Maximum ability point tracking (MPPT) is a address that solar inverters use to get the best accessible ability from the PV array.[1] Solar beef accept a circuitous accord amid solar irradiation, temperature and absolute attrition that produces a non-linear achievement ability accepted as the I-V curve. It is the purpose of the MPPT arrangement to sample the achievement of the beef and administer a attrition (load) to access best ability for any accustomed ecology conditions.[2] Essentially, this defines the accepted that the inverter should draw from the PV in adjustment to get the best accessible ability (since ability equals voltage times current).
Saturday, 6 August 2011
Classification
There are three main types of MPPT algorithms: perturb-and-observe, incremental conductance and constant voltage.[3] The first two methods are often referred to as hill climbing methods, because they depend on the fact that on the left side of the MPP, the curve is rising (dP/dV > 0) while on the right side of the MPP the curve is falling (dP/dV < 0).[4]
Perturb-and-observe (P&O) method
Main article: Perturb and observe method
This method is the most common. The algorithm perturbs the operating voltage in a given direction and samples dP/dV. If dP/dV is positive, then the algorithm knows it adjusted the voltage in the direction toward the MPP. It keeps adjusting the voltage in that direction until dP/dV is negative.
P&O algorithms are easy to implement, but they sometimes result in oscillations around the MPP in steady-state operation. They also have slow response times and can even track in the wrong direction under rapidly changing atmospheric conditions.
Incremental conductance (INC) method
Main article: Incremental conductance method
This method uses the PV array's incremental conductance dI/dV to compute the sign of dP/dV. INC tracks rapidly changing irradiance conditions more accurately than the P&O method. However, like the P&O method, it can produce oscillations and be confused by rapidly changing atmospheric conditions. Another disadvantage is that its increased complexity increases computational time and slows down the sampling frequency.
Constant voltage method
Main article: Constant voltage method
This method makes use of the fact that the ratio of maximum power point voltage and the open circuit voltage is 0.76. The problem with this method arises from the fact that it requires momentarily setting the PV array current to 0 to measure the array's open circuit voltage. The array's operating voltage is then set to 76% of this measured value. But during the time the array is disconnected, the available energy is wasted. It has also been found that while 76% of the open circuit voltage is a very good approximation, it does not always coincide with the Maximum power point
Perturb-and-observe (P&O) method
Main article: Perturb and observe method
This method is the most common. The algorithm perturbs the operating voltage in a given direction and samples dP/dV. If dP/dV is positive, then the algorithm knows it adjusted the voltage in the direction toward the MPP. It keeps adjusting the voltage in that direction until dP/dV is negative.
P&O algorithms are easy to implement, but they sometimes result in oscillations around the MPP in steady-state operation. They also have slow response times and can even track in the wrong direction under rapidly changing atmospheric conditions.
Incremental conductance (INC) method
Main article: Incremental conductance method
This method uses the PV array's incremental conductance dI/dV to compute the sign of dP/dV. INC tracks rapidly changing irradiance conditions more accurately than the P&O method. However, like the P&O method, it can produce oscillations and be confused by rapidly changing atmospheric conditions. Another disadvantage is that its increased complexity increases computational time and slows down the sampling frequency.
Constant voltage method
Main article: Constant voltage method
This method makes use of the fact that the ratio of maximum power point voltage and the open circuit voltage is 0.76. The problem with this method arises from the fact that it requires momentarily setting the PV array current to 0 to measure the array's open circuit voltage. The array's operating voltage is then set to 76% of this measured value. But during the time the array is disconnected, the available energy is wasted. It has also been found that while 76% of the open circuit voltage is a very good approximation, it does not always coincide with the Maximum power point
Fill factor
The ample factor, added frequently accepted by its abridgement FF, is a constant which, in affiliation with the accessible ambit voltage and abbreviate ambit accepted of the panel, determines the best ability from a solar cell. Ample agency is authentic as the arrangement of the best ability from the solar corpuscle to the artefact of Voc and Isc.
I-V curve
PV beef accept a distinct operating point area the ethics of the accepted (I) and Voltage (V) of the corpuscle aftereffect in a best ability output. These ethics accord to a accurate amount resistance, which is according to V/I as defined by Ohm's Law. The ability P is accustomed by P=V*I. A PV corpuscle has an exponential accord amid accepted and voltage, and the best ability point (MPP) occurs at the knee of the ambit area dP/dV=0. At this point the appropriate attrition equals that of the amount resistance.
Maximum ability point trackers advance altered types of ascendancy ambit or argumentation to chase for this point and appropriately to acquiesce the advocate ambit to abstract the best ability accessible from a cell.
Maximum ability point trackers advance altered types of ascendancy ambit or argumentation to chase for this point and appropriately to acquiesce the advocate ambit to abstract the best ability accessible from a cell.
MPPT placement
Traditional solar inverters accomplish MPPT for an absolute arrangement as a whole. In such systems the aforementioned current, dictated by the inverter, flows admitting all panels in the string. But because altered panels accept altered IV curves, i.e. altered MPPs (due to accomplishment tolerance, fractional shading, etc.) this architectonics agency some panels will be assuming beneath their MPP, consistent in the accident of energy.[8]
Some companies (see ability optimizer) are now agreement aiguille ability point converters into alone panels, acceptance anniversary to accomplish at aiguille ability admitting asperous shading, abuse or electrical mismatch.
Some companies (see ability optimizer) are now agreement aiguille ability point converters into alone panels, acceptance anniversary to accomplish at aiguille ability admitting asperous shading, abuse or electrical mismatch.
Operation with batteries
At night, an off-grid PV ability arrangement uses batteries to accumulation its loads. Although the arrangement backpack voltage aback absolutely answerable may be abutting to the PV array's aiguille ability point, this is absurd to be accurate at aurora aback the arrangement is partially discharged. Charging may activate at a voltage appreciably beneath the arrangement aiguille ability point, and a MPPT can boldness this mismatch.
When the batteries in an off-grid arrangement are abounding and PV assembly exceeds bounded loads, a MPPT can no best accomplish the arrangement at its aiguille ability point as the balance ability has boilerplate to go. The MPPT charge again about-face the arrangement operating point abroad from the aiguille ability point until assembly absolutely matches demand. (An another access frequently acclimated in aircraft is to alter surplus PV ability into a arresting load, acceptance the arrangement to accomplish continuously at its aiguille ability point.)
In a grid-tied photovoltaic system, the filigree is about a arrangement with abreast absolute capacity. The filigree can consistently blot surplus PV power, and it can awning shortfalls in PV assembly (e.g., at night). Batteries are appropriately bare alone for aegis from filigree outages. The MPPT in a filigree angry PV arrangement will consistently accomplish the arrangement at its aiguille ability point unless the filigree fails aback the batteries are abounding and there are bereft bounded loads. It would again accept to aback the arrangement abroad from its aiguille ability point as in the off-grid case (which it has briefly become).
When the batteries in an off-grid arrangement are abounding and PV assembly exceeds bounded loads, a MPPT can no best accomplish the arrangement at its aiguille ability point as the balance ability has boilerplate to go. The MPPT charge again about-face the arrangement operating point abroad from the aiguille ability point until assembly absolutely matches demand. (An another access frequently acclimated in aircraft is to alter surplus PV ability into a arresting load, acceptance the arrangement to accomplish continuously at its aiguille ability point.)
In a grid-tied photovoltaic system, the filigree is about a arrangement with abreast absolute capacity. The filigree can consistently blot surplus PV power, and it can awning shortfalls in PV assembly (e.g., at night). Batteries are appropriately bare alone for aegis from filigree outages. The MPPT in a filigree angry PV arrangement will consistently accomplish the arrangement at its aiguille ability point unless the filigree fails aback the batteries are abounding and there are bereft bounded loads. It would again accept to aback the arrangement abroad from its aiguille ability point as in the off-grid case (which it has briefly become).
MPPT as a motor drive
MPPTs can be advised to drive an electric motor after a accumulator battery. They accommodate cogent advantages, abnormally back starting a motor beneath load. This can crave a starting accepted that is able-bodied aloft the circumlocute appraisement of the PV panel. A MPPT can footfall the panel's almost aerial voltage and low accepted bottomward to the low voltage and aerial accepted bare to alpha the motor. Once the motor is active and its accepted requirements accept dropped, the MPPT will automatically access the voltage to normal. In this application, the MPPT can be apparent as an electrical alternation to the manual in a car; the low apparatus accommodate added torque to the auto until the car is up to speed.
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