What is a string inverter?

What is a string inverter?

String inverters are mainly used in small and medium-sized photovoltaic power generation systems. They refer to single-phase or three-phase output inverters that can be directly connected to the string and used for outdoor hanging installations based on the modular concept. It is a kind of inverter that has a maximum power peak tracking at the DC end and is connected in parallel at the AC end. It has formed some fixed characteristics: the DC input terminal can be directly connected to the photovoltaic string without passing through the DC combiner box. The output voltage range is wide, and the output AC phase voltage is mostly 180~280V, which can be directly connected to the local single-phase or three-phase power grid; The number of MPPT channels is usually 2 or more. MPPT control is more precise, efficient, and flexible in design. It can adapt to a variety of different application scenarios. Power devices use low-current IGBTs. The system topology uses DC/DCBoost boost and DC-AC full Bridge inverts the way of two-stage power electronic device conversion and power frequency isolation transformer.

Many large-scale photovoltaic power stations use string inverters. The advantage is that they are not affected by module differences and shadows between strings. At the same time, it reduces the mismatch between the optimal working point of photovoltaic modules and the inverter, and increases the power generation. These technical advantages reduce the cost of the system. At the same time, the concept of “master-slave” is introduced between the strings, so that the system connects several sets of photovoltaic strings in the case that a single string of electric energy cannot make a single inverter work. Together, let one or several of them work to produce more electricity. Photovoltaic power generation systems using string inverters generally include: photovoltaic modules, DC cables, inverters, AC cables, AC combiner boxes, AC cables, isolation transformers, and AC power distribution.

What is a string inverter?
Figure 1 String inverter scheme

The latest concept is that several inverters form a “team” to replace the concept of “master-slave”, which further increases the reliability of the system. At present, transformerless string inverters have occupied a dominant position.

The most common reasons for choosing string inverters are mainly the following two points:

(1) Avoid the barrel effect of the centralized inverter power station. In the centralized grid-connected inverter system, the square array of components arrives at the inverter after two confluences. The maximum power tracking function (MPPT) of the inverter cannot monitor the operation of each component, so it is impossible to make every component in the The best operating point, if a component fails or is blocked by shadows, it will affect the power generation efficiency of the entire system. For string-type power stations, only the corresponding one of the blocked MPPTs is affected. Under normal circumstances, the installation distances and installation angles between the various components are different. Partial shading will inevitably occur during a certain period of the day, especially when the sun altitude is low in the morning and evening, or some vegetation may block some of the cells. If a 500kW square array solar panel uses one MPPT to track, a certain amount of power generation will be lost. This situation is also applicable when the battery components are dirty, shaded, aging, heating up, and hot spots.

What is a string inverter?
Figure 2 The barrel effect of a centralized inverter power station

Power stations that use string inverters can use components of different orientations in the same project. In mountain projects, due to the complex topography of the area, there is little flat land, land leveling cannot be done, and the topography facing the south is limited. Therefore, to ensure capacity, the southeast and southwest slopes, and east and west slopes must be fully utilized. At this time, the photovoltaic modules cannot be arranged completely facing south.

Power stations using string inverters can use different types of components in the same project, which is not possible in traditional centralized inverter power stations.

(2) String inverters also have the advantages of low self-consumption, small failure effects, and convenient replacement and maintenance. The centralized inverter itself consumes a lot of power and the ventilation and heat dissipation of the computer room. The system maintenance is relatively complicated. When a fault occurs, the entire power station will be paralyzed. When a string inverter fails, only one inverter stops generating power. The power station can operate as usual, thus greatly reducing losses. In addition, the string-connected grid-connected inverter is small in size, light in weight, and very convenient to transport and install. It does not require professional tools and equipment, and does not require a special power distribution room. It can simplify construction and installation in various applications. Reduce the land occupation, and the DC line connection does not require DC combiner boxes and DC distribution cabinets. This means that the repair cycle of string inverters is shorter than that of centralized inverters.

The above two reasons should be the most common reasons for choosing string inverters at present. In addition, a flexible Internet monitoring solution can allow power stations using string inverters to accurately monitor each string, making it easier to find out where there is The component of the problem, and real-time control of the detailed information and historical records of each group of strings.

What is a string inverter?
Figure 3 Comparison of the repair time period of centralized and string inverters

Of course, string inverters also have disadvantages:
(1) There are many electronic components, and the power device and signal circuit are on the same board, which is difficult to design and manufacture, and the reliability is slightly worse.
(2) The electrical clearance of the power device is small, which is not suitable for high altitude areas. Outdoor installation, wind and sun can easily lead to aging of the housing and heat sink.
(3) The design without isolation transformer has poor electrical safety and is not suitable for the negative grounding system of thin film components. The direct current component is large, which has a large impact on the power grid.
(4) When multiple inverters are connected in parallel, the total harmonics are high, and the THD (total harmonic distortion) 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 more difficult to suppress.
(5) With a large number of inverters, the total failure rate will increase, making system monitoring difficult.
(6) Without DC circuit breaker and AC circuit breaker, it is not easy to disconnect when the system fails.
(7) A single inverter can realize the zero voltage ride through function, but when multiple machines are connected in parallel, it is difficult to realize the zero voltage ride through function, reactive power regulation, active power regulation and other functions.

Compare items50kW string inverter1MW centralized inverter
Combiner boxNo need for combiner box, DC input is subdivided into each stringNeed a combiner box to centralize the flow
DC wiringThe DC side wiring is simple, distributed on-site and grid-connected; the DC cable is short and the cost is lowThe DC side wiring is relatively complicated and the distance is long. If necessary, multi-level bus is required, and the cost is relatively high.
AC wiringThe AC side cable connection distance is long, and each inverter needs an AC circuit breaker, which can be connected to the grid on site or AC converged to the gridThe distance between the AC side and the transformer is short, the line loss is small, and the AC wiring is simple and low in cost
The output voltageThe output is three-phase AC 400V, which can be connected to the grid at low voltage and does not need an isolation transformerOutput three-phase AC 270V, and need to add 400V isolation transformer or step-up transformer for grid connection
Protection levelProtection grade IP65, can be installed outdoors, can be installed nearby the moduleThe degree of protection is IP20, indoor installation, or construction of outdoor rooms
cooling methodNatural cooling or air coolingForced air cooling, requiring a large flow air duct
Operating voltage rangeWide range of MPPT voltage, 250 ~ 800V, can generate electricity in low illuminance weather such as rainy and cloudy weatherMPPT range is 450 ~ 820v, the power generation range is narrow
efficientThe highest efficiency is 98.2%, and the overall efficiency is 97.6%The highest efficiency without isolation transformer is 98.0%, and the overall efficiency is 97.5%, and the highest efficiency with isolation transformer is 97.0%, and the overall efficiency is 96.5%.
Power QualityA single unit has a THD of 3%, and 25 units together have a total THD of over 5%. No isolation transformer, large DC componentSingle unit THD<3%, two units connected in parallel is about 3%, and there is no DC component with isolation transformer
Grid regulationThere is no low voltage ride-through function, and the power factor adjustment function of the grid is weakWith low voltage ride-through function, the power grid can adjust the power factor, and the active and reactive functions are weak
SafetyNo DC circuit breaker and AC circuit breaker, less safetyThere are DC circuit breakers and AC circuit breakers, which can be disconnected according to the different conditions of the fault, with good safety
Table 1 Comparison of string 50kW inverter and centralized 1MW inverter

Table 1 is a comparison between string 50kW inverters and centralized 1MW inverters. Both have their own advantages and disadvantages.