◊ This is part of the ‘Electricity Generation’ series of articles ◊
The technology which underpins electricity generation has been evolving for over a century. Beginning with the Faraday coil and progressing to semiconductors and photovoltaic cells, they have all contributed to modern energy production.
Every generator utilizes a combination of technologies to produce their energy output. This article looks at the building blocks which can be assembled in various combinations to produce electricity.
Some history
Early electricity generation in the late 18th century was from batteries. A battery produces dc from a galvanic chemical reaction. The discovery of the effect of a moving magnetic field on a coil of wire was credited to Faraday in 1831. The Faraday coil as it was called, was the start of the ac generation revolution. In 1866, a German scientist named Werner Von Siemens produced the first self-excited generator which was called a Dynamo. Nikola Tesla invented a multi-phase AC generator in 1887 which facilitated electricity production on a large scale. Rotating machines provide most of the electricity generation in the world today.
The semiconductor era
The refinement in power semiconductors, including the invention of the thyristor in 1950 by William Shockley, provided a new way for energy sources to couple to the grid. A thyristor is also called a silicon controlled rectifier (SCR). The SCR led to the development of inverters which convert a dc signal to ac. Decades later it became economical to use these semiconductors on a commercial scale to connect dc systems with the ac power grid. Today, inverter-based equipment is used to couple solar PV to the grid on a large scale. Inverters also facilitate the coupling of a variable speed wind turbine to the fixed frequency ac power grid.
Rotating machines
Rotating machines provide the majority of energy supply to the grid. A rotating machine utilizes a prime mover (turbine or reciprocating engine) to spin an electricity generator. The combination of prime mover and generator may be on a vertical or horizontal axis depending on the implementation.
Turbines may be driven by water, wind, fossil-fuel internal combustion or a thermodynamic cycle (i.e. steam).
The most common reciprocating engines may use fossil or biofuel for internal combustion. A Stirling heat engine has also been used to drive a generator in solar dish applications.
Rotating generators are either synchronous or induction. Induction generators are asynchronous (also termed non-synchronous).
Synchronous and Asynchronous (non-synchronous)
Synchronous generators operate at the grid frequency while asynchronous generators operate at a frequency determined by their design characteristics. Asynchronous or non-synchronous generators do not operate at the same frequency as the grid, or couple to the grid via inverter-based equipment.
Turbines
A turbine is a rotating mechanism designed to extract energy from a medium. The medium may be water, steam, organic fluid, wind or a combustion gas. A turbine is not an electricity generator. It provides the energy to drive a generator which produces electricity. The speed of a turbine in synchronous applications is regulated by a governor system which maintains constant speed when the load changes.
Thermodynamic cycles – Rankine and Brayton
There is an extensive variety of turbines in use to drive generators, however, the most common implementations are based on thermodynamic processes.
The Rankine cycle
1 – Pump
2 – Boiler
3 – Turbine
4 – Condenser
Q is heat transfer
W is work
Figure 2 – The Rankine cycle process diagram – Image credit: Andrew Ainsworth, Wikimedia, under the Creative Commons Attribution-Share Alike 3.0 Unported license. Modified by author.
The Rankine cycle forms the basis for many different types of generation including fossil fuel, geothermal and solar thermal power plants. A variation of the Rankine cycle is used for binary cycle geothermal power plants which use an organic fluid for heat transfer instead of steam. The binary cycle is also referred to as an Organic Rankine cycle.
The Brayton cycle
The Brayton cycle in the context of electricity generation, is a thermodynamic cycle referring to the operation of a combustion turbine. The Brayton-type engine consists of an air compressor, a combustion chamber and a gas turbine. The Brayton cycle is also referred to as a simple cycle.
Hydro turbines
Hydro turbines have been used for over a century and may be of many different kinds:
- Bulb
- Cross-flow
- Impulse
- Pelton
- Reaction
- Propeller (including Kaplan-type)
- Francis
- Kinetic
- Straflo
- Tube
Additional information on hydropower turbines is available from the US Department of Energy.
Wind turbines
Wind turbines have two main types based on the axis of orientation.
- Horizontal axis
- propeller
Wind turbine speed regulation can be achieved through blade aerodynamics (stall regulation) or by controlling blade pitch (pitch regulation).
Stall regulation utilizes a blade design that causes them to ‘stall’ at higher wind velocities, reducing rotational speed and torque. It is a simple and reliable solution to manage turbine speed in high wind conditions.
Pitch regulation requires a special active control system and blade mechanism that can alter the blade angle according to wind conditions. Pitch control can maximize the turbine efficiency and prevent over speed in high wind conditions.
Additional information is available from my series on wind generation and the U.S. Energy Information Administration.
Internal combustion engine prime movers
A diesel, biofuel, gasoline, propane, natural gas or oil reciprocating internal combustion engine may also be used to provide the energy to drive a generator. Gasoline engines are used for small portable generators from a few kW capacity up to 20 kW. Natural gas and propane reciprocating engines are readily available in sizes up to 22 kW. Diesel engines are used for larger applications up to approximately 2.5 MVA.
Reciprocating internal combustion generators are primarily used for emergency auxiliary power and remote communities in grid applications.
A combustion turbine is an engine which converts fossil fuel (natural gas, oil) energy to rotation through internal combustion. Combustion turbine generating facilities have capacities in the hundreds of megawatts.
Steam turbines
Steam turbines types include:
- backpressure
- condensing
- extraction
- impulse
- reaction
- reheat
Additional reading: ‘Exploring Types of Steam Turbines’, Allied Power Group
Generators
Synchronous generators
A synchronous generator is a rotating machine that is designed to operate at a single speed and provide a sinusoidal output at the system frequency. It operates synchronized to the grid to which it is connected. Synchronous generators produce the majority of grid energy supply and play a critical role in ensuring reliability. They provide voltage and frequency support as well as much needed inertia to manage transient disturbances.
A synchronous generator uses a rotating magnetic field coupled to a stationary winding to produce a voltage on the stationary winding. The rotating magnetic field is produced by an armature (rotor) with copper windings fed by a dc current through slip rings mounted to its shaft. The rotor windings wrap around laminated iron cores which function as electromagnets. The rotor’s magnetic poles are coupled through an air gap to the stationary coils of the stator. The output voltage, frequency and power is determined by the number of poles in the generator, the rotational speed and the flux density produced by the rotor. The dc current for the rotor is provided by an excitation system which typically requires a separate power source.
In 1920. the Queenston Generating Station (now called Sir Adam Beck #1) synchronous hydroelectric generator units at Niagara Falls Ontario were purchased as 25 Hz 16 pole (8 pairs) with a rated output of 45 MVA at 12 kV and 80% power factor. They ran at 187.5 rpm.
Ref HEPCO 1920 Annual Report.
A synchronous generator may also use permanent magnets for the rotor, however this compromises its ability to regulate the voltage output. Brushless designs are possible for synchronous generators to eliminate slip rings with the trade-off being complexity and cost.
Induction generators
An induction generator is a rotating machine that relies on a stationary winding (stator) to induce current in an armature (rotor). Unlike a synchronous generator’s rotor, the induction generator is self-excited without any separate power supply or slip rings. The rotor must have a higher speed than the system it is coupled to in order to generate power. Since the induction generator operates at a speed higher than the system, it is considered an asynchronous generator.
The induction generator requires reactive power (VARs) from the system in order to generate electricity. Without a connection to a system capable of supplying reactive power, it will be unable to generate real power.
Synchronous vs induction generators
What is the difference between synchronous and induction generators?
A synchronous generator operates at a fixed speed synchronized with the power grid, while an induction generator operates at a speed higher than the synchronous speed. The induction generator’s speed is not directly locked to the grid frequency. The speed difference is due to the way each generator creates its magnetic field. Synchronous generators require a separate DC excitation system for the rotor. Induction generator rotors rely on the magnetic field induced from the power grid itself.
| Parameter | Synchronous Generator | Induction Generator |
| Speed Control | Operates at a constant synchronous speed, meaning the output frequency is directly related to the rotor speed | Must rotate faster than the synchronous speed to generate electricity |
| Excitation | Requires a separate DC excitation current to create the magnetic field on the rotor, usually supplied through slip rings and brushes | No separate excitation system, relies on the magnetic field induced from the power grid |
| Power Factor Control | Can actively control its power factor by adjusting the excitation current | Typically draws reactive power from the grid, leading to a lagging power factor |
| Applications | Large power plants, applications needing precise speed control and power factor regulation | Wind turbines, situations where simple construction and low initial cost are priorities |
Direct current (dc) generators
A dc generator is a rotating machine consisting of an armature with coils and either fixed permanent magnets or a stator winding to produce the magnetic field. The output is direct current produced by using mechanical switching or commutation. The commutation can be provided though stationary carbon brushes that contact the generators rotating armature. The rotating armature coil has isolated contacts that rotate past fixed brushes to switch the armature windings and maintain direct current output. There are various configurations used for dc generators including permanent magnet and dc current excitation. The dc generator requires relatively high maintenance due to the implementation of brushes on the rotating armature and are not used often in grid applications. The dc generator requires fully rated inverters to couple to the ac grid.
Inverter-based resources (IBRs)
Inverter based resources couple to the grid using switched power semiconductors. The energy may be from entirely stationary equipment (solar PV) or a combination of stationary and rotating machines (wind turbine).
Combined technology generators: inverter-based and doubly-fed resources
There are generators (i.e. wind) that cannot run at a single speed and yet connect to the ac grid. Power semiconductors can be used along with innovative generator design to provide a solution.
Variable speed wind turbines can use doubly fed generators to couple to the grid. Doubly fed generators use a stationary winding that is grid connected, and a rotating winding which is fed a variable frequency ac current through slip rings. These generators are called Doubly Fed Asynchronous Generators (DFAG – type 3). They may also be referred to as Doubly Fed Induction Generators (DFIG). An example of this technology is the GE-Verona 3MW wind turbine which is a DFIG design.
Inverter based equipment can be used to take the variable speed output of a generator, rectify it and convert it to the fixed grid frequency. This solution is referred to as type 4 – variable speed turbine with fully rated power converter.
Wind generator types are described in the Generation by primary energy source article.
That’s a high level view of the different technologies used to generate electricity on the modern grid. These technologies are used in various combinations to construct generating facilities that produce power for the grid.
previous article > Advanced concepts of grid supply
next article > Generation by primary energy source
Derek
