Total Harmonic Distortion and Effects in Electrical Power Systems
Total Harmonic Distortion and Effects in Electrical Power Systems
Total Harmonic Distortion and Effects in Electrical Power Systems
Introduction
The power quality of distribution systems has a drastic effect on power regulation and
consumption. Johan Lundquist of the Chalmers University of Technology in Goteberg,
Sweden put it best, stating “The phrase ‘power quality’ has been widely used during
the last decade and includes all aspects of events in the system that deviates from
normal operation.”1 This has been especially true after the second half of the 20th
century when
new types of electronic power sources caused distortion in waveforms of the power
system.
Power sources act as non-linear loads, drawing a distorted waveform that contains
harmonics. These harmonics can cause problems ranging from telephone transmission
interference to degradation of conductors and insulating material in motors and
transformers. Therefore it is important to gauge the total effect of these harmonics. The
summation of all harmonics in a system is known as total harmonic distortion (THD).
This paper will attempt to explain the concept of THD and its effects on electrical
equipment. It will also outline the low THD of the Associated Power Technologies (APT)
line of programmable sources and how these can be used to more effectively test
equipment.
Imagine a power system with an AC source and an electrical load (Figure 1).
Now imagine that this load is going to take on one of two basic types: linear or non-
linear. The type of load is going to affect the power quality of the system. This is due to
the current draw of each type of load. Linear loads draw current that is sinusoidal in
nature so they generally do not distort the waveform (Figure 2). Most household
appliances are categorized as linear loads. Non-linear loads, however, can draw current
that is not perfectly sinusoidal (Figure 3). Since the current waveform deviates from a
sine wave, voltage waveform distortions are created.
As can be observed from the waveform in Figure 3, waveform distortions can drastically
alter the shape of the sinusoid. However, no matter the level of complexity of the
fundamental wave, it is actually just a composite of multiple waveforms called
harmonics.
Harmonics have frequencies that are integer multiples of the waveform’s fundamental
frequency. For example, given a 60Hz fundamental waveform, the 2nd, 3rd, 4th and 5th
harmonic components will be at 120Hz, 180Hz, 240Hz and 300Hz respectively. Thus,
harmonic distortion is the degree to which a waveform deviates from its pure sinusoidal
values as a result of the summation of all these harmonic elements. The ideal sine wave
has zero harmonic components. In that case, there is nothing to distort this perfect wave.
Total harmonic distortion, or THD, is the summation of all harmonic components of the
voltage or current waveform compared against the fundamental component of the voltage
or current wave:
The formula above shows the calculation for THD on a voltage signal. The end result is a
percentage comparing the harmonic components to the fundamental component of a
signal. The higher the percentage, the more distortion that is present on the mains signal.
The limits on voltage harmonics are thus set at 5% for THD and 3% for any single
harmonic. It is important to note that the suggestions and values given in this standard are
purely voluntary. However, keeping low THD values on a system will further ensure
proper operation of equipment and a longer equipment life span.
APT power sources are measured for THD up to the 40th harmonic for the various
frequency outputs of the source (mains frequency up to 1000Hz). This ensures a low
THD value over the entire operating frequency range of the instrument. Utilizing an APT
source will provide a clean signal with low THD and isolation from local supply
interference.
References
1
Lundquist, Johan. On Harmonic Distortion in Power Systems. Chalmers University of Technology:
Department of Electrical Power Engineering, 2001.
2
Vic Gosbell. “Harmonic Distortion in the Electrical Supply System,” PQC Tech Note No. 3 (Power
Quality Centre), Elliot Sound Products
3
“Harmonics (electrical power).” Wikipedia, The Free Encyclopedia. Wikimedia Foundation, Inc. 4 April
2011. Web. 5 April 2011.
4
IEEE Std 519-1992, IEEE Recommended Practices and Requirements for Harmonic Control in Electrical
Power Systems, New York, NY: IEEE.