Page 249 - Advances in Renewable Energies and Power Technologies

222 CHAPTER 6 Flexible Power Control of Photovoltaic Systems

MPPT operation * MPP
PRRC operation R r(t) =R r C
(PRRC)
(kW) * B MPP R r
*
P pv R r(t)≤ R r A
(MPPT)
power MPP
PV Irradiance level:
1000 W/m 2
700 W/m 2
500 W/m 2
PV voltage v pv (V)
FIGURE 6.14
Operational principle of the Power Ramp-Rate Control (PRRC) algorithm: MPPT mode
(A / B) and PRRC mode (B / C), where R r (t) is the PV power ramp-rate and R is the

r
ramp-rate limit.
ramp-rate constraint (i.e., R r (t) > R ). In that case, the extracted PV power needs to

r
be reduced to regulate the change rate of the PV power [57]. This can be achieved by
perturbing the PV voltage v toward the left side of the MPP until the PV power
pv

change rate is equal to the set-point (i.e., R r (t) ¼ R ), as illustrated in Fig. 6.14.
r
The performance of the PV system with the PRRC strategy is demonstrated in
Fig. 6.15A with a clear-day solar irradiance and ambient temperature condition.
The available power and the extracted PV power with the PRRC algorithm are
shown in Fig. 6.15A, where it can be seen that the PV power follows a ramp chang-
ing manner. The corresponding power ramp-rate with the PRRC strategy is shown in
Fig. 6.16A, which verifies that the change rate of the PV power is kept below the

maximum limit (i.e., R r (t) R ). Another experimental result with a fluctuating so-
r
lar irradiance (e.g., during a cloudy day) is shown in Fig. 6.15B. In this case, it is
more challenging to control the change rate of the PV power because of the rapid
change in solar irradiance condition. It can be seen from the ramp-rate result in
Fig. 6.16B that the change rate of the PV power can be limited according to the refer-
ence in most cases. The PV power ramp-rate limit is only exceeded during very fast
transients, whereas the PRRC algorithm requires a number of iterations to reduce the

PV power change rate according to the set-point R . Nevertheless, the experimental
r
results have confirmed that the PRRC algorithm can effectively control the change
rate in the PV power according to the demand.
5.3 POWER RESERVE CONTROL ALGORITHM

The idea of the PRC is to keep the PV output power follow the available PV power
dynamic with the power difference corresponding to the amount of power reserve
DP [58,59]. The basic concept of this operation can be expressed as
P pv ¼ P avai DP