Maximum power point tracking ( MPPT ) or sometimes just power point tracking ( PPT )) is a commonly used technique with wind turbines and photovoltaic solar systems (PV) to maximize power extraction in all conditions.
Although solar power is primarily covered, this principle applies generally to sources with variable power: for example, optical power transmission and thermophotovoltaics.
The solar system of PV exists in a variety of different configurations related to their relationship with the inverter system, external network, battery bank, or other electrical load. Regardless of the ultimate goal of solar power, the main problem handled by MPPT is that the power transfer efficiency of the solar cells depends on both the amount of sunlight that falls on the solar panel and the electrical characteristics of the load. Because the amount of sunlight varies, the load characteristics that provide the highest power transfer efficiency change, so that system efficiency is optimized when load characteristics are changed to keep power transfer at highest efficiency. This load characteristic is called the maximum power point (MPP) and MPPT is the process of finding this point and keeping the load characteristics there. Electrical circuits can be designed to present a random charge to a photovoltaic cell and then change the voltage, current, or frequency to adjust the device or other systems, and MPPT solves the problem of selecting the best load to be presented to the cell to obtain the most usable power.
Solar cells have a complex relationship between temperature and total resistance resulting in a non-linear output efficiency that can be analyzed based on the I-V curve. It is the purpose of the MPPT system to sample the PV cell output and apply the proper resistance (load) to obtain maximum power for any given environmental conditions. MPPT devices are typically integrated into electrical power converter systems that provide voltage or current conversion, filtering, and regulation to drive a variety of charges, including power grids, batteries, or motors.
- The solar inverter converts DC power into AC power and can incorporate MPPT: an inverter as it samples output power (curve IV) of the solar module and implements proper resistance (load) to obtain maximum power./li>
- Power on MPP (P mpp ) is the product of MPP (Vp subp subp) mpp ) and MPP current (I mpp ).
Video Maximum power point tracking
​​â € <â €
Photovoltaic cells have a complex relationship between their operating environment and the maximum power they can generate. The fill factor, abbreviated FF , is a parameter that characterizes the non-linear electrical behavior of the solar cell. The fill factor is defined as the maximum power ratio from the solar cell to the open circuit voltage product V oc and short circuit current I sc . In tabulated data, it is often used to estimate the maximum power a cell can provide with optimal load under certain conditions, P = FF * V oc * I sc . For most purposes, FF, V oc , and I sc are sufficient information to provide a useful approximate model of the electrical behavior of photovoltaic cells under typical conditions.
For each set of operational conditions, the cell has a single operating point where the current value ( I ) and the voltage ( V ) of the cell produce the maximum output power. These values ​​correspond to the particular load resistance, which is equal to V /I as determined by Ohm's Law. The power of P is given by P = V * I . Photovoltaic cells, for most useful curves, act as a constant current source. However, in the photovoltaic MPP region, the curve has an inverse exponential relationship between current and voltage. From the basic circuit theory, power sent from or to the device is optimized in which the derivative (graphically, slope) dI/dV of curve IV is the same and is contrary to the I/V ratio (where d P/dV = 0). This is known as the maximum power point (MPP) and corresponds to the "knee" of the curve.
A load with resistance R = V/I is the exact opposite of this value drawing maximum power from the device. This is sometimes called the 'resistance characteristic' of the cell. This is a dynamic quantity that changes depending on the level of illumination, as well as other factors such as temperature and age of the cell. If the resistance is lower or higher than this value, the power drawn will be less than the maximum available, and thus the cell will not be used as efficiently as possible. The maximum power point tracer utilizes various types of control or logic circuits to locate this point and thus allow the converter circuit to extract the maximum power available from the cell.
Maps Maximum power point tracking
Implementation
When the load is connected directly to the solar panel, the operating point of the panel will rarely reach its peak. The impedance seen by the panel obtains the operating point of the solar panel. Thus by varying the impedance seen by the panel, the operating point can be moved to the peak power point. Since the panel is a DC device, a DC-DC converter must be used to convert the impedance of one circuit (source) to another circuit (load). Change the task ratio of the DC-DC converter results in impedance changes as seen by the panel. At a given impedance (or task ratio) the operating point will be at the peak power transfer point. The I-V curves of panels can vary greatly with variations in atmospheric conditions such as jets and temperatures. Therefore, it is not feasible to improve the ratio of tasks to dynamically changing operating conditions.
MPPT implementation uses algorithms that often follow the voltage and current panels, then adjust the ratio of tasks as needed. Microcontrollers are used to implement the algorithm. Modern applications often take advantage of larger computers for analytics and load forecasting.
Classification
Controllers can follow several strategies to optimize the power output of an array. The maximum power point tracer can apply different algorithms and switch between them based on array operating conditions.
Disrupt and observe
In this method the controller adjusts the voltage with a small number of arrays and measures power; if power increases, further adjustment in that direction is attempted until energy is no longer increased. This is called interference and observation method and most commonly, although this method can produce oscillation of power output. This is referred to as the method of climbing a hill, because it depends on increasing the power curve to the voltage below the maximum power point, and falling above that point. Perturb and observe are the most commonly used MPPT methods because of their ease of application. Distracting and observing methods can produce top-level efficiencies, provided appropriate predictive and adaptive hill climb strategies are adopted.
Additional conductance
In a method of increasing conductance, the controller measures additional changes in the current and voltage of the PV array to predict the effect of the voltage change. This method requires more calculations in the controller, but can track conditions that change faster than interference and observation methods (P & amp; O). Like algorithm P & amp; O, it can generate oscillations in the power output. This method utilizes incremental conductance (dI/dV) of the photovoltaic arrays to calculate power change signals with respect to voltage (dP/dV).
The incremental conductance method calculates the maximum power point with an additional conductance ratio (I ? /V ? ) to the array conductance (I/V). When both are the same, the output voltage is the MPP voltage. The controller maintains this voltage until the irradiation and process changes are repeated.
The incremental conductance method is based on the observation that at the point of maximum power dP/dV = 0, and P = IV. The current of the array can be expressed is a function of voltage: P = I (V) V. Therefore, dP/dV = VdI/dV I (V). This setting is equal to zero result: dI/dV = -I (V)/V. Therefore, the maximum power point is reached when the additional conductance equals the negative of the instant conductance.
Current sweep
The current sweeping method uses sweep waves for the current PV array so that the I-V characteristic of the PV array is obtained and updated at fixed time intervals. The maximum power point voltage can then be calculated from the characteristic curve at the same interval.
Constant voltage
The term "constant voltage" in MPP tracking is used to describe different techniques by different authors, one in which the output voltage is set to a constant value in all conditions and one in which the output voltage is set based on a constant ratio to an open circuit voltage measured (V OC ). This latter technique is referred to as the "open stress" method by some authors. If the output voltage remains constant, there is no effort to trace the maximum power point, so it is not the technique of tracing maximum power point in the strict sense, although it has some advantages in cases when MPP tracking tends to fail, and is therefore sometimes used to complement method of MPPT in such cases.
In the "constant voltage" method of MPPT (also known as "open voltage method"), the power delivered to the load is interrupted instantaneously and the open circuit voltage with zero current is measured. The controller then resumes operation with a controlled voltage at a fixed ratio, such as 0.76, of the open circuit voltage V OC . This is usually a predetermined value to be the maximum power point, either empirically or by modeling, for expected operating conditions. The operating point of the PV array is thus stored near the MPP by adjusting the array voltage and matching it with a fixed reference voltage V ref = kV OC . The value of V ref may also be chosen to provide optimal performance relative to other factors as well as MPP, but the central idea in this technique is that V ref is defined as the ratio to V < sub> OC .
One approach inherent in the "constant voltage" ratio method is that the ratio of MPP to V OC is only approximately constant, thus allowing room for further optimization.
Comparison of methods
Both interfere and observe, and additional conductance, are examples of the "climbing hill" method that can find the local maximum power curve for the operating conditions of the PV arrangement, and therefore provide the correct maximum power point.
Interference and observation methods require oscillating power output around the maximum power point even under steady state irradiation.
The incremental conductance method has the advantage through interference and observation methods (P & amp; O) that can determine the maximum power point without oscillating around this value. It can track maximum power points under various irradiation conditions with higher accuracy than interference and observation methods. However, additional conductance methods can produce oscillations (unintentional) and can work irregularly under rapidly changing atmospheric conditions. Sampling frequency decreases because of the higher complexity of the algorithm compared to the P & amp; O.
In the constant voltage ratio (or "open voltage") method, the current from the photovoltaic array must be set to zero for a moment to measure the open circuit voltage and then after it is set to a predetermined percentage of the measured voltage, typically around 76%. Energy can be wasted as long as the current time is set to zero. An approximation of 76% because the MPP/V OC ratio is not necessarily accurate. Although simple and low cost to implement, the interrupt reduces the efficiency of the array and does not ensure finding the actual maximum power point. However, the efficiency of some systems can reach above 95%.
MPPT Placement
Traditional solar inverters perform MPPT for entire PV arrays (module associations) as a whole. In such systems the same stream, dictated by an inverter, flows through all modules in a string (series). Because different modules have different I-V curves and different MPPs (due to manufacturing tolerances, partial shading, etc.) this architecture means some modules will work under their MPP, resulting in lower efficiency.
Some companies (see power optimizer) now put the tracker of the maximum power point into individual modules, allowing each to operate at peak efficiency despite uneven shading, dirtiness or mismatch of electricity.
The data suggest having one inverter with one MPPT for projects that have east and west modules no loss when compared to having two inverters or one inverter with more than one MPPT.
Operation with battery
At night, off-grid PV systems can use batteries to supply loads. Although a fully charged battery voltage may be close to the maximum PV point power point voltage, this is not possible at sunrise when the battery is partially released. Charging can start at a voltage well below the maximum power point voltage of the PV panel, and MPPT can resolve this mismatch.
When the battery in the off-grid system is fully charged and PV production exceeds local load, MPPT can no longer operate the panel at its maximum power point because excess power has no load to absorb it. MPPT should then shift the PV panel operating point from the peak power point until production is exactly the same as the demand. (An alternative approach commonly used in spacecraft is to divert excess PV power to resistive loads, allowing the panel to operate continuously at its peak power point.)
In a photovoltaic system connected to the network, all the power sent from the solar module will be sent to the power grid. Therefore, MPPT in a connected PV grid system will always strive to operate the PV module at its maximum power point.
References
External links
Media related to the maximum power point Tracer in Wikimedia Commons
- MPPT tracker by Daniel F. Butay (Microchip-based PIC)
Source of the article : Wikipedia
0 Comments