Advantages of Operating Generators in Parallel

Advantages of Operating Generators in Parallel

• Several generators can supply a bigger load than one machine by itself.
• Having many generators increases the reliability of the power system, as the failure of one of them does not cause a total power loss to the load.
• Having many generators operating in parallel allows one or more of them to be removed for shutdown and preventive maintenance.
• Operating multiple smaller generators in parallel allows for more efficient operation, as only the required number of generators need to operate near full load.

Conditions Required for Paralleling

• The rms line voltages of the two generators must be equal.
• The two generators must have the same phase sequence.
• The phase angles of the two a phases must be equal.
• The frequency of the oncoming generator must be slightly higher than the frequency of the running system.

Frequency-Power and Voltage-Reactive Power Characteristics of a Synchronous Generator

• Prime movers tend to behave similarly, with speed decreasing as power drawn from them increases.
• Governor mechanisms are used to make the decrease in speed linear with an increase in power demand.
• The speed droop (SD) of a prime mover is defined by the equation SD = (n nl - n fl) / n nl.

Synchronous Generator Types

• Salient-pole synchronous generators have a non-uniform air-gap and are typically used for hydroelectric power generation.
• Cylindrical-rotor synchronous generators have a uniform air-gap and are typically used for turbogeneration.

Operation Principle

• The rotor of the generator is driven by a prime-mover.
• A dc current flowing in the rotor winding produces a rotating magnetic field within the machine.
• The rotating magnetic field induces a three-phase voltage in the stator winding of the generator.

Electrical Frequency

• The electrical frequency produced is locked or synchronized to the mechanical speed of rotation of a synchronous generator: fe = (120 * P) / nm.

Parallel Operation of AC Generators

• Synchronous generators are operated in parallel to provide a larger total power output and to improve system reliability.
• The relationship between frequency and power can be described quantitatively by the equation.
• The relationship between reactive power Q and terminal voltage VT can be derived similarly.

Operation of Generators in Parallel with Large Power Systems

• An infinite bus is a power system so large that its voltage and frequency do not vary regardless of how much real and reactive power is drawn from or supplied to it.
• The power frequency characteristic of such a system is shown in Figure.
• When a generator is connected to an infinite bus in parallel, the generator's prime mover and field controls determine the operating frequency and terminal voltage.