What is a String Inverter? Advantages and Disadvantages of String Photovoltaic Inverter

The string photovoltaic inverter directly converts the direct current generated by the photovoltaic modules into alternating current and then boosts it and connects it to the grid. Transformer.

The string inverter is based on the concept of modularization. Each photovoltaic string (1-5kw) passes through an inverter, and several panels form a string with a power of several hundred watts to thousands of watts. The input low-power single-phase inverter has the maximum power peak tracking at the DC end, and is parallel-connected to the grid at the AC end. It has become the most popular inverter in the international market. So what are the advantages and disadvantages of string inverters?

1. Advantages of string inverters

1.1 The string-type grid-connected inverter is small in size and light in weight, and it is very convenient to carry and install. It does not require professional tools and equipment, nor does it require a special power distribution room. It can simplify construction in various applications. Reduced footprint, DC line connection does not require DC combiner boxes and DC power distribution cabinets.

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

1.3 The string inverter adopts a modular design, the DC end has multi-channel MPPT function, and the AC end is connected in parallel to the grid. The best operating point does not match the inverter, which maximizes the power generation.

1.4 The string type also has the advantages of low self-consumption, small failure impact, and convenient replacement and maintenance.

2. Disadvantages of string inverters

2.1 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 , prone to resonance.

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

2.3 Large-scale ground power stations use a large number of string inverters, the total failure rate will increase, and the system monitoring is difficult.

2.4 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 , prone to resonance.

The function of the photovoltaic inverter is to convert the variable DC voltage generated by the photovoltaic module into the AC frequency of the mains frequency, which is one of the important system balances in the photovoltaic array system. At present, the common inverters on the market are mainly centralized inverters and string inverters. Let’s take a look at the advantages and disadvantages of photovoltaic centralized inverters and string inverters and their adaptation scenarios.

3. Brief introduction of two inverters

3.1 Centralized inverter: The centralized inverter converts the aggregated direct current into alternating current. Therefore, the power of the inverter is relatively large. Generally, a centralized inverter of 500KW or more is used in a photovoltaic power station.

3.2 String inverter: The string inverter directly converts the direct current generated by the components into alternating current, and then aggregates it. Therefore, the power of the inverter is relatively small, and photovoltaic power stations generally use less than 100KW String inverter.

4. Advantages and disadvantages of centralized inverters and applicable scenarios

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.

4.1 Main advantages: (1) Easy maintenance and management; (2) High integration of inverter and high power density; (3) Complete protection functions of inverter, high safety of power station; (4) Adjustment of active power factor function, high and low voltage ride through function.

4.2 Main disadvantages:

4.2.1 The DC combiner box has a high failure rate, which affects the entire system;

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

4.2.3 The power consumption of the inverter itself and the noise of ventilation and heat dissipation in the machine room are large, and the system maintenance is relatively complicated

4.2.4 In the centralized grid-connected inverter system, the square matrix of photovoltaic modules reaches the inverter after two confluences, and the maximum power tracking function of the inverter cannot monitor the operation of each module, so it is impossible to make each module At the best working point, when a component fails or is blocked by shadows, it will affect the power generation efficiency of the entire system

4.2.5 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.

5. Advantages and disadvantages of string inverters and applicable scenarios

String inverters are suitable for photovoltaic power plants in various scenarios

5.1 Main advantages

5.1.1 The string inverter adopts a modular design. Every two strings corresponds to one MPPT. The DC end has the maximum power tracking function, and the AC end is connected in parallel to the grid. The advantage is that it is not blocked by module differences and shadows between photovoltaic modules. At the same time, it reduces the mismatch between the optimal operating point of the photovoltaic module and the inverter, and maximizes the power generation.

5.1.2 The MPPT voltage range of the string inverter is wide, generally 200-1000V, the maximum currently reaches 600-1500V, and the component configuration is more flexible. In rainy and foggy areas, the power generation time is long.

5.1.3 The string inverter is small in size, light in weight, and very convenient to transport and install. It does not require professional tools and equipment, nor does it require a special power distribution room; it can simplify construction and reduce occupation in various applications. Ground; DC line connection does not require DC combiner box and DC power distribution cabinet, etc.

5.1.4 The string inverter has low self-consumption, low failure rate and more convenient maintenance.