العوامل التى تحدد جودة منبع القدرة الكهربية Power Quality

Harmonics What You Should Know INTRODUCTION This summary is an extracted 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: 1.   Blinking of Incandescent Lights - Transformer Saturation 2.   Capacitor Failure - Harmonic Resonance 3.   Circuit Breakers Tripping - Inductive Heating and Overload 4.   Computer Malfunction or Lockup - Voltage Distortion 5.   Conductor Failure - Inductive Heating 6.   Electronic Equipment Shutting down - Voltage Distortion 7.   Flickering of Fluorescent Lights - Transformer Saturation 8.   Fuses Blowing for No Apparent Reason - Inductive Heating and Overload 9.   Motor Failures (overheating) - Voltage Drop 10.                Neutral Conductor and Terminal Failures - Additive Triplen Currents 11.                Electromagnetic Load Failures - Inductive Heating 12.                Overheating of Metal Enclosures - Inductive Heating 13.                Power Interference on Voice Communication - Harmonic Noise 14.                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: 1.   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). 2.   Inductive heating of conductors, breakers, fuses, and all other devices that carry current (because of eddy currents, skin effect, and hysteresis). 3.   Inductive heating of metal parts such as raceways, metal enclosures, and other ferrous (iron or steel) metal parts (because of eddy currents and hysteresis). 4.   Voltage distortion resulting in unpredictable equipment operation because of harmonics. 5.   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 helpfull. If you want to know more about this subject, please attend our seminar or order our home study video program today. Harmonics (8 Hours CEU Credit) Unusual problems have been developing recently in electrical installations such as: overheating of transformers, motors, conductors, equipment, and even raceways and boxes! Circuit breakers are tripping, fuses are blowing for no apparent reason, and electronic equipment is suddenly shutting down! The culprit is likely harmonic currents resulting from modern electronic non-linear loads such as personal computers, laser printers, electronic ballast, dimmers, welders, adjustable speed drives, etc. This program includes one six-hour video with one Textbook to help you understand the problems and the solutions that are available. Textbook with one 6-hour video - $109 [HARV], Book only $25 [HARB].