Power system harmonics distort the shape of the perfect voltage and current sinusoidal waveforms ideal to the power grid, and are multiples of the fundamental grid frequencies of 50 or 60 hertz found throughout the world. Problems caused by harmonics include overloaded circuits and higher system losses that can lead to premature equipment failure in utility and customer systems. Low utility power factor is generally associated with harmonics in electric metering.
Causes of Harmonic Distortion
Non-linear power electronic devices such as adjustable speed drives, switching power supplies and diode rectifiers are the main culprits of harmonic distortion. These devices operate by either chopping up the alternating current (AC) waveform or converting it into another form known as direct current (DC). In the process, power is switched on for only part of the time and is off during the rest. This non-linear action introduces harmonic elements that alter the shape of the original waveform. If severe enough, and left unchecked, harmonics can find their way into other parts of the system and cause damage.
Engineers use a tool called Fourier analysis to mathematically demonstrate a distorted yet periodic waveform is actually made up of a fundamental plus a series of sine wave components combined in the right proportions. For example, the half-moon waveform output of a half-wave diode rectifier is very rich in harmonics. Even-ordered harmonics (e.g., 100/120 hertz) are usually not a problem compared to odd-ordered harmonics (e.g., 150/180 hertz), because most electronic loads have half-symmetric waveforms that do not contain even harmonics.
In power systems the triplen harmonic series, which are odd multiples of the third harmonic (e.g., 3rd, 9th, 15th, 21st, etc.), cause the most concern. Three-phase power is often supplied as a four-wire service that contains a neutral, or fourth, conductor. Under normal balanced conditions, when each of the three phases are carrying an equal share of the load, the current in the neutral wire is zero, or nearly so. However, when triplens are present, the net effect is additive and the neutral conductor may end up carrying more current than it was designed for, resulting in overheating and possibly fire. Triplens can also cause problems in three-phase delta transformers. Unless the transformer was specially designed to accommodate harmonics, the circulating triplen currents can overheat the transformer windings and reduce its nameplate rating.
Total current harmonic distortion and total voltage harmonic distortion, represented as THDV and THDI respectively, are two important figures of merit used to quantify the presence of harmonics in a system. Using the fundamental voltage or current quantity as a baseline, the sum of the contribution of all harmonic elements (excluding the fundamental) is compared against it. For example, a THD equal to 100 percent means the harmonic contribution is just as large as the fundamental. Current distortion can range from a few percent to more than 100 percent, while voltage distortion is generally less than five percent.
Power factor is another key performance metric utilities use to determine how efficiently customers use power. True power factor is a combination of both a displacement power factor, associated with the fundamental component, and also a distortion power factor associated with harmonics. When no harmonics are present, true and displacement power factor is the same. However, with harmonics, true power factor generally decreases due to distortion.
Power Quality Metering
Modern solid-state digital utility meters today have advanced capabilities that can measure the magnitude of individual harmonics present in a system and also compute total harmonic distortion. A version of Fourier analysis, known as the Fast Fourier or Discrete Fourier Transform methods, can be coded into the meter’s firmware to perform these tasks. Power factor is commonly metered by utilities to determine how efficiently customers use power and the burden they place upon the power grid. Both the local utility and regulators set minimum power factor standards to ensure efficiency. Customers who fall below the threshold may wind up paying extra in the form of demand charges incurred on their bill.