INTRODUCTION
This summary is an extraction from Mike Holt's book
and video on the subject.
In years past, most electrical equipment operated on an ideal voltage
and current waveform. However, in the past 25 years (particularly since
the late 1980's) there has been an explosion in the use of solid-state
electronic technology. This new, highly efficient, electronic technology
provides improved product quality with increased productivity by the use
of smaller and lighter electrical components. Today we are able to
produce products at costs less than in years past, but . . . this new
technology requires clean electric power and is highly sensitive to
power distortions.
Electronic devices convert 60 Hz alternating current to direct
current by the use of switching power supplies that contain rectifiers
and often capacitors. In addition to converting alternating current to
direct current, sometimes the current is converted back to alternating
current but into a different frequency.
Electronic equipment (switching power supplies) draws current
differently than non-electronic equipment. Instead of a load having a
constant impedance drawing current in proportion to the sinusoidal
voltage, electronic devices change their impedance by switching on and
off near the peak of the voltage waveform. Switching loads on and off
during part of the waveform results in short, abrupt, nonsinusoidal
current pulses during a controlled portion of the incoming peak voltage
waveform. These abrupt pulsating current pulses introduce unanticipated
reflective currents (harmonics) back into the power distribution system.
The currents operate at frequencies other than the fundamental 60 Hz.
Harmonic currents can be likened to the vibration of water in a water
line when a valve is open and closed suddenly.
Harmonics affect us all; from the secretary operating a computer, the
electrician trouble shooting equipment failure, the electrical
contractor having to absorb the cost of equipment replacement, the
inspector who must investigate the cause of electric fires, to the
facilities management interested in effective and efficient equipment
operation and the avoidance of down time. The scope of harmonics impacts
architects, engineers, designers, property managers, building
maintenance personnel, suppliers, equipment manufactures, and (of
course) private industry.
WHAT TYPES OF LOADS CAUSE THE PROBLEM?
The largest contributor of reflective harmonic currents for
commercial buildings is the personal computer. There are, however, other
large contributors too, such as: Arc Equipment Audio Recorders Battery
Chargers Computer Power Units (CPU) Copy Machines Discharge Lighting
(fluorescent, mercury, sodium, etc.) Electronic Dimmers Electronic
Ballasts Elevators Facsimiles (FAX) File Servers Laser Printers Local
Area Networks (LAN) Personal Computers (PC) Rectifiers Telecommunication
Equipment Uninterrupted Power Supplies (UPS) Variable Frequency Drives
(VFD) Video Recorders Video Display Units Welders
CLEAN POWER IS REQUIRED FOR TODAY'S EQUIPMENT
Electronic microprocessor equipment requires clean power. This type
of equipment needs undistorted voltage to function properly, and it is
particularly sensitive to voltage transients (notches or spikes) and
flat topping of the voltage waveform caused by the large pulsating
currents. High frequency harmonic currents can introduce voltage (noise)
in electronic cables or components. They can add zero voltage crossing
to the voltage waveform, which can cause havoc for microprocessors and
other electronic devices that depend on 60 Hz frequency (120 zero
crossings per second) for the clock oscillator timing circuit.
Electronic equipment installation manuals often require the total
voltage distortion to be no more than 10%. Voltage distortion can cause
poor product performance, but in general, it is not a safety hazard.
Strangely, electronic equipment requires clean power, but its power
supplies generate the reflective harmonic currents that cause the
voltage distortions!
PROCESSING SPEEDS
In the past 10 years the processing speed, the volume of data that is
transmitted, and the amount of data stored on computers has increased by
leaps and bounds. As the processing speeds of computers are increasing,
the machines become more sensitive to voltage distortions. Over the next
decade it is projected that personal computer processing speeds will
increase by at least 15 times; multi-user and work station computers by
10 times; and graphic super computers by more than five times.
EXACTLY WHAT IS THE PROBLEM?
The actual problems of any building will vary, depending on the types
and number of installed harmonic producing loads. Most buildings can
withstand nonlinear loads of up to 15% of the total electrical system
capacity without concern, but, when the nonlinear loads exceed 15% some
non-apparent negative consequences can be expected. For buildings that
have nonlinear loading of more than 25%, particular problems can be
become apparent. The following is a short summary of most problems
caused by harmonics:
- Blinking of Incandescent Lights - Transformer Saturation
- Capacitor Failure - Harmonic Resonance
- Circuit Breakers Tripping - Inductive Heating and Overload
- Computer Malfunction or Lockup - Voltage Distortion
- Conductor Failure - Inductive Heating
- Electronic Equipment Shutting down - Voltage Distortion
- Flickering of Fluorescent Lights - Transformer Saturation
- Fuses Blowing for No Apparent Reason - Inductive Heating and
Overload
- Motor Failures (overheating) - Voltage Drop
- Neutral Conductor and Terminal Failures - Additive Triplen
Currents
- Electromagnetic Load Failures - Inductive Heating
- Overheating of Metal Enclosures - Inductive Heating
- Power Interference on Voice Communication - Harmonic Noise
- Transformer Failures - Inductive Heating
The heating effects of harmonic currents can cause destruction of
equipment, conductors, and fires. The results can be unpredictable legal
and financial ramifications. Voltage distortions can lead to overheating
of equipment, electronic equipment failure, expensive downtime, and
maintenance difficulties. Harmonic currents and voltage distortion are
becoming the most severe and complex electrical challenge for the
electrical industry. The problems associated with nonlinear loads were
once limited to isolated devices and computer rooms, but now the problem
can appear throughout the building and utility system.
PAST, PRESENT, AND FUTURE TRENDS
In the past, most electric power was consumed by "linear loads."
Reflective harmonic currents from nonlinear loads (fluorescent lighting)
were a relatively minor component of the total building power usage. The
Electric Power Research Institute (EPRI) estimates that in 1992, 15 to
20% of the total load was nonlinear, and by the year 2,000 it is
expected that 50 to 70% of all loads will be nonlinear. As we can see
from the EPRI's projection, the problems (or opportunities) of harmonics
will be growing with the expanded used or electronics. Few people in the
trade understand the basics of harmonics; much less have a working
knowledge of the problems.
IS THERE ANYTHING WE CAN DO?
Be sure the electrician who performs any work on your facility has
been completely trained (ask for a certificate on harmonics) on the
causes, the effects, and the solutions of harmonic currents. Because
harmonics are here to stay, we must adjust our thinking on electrical
system design, installation, inspection, and maintenance. We must
anticipate the non-apparent overload of the electrical system and the
associated distortions to the voltage waveforms.
Think of harmonic currents as the symptoms of the common cold; there
is no cure, but we can treat the symptoms. Before we apply any
treatments or preventive measures, we must understand the symptoms and
their cause.
How can you tell if the person you're talking to understands the
problem? Simply ask what type of ammeter they use to measure current. If
the answer is not, "a True-RMS meter," then you can be sure this person
will not solve your problems and might actually contribute to further
destruction and unsafe practices. The average electrician or electrical
contractor does not even know that there is a problem.
Having the right meter is part of the solution, but understanding the
use of the meter and harmonic currents is critical!
WHY AN AVERAGE RESPONSE AMMETER IS USELESS!
Average response ammeters are only accurate when measuring 60 Hz
loads that have sinusoidal current waveforms and cannot accurately
measure the current of nonlinear loads. The reason is that nonlinear
loads draw current in a nonsinusoidal manner, which produces reflective
harmonic currents that operate above 60 Hz; both of these conditions are
beyond the meter's design criteria. When an average response ammeter is
used to measure nonlinear load current, the results can be inaccurate
readings of as much as 25% to 50% below the actual true-RMS current. As
a result, the actual current of a circuit can exceed the rating of
conductors and equipment. The actual current cannot be detected with the
average-responding ammeter!
In order to perform basic electrical trouble shooting for today's
electrical systems, we must have an ammeter that provides true-RMS and
instantaneous peak current ratings of the circuit. This meter must have
the capacity of measuring the electrical characteristics of the waveform
by sampling many points along the waveform. True-RMS meters are designed
for just that, and they are accurate for both simple (sinusoidal) and
complex (nonsinusoidal) alternating and direct current waveforms.
Average response meters are only accurate with simple sinusoidal
alternating current waveforms; not the complex waveforms resulting from
nonlinear loads.
To say it bluntly, if you have an average responding ammeter you
might as well make a lamp out of it because it is useless! If you're
trying to convince your superiors to purchase a true-RMS meter that
costs $300 to $400 and they don't understand why; make them a copy of
this paragraph. You must have a True-RMS meter to properly measure
electrical currents from today's loads. An average meter is useless!
WHAT TYPES OF LOADS CAUSE HARMONIC CURRENTS?
Let's understand the difference between linear and nonlinear loads. A
linear load is a load that opposes the applied voltage with constant
impedance resulting in a current waveform that changes in direct
proportion to the change in the applied voltage. Examples of these loads
are resistance heating, incandescent lighting, motors, etc. If the
impedance is constant, then the applied voltage is sinusoidal, and the
current waveform will also be sinusoidal.
A nonlinear load, on the other hand, is a load that does not oppose
the applied voltage with constant impedance. The result is a
nonsinusoidal current waveform that does not conform to the waveform of
the applied voltage. Nonlinear loads have high impedance during part of
the voltage waveform, and when the voltage is at or near the peak the
impedance is suddenly reduced. The reduced impedance at the peak voltage
results in a large, sudden, rise in current flow until the impedance is
suddenly increased resulting in a sudden drop in current.
Because the voltage and current waveforms are no longer related, they
are said to be "nonlinear." Nonlinear loads are loads that have
diode-capacitor power supplies such as: computers; laser printers;
welders; variable frequency drives; UPS systems; fluorescent lighting;
etc., which draw current in short pulses during the peak of the line
voltage. These nonsinusoidal current pulses introduce unanticipated
reflective currents back into the power distribution system, and the
currents operate at frequencies other than the fundamental 60 Hz.
Harmonic is a term that describes sinusoidal waveforms that operate
at a frequency that is a multiple of the fundamental 60 Hz frequency.
When a current, or voltage, operates at other than the fundamental 60 Hz
frequency it is said to operate at a specific harmonic order (3rd
harmonics operate at 180 Hz; 5th harmonics operate at 300 Hz).
Because reflective harmonic currents operate at frequencies higher
than the fundamental, we must be concerned with their effect in the
electrical distribution system. The most significant effects of high
frequency harmonic currents are as follows:
- Inductive heating of transformers, generators, and other
electromagnetic devices such as motors, relays, and coils (due to the
inductive heating effects of eddy currents, skin effect, and
hysteresis).
- Inductive heating of conductors, breakers, fuses, and all other
devices that carry current (because of eddy currents, skin effect, and
hysteresis).
- Inductive heating of metal parts such as raceways, metal
enclosures, and other ferrous (iron or steel) metal parts (because of
eddy currents and hysteresis).
- Voltage distortion resulting in unpredictable equipment operation
because of harmonics.
- Excessive neutral current resulting in equipment overheating or
failure because of additive harmonic currents, excessive voltage drop,
and distortion.
HOW SERIOUS IS THIS PROBLEM?
The effects of harmonic currents on electrical distribution systems
are not understood by most in the electrical industry. The number one
hazard with harmonic currents is equipment failure because of current
overload that result in fires. In addition to the electrical safety
aspects, harmonics cause voltage waveform distortions that affect many
different types of loads in different ways.
Research on the problems and solutions is still in its infancy;
solutions recommended today may not be viewed as correct in the future.
Because of the research that is continuing we must all keep ourselves
current on this subject.
I hope this short summary was helpful. If you want to know more
about this subject, please attend our seminar or order
our home study
video program today.
Special
thanks to Mike Holt, renowned author and educator, for
allowing us to share this information with you. You can learn more on
this and other subjects through his excellent educational materials and
seminars.
