Using an "offshore" approach to control harmonics can save refiners design dollars and space, says power systems pro.
By making various on-site processes more automated, refiners seek to achieve greater efficiency, productivity and safety in their operations. However, the computer-controlled equipment directing these automated processes may be prematurely degrading and delivering just a small fraction of its intended service life. The reason? Harmonic currents.
"Harmonics are currents generated by today's modern equipment in refineries and other industrial and commercial facilities," said Dan Carnovale, manager of Eaton's Power Systems Experience Center near Pittsburgh. He explained that power electronic loads such as variable frequency drives (VFD), computers and energy-efficient lighting systems produce harmonic currents.
"Refineries are continually adding more automation to their processes and these loads are often susceptible to the 'new normal,' which includes some harmonic distortion," he said. "Overheating of equipment, oversizing of equipment and damage from these harmonics cost both capital dollars and operating expense to refineries."
A Potential Problem for Neighbors
Harmonics produced at a refinery can create problems beyond that facility. Unlike self-contained offshore power systems, refineries and other large land-based industrial plants tie into to the power grid. As a result, harmonics from a single facility can "infect" the grid and the voltage distortion can diminish the power quality for a utility's other customers. In effect, harmonics issues that cause equipment to degrade at a refinery can lead to similar problems at other facilities.
Like Bad Fuel In An Automobile
Harmonics, classified as "non-linear loads," draw a current that does not resemble the 60-hertz (Hz) voltage source, Carnovale continued. As a point of reference, North America's electric power supply is designed to deliver homes with 120 volts of electricity at a frequency of 60 Hz – or 60 cycles per second. Refineries receive four times the voltage. Although this combination of voltage and frequency is not unique to North America, power grids in many other parts of the world are designed to supply 230 volts at 50 Hz.
"Basically, any equipment that converts the power from alternating current to direct current or converts the frequency from 60 Hz to another frequency creates harmonics," Carnovale said. Like bad fuel in an automobile, the distorted electrical waveforms make an electrical system run less efficiently, he added. Moreover, he pointed out the poor-quality electricity makes motors, drives and other equipment run harder and hotter, hiking power costs and causing premature wear and tear.
"Systems designed for 60 Hz currents may overheat and degrade from excessive harmonic current flowing through the system," Carnovale explained. "The insulation on transformers, for example, designed for a 20- to 30-year life may degrade and the transformer may prematurely fail after only three to five years in the presence of high harmonic currents."
"Harmonic currents also cause harmonic voltage distortion," he added, noting that computer-controlled processes are most susceptible to the problem at a refinery. "Processes in a refinery may mis-operate when the voltage is distorted significantly. This problem is exacerbated when the system is operating on backup generators under contingency situations."
Refiners have traditionally employed a variety of approaches – such as installing "oversized" and heavier "hardened" transformers and backup generators – to compensate for power losses stemming from harmonics, said Carnovale. However, he pointed out that taking such measures can be expensive and may result in the loss of valuable space. As a result, refiners are increasingly employing "offshore" approaches to control harmonics, he said.
A Dramatic Illustration of Harmonics
On Nov. 7, 1940, less than five months after it opened to traffic, the first Tacoma Narrows Bridge collapsed into Puget Sound in Washington State. The spectacular failure of the suspension bridge followed regular vertical movement of the deck in windy conditions – a phenomenon that led to the bridge's nickname "Galloping Gertie." According to the Washington State Department of Transportation, the collapse led to a major change in bridge engineers' understanding of "aerodynamics and the dynamic effects of wind forces."
The failure of "Galloping Gertie" also provides a clear visualization of the damaging effect of harmonics, said Dan Carnovale, manager of Eaton's Power Systems Experience Center.
"In that case, the wind 'excited' the mechanical oscillations - called 'resonance' – and eventually destroyed the bridge because there wasn't enough damping in the system to control the resonant condition," he said.
"With harmonic-producing loads, the harmonics – like the wind – excite resonance with power factor correction capacitor banks and amplify the harmonics to a dangerous level," Carnovale continued. "Unfortunately, this is a self-correcting problem. Either the capacitor bank will blow up, the fuses will blow or another main component like a transformer may fail, taking the system out of resonance – a less-than-ideal situation!"
Controlling Harmonics, Offshore Style
Unlike refineries and other land-based facilities linked to the power grid, offshore oil and gas drilling rigs and production platforms rely exclusively on onboard generators to produce electricity. Moreover, space is at a premium on these installations. As a result, offshore operators have long deployed specialized, low-profile devices that screen out much of the harmonic voltage distortion, noted Carnovale.
'The 'strength' of the power system is limited by the generators on the platform," he said. "Therefore, many customized harmonics filters and reactive compensation devices have been employed to control the harmonic currents offshore … Active filters have become more prevalent as detailed analysis is not required for their use and they (need) much less weight and footprint compared to traditional harmonic filters."
Where space is less of an issue, phase shifting – canceling out harmonics produced by transformers with step up/down connections to generators – is another route to mitigating voltage distortion, he added.
Refineries have taken note of these offshore solutions and are more frequently deploying power electronic solutions such as active filters, Carnovale continued. In addition to requiring minimal design consideration, using the space-saving harmonics-control equipment protects an electrical load within a refinery as well as the power system feeding the facility, he said.
"Any process industry is only as good as its weakest link," Carnovale concluded. "As we rely more on automation and control, we must eliminate or at least minimize the susceptibility of our critical loads to harmonic current and voltage issues."
The original Tacoma Narrows Bridge: a victim of harmonics. Photo courtesy of the Washington State Department of Transportation.
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