I apologize for delay, I was very busy last week, our second daughter (Ava) was born and we involved it.
Attached Figure is a typical circuit diagram of the grid-connected PV power system. It interfaces with the local utility lines at the output side of the inverter as shown. A battery is often added to meet short-term load peaks. In recent years, large building-integrated PV installations have made significant advances by adding grid connections to the system design. The PV systems interface the grid at the output terminals of a synchronizing breaker after the inverter. The power flows in either direction depending on the site voltage at the breaker terminals. The synchronizing breakers in Figure have internal voltage and phase-angle sensors to monitor the site and grid voltages and signal the correct instant for closing the breaker. As a part of the automatic protection circuit, any attempt to close the breaker at an incorrect instant is rejected by the breaker. A small unavoidable difference between the site and grid voltages results in an inrush current flowing between the site and the grid. The inrush current eventually decays to zero at an exponential rate that depends on the internal resistance and inductance. Once synchronized, the voltage and frequency of the PV system need to be controlled.
Indeed the output voltage of PV system have same role of generator emf in conventional system and interconnection impedance have same role of generator internal impedance. Also the link connecting a renewable power site with the area grid introduces an operating limit in two ways, the voltage regulation and the stability limit. In most cases, the link can be considered as an electrically short transmission line. The ground capacitance and the ground leakage resistance are generally negligible. The equivalent circuit of such a line, therefore, reduces to a series leakage impedance Z.
The direction of power flow depends on the sending- and receiving-end voltages and the electrical phase angle between the two. However, the maximum power the line can transfer while maintaining a stable operation has a certain limit. We derive in the following text the stability limit, assuming that the power flows from the renewable site to the grid, although the same limit applies in the reverse direction as well. The series resistance in most lines is negligible and hence is ignored here.
The power transferred to the grid via the link line is as follows:
P = (Vs.Vr).sind/X
Thus, the magnitude of the real power transferred by the line depends on the power angle d. If d> 0, the power flows from the site to the grid. On the other hand, if d< 0, the site draws power from the grid.
The reactive power depends on (Vs - Vr). If Vs > Vr, the reactive power flows from the site to the grid. If Vs < Vr, the reactive power flows from the grid to the site. Obviously, the power flow in either direction is maximum when d is 90°. Beyond Pmax, the link line becomes unstable and falls out of synchronous operation. That is, it loses its ability to synchronously transfer power from the renewable power plant to the utility grid. This is referred to as the steady-state stability limit. In practice, the line loading must be kept well below this limit to allow for transients such as sudden load steps and system faults. The maximum power the line can transfer without losing the stability even during system transients is referred to as the dynamic stability limit. In a typical system, the power angle must be kept below 15 or 20° to maintain dynamic stability at all times.
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