The Problem: Harmonics
To begin, when discussing the topic of harmonics, it is important to note that we almost exclusively are discussing three-phase power systems. Harmonics found within single-phase power systems are typically ignored due to their small size.
Stated previously, harmonics are electric voltages and currents that appear on the electric power system because of non-linear electric devices. A non-linear device is one in which the current is not proportional to the applied voltage. Examples of non-linear loads include:
- Arcing devices such as arc furnaces
- Power electronic equipment such as variable-speed drives, battery chargers and rectifiers
- Office equipment such as photocopiers, laser printers and fax machines
Ideally, voltage and current waveforms are perfect sinusoids. With non-linear loads, these waveforms are often distorted. This deviation from a perfect sine wave can be represented by harmonics which are sinusoidal components having a frequency that is an integral multiple of the fundamental frequency.
Thus, a pure voltage, or current sine wave, has no distortion and no harmonics; a non-sinusoidal wave has distortions and harmonics. To quantify the distortion, the term Total Harmonic Distortion (THD) is used. THD expresses the distortion as a percentage (%) of the fundamental (pure sine) of voltage and current waveforms.
High harmonic currents can have several negative effects on a facility. High levels of distortion can lower power factors, overheat equipment, and lead to penalties from the local utility for exceeding recommended limits. Each of these effects can result in higher cost to the facility.
Poor Power Factor
The harmonic currents caused by the non-linear loads do not carry any real power (kW) even though they increase the volt-amperage (kVA). Because true power factor is equal to the watts divided by the volt-amperes:
Factor = W/VA
with any increase in volt-amperes without a corresponding increase in watts will lower the power factor. A lower power factor will affect facilities in two ways: Losses inside the facility will increase due to the higher level of current required to perform the work.
Utilities will charge a penalty if the power factor falls below a predetermined level. Both of these will increase utility bills.
Overheating of Transformers
Overheating of transformers is another issue associated with harmonic currents. Overheating shortens the life of the transformer. ANSI/IEEE Standard C57 states that a transformer can only be expected to carry its rated current if the current distortion is less than 5%. If the current distortion exceeds this value, then some amount of de-rating is required. The overheating is primarily due to the higher eddy-current losses inside the transformer than were anticipated by the designer. The overheating can be avoided by either de-rating the transformer or by specifying a “k-rated” transformer that is designed for the higher levels of eddy currents.
Large Currents in Neutral Wires
Another effect of harmonic currents on the power system is the overheating of neutral wires in wye-connected circuits due to the third harmonic and any multiples thereof that do not cancel in the neutral, as do other harmonic currents. The result is a large 180-Hz current in the neutral conductor—if there are significant non-linear loads connected to the wye source. Usually the higher multiples of the third harmonic are of small magnitude. The attended increase in the RMS value of the current, however, can cause excessive heating in the neutral wire. Currents as high as 200% of the phase conductors have been seen in the field. This large level of current can easily burn up the neutral creating an open neutral environment with very serious consequences.
Harmonic currents can distort the voltage waveform and cause harmonic voltages. Voltage distortion affects not only sensitive electronic loads but also electric motors and capacitor banks. In electric motors, negative sequence harmonics (i.e. 5th, 11th, 17th), are so called because their sequence (ABC or ACB) is opposite of the fundamental sequence, produce rotating magnetic fields. These fields rotate in the opposite direction of the fundamental magnetic field and could cause not only overheating but also mechanical oscillations in the motor-load system.
Problem on Capacitor Banks
On facilities with capacitor banks, the reactance (impedance) decreases as the frequency increases. This causes the capacitor bank to act as a trap for higher harmonic currents from the surrounding utility system. The effect is an increase in current, in heating and an increase in dielectric stresses that could lead to capacitor bank failure.
Other Problems Caused by Harmonics:
- Unacceptable neutral-to-ground voltages
- Breakers and fuses tripping
- Interference on phone and communications systems
- Unreliable operation of electronic equipment
- Erroneous register of electric meters
- Wasted energy/light electric bills - kW & kWh
- Wasted capacity - inefficient distribution of powerIncreased maintenance of equipment and machinery