Working Principle of an Induction Generator:
Let us see, What is the working principle of an Induction generator?
For a three-phase Induction motor if the rotor is coupled to the prime mover and if the Prime-mover is capable of driving the induction machine rotor at a speed higher than the synchronous speed, it will be observed that energy is returned to the AC Mains. The machine will then work as an Induction generator.
For induction generators, the value of the slip is negative as the prime mover rotates the rotor at a speed higher than the synchronous speed. An induction generator is also called an Asynchronous generator. Since the speed of the rotor is different from the synchronous speed.
Operation of an Induction Generator:
The torque-speed characteristic curve of an induction motor is shown in the figure below. The machine will work as a motor when the rotor speed is below synchronous speed and the generator when speed is above synchronous speed.
The operating range of the machine as a generator is limited to the maximum value of torque corresponding to a slip of OP as indicated in the figure.
There is a practical upper limit to the speed at which an induction generator can be operated above synchronous speed. The speed of the generator should be below breakaway torque. If the speed crosses above the breakaway torque the real power generates quickly to a low value.
Protection required for an Induction Generator:
- Over speed protection usually by governors or speed limit switches is required to prevent the generator from reaching the pushover point and to generate real power efficiently.
- Reverse power relays are required to prevent the motor operation. These relays sense the direction of power flow and disconnect the induction generator when it begins to operate as a motor.
- Under/over speed conditions the power factor capacitors must be disconnected from the generator using a separate breaker to prevent over excitation and causes to generate uncontrolled high voltages.
Induction Generator Types:
- Grid connected Induction generator
- Isolated Self-Excited Induction generator
- Doubly fed Induction generator-DFIG
1. Grid Connected Induction Generator Working
When an induction generator is connected to the grid, the system supplies reactive power to operate the generator.
The stator of the Induction motor is connected to the line power source that provides excitation. The rotor is a squirrel cage made with either aluminium or copper bars. If the shaft is forced to rotate at a speed higher than synchronous speed, the slip becomes negative and the induction generator starts delivering energy to the supply mains.
2. Isolated Self-Excited Induction Generator Working
Delta connected capacitor bank is connected across the terminals of the induction machine as shown in the below figure.
These Delta-connected capacitors provide the excitation current required for the machine to work as a generator. These capacitors supply necessary reactive power to the induction generator to build voltage. These capacitors also supply reactive power to the load.
When the prime-mover rotates the rotor, a small amount of emf is induced in the stator due to residual magnetism present in the rotor. The frequency of the induced emf depends upon the rotor speed. This way the machine works as a self-excited induction generator.
The prime mover may be a windmill here.
3. Doubly Fed Induction Generator Working [DFIG]
DFIGs are variable speed generators used in Wind turbines due to their several advantages.
Operation of Doubly Fed Induction Generator:
Doubly fed Induction generators fed ac currents into both stator and rotor windings. The wind power captured by the turbine is converted into electric power by the generator and is transferred to the grid by stator and rotor windings.
The major advantage of DFIG is that it allows the amplitude and frequency of their output voltages to be maintained at a constant value, irrespective of the speed of the wind turbine rotor. Hence DFIG can be directly connected to the ac power network and remained synchronized at all times.
Parts of Doubly Fed Induction Generator:
1. Three-phase Wound rotor Induction Machine:
The Doubly fed Induction generator is a three-phase wound rotor induction machine. The rotor windings of the DFIG-based wind turbines are connected with the use of two back-to-back converters, while the stator windings are connected directly to the network via a power transformer.
2. Back to Back converters:[AC/DC/AC Converter]:
The two back-to-back converters are namely RSC- Rotor Side Converter and GSC- Grid Side Converter. These converters are used to control the terminal voltage and power factor by injecting reactive power into the system.
- Rotor Side Converter-RSC: [AC/DC]
The generator slip rings are connected to the rotor side converter, which shares a DC link with the grid side converter.
By receiving Pitch angle and Voltage commands RSC is used to control the torque/speed of the generator and also control the power factor at the stator terminals. RSC controls the rotor currents. Controlling the rotor currents and their frequency controls the slip and hence the speed of the machine.
- Grid Side Converter-GSC: [DC/AC]
By receiving voltage command keeps the DC-link voltage constant and controls the reactive power or voltage at grid terminals.
3. DC-link Capacitor:
A DC-link capacitor is placed between the two converters. It acts as a DC voltage source in order to keep the voltage variations in the DC-link.
Applications of Induction Generators:
- Existing Supply or Grid connected induction generators used for Small and variable amounts of Water power generated hydraulic turbines, Load driving hoists for generating braking torque.
- Self-excited induction generators are used for power generation using renewable energy sources like windmill generators, in isolated places. Induction generators are also used as stand-alone power supply units for loads in remote places.
- Doubly fed induction generators used in wind power plants. The advantage of DFIGs is to maintain a reliable operation of the network with improved power quality.
Differences between Synchronous and Induction Generators:
- Induction generator needs no DC excitation.
- Grid connected Induction generator will generate only when it is connected to the three phase supply.
- For self-excited Induction generators those are not connected to grid require a source of reactive power to operate, for this Parallel capacitors are used to supply reactive power.
- For an induction generator, no synchronising is required since the machine will generate only when the slip becomes negative.
- Induction generators rotors designed with lower resistance in order to reduce losses and slip.
- The output voltage and frequency are independent of speed variations and hence no voltage regulator is required. Voltage and frequency are controlled by the power system.
- Compared to synchronous motor the construction is simple. It requires less auxiliary equipment. No brushes, diodes and collector rings used.
- Maintenance cost is less.
- Size is small per KW output power, due to its High energy density.
- Unlike a synchronous generator, large power swings do not pull the generator out of synchronisation with the connected network.