In a recent webinar from our Exploring North Carolina’s Clean Energy Transition program, we learned about power quality issues and solutions in the large business and industry sector. The session featured Matt Davey, lead power quality engineer at Duke Energy; Robert (Bob) Beadle, director of grid infrastructure at the North Carolina Electric Membership Corporation; Tyler Harriett, system engineer at Halifax Electric Membership Corporation; and our own Michael Lyda, technical supervisor for Advanced Energy’s large business and industry team.
The presenters discussed the relationship between power quality and reliability, how power quality can affect large utility customers and ways to mitigate the impact of power quality disturbances and interruptions.
Power Quality from Duke Energy’s Perspective
Duke Energy’s Matt Davey began by explaining how grid reliability and power quality are two sides of the same coin. Reliability metrics and measurements — such as SAIFI, SAIDI, CAIDI and MAIFI — generally center on the frequency and duration of power outages. Power quality emphasizes the usability of power that is available. Even with available power, equipment may still run improperly for a variety of reasons.
Because of the diversity of industrial equipment and the demands of different industries, power quality issues — fluctuations in available and reliable power that can impact facility equipment — are varied and potentially complicated. They can include the following:
- Momentary interruptions that produce dips or increases in voltage (known as voltage sags and swells, respectively)
- Under or overvoltage, in which the delivered voltage fails to reach or exceeds the voltage requested by a customer
- Voltage unbalance in facilities with three-phase service, in which one phase is substantially higher or lower than the others
- Very brief — on the order of microseconds — voltage transients; programmable logic controllers (PLCs) and variable frequency drives (VFDs) are particularly sensitive to these
- Harmonic distortion, which is appearing more frequently with the arrival of more efficient and inverter-based equipment, including VFDs
Power quality also encompasses how end-use equipment interacts with the grid, and how events on one part of the system can impact another. For example, consider a vehicle accident that causes a grid fault or short circuit. The directly affected circuit might experience a brief power outage. The adjacent or unaffected circuit, while maintaining power, could itself experience a voltage sag, maybe dipping to 60% of the nominal, expected voltage.
Even brief power quality events can make it difficult for machines to operate properly and can have severe consequences. In general, poor power quality can result in significant impacts for businesses in the form of equipment damage, increased maintenance costs, scrapped product, lost production time and the implementation of mitigation strategies.
Power Quality for Rural North Carolina Electric Cooperatives
Commercial and industrial members make up only about 6% of the member base of the North Carolina Electric Membership Corporation, but power quality remains a key area of focus. In fact, as electrification increases — along with the growth of facilities such as data centers with specialized equipment — it’s becoming even more important.
The “typical” rural co-op system uses long, smaller feeders, which can amplify voltage problems. A particular goal for co-ops is to make sure that members at the ends of lines — furthest from the substation — avoid issues due to voltage drop or general line exposure (which increases the likelihood of power disruptions caused by vegetation or animals).
In large manufacturing facilities, another concern is limiting what’s known as across-the-line starts. In these situations, all motors come online at once, causing voltage to drag due to high in-rush current. Staggering motor startups is one way to stabilize the system. Other power quality issues experienced by co-op members include low power factor, which could require higher-capacity wires and facilities; grounding problems; intermittent voltage swings (poor voltage regulation); harmonics; and higher system losses.
In addition to impacts on system performance, poor power quality can have financial consequences as well. For instance, co-ops may incur penalties as part of power purchase agreements and transmission service from Duke Energy and Dominion Energy.
A specific power quality example was illustrated by Halifax Electric Membership Corporation’s Tyler Harriett. A few years ago, a medium-size sawmill approached the co-op with voltage problems, as its saws were stalling during cuts.
Halifax visited the site and installed a power quality meter on the pad-mounted transformer. This device provided high-speed sampling of electrical quantities at the transformer to gather data and assess what was occurring (think of it like a heart monitor that might be worn by a hospital patient).
Using the power quality meter, the team realized that the site was seeing a power factor as low as 0.4. Ideally, this value, which is the relationship between a site’s real power in kilowatts and its apparent power in kilovolt-amperes, should be unity, or 1.0.
To bring the sawmill’s power factor closer to unity, Halifax added power capacitors. Power capacitors help balance out the inductive load of motors, which are common in sawmills, to improve the efficiency of the distribution system. They reduce the total current drawn from the generator and flowing through the system.
The power capacitors worked, and with power factor improved — it was corrected to around 0.85 — there was less reactive current flowing, which reduced system losses by nearly 78%. Broadly speaking, other benefits of power factor correction include the following:
- Voltage support and less power generated for the same facility output
- Potential for lower utility bills by avoiding penalties
- Reduced loading on transformers and distribution lines, leading to more availability for other members without needing to increase capacity/capital investment
- Potentially extended motor life thanks to reduced load current flowing through the motor windings
Simulating Power Quality Events
Advanced Energy’s Michael Lyda concluded the webinar by discussing the organization’s recently procured voltage sag generator. A sag generator can produce voltage sags and swells to emulate what might be seen in the real world. Advanced Energy’s model is a three-phase unit rated for up to 480 volts and 60 amps. It can produce sags down to 30% of the rated voltage and swells up to 125%, and can implement sag durations from ¼ cycle (~4 milliseconds) to up to 10 seconds. Furthermore, the three phases are independently controlled, allowing for unbalanced sags and swells on each phase.
Over the next few months, Advanced Energy plans to use the sag generator to conduct exploratory lab testing on various pieces of polyphase and single-phase equipment, including motors, drives, contactors, relays, etc. Later, the team would like to bring it to customer facilities, helping component and equipment manufacturers evaluate the sag tolerance of their devices.
Overall, the sag generator should be a great addition that supports manufacturers, end-use customers and Advanced Energy’s utility members: Duke Energy, North Carolina’s Electric Cooperatives and Dominion Energy.
As power delivery and electrification continue to evolve, power quality will remain at the forefront of utility and customer concerns. Fortunately, there are ways to mitigate issues that do arise. If you have a question about power quality in your facility, reach out to your local utility or Advanced Energy for guidance. To learn more about power quality and other aspects of North Carolina’s clean energy transition, visit www.ncenergytransition.org, and stay tuned for future webinars.