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E L E CT R I C AL CONNE CT I ON
W I NT E R 2 01 5
SOLAR INVERTERS
On the grid
S
olar photovoltaic inverters differ
from other types, often battery
driven, that are used for marine,
outdoors and emergency supplies.
The main difference is that the DC input
side, the DC link, has integrated maximum
power point tracking (MPPT) circuitry.
As discussed in the overview article, MPPT
circuitry is necessary to extract maximum
power from solar panels under conditions
of varying insolation, overshadowing,
panel surface temperature and the gradual
changing of the sun’s elevation and azimuth
angles during the day.
This article deals with grid-connected
inverters. Although the single-phase
versions are discussed in some detail, many
of the circuit considerations apply equally
to three-phase inverters.
Grid-connected inverters are frequency
constrained because the switching circuits
are locked in by the zero crossover points
of mains voltage.
TRANSFORMER OR NOT?
There are two main types of inverters:
transformer connected and transformer-less.
These days the latter type accounts
for more than 80% of solar inverters sold
worldwide.
The transformer category is divided
into high-frequency (HF) and line-frequency
types.
HF transformer inverters have an
intermediate stage between the solar panels
and the DC link of the inverter, and these are
coupled by the transformer. However, the
inverter output connects directly to the grid.
Line-frequency transformers couple the
output of inverters to the grid.
Line-transformer types have
disadvantages. They are heavy because of
the large cores necessary for the transformers,
and because the inverters have to supply
magnetising current, thus causing low power
factor as power demand drops.
HF transformer types have small cores, are
much lighter and have little or no effect on
inverter output.
Transformers are used because of the
galvanic isolation they provide. As we delve
further into the circuitry, the advantage of
isolation will become apparent. However,
most of the more detailed discussion here is
on transformer-less types, as they are by far
the most popular in Australia.
INVERTER PRINCIPLES
In Figure 1 a basic H4 bridge inverter is
shown – for simplicity, using switch symbols
rather than transistor symbols.
S1 and S4 close alternately to S3 and S2.
This mode is called bipolar ‘modulation’.
The other mode, unipolar, uses a different
switching scheme whereby S3 is closed for
one half-cycle, and S4 is closed for the next
half-cycle. S1 and S2 are the ‘within half-
cycle’ modulating switches.
The H4 bridge is the most basic of inverter
topologies. It is popular because it has a low
component count and is therefore lower in
cost. The inductors in the circuit diagram are
there to smooth the basically rectangular
inverter current pulses.
The common way of generating quasi
sine waves is by pulse width modulation.
In Figures 2a and 2b the modulation
methods for bipolar and unipolar
modulation are made clear.
The operating principle involves a triangular
voltage wave (the carrier) intersecting with
a grid-synchronised sine wave.
For the unipolar mode, a 180° phase-
shifted sine wave is generated in addition.
Carrier and sine waves are generated in the
control circuitry of the inverter.
The unipolar modulated inverter has
smaller current ripple then the bipolar
one, and thus requires smaller smoothing
inductors.
It is therefore a popular choice for
solar inverters. However it exhibits higher
common mode voltage and this can
result in high leakage currents. These arise
because the PV panels have significant
parasitic capacitance to earth. The leakage
current path is shown in Figure 3.
LEAKAGE CURRENT
In July 2010 there was an incident in
Queensland in which a home occupier got
a shock from a ladder leaning against the
gutter of his house.
Safety testing did not reveal any fault, and
the conclusion was that the array capacitance
was coupling leakage current into the roof
and onto the ladder via the gutter.
Overcoming leakage problems is very
much a subject of topology, but first let’s
consider ‘freewheel current’ which is the
primary contributor.
In Figure 1, diodes are connected across
the switches representing transistors
(eg: IGBT types).
The diodes feed the freewheel current
back into the DC link capacitor and PV
panels. The freewheel current flows because
of inductive kickback when switches open,
thus building up common mode voltage
at the PV panel and therefore leakage
current back to earth through the parasitic
capacitance of the PV panels.
Tests have been conducted in Australia,
indicating that currents of 30mA or more are
quite possible.
Solar panel arrays can be
better distributed for
maximum ‘collectible’
characteristics.
Phil Kreveld
discusses the principles.