Practical Solutions to Industrial Power Quality Issues
Power delivery is changing. New technologies, devices and loads are coming online, and utilities cannot always guarantee flawless electricity distribution to industrial facilities. This combination can lead to power quality issues — stemming from both sides of the electric meter — that can damage equipment and slow operations.
Here are a few power quality events and what you can do about them.
Voltage unbalance occurs when there is a discrepancy between the phase voltages in a three-phase power system. Many issues can lead to voltage unbalance, but it is primarily caused by uneven distributions of single-phase loads over a three-phase power system.
The National Electrical Manufacturers Association (NEMA) sets limits for the maximum unbalance electric motors should experience, and exceeding that limit can damage equipment. Even a voltage unbalance of just 1% can weaken motor efficiency, and increased unbalance adds heat to motors and is detrimental to motor life as well.
Solutions for managing voltage unbalance include using phase monitors. These devices, when working correctly, are designed to protect motors by managing the amount of voltage unbalance the motors see. If the incoming voltage exceeds the established level, the phase monitor sends a signal to a three-phase contactor, which can shut the equipment down. This functionality helps restrict the amount of unbalance and under/overvoltage that make it to the motor terminals.
Phase monitors are also beneficial because they are relatively inexpensive, and once they are initially set, they can be left alone. However, they may nuisance trip if not configured correctly, so be sure to have an electrical professional add them to your equipment to verify proper setup.
With advancements in technology, more equipment, particularly high-frequency equipment, is able to react quickly to changes in power input. While beneficial, this ability also increases sensitivity to voltage sag, which, as it sounds, is a short reduction in voltage (as short as several microseconds). Even these brief dips can be enough to trip equipment.
Voltage sag is caused by abrupt increases in load on a power grid and can either stem from a utility or facility equipment. Voltage sag is particularly detrimental for processes that take a long time to restart, or where a shutdown causes a long delay in production, such as server farms and paper or plastic rolling applications.
Testing with a sag generator can help identify if the problem is arising, and ride-through devices can be placed on equipment to put a slight delay on response times, which decreases sensitivity to sags. These devices can save money and labor hours if used correctly.
The inverse of voltage sag is voltage surge, also known as voltage swell. Though less common, a surge occurs when there is a brief increase in voltage. This can happen, for example, when a large load is turned off or due to a lightning strike that is not directed to ground.
Just like surge protectors are used in homes to protect against voltage surge, surge arresters can be deployed in industrial facilities. These devices can be placed on equipment or on lines to help mitigate harmful effects.
When we plug something in, the current and voltage produce a sine wave. In a perfect world, this sine wave would only contain the first order wave (or 60 Hz portion in the U.S.). However, non-linear technologies that rely on capacitive reactive power — such as CFLs, LEDs, computers and semiconductors — are associated with more than just the first order wave (higher than 60 Hz), and these are growing in popularity. These devices have multiple transistors with non-linear semiconductors, and their sine waves are made up of many other sine waves, producing harmonic distortion.
Harmonic distortion refers to the percentage of all waves going to an electric load that is not the first order harmonic (think the buzzing sound you hear over headphones when there’s poor audio quality). Electric equipment can have both voltage and current distortion, and all non-linear devices experience at least some distortion. While they still function, they could fail prematurely.
Combating harmonic distortion requires identifying what should be added to a circuit to eliminate the issue. A variable frequency drive (VFD) on a motor that utilizes capacitors to store voltage for the DC bus, for example, can offset the motor’s inductive load but add current distortion in the process. As a remedy, more inductance would be needed to limit this distortion, which could come in the form of a line reactor (added before the VFD), a load reactor (after the VFD) or DC choke (on the VFD). Each type of reactor has benefits and should be considered based on application needs.
Power quality events have the potential to disrupt equipment and plant operations. Knowing what to look for and ways to manage issues can go a long way toward ensuring your facility stays up and running.