Page 323 - A Comprehensive Guide to Solar Energy Systems

328 A COMPrEhENSIVE GUIdE TO SOlAr ENErGy SySTEMS

FIGURE 15.6 The behavior of two solar installations with the same export capacity but with different generating
capacities [6]. Used with permission from the National Grid.

The GTI must match the phase of the grid voltage and maintain the output voltage
slightly higher than the grid voltage at any instant. It does this by measuring the current
AC grid waveform and manipulating its power electronic components to create an output
with a voltage to match and therefore correspond with the grid. A high-quality modern
GTI has a fixed unity power factor, which means its output voltage and current are per-
fectly lined up, and its phase angle is within 1 degree of the AC power grid.
The electricity grid voltage will fluctuate slightly as it is reconfigured as necessary in
normal operation. larger disturbances occur when electrical equipment failures happen
and are dealt with. during a voltage disturbance the inverter needs to decide whether to
keep generating an output matched to the network or to shut down and has to make this
decision quickly based on the voltage it measures. This behavior needs to be carefully
specified and can be tricky for manufacturers to design in to their inverters. It differs
from a conventional “synchronous” generator, which has a inherent tendency to keep
generating and supporting the network because it is electromagnetically coupled to the
network.
The inverters used to link solar PV generators to the grid have not needed to pro-
vide support to the network thus far, as loss of their output hasn’t been significant to the