Answers to Frequently asked Questions
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Q What is IEEE-519 ? A. It is the North American Standard that describes acceptable limits of harmonics in electrical systems. The proper reference is IEEE Std 519-1992, entitled "IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems". |
Q What will happen to my loads and electrical system if I operate outside the IEEE 519 guidelines? A. Perhaps nothing or maybe disaster! Systems that operate above the guidelines are operating in a less reliable environment. More problems occur than not. Some issues may be immediate other over the long term. |
Q What is the Maximum Possible Neutral Current? A. In a system feeding single-phase (phase-neutral) loads using switching power supplies, such as 120V personal computers and other electronic equipment, the worst case scenario for neutral current is when the pulses drawn by the loads on each phase do not overlap. The total resulting current in the neutral conductor is the arithmetic sum, which is the root of the sum of the squares: |
 The maximum is reached when the loads are balanced, so on a per unit basis, the result is:
 times the phase current. Any overlap of pulses between the phases, and typically there is some, reduces the amount of neutral current compared with the phase current. |
Q Why do some people say neutral current can reach 3 times phase current ? A. Over the past ten years, there have been many articles published that mention neutral current, some mention 1.73 times at the maximum possible level, some say 3 times, others say it can be 1.73 or 3 without any back-up. We have described how neutral current can reach 1.73 times the phase current. The 3 times number comes from simply summing the current from the 3 phases into the neutral conductor (1+1+1 = 3). This reflects either a lack of understanding of the requirement to do the root of the sum of the squares, or contemplates that all loads draw only zero sequence 180Hz, not consuming any fundamental (or other) current, obviously not possible in our electrical systems. |
Q What is a typical level of Neutral Current for an Office Environment? A. Today’s offices are filled with desktop computers and laser printers. Where these are the only loads fed from the local electrical panel, neutral current can be expected to be in the range of 1.3 to 1.5 times the phase current. In many offices, the workspace cubicles have a fluorescent task light that is plugged into the same system. Since the harmonic content of an average fluorescent lamp is lower than for a computer, it lowers the ratio of harmonics amps to RMS amps, and this dilution reduced the expected neutral current to about equal to the phase current. However, it should be noted that if the task lighting uses compact fluorescent lamps, which have a rich harmonic spectrum like a desktop computer, neutral current can be expected to be in the 1.3-1.5 times the phase current. |
Q Phase-shifting is great but what if I have a single nonlinear load, like one large motor drive? A. The phase-shift technique relies on using the harmonic currents produced by one load to cancel with the harmonic currents produced by a similar but phase-shifted load. Phase-shifting will therefore not work with a single harmonic producing load. Options for harmonic treatment include active filters, tuned filters, reactors, or moving the load to a dedicated feeder. Other treatments are possible. The choice will depend on the results of an analysis of many variables such as the electrical system characteristics, the load profile and duty cycle. |
Q What are harmonics A. Harmonics are multiples of some base frequency, each having a frequency and amplitude. For example our electrical system is 60 hertz so the 3rd harmonic is 180 hertz and the 5th harmonic is 300 hertz. Each harmonic is sinusoidal and as such repeats through a system. |
Q What causes harmonics? A. Harmonic in our electrical systems today are caused by nonlinear loads such as personal computers and variable speed drives. These harmonics are introduced to the system through the way the load draws current. |
Q What harmonics are most prevalent in today's electrical systems? A. The loads that we place on our systems today are computers, laser printers, video monitors, electronic ballasts, variable speed drives and ups. Loads that are connected phase to neutral ( equipment typically plugged into a wall receptacle) produce an abundance of 3rd, 5th and 7th harmonic currents. Phase to phase and three phase loads ( variable speed drives for example ) that are 6 pulse produce mostly 5th and 7th harmonics. |
Q What are the effects of harmonics in an electrical system? A. The most common issue is extra heat due to the additional losses. Excessive neutral current will result in systems that have high density of single phase loads due to the triplens (3rd harmonic and its odd multiples) and the imbalance of the 60 hertz and the other non triplen harmonic currents such as 5th and 7th. In system In systems where the impedance is high voltage distortion will also become an issue. |
Q Why do neutral conductors need to be oversized? A. Single phase nonlinear loads carry a high amount of 3rd harmonic. The 3rd harmonic is in phase in all three phases so they add into the neutral. This can result in neutral current with a higher value than the phase current. The large conductor is designed to carry the additional expected current. |
Q How do I test for harmonics? A. The most thorough way to test for harmonics is to take measurements using a harmonic analyzer. Simple tests would include measuring neutral to ground voltage at the panels and at the wall receptacles. Take a simple meter and measure the neutral current. Also you can compare the RMS readings to the non RMS readings. |
Q After collecting data how do I relate the measurements to an acceptable standard? A. IEEE 519 is the North American recommended practice for harmonic levels. This practice recommend voltage distortion to be less than 5% thd and no individual voltage to greater than 3%. It also provides levels for current but only at the point of common coupling ( the point where the utility stops and in-house distribution begins). |
Q What strategies are available for harmonic reduction?
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Q Will a K factor transformer reduce harmonics? A. A k factor transformer is derated to survive in a harmonic rich environment. It will not cancel any harmonic. |
Q What are harmonic cancellation transformers? A. These transformers if designed properly will incorporate low zero sequence impedance and phase shifting, noise reduction and higher efficiencies. They are used in place of isolation and k rated transformers to cancel 3rd, 5th and 7th harmonic distortion. |
Q What should I look for in a supplier?
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