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E L E CT R I C AL CONNE CT I ON
W I NT E R 2 01 5
LET’S GET TECHNICAL
Shine a light on solar
L
ots of companies sell items that
make up a solar system, but their
mission in life is not necessarily
doing the installation. That falls upon the
humble electrician.
These companies don’t necessarily
have accreditation with the Clean Energy
Council (CEC). However, they can employ
contractors, who must be accredited.
The company supplying the equipment
will carry the warranty obligation, but the
electrician installing the system has to sign
off, and green energy rebates are awarded
on that basis.
Electrical Connection
is providing the
necessary information to bring non-
specialists up to speed and closer to
accreditation.
The focus is on rooftop, grid-connected
PV systems, but because storage batteries
are becoming more economical – lithium-
ion types in particular – increasing adoption
is likely as feed-in tariffs fall or disappear.
PV PANELS
The technology behind PV panels is well
developed and there are more than 50
major suppliers worldwide, most of them
in China.
In Australia the main panel types for
rooftops are polycrystalline and crystalline
silicon. Each has its advantages, but these
are not major performance differentiators.
The customer for a grid-connected
system is interested only in saving as many
kilowatt hours as possible. In practice
there are enough variables having little or
nothing to do with PV panel performance
that influence the result – the AC kilowatts
and kilowatt-hours.
Figure 1 shows a typical rooftop panel
with 60 photovoltaic cells connected
in series. The inset shows an equivalent
circuit diagram of a single cell. It is a current
generator with a diode across it, plus a bit
of lead resistance.
The higher the intensity of sunlight
(W/m
2
– tech name: insolation), the more
current is generated. The complete panel
will crank out about 300W under maximum
conditions (standard temperature
condition, or STC), or something close
to that, with a voltage of about 30V. The
current is therefore 10A and the voltage per
cell about 0.5V.
The circuit diagram for the complete
panel can be visualised as a 10A current
generator with a single diode (made up of
60 in series, a ‘lumped’ diode) across the
current generator.
HOW IMPORTANT IS VOLTAGE?
A single-phase, grid-connected PV
system has to supply 240V AC (that’s rms
voltage), and the peak voltage is therefore
142% of that, or 340V.
The DC link voltage supplying the
inverter can’t be any lower than the peak
AC voltage (unless the inverter connects to
the grid via a step-up transformer).
The smallest commercial PV system
is 1.5kW (six panels), which at 300W
each yields 1.8kW. However, with all six
connected in series, the voltage output
is 180V DC. It’s not a problem, because
inverters can cater for lower DC output
using a boost circuit. More of this in the
inverter section.
SERIES OR PARALLEL
PV panels can be connected in series or
parallel, and either way it’s the total number
that determines the aggregate power.
For battery charging, the connection is
in parallel, i.e. current up and voltage down.
Otherwise there would have to be a buck
circuit to bring voltage down closer to the
battery voltage.
MAXIMUM POWER TRACKING
In inverter specifications there’s always
reference to maximum power point
tracking (MPPT).
The idea is to have the right combination
of voltage and current for the highest DC
watts output.
If the PV array is open circuited, you’ll get
maximum voltage, zero current and zero
power. If short-circuited, there will be zero
voltage and maximum current, and again
zero power output.
Somewhere in between is the maximum
power point. Figure 2 shows a set of
current-voltage curves at increasing
insolation for a typical panel. The power
curve is in black and, as can be seen,
maximum power is close to the open
circuit voltage but moves to lower voltages
as insolation decreases.
MPPT control is often complicated, but a
The basic principles of
photovoltaic systems
make useful reading for
non-specialist electricians
installing all or part of a system.
Phil Kreveld
sets them out.
Figure 1.