MPPT is the very core technology of the inverter, and the MPPT voltage is a very critical parameter in the design of photovoltaic power plants.
What is MPPT?
In the figure above, the output voltage and current of the photovoltaic module follow the V curve (green) and the PV curve (blue). If you want the inverter to output the most power. If it is large, the DC voltage needs to run at the maximum point where the red point is, and this point is the maximum power point. If the maximum power point is 550V, 550V. The power is 200W. At this time, the power when running at 520V is about 190W, and the power at 580V is about 185W. There is no power at 550V.big. If the inverter tracks less than 550V, it will lose power generation, but it will not have other impacts on the system.
So why keep tracking? Because this curve changes with different light intensity, temperature and shading, the maximum power point is also at Changes, maybe the maximum power point voltage is 560V in the morning, 520V at noon, and 550V in the afternoon, so the inverter needs to constantly look for this The maximum power point, that is, the maximum power point is tracked, so as to ensure that the energy of the battery panel can be maximized throughout the day without wasting solar resource.
On the basis of understanding the above basic knowledge, let's talk about MPPT.
MPPT, the abbreviation of MaximumPowerPointTracking, Chinese for "maximum power point tracking", it refers to the inverter phase according to the outside world The output power of the photovoltaic array is adjusted according to different ambient temperature, light intensity and other characteristics, so that the photovoltaic array always outputs the maximum power.
What is the role of MPPT?
Due to the influence of external factors such as light intensity and environment, the output power of the solar cell changes, and the power emitted by the light intensity is more, bringing. The MPPT maximum power tracking inverter is to make full use of solar cells and make them run at the maximum power point. That is to say, in the sun In the case of constant radiation, the output power after MPPT will be higher than that before MPPT, which is the role of MPPT.
For example, suppose MPPT has not started tracking, and the output voltage of the module is 500V at this time, and then after MPPT starts tracking, it starts to pass. The internal circuit structure adjusts the resistance on the loop to change the output voltage of the component and at the same time change the output current until the output power reaches the maximum (assuming it is 550V maximum), and then it has to track continuously, so that the output voltage of the module at 550V is constant under the condition of constant solar radiation In some cases, the output power will be higher than that of 50OV, which is the function of MPPT.
Influencing factors of MPPT of a single photovoltaic module
Generally speaking, the impact of illuminance and temperature changes on output power is most directly reflected in MPPT, that is to say, illuminance and temperature are important factors affecting MPPT.
As the irradiance decreases, the output power of photovoltaic modules will decrease: as the temperature increases, the output power of photovoltaic modules will decrease:
1. Effect of irradiance on MPPT
2. Effect of temperature on MPPT
MPPT of photovoltaic power generation unit and its principle and algorithm
1. MPPT of photovoltaic power generation unit
Both pictures are the MPPT trace of a photovoltaic module, but in actual engineering, a 500kW inverter often needs to connect 80~90 photovoltaic strings.
The photovoltaic array is susceptible to interference from the surrounding environment (such as floating clouds, buildings, tree shade, etc.) and the dust on the surface of the battery panel during use, resulting in a decrease in the output power of the photovoltaic array and a complex output characteristic curve. The output characteristic curve presents multi-extreme points. If there is an air-conditioning unit behind a certain string in a power station: another leaf covers a certain battery slice: another tree shade covers part of the components. Then there must be an output power deviation between different strings. Therefore, the output characteristic curve of the photovoltaic strings connected to each inverter becomes complicated, showing multiple extreme points. How to find the highest point requires MPPT!
2. The principle of maximum power point tracking
With the development of electronic technology, the MPPT control of the current solar cell array is generally completed through the DC/DC conversion circuit. Its principle box is shown below.
The photovoltaic cell array is connected to the load through a DC/DC circuit, and the maximum power tracking device continuously detects the current and voltage changes of the photovoltaic array, and adjusts the duty cycle of the PWM driving signal of the DC/DC converter according to the change.
For linear circuits, when the load resistance is equal to the internal resistance of the power supply, the power supply has the maximum power output. Although both photovoltaic cells and DC/DC conversion circuits are strongly nonlinear, they can be considered as linear circuits in a very short time. Therefore, as long as the equivalent resistance of the DC-DC conversion circuit is adjusted so that it is always equal to the internal resistance of the photovoltaic cell, the maximum output of the photovoltaic cell can be achieved, and the MPPT of the photovoltaic cell can also be realized.
3. MPPT algorithm
At present, the maximum power point tracking (MPPT) technology of photovoltaic arrays has been studied at home and abroad, and various control methods have been developed. The following are commonly used: Constant Voltage Tracking (CVT for short), interference observation Law(PerturbationAndObservationmethod referred to as P&O), incremental conductance (IncrementalConductancemethodNC for short), conductance incremental method based on Faldo's variable step size, and so on. (These algorithms can only be used under unoccluded conditions)
1) MPPT algorithm for single peak power output
At present, under the condition of no shading, the control methods of maximum power point tracking (MPP) of photovoltaic arrays are commonly used as follows:
Constant Voltage Tracking (ConstantVoltageTracking referred to as CVT)
Interference observation method (PerturbationAndObservationmethodi referred to as P&O)
Incremental Conductance method (Incremental Conductance method referred to as lNC)
Conductance incremental method based on gradient variable step size, etc.
2) Multi-peak power output MPPT algorithm
Ordinary maximum power tracking algorithms, such as disturbance arc observation and conductance increment method, may fail under the cover of a cloud, and cannot achieve realSignificance of maximum power tracking. At present, some people in the world have proposed a multi-peak MPPT algorithm, which mainly includes the following three types:
Composite MPPT Algorithm Combined with Conventional Algorithm
Fibonacci method
short circuit current pulse method
Summarize:
At present, the industry has realized the importance of multiple MPPT channels in the inverter, and the multi-MPPT string inverter has been widely recognized.
