Induction Generator or Asynchronous Generator: Construction & Working
Synchronous Generator or Induction Generator – Working, Types, Characteristics, Advantages and Applications
Electricity is a form of energy that we use in almost every aspect of our life and we cannot imagine our life without it. In the world of technology and power generation, induction generators play a vital role in harnessing mechanical energy and convert it into electrical energy. Be it wind energy or energy in flowing water, it efficiently captures it and converts it into the electrical power that we use in our daily life. In this article, we will discuss the induction generator, its working principle and advantages, etc.
Generator
A generator is essentially an electrical machine that converts mechanical energy into electrical energy. The mechanical power to the rotor of the machine is provided using a prime mover such as a water turbine, steam turbine, wind turbine, etc. The stator of the machine is used to provide output electrical power to a given circuit.
The generators are classified into AC and DC generators. Where the AC generator is further classified into alternator or synchronous generator and asynchronous (induction) generator.
Induction Generator
An induction generator (also known as synchronous generator) is a type of AC generator that converts mechanical energy into AC electrical energy. It is also known as an asynchronous generator since it operates at a speed other than the synchronous speed. Initially, it runs as an induction motor with a speed less than the synchronous speed and positive slip. When the prime mover speed is increased above the synchronous speed, the slips become negative and a reverse rotor current is generated that induces a voltage in the stator generating output stator current.
It is widely used in renewable energy such as wind turbines, hydro turbines, etc. due to its efficient operation at variable speeds and almost constant output frequency. It is also self-exciting.
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Construction of Synchronous Generator
The construction of an induction generator is similar to an induction motor since an Induction machine can be used as either an induction motor or induction generator except for the difference in the operating speed or slip.
It has two main parts
- Stator
- Rotor
Stator
The stator is the stationary part of the machine that carries the windings called stator windings. The windings are placed inside a core made of laminated steel to reduce Eddy current loss. There are two stator windings i.e. main winding and auxiliary winding.
Rotor
The rotor is the rotating part of the generator. The rotor (either salient or non-salient (cylindrical pole)) used in the induction generator is the squirrel cage rotor. Instead of rotor windings, copper bars are used that are short-circuited at the end using end-rings. Therefore, the copper bars complete the circuit to provide a path for the flow of induced current.
Working Principle of Induction Generator
The induction generator as its name suggests operates on the principle of electromagnetic induction.
To better understated the operation of an induction generator, you need to know about the slip. Slip is the difference between the synchronous speed Ns and the speed of the rotor N. It is given by
Slip , s = (Ns – N)/Ns
The synchronous speed, Ns is the speed of the stator’s revolving magnetic field that only depends on the supply frequency and the number of poles.
The slip of a machine depends on the speed of the rotor N. It can be either positive, zero, or negative depending on the rotor speed.
The slip is positive when the rotor speed is lower than the synchronous speed. The rotor tries to catch up to the synchronous speed, and as a result, it generates torque. Therefore it operates as an induction motor.
The slip is zero when the rotor revolves at or near synchronous speed. It occurs at no load situation. There is no rotor current as well as no torque generated.
The slip is negative when the rotor speed is higher than the synchronous speed. It only occurs when the rotor is driven by a prime mover. In such case, the induction machine will deliver power to the electrical load thus operating as an induction generator. It is also called an asynchronous generator because it runs at a speed other than the synchronous speed.
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Operation of Induction Generator
Step by step operation is explained below
Initially, the induction machine stator is connected to the AC power supply. It generates a revolving magnetic field (RMF) in the stator. Due to this revolving magnetic field, a current is induced in the rotor called rotor current. The rotor current flows in the short-circuited bars of the rotor and starts generating its own magnetic field called the rotor field. The rotor field interacts with the stator field and starts revolving in its direction. It draws power from the source and acts as an induction motor as long as the rotor speed N is less than the synchronous speed Ns or the slip is positive.
Now the prime mover is used to gradually increase the rotor speed to match the synchronous speed. The rotor reaches the synchronous speed at which point there is no difference between the rotor speed and synchronous speed. Hence the slip will become zero and there will be no rotor current. Therefore, there will be no torque generated.
The next step is to increase the rotor speed above the synchronous speed using the prime mover. The slip becomes negative. The stator RMF starts cutting the rotor bars and generates rotor current in the opposite direction. This rotor current start generating its own magnetic field that opposes the stator RMF. This causes the stator current to flow out of the stator winding against the applied voltage. At this point, it starts delivering power to the circuit connected to its stator. Thus it acts as an induction generator
An induction machine is not self-exciting. It requires reactive power to produce the rotating magnetic field. Therefore the AC line provides the necessary reactive power whereas the generator provides the active power to the AC line.
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Types of Induction (Synchronous) Generators
There are a few types of induction generator
Grid Connected Induction Generator (GCIG)
As its name suggests, such an induction generator is connected to the grid which provides the necessary reactive power for the stator excitation. It works in parallel with the electrical grid. It is commonly used in renewable energy such as hydroelectric power stations and wind turbines. The diagram of the grid-connected induction generator is given before.
The stator of GCIG is connected to the grid that provides the excitation power. The grid voltage generates the rotating magnetic field in the stator. A squirrel cage rotor is used that is connected to the prime mover that rotates the rotor. When its speed increases above the synchronous speed, the slip becomes negative and the stator winding starts delivering power to the connected grid.
Since it is directly connected to the grid, it must be synchronized with the grid before it starts generating power. Its output frequency and phase must match the frequency and phase of the grid. A control system is used to synchronize the GCIG.
Self-Excited Induction Generator (SEIG)
An induction machine requires reactive power for excitation whether it operates as a generator or motor. In a grid-connected induction generator, it draws reactive power from the grid. A self-excited induction generator does not require an external power source to operate. It is used in small-scale power plants where the grid connection is not available.
In SEIG, a capacitor bank is used to provide the necessary reactive power for excitation. It is connected in delta configuration across the stator’s terminals. The prime mover rotates the rotor with enough speed to induce a voltage across the stator’s terminals. As a result, the capacitor current starts following providing the reactive power for the excitation. The capacitor also supplies reactive power to the load.
Wound Rotor Induction Generator (WRIG)
As its name suggests, such induction generators include wound-type rotor. In squirrel cage rotor, the design does not allow external access to change electrical characteristics and the rotor current only depends on the slip of the generator. On the other hand, the WRIG wound rotor is externally accessible using a slip ring. The slip ring assembly allows to control the rotor current and thus provides reactive power compensation. It is used in wind turbines and hydroelectric power plants where the operating speed is variable.
The rotor current is controlled by using a network of resistors connected through the slip ring. It allows the generator to provide constant output power even if there is high-speed wind.
Doubly Fed Induction Generator (DFIG)
In DFIG, AC power is supplied to the stator as well as the rotor. Thus the name doubly fed. The rotor is electrically accessible which means DFIG uses a wound-type rotor with slip rings. It maintains a constant amplitude and frequency of the delivering power at variable speed.
DFIG includes a back-to-back converter that converts AC into DC and back into AC. Basically, there are two converters AC-to-DC (grid side) and DC-to-AC (rotor side). The advantage is its amplitude and frequency are adjustable. The stator is directly connected to the grid whereas the rotor is connected through a back-to-back converter controlled by the prime mover. A DC link capacitor is placed between the two converters that act as a DC voltage source.
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Protection in Induction Generator
There are a few necessary protections required in induction generators.
Reverse Power Protection Relay
The induction generator draws power instead of delivering it when it operates as a motor. These relays sense the direction of power flow and break the circuit when power flows in the reverse direction. It is used to disconnect the power to the induction generator when it starts operating as a motor
Over Speed Protection
Stored limit switches are used to prevent or limit the generator from reaching its maximum speed limit. It helps in efficient power generation.
Under/Over Speed Relay
The under/over speed conditions cause over-excitation therefore, the capacitors much be disconnected to prevent uncontrolled voltage generation due to over-excitation.
Advantages & Disadvantages
Advantages
Here are some advantages of induction generator
- It does not require synchronization as the main supply line.
- It has robust construction and requires less maintenance.
- It has a higher energy density (size per KW)
- Prime mover speed variation does not impact its performance
- They can operate in parallel.
- It has a lower relative cost.
Disadvantages
Here are some disadvantages of induction generator
- It cannot generate reactive power on its own that is required for excitation. Therefore it draws reactive power from the grid.
- The speed must be above synchronous speed to operate as a generator otherwise it will operate as a motor.
Applications of Synchronous (Induction) Generators
Here are some applications of induction generators.
- Wind Power Generation: They are widely used in wind power generation. In wind turbines, the mechanical energy of the wind rotates the rotor at a high speed above the synchronous speed. As it is an asynchronous generator, it is suitable for variable wind speed operation.
- Hydroelectric Power Plants: It is also used in hydroelectric power generation in hydro dams. The mechanical energy from the flowing water is converted into rotational energy to rotate the rotor beyond synchronous speed. The energy is converted into electrical energy. It can be used in both small-scale and large-scale power plants.
- Rural electrification: Rural areas lack access to proper electrical grid. Therefore induction generators are used in rural areas to provide electricity. Several small-scale induction generator power plants are used to meet the energy demand.
- Isolated power generator: self-excited induction generator is used in remote locations having no electrical grid such as mining sites, camping, etc.
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