Paralleling Generators for Large Power Plants

Methods of Generating Power

The United States and Canada share a combined power grid. This allows Canada to sell power to the US when demands are high. Multiple power plants supply power to the grid. There is more than one method to generate power to the utility grid. Below are some common methods are listed:

• Water & Wind Power Plants - Power created from renewable energy sources such as hydroelectric and wind farms. Hydroelectric is water powered from dams and streams.
• Fossil Fuel & Nuclear Power - Water converted into super-heated steam by coal, fuel or splitting of atoms. Super-heated steam is routed to steam turbines that alternators.
• Industrial Generators - Remote Provinces in Canada use banks of diesel generators to supply power to villages and towns. 

In each of the above methods, a prime mover (water, steam, industrial engine) drives an alternator. The combination of the prime mover and the alternator is considered to be a generator or generating unit. Every plant is constructed with many generating units.

The output voltage of a generator is controlled by changing rotor magnetic field strength by use of a voltage regulator.

Generators are placed in parallel with one another to accomplish grid load demands. This article will describe some basic principles of paralleling generators of large power plants. 

Hydroelectric Power Plants

Hydroelectric power plants are among the most widely power plants that use natural renewable resources. These power plants use pressurized water to turn generator turbines. They can be located on rivers or reservoir dams. Dams have intake tubes located towards the bottom of the structure that route water through a penstock. This fast-moving water turns the turbine. For more information on hydroelectric power creation go to Hydroelectric Power published by the United States Geological Survey (USGS).

Paralleling is placing an off-line generator onto the electrical grid with other operating generator(s). Automatic controls allow a generator to be placed on the grid or removed depending on power demands. The manual method requires an operator to establish parallel operations. In this method a synchro scope or a light flashing method can be used. All require sine wave, phase sequence, frequency, phase angle difference and voltage difference parameters to be met. Sine wave, phase angle and voltage are all specific internal alternator controls, while frequency is the speed of the rotor controlled by the amount of and pressure of the water to the turbine. If any of the parameters are not met during paralleling activity, the incoming generator will not synchronize and off going will complete its shutdown process. 

Fossil Fuel Power Plants

Coal or natural gas is used to heat water into steam. Steam contains water vapor. An additional process heats the steam above the saturation point, often called dry steam. The dry steam is then routed to steam turbine generators. If water vapor is contained in the steam it acts like a cutting torch to the turbine vanes. 

Turbine vanes turn the generator to produce electricity. The paralleling parameters are the same as with a hydroelectric plant. A centralized control room contains panels responsible for fuel supply to boilers and steam cycle.

The amount of steam that is routed to the turbines determines the speed of the rotor and frequency of the generator. Once all parameters are met the oncoming generator can be placed on line. Control panels feature both automated and manual generator paralleling functions.

Power Plants with Industrial Generators

Remote and off-grid communities in Canada are a good example of the use of diesel generators for primary power. The diesel generator is an extremely dependable source of power and widely used in areas that do not have fossil fuel, nuclear, and hydroelectric plant capabilities. The Canadian Government conducted the Status of Remote/Off-Grid Communities in Canada and created a publication in August of 2011. The study is proof positive of the reliability and use of diesel generators as primary power source.

Multiple diesel generators are operated in parallel to provide power. Paralleling an industrial diesel generator shares the same electrical parameters of all of the other methods of creating power. The diesel engine is the prime mover connected to alternator that creates the electricity. The engine and alternator communicate with one another to reach generator operating parameters. When paralleling an oncoming generator, engine speed (generator frequency) is controlled by the engine Electronic Control Module (ECM). Main controlling systems interface the generators operating in parallel. For more information go to Paralleling Industrial Generators.

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Nuclear Power Plants

These power plants require a fuel source to operate. Some argue that nuclear is so clean that it should be classified as a renewable source. However, the International Renewable Energy Agency (IRENA) will not support the move because of long complicated process, risks, and nuclear waste management. For more information on debate go Is Nuclear Energy Renewable Energy? published by Navid Chowdhury, Stanford University.

The basic principles of nuclear power creation are:

• Water circulates through a nuclear core reaching temperatures of over 500 degrees (super-heated steam). Water is heated by the fission process
  • The temperature of the water is maintained by inserting control rods into the bottom of the fuel core
  • Reactor vessel is where the super-heated steam is created
• Super-heated steam is routed to the generator steam turbine. 

The above list is a simplified look at the generation of steam. Nuclear power plants are closely regulated for operation and maintenance of the systems. Steam turbine generators supply power the same way as fossil fuel power plants. The creation of the steam is the big difference. 

While the concept of paralleling remains the same as all of the other methods, the process is a world apart. Control systems could be divided into the three categories below:

1. Creation of Steam - Maintaining proper steam temperatures with advanced control panels
2. Distribution of Steam - Amount of steam routed to turbines for generator speed (frequency) control
3. Electrical Distribution - All distribution and controls panels and associated equipment

Operating the generators in parallel require multiple systems interface. The speed of the turbine is controlled by the amount of steam that is supplied to it. When placing a generator on-line, the equipment and auxiliary systems supplying steam to the turbine communicate constantly. The paralleling operation of generators and supplying the proper amount of power to the grid requires constant monitoring and maintenance of off-line systems. Each plant has established operating procedures to adhere to regulations.





 
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