酒英语功率因数英文文献翻译

2021年1月20日发(作者：英语论坛)



电气专业本科毕业设计英文翻译









学院
(
部
)
：电气与信息工程学院

专业班级：

电气
09-6
班

学生姓名：

李

鼎

指导教师：

李红月副教授




年

月

日

POWER FACTOR
The power factor of an AC electric power system is defined as the ratio of the
real
power
flowing
to
the
load
to
the
apparent
power
in
the
circuit,
and
is
a
dimensionless number between 0 and 1 (frequently expressed as a percentage, e.g. 0.5
pf
=
50%
pf).
Real
power
is
the
capacity
of
the
circuit
for
performing
work
in
a
particular time. Apparent power is the product of the current and voltage of the circuit.
Due to energy stored in the load and returned to the source, or due to a non-linear load
that distorts the wave shape of the current drawn from the source, the apparent power
will be greater than the real power.

In an electric power system, a load with a low power factor draws more current
than a load with a high power factor for the same amount of useful power transferred.
The
higher
currents
increase
the
energy
lost
in
the
distribution
system,
and
require
larger
wires
and
other
equipment.
Because
of
the
costs
of
larger
equipment
and
wasted
energy,
electrical
utilities
will
usually
charge
a
higher
cost
to
industrial
or
commercial customers where there is a low power factor.

Linear loads with low power factor (such as induction motors) can be corrected
with a passive network of capacitors or inductors. Non-linear loads, such as rectifiers,
distort
the
current
drawn
from
the
system.
In
such
cases,
active
or
passive
power
factor correction may be used to counteract the distortion and raise the power factor.
The devices for correction of the power factor may be at a central substation, spread
out over a distribution system, or built into power-consuming equipment.

Power factor in linear circuits .
Instantaneous and average power calculated from AC voltage and current with a
unity power factor (φ=0, cosφ=1). Since the blue line is above the axis, all power is
real power consumed by the load.

Instantaneous and average power calculated from AC voltage and current with a
zero
power
f
actor
(φ=90,
cosφ=0).
The
blue
line
shows
all
the
power
is
stored
temporarily in the load during the first quarter cycle and returned to the grid during
the second quarter cycle, so no real power is consumed.

Instantaneous and average power calculated from AC voltage and current with a
lagging power factor (φ=45, cosφ=0.71). The blue line shows some of the power is
returned to the grid during the part of the cycle labelled φ
.

In a purely resistive AC circuit, voltage and current waveforms are in step (or in
phase), changing polarity at the same instant in each cycle. All the power entering the
loads
is
consumed.
Where
reactive
loads
are
present,
such
as
with
capacitors
or
inductors, energy storage in the loads result in a time difference between the current
and
voltage
waveforms.
During
each
cycle
of
the
AC
voltage,
extra
energy,
in
addition
to
any
energy
consumed
in
the
load,
is
temporarily
stored
in
the
load
in
electric or magnetic fields, and then returned to the power grid a fraction of a second
later
in
the
cycle.
The

and
flow
of
this
nonproductive
power
increases
the
current in the line. Thus, a circuit with a low power factor will use higher currents to
transfer a given quantity of real power than a circuit with a high power factor. A linear
load does not change the shape of the waveform of the current, but may change the
relative timing (phase) between voltage and current.

Circuits
containing
purely
resistive
heating
elements
(filament
lamps,
strip
heaters, cooking stoves, etc.) have a power factor of 1.0. Circuits containing inductive
or
capacitive
elements
(electric
motors,
solenoid
valves,
lamp
ballasts,
and
others
)
often have a power factor below 1.0.

Definition and calculation

AC
power
flow
has
the
three
components:
real
power
(also
known
as
active
power)
(P),
measured
in
watts
(W);
apparent
power
(S),
measured
in
volt-amperes
(V
A); and reactive power (Q), measured in reactive volt- amperes (var).

The power factor is defined as
In
the
case
of
a
perfectly
sinusoidal
waveform,
P,
Q
and
S
can
be
expressed
as
vectors that form a vector triangle such that: If is the phase angle between the current
and voltage, then the power factor is equal to the cosine of the angle, , and: Since the
units are consistent, the power factor is by definition a dimensionless number between
0 and 1. When power factor is
equal
to 0, the energy
flow is
entirely
reactive,
and
stored energy in the load returns to the source on each cycle. When the power factor is
1, all the energy supplied by the source is consumed by the load. Power factors are
usually
stated
as

or

to
show
the
sign
of
the
phase
angle.
If
a
purely resistive load is connected to a power supply, current and voltage will change
polarity in step, the power factor will be unity (1), and the electrical energy flows in a
single direction across the network in each cycle. Inductive loads such as transformers
and motors (any type of wound coil) consume reactive power with current waveform
lagging the voltage. Capacitive loads such as capacitor banks or buried cable generate
reactive power with current phase leading the voltage. Both types of loads will absorb
energy during part of the AC cycle, which is stored in the device's magnetic or electric
field,
only to
return this energy back to
the source during the rest
of the cycle. For
example, to get 1 kW of real power, if the power factor is unity, 1 kV
A of apparent
power needs to be transferred (1 kW ÷
1 = 1 kV
A). At low values of power factor,
more apparent power needs to be transferred to get the same real power. To get 1 kW
of real power at 0.2 power factor, 5 kV
A of apparent power needs to be transferred (1
kW
÷

0.2
=
5
kV
A).
This
apparent
power
must
be
produced
and
transmitted
to
the
load in the conventional fashion, and is subject to the usual distributed losses in the
production and transmission processes. Electrical loads consuming alternating current
power
consume
both
real
power
and
reactive
power.
The
vector
sum
of
real
and
reactive power is the apparent power. The presence of reactive power causes the real
power to be less than the apparent power, and so, the electric load has a power factor
of less than 1.

Power factor correction of linear loads

It is often desirable to adjust the power factor of a system to near 1.0. This power
factor
correction
(PFC)
is
achieved
by
switching
in
or
out
banks
of
inductors
or
capacitors. For example the inductive effect of motor loads may be offset by locally
connected capacitors. When reactive elements supply or absorb reactive power near
the load, the apparent power is reduced.

Power factor correction may be applied by an electrical power transmission utility
to
improve
the
stability
and
efficiency
of
the
transmission
network.
Correction
equipment
may
be
installed
by
individual
electrical
customers
to
reduce
the
costs
charged
to
them
by
their
electricity
supplier.
A
high
power
factor
is
generally
desirable in a transmission system to reduce transmission losses and improve voltage
regulation at the load.
Power factor correction brings the power factor of an AC power circuit closer to 1
by supplying reactive power of opposite sign, adding capacitors or inductors which
act to cancel the inductive or capacitive effects of the load, respectively. For example,
the inductive effect of motor loads may be offset by locally connected capacitors. If a
load
had
a
capacitive
value,
inductors
(also
known
as
reactors
in
this
context)
are
connected to correct the power factor. In the electricity industry, inductors are said to
consume reactive power and capacitors are said to supply it, even though the reactive
power is actually just moving back and forth on each AC cycle.

The
reactive
elements
can
create
voltage
fluctuations
and
harmonic
noise
when
switched
on
or
off.
They
will
supply
or
sink
reactive
power
regardless
of
whether
there is a corresponding load operating nearby, increasing the system's no-load losses.
In a worst case, reactive elements can interact with the system and with each other to
create
resonant
conditions,
resulting
in
system
instability
and
severe
overvoltage
fluctuations. As such, reactive elements cannot simply be applied at will, and power
factor correction is normally subject to engineering analysis.
An automatic power factor correction unit is used to improve power factor. A power
factor correction unit usually consists of a number of capacitors that are switched by
means
of
contactors.
These
contactors
are
controlled
by
a
regulator
that
measures
power
factor
in
an
electrical
network.
To
be
able
to
measure
power
factor,
the
regulator uses a current transformer to measure the current in one phase.


Depending
on
the
load
and
power
factor
of
the
network,
the
power
factor
controller
will
switch
the
necessary
blocks
of
capacitors
in
steps
to
make
sure
the
power factor stays above a selected value (usually demanded by the energy supplier),
say 0.9.

Instead of using a set of switched capacitors, an unloaded synchronous motor can
supply
reactive
power.
The
reactive
power
drawn
by
the
synchronous
motor
is
a
function of its field
excitation. This
is
referred to as
a synchronous condenser.
It
is
started and connected to the electrical network. It operates at a leading power factor
and
puts
vars
onto
the
network
as
required
to
support
a
system’s
voltage
or
to
maintain the system power factor at a specified level.








The condenser’s installation and operation are identical to large electric motors. Its
principal advantage is the ease with which the amount of correction can be adjusted; it
behaves
like
an
electrically
variable
capacitor.
Unlike
capacitors,
the
amount
of
reactive
power
supplied
is
proportional
to
voltage,
not
the
square
of
voltage;
this
improves voltage stability on large networks. Synchronous condensors are often used
in
connection
with
high
voltage
direct
current
transmission
projects
or
in
large
industrial plants such as steel mills.

Non-sinusoidal components

Non-linear
loads
change
the
shape
of
the
current
waveform
from
a
sine
wave
to