Induction Generator Working Principle, Induction Generator Types
Table of Contents
Induction Generator Working Principle & Types
Working Principle of an Induction Generator:
What is the working Principle of an Induction Generator?
For a Three-phase Induction Motor, it will be Observed that energy is returned to the AC Mains 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. After that, the device will function as an Induction Generator.
As the prime mover rotates the rotor faster than the Synchronous speed in Induction Generators, the value of the slip is negative. Asynchronous Generator is another name for an Induction Generator. because the rotor’s speed and the Synchronous speed are different.
Operation of an Induction Generator:
The graph below displays the Torque-speed Characteristic curve of an Induction Motor. When the rotor speed is below Synchronous speed, the device will operate as a motor, and when it is above Synchronous speed, it will operate as a Generator.
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.
The speed above Synchronous speed that an Induction Generator can operate has a practical upper limit. The generator’s speed ought to be lower than the torque breakaway. The real power generates quickly to a low value if the speed exceeds the Breakaway torque.
Protection required for an Induction Generator:
- To prevent the Generator from Reaching the Pushover point and to efficiently produce real power, overspeed protection, Typically provided by governors or speed limit Switches, is necessary.
- Relays for reverse power are necessary to stop the motor from running. When the Induction Generator starts acting like a motor, these relays detect the direction of power flow and cut off the power.
- To avoid overexcitation and causes to Generate uncontrolled high voltages, the power factor Capacitors must be disconnected from the Generator using a separate breaker during under/overspeed conditions.
Induction Generator Types:
There are three types of Induction Generators –
- Grid connected Induction Generator
- Isolated Self-Excited Induction Generator
- Doubly fed Induction Generator-DFIG
1. Grid Connected Induction Generator Working
Reactive power is provided by the system to run an Induction Generator when it is connected to the grid.
The line power source that provides excitation is connected to the stator of the induction motor. The rotor is a squirrel cage constructed from copper or aluminium bars. When the slip becomes negative and the shaft is forced to rotate faster than synchronous speed, power is sent to the supply mains by the induction generator.
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.
For the machine to function as a Generator, these Capacitors connected in a delta Configuration supply the necessary Excitation current. These Capacitors provide the Induction Generator with the reactive power it needs to build voltage. Additionally, the load receives reactive power from these Capacitors. Due to Residual Magnetism in the rotor, when the Prime-mover rotates the rotor, a small amount of emf is induced in the stator. The rotor speed affects the Frequency of the induced emf. The device functions as a Self-excited Induction Generator in this manner.
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:
Hardly fed Ac currents were fed into the stator and rotor Windings of Induction Generators. The Generator transforms the wind energy captured by the turbine into electricity, which is then sent to the grid via the stator and rotor windings. The main benefit of DFIG is that it enables output voltage amplitude and frequency to be kept constant regardless of the speed of the wind turbine rotor. As a result, DFIG can be connected directly to the ac power network and always maintain synchronisation.
Parts of Doubly Fed Induction Generator:
1. Three-phase Wound rotor Induction Machine:
A wound rotor induction machine with three phases is the Doubly fed Induction generator. The stator windings of the DFIG-based wind turbines are connected directly to the network via a power transformer, while the rotor windings are connected using two back-to-back converters.
2. Back to Back converters:[AC/DC/AC Converter]:
Rotor Side Converter (RSC) and Grid Side Converter (GSC) are the two back-to-back converters. By adding reactive power to the system, these converters are used to regulate the terminal voltage and power factor.
- 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.
RSC is used to control the torque/speed of the generator as well as the power factor at the stator terminals by receiving commands for pitch angle and voltage. Rotor currents are under RSC’s control. The slip and subsequently the machine’s speed are controlled by managing the rotor currents and their frequency.
- 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:
Between the two converters is a DC-link capacitor. To maintain the voltage variations in the DC-link, it serves as a source of DC voltage.
Applications of Induction Generators:
- In remote locations, self-excited induction generators are used to generate electricity using windmill generators and other renewable energy sources. Induction generators can also be used to power loads independently in remote locations.
- Existing supply or grid-connected induction generators are used to power hydraulic turbines that produce small, intermittent amounts of water power as well as load-driving hoists that produce braking torque.
- Used in wind power plants are doubly fed induction generators. The benefit of DFIGs is improved power quality while maintaining the network’s dependable operation.
Differences between Synchronous and Induction Generators:
- linked to the grid Only when the Induction Generator is connected to a Three-phase supply will it start to produce.
- Induction Generators needs no DC Excitation.
- For an Induction Generator, no synchronising is required since the machine will generate only when the slip becomes negative.
- Parallel Capacitors are used to provide reactive power to Self-excited induction generators that are not connected to the grid in order for them to operate.
- 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.
- Induction generators rotors designed with lower resistance in order to reduce losses and slip.
- The construction is straightforward when compared to synchronous motor. There is less auxiliary hardware needed. No collector rings, diodes, or brushes are employed.
- Size is small per KW output power, due to its High energy density.
- Maintenance cost is less.
- Large power swings do not cause the generator to lose synchronisation with the connected network, unlike synchronous generators.