Isolated induction generator and renewable energy role

An isolated (self-excited) induction generator is a 3-phase induction machine driven above synchronous speed by a prime mover, with a capacitor bank across its stator terminals to supply the reactive power it needs, feeding a local load with no grid.

Operation in isolated mode

 Turbine   +---------+
 (N > Ns)==| Ind.    |===+=========> Local load
           | machine |   |
           +---------+  === C (3-phase bank)

1. Need for capacitors: an induction machine cannot make its own magnetizing current. With no grid, the capacitors supply the leading current that acts as the machine's lagging magnetizing current.
2. Voltage build-up: the rotor has a little residual magnetism. When driven, it induces a small emf in the stator. This sends a small leading current through the capacitors, which strengthens the air-gap flux, which increases the emf. This cumulative process continues until the magnetization curve ($V$ vs $I_m$) meets the capacitor line $V = I_c/(\omega C)$.

 V |          capacitor line
   |        /   V = Ic.Xc
   |      / .----- magnetization
   |    /.'          curve
   |  .'/  <- operating point
   |.' /      (intersection)
   |__/_____________ I

3. Generation: slip is negative ($N > N_s$), so power flows from the shaft to the stator and load.
4. Voltage and frequency depend on speed, capacitance and load. Too small C (line steeper than the curve's air-gap line) means no build-up. A lagging load takes extra VAr and lowers voltage; so an electronic load controller or extra capacitors are used for regulation.

Role in renewable energy

• Wind energy: squirrel-cage (fixed speed) and doubly-fed (slip-ring) induction generators are widely used in wind turbines; they tolerate speed variation and need no synchronizing.
• Micro and small hydro: self-excited induction generators (often motors used as generators) supply villages in remote hilly areas, e.g. many micro-hydro schemes in Nepal.
• Advantages: rugged brushless cage rotor, low cost, little maintenance, easily available as standard motors, inherent short-circuit protection (voltage collapses on fault).
• Limitations: need for reactive power, poor voltage and frequency regulation in isolated mode.