An induction generator or asynchronous generator is a type of alternating current (AC) electrical generator that uses the principles of induction motors to produce electric power. Induction generators operate by mechanically turning their rotors faster than synchronous speed. A regular AC induction motor usually can be used as a generator, without any internal modifications. Induction generators are useful in applications such as mini hydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure, because they can recover energy with relatively simple controls.
An induction generator usually draws its excitation power from an electrical grid. Because of this, induction generators cannot usually black start a de-energized distribution system. Sometimes, however, they are self-excited by using phase-correcting capacitors.
Principle of operation[edit]
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An induction generator produces electrical power when its rotor is turned faster than the synchronous speed. For a typical four-pole motor (two pairs of poles on stator) operating on a 60 Hz electrical grid, the synchronous speed is 1800 rotations per minute (rpm). The same four-pole motor operating on a 50 Hz grid will have a synchronous speed of 1500 RPM. The motor normally turns slightly slower than the synchronous speed; the difference between synchronous and operating speed is called 'slip' and is usually expressed as per cent of the synchronous speed. For example, a motor operating at 1450 RPM that has a synchronous speed of 1500 RPM is running at a slip of +3.3%.
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In normal motor operation, the stator flux rotation is faster than the rotor rotation. This causes the stator flux to induce rotor currents, which create a rotor flux with magnetic polarity opposite to stator. In this way, the rotor is dragged along behind stator flux, with the currents in the rotor induced at the slip frequency.
In generator operation, a prime mover (turbine or engine) drives the rotor above the synchronous speed (negative slip). The stator flux still induces currents in the rotor, but since the opposing rotor flux is now cutting the stator coils, an active current is produced in stator coils and the motor now operates as a generator, sending power back to the electrical grid.
Excitation[edit]
An induction machine requires an externally-supplied armature current. Because the rotor field always lags behind the stator field, the induction machine always consumes reactive power, regardless of whether it is operating as a generator or a motor.
A source of excitation current for magnetizing flux (reactive power) for the stator is still required, to induce rotor current. This can be supplied from the electrical grid or, once it starts producing power, from the generator itself. The generating mode for induction motors is complicated by the need to excite the rotor, which begins with only residual magnetization. In some cases, that residual magnetization is enough to self-excite the motor under load. Therefore, it is necessary to either snap the motor and connect it momentarily to a live grid or to add capacitors charged initially by residual magnetism and providing the required reactive power during operation. Similar is the operation of the induction motor in parallel with a synchronous motor serving as a power factor compensator. A feature in the generator mode in parallel to the grid is that the rotor speed is higher than in the driving mode. Then active energy is being given to the grid.[1]Another disadvantage of induction motor generator is that it consumes a significant magnetizing current I0 = (20-35)%.
An induction machine can be started by charging the capacitors, with a DC source, while the generator is turning typically at or above generating speeds. Once the DC source is removed the capacitors will provide the magnetization current required to begin producing voltage.
An induction machine that has recently been operating may also spontaneously produce voltage and current due to residual magnetism left in the core .
Active power[edit]
Active power delivered to the line is proportional to slip above the synchronous speed. Full rated power of the generator is reached at very small slip values (motor dependent, typically 3%). At synchronous speed of 1800 rpm, generator will produce no power. When the driving speed is increased to 1860 rpm (typical example), full output power is produced. If the prime mover is unable to produce enough power to fully drive the generator, speed will remain somewhere between 1800 and 1860 rpm range.
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Required capacitance[edit]
A capacitor bank must supply reactive power to the motor when used in stand-alone mode. The reactive power supplied should be equal or greater than the reactive power that the machine normally draws when operating as a motor.
Torque vs. slip[edit]
The basic fundamental of induction generators is the conversion from mechanical energy to electrical energy. This requires an external torque applied to the rotor to turn it faster than the synchronous speed. However, indefinitely increasing torque doesn't lead to an indefinite increase in power generation. The rotating magnetic field torque excited from the armature works to counter the motion of the rotor and prevent over speed because of induced motion in the opposite direction. As the speed of the motor increases the counter torque reaches a max value of torque (breakdown torque) that it can operate until before the operating conditions become unstable. Ideally, induction generators work best in the stable region between the no-load condition and maximum torque region.
Rated current[edit]
The maximum power that can be produced by an induction motor operated as a generator is limited by the rated current of the machine's windings.
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Grid and stand-alone connections[edit]
In induction generators, the reactive power required to establish the air gap magnetic flux is provided by a capacitor bank connected to the machine in case of stand-alone system and in case of grid connection it draws reactive power from the grid to maintain its air gap flux. For a grid-connected system, frequency and voltage at the machine will be dictated by the electric grid, since it is very small compared to the whole system. For stand-alone systems, frequency and voltage are complex function of machine parameters, capacitance used for excitation, and load value and type.
Uses[edit]
Induction generators are often used in wind turbines and some micro hydro installations due to their ability to produce useful power at varying rotor speeds. Induction generators are mechanically and electrically simpler than other generator types. They are also more rugged, requiring no brushes or commutators.
Limitations[edit]
An induction generator connected to a capacitor system can generate sufficient reactive power to operate on its own. When the load current exceeds the capability of the generator to supply both magnetization reactive power and load power the generator will immediately cease to produce power. The load must be removed and the induction generator restarted with either a DC source, or if present, residual magnetism in the core.[2]
Induction generators are particularly suitable for wind generating stations as in this case speed is always a variable factor. Unlike synchronous motors, induction generators are load-dependent and cannot be used alone for grid frequency control.
Example application[edit]
As an example, consider the use of a 10 hp, 1760 r/min, 440 V, three-phase induction motor as an asynchronous generator. The full-load current of the motor is 10 A and the full-load power factor is 0.8.
Required capacitance per phase if capacitors are connected in delta:
- Apparent power S = √3 E I = 1.73 × 440 × 10 = 7612 VA
- Active power P = S cos θ = 7612 × 0.8 = 6090 W
- Reactive power Q = = 4567 VAR
For a machine to run as an asynchronous generator, capacitor bank must supply minimum 4567 / 3 phases = 1523 VAR per phase. Voltage per capacitor is 440 V because capacitors are connected in delta.
- Capacitive current Ic = Q/E = 1523/440 = 3.46 A
- Capacitive reactance per phase Xc = E/Ic = 127 Ω
Minimum capacitance per phase:
- C = 1 / (2*π*f*Xc) = 1 / (2 * 3.141 * 60 * 127) = 21 microfarads.
If the load also absorbs reactive power, capacitor bank must be increased in size to compensate.
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):
- if Ns = 1800, one can choose N=Ns+40 rpm
- Required prime mover speed N = 1800 + 40 = 1840 rpm.
See also[edit]
Notes[edit]
- ^Citation error. See inline comment how to fix.[verification needed]
- ^Huassain, Ashfaq. Electric Machines. Dhanpat Rai and Co. p. 411.
References[edit]
- Electrical Machines, Drives, and Power Systems, 4th edition, Theodore Wildi, Prentice Hall, ISBN0-13-082460-7, pages 311–314.
External links[edit]
This is a micro but stable implementation of an asynchronous 'Key-Value' cache grid with an option for persistence.In short if you have 5 instances running, 'Key-Value' pair added to one instance gets replicated automatically to other instances.
Please refer wiki for more information.
For sbt add the below in your build.sbt:
For Maven add the below in your pom.xml file:
Latest Snapshots can be downloaded from: https://oss.sonatype.org/content/repositories/snapshots/com/jatinpuri
External Jar can be downloaded from repo. You will manually have to download the dependencies. (Will update the part with link to all necessory links in future)
Current implementation is based on Akka Actor's and hence can it only be used using message communication with Actors. This constraint might be removed in future.
The current version system includes a primary node, which is responsible for replicating all changes to a set of secondary nodes where secondary nodes might join and leave at arbitrary times. Internally all the changes are persisted locally both by primary or secondary node. (Persistence is loosely coupled and can be done using any SQL or NOSQL based database or file for that matter).
Clients contacting the primary node directly can use all operations on the key-value store, while clients contacting the secondaries can only use lookups.
The two set of operations are:
Update Commands
Insert(key, value, id) - This message instructs the primary to insert the (key, value) pair into the storage and replicate it to the secondaries.
Remove(key, id) - This message instructs the primary to remove the key (and its corresponding value) from the storage and then remove it from the secondaries.
A successful Insert or Remove results in a reply to the client in the form of an OperationAck(id) message where the id field matches the corresponding id field of the operation that has been acknowledged.A failed Insert or Remove command results in an OperationFailed(id) reply. A failure is defined as the inability to confirm the operation within '1 second'. See the wiki for more details
Lookup
Get(key, id) - Instructs the replica to look up the 'current' (what current means is described in detail in the next section) value assigned with the key in the storage and reply with the stored value.
A Get operation results in a GetResult(key, valueOption, id) message where the id field matches the value in the id field of the corresponding Get message. The valueOption field contains None if the key is not present in the replica or Some(value) if a value is currently assigned to the given key in that replica.
System Behavior - Consistency Guarantees
Ordering is maintained.
If the following command is sent to the primary replica, waiting for successful acknowledgement of each operation before proceeding with the next:
- Ordering is guaranteed for clients contacting the primary replica:
A second client reading directly from the primary will not see:
- Ordering is guaranteed for clients contacting the secondary replica:
For a second client reading from one of the secondary replicas, the exact same requirements apply as if that client was reading from the primary, with the following addition:
It is guaranteed that a client reading from a secondary replica will eventually see the following (at some point in the future):
key1 containing bkey2 containing 2
- Ordering guarantees for clients contacting different replicas
If a second client asks different replicas for the same key, it may observe different values during the time window when an update is disseminated. The client asking for key1 might see:
Eventually all reads will result in the value b if no other updates are done on key1. Eventual consistency means that given enough time, all replicas settle on the same view.
Durability & Persistence
Whenever the primary replica receives an update operation (either Insert or Remove) it replies with an OperationAck(id) or OperationFailed(id) message, which is sent at most 1 second after the update command was processed.
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A positive OperationAck reply is sent as soon as the following is successful:
Persistence trait has been implemented using which the data can be persisted using any SQL or NOSQL based database or file for that matter.
Synchronous Generator Basics
See Wiki for more information.