Identifying Common Power Quality Disturbances

No other industry places more demands on electrical power distribution systems than mining. Much of the facility’s infrastructure relies heavily on electric power. The harsh conditions, non-linear loads, extensive use of long cables for mobile equipment, complex grounding, and the adoption of ever larger machinery with higher power requirements all challenge the reliability of the power that mines depend on to keep operations running 24/7, 365 days a year.

While mining engineers and mine managers clearly understand the importance of reliability, they should also know how power quality issues impact the total cost of operations. Mines typically experience four types of power quality disturbances. Identifying those problems is the first step toward developing mitigation tactics that can improve a mine’s safety, reliability, productivity, and cost efficiency.

Power quality is critical in mining, since even short interruptions can lead to safety hazards, equipment damage and production losses costing hundreds of thousands of dollars per hour.

Mining Power Systems

Mines consume tremendous amounts of power, as much as 120 megawatts per 100,000 tons of mined product each day.⁴

Mines draw electric power from a variety of sources, sometimes resulting in a patchwork system. When AC grid access first became available, operators began purchasing much of their electrical power from utility plants. High-voltage DC power remains in use, mainly for draglines and rope shovels, uninterruptible power supplies (UPS), and emergency systems, supplied by AC-to-DC conversion or by onsite DC microgrids. Fuel-driven generator sets (gensets) are also commonly used, as are gas- and coal-fired boilers, and renewable energy sources.

Transmitting electricity over long distances to a mine can result in power loss, voltage drop, interference, and weather-related outages from high winds, lightning strikes, or ice storms. Utility supply switching (breakers, contactors, or taps) can also give rise to damaging voltage surges.

Power systems in mining operations vary greatly but in general, they will follow a basic design: high-voltage power coming off the electric grid feeds high-voltage transformers that supply a main substation. In turn, the main substation distributes energy to multiple secondary substations as well as directly to the mine’s largest motor loads. Secondary substations supply power to medium voltage loads and to medium voltage/low voltage transformers that connect to motor control centers. At any stage in this distribution there are power quality threats that can enter the system.

Internal Power Disturbances

While the first inclination of some mine operators is to blame the utility for power quality issues, that view is frequently inaccurate. It’s more common that power quality problems arise from dynamic conditions within the mining facility, manifesting in the form of overvoltage swells, undervoltage sags, harmonic distortion, or total power interruption.

All mine power systems face the challenges of poor power quality. The costly consequences range across operational interruptions, lost data, damaged equipment, and energy inefficiency, to utility fines, reduced productivity, and even worker safety. Due to a mine power system’s complexity, a single power quality solution is rarely adequate to ensure reliable power. The variability of power disturbances therefore requires a multi-tiered approach that often starts by identifying power-offending devices, such as drives, motors, and pumps.

The digital age has come to mining and it brought with it new power quality challenges. Over the past 20 years, the mining industry has implemented more and more automation control within processes. Most of the sophisticated electronics being installed are highly sensitive to power quality disturbances, much more so than traditional machinery. Digital equipment requires protection using tracking filters, surge protective and power conversion products, including K-Factor and Drive Isolation Transformers. Last, but not least, all control panel points should employ surge protection devices.¹

Common Power Quality
Disturbances in Mines

Voltage Sags and Undervoltage—Voltage sags and extended undervoltage conditions are the most common power quality disturbances in underground longwall mining. They occur when a large increase in load current stresses the electrical supply system, causing the supply voltage to drop below levels at which equipment is designed to operate. Voltage sag, as defined by IEEE, is a reduction in voltage for a short time. The voltage reduction magnitude is between 10% and 90% of the normal root mean square (RMS) voltage at 60 Hz. The duration of a voltage sag event, by definition, is less than 1 min and more than 8 msec, or a half cycle of 60-Hz electrical power.² Undervoltage events are similar to voltage sags but extend over one minute.

In mines, the underlying causes of sags and undervoltage may include the start-up of high-power motors, transformer inrush, ground faults, short circuits, or tripping and reclosing circuit breakers. Contributing factors may include the long cables commonly used for mobile equipment, and the incorrect specification of transformers.

Both sags and undervoltage may cause nuisance tripping of breakers, equipment malfunction and shutdown, or premature equipment failure. Operation at reduced voltage will increase heating effects at high-resistance connections, elevating the risk of combustion or explosion in classified areas. Signs of these issues include dimmed or flickering lights, poorly operating HVAC units, and motors running hot. PLCs, automation control systems, and computers may lock up or power down, resulting in lost data, increased production costs, and a greater probability of equipment failure.

Overvoltage/Swells—Overvoltage and voltage swells are also known as transients, spikes, or impulses. The control of overvoltages is critical to maintaining mine uptime. An overvoltage condition may occur by accidental contact of equipment with a higher voltage system, or from transient phenomena due to lightning strikes, intermittent ground faults, autotransformer connections, or switching surges. The maximum ratings for cable insulation, transformer windings, relay contactors, and so forth may be temporarily exceeded in these cases. This does not usually result in an immediate breakdown of equipment, but component parts of the electrical system are successively overstressed and weakened by repeated exposure. This leads to premature failures, reduced component life, and mysterious “nuisance trips,” which can occur without apparent reason.¹

One of the serious consequences of overvoltage is insulation degradation. Deteriorating insulation jeopardizes the safe operation of the entire mine power system. Weakened or failed insulation can serve as a catalyst for electric faults, power outages, fires, and the explosion of methane and coal dust. Insulation is designed for its safe maximum applied voltage along with a transient overvoltage rating indicating the peak voltage it can withstand. If regularly subjected to above peak voltage, insulation will progressively weaken until it fails, resulting in a line-to-ground or line-to-line fault. Weakened insulation in a portable mining cable represents a safety hazard since the insulation appears to be functional when a lethal potential may exist on its surface.

Harmonics—Harmonics refer to voltages or currents having frequencies that are integer multiples of the frequency at which the supply system is designed to operate. Harmonics combine with the fundamental voltage or current and produce waveform distortion. Harmonic distortion exists due to the nonlinear characteristics of devices — such as variable speed drives (VSDs) — and loads on the power system.³ Harmonics can lead to increased heating in equipment and conductors, misfiring of VSDs, and torque pulsations in motors. Other symptoms of harmonic distortion in a mining power system are interference in the mine’s communication system, flickering lights, tripped breakers, and even loosened electrical connections.

The vast majority of the loads in a mine are electric motors, most of which today feature non-linear VSDs, making VSDs the main source of harmonics in a mining operation. Standard 3-phase power VSDs have a full wave rectifier that improves motor efficiency but that generates considerable harmonics.

Total Power Interruption—In remote areas of the world, local utility grids may struggle with the capacity to reliably support the power demands of a modern mine. Weather can wreak havoc on long suspended power lines supplying a mine. Gensets can fail, as can microgrids, leaving a mine in the dark.

In the event of a total power disruption all mining production comes to a halt. Safety standards require emergency diesel generators to be on-site to supply power to critical loads in case of blackouts, ensuring that mine workers can safely evacuate the mine.

Electrical equipment is another story. If left unprotected, the lifespan for even the most rugged machinery will be shortened as the result of sudden power interruptions. Blackouts are especially devastating to devices where normal shut down sequences are critical. Sudden power loss for computer-based equipment, such as PLCs, automation systems, and industrial robots can result in corrupt files, lost data, and possible damage to the operating systems. When this type of equipment is brought back online, the operating system may fail to boot, or critical operational data may be lost. Furthermore, the physical lifespan of the equipment can be reduced by frequent power losses.

Applying a multi-tier power quality and protection strategy has proven to lower mining costs, minimize hazards, and extend equipment longevity, helping to ensure that operations remain profitable and sustainable over time. Today’s mines have greater power requirements than ever before. These increased and varied loads have come at the same time as the adoption of sensitive electronics in mines for automation. The increased number of devices drawing power from the utility grid has led to greater dependency on the level of power quality required in today’s highly competitive, efficiency-driven mining industry.

This article was written and submitted by Emerson.

References:

1. Mine Power Systems, Lloyd A. Morley, January 1990. www.cdc.gov/niosh/mining/works/coversheet624.html; www.cdc.gov/niosh/mining/UserFiles/works/pdfs/ic9258.pdf.

2. Voltage sags and what to do about them, Jack Smith, August 2002. www.plantengineering.com/articles/voltage-sags-and-what-to-do-about-them/.

3. IEEE Std 1159-1995(R2001) IEEE Recommended Practice for Monitoring Electric Power Quality.

4. Power Quality Issues of Distorted and Weak Distribution Networks in Mining Industry: A Review, Jalil Yaghoobi, Ahmad Abdullah, Dinesh Kumar, Firuz Zare, and Hamid Soltani, November 18, 2019. https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8889719.