In the vast ocean of power quality issues, harmonics are often overlooked. However, harmonics can be costly in terms of reduced efficiency and shorter equipment lifespan. Understanding the causes of harmonic distortion and how it can affect your electrical equipment can help you find the right solution for your facility.
A higher frequency
Surfers are always looking for the perfect wave and so are many electrical devices. For electric current, perfection is the sine wave, in which voltage delivered to your facility changes smoothly from positive to negative and back again 60 times per second (60 Hertz). Commonly used electrical devices, such as incandescent lights and resistance heaters, are called "linear" because their amperage rises and falls in direct proportion to the undistorted voltage wave.
In recent decades, the proliferation of "nonlinear" devices, such as computers, electronic ballasts and variable frequency drives, has led to harmonics problems in many facilities. Nonlinear devices draw current in pulses rather than continuously, adding levels of (harmonic) current waves onto the 60 Hertz sine wave. These harmonic waves operate at much higher frequencies and their addition results in a distorted waveform.
Harmonic levels and the problems they cause vary depending on the facility and the type of equipment used. Typical issues include:
- Overheating in neutral conductors, transformers or induction motors
- Malfunctioning of computers and electronic equipment
- Failure of power factor correction capacitors
- Nuisance tripping of circuit breakers
Harmonics problems can cost your facility in a number of ways. Downtime from equipment failure or malfunction can result in lost productivity and increased maintenance. Overheating of transformers and motors can shorten equipment life and require repairs. The extra heat can also result in increased cooling costs in air conditioned areas.
Leveling the field
There are a variety of ways to deal with harmonics, but not every solution is appropriate for your situation. Commonly used strategies for mitigating harmonic distortion include:
Oversizing neutrals oversize the neutral conductor in shared circuits to twice the size of the phase conductor when the load includes nonlinear equipment.
Oversizing transformers upgrade equipment size for increased thermal capacity for transformers serving harmonics producing loads.
Zig-zag transformers can be installed near equipment that produces large harmonic currents. The transformer traps the harmonic currents and prevents them from traveling upstream.
Phase-shifting transformers and multi-pulse VFDs use a combination of delta- and wye-wound transformer secondaries which results in cancellation of harmonic frequencies.
Passive harmonic filters recirculate the harmonic currents back to the load without causing voltage distortion at the chosen frequencies.
Active harmonic filters inject equal and opposite harmonics onto the power system to cancel those generated by other loads.
K-rated transformers are specially designed to tolerate harmonics. The K-rating (usually 1 to 50) indicates the transformer's ability to withstand degradation from harmonics effects.
The first step in solving a harmonics problem is to carefully evaluate your electrical system and equipment to determine the source and extent of the problem. Armed with this information, you can find the right harmonics solution for your application.