Large industries and
electrical utilities are very concerned about the presence of nonlinear loads
in their electrical power systems. This article describes the causes of
harmonics and their effects, as well as the means to improve power quality and
to protect the equipment.
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.
There is great concern about
the distribution transformers that supply the nonlinear loads, since they
suffer from: (1) overheating of windings, insulation, and oil; (2) additional
eddy current heating in metallic parts; (3) higher stress in tap changers,
bushings, and cable-end connections. The overheating of general wiring due to
skin and proximity effects is not so serious as the heating of the neutral
conductor in a 3-phase system with a line-to-neutral connection. The harmonic
spectrum of the current must be analyzed in order to determine the excess
losses in transformers.
Transformers supplying
nonlinear loads must have additional overcurrent protection. Linear loads that
are supplied with harmonic voltage distortion draw a nonlinear harmonic
current. The life of a motor is reduced due to overheating. Some equipment,
like communications and data processing devices, may fail when connected to systems
that contain a large amount of nonlinear loads and, consequently, harmonics.
The K-factor is a rating
system that was conceived to indicate how well a transformer can handle
harmonics generated by nonlinear loads. A linear load provides a K-factor of 1 (one).
The higher the K-factor, the greater the harmonic heating effects. The main
objective of creating this indicative factor is to design a distribution
transformer which can operate under certain conditions without life loss.
Since harmonics cause much
more problems than simply overheating the transformers, it is more appropriate
to make use of a resource that is able to reduce the system voltage distortion.
Harmonic Mitigating Transformers are efficient in solving overheating and the
power quality problems generated by harmonics. The voltage distortion at the
output is kept very low because of the cancellation of the harmonic magnetic
fluxes within the transformer’s windings.
The Institute of Electrical
and Electronics Engineers (IEEE) prepared the guide "IEEE Recommended
Practices and Requirements for Harmonic Control in Electrical Power Systems",
which provides guidelines for the power quality that the utility must supply
and the users can deliver back to the power distribution system. The guide
suggests corrections in the system that are able to eliminate or reduce
specific orders of harmonics, such as line reactors and the use of a 12-pulse
converter front-end.