Environmental Engineering Reference
In-Depth Information
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PV
Array
Figure 4.38
Basic grid - connected PV inverter
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PV
Array
Figure 4.39
Transformerless grid - connected PV inverter
Basic Grid - connected PV Inverter
The grid-connected PV inverter shown in Figure 4.38 is based on the transistor bridge circuit
of Figure 4.29. It is required that the DC voltage of a PV array is inverted and connected to
a grid of, say, 230 V. For such a simple, e.g. domestic application, the array voltage would
have to be well over 350 V DC at the maximum power point, which would be impractical in
many cases. The transformer is therefore required in order to raise the output inverter
voltage.
The control could be implemented as described earlier under the heading 'Output control
in a grid-connected inverter' in Section 4.5.6. Control of the AC output current would provide
control of the DC current drawn from the array and hence allow maximum power point
tracking. The main drawback of the circuit is the size, weight and cost of the output 50 or
60 Hz transformer.
Transformerless Grid - connected PV Inverter
The addition of a DC booster stage, at the input of the inverter, as shown in Figure 4.39,
removes the need for the mains frequency transformer, and reduces the current in the inverter
transistors as they operate at a higher voltage than in Figure 4.38. The converter in Figure
4.39 has two stages, both of which require to be controlled.
One possibility would be to operate the DC booster at a fi xed duty cycle, which would
give a fi xed ratio between the average DC currents and allow the maximum power point
tracking to be implemented by controlling the AC output current as it was in the previous
example. In practice, it may be more effi cient to fi x the voltage of the DC bus (the DC con-
