Induction Generators
Content:
1. Introduction to induction generators
2. Types of Induction generators
3. Grid connected induction generator
4. Self excited induction generator
5. Role of capacitor bank in SEIG
6. Doubly Fed Induction Generator (DFIG)
Introduction to Induction Generators
Torque speed characteristics of induction machines
Nr < 0 Nr < Ns Nr > Ns
S>1 0<S<1 S<0
Machine brakes Machine operates Machine operates
as motor as generator
Introduction to Induction Generators
➢ Two conditions for IG to produce electric power :
1. The generator speed is turned faster than the synchronous speed (Ns)
2. Magnetic field ( excitation ), Since it does not have external excitation circuitry and
it cannot generate reactive energy (Q).
Speed > Ns
Induction Electric power
Excitation
generator
Types of Induction Generators
Grid connected induction Self Excited induction
generator generator (SEIG)
▪ Induction machine is connected ▪ Induction machine is connected
to the grid or power supply directly to the load
Grid Connected Induction Generator
❑ Operation
Consider, AC supply is connected to the stator
terminals of an induction machine
The machine is acting as a motor (Nr < Ns)
Now, if the rotor is accelerated to the synchronous speed by means of a prime mover
Nr = Ns The net torque will be zero
Grid Connected Induction Generator
❑ Operation
If the rotor is made to rotate at a speed more
than the synchronous speed (Nr > Ns)
Speed of Rotor magnetic field now is
higher than speed of stator magnetic field
This causes a stator voltage which pushes current flowing out of the stator winding against
the applied voltage
Grid Connected Induction Generator
❑ Operation
• The induction generator takes reactive
power from the supply for providing
excitation
• The induction generator injects active
power to the grid (or supply)
• As rotor speed increases, the injected
active power increases
Application: Wind Turbines
• Turbine rotor converts wind energy into mechanical energy at low rotational speeds
• Gearbox is used to increase the speed of rotation higher than synchronous speed to
operate the induction machine as generator
Application: Wind Turbines
• The generator converts the mechanical energy to electric energy along with the
power electronic converter
• Power electronic converter is used for producing controlled power with desirable
voltage and frequency
Application: Wind Turbines
reactive power (Q)
Active power (P)
• Step up transformer is used to raise the voltage to be connected with the transmission
and utility grid
• Electric power is transmitted and distributed to the load centers
Self Excited Induction Generator (SEIG)
• It is also called “ islanded generator” or “stand-alone generator”
• Induction generator needs reactive power for excitation
• For SEIG, a capacitor bank can be connected across the stator terminals to supply reactive
power to the machine as well as to the load
Self Excited Induction Generator (SEIG)
❑ Magnetization curve of induction machine
• This curve represents the magnetizing current IM essential for excitation as a function of
terminal voltage (Vt)
• It can be obtained by running the machine as a
motor at no load and calculating its stator
current (Im) as a function of the terminal voltage
Self Excited Induction Generator (SEIG)
❑ Capacitor voltage-current characteristics
• The current which is provided by the capacitor banks is directly proportionate to the
voltage supplied to it
Self Excited Induction Generator (SEIG)
❑ Operation
• When the rotor is rotated at an enough speed, a small voltage is generated across the
stator terminals due to residual flux
Øres
• Due to this small generated voltage, capacitor
current is produced
Ic
• Ic increases Ø Voltage (V) increases
Eres
Ic Ø V
➢ Until V= rated voltage
Self Excited Induction Generator (SEIG)
Operating point
Mag.curve
Capacitor line
Build up voltage of SEIG
Self Excited Induction Generator (SEIG)
❑ reasons for build up voltage failure in SEIG
1. No residual flux in the machine
The machine is brand new and has nor residual flux
➢ Solution
1. Operate the machine first as grid connected machine
2. Or, Connect DC source across the stator winding to excite it
Self Excited Induction Generator (SEIG)
❑ reasons for build up voltage failure in SEIG
2. Capacitor bank is very small (C< Ccr)
If C< Ccr , No intersection between
the 2 characteristics
No operating point and the machine
fails to build up the voltage
➢ Solution
• The value of the capacitor should be
higher than the critical capacitor
Self Excited Induction Generator (SEIG)
❑ reasons for build up voltage failure in SEIG
3. Mototr speed is very small (N< Ncr)
If N < Ncr , No intersection between
the 2 characteristics
No operating point and the machine
fails to build up the voltage
➢ Solution
• The motor speed should be higher than
the critical speed
Self Excited Induction Generator (SEIG)
➢ Example
𝑆 = 3 ∗𝑉 ∗𝐼 𝑆 = 3 ∗ 440 ∗ 41 𝑆 = 31.246 𝐾𝑉𝐴
𝑃 = 𝑆 ∗ 𝑃. 𝐹 𝑃 = 31.246 ∗ 0.84 𝑃 = 26.24 KW
𝑄= (𝑆)2 −(𝑃)2 𝑄= (31.24)2 −(26.24)2 = 17 KVAR
Self Excited Induction Generator (SEIG)
➢ Example
3 ∗ 𝑉2
𝑄= 𝑄 = 3 ∗ 𝑉 2 ∗ 𝑊𝐶
𝑋𝑐
17000 = 3 ∗ 4402 ∗ 2 ∗ 𝑝𝑖 ∗ 60 ∗ 𝐶 𝐶 = 78 micro farad
• If the load also absorbs reactive power, capacitor bank must be increased in size to compensate
Self Excited Induction Generator (SEIG)
➢ Example
• Prime mover speed should be used to generate frequency of 60 Hz:
• Typically, slip should be similar to full-load value when machine is running as motor, but
negative (generator operation):
Required prime mover speed N
• if Ns = 1800, one can choose N=Ns+40 rpm
= 1800 + 40 = 1840 rpm.
