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220 CHAPTER 6 Flexible Power Control of Photovoltaic Systems

these solutions may introduce more complexity of the algorithm and may reduce the
control robustness. As a consequence, the conventional P&O MPPTalgorithm is still
being widely adopted MPPT algorithm in industry.

5. FLEXIBLE ACTIVE POWER CONTROL OF PV SYSTEMS
In this section, the realization of the flexible active power control strategies by modi-
fying MPPT algorithms is discussed. Following the control structure in Fig. 6.9, the
active power control strategy is implemented in the dcedc converter. This is
achieved by determining an appropriate reference PV voltage v for a certain active
pv
power control strategy (e.g., PLC, PRRC, PRC), which will be discussed in the
following section.

5.1 POWER-LIMITING CONTROL ALGORITHM

For the PLC strategy, the PV system is not allowed to deliver the output power
higher than the power limit value P limit . This condition can be easily fulfilled as
long as the available PV power is less than the power limit value (i.e.,
P avai P limit ), which usually occurs during the low solar irradiance condition. In
that case, the typical MPPT operation is employed, where the reference PV voltage
v is set from the MPPT algorithm (e.g., P&O MPPT), to allow the PV system to
pv
inject the maximum available power to the grid (i.e., A/Bin Fig. 6.12). However,
once the available PV power is reaching and exceeding the level of power limit (i.e.,
P avai > P limit ), e.g., because of the solar irradiance level increase, the operating point
of the PV system in the PeV curve needs to be regulated below the MPP. More spe-
cifically, to keep the PV power to be constant at a certain value, the operating point
of the PV system has to be regulated along the horizontal line during the solar

MPPT operation MPP
PLC operation
(kW) P pv = P limit MPP
power P pv MPP

PV Irradiance level:
1000 W/m 2
700 W/m 2
500 W/m 2
PV voltage v pv (V)
FIGURE 6.12
Operational principle of the Power-Limiting Control (PLC) algorithm: MPPT mode
(A / B) and PLC mode (B / C), where P limit is the power limit.