Generators - Wind Energy Systems for Electric Power Generation - page 33

a)

Fig. 3.6 Performance in steady-state ( a ) current locus (Ossanna's circle diagramme); ( b ) vector
diagramme (Generator operation)

The current locus curve for steady-state operation can be drawn in the complex
plane. For the machine modelled as in Fig. 3.2, and with secondary winding short-
circuited, this is a circle. Figure 3.6a shows the so-called Ossanna diagramme. The
circle may be defined by three points:

- P 0 for no-load operation, s = 0, where current I 0 is mainly inductive with only a
small active component,
- P k for short-circuit, s = 1; where current I k is typically 5 ... 8 times the rated
value,
- P ∞ for unlimited positive or negative slip, rendering a purely inductive secondary
side impedance.
The circle diameter is ( P 0 P Φ ) and its centre point m, which is located slightly above
the horizontal axis. Note that in the complex plane points of motoring operation
are in the upper half-plane, while generator operation is in the lower half-plane. All
points of operation are in the right half-plane indicating reactive current consump-
tion (inductive currents), characteristic for an induction machine.
In the figure P NM denotes the rated current in motor operation, showing a phase
displacement ϕ N with respect to the voltage vector U 1 . In generator operation the

rated current is I NG , as indicated by point P NG .