Transcript Parallel Operation of Synchronous Generators

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:
SD 
nnl - nf l
100%
nf l
nnl is theno  load primemoverspeed
nfl is thefull load primemoverspeed.
nnl
nnl
nfl
nfl
r/min
Hz
Pfl
Power KW
Pfl
Power kW
The relation between frequency and power
P  S P  f nl - f sys 
P  power output of thegenerator
f nl  no - load frequencyof thegenerator
f sys  operatingfrequencyof system
S P  slope of curve,in kW/Hzor MW/Hz.
Operation of generators in parallel with large
power systems
VT
fe
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.
fe
fnl
Pinf bus
PG
Pload
Operation of generators in parallel with other
generators of the same size
The power house
Generator1
fe
Generator2
60 Hz
PG1
Ptot  Pload  PG1  PG 2
PG2
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
Unity power factor
I
Ef
Im
Underexcited
Ef
V = fixed in magnitude,
phase, and frequency
Infinite bus
Per-phase model of a synchronous generator
operating into an infinite bus.
Overexcited


I
V
Re