A nonlinear load in a power system is characterized by the introduction of a switching action and consequently current interruptions. This behavior provides current with different components that are multiples of the fundamental frequency of the system. These components are called harmonics. The amplitude and phase angle of a harmonic is dependent on the circuit and on the load it drives. For a fundamental power frequency of 60 Hz, the 2nd harmonic is 120 Hz, the 3rd harmonic is 180 Hz, and so on. The harmonic currents flow toward the power source through the path of least impedance.
Some examples of nonlinear loads that can generate harmonic currents are computers, fax machines, printers, PLCs, refrigerators, TVs and electronic lighting ballasts. Personal computers constitute nonlinear loads since they incorporate switched-mode power supplies. The PC current is mainly dominated by the third and fifth harmonic components. Current harmonics deteriorate the power factor of the system, what is the ratio between the average power of a certain load and the average power calculated for a pure resistive load with equal voltage amplitude. Differently from the reactive power drawn by a linear load, the harmonic currents cannot be corrected by the use of capacitors and inductors, but by the use of Harmonic Mitigating Transformers.
Current distortions can produce voltage distortions. When currents with harmonics flow through electrical generation systems and transmission lines, additional distortions take place because of the impedance of the electrical network.
Power systems that are conceived to operate at the fundamental frequency are susceptible to erroneous behavior as more and more nonlinear loads are connected to the network. Harmonics increase the resistances of the conductors due to skin effect and cause an abnormal neutral-ground voltage difference. The more nonlinear loads connected, the higher the overall sum of harmonics, though the total sum is less than the sum of the individual magnitudes. Harmonics can damage components like fuses and circuit breakers, and can cause utility meters to record wrong measurements.
Filters can be used when equipments are connected in a non-sinusoidal system. Nonlinear loads can be modeled, and these models can be used to evaluate the voltage and current harmonics in the loads.