# Maximum power point tracking

Maximum power point tracking   ( MPPT or sometimes just PPT   ) is a technique commonly used with wind turbines and photovoltaic (PV) solar systems to maximize power extraction under all conditions.

Although solar power is mainly covered, the principle applies to sources of power, and optical power transmission and thermophotovoltaics .

PV solar systems exist in many different configurations with their links to inverter systems, external grids, battery banks, or other electrical loads.  Regardless of the ultimate destination of solar power, though, the central problem addressed by MPPT is that the efficiency of solar energy depends on the solar panel. load. As the amount of sunlight varies, the load characteristics of the highest power transfer efficiency, the efficiency of the system is maximized. This load characteristic is called the maximum power pointand MPPT is the process of finding this point and keeping the load characteristic there. Electrical circuits can be designed to present arbitrary charges to the photovoltaic cells and then convert the voltage, current, or frequency to suit other devices or systems, and MPPT solves the problem of choosing the best load the most usable power out.

Solar cells have a complex relationship between temperature and total resistance that produces a non-linear output efficiency that can be analyzed based on the IV curve .   It is the purpose of the MPPT system to sample the output of the cells and apply the proper resistance (load) to obtain maximum power for any given environmental conditions.  MPPT devices are typically integrated into an electric power converter that provides voltage or current conversion, filtering, and regulation for driving various loads, including power grids, batteries, or motors.

• Solar inverters convert the DC power to AC power and may include MPPT: such inverters sample the power output (IV curve) from the solar modules and apply the proper resistance (load) so as to obtain maximum power.
• The power at the MPP (P MPP ) is the product of the MPP voltage (V MPP ) and MPP current (I MPP ).

## IV curve

Photovoltaic solar cell IV curves where a line intersects the knee of the curves where the maximum power transfer point is located.

Photovoltaic cells have a relationship between their operating environment and the maximum power they can produce. The fill factor , abbreviated FF , is a parameter which characterizes the non-linear electrical behavior of the solar cell. Fill factor is defined as the ratio of the maximum power from the solar cell to the product of Open Circuit Voltage V oc and Short-Circuit Current I sc . In this paper, it is often used to estimate the maximum power of an optimal load under given conditions, P = FF * V oc * I sc . For most purposes, FF, V oc , and I sc are required to provide a relevant approximate model of the electrical behavior of a photovoltaic cell under typical conditions.

For any given set of operational conditions, the values ​​of the current ( I ) and Voltage ( V ) of the cell result in a maximum power output.  These values ​​correspond to a particular load resistance , which is equal to V / I as specified by Ohm’s Law . The power P is Given by P = V * I . A photovoltaic cell, for the majority of its useful curve, acts as a constant current source . However, at a photovoltaic cell’s MPP region, its curve has an inverse inverse exponential relationship between current and voltage. From basic system theory, the Power Delivered from gold to a device is optimized Where the derivative (Graphically, the slope) dI / dV of the IV curve is equal and opposite 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 equal to the reciprocal of this value draws the maximum power from the device. This is sometimes called the ‘characteristic resistance’ of the cell. This is a dynamic quantity which changes depending on the level of illumination, as well as other factors such as temperature and the age of the cell. If the resistance is greater than this value, the power drawn would be less than the maximum available, and thus the cell would not be used as it could be. Maximum power point trackers utilize different types of control circuit or logic to achieve this point and thus to allow the converter circuit to extract the maximum power available from a cell.

## MPPT Implementation

When a load is directly connected to the solar panel, the operating point of the panel will rarely be at peak power. The impedance seen by the panel derives from the operating point of the solar panel. Thus by varying the impedance seen by the panel, the operating point can be moved towards peak power point. Since panels are DC devices, DC-DC converters must be used to transform the impedance of one circuit (source) to the other circuit (load). Changing the duty ratio of the DC-DC converter results in an impedance change as seen by the panel. At a particular impedance (or duty ratio) the operating point will be at the peak power transfer point. The IV curve of the panel can vary with variation in atmospheric conditions such as radiance and temperature. Therefore,

MPPT implementations utilize algorithms that frequently sample panel voltages and currents, then adjust the duty ratio as needed. Microcontrollers are employed to implement the algorithms. Modern implementations often utilize larger computers for analytics and load forecasting.

## Classification

Controllers can follow several strategies to optimize the power output of an array. Maximum power point trackers may implement different algorithms and switch between them based on the operating conditions of the array. 

### Perturb and observe

In this method the controller adjusts the voltage by a small amount of the array and measures power; if the power increases, further adjustments in that direction. This is called the most common method in the field of oscillations of power output.   It is referred to as a hill climbing method, because it depends on the rise of the curve of power, and the fall above that point.  Perturb and observe is the most commonly used MPPT method. This method is suitable for a predictive and adaptive hill climbing strategy.  

### Incremental conductance

In the incremental conductance method, the controller measures incremental changes in the current array and voltage to predict the effect of a voltage change. This method requires more computation in the controller, but can change and observe method (P & O). Like the P & O algorithm, it can produce oscillations in power output.  This method utilizes the incremental conductance (dI / dV) of the photovoltaic array to compute the signal of change in power with respect to voltage (dP / dV). 

The incremental conductance method computes the maximum power point by comparison of the incremental conductance (I Δ / V Δ ) to the array conductance (I / V). When these two are the same (I / V = ​​I Δ / V Δ ), the output voltage is the MPP voltage. The controller maintains this voltage until the irradiation changes and the process is repeated. 

The incremental conductance method is based on the observation that the maximum power point dP / dV = 0, and that P = IV. The current from the array can be expressed as a function of the voltage: P = I (V) V. Therefore, dP / dV = VdI / dV + I (V). Setting this equal to zero yields: dI / dV = -I (V) / V. Therefore, the maximum power factor is achieved when the incremental conductance is equal to the negative of the instantaneous conductance.

### Current sweep

The current sweep method uses a PV array which is such that the characteristic of the PV array is obtained at fixed time intervals. The maximum power point voltage can then be computed from the characteristic curve at the same intervals.  

### Constant voltage

The term “constant voltage” in MPP is used to describe the voltage in the field of constant voltage. measured open circuit voltage (V OC ). The latter technique is referred to in contrast as the “open voltage” method by some authors.  If the output voltage is held constant, there is no attempt to maximize power point, so it does not have a maximum power point tracking technique in a strict sense, although it does have some advantages in cases when the MPP tracking tend to fail, and thus it is sometimes used to supplement an MPPT method in those cases.

In the “constant voltage” MPPT method (also known as the “open voltage method”), the power delivered to the load is momentarily interrupted and the open-circuit voltage is measured. The controller then resumes operation with the voltage controlled at a fixed ratio, such as 0.76, of the open-circuit voltage V OC .  This is usually a value that has been determined for the maximum power point, or empirically based on modeling, for expected operating conditions.   The operating points of the PV array is THUS kept near the MPP by regulating the voltage array and matching it to the fixed reference voltage V ref = kV OC . The value of V refmay be chosen to give optimal performance to other factors as well as the MPP, but the central idea in this technique is that V ref is determined as a ratio to V OC .

One of the inherent approximations to the “constant voltage” ratio is that the ratio of the MPP voltage to V OC is only approximately constant, so it leaves room for further possible optimization.

### Comparison of methods

Both perturbations and observations, and incremental conductance, are examples of “hill climbing” methods that allow maximum power point analysis.   

The disturbance and observance method requires oscillating power output around the maximum power point even under steady state irradiance.

The incremental conductance method has the advantage over the disturbance and observation (P & O) method that it can determine the maximum power point without oscillating around this value.  It is possible to perform a better performance under the conditions of higher incidence of irradiation.  However, the incremental conductance method can produce oscillations (unintentionally) and can perform erratically under rapidly changing atmospheric conditions. The sampling frequency is decreased to the higher complexity of the algorithm compared to the P & O method. 

In the constant voltage ratio (or “open voltage”), the current of the photovoltaic array must be set to zero, and the rate of change is generally high, usually around 76%.  Energy can be wasted during the time is set to zero. The approximation of 76% as the MPP / V OC ratio is not necessarily accurate.  Although simple and low-cost to implement, the interruptions reduce array efficiency and maximize the power point. However, efficiencies of some systems may reach above 95%. 

## MPPT placement

Traditional solar inverters perform MPPT for the whole PV array (module association) as a whole. In such systems the same current, dictated by the inverter, flows through all modules in the string (series). Because MPPs (due to manufacturing tolerance, partial shading,  and so on) result in different MPPs, resulting in lower efficiency. 

Some companies (see power optimizer ) are in the process of being able to operate on individual modules, allowing them to operate at peak efficiency, even after shading, soiling or electrical mismatch.

Data suggests having one inverter with one MPPT for a project that has east and west-facing modules presents no disadvantages when compared to having two inverters or one inverter. 

## Operation with batteries

At night, an off grid PV system can use batteries to supply loads. Although the battery pack may be close to the PV panel’s maximum power point voltage, this is unlikely to be true at sunrise when the battery has been partially discharged. The maximum power point voltage panel can be determined at this point, and an MPPT can resolve this mismatch.

When the batteries in an off-grid system are fully charged and the production of local power is higher than that, it can not be used to the maximum power. The MPPT must then shift to the point of production. (An alternative approach commonly used in spacecraft is to divert surplus power to a resistive load.

In a grid connected photovoltaic system, it will be delivered to the grid. Therefore, the MPPT in a grid connected PV system will always be able to exploit the PV modules at its maximum power point.

## References

1. Jump up^ Seyedmahmoudian, M .; Horan, B .; Soon, T. Kok; Rahmani, R .; Than Oo, A. Muang; Mekhilef, S .; Stojcevski, A. (2016-10-01). “State of the art artificial intelligence-based MPPT techniques for mitigating partial shading effects on PV systems – A review” . Renewable and Sustainable Energy Reviews . 64 : 435-455. doi : 10.1016 / j.rser.2016.06.053 .
2. Jump up^ Seyedmahmoudian, Mehdi; Horan, Ben; Rahmani, Rasul; Maung Than Oo, Aman; Stojcevski, Alex (2016-03-02). “Efficient Photovoltaic Maximum System Power Point Tracking Using a New Technique” . Energies . 9(3): 147. doi : 10.3390 / en9030147 .
3. Jump up^ “What Is Maximum Power Point Tracking (MPPT)” .
4. Jump up^ “A Survey of Maximum PPT techniques of PV Systems – IEEE Xplore”(PDF) . Retrieved 2016-10-04 .
5. Jump up^ Seyedmahmoudian, M .; Rahmani, R .; Mekhilef, S .; Maung Than Oo, A .; Stojcevski, A .; Soon, Tey Kok; Ghandhari, AS (2015-07-01). “Simulation and Hardware Implementation of New Maximum Power Point Tracking Technique for Partially Shaded PV System Using Hybrid DEPSO Method” . IEEE Transactions on Sustainable Energy . 6 (3): 850-862. doi : 10.1109 / TSTE.2015.2413359 . ISSN  1949-3029 .
6. ^ Jump up to:b Seyedmahmoudian, Mohammadmehdi; Engineering, School of; Science, Faculty of; Environment, Engineering & Built; University, Deakin; Victoria; Australia; Mohamadi, Arash; Kumary, Swarna. “A Comparative Study on Procedure and State of the Art of Conventional Maximum Power Point Tracking Techniques for Photovoltaic System” . International Journal of Computer and Electrical Engineering . 6 (5): 402-414. doi : 10.17706 / ijcee.2014.v6.859 .
7. Jump up^ Seyedmahmoudian, Mohammadmehdi; Mekhilef, Saad; Rahmani, Rasul; Yusof, Rubiyah; Renani, Ehsan Taslimi (2013-01-04). “Analytical Modeling of Partially Shaded Photovoltaic Systems” . Energies . 6 (1): 128-144. doi : 10.3390 / en6010128 .
8. Jump up^ Surawdhaniwar, Sonali; Mr. Ritesh Diwan (July 2012). “Study of Maximum Power Point Tracking Using Perturb and Observe Method”. International Journal of Advanced Research in Computer Engineering & Technology . 1 (5): 106-110.
9. Jump up^ Seyedmahmoudian, Mohammadmehdi; Mekhilef, Saad; Rahmani, Rasul; Yusof, Rubiyah; Shojaei, Ali Asghar (2014-03-01). “Maximum power point tracking of a partial photovoltaic array using an evolutionary algorithm: A particle swarm technical optimization” . Journal of Renewable and Sustainable Energy . 6 (2): 023102. doi : 10.1063 / 1.4868025 . ISSN  1941-7012 .
10. Jump up^ “University of Chicago GEOS24705 Solar Photovoltaics EJM May 2011″(PDF) .
11. Jump up^ Sze, Simon M. (1981). Physics of Semiconductor Devices (2nd ed.). p. 796.
12. Jump up^ Rahmani, R., M. Seyedmahmoudian, S. Mekhilef and R. Yusof, 2013. Implementation of fuzzy logic. Am. J. Applied Sci., 10: 209-218.
13. ^ Jump up to:e “Maximum Power Point Tracking” . zone.ni.com . zone.ni.com . Retrieved 2011-06-18 .
14. Jump up^ “ADVANCED ALGORITHM FOR MPPT CONTROL OF PHOTOVOLTAIC SYSTEM” (PDF) . solarbuildings.ca . Retrieved 2013-12-19 .
15. ^ Jump up to:b “Comparative Study of Maximum Power Point Tracking Algorithms”. doi : 10.1002 / pip.459 .
16. Jump up^ “Performance Improvement of Maximum Point Power Tracking Perturb and Observe Method” . actapress.com . Retrieved 2011-06-18 .
17. Jump up^ Zhang, Q .; C. Hu; L. Chen; A. Amirahmadi; N. Kutkut; I. Batarseh (2014). “A Center Point Iteration MPPT Method With Application on the Frequency-Modulated Microinverter LLC”. IEEE Transactions on Power Electronics . 29 (3): 1262-1274. doi : 10.1109 / tpel.2013.2262806 .
18. ^ Jump up to:c “Evaluation of Micro Controller Based Maximum Power Point Tracking Methods Using dSPACE Platform” (PDF) . itee.uq.edu.au . Retrieved 2011-06-18 .
19. ^ Jump up to:f “MPPT ALGORITHMS” . powerelectronics.com . Retrieved 2011-06-10 .
20. Jump up^ Esram, Trishan; PL Chapman (2007). “Comparison of Maximum Array Photovoltaic Power Point Tracking Techniques”. IEEE trans. on Energy Conv . 22 (2).
21. Jump up^ Bodur, Mehmet; M. Ermis (1994). “Maximum power point tracking for low power photovoltaic solar panels”. Proc. 7th Mediterranean Electrotechnical Conf. : 758-761.
22. Jump up^ “Energy comparison of MPPT techniques for PV Systems” (PDF) . wseas . Retrieved 2011-06-18 .
23. ^ Jump up to:b “Design and Simulation of an open voltage algorithm based maximum power point tracker for battery charging PV system” . ieee.org .
24. Jump up^ Seyedmahmoudian, M .; Mekhilef, S .; Rahmani, R .; Yusof, R .; Renani, Analytical Modeling of Partially Shaded Photovoltaic Systems. Energies 2013, 6, 128-144.
25. Jump up^ “Invert your thinking: Squeezing more power out of your solar panels” . blogs.scientificamerican.com . Retrieved 2015-05-05 .
26. Jump up^ “InterPV.net – Global PhotoVoltaic Business Magazine” . interpv.net . 