Parallel Operation of Synchronous Generators Based on Sections 5.9 of the Textbook

Generators being paralleled with a running power system G1 G2 Gn Load

Why are synchronous generators operated in parallel? Many generators can supply a bigger load than one machine by itself. Having many generators increases the reliability of the power system, since failure of any 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. If only one generator is used and it is not operating at near full load, then it will be relatively inefficient. But with many machines it is possible to operate only a fraction of them. The ones that do operate near full load and therefore more efficiently.

What are the conditions 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 new generators, called the oncoming generator, must be slightly higher than the frequency of the running system.

The three-light-bulb method for checking phase sequence. G1 G2 Gn Load

Frequency-Power and Voltage-Reactive Power Characteristics Prime movers: Steam turbine, diesel turbine, wind turbine, water turbine, and gas turbine. As the power drawn from prime movers increases, the speed at which they turn decreases. The decrease is usually nonlinear. Some form of governor mechanism is usually included to make the decrease in speed linear with an increase in power demand. Whatever governor mechanism is present on a prime mover, it will always be adjusted to provide a slight drooping characteristics with increasing load. The speed droop (SD) of a prime mover is defined by the equation:

r/min Hz Power KWPower kW n nl n fl n nl n fl P fl

The relation between frequency and power

Operation of generators in parallel with large power systems fefe VTVT Power P, kW (supplied) Power Q, kVAR (supplied)

The concept if infinite bus An infinite bus is a power system so large that its voltage and frequency do not vary regardless of how much real and reactive is drawn from or supplied to it. The power frequency characteristic of such a system is shown in the previous figures. f nl fefe PGPG P inf bus P load

Operation of generators in parallel with other generators of the same size The power house fefe 60 Hz P G1 P G2

In the case of two generators are operating together The system is constrained in that the total power supplied by the two generators together must equal the amount consumed by the load. To adjust the real power sharing between generators without changing f sys, simultaneously increase the governor set points on one generator while decreasing the governor set points on the other. The machine whose governor set point was increased will assume more of the load. To adjust f sys without changing the real power sharing, simultaneously increase or decrease both generator’s governor set points. To adjust the reactive power sharing between generators without changing V T, simultaneously increase the field current on one generator while decreasing the field current on the other. The machine whose field current was increased will assume more of the load. To adjust V T without changing the reactive power sharing, simultaneously increase or decrease both generator’s field currents.

Important! from the textbook See Figure 5-38 Solve Example 5-5 Solve Example 5-6

EfEf I V = fixed in magnitude, phase, and frequency Infinite bus Per-phase model of a synchronous generator operating into an infinite bus. Unity power factor Underexcited Overexcited V EfEf I Re Im