Environmental Engineering Reference
In-Depth Information
A downstream filter suppresses all high-frequency portions of this signal, so that,
at the exit, only the desired sine-shaped voltage is present. A downstream trans-
former adjusts the voltage to the required 230 V. This system component can be
omitted in case of sufficiently high input voltage (> 350 V). This allows for in-
verter-related savings as well as for considerably higher efficiencies especially
within the lower partial-load range. Inverters without transformer are also signifi-
cantly reduced in weight.
For stand-alone systems or isolated grids, such high input voltages are cur-
rently only applied for large plants. This is mainly due to the fact that also batter-
ies would need to be connected in series to obtain such high voltages. However,
due to the risk of individualisation of different cells, it is much more difficult to
operate a battery at a high voltage level than at low voltages. For plants of in-
stalled capacities of up to 10 kW input voltages usually vary between 48 and
60 V.
To avoid these shortcomings another typology includes a DC/DC converter be-
tween battery and the inverter that supplies the appropriate voltage level. This
concept allows designing transformerless inverters for any type of photovoltaic
system. A disadvantage of all typology without transformers is the fact that they
have no galvanic interruption and therefore some additional safety features must
be integrated.
S1
S1
S1
S1
S3
S3
S3
230 V
50 Hz
230 V
50 Hz
230 V
50 Hz
t
t
S2
S4
S4
S4
S2
S2
S2
t
t
Bridge circuit
Filter
Transformer
Fig. 6.19 Circuit principle of a pulse-width modulated inverter
Depending on the kind of application the following requirements are relevant
for island inverters /6-29/.
High efficiency. The efficiency of island inverters should be as high as possible
and should be high already within the lower section of the partial-load range.
However, if the inverter is only occasionally turned on within a DC grid to
supply an assigned AC consumer, the internal power consumption and the effi-
ciency of the inverter are of minor importance. Yet, if the inverter is continu-
ously operated, to provide, for instance, power to the grid of a dwelling house,
its self-consumption is a critical variable. Every percent of inverter internal
consumption reduces the mean annual efficiency by approximately 10 %. Thus,
inverter self-consumption should be below 1 % of the nominal output power,
corresponding to an efficiency of above 90 % at 10 % of the nominal output
 
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