The Impact of Solar Inverters on Photovoltaic Systems

In a photovoltaic system, the cost of a solar inverter accounts for less than 5% of the system's total power generation efficiency, yet it is a key factor in determining power generation efficiency. Even if the components and other accessories are identical, choosing different inverters can result in a 5% to 10% difference in total system power generation.

This difference is primarily due to the inverter. MPPT efficiency is a key factor in determining the power generation capacity of a photovoltaic inverter, even more important than the efficiency of the inverter itself. MPPT efficiency is calculated by multiplying hardware efficiency by software efficiency. Hardware efficiency primarily depends on the accuracy of the sampling circuit, the MPPT voltage range, and the number of MPPT paths.

The Impact of Solar Inverters on Photovoltaic Systems

Maximum Power Point Tracking (MPPT) is a core technology in photovoltaic power generation systems. It adjusts the output power of a photovoltaic array based on varying ambient temperature and light intensity, ensuring that the array consistently outputs maximum power.

MPPT Sampling Circuit Accuracy

There are many ways to implement MPPT. However, regardless of the method, it is necessary to first measure changes in component power and then react to those changes. The most critical component in this process is the current sensor. Its measurement accuracy and linearity error directly determine hardware efficiency. There are two main types of current sensors: closed-loop and open-loop. Open-loop current sensors are generally voltage-based and feature small size, light weight, no plug loss, low cost, and a linear accuracy of up to 99%. Their total measurement error is approximately 1%. Closed-loop current sensors offer a wide frequency band, high accuracy, fast response time, and strong anti-interference capabilities. Their linear accuracy is approximately 99%.

Closed-loop sensors are advantageous when weather conditions fluctuate drastically.

Closed-loop current sensors

Open-loop current sensors

MPPT voltage range

The operating voltage range of a power inverter is related to the inverter's electrical topology and output voltage. String inverters and distributed inverters use a two-stage electrical topology. The MPPT operating voltage range is 250-850V.

Central inverters use a single-stage structure, with output voltages ranging from 270V, 315V, and 400V. Input MPPT voltage ranges include 450-850V, 500-850V, and 570-850V. Single-stage structures include string inverters, which consist of a single DC-AC inverter. Their output voltage is 400V, and their MPPT input voltage range is 570-850V. From an application perspective, each inverter type has its own advantages and disadvantages.

From an inverter perspective, for inverters with higher output voltages, lower currents yield higher efficiency at the same power level. Compared to two-stage structures, single-stage structures offer simpler construction, higher reliability, and lower costs.

From a system perspective, an inverter with a wider MPPT voltage range can operate earlier and later, extending power generation time.

According to the principle of voltage source series connection, the system output voltages add up, but the current remains constant. When PV panels are connected in series, the output current is determined by the minimum number of panels. Due to factors such as panel materials, processing techniques, shadows, and dust, reducing the output of one panel will also reduce the output of the entire series. Therefore, the number of series components should be minimized and the number of parallel components increased to reduce the impact of component inconsistency.

MPPT Cycle

Currently, string inverters have one to five MPPT circuits, and power-frequency centralized inverters also have one to three MPPT circuits. Distributed inverters integrate combiner boxes and MPPT boosting. There are multiple MPPTs, and there is also a high-frequency modular centralized inverter. Each module has an MPPT. This solution was introduced by Emerson in 2010, but perhaps due to the immaturity of the technology at the time, the market response was less than ideal.

From the perspective of addressing mismatch issues, more MPPTs are beneficial. From the perspective of stability and efficiency, fewer MPPTs are better, as increasing the number of MPPTs often leads to higher system costs, poorer stability, and greater losses. Therefore, it is necessary to select an appropriate solution based on the actual terrain requirements. Theoretically, component inconsistency should exceed 0.5%, which provides some operational value.

Functional Loss: There are many MPPT algorithms, such as the interference observation method, incremental conductance method, and incremental conductance method. Regardless of the algorithm used, solar insolation intensity is determined by the constantly changing DC voltage, so errors are inevitable. For example, even when the voltage is actually at the optimal operating point, the inverter will still attempt to change the voltage to determine whether it is at that point. If the number of MPPT loops increases, losses will increase.

Measurement Losses: When MPPT is operating, the inverter must measure both current and voltage. Generally speaking, higher currents provide greater interference immunity and lower errors. A two-loop MPPT can handle twice the current of a four-loop MPPT, significantly reducing errors. For example, a company's 50kW solar inverter uses the HLSR20-P open-loop DC current sensor, which measures 20A with an error of 1%. Errors often occur when the input current is less than 0.5A, and operation is essentially non-existent below 0.2A.

Circuit Losses: The MPPT main circuit includes an inductor and a switching transistor, which also generate losses during operation. Generally speaking, higher currents and lower inductances reduce losses.

The following figure shows the actual power generation of different MPPT inverters at two different locations. As can be seen from the figure, in unobstructed, flat areas, the power generation of the two inverters is similar. In mountainous areas or on rooftops with moderate sunshade conditions, a two-stage, multi-channel MPPT inverter offers higher power generation capacity.

Summary

The diversity of inverter MPPT technology greatly facilitates power plant design. By combining practical and scientific design, different inverters can be selected for different terrains and lighting conditions, reducing power plant costs and improving economic efficiency. Mountainous power plants and rooftop power plants exhibit inconsistent and partial shade. Different mountainous areas have different shade characteristics, leading to component mismatch issues. It is recommended to choose a two-stage inverter with multi-channel MPPT and a wide voltage range to extend power generation time in the morning and evening. This is especially true in flat areas with no obstructions and good lighting conditions.