What power quality means
Ideally, the voltage supplied to a plant would be a smooth sine wave at a steady magnitude and frequency. Power quality describes how closely the real supply matches that ideal, and how well equipment copes when it does not. Many problems that look like equipment failures, such as random drive trips, overheated transformers or flickering lights, turn out to be power quality issues.
This article is a general introduction. Measuring and correcting power quality problems requires proper instruments and qualified engineers, and the right answer depends heavily on the specific site.
Harmonics: distorted waveforms
Harmonics are currents or voltages at multiples of the basic supply frequency. If the supply is 60 Hz, for example, the third harmonic is 180 Hz, and the fifth is 300 Hz. These add to the fundamental wave and distort its shape.
Harmonics come mostly from nonlinear loads, which draw current in pulses rather than smoothly. Common sources in plants include variable speed drives, rectifiers, switching power supplies, welding equipment, arc furnaces and some lighting. One such load is rarely a problem on its own, but a plant full of them can add up.
The current distortion flows through the plant impedance, which in turn distorts the voltage seen by other loads. That is how one machine can affect its neighbors.
What harmonics can do
Typical effects include:
- Extra heating in transformers, cables and motors, which can shorten insulation life.
- Overheated neutral conductors in some three-phase, four-wire systems, because certain harmonics add up in the neutral rather than canceling.
- Nuisance operation of breakers, relays or sensitive electronics.
- Interference with metering and communications.
- Stress on capacitors, especially when a resonance condition amplifies a harmonic.
How severe the effect is depends on the proportion of nonlinear load, the strength of the supply and the design of the system.
Voltage sags and swells
A sag is a short drop in voltage, lasting from a fraction of a second to a few seconds. Sags are usually caused by faults elsewhere on the utility network or in the plant, or by starting large motors. A swell is a short rise in voltage, which can occur after a fault clears or when large loads drop off.
Sags are a major cause of production interruptions. A drive, a contactor or a controller may drop out even though the lights barely dim. Equipment varies in how well it rides through, and the manufacturer’s data is the best guide to its tolerance.
Other disturbances
Several other issues fall under the same heading:
- Transients: very brief spikes, often from switching or lightning, which surge protection is meant to limit.
- Interruptions: complete loss of voltage, short or long.
- Voltage imbalance: unequal voltages across the three phases, which can cause overheating in three phase motors.
- Flicker: repeated small voltage variations, often caused by fluctuating loads like arc welders.
- Frequency variation: more of a concern with on-site generation than with a strong utility.
Finding the cause
Because many disturbances are brief, investigations typically rely on power quality analyzers that record waveforms and events over days or weeks. Staff often start with simple questions. When do the problems occur? Do they line up with a particular machine starting? Do only some devices on the network misbehave? Records of events, along with a single-line diagram, help narrow it down. Interpreting recordings requires experience, so many plants involve specialists.
Common ways to reduce problems
The remedies depend on the cause, and a few examples show the range. Line reactors or filters can reduce harmonic currents from drives. Drives with built-in reactors or low-harmonic designs reduce the issue at the source. Dedicated harmonic filters, transformers with special windings and separating sensitive loads onto their own feeders are other approaches. Ride-through features in drives and in programmable logic controllers help with sags, and uninterruptible power supplies can protect critical control equipment. Correcting power factor with capacitors needs particular care in the presence of harmonics, because the capacitors can interact with them.
None of these is automatically right. Adding a filter without understanding the problem can make things worse, so measurements should come first.
Links to energy and monitoring
Power quality data is often collected alongside other consumption data in energy management systems, which helps connect unexplained stoppages with electrical causes. Motors fed from distorted supplies can run hotter, which is one more reason industrial electric motors are checked during investigations.
Why it matters economically
Power quality events can cost a plant production time, scrap and wear on equipment even when no single event is dramatic. A practical approach is to treat power quality as part of reliability planning, alongside other topics such as power factor and surge protection. Monitoring trends also helps connect unexplained stoppages with electrical causes.
Safety note
Power quality investigations often involve connecting instruments to live equipment. That should be done only by qualified personnel using appropriate protective equipment, following manufacturer instructions, site procedures and local regulations.