Photovoltaics - Understanding Renewable Energy Systems - page 172

I NVERTERS

Inverter technology
It was assumed until now that loads are operated by direct current (DC);
however, most consumer applications use alternating current (AC). An island
grid inverter can operate AC devices with a photovoltaic stand-alone system.
If the system is to be connected to a national grid, a grid-connected inverter is
used. Both inverter categories are similar, but there are some differences in the
details.
Power electronic devices are used today to convert DC to AC. Different
types of semiconductor elements that can switch voltages higher than 1000 V
or even currents higher than 1000 A are:

• power MOSFET (power field effect transistors)

• bipolar power transistors

• insulated gate bipolar transistors (IGBT)

• thyristors (controllable diodes)

• triacs (two-direction thyristors)

• gate turn-off (GTO) thyristors (switchable thyristors).

A short look at the semiconductor thyristor will cover the functionality of
power electronic switches. Figure 4.52 shows the thyristor symbol with three
contacts: A (anode), C (cathode) and G (gate).
If the control current i G is equal to zero, the thyristor blocks at negative
and positive voltages v . A control current i G exceeding a positive maximum
voltage v or, exceeding a maximum voltage increase d v /d t , switches the
thyristor into forward mode. Then, a current i can flow through the thyristor
if there is a positive voltage v . Normally, this is caused by a control current i G .
While the forward current i is above the holding current i H , the thyristor
remains conductive. A triac can be operated in both directions and a GTO
thyristor can be turned off by a negative control current.
An inverter must periodically direct the current from one branch to
another; this is called commutation . Therefore, different thyristors must switch
alternately. If the energy needed to change the state of the thyristor comes from
outside, for instance from the grid, the commutation is called external or line

Figure 4.52 Thyristor Symbol