By Stu Accola, product marketing manager, MTE Corporation
In certain rural and rugged areas in North America, single-phase power is used to help power operations of commercial customers, such as those in the agriculture and oil and gas industries. Even though three-phase power is most efficient, single-phase power often is installed in these remote areas because the cost is significantly less, and still offers enough power to run the necessary applications.
Just like three-phase systems, however, single-phase power can cause a variety of power quality issues, which ultimately costs companies money in repairs and production downtime.
Differences Between Single Phase and Three Phase Power
For companies operating in rural areas, it is important to realize that access to power may not be the ultimate barrier. Rather, you want to consider the quality of the power used in your operations. It's essential to verify that the power is not over burdened with harmonic distortion.
In three-phase power systems, the power circuit uses a combination of three alternating currents that differ in phase by 120°. As a result, the power never drops to zero, and thus has the capability to carry more load. The current in each conductor reaches its peak instantaneous value sequentially, so the waveforms of the three supply-conductors are offset from one another in time by one-third of their period.
Therefore, the key difference between single-phase and three-phase AC power is the constancy of delivery.
In comparison to single-phase, three-phase power distribution involves less conductor material for transferring the same amount of power, which makes it more efficient in terms of power delivered per cable used and more suited for industrial and large commercial customers.
Another advantage of three-phase power is that the DC rectified from the three-phase supply has less ripple factor than the DC rectified from single-phase supply, which requires large filter capacitors to obtain smooth DC output.
Single-Phase Power Challenges
Harmonics can occur in both three-phase and single-phase systems. However, single-phase power can sometimes produce higher levels of harmonics than three-phase power. It can also be harder to remove these harmonics in single-phase applications.
What Are Harmonics?
Harmonics can be best described as the shape or characteristics of a voltage or current waveform relative to its fundamental frequency. But what does that mean?
Well, the ideal power source for all power systems are smooth, sinusoidal waves. These perfect sinewaves don't contain harmonics. When waveforms deviate from a sinewave shape, they contain harmonics. These current harmonics distort the voltage waveform and create distortion in the power system, which can cause many problems.
Harmonics typically are caused by nonlinear loads. When current isn't proportional to the voltage, the load is classified as nonlinear. Nonlinear loads are most associated with modern electronic equipment that often relies on line-operated switch mode power supplies. Examples of nonlinear loads include variable-frequency drives (VFDs), arc furnaces and other power supplies.
Linear loads have a current waveform that is proportional to the amount of voltage applied. Examples of the types of linear loads are incandescent lamps, heaters and resistors.
Effects of Harmonics in a System
A power system's ability to perform at optimal levels is compromised when harmonic distortion enters the system. It creates inefficiencies in equipment operations because of the increased need for power consumption.
The most commonly observed effects of harmonics in the system are:
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Components overheating.
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Equipment malfunctions.
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Tripping of circuit breakers and VFDs.
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Sensor measurements becoming inaccurate.
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Telecommunication interference.
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Disruptions to operations.
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Decreased life of components.
Mitigating Harmonic Distortion
The following measures are being adopted to reduce the effects of harmonic distortion:
- For mitigating the effects of harmonics, filtering is used to reduce the magnitude of the harmonic waveforms. Harmonic filters built specifically for single-phase power can provide reliable harmonic protection, which allows equipment to run with increased energy efficiency and better overall THID performance.
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When nonlinear loads are a significant part of the total load in the facility, it's important to choose electronic equipment with total harmonic distortion (THD) within prescribed limits. For example, avoiding electronic ballasts with more than 20% current THD can be helpful. A line reactor rated at 3 to 5% impedance in series with an adjustable-speed drive can be a low-cost solution to significantly reduce total harmonic distortion. Choosing electronic equipment with low THD will help decrease harmonics, improve power factor and reduce losses in power system components.
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Wherever nonlinear devices are abundant in a system, it's important to detect and diagnose the level of harmonics present in the system. Some manufacturers offer a single-phase power analyzer that can digitally capture waveforms and perform many kinds of analysis functions, including Fourier analysis, to determine harmonic content.
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Inverter produced harmonics in the output voltage can be mitigated by using preselected switching angles. This can be a cost-effective solution.
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Increasing the size of neutral conductors to better accommodate triplen harmonics (which only exist in single phase) can be helpful. Using improved system components that can handle the harmonics more effectively, such as finely stranded conductors and k-factor transformers, also can mitigate excessive heating caused by harmonics.
Planning Ahead is the Key
For companies operating in rural areas, it is important to realize that access to power may not be the ultimate barrier. Rather, you want to consider the quality of the power used in your operations. It's essential to verify that the power is not over burdened with harmonic distortion.
Companies working in rural areas should proactively research the level of harmonics in their systems, rather than waiting for issues like nuisance tripping or overheating of components, which can disrupt operations and loss of revenue or production.
MTE Corporation is a participating Encompass™ Product Partner in the Rockwell Automation PartnerNetwork™. Based in Menomonee Falls, Wisconsin, MTE Corporation specializes in power quality solutions.
The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.
