Identifying and Mitigating Power Quality Issues

Industrial operations consume about one half of all electricity generated worldwide. That's largely due to the massive and constantly moving mechanical operations that take place in plants in the form of industrial motors, pumps and fans.

And the demand isn't dwindling. The U.S. Energy Information Administration estimates that industrial energy consumption will increase from a level of 191 quadrillion Btu five years ago to 288 quadrillion Btu in 2035.

For plant operations, equally as important as the quantity of the power is the quality of the power. Unfortunately, power quality remains too much of an unknown factor for many plant operators. Coupled with the aging grid and growing grid destabilization, power quality can be unpredictable. In an era of advanced metrics being available for virtually every process and machine on the plant floor, power quality largely remains a black hole.

Better understanding power quality – and the ability to dynamically monitor and control it – can have an impact on the overall efficiency of a machine, process or plant and also reduce operational costs. How so? Manufacturers today design plant equipment to operate smoothly with AC power. But the electronics in many cases are digital, converting the AC to DC.  These digital operations can be very sensitive and occurs thousands of times each second.  Due to the way electricity is distributed, there may be some interference in your electric service, whether from storms knocking down power lines or the way other customers in your area use electricity.  When this happens, the AC wave coming into your plant is distorted and may not convert easily into DC. The result may be flickering lights, loss of data on your computer, undetermined equipment shutdowns or even burned-out circuit boards.

There are three main categories of power-quality issues that plant operators should focus their time and resources on:

1. Sags and swells
2. Harmonics
3. Power factor

Sags and swells

The utilities have a very sophisticated power system that delivers a reliable supply of power that satisfies national voltage standards.  But occasionally, electrical systems experience voltage disturbances from natural or man-made causes. This puts the onus of mitigating risk on end users, such as plants, to ensure they have the power quality they need to sustain their operations.

Machines and devices used in a plant are built to operate within a certain range of voltage variation. But they can only accept so little or so much, and voltage outside the “safe” range can either halt equipment due to a drop in voltage or result in damage due to a spike in voltage. These variations are known as sags and swells, and it’s the responsibility of plant operators to make the necessary adjustments to account for them.

Sags occur when an excess in voltage consumption, which takes place either inside or outside a plant, results in too little voltage to power a plant's machines or devices.

Equipment dropping offline is the most common effect of a sag. Additionally, when the voltage returns, a high current inrush can occur because it bypasses the soft-charge circuit. This can generate heat and even damage equipment.

Swells, on the other hand, can send a significant jolt to a system and fry power supplies and components or even destroy equipment.

A number of factors, both inside and outside the plant walls, can cause voltage events. They include:

Despite the impacts that these voltage incidents can have on a plant’s operations, too many plant operators can be figuratively in the dark that they've even occurred. As a result, operators may not know that damages to their machines or operations resulted from power-quality issues. Instead, they assume that any failures are the result of the equipment itself. 

A good rule of thumb is: If a machine failure happens twice and a plant operator or technician can’t identify the root of the problem, odds are strong that the root cause was a voltage incident.

Plant operators should implement a voltage-monitoring system to track and even help predict voltage events across their operations. Voltage monitors are easy to use and install, and they’re virtually maintenance free.

Additionally, plants need mitigation systems in place to protect their plants and machines when voltage events take place. Such solutions for sags include:

• Uninterruptable power supplies (UPS) – Ideal for protecting personal computers, PLCs, data networks and servers, UPS solutions deliver consistent and clean power by correcting low or high voltage conditions in critical process applications. Connected to a plant’s network, UPS solutions also can be managed remotely.

• Adaptive power supplies – An adaptive power supply optimizes the remaining power supply during a sag to maintain an uninterrupted flow of optimal voltage to the load. This is a scalable solution that can provide up to 5 seconds of ride-through voltage during a sag.
• Capacitors – Often localized to a high-value process or asset, capacitors can assist with monitor voltage and then discharge in the event of a sag to provide the necessary voltage for up to two minutes.
• Back-up generators – If a sag is longer and leads to a power-outage event, plants can use back-up generators to provide the necessary voltage for longer periods of time.

The primary solution for swells is soft-start equipment. This allows motors to slowly ramp up and ease into their operations. Soft-start equipment can not only protect equipment but help optimize energy consumption when swells take place.

Harmonics

Harmonics are distortion to a voltage or current waveform. In essence, it’s noisy power. But that noise can cause some serious problems in a plant’s operations, such as increased heat within a system, improper equipment operation, incorrect meter readings and even motor failures. Loads that can produce harmonics include inverters, electric-arc furnaces, switch-mode power supplies, DC converters, and AC or DC motor drives.

The recommended standard for harmonics in North America is IEEE Standard 519-1992. It recommends that buildings keep their percent of voltage distortion at 5 percent or below.

To begin, plant operators should consider having a harmonic modeling analysis done – particularly if they’re adding a large non-linear load to a plant power system. This analysis can predict harmonic distortion levels based on existing power-system data and also confirm if harmonic levels fall within the IEEE standard’s recommended distortion level or other utility limits. Beyond this analysis, plants should use power metering to provide harmonics analysis on an ongoing basis.

The best method for mitigating harmonics is with harmonics filters, which are available in multiple variations:

• Passive – a static filter that mitigates a specific harmonic frequency

• Active – monitors the harmonic frequency and actively cancels out any changes in frequency that result from abrupt load changes

• Notch – adaptive filter that estimates and mitigates harmonic frequencies

Choosing the right filter is crucial to reaching the desired reduction in harmonics at a minimal cost, and a harmonic analysis and power metering can help plant operators determine the best solution.

Power Factor

A significant source of power consumption in plants comes from inductive loads. These loads include motors, generators and transformers, among other things, and are the guilty party when the utility bill arrives with a low-power-factor penalty.

But addressing power factor can impact more than a plant's utility fees. As the U.S. Department of Energy says: “Uncorrected power factor will cause power losses in your distribution system. You may experience voltage drops as power losses increase. Excessive voltage drops can cause overheating and premature failure of motors and other inductive equipment.”

The goal is to achieve unity, which is a power factor of 1, although utilities typically will only send fees if a facility's power factor is less than 0.95. An assessment is a good place to start, particularly in a new facility or after a significant upgrade. This allows a plant operator to identify what or where their power-quality issues may be, and then address them.

But plants are rarely static operations. Processes can change, machines can be upgraded and operations can expand. Unless plants operate in cruise control, their power factor will fluctuate. Because of this, assessments should only be considered the first step in a larger effort to monitor and mitigate low power factor.

Plant operators have two options when it comes to knowing what their power factor is. The first option is to wait for a utility bill to arrive in the mail so they can find out how much they might owe for power-factor correction. The second option is to install power meters. This proactive approach allows plants to independently address low power factor issues before they show up in the form of fees.

If a low power factor has been identified, the only way to bring a plant back toward unity is with a capacitor bank. These simple and pre-engineered banks can be automated to use real-time power and energy data to meet the needs of a plant's power correction.

The Right Connection

All of these power-quality technologies are connected via a plant's network architecture. Utilizing an open standard network architecture like EtherNet/IP eases the integration of these technologies not just across a plant floor, but also across the entire enterprise via the Internet Protocol.

While a proprietary network technology restricts certain data and requires the use of switches, gateways and routers to integrate new technologies, a network technology like EtherNet/IP allows plants to quickly and easily utilize the latest innovations for power quality.

It also takes advantage of the “Internet of Things” and the billions of devices that are connected to it. A direct benefit can be seen in the aforementioned voltage monitors, which can be connected to a Web- or cloud-based power-grid monitoring network. This enables plant operators or other subscribers to follow voltage events on a computer, smart phone or tablet, whether they’re physically at the plant or not.

Such connectivity also enables automatic data management and near-instant notifications via SMS or email. Additionally, it even allows the voltage monitors to be connected to the national weather database to help predict when voltage events might occur.

As plants become more connected and as new technologies become available for the monitoring and mitigation of power-quality issues, plant operators will want to ensure they're prepared to leverage the best of both worlds.

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Power meters should be placed where the utility feeds in and where most of a plant's power is consumed:

• Mains
• Switchgears
• Substations

This enables plant operators to get a more granular view into where power-quality issues may be so they can proactively make any needed fixes. Meters have even enabled plant operators to realize they were being overcharged by their utility company, allowing them to get rebates in the range of tens of thousands of dollars.

If putting metering throughout these three key areas isn't feasible, meters should either be distributed at high-value assets – namely those that would have the highest maintenance costs if they're damaged in a voltage event – or across the 20 percent of assets that account of 80 percent of a plant's energy usage.