Civil Engineering Reference
In-Depth Information
the speed in ratio exceeding four to one. In renewable power appli-
cations, such commutation difficulty is unlikely as the speed varies
over a narrow range.
The variable DC link voltage is obtained two ways:
•
one way is to connect a variable ratio transformer on the input side
of the rectifier. The secondary tap changing is automatically
obtained in a closed-loop control system.
•
the other way is to use the phase-controlled rectifier in place of the
uncontrolled rectifier in
Figure 11-5
.
At reduced output voltage,
this method gives poor power factor and high harmonic content,
and requires filtering the DC voltage before feeding to the inverter.
11.4.2
Frequency Control
The output frequency of the inverter solely depends on the rate at which
the switching thyristors or transistors are triggered into conduction. The
triggering rate is determined by the reference oscillator producing a contin-
uous train of timing pulses, which are directed by logic circuits to the thy-
ristor gating circuits. The timing pulse train is also used to control the turn-
off circuits. The frequency stability and accuracy requirements of the inverter
dictate the selection of the reference oscillator. A simple temperature com-
pensated R-C relaxation oscillator gives the frequency stability within
0.02 percent. When better stability is needed, a crystal-controlled oscillator
and digital counters may be used, which can provide stability of .001 percent
or better. The frequency control in a stand-alone power system is an open-
loop system. The steady state or transient load changes do not affect the
frequency. This is one of the major advantages of the power electronics
inverter over the old electromechanical means of frequency controls.
11.5
Battery Charge/Discharge Converters
The stand-alone photovoltaic power system uses the DC to DC converter
for battery charging and discharging.
11.5.1
Battery Charge Converter
Figure 11-6
is the most widely used DC-DC battery charge converter circuit,
also called the buck converter. The switching device used in such converters
may be the BJT, MOSFET, or the IGBT. The buck converter steps down the
input bus voltage to the battery voltage during battery charging. The tran-
sistor switch is turned on and off at high frequency (in tens of kHz). The
duty ratio D of the switch is defined as the following:
