Asynchronous Generators Connected to the Grid
Asynchronous generators, also known as induction generators, are becoming increasingly popular in various renewable energy applications, particularly in small-scale hydroelectric and wind power installations. Their robust construction, ease of maintenance, and ability to operate without a separate DC excitation system make them an attractive option. This article delves into the basic operation and key design principles and considerations for asynchronous generators connected to the grid.
- Basic Operation
An asynchronous generator operates by converting mechanical power from a prime mover, such as a turbine, into electrical power. To generate electricity, the prime mover must drive the generator at a speed higher than its synchronous speed, creating the necessary slip. Unlike synchronous generators, asynchronous generators do not require a separate excitation system; they draw the necessary reactive magnetizing current from the grid or from a capacitor bank in standalone applications.
The rotor, typically a squirrel-cage design. This type is robust, reliable, and relatively inexpensive compared to synchronous generators.
- Equivalent Circuit
The equivalent circuit of an induction generator is similar to that of an induction motor, comprising resistances and reactances representing the stator and rotor windings, core losses, and magnetizing inductance.
The need for the equivalent circuit of an induction generator is not as much in application as for an induction motor. Namely, it is often necessary to provide reactances similar to those for a synchronous generator, such as subtransient and direct axis reactances, which determine the impact on the grid and possible short-circuit currents.
- Saturation and Core Losses
Magnetic saturation of the core is a crucial factor in the design of induction generators. The design must ensure that the generator operates below the saturation point under normal conditions. Saturation becomes especially critical when an induction motor is converted to operate as an induction generator. Since the generator operates with higher induced voltages (air gap voltage), the risk of saturation is greater than in motor operation. Therefore, initial design considerations often involve setting nominal voltages slightly higher to avoid saturation issues.
- Efficiency and Losses
Efficiency in an induction generator is influenced by various losses, including copper losses in the stator and rotor windings, core losses due to hysteresis and eddy currents, friction and windage losses, and stray load losses. Design strategies to minimize these losses include using high-quality core materials with low hysteresis loss, optimizing the winding design to reduce resistance, and ensuring proper ventilation to manage temperature rise and reduce friction losses.
The generator is designed to achieve maximum efficiency at nominal power, but efficiency significantly drops at reduced power levels. In an induction generator connected to the grid, motor start-up curves and torque, or start-up current are not critical. Thus, the rotor winding is dimensioned primarily for maximum efficiency relative to the previously mentioned parameters.
- Torque and Slip
The performance of an induction generator is characterized by its torque-slip relationship. As the rotor speed increases beyond the synchronous speed, the slip becomes negative, and the machine operates as a generator. In this case, mechanical energy is transferred from the prime mover to the electrical system. Designing the torque-slip characteristics is crucial for the efficient handling of mechanical power input and stable operation. The generator should have high pull-out torque (breakdown torque) to ensure stability under varying load conditions. As the rotational speed increases, so does the slip and torque from nominal torque to maximum torque (pull-out). With further increase in speed, the machine falls out of synchronism with the grid. In this case, the machine goes into overspeed, creating a demanding situation where speed can be 2x nominal speed (sometimes more). At maximum power, slip can usually be up to 1% or 3%. A more efficient machine has less slip.
Asynchronous generators have limited overload capacity compared to synchronous generators, making precise design even more essential.
- Overspeed
As mentioned earlier, an asynchronous generator must mechanically satisfy overspeed requirements. For such higher speeds, bearings and the shaft need to be designed accordingly. The critical speed for shaft bending should be at least 30% away from the overspeed.
- Voltage Control
In parallel operation with the grid, voltage control is not as critical since this parameter is regulated by the grid. However, for standalone systems, additional components for voltage control are required, such as automatic voltage regulators (AVR).
- Short Circuit and Power Factor Performance
The X/R ratio will determine the decay rate of the fault current contributed by the induction generator. When a fault occurs on the grid, the generator cannot supply the fault location with the sustained short circuit as it does with a synchronous generator. The power factor is generally worse for an asynchronous generator, especially with multipole machines compared to a synchronous generator, and the power factor cannot be regulated by excitation current as it is with a synchronous generator.
The power factor cannot be regulated by excitation current as it is with a synchronous generator.
- Induction Motor Used as Asynchronous Generator
A standard induction motor is often chosen as an asynchronous generator. This choice significantly reduces the initial investment and maintenance costs. However, some limitations need to be considered. One limitation is the ability to mechanically satisfy overspeed. Another limitation is the risk of machine saturation, as previously mentioned.
Conclusion
Designing an asynchronous generator connected to the grid involves a comprehensive understanding of both electrical and mechanical principles. Key considerations include managing core saturation, minimizing losses, and ensuring high pull-out torque for stability under varying loads. By adhering to these principles, engineers can design efficient and reliable induction generators suitable for various renewable energy applications. Asynchronous generators offer many advantages, such as simpler control and lower maintenance requirements compared to synchronous generators, making them an excellent choice for renewable energy systems.
Electrical machines
August 28, 2024
