Maximum Power Point Tracking

Introduction

Solar panels have a nonlinear voltage-current characteristic, with a distinct maximum power point (MPP), which depends on the environmental factors, such as temperature and irradiation. In order to continuously harvest maximum power at any point of time from the solar panels, MPPT algorithms need to be employed. The calculations result in an output that delivers maximum current at the required voltage at any point in time. During low light level situations it will compensate for the low light level and find the new point at which the solar cell delivers its maximum power output.

MPPT Algorithms

Over the past decades many methods to find the MPP have been developed and published. These techniques differ in many aspects such as required sensors, complexity, cost, range of effectiveness, convergence speed, correct tracking when irradiation and/or temperature change, hardware needed for the implementation or popularity among others. A complete review of 19 different MPPT algorithms can be found

Among the above discussed methods, three methods have been studied and analysed in detail.

Classification of MPPT algorithms

Constant Voltage Tracking

This is comparatively an easy and inefficient method to find the maximum power point of any solar photovoltaic module. It is assumed that the maximum power point of solar PV module lies at about 0.75 times the open circuit voltage (Voc). So by measuring the open circuit voltage of the PV module, a reference voltage can be generated and feed forward voltage scheme can be implemented to bring solar PV module voltage to a point of maximum power. The drawbacks of this method are:

• The maximum power point of a solar PV module does not always lies at 0.75*Voc. Hence the tracking efficiency is low.

• The open circuit of the solar PV module varies with the temperature. Hence, open circuit voltage is to be measured continuously for temperature variations.

The Perturb & Observe (P&O) Algorithm

The Perturb and Observe method is a widely used approach to MPPT. As the name suggests, this method works by perturbing the system by increasing or decreasing the PV module operating voltage and observing its impact on the output power supplied by the module. As shown by the flow chart in Figure 1, PV system controller change PV module output with a small step in each control cycle. The step size is generally fixed and it can be increased or decreased. Both PV module output voltage and output current can be the control object, so this process is called "perturbation". Then, by comparing PV array output power of the cycles before and after the perturbation, this method determines the maximum power point.

If the power output is increased at a particular cycle, then according to this method, the system controller will change the step in the same direction as the previous cycle and checks for further increase in power of PV module. While if the output power observed is decreased, then the system controller change the step in direction opposite to the previous cycle. In this way, the actual operating point of PV module can move closer to the maximum power point, and finally in steady state, oscillates around the maximum power point in a very small area. This causes a power loss which depends on the step width of a single perturbation. If the step width is large, the MPPT algorithm will be responding quickly to sudden changes in operating conditions with the trade-off of increased losses under stable or slowly changing conditions. If the step width is very small the losses under stable or slowly changing conditions will be reduced, but the system will be only able to respond very slowly to rapid changes in temperature or insolation. The value for the ideal step width is system dependent and needs to be determined experimentally.

Figure 1

The Incremental Conductance Algorithm

The disadvantage of perturb and observe method to track the peak power under fast varying atmospheric condition is overcome by IC method. The IC can determine that the MPPT has reached the MPP and stop perturbing the operating point. If this condition is not met, the direction in which the MPPT operating point must be perturbed can be calculated using the relationship between dl/dV and –I/V (Eq. 1). This relationship is derived from the fact that dP/dV is negative when the MPPT is to the right of the MPP and positive when it is to the left of the MPP. This algorithm has advantages over P&O in that it can determine when the MPPT has reached the MPP, where P&O oscillates around the MPP. Also, incremental conductance can track rapidly increasing and decreasing irradiance conditions with higher accuracy than perturb and observe. One disadvantage of this algorithm is the increased complexity when compared to P&O.

Eq. 1

The flowchart depicts the working of Incremental Conductance method.