How does inverter MPPT selection affect power generation?

Choosing different MPPT routes will have a certain impact on the power generation of the system. From the perspective of solving the mismatch problem, the fewer strings behind one MPPT, the better; from the perspective of stability and efficiency, the more strings behind one MPPT, the better, because the more MPPTs there are, the higher the system cost, the worse the stability, and the more losses. In practical applications, it is necessary to choose the appropriate solution based on the actual terrain.

1. Advantages of MPPT with fewer strings and more

(1) Low functional loss: There are many MPPT algorithms, including interference observation method, incremental conductance method, incremental conductance method, etc. No matter which algorithm is used, it continuously changes the DC voltage to judge the change in sunlight intensity, so there will be errors. For example, when the voltage is actually at the optimal working point, the inverter will still try to change the voltage to determine whether it is the optimal working point. One more MPPT will result in one more loss.

(2) Less measurement loss: When MPPT is working, the inverter needs to measure current and voltage. Generally speaking, the larger the current, the greater the anti-interference ability and the smaller the error.

(3) Less circuit loss: The MPPT power circuit has an inductor and a switch tube, which will generate losses during operation. The more MPPT paths there are, the greater the loss. Generally speaking, the larger the current, the smaller the inductance can be, and the less the loss.

2. Advantages of MPPT with multiple strings and few strings

(1) Each MPPT circuit of the inverter operates independently without interfering with each other. The strings can be of different models, different quantities, and different directions and tilt angles. Therefore, the number of strings is small and the system design flexibility is greater.

(2) Reduce DC side fuse failure: The most common failure of photovoltaic systems is DC side failure. An MPPT is configured with 1 to 2 strings. Even if a component in one string is short-circuited, the total current will not exceed 15%, so there is no need to configure a fuse. Common failure modes of fuses are overcurrent melting, aging melting, and overtemperature melting. Overcurrent melting is a protective melting that occurs when the load exceeds the rated value, such as overload or short circuit; aging melting refers to the fault melting that occurs in the absence of overcurrent due to the decrease of the current interception capacity due to long-term operation; the current of the fuse is closely related to the temperature. If the fuse works at high temperature, the current interception capacity decreases, and the possibility of fault melting is relatively high.

(3) Accurate fault location: The inverter independently detects the voltage and current of each input, and can sample the string current and voltage in real time to promptly detect line faults, component faults, shading and other problems. The detection accuracy can be improved through horizontal comparison of strings, meteorological conditions, and historical data.

(4) Matching power optimizer is more suitable: Currently, one of the solutions to eliminate the impact of mismatch at the component end is to use a power optimizer. The photovoltaic optimizer can convert low current into high current according to the needs of the series circuit. Finally, the output ends of each power optimizer are connected in series and connected to the inverter. Multiple strings are connected to the optimizer. According to the principle of consistent voltage in parallel circuits, when a string is blocked by shadows and the power decreases, the optimizer changes the voltage, the total voltage of this circuit will decrease, and it will also affect the voltage drop of other circuits of the same MPPT, resulting in a decrease in total power.

Summarize

Combined with the actual situation, scientific design. According to different terrains and component shielding conditions, select inverters with different MPPT architectures to reduce the procurement and maintenance costs of power stations and improve economic benefits.

(1) In areas with good lighting conditions and no obstructions on flat ground, it is recommended to choose a single-channel MPPT, single-stage inverter to improve system reliability and reduce system costs;

(2) For power stations on hills with complex terrain, such as large power stations such as the Leader Base, there are problems of inconsistent orientation and partial shielding. In addition, different hills have different shielding characteristics, which leads to component mismatch problems. Therefore, multiple MPPTs have to be selected. In this case, an inverter with two string inputs per MPPT is a better choice. It has no fuses and vulnerable parts, high fault location accuracy, and simpler maintenance.

(3) For hilly power stations and rooftop power stations with not very complex terrain and no obstruction of components, it is recommended to choose an inverter with two MPPTs and multiple strings, which can take into account both string mismatch and high efficiency and has more flexible design.