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Chapter 4 Evaluation of Combined Heat and Power Technologies for Wastewater Treatment Facilities 4.3 Electrical Considerations Most CHP applications located at POTWs are forms of distributed generation that operate in parallel with the electric utility. Electric power generation systems are generally one of four types: 1. Alternating current synchronous generators 2. Alternating current induction generators 3. Direct current generators 4. Direct current devices with an inverter to produce alternating current In power generation, an electrical generator is an energy conversion device or machine that transforms rotational mechanical energy to electrical energy generally using electromagnetic force. Most generators produce alternating current (AC) and are either synchronous or induction-type generators. The following paragraphs describe some of the characteristics of AC generators. Synchronous generators: Synchronous AC generators are the most common type of electricity-producing machines. Synchronous AC generators independently control speed and voltage and must therefore be carefully synchronized with each other and the utility grid when operated in parallel. Synchronous AC generators are extensively used for power production and are available in unit sizes from less than 1 kW to over 500,000 kW. Induction generators: Induction AC generators are common in sizes smaller than about 200 kW. Unlike synchronous generators, induction generators require external excitation, usually from the electric utility grid to which they are connected, and thus they are not suitable for grid-isolated, stand-alone duty, or for standby power duty. Induction generators are similar to induction motors and are available in similar sizes. Table 4-1 summarizes some of the differences between induction and synchronous generators. Table 4-1. Comparison of Induction and Synchronous Generators Item or Issue Initial capital cost Electrical generation efficiency Operation in stand alone or island mode Excitation Power factor (PF) Overall complexity Possibility of energizing electrical utility’s incoming dead lines The prime mover’s starting system Electric utility interconnection Operational cost savings Induction Generator Slightly less, if smaller than about 500 kW Up to about 97% Must operate in parallel with the electric utility Provided by the electric utility grid Decreases PF Simplest Normally not an issues since induction machines require utility power excitation Can act as a conventional motor to start the prime mover Simplified Typically lower due to less efficient operation and PF penalty Synchronous Generator Slightly less, if larger than about 500 kW Up to about 98% Can operate in parallel with the utility or can stand alone. Can operate as a standby generator Requires its own excitation Can improve PF, if operated appropriately Slightly more complex Requires interconnection protection from energizing dead utility source lines Prime mover is typically started by a compressed air motor or by a battery-powered electric starter motor Required and is more complex Typically higher due to more efficient operation and lack of PF penalty 4-9PDF Image | Combined Heat and Power Technologies for Wastewater Facilities
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