doubly fed induction generators in wind energy

A Seminar onDoubly Fed Induction

Generators(DFIG)

Presentation by M.D.Udaya kumar

(M.E. Power Management)

• Turbine’s rotor absorbs the mechanical power fluctuations.

• Output power curve is smoother.

• Since variable speed operation produces a variable frequency voltage, a power electronic converter must be used.

Stator RMF causes the rotor to spin.

As rotor approaches synchronous speed of the rmf, the relative motion becomes less and less.

Thus, an induction machine as a motor always spins somewhat slower than synchronous speed.

Slip:The difference in speed between the stator and the rotor

s = rotor slip : positive for a motor, negative for a generatorNS = no-load synchronous speed (rpm) f = frequency (Hz) p = number of polesNR = rotor speed (rpm)

1 (6.28)S R R

S S

N N Ns

N N

The stator requires excitation current from the grid if it is grid-connected or by

incorporating external capacitors Wind speed forces generator shaft to exceed

synchronous speed

Slip is negative because the rotor spins faster than synchronous speed

Slip is normally less than 1% for grid-connected

Single-phase, self-excited, Induction Generator

Simple and robust construction. Can run independently. Inexpensive. Minimal maintenance. Inherent overload protection. At high speed, reduces size and weight

of machine and filter components.

The stator winding is directly connected to the grid.

The rotor winding is fed through a converter which can vary the electrical frequency as desired by the grid.

Energy capture is more efficient.

A cycloconverter is used as inverter.

So the frequency deviation is restricted to approximately ± 40% of the nominal speed

The slip power of the induction generator is not only fed into the grid, but the rotor is also supplied with power from the grid.

Both oversynchronous and subsynchronous operation of the generator is possible.

1.Current-mode Control (PVdq) This technique is often used for the electrical control of the DFIG

The rotor current is split into two orthogonal components, d and q.

The q component of the current is used to regulate the torque and

The d component is used to regulate power factor or terminal voltage.

To modify the electromagnetic torque of the generator according to wind speed variations.

Driving the system to the required operating point reference.

The reference torque is manipulated to generate a reference value for the rotor current in the q axis.

To provide terminal voltage or power factor control using the rotor-side converter.

Reactive power injection for voltage control schemes, the rotor-side converter is likely to be preferred.

It is because the rotor circuit is effectively amplified by a factor of 1/s.

It is done by by adjusting the magnitude and angle of the rotor flux vector.

Advantages: Provides low interaction between the

power and voltage control loop. Also provides enhanced system damping

and voltage recovery during faults.

The difference in the magnitude of Vs, from Vsref , forms an error signal.

Then it is processed via the AVR compensator to produce the reference value for |Eg|ref.

In this, Peref is determined by the wind turbine P–N characteristic for maximum power extraction from the prevailing wind velocity.

The difference in the magnitude of Pe, from Peref , forms an error signal.

Then it is processed by the compensator to produce the reference value, δgref with respect to the stator voltage vector.

Thank you