Types and advantages and disadvantages of photovoltaic inverters
Photovoltaic inverter is an inverter dedicated to the field of solar photovoltaic power generation. It is an indispensable core component in the photovoltaic system. Its biggest role is to convert the direct current generated by the solar cell into a The AC energy of the load.
At present, the common types of photovoltaic inverters are: centralized inverters, string inverters, and micro inverters.
Centralized Inverter
Centralized inverters are generally used in large-scale power generation systems such as large factories with uniform sunshine, desert power stations, and ground power stations. The total power of the system is large, generally above the megawatt level. Comparison of main components of photovoltaic power plant systems using centralized inverters: photovoltaic modules, DC cables, combiner boxes, DC cables, DC confluence distribution, DC cables, inverters, isolation transformers, AC power distribution, and power grids.
Advantages of centralized inverters:
1. The number of inverters is small, which is easy to manage.
2. The number of inverter components is small and the reliability is high.
3. Less harmonic content, less DC component and high power quality.
4. The inverter has high integration, high power density and low cost.
5. Various protection functions of the inverter are complete, and the safety of the power station is high.
6. It has power factor adjustment function and low voltage ride through function, and the power grid has good 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 continuous fire, because the solar panels will be in working condition as long as there is light. For distributed roof plants, it will bring serious safety hazards. Not only the economic benefits of the power station itself are affected, but more importantly, other equipment in the plant will be affected, causing very large losses to the owner.
2. Irregular roof, using a single 500KW inverter cannot make full use of the roof area. Inverters are often in overload or light load or over-allocation or under-allocation.
3. For roofs with multiple orientations, partial shadows of the battery panels lead to inconsistency of the strings, and a single MPPT leads to relatively low power generation; at the same time, the mismatch loss of each string will also lead to a loss of power generation.
4. Inverters need 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 for fault location and repair to seriously affect Customer power generation income. The failure rate of DC combiner boxes is high, and each string cannot be monitored, which increases the time for fault location. Due to the volatilization of fuses, the failure rate and maintenance cost are high, and regular replacement and maintenance are required; the wiring is complicated, and there are many joints processed on site, resulting in a high failure rate; After 1-2 years of current operation of some projects, the effective power generation rate is lower than 90%.
5. The centralized solution requires the inverter room and corresponding civil works, and at the same time needs to be equipped with corresponding fans, air ducts, smoke detectors, temperature sensors and other equipment. If a photovoltaic power station is installed on the roof of a traditional factory building, its power distribution room is not necessarily It can accommodate bulky 500K Wp centralized inverters. At the same time, it not only increases the construction complexity, but also increases the initial investment and operation and maintenance costs.
6. The centralized inverter needs forced air cooling, and the computer room consumes a lot of power, with an average of at least 300W or more. Regular dust cleaning, fan maintenance and dust filter replacement are required.
7. The MPPT voltage range of the centralized inverter is narrow, generally 450-820V, and the component configuration is not flexible. In rainy days and foggy areas, the power generation time is short.
8. There is no redundant capability in the centralized grid-connected inverter system. If there is a shutdown due to failure, the entire system will stop generating power.
String inverter
String inverters are suitable for small and medium-sized rooftop photovoltaic power generation systems and small ground power stations. The photovoltaic power station system using string inverters includes components, DC cables, inverters, AC power distribution, and grids.
Advantages of string inverters:
1. The network structure of the string inverter is simple, and it can be installed outdoors nearby. According to the capacity of the roof, the flexible combination of inverters can be selected to make full use of the roof area and increase the return on investment.
2. The types of equipment are simple and save investment. The DC combiner box, DC power distribution cabinet, isolation transformer, machine room and related civil engineering can all be omitted, while reducing the difficulty of on-site construction organization and shortening the construction period. Comprehensive calculation, the civil engineering workload of the string inverter system is only 1/4 of that of the centralized inverter system. In terms of safety and reliability, it avoids the risk of a fire on the DC side of the traditional centralized solution that cannot be extinguished.
3. The inconsistency of components has little impact on power generation. The self-consumption power of the inverter is small, and the self-consumption power of the string type is 20W, which is only 1.3% of the centralized type. According to the test of the actual project, the system efficiency of the string type scheme About 5% higher than the centralized solution.
4. Each inverter has 3 channels of independent MPPT, each channel is tracked separately, the impact of a single channel failure is small, the output of each panel is finely managed, and the power generation of the whole system is high. In the event of partial shading, partial fouling, or partial failure, except for the impact on the power generation of the affected part, the remaining parts can still guarantee the maximum power output.
5. The string inverter does not require professional engineers to maintain, the equipment is modular, and the on-site installation and commissioning are simple. The replacement of an inverter can be completed in 20 minutes without the need for professional personnel on duty, realizing "fool-style" maintenance. The failure of a single inverter has a low impact on power generation, the system reliability and annual availability are high, and the annual failure rate of inverters is less than 0.5%.
6. Each inverter can realize intelligent monitoring of 6 strings, reducing fault location time by 80%, independently detecting the voltage and current of each input, sampling the current and voltage of strings in real time, and discovering line faults and component faults in time , occlusion and other issues. Improve detection accuracy through horizontal comparison of strings, comparison of weather conditions, comparison of historical data, etc. Moreover, it can cooperate with the background network management to provide automatic operation and maintenance suggestions, such as cleaning, string matching optimization, and inverter coordination.
7. The MPPT voltage range of the string inverter is wide, generally 250-800V, and the component configuration is more flexible. In rainy days and foggy areas, the power generation time is long.
8. The inverter is maintenance-free, with natural heat dissipation, low self-consumption, IP65, and can operate reliably in rain, sand and salt spray environments. At present, the mainstream string inverters in China have reached IP65. For example, Huawei’s string inverter body adopts an all-aluminum one-piece closed casing, no external fan design, and the inverter casing is sprayed with high weather-resistant outdoor coating. Layer protection, the radiator adopts thickened anodic oxidation oxidation process, and all installation parts are made of corrosion-resistant materials such as stainless steel, which truly realizes IP65 protection, so as to meet the application in salt spray and high humidity environment.
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, which is not suitable for high altitude areas. Outdoor installation, wind and sun can easily lead to aging of the shell and heat sink.
3. The design without an isolation transformer has poor electrical safety, and is not suitable for the negative grounding system of thin-film modules. 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%, but if more than 40 inverters are connected in parallel, the total harmonics will be superimposed, and it is difficult to suppress .
5. The number of inverters is large, the total failure rate will increase, and the system monitoring is difficult.
6. There is no DC circuit breaker and AC circuit breaker, and there is no DC fuse. When the system fails, it is not easy to disconnect, and the short circuit of the 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 adjustment, active power adjustment and other functions.
8. A 10MW power station requires 300-400 string inverters. According to the reliability theory of electrical power supply, the failure rate of a power station is more than 10 times that of a 500KW inverter.
9. Due to the limitation of domestic manual construction level, there will be problems such as wrong connection, reverse connection, loose connection, virtual connection, etc., and a single inverter has 10 contacts, and 400 inverters have 4000 contacts. As long as there is a certain If these inspections are not in place, under the condition of 1000 V DC voltage, the phenomenon of DC arcing 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, and the integrated relay protection system needs to be redesigned.
11. The string inverter can only achieve full operation under the ambient temperature of 4CTC, and the derating will be reduced by 5% for every 10°C increase. The 500KW inverter can operate under the ambient temperature of 55°C1: 1.1 Full operation.
12. The cost of the string inverter is 0.15-0.2 yuan higher per W than the centralized inverter.
13. Due to the control license of the grid-connected inverter 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, and the grid company is very repulsive.
micro inverter
In a traditional PV system, about 10 photovoltaic panels are connected in series to the DC input end of each string inverter. When one of the 10 battery panels connected in series fails to work well, the string will be affected.
Advantages of microinverters:
1. Simple installation: the 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 cost and no related DC terminal line loss.
3. System safety: no DC high voltage, high system safety, no personal and fire hazards. After the photovoltaic power station system using centralized and string inverters is disconnected from the grid, although the output of the AC terminal is 0V, the DC terminal voltage still maintains 600-1000V, while the DC terminal voltage of the photovoltaic power station using micro-inverters 30-40V only.
4. Management intelligence: worldwide, the working condition of each component can be monitored through the network. The faulty components and micro-inverters can be observed in time and directly, and the system fault can be solved in time.
5. Use environment: The micro-inverter and monitoring equipment do not need a dedicated computer room, and the micro-inverter is used in an outdoor environment, saving site costs and requiring no special care.
6. Multiple power generation: The single-component MPPT function of the micro-inverter system can effectively solve the mismatch between components. It can be multi-oriented, and various types of components can be connected to each other to maximize the power generation of the system. Effectively solve the problem of power drop of the corresponding system due to building structure, stains on some components, and shading, and the system efficiency can reach up to 90%.
7. The stability of the micro-inverter system is better, and the service life of a single machine can be as long as 25 years, reducing the relevant DC terminal connectors, correspondingly reducing the hidden dangers of installation and equipment failure, and the failure of a single component has no effect on the entire system, improving system stability 100% working time guarantee.
Disadvantages of microinverters:
1. The unit cost of the system is significantly higher than that of centralized inverters and string inverters.
2. The maintenance cost of the inverter is relatively high.
