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210 CHAPTER 6 Flexible Power Control of Photovoltaic Systems

(i.e., 70% of the rated power). Similar requirements have also been defined in the
grid codes of other countries (e.g., Denmark and Japan).

2.2 GRID VOLTAGE FLUCTUATION BECAUSE OF INTERMITTENCY
OF PV ENERGY

Another potential problem caused by PVs is because of the intermittency nature of
solar energy. It is well known that the PV power can be fluctuating considerably in
the case of cloudy days, where the power production can suddenly drop because of
passing clouds. Actually, the change rate of the PV power is also correlated with the
system size [19]. For instance, this problem is usually pronounced in small-scale PV
systems (rooftop PV applications) because a passing cloud can easily cover a major
area of the PV panels [20]. In the case of a wide-scale grid-connected PV system,
those sudden changes in the PV power can potentially induce severe grid voltage
fluctuations [7], which thus should be addressed.
To reduce the power fluctuation from the PVs, the PRRC is introduced to limit
the PV output power change rate [14,17]. Namely, as long as the PV output power
changing rate is below a certain limit, the PV system is allowed to continuously
operate in the Maximum Power Point Tracking (MPPT) mode (i.e., normal opera-
tion). However, once the PV power changing rate reaches the maximum limit, the
PV output power should be controlled in a ramp manner, as shown in Fig. 6.1.

2.3 LIMITED-FREQUENCY REGULATION CAPABILITY TO STABILIZE
THE GRID DURING FREQUENCY DEVIATION
In the conventional power systems, the grid frequency is normally regulated by large
conventional power plants (e.g., coal, gas turbine), which are considered as dispatch-
able sources of electricity. However, as the installation of grid-connected PV system
increases, the system operator will have less capability to stabilize the grid in the
case of frequency deviations, as a large portion of PV systems cannot be easily
controlled by the system operator. Initially, some grid regulations require the PV sys-
tems to be disconnected from the power grid during the frequency deviation. How-
ever, as the penetration level of grid-connected PV systems increases, disconnecting
the large amount of PV systems during the frequency deviation will challenge the
grid stability because of a sudden loss of large power generation [21]. This is known
as the 50.2-Hz problem, which is highly concerned in countries with high PV pene-
tration (e.g., Germany) [12].
With the above concerns, the frequency regulation has been implemented in
grid codes, where the PV systems are not allowed to immediately disconnect
from the grid in response to frequency deviations. Instead, the PV system needs
to provide a power reserve by reducing its output power to a certain level (specified
by the grid codes), as shown in Fig. 6.1. In this way, the PV systems are requested
to contribute to frequency regulations andsupport the gridduringfrequency
deviation.