Electrical Equipment - Wind Energy Systems for Electric Power Generation - page 54

• A.c./a.c. inverters

They transform a.c. current of a given voltage, frequency an number of phases
into a.c. current of another voltage, frequency and number of phases.
Inverters with intermediate circuit combine an uncontrolled or controlled
rectifier and an external or self-controlled d.c./a.c. inverter. Both inverters are
coupled by the intermediate circuit, either with impressed voltage (voltage-
source-inverter, VSI), or with impressed current (current-source inverter, CSI).
Cyclo converters are built without intermediate circuit; the power is trans-
formed in the same inverter
A newer concept is the matrix-inverter where fast switches couple input and
output side, using no energy storage elements.

• D.c./d.c. inverters (choppers)

They transform d.c current of a given voltage and polarity to d.c. currennt of
another voltage and polarity. Inverters using an energy storage element and a
pulse-control scheme are usually called choppers.
Depending on the ratio of output-/input-voltage, we have step-up (boost) in-
verters for ratios > 1 and step-down (buck) inverters for ratios < 1. Specific
inverter circuits are capable of both ways of operation (buck-boost).
Note that in this topic resonant and other special inverters which are normally not
used in wind energy application are not discussed.

4.3.2 External-Commutated Inverters

4.3.2.1 Thyristor Bridge Inverter

Figure 4.2 shows a fully-controlled rectifier circuit containing a three-phase inverter
transformer, a six-pulse bridge (B6) circuit of thyristors and a choke in the d.c.
output. Due to the gate-turn-on capability without gate turn-off of the thyristors the
commutation voltage must be supplied by the grid. This is the classical a.c./d.c.

Fig. 4.2 Six-pulse bridge (B6) thyristor rectifier circuit