I. Maximum Power Point Tracking Technology
The specific implementation method is to use a maximum power point tracking controller to convert the DC power output from the solar panel into AC power input to the inverter, which can generally increase the inverter output power by about 5% to 25%.
II. Voltage Boost Technology
Voltage boosting technology refers to increasing the inverter's output power by raising its input voltage. Specifically, it involves using an electronic transformer or a DC-DC boost converter to increase the input voltage to a range acceptable to the inverter, thereby increasing its output power. This method is suitable for situations in power generation systems where the inverter's input voltage requirements cannot be met.
III. Current Enhancement Technology
Current enhancement technology refers to increasing the output power of an inverter by adjusting its output current. Specifically, it involves using electronic switches or capacitors to increase current flow and then feeding it into the grid, thereby increasing the inverter's output power. This method is suitable for situations where solar panel power generation is low or the inverter's voltage is low during operation.
IV. Optimize Inverter Structural Design
Optimizing the inverter's structural design is also an effective way to improve its output power. In inverter design, efficiency can be improved by rationally arranging circuits, adding a heat dissipation system, and reducing transmission losses, thereby increasing the inverter's output power. Furthermore, when selecting an inverter, priority should be given to products with high efficiency, high power, and high reliability.
In the diagram above, the output voltage and current of the photovoltaic module follow the IV curve (green) and the PV curve (blue). To maximize the inverter's output power, the DC voltage needs to operate at the maximum point indicated by the red dot; this point is the maximum power point. If the maximum power point is 550V, the power output at 550V is 200W. At this point, the power output at 520V is approximately 190W, and at 580V it is approximately 185W, neither of which is as high as the power output at 550V. If the inverter cannot track 550V, it will lose some power generation, but this will not have any other impact on the system.
So why is continuous tracking necessary? Because this curve changes with varying light intensity, temperature, and shading, causing the maximum power point (MPPT) to also change. The MPPT voltage might be 560V in the morning, 520V at noon, and 550V in the afternoon. Therefore, the inverter needs to continuously locate this MPPT, a process known as maximum power point tracking, to ensure that the solar panels output maximum energy throughout the day, avoiding waste of solar energy resources. Building on this basic understanding, let's discuss MPPT.
MPPT, short for Maximum PowerPoint Tracking, refers to the inverter adjusting the output power of the photovoltaic array according to different external environmental characteristics such as temperature and light intensity, so that the photovoltaic array always outputs maximum power.
The photovoltaic array is connected to the load via a DC/DC circuit. The maximum power point tracking device continuously detects changes in the current and voltage of the photovoltaic array and adjusts the duty cycle of the PWM drive signal of the DC/DC converter according to these changes.
For linear circuits, the power supply achieves maximum power output when the load resistance equals the internal resistance of the power source. Although both photovoltaic cells and DC/DC conversion circuits are highly nonlinear, they can be considered linear circuits for a very short time. Therefore, by adjusting the equivalent resistance of the DC-DC conversion circuit to always equal the internal resistance of the photovoltaic cell, the maximum output of the photovoltaic cell can be achieved, thus realizing the maximum power point test (MPPT) of the photovoltaic cell.
Because solar cells are affected by external factors such as light intensity and environment, their output power varies. Higher light intensity results in more electricity output. Inverters with maximum power point tracking (MPPT) are designed to fully utilize solar cells, allowing them to operate at their maximum power point. In other words, with constant solar radiation, the output power after implementing MPPT will be higher than before, which is the function of MPPT.
Let's assume that the MPPT hasn't started tracking yet, and the module's output voltage is 500V. After the MPPT starts tracking, it adjusts the resistance in the circuit through its internal circuit structure to change the module's output voltage and output current, until the output power is at its maximum (let's say 550V). It then continues to track continuously. In other words, with constant solar radiation, the module's output power will be higher at 550V than at 500V. This is the function of the MPPT.
Currently, maximum power point tracking (MPPT) technology for photovoltaic arrays has been researched to some extent both domestically and internationally, resulting in the development of various control methods. Commonly used methods include: Constant Voltage Tracking (CVT), Perturbation and Observation (P&O), Incremental Conductance (INC), and gradient-based variable step size incremental conductance method, etc. (These algorithms can only be used under unobstructed conditions.)
1) Algorithm for MPPT of single peak power output
Currently, the commonly used control methods for maximum power point tracking (MPPT) of photovoltaic arrays under unobstructed conditions include the following:
Constant Voltage Tracking (CVT)
Perturbation and Observation (P&O) method
Incremental Conductance Method (INC)
l Based on gradient variable step size incremental conductance method, etc.
2) Multi-peak power output (MPPT) algorithm
Conventional maximum power point tracking (MPPT) algorithms, such as the perturbation observation method and the incremental conductance method, may fail under cloud cover, thus failing to achieve true MPPT. Currently, multi-peak MPPT algorithms have been proposed internationally, mainly including the following three types:
Composite MPPT Algorithm Combining Conventional Algorithms
Fibonacci method
Short-circuit current pulse method
In photovoltaic (PV) systems, the cost of the inverter accounts for less than 5% of the total cost, yet it is one of the decisive factors in power generation efficiency. When components such as modules are identical, choosing different inverters can result in a 5% to 10% difference in the total power generation of the system. The main reason for this difference is the inverter itself. MPPT efficiency is a key factor determining the power generation of a PV inverter, and its importance even exceeds that of the inverter itself. MPPT efficiency equals hardware efficiency multiplied by software efficiency. Hardware efficiency is mainly determined by the accuracy of the sampling circuit, the MPPT voltage range, and the number of MPPT channels, while software efficiency is mainly determined by the control algorithm.
Maximum Power Point Tracking (MPPT) is a core technology in photovoltaic power generation systems. It refers to adjusting the output power of a photovoltaic array according to different external environmental characteristics such as temperature and light intensity, so that the photovoltaic array always outputs maximum power.
The explosive growth of China's photovoltaic market has spurred the development of photovoltaic inverters, leading to a proliferation of various technologies. Currently in use are centralized inverters, single-stage string inverters, two-stage string inverters, distributed inverters, high-frequency modular inverters, and MPPT (Multi-Level Transmission) technology, among others.
Introduction to MPPT in Inverters_What is the Use of MPPT in Inverters
There are many ways to implement MPPT (Multi-Level Testing), but regardless of the method used, the first step is to measure the change in component power and then react to the change. The most critical component in this process is the current sensor, whose measurement accuracy and linearity directly determine the hardware efficiency. Manufacturers of high-quality current sensors include LEM (Switzerland), VAC (USA), and Tamura (Japan). They offer both open-loop and closed-loop types. Open-loop current sensors are generally voltage-type, small in size, lightweight, have no insertion loss, low cost, 99% linearity accuracy, and a total measurement error of around 1%. Closed-loop current sensors have a wide bandwidth, high accuracy, fast response time, strong anti-interference capability, 99.9% linearity accuracy, and a total measurement error of 0.4%.
Using closed-loop sensors is advantageous when the weather changes drastically.
The operating voltage range of an inverter is related to its electrical topology and output voltage. String inverters and distributed inverters have a two-stage electrical topology, with an MPPT operating voltage range of 250-850V. Centralized inverters have a single-stage structure, with output voltages of 270V, 315V, and 400V, and input MPPT voltage ranges of 450-850V, 500-850V, and 570-850V. There is also a single-stage string inverter, which has only one DC-AC inverter stage, with an output voltage of 400V and an MPPT input voltage range of 570-850V. From an application perspective, each has its advantages and disadvantages.
1) From the inverter's perspective, the higher the output voltage of an inverter, the lower the current and the higher the efficiency for the same power rating. Single-stage inverters are simpler, more reliable, lower in cost, and cheaper than two-stage inverters.
2) From a system perspective, the wider the MPPT voltage range of the inverter, the earlier it can start up, the later it can shut down, and the longer the power generation time.
3) According to the principle of voltage source series connection, the system output voltages are added together, and the current remains unchanged. After photovoltaic modules are connected in series, the output current is determined by the minimum number of solar panels. Affected by the raw materials of the modules, the processing technology, shading, dust, etc., if the power of one module decreases, the power of the modules in the string will decrease. Therefore, the number of modules connected in series should be minimized, and the number of modules connected in parallel should be maximized to reduce the impact caused by the inconsistency of the modules.
Currently, string inverters have 1 to 5 MPPT channels. Centralized inverters generally have 1 MPPT channel. Distributed inverters integrate the combiner box and MPPT boosting into one unit and have multiple MPPT channels. There is also a type of high-frequency modular inverter where each module has one MPPT channel.
From the perspective of resolving mismatch issues, a higher number of MPPTs is more advantageous; however, from the perspective of stability and efficiency, a lower number of MPPTs is better, because a higher number of MPPTs increases system cost, reduces stability, and increases losses. Therefore, it is necessary to choose an appropriate solution based on the actual terrain requirements. Theoretically, the component inconsistency must exceed 0.5% to be considered worthwhile.
1) Functional loss: There are many MPPT algorithms, such as interference observation method, incremental conductance method, and incremental conductance method. Regardless of the algorithm, they all judge the change in sunlight intensity by continuously changing the DC voltage. Therefore, there will be errors. For example, when the voltage is actually at the optimal operating point, the inverter will still try to change the voltage to judge whether it is the optimal operating point. Each additional MPPT will result in an additional loss.
2) Measurement of Losses: When the MPPT is working, the inverter needs to measure current and voltage. Generally speaking, the larger the current, the greater the anti-interference capability and the smaller the error. A 2-channel MPPT has twice the current and half the error compared to a 4-channel MPPT. For example, a company's 50kW inverter uses an open-loop DC current sensor HLSR20-P with a current rating of 20A and an error of 1%. When the input current is less than 0.5A, errors frequently occur, and when the input current is less than 0.2A, it basically cannot work.
3) Circuit losses: The MPPT main circuit has an inductor and a switching transistor, which also generate losses during operation. Generally speaking, the larger the current, the smaller the inductor can be, and the less the loss.
The diagram below illustrates the actual power generation of different MPPT inverters (single-pole single-path and dual-pole multi-path) in two different locations. As can be seen from the diagram, in flat, unobstructed areas with good sunlight, the power generation of the two types of inverters is similar. The single-pole single-path inverter generates power for shorter periods in the morning and evening, resulting in some power loss. However, due to its low loss and high efficiency, its output power is greater than that of the dual-pole multi-path inverter once the sunlight reaches the starting voltage. Therefore, overall, they are roughly equivalent.
In mountainous areas or regions with limited sunlight due to rooftop shading, dual-stage multi-MPPT inverters generate high power. This is because the low-power generation period is longer, while the high-power generation period is shorter.
