Parallel Operationof Synchronous Generators

Based on Sections 5.9of 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:

.speedmover prime load full theis speedmover prime loadno theis

%100-

SD

fl

nl

lf

lfnl

nn

nnn

r/min Hz

Power KW Power kW

nnl

nfl

nnl

nfl

Pfl Pfl

The relation between frequency and power

MW/Hz.or kW/Hzin curve, of slope

system offrequency operating generator theoffrequency load-no

generator theofoutput power

-

sys

nl

sysnl

P

P

S

ffP

ffSP

Operation of generators in parallel with large power systems

fe VT

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.

fnl

fe

PGPinf bus

Pload

Operation of generators in parallel with other generators of the same size

The power house

21 GGloadtot PPPP

fe

60 Hz

PG1 PG2

1Generator 2Generator

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 fsys, 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 fsys 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 VT, 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 VT 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

Ef

I

V = fixed in magnitude, phase, and frequency

Infinite bus

Per-phase model of a synchronous generatoroperating into an infinite bus.

Unity power factor

Underexcited Overexcited

V

Ef

I

Re

Im