently for household grid-connected inverters in terms of technology, product functionality, etc.

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

In the "VDE-AR-N 4105" standard, the latest German version of the inverter incorporated into the low-voltage power grid in 2012, there are two important technical requirements. The first is to explicitly require the grid administrator to reduce the active power. The main points are as follows:

For safety reasons: Grid overload.

Grid managers can remotely reduce active power.

The inverter needs to be active in the technology with a step size of less than 10% Pn.

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

Response time is within 1 minute.

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

The maximum total apparent power of the system is ΣSmax ≤ 3.68 KVA, no reactive power adjustment is required, and the power factor range is ±0.95.

The maximum total apparent power of the system is 3.68KVA< ΣSmax ≤13.8KVA, and the grid operator provides characteristic curves within ±0.95.

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

The VDE4105 standard described above clearly requires the adjustment and control of reactive power and active power, and proposes adjustment methods at the same time. These put forward new requirements for inverter manufacturers, system providers and other suppliers of auxiliary products, especially Inverter. In order to meet the new standards, low-voltage grid-connected inverters must be technically upgraded. These upgrades include specific details in several aspects. The first is to increase the controllable output power limit function, and the second is the power factor. The cos θ can be adjusted within a certain range, that is, the reactive power adjustable function is increased. Generally, the cost of adjusting the cos θ in the range of ±0.9 is not large. Third, in order to meet the inverter can accept the grid instructions to complete the above required parameter adjustment, the inverter must be able to provide an interface to receive grid instructions in the communication, the communication protocol can be compatible with some supporting monitoring equipment, of course, inverter manufacturers and This product may not be available. Therefore, after the VDE4105 standard was issued, it immediately created a new technology threshold for domestic and foreign manufacturers of low-power inverters and could not meet the new standards, which means that such products will no longer be sold in Germany, and other European countries will also refer to them. The new German standards set national standards, and similar requirements will be issued at these technical points. These standards will soon be promoted throughout the European countries and even globally.

As a domestic inverter manufacturer, the market for household-use products has been abroad, so new grid-connected technology standards, including the VDE4105 standard, pose new challenges to these companies. At present, some domestic companies, including Beijing Quinlan, are Both have already passed the certification, but many companies also adopt a wait-and-see attitude and have not provided similar functions or have not done such certification. Considering short-term interests, many countries in Europe and other countries currently do not explicitly put forward technical requirements similar to the VDE4105 standard. Therefore, these enterprises with a wait-and-see attitude have no influence or little influence on the current interests, and they are driven by Did not seriously consider these technical requirements. However, from the analysis of future development trends, Germany, as the country with the most advanced photovoltaic technology, standards, and market development, tends to play a role as a weathervane. Its policies and standards have great influence. Other European countries and countries outside of Europe have To follow and follow the trend, once the standard is widely promoted, those enterprises that cannot meet the new requirements of the standard will suffer setbacks and losses.

On the other hand, Germany's introduction of new standards is technically a requirement for distributed power generation to be a constraint and contribution. Constraints are reflected in the ability of power grid operators to be more tolerant to the power grid, the stability of local power grids, and other reasons. Distributed generation devices reduce generation power and dynamically adjust output power. The contribution is reflected in that in addition to output active power, the inverter can also output reactive power to the grid, and the output quota, output capacitance type active or inductive reactive power can be Regulated, this requirement is entirely based on the fact that modern power electronics can achieve reactive power compensation. In earlier power electronics, uncontrollable and semi-controllable devices and backward control technologies were constrained. Similar requirements were implemented. Domestic inverter manufacturers should also fully understand the deep technical background behind the new standards requirements. Distributed generation should be used as a supplement to the power grid. The first thing to do is to be safe, reliable, controlled, and clean. (The cleanliness of the power grid refers to Harmonic content is as small as possible. Therefore, it is absolutely necessary and necessary to accept commands from the grid and complete the corresponding parameter adjustment. One of the advantages of the micro power supply lies precisely in the bidirectional controllability.

Multi-channel MPPT tracking improves system power generation efficiency

In the first technology competition of PV inverters, the pursuit of single-machine conversion efficiency, MPPT tracking efficiency has become a technical point of competition, and inverters with higher conversion efficiency will undoubtedly win in such competition. At present, on the other hand, string inverters are still moving toward improving the overall efficiency of stand-alone machines, single-channel MPPT efficiency, and the fact that we must also pay attention to the fact that in power electronics, a new and mature topology structure In the absence of a major breakthrough within a relatively long period of time, there is little room for improvement in efficiency. The products of domestic low-power inverter manufacturers and foreign mainstream inverter manufacturers have already been in line with certain efficiency parameters. Even more than foreign products on the model. However, the inverter efficiency of the string-type and transformerless design has reached more than 98%. The inverter manufacturers have begun to struggle at the "decimal point". The improvement of stand-alone efficiency has reached a saturation line, and further improvement has been achieved. The space is not big, but the cost may be great. For example, multi-level technology can improve certain system efficiency, but at the cost of increasing the use of single-tube, three-phase three-level if using a traditional two-level full-bridge, use six IGBTs, if you change to three Flat topology, it is necessary to increase the number of inverters to 12 IGBTs, adding devices means increasing the cost of the whole machine, but the efficiency can be improved in the single digits below, these performance improvements are often not be ploughed by customers, Customers prefer the choice of cost-effective and comprehensive machines, and they will not simply pursue single-machine efficiency.

In fact, it is very difficult to increase the efficiency of solar inverters by 1%. However, due to the improper design of the system, the power generation effect is reduced by 10%. However, it is not uncommon. Some of them may even cause the system to fail to operate normally for a long time. Starting from this fact, new requirements have been put forward for inverter design, namely to meet more flexible system configurations. In the case of single-channel MPPT tracking, all PV modules are connected in series and connected to the inverter. If a PV module failure occurs in the string, the overall MPPT efficiency of the circuit will be affected, which may make the system The efficiency is greatly reduced, and no matter how high the inverter's conversion efficiency and tracking efficiency, it will not work. In order to reduce the risk of system efficiency degradation caused by this failure, the trend of multi-channel independent MPPT tracking has emerged in the household-type low-power inverters. The multi-channel MPPT can reduce the losses caused by the single-channel MPPT failure and give the system a loss. The configuration brings more flexibility.


Figure 2 A Two Independent MPPT Inverter Topology Circuit

Thanks to the new topology design, multi-path 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 more flexible system configuration and reduces the risk of potential loss of system efficiency. 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 requirements

The installation of household inverters is relatively complex and can be applied to the environment of residential buildings, warehouses, public buildings, office buildings, carports, etc. This also puts forward more flexible and varied demands on the monitoring of inverters. In the previous design of inverters, communication was mostly wired, such as communication interfaces based on RS485, customers would inevitably need to perform wiring during installation, and at the same time, the means of client monitoring were mostly based on PC-based application software, which could achieve one-to-many monitor. Now this model has been greatly challenged. The customer is not satisfied with the wired way, but more demands that the inverter can provide the wireless monitoring scheme. This change requires the inverter itself to be compatible with the wireless communication protocol. And interfaces, on the other hand, can provide overall monitoring solutions, especially client-side wireless monitoring solutions. For the design of inverters, the technical route that breaks through the design of traditional industrial system communication interfaces and system monitoring solutions will pose challenges to many inverter companies that have transformed from the traditional power industry. The new communication and monitoring solutions are more based on technology development in the field of IT communications, which is totally different from traditional electrical technology development. These products and solutions have a certain degree of independence and can be sold separately. Therefore, inverter manufacturers can fully 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 vendors. This cooperation model is in the current inverter industry. The middle one is more common.

The monitoring product is not limited to the monitoring of photovoltaic system products such as inverters. The previous analysis mentioned that the inverter also receives instructions from the grid and completes some electrical control actions, that is, communication between the monitoring system and the inverter. It is bidirectional and the down-link instructions (monitoring system to inverter) have certain real-time requirements. Therefore, monitoring products are gradually more professional, and companies offering such products should understand the new technical standards more extensively and deeply, cooperate more closely with downstream products such as inverters, and constantly improve products and solutions. Figure 3 shows the home system monitoring solution provided by KLNE.


Figure 3 household system monitoring program

Pay more attention to appearance and structure design

In recent PV exhibitions, fewer and fewer new products have been introduced. Each inverter manufacturer tends to have the same key factors in terms of product features, performance, etc. The difference in competition is not in the core technology but in the direction of Differentiation of non-core technologies such as product design, display panels, and institutional design.


Fig. 4 The design of a new type of household inverter

The end users of household inverters are individuals and families. Therefore, the appearance design of inverters is closer to the design of home appliances. The appearance is beautiful, the volume is small, the installation is convenient, the weight of the whole machine is light, the workmanship is delicate, and the display panel is graphical. Intuitive. These characteristics are precisely in line with people's demand for a household product that enters the home. In many cases, home-use inverters are placed indoors, so the product itself should meet low noise, aesthetics, small size, and intuitive display interface. With the characteristics of beauty and small weight, for inverter manufacturers, the needs of users of the target market in such aspects as appearance, aesthetics and installation should be fully studied to meet the differentiated demands of customers and make inverter products. In addition to security, high efficiency and other conditions, it also meets users' needs for appearance, interface display, installation, etc. to better meet customer needs, promote market development and product replacement.

The domestic inverter manufacturers have already reached the same starting line with their counterparts in other countries in terms of product performance. However, the stability of the products, especially the appearance of the products, is still lagging behind that of foreign products. The product design is lacking in creativity and appearance. The old-fashioned, simple and simple display panel, the color material is also very single, so that in the same market and foreign similar products in the lack of competition.

There are many reasons for this result. There are both subjective and objective reasons. The subjective reason is that the domestic inverter products are developed after the foreign inverter products. The beginning is the stability of the product. In terms of performance, a great deal of effort and attention has been devoted, and there has not been a great emphasis on non-core technical product features. 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 talent in this area is not an example of inverter products. Actually, many domestic products have a large gap in appearance and customer experience with foreign products, and this is one of the reasons for this result.

In the current situation where the domestic product transformer products are increasingly fierce in the international market, especially when the functions, performance, etc. of the products converge, the domestic inverter products should pay more attention to the design of the products and cultivate and introduce products. Appearance, structural design professionals, in-depth study and understanding of the needs of foreign users, learn from the experience of foreign counterparts in this area, and strive to narrow this gap.

The above summarizes some of the new technical features of the 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. The technical constraints and requirements from various aspects make this type of inverter To further emphasize the design of the complete machine, the volume weight will be further reduced, the power density will increase, the advanced display panel and the graphical display interface, the interface for wireless communication, multiple MPPT tracking and wider MPPT Voltage range, can accept the adjustment command of the grid side to perform real-time adjustment of reactive power, active power, and low-voltage ride-through (German BDEW technical standard requirements). Domestic inverter products have a strong price advantage in foreign sales, but one's pursuit of low prices is not a strategy for long-term development of the company, and it is not conducive to the overall competition of domestic photovoltaic inverter enterprises in foreign countries, blindly low. The price will inevitably lead to "double reverse" punishment similar to the battery components, which will bring crisis to the development of the entire industry. Therefore, domestic inverter companies should make product details, take the initiative to adapt to the latest technical requirements and specifications, and work hard to improve the overall competitiveness of products.

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