One Question — three answers, based on the person asking…

Transcription

Green Revolution EMS™
8794 Easton Road, Unit F3
Ottsville, PA 18942
P – 800.655.1033
F – 610.8547.1049
[email protected]
What is a Capacitor?
One Question — three answers, based on the person asking…
For the CEO and Business Owner or Manager
Capacitors are electrical devices that come in many shapes and sizes for all manner of
powered systems. Electric utility companies have employed the role played by the capacitor in
reducing energy usage for many years.
Costs are the primary focus for a CEO and business owner or manager. The cost of billed
electric power and the impact of power quality problems affecting operations can be reduced
with capacitance-based products such as Green Revolution’s EMS (Energy Management
Solutions). Power capacitors are well understood and have been used in large industrial facilities
for many years.
Continuously increasing energy costs and concern for the environmental impact brings new
urgency to the question of investing in systems and equipment to reduce energy bills and get
useful financial return. Capacitor based solutions are now proving to be one of the fastest ROI
measures to reduce energy bills. Replacing equipment for higher efficiency and alternative or
renewable sources have much longer payback. Energy management solutions do have some
controversy. This controversy is largely due to power company experts considering that power
factor penalties have not been charged to most medium, small, and residential electric
customers, in the past. Energy savings of 10% or less were considered insignificant because of
the difficulty in accurate measurement.
One oncoming “force”, that will change everything, is the computerized, digital 'smart' electric
meters. The true cost of delivered electrical power is measured as KVA, apparent power, which
can now be billed to all electric customers. As power companies persuade government
regulators to allow flexible billing to include power factor, KVA, peak demand, and time-of-use
rates, capacitor-based energy management may become mandatory.
At Green Revolution EMS, our business focus is helping you to save on the electric bill. Many
electric customers are wasting 10% or more of their electrical costs from multiple causes. The
exact savings for any particular facility, by installing power capacitor systems, depends on many
factors. How our local power company measures electrical power usage and the specifics of
their rate structure has a great effect on how much cost savings can be achieved.
Our electrical power “grid” uses AC (alternating current) to allow high voltage on long
transmission lines to be stepped down to end customers through transformers. One
consequence of AC power is a loss of efficiency arising from inductive loads, mainly electric
What is a Capacitor 4_12_13.docx
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motors. Electric, AC induction motors create a load in the electrical system called “lagging
reactive power”. This “reactive power” is found to reverse direction of power flow in each cycle
at 60Hz (50Hz in some other nations). When measured over a period of time, the reactive power
has an average energy transfer of zero, though it causes current to flow in wires and causes real
energy to be lost while consuming some capacity of the electrical system.
Another term, power factor, is one measure of the percent of the electrical power that is real
power (not reactive power or waveform distortion). For instance, a power factor of 0.95 tells us
that real power is 95% and reactive power is 5% of the total “apparent” power usage. Another
closely related concept is power quality, the impact of any and all forms of distortion in electric
power, which affects efficiency and proper operation of your electrical equipment. Reactive
power is considered to cause “displacement power factor”, because it results from a phase shift
between incoming voltage and load current.
Capacitors are tied to power systems to produce a correcting or compensating effect for both
power factor (reactive power) and power quality. Electric utility companies use high voltage
capacitors to compensate for reactive power to reduce the apparent power load all the way
back to the generating plants and also reduce voltage drop in long power lines.
Electrical AC motors have a power factor (percentage of real power in total apparent power)
of 0.75 to 0.85, typically, and must be compensated or corrected by capacitors for efficient use
of the electrical system. Capacitor-based energy management systems, such as the Green
Revolution PACS (Power Application Control System), provide the same service inside our
facilities, improving power factor, reducing voltage drop, and improving power quality. Power
quality problems include power surges, voltage stability, and interference frequencies, which
affect electrical equipment and motor efficiency. Most electrical equipment produces some
irregular current load, which contributes to poor power quality, depending on the amount of
energy used and other factors, such as maximum current load in electrical systems and length
of power wires. Harmonics (frequencies in multiples of 60Hz) and EMF (electromagnetic
frequencies much higher than 60Hz) are measured by engineers to analyze power quality
problems.
The ideal power system is a pure 60Hz “Sine Wave” which provides the maximum power for its
users. Any extra frequencies or irregular waveform pattern causes a loss of efficiency.
Capacitors in energy management equipment absorb and bypass a portion of the interfering
frequencies to improve power quality.
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For The Plant Manager
Managing electric power systems is one responsibility for plant managers. Power factor
correction capacitors are common in large commercial and industrial facilities. Capacitors in
AC power systems provide a compensating effect, countering the reactive power from AC
electrical motors, which adds to circuit loading and voltage drop. These capacitors are very
different from what we find in power supplies and electronics.
The steady rise in electric bills at many facilities leads us to go after cost reductions and
efficiency problems, more now than in the past. Power capacitors are used as a practical
method to reduce electric bills and to get the most utilization out of existing power distribution
networks. Consider that each electric motor that runs directly across-the-lines on AC power
could offer an opportunity of up to 10% reduction in Amps with capacitors installed in the power
system. Generally, a capacitor system is considered to compensate from its point of connection
and backward through the electric meter toward the generating source. In theory, the closer a
capacitor is to each AC motor, the greater the amount of wiring and electrical elements are
benefitted. The most practical method is to install a power capacitor unit or 'capacitor bank' at
an electrical panel rather than at each motor. This is a trade-off for convenience and costeffectiveness. Like other parts of the electrical system, the goal is always to right-size
capacitance to the electrical load.
Each branch circuit supplied through an electrical panel, wiring center, or motor control center
(mcc), introduces some amount of lagging reactive current (Amps) when an AC motor is
running. A typical electrical panel could have 10 or 20 motors all running, compounding
together so that a single, properly-sized capacitor 'bank' will usefully compensate out some or all
of the unwanted “reactive current” load, drawn from the electrical system. Capacitors draw
current, described as leading reactive current, which negates with lagging reactive current in
the electrical system, back toward the generating source.
An AC induction motor running “across-the-lines” will always draw as much amperage and
power—whatever it takes—to keep running at its synchronizing the speed of the 60Hz power line.
Whether a motor is heavily loaded or lightly loaded at the shaft, the reactive power will be
about the same, since it is the result of inductive windings in the motor's construction.
Power Factor is the measure of “useful” power as a percent of total power used in the electrical
system. A 0.95 power factor suggests a 95% “clean” power system and the goal of most power
companies. Many AC motors operate at about 0.75 to 0.85 power factor. Reactive power is
considered to be a “lagging” effect, where the electrical current is a quarter cycle delayed
from AC voltage waveform. Capacitors give an opposing effect, so the electrical current leads
the voltage by a quarter cycle. We never try to completely compensate the system to power
factor 1.0, since the risk of over-compensation is a safety concern with resonance and possible,
excessive high voltage.
Power capacitors are measured in kVAR (kVA reactive), which is thought of as “reactive Kw”.
When the new technology smart meters are fully used, the power companies need to charge us
for every kVAR of load we draw from the electric service at the electric meter.
True energy savings with capacitor-based energy management solutions comes in an indirect
way, by reducing voltage drop and improving the voltage waveform "power quality". Total
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reduction in real energy use depends on many factors, though the benefits are greatest at times
of maximum load or peak demand. AC induction motors are strongly affected by voltage drop
and power quality. Any voltage waveform distortion or interference that diminishes the
magnetic field, or does not contribute to shaft torque, is wasted energy and lost as heat. Poor
power quality or reduced, “dropped” voltage causes an AC induction motor to draw more
current to continue running at full speed. These conditions produce more heat dissipation in
motors, which reduces work capacity and impacts the service life if motors run continuously at
this level.
The Bottom Line: We need to run our electrical systems at best efficiency by compensating-out
the reactive power load from AC motors with the installation of power capacitors, also known as
power factor correction capacitors.
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For The Engineer
Capacitors are one of the three most common electrical components, along with resistors and
inductors. The response in an AC power system for capacitors and inductors is markedly different
than DC equipment. In DC equipment, capacitors are used to stabilize voltage in power supplies
and filter out ripples and noise by providing a bypass path for higher frequencies. AC voltage
changes our focus to the shape of the power waveform, desired to be a perfect sine wave.
Inductors give us the relation, V=L di/dt (voltage is proportional to rate of change in current) so
the current through an inductor is the integral, shifted 90 degree lagging, compared to the
voltage waveform. Capacitors give us the relation I = C dv/dt (current is proportional to the rate
of change in voltage) so the current (derivative) leads the voltage 90 degrees compared to the
voltage waveform. In the case of AC systems, an inductor and capacitor both have a portion of
each 60Hz power cycle where energy is “stored and released” every few milliseconds.
On average, over time, the power consumed by a theoretical inductor or capacitor is zero. In
each power cycle, electrical current and power flows in both directions, loading and heating
the wires and distribution circuits.
An AC induction motor is equivalent to an inductor and a resistor in its simplest terms. The
inductor represents the windings that produce the air gap magnetic field between the stator
and rotor. Inductive or magnetizing current is typically 10% or more of the total motor current.
When a motor is mechanically loaded, the slip in RPM causes a current draw that is represented
as a resistance that varies with shaft load.
Without capacitors in the system, the current waveform at the electrical meter is lagging in
phase angle versus the incoming voltage waveform. The cosine of the lagging phase angle of
the current waveform, compared to the voltage waveform, is the power factor. Conventional,
old-technology electric meters only allowed for electric billing of the effective or working "real"
power that produces torque in motors. However, the current in Amps is drawing from the power
line and must be supplied by the power company. This is why all electrical distribution
equipment is rated in kVA, apparent power, and is not dependent on how efficiently the user’s
electrical loads use the power.
Adding a capacitor to the system has the effect to neutralize or diminish the lagging-phaseangle current by adding leading-phase-angle current. The ultimate power correction would be
a net zero lagging phase angle, with supplied voltage and current load perfectly “in phase”. (A
1.0 power factor).
There is a percentage of increased heat loss from reactive current, though the more interesting
effect is how this "reactive current" consumes a portion of the load carrying capacity of the
distribution system, down to each motor load. Wiring, connections, and transformers all
contribute to voltage drop and voltage waveform distortion according to the flow and
waveform of current drawn through. The longer the wiring path and the nearer to rated
capacity, the more impact on the voltage level and waveform. The impedance of the power
system is the electrical measure of how the wiring and electrical distribution can be represented
as an actual electrical load, rather than the idealized perfect electrical conductor.
One more effect to understand with capacitors is the influence to shunt or bypass some portion
of harmonics and higher EMF (electromagnetic frequencies) away from the working loads, such
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as motors. A capacitor's impedance, resistance, which varies with frequency, approaches a
short circuit (low resistance) with increasing frequency. The benefit for an electrical system is
that some amount of interference energy will be passed through the capacitor as a current load
and reduces the voltage waveform distortion to our working equipment. This is actual current
load and heat dissipating in a capacitor, which could cause overheating and overloading, if
excessive. A concern in electrical systems is that total harmonic frequencies, energy from power
rectifiers and other sources, can cause failure in capacitors, if not properly protected. The
benefit of improving power quality, absorbing harmonics, EMF, and surge transients can be a
good effect, if the amount of total distortion energy does not become excessive. In most
situations, reduction in EMF and harmonic energy improves the efficiency of motors and other
equipment, to reduce the electric costs. Some researchers have found the reduction in EMF
frequencies will benefit people with acute sensitivities and medical electronic devices.
Clearly, any real-world facility is far more complex, with many different types of motors, lighting,
and various machinery and powered equipment. Electrical power systems are built in a very
structured way in most facilities. We buy from a few suppliers and they have limited catalogs of
standard sizes of equipment. The practical location to install power capacitors is to be directly
wired near an electrical panel. The principal challenge and caution with power capacitors is to
avoid “over compensating”, which could cause over-voltage and exceed the safe voltage
rating of the wiring. Large installations may require a switched bank, or automatic control
system, because of the unpredictable nature of many motors operating at different times. One
"rule of thumb" is to use fixed capacitors up to twenty-five percent of the transformer rating
(capacitor kVAR versus the transformer kVA). A fixed capacitor is the most cost-effective means
of power factor compensation, expected to give a long service life without maintenance.
The planning and calculations for power capacitors must also consider other electrical and
electronic equipment in the same panel or branch circuits. Any type of variable speed drive,
UPS (uninterruptible power system), or non-motor load can produce an array of other waveforms
and frequencies in the power system. It may be necessary to add a line reactor to avoid
harmonic frequency energy (multiples of the 60 Hz line frequency) from overloading the
capacitor, since the current through the capacitor increases in proportion to the frequency. A
line reactor could limit harmonic current flow and “detune” a resonant frequency condition. The
planning and installation process must comply with all safe practices and electrical codes. Every
facility and installation should be evaluated to plan the best system of power factor correction
capacitors.
Our financial business goal is to get the best-cost savings in your electric bill. When installing
power factor capacitors, we target at least 8% or greater cost savings from any and all benefits:
reduced kVA, total energy in kWh (kilowatt hours), and lower kW or kVA peak demand.
However, it is always an effort to understand the interior and exterior environments and other
variations in power usage over a period of time, and same month, previous year comparisons.
This is the difficulty to measure total savings over a long time period such as one year. Power
quality instruments are commercially available to log the power usage over a period of time
and also to examine the power waveform at any moment in time.
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One Question — three answers, based on the person asking…

Transcription

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