Types, advantages and disadvantages of photovoltaic inverters

The photovoltaic inverter is an inverter specially used in the field of solar photovoltaic power generation. It is an indispensable core component in the photovoltaic system. Its biggest role is to convert the DC power generated by the solar cell into a DC power that can be directly integrated into the power grid through power electronic conversion technology. The AC energy of the load.

Currently, the common types of photovoltaic inverters are: centralized inverters, string inverters, and micro-inverters.

Centralized inverter

Centralized inverters are generally used in large power generation systems such as large factories with uniform sunshine, desert power stations, ground power stations, etc. The total power of the system is large, usually above the megawatt level. Comparison of the main components of photovoltaic power station systems using centralized inverters: photovoltaic modules, DC cables, combiner boxes, DC combined power distribution, inverters, isolation transformers, AC power distribution, and power grids.

Advantages of centralized inverters:

1. The number of inverters is small and easy to manage.

2. The inverter has a small number of components and high reliability.

3. Low harmonic content, low DC component and high power quality.

4. The inverter has high integration, high power density and low cost.

5. The inverter has various protection functions and the power station has high safety.

6. It has power factor adjustment function and low voltage ride-through function, and has good power grid regulation.

Disadvantages of centralized inverters:

1. Safety issues: The DC combiner box is used in the centralized solution. Due to the built-in DC branch fuse, there is a risk of frequent fires, because the solar panels will be in working condition as long as there is light. For distributed rooftop factories, it will bring serious safety risks. Not only will the economic benefits of the power station itself be affected, but more importantly, it will affect other equipment in the factory, causing very large losses to the owners.

2. For irregular roofs, a single 500KW inverter cannot make full use of the roof area. The inverter is often overloaded or underloaded, or overmatched or undermatched.

3. On roofs with multiple orientations, the panel is partially shaded, causing inconsistency in the strings. Single-channel MPPT results in relatively low power generation; at the same time, the mismatch loss of each string will also lead to a loss of power generation.

4. The inverter requires maintenance by professional engineers. A single inverter failure has a great impact on power generation and poses a huge challenge to the safety of maintenance personnel. At the same time, there are many types of spare parts, and it takes more than 3 days to locate and repair the fault, which has a serious impact. Customer power generation revenue. The DC combiner box has a high failure rate and cannot monitor each string, which increases fault location time. Due to the volatilization of fuses, the failure rate and maintenance costs are high, requiring regular replacement and maintenance. The lines are complex and there are many joints processed on site, so the failure rate is high. After some projects were operated for 1-2 years, the effective power generation rate was lower than 90%.

5. The centralized solution requires an inverter room and corresponding civil engineering, and it also needs to be equipped with corresponding fans, air ducts, smoke sensors, temperature sensors and other equipment. However, if a photovoltaic power station is installed on the roof of a traditional factory, its power distribution room may not necessarily be It can accommodate a large 500K Wp centralized inverter. At the same time, it not only increases the construction complexity, but also increases the initial investment and operation and maintenance costs.

6. Centralized inverters require forced air cooling, and the computer room consumes a lot of power, with an average of at least 300W. Regular dust sweeping, fan maintenance, and dust filter replacement are required.

7. The central inverter MPPT voltage range is narrow, generally 450-820V, and the component configuration is not flexible. On rainy days and in areas with a lot of fog, the power generation time is short.

8. There is no redundancy capability in the centralized grid-connected inverter system. If a failure occurs, the entire system will stop generating electricity.

String inverter

String inverters are suitable for small and medium-sized rooftop photovoltaic power generation systems and small ground power stations. Photovoltaic power station systems using string inverters include components, DC cables, inverters, AC power distribution, and power grids.

Advantages of string inverter:

1. The string inverter has a simple network structure and can be installed outdoors nearby. The appropriate inverters can be selected and combined flexibly according to the roof capacity to make full use of the roof area and increase investment returns.

2. The equipment types are simple and save investment. DC combiner boxes, DC distribution cabinets, isolation transformers, computer rooms and related civil engineering can all be omitted, while reducing the difficulty of on-site construction organization and shortening the construction period. Comprehensive calculations show that the civil engineering workload of a string inverter system is only 1/4 of that of a centralized inverter system. In terms of safety and reliability, it avoids the risk of fire on the DC side that cannot be extinguished in traditional centralized solutions.

3. Component inconsistency has little impact on power generation. The inverter consumes less power. The self-consumption power of the string type is 20W, which is only 1.3% of the centralized type. According to the test of actual projects, the system efficiency of the string type scheme is It is at least 5% higher than the centralized solution.

4. Each inverter is equipped with 3 independent MPPT channels. Each inverter is tracked separately. The impact of a single channel failure is small. The output of each panel is carefully managed, and the power generation of the whole system is high. When partial occlusion, partial pollution, or partial failure occurs, except for the power generation of the affected part, the remaining parts can still ensure maximum power output.

5. The string inverter does not require maintenance by professional engineers. The equipment is modular and easy to install and debug on site. The replacement of an inverter can be completed in 20 minutes, without the need for professional personnel to be on duty, achieving "fool-like" maintenance. The failure of a single inverter has a low impact on power generation, the system reliability and annual availability rate are high, and the annual inverter failure rate is less than 0.5%.

6. Each inverter can realize intelligent monitoring of 6 strings, reducing fault location time by 80%. It can independently detect the voltage and current of each input, and can sample string current and voltage in real time to detect line faults and component faults in time. , occlusion and other issues. Improve detection accuracy through horizontal comparison of group strings, comparison of meteorological conditions, comparison of historical data, etc. And it can cooperate with the backend network management to provide automatic operation and maintenance suggestions, such as cleaning, string matching optimization, inverter collaboration, etc.

7. The string inverter MPPT voltage range is wide, generally 250-800V, and the component configuration is more flexible. On rainy days and in areas with a lot of fog, the power generation time is long.

8. The inverter is maintenance-free, has natural heat dissipation, has low power consumption, is IP65 rated, and can operate reliably in rain, sand and salt spray environments. At present, domestic mainstream string inverters have reached IP65. For example, Huawei's string inverter body adopts an all-aluminum integrated closed casing with no external fan design. The inverter casing is sprayed with high weather-resistant outdoor coating. Layer protection, the radiator adopts thickened anodizing oxidation process, and all installation parts are made of stainless steel and other corrosion-resistant materials, truly achieving IP65 protection, thus meeting the application in salt spray and high humidity environments.

Disadvantages of string inverters:

1. There are many electronic components, and the power devices and signal circuits are on the same board. It is difficult to design and manufacture, and the reliability is slightly poor.

2. The electrical clearance of power devices is small and is not suitable for high altitude areas. When installed outdoors, exposure to wind and sun can easily cause aging of the casing and heat sink.

3. Designed without an isolation transformer, the electrical safety is slightly poor, and it is not suitable for thin film module negative grounding systems. The DC component is large and has a great impact on the power grid.

4. When multiple inverters are connected in parallel, the total harmonics are high, and the THDI of a single inverter can be controlled to more than 2%. However, if more than 40 inverters are connected in parallel, the total harmonics will be superimposed and difficult to suppress. .

5. With a large number of inverters, the total failure rate will increase, making system monitoring difficult.

6. There are no DC circuit breakers, AC circuit breakers, and DC fuses. When the system fails, it is not easy to disconnect. Short circuit of component strings will cause serious accidents.

7. A single inverter can realize the zero voltage ride through function, but when multiple inverters are connected in parallel, it is difficult to realize the zero voltage ride through function, reactive power regulation, active power regulation and other functions.

8. A 10MW power station requires 300-400 string inverters. According to the electrical power supply reliability theory, the power station failure rate is more than 10 times that of a 500KW inverter.

9. Due to the limitation of manual construction level, problems such as wrong connection, reverse connection, loose connection, and virtual connection may occur. Moreover, a single inverter has 10 contacts, and 400 inverters have 4,000 contacts. As long as there is a certain If these inspections are not in place, under the condition of 1000V DC voltage, the DC arcing phenomenon will be very serious, and the phenomenon of burning the machine will often occur.

10. The larger the scale of the string power station, the more communication nodes there are, and the comprehensive relay protection system needs to be redesigned.

11. The string inverter can operate at full capacity only when the ambient temperature is 4CTC, and will be derated by 5% for every 10°C rise. A 500KW inverter can operate at an ambient temperature of 55°C1: 1.1 Operating at full capacity.

12. The cost of string inverters is $0.02-0.03 higher per W than that of centralized inverters.

13. Since the control of the grid-connected inverter is designed to track the grid voltage in real time, the power quality of the power grid of the Northwest Power Station is very poor, so the harmonics will be relatively large, which is very objectionable to the power grid company.

Microinverter

In a traditional PV system, the DC input end of each string inverter is connected in series by about 10 photovoltaic panels. When one of the 10 battery panels connected in series does not work well, the whole string will be affected.

Advantages of microinverters:

1. Simple installation: building block installation method simplifies the design and on-site installation steps, and a single cable is connected to the power distribution cabinet.

2. Investment saving: There is no DC part design and related parts, saving system costs and no related DC end line losses.

3. System safety: There is no DC high voltage, the system is highly safe, and there are no personal or fire hazards. After the photovoltaic power station system using centralized and string inverters is disconnected from the grid, although the AC output is 0V, the DC voltage still remains 600-1000V, while the DC voltage of the photovoltaic power station using micro-inverter 30-40V only.

4. Management intelligence: The working status of each component can be monitored through the network worldwide. Faulty components and micro-inversions can be observed in a timely and direct manner, and system faults can be resolved in a timely manner.

5. Usage environment: Micro-inverters and monitoring equipment do not require a dedicated computer room. The use environment of micro-inverters is outdoors, which saves space costs and does not require special care.

6. Multi-generation: The MPPT function at the single component level of the micro-inverter system effectively solves the mismatch between components. It can be oriented in multiple directions, and multiple types of components can be connected to each other to maximize the power generation of the system. Effectively solve the problem of corresponding system power reduction caused by building structure, stains on some components, and occlusion, and the system efficiency can reach up to 90%.

7. The micro-inverter system has better stability. The designed service life of a single machine can be as long as 25 years. It reduces the related DC connectors and correspondingly reduces the hidden dangers of installation and equipment failure. A single component failure has no impact on the entire system, improving system stability. Safety, 100% working time guaranteed.

Disadvantages of microinverters:

1. The unit cost of the system is obviously much higher than that of centralized inverters and string inverters.

2. Inverter maintenance costs are high.