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2. Demand to Grid-Connected PV Systems 209

production from the PV systems is limited to a certain set-point P limit during the
operation.
• Power Ramp-Rate Control (PRRC) (also called Power Gradient Control): The
maximum change rate of the active power production from the PV systems is

limited to a certain rate R .
r
• Power Reserve Control (PRC) (also called Delta Power Control): An active
power from the PV system P pv is regulated below the maximum available power
P avai to provide a certain amount of power reserve DP.
By implementing such control functionalities, the PV systems not only are acting
as a purely power-generating unit but also are involved in grid supporting by
providing flexible power delivery. Therefore, “grid-friendly” integrations can be
ensured, and the above technical challenges caused by the intermittency nature of
solar energy can be resolved.
In this chapter, the demands for flexible power control in grid-connected PV sys-
tems are briefly discussed. An overview of possible control solutions to realize flex-
ible active power control for the PV system is provided. Then, the rest of the chapter
focuses on the flexible power control solutions through the modification of the power
converter control algorithm. Finally, concluding remarks are provided.

2. DEMAND TO GRID-CONNECTED PV SYSTEMS
Several challenges regarding grid integration of PV system have been reported in
recent studies. In this section, three commonly concerned issues related to active po-
wer control of the PV system are discussed.

2.1 OVERLOADING OF THE GRID (OVERVOLTAGE) DURING PV
PEAK-POWER GENERATION PERIOD
Under a large PV installation scenario, overloading of the grid is one of the associ-
ated and the most concerned issues [9]. When many PV systems are connected to the
grid, they can introduce a significant peak surplus power during midday (i.e., when
the PV power production is highest). This will increase the power losses and lead to
overvoltage (if the grid capacity remains the same) and thus should be avoided as it
will overstress the equipment in the systems. This issue has been increasingly of
concern recently. For instance, it has been reported that parts of the distribution
grid in Northern Ireland have experienced a severe overloading because of a high
number of PV systems connected to the grid [18]. To solve this problem, the PLC
scheme has been introduced in grid regulations, where the active power injected
from the PV systems has to be limited to a certain value if demanded, as illustrated
in Fig. 6.1. By doing so, the peak-power from PV systems can be avoided. This
requirement is currently adopted in Germany through the grid codes [15], where
the newly installed PV systems have to be able to limit their maximum feed-in power