Wind Energy Systems - Wind Energy Systems for Electric Power Generation - page 107

Fig. 5.29 Stand-alone system with synchronous generator and battery storage

The concept of a stand-alone system in the kW-range is illustrated in Fig. 5.29.
The synchronous, three-phase PM excited generator 2 is driven by the wind turbine
1 . A first d.c., variable voltage intermediate circuit is fed via the diode bridge recti-
fier 3 . A step-up inverter (boost chopper) 5 supplies the second d.c. circuit contain-
ing a storage battery 6 . An active front-end inverter 7 feeds the single-phase local
grid 10 . In practice a transformer 8 will be necessary to match battery and grid volt-
age; 9 is a passive filter for harmonics reduction, and 4 indicates a short-circuiting
device. Note similarities to the circuit in Fig. 5.15.

5.4.2.3 Systems with Induction Generator

The self-excited induction generator concept (SEIG) with capacitors as reactive
power source has often been recommended in literature. Simple solutions with con-
stant or switched capacitor banks are, however, not suitable for practical use due to
the load and speed dependence of the terminal voltage; see Fig. 3.12. An acceptable
solution would require appropriate voltage and reactive power control.
A simple possibility is to use a phase-controled inductance (see Fig. 3.10a) to-
gether with a capacitor bank. A pilot set-up is shown in Fig. 5.30, where the output
voltage is controlled by means of a thyristor inverter, adjusting the phase angle α .

The passive LR-load is connected to the generator system by closing the switch S.
More sophisticated SEIG control concepts use active filters [chat06].
The supply of magnetizing current to a cage induction machine used as a genera-
tor can be achieved by means of an active front end inverter, see 4.3.3.3. Figure 5.31
indicates a stand-alone system, where C 1 is designed to provide a basic compensa-
tion to take some of the inverter load, but not sufficient for creating self-excitation.
The scheme contains a buck-boost dc-dc inverter with an inductive storage element;
hence the polarity from left to right side is reversed. This solution with two inter-
mediate dc-circuits is suitable to cover a wide speed range, however at considerable