PVGS - Introduction

In this section, the PVGS and its functional operation are discussed.
Figure 1 shows the one-line diagram of a photovoltaic generation system, which is then connected to a distribution system (see figure 2).
The distribution system, as shown in figure 2, consists of a 100 MVA three-winding transformer that connects a high voltage (154 kV - wye floating) transmission system to a medium (22.9 kV - wye grounded) distribution feeder.
The tertiary winding is 5.7 kV delta. There is a 5 MVAr capacitor bank connected to the MV (medium voltage) winding of the transformer.
A load of 0.5244 MW and 0.1669 MWAr is connected at the middle of the feeder.
The far end of the feeder has the PVGS connected to the point of common coupling (PCC).

Figure 1: Photovoltaic Generation System

Figure 2: Distribution System Connected with a Photovoltaic Generation System

In the simplest case, a PV cell can be described by a diode (see figure 3).
The current-voltage characteristic can be determined using

and it is shown in figure 4.
The photocurrent I_Ph = c₀ · E depends on the irradiance E (see figure 4) and the coefficient c₀. I_S ≈ 10⁻¹⁰ is the saturation current.
And the diode factor is m = 1 in the case of an ideal diode.
At a temperature of ϑ = 25 °C, U_T is equal to 25.7 mV.

Figure 3: Photovoltaic Cell (cf. [3])

Figure 4: Irradiance Dependency of I-U-characteristics of a PV Cell [3]

Figure 5: I-U-characteristic (in blue) and P-U-characteristic (in red) of a PV Cell [3]

The maximum of the P-V-characteristic in figure 5 is called Maximum Power Point (MPP).
The MPP tracker determines the current MPP and enables tracking it.
In contrast, in the curtailment mode the maximum active power is not aimed, but a reference value P_ref should be reached.