Perturb, observe and check algorithm

Perturb, observe and check (POC) maximum power point tracking algorithm has two stages. First stage is to use perturb and observe maximum power point tracking algorithm to find a local maximum power point, P_MPP with voltage U_MPP and current IMPP. In second stage, the tracking algorithm checks, if there are other maximum power points by scanning the output characteristics of the photovoltaic generator to ensure that the found maximum power point PMPP is in fact the global MPP. If it is, the control is returned to PO algorithm to continue the tracking process around UMPP. In opposite case, the greatest maximum power point found, PNEW with voltage UNEW , becomes the reference point to track. U_NEW is given to the PO algorithm as a reference voltage U_ref in order to track the global maximum power point. The flowchart of the algorithm is shown in Figure 5.4. UMPP and IMPP are the saved values of the greatest maximum power point found in the previous time the PO algorithm was performed.

Figure 5.4. Flowchart of the Perturb, observe and check maximum power point tracking algorithm.[Alo09, p. 4]

Main advantage of this checking algorithm is that it doesn’t scan the whole P-U curve. When operating in some arbitrary maximum power point on the P-U curve, it is not necessary to check if there is another maximum power point just next to it. In series connection of PV modules, bypass diodes are the reason for multiple maximum power points. If there were no bypass diodes, photovoltaic module in worst conditions would simply limit the current of the whole string. I-U and P-U curves of the photovoltaic generator can be divided into several voltage ranges according to the number of bypass diodes. It is then easy to notice that there is some minimum distance UDIS between two maximum power points. This can be approximated as:

n

U_DIS U^MPPOC , (18)

where UDIS is the minimum distance between two maximum power points on the P-U curve, UMPPOC is the approximated voltage of the maximum power point in normal operating conditions with uniform irradiance and n is the number of bypass diodes in the generator.[Alo09, p. 5]

Checking algorithm starts from U_MIN, the lowest voltage in which the tracker can operate. UMIN is located near the short-circuit condition. Voltage and current are monitored and power is calculated and compared with PMPP, the power of the maximum power point saved during the running of the PO algorithm. If the power is greater than the saved value from PO algorithm, the Checking algorithm returns the control to PO

PO algorithm

Save UMPP, IMPP and PMPP

Checking algorithm

Is there UNEW for which

PNEW > PMPP

Uref = UMPP Uref = UNEW

No Yes

algorithm to track the new maximum power point and saves the new maximum power point PNEW as PMPP. If the power is lower than the saved value, a new operating point to be measured is calculated with following expression:

LAST MPP

NEW I

U P , (19)

where UNEW is the new voltage to be measured, PMPP is the power of the maximum power point found by PO algorithm and ILAST is the last monitored current. Then new operating point is measured and compared to the saved maximum power point. These steps are repeated through the whole P-U curve from lowest feasible voltage to the last possible maximum power point voltage, which is approximated as U_MPPOC. Flowchart of the checking algorithm is shown in Figure 5.5.

Figure 5.5. Flowchart of the Checking algorithm.[Alo09, p. 5]

In Figure 5.6 there are 10 photovoltaic modules connected in series with bypass diodes in parallel with each of the modules. The series connection of modules is exhibiting three different irradiance values. Two of the modules have irradiance of 800 W/m², five of the modules have irradiance of 400 W/m² and three of the modules have irradiance of 200 W/m². The P-U characteristics of the generator and the operation of

Start Checking

Checking algorithm are shown in figure 5.6. The saved MPP from the previous time the PO algorithm was performed is assumed to be MPP1.

0 50 100 150 200 250 300

Figure 5.6. The operation of the Checking algorithm.

The algorithm starts from the lowest feasible voltage UMIN, which is marked as 1 in figure 5.6. Because the power in UMIN is lower than in MPP1, algorithm calculates new operating point U_NEW, as shown in Equation (21), to be checked. This is marked as 2 in Figure 5.6. In point 3, the power is still lower than in MPP1. In point 4, however, the power is greater than in MPP1 and the Checking algorithm gives the control to PO algorithm in order to track the maximum power point. Once the MPP3 is reached, PO algorithm saves the value of voltage in MPP3 as UMPP, the value of current as IMPP and the value of power as PMPP. Next, the Checking algorithm is reactivated and it checks if the voltage of the operating point U_NEW is greater than the voltage of the saved MPP subtracted by minimum distance between two MPPs, U_DIS. In this case, when the operating point is MPP3, the voltage is obviously more than voltage of MPP3 subtracted by UDIS. This way the Checking algorithm obtains the information about the location of the operating point, if it is at lower voltages or at higher voltages than the last saved MPP. Finally the Checking algorithm checks the points from 6 to 8 and concludes that the MPP3 is in fact the global MPP. Checking algorithm stops, when the operating point voltage U_NEW becomes higher than the maximum voltage of the MPP UMPPOC, which is 240 V in the case of Figure 5.6.