As an important part of photovoltaic power generation, the inverter mainly converts the direct current generated by photovoltaic modules into alternating current. At present, common inverters on the market are mainly divided into centralized inverters and string inverters, as well as trendy distributed inverters.
1. Centralized inverter
As the name implies, the centralized inverter converts the direct current generated by photovoltaic modules into alternating current for step-up and grid connection. Therefore, the power of the inverter is relatively large. Centralized inverters of more than 500kW are generally used in photovoltaic power plants.
(1) The advantages of centralized inverters are as follows:
1. High power, small quantity, easy to manage; few components, good stability, easy to maintain;
2. Less harmonic content, high power quality; complete protection functions, high safety;
3. With power factor adjustment function and low voltage ride-through function, the power grid can be adjusted well.
(2) The centralized inverter has the following problems:
1. The MPPT voltage range of the centralized inverter is narrow, and the operation of each component cannot be monitored, so it is impossible to make each component at the best working point, and the component configuration is not flexible;
2. The centralized inverter occupies a large area, requires a dedicated machine room, and the installation is not flexible;
3. The power consumption of itself and the ventilation and heat dissipation of the computer room are large.
2. String inverter
As the name suggests, the string inverter converts the direct current generated by photovoltaic modules directly into alternating current, then boosts the voltage and connects to the grid. Therefore, the power of the inverter is relatively small. PV power plants generally use string inverters below 50kW.
(1) Advantages of string inverters:
1. It is not affected by module differences between strings and shadow shading, and at the same time reduces the mismatch between the optimal operating point of the photovoltaic cell module and the inverter, and maximizes the power generation;
2. The MPPT voltage range is wide, and the module configuration is more flexible; in rainy days and areas with a lot of fog, the power generation time is long;
3. Small size, small footprint, no special machine room, flexible installation;
4. Low self-consumption and little impact of failure.
(2) There are problems with string inverters:
1. The electrical clearance of power devices is small, which is not suitable for high altitude areas; there are many components, integrated together, and the stability is slightly poor;
2. Outdoor installation, wind and sun can easily lead to aging of the shell and heat sink;
3. The number of inverters is large, the total failure rate will increase, and the system monitoring is difficult;
4. Without the design of isolation transformer, the electrical safety is slightly poor, and it is not suitable for the negative grounding system of thin film modules.
3. Distributed inverter
The distributed inverter is a new inverter form proposed in the past two years, and its main features are "centralized inverter" and "decentralized MPPT tracking". The distributed inverter is a product that combines the advantages of the centralized inverter and the string inverter, and achieves the low cost of the centralized inverter and the high power generation of the string inverter. ".
(1) Advantages of distributed inverters:
1. Compared with the centralized type, "decentralized MPPT tracking" reduces the probability of mismatch and increases the power generation;
2. Compared with the centralized type and the string type, the distributed type inverter has a boost function, which reduces the line loss;
3. Compared with the string type, "centralized inverter" has more advantages in terms of construction cost.
(2) Distributed inverter problem;
1. Less engineering experience. Compared with the first two types, it is still a new form, and its application in engineering projects is relatively small;
2. Features such as safety, stability and high power generation still need to be tested by engineering projects;
3. Due to the use of "centralized inverter", the shortcomings of large footprint and the need for a dedicated machine room also exist in the distributed inverter.
The principle of the inverter:
1. Current source
Current source is relative to voltage source.
For a voltage source, the voltage output from the power supply to both ends of the load tries to remain constant. That is to say, the voltage on the power supply is constant. From the perspective of Ohm's law, the power supply voltage V is constant, and I and R can be changed, that is, V= IR.
For a current source, the current output from the power supply to the load tries to be constant, that is, the current from the power supply is constant. This is not common, but it does exist, and is applied in many occasions, and also obeys Ohm's law, that is, V=IR.
In fact, in actual operation, the voltage source and current source are not well distinguished, but there are many characteristics to follow. In power electronic equipment, you can distinguish between voltage source and current source by looking at its control object. The voltage source is controlled by voltage. Object, while the current source uses current as the control object, which is often referred to as direct current control. The photovoltaic grid-connected inverter uses the three-phase AC output voltage as the main control object plus DC voltage to realize MPPT maximum power control.
2. Active inverter
The active inverter was initially applied to the braking of the motor, and the excitation energy in the rotor when the motor was braking was fed back to the grid by switching off the thyristor to achieve energy saving.
A typical feature of an active inverter is that its output is also connected to a power source, so the image becomes an active inverter. The inverter is used as a power source to transmit its own energy to another power source, which is grid-connected power generation.
3. Grid-connected power generation