Introduction - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 38

Figure 1.36 Typical protection circuit

After determining the voltage reference V REF , R 1 and R 2 can be selected to scale V s to the
range − V REF ∼ V REF . Then, R 4 and R 7 can be determined to scale 0 ∼ 2 × V REF to the

range of the analog inputs. For example, if the range of analog inputs is 0 ∼ 3 V, then V REF

can be chosen as 3 V and R 4 and R 7 can be chosen to satisfy R 4 = 2 × R 7.

1.3.6 Protection
It is very important to equip an inverter with as many protection mechanisms as possible, at a
reasonable cost. For the currents and voltages already measured, it is straightforward to add
over-voltage and over-current protection. This can be done in the control algorithm and/or
with hardware circuits.
Figure 1.36 shows a typical circuit for protection when a signal exceeds a certain value.
This can be used for over-voltage and over-current protection. It is basically a comparator and
the threshold can be easily adjusted with a potentiometer R 1. The output signal V ip changes
from 1 to 0 when

R 2

V s >

R 2 V R , R 1 +

according to which appropriate actions can be taken.

1.3.7 Central Controller
Because of the complex functions of an inverter and the requirement of a high sampling
frequency, e.g. to handle harmonics up to a certain order, it is often necessary to use a powerful
micro-controller as the core of the electronic part of an inverter. Moreover, it is also important
that many development tools and the maximum support possible are available to speed up the
design process. There are many options but, in this section, the TMS320F28335 digital signal
controller from Texas Instruments is described because (TI 2007)

1. it is a 32-bit single-precision floating-point processor compatible with IEEE-754 that
is dedicated to demanding control applications, e.g. power and energy conversion
applications;