Synchronous Generator

A synchronous generator is a fundamental electrical machine that converts mechanical energy into alternating current (AC) electricity through the principle of electromagnetic induction, characterized by its ability to operate at a constant speed or "synchronously" with the frequency of the electrical grid. Synchronous generators are widely used in power plants—from fossil fuel to nuclear and hydroelectric facilities—due to their efficiency, reliability, and ability to contribute to grid stability by supporting voltage levels and providing reactive power.

A synchronous generator operates at a constant speed, in sync with the line frequency. This synchrony means that the rotor's speed is identical to the rotating magnetic field generated by the stator’s AC winding. The relationship between the synchronous speed (ns), the frequency (f), the number of pole pairs (p), and the rotor's mechanical speed (n) is described by the equation:

More generally, synchronous machines can operate in various modes, such as no-load, motoring, or generating.

A synchronous condenser operates in no-load mode, functioning not to convert energy but to manage the electric power grid conditions. Its roles include voltage regulation by controlling reactive power, power factor correction through field excitation adjustments, and enhancing grid stability by contributing inertial mass for frequency regulation.

Synchronous motors are widely used across various industries to maintain a constant speed regardless of load fluctuations and for high efficiency at low speeds. Key applications include driving pumps, compressors, fans, blowers, and conveyors where precise speed control is critical, as well as in rolling mills, paper and textile mills, and marine propulsion due to their high starting torque and steady operation.

History

In the 1830s, Michael Faraday discovered the principles of electromagnetic induction. He demonstrated that a changing magnetic field induces an electric current in a conductor.

In the 1860s, the first practical generators that converted AC into DC using a commutator appeared.

In the 1870s-80s, Alternating Current (AC) systems were developed, including generators, transformers, and motors. The first three-phase synchronous generators went into operation during this time.

In the 1890-1900s, synchronous generators replaced DC dynamos in large power plants, becoming the main form of power generation.

In the 1910s-1920s, feedback control systems led to the first Automatic Voltage Regulators (AVRs), which adjusted the excitation of a generator to maintain a constant voltage level despite changes in load or other system conditions. Mathematical models of synchronous generators were also developed during this time.

In the 1920s-1930s, with synchronous generators largely replacing all early forms of power generation, utilities started to interconnect many generators on the regional power system into larger power grids. This was only possible due to the synchronous generator’s ability to produce constant frequency and voltage, allowing for the sharing of power resources over larger geographical areas.

In the 1940s-1960s, improvements in materials and better cooling systems, such as hydrogen cooling, allowed larger and more efficient generators to be built. Power system analysis also became more critical to understanding the behavior of the connected grid.

In the 1970s-2000s, the introduction of digital control systems allowed more precise control and monitoring of synchronous generators. The introduction of tools like finite element analysis (FEA) allowed for a more sophisticated design of magnetic circuits within the generator.

Electrical Phases

In a synchronous generator, the electrical phases refer to the distinct sets of windings or circuits in the stator, where each set generates an AC voltage out of phase with the others. The most common configuration in synchronous generators is a three-phase system with three separate windings spaced evenly around the stator.

Each phase in a three-phase system is typically labeled as Phase A, B, and C. These phases are electrically spaced 120 degrees apart from each other. This spatial and electrical separation ensures that the AC in each phase reaches its maximum value at different times. The three-phase system is the standard in power generation and distribution because it provides a more efficient and stable power supply than single-phase systems. It also allows for simpler and more economical motor designs and enables more power transmission over longer distances with less material.

A three-phase system facilitates the smooth conversion of mechanical energy (from turbines or other prime movers) into electrical energy by ensuring a constant power transfer. Since each phase reaches its peak voltage at one-third of a cycle apart from the others, one of the three phases is nearing its peak at any given time. As a result, the combined power output of all three phases tends to be more constant and smoother over time compared to a single-phase system, where power delivery is more pulsating due to its reaching zero twice in each cycle. In a perfectly balanced system, the sum of the instantaneous powers in a three-phase system is constant.

Types of Synchronous Generators

Turboalternators (Turbogenerators): These are large-capacity generators driven by steam turbines, typically operating at high speeds ranging from 1500 to 3600 rpm. They are employed in various power generation settings, including conventional thermal power stations and nuclear power stations. Turboalternators can deliver immense power, with ratings up to 1300 MVA for two-pole configurations and up to 1700 MVA for four-pole configurations found in nuclear power stations. Their high-speed operation and substantial power output make them crucial in large-scale electricity generation.

generator for large power plants (Siemens Energy)

Hydroalternators: These generators are driven by hydropower turbines, harnessing energy from flowing water. Depending on the specific hydropower application and turbine design, they operate at a broader range of speeds, from as low as 60 rpm to as high as 1000 rpm.

Hover Dam Generators

Combustion Engine-Driven Generators: These generators are powered by various internal combustion engines, with Diesel engines being the most common. They can operate at speeds up to 3600 rpm and achieve power ratings up to 200 MVA. These generators offer versatility and reliability, especially when a stable grid connection is unavailable.

Industrial Diesel Generator (Generac)

Gas Turbine Driven-Generators: These generators are powered by gas turbines. Smaller gas turbines may operate most efficiently at speeds higher than the synchronous speed of the generator. A gearbox matches the high rotational speed of the gas turbine to the stable speed required for the synchronous generator. This arrangement ensures efficient operation of both the turbine and the generator. Large power plant gas turbines are often designed to operate at the synchronous speed of the generator to remove the gearbox, improving both efficiency and reliability. They can deliver power ratings up to ~500 MW for simple cycle operation and ~880 MW for combined cycle operation. Gas turbine generators are known for their rapid start-up capabilities and are often used to supply rapidly changing loads.

Hydrogen-cooled generator (GE Gas Power)

Microturbines: These are compact generators integrated with gas turbines, with typical sizes ranging from 30kW to 600kW, depending on the application. Large systems can be built by paralleling smaller microturbines. They operate at very high speeds, reaching up to 120,000 rpm. Microturbines are utilized for distributed power generation and are valued for their compact size, efficiency, and ability to operate on various fuels.

Microturbine Illustration (Capstone)

Wind Turbine Generators: Driven by the wind's kinetic energy, these generators operate at very low speeds, ranging from 12 to 30 rpm. The latest offshore wind turbine can exceed 10 MW. Since the speed of the rotor varies and is determined only by the wind power, the generator output is usually connected to power electronic circuits that resolve the frequency mismatch between the generator output and the power system.

Components of a Synchronous Generator

Generator Example (Siemens Energy)

Stator

In operation, the rotor's magnetic field induces an alternating current in the stator windings due to Faraday's law of electromagnetic induction. The armature (stator) windings are subjected to this time-varying magnetic field, generating voltages across the windings. Because of the rotor's rotation and the stator windings' distribution, the winding voltages reach their peak values at different times, producing a three-phase AC output.

Stator Core: The core is generally made up of laminated, high-grade silicon steel. Silicon steel's high magnetic permeability helps reduce hysteresis losses while increasing steel's electrical resistivity, which reduces eddy current losses. The thin lamination between silicon steel sheets further increases electrical resistance, which minimizes the flow of eddy currents.

Armature Windings: These are the conductors where the output voltage is generated. Synchronous generators usually have three sets of windings corresponding to the three phases of AC power. They are evenly distributed in slots over the inner periphery of the stator core. Each slot contains segments of all three phase windings - Phase A, B, and C. Depending on the generator's design, these three windings are typically arranged in either a "Y" or "Delta" configuration.

Insulation: Insulation is used around the windings to prevent short circuits with the core and other phase windings.

Rotor

The rotor is the rotating part of the generator, which produces the rotating magnetic field. The rotor is typically an electromagnet, but in some cases, it may be a permanent magnet. There are two primary rotor types in wound-field synchronous machines: the non-salient pole (or cylindrical) rotor and the salient-pole rotor. The former is typically a solid steel two-pole or four-pole rotor. The latter features a laminated pole core and shoes, with an additional damper winding near the surface of the pole shoes. In synchronous generators, this damper winding helps to dampen oscillations and counteract the backward component of the armature's magnetic field, ensuring stable operation. Nearly all synchronous generators driven by steam turbines are non-salient pole machines. In contrast, hydro generators that operate at low speeds are usually salient-pole.

•   Field Windings: These are the windings where a DC current is supplied to produce a magnetic field. The field windings are usually made up of copper or some other highly conductive material. The strength of the magnetic field generated by the rotor is proportional to the amount of current flowing through the field windings.
• Pole Cores: These are made of laminated silicon steel and provide a path for magnetic flux. Depending on the design, the rotor can have a different number of poles - standard configurations are 2-pole or 4-pole, but it could have more.
• Slip Rings: These are rings found in older generators that connect to the rotor field windings and rotate along the rotor. The DC current is supplied to the rotor through slip rings and brushes. In newer generators, brushless or static excitation systems have replaced slip rings and brushes.
• Cooling System: The rotor also usually includes a cooling system, as it can get quite hot from the current in the field windings and mechanical losses.
• Damping Windings or Amortisseur Windings: These are additional windings embedded in the rotor pole faces. These windings aren't connected to any power source but are shorted on themselves. They provide stability to the generator during transient conditions.
• Flywheel effect (Rotor Inertia): The rotor's mass provides a flywheel effect, storing kinetic energy. This stored energy helps the generator maintain a stable frequency in the short term during rapid load changes.

Excitation System

One major issue associated with synchronous generator design is how to generate the rotor DC, which controls the strength of the rotor magnetic field. Older synchronous generators used mechanical brushes to maintain an electrical connection between the rotor shaft and an external source that provided DC power. The brushes rubbed against a set of metal slip rings on the rotor shaft to maintain electrical contact. While this design was straightforward, the high-speed rotation can wear out the brushes, requiring regular maintenance and replacement. Additionally, the friction between the brushes and slip rings causes losses and can lead to sparking, which may harm the generator.

To overcome these issues, newer large synchronous generators are usually designed with brushless excitation systems. They have a set of permanent magnets evenly distributed on the rotor shaft that spins with the rotor, called the pilot exciter. This permanent magnet design eliminates the need for mechanical brushes. Instead, this smaller magnetic field from permanent magnets induces an AC voltage on a set of stator coils, which power up a regulator circuit. The regulator circuit then uses this power to generate a larger magnetic field in the stator. This second controllable magnetic field induces AC voltages onto another set of coils mounted on the rotor. These AC voltages are then rectified with semiconductor diodes to generate the main rotor DC current and the associated magnetic field of the generator.

The brushless excitation system has a couple of key advantages. It eliminates the need for regular maintenance and replacement of brushes and removes the issue of sparking and power losses associated with brushes. However, the design and control of these systems are more complex than the traditional brush and slip ring method, which translates into higher costs. Controlling the field current in a brushless excitation system requires electronic control and rectification systems, which can potentially fail. Since the pilot exciter is mounted on the rotor, it is subjected to high centrifugal forces and thermal stresses, which can lead to mechanical failures or degradation over time.

Static Excitation System

Another variation to the brushless excitation system is a static excitation system where the permanent magnets on the rotor are removed to simplify the design and improve reliability. A static excitation system typically requires an external power source, such as the power grid, to initiate the excitation process. Once the generator reaches steady state grid-tied operation, the excitation is then maintained by the generator's output. Static excitation systems typically use power electronic switches to speed up rotor excitation current adjustments.

Permanent-Magnet Generators

In smaller synchronous generators, where the rotor magnetic field is 1.4 Telsa or less, permanent magnets may provide the main rotor magnetic field. Permanent Magnet Synchronous Machines (PMSMs) are increasingly used in various generation and motor applications due to their high efficiency, high power density, and reliable performance. They are highly reliable due to the absence of brushes and complex excitation systems of large generators.